Cement Chapter 6

8/11/2019 Cement Chapter 6

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6. Cement Milling

Performance

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 72

6 . 1 I N T R O DU C T I O N

6 . 2 G R I N DA B IL I T Y

6 . 3 F I N E N E S S V E R S US K W H / T O N N E

6 . 4 C I R C U I T P E R F O R M A N C E

6 . 5 S E P A R A T O R P E R F O R M A N C E

6 . 5 . 1 I N T R O D U C T I O N

6 . 5 . 2 T R O M P C URV E A N D S E PA R AT O R E F F I C I E N CY6 . 5 . 3 S E PA R AT O R B Y- PA S S

6 . 5 . 4 I N F L U EN C E O N CI R C UI T P E R F O RM A N C E

6 . 6 M I L L H O L D - U P

6 . 7 M E D I A S I Z E S

6 .8 M I L L S IM UL AT I ON

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6.1 INTRODUCTION

The preceding section aimed at examining the parametersinvolved in ball mills and the mill systems used for the grindingof cement.

In this section we shall consider some of the factors thatinfluence the efficiency of grinding, and attempt to understand

some of their interactions in the milling circuit.

6. CEMENT MILLING PERFORMANCE

contents chapter 6 chapter 7


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6.2 GRINDABILITY

The grindability of feed materials generally refers to the materialproperties which influence the SSA: kWh/tonne relationship. Ifmaterial becomes harder then the SSA will be lower for a givenkWh/tonne, or, more realistically, the kWh/tonne will be higherfor a given SSA.

The grindability can be derived in various laboratory tests, themajority of which use a laboratory ball mill test in which SSA ismonitored against grinding time. The grindability curve (SeeFigure 56) can then be derived for a given clinker, or feedmaterial, and compared to a standard or reference curve.

Figure 56. Average Cement Grindability Curve.

Notes: For 0 200/250m2/kg, the relationship is close to linearAbove this SSA, the deviation from linear graduallyincreasese.g.from 100 to 200m2/kg requires 10.3kWh/tonne

from 300 to 400m2/kg requires 17.7kWh/tonnei.e. ~70% more kWh/tonneGrinding end-point effectively reached around

500 - 700m2

/kg

The curve shown in Figure 56 represents a typical cement(clinker and gypsum only) grindability curve. At 300, 350 and400 m2/kg the kWh/tonne are:

300 m2/kg 33.7 kWh/t350 m2/kg 41.0 kWh/t400 m2/kg 51.4 kWh/t

Other clinkers can be compared at a particular SSA, either anominal one, e.g. 300 m2/kg, or at an actual production SSA.

For example, if another clinker gave 36.5 kWh/t at 300m2/kgthen its grindability would be 108% of the reference.

The curve can also be used when assessing plant operating data

where there have been small differences in SSA. (See TIS MS014)

For example:To assess a change in grinding efficiency, for example as aresult of changing grinding additiveExample A: 370m2/kg 66.4 t/hrExample B: 360m2/kg 69.7 t/hr

From the graph we would expect 44.9 and 42.8 kWh/trespectively. Therefore can expect a 44.8/42.8 increase inoutput for the reduction in SSA from 370 to 360m2/kg.

i.e. 66.4 x 44.9/42.8 = 69.7 tonnes/hour

Hence, in this example, the difference in B could be entirelyattributed to the change in SSA.

Clinker grindability becomes more difficult for:-- higher C2S level- harder burning (larger crystal sizes)- higher clinker SO3 (some of this is simply the effect on

a lower gypsum content)- denser clinker

The grindability will appear easier where materials thatcontribute to the SSA are present e.g.:-

- gypsum- limestone

- pozzolan- fly ash

As an example, a 1% gypsum increase will produce an additional12 m2/kg for a constant kWh/t. (1% SO3, constant clinker SO3and constant kWh/t = 30m2/kg). (See also Section 1).




