Cement Chapter 5

7/21/2019 Cement Chapter 5

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5. Cement Milling

Systems

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 4 54

5 . 1 I N T R O DU C T I O N

5 . 2 O P E N - C I R C U I T M I L L I N G

5 . 3 C L O S E D - C I R C U I T MI L L I N G

5 . 3 . 1 I N T R O D U C T I O N

5 . 3. 2 S E P A R AT O R S

5 . 3 . 3 C I RC U I T C O N T R O L

5 . 4 R O L L P R E S S

5 . 4 . 1 I N T R O D U C T I O N

5 . 4 . 2 B A S I C O P E R AT I O N

5 . 4 . 3 R O L L P R E S S T E R M I N O L O G Y

5 . 4 . 4 M I L L I N G C I RC U I T S W I T H R O LL P R E SS E S

5 . 5 R O L L E R M I L L

5 . 6 P R E - C R U S H I N G

5 . 7 H O R O M I L L

contents chapter 6
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5.1 INTRODUCTION

In the simplest system, the ball mill is used in open-circuit wherefeed materials are introduced to the mill inlet and cementproduct leaves at the mill exit.

Increasingly mills were used in conjunction with an airseparator in which the mill exit material was classified

according to size, the fines becoming the product and the coarsebeing returned to the mill inlet. Hence the ball mill wasoperated in closed-circuit.

In recent years the roll press was developed and a number ofmill circuits using a combination of roll press, ball mill and airseparator have emerged. In some instances there are circuitswithout the ball mill in which either a roll press or a verticalroller mill are used.

Use of pre-crushing devices is now also re-gaining interest.

5. CEMENT MILLING SYSTEMS

contents chapter 5 chapter 6


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5.2 OPEN-CIRCUIT MILLING

The use of open-circuit mills for cement grinding is largelyrestricted to smaller capacity and/or low product Blaine (whereless than 300m2/kg).

The circuit usually consists of: (See Figure 55).- ball mill (or tube mill, as sometimes referred, due to

high length to diameter ratio).- feed conveyor- discharge hood- fan- dust f il ter- product conveyor- water cooling (external or internal)

Figure 55. Open Circuit Ball Mill Layout.

The mill itself can be 3 chambers, or 2 chambers withclassifying liner or sometimes 4 chambers. The length todiameter ratio will usually be at least 3:1, but often is higher at5 or even 6:1.

There are many mills of 2.5-3m diameter at around 1000kW,but larger open-circuit mills of 2200 kW are also relativelycommon.

Mill control is relatively simple and product fineness (usuallythe Blaine) is controlled by the feed rate. The particle size

distribution is essentially only a function of the feed rate andproduct Blaine for any given mill. However the psd is influencedby the mill configuration (e.g. media grading, see section 6). Ingeneral an open-circuit mill will produce a noticeably wider psdthan that of cement produced by a closed-circuit system.

Because of the relatively low efficiency of such a system, and the

lack of separator rejects, temperature rise in the mill can be verysignificant.

Cooling can be achieved by shell cooling water, internal waterinjection and ventilation airflow.

One important feature of an open-circuit mill is that the productfineness is the same as that of the material leaving the mill.Naturally, as we shall see, this differs considerably to thesituation in closed-circuit mills. As a result of this the 'in-mill'fineness, i.e. fineness of material leaving the mill, can be veryhigh. Thus the problems associated with fine particle attractionare much more significant in open-circuit mills. The resultantagglomeration and coating of material on the media charge andliners results in a significant reduction in grinding efficiency.This is further exacerbated by the often high temperatures.

The main advantage of open-circuit mill systems lies in theircomparatively lower capital cost, since there is no need for theseparator and associated ancillaries. Also, the mill house canoften be very much smaller.

Because the mill is effectively "one-pass", any residual coarsenibs (e.g. 3-5mm or larger) need to be removed from the circuit,or they would end in the product and potentially causeproblems in packing and customer materials handlingequipment. One method is to pass the mill exit material over a

screen to separate the nibs. These are then discarded, usuallydaily or per shift. Nibs usually only represent around 0.1% ofthe product.

The mill ventilation air passes through ducting to a filter (eitherfabric bags or electrostatic precipitator) where the entrained

particles are removed. This dust is then usually passed directlyto the product. Sometimes there is a static separator, to removethe bulk of the dust, prior to the filter. The ducting at the milldischarge hood is kept relatively large to keep the dust burdento a low level. This requires velocities of only 1-2m/sec. In theducting transport velocities (say 20m/sec) are used.

Care should be taken when sampling the product from the mill,noting where the dust is returned (usually into the dischargehood).

Product conveying is usually of screw-pump design, but can bescrew conveyor, belt conveyor or blow tank.


