Cement Clinker Grinding Practice and Technology

Hakan Benzer*, Alex Jankovic†, Levent Ergun*

* Hacettepe University, Ankara, Turkey† Metso Minerals Process Technology Asia Pacific, Brisbane, Australia

ABSTRACT

The current world consumption of cement is close to 2 billion tonnes per annum and it is

increasing at about 1% per annum. Conventional cement grinding circuits consist of two-

compartment tube mills and the air separators. Alternative mills such as High Pressure Grinding

Rolls, Vertical Roller Mill and Horomill has been applied in recent times in order to improve the

grinding efficiency. Air separators play crucial role in improving overall energy efficiency of the

cement grinding circuits and has been improved continuously over the decades.

Introduction of clinker pre-crushing stage can significantly improve the cement grinding energy

efficiency. Due to relatively low capital cost the Barmac crusher is an attractive upgrade option.

Hybrid grinding circuits with HPGR are being widely used primarily due to higher energy

efficiency, with specific energy consumption reduced to almost 50% compared to some

conventional circuits.

INTRODUCTION

The current world consumption of cement is close to 2 billion tones per annum. During the last

10 years the cement production has increased 38 %. Different types of Portland cement are

manufactured to meet different physical and chemical requirements for specific purposes. The

American Society for Testing and Manufacturing (ASTM) has designated five types of Portland

cement as given in Table 1.

Table 1. Portland cement classification with its constituents and fineness

Types Clinker%

Admixture %

Minor component%

Fineness +45µm %

CEM I 95-100 - 0-5 11.4CEM II 80-94 6-20 0-5 14.2CEM III 35-64 36-65 0-5 5.9CEM IV 65-89 11-35 0-5 11.6CEM V 40-64 18-30 0-5 17.0

Grinding Portland cement is made from exact proportions of materials containing calcium, silica,

alumina and iron. Approximately 1.5 tonnes of raw materials are required to produce 1 tonne of

finished cement. Grinding operation is the major operation and occurs at the beginning and the

end of the cement making process. The last step in the process of manufacturing portland cement

is the finish grinding of clinker together with small amount of gypsum and some admixtures. The

principal objectives of clinker grinding are to promote the hydration of cement and to ensure

complete coating of inert aggregates. The fineness of the cement effects on the concrete

properties in terms of the placeability, strength and permeability. The finer the grind the more

reactive is the finished cement. Therefore, every type of cement has got its own fineness to meet

the required quality. In Figure 1 the particle size distribution of the different cement types are

presented.

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1

Particle Size (mm)

Cum

. % p

assi

ng

CEM I

CEM II

CEM III

CEM IV

CEM V

Figure 1 Size distribution variation among the different cement types

The electrical energy consumed in the conventional cement making process is in order of 110

kWh/tonne and about 30% of which is used for the raw materials preparation and about 40% for

the final cement production by cement clinker grinding. Figure 2 shows the electrical energy

consumption split in a typical plant (Fujimoto, 1993). Production costs and environmental

concerns are emphasizing the need to use less energy and therefore the development of more

energy efficient machines for grinding and classification.

Figure 2 Energy consumption for different stages of cement production

EQUIPMENT USED FOR CLINKER GRINDING

Tube Ball Mills

The continuous ball mill has been used for just over one hundred years and it still the most widely

installed grinding equipment in cement manufacturing. Cement is ground in tube ball mills

operating either in open or closed circuit. The tube mills are characterized by the length/diameter

(L/D) ratio and it is found that the best alternative for the ratio is 3 in terms of energy expenditure

(Schnatz and Knobloch, 2000). The tube ball mills can be operated as one, two or three

compartment and the length of the each compartment should be evaluated based on the size

distribution variation from feed end to the discharge end.

Special diaphragms divide the cylinders of multi compartment mills. The diaphragms are

primarily designed to prevent passing of the balls to the next compartment and to allow the flow

Quarry crushing and prehomo

5%Raw material

grinding24%

Feed homogenization

6%Burning and

cooling22%

Conveying, packing and

loading5%

Finish cement grinding

38%

and pre-homogenization

of ground material through the mill. The design of the diaphragms influence the fineness of the

ground material (Duda, 1985).