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6.3 FINENESS VERSUS KWH/TONNE

We have already seen in Sections 5 and 6.2 that the relationshipbetween SSA and kWh/tonne is not linear (as postulated byRittinger). The deviation concerns the increasing negativeinfluence of agglomeration and coating as fineness increases. Ineffect the slope of the relationship decreases as fineness increases.

In the previous section we saw the following values from thegrindability curve (Figure 56):-

300 m2/kg 33.7 kWh/t350 m2/kg 41.0 kWh/t400 m2/kg 51.4 kWh/t

Thus the overall slope (from 0 m2/kg) is:-300 m2/kg 8.90350 m2/kg 8.53400 m2/kg 7.78

The units are m2/kg kWh/tonne, and these can be readily re-arranged to m2/kWh, i.e. 8900, 8530 and 7780. Alternatively,other units can be used, such as cm 2/kWh and m2/kW.min.However, one recognised unit is cm2/j (a joule is a watt. second)

Therefore we have:-300 m2/kg 33.7 kWh/t = 24.7 cm2/j

350 m2

/kg 41.0 kWh/t = 23.7 cm2

/j400 m2/kg 51.4 kWh/t = 21.6 cm2/j

Thus if a mill operates at a lower level of fineness the energyefficiency (cm2/j) is higher. A typical relationship between theefficiency in cm2/j and the mill exit fineness is shown in Figure 80.

Figure 80. Influence of Mill Exit Fineness on Ball Mill Efficiency




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If we examine the lower line, then we can clearly see that as themill exit fineness increases the efficiency decreases.

As already discussed, the reduction in grinding efficiency is as aresult of:-- agglomeration and subsequent disagglomeration of particles- the resultant adhesion of particles to mill internals, i.e.

coating (See Figure 81)

Agglomeration is caused by Inter-Particle Attractive Forces(See Figures 82 and 83):-

- mechanical packing of particles- chemical bonding, e.g. hydration bridges- thermodynamic, reduction in surface energy- physical, e.g. surface charges

Note: 1 cm2/j = 360 m2/kWh

kWh/tonne = 1000 x m2/kg m2

kWh

where m2/kg refers to the product fineness, f.

Figure 81. Influence of Agglomeration and Coating.

Figure 81. Influence of Agglomeration and Coating (continued).

Figure 82/83. Causes of Agglomeration.


Compressibility of Agglomerates

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6.4 CIRCUIT PERFORMANCE

6.4.1 OPEN CIRCUIT

Clearly, for an open-circuit mill, the mill exit fineness will haveto be virtually the same as that of the desired cement productfineness. If we assume a grindability behaviour the same as thatshown in Figure 80, then for a product SSA of 370m 2/kg wehave an efficiency of around 22.9 cm2/j.

Therefore the m2/kWh = 8244Therefore the kWh/tonne = 44.9(See Section 6.3 for equations)

6.4.2 CLOSED-CIRCUIT

In a closed circuit system the mill exit (or separator feed) SSAwill be lower than that of the product and will depend on thecirculating load and the separator performance.

If we now have a mill operating with a circulating load of 300%(A/F) with a conventional separator producing a rejects SSA of220m2/kg, the mill exit fineness would be: (See TIS MS013).

a = Ff + RrA

Assume F = 100, then A = 300 and R = 200From above, f = 370 (same as in 6.4.1) and r = 220

Therefore a = 270 m2/kg

From figure 80, we would expect the efficiency to increase to26.2cm2/j. Hence, as before, the kWh/tonne = 39.2.

Thus, compared to open-circuit operation we have a lowerkWh/t equivalent to a 44.9/39.2 increase in output, i.e. 14%.

The higher output results from the effect of the circulating loadreducing the mill exit fineness (and hence the in-mill fineness).This results in less coating and agglomeration and thus anincrease in the grinding efficiency.This represents the simple theoretical approach to closed-circuitmilling.