Filter

Air

Dust

Fan

Feed Product

Mill



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5.3 CLOSED-CIRCUIT MILLING

5.3.1 INTRODUCTION

As discussed in section 3, Rittinger postulated that the increasein SSA is proportional to the energy input. However we shallsee that for higher fineness levels there is an increasing deviationfrom this linear relationship (see section 6). Thus the kWh/tonnerequired for an increase in the Blaine of 10m 2/kg increases as the

Blaine increases. In fact increasing the Blaine from 300-400m2/kg requires some 50-100% more kWh/tonne thanincreasing from 100 to 200m2/kg. (See figure 56).

Figure 56. Average Cement Grindability Curve.

Notes: For 0 200/250m2/kg, the relationship is close 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

As we have already seen in open-circuit mills the in-mill finenesshas to reach that of the product. At high SSA's the grindingefficiency reduces. Thus, by grinding to a lower in-mill finenessthe grinding efficiency can be enhanced.

In a closed-circuit milling system the total mill throughput issubstantially higher than that of an open-circuit mill and thusthe material leaving the mill has a much lower SSA than that ofthe product from an open-circuit mill. This coarser materialthen passes to an air separator where the coarser particles arereturned to the mill for further grinding. The finer particles arecollected as the cement product.

The main elements of a closed-circuit system are shown infigure 57. These are similar to those of an open-circuit mill (See5.2) with the addition of:

- Material Elevator (Bucket)- Air Separator (or classifier)- Separator rejects handling system (air slide, screw

conveyor)- Separator Fan

The most important element is of course the separator.

Figure 57. Closed Circuit Ball Mill Layout.

5.3.2 SEPARATOR

The 1980's saw a rapid introduction of so-called high efficiencyseparators. In the past, the move to closed-circuit operation hadnot resulted in the expected reduction in power consumption forOPC. In general, at modest Blaine, the additional 10% or socapacity achieved over open-circuit operation was matched byaround a 10% increase in total kW in the circuit (e.g. elevator,

separator and fan).

The limited benefits for conventional separators (often referredto as 1st generation) are due to their inherent poor efficiencywhich results in excessive recycling of fines and failure tocontrol product residue levels.

Such types of separator include:- Heyd- FLS CV- Polysius Turbo- Sturtevant Whirlwind

Their poor separation efficiency can be attributed to:- Poor dispersion of feed- By-pass of feed directly into the rejects stream- Ill-defined and weak separating forces, which

prevent a clear separation of fine and coarsematerial

- Poor product/airflow separation

The conventional separator (See Figure 58) essentiallyconsists of:

- Two concentric cyclones- Feed distribution plate- Main Fan- Auxiliary fan (or means of control)

- Return louvres for the air


Dust

Product

Separator

Fines

F (f)

SeparatorFeed

A (a)

Filter

Air

Fan

FeedMill

Returns

R (r)

Mill Outlet



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Figure 58. 1st Generation Separator.

The feed is introduced, from the side or top, into the inner coneand onto a rotating distribution plate. A main fan, inside theseparator casing, causes the air to flow up in the inner cone,through the fan, and down the annulus of the two cones. Theair is then returned to the inner cone through a set(s) of louvresor vanes. The airflow also has a strong tangential component toits flow.

Between the distribution plate and the main fan is the separatingzone. Here particles are subjected to (See Figure 59):

- A drag force due to the airflow

- A centrifugal force (tangential component)- Gravity

Figure 59. Principal of Air Separator.

Larger particles are influenced by the centrifugal forces andgravity to a greater degree and these descend down the inner coneto the base where they pass to the returns conveying system.

Smaller particles are more strongly influenced by the drag forceof the airflow and these become entrained passing through thefan and into the annulus between the cones. Separation ofmaterial from the air occurs at the louvres as the air is drawnsharply back through the louvres.

The 'cut size' (See Section 6) is controlled by the airflow and itsangular (tangential component). Separator efficiency isdiscussed in section 6.

Given some of the recognised areas of inefficiency variousattempts were made to improve separator efficiency. One ofthese concerned the product/air separation in the annulus of thecones. Here, the efficiency was low and thus fines were recycledto the inner cone. The development of the cyclone separator(often referred to as 2nd generation) effectively carried out thisproduct/air separation externally in high efficiency cyclones.

(See Figure 60). The separator airflow is generated in a higherefficiency clean (virtually) fan.

As a result of this development separator efficiency was greatlyimproved.

Figure 60. 2nd Generation Separator.

During the early 1980's Onoda developed the O-Sepa separatorwhich has been termed high efficiency (or 3rd generation). Thisis marketed by Onoda, FLS-Fuller and CLE. Following this,designs emerged from Sturtevant, Polysius, FLS, KHD, O+K, etc.

All of these newer designs feature:-- A feed dispersion system which ensures that all the

feed material passes through the separating zone.- Stronger, well defined, separating forces (directly

opposing)- External fan and high efficiency product/air

separation- Generally a more compact design

Examples of these are shown in Figures 61-64.


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Figure 61. O-Sepa 3rd Generation Separator.

Figure 62. Sturtevant SD 3rd Generation Separator.

