fine coal circuitry considerations in treatment of soft coal with difficult washabilities

y 88 (2007) 759–769www.elsevier.com/locate/fuproc

Fuel Processing Technolog

Fine coal circuitry considerations in treatment of softcoal with difficult washabilities

Yunkai Xia a,⁎, JianGuang Li b

a Taggart, LLC, Pittsburgh, PA, USAb Kailuan Coal Group, TangShan, Hebei, China

Received 12 November 2006; received in revised form 4 March 2007; accepted 15 March 2007

Abstract

A typical process used in Chinese metallurgical coal preparation plants employs heavy-media separation to treat the coal coarser than 0.5 mm.The −0.5 mm fine coal is treated with froth flotation. A major disadvantage of this process is that a large quantity of fine coal is recycled in theheavy-media cyclone circuit, which results in high magnetite losses. The −0.5 mm fine coal in the media is a result of poor raw coal deslimescreen efficiency and the continuous breakage associated with the processing of soft coal. Another disadvantage of this typical process is that somecoarse clean coal particles are lost to the froth flotation tailings. This investigation focuses on the simulation of processing fine soft coal withwater-only cyclone (WOC) and spirals. WOCs and spirals have become popular devices for treating fine coal. WOCs can operate at low specificgravity cut points to produce low ash clean coal while spirals tends to operate at high specific gravity cut points and act as a scavenger to rewashthe underflow (refuse) from WOCs. The combination of WOCs and spirals can compete with a heavy-media cyclone with respect to bothefficiency and clean coal yield when treating 1 mm×100 Mesh fine coal. This fine coal processing circuit can subsequently increase the bottomsize of the heavy-media cyclone feed from 0.5 mm to 1 mm which will reduce the loading of the heavy-media cyclone circuit. This change incircuitry thus reduces magnetite consumption without scarifying separation efficiency of the 1 mm×0 size fraction. Furthermore, the reduction ofthe nominal top size of the froth flotation feed from 0.5 mm to 0.15 mm will greatly decrease or eliminate the loss of clean coal to flotationtailings.© 2007 Elsevier B.V. All rights reserved.

Keywords: Coal; Gravity separation; Simulation; Fine particle processing

1. Introduction

1.1. Importance of fine coal circuits

ROM (Run of mine) coal is following a trend towardincreased −1 mm fine coal. The increasing use of continuousminers underground aggravates this problem in that thesemachines commonly produce a raw coal with 50% or more ofthe solids being finer than 6.35 mm. Magnetite losses in aheavy-media cyclone increase as the quantity of fine coal in thefeed increases. This is due to the increasing specific surface areaof fine particles. Fine clean coal also has much higher surfacemoisture than coarse clean coal. The subsequent increase in thesurface moisture of the overall clean coal product negatively

⁎ Corresponding author. Tel.: +1 412 352 6911; fax: +1 412 429 9800.E-mail address: [email protected] (Y. Xia).

0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2007.03.002

affects sale prices. Fine coal processing also consumes largequantities of reagents such as flocculants, as well as collectorand frothers when froth flotation is utilized. The usage of thesechemicals might not only increase the operating costs, but mayalso cause environment concerns. Therefore, when fine coalconsists of a large percentage of the raw coal feed, carefulconsideration must be taken into the selection of fine coalcircuits whose separation efficiency will have a heavy impact onoverall plant performance. For the fine coal studied in thisinvestigation, the flotation performance is poor due to thesurface oxidation during storage and coarse coal losses in theflotation tailings. The flotation test results (0.5 mm×0) show aclean coal yield of 57% with a clean coal ash of 12.0% and theflotation tailing ash content is as low as 36.65%. An alterna-tive to processing this 0.5 mm×0 fraction with froth flotationis to use gravity separators, such as water-only cyclones andspirals.