Figure 3 Example of mill liners in the first and second compartment of a cement ball mill

Various shapes of mill liners have been developed for the cement mills (see Figure 3). The

classifying liners are special application in clinker grinding. This lining causes a classification of

the grinding ball sizes resulting in a decrease in size along the grinding path. The grooved liner

application also favours the slipping motion in the second compartment of the mills where

abrasion breakage is dominant.

Operation of the tube ball mills is relatively well understood and there are several design and

operating parameters of the ball milling operations, which affect the mill efficiency and the

quality of the cement produced (Gouda, 1981).

Vertical Roller Mill (VRM)

Vertical roller mills have been in use for limestone and coal grinding in cement industry for many

years thanks to high drying capacity, low energy consumption, compactness and reliability in

operation. The largest mill in operation has an installed power of 6 MW grinding 840 t/h from the

lump feed size down to 85 % 90µm. Cement grinding by a vertical roller mill is applied in

pregrinding systems, advanced pregrinding systems and finish grinding systems (Schimoide,

1996).

In a VRM the interparticle comminution takes place in a material filled gap between the rotating

table and the grinding rollers. The mill feed is charged to the center of the table and moves

affected by centrifugal forces and friction towards the table’s edge. On it is way it is nipped by 2,

3, 4 or 6 conical rollers installed at the outside rim of the table. The rollers are attached to

hydraulic cylinders which provide grinding forces for comminution of the material. The ground

particles leave the table by an air stream and taken up to the separator incorporated in the casing

of the mill. The fine product is taken as the mill discharge and the coarse reject of the separator

falls back on to the table as the circulating load.

Finish grinding of cement by means of a VRM was first put into commercial operation in 1984

(Shimoide, 1996). Since then, however, further applications were relatively limited. One reason is

that a portion of the power savings achieved in the VRM (attributed to higher grinding efficiency)

is lost due to a higher power consumption from the fan (FLS, 1998). The second one is the wear

problem, however in the recent years plant trials indicated that the problems can be reduced with

new roller design. The wear rate and the throughput of the system depends very heavily on the

consistency of the materials being ground (Nobis, 2001). Effective comminution largely depends

on the formation of a stable grinding bed between the rollers and the grinding table.

The main operational bottle neck’s of the VRM is the high circulating loads from separator

rejects. These cause an inefficient grinding operation because of the high load accumulation

inside the mill. In order to overcome this problem the roller mills are operated with external

material circulation. It was reported that the specific power consumption for producing portland

cement with external material circulation was 30 % less than for producing these cements in tube

mills (Feige, 1983). CKP mill is an example for this type machine and CKP mills were developed

on the basis of the proven technology of vertical roller mills (Suton et al., 1992). In CKP system,

materials are fed through a central chute. Centrifugal force, combined with rotation of the table,

distributes the product over the table surface. After grinding, which is carried out between the

table and rollers, the material is extracted from the CKP by gravity with the assistance of the

scrapers (Miranda et.al, 1998). CKP mills are generally used as pregrinders and the grinding

energy efficiency of using CKP mill as a pregrinder resulted with grinding energy saving of 17

% (Dupuis and, Rhin, 2003).

Horizantal Roller Mills (HOROMILL)

The HOROMILL consists of a horizontal cylinder supported onslide-shoe bearings and driven

through an open gear train. The principles of HOROMILL are briefly summarized as a bed

material compression mill, a multi-compression mill and as high capacity mills (Cornille, 1999).

The simplified diagram is given in Figure 3.

Figure 3 Comminution principle in Horomill

The material passes into the mill at one end of the cylinder and, because of the centrifugal effect

caused by operating the cylinder above the critical speed, is carried as a uniformly distributed

layer of material on its inner surface. The finished product is collected in a dust filter, while the

coarse particles are recycled to the mill. The grinding force is transmitted to the roller by

hydraulic cylinders. Internal fittings are provided to control the material recirculation. It’s been

reported that the grinding process based on multiple compressions give the machine a high

stability and also the recirculating load can be adjusted to suit the quality target (Cordonnier,

1994).