Figure 63. Polysius Sepol 3rd Generation Separator.

Figure 64. FLS Sepax 3rd Generation Separator.

The principal design data for the main separators (O-Sepa, SD,Sepax, Sepol, Sepmaster, O+K, Pfeiffer) is given in figure 65.

Some of the important design differences for each supplier are:

O-Sepa: Top feed inletTangential air inletVertical guide vanesRotor cage with bladesTop extraction of air + fines

Sturtevant SD: Top feed inletTangential air inletHorizontal guide vanesRotor cage with rodsBottom extraction of air + fines

FLS Sepax: Bottom/Tangential feed inletTangential/bottom air inletVertical guide vanesRotor cage with bladesTop extraction of air + fines

Polysius Sepol: Top feed inletTangential air inletVertical guide vanesRotor cage with rodsBottom extraction of air + fines




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Figure 65. Principal Separator Dimensions.O-Sepa

Sturtevant SD

FLS Sepax Polysius Sepol


Type

Rotor Dimensions Separator

Airflow

(m3/h)

Air Speed

Rotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

N - 500 1.045 0.58 1.80 1.90 30,000 4.38

N - 750 1.290 0.73 1.77 2.96 45,000 4.23

N - 1000 1.490 0.85 1.75 3.98 60,000 4.19

N - 1500 1.830 1.08 1.69 6.21 90,000 4.03

N - 2000 2.100 1.24 1.69 8.18 120,000 4.07

N - 2500 2.360 1.39 1.70 10.31 150,000 4.04

N - 3000 2.590 1.53 1.69 12.45 180,000 4.02

N - 3500 2.800 1.66 1.69 14.60 210,000 3.99

N - 4000 2.980 1.78 1.67 16.66 240,000 4.00

N - 4500 3.160 1.90 1.66 18.86 270,000 3.98

Type


Airflow

(m3/h)

Air Speed

Rotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

190 1.270 0.58 2.19 2.31 26,000 3.12

212 1.420 0.73 1.95 3.26 36,000 3.07

236 1.550 0.89 1.74 4.33 48,000 3.08

250 1.640 0.99 1.66 5.10 56,000 3.05

265 1.730 1.10 1.57 5.98 66,000 3.07

280 1.810 1.20 1.51 6.82 76,000 3.09

300 1.940 1.34 1.45 8.17 91,000 3.10

315 2.030 1.45 1.40 9.25 102,000 3.06

335 2.150 1.59 1.35 10.74 118,000 3.05

355 2.270 1.73 1.31 12.34 136,000 3.06

375 2.390 1.88 1.27 14.12 154,000 3.03400 2.540 2.06 1.23 16.44 179,000 3.02

425 2.690 2.24 1.20 18.93 207,000 3.04

450 2.840 2.42 1.17 21.59 235,000 3.02

475 2.990 2.60 1.15 24.42 266,000 3.03

Type

1.700 Separator

Airflow

(m3/h)

Air Speed

Rotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

HS 60 0.600 0.40 1.50 0.75 9,000 3.32

HS 70 0.700 0.47 1.49 1.03 13,000 3.49

HS 80 0.800 0.53 1.51 1.33 17,000 3.55

HS 90 0.900 0.60 1.50 1.70 21,000 3.44

HS 100 1.000 0.67 1.49 2.10 26,000 3.43

HS 110 1.100 0.73 1.51 2.52 32,000 3.52

HS 120 1.200 0.80 1.50 3.02 38,000 3.50

HS 130 1.300 0.87 1.49 3.55 44,000 3.44

HS 140 1.400 0.93 1.51 4.09 51,000 3.46

HS 150 1.500 1.00 1.50 4.71 59,000 3.48

HS 160 1.600 1.07 1.50 5.38 67,000 3.46HS 170 1.700 1.13 1.50 6.03 76,000 3.50

HS 185 1.850 1.23 1.50 7.15 90,000 3.50

HS 200 2.000 1.33 1.50 8.36 105,000 3.49

HS 215 2.150 1.43 1.50 9.66 121,000 3.48

HS 230 2.300 1.53 1.50 11.06 138,000 3.47

HS 250 2.500 1.67 1.50 13.12 164,000 3.47

HS 270 2.700 1.80 1.50 15.27 191,000 3.47

HS 290 2.900 1.93 1.50 17.58 220,000 3.48

HS 310 3.100 2.07 1.50 20.16 252,000 3.47

HS 330 3.300 2.20 1.50 22.81 285,000 3.47

Type


Airflow

(m3/h)

Air Speed

Rotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

SD 20 0.400 0.30 1.33 0.38 4,000 2.95

SD 30 0.750 0.45 1.67 1.06 12,000 3.14

SD 40 1.000 0.60 1.67 1.88 22,000 3.24

SD 60 1.500 0.85 1.76 4.01 50,000 3.47

SD 80 2.000 0.95 2.11 5.97 75,000 3.49

SD 100 2.500 1.15 2.17 9.03 115,000 3.54

SD 120 3.000 1.35 2.22 12.72 165,000 3.60

SD 150 3.750 1.65 2.27 19.44 250,000 3.57

SD 150S 3.750 1.90 1.97 22.38 290,000 3.60

SD 180 4.500 1.95 2.31 27.57 380,000 3.83

SD 210 5.250 2.20 2.39 36.29 500,000 3.83



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KHD Sepmaster SKS Pfeiffer QDK

O & K QS

In most cases closed-circuit systems using these separatorsoperate with a separate mill venting air fan. The dust from thisis either returned to the product or the separator feed.