mailto:[email protected]

http://dx.doi.org/10.1016/j.fuproc.2007.03.002

760 Y. Xia, J. Li / Fuel Processing Technology 88 (2007) 759–769

1.2. The functions and performance of water-only cyclone(WOC)

The generalized WOC separator consists essentially of acylindrical column with a short conical bottom. Typically, 15 in.(350 mm) diameter WOCs are used effectively in preparationplants to wash the 1 mm×0.15 mm raw coal. Raw coal(1 mm×0) slurry enters the cyclone tangentially through aninlet in the upper part of the cylindrical column. Refuse particlesaccumulate in the cone of the cyclone and create a dense bed ofparticles above the apex of the cyclone, which makes the WOCa gravity separator rather than a classifier. Clean coal (thecyclone overflow) leaves through a central vortex finder pipeand the refuse able to penetrate the dense bed of particles isdischarged through the apex. In general, the major operatingvariables of the WOC include the geometry of cyclone (cyclonediameter, ratio of vortex finder to apex diameter, vortex finderlength, and apex size), solid concentration of the feed, and inletpressure.

WOC shows many advantages in fine coal processing, thesesbenefits include: no moving parts; low operating costs; specificgravity cut points as low as 1.26; very low ash clean coalwithout the use of heavy media; does not require pre-screeningand thus all −1 mm fine coal can be fed directly to the WOC;efficiently clean oxidized raw coal and removes free pyritedown to 100 mesh, whereas flotation will not; and requires littlespace for operations [1,2].

1.3. The functions and performance of spirals

The use of spiral separators in the treatment of fine coals isnow a popular option in fine coal circuitry design. Spiral can beeffectively clean 1 mm×0.15 mm coal at very low operatingcosts. Unfortunately, due to the classification effect of particlesin flowing films, spirals tend to have a high specific gravity cutpoint (1.7 to 2.1) and misplace significant amounts of high ashfines into the clean coal. The specific gravity cut point can bereduced, but it is not economical because the feed rate to spiralmust then be greatly reduced [3,4]. The separation efficiency ofa spiral is also worse than that of a heavy-media cyclone whencleaning 1 mm×0.15 mm coal. Spirals are often used as ascavenger in multi-stage circuits to rewash the refuse fromWOCs [5]. From a process design viewpoint, it is clearlyadvantageous to be able to forecast the results of the selection offine coal circuitry. Different fine coal circuits were comparedand the separation performance computed and predicted in thisinvestigation.

The objectives of this investigation were to maximize theseparation efficiencies in the fine coal circuits, to decrease themagnetite loses in the dense media cyclone circuits, and toeliminate the coarse clean coal lost to the flotation tailings.Specific aims were to:

1. Quantify and determine the cause of significant magnetitelosses in the heavy dense medium cyclone circuit;

2. Quantify the coarse clean coal lost in traditional flotationprocess;

3. Simulate the separation performance of different fine coalcircuits;

4. Quantity the contributions of new WOC–spiral fine coalcircuits to the existing heavy-media cyclone circuit;

5. Increase the plant capacity by the addition of a new fine coalcircuit.

2. Analysis of existing flowsheet

2.1. Existing flowsheet

The flowsheet for treating 13 mm×0 coal in the Qianjiayincoal preparation plant is shown in Fig. 1. The plant feed(13 mm×0) is fed to raw coal banana screens with a 0.5 mmdeck aperture. The +0.5 mm coal feeds the primary heavy-media cyclone (HMC) circuit. The primary HMC refuse isrewashed in the secondary HMCs to produce middlings andrefuse products. The misplaced 0.5 mm×0.3 mm fine clean coalis recovered from the primary magnetic separator tailings by abank of classifying cyclones; the classifying cyclone underflowflows to a bank of sieve bends. The sieve bend oversize isdischarged into fine clean coal centrifuges. The raw coaldeslime screen undersize (0.5 mm×0), the classifying cycloneoverflow, and the sieve bend undersize feed the froth flotationcells. The flotation concentrate is dewatered by a bank ofvacuum filters. The flotation tailing is pumped to a bank ofclassifying cyclones to recover the lost coarse clean coalparticles from trapped in the flotation tailings. Coarse clean coalis lost to the flotation tailings due to the poor kinetics associatedwith floating coarse particles. The underflow from theclassifying cyclone flows to a bank of screen bowl centrifuges.The fine coal centrifuge and vacuum filter effluents recycle tothe flotation cells. The screen bowl centrifuge effluents, whichconsist of high ash fines, flow to the refuse thickener. Thethickener underflow is dewatered with frame filter presses.