There are two ways in which the mill exit fineness, and hencethe efficiency can be further influenced, namely:

- increase of circulating load- increase of separator efficiency

Using the same values as before, a circulating load of 500% gives:-

a = 100 x 370 + 400 x 220 = 250 m2/kg500

The equivalent cm2/j (Figure 80) are 26.8 and the kWh/tonnereduces to only 38.3.

For a high efficiency separator we shall see later that there are lessfines in the rejects and thus the SSA is lower, for example 90m 2/kg.

a = 100 x 370 + 200 x 90 = 183 m2/kg300

From Figure 80, the cm2/j = 27.5 and the corresponding kWh/tare 37.4.

These calculations are summarised in Figure 84.

In fact, if we examine the circulating load only, we wouldexpect the mill exit fineness to continue to reduce for increasingcirculating load. Hence we would expect marginal gains inefficiency (cm2/j), as shown in Figure 85, and thus continualreductions in kWh/tonne.

However, in reality, there are a number of factors that limitthis:-- physical limitation of materials handling (e.g. elevator)- overloading of the separator- overfilling of the mill

The latter two are discussed in the following sections.

Figure 84. Closed Circuit Operation, Influence of CirculatingLoad.


Mill Circuit Open Closed Closed Closed

Separator

Product SSA

Rejects SSA

A/F %

Mill Exit SSA

cm2/joule

kWh/tonne

Output %

N/A

370

N/A

100

370

22.9

44.9

100

Conv.

370

220

300

270

26.2

39.2

114

Conv.

370

220

500

250

26.8

38.3

117

H/E

370

90

300

183

27.5

37.5

120



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Figure 85. Influence of Circulating Load on Mill Efficiency.



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6.5 SEPARATOR PERFORMANCE

6.5.1 INTRODUCTION

The principal function of the separator is to correctly place thefiner sizes to the final product and the coarser sizes back to themill. We have already seen in Section 5 that the classificationresults from a balance between centrifugal, drag and gravityforces.

In practice, under steady-state operation, a separator willoperate at a "cut-point" size above which the particles willpredominantly pass to the returns and below which they willpredominantly pass to the product.

However there will be a degree of misplacement of material, i.e.some larger sizes will pass to the product and more significantlysome finer particles will pass to the returns.

A convenient means of describing the performance of aseparation is to determine the grade efficiency curve (alsoknown as the Tromp Curve or Size Selectivity Curve).

6.5.2 TROMP CURVE AND SEPARATOR EFFICIENCY

The Tromp curve is determined as follows:-1. Determine the particle size distributions for the separator

feed, fines and rejects (a, f, r). Present the results ascumulative percent finer (See Figure 86).

2. Sum the 3 psd's and determine the mass balance, i.e. A, F,R. In the example F = 50.8, therefore R = 49.2 and A/F =197% (See also TIS MS013).

3. This is the A/F assumed to be closest to reality. However itcan also be calculated for the individual size intervals asshown in Figure 86 using the incremental psd's.

4. The actual psd for the fines and rejects (f, r) are then usedwith the calculated F and R to back calculate the psd forthe feed (a).

i.e. ai = Ffi + RriA

where ai, fi and ri are the incremental psd's

5. The Tromp curve is defined as:-The mass of material at size i in the separator Rejects x 100%The mass of material at size i in the separator Feed

i.e. Trompi = Rri x 100%Aai

The resultant curve is shown in Figure 87.

Figure 86. Particle Size Analysis and Separator Performance

Figure 87. Separator Performance, Coarse Grade Efficiency Tromp Curve.

6. The Grade efficiency of 50% represents the size at whichparticles have an equal chance of being placed in the finesor rejects - hence the term equi-probable cut size, e. In theexample this is around 35 microns.

7. The "imperfection" can be assessed by the degree to which

Circulating Load: A/F = 197%R/F = 97%

Equi-probabl Size e = 35 umSeparator By-pass S = 19%Imperfection I = 0.32

Recovery at 48 micron Fines = 65%Coarse = 83%


Particle

Size

(microns)

Percentage Finer Mid

Int.

