mill testing optimisation and targeting
TRANSCRIPT
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MILL TESTING,OPTIMISATION AND
TARGETING
D BAIRD
MILL TESTING, OPTTMISATION AND TARGETING
OVERVIEW
This paper is divided into three sections which relate to the TESTING,OPTIMISATION and performance TARGETING of cement mills.
SECTION A : BALL MILL TESTING
The various methods used for testing ball mills are described, especially mill axialsamplingtesting. Whilst the paper concentrates primarily on cement mills, thesetest methods can be equally applied to raw mills. Hence some of the examplesshown include raw mills.
SECTION B: CEMENT MILL OPTIMISATION
This section provides general guidelines on how to optimise the internalcordiguration of cement millsto suit the typeshnge of cements produced, Thesenotes include recent feedback on several different types of mill internal liners anddiaphragms.
SECTION C : CEMENT MILL PERFORMANCE TARGETING
This section examines the various methods by which the target petiormance foropen/closed circuit cement mills can be established using a knowledge of thefollowing:-
● Cement grindability data
● The results from mill inspections and axial sampling tests.
The results of the BCI cement millbenchmarking exercise are also reviewed in thissection.
SECTION A
BALL MILL TESTING
SECTION A
1.
2.
3.
4.
5.
6.
BALL MILL TESTING
CONTENTS
INTRODUCTION
MONITORING MILL PERFORMANCE
2.1 Mill Throughput Tests2.2 Power Drawn2.3 MU Product QudIty2.4 Feed Grindability2.5 Temperature2.6 Air Flow
AXIAL SAMPLING TESTS
3.1 Full Test Procedure for an Aid Test3.2 The “Quick Test” Method
AXXAL TEST FOR A CEMENT MILLPRACTICAL EXAMPLE
4.1 Mill Performance Before and After Tuning4.2 Background to Testwork4.3 Mill Inspection4.4 Axial Sampling Test Results4.5 Gas Circuit Tests4.6 Power Drawn ‘4.7 Medium/Long Term Optimisation
MILL INSPECTION AND MAINTENANCE
5.1 Lining Plates5.2 Diaphragms5.3 Meda5.4 Voidage Filling5.5 The Importance of Regular Mill Maintenance
and the use of Axial Tests
FINE MEDIA IN CEMENT MILLS
APPENDIX IA
APPENDIX HA
APPENDIX IIIA
APPENDIX IVA
APPENDIX VA
APPENDIX VIA
APPENDIX VHA
Cement Mill Faults
The Fineness of Samples taken immediately prior to the First ChamberDiaphragm
Essential Preconditions for use of Finer Media Gradings in ChamberTwo of Cement Millstial Sampling Curves for Cement Mill - Second Chambers - SieveResidues
Example of How the Volume Loading within a Mill can be tiected bythe Accumulation of Unground Material (Nbs)
Mass Balance on a Raw Milling Circuit
Example of the “Quick Test” method for PucialSampling Tests
1. INTRODUCTION
The efficiencyof grinding depends upon a number of factors, and a variation of one or more of.these causes deterioration of mill performance. If this goes unchecked very inefficient grindingoccurs resulting in a very poor quality mill product.
CarefiJ routine observation of mill residues and power used for the grinding process will showwhen efficiency begins to fall off and whether a thorough check on performance is necessary.
2. MONITORING MILL PERFORMANCE
In order to monitor a mill’s perfimrmnce, the following data is required:-
● Mill Throughput
● Power Drawn
● Mill Product Quality
● Feed Grindability
● Mill Temperature/Product Temperature
● Mill Air Flow/Cooling
Much of the above information should normally be recorded as part of the Works routineprocedures. When routiie data is unobtainableor is suspect then the following tests and checksmay be carried out.
2.1 Mill Throu~hDut Tests
2.1.1 Weigh Feedem
All too ofien weigh fkeders can give a misleadmgpicture of a mill’s throughput. Direct readingsof the mill throughput from a weigh feeder or totaliser are subject to possible errors in thecalibration of the feeder. Regular checks on the calibration of feeders in accordance withmanufacturers recommended procedures can reduce the degree of error.
A simple method of checking the accuracy of a weigh feeder is by measuring the weight ofmaterial over a known length of belt under steady feed conditions, knowing the belt speedenables the throughput to be estimated. Sufficient length of belt must be sampled and accounttaken of any cyclic variation in feed rate if this method is to be accurate.
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Chain tests can be used to check the accuracy of weigh feeders. After zeroing the scale, theweigher belt is loaded up with a set of chains which are calibrated to cover the weighing rangeof the scales. The scale indicator and recorder can be checked from a knowledge of the chainloading and belt speed.
2.1.2 Salt Tests
Another method for determiningthroughput is the salt t~ where salt is used as a tracer throughthe mill. Under steady conditionsa constant amount of salt is added to the mill feed and the risein chloride level of the millproduct is obsemed. Atypical procedure for a Salt Test on a cementmill would be as follows:-
●
●
●
●
●
●
Sample the mill feed and finished cement for approximately 30 minutes before startingthe test in order to establish control conditions.
When the mill is running steadily add accurately measured equal quantities of salt atregular intervals for a period of two hours. The addition rate of rate should beapproximately 0.5°/0of the mill output per minute.
Sample the mill product at regular intervals for up to 3 hours.
Anaiysethe samplesfor chlorideusing the chromate direct titration method. Determinethe purity of the salt added and determinethe chloride content of the clinker and gypsumfeed.
Plot a-graph of% Cl against time and note the steady average value to which thechloride level rises. (M) as shown in F@ure 1.
Determine the output by the following mass balance where:-
X = Chloride entering mill in kghnin
Y = Cement Mill Output
z=’ 0/0Chloride in the cement prior to salt addition
M= ‘YoChloride in the cement after salt addition ,=
From which y maybe calculated:
~ _ x. (loo - M.(NaCl/Cl)
M- 100.Z’
This test method is more complicated than belt sectioning/weigh offs and is also prone tosampling and analytical errors. Hence this test is not commo@y used. ‘,
2.1.3 Cement Weigh Off
In this test the cement mills product is diverted into an empty clean silo where it can beseparately packed off and weighed. For valid results, the test must be run for sufficiently longtime, i.e. at least 24 hours. Errors will arise if the silo used cannot be effectively emptied outbefore and afier the test due to build up.
2.1.4 Clinker Drop Tests and Volume Measurements
In cases where space allows for the collection of feed belt material, a drop test maybe carriedout by diverting the material through some form of by-pass into a pre-weighed dumper. Bycollecting the fed material over a known period the mill throughput can be estimated. ,
Another method which is not particularly accurate but which can be used to give a rough guideto mill output, is the method of measuring the fall in level of clinker in a feed hopper, whilst themill is running with a steady feed.
Samples are taken during the test to determine the clinker bulk density and the S03 level in theclinker and fished cement. An SOSmass balance then enables the gypsum addition rate to becalculated whilst the clinker throughput is estimated from the bulk density and fdl in volume inthe hopper. Errors arise in this method from level measurements and differences in the degreeof compaction and segregation effects which may alter the bulk density of clinker in the,hopperfrom that measured on the feed belt.
2.2 Power Drawn
The most usefid method of checking the power drawn by a mill is by taking routine readingsfrom an integrating kWh meter. Such readings are vexy often taken on a weekly basis. Spotchecks can be made by timing a number of revolutions of the disc of the kWh meter and applyingthe appropriate correction factor for the meter.
If neither of these tests can be carried out, then an estimate of the power drawn can be madeform ammeter readings. A knowledge of the voltage and power factor enables the power drawnto be deduced though such estiniates are often subject to large errors. From records of the millthroughput and the power drawn, the power consumption in kWh/tonne is calculated.
Gross Power =43 V.I Cos#’
where V = Voltage, I = Current, Cos ~ = power factor
4
Gross power is the power to the mill motor, Motor and gearbox losses are normally between5 and 10% depending upon the drive systemused. Hence the nett power absorbed by the chargein the mill is typically 90 to 95°/0of the gross power absorbed. See notes in section 3.1.
2.3 Mill Product (Mudity
When referringto a mill’soutput, reference should also be made to those quality aspects whichcan affect the output. It is normal to check a cement mill’s product for suxface area and sieveresidues at 90 and 45 microns. A record of S03 content is important as the form of sulphateaddition, whether it be gypsum or anhydnte can have a significant effect on mill outputs byaltering the grindability of the feed. Factors which influence the grindability characteristics ofclinker are summarised below:-
2.3.1 Clinker Grindability - Effect of Clinker Chemistry Variations
The following factors have been shown to have a detrimental effect on clinker grindability andhence cement mill output.
● A lower LSF clinker ie a higher proportion of CJ in place of CJ.● Lower free lime● High liquid phase● Lower gypsum addition● Higher clinker SOJ
With any source of clinker supply there will be natural variations in the clinker chemistry andhence grindability. If a mill automatic control circuit works correctly then it should enable
..additional output to be achieved during periods when grinding lower grindability clinker.
Figure 2 shows the relationshipbetween clinker chemistry and mill only power consumption fortwo closed circuit cement mills grinding OPC and SRC clinker when grinding to a constantsurface area of 306m2/kg. This illustrates the effect of LSF, C3S and C2S on clinkergrindability.
Hence, variations in the quality of clinker produced can have a significant effect on thepetiorrnance of any cement milliig circuit. This aspect needs to be considered when evaluatingthe performance of any mill circuit.
2.4 Feed Grindabilitv
Changes in the grindability of the clinker can tiect mill performance and so it is advisable tocany out grindability tests on the clinker at regular intervals. When carrying out axial tests ona mill, as well be described in greater detail later on in this paper, it is recommended thatapproximately 50kg of average clinker sample is taken for a grindability test to be carried out.
5
56
Cn 54
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52
50
22
1894
93
- Relationship between Specfic Power Consumption and CementChemistrv for two Closed Circuit Cement Mills
[email protected] Mill (S.R.C)
-... No.1 Mill (OPC)-...*.-““””......<%.-%.
●..*-..*%..-..“*------...%.-%.-
.-.#---.*.*--*.-”-*-...4”
...---”-”.*-*-.**.. .-““-- No 1..-.-””-” ....---a”-
------------- . . . . %*.- %..m------ No.1 Mill (OPC)..-----=------.....*.-{●-...*--..*-%...-
LSFNo.2 Mill (S.R.C)
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9032.5 33 33.5
The result of this test enables the actual mill performance to be compared with the theoreticalperformance and is usefid in showing how efficiently the mill and individual chambers areperforming highlightingareas of the millwhere the performance can be improved by alterationsto the mill charge etc.
Gypsum addition has a significant impact upon the grindability characteristics of cement.Additivessuch as limestonealso afkcts the cement grindability characteristics and the following“rules of thumb” can be applied.
+ 10?0
+ 1‘??0
2.5
Limestone addition reduces the B.C.I. grindability figure by approximately 2.5%
Gypsumaddition can equate to+ 5% mill output u + 12m2Agsurface area at the sameoutput.
Temperature
Problems arise when hot clinker is fd to a millor the mill’s cooling system, i.e. induced draughtor water injection do not fimction properly. Thermocouples can be used to monitor both feedand product temperatures and the latter can be used to control a water injection control loop.
2.6 Air Flow
For adequate ventilatio~ the quantity of air through an open circuit mill should be 2-3 volumechanges of air per minute. Here the term volume refers to the free volume above the charge inthe mill and estimates are made using a standard temperature of 11OdegC.
Whh more recent and larger closed circuit mill installations sufficient ventilation for up to 7 airchanges per minute is oilen provided. However, at this high level of airflow, there is a risk thatthe cement cmied out of the mill to the filter plant is much coarser than the finished product.Thus mill airflows are usually reduced to a level below 7 air changes per minute.
There are a number of difficulties involved in making measurements of air flow through a mill.Measurements taken around the ducting leading to the dust filtering plant can be meaninglessif the millhas poor seals as the resultant air flow figures are more likely to indicate inleak ratherthan ventilation air flow.
Measurements recently taken at one UK Works indicate that whilst the air flow through thecement mill filtering plants was adequate only 200/0of this air flow was actually being drawnthrough the mill owing to poor mill outlet seals. Pitot measurements in this region suffer fromproblems of blocking Pitot tubes due to dust and humidity. To measure the quantity of airactually flowingthrough the mill, an anemometer can be used at the mill inlet with the feed rateoff the mill. Draught irdcators maybe provided at the mill inlet to give some rough idea of thequantity of cooling air through the mill. The differential pressure across a cement mill beingtypically40-60 mmwg (for open circuit mill with moderate cooling airflow) and 75-100 mmwg(large closed circuit mills with high mill ventilation).
7
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Samplingp(obe
Vent pipe
\
$
Hood
l!i’R@-13usttrap Drying Ein
Mill
Inlet pipe
&Thermometer
~ I?otometer
: !$Zodetector-lJ-1“N20bottle
~IGURE 3 NzO Tracer Method
Another method of assessing the air flow through the mill is to use nitrous oxide into the millinlet as a tracer. The concentration of N20 in the exit air is measured using an infrared detector.FQure 3 shows the arrangement for N20 tracer testing on a mill. A similar technique has beenapplied, in a limited number of cases, using Carbon Monoxide as the tracer gas.
If either the routine performance data or any of the above tests show that there has been adeterioration in the perfiorrnanceof a particular mill, then it is advisable to carry out a moredetailed examination into the internal state of the mill as well as an axial test.
3. AXIAL SAMPLING TESTS
An axial sample testis means of determining how well a mill is grinding along its length. Sucha test can highlightarm withinthe millwhere the grinding is not being carried out as efficientlyas it should be. When coupled with the results for the grindability test it is then possible tocompare the overall performance of the mill as well as individual mill chambers with thetheoretical performance predicted.
To ensure that correct conclusions are drawn from any axial sampling test, it is essential thatrepresentative samples are taken throughout the mill othenvise results can ofien be misleading.The full test procedure for an axial sampling testis as follows.
3.1 Full Test Procedure for an Axial Test --
● Sample the mill fd and product under steady conditions for approximately 40 minutesprior to stopping the mill. Record the mill output and power consumption
In the case of a closed circuit milling system. take samples of the mill and separatorcircuit before crash stopping in order to construct a mass balance and to estimate theseparator efficiencies. The mass balance can be used to estimate the mill recirculatingload and allow any rejects weighing device to be cross-checked. Mass balances shouldbe based on 45 micron residues for cement mill circuits and 90 micron residues for rawmill circuits. Appendix VIA shows a typical mass balance/separator efficiencycalculation for a raw mill circuit.
● Stop both the mill and the f~ simultaneously. Ifthe fd is stopped before the mill thenthe residual material within the mill will be ground finer than normal and this will makethe overall mill efficiency appear higher than it actually is.
● After allowing sufficient time for cooling, enter the mill and take axial samples. Divideup the mill internally into sampling points typically 0.5-1 m apart. Samples should alsobe taken at the diaphragms. At each point on the axis, an average sample should betaken of the material along a line at right angles to the axis of the mill. The materialshould be taken from points a few inches below the ball charge and not from the surface.In the case of a three chambered mill, take larger samples in the first and secondchambers than in the third chamber.
9
CLINKER GRADE mm SURFACE AREAB.S. SIEVE mz I kg
.$ +19 0“2
+“y
‘8 - 19 +9’5 0“3
~3“~.
–- — -9”5 +4.8 0.60 + 163’8 +7 -4”8 +2-4 1“1
‘m-7 + 14 -2.4 +1”2 183
-i4 + 25 -1”2 +0”6 3s46
-25 + 52 -0-6 +0”3 6.42
-52 +100 -0’3 +0 .15 17”8
-1oo -(j .15 Measured directly(Lea Nurse)
PIGW 4 surface Area of Different ClinkerGrades
Ensure that allsamples taken are filly representative ofmatetial tittin the mill. Forexampleif nibs of unground material are present, sample these and do not simple discard them. Takelarger sample quantities if large quantities of nibs are present.
● Allow the samplesto cool before measuringthe surfice area in the case of a cement mill,ardor sieve residues in the case of a raw mill. The coarser samples whose sutiace areacannot be measured directly must be graded and their surface areas calculated fromFigure 4. For a cement mill, a check should also be made on sieve residue of samplesthroughout the mill: of particular importance are the residues at 45 and 300P m, and at2.36mm sieve sizes. (see Appendix IVA)
● Measure the height above the charge and calculate the VOvolume loading from Figure5. From this the weight of me&a in each chambercan be calculated using a value for theaverage media bulk density, if none is available the use a media density ofapproximately:-
4.3 t/m3for Chamber 14.5 t/m3for Chamber 2
Ifa third chambered mill is tested the media density figures are 4.3,4.4 and 4.5t./m3forchamber 1,2and 3 respectively. Figure 5. is an approximation to the formula shown inFigure 6. Normally a computer spreadsheet approach should be used to provideconsistent calculation method with the accurate date for each mill.
The height above the charge is best measured with the filling slightly run down,otherwise a false high value for volume load willbe obtained. In addition large quantitiesof unground nibs can cause the charge to “bulk” hence giving a false impression of theactual volume loading in each chamber (see example in Appendix VA)
● Using the power equation, calculate the power absorbed by each mill chamber and for..the mill overall. Compare thk with the figures obtained from the kwh meters.
Nettwhere
Note:-
kW = 0.2846 D.A.W.N.D = Mill diameter inside the lining in metresA = 1.073-J where J is the fractional volume loadingW = Weight of media in tonnesN = Mill speed in rev/rein
nett kW = Gross kW x n where n = 0.95-0.9 depending on losses inmotor/gearbox etc.
11
0-s0 0-55
FIGURE 5 Ball Mill -
0-60
Volume
O-65
Ratio
Loading
0-70 0-75 0-80 0-85
FIGURE 6 VOLUME LOAD FORMULA
H = Height above charge (m)
D = Mill internal diameter inside lining (m)
% Volume Load = ~“
[ 1(0.25.Cos(2m(WD - 0.5))) - ((HID - 0.5). (S2)))
,,
The factor of net/gross power is very important since it can give an indication of what ishappening inside a ball mill. Some typical factors found with different mills areas follows:
Net/Gross Factor Mill Tme
0.94-0.95 Modem efficientmillwith central drive, efficient gearboxand drive motor. Good material flow through millwithout hold up and charge expansion.
0.93-0.90 Ghth gear driven mill of older design with higher drivesystem losses
0.88-0.89 Check to see whether mill could be running withminimum powder hold up causing charge to pull higherkW.
Generally, if the net/gross ratio exceeds 0.95-0.96, then it is likely that the volumeloading estimate is affectedby charge expansion due to hold up of material or excessivequantities of nibs etc. This condition is very common in the following cases.
Cement mills using Fine (25-15rnm) media in chamber 2.