In many cases the separator will have an open-circuit airflow(straight through) which significantly contributes to the mill

circuit cooling. Airflow is often designed to give a solids loadingof around 2kg/m3 (kg of total separator feed).

The product is usually collected in a high efficiency bag filter,(See Figure 66), with either pulse jet cleaning or reverse air.

Figure 66. Bag Filter.


Type

Rotor Dimensions SeparatorAirflow

(m3/h)

Air SpeedRotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

SKS 12 12,000

SKS 18 18,000SKS 25 25,000

SKS 36 1.180 0.73 1.62 2.71 36,000 3.70

SKS 45 1.310 0.82 1.60 3.37 45,000 3.70

SKS 60 1.590 0.97 1.64 4.85 60,000 3.44

SKS 75 1.700 1.06 1.60 5.66 75,000 3.68

SKS 95 1.910 1.10 1.74 6.60 95,000 4.00

SKS 120 2.150 1.34 1.60 9.05 120,000 3.68

SKS 145 145,000

SKS 175 2.600 1.62 1.60 13.23 175,000 3.67

SKS 210 2.840 1.78 1.60 15.88 210,000 3.67

SKS 250 3.100 1.94 1.60 18.89 250,000 3.68

SKS 290 290,000

Type


(m3/h)

Air SpeedRotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

QDK 6 0.480 0.32 1.50 0.48 4,000 2.95

QDK 8.5 0.650 0.44 1.48 0.90 12,000 3.14QDK 11 0.860 0.58 1.48 1.57 22,000 3.24

QDK 12.5 0.960 0.64 1.50 1.93 50,000 3.47

QDK 14.5 1.150 0.77 1.49 2.78 75,000 3.49

QDK 16.5 1.270 0.85 1.49 3.39 115,000 3.54

QDK 19 1.500 1.00 1.50 4.71 165,000 3.60

QDK 22 1.700 1.14 1.49 6.09 250,000 3.57

QDK 25 1.950 1.30 1.50 7.96 290,000 3.60

QDK 29 2.250 1.50 1.50 10.60 380,000 3.83

QDK 31 2.400 1.60 1.50 12.06 500,000 3.83

Type


(m3/h)

Air SpeedRotor

(m/s)Diameter

(m)

Height

(m) D/H

Area

(m2)

QS 50/2 0.50 0.50 1.00 0.79 4,000 2.95

QS 63/2 0.63 0.63 1.00 1.25 13,000 2.90

QS 80/2 0.80 0.80 1.00 2.01 20,000 2.76

QS 100/2 1.00 1.00 1.00 3.14 32,000 2.83QS 125/2 1.25 1.25 1.00 4.91 50,000 2.83

QS 160/2 1.60 1.60 1.00 8.04 80,000 2.76

QS 180/2 1.80 1.80 1.00 10.18 104,000 2.84

QS 200/2 2.00 2.00 1.00 12.57 130,000 2.87

QS 225/2 2.25 2.25 1.00 15.90 164,000 2.86

QS 250/2 2.50 2.50 1.00 19.63 200,000 2.83

QDK 33 2.550 1.70 1.50 13.62 187,000 3.83

QDK 38 2.950 1.97 1.50 18.26 248,000 3.77



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In some older systems some of the separator air was drawnthrough the mill and thus there was only one fan. In thesesystems care is required when sampling the separator feed, sinceonly a proportion will be mechanically conveyed from the mill.In general, separate mill ventilation systems are preferred.

In some cases the separator product is collected in cyclones and

the air is recycled to the separator (similar to the cycloneseparator air circuit).

The higher separation efficiency of these separators results inless misplaced material, i.e. less fines are returned to the milland less coarse particles are placed into the product. Thereforethe in-mill fineness is lower (lower Blaine of mill exit/separatorfeed) and the final product has a lower residue/narrower particlesize distribution.

The reduced level of fines in the separator rejects can clearly beseen from determination of the Blaine. For OPC production ataround 350m2/kg the typical Blaine values for the rejects are: -

Conventional Separator (1st Generation) 150-250 m2/kgCyclone Separator (2nd Generation) 100-200 m2/kgHigh Efficiency Separator(3rd Generation) 50-150 m2/kg

For simplicity we can assume that a Blaine mass balance can bemade for a closed-circuit system. This assumes, for example,that if we had 1kg of cement at 400m 2/kg and 1kg of cement at300m2/kg and we careful ly mixed them together, we wouldproduce 2kg at 350m2/kg.