Size

Calc.

F

Calc.

Feed

a

Coarse

Grade

Efficiency

Feed

a

Fines

f

Coarse

r

(microns)

192

128

96

64

48

32

24

16

12

8

6

4

3

2

1.5

1

99.4

97.3

93.5

82.4

70.2

52.0

41.8

31.5

26.2

20.3

16.7

12.3

9.7

6.7

5.0

3.1

100.0

99.8

99.2

96.1

89.7

74.3

62.1

47.2

38.8

29.6

24.1

17.7

14.0

9.8

7.4

4.6

98.2

94.3

87.2

67.7

49.0

28.2

20.7

15.5

13.5

11.2

9.6

7.4

5.9

4.0

3.0

1.8

160.0

112.0

80.0

56.0

40.0

28.0

20.0

14.0

10.0

7.0

5.0

3.5

2.5

1.75

1.25

0.5

avg =

49

51

51

53

48

57

53

52

52

51

52

50

48

50

44

46

50.2

99.1

97.1

93.3

82.1

69.7

51.6

41.7

31.6

26.3

20.5

17.0

12.6

10.0

6.9

5.2

3.2

95.0

92.0

85.9

73.9

56.7

37.3

25.3

18.8

19.5

22.0

25.0

28.2

30.5

28.8

29.4

27.5

SUM

SSA(m2/kg)Alpine45um

668.1

205

29.3

814.4

330

6.6

517.2

90

46.4

F =

F =

F =

50.8

==47.9

43.0



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the curve deviates from the vertical. (Perfect separation, nomisplaced material, would be a vertical line at theequiprobable size).

One method proposed for Imperfection is, I = D75- D502 x D50

where D50 is the equiprobable cut size and D75 is the sizecorresponding to a Tromp efficiency of 75%.

In the example, this is 0.32.

The result, does however depend on the cut size.

8. The so-called "acceptance" and "rejection" efficiencies canthen be calculated from the psd and the cut size (e).

These define the efficiency of correctly placing material, i.e.

Acceptance efficiency:Ea = mass of material in the fines less than size e x 100%

mass of material in the feed less than size e

Rejection efficiency:Er = mass of material in the rejects greater than size e x 100%

mass of material in the feed greater than size e

These two measures should aim to be as high as possibleand close to 100%.

9. "Recoveries" can be calculated in a similar manner for anyspecified size. For example, in Figure 86, 48 microns waschosen. The Fines Recovery was 65%, whilst the coarserecovery was 83%.

This means that in the separation in question, 65% of thematerial less than 48 microns was placed in the productand 83% of the material greater than 48 microns wasplaced in the rejects.

10. Finally we can determine the by-pass.

These parameters are summarised in TIS MS015.

6.5.3 SEPARATOR BY-PASS

The majority of the terms for separator efficiency discussed inSection 6.5.2 are either relatively complex to compare or notpractical enough. For these reasons the term of by-pass, S, has

become increasingly referred to within the cement industry.

One of the deficiencies of conventional separators was thecomparative ease to which feed material could fall directly intothe rejects cone (See Figure 58 and Section 5.3).

This is an immediately obvious way that material can by-passthe so-called separating zone.

The quantity of material that fails to be separated by the forcebalance in the separating zone is referred to as the by-pass (SeeFigure 88).

This can be directly interpreted from the Tromp curve (SeeFigure 88), i.e. the by-pass is effectively the minimum coarsegrade efficiency (usually around 3-10 microns), in this case19%.

The by-pass, in practice, describes the amount of feed material

being incorrectly placed directly into the rejects stream.

Many separations also have a noticeable "fish-hook", i.e. thegrade efficiencies increase after the minimum, for smaller sizes(e.g. less the 5 microns), indicating recycle of fines at a higherlevel than indicated from the by-pass alone.