The comparison with a ball mill indicate that HOROMILL operates with a large grinding bed

thickness and moderate pressures which leads to energy savings of 35 to 40 % for cement

grinding. The operational experience indicates that the specific cost concerning the liners and

wear parts are higher than an equivalent ball mill (Brunelli, 2001). Mechanical problems with a

Horomill are reported in Konya cement plant in Turkey (Fochardiere, 1999).

High Pressure Grinding Rolls (HPGR)

The High Pressure Grinding Rolls (HPGR) developed by Professor Schoenert has been offered as

a comminution technology with claims of improved performance relative to conventional

grinding technology. In particular, it has been claimed that the advantage of the high pressure

grinding roll is its lower specific energy consumption (Schoenert, 1979).

The material to be ground is compressed in a gap between two counter rotating grinding rolls (see

Figure 5) with circumferential speed of 1 to 1.8 m/s to form a compacted cake. The compacted

cake contains fine particles, coarser particles with larger numbers of incipient cracks and weak

points, which greatly reduce the energy expenditure during further comminution (Ellerbrock,

1994).

Fixed roll

Feed

Moveable rollOil cylinders

Product

Nitrogen cylinder

Figure 5 The principle of operation of high pressure grinding rolls

HPGR can be used at several stages in cement grinding. These configurations are named as pre-

grinding, finish grinding, hybrid grinding and semi finish grinding. In pregrinding configuration,

reductions of overall energy consumption in the range of 20 % have been achieved (Kellerwessel,

1986). The hybrid grinding configuration is achieved by splitting the coarse fraction from the air

classifier to the high pressure grinding rolls and ball mill respectively. In the semi-finish grinding

configuration the high pressure grinding roll is operated in closed circuit with the air classifier

and the fines from the separator are finally ground in a tube mill circuit. In the finish grinding

configuration the high pressure grinding rolls operate with an air classifier in closed circuit. In

this alternative the energy saving potential is up to 50 % (Kellerwessel 1996). Hovewer, in finish

grinding application the water requirement to make mortar increases significantly as result of the

narrow size distribution (Roseman, 1989 ; Odler and Chen, 1995).

Air Classifiers

Classification in the clinker grinding circuits is achieved using the air separators. Development of

air classifiers was based on the principles of two devices, the simple expansion chamber and

Mumford and Mood separator, patented in 1885 (Klumpar et al., 1986).

There are two types of air classifiers, dynamic and static. Dynamic classifiers have moving and

fixed internal parts respectively. Dynamic classifiers have evolved through three generations,

each being significantly better than its predecessor.

Static air classifiers does not have moving parts and classification achieved is by changes in air

velocity and direction were an early invention. The principle of operation is shown in Figure 5a.

The air stream carrying the particles is converted from a directional flow through the outer cone

into a rotating flow by guide vanes. The particles are subject to centrifugal force, the coarse

particles move to the outer wall of the inner cone and are collected in a bin, and the fine particles

leave with the air and are sent to a dust collector. The product size can be altered to some extent

by changing the angle of the vanes but the efficiency is low and static classifiers can be regarded

more as grit separators than efficient classifiers.

a. b.

Figure 6 a - schematic view of the static air classifier, b - separation mechanism in a

dynamic air classifier

The dynamic air classifiers utilize a distribution plate to disperse the feed material into the

separation zone. Thus a particle of material is subjected to three forces: centrifugal force from the

distribution plate, uplift from the air current and gravity. Figure 5b indicates the forces acting on

a particle in a dynamic air classifier.

The first generation classifier had a distributor plate and the air circulation in the classifier was

provided by a vertically supported rotor. The main problems with the first generation classifiers

were the circulating air becomes very hot, fine particles were not removed from the recycling air

and the control of the product is very difficult. Figure 7a shows a simplified sketch of a first

generation air separator.

a. b.

Figure 7 First (a) and second (b) generation dynamic air separator

In the second generation type (see Figure 7b), the main difference from the first generation was

the external fan replaced to circulate the air and these are equipped with a cyclone for the fines.

The product control can be achieved individually by adjusting the rotor speed and air velocity

separately.

