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Cement grinding optimisation
Minerals Engineering 17 (2004) 1075–1081 This article is also available online at: www.elsevier.com/locate/mineng
Cement grinding optimisation Alex Jankovic a
a,*
, Walter Valery a, Eugene Davis
b
Metso Minerals Process Technology Asia-Pacific, Brisbane, Australia b Metso Minerals Asia-Pacific, Perth, Australia Received 28 April 2004; accepted 20 June 2004
Abstract The current world consumption of cement is about 1.5 billion tonnes per annum and it is increasing at about 1% per annum. The electrical energy consumed in cement production is approximately 110 kWh/tonne, and around 40% of this energy is consumed for clinker grinding. There is potential to optimise conventional cement clinker grinding circuits and in the last decade significant progress has been achieved. 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. This paper describes the tools available for the analysis and optimisation of cement grinding circuits. The application of the Bond based methodology as well as Population Balance Models (PBM) is presented using a case study. The throughput of current conventional closed grinding circuit can be increased by 10–20% by pre-crushing the clinker using the Barmac crusher. A potential for application of stirred milling technology for fine cement grinding was also discussed. 2004 Published by Elsevier Ltd. Keywords: Dry grinding; Process optimisation; Modelling
1. Introduction For all dry grinding applications, cement production is certainly the most important. The estimate for the world energy consumption for cement production is 18.7 TWh which is approximately 0.02% of total world energy consumption per year. The world consumption of cement was about 1.72 billion tonnes in 2002 and it is increasing at about 1% per annum. Cement production process typically involves: • grinding limestone (and other raw materials to achieve the right chemical composition) to about 90% passing 90 lm in a dry circuit,
* Corresponding author. Present address: 24 Lavarack Avenue, Eagle Fram Qld 4009, Australia. Tel.: +61 7 3868 2144; fax: +61 7 3868 2105. E-mail address: [email protected] (A. Jankovic).
0892-6875/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.mineng.2004.06.031
• making cement by the chemical reaction between the components of the ground mixture. This chemical reaction occurs at high temperature in a rotary kiln, • grinding the cement clinker nodules to 100% passing 90 lm in a dry circuit. Grinding occurs at the beginning and the end of the cement making process. Approximately 1.5 tonnes of raw materials are required to produce 1 tonne of finished cement. The electrical energy consumed in the 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. 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. The world energy consumed for cement production is similar to energy used for grinding in US mining industry, which is a significant fraction of the world total. In
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A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
the minerals industry, research in modelling and simulation of the grinding process has a long and successful history. However, in the cement industry, the grinding process is more of an ‘‘art’’ than engineering and the equipment manufacturers exclusively hold the ‘‘knowhow’’. The process is designed and operated using carefully guarded ‘‘recipes’’ and rules. In such as environment there is little or no room for process understanding and improvement. This paper discusses how techniques developed in minerals processing can be applied in cement grinding optimisation. Only the cement clinker grinding is discussed and area of raw material preparation is not covered.
2. Cement 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. It is not uncommon to produce the cement in an open circuit. Advances in cement grinding technology is slow and these advances are limited to more developed countries. Approximately 95% of the feed to the cement grinding circuit are clinker and the rest of the feed are ‘‘additives’’ which includes grinding aids. The quality of cement is measured by the surface area or the Blane index. The unit of the Blane index is m2/kg, and this index is determined by the Blane air permeability test. The surface area of the cement powder depends on size distribution of cement particles; smaller particles have larger surface area. If the particle size distribution is known, the Blane index can be successfully predicted (Zhang and Napier-Munn, 1995). The cement clinker grinding circuit reduces the feed from 80% passing size between 10 and 20 mm to 100% passing 90 lm. The size reduction takes place in a twocompartment tube mill; the first compartment of the mill is shorter than the second compartment. The coarse clinker is ground in the first compartment where larger balls (80, 60, 50 mm) are used and the fine grinding is done in the second compartment where smaller balls (below 25 mm) are used. A diaphragm (see Fig. 1) separates the two compartments and allows only particles below a certain size to pass to the second compartment. Ground material exits the mill through the discharge grate which prevents grinding balls from leaving the mill. A proportion of material, mostly fines, is ‘‘airswept’’ out of the mill. The final product is the fine fraction of the air classifier and the coarse fraction returns to the mill. In the past 20 years, high pressure grinding roll (HPGR) technology has been used in pre-crushing of clinker. Presently, many American and European cement grinding circuits have HPGR which increases grinding capacity and energy efficiency.