Applying this to a closed-circuit mill we can see the effect ofreducing the separator rejects Blaine on the separator feed Blaine.

Referring to TIS MS013 if we have a closed-circuit millproducing 50 tonnes/hour with a circulating load of 300%

(rejects of 100 tonnes/hour, total feed of 150 tonnes/hour), thenfor a product Blaine of 350 m2/kg we find the following:-

Rejects Blaine Mill exit/separator feed Blaine200 250150 217100 183

Thus as we return less fines to the mill the mill exit finenessreduces. Hence the in-mill fineness also reduces and the grindingefficiency is increased. This is quantified in Section 6.

This higher grinding efficiency means that an increase in outputis achieved.

The narrower psd results in an enhanced strength development, asdiscussed in Section 3, and this can often result in a reduction in theBlaine target with additional associated increases in mill output.

In general, the application of high efficiency separators ontoexisting mills with conventional separators (or even open-circuit) can be expected to result in:-

- a reduced SSA (by 20-50m2/kg)- a narrower psd (lower residues, e.g. 45 microns of

0-5% compared to 5-15%)- poorer packing behaviour of the cement product- marginally lower bulk density (2-5%)- higher paste water demand (28-34%, compared to

24-28%)- longer setting times (higher paste water demand,

poorer packing and effective larger spacing)- a reduced early strength (lower Blaine related)- a lower milling temperature and hence less gypsum

dehydration

- some change in concrete slump behaviour, dependingon:-- change in SSA- clinker chemistry (C3A, alkalis, SO3)- change in milling temperature- gypsum source, cement SO3, clinker SO3- existing degree of optimised slump properties

The main advantages of these separators are:-- the ability to increase existing mill capacity by

around 10-30%- a reduction in kWh/t of 5-20%- the ability to produce cool cement (60oC and below)- increased product fineness flexibility (e.g. super fine

cements of 500-1000 m2/kg)

In some closed-circuit systems, particularly where there is a highmill airflow, a static separator is used in the mill air circuit toremove the dust. These separators have no moving rotor andonly static guide vanes (See Figure 67). However, the principleof operation is similar to that shown in Figure 59.


Figure 67. Static Separator.



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5.3.3 CIRCUIT CONTROL

To ensure optimum mill operation and efficiency it is importantto have a suitable means of control.

The most usual control system is called "total feed control". Inthis the objective is to maintain a constant total tonnes/hourthrough the mill (and thus separator).

The signal for the total feed can be:-- elevator amps- rejects and feed mass flow signals (e.g. impact or belt weigher)- mill power

The first two are most commonly used.

The principle involves a total feed set-point, which can bederived by experimental trials during commissioning. Duringcontrolled operation the signal of total feed is matched againstthe set-point and the fresh feed is periodically altered. Anincrease in fresh feed rate will result in more rejects and areduction in less rejects. However 1 tonne/hour of rejects DOES

NOT equate to 1 tonne/hour of fresh feed. A ratio of between3:1 and 10:1 is used i.e. at 10:1 if the rejects increases by 10tonnes/hour, the feed is reduced by 1 tonne/hour.

The same total feed set-point is used for all products, theproduct fineness being adjusted by the separator settings only(usually rotor speed or airflow change - a higher speed giving afiner product and a higher airflow giving a coarser product).These settings are also found during commissioning.

As an example we can consider the following mill:-

1. OPC production, 350m2/kg, 100 tonnes/hour, set-point of

280 tonnes/hour, separator setting A.

i.e. A/F = 280 = 280%100

2. To change to RHC, 450m2/kg, the new separator setting (B)is made (i.e. increased rotor speed and/or reduced airflow).The higher separator setting will cause an increase in therejects rate and thus the total feed will be above the set-point of 280. Therefore the automatic control will reducethe feed rate until the mill is once again in steady-state, e.g.RHC production, 450m2/kg, 65 tonnes/hour, set-point of

280, separator setting B.i.e. A/F = 280 = 431%

65As you can see, with such a control system, higher circulatingloads will result for higher product fineness levels.

The principle of total feed rate control means that the loadingto the separator (e.g. kg/m3) is constant for all products andthat the influence on powder filling in the mill is also constant(See Section 6).

In a situation where the feed grindability becomes more difficultthe rejects rate would increase and this would result in anautomatic correction (reduction) in the feed rate.

One problem of such a system is the situation if theintermediate diaphragm becomes blocked or the first chamberstarts to overfill. In this case the rejects rate would reduce andthe total feed control system would automatically increase thefresh feed rate, i.e. further exacerbating the problem. Toovercome this eventuality it is common to have noise detection("ears") for the 1st chamber. This can detect a filling of thechamber and then override the total feed control system.