2.2. Raw coal washabilities

The expected feed to the preparation plant is Seam 5#, 9#and #12. The analysis of the washability characteristics of thesecoals indicate that a separation specific gravity of 1.50 isrequired to achieve a primary product target ash content up to11.0%. This coal is very difficult to wash because the amount ofnear gravity material (+/−0.1 SG units) consists of 20–25% ofthe feed. Typical coal preparation plants operating at a 1.50specific gravity can efficiently process down to 1 mm in a singleheavy-media cyclone. Float-sink test results of the feed, cleancoal product, and refuse sampled from the 840 mm diameterheavy-media cyclone were used to generate the partition curvesfor different size fractions processed in the heavy-mediacyclone (Fig. 2). The data was collected when processing670 metric tons per hour (MTPH) of 13 mm×0.5 mm feed.Fig. 2 illustrates how separation efficiency in the heavy-mediacyclone (HMC) decreases as the feed particle size decreases,which is in agreement with the results from other investiga-tors [6]. As shown in Table 1, for a 13 mm×1 mm coal, ifthe particles are relatively coarse, a good separation can be

Fig. 1. Existing flowsheet of Qianjiayin coal preparation plant.

761Y.

Xia,

J.Li/Fuel

Processing

Technology88

(2007)759–769

Table 2Particle size distribution in HMC cyclone

Deslimescreen feed

HMC feed HMCoverflow

HMCunderflow

Size, mm Wt.% Ash% Wt.% Ash% Wt.% Ash% Wt.% Ash%

13–3 48.04 31.41 58.58 34.19 17.64 10.73 13.48 60.813–1 16.53 23.33 20.16 23.33 14.96 9.24 39.84 55.431–0.5 15.34 21.31 17.37 21.31 34.78 9.77 15.78 45.480.5–0.25 4.68 19.02 1.75 19.02 7.87 11.23 7.92 41.630.25–0.125 4.31 17.84 0.90 17.84 11.98 13.34 10.61 39.940.125–0.075

2.29 19.70 0.77 19.70 10.11 14.02 10.39 40.31

b0.075 8.83 19.97 0.47 19.97 2.66 15.34 1.98 39.42Sum 100.00 26.09 100.00 29.17 100.00 10.98 100.00 49.96

Fig. 2. Partition curves for different size fractions in HMC separation.


expected. However, with a significant amount of −1 mm finesin the feed, a poor separation is obtained.

When a coal with broad size distribution is washed in asingle heavy-media cyclone, the separating gravity or “offset”increases as the particle size decreases. As the separating gravityincreases, the probability of particles being misplaced (or“cleaning” in-efficiency) also increases. As shown in Table 1,the separating gravity of a heavy-media cyclone circuit washing13 mm×3 mm raw coal is 1.4845 while the 0.5 mm×0.25 mmfraction separates at a specific gravity greater than 1.64. Theseparation density for the 0.25 mm×0.125 mm fraction is ashigh as 1.83 which means high ash particles within this sizefraction are misplaced to clean coal. The high ash finesmisplaced to clean coal will increase the overall product cleancoal ash. In order to meet the product target ash, the heavy-media cyclone must run at a lower specific gravity at theexpense of overall product yield.