Fig. 1. Diaphragm between the two compartments of the twocompartment mill—view from first compartment.
3. Cement grinding simulation To optimise cement grinding, standard Bond grinding calculations can be used as well as modelling and simulation techniques based on population balance model (PBM). Mill power draw prediction can be carried out using MorrellÕs power model for tumbling mills. 3.1. Bond method The established technique for determining power requirements for ball mills is the Bond method (Bond, 1961). This method also involves the application of some Ôefficiency factorsÕ as described by Rowland (1975). BondÕs equation describes the specific power required to reduce a feed from a specified feed F80 to a product with a specified P80: 10 10 W m ¼ W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð1Þ P 80 F 80 where: Wm Wi P80 F80
mill specific motor output power (kWh/t) Bond ball mill work index (kWh/t) sieve size passing 80% of the mill product (lm) sieve size passing 80% of the mill feed (lm)
The ‘‘efficiency factors’’ (Rowland, 1975) modify Eq. (1) so that it caters for circuit conditions which are different from that Bond used to develop his original equation. There are efficiency factors for dry grinding, open circuit ball milling, mill diameter, oversize feed, grinding finer than 75 lm and too large or too small reduction
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
ratios. For cement application, the dry grinding (EF1), mill diameter (EF3), oversize feed (EF4) and fine product (EF5) factors are relevant. Therefore, the equation for the specific power requirement is: 10 10 W m ¼ EF1 EF3 EF4 EF5 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð2Þ P 80 F 80 According to Bond the specific power (calculated using Eq. (1)) should be multiplied by EF3 where the mill diameter exceeds 8 ft. Rowland (1975) modified the application of EF3 and stated it should be used up to mills of 12 ft. For mills larger than 12 ft the value of EF3 remains constant at the value for 12 ft mills. 0:2 2:44 EF3 ¼ ð8=Dft Þ0:2 ¼ ð3Þ D
The oversize feed factor (EF4) caters for situations where the feed size is coarser than a specific size limit (F0), which is a function of ore hardness. Bond argued that if the feed ore were coarser than this, bigger balls would be needed to break the coarser feed particles at the expense of grinding of smaller particles. Conversely, if smaller balls were used to grind the finer particles, the smaller balls would not break the coarser particles. Either way a grinding inefficiency would result. The EF4 factor is applied only when the F80 is greater than F0 and has a value greater than 1. The correction factor, EF4, for the ore feed size is calculated as follows (Rowland, 1975): F 80 F 0 EF4 ¼ Rr þ ðW i 7Þ ð4Þ Rr F0 13 P 0 ¼ 4000 Wi
0:5 for ball mills
Rr ¼ F 80 =P 80 where: Rr F0 F80 P80
ð5Þ ð6Þ
size reduction ratio optimum mill feed size (lm) actual mill feed size (lm) mill product size (lm)
The correction factor EF5 for the products finer than 75 lm (Rowland, 1975) is determined by: EF5 ¼
P 80 þ 10:3 1:145 P 80
Bond calculations and therefore empirical corrections were introduced. The following modified Bond equation was proposed for crushing (Magdalinovic, 1990): A 10 10 ð8Þ W c ¼ pffiffiffiffiffi W i pffiffiffiffiffi pffiffiffiffiffi Pc Pc Fc where: energy consumed for clinker crushing (kWh/t) Wc Wi Bond ball mill work index (kWh/t) Pc sieve size passing 80% of the clinker after crushing (lm) Fc sieve size passing 80% of the clinker before crushing (lm) A is an empirical coefficient, dependant on clinker and crusher properties Based on the above considerations for crushing and grinding, the energy consumption for the clinker precrushing and ball milling can be estimated using the following Bond based model:
where: is the mill diameter inside liners (ft) Dft D is the mill diameter inside liners (m)
1077
ð7Þ
It was found in the crushing area that there are significant differences between the real plant data and the
W ¼WcþWm
ð9Þ
As pre-crushing product size Pc is equal to the mill feed size F80 then: 0:2 A 10 10 2:44 W ¼ pffiffiffiffiffiffiffi W i pffiffiffiffiffiffiffi pffiffiffiffiffi þ 1:3 D F 80 P 80 Fc F 80 F 0 Rr þ ðW i 7Þ Rr F0 P 80 þ 10:3 10 10 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð10Þ 1:145 P 80 P 80 F 80 3.2. PBM models In the area of dry particle reduction, the population balance models (PBM) for crushers, HPGR, ball mills, air-swept ball mills and air separators have been developed (Lynch, 1977; Austin et al., 1980; Zhang et al., 1988; Morrell et al., 1997; Benzer et al., 2001, 2003). These models can be used to simulate cement grinding circuits and to assist their optimisation. The ‘‘work horse’’ of the cement grinding plant is the two-compartment ball mill, commonly called the tube mill. Significant advances in model development were achieved in recent years (Benzer et al., 2001, 2003) through research on industrial scale. The breakage and transport mechanisms are better understood as well as the classification action of the diaphragm. Further advances are expected in modelling the effect of diaphragm design on classification and powder transport. The basis for modeling the two-compartment ball mill is the perfect mixing ball mill model. It can be illustrated by the following equation:
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fi þ
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
i X aij rj pj j¼1
dj
¼ pi þ
ri pi di
ð11Þ
where: feed rate of size fraction i (t/h) fi pi product flow of size fraction i (t/h) aij the mass fraction of size that appear at size i after breakage ri breakage rate of particle size i (h1) si amount of size particles inside the mill (tonnes) di the discharge rate of particle size (h1)
Barmac B-series VSI (vertical shaft impactor) crushers are applied to a broad range of materials in minerals and aggregate industry. Due to the ‘‘autogenous grinding action’’ it is especially efficient for high abrasive materials such as cement clinker. The crushing action is schematically presented in Fig. 3. When the incoming rock passes over the distributing plate the rock is divided in three separate streams and is forcefully impacted on the rock lining at the distributing
The model consist two important parameter, the breakage function (aij) that describe the material characteristic and breakage/discharge rate function (ri/di) which defines the machine characteristics and can be calculated when feed and product size distribution are known and breakage function is available. The air classifier controls the final product quality. Therefore, the air classifier has a crucial role in the circuit and a lot of attention is paid on the design and operation of the air classifier. The classification action is modeled using the efficiency curve approach. Effect of the classifier design and operational parameters on the efficiency is complicated and work is in progress to improve the current models.
4. The case study Potential benefits of using the Barmac crusher for clinker pre-crushing were studied for a cement plant. Fig. 2 shows the proposed cement grinding circuit.
Fig. 3. Schematic of Barmac crusher operation.
Fig. 2. Simplified cement grinding circuit with precrushing stage.
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
cum%pass
100 90
Clinker
80
case A
70
case B
60
case C
50 40 30 20 10 0 0.01
0.1
1
10
100
size (mm)
Fig. 4. Clinker and the crushed clinker size distribution (case A, B, C) using Barmac crusher.
Table 1 Bond method power calculation for two-compartment mill Mill feed F80 (mm)
EF4
Power required (kW)
Difference (%)
15.5 4.5 3.0 1.8
1.06 1.01 1 1
3564 3251 3133 3018
0.0 8.8 12.1 15.3
35 Total grinding kWh/t
plate. The material is being rapidly accelerated by the centrifugal force of the rotor action and is compressed against the rock lining which is formed in the crushing chamber. Multiple events occur and a variety of forces act on the individual particles as they proceed. The crushed rock is discharged through the clearance between the crusher chamber and the rotor wall. Fig. 4 shows the clinker size distribution and predicted Barmac crusher size distributions. These data are based on Barmac pilot testing and the simulations. It can be observed that the crushed clinker has a significant amount (10–20%) of 75 lm material. As the cement size is in this size range, it could be concluded that the Barmac crusher produces a significant amount of finely ground cement (see Table 1). It can be seen that a reduction in two-compartment mill power in the order of 9–15% is calculated for different crushed clinker feed F80 sizes. Corresponding to the change in factor EF4, a 5–6% reduction comes from improving the milling efficiency with finer feed. In order to get this improvement, the ball charge size distribution in the first compartment needs to be adjusted for finer feed. As energy is consumed in pre-crushing stage, the total power required will be sum of two-compartment ball mill and Barmac crusher power. Fig. 5 shows how the total comminution energy consumption depends on the Barmac crusher product size. It can be seen that the total energy consumption would reduce with reducing the product size of the Barmac crusher. This indi-
1079
34 33 32 31 30 29 28 27 26 25
0
2
4
6
8
10
12
14
16
Barmac product P80 (mm)
Fig. 5. Specific power requirement for cement grinding circuit including precrushing stage with Barmac crusher.