First chambers of cementkaw mills with coarsehrd feed materials, worn ballcharge and liners giving poor crushing action.
Inadequate ventilatiotilocked diaphragms causing excessive material hold upwithin the mill.
● Plot axial graphs as follows:-
(a) Cement Mill
Surface area versus position along mill axisResidue versus position along mill axis
(b) Raw Mill.
Residue versus position along mill axisIfthe volume loadiigs vaxysignikantly then it can be usefid to plot cement surface areaversus the cumulative power drawn along the axis of the mill. However, current BCTCguidelines put more emphasis on the shape of the cement.lmeal residue curve changesalong the mill axis. Appendix DA gives details of the recommended sieve residues whichshould be examined when inteqweting axial sampling test results.
The axial samplingcurves should show a steady rise in cement surface area and an evenreduction in cement (or raw meal) residue along the axis of the mill chambers.
Graph contains any flat sections or sections where the rate of surface production is lowas indicated by a shallow slope of the graph, then this indicates areas of the mill wherethe efficiency is low due to:-
Incorrect ball size
Insufficient charge
Blocked diaphragms
The cement/raw meal residue versus position along mill axis curve is usefil for showing thefollowing:-
Whether or not the material leaving say, chamber 1, is sufficiently fine for finegrinding in chambers 2/3. See the guideline figures for cementhaw mills givenin Appendix 11A.
Whether or not there is any accumulation of oversized materkd (ie nibs orspitzers) in the chambers due to coarse feed size, inadequate first chambercrushing, classification of nibs by classi&ing liners in chamber 2 etc.
Whether or not the secondhhkd chamber media is sufficiently fine to allow asteady reduction in 45 micron residu-ein a cement mill secondhhird chamber.
● Calculate the gross power drawn by each chamber of the mill and work out the grosskWh/tonne figure for each chamber. Compare these results with the guidelines given inSection B.
● Measure the media grading in chamber by the following means:-
● First Chamber
Remove media from the charge, record its size and number of balls in each sizerange 90,80,70, 60mm etc. Try to measure as many balls as possible with aminimum around 50. Take 2-3 measurements in the first chamber.
This will only give a rough guide to the media grading, but can help to decide ifthe charge is say, too worn and in need of regrading.
● Second Chamber
When taking the axial samples it is often convenient to take samples of the mediaat the same time. When taking every other sample in a mill with a classi&ing
15
lining the sample and media can be dug out of the mill together. Load thesamples into, say, cement bags and remove these from the mill.
Sieve off the material, sample and record the total weight of the media. Countthe number of balls present and estimate the average ball weight. Plot a graphof average ball weight against position in the chamber. If the classifying liningis working correctly then obviously there should be a gradual reduction inaverage ball weight along the length of the chamber.
● Cany out a sii grading of the fd clinker. This is important when determining the sizeof media to be added to the mill’slist chamber. Usually the maximum feed size dictatesthe maximumball sii that shouldbe added to the first chamber of the mill, for example,for 19mmcIinkerthe typicalmaximummedia size is approximately 90mm. If the clinkerproves particularly hard to grind then media up to 100mrn size may even be necessaryin the extreme case. Normally one varies the proportions of 90mm media in the chargeto suit different types of cement milling mndhions and different mill configurations.Please refer to the media grading guidelines given in Section B.
3.2 The “(hick Test” Method
tial sampling tests are only of any use if the results are correctly interpreted and acted uponwith a short/medium/long term action plan formulated.
To have any impact, the results need to be reported as soon as possible after the tests have beencarried out.
To speed up this process, BCTC use a “Quick Test” method which fmtures
Taking the minimum number of essential samples.
Carries out the minimum analysis based primarily on the key sieve residues.
The “Quick Test” procedure is not rigidlyfixed and very ofkq the number of samples taken mayvary to suit milling condhions. for example, usually 3 samples would be taken at the inletmiddle and before the diaphragm in chamber 1.
If there is evidence of nibs accumulation prior to ‘the intermediate diaphragm then it maybeusefid to take an extra sample at O.5m prior to the diaphragm. This shows whether or not thecoarse material at the diaphragm is common throughout the chamber or is due to build up.
Another usefid type of test which can be carried out is the “Quick Test” applied to a mill inwhich the mill output has been “pushed” prior to the crash stop. This method is particularlyuseiid when testing raw millsto determine what the limiting factor on performance is. Care hasto be taken not to push the millf~ rate too Ibr if the circuit is unstable. However, some useiidMormation can be shown up by the test. A recent test on a raw mill showed that as the f=d rate
16
was increased, the material leaving chamber 1 coarsened up and nibs started to buildup at thediaphragms. These nibs then started to blind the diaphragm slots and this resulted in coarsematerial spilling into chamber 2 via the central ventilation grid.
Solution - Coarsen up chamber 1, charge by adding extra 90mrn media andreducing 60/70mm media quantity.
,. Reduce size of central vent grid to permit higher volume load andto cope with charge expansion.
4. AXIAL SAMPLING TEST FOR A CEMENT MILL
Practical ExamDle
The following example demonstrates how the results of axial sampling tests can be interpreted,acted upon and improved performance obtained.
4.1 Mill Performance Before and After Tuning
Mill Absorbed Outr.)ut B.C. ConventionalCase Power (kW\ tQl mill efflciencv 0/0 M. S.P.F.
Before 2714 65.3 138 1.021
After 2830 70.7 144 1.061
The BC conventional mill efficiency and M.S.P.F. (mill surface production factor) are bothmeasures of the mill efficiency compared with the BC cement grindability test estimate. Thesefactors are described in section C.
4.2 Background to Testwork
The closed circuit mill is a USA mill equipped with a large conventional separator and largeelevator transport capacity. This design has been proven to give a good efficiency when usedin conjunction with grinding aid.(see sections B and C)
Representatives samples of clinker and gypsum were taken over a period before and after thetest. These were used for cement gnndability testing at BCTC. The test results showed anincrease in cement grindabilityfrom 125°/0previouslyto the current level of 129°/0. This increasewas due to higher C# levels in the clinker and alone would have accounted for 3°Aloss of milloutput .
17
The plant urgently needed to maximise the cement milling capacity to cope with high seasonaldemand. This prevented stoppages for major mill changes and the aims of the testwork were asfollows:-
Maximise output by means of minimum changes to ball chargeiintemals withoutmajor internal replacement.
Obtain output gain NOW!
Identi@ mediudlong term plan to optimise mill petiormance.
The mill was equipped with a 3356Kw motor but was limited to drawing 2714Kw by volumeloading constraints. There were problems with the design of the stepped chute feeder. Thisregularly backspilledclinkerlgypsumat the mill inlet and thus limited chamber 1 volume loadlng.
4.3 Mill Inspection
Details of the mill internal inspection report are shown in Table 1. The key points areasfollows:-
a)
b)
c)
d)
e)
9
d
The mill produces high surface area cements at 391m2/Kg (3800 cm2/gBlaine)
The millcritical speed is very high at 80.79?! compared with a typical value fora modem mill of 76°/0. The discharge side of the intermediate diaphragmfeatured a915mm diameter outlet. Thk limited the volume loading to around35’%under static load conditions.
The mill was equipped with wave type liners in chamber 1 compared with ournormal recommended liners i.e. step liner or Lorain bar lift liner. However, itwould be unwise to change the liner to these types as the lifting action couldprove too severe due to the high critical speed:
The interm~late diaphragm was badly blocked by small broken media with only2.09V0 effective slot area. Cleaning of the slots could increase this to 5.27V0which is acceptable.
The first chamber charge contained worn undersized media which neededregrading to avoid it breaking up and blocking the slots.
Material levels in chamber 1 showed the chamber to be running very light ofmaterial.
The first chamber was charged to 37.3V0volume load but this included someexpansion of the charge which is allowed for by using a lower media density of4.097 = t/m3 in chamber 1.
— —..-m _=-
MILL INSPECTION
, *TIONS- M~~l sampling teaL13r7/94
$ b?&RE&:- E%AMPLEMILL NO.:- Cemant MN No.2
NOTE5- Before charging,the mill drew 3640 HP or 2714 kw.Approxtmate)y10-12 stons of I?Wd@ was addad to Chambar 2 after the teat.Mediiedded wee W4tol.5inchapprox
MILL MOTOR. - 33S6KW MILL SPEED (RPM)= 17.42 M53wrWt
MILL MOTOR:. 4530 HP MILL SPEED (RPM)= f7,44 Mlnakeed for CbfNMNMEAN INTERNAL DIAMETER= 3.622 m CRITICAL SPEED = 21.69 rpm
PERCENT CRITICAL=0h6rnber 1
60.79 %
-==x-—
EffSCtii LSWthm . $. Wmerl@a cftdal mill length. 30.39 %Internal tiameter m s Total chambarhsngthm= lt.090Internal volume m3 = 3602
Liner type.- Wavelinerwith nomine16U-65 mmprofiiencwwrn tounder(%3mm.culditikrndlinera- Liners inthefirat 7rmwareworn togrowadprdite.
WPhfwmu=mk 1 OuM—-.——
Diaphwn ty*.- Modem IWffergratesinatalled on~iiftertype~ support~.Circumferential dote with self clearing prcfle which ara partially worn.The outer segments surfaces were @ttadThe d@ragm has slotted gratea on its dwharge *.
Ixaphm e- Tha outer ssqments are badly bkxked by srnall/bmkan made end the slda ~denibbad duringthe inapectlonHwew,tha methcd IX cleaning these alotaallowsthetrap macha to drop into the charge and reblockthe d@ragm,There is a signikant quantity of small rnada m the first chamber Wctrwill tend to braak up and reblock the mtennadii diaphiagm.
Dk@wagm SW a&e8 and area:-Slct lengths:- 12 eegmerbWTER-Twtn ekXe mm 1!50 125 65 60 140 M6 156
115 1s0 165 fro 476 17!3 185
Sloteizesmm= 6 6PeftwY &%w M & Jdia.
6 0Cond!tii of StOtS=Percentage blookage=Maxtmumslot area without Mookage =-i@Ot SfWt With_=
0.3377 m20.01s9 m2
1651%
6
INNER.Twirlemsmm lm 115 ?30 140 150E 115 105 190 160 I@
Single elota mm 145 150 It%S&tsizesmm= 6 e 6condition of elota = PwtkMyt&d Lw LmediaPercentage MOckage=Mexlmum slot area without bbckage = 0.2570 m2ERecuve slot area with bbckaga = 0.2165 m2Total slot length m=Diaphm@rneffectii cross sectional ar6a (w@ 11.262 WEIGHTEO MEAN SLOT SIZE= 6.(H) mmDmphragmelc4 areesaaparagnt c#CSA= 5.27 % If ail slots are clear.
2.CQ % Wkh aiota bbckage.
Maximum slot aim= 6mm Minimum slot eizw %InmRacammanded slot size= 6 mm
DISCHARGE OF INTERMEDIATE DIAPHRAGM IN CHAMBER 2:-lNNER-Slot sizes mm = 6 6
waaIa L.e 6
Condition of slots=OUTER-Slot SiZeSmm = 6 6Conditionofslota= &e4Y46%bto&dbY bmkdeldrJiR
l~~mvcentralwgrtd————-.Diameter Qfcentral vent grid= 670 mm Meeheb= 25Areaofcerrtral wwgridm2= 0.353Minimum heiiht abow charge if media is aama level as wnt grid (m)=Voktmebadat whiimedi aissameteve ieewmtgnd%= XK) lb
Medh gradirrg-chambe$ 1—-—Sectoc- outlet ——.— —
Oe#eke Nodmm balls
—— —z
100 0m oso o66 3
2:
:: 3w 356 450 0
0z 23530 :
-z ;
T@alNod x)
.—rim SALL 62.0SUE MM
Intd
No.ofbatle
o03
120762
73.5
OVERAUCHAMSER 1 MEAN BALL SUE=
2.230 HID= 0.56s
67.8 MM
corrMkmdrr@te:- Char9econtainaebniflcant cwentiiy of fine media and naada wmening.The average ball size should be Increased to S0 Guidelines.Sas DAWN Cabuwrxrs Wihirskateathetaomechergeexgemk mhaaomurud.
tab
-totat=
46mmmperaegmed
emm
3120mmpereagment
450QPaf-
6.m nun
6.00 mm
TABLE 1
-Mullnlet— ——...———
3 Diameter d inlet insole Iinmg (m) =Diamaterof inletcone beforeendwall Wars (m)=
0.80.6
~ Minimum hewghtabowJchaqe if media ia same ISWIIaa inletlnmnbn (m)- 2.296 WD= o.m6volume bad*whichrnedii iaeemelmml eainletcone%= s.m %
-al M.— —Rmaida= +20mmabowcherge (25Uoftddchafnb@Middle =%me level (50% C4chamber)Fsllingside=Eabmated-120 mm E&ow charga(25% dchamb@)(XmraIlswags matwialMight= -25 mm belowchew.
chamber 2-aaa—
Ethxtiw Lengthm = 7.72 ~ *M till w:- @41.81internaldiarnatarm = S636 Aw!rags-mirw3.82m; rnmF3.865 m.llnas=2 by 3W mm.Inbsmalvohmmm3 = 8922
Mnertypw- wmlypawith eomm WvspldBa.Lineris In goodcondti.
MWm w- Urcumfafeniiel aids d Mersnt design tointsnnediste d@MsagrnThareiea soliiaecbrm whhnoabtain c41.69mdwwtar.There iss gap baksan the solidcenterandthe inrmarsegments.
~ -“ The outerrowof segmentsare badlyblockedbybmlrenfundemkadmsdkTtraarSaare badiywzfn OapecWylnttra innerSOgmOntO.
~e~y Siza Uld am-12aqmenls
OUTER- H mm 75 mm sldw 28123mms~ 22
SMsimemms 16 16 18 17 Ie18
cendiidel@s= PslUly:&Jed by Ln rLs.Percentage biockage=?ilaximumslc?ar eawithoutbbckage= O.mm m2Effac4kalatarea with bkmkage= 0.1814 m2
S&t lengitw. 12 SegmmisINNER- el@emm 60mm alda= 2 75mmekHa=
85mm slots= 2 120mm ebts=70mm alOb@ 1 126mm alota=
ac4@zsemm= 15 14 12 1214 14 :: 13 3s
302
48
1s
cendiidakAa= PadwaybY bdummneli midia. -Percentageblocke@=Maxh-numslatareewithoutbbckege= o.6m9 M2Effactkask4 areawittrbkckega= 0.3643 m2Diaphragma?hxUVScme eectimsl area (m2J= 11.557DiephragmaJ@areaaaaparcantc4CSA= 14,82 % mardmumifallatdesre-. ‘- -–CX@wa9mslc4araeeesrnmmtafCSA= 6.45 %aOtualwkh blOckadakASMaxtmui SW size= 18 mm MWmumakxaiza= 12 mmRammmW@eld- 8mm
Chemk20utletd@hragm antralvef4gftd
okmeterd-Ventgnd= @ii CeMar-nd @lceble
MedagmMgamk 2—.. -------- —. -6a@$x- Iniet MirtrSa.— —
BallSize Nod Nodmm belle
—.
m o 065 0 0m o75 : 0m o 066 0 0
z : ;50 245 ;40 1: e
: ; 225 0 2320 0 4
ToteiNod 27 im
bstb—.-—MEAN 6ALL 38.7 30.2S&X MMOVemllrnaanbaneize= 20S4 mmconditiond madiw- Themedksgmdin ginctudeaewnewwaizedmediid@m40mrnslze.
Lightcoatingon rnedWtinin@abutdiephmgmwe moracosted.~w
23.8
tMrSltc+lsmbUrnakdal ls!ml=-12mm bek4vcharge.
l%odbek~hout-==-==============
Beltspeed(fLWn)= Weightd Ssmple(lba)=6ettspeed (rn/min)= 4H Weight C4ample= n’:Feadtonnagatph= 73.19 Bettsectionlength(m)= 25Fssd_st@r* 80.70 Rlehta= 8t%30cln@
Feed sisegmdlng-===-.= ==S==
Size(ii)
21
0.750.s
0.375
Idnue 0°%%
AUTHOR-D. BAIRO VEmiolt-1 DAIE-le/7m4
- Wekwm)— —-50.8025.40 1.:19.05 0.41;7CJ 0.6
o.e6.35 1.1
20.1
waigMlb
tabP 6060mmpsra8gmawt
19.27mm
,——- — --
TABLE 1- CONTINUED
h) There was no evidence of nibs leaving chamber 1 so the first chamber was doingadequate fd siie reduction despite its worn charge. Hence to maximise the mill output,it is necessaryto maximise the charge in chamber 2 as far as possible. Hence some 10,9tonnes of fine me&a was added to chamber 2 to give 37.3’XOvolume ioad. To permit thishigher volume loading in chamber 2, a steel retaining ring was fitted to chamber 2 sideof the intermediate diaphragm. This reduced the vent grid diameter from 915 to 675mrn.This would permit the volume loading to be raised fi.u-therto 38.9?40using 25-20mmmedia once this media was available
i)
j)
k)
Chamber 2 contained some media of 38rnm size and above. To maximise thehe grindingpdormance of the charge, it is necessary to use a finer top up sizeof around 25mm.
The inlet step chute feeder designwas modified to permit higher volume loadingat a fiture date if required.
The axial sampling test results (see section 4.3) and the mill inspection showedthat there was no need to add extra media to chamber 1 in the short term. Oncechamber 2 charge is maximized the mill still had spare motor capacity to permithigher volume loadings in chamber 1 and 2.
Chamber 2 has a worn Magotteaux classi&ing liner. Unfortunately theclassi&ing Iiner design tends to reduce the mill internal diameter and fhrtherlimits the maximum power that can be drawn. The mill is effectively“overmotored” for the size of its shell.
4.4 Axial Samding Test Results
The attached data (Table 2) and axial curves (Figures 7 & 8) show the following:-
a) The size reduction in chamber 1 is good with acceptable residues in the sampletaken prior to the intermediate diaphragm.
b) The 45 micron sieveresidue reduction in chamber 2 is only average and the millwould benefit flom a higher proportion of finer media of 25-20mm size. Pleasenote that USA cement mill ball charges do not tend to be as fine as UK ballcharges using the BCTC guidelines for fine media. Section B gives fbrtherdetails. The USA mills operate ve~ efficiently with high circulating loads andit is known that very fine media (15-17mm size) tends to increase hold up withinthe ndl.
Once the millchamber2 charge was optimised, fi.u-theraxial sampling tests werenecessary to decide on what action is required concerning chamber 1 chargegrading.
21
24
20
16
12
8
4
0
Fiqure 7 - AXIAL SAMPLING TEST CURVE - ExamplePercent Retained on 2.36mm, 1.18mm and 300um Sieves
~–k’ v v’
— —3 6 9 14 19 24 29 34
Distance from Feed Spout (ft.)