It is also important to control the mill to a desired temperature.Typically this will usually involve:-

- 100-105C at intermediate diaphragm

- 105-125C at mil l exit

In systems with high efficiency, open-circuit separators the millwill often be at around 100-110C whilst the product can bebelow 70C.

Where cement product temperatures are too high cement coolers(heat exchangers) can be used.




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5.4 ROLL PRESS

5.4.1 INTRODUCTION

As discussed in Section 3.4, Rittinger's theory concerned therelationship between kWh/tonne and new surface produced. Ifthe only energy involved in size reduction was associated withthe material surface energy, then grinding efficiency could betermed to be 100% efficient. With this level of efficiency cement

production to 300m2

/kg would only involve around 0.1kWh/tonne! (desirable, but unattainable). (See Figure 68 andSection 6).

However in practice there is an associated amount of energyrequired to maintain crack propagation and this reducesefficiency, compared to the surface energy, to about 10%.

In order to bring about the application of just the correctamount of energy for breakage, it has long been recognised thatslow compression of individual particles is the most efficientmethod. However this involves a further loss of efficiency as aresult of internal friction losses. Efficiency would be reduced toaround 1% of the surface energy. The kWh/tonne for 300m2/kg

would have increased to around 8.

Figure 68. Efficiency of Grinding Equipment.Cm2/j Efficiency Approx. kWh/t to

300m2/kg Cement104 100% Surface Energy of Solid


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Figure 71. Roll Press Product Particle Size Distribution. 5.4.3 ROLLPRESS TERMINOLOGYThe actual equipment is usually referred to as:-

Roll PressRoller PressPress

Terms such as "crushing rolls", "roller mill", "pre-stressing

rolls" should be avoided.

Below is some of the terminology associated with the roll pressand the milling circuits involving them.

Cake: Compacted product from the press

Slabs: As for cake

Pre-Grinding: System where the roll press is used in frontof a milling system (open or closed circuit)

Cake re-circulation: Operation of a press with recycle of itsown product, often collected from thesides of the material leaving the gap

Edge material: Material passing the roll press gap at theedges which is subjected to a lowerpressure

Gap: Distance between the rolls

Cake thickness: Thickness of the roll press product. Thisis typically 2% of the roll diameter, e.g.10mm to 40mm. Thickness reduces forhigher pressures

Roll Speed: This is usually the peripheral speed of therolls, typically 1-2m/sec. Lower speeds areneeded for stable operation when theBlaine is higher or there is a greater

proportion of material re-circulation (e.g.cake or separator rejects)

Operating pressure: Measure of the force applied, e.g. between5000-8000 kN/m2, or up to 300 MPa

Roll diameter: Diameters of the rolls can be up to 2m

Roll width: Width or length of roll. TypicalWidth:Diameter are between 1:1 and 0.5:1

Cake density: Bulk density of the press product.This is typically 2.2-2.5, from a feed of1.6-1.8 tonnes/m3

Installed Power: Total power installed can be as high as2500 kW. Drawn power will often onlybe 70-80% of this

Specific Power The roll press operates at between 2 and 4Consumption: kWh/tonne, depending on the operating

pressure, e.g.

350-450 MPa 4 kWh/t320 MPa 3.2 kWh/t200-220 MPa 2.7 kWh/t

Fixed Roll : Rotates, but does not move in towards gap

Moveable Roll: Rotates and moves under the hydraulicpressure

Wear: The roll surface is wear-protected, eitherby welded surface or sometimes segmented

plates. The wear is caused by the relativemovement between the material and theroll, as material is drawn into the gap

Roll damage: Damage, as opposed to wear, for exampleby a foreign body, can result in surfacecracking. Higher pressures increase the riskof damage

Coefficient of This refers to the equivalent kWh/t in aSubstitution: ball mill replaced by the press, e.g.

Mill only, 2500 kW 100 t/hr 25 kWh/tMill+Press, 2500 kW 150t/hr 16.7 kWh/tPress, 450 kW 150t/hr 3.0 kWh/t

Coefficient = 25-16.7 = 2.83

Coefficients are typically:-Clinker = 2-2.8Slag = 2.5-3.5Limestone = 1.7-2.0


Particle Size % Finer

Range Average

10000 99 100 90

5000 95 99 90

2000 78 93 80

1000 65 82 72

500 54 73 63

250 45 65 55

125 37 55 45

90 34 49 41

63 28 40 34

45 23 33 28

30 18 24 21

20 12 17 14

15 10 14 12

10 8 11 9

5 3 5 4

2 1 2 2

SSA 70 100 85



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Reaction angle or Angle from the horizontal at which theForce angle: maximum pressure occurs (1). This is

typically 2 to 3. (See Figure 69a)

Nip angle : Angle from the horizontal where thematerial begins to move at the roll speed.This is 7-9. (See Figure 69a)

