17-determination of fineness

THE CEMENT FACTORY

17. DETERMINATION OF FINENESS

The Cement factory17. Determination of Fineness

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Table of Contents

17.1 SIEVING......................................................................................................... 417.2 SIEVES........................................................................................................... 717.3 WET SIEVING................................................................................................. 817.4 FLS-FLOURMETER ........................................................................................ 917.5 BLAINE AIR PERMEABILITY ......................................................................... 1117.6 FINENESS FROM SEDIMENTATION ................................................................ 12

Table of Figures

FIGURE 17-1 ............................................................................................................................................5FIGURE 17-2 ............................................................................................................................................6FIGURE 17-3 ............................................................................................................................................7FIGURE 17-4 ............................................................................................................................................8FIGURE 17-5 ............................................................................................................................................9FIGURE 17-6¨ .........................................................................................................................................11FIGURE 17-7 ..........................................................................................................................................12FIGURE 17-8 ..........................................................................................................................................13


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17 DETERMINATION OF FINENESS

17.1 SievingThe most important criterium for fineness in the cement industry is the sievefineness, expressed in per cent residue of a weighed sample on a sieve with givenmesh. Thus it is often required to grind raw materials to a fineness corresponding to10% residue on a sieve with 90µ openings (1 µ = 1 micron = 0.001 mm ). Written :10% + 90µ .

A significant picture of the fineness of a material is obtained by sieving a sample onsieves with decreasing openings and introduce the values in a coordinate system. Theensuing figure is called a granulometric curve.

Below is shown a granulometric curve (A) for a portland cement, obtained by sievinga sample on sieves with openings 20µ , 44µ , 63µ , 90µ , 150µ , and 200µ . Theseare the commonly employed values for sieving raw meal, cement and coal meal. Theresidues on the lower part of the curve are obtained by sedimentation analysis.

Alternatively the granulometric curve may be drawn in the Rosin-Ramler-Sperlingdiagram (B), a quasi-logarithmic system which will generally portray the curve as astraight line from the equation

log)(

1kR

= n

kk⎟⎠⎞

⎜⎝⎛

where R(k) is the residue on the k-micron sieve, k’ is the mesh giving a residue of10% and n=tando(is the slope of the graph.


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Figure 17-1

If two points with residues R(k1) og R(k2) are known the equation can be written

))1(/1log())1(/1log(

kRkR =

n

kk

⎟⎠⎞

⎜⎝⎛

21

from which the slope of the curve : n=

21log

))2(1log())1(1log(log

kk

kRkR

Example :Residues from cement. 11%+90u and 0.6% on210u.

R(k1) = R (90) = 0.11 and k1 = 90R(k2) = R(210) = 0.006 and k2 = 210

From the above : n = tan = 0.992 slope = 44.70.

When only the residue on one size of sieve is known the curve may as anapproximation be drawn at a slope of 450 through the coordinate.

Example :A Portland cement is ground to 3000 cm2/g in closed, respectively in open circuit.The residues are :

Open circuit 45%+25u and 8.5%+90u


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Closed circuit 36%+25uand 2.5%+90u

The slopes found from the formulas are :

Open circuit: n = 0.88, corresponding to a slope of 410.Closed circuit: n = 1.00, corresponding to a slope of 450.

It will be noted that the curve for the cement ground in closed circuit is slightlysteeper than the curve for the cement ground in open circuit. This is the patternnormally met with, and it would imply that the strength properties of cement groundin closed circuit are superior to those of cement ground in open circuit.

Figure 17-2

Example : Determination of the circulation factor for a separator on basis of sieveresidues on four different sieve sizes. Refer section 5.10.


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% Residue on s ieve150µ 90µ 43µ 20µ

Mill discharge Rm

Coarse return Rg

Fines Rf

4.5

7.5

0.1

15

25

2

38

56

12

63

80

40

150µ : C = 5.45.71.05.7

−− = 2.50

90µ : C= 1525225

−− = 2.30

43 µ : C = 38561256

−− = 2.44

20 µ : C = 63804080

−− = 2.35

C = 4.24

35.244.230.250.2≅

+++

Circulation factors calculated from the formula

C = RmRgRfRg

−−

Figure 17-3

17.2 SievesSieves for pulverulent materials are normally made in cylindrical form from brasssheet-metal, as shown in figure 17.2(A). Sieves are standardised today, as well inrespect of external dimensions as in actual mesh. The sieves mentioned on theprevious page are standard sizes in most countries.

The most common sieve is that of mesh 90µ , often termed the 4900- sieve afterWest German standards. The number has a relation to the number of openings persquare-unit. In the same way the 200µ sieve is sometimes termed the 900-sieve.

Sieving is often done in automatic sieving apparatus, which will sieve the sampleover a given interval of time, under constant shaking. The apparatus permits stackingof several sieves on the top of each other, as shown, whereby a completegranulometric curve can be obtained in one sieving operation. A commonly usedsieving apparatus is the ALPINE-sieve.