D 2360pm + 1180pm o 300pm
1
70
60
50
40
30
20
10
Figure 8 - AXIAL SAMPLING TEST CURVE - ExamplePercent Retained on 45pm and 90Bm Sieves
.
—
u
3 6 9 14 19 24 29 34Distance from Feed Spout (ft.)
/
~ 90pm + 45pm/
TABLE 2
AXIAL SAMPLING TEST - SIEVE RESIDUE RESULTS ,
0/0Cumulative residue at sieve size:-
Chamher 2.36mm 1.18mm 300mm 90mm 45mm
1 InletAl 12.9 17.8 24.8 46.5 ‘55.5
InletB1 2.2 4.3 13.6 46.7 52.1
OutletCl 0.9 2.6 11.5 39.7 58.5
2 InletM 0.8 1.9 9.9 38.7 58.1
MetB2 0.1 0.2 3.7 32.1 56.0
InletC2 o 0.1 1.2 25 55.3
InletD2 0.1 0.5 1.2 21.5 54.1
OutletE2 0.3 0.4 1.3 20.0 53.7
Comparisonofmatial fnenessleavingChamber1
0/0CumulativeResidueSieveSize SampleC1 I BCTCGuideline
2.36rnrn 0.9 1
1.18mm 2.6 6
3ooum 11.5 20encematerialleavingChamber1issuillcientlyfmetobehandledbyfmemediainChamber2.
5 Gas Circuit Tests
Measurements of the mill airflow plus inieaking air levels showed that these were below theoptimum. The poor design of the stepped chute feeder and high inleakingair over the mill dischargeresulted in low airflow through the mill. Hence these areas needed to be improved by redesigningthe step chute feeder and sealing up the mill discharge hood, bag filter doors etc.
4.6 Power Drawn
Table 3 shows the mill power drawn estimates for both chambers, before adding the extra 10.9tonnes media to chamber 2.
The mill first chamber was drawing approximately 12.33 kWh/tome cement at 65.3tph output.Op_g cbber 2 charge allowed the output to be increased to 70.7tph which is equivalent to11.39 kWMtonne cement. Both figures are higher than the recommended 9-11 kWhAome forchamber 1. This data plus the axial samplingtest data showed that chamber 1 had adequate chargeto handle higher outputs. The mill was lacking power for fine grinding in chamber 2,
24 .
TABLE 3
MILL VOLUME LOAD AND POWER DRAWN
CHAMBER NO. I 1 I 2 I TOTAL/AVEI
Effective Length 3.37 7.66 11.03r
Percent of total length 30.55 69.45
Diam. inside lining (m) 3.7900 3.8360
Mean Diameter (m) 3.82
Mill Speed (rPm) 17.44
Criticid Speed (rPm) 21.59
Percent Critical Speed 80.79
VOLUME LOADESTIMATION
Ave Ht above charge (m) 2.2767 2.3925
0.601 0.624
0/0Volume load 37.27 34.41 35.28
Chamber Volume (m3) 38.02 88.53 126.55
Media Density (t/m3) 4097 4.288
Media Volume (m3) 1417 3046 44.63
Media Weight (t) 5805 130.63 188.68
Chamber Free Volume (m3) 23.85 58.06 81.91
MILL POWERCONSUMPTION
Chamber nett KW (DAWN) I 765 I 1813 I 2578
Total gross KW (Meter) ! 2714i I
Ratio nett.lgross power : 0.950I I I
Estimated power losses (%) 5.039
Gross KW per chamber I 805 I 1909 I 2714
MILL OUTPUT I I ICement tph 65.3
Gross kWh/tonne per chamber 12.33 29.23 41.56
Cement M2/Ktz 391
I
25
FIG.9 BLUE CIRCLE INDUSTRIES - PLYMSTOCKWORKSTOOL FOR CLEANING DIAPHRAGM SLOTS
Insert end into slots and level outblockage material.Don’t allow nibs etc. to fall back ontocharge - put downsheets /collect any material falling
bags toout of slots.
SECTION A - A
I4 b 65mm barI ● ‘welded tp
~—6mm plate
m
plate
80mm
J A*—90mm~
+230mrn+’30mm----575mm ●
30mm
4.7 Medium/Lonz Term OWimisation
4.7.1 Mill Diaphragms
a)
b)
c)
d)
Due to the breakage of small media in chamber 1 and the lack of any nibs trap,there is a high tendency for diaphragm slot blockages. Solving this problem iscritical if the mills are to be optimised and if the circulating loads are increased.Hence a correctly designed nibs trap is required with the following features:-
Locate aerated nibs trap in airslide close to elevator discharge.
Use double slide gates to discharge nibshroken media.
screen out nibshoken rneda and return cement to elevator using a smallvibratory screen beneath nibs trap.
Despite the diaphragm blockage problems, the material levels in both chamberswere low. Hence material flow through the mill in is not a major problem atpresent. It could become a problemif the slots are not kept clean at higher outputs.
The outlet diaphragm design needs to be improved using a modem Pfeiffer stylediaphragm with 8mm circumferential slots. The replacement diaphragm shouldhave a cental ventilation grid of no more than 675mm diameter to allow up to38.9’%0volume load (speci& 4070 volume load in any enquiry). The presentdiaphragm has oversized slots of 12-18mm size.
Use BCI diaphragm denibbing tool for cleaning diaphragms as per Figure 9.
4.7.2 Liner Plates
a) When liner plates need replacing in chamber 1- use a simple wave type liner.
Note:- this recommendation is not the normal liner recommendation (see sectionB for details) but is necessary here due to the untypically high mill critical speed.
b) Chamber 2- the classifying liner reduces mill internal diameter. When this linerrequires replacement - consider a simple wave or ripple profile liner to increase themill internal dkrneter and maximise ball charge.
4.7.3 Media
a) Increase volume loading to 38.9?? in chamber 2 using 25-50 mm media top up.Use typical USA mill me&a grading as per section B guidelines.
27
b) Top up charge in chamber 2 with 25mm instead of 38-50mm media in the fhture.
c) Regrade chamber 1 and review grading after retesting mill with extra charge inchamber 2. See Section B for chamber 1 media grading guidelines.
5. MILL INSPECTION AND MAINTENANCE
During an axial test or as part of a programme of routine mill maintenance, it is usual to carry outan examination of the mill internals. Regular inspection of mill internals is an essential part ofmaintaining optimum performance. A carefid inspection will help to support the findings andconclusions of an axial sampling test. The two processes should always be considered together.
Special attention should always be paid to the following points during an inspection:-
5.1 Lhinp Plates
Examine the plates for any $gns of wear, cmting and breakage. Normally one expects a reasonablylong life from lining plates and it is important to keep an eye out for any unexpected wear orbreakages so that suppliers quahty can be checked.
5.2 DiaDhrapms
Examine the diaphragm for any breakages, wear and blockage. If the diaphragm shows signs ofblockage thenit is important to determine what has caused the blockage as this can tiect whataction needs to be taken, for example, the presence of nibs could indicate the absence of sufficientquantities of larger size media.
5.3 Media ,,
Inspect the mdla for wear and breakages. From the results of the axial test ball grading, check the-g at the s@ed grading if this exists and from this determine what sized media shouldbe added or whether or not the charge should be re-graded. Note any differences between themedia levels in each chamber since too great a step up in level can cause hold ups along the millunless a lifter type diaphragm is used. Note any coating of the media due to poor mill ventilationor moisture.
....
5.4 Voida~e Filling
During an axial test check whether the feed material fills the voids of the balls. Over-filling mayindicate diaphragm blockages and a restriction to flow whilst under-filling could be causingexcessive ball wear and heat generation.
28
Ifa millhas been brought down for examination due to a specific fault, for example, its output hasfidlenor nibs are present in the product, then there area number of possible explanations for this.Appendix I lists some common cement millfaults together with their possible causes and remedies.
5.5 The Importance of Remdar Mill Maintenance and the Use of Axial Tests
Figure 10 illustratesthe importance of regularly maintaining the correct level of charge in a mill byindicating what happens when the charge is allowed to run down in a d~ raw mill, over a periodof time. It can be seen that as the power drawn by the mill has fallen due to wear on the charge,the tonnage has fallen and the kWh/tonne have risen.
Approximately f40,000 per annum could be saved on power costs by restoring the mill to itsprevious performance. In additionto power savings there would have been additional benefits dueto increased raw meal availability.
F@re 9 demonstrates the point that maintaininga millat its optimum performance requires routingrecharging of the media with periodic regrading to remove tramp or undersized charge. Changesin milling systems take place gradually over periods of years. Initially, lack of attention tomaintenancewill save repair costs but then gradually starts to increase operating costs as the plantefficiencytails off. The plant may then be faced with the problem of having high revenue cost formill internals tier a period of say, 5-7 years time when Iinerskiiaphragmsneed a major replacementprogramme. Cement/raw mill performance can be maintained closer to the optimum by havingshort/mediurn/long term action plans with planned replacement schedules. Guidelines for theselection of mill internals (liners, diaphragms, ball gradings) are reviewed in Section B and theseguidelines should be consulted before ordering replacement mill internals.
By regularlymonitoringthe mill’sperformance and by carrying out axial tests from time to time, itshould be possible to determine the optimum performance from a mill. In addition to providinginformation on how efficiently the grinding process is being carried out within a given mill, axialtests also enable an insightto be given into the effect of other process changes which can affect themill’sperformance. For example, the effects of feed pre-crushing, changes in gypsum addition rateand feeding cooler clinker to cement millscan all be investigated more thoroughly by means of axialtests.
There is a tendency to only consider carrying out an axial test and other mill tests when somethinghas “gone wrong” and a millis not performing as well as it should. However, it is equally importantto cany out axial tests when a mill is performing well so that we can establish why it is performingwell. By carrying out axial tests on a regular basis it is possible to build up a record of milloperating data, thereby enabling factors such as optimum charge grading to be determined.
29
.,,
,,
,.
II
,t
1
08.
0m
*N
w“
o
no
IEc’
+T.PH.
4-0/0gopm
res.
6. FINE MEDIA IN CEMENT MILLS
In order to maximise the effectiveness of fine grinding by the use of fine media (ie 25-15mm size)in cement mills, a checklkt of essential preconditions is attached (as Appendix IIIB). Specialattention should be paid to these points when testing mills with fine charges. All too ofien, finemedia is not as effective as it might be, due to the reasons:-
● Poor first chamber performance resulting in nibs/coarse material entering second chambers.
● Lack of adequate mill cooling and ventilation leading to coating.
● Excessive water injection plus (ii) leading to media coating.
● D]aphragmblockage by nibshramp metalhoken media causing overfilling of the chamberand “cushioning” of the fine charge.
31
1.
2.
3.
4.
5.
6.
7.
8.
APPENDIX 1A
CEMENT MILL FAULTS
Output Decreases
Cement Too Coarse
Cement Too Fine
Mill Fills
Mill Empties
Choked Diaphragms
Clinker Nibs in Cement
Formation of Coating
APPENDIX IA
CEMENT MILL FAULTS
1. OUTPUT DECREASES
POSSIBLE CAUSIJ REMEDY
a. Increase in clinker hardness and size Examine raw fed composition and kilnburning conditions.
b. Mill charge too worn Dump charge, re-grade and makeup newcharge.
c. Mill volume loadlng too low Measure each chamber and add correcttonnage of new bodies.
d, Too smali bodies used for makeup Charge should be dumped and re-graded,or (less satisfactory) use largest bodies formake up.
e. Diaphragm slots partially blocked Inspect Diaphragms clean slots. Furtheraction as in Section 6
f. Fractured diaphragm Replace section(s).
+
2. CEMENT TOO COARSE
POSSIBLE CAUSE REMEDY
a. Increase in clinker hardness and size Examine raw fd composition and kilnburning conditions.
b. Grinding media too large Make up with smaller media e.g. 50/50 ofthe two smallest sizes.
c. Mill volume loading too low Measure each chamber and add correcttomage of new bodies.
d. Diaphragm slots too worn Replace as soon as possible (with steeldiaphragms, weld bar into worst slots as atemporafy measure).
e. Coating forming Reduce mill temperature. Use dryer feed.Increase mill ventilation.
1
3. CEMENT TOO FINE
POSSIBLE CAUSE REMEDY
a. Output decreased See Section 1.
b. Last chamber media too small Dump and re-charge or make up with largemedia.
c. Diaphragm slots practically blocked Locate source of blockage by sound andexamine diaphragm. Further action seeSection 6.
....
4. MILL FILLS IPOSSIBLE CAUSE I REMEDY
a. Harder feed clinker Check raw’feed composition and kiln ~burning conditions. Increase 1st chamberpiece weight.
b. Too much feed Run without feed until chamber sound isnormal. Then use slightly reduced feed. Ifmill fills again consider (a), (c) and (d).
c. Worn gn‘ridingmedia in 1st chamber I See Section lb.
d. Choked diaphragm(s) See Section 6.
e. Coating forming I See Section 8.
5. MILL EMPTIES
POSSIBLE CAUSE REMEDY
a. Insufficient feed Increase feed slowly.
b. First diaphragm slots worn See Section 2d.
c. Decrease in clinker hardness and size Check raw feed composition and kiln.,burning conditions decrease 1st chamberpiece weight.
d. Media in last chamber too large Dump charge and refill with charge ofsmaller piece weight.
Note: I.famillfillsseverely,cleiuingisMIicultduetotheexcessivetemperaturerisethatresultsfromrunrihgtithreducedventilationandfeed.Alittlegrindingaidpouredintoeachchamberthroughthemaindoorintum(startingwiththelastchamber)acceleratestheclearing.
,.
2
.-...,,
6. CHOKED DIAPHRAGMS
POSSIBLE CAUSE REMEDY
a. Steel diaphragm slots closed by Burn/grind out slots to original width,metal flow replace diaphragm soonest.
b. Tramp metal in feed Greater care and tidiness when scrap metalis handled and repairs are carried out inproximity to clinker store and clinkerhandling systems.
c. Grinding body pieces formed in Examine cast iron and very hard grindingmill rneda for casting faults and excessive
brittleness. Inspect mill for very largebodies mixed with very small bodies (iffound, dump charge).
d. Over worn grinding media Dump charge and make up with new.
e. Clinker nibs See Section 7.
f Coating forming in slots See Section 8.
7. CLINKER Nl_BSIN CEMENT
POSSIBLE CAUSE REMEDY
a. 1st chamber diaphragm slots too-.
Replace soonest.worn
b. 1st chamber diaphragm has open Repair as necessary.crack/k ill fitting or loose
c. Exceptionally hard and/or size Examine raw meal quality and burningfeed control (if permanent, increase ball size in
1st chamber).
d. Maximum ball size in 2nd Increase piece weight by adding 70mmchamber too small balls.
3
8. FORMATION OF COATING ,’
POSSIBLE CAUSE REMEDY
a. Excessive moisture in feed Mix dry feed with moist feed to reducemoisture content.
b. Inadequate ventilation Increase by opening fan damper ,or identifjsource of in-leaks after mill and reduce.Inspect mill to find possible blockagewithin mill. Differential pressure acrossmill should be around 40-60mmwg.
c. Too high milling temperature Reduce clinker temperature, increase millcooling and ventilation.
d. Grinding body charge size Use grinding aid (including internal watergenerally too large ~ cooling) if charge is otherwise petiorrning
satisfactorily i.e. ,when grinding RHC(rapid hardening cement) in and OPC(Open Circuit Mill).
I
,.
4
APPENDIX HA
THE FINENESS OF SAMPLES TAKEN IMMEDIATELY PRIORTO THE FIRST CHAMBER DIAPHRAGM
1. Samdin~ Method
Itis important to use common sense when taking samples within a mill. If visual inspection of thematerial shows it to be relativelyfine, then small(200 gm) sampleswill suffice for carrying out sieveanalysis. However, if large quantities of nibs are present, it is advisable to take larger samples, i.e.around 0.5-1 kg in weight.
2. Samrde Analysis
Coarse samples should berecommended range for raw
Sieve Size
76 mm50 mm25 mm12 mm6mm
Microns
2,360
1,180
300
9075
45
))))
graded through the convenient sieve sizes available on site. Amill and cement mill testing are as follows:-
Notes
Choose coarse sieve sizes to suit typical feed size of stone orclinker to the mill. Of particular importance for raw millassessment.
Important for anaiysingsamples prior to the intermediate diaphragm inraw/cement mills.
Important for analysingsamples prior to the intermediate diaphragm inraw/cement mills.
Important for analysingsamples prior to the intermediate diaphragm inraw/cement mills.
) Use for mass balance calculations around raw mill circuits.
)
Important for analysing samples ex cement mill second chambers. Donot normallyuse for raw mdltesting. Use for mass balance calculationaround cement mill circuits.
1
3. Guideline values for material samded before intermediate diaRhra~m ie. leavingchamber 1.
● Cement Mills .,. ..
See Appendix III section (1) for details
● Raw Mills
Check for any accumulation of material in the plus 12* size since residues can increaseif there is a buildup of coarse material at the diaphragm due to poor crushing action etc.
The 300 micron residue is also a usefbl gl.lideto ~-- -’-–I--- --A-—-.-..:.111S1QlldlllUGl~Gllu1lllaluu 1=.
Cumulative % Residueon 300 Micron Sieve ~
33 (A)
43 03)
72 (c)
Efficientunderrun
first chamber handling fine stone - mill- capable of slightly higher oytput.
Typical mill installation - efficient first chamberhandling reasonable size stone.
Mill with first chamber over-filled due to coarsefeed_size and inadequate ball size. Extra 90mm oreven 100mm media required to improve crushingaction.
..
As can be seem in the case of this design of a mill a more realistic target residue would be around40% retained on 300 micron i.e. twice the residue considered acceptable for a cement mill.
2
APPENDIX HL4
ESSENTIAL PRECONDITIONS FOR THE UTILISATION OF SMALLER MEDIASIZES IN SECOND AND THIRD CHAMBERS OF CEMENT MTLLS
1.
2.
3.
4.
5.
6.
7.
8.
The first chamber charge should be in good condition andfineness of samples prior to the intermediate diaphragm.
Sieve Size O/OCumulative Residue
2.36 mm l%1.18mm 6’%200 urn 20%
When determiningthis fineness, it is necessary to take large
produce the following typical
samples and not to ignore anyunground clinker nibs present. If nibs are present, the first chamber media grading shouldbe checked and adjustedwith extra 90/80mm instead of 70rnm media. Any media less than55mrn should be removed.
The first chamber should have an efficient reverse step/lifter type lining.
The diaphragm slots must be clear and even with no excessive gaps which can allow coarsematerial to enter the second chamber from chamber 1.