cates that the cement grinding efficiency may be improved up to 10% compared to a conventional circuit without pre-crushing. The alternative benefit of introducing the Barmac crusher in pre-crushing stage is increasing the circuit capacity, as the capital investment is relatively low compared to HPGR. The PBM modelling of the clinker grinding was carried out using the principles described earlier. It should be noted that in order to obtain the ‘‘site specific’’ model constants, detailed surveys of the milling circuit are required: the size distribution of the material in each stream as well as from different points inside the mill. In this study the model constants published in the literature (Benzer et al., 2001, 2003) were used. The base case flowsheet generated in the JKSimMet grinding simulation software is shown in Fig. 6. The first compartment was modeled as two ball mills in series, diaphragm between the first and second compartments was represented as a screen and the second compartment is represented as one ball mill. Information given in the flowsheet is solids throughput (t/h), 80% passing size and % passing 0.01 mm. Using the above base case model, simulations with different feed size distributions (raw clinker, and precrushed clinker using the Barmac crusher) were carried out, keeping the product size constant at P80 = 0.038 mm. The resulting increase in throughput is shown in Table 2. Table 3 summarises the potential increase in throughput from using a Barmac crusher to pre-crush the cement clinker. It should be noted that this result does not consider any circuit physical limitations such as conveying, aeration and air classifier capacity. It can be observed that Bond calculations gave less throughput increase than PBM simulations. This could be because the PBM simulation results may have been overoptimistic as the separator performance was kept the same. A model capable of simulating changes in performance with different air separator loads was not available.
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A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
Fig. 6. Greens Island base case JKSimMet simulation flowsheet.
Table 2 Simulated increase in throughput F80 (mm)
Simulated throughput (t/h)
Increase (%)
15.5 4.5 3.0 1.8
110 125 135 140
0.0 13.6 22.7 27.3
Table 3 Predicted increase in throughput using the Bond and grinding modelling method—base case throughput 110 t/h Feed F80 (mm)
minus 75 l in Barmac product (%)
% throughput increase, Bond method
% throughput increase, PBM
15.5 4.5 3.0 1.8
0 12.1 15.4 20.5
0 9.1 14.3 18.2
0 13.6 22.7 27.3
partment is usually double that of the first compartment. Smaller grinding balls are also used in the second compartment. The efficiency of fine grinding in the second compartment is largely controlled by the size of grinding balls. Due to limitation of the millÕs rotational speed, the smallest ball size is usually restricted to about 15 mm. Cement grinding using stirred mills (Pilevneli and Azizli, 1999) indicates that using smaller media (5–8 mm range) improves grinding energy efficiency up to 50% using stirred mills. For specialized types of cement, which are finer than Portland, this figure would be even higher. Significant benefits of using Tower mill were also reported (Shibayama et al., 2000) as well as the industrial applications for production of fine 12 000 Blane cement. The stirred mills can be used in different roles and it is expected that their application in the future will be significant.
6. Conclusion 5. Stirred milling potential As the product size decreases the energy required for particle breakage increases rapidly. The pre-crushing stage increases the milling efficiency in the first compartment of the two-compartment mill where coarse milling takes place. Pre-crushing however does not affect milling in the second compartment apart from producing significant amount of final product size (10–15%). In order to produce the final product size, the length of second com-
There is a scope for significant optimisation of the traditional cement grinding circuits. Grinding process modelling and simulation methods can be used for optimisation. A case study conducted using the data from an industrial cement plant indicates that: • Pre-crushing of cement clinker using a Barmac crusher offers realistic benefits to a cement plant in terms of process efficiency.