This arises since these particles are treated as coarser particleson account of:-

- agglomeration- adhesion to larger particles- entrainment

6.5.4 INFLUENCE ON CIRCUIT PERFORMANCE

Figure 88. Separator By-Pass.




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Naturally as the by-pass increases, so the amount of finesreturned to the mill increases. The by-pass is increased for:-

- a reduced separation efficiency (e.g. conventionalseparation compared to high efficiency)

- a higher circulating load

The latter is important for any given circuit since this can be

controlled over a large range of conditions.

In Section 6.4 we saw that, in theory, as circulating loadincreases, the mill exit fineness (and hence in-mill fineness)decreases and the overall efficiency should increase. Howeverfor a given circuit, as the circulating load increases, the totalfeed rate to the separator increases and the by-pass will increase.

Basically, separator efficiency decreases for an increase in feed rateand hence an increase in loading. The loading can often berepresented simply in terms of tonnes/hour, or, for wider comparison,in terms of kg/m3 (solids to air loading) or tonnes/hour/m2 (solids tocross sectional area of either casing or rotor).

Thus the maximum efficiency of a separator is only when thereis effectively no feed (not much use). The deterioration ofefficiency is shown in Figure 89a, where 1-S is plotted againstthe solids loading. The increase in by-pass is seen to be morepronounced for conventional separators.

This increase in by-pass as circulating increases means that anincreasing level of fines are returned to the mill. In effect theBlaine of the rejects increases and thus the reduction in Blaine atthe mill exit is not so great. This then results in lower grindingefficiency (See Figure 89b)

Figure 89a. Influence of Solids Loading on SeparatorPerformance.

Figure 89b. Influence of By-Pass on Grinding Efficiency.




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Hence the expected beneficial influence of higher circulatingload on overall mill efficiency (cm2/j) is limited. See Figure 90.

In effect, there is an optimum circulating load due to acompromise between two opposing effects, i.e.

- benefits of higher circulating load on mill exit fineness- decrease in separator efficiency for higher loadings

However we shall now see that there is a further limitation onthe circulating load, that of mill hold-up.





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6.6 MILL HOLD-UP

In Section 4 we examined the parameter of mill hold-up, i.e.powder loading or residence time. The hold-up was discussed interms of the percentage of media voidage occupied by material.This is shown schematically in Figure 91.

Figure 91. Powder Loading Void Filling.

For very low powder loadings (say less than 60% void filling) itis reasonable to expect inefficient grinding since there will be alarge proportion of energy expended on media to media impactswithout any material comminution. Conversely, for very highpowder loadings (say more than 120% void filling) it would bereasonable to expect cushioning and hence absorbance, of theenergy of media impacts in the material bed. Again, energyefficiency would not be optimised.

Austin and co-workers investigated the influence of powderfilling and derived the following type of relationship:-

E = 3.26258e-1.2U

where E represents grinding efficiency and U is the fractionalvoid filling. This shows higher levels of efficiency for lowerpowder filling levels (See Figure 92a).

However, in a continuous mill insteady-state, lower levels ofpowder filling will mean lowerresidence times. Overall grindingachieved will be a product ofgrinding efficiency (in effect rate ofgrinding) and residence time.

Hence overall efficiency should beseen as:-

Eff = 3.26258e-1.2U . U

The relationship is shown in Figure92.

Figure 92a. Effect of Hold-Up onRate of Breakage.

Figure 92. Influence of PowderFilling on Mill Efficiency.




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The optimum void filling is seen to be at 85%. Experimentationhas also confirmed that the resultant optimum powder fillingoccurs at 85% of void filling. (Note: this is for optimum voidfilling alone and does not take into account optimisation ofother parameters. See later in this section). A number of millingparameters have a strong influence on the hold-up in the mill,and these include:-

- mil l venti lation rate- media grading- diaphragm design and condition- no. of chambers (no. of diaphragms)- separator type and efficiency and circulating load- volume loading- mill speed- use of additive (type and dosage?)- material characteristics

It can be expected that higher mill airflows will assist in thetransport of material through the mill, thereby allowing a lowerpowder filling. If the diaphragm becomes partially blocked, asoften happens, then the available open area to transport

material is lower, which dictates a higher powder level in themill to maintain steady-state flow through the diaphragm andthus through the mill.