The outlet diaphragm must be clear of blockages, etc, to avoid chamber overllling andhence cushioning of the fine charge.
Check the second chamber axial sampling cume for any signs of nibs/coarse material fromthe first chamber which can inhibitthe fine grinding characteristics. Figure IIIA shows andexample of two rni.11%one with an efficientand one with inefficientfirst chamber. It is notedhow the second chamber of the latter mill has to carry out some of the crushing actionwhich should have been carried out in the first chamber.
The mill must be adequately vented, i.e. with typically three air changes per minute (or0.25kg air per kg cement) for open circuit mills or typically five changes (or 0.4kg air perkg cement) for closed circuitmills. These airtlows refer to air flow actually entering the millinlet which is oflen lower than at the filter due to inleaking air.
Avoid high millingtemperatures above 120”C, which can cause coating in the second (andthird) chambers. Coating can adversely affect the pefionnance of media especially whenusing finer charges.
Excessive water injection to control otherwise high milling temperatures should also beavoided wherever possible in order to prevent hard coating on media.
50
40
30
20
10
0
FIGURE lIIA
!------------------------- --,--.O- --
----*
+ 45 micron -*-●9 MILL B
●99... -.-..=* -mm-m-------- ------
b---- ---0 ----●
+ 2.36 mryc.●e* .*
●9**** B/
Nibstransfenedftom●e. ●*-
..* . chamberone of Mill B-.--
A“0- 0---- 0-----------
n I
FIGURE A - AXAIL SAMPLINGTEST - SIEVE RESIDUES FOR
CHAMBER TWO
ILL AI ) Efficient first chamber
charge and lining.
ii) Fine ( 25- 15mm ) mediain chamber two.
J!!llLui)
ii)
iii)
I I
20 40 6i 800
% CHAMBER LENGTH
100
inefficient first chambernibs prior to
intermediate diaphragm
pooriiningand charge.Conventional charge(60 - 17mm ) in chamber two.Poor classifying iining givesreverse classification.
II
APPENDIX IVA
AXIAL SAMPLING CURVES FOR CEMENT MILLSECOND CHAMBERS - SIEVE RESIDUES
General
In addition to examinhg the surface area versus nett kW drawdchamber length relationship for thesecond chamber of a cement mill, it is also usefid to examine the sieve residue relationship.
This is best examined by using a 45 micron sieve or even finer sieves such as 32 and 25 micronsieves if these are available.
Very oflem the ti~ sampling curves for second chambers show a steady reduced in 90 and 300micron residue throughout the chamber. This gives the impressionof efficient grinding taking place.However, cement is predominantlyfiner than 45/32 micron and to assess the fine grading efficient,these sieve residues should also be examined.
Figure IIIA shows axial sampling curves for two closed circuit cement mills. Mill ‘A’was anefficient mill using fine media (25-1 5mm) in its second chamber. The material leaving the firstchamber of this mill was fine and within the guidelines shown in Appendix III.
Mill B’ used a ftily conventionalmedia gradhg consisting of 60mm to 17mm media. The mill alsohad a form of class@ing liningwhich did not fi.mctionwell and resulting in reverse classification ofthe media.
Comparing the 45um residue shows that the material at the inlet to the chamber was of similarfineness i.e. around 50% cumulative residue.
However, the chamber outlet samples were as follows:-
Mill Outlet Residue Reduction in Residue~ 45 micron Across Chamber
A 46% 4%B 28% 22%
This shows the superior grinding characteristics of the finer mdla used in conjunction with anefficient first chamber charge in Mill ‘A’.
To improve Mill ‘B’to give a similar performance would have involved costly replacement of theliner/charge. It was therefore decided to reduce the average replacement media size from 60mmto 25mm size and replace some of the coarser 60mm media by the available 25-17mm media.
APPENDIX VA
EXAMPLE OF HOW THE VOLUME LOADING WITHIN A MILL CAN BEAFFECTED BY THE ACCUMULATION OF UNGROUND MATERIAL (NIBS)
● Background
The following results were obtained on a 4200 KW raw mill, which was designed toproduce 220 tph raw meal. The mill was crash stopped afier running at 75% of its designload during commissioning. It was decided to cany out an axial sampling test on the millbefore raisingthe charge to 90% load. The millwas crash stopped and it was noted that thefirst chamberwas very fill of unground stone. Later on the mill was run out and the heightabove charge measurements were rechecked.
● ADDiirent First Chamber Volume Loadinp Followinp Crash StOD
Average height above charge (H) =
Inside lining diameter of chamber (D) =
=
“Apparent” volume load =
● Volume Loading After Mill was Run Out
Average height above charge (H) =
=
Volume load =
Tomage of media loaded into mill =
Chamber internal volume =
Volume of media =
Media Density =
3.07m
4.42m
3.07— = 0.6954.42
25.5%
3.275m
0.74
20%
88 tonnes
92.1m3
3251 x 0.02= 18.42m3
88.0= 4.78 tlm3
18.42
● Amarent Media Densitv
Due to the presence of nibs occupying approximately 25.5- 20.0= 5.5% of the ,internalvolume of the mill, the apparent media density is less than that shownabove.
i.e. Volume of media plus nibs = 92.1 X0.255= 23.4@
88.0Apparent media density = — = 3.75t/m3
23.49
● Conclusions
Hence, unless the volume loading of the chamber had been checked with themill run out it would have appeared that the chamber contained more mediathan it actually did, i.e:-
92.1 X0.255X 4.78 = 112.3 to~es
Another means of cross-checking the actual media loaded is to calculate theKW drawn for each chamber using the power formula and cross-checkingthese results against the figures from the mill kwh meter.
2.
APPENDIX VIA
MASS BALANCE ON A RAW MILLING CEMENT
General
The example chosen to demonstrate mass balance calculations on a milling circuit is that of adouble rotator mill. It was felt that this represents one of the more complex closed circuitmillingsystems availablewhich couldbest illustrate some of the techniquesJproblems involved.Generally speaking, most closed circuit cementlraw milling systems should be simpler toevaluate than the example shown.
Mill Circuit Samples
Rejects from static separator
Fines from cyclone atler static separator
Rejects from Wedag separator (feed tochamber 2)
Feed to Wedag separator
Mill central discharge
Fines from Wedag separator
.. Finished raw meal
Mass Balance Calculations
● Mill Throu~hDut
Mill feed rate
Feed moisture
DW tonnage
● Finished Raw Meal
0/0Cumulative retainedon 90 micron sieve
66.00
9.24
76.22
52.90
49.50
2.70
6.30
= 42.8 tph(from totaliser readings
.. prior to test)
= 3.1’%0
= 42.8 X 0.969= 41.5 tph
Dry tonnage = 41.5 tph
The finished raw meal consists of (a)
(b)
:. (41.5 -x)
Mass balance on 90 micron residue
(2.7) + (41.5 -X) 9.24
x
Hence, fines ex cyclone
fines ex cyclonefines ex Wedag separator
= tph of iines ex Wedag separator
= tph fines ex cyclone
= 41.5 (6.30)
= 18.7 tph
= 41.50 -18.7 = 22.8 tph
● Reiects Feed to the Seuarator
The fd to the separator consists of milldischarge material together with rejects fromthe static separator.
Basis 1 tph of separator feed
let y = tph of material ex millcentral discharge
Mass balance on 90 micron residue
y (49.5)+ (l-y) 66.0 = 52.90 X 1.0
Y = 0.79
i.e. 79% of the feed to the separator consists of material from the mill centraldischarge.
● SeDarator Performance
(a)
(b)
(c)
2
o/ORecirculating Load
76.22 -2.70 = s 15Circulating factor C =
76.22 -52.90 “
Recirculating load = C-1 = 2.15 i.e. 215%
Fines Efficiency100 (100 - 2.7)= = 65.6?40
315- (2.70 + 2.15 X 76.22)
Coarse Efficiency100 (2. 15 ‘ 76.22) = 984%=2.7 + 2.15 X 76.22 “
The coarse (or rejection) efficiency is good. The fines efficiency is on the low side due to thefine cut point of the separator (2-3’?40plus 90 micron as opposed to 5.0%+90 micron target -see comments in conclusions section). The recirculatingload estimate does not agree with thatindicated by the rejects weigher (1090A). However, the two cannot be directly compared.
● Mass Balance Over SeDarator
Fines ex separator =
Let rejects =
Balance on 90 micron residues
(R+ 18.7) 52.9 =
R= 40.3 tph
Hence, separator feed =
● Reiects ex static SeDarator
These form 21% of feed to separator =
Hence, mill central discharge =
● Indicated Versus Actual Recirculating Load
(a) Indicated
Mill rejects weigher =“!- Mill feed dxy =
Hence, indicated recirculating load =
0) Estimated
Calculated rejects ex Wedag =Mill feed =
Estimated recirculating load =
18.7 tph
R tph
R (76.22)+ 18.7 (2.7)
40.3 + 18.7= 59.0 tph
59.0 X0.21 = 12.4 tph
59- 12.4= 46.6 tph
45.1 tph41.5 tph
45.1— x 100 = 109?/041.5
40.3 tph41.5 tph
40.3— = 97%41.5
These are within the emors of accuracy that can be expected.
3
( Figures in brackets are 90 micron sieve residues) b
Fresh Feed _41.5 tph42.8 tph
Separator
4I
‘TI
(6&.0%)
\
12.4 tph
11
n
(76:22%)40.3 tph
FinesCyclone
18.7 tph
(9.24%)22.8 tph
(6.30%)
Finished raw
FIG. VIA - MASS BALANCE ON RAW MILL CIRCUITmeal 41.5 tph
Mass Balance and Conclusions
Figure VIA gives the completed Mass Balance.
The above mass balance must be treated cautiously for the following reasons. These illustratesome of the problems with calculating circuit mass balances.
1. The estimated finesfrom the cyclone is 22.8 tph or 55% of the finished raw meal. Thisis most probably an over-estimate. Normally, one would expect only 30’XOof theproduct to come from this cyclone.
2. The mass balances are carriedout usiig 90 micron residues. Considering the very fineproduct produced by the Wedag separator it maybe advisable to use a finer sieve sizefor constructing the mass balance. It is best to carry out fbrther sieve gradings on say45 or 75 urn residues to see which results relate best to the measured rejects flow rate.
3. In the example showq the Wedag separator was producing a finer product in order tocompensate for the coarser product from the cyclone after the static separator. Thetarget residue was 5V0on 90 micron. This proved to be usefid information. Atler thete~ the static separator was inspected internally and it was found that the suspended“bob”beneath the central cone of the separator was misaligned. This was subsequentlyput right and finer residues resulted.
4. To cany out a fill assessment of the separator performance the efficiency figuresshould be estimated over the fill range of particle size results. Reference to a singleresidue efficiency will only give a very rough guide to separator performance. Whencomparing results from the same mill, reference should be made to the level ofrecirculating load, i.e. an apparentlypoor fines efficiencyresult may simply be a featureof operation with high recirculatingloads rather than any fault with the separator-itself.Construct TROMP curves for the separator product using fi.dlpsd analysis.
5. When two fines or rejects streams are mixed (as in the above example) if the analysisof the materials before and after mixing is similar then it becomes very difficult toestimate their respective tonnages. If sieve residues are no help - try tracer techniquesor check for any chemical variation.
6. To improve accuracy - several samples should be taken horn the circuit and analysed.
APPENDIX VHA
EXAMPLE OF THE “OUICK TEST” METHODFOR AXIAL SAMPLING TESTS
1. MILL INSPECTION REPORT
Tables VII (i) and VII (ii) and Figure VIIA sudse the essential data from the millinspection concerning media gradings, liners, diaphragms, etc.
1.1 Observations
The following points were noted during the test and inspection.
●
●
●
●
1.2
There is an excessive quantity of unground nibs at the intermediate diaphragms.
The first chamber lining has a relatively shallow step of only 45mm compared with arecommended 70mm. This gives a poor crushing action. There is an identical mill atthis Works which has a conventional 70mrn reverse step lining. The latter mill has noevidence of nibs before the diaphragm. Both mills have a good “coarse” charge inChamber 1 which is more than adequate for the fine clinker feed size.
The estimated gross mill kW is too high using the DAWN formul~ i.e. 2390 kW asopposed to an actual 2200 kW absorbed. This over-estimate is most probably due tothe presence of large quantitiesof nibs in Chamber 1which causes the charge to “bulk”and give a fhlseover-estimate of the volume load. It is best to re-check volume loadsafter the mill has been run out.
The intermediate diaphragm has gaps between its adjacent segments which allowcoarse material to enter Chamber2. The outlet diaphragm slots are badly blocked andthe second chamber should be tipped and regraded to remove undersized and brokenme&a (i.e. -12mm size). The effective area of the outlet diaphragm is low and fiutherblockages could cause hold-up problems. All slots should be cleaned out and all gapsbetween segments should be sealed up. New diaphragm segments are necessary toreplace all the badly worn and burned outslots which have jagged profiles.
Test Method
The number of samples taken was nine in total (four in chamber 1 and five in ‘;hamber 2).TableV’5(i) shows the sievegradings analysed. Note that greatest importance is attached tosieve residues at 2.36 urn, 300 urn and 45 um. Surface area results in chamber 2 could alsobe carried out. The mill was crash stopped in the early hours and allowed to cool for 1-2hours using modest mill cooling airflow. All samples were analysed and the results, plusconclusions, drawn up (axial sampling cumes, media gradings, belt weigh off results) withinthe same day.
Samples of the separator f- fines and rejects were also analysed. These confirmed that theseparator efficiency was reasonably good and was typical of “medium efficiency separation”using a Wedag type separator.
1
TABLE VII (i) : MILL INSPECTION REPORT
CHAMBER- .1 2
ChamberLcnglh(m) 3.76 8.484
El%ctivediamekr(m) 3.63 3.658
Averagehcighl above chwgc (m) 2.354 2.358
‘/o %hrmc load 31.38 31.85
Chamber volume (m’) 38.91 89.16
Media volume (m’) i2.21 28.40
Media density assumed (Vm’) 4.3 4.5
Estimafc~weightm~la(1) 52.50 127.80
Chamber ficc volume (m’) 26.70 60.76
Wctt kW drawn fA6.48 1576.04
3ross kW drawn 695.14 1694.67
rotrrl 8rosa kW drawn (2389.81 - too high)
4ctual grow kW drawn
Factor formcdirrdensity, eta to
~chicve richra[ gross kW
3cpth of ccmcnt above charge Material below chmga Material ●t acme Icvel
hnrling 0( mcdirJlinin@
.incr type Bolkcc Rcvcrai Step Ckasifyitg
Mpth atcp on Chamber I lining NtA
Mill speed -15.7 rpmWeighted avcrcgc internal dinrnclcr (m) = 3.649m
‘X0critical -71 .lOA
CHAMBER
Orrtkt dianhramq 1 2
mm slot size (ran8e) ●vcraga 7 (range 4-8) 9 (range 8-9)
Maximum clot ma with sII slots ckar (m’) cffkdivc 0.564 0.67912
Actual slot ara (m? cfktivc 0.34304 0.41264
D@rhragm C.!LA. (m~ 10.349 10.509
●/0 slot am maximum 5.45 6.46
●/. slot ma actual 3.31 3.93
~cdia DistributionAverage
Inkt Ball !%m
Size (m) Weight % Sample pt. (mm)
Mcdii sin distribution by either random 90 23.2 31.0rampling ((X) ot ball weighthize CIl) 85 25.0 : 27.5
80 !3.9 3 27.5
75 13.5 4 19.570 10.9 s 18.065 10.0 outlet
60 2.9-60 0.6
x
3vcrall wcra8c ball aim in chamber (mm) 77 .21.5
h pqmliOrr+30mm media in CD NIA 3.78
~mpchcs of tramp matcnal or broken media in chmgc Negligible
~ proportion of undcrsi?xd (@mm) mcdii in CI 0.6Va NIA
II
1
2
3
4
1
2
‘3
4
5
\
TABLE VII (u) . AXIAL SA.. . MPLING TEST SAMPLES$
CUMULATIVE h RETAINEDo
+ 10mm +5mm +2.36mm +1.18mm +300/A +90/A +45/z
2.96 8.17 14.62 20.76 34.63 50.16
3.18 5.26 9.15 15.56 35.40 49.17
1 3.09 .3.53 4.77 8.81 27.32 ,49.75
5.23 5.44 5.77 7.85 24.25 45.17
. 0.74 1.84 16.66 41.10 67.02
. 0.46 0.62 6.68 31.43 59.65
2 . 0,32 0.38 3.07 20.20 54.67
. . 0.29 0.31 1.31 12.57 43.48
2.58 2.60 3,48 13.22 57.34
1!!Ill
II
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II1
.
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3mvm
ma
%
SECTION B
CEMENT MILL OPTIMISATION
SECTION B
GENERAL GUIDELINES TO OPTIMISEPERFORMANCE OF OPEN AND CLOSED CIRCUIT
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
CEMENT MILLS
CONTENTS
INTRODUCTION
MILL TESTING/ROUTINE INSPECTION
MILL CONFIGURATION -CHAMBERLENGTHS
MILLLINERPLATE DESIGN
4.1 First Chamber Liner Design4.2 Second Chamber Liner Design
MEDIA GRADINGS
5.1 Chamber 1 Media.5.2 Chamber 2 Media
DIAPHWGM DESIGN
MILL VENTILATION
MILL POWER DRAWN, MILL CRITICAL SPEED AND MEDIA LOAD
GUIDELINES FOR MILL FIRST CHAMBER POWERCONSUMPTION
CLOSED CIRCUIT MILL OPTIMISATION
THE PURCHASING OF NEW CEMENT MILL INTERNALS
APPENDIX lB: Recent experiences with cement mill internals
APPENDIX IIB: Notes on the performance of USA cement mills
...—.—.
1. INTRODUCTION
This paper is intended as a set of generid guidelines to assist plant personnel in optimizing theperformance of open and closed circuit cement mills. Very oilen, when efforts are made tooptimise a mills’performance, certain compromises may have to be made in the following areas:-
●
●
● ✚
Me&a grading - available stocks of media plus the wish to avoid excessively high mediareplacement costs often results in “compromise” charges being used.
Existing plant design limitations - older mills often have inadequate ventilation/filterplants, etc. Whhout major redesign/capital expenditure it is often not possible to achievecurrent ventilation standards.
Factors such as very highclinkerfd temperatures and inadequate mill cooling will oftenlimit the extend to-which, say, fine media can be used in secondhhird chambers.
Hence, common sense has to be applied when mill optimisation is carried out and all too oftencompromises are necessary. It is important to be aware of the factors which limit a mills’petiormance so that on-going improvements can be made in key areas. Routine mill testing andinspection is essential to any programme of mill optimisation. When the opportunity arises torepla% diaphragmdiners, etc., it is essential’to make sure that outdated designs are not repeatedand that the latest/optimum designs are applied.