The float-sink test shows that this coal seam containshigh quantities of near gravity material present in the 1.40–1.70 gravity fraction range. Treating a coal with such a difficultwashability results in decreased HMC performance and morehigh ash material will report to the clean coal product. As a

Table 1Separation performance in HMC cyclone

Particle size SG50 Ep Yield%

Ash%

Separating gravitydifferences(or “offset”) fromSG50 for13 mm×3 mm

13 mm×3 mm 1.4650 0.0419 51.27 10.93 03 mm×2 mm 1.4831 0.0421 70.04 9.13 0.01812 mm×1 mm 1.4884 0.0420 73.05 9.13 0.02341 mm×0.5 mm 1.5048 0.0707 66.63 9.56 0.03980.5 mm×0.25 mm 1.6972 0.1133 86.72 11.35 0.23220.25 mm×0.125 mm 1.9037 0.1202 94.46 13.26 0.4387

result, the separating gravity must be lowered to compensate. Insuch cases, the size range of the feed to the HMC's should belimited and the finer size fractions should be washed separatelyfrom the coarse material so that the separating gravities of bothfractions can be controlled. This allows both circuits to beoptimized and maximum recovery is achieved.

2.3. Grinding of soft coal in heavy-media cyclone processing

Size distribution analysis of the HMC feed and productsshows that this coal is very soft. In Table 2, the HMC feedconsists of approximately 58.0% 13 mm×3 mm particles;however, the clean coal product contains only 10.73% of+3 mm particles, and the refuse contains 13.48% of +3 mmparticles. Most of the 13 mm×3 mm particles are broken orground into −3 mm particles. It is clearly shown in Fig. 1 that afeed coarser than 1 mm is expected where a poor gravityseparation is unavoidable. In order to decrease the breakage ofcoal, feed rate should be reduced and the feed particle size rangeshould be limited to be as narrow as possible.

2.4. Flotation results

The loss of coarse coal in the flotation tailing was analyzedand the results are listed in Table 3. It is shown thatapproximately 38.6% of the flotation tailings are coarser than0.125 mm, with an approximate ash of 25.6%, which is lowerthan the raw coal feed. The flotation process failed to clean the+0.25 mm size fraction and it should be diverted from flotationand processed by gravity concentration.

Table 3Tailing sizing in the Qianjiayin coal preparation plant

Size Individual Cumulative

Mm Wt.% Ash% Wt.% Ash%

+0.5 4.25 28.01 4.25 28.010.5–0.25 17.92 23.24 22.17 24.150.25–0.125 16.38 27.46 38.55 25.560.125–0.074 14.73 32.34 53.28 27.430.074–0.045 19.62 42.56 72.90 31.50−0.045 27.1 50.48 100.00 36.65Sum 100.00 36.65

Fig. 3. Fine coal circuits with single-stage spiral.

763Y. Xia, J. Li / Fuel Processing Technology 88 (2007) 759–769

2.5. Problems with the existing process

The problems with the existing process are summarizedbelow:

• High magnetite losses. The magnetite consumption is as highas 3.0 kg per ton of raw coal. From the size analysis of theHMC products, there exists a large quantity of −0.5 mm finecoal which will decrease the magnetite recovery. This largequantity of fine coal comes from misplaced fine coal causedby blinded raw coal deslime screens or from the breakage ofthe +0.5 mm coal in HMC circuit.

• Low clean coal yield. The gravity separation of the −0.5 mmcoal is not satisfactory as shown in Fig. 2. The offset betweenthe separating gravities of the +0.5 mm and −0.5 mmmaterialin the HMC increases the ash of the HMC product. In order tosatisfy the overall clean coal ash requirements, the HMC mustoperate at a lower specific gravity cut point to compensate forthe offset which decreases the overall clean coal yield.