It has been shown that a higher mill throughput requires ahigher head for material to push itself through the media toreach the discharge end of the mill. A higher throughput willalso need to cover a larger open area of the diaphragm in orderto transport a higher rate of material (as there is a limit to thevolume of material that can be transported per unit area ofslots). Thus higher total throughputs, e.g. higher circulatingload, will result in a higher powder filling level. Such arelationship is shown in Figure 93.

Figure 93. Influence of Mill Throughput on Powder Filling Level.

Therefore, if we re-consider the effect of a highercirculating load we now have:-

(i) benefits of a lower mill exit fineness leading to highergrinding efficiency

but (ii) a decrease in separator efficiency as a result of ahigher loading, hence an increase in the returns offines, and thus opposing the benefits of (i)

and (iii) an increase in the total mill throughput and thus anincrease in the mill hold-up and hence a decrease in

the grinding efficiency (once above the optimum),thereby opposing the benefits of (i) further.

The revised relationship for the influence of the circulating loadis shown in Figure 94




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From these relationships it is possible to understand why anoptimum circulating load exists for any given mill. Theoptimum circulating load will depend on the influence of:

- product fineness.- in-mill fineness (mill exit Blaine) and hence the degree

of particle agglomeration and coating.- separator loading and hence separation efficiency.- mill void filling and hence grinding efficiency.

As already discussed, the optimum void filling is 85%, however

once the effect of optimum circulating load is accounted for theoverall optimum void filling is typically around 100%.

Below is a typical sequence of changing parameters as thecirculating load is increased:-

Thus there is an optimum circulating load for maximum output.

However, there is not an optimum separator efficiency, only aseparator by-pass which coincides with the overall optimumconditions.

In this example, the optimum circulating load is around 360%with a by-pass of 22%, a void filling of 100% and a mill exitBlaine of 175m2/kg.

In general, a low circulating load will result in poor millefficiency due to high mill exit Blaine and high levels ofagglomeration and coating. A high circulating load will alsoresult in poor mill efficiency due to high separator loading, andhence high by-pass and recycle of fines, and high mill loading

and high void filling, and hence low grinding efficiency.

The mill loading can be defined in terms of the total millthroughput (feed + separator rejects) in tonnes/hr divided by themill cross-sectional area in m2. Hence the units aretonnes/hour/m2. This value is usually in the range 20-30, with atypical target of 20.


Mill kW

Mill Diameter, m

Mill x. sectional

area, m2

2100

3.6

10.2

Product Blaine,

m2/kg

Circulating Load,

A/F, %

Mill loading, t/hr/m2

Mill Exit Blaine,

m2/kg

Separator By-pass, %

Mill Void Filling, %

Mill Output, t/hr

Mill kWh/t

Mill Output, %

150

8.0

280

9

77

54.6

38.5

94.5

200

11.1

235

12

85

56.5

37.2

97.8

300

17.0

190

18

96

57.7

36.4

99.8

360

20.4

175

22

100

57.8

36.3

100

370

500

28.2

155

28

109

57.5

36.5

99.5

1000

54.3

125

48

125

55.3

38.0

95.7



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6.7 MEDIA SIZES

In chamber 1, the media is required to sufficiently reduce feedsizes to allow grinding by relatively smaller media sizes in thesecond chamber (and maybe subsequent chambers).

The top size is generally 90mm, although 100mm can be used insome circumstances, so long as liner breakage does not become

an unacceptable risk. There are various relationships betweenmaximum feed particle size and ball size, such as that of Bond.