2.
●
●
●
●
3.
MILL TESTING/ROUTINE INSPECTION
Axial samplingtests - make use of the “quick test” method which concentrates on gettingthe maximum amount of essential information out of the minimum amount ofsampling/analysis. (See 3.2 in section A).
A minimum of one axial sampling test per mill per year is essential. Preferably 2-3 testsshould be carried out per annum on each mill. there is no point carrying out tests if firmconclusions are not drawn and if recommendations are not acted upon.
Make greater use of cameras to obtain a photographic record, of mill inspections.Photographs are usually more convincing than words.
Routine mill inspections should be carried out on a monthlybasis in order to check for anyobvious failures or problems plus charging levels as a preventative measure. This willensure that mill internals are maintained.
MILL CONFIG~TION - CHAMBER LENGTHS
Typical chamber lengths for open/closed circuit cement mills are as follows:-
‘/o OF OVERALL MILL EFFECTIVE LENGTH
MILL CLOSED
Chamber 1 30 28-34Chamber 2 20 72-66Chamber 3 50 Not applicable(plus 4 if fitted)
I 100% I 100%
The above is a general guide only. Certain ding systems, such as closed circuit mills for whitecement, ofien use much shorter first chamber lengths (eg Ras Al Khaimah - first chamber = 14Yo).Similarly, the use of pre-crushing equipment such as Roll Presses often makes it necessary toadjust chamber conjurations/charges etc. Chamber lengths are often a compromise, especiallyif the mill is used for more than one cement type, eg RHC and OPC. First chamber lengths haveto be suitable for the worst case, ie highest throughput at the lowest surface area and the hardestgrindability.
2
The followinggeneralised guidelinescan also be appliedwhen selectingthe optimum first chamberlength:-
QisJ3 % of total effective lemzth
High critical speed (75%)Stepliner, Coarse Ball charge 28’?40Average surface area (350 mz)
Low critical speed (70-72%)Low surface area (280-300m2/kg) 34%
Reference is drawn to section 9 of this paper in which the power input requirements of chamber1 are reviewed.
3
4. MILL LINER PLATE DESIGN
4.1 First Chamber Liner Design
● The standard recommended liner design for Chamber 1 is the reverse step liner with aripple profilewhich is availablefrom severalmanufacturers, e.g. Magotteau~ Bradley andFoster, etc. See Figure 1 for details of this liner design.
●
●
●
●
When this liner wears, the “step” distance is reduced and there is less separation of thecharge. Some older liners were often provided with a relatively shallow step of only 40-50mm. This is insufficient and the recommended step sizes areas follows:- ,,
Minimum = 60mrnMaximum = 80mmTypical = 70mm
The optiium step size is a fi.mctionof mill speed. With slower speed mills, it is desirable Ito use the maximum step size in order to compensate for the loss of lifting action.
Reverse step liners are suitable for ball mills with critical speeds in the range 69-77Y0.However, be carefi,dof using this liner design if the mill has a very high critical speed of80-81%. In the later case a simplewave type liner is suitable as referred to in the examplegiven in section ~ part 4.
Lorain liners (see Appendix IB) are usefid for USA style cement mills featuring highcirculating loads with grinding aid usage. Thk liner gives high lift characteristics whichcan help to overcome the cushioning effects of very high material throughputs. Thereagai~ avoid using this liner if the mill critical speed is very high at 80-810/o.
If a reverse step liner is installed to replace a smooth type liner - remember to take intoaccount any limitationsto the maximum mill loading/power drawn (due to any reductionin mill internal diameter).
A recent development in liner design is the DUOLIFT liner available from Magotteaux(see Figure 1.1). This liner is claimed to give more effective lifling/crushing action thanreverse step liners. However, following installations in the UK and overseas, we haveserious reservations concerning this liner design (see Appendix IB).
Under no circumstances use a “smooth” type liner such as the Voest-Alpine grooved linerdesign, which gives little lifting action and tends to result in high levels of nibs. Thisdesign has recently been altered to incorporate “activator”plates, although there is no datato show whether its petiormance has been improved.
4
FIGURE 1: BOLTED REVERSE STEP LINER FOR CHAMBER ONE
N OFROTATION
i
NOTE - REVERSED STEP LINING PLATE DIMENSIONS ARE TYPICAL ONLY
FIGURE 1.1: DUOLIFT LINER
L,,,/./ I
I wb
\\
mo
v
4.2 Second Chamber Liner Desire
● Classifying liners - several designs are available and these give varying results. Someliiers (e.g. UBE type) can even give complete reverse classification! C1assif@g liningscame into vogue when open circuit mills were converted Ilom say 3 to 2 chambers usingconventional (60- 17mm) media in Chamber 2.
● It has been found that classiig liningsoften class@ well on conventional mediL but lesswell on finer sized media (i.e. 25- 15mm). Hence, with the current tendance to use finerreed% there seems littlejustificationinusing classifying liners. A simple ripple type liner,such as the FLS DRAGPEB, is usually adequate with finer sized media.
● A disadvantage of classi@ingliners is that they can allow clinker nibs to be classified assmall media, causing build-up of material and blockages of second chamber outletdiaphragms.
● Another disadvantage of classi~ng linings is their greater overall depth and the loss ofeffective mill diameter/power drawn which results. In addition, where mill shell watercooking is used, the thickness of this liner limits heat transfer. Potential 10SSof milldiameter is particularly significant on smaller diameter mills.
● In cases where the mill is not restricted by volume load considerations (ie, the fill motorload can be drawn at around 28-32V0volume load) classifying linings can be an effectivelining with a long effective life.
● For USA cement mills, the Lorain liner can prove effkctive in chamber 2 since it allowssome lifl characteristics to handle the high material throughput. The Lorain liner alsodoes not cause the same loss of internal diameter as a classifying lining. This is anessential feature for mills operating with up to 40°/0volume loading in chamber 2.
..● See Appendix IB for fi.uther details of current chamber 2 liner designs
7
5. MEDIA GRADINGS
5.1 Chamber 1 Media
Typical recommended first chamber media gradings are within the following range of “fine” and“coarse” gradings. All media should be “H*d” with matching liner/diaphragm materials.
I WEIGHT VOIN CHAMBER
Charge Type: Fine Coarse.
Ball Sizemm
90 23 36.580 32 29.570 20 24.060 25 10.0
I 100’%0 I 100%0
,,
The choice of media gradings for Chamber 1 will depend upon several factors, ie:-
● Size and hardness of clinker feed to mill, together with the length of the first chamber.
● Type of liner - if a poor first chamber liner is fitted, it may be necessary to use a verycoarse grading to compensate for the lack of any lifting action. With the new DUOLIFTliner design there has been a tendency to use a relatively fine first chamber charge. Thisis not yet proven as validwithin BCI. As a general rule, the coarser grading shoivn aboveis probably more applicable within BCI UK plants.
.,.● As a general rule, the “Coarse” grading is more suitable for
(a) Low critical speed (68-72’Yo)
(b) Worn step liners
(c) Coarse clinker (50mm plus)
(d) Hard clinker
(e) Short first chamber (28Yo)with significant production of lower SSA cement ie,300m2/kg.
The “Fine” grading may prove more suitable for mills which have well sized firstchambers, grinding fine clinker fked to higher cement surface areas (3’70-440 m2/kg). Theabove are rough guidelines and have to be applied with common sense. Hence, if a
8
cement millhas to produce a wide range of cement types o~ say, 300 to 440 m2/kg S.S.~some compromiseswith respect to grrdng may have to be made. Hence a coarser ,chargemay be needed to suit grinding at 300 m2/kginstead of the optimum finer media gradingrequired for 440 m2/kg. Hence the tendency is towards the coarser grading in many cases.
.● As a general guide, the first chamber charge should be adjusted to give the following
typical fineness of material sampled immediately before the intermediate diaphragm.
o/OCumulative ResidueSieve Size
2.36rnm 170
300 urn 20%
When taking these samples, it is essential to take large samples (1 kg) and sieve all thematerial at 2.36mm in order to detect any nibs of unground material (see Mill Testingpaper). If nibs are present coarsen the charge and check effectiveness of liner design.
5.2 Chamber 2 Media
● Current practice favours the use of liner media (i.e. 25-15mm size) in second/thirdchambers instead of conventional 60-17mm media gradings since fine media is moreefilcient for fine grinding. Under the correct milling conditions, fine media has beenproven to give increases in output of up to 25V0,subject to the initial condition of themedia grading as well as the mill itself
● Under the followingadverse conditions, the effectivenessof fine media can be limited by:-...
Inadequate mill ventilation
Poor cooling within the mill
.- Excessivelyhigh millingtemperatures and/or high water injection rates leading tocoating problems
Chamber overfilling problems due to blocked outlet diaphragms, etc., causingexcessive material hold-upExcessive coarse material entering ex Chamber 1 due to factors such as poordiaphragms, liners and/or media in Chamber 1
Under these conditions, fine media will tend to become coated and its fine grindingpetiormance becomes limited due to cushioning effkcts, etc.
In the Mill Testing Paper (Appendix 111A)a list of essential pre-conditions fpr the optimum useof fine media has been drawn up. These preconditions should be examined for individual millswhere it is proposed to use fine media. ,,
● ✚ As a general guideline, typical fine chamber charges for two chambered open and closedcircuit mills for UK style cement mills are as follows:-
WEIGHT VOIN CHAMBER
Ball Sizemm Open Closed
=-E-EI 100% ,1 1(.)Ovo I
,,
This applies for closed circuit millswith 150-200% circulating load. Whh three chambered opencircuit millsthe media is distributed with typically, 30-25mm media in Chamber 2 and 20-15mmin Chamber 3.
The above gradings are only typical and in practice may be finer/coarser according tocircumstances. In most cases the grading actuallyused in a mill will be a compromise which takesinto account the following factors:-
The quantity/cost of rejecting +30rnm media horn the charge. Obviously the finer thecharge is made, the greater the cost of replacing “ovetsized” media becomes.
Adverse milling conditions such as coating may make very fine media less effective dueto coating and cushioning. Under these circumstances a slightly coarser or conventionalcharge may be used. Common sense has to be used when deciding upon charge fineness.
I.fa xnilIalready has problems with material transportatio~ then finer media may worsenthese problems. Attention to diaphragms and possible use of grinding aids can help toovercome some transportation problems. ~ ,,
● USA cement mill - due to the high circulating loads (350-400%) u~ the second chambermedia gradings do not tend to be as fine as shown above. A problem with fine media isits tendency to expand and incr~e the apparent volume loading. Thk makes diaphragmdesign more critical and can limit the maximum power drawn by the mill.
10
Ball Size(mm)
3832252219
Coarse GradingWeight YO
171829279
Fine GradingWeight %
o15303025
II 1 I
1~ 100% 1Oovo
Note that the proportion of 19rnrnmedia is significantly less than the typical UK mill guidelinegradings. h additio~ very fineme&a of 15mrnsize is not normally recommended. Although thehigh usage of grinding aids can help to offset some of the problems associated with using finemedia, we have not yet tended to use very fine media gradings in the USA mills. Some notesconcerning the performance of USA cement mills are contained in Appendix IIB.
11
6.
●
●
●
DIAPHRAGM DESIGN
All diaphragms used should be of the double diaphragm/lifter/flow control type, i.e. thelevel of material in the chambers is controlled by a positive conveying action. Anadjustable form of flow control is highly desirable to give variable control of materiallevels within the chambers.
All diaphragm segments should be constructed of wear resistanthard wearing materials.Avoid use of steels which can spread thereby causing “peening over” of the diaphragmslots leadhg to closures and blockages.
Optimum slot sizes and slot areas depend upon the mill lengthkliameter ratio as well asthe circuit design (i.e. open or closed).
Typical guidelines are as follows:-
Ot)en Circuit (Typical 4 to 5.5 L/D Ratio)
Chamber Outlet Diaphragm ~ zSlot size (mm) 67Slot area (%) 4-5 5-6
Closed Circuit (Typical under 3.5 L/D Ratio)
Chamber Outlet Diaphragm 1 zSlot size (mm) 68Slot area (’?!) 5-6 6-8
Note the followk
● There should be a steady increase in slot size and area on progression through themill. This is to avoid excessive material holdup due to, say, the build-up of nibsin diaphragms. It ensures that any coarse material ex Chamber 1 will not causeblockages in subsequent chambers.
● When former open circuit mills of 4-5.5 L/D ratio are closed circuited, it isessentialto maxiniise slot areas to avoid material transportation problems throughthe mill. If slot areas are not maximised,this can otlen impose a limit on achievingthe optimum mill recirculating load as well as the minimum cement residues (i.e.narrowest particle size distribution).
● In the above tables the quoted areas are for the slots only, they do not include thearea of the central ventilation grid. This point must be made clear to supplierswhen orderingkpecifing new diaphragms.
12
● Slots should preferably be circumferential and the slot profile should not be of a typewhich encourages blockages by tramp metal/clinkernibs, eg, provide a “self cleaning” slotprofile as per the Pfeiffer and Magotteaux Opticontrol diaphragms (see Appendix IB)
7. MILL VENTILATION
s Recommended mill ventilation airflows are as follows:-
Ckcuit Open ClosedAir changeshnin 3 5kg aidkg cement 0.25 0.4
● When designingventilation systems, filters should be seized with adequate allowance forinleakingair over the ventilation circuit. Filter plants should be sized for at ieast 5 (opencircuit) and 7 (closed circuit) air changes per minute. All air volumes are related tochanging the free volume above the charge. Air volumes are based upon a standard airtemperature of110”C.
● The millinlet, outlet and diaphragmdesign should not impose a high restriction to airflowthrough the mill. Ensure that the following design features are incorporated:-
● Use a stepped chute mill inletwith an inlet trunnion scroll as per Figure 2. Avoidthe use of drum type feeders which restrict airflow.
● Maximise the hood discharge area in order to reduce air velocities to allow dust-drop out prior to the filter. Keep ductwork runs to the mill ventilation system asshort as possible with the minimum number of bends and horizontal runs. Fltadequate sealing before airslides, etc., at the discharge end. Wkh closed circuitmills, design the filter system such that filter dust is returned to the separator.This prevents the ventilationairtlow being cut back to match final cement fineness.In open circuit millingsystemsthis filter dust obviously cannot be re-classified andwill thus affect the finished cement surface area.
● Use double diaphragms with central ventilation grids. On smaller diameter mills(e.g. 895 kW or less) ensure that the area of the ventilation grid is not excessive,thereby restricting volume loading. When central ventilation grids are used it isessential to make sure that diaphragm slots are kept clean and-are adequatelysized. Ifno~ there is a risk of coarse material overfilling Chamber 1 and floodingthrough the central grid into Chamber 2.
● Low airflows through cement mills often result from the following:-
● High restriction/blockages of ductwork to filter plants
● High irdeaking air over mill hood, filter plant and ductwork
● Blinded filter bags/poor cleaning, etc.
13
-...
......“
.●
✎
“.\.
“..
1I
..
.
q
● Measurement of millventilation airflows can be earned out by means of relatively simpletest work as follows:-
‘- Measure all sources (where possible) of mill inlet airflow using a hot wireanemometer.
Measure the mill exit cement temperature plus the exit air and ambient airtemperature.
Measure the exit airflow to the filter using a Pilot tube.
Carry out a heatimass balance over the mill exit airflow. If there is a largedifferencebetween the temperature of the exit cement and the air to the filter, thenthis will indicatea high levelof inleakingair which can be calculated from a simpleheat and mass balance.
Compare the measured mill inlet airflow with that estimated using heatimassbalance techniques over the mill exit. An average of the two results will give anapproximate indicator of the actual airflow through the mill itself
Calculate the air changes per minute 170mthe measured volume loadings/freevolume above charge data.
Using this method on a UK 746 kW cement mill gave the following results:-
Measured mill iniet airflow = 18.7 kg/reinEstimated mill exit airflow (by heat balance) = 25.7 kglmin
Average airilow = 22.2 kglrninFilter exit airflow = 52.7 kglmin
Mill inlet airflow as a percentage of thefilter exit airflow = 42. 1?40,i.e.
Inleaking air = 57.9% of total filter exit airflow!
15
8. MILL POWER DRAWN. MILL CRITICAL SPEED AND MEDIA LOAD,,
The power drawn by a millcan be estimated using various formulae such as the DAWN formula.In order to optimise mill performance, in particular the lifiing/cmshing action in Chamber 1, thefollowing conditions are preferred.
High mill critical speed - typically 75-77%
Adequate reverse step liner fitted in Chamber 1
No chamber overilllingor excessivenibs which reduce the effective media density
Efficient charge in all chambers
Under these conditions, the overall power drawn per tonne media in the mill is typically 14-16gross kW/tonne media— .
w - DOnot confbse this with the guidelines for power drawn per tonne cement as outlinedin section 9 below)
For older mills which have:-
Low critical speed i.e. typically 68-70’?40
Poor chamber 1 liner which allows slippage of the charge
Chamber overfillinghnefficient media, etc.
The gross kW/tonne media maybe only 10-12 gross kW/tonne media.
Hence, it is worthwhile checking the critical speed and gross kW drawn per tonne media loadedfor each mill. It maybe possible to compensate for the low critical speed of some older mills byoptimizing media gradingdliner designs, etc, as outlined within these guidelines.
16
9.
The optimum configuration of a mill will depend upon the types and range of cements to re-produced. For example, a mill producing high surface area (380-400 m2/kg) cements will tendto require less power to be absorbed in its first chamber than a mill which is optimised forproducing coarse cements (280-300 m2/kg). However, if the same mill has to be used for bothcement types, a compromiseis needed. Hence the “chargein the first chamber cannot be reducedtoo far or the mill will tend to overfill and block when producing higher outputs at the lowercement surface areas.
The recommended guideline for first chamber gross power consumption is as follows:-
9-11 gross kWh/tonne cement
Current BCTC mill optimisation philosophy is as follows:-
Put sufficient media/power into chamber 1 to achieve adequate crushing ofmaterial leaving the chamber.
Avoid exceeding the above guideline first chamber power consumption unlesshaving to compensate for less efficient mill design ie low critical speed or wornchamber 1 liners.
Maximise fine grinding charge in chamber 2.
Note, if the mill has to produce a wide range of cement surface areas then it is necessary tocompromise ie,
● ~~ed OPC 2238kW mill
65tph at 300 m2/kg
Chamber 1 power = 670 kW
Chamber 1 gross power consumption = ~ = 10.3 kWh/tonne cement65
● R.H.C.