Pressure profile: Refers to the pressure applied to thematerial as it passes through the rolls.Pressure gradually increases from the nipangle to a maximum at the force angle andis then rapidly reduced. (See Figure 69a)

Stable operation: For press stability there needs to be:-- Friction between material and rolls- Drawing in of material, nipping- Formation of a column of material- Smooth pressure profile

Note (1) The maximum pressure occurs above the horizontal, i.e.not at the minimum gap. This is because the material in the gapshows some plasticity. The downwards velocity near the minimumgap can be up to 4x the roll speed, hence the pressure is reduced.Instability: Instability can arise as a result of the

following sequence:-- escaping air- some fluidisation- loss of grip- pressure profile concentrated over a

smaller angle, reaction angle is lower- material re-gripped with a higher

reaction angle- cycling then occurs (10-20Hz) with

reaction angles of between 3x and -1 ofnormal levels

- in the extreme escaping air can causefluidisation in the material column.

High levels of recirculation of fines (e.g. separator rejects) canbe the most significant cause of entrapped air and instability.

Figure 69a. Roll Press Pressure Profile.

5.4.4 MILLING CIRCUITS WITH ROLL PRESSES

5.4.4.1 PRE-GRINDING

In this circuit (See Figure 72) the roll press is only utilised infront of the ball milling system. It can be operated with singlepass of feed or more often with recirculation of cake. The millcan be open or closed-circuit. There is not normally adisagglomerator.

The press kW will be typically up to 25% of the mill power.Production can be increased by approximately 20-50%,depending on the level of cake recirculation.

Figure 72. Pre-Grinding Circuit.

5.4.4.2 HYBRID GRINDING

In this circuit (See Figure 73) the press is operated with somerecirculation of separator rejects. However, to avoid instability itis increasingly often that cake re-circulation is used inpreference.

Material is fed to the press and all of the product passes to the

ball mill. The ball mill product then passes to a separator andthe rejects are split between the roll press and the ball mill.

The degree of recirculation to the press dictates the size of pressfor a given mill and thus the ratio of kWh/tonne in the press tothat in the mill.

The press kW can typically be 25-75% of the mill kW.Production can be increased by up to 100%. Sometimesreferred to as "mixed" or "split" circuit.

Figure 73. Hybrid Grinding Circuit.

5.4.4.3 SEMI-FINISH GRINDINGThis circuit (See Figure 74) differs to hybrid in that the productfrom the press passes to the separator, and not the mill, after adisagglomeration stage. Sometimes referred to as "partialfinish" grinding, since some of the product will not have passedthe ball mill.


Air

Dust

Filter

Fan

Feed

Product

Mill

Roll Press

Returns

R (r)

"Cake"Recycle

Separator

Fines

F (f)Product

A(a) SeparatorFeed

Mill Outlet

Air

Dust

Filter

Fan

Feed

Product

Mill

Roll Press

"Cake"Recycle

Returns

R (r)

Separator

Fines

F (f)Product

A(a) SeparatorFeed

Mill Outlet



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The separator rejects are split between the ball mill and thepress.

Disagglomeration can be achieved in a number of ways, e.g.:-- integral in separator- separate unit, e.g. hammer mill, Barmac type- part of a ball mill, e.g. chamber of a double rotator

mill- single chamber ball mill (low volume loading)

Press sizes, and increases in capacity are similar to those ofhybrid systems.

One advantage of this system is that the material transportthrough the mill is substantially lower than that in a hybridsystem, where all of the material leaving the press passes themill. Total mill throughputs in excess of 30 tonnes/hour/m2

could be required for hybrid operation.

Figure 74. Semi-Finish Grinding Circuit.

5.4.4.4 FINISH (OR INTEGRAL) GRINDING

In this circuit (See Figure 75) there is no ball mill. The pressoperates in closed-circuit with a disagglomerator and aseparator. A high circulating load is required. (See Section5.4.4.6).

This circuit produces a narrow psd and usually involves low

milling temperatures. To date, there has been a conservativereaction to such systems, especially where clinker reactivity isdeemed to be high.

Figure 75. Finish Grinding Circuit.

5.4.4.5 SEMI INTEGRAL OR COMBI GRINDING

This system (See Figure 76) combines the advantages of theenergy efficiency of the finish system with the productcharacteristics of a ball milling circuit.

Material is fed to a closed-circuit roll press. The separatorrejects all pass back to the press. The separator product, whichwill typically have a Blaine of 180-220 m2/kg, is then finished ina ball mill (open or closed-circuit). At least 120-150m2/kg of thefinal product SSA is achieved in the ball mill.

The finishing ball mill can be open-circuit with only small mediasince the feed does not contain any large sized particles.

A relatively recent development concerns the use of the Cascadeor V-separator in the press part of the circuit. This is acombination of a crossflow and upstream classifier and has nomoving parts. (See Figure 76a.).

Figure 76. Semi Integral or Combi Grinding Circuit.

Figure 76a. Cascade Separator, V-Separator.