The third generation separators are known as high efficiency separators (see Figure 8). The

feeding of the material to the separator is achieved as dispersed curtain of particles and the

horizontal air flow to the separator gives a uniform separation performance. The fine particles

passes through a rotating cage before goes to the fine product. The bars of the cage assists in the

performance of the separator.

Figure 8 Schematic of a third generation dynamic air separator

CIRCUIT CONFIGURATION FOR IMPROVED ENERGY EFFICIENCY

For most of the twentieth century, the common dry grinding circuits for the production of

finished cement from cement clinker, consist of two-compartment tube mills with or without the

air separators. The advantage of this circuit is its simplicity and easy operation; however, the

energy consumption is high especially for the open circuit operation.

The circuit with two-compartment tube mills has limited energy efficiency partly due to high

reduction ratio required in a single comminution/classification step. Clinker feed size can vary

from F80=10–40 mm and the final product size is P80=35-40 microns and the size reduction ratio

could be in order of 250-1000. Large balls (up to 100mm) are used in the first compartment of the

tube mill to crush the coarse clinker, however the ball mill energy efficiency reduces for feed

sizes larger than F80=2-3 mm. It should be therefore more energy effective to pre-crush the

clinker. Recent work indicates that introduction of the Barmac crusher for clinker pre-crushing

can increase the cement circuit throughput in order of 10-20%. Alternatively, the total energy

consumption of the circuit can be reduced in order of 5-10% (Jankovic et al, 2004). This is an

attractive upgrade option due to relatively low capital investment for the Barmac crusher.

Clinker pre-crushing can be carried out with different crushers. Figure 9 shows the product size

distribution of the closed Barmac and HP cone crusher circuits with 4.75 mm screen at 2.3 kWh/t

specific energy input. Although the 80% passing size for the HP cone crusher is finer, the Barmac

product is potentially more favorable due to higher content of fines. This advantage however is

not crucial for the selection as the clinker feed size, hardness and abrasivity, as well as the

required capacity, may favor the selection of the particular crusher.

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

size (mm)

cum

ula

tive

% p

assi

ng

HP cone, 2.3 kWh/t, productBarmac, 2.3 kWh/t, product

HP cone feedBarmac feed

Figure 9 Product size distribution from the closed Barmac and HP cone crusher circuit

In order to obtain the most efficient breakage in the first compartment of the ball mill after

introduction of the pre-crushing stage, the ball size distribution should be selected according to

the particle size distribution of the material fed to the mill. The particle size distribution of the

combined ball mill feed (new feed + 150% recycle) with raw and pre-crushed clinker is shown in

Figure 10. It can be observed that there is a significant fraction of the material coarser than 5 mm

in the feed containing the raw clinker. For such feed, top size ball required would be 90-100 mm

according to Bond formula. For pre-crushed feed the top ball size would be 35-40 mm due to

absence of coarse particles. Therefore, if the feed to the ball mill is pre-crushed, ball charge in the

first compartment has to be optimized to maximize grinding efficiency.

In addition to ball size optimization, the optimum length ratio between the first and second

compartment would be affected. Design of the transfer grate, mill liners and sweep air velocity

should be also reviewed for reduced ball size in order to provide efficient removal of fine

particles.

0

2

4

6

8

10

12

14

16

18

0.01 0.1 1 10 100size (mm)

% r

etai

ned

raw clinkerpre-crushed clinker

Particle size (mm)

Optimum ball size

(mm) Raw clinker

(% ret)

Pre-crushed clinker (% ret)

37.5 142 0.20 0 25 116 0.53 0 19 101 3.71 0

13.7 90 6.30 0 9.5 71 4.15 0

4.75 50 10.28 0.40 2.36 36 7.50 9.65 1.18 25 2.86 11.26 0.6 18 0.94 4.74 0.3 13 4.63 4.40

0.15 9 3.821 6.83 0.075 6 13.92 16.12 0.053 / 9.82 9.89 0.038 / 8.17 8.63 0.032 / 3.41 3.84 0.025 / 3.97 4.60 0.01 / 8.27 9.53

0 / 7.49 10.10

0

2

4

6

8

10

12

14

16

18

0.01 0.1 1 10 100size (mm)

% r

etai

ned

raw clinkerpre-crushed clinker

Particle size (mm)