Same tonnage media/power in chamber 1 = 40tph at 400 m2/kg
Chamber 1 gross power consumption = ~ = 16.8 kWh/tonne cement40
17
Hence, when producing RHC cement, the first chamber power is too high. However, if the mediais reduced in chamber 1 (or chamber 1 is shortened), there will be insufficient power available tohandle 65tph OPC at 300 m2/kg. Hence a decision has to be made on whether the millperformance is to be optimised for Bagged OPC of R.H,C production.
Possible compromise for Bagged OPC
Chamber 1,’
9 kWh/tonne at 65tph requires 585 kW
Chamber 2 1653 kW available
25.4 kWh/tonne on Bagged OPC41.3 kWh/tome on RHC
Any such compromise must be based upon a detailed working knowledge of the mill frominspectionskixkdsamplingtests etc. It is known that certain milling systems can perform well ononly 8 kWh/tonne cement in chamber 1 whilst others may have to handle clinker and extendersneeding around 12 kWh/tonne.come to a sensible compromise.
Hence, common sense and factual testwork data is required to
,,
18
10. CLOSED CIRCUIT MILL OPTIMISATION
Testwork can be carried out on closed circuit milling systems in order to establish the optimummill recirculating load. The aim of the tests is to minimise the overall circuit ldHw/tonne andmaximise output. In additio~ quality parameters such as cement residues are also related torecirculating load levels and the effects of changes in residue also has to be taken into account.
Test work aimed at optimizing closed circuit milling systems is often a lengthy procedure andresults are not always conclusive. In order to cany out these tests, fill particle size analysisresults of the separator feed, fines and rejects streams should be carried out using analysers suchas the SEDIGRAPH or C.I.L.A.S. type. These allow the parameters such as separator by-pass,Rosin-Ramler slope, etc., to be determined. When trying to establish the minimum powerconsumption for the millingsystem the effectsof varying recirculating load upon ancillary powerconsumption must also be taken into account. In some cases, there maybe physical limitationswhich restrict the optimum recirculating load from being achieved such as diaphragm slotsize/are~ elevator capacity limitations, etc. ,
Ideally, fi,dlparticle size analysis equipment should be available at the Works to allow routinetesting of cement psd as well as the separator performance parameters. This provides usetld datawhich can be logged along with the normal routine quality/petiormance data such as:-
● outputs● kW absorbed, mill and ancillaries● Cement surface areas, strengths, etc● Cement residues● Clinker chemistry e.g. CZS,CqSand S03● Cement grindability
Computer based mill modelling techniques are used within BCC Technical Services in order tosimulate mill operation. These techniques can prove very usefid for predicting the theoreticalgains to be achieved by:-
● Optimizing the recirculating load for existing milling systems● Optirnising the media gradings● Conversion from either open circuit or closed circuitlconventional separator
operation to high efficiency separator operation
MN modelling techniques are most usefid when used in comection with normal mill/separatortesting procedures. They can help to quanti@ the benefits to be gained through plantimprovement/modifications. bother use of the model is that of mill conversion tenderevaluation. The mill model has been used to cross-check suppliers claims for increased outputby conversion to high efficiency separation operation at Cauldon, Dunbar, El Melon, etc.
19
11. THE PURCHASING OF NEW CEMENT MILL INTERNALS
Use of the various mill efficiencytargeting methods can help to formulate any cases for changingmill internals in the short and long term. It is recommended that the current mill internals becompared with BCI guidelines as contained in the Cement Optimisation paper. This,paper isupdated at regular intervalsto reflect current user experiences with new and existing designs, etc.
Avoid buying mill internals based upon suppliers claims. Ml equipment suppliers have a vested
interest in sellihgworks their equipment which may not be the qptimum for a given duty. Therehave been severalbad examplesof new millinternals being ordered which have not produced anyimprovement in millingefficiency. In certain caseq internals have been ordered which contravenethe BCI guidelines and have resulted in a reduction in milling efficiency.
Generally speaking, any renewal of mill internals should give an improvement in millingperformance as worn linersldiaphragms, etc. are replaced. Unfor@nately, suppliers oftenrecommend changes to
● Mill Chamber lengths● Media gradings● Lining thicknesses
which can prove detrimental to the efficiency of a mill. Whilst a short term benefit may beachieved by the new internals, there is a risk that the long term efficiency is adversely affected.Hence the message is Consult the BCI guidelines and speak to BCTC Technical Centreengineers.
These guidelines are generalised guidelines only and have to be applied with common sense inorder to arrive at cost effective solutions to optimizing mills.
..
20
APPENDIX Ill
RECENT EXPERIENCES WITH CEMENT MTLL INTERNALS
1. ALTERNATIVE LINER PLATE DESIGNS FOR CHAMBER 1
1.1 Magotteaux Duolift Liner1.2 Voest Alpine1.3 Lorain Liners1.4 Step Liners
2. CHAMBER 2 LINERS
2.1 Magotteaux Classi~ng Linings2.2 Lorain Liners2.3 Christian Pfeiffer Classi@g Liners
3. DIAPHR4GM DESIGNS
3.1 FLS Combidan Diaphragms3.2 Magotteaux Opticontrol Diaphragm3.3 Pfeiffer Third Generation Diaphragm
.. 3.4 Magotteaux Airfeel Diaphragm
1. ALTERNATIVE LINER PLATE DESIGNS FOR CHAMBER 1
1.1 Ma~otteaux Duolift Liner
There have been several installations within the BC Group. This liner design has been installedat Weardale, Cauldon, Cemento Melon and Aberthaw Works with varying results. The mainconclusions drawn are summarised below and the liner design is shown in Figure 1.1.
●
●
●
●
●
1.2
No proven advantage over normal (i.e. 60-80mm) step type liners, apart from themaintenance advantage of having a beltless design (less spillage, bolt breakages, etc).
The DuoMIlinerwas removed from Weardale No. 3 cement mill following a decrease inmillpefiormance and less stable mill operation. Other factors (coarser ball charge, etc.)also contributed to this problem.
Magotteaux subsequently fitted a step liner replacement.
There is some evidenceto suggest that the Duolifi is not suitable for mills of high criticalspeed (75°/0plus) and can cause higher power losses.
The Duolift liner in No. 19 raw mill at Cemento Melon was seen to give no advantageover FLS flat place liners. WMst the Duolift lining helped to reduce the level of nibsbefore the intermediate diaphragm, the lining (plus chamber 2 classifyhg liner) caused areduction in millinternaldiameter. There was no evidence of any improvement in outputover the similar No. 18 raw mill with its older FLS internals.
Aberthaw Works No. 1-3 mills - improvements to the mill ventilation and the installationof Duolii linershave generallyincreasedproduction. However, the first chamber absorbsa relativelyhigh power (16 kWh/t) to achieve a satisfactory fineness. This could limit themaximum mill potential output.
Voest A]Dine - Activator plate Lining
This is a development of the VA grooved liner plates which were unsuitable due to high chargeslippage/excessive nibs problems. Test work on the liners used at Fujairah cement showed thatthe liner gave poor Iifiing action. The design has been modified to include raised sections or“activators” to increase lift.
.
Since experience with this design in cement mills is limited, the following experiences with rawmilling have been included:-
This design has been installedin the limestone mill at Rawang Works and the raw mill at KanthanWorks.
At Rawang Works, whilst improved power consumption is claimed, any improvement inpetiormance must be judged in the light of the poor state of the liners which were replaced. An
1
area of concern is the reduced millpower tid whether or not this represents a real power saving.The inference is that the design could still allow charge slippage and thereby’limit the maximumusefid power drawn by a mill.
The MFL liners installed in Kanthan Works raw mill gave reasonable performance, but tended tobe prone to build-ups when handlingwet materials, When wet rriaterialscoats up the gap ,betweenadjacent lifters, the Iiner becomes inefficient., For contractual reasons these liners ,were replacedby conventional s when the mill was uprated by Roll Press addition in 1993-4.
The Activator plate design offered by Christian Pfeiffer is also available for cement mills.However, the raised step on these plates is only 40mm and the lining does not appear to promotegood crushing action in Chamber 1. Hence, this design cannot be recommended especially whenused with mills of low critical speed (68-720/0).Figure 3 shows de@ls of the MFL Activator platedesigns used in Malaysia.
1.3 Lorain Liners
This bar type lifter is common in the USA but less common in the UK. The liner seems toachieve good charge separation/liRhg action in the mill. Hence, we would not advocatereplacement of this liner design by newer designs. The high lift characteristics may well be themost appropriate lining type for mills such as those commonly found in the ,US~ i.e. with:
high volume loadings (38-40%),’ ‘,
high recirculating loads (300%) –1’
See Figure 4 for details of the Lorain liner.,,
.,Beware of usiig a high lifl liner design, such as the Lorain, with high critical speed mills, i.e. 78-80V0critical speed.
1.4 SteD Liners ,.
Avoid the use of the Magotteaux low step liners (step 35-45mm) such as installed in Malaysia(Kanthan No. 3 and Rawang No. 5 cement mills). These give poor crushing actio~ even whenused with a coarse first chamber ball grading.
Some Magotteaux step liners have medium step distances, i.e, 50-60mm which may work wellwhen new and in millswith higher critical speeds (75°/0plus). However, when the step wems toaround 40mm it is less effective. Hence, the prefemed step distance is 60-80mm.
,,
Avoid using a very deep step distance (80mm) if the mill ‘Mo critical speed is 76?40or above.
2
FIGURE 3 COMPARISON BETWEENACTIVATING LINER PLATES
F-l
3$9I
1-
C)RIGINAL-tiFL ACTIVATING LINER
i,
___.. -——.—.
FIGURE 4 LORAIN LINER PLATES FORCHAMBER 1 AND CHAMBER 2
,..
CHAIIBER 1 cHAllBER 2
1.4.1 Beltless versus Bolted Step Liners
Following installationof a bokless first chamber step liner in Plymstock No. 2 cement mill, BCCwere concerned over the efficiency of thk design compared with the conventional bolted linerdesign.
Comparative tests were carried out in Magotteaux’s pilot plant and these showed the following:-
For volume loads below 32%, the beltless liner absorbs more power than thebolted liner.
For volume loads of 29%, the liftingangle (see sketch) is 4% higher for the boltedliner.
Overall the difference in absorbed power is 1.7% higher for the beltless liner.
However, Magotteaux concluded that in operation, there is little actual difference between thetwo designs. BCTC view is as follows:- -
The beltless is less efficient indesign.
. The beltless design tends toaccumulated in chamber 1.
actual operation when compared with the bolted
produce a tailing of oversized nibs which can
-The beltless liner should not be used with slower speed mills (ie, 68-72%criticalspeed) especially if such mills have a small first chamber and have to grind lowerS.S.A. cements or hard/oversized clinker.
At Cemento Melon, No. 16 cement mill is fitted with the Magotteaux beltless liner design. Themill speed was increased from 70 to 78.6°/0,following gearbox repairs. This allowed the milloutput to be increased from 23 to 24tph at a similar power consumption. The higher speedallowed chamber 1 to be shortened and at this higher speed the beltless liner gave reasonablecrushing action. Whilst the beltless step liner is considered to be inferior to the bolted design itsperformance is acceptable at higher critical speeds (76-78%).
Figure 5 shows the comparative design of Magotteaux Bolted and Beltless lineqs.
5
oz
LL
l
2jnozzi
2. CHAMBER 2 LINERS
2.1 Mwzotteaux Classifying Linings
Mention has already been made of how classiijing linings can limit output on mills which arevolume loading limited. This is due to the 10SSof internaldiameter and reduction in critical speed.Magotteaux are aware of BCTC views on this and have developed a thinner classi&ing lining”which was recently fitted to the new larger mill shells of Cookstown 1 and 2 mills. The linerappears to give good classi$ing action Cookstown but was also reported as giving some reverseclassificationelsewhere. Figure 6.1 shows a comparison between classifying liners and Dragpebliners. Figure 6.2 shows the Cookstown Lher design.
2.2 Lorain Liners
These linersare suitablefor second chambers of USA mills where high material throughputs andhigh volume loadings require a more aggressive action to the charge.
2.3 Christian Pfeiffer Classifvinp Liners
The design of cktssi&inglinerplate f~tures a very deep (190mm) wedge liner plate coupled witha flat plate with grooved profile and activator section at one edge. It is fitted to Ravena No.4cement mill.
Whilst the liner appears to give reasonable ball classification this design cannot be recommendedsince it limits the mill internal diameter and runs the risk of classif@g nibs to the outlet ofchamber 2. In the USA mill application, great care has to be taken to avoid nibs and maintaineven 6mm gaps in the intermediate diaphragm to avoid nibs.
7
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a
53
n
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3m
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&m.-IA
L-o
m
*
0
S
3. DIAPHRAGM DESIGNS
3.1 FLS Combidan Diaohra~m
This diaphragm design features large armour castings which have gaps between them to allowmaterial flow. The diaphragm slots consist of a screen with 6 or 8mm pefiorations.
This design is installedin the new cement millsat Aberthaw and Kanthan Works. Problems arosewith the Abefihaw mill which are associated with the lack of any means of regulating the flowcontrol through the diaphragm. The diaphragm was modified by cutting away four of the ninelifter plates and converting these to an adjustable plate.
This modification proved reasonably successful and allowed greater material retention inChamber 1. See Figure 7.1 and 7.2 for details.
However, the Combidan diaphragmis not seen to be as good as either the Pfeiffer or MagotteauxOpticontrol design. hy fbture Combidan installations must be provided with the flow controlmodifications as per the Aberthaw mill.
The Combidan diaphragm also relies on a good first chamber liner and ball charge to avoid nibs.Clinker nibs can quickly block the perforated screens and there is little self-cleaning action sincethe media is not normally in contact with these screens. The vent grid slot sizes tend to be thesame size as the screens which makes them prone to blinding.
The FLS outlet diaphragm design used at Kanthan and Aberthaw Works also had a tendency tostrip material out of the second chamber. Hence, it was necessmy to block the outer row of slotsto improve material retention.
Generally we note that when using the combidan diaphragm design, FLS/Fuller tend to use acomparatively long first chamber in order to avoid potential nibs blockage problems. This oftenrisks achievingoptimum efficiency from the mill especially when grinding higher S.S.A. cementsas witnessed at Aberthaw.
3.2 Mmzotteaux opticontrol DiaDhra~m
The Slegten diaphragm has been refined to use circutierential slots in a pattern similar to thePfeiffer third generation diaphragm. This improves the self-cleaning action of the slots. Adiaphragm of this design was installed in Cemento Melon No. 21 cement mill in 1992 andappeared to perform well after modifications were made to reduce the size of the centralventilation grid to allow for charge expansion. Some problems have arisen with second chamberfine media back spillage. Care has to be taken when selecting diaphragms for high volumeloadings or when using fine media which can expand. See Figures 8.1 and 8.2 for details of thediaphragm slots whilst Figure 9 gives details of the adjustable scoops used for materials flowcontrol.
During 1994, there were problems with the Cemento Melon No. 21 diaphragm support structure.Severe cractig of the Ih.me segments has occurred and this has been phrtially blamed upon smallmeda backspillage from chamber 2. We, therefore, have reservations about recommending ttis
10
..
FIGURE 7.1- FLS COMBIDAN DIAPHRAGM
..
●9
. 8
Centrepiece 1 Scfeenifgpbte●
Residue space ~ 1 /~ Liftem I
I%=..
““ ●*” .“ “~%.. ●X.●
# 6-. .“”. .“: -. ”..- -. > ..- .
R . . . . . . . “0 IF - “ Ull
.
.
FIGURE 7.2 MODIFICATION TO COMBIDANDIAPHRAGM AT ABERTHAW WORKS
MODIFICATION - REPLACE 4 OUT OF 9 LIFTERS [FIXEDIBY MOVEABLE LIFTERS
.,.
I
I
[/////////
I I I
i
/
INVERTED PLAN.
FIGURE 8.1- MAGOTTEAUX - OPTICONTROL DIAPHRAGM
.“
. —————— .
L ~-’—. —————
&_ -—
=.,E—
—.
_——
—
..—-~ .—.
—3k=
—-.——_—.~—-
——
.
—
SELFCLEANING
SLOTPROFILE
FIGURE 8.2
(
A) PROBLEM - MATERIAL FLOW THROUGH VENT GRID
SOLUTION
>,-
B) CLOSURE PLATE - INSIDE VENT GRID.. ,
FIGURE 9- FLOW CONTROL SCOOPS
The scoops can easily be adjusted byturningthem on their axes,
Position A will not allow the scoop topick up material, thus the diaphragmfills up with material,Position B shows the scoop partiallyopen,If set at position C the scoop is fullyopened and will pick up the maxi-mum amount of material. If severalscoops are in this position, the levelof material in the diaphragm willdecrease,
The differentsettingsof the scoopswillnot affect the mill output:
Q scoop output= S usetd area X H active height
= constant
•1AmI!!#l
U.-,,*H.<,
I
diaphragmfor larger mills (ie above 4.OM diameter) especially thnse with fine media and chargeexpansion characteristics. ,!
‘,
Magotteaux have modified their design and their latest Opticontrol diaphragm design will beinstalled in the new Kanthan CM5 and Pasir Gudang cement mills. Before ordering ,my newOpticontrol diaphragms it is important to check that the latest improvements are included.
Before ordering this diaphragm always check the required volume loadings and whether or notthe millhas a tendency for charge expansion. For example, using the DAWN spreadsheet, we canestimate the followingmaximumventilation@d sizeswould be n@ed for Cauldon No. 4 cementmill with its Magotteaux intermediate and outlet diaphragms.
Mill Chamber 2 Height Above Maximum VentkW Volume Load Charge Grid Diameter
‘/0 (m) (mm),,
1418 38 1.726 5521406 37 1.748 5961394 36 1.771 6421368 34.15 1.815 -’ 749
(Present) (Actual)
Allowances for charge expansion when using fine media (25-1 5mm) in chamber 2.
Typical = 2-4% higher v~lume load
Worst cases = 7% higher volume load
3.3 Pfeiffer Third Generation Dia~hragm
This diaphragm has given good results in USA and Malaysia. The flow control is easy to adjust.,There is only one area of concern and that relates to the diaphragm segments. As with earlierPfeiffer diaphragms, where the segments meet there are areas where any movement of thesegments can allow gaps to occur. Whhout any backing plates behind these points, the gaps canallow coarser materialshibs to enter chamber 2.
Ailer discussions between BCTC Chiistian Pfeiffer have a~eed to modiijr their diaphragm!Details of the diaphragndmodifications are shown in Figures 10.1 and 10.2.
Whh this modificatio~ the Pfeiffer design is believed to be very suitable for closed circuit milk,especially when operating with high circulating loads. Care has to b,etaken with open circuitmil.1$since many of the modem lifter diaphragm designs can tend to strip material out of the firstchamber.