• Losses of coarse coal to froth flotation tailing. The −0.5 mmfine coal is fed to flotation cells directly. Little cleaning occurson the +0.125mm fraction in the flotation process.Most of thecoarse clean coal is lost in the tailings due to oxidation of thefeed and the poor kinetics of the flotation of coarse particles.The cumulative ash content for the +0.125 mm size fractionis actually lower than that of the raw coal feed.

3. Simulation of fine coal circuits for 1 mm×0.15 mm coal

3.1. Description of fine coal circuits

A new fine coal circuit using WOCs and spirals for the treat-ment of 1 mm×0.15 mm coal is proposed to improve the overallplant separation performance. Six commonly used fine coal circuitshave been simulated and compared. These processes include thesingle-stage spiral and different two-stage separation scenarioswhich combine WOCs and spirals. The standard partition curvesare plotted to evaluate the density separation performance of eachfine coal circuit option. The partition curve is a commonly usedmethod of portraying the performance of a gravity separator inwhich the yield of the clean coal from a given density fraction of afeed is plotted against the midpoint of the density range of thatfraction. The specific gravity cut point, SG50, signifies the specificgravity of those particles in the feed that have a 50% chance ofreporting to either with the overflow or the underflow in separator.The efficiency criterion used is probable error, Ep. Numerically,Ep, for a distribution curve is half of the density interval requiredfor the curve to pass through the ordinates of 25% and 75%.

3.1.1. Single-stage spiral (SP)The simplest fine coal circuit is made of single stage of

spirals. As shown in Fig. 3, the −1 mm raw coal that passesthrough the raw coal deslime screen is pumped to a bank of350 mm diameter raw coal classifying cyclones. The cycloneswill make a nominal separation at approximately 0.15 mm. Theclassifying cyclone underflow is diluted to the correct percentsolids and feed by gravity to a bank of triple-start spirals. The

spiral will separate the 1 mm×0.15 mm raw coal into clean coaland refuse. This circuit is simple and straight-forward. Thespiral feed is deslimed with raw coal classifying cyclones toreduce the quantity of misplaced fines in the spiral product.However, the separation efficiency is not satisfactory and thedensity cut point is usually high and a low ash clean coalproduct is difficult to achieve.

3.1.2. WOC rougher and spiral scavenger without recycling(WOC–SP-NoRec)

As shown in Fig. 4 the −1 mm raw coal feeds a WOCwithout being deslimed first. The WOC underflow is diluted tothe correct percent solids and fed by gravity to a bank of triple-start spirals. The clean coal from the spirals joins with the WOCoverflow. Neither clean coal nor middlings from spirals arerecycled. The 0.15 mm×0 raw coal must be removed from the1 mm×0.15 mm clean coal with classifying cyclones.

3.1.3. WOC rougher and spiral scavenger with spiral cleancoal recycling (WOC–SP-Rec)

This circuit in Fig. 5 is similar to the one in Fig. 4 except thatthe clean coal from the scavenger spirals is recycled to the WOCfeed sump. When WOCs are used as scalping devices in com-bination with spirals, a low gravity separation can be made atefficiencies almost equivalent to those obtained by heavy-mediacyclones alone. The cleaning efficiency is enhanced even furtherwhen the spiral middlings are re-circulated back into the primarycyclone feed. Washing efficiency improves as the cut point of theindividual separators diverges, however, with spiral clean coal ormiddlings recirculation, cyclone throughput is decreased.

3.1.4. WOC rougher and spiral scavenger with spiralmiddlings cycling (WOC–SP-MidRec)

These circuits in Fig. 6 are similar to the circuits introduced inFig. 5, however, in this situation, spiral middlings are recycled

Fig. 4. Fine coal circuits with WOC and spirals without recycling.


instead of clean coal. The spiral clean coal will still join WOCoverflow.