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
• The introduction of the Barmac crusher can increase the cement circuit throughput in order of 10–20%, providing that there is no capacity limitation in other parts of the circuit. This is an attractive option due to relatively low capital investment of the Barmac crusher. The overall energy efficiency of the circuit can also be improved in order of 5–10%. The stirred milling technology could further improve energy efficiency of cement grinding. Initial work indicates great potential and significant development in this direction should be expected.
References Austin, L.G., Weymont, N.P., Knobloch, O., 1980. The simulation of air-swept cement mill. In: Proceeding of European symposium. Particle Technology, Amsterdam. Benzer, H., Ergun, L., Oner, M., Lynch, A.J., 2001. Simulation of Open Circuit Clinker Grinding. Minerals Engineering 14 (7), 701– 710. Benzer, H., Ergun, L., Oner, M., Lynch, A.J., 2003.Case studies of models of tube mill and air separator grinding circuits. In: Lorenze, V., D.J. Bradshaw, D.J. (Eds.), Proceedings: XXII International Mineral Processing Congress, pp. 1524–1533. Bond, F.C., 1961. Crushing and Grinding Calculations Parts I and II. British Chemical Engineering 6 (6&8). Lynch, A.J., 1977. Mineral Crushing and Grinding Circuit – Their Simulation, Optimisation, Design and Control. Elsevier Scientific, Amsterdam.
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Magdalinovic, N., 1990. Mathematical Model for Determination of an Optimal Crusher Product size. Aufbereitungs-Technik 31, 5. Morrell, S., Shi, F., Tondo, l997. Modelling and Scale-up of High Pressure Grinding Rolls. In: Proceedings of the XX International Mineral Processing Congress(IMPC). Aachen, Germany, September 1997. Pilevneli, C.C., Khairun Azizi Mohd Azizli, 1999. Semi-Batch Dry Grinding Tests of a Pilot Scale Vertical Stirred Mill. In: Proceedings of VIII Balkan Mineral Processing Conference. Belgrade, Yugoslavia. Rowland, C.A., 1975. The tools of power: How to evaluate grinding mill performance using Bond Work Index to measure grinding efficiency AIME Annual Meeting. Tucson, Arizona. Shibayama, A., Mori, S., Bissombolo, A., 2000. Studies on Comminution Mechanism of the Dry Tower Mill KD-3. In: Proceedings of the XXI International Mineral Processing Congress. Rome. Zhang, Y.M., Napier-Munn, T.J., 1995. Effects of particle size distribution, surface area and chemical composition on Portland cement strength. Powder Technology 83, 245–252. Zhang, Y.M., Napier-Munn, T.J., Kavetsky, A., 1988. Application of comminution and classification modelling to grinding of cement clinker. Transaction Institution of Mining and Metallurgy (Section C: Mineral Processing and Extractive Metallurgy). C207–C213.
Further reading Bond, F.C., 1985. Testing and Calculations. SME Mineral Processing Handbook, Norman L. Weiss, Editor in Chief. Bond, F.C., 1962. Crushing and Grinding Calculations—April 1962 Additions and Revisions. Allis-Chalmers Manufacturing Co., Milwaukee, Wisconsin. Brugan, M.J., 1991. Mills at grinding edge. Pit and Quarry, 34–41.
Cement grinding optimisation Alex Jankovic a
a,*
, Walter Valery a, Eugene Davis
b
Metso Minerals Process Technology Asia-Pacific, Brisbane, Australia b Metso Minerals Asia-Pacific, Perth, Australia Received 28 April 2004; accepted 20 June 2004
Abstract The current world consumption of cement is about 1.5 billion tonnes per annum and it is increasing at about 1% per annum. The electrical energy consumed in cement production is approximately 110 kWh/tonne, and around 40% of this energy is consumed for clinker grinding. There is potential to optimise conventional cement clinker grinding circuits and in the last decade significant progress has been achieved. 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. This paper describes the tools available for the analysis and optimisation of cement grinding circuits. The application of the Bond based methodology as well as Population Balance Models (PBM) is presented using a case study. The throughput of current conventional closed grinding circuit can be increased by 10–20% by pre-crushing the clinker using the Barmac crusher. A potential for application of stirred milling technology for fine cement grinding was also discussed. 2004 Published by Elsevier Ltd. Keywords: Dry grinding; Process optimisation; Modelling
1. Introduction For all dry grinding applications, cement production is certainly the most important. The estimate for the world energy consumption for cement production is 18.7 TWh which is approximately 0.02% of total world energy consumption per year. The world consumption of cement was about 1.72 billion tonnes in 2002 and it is increasing at about 1% per annum. Cement production process typically involves: • grinding limestone (and other raw materials to achieve the right chemical composition) to about 90% passing 90 lm in a dry circuit,
* Corresponding author. Present address: 24 Lavarack Avenue, Eagle Fram Qld 4009, Australia. Tel.: +61 7 3868 2144; fax: +61 7 3868 2105. E-mail address: [email protected] (A. Jankovic).