,’
16
FIGURE 10.1- PFEIFFER INTERMEDIATE DIAPHRAGM
Self-Cleaning Feature
The front side of the slottedplates have continuousconcentric grooves whichcoincide with the slots. Ballsare guided by these grooves.Material which wouldotherwise plug the slots isforced through. This self-cleaning effect assures the freearea for material flow.
III I
II1I18I1#itII11I1,I1IItI1I
FIGURE 1092 PFIEFFER DIAPHRAGM MODIFICATION
SKETCH OF
ADDITIONAL
DIAPHRAGM SUBSTRUCTURE
BETWEEN RINGS OFTO CLOSE UP GAP
SLOTTED PLATES
Please note that the earlier first and second generation Pfeiffer diaphragms tended to be lesssuitable due to the lack of flow control which tended to result in underfilled chambers. Whenordering replacement Pfeiffer diaphragms ensure that:-
(a) Design is third generation type.
(b) Specifi maximum volume loading required in both chambers. The design issuitable for volume loadings up to 40°/0.
3.4 Ma~otteaux Airfeel Dia~hragm
A recent Magotteaux development,the firfeel diaphragm has yet to be evaluated, but is probablynot suitable for open circuit applications. We have concerns over the flow control action of thisdesign, which is mill ventilation rate dependent.
19
APPENDIX ITB
NOTES ON THE PERFORMANCE OF USA CEMENT MILLS
We are canying out ongoing surveys of the USA plants which have shown the following:-
s When testing the cement millsgenerallygive good efficiencyfigures when compared withthe BC grindability test.
● whilst there is no singIecommon design basis and there are many different combinationsof mill internals, separators, etc., some aspects of the mills are similar i.e.:-
Common use of grinding aids.Mills operate at high circulating loads, i.e. 300V0.Conventional separators are of large size in order to cope with high circulatingload.Separator efficiency is good considering ageldesign.Use of high liil liners such as the Lorain type.
● As an example, at Tulsa Works, the millswere tested and were found to be operating witha mill efficiency of around 125°Abased upon the Blue Circle grindability test data. Thisefficiencylevelwas the same as our target value for closed circuit mills. However, it wasapparent that the mill internal conditions were very poor with poor linings in Chamber 1and excessive levels of nibs. The mills were producing around 34.5 stph (31.3 tph) ofType 1 cement at 3500 Blaine and grinding aids were used. Hence, although it was to beexpected that the mill pefiormance could be improved, it was difficult to predict to whatlevel.
“’Themills were subsequently modified by installation of the following:-
Pfeiffer intermediate diaphragms with high transport rates
Lorain lifters (simple bar lifier type desi~ non classi~ng)
Ball charge
Separate mill ventilation installed plus minor circuit modifications’
Mer optimisation, the mills achieve 36-38 stph output, which implies a mill efficiency of 130-138’XO.
The milloutput was higher than expected due to the practice of running over the rated motor load(these millsare typically run at 1240 kW compared with nominal mill motor ratingof1119 kw)with high media volume loads.
NOTE:
When ordering new diaphragms for USA mills, etc., ,wherehigh volume loadings are used (36-I~
40??) - always speci@this to the diaphragm supplier so that their design can be suitably modified.The Pfieffer diaphragm has various sizes of inner diaphragm segments available to suit differentvolume loadings.
Hence, a combination of the following factors:-,,
Use of grinding aids.
Mill design fatures as summarisedabove appears to permit an increase in the BCtarget mill efficiency to around 135% for closed circuit mills.
‘1
,’
,.
2
SECTION C
CEMENT MILL PERFORMANCE TARGETING
SECTION C
CEMENT MILL PERFORMANCE TARGETING
CONTENTS
1. TARGET PERFORMANCE FOR CEMENT MILLS
1.1 Mill Power Drawn1.2 Mill Running Time1.3 Mill Charge Levels
2. CEMENT MILLING TARGET OUTPUT AND CEMENT RESIDUEFOR A GIVEN SURFACE AREA
2.12.22.32.4
2.52.6
2.7
Target Performance for an Open Circuit Cement Mill - ExampleTarget Performances for a Closed Circuit Cement MillOpen to Close Circuit Mill ConversionMill Surface Production Factor Method for Targeting Closed CircuitCement Milling PerformancesMill Net to Gross Power RatioMill Surface Production Factor - Target Values for different closed circuitmilling systemsSumma~ of Closed Circuit Mill Pefiormance Targeting
3. HIGH EFFICIENCY SEPARATOR CONVERSION EXAMPLE
3.1 Results Obtained from Conversion3.2 Effect of Changes on Mill Output
4. BENCHMARKING CEMENT MILL PERFORMANCE
4.1 Methods Used for Benchmarking Cement Mill Performance4.2 The 1994 Benchmarking Results for Cement Milling4.3 How To Improve Cement MNing Performance4.4 How to Improve Benchmarking Methodology4.5 Conclusions
1. TARGET PERFORMANCE FOR CEMENT MILLS
It is important to set a target performance level for any milling system in order to identifi itspotential performance. This paper sets out some guidelines on how this can be achieved. Thefactors influencing the overall annual average performance of a mill are considered individually.
1.1 Mill Power Drawn
With adequate maintenance,it shouldbe possibleto maintaina millat its fill absorbed power overlong periods. However, in practice, there can be limitations to achieving this, such as:-
Peak demand periods resulting in mills having to run continuously without maintenance,e.g. charge replenishment, diaphragm cleaning, liner replacement etc.
Resource limitations, e.g. lack of Iabour, limitations on capital/revenue expenditure onreplacement mill internals etc.
Other plant limitations, e.g. mill gearbox, product transportation, mill circuit designlimitationsetc. A combinationof these factors could mean that the installed motor powerof any mill may not be filly utilised.
For example, there is no merit in charging a mill to draw till power if (a) the additionalmillthroughput cannot be handled by, say, the cement transportation system or if (b) theperiod between major gearbox repairs is reduced to unacceptable levels. (Note the aboveare real limitations currently applying to mills within the Group).
For target purposes, the following figures can be considered.
Target annual average power =97% of installed motor power
Typical “Best” annual average = 100?40installed power
Many mills,especiallyin the USA consistently run above the design motor load! Check on typeof motor, type and design of gearbox, motor cooling, start up loading etc before giving anyconsideration to this method of operation. The long term effects upon motor/gearbox life etchave to be considered.
The maximum power input to a ball mill will depend upon the efficiency of the motor andgearbox.
A typical breakdown of these is as follows:-
Mill motor
Mill motor efficiency
Maximum mill motor input power
Gearbox efllciency
Net power to mill
Overall power losses
Net/gross power ratio
~ ie. 6°/0power losses,
= 2238kW
= 95’?40
,’
= 2356kW
= 99??
= 2238 X 0.99
= 2216kW
= 2356-2216 = 140kW
,2216—=0.94’= 2356
-.
Hence, the maximum input power to the mill gearbox and the mill motor efficiency must bechecked before deciding the maximum input power. At the same time, the mechanical conditionof the gearbox must be considered
1.2 Mill Running Time
● This is subject to considerations such as maximum demand and the other plant equipmentwhich has to be run. Maximum demand considerations may typically make it desirable tostop the mill for, say, 4-5 hours per day for periods of time.
● Atypical target annual average mill running time figure is 85% assuming no major M.D.limitations. ,.
● “Best” annual average mill running times are targeted at 90Y0.
● Do not confhse the difference between peak and average running time, i.e. during peakdemand periods, many millsmay exceed 90% ruining time for several weeks. However,even when there is consistent high demand all year round, an annual average figure of90% is a high target to achieve.
2
1.3 Mill Chmwe Levels
● As the charge level in a mill is increased, the power drawn per additional tonne of medialoaded reduces since the centerline of the media gets closer to the centerline of the mill.
● In addition to the limitations of mill motor and gearbox sizing, there may be certaininternal limitations which limit the maximum charge level, i.e.
Size and design of mill inletioutlet trunnions.
Lining plate type/design and lining thickness.
Type and design of diaphragms.
Do diaphragmshave centralventilation grids or solid centres? A large central ventilationgrid may limit the maximum volume loading - since above a certain level, the media andmaterial may be above the levelof the vent grid. In this condition, there is a risk of coarsematerial entering chamber2 via the vent grid instead of being held back by the diaphragmslots.
Certain types of smooth lining plates allow slippage of the charge (e.g. Voest-Alpinegrooved liner plates) which can reduce the maximum usefi.dpower drawn antior resultin high volume loads to draw fill motor power!
Relativelythick liner plates such as classivlng linings, can result in a loss of mill internaldiameter. This in turn can result in higher volume loadings to draw fill motor power.
Check the design of any mill trunnion liners. A scroll should be fitted to help transportmaterial (and any media thrown back) into the mill.
Expansion of the mill charge caused by accumulation of unground material or by highthroughputifine media can also limitmaximumcharging levels. Always check the volume.loading with the mill run out and compare this figure with the levels taken afier a crashstop (see example in i4ppendix VA of Mill Testing paper A).
● Hence, the above factors need to be taken into account before considering the maximumvolume loading/power drawn for any mill. Always be aware of mills which have been“over-motored” such as the 1600 HP (1 194 kW) mills which have 1200 HP (895 kw)shells and a usefid power drawn limit of around 1000 kW. Just because the mill motorplaque gives a figure of X kW installed power, this does not inevitably mean that 100%of X kW can be usefully absorbed during operation! If there are any limitations whichlimit the maximumpower drawn (e.g. liner/diaphragm type etc) - then consider ways andmeans of overcoming these by, say, the use of alternative mill internals. In the case ofCookstown Works two 1600HP cement mills, the mill shells were replaced by largerdiameter shells in order to allow the maximum motor power to be used.
3
2. CEMENT MILLINGTARGET OUTPUT AND CEMENT RESIDUE FOR A GIVEN SURFACE AREA
● The cement particle size distribution for a given surface area can be characterised by theRosin Ramler slope (taken from the slope of the psd plot on Rosin Ramler graph paper).
Efficient operation of a cement milling system is usually indicated by a steep R-R slope,and lower cement 45 micron residue for a given surface area.
Table 1 shows how the cement residue and sutiace area can be used to predict the R-Rslope. Table I also shows typical cement residues and R-R slopes for open and closedcircuit cement mills.
This comparison can give an indication of the relative efficiency of a milling system.Please note that the comparison applies to neat (i.e. clinker plus gypsum only) cements.When @ding extended (e.g. limestonefilledor pozzolanic) cements, the relationship willnot be the same as shown in Table 1.
Ifa cement mill produces a range of cement types, examine the SSA/R-R slope data foreach cement type. This can help identifi those milling systems with (a) reasonablyefficient performance at low surface areas, (b) poor efficiency at high surface areacements. This is typicalof the 6000 HP cement millswith FLS-CV separators which havepoor performance at high surface areas (380-400 m2/kg plus) due to poor separatorperformance.
● Cany out a cement gnndability test to estimate the theoretical performance of a mill.
The BCI gnndability test provides ve~ usefid data which needs carefil interpretationbefore it can be used to target a mills petiormance. However, the results are open to awide degree of interpretation and care has to be exercised when applying the data todifferent milling circuits.
The following example has been chosen to demonstrate how an open circuit millperformance can be assessed and targets established.
5
2.1 Tar~et Performance for an O~en Circuit Cement Mill - Examde
Using the BCI cement grindability data, together with the actual mill operating data, theperformance of an open circuit mill can be established thus:-
Mill output =
Mill power drawn =
Cement sufiace area =
Cement 45 micron residue =
Rosin Ramler slope =
BCI cement grindability =
From grindability curve -Revs/lb at 380 m2/kg =
12.3 tph
640 kW gross
380 m2/kg
14% retained
0.97
109% (at 300 mz/kg)
105 revdlb
Hence, Theoretical Gross power consumption
Revs/lb x 0.381= kwhh0.9
= 44.5 kWh/t (Gross)
Note: In this method the power losses for the mill motor and gearbox areassumed to be 10O/O.Hence a netigross factor of 0.9 is used above.
Actual mill Gross power consumption
640= =’52.0 kWh/t (Gross)12.3
Mill efficiency100 x Theoretical gross kWh/t=
Actual gross kWh/t
100 x 44.5=52
..
This compares with the following Target open circuit milling eficiency:-
0/0 Mill Efflciencv
Open circuit mill without grinding aid
(See notes in section 2.2 for fhtiher details)
Hence, the performance of this mill at 85.6?40efficiency
115’%0
is very poor!
Using a target figureof115?40 milling efficiency for this open circuit mill,
Target Gross kWh/t (mill only)
If the millis chargedoutput would be:-
44.5= 38.7 kwhh
1.15
to its design motor load of 709 kW, the potential maximum
709 = 18.3 tDh (Maximum)38.7 ‘“’ ‘
For OPC cement, an efficient open circuit mill could achieve 7% residue at 380m3/kgsurface area, i.e. half of the current cement residue.
Target residue = 7% at 380 m2/kgat 1.1 R-R slope
Action reauired
● Carry out fill detailed inspection of mill internals.
● Cany out axial sampling tests and use datahesults to optimise mill mediagrading.
● Check mill ventilation and mill cooling systems.
● Draw up action plan and establish improvement programme, i.e.modernisation of mill liners, diaphragms, feed system, installation of newmill ventilation bag filters etc. Use BCTC cement mill optimisationguidelines and per Section B.
● Repair mill shell and overhaulmotor load to be drawn.
mill gearbox in order to allow the fill mill
7
2.1.1 Limitations when targeting Open Circuit Cement Mill Performance
The BCI Target open circuit mill efficiency is not a fixed parameter for the following reasons:-
● The use of grinding aid can increase the target efficiency figure by 5-1O%.
● It is possible to achieve higher efficiency levels in certain mills with the followingcharacteristics: -
Under
Clinker feed of small size and relatively soft to crush
Mill optimised for fine grinding with the minimum power consumption requiredfor first chamber crushing. ,’
these circumstances, open circuit mill efficiencies of 120’%have been achieved.
● The target efficiency is dependant upon the mill design and the finished cement surfacearea. Hence, the production of higher surface area cements usually results in a lowertarget efficiency figure for the following reasons:-
More cushioningof charge, coating problems due to finer material within the mill.
Remember the shape of the grindability curve and the reasons why the slope ofthe cuwes is less at higher surface areas!
Reduced output and limitations to mill cooling airflow/water ‘injectioncan causeproblems with milltemperature control at higher surface areas. This worsens thepotential coating “kndcushioning problems.
The target mill efficiency figure could thus be modified to allow for higher surface area...cements as follows:-
-
115 300110 325,105 350100 375
8
The “best” open circuit mill data for BCC UK works shows a maximum milling efficiency of120?40for 350 m2/kg. Hence, in the example given in Section 2.1, whilst the mill could achievea target efficiencyof 1150A,it would be safer to assume a figure of 100°41giving the following:-
Target Gross kWh/t (mill only)
_44J=— 44.51.00
Mill output at the design 709 kW
709= — = 15.9 tph44.5
This is still 29.3% higher than the current mill output of 12.3 tph.
2.2 Target Performance for a Closed Circuit Cement Mill
Here agai~ a simplistic approach which can be used is to takemilling efficiencies.
the following standard target
SYN!an Target ‘AMill Efficiency
Closed Circuit Mill- no gfinding aid 125%
Closed Circuit Mill- with grinding aid 135%
Urdiortumtely,there are limitationswith this approach and these can be shown up by the followingcases: -
● Closed circuit mills with older separators or undersized separators - these mills tend tosuffer a deterioration in milling efficiency at higher surface areas. The problem can oftenbe compounded by millcoolig and coating problems. Examples of this are the 6000 HPFLS cement mills with poor C.V. type separators and with poor mill cooling andventilation facilities.
● U.S.A mills or modem mills designedwith large efficient separators, high mill circulatingloads, etc. Under these circumstances the target mill efficiency will tend to increase withincreasingcement surface area. Allowancecan be made for this within the formula for netpower consumptio~ i.e. using the FLS formula with the grindability test data for netpower consumption:-
Open circuit Net power consumption = 34.0 kWh/tonne
Cement sufiace area = 345 m21kg
Equivalent power consumption for closed circuit milling with a high efficiency separator
=
Where factor =
=
Hence, closed circuit power consumption
34.0 x factor
1.2- (0.001 x m2/kg)
1.2- (0.001x 345)
0.855
0.855 X 34.0
29.1 kWh/tonne net
10
2.3 O~en to Closed Circuit Mill Conversion
The above formula is useful in so far as it demonstrates the benefits of Open to Closed circuitconversion. The formula shows that greater mill power ,consumption savings are achievable ‘athigher cement surface areas. The above formula demonstrates the known facts that:-
●
●
Open to Closed circuit mill conversion will not produce significant power consumptionsavings at low surface areas (unless the conversion also improves milling efficiency byoptimizing the mill internals or coolinghentilation aspects).
Open to Closed circuit mill conversion is most beneficial when producing high sufiacearea cements. Part of the benefit can arise from the “BktineBonus” in which the same 28day strengths can be achieved at a lower surface area due to the lower cement residues.Care has to be taken to ensure that any loss of early strengths does not cause problemsor limit the “Blaine Bonus”.
Any Mill power savingsmust be offset by higherancilkq power consumption costs whichcan often result in higher overall milling energy costs, i.e.
Ancillarv kWh/tonne cement
Conventional separatorMechanical conveying ofcement 4-5
—High Efficiency separatorwith pneumatic conveying 8-10
The claims for outputipower consumption benefits must always be tested. Appendix I,shows an example in which a 16.2V0gain in output was claimed from converting a millwith a medium efficiencyseparator to a high efficiency type. However, when the benefitsof the conversion are examined more closely, it was found that the separator conversiongave only 3.6’?40of the overall output benefit and the major gains were due to optimizingthe mill charge and media grading!
11
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2.4 Mill Surface Production Factor Method for Tarzetin~ ClosedCircuit Cement Milling Performance
BCTC currently use a different approach to targeting closed circuit cement mills with efficientseparators. This method uses two approaches, as follows:-
● From the cement gtindalility cuwe, estimate the current millingefficiency using the simpleapproach shown in the open circuitmillexample. Compare the milling efficiency with theabove targets of 125% (no grinding aid) and 135°A(with grinding aid). In thk method,the mill power losses are always fixed at 10’?4o,giving a net/gross power ratio of 0.9.
,s Estimate the Mill Surface Production Factor (M.S.P.F.) for the milling system. TheM. S.P.F. is a measure of how efficient a closed mill is when compared with the BCIgnndability cuwe.
Whh reference to Figure 1, the BCI grindability curve, it can be seen that the curve is usually astraight lineUPto approximately200 m2/kg. Extendingthe same slope of this line up to the targetcement surface area would give a graph of net power consumption versus surface area. If theACTUAL net power consumption of the mill fits on this extended line, then the Mill Surfaceproduction factor is 1.0.