3.1.5. Spiral rougher and spiral cleaner with cycling (SP–SP)In Fig. 7, the −1 mm raw coal that passes through the raw

coal deslime screen will flow to the rougher spiral feed sump,which will then be pumped to a bank of triple-start spirals. Theclean coal from the primary spirals is fed to the secondaryspirals. The secondary spiral refuse will be recycled to theprimary spiral sump. In this process, re-cleaning of the large

Fig. 5. Fine coal circuits with WO

quantities of primary spiral clean coal increases the number ofsecondary spirals required. Although the separation efficiency ishigher than that of single stage of spirals, it is not an econom-ically attractive option.

3.2. Simulations and discussions

The simulation results of all of the above fine coal optionsare shown in Figs. 8–10. Fig. 8 illustrates the simulatedseparation performance results for different fine coal circuitries

C and spirals with recycling.

Fig. 6. Fine coal circuits with WOC and spirals with spiral recycling.


when processing 1 mm×0.15 mm coal. Higher performance isobtained by reprocessing WOC underflow in spirals than byusing spirals alone. WOC–SP-recycle circuitry has the bestseparation performance which is presented by the highest cleancoal yield at same clean coal ash of the other options. Thecircuitry consisting of a single-stage spiral has the lowest cleancoal yield. The circuitries made of two-stage WOCs, two stagesof spirals, and of WOCs and spiral without recycling all have

Fig. 7. Fine coal circuits with two stages

similar performances. The clean coal yields are between thoseof the WOC–SP-Rec and SP. In Fig. 9, for all circuitries, theSG50 in WOC is fixed at 1.45, and the overall SG50 is calculatedwhen the spiral SG50 changes. It was found that the WOC–SP-Rec circuitry has the lowest overall specific gravity cut point(SG50) values at the same spiral SG50. The overall change inSG50 values is least sensitive to the changes of the spiral. Whenthe spiral SG50 increases from 1.65 to 2.20, the overall circuit

of spirals with recycling (SP–SP).

Fig. 8. Clean coal yield vs. clean ash on dry basis. Fig. 10. Overall Ep changes with spiral SG50.


SG50 increases from 1.61 to 1.78, this SG50 cutting range willmaintain a reasonable high clean coal yield at a reasonable lowclean coal ash. The overall circuitry separation efficiencies withchanges of spiral SG50 are shown in Figs. 10. It is noted that theWOC–SP-Rec circuitry has the lowest Ep values and thesmallest slope among all of the curves (Fig. 11). That hints thatthis fine circuit has the highest separation efficiencies and canmaintain a low Ep values over a wide change of SG50 values inspirals.

The overall separation performances of processing13 mm×0 coal are shown in Fig. 12 using the proposedprocess and by the existing process. The new process has a

Fig. 9. Overall SG50 changes with spiral SG50.

4.37% higher clean coal yield than the existing process. Thiscorresponds to a 2.84% higher clean coal yield for the wholeplant since the 13 mm×0 fraction comprises 65% of the plantfeed.

4. Suggested plant flowsheet

The suggested flowsheet is shown in Fig. 12. The upgradedfine coal circuit includes a WOC–spiral circuit for the1.0 mm×0.15 mm size fraction. The deck opening of thedeslime screen is changed to 1 mm. The −1 mm raw coal whichpasses through the raw coal deslime screen flows to the WOCsump and is pumped to a bank of 350 mm diameter WOCs.WOCs are capable of separating at low gravities (1.40s);

Fig. 11. Overall clean coal yield vs. clean coal ash.

Fig. 12. Upgraded flowsheet of Qianjiayin coal preparation plant.

767Y.

Xia,

J.Li/Fuel

Processing

Technology88

(2007)759–769


however, they misplace some clean coal to the refuse streambecause a WOC has lower separation efficiency than that of aheavy-media cyclone. To overcome this inefficiency the WOCunderflow is rewashed with spirals. The WOC underflow isdiluted to the correct percent solids and fed by gravity to twobanks of triple-start spirals. The refuse from the spirals isdewatered and discharged onto the refuse collecting conveyor.The clean coal from the spirals is recycled to the WOC sump.This circuit, with the middling stream recycled, provides systemseparating gravities of approximately 1.61 at an Ep of 0.10. Themost important feature is that this fine circuit can achieve aproduct ash of approximately 11.1% at a clean coal yield of75.4% from the 1 mm×0.15 mm raw coal fraction, whichallows the gravities in the heavy-media circuits to be raised,optimizing the overall plant yield.