Optimum ball size

(mm) Raw clinker

(% ret)

Pre-crushed clinker (% ret)

37.5 142 0.20 0 25 116 0.53 0 19 101 3.71 0

13.7 90 6.30 0 9.5 71 4.15 0

4.75 50 10.28 0.40 2.36 36 7.50 9.65 1.18 25 2.86 11.26 0.6 18 0.94 4.74 0.3 13 4.63 4.40

0.15 9 3.821 6.83 0.075 6 13.92 16.12 0.053 / 9.82 9.89 0.038 / 8.17 8.63 0.032 / 3.41 3.84 0.025 / 3.97 4.60 0.01 / 8.27 9.53

0 / 7.49 10.10

Figure 10 Combined ball mill feed size distribution with raw and pre-crushed clinker

In last 20 years High Pressure Grinding Rolls (HPGR) are being extensively used in cement

grinding circuit mainly due to higher grinding efficiency compared to conventional two

compartment tube mills. HPGR can be used for pre-crushing, finish grinding, hybrid grinding and

semi finish grinding. Table 2 shows the energy consumption of five cement grinding circuit with

different HPGR application (Aydogan et al, 2003). It can be observed that the overall circuit

specific energy consumption decreases when larger portion of size reduction (higher HPGR

kWh/t) is done by HPGR. Compared to circuit utilizing the ball mill only for grinding, energy

savings in excess of 40% are achievable providing that circuit is optimized and automated

process control is applied.

Table 2 Specific energy consumption in different cement grinding circuit utilising HPGR

Cement Grinding Circuit DescriptionHPGR

specific energy consumption

(kWh/t)

Circuit overall specific energy consumption

(kWh/t)Open circuit HPGR, closed circuit ball mill 4.05 34.2Open circuit HPGR with partial recycling, closed circuit ball mill

8.9 29.6

Hybrid grinding / 29.9Closed circuit HPGR, closed circuit ball mill 8.0 21.7Semi-Finish grinding 9.8 23.0

In order to assess the performance of a particular cement grinding circuit and to compare

efficiency of different circuit configurations, complete audits are required. The audit includes

monitoring and sampling different circuit streams during the steady state operation, as well as

mill inspection and sampling after the crush-stop. Based on information obtained from the audit

circuit mass balance can be carried out to determine material flows (solids and air) around the

circuit. Only after this the detailed performance of the circuit as well as individual equipment can

be assessed and potential bottlenecks identified. To assist with circuit optimization, site and

equipment specific models are calibrated based on the results from the audit. Models can be then

used to simulate different operating conditions and circuit scenarios (Benzer et al, 2001, 2003).

CONCLUSION

The current world consumption of cement is close to 2 billion tonnes per annum and it is

increasing at about 1% per annum. The electrical energy consumed in the conventional cement

making process is approximately 110 kWh/tonne, and around 40% of this energy is consumed for

clinker grinding.

For most of the twentieth century, the dry grinding circuits for the production of finished cement

from cement clinker consist of two-compartment tube mills and the air separators. Alternative

mills such as High Pressure Grinding Rolls (HPGR), Vertical Roller Mill and HOROMILL has

been applied in recent times in order to improve the grinding efficiency. Significant energy

savings are reported in applications of these mills, HPGR being the most widely used.

Air separators were improved over the time from very inefficient static separators to high

efficiency dynamic separators. They play crucial role in improving overall energy efficiency of

the cement grinding circuits.

The increasing demand for “finer cement” products, and the need for reduction in energy

consumption and green house gas emissions, reinforces the need for grinding optimisation. In the

last two decade significant progress has been achieved by new equipment design and new circuit

configuration. Introduction of clinker pre-crushing stage can significantly improve the energy

efficiency. Due to relatively low capital cost Barmac crusher is an attractive upgrade option.

Hybrid grinding circuits with HPGR are being widely used primarily due to higher energy

efficiency, with specific energy consumption reduced to almost 50% compared to some

conventional circuits.

In order to optimize grinding circuit, detailed knowledge of circuit operation is required.

Modelling and simulation techniques can be effectively utilized to assist in process optimization.

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