Feed

Roll Press

"Cake"

Recycle

Returns

R (r)

Separator

Fines

F (f)Product

A(a) Separator

Feed

Disagglomerator

Feed

Roll Press

"Cake"ecycle

Returns

Separator

Fines

SeparatorFeed

Disagglomerator

Dust

Filter

Fan

Product

Mill

Returns

Separator

FinesProduct

SeparatorFeed

Air

Dust

Filter

Fan

Feed

Mill

Roll Press

"Cake"

Recycle

Returns

R (r)

Separator

Fines

F (f)Product

A(a) Separator

Feed

Mill

Outlet

Disagglomerator



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5.4.4.6 CIRCUIT EXAMPLES

CCB: This is the world's largest cement mill installation(1992).Roll Press: 2m dia x 1m 2500 kWBall Mill: 5.8 x 17.0m 8700 kWSeparators: 4 Sepol

For 300-310 m2

/kg, production exceeds 450tonnes/hour470 m2/kg production around 180-200 tonnes/hour

Only press cake is recycled, since instability is aproblem with separator rejects. Total throughput hasbeen as high as 1000 tonnes/hour (equivalent to 38tonnes/hour/m2). Thus, whilst the system is termedhybrid, in practice it is pre-grinding with cakerecirculation.

Cormeilles: This was the first finish system for cement(1991). (See Figure 77).

CPA was produced at relatively low Blaine at around40 tonnes/hour. Total feed rate was around 240-300tph (A/F = 600-700%). Disagglomeration is in thetop part of the separator. Hot secondary air is used toraise the temperature in the separator air circuit toachieve some dehydration of the gypsum. Temperaturesof 80C are achieved.

The press draws around 700kW. Operates with totalfeed rate control. Clinker is not considered reactive -1.5% SO3,


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5.5 ROLLER MILL

The roller mill, or vertical spindle mill, is commonly used forthe grinding of raw materials (See Section 2, Figure 27) but hasnot been so widely applied to the grinding of cement.

Initially the application of roller mills for cement wasconsidered inappropriate, on account of:-

- high wear rate (clinker more abrasive than limestone)- excessive vibrat ion- poor product characteristics (narrow psd, poor

workability)

However there are advantages of lower kWh/tonne and, in somecases, the inherent drying capability.

Recent years have seen improved metallurgy (to overcome wearproblems), improved operating experience (improvedunderstanding of vibration) and a better understanding ofcement characteristics as influenced by psd and temperature (asa result of the experience gained from high efficiency separatorsand roll presses).

The grinding of slag and slag cements in roller mills is relativelycommon, particularly in the Far East.

Teutonia (Germany) has a 70 tonnes/hour roller mill which hasoperated successfully since the 1980s.

Figure 27. Roller Mill Figure 27a. Roller Mill for Cement




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5.6 PRE-CRUSHING

With the introduction of the roll press there was a re-emergenceof the interest in pre-crushing (as opposed to pre-grinding). Pre-crushing devices operate at similar or lower kWh/tonne to theroll press and produce a crushed product, similar in the coarsersizes but absent of fines when compared to that of the press.They are used in circuits similar to those of the roll press.

The pre-crushers can be:-- simple crushers (e.g. hammer)- vertical shaft impactor (e.g. Barmac)- roller mill

The vertical shaft impactor (See Figure 78) concerns passingfeed material into a closed rotor assembly which acceleratesmaterial out of the rotor where it impacts either impact platesor a bed of rock (e.g. Barmac). Some fines generation exists butmainly as a result of the natural breakage of material.

Throughput increases for ball mill circuits are sometimesclaimed to be similar to that achieved by roll presses, but are

likely to be lower, due to the absence of fines. Increases inoutput of 25% have been achieved, but are more typically 10-15%. The crushers have a significantly lower capital cost andare less prone to problems of damage when compared to theroll press.

In recent years, the roller mill is also finding applications as apre-grinding device in front of a ball mill circuit.

Figure 78. Vertical Shaft Impactor.




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5.7 HOROMILL

The Horomill was developed in the early 1990s. It aims tocombine all the benefits of a roll press with all the reliability ofa ball mill. In effect it is a combination of roll press, roller milland ball mill. (See Figure 79).

The HOROMILL consists of a horizontal cylindrical shell. The

shell is relatively short, with a length to diameter ratio of lessthan 1. The shell is only lined in the grinding zone. Inside thecylinder there is a roller which is horizontally supported andhydraulically loaded. The roll is only driven by the frictionalaction of the compressed material bed in the stressing gapbetween the two surfaces.

A patented system allows the material to be scraped off the millshell and to be moved forward towards the mill outlet. Thegrinding action is exerted through a varying number of passagesfrom 4 to 6 between the roller and mill shell.FLS worked for a time in the development of the CEMAX mill,but this never reached full commercialisation.

Figure 79. HOROMILL Horizontal Roller Mill.



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Cement Chapter 5