The clean coal from the spirals joins with the WOC overflowand feeds to the clean coal classifying cyclone sump. The cleancoal is then pumped to two banks of six (6) 380 mm diameterclassifying cyclones to remove misplaced fines in the cleancoal. The cyclones will make a nominal separation at approx-imately 0.15 mm. The 1 mm×0.15 mm from the cycloneunderflow feeds onto (2) 1.8 m wide sieve bends. The sievebend oversize discharges into two fine clean coal centrifuges.The 0.15 mm×0 from the cyclone overflow combines withsieve bends undersize and the effluent from the fine clean coalcentrifuges and the resulting slurry flows to the flotation cells.

4.1. Heavy-media circuitry

The selection of a heavy-media circuitry for processing the+1 mm raw coal is very typical in the North America. Water-based circuits generally have lower performance efficiencies,and have been found over the years to be nearly equivalent interms of installed capital costs. Furthermore, the quantity ofnear gravity material in the investigated plant feed (approxi-mately 15–25% by weight) at a 1.50 separating gravity is highfor a water-based type circuit, therefore this type of circuitry isnot recommended for processing +1 mm coal. In the newprocess, the most of the 1 mm×0.5 mm raw coal never entersthe heavy-media circuit, so the average size of the coal feed tothe HMC is increased and the feed load on the HMC isdecreased. The magnetite loss is greatly reduced. Capital andoperating costs with respect to drain and rinse screens, pumps,and magnetic separators and all similar equipment are alsoreduced significantly. Since the bottom size of the HMC feed isincreased from 0.5 mm to 1.0 mm, the average grain size of feedincreased, which in turn increases the deslime screen, and D&Rscreen capacities because screens can treat more coarse coalthan fine coals on a weight basis. Finally, the quantity ofwashing water is also decreased.

4.2. Fine coal washing circuitry

The proposed process includes both water-only cyclones andspirals for processing the 1 mm×0.15 mm raw coal. Theprimary water-only cyclone/secondary spiral option is selectedbased on the flexibility of the operating parameters provided by

the system. The water-only cyclones are generally operated at alower cut point. As described, the water-only cyclone can beoperated at separating gravities typically as low as 1.40–1.45and as high as +2.0. Operating the water-only cyclones at alower specific gravity minimizes the potential carry over ofhigher ash fines to the clean coal product. The water-onlycyclone reject is then re-cleaned in a coal washing spiral circuit.

This circuitry is selected in conjunction with a heavy-mediacyclone chosen to process down to 1 mm for several reasons:

1. When testing heavy-media cyclone circuits containing a widerrange of feed sizes, the separating gravity of the 1 mm×0.5 mm size fraction is typically 0.05–0.10 specific gravityunits higher than the coarser size fractions. (i.e. if the coarserseparating gravities are approximately 1.55, then the cut pointof the fine coal would be approximately 1.60–1.65 S.G.)

2. The performance of the two-stage fine coal circuit is nearlyequivalent to the heavy-media cyclone circuit; but moreimportantly, the losses associated with the 0.5 mm×0.15 mmreporting to the froth flotation cell are eliminated.

3. The proposed fine coal circuitry is capable of moreefficiently processing misplaced coal. For example, as thescreening panels of deslime and product screens wear out,the aperture increases and misplaces coarser sized coal to thefine coal circuitry. The proposed plant design can effectivelyhandle coal sizes up to 2 mm. With typical two circuitoperation (HMC and froth flotation), the coarser sized coalsent to froth flotation is lost to the refuse stream.