However, in general the media sizes in chamber 1 should be inthe following ranges:-

Ball Size Weight %(mm) Coarse Medium Fine90 40 25 1580 29 36 4170 19 24 2760 12 15 17

Mean Size (mm) ~80 ~77 ~75

The actual choice of grading used will depend on:-- Top size of clinker (or other)- Hardness of clinker (or other)- Liner design and condition- Volume loading

In chamber 2, the media sizes will have a much greater influenceon the overall mill efficiency.

In many mills, the philosophy is to use a grading of ball sizes inthe 60-17mm size range, together with a classifying liner. Theclassifying liner has the objective of placing the larger sizestowards the inlet of the chamber, and thus the finer sizes at the

chamber outlet.

However, in theory, and in practice when all other parametersare constant, grinding efficiency is greater for smaller ball sizes.One explanation lies in the significantly higher number of ballsas the size decreases, which result in many more ball/particlecontacts. The relationship between specific rate of breakage (See

Section 3.4), i.e. grinding efficiency, and particle size is shown inFigure 95.

For large particle sizes (above 1mm) larger ball sizes are moreeffective. However for most particle sizes (e.g. below 0.5mm)small ball sizes provide a higher rate of breakage. In fact, forparticles below 1mm, the optimum ball size is below 25mm.

(See Figure 96).

Thus, if particles can be efficiently crushed so that everythingpasses 1mm, a small range of ball sizes should provide a greaterlevel of mill efficiency. For this reason somemanufacturers/suppliers have favoured a smaller sized mediagrading of 25-17mm only. In this case, a classifying linerbecomes less important.

Figure 95. Relationship Between Particle Size and Ball Size forMaximum Breakage Rate.




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Figure 96. Relationship Between Particle Size and Breakage Ratefor Different Ball Sizes.

However, to ensure effective use of small media it is veryimportant to avoid:-

- high powder filling levels (high void filling)- oversize feed particles- mill internals coating

The efficiency of small ball sizes rapidly reduces if the powder

filling becomes excessively high (small ball sizes in effect "float",and their impact energy is cushioned by the bed of material). Thepresence of coarse particles (1mm and above) will not beadequately treated by smaller ball sizes and hence can persist inthe second chamber and eventually result in excessive powderfilling. Once coated, small ball sizes more rapidly reduce inefficiency of grinding than larger ball sizes.

Hence to successfully use smaller ball sizes, it is essential to have:-- good chamber 1 performance, i.e. adequate crushing of

material and low levels of material larger than 1mmpassing to chamber 2.

- good intermediate diaphragm condition (no largeopenings or defects)

- low void filling levels (good mill ventilation, not

excessive mill throughput)

The latter has to be examined with particular care, since smallermedia sizes result in an increased resistance to material flowthrough the mill. This leads to a higher powder filling level (SeeFigure 97).

For this reason smaller media sizes may not always provide theexpected benefits in grinding efficiency, since the negativeinfluence of a higher hold-up maybe more important than thebenefits of small media sizes.

Figure 97. Influence of Mean Ball Size on Void Filling




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6.8 MILL SIMULATION

Given some understanding of the relationships discussed for millcircuits it is possible to simulate the mill performance. Therelationships used, include:-

- Influence of separator size or airflow and throughputon the separator by-pass level

- Relationship between actual feed passing to the"separating zone" and the total separator feed

- Relationship between product blaine and separatorfeed fineness

- Relationship between separator rejects, product andfeed fineness

- Relationship between mill inlet, rejects and freshfeed fineness

- Relationship between mill performance (kWh/tonne)

and mill inlet and outlet fineness

- Influence of mill throughput on mill hold-up

- Influence of mill hold-up on efficiency of grinding

- Influence of mill feed grindability

With such a model it is possible to assess, to some degree,:-

- Influence of circulating load

- Influence of separator efficiency

- Influence of mill hold-up



Date post:	03-Jun-2018
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Cement Chapter 6

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