0892-6875/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.mineng.2004.06.031
• making cement by the chemical reaction between the components of the ground mixture. This chemical reaction occurs at high temperature in a rotary kiln, • grinding the cement clinker nodules to 100% passing 90 lm in a dry circuit. Grinding occurs at the beginning and the end of the cement making process. Approximately 1.5 tonnes of raw materials are required to produce 1 tonne of finished cement. The electrical energy consumed in the 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. 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. The world energy consumed for cement production is similar to energy used for grinding in US mining industry, which is a significant fraction of the world total. In
1076
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
the minerals industry, research in modelling and simulation of the grinding process has a long and successful history. However, in the cement industry, the grinding process is more of an ‘‘art’’ than engineering and the equipment manufacturers exclusively hold the ‘‘knowhow’’. The process is designed and operated using carefully guarded ‘‘recipes’’ and rules. In such as environment there is little or no room for process understanding and improvement. This paper discusses how techniques developed in minerals processing can be applied in cement grinding optimisation. Only the cement clinker grinding is discussed and area of raw material preparation is not covered.
2. Cement 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. It is not uncommon to produce the cement in an open circuit. Advances in cement grinding technology is slow and these advances are limited to more developed countries. Approximately 95% of the feed to the cement grinding circuit are clinker and the rest of the feed are ‘‘additives’’ which includes grinding aids. The quality of cement is measured by the surface area or the Blane index. The unit of the Blane index is m2/kg, and this index is determined by the Blane air permeability test. The surface area of the cement powder depends on size distribution of cement particles; smaller particles have larger surface area. If the particle size distribution is known, the Blane index can be successfully predicted (Zhang and Napier-Munn, 1995). The cement clinker grinding circuit reduces the feed from 80% passing size between 10 and 20 mm to 100% passing 90 lm. The size reduction takes place in a twocompartment tube mill; the first compartment of the mill is shorter than the second compartment. The coarse clinker is ground in the first compartment where larger balls (80, 60, 50 mm) are used and the fine grinding is done in the second compartment where smaller balls (below 25 mm) are used. A diaphragm (see Fig. 1) separates the two compartments and allows only particles below a certain size to pass to the second compartment. Ground material exits the mill through the discharge grate which prevents grinding balls from leaving the mill. A proportion of material, mostly fines, is ‘‘airswept’’ out of the mill. The final product is the fine fraction of the air classifier and the coarse fraction returns to the mill. In the past 20 years, high pressure grinding roll (HPGR) technology has been used in pre-crushing of clinker. Presently, many American and European cement grinding circuits have HPGR which increases grinding capacity and energy efficiency.
Fig. 1. Diaphragm between the two compartments of the twocompartment mill—view from first compartment.