4. Moving the 1 mm×0.5 mm coal from the heavy-mediacyclone circuit to a fine coal circuit by increasing the bottomsize of the deslime screen from 0.5 mm to 1.0 mm providesseveral additional advantages:• Lower magnetite consumption. Removing the 1 mm×0.5 mm size fraction (increasing the average particle size)reduces both the surface moisture of the feed to the heavy-media cyclones (reducing bleed requirements) and theamount of adhering media to be washed of the clean coaland reject drain and rinse screens. The combined effectreduces the total tonnage of magnetite reporting to andrecovered from the magnetic separators; thus reducingpotential magnetite losses.

• Lower coal breakage and fine creation by using a reducedfeed rate to the HMC.

• Lower product moisture. Moving the fine coal from thelower “g” coarse coal centrifuge to the higher “g” fine coalcentrifuge reduces total plant product moisture.

• Reduced flotation feed load. This increases the retentiontime of −0.15 mm coal in the flotation cells which resultsin higher yield and better selection for the application.

4.3. Froth flotation (0.15 mm×0)

The proposed process washes the −0.15 mm with frothflotation cells. Significant coal loss occurs in the +0.25 mm sizefraction in froth flotation units. Typically, the froth bed is notcapable of supporting the coarser size particles and thus thecoarser clean coal sinks to the bottom of the cells and reports as


tailings. Historically, it was not uncommon for coal losses in therange of 20–25% of the available yield on the 0.5 mm×0.25 mm size fraction. In the specific case of the Qianjiayinplant, based on the Seam 5# adjusted feed size distribution andwashability, a potentially significant clean coal loss was avoidedby diverting the +0.15 mm fine coal to WOC–SP circuitry.Flotation reagent consumption and related costs are subse-quently decreased.

5. Conclusions

From the analysis of the Qianjiayin coal preparation plant, asimulation of fine coal circuitries was undertaken in orderimprove the separation performance of the plant. The problemswith the existing process include significant magnetite lossesand coarse clean coal loss in the tailings from inefficientflotation. Due to difficult washabilities of this soft coal, theexisting fine coal separation does not show satisfactoryperformance as evidenced by a high Ep and high clean coalash content. A simulation of fine coal circuitries for 1 mm×0.15 mm coal shows that when WOC are used as scalpingdevices in combination with spirals as scavenger, low gravityseparation with operation flexibility can occur at equivalent toor even exceeding those obtained by heavy-media cyclones.

Acknowledgment

The authors thank KaiLuan Coal Group, Tangshang City,Hebei, China for its financial support of this work.

References

[1] E.J. O'Brien, K.J. Sharpeta, Water-only cyclones: their functions andperformance, Coal Age (Jan. 1976) 110–115.

[2] R. Hochscheid, The evolving role of water-only cyclones in fine coal cleaning,SME, Denver, CO, 1981, pp. 1–6, Preprint Number 81-395, Nov. 18–20,1981.

[3] A.B. Holland-Batt, The effect of feed rate on the performance of coal spirals,Coal Preparation 14 (1994) 199–222.

[4] G.H. Luttrell, J.N. Kohmuench, F.L. Stanley, G.D. Trump, Improvingspiral performance using circuit analysis, Minerals & MetallurgicalProcessing 15 (4) (1998) 16–21.

[5] E. Gallagher, R. Ellis, G. Pitt, A. Partridge, Operation of a 300 tph spiralconcentrator circuit at the German Creek Preparation plant, 10thInternational Coal Preparation Congress Proceedings, Canada, Edmonton,1986, pp. 1–10.

[6] R.P. King, A.H. ad Juckes, Performance of dense-medium cyclone whenbeneficiating fine coal, Coal Preparation 5 (1988) 195–210.

fine coal circuitry considerations in treatment of soft coal with difficult washabilities

Documents