● When the perfiorrnanceof several U.S.A. cement mills was compared, the following wasnoted
A plot of actual net mill only kWh/tonne versus surface area on the grindabilitycurve gave many points above the line for U.S.A. cement mills..—
The slope of the line obtained through the actual operating data was typically 5-10% steeper than the linear part of the gnndability curve, i.e.
M.S.P.F. = 1.05 tol.lo
The millsexaminedall used grindingaids and operated with high circulating loads—with large efficient separators.
● Examination of some conventional UK closed circuit milling installations withI@h/mediumefficiencyseparators, low circulating loads(150Yo) and no grinding aid gaveactual operating points some 5-10’%0below the slope of the linear part of the grindabilitycuxve, i.e.
.
M.S.P.F. = 0.90-0.95
13
2.5 Mill Net to Gross Power Ratio
In this method, the ACTUAL net power of the millis used rather,than the fixed factor of 0.9 usedfor B.C.I. millingefficiency. The actual net to gross power ratio will depend upon the type of milldrive system and is typically as follows:-
Mlll Drive ~Net/Gross Power Ratio
Efficient Central Drive Mill 0.94-0.95
Efficient Girth Gear Drive Mill 0.93
Inefficient/older Gh-thGearDrive Mill 0;90
For fimther details please refer to the additional notes in Section 3.1 of part A (Mill Testing)
2.6 Mill Surface Production Factor Tawet Values forDifferent Closed Circuit Millin~ Svstems
From the above study, the following Target Mill Surface Production factors have beenestablished.
MILL SYSTEM TARGET MILL SURFACEPRODUCTION FACTOR
Conventional Closed Circuit Mdling SystemCirculating load 100- 200%No grinding aid 0.9-0.95Conventional Separator
Medium/High Efficiency SeparatorNo grinding aid 0.95-1.05Circulating load 100- 200%
MediudHigh Efficiency Large SeparatorHigh circulating load (300 - 400%) ..
Grinding aid 1.05-1.10Mill internals optimised for above method ofoperation with high material transpofiation rate
BCTC have developed computer spreadsheets for use with the above mill targeting methods.Care has to be taken when establishing targets and fin-thermethods are also available to assist inthis process, e.g. the B.C.I. closed circuit mill and the mill media models.
14
2.7 Summarv of Closed Circuit lt4ill Performance Tar~eting
Hence, in summary, BCI recommend that ~ methods for targeting mill performance levelsshould be used for closed circuit milling systems, i,e.
● BCI Milling Efficiency
s Mill Surface Production Factor.
The target efficiency should then be established by comparing both sets of results and making abalanced judgement.
All targets for-milling performance must be realistic and achievable. Hence, the targeting datacannot be considered alone and supporting data from axial sampling tests and mill inspections isalso essential. Avoid setting too high a target performance if the evidence from axial samplingtests and inspections does not reveal any obvious areas of inefficiency. Bear in mind that millinternals have finite lives before wear/tear affect their efficiency. There is no point setting thetheoretical maximum efficiency target now if it is not planned to replace the mill internals forseveral years.
15
..
3. HIGH ElWIClENCl’ SEPARATOR
CONVERSION EXAMPLE
3.1 Results Obtained From Conversion
Before Afler 0/0Increasein @tDUt
Output (tph) 126.3 146.8 16.2%
Surface Area 391 382M2~g
KW mill motor 3900 4230 ~
KwH/t30.9 28.8
Separatorbypass %
20 10Rejects surfacearea m21kg
115 86I Rosin-Rammler
slope 1.15 1.17
.
16
3.2 Effect Of Changes On Mill Outrmt
@ Increasing mill charge
Uprate pneumatic conveying system
Repair mill gearbox
Mill motor load increases
Predicted output
Output gain
s Immove mill efllciency
By use of coarser media
Mill efficiency
Output gain
3900-4230kW
= 126.3 X = = 137 tph3900
= 8.5V0
n Chamber 1 and finer media in Chamber 2.
123- 128?40
= 4.1’%0
TOTAL DUE TO MILLINGIMPROVEMENTS = 12.6°A A,
● HiizhEfflciencv Separator
Reduced surface area of cement (Blaine Bonus) for same 28-day strength.
Predicted output = 126.3 ~’”5(381)
= 130.8 tph
I OUTPUT GAIN = 3.6% I
17
4 BENCHMARKING CEMENT MILL PERFORMANCE
Blue Circle Tectilcal Centre carry out an annual review of performance of the cement millsoperating within the Blue Circle group and its associated companies. For this exercise the BlueCircle conventional mill efficiency method is used together with a simple formula for thegrindability curve. The results of this exercise and the conclusions drawn are presented in thissection.
4.1 Methods Used For Benchmarking Cement Mill Performance
,The first attempt at benchmarking of cement mills (i.e. primary benchmarking) that was carriedout was a simple comparison of specific power usage per tonne of cement for the whole millingsystem. This approach could give a fake impressionas to the milling efficiency as no account wasmade of the following factors:-
● Type of milling system (open/closed)● Cement grindability● Target surface area● Use of grinding aid
In order to make a more realistic comparison between individual mills, a more comprehensivemethod of secondq benchmarkingwas used to assess the 1994 performance data. This methodtakes into account the following factors.
4.1.1 Mill Absorbed and Installed Power
To achieve m@mum output from the mill, it is essential to rpn the mill motor at it’s maximumabsorbed power. Practically, this is not always possible but for the purpose of benchmarking thisshould be assumed to set the production target.
It should be noted that certain B.C. Cement Mills (e.g. Plymstock CM2, Cauldon CM4) cannotdraw the maximum motor load due to their internal design i.e classifying liners reduce internaldiameter.
Other B.C. cement mills (e.g. Tul~ Aberthaw CM1O)tend to run with an absorbed power abovethe installed power. Remember that the motor input power may be 5-6% higher than the ratedoutput power.
4.1.2 Surface Area and Grindability
Milling systems throughout the group have both significantlydifferent feeds, both in terms of feedmixture and grindability, and the cement from these systems has a wide variation in productspecific surface area. To take account of these variations and to allow an appropriate target tobe set, B.C. cement grindability tests are earned out on the clinker, gypsum and additives used.BCTC have built up historical data concerning individual works ,cement “grindabilitydata. Recentdata is lacking for some plants. Hence the historical data has been used with the appropriatecorrection factors for any changes in S03 levels, additive dosage rates etc.
18
Prediction of the theoretical power consumption is based upon the grindability curve (see typicalexample in Figure 1) which relates the specific surface area of the cement to Revs/lb of the testmill. The latter is directly related to the theoretical mill power consumption by the formula
TheoreticalMill onlyGross ,kh/tonne — (Revs/lb at product SSA) x 0.3815—
0.9
The mill only power consumption is compared with the theoretical kwhhonne to give the B.C.conventional milling efficiency which is defined as follows:
Conventional B.C.Milling Efficiency YO = ~~. x Theoretical Kwh/t
Actual kwhlt
4.1.3 TvDe of Millinp Svstem and the target B.C. Millinp Efflciencv
The BCI grindability test is an approximation of the open circuit milling situation, although theactual specific power is expected to be less than that predicted in the laboratory. Similarly, thespecific power for a closed circuit mill is also expected to be less than for open circuit. Inaddition, the use of third generation separator will yield lower specific power than for first orsecond generation separators. The specific power value obtained from the grindability curve canthus be converted to the target kwhhonne for open or closed circuit mills by using the followingfactors.
0/0 B.C. Milling EfficiencyWithout ~
MlllinRSvstem Grinding Aid Grinding Aid -
Open Circuit 115Closed Circuit with
125
conventional separator 120 130Closed Circuit with
high etliciency separator 125 135
This assumes that using a conventional separator or a high efficiency separator increases thepotential millingefficiencyof an open circuit mill by around 5 and 10% respectively; while use ofa grinding aid in all three millingsystemsincreases the potential milling efficiency by around 10O/O.
If the VOgrindability figure is known, the BC grindability curve can be related to a simple cuwefit. The conventional mill efficiency (E) can then be determined using the following formula:-
E[
GxS
1K
11.5 - [0.044 (s/100)3]
19
Where K = actual energy consumption mill only (kWh/tonne)G= BCI grindability (??) ,’
s = Lea and Nurse Specific $.uface area (m2/kg)
4.1.4 Overall Targeting of the Cement Mill Output
The above factors are combined together as follows:-
InstalledActual Output x Motor (Power) x Target(tph) (P) Efficiency (Yo)
Target MillOutput (tph) =
Absorbed motor kw x Actual % mill efficiency (E)
Hence, to be a “top petiormer” a cement mill must be kept up to charge to draw its rated powerand must have the optimum configuration of mill internals with adequate ventilation and cooling.
4.2, The 1994 Benchmarking Results For Cement Milling
The results for open and closed circuit cement rnllls are show in Figures 2 and,3 respectively(secondary benchmarking data). In overall energy consumption terms, the cement millingkwh/tonne figures for 1992-94 are shown in Figure 4. This data is primary benchmarking dataand does not distinguish between types of mill, surface area targets etc.
4.2.1 Interpretation of Results
The performance data is expressed as “?40 of target” based upon the above parameters. Cautionhas to be exercised when interpreting results and a detailed knowledge of individual mills isnecessary before leaping to conclusions. Some conclusions which can be drawn are as follows:-
,’
The closed circuit cement mills 1,2 and 3 at Ravena Works have good milling efficienciesbut are “overrnotored. Hence the maximum mill usefhl power is taken as 4000 HP andnot the installed 4,500 HP.
● ’
●
●
The Ewekoro/Shagamu millingefficienciesare too high ‘dueto the high surface area fromprecipitator dust addition.
As mills NOS. 1, 2 and 3 at Rawang were converted in the latter half of 1994 the targetoutputs were calculated assuming a high efficiency separator was in operation.
The pozzolanic cement ground at Cemento Melon, Chile, tends to reduce mill internalcoating problems and milling efilciency levels tend to be high.
20
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As the absorbed mill powerpowers were assumed:-
●
●
●
I
●
●
●
No. 21 millNo. 16 mill
appears higher than the actual at Melon the following absorbed
4230 kW890 kW
No. 10, 14 and 15 mills 700 kW
Note that the age/mechanical condition of several cement mills ‘hasresulted in the millpower being limited due to shell cracking problems. This applies to Cemento Melon 10,14 and 15 mills.
The use of classif@g liners in certain cement mills prevents these mills from drawing themaximum motor power. Examples are:-
Ravena MillsCauldon CM4Plymstock CM2
.TheBenchmarking exercise is based upon data for the ,maincement produced. In casessuch as Aberthaw CM1O,it is not possible to fhlly optimise the mill for Bulk OPC whenit also has to produce Bag OPC and RHC. This results in a compromise to millingefficiency i.e. chamber 1 has to have sufficient power to cope with 90 tph of Bag OPC.This results in power wastage when grinding Bulk OPC as 78 tph.
Hence some compromise of milling efficiency is unavoidable where a single fill has toproduce several cement types with a wide range of outputs.
The r~orted grindability from Aberthaw of 127% corresponds to a neat clinker/gypsumcement. In practice as around 4°/0limestone is added agrindtillityof117°/0 was assumed.
The reported grindabdityfrom Dunbar is regarded as being slightly higher than typical andfor the calculations a lower value of 139% was assumed.
The reported grindabilities for Rawang and Kanthan were determined in Malaysia.Previous results showed that BCTC obtained lower values and consequently the current -reported values were reduced accordingly.
The swing mill at Tulsa was not included in this exercise as it is also used to supplementraw fked production.
Technical survey data has shown that the efficiency of the SVeardale cement mills isamongst the highest for open circuit mills. However, when the rpill installed power figuresare examined, these are generally just below the installed power.
The Westbury cement mills tend to use more power than the installed motor power (notexceedng the motor input power liinitations). Hence the milling efficiency compares wellon an installed power basis.
24
● Detailed testwork on the Weardale and Westbury open circuit mills do not support theirrelative rankhg. Whilst the Westbury mills tend to run at higher power consumptionthan Weardale, the Westbwy milling efficiencies are generally lower than Weardale’s.
● Hope and Northfleet works have 6000 HP closed circuit cement mills. It is known thatthe efficiency of these mills are good when producing low surface area cements.However, millingefficiencylevelstail off when the millshave to produce high surface areacements due to the very poor pefiorrnanceof the CV. separators and mill cooling/coatingproblems.
4.3 How Can We Act On These Results To Imwove Cement Millhw Performance
To illustrate how the benchmarkingresults can be used to establish targets, an example of a 1491kW closed circuit cement mill is given below:-
4.3.1 Current Performance
Mill output =
Absorbed Power =
Cement Surface area .
BC Cement grindability =
BC Conventional mill efficiency =
37 tph1368 kW320 m2/kg122%109VO
4.3.2 Potential Mill Performance
Target millefficiencyfor closed circuit millwith high efficiency separator without grinding aid =125V0
Potential output at 1491 kWand 125°/0eflciency
Actual output as% of target
.37 XM2XE1368 109
= 46.2 tph
= 100” x x- = 80.lVO46.2
25
FIGURE 5
CM Axial Test
Residue (%)80
60
40
20
SSA (cm’/g)
let Chamber - central 2nd ChamberDia
\!/
/l\
.
.
.
.
!-,
.
IoABCD
Av.
400
80
60
40
20
0
ABCDEF
2.36mm 1.18mm 300 micron 90 micron 45 micron SSA+- * “+
media size (mm)
let Chamber
67.5
ABCD
Bntra
Dia
240
220
200
180
- SSA (cm’/g)
I SSA Av.mediasize (mm) I
160
140
120
%d Chamber
40.3
ABCDEF
220
200
180
160
140
120
4.3.3 Mill Inspection and Axial Sampling Test Results
Figure 5showsthe fill tidsampling test results and media grading. Theconclusions drawnfrom this data plus the mill internal inspection showed the following:-
● The mill first chamber is too long at 34’%0. This should be shortened to 28’%0of the totallength. This will allow more fine media to be added in chamber 2 (LONG TERM).
● The ixiterrnediatediaphragms have large diameter central ventilation grids. Modi~ theseby fitting internal retainingrings to allowthe volume loads to be increased - note that millcurrently only 91.8?40of its rated motor power. (SHORT TERM).
c Replace the classifyingliner in chamber 2 by a Dragpeb liner to permit higher ball charge.The classifyingIinerreduces the internaldiameter and this also limits the maximum powerdrawn (LONG TERM).
● Regrade chamber media to BCTC recommended grading (SHORT TERM).
● Regrade chamber 2 media and remove media of 60- 30mm size. Replace this by 25-15mm media (SHORT TERM).
Hence, the benchmarkingresults indicate the potential to make a significant improvement to thismillsperformance. Once the targets are established- detailedtestwork and inspections are neededto identifi what we must do to improve the mill. Please note that the improvement programmeshown above identifies short and long term proposals. Obviously there is no sense in replacingexisting Iinersif these are in good condition. However, when the mill requires major repair, usethis opportunity to move the diaphragm/change liner designs etc. Hence the optimisation of anymill has to be an economic compromise.
27
4.4 How We Can ImDrove Our Benchmarking Methodolon
4.4.1 Targeting Methods
The BC conventional mill efficiency method wassimplicity and greater flexibility when dealing with
‘,
chosen for the 1994 data review due to itsmany types of milling system.
However, this simplemethod does have several limitations,in particular, the method tends to set:-
i) a too high a target performance may be set when producing low surflacearea cements(280-300 m’/kg)
ii) a too low a target pefiormance maybe set in mills producing a mediumhigh surface areacement (350-380 m2/kg) if grinding aids are used and large separators are operatedresulting in high circulating loads.
BCTC have therefore recently developed the Mill Surface Production Factor (M.S.P.F) methodfor targeting cement mills. This method is usefid for targeting cement mills in U.S.A. whichfeature:-
Use of grinding aidHigh mill circulating loadsMedium/Mgh cement surface areasLarge conventional separators.
Using the BC conventionalmillefficiency,figures of 140-150% efficiency can be achieved whichare much higher than the target figures shown in section 1.3. The M. S.P.F. method is moreappropriate for targeting mills in USA and Chile but this method also has its limitations.
In the fiture, we may use a combination of these two methods for targeting purposes. Inadditio~ the current technical suweys being carried out on U.K./overseas works will help BCTCto establish a more accurate data base concerning individual mills, their performance levels, thefactors affecting their efficiency as well as their performance targets.
4.4.2 Accuracy of the Reported Data
The 1994 Benchmarking exercise has shown up several anomalies in the data presented. Furtherplant surveys have revealed that some of the data reported by individual works is misleading dueto factors such as:
● Inaccurate assessment of individualmilloutputs due to inaccurate weigh feeders, bookingmethods.
● Mill only power consumption figures are inaccurate due to lack of accurate meteringandlor inclusion of some ancillary plant.
28
● Additives such as Limestone precipitator dust causing major reduction in cementgrindability.
● Artificiallyhigh cement surface areas dtie to dust addition and subsequent hydration of thecement which results in an artificially high cement surface area (if the artificially highperformance of the Ewekoro and Shagamu cement mills).
Hence, the results obtained from the Benchmarking exercise need to be read with some cautionat present. These results can onlybe as accurate as the data reported. BCTC will amend obviousdata errors as far as possible but we do urge each works to report the data as accurately aspossible.
4.4.3 Targeting According to Cement Type
Please note the comments contained in section 2.2 paragraph (vii) concerning the efficiencycompromises which have to be made when a single mill has to produce several types of cement.
The Benchmarkingdata is based upon the ~ product cement only. In time it may be possibleto extend the evaluation to examine individual mills producing several types of cement. Thiscould show up how much efficiency is lost by the need to compromise when producing severaldifferent cements.
..
29
4.5,
●
●
●
●
●
●
●
Conclusions
The benchmarking exercise provides a valuable tool for comparing the performance ofcement mills within the BC Group.
Having identified those mills which need improving, we have to set realistic targets fortheir petiormance.
The target performance should be based upon the following:-
Detailed axial sampling test data.Cement grindability data.A working knowledge of any mill plus ancillary plant limitations e.g. FK pump,bucket elevator, separator capacity.
The target performance should be judged using the above data, common sense as well asthe estimates of- 9
- B.C. conventional mill efficiencyMill surface production factor. 1’
Reference should be made to the latest Cement Mill Optimisation guidelines which areupdated regularly by BCTC.
BCTC are continually updating our know-ledge base on the performance of new millinternal designs as well as the methods for targeting mill performance.
Please consult BCTC engineers prior to mill refurbishment programmed so that theopportunity for improvement is used. Please do not simply repeat old internaldesigns/original mill configurations if these do not give the optimum performance.
30