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Coarse sieve for raw materials, raw coal and clinker are often mounted in rectangularwooden frames as shown in(B). They may be designed for stacking, but it is notusual to encounter automatic sieving apparatus for this type of sieve.

Frame-sieves for coarse sieving are normally seen in sizes between 5 mm and 50mm, although coarser as well as finer occur.

Residues on mesh under 20µ is usually found by sedimentation methods, the sieveapertures being too small to permit accurate sieving.

Figure 17-1

17.3 Wet sievingWet sieving assures uniform sieving conditions from one operation to the other andis especially suited for sieving on very fine mesh. The material should not exceed 1mm and grain size must not react with water. It is, however, perfectly possible towet-sieve cement since its reaction with water is sufficiently slow.

The apparatus consists of a carrousel (5) with detachable sieve (6) which is drivenwith a rocking movement by the motor (4) while a fine jet of water hits the sieveface. The water pressure, normally 0.7 bar, registers on the pressure guage (3) and isregulated with the value(1). The relay (7) controls the sieve interval which isnormally 3 minutes, after which the magnetic valve (2) stops the water supply andthe motor.


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The size of the sample depends on the employed sieve. With the mesh of 44µ thesample 5g is most often used and with 90µ a sample of 10g, in both cases drymeasure.

The weighed material is placed on the sieve which is then fitted on the carrousel. Thetime clock is started and the water pressure checked. Sieving has commenced. Afterthe automatic process the sieve is removed and the residue is poured into the beakerand dried in an oven or a hot-plate. Finally the dry residue is weighed and calculated.

The sieve should be cleaned periodically in a three percent nitric acid solution or a20% acetic acid solution with subsequent rinse in water. The nozzle is cleaned fordeposits in a similar solution.

Figure 17-1

17.4 FLS-FlourmeterThe Flourmeter is a laboratory air separator used for determining a fineness ofcement, a quantity of 5g being air separated for 25 minutes. Afterwards the residue isweighed and calculated as a percentage of the original sample. The residuecorresponds to the residue from a sieving test on the 25µ sieve.


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The apparatus consists of a glass funnel (1) with detachable bottom part (2) ,connected to a fan (6) through flow nozzle (3) and regulating valve. The latter servesto cut off the air flow to the funnel when the timer (5) has switched off the current tothe fan (6).

The fan is started by first depressing the red button (11), marked “stop”, and then theyellow button (7) “start” and the air is adjusted by (4) until the pressure given in thecertificate is read on the pressure gauge (12). The material is now introduced with thecharging device and the timer is activated with the green button (8), marked“timing”. The air flows up through the funnel.

The material will float in the air stream, the finest particles being carried out at thetop of the funnel and collected in the filter (9), while the cleaned air is re-introducedat the foot of the funnel. A rapping device (10) prevents fine particles from adheringto the glass.

When the pointer (5) has reached 0-position after a time interval of 25 minutes thegreen pilot light (8) is extinguished and fan and rapping device stop. The bottom partis removed and the residue weighed .

The Flourmeter is adjusted with a standard flourspar sample supplied with theapparatus.


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Figure 17-1¨

17.5 Blaine air permeabilityThe fineness of cement is most often expressed through its specific surface in cm2/g.since the test is normally carried out with a Blaine apparatus we frequently speak ofthe cement, but it must be borne in mind that there are other instruments fordetermination of specific surface.

The fineness of a cement is closely related to the number and size of the poresbetween the grains, and thereby to the time it takes for a given quantity of air topenetrate a sample. The air permeability apparatus functions by drawing a givenquantity of air through a sample and measure the elapsed time interval.

The weighed sample (8) is placed in the cell (1) between two layers of filter paperand compressed with a plunger (7). The cell is then placed in the seat of themanometer pipe (2) and the fluid is drawn up to the top mark (3) with the rubber ball(4). Finally valve (5) is closed.


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The level of the fluid will now fall in the right branch of the manometer pipe as theair penetrates the sample, and the time interval of its passage between the twoengraved marks (6) is timed on a stop watch. When the apparatus is calibrated andthe size of the sample uniform from one test to another the specific surface can betabulated as a direct function of the measured time interval of the passage betweenthe measuring points (6).

Portland cement is characterised with fineness around 2800-3000 cm2/g while thefineness of rapid-hardening and super rapid-hardening cements may go up to 5000-6000 cm2/g specific surface.

Figure 17-1

17.6 Fineness from sedimentationThe fingered fraction of the granulometric curve is obtained by sedimentationanalysis, for example by the Andreasen apparatus. The process is based on the factthat particles of different sizes have different rates of sedimentation.


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The apparatus consist of a measuring cylinder with pipette, containing a carrier fluid.The sample is mixed in the fluid and the rate of sedimentation, i.e. the particle size, isdetermined by extraction of part-samples from a given depth at fixed fine intervals.

The particle size is then calculated from the specific weight of the sample, the rate ofsedimentation and the viscosity and specific weight of the carrier fluid.

The method is ideal for determination of the size of extremely finegrined particles,but it is time-consuming and requires accuracy. It is used in laboratory work andrarely during the daily routine.

Figure 17-1

17-determination of fineness

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