3. Cement grinding simulation To optimise cement grinding, standard Bond grinding calculations can be used as well as modelling and simulation techniques based on population balance model (PBM). Mill power draw prediction can be carried out using MorrellÕs power model for tumbling mills. 3.1. Bond method The established technique for determining power requirements for ball mills is the Bond method (Bond, 1961). This method also involves the application of some Ôefficiency factorsÕ as described by Rowland (1975). BondÕs equation describes the specific power required to reduce a feed from a specified feed F80 to a product with a specified P80: 10 10 W m ¼ W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð1Þ P 80 F 80 where: Wm Wi P80 F80
mill specific motor output power (kWh/t) Bond ball mill work index (kWh/t) sieve size passing 80% of the mill product (lm) sieve size passing 80% of the mill feed (lm)
The ‘‘efficiency factors’’ (Rowland, 1975) modify Eq. (1) so that it caters for circuit conditions which are different from that Bond used to develop his original equation. There are efficiency factors for dry grinding, open circuit ball milling, mill diameter, oversize feed, grinding finer than 75 lm and too large or too small reduction
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
ratios. For cement application, the dry grinding (EF1), mill diameter (EF3), oversize feed (EF4) and fine product (EF5) factors are relevant. Therefore, the equation for the specific power requirement is: 10 10 W m ¼ EF1 EF3 EF4 EF5 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð2Þ P 80 F 80 According to Bond the specific power (calculated using Eq. (1)) should be multiplied by EF3 where the mill diameter exceeds 8 ft. Rowland (1975) modified the application of EF3 and stated it should be used up to mills of 12 ft. For mills larger than 12 ft the value of EF3 remains constant at the value for 12 ft mills. 0:2 2:44 EF3 ¼ ð8=Dft Þ0:2 ¼ ð3Þ D
The oversize feed factor (EF4) caters for situations where the feed size is coarser than a specific size limit (F0), which is a function of ore hardness. Bond argued that if the feed ore were coarser than this, bigger balls would be needed to break the coarser feed particles at the expense of grinding of smaller particles. Conversely, if smaller balls were used to grind the finer particles, the smaller balls would not break the coarser particles. Either way a grinding inefficiency would result. The EF4 factor is applied only when the F80 is greater than F0 and has a value greater than 1. The correction factor, EF4, for the ore feed size is calculated as follows (Rowland, 1975): F 80 F 0 EF4 ¼ Rr þ ðW i 7Þ ð4Þ Rr F0 13 P 0 ¼ 4000 Wi
0:5 for ball mills
Rr ¼ F 80 =P 80 where: Rr F0 F80 P80
ð5Þ ð6Þ
size reduction ratio optimum mill feed size (lm) actual mill feed size (lm) mill product size (lm)
The correction factor EF5 for the products finer than 75 lm (Rowland, 1975) is determined by: EF5 ¼
P 80 þ 10:3 1:145 P 80
Bond calculations and therefore empirical corrections were introduced. The following modified Bond equation was proposed for crushing (Magdalinovic, 1990): A 10 10 ð8Þ W c ¼ pffiffiffiffiffi W i pffiffiffiffiffi pffiffiffiffiffi Pc Pc Fc where: energy consumed for clinker crushing (kWh/t) Wc Wi Bond ball mill work index (kWh/t) Pc sieve size passing 80% of the clinker after crushing (lm) Fc sieve size passing 80% of the clinker before crushing (lm) A is an empirical coefficient, dependant on clinker and crusher properties Based on the above considerations for crushing and grinding, the energy consumption for the clinker precrushing and ball milling can be estimated using the following Bond based model:
where: is the mill diameter inside liners (ft) Dft D is the mill diameter inside liners (m)
1077
ð7Þ
It was found in the crushing area that there are significant differences between the real plant data and the
W ¼WcþWm
ð9Þ
As pre-crushing product size Pc is equal to the mill feed size F80 then: 0:2 A 10 10 2:44 W ¼ pffiffiffiffiffiffiffi W i pffiffiffiffiffiffiffi pffiffiffiffiffi þ 1:3 D F 80 P 80 Fc F 80 F 0 Rr þ ðW i 7Þ Rr F0 P 80 þ 10:3 10 10 W i pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ð10Þ 1:145 P 80 P 80 F 80 3.2. PBM models In the area of dry particle reduction, the population balance models (PBM) for crushers, HPGR, ball mills, air-swept ball mills and air separators have been developed (Lynch, 1977; Austin et al., 1980; Zhang et al., 1988; Morrell et al., 1997; Benzer et al., 2001, 2003). These models can be used to simulate cement grinding circuits and to assist their optimisation. The ‘‘work horse’’ of the cement grinding plant is the two-compartment ball mill, commonly called the tube mill. Significant advances in model development were achieved in recent years (Benzer et al., 2001, 2003) through research on industrial scale. The breakage and transport mechanisms are better understood as well as the classification action of the diaphragm. Further advances are expected in modelling the effect of diaphragm design on classification and powder transport. The basis for modeling the two-compartment ball mill is the perfect mixing ball mill model. It can be illustrated by the following equation:
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fi þ
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i X aij rj pj j¼1
dj
¼ pi þ
ri pi di
ð11Þ
where: feed rate of size fraction i (t/h) fi pi product flow of size fraction i (t/h) aij the mass fraction of size that appear at size i after breakage ri breakage rate of particle size i (h1) si amount of size particles inside the mill (tonnes) di the discharge rate of particle size (h1)
Barmac B-series VSI (vertical shaft impactor) crushers are applied to a broad range of materials in minerals and aggregate industry. Due to the ‘‘autogenous grinding action’’ it is especially efficient for high abrasive materials such as cement clinker. The crushing action is schematically presented in Fig. 3. When the incoming rock passes over the distributing plate the rock is divided in three separate streams and is forcefully impacted on the rock lining at the distributing
The model consist two important parameter, the breakage function (aij) that describe the material characteristic and breakage/discharge rate function (ri/di) which defines the machine characteristics and can be calculated when feed and product size distribution are known and breakage function is available. The air classifier controls the final product quality. Therefore, the air classifier has a crucial role in the circuit and a lot of attention is paid on the design and operation of the air classifier. The classification action is modeled using the efficiency curve approach. Effect of the classifier design and operational parameters on the efficiency is complicated and work is in progress to improve the current models.
4. The case study Potential benefits of using the Barmac crusher for clinker pre-crushing were studied for a cement plant. Fig. 2 shows the proposed cement grinding circuit.
Fig. 3. Schematic of Barmac crusher operation.
Fig. 2. Simplified cement grinding circuit with precrushing stage.
A. Jankovic et al. / Minerals Engineering 17 (2004) 1075–1081
cum%pass
100 90
Clinker
80
case A
70
case B
60
case C
50 40 30 20 10 0 0.01
0.1
1
10
100
size (mm)
Fig. 4. Clinker and the crushed clinker size distribution (case A, B, C) using Barmac crusher.
Table 1 Bond method power calculation for two-compartment mill Mill feed F80 (mm)
EF4
Power required (kW)
Difference (%)
15.5 4.5 3.0 1.8
1.06 1.01 1 1
3564 3251 3133 3018
0.0 8.8 12.1 15.3
35 Total grinding kWh/t
plate. The material is being rapidly accelerated by the centrifugal force of the rotor action and is compressed against the rock lining which is formed in the crushing chamber. Multiple events occur and a variety of forces act on the individual particles as they proceed. The crushed rock is discharged through the clearance between the crusher chamber and the rotor wall. Fig. 4 shows the clinker size distribution and predicted Barmac crusher size distributions. These data are based on Barmac pilot testing and the simulations. It can be observed that the crushed clinker has a significant amount (10–20%) of 75 lm material. As the cement size is in this size range, it could be concluded that the Barmac crusher produces a significant amount of finely ground cement (see Table 1). It can be seen that a reduction in two-compartment mill power in the order of 9–15% is calculated for different crushed clinker feed F80 sizes. Corresponding to the change in factor EF4, a 5–6% reduction comes from improving the milling efficiency with finer feed. In order to get this improvement, the ball charge size distribution in the first compartment needs to be adjusted for finer feed. As energy is consumed in pre-crushing stage, the total power required will be sum of two-compartment ball mill and Barmac crusher power. Fig. 5 shows how the total comminution energy consumption depends on the Barmac crusher product size. It can be seen that the total energy consumption would reduce with reducing the product size of the Barmac crusher. This indi-
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34 33 32 31 30 29 28 27 26 25
0
2
4
6
8
10
12
14
16
Barmac product P80 (mm)
Fig. 5. Specific power requirement for cement grinding circuit including precrushing stage with Barmac crusher.
cates that the cement grinding efficiency may be improved up to 10% compared to a conventional circuit without pre-crushing. The alternative benefit of introducing the Barmac crusher in pre-crushing stage is increasing the circuit capacity, as the capital investment is relatively low compared to HPGR. The PBM modelling of the clinker grinding was carried out using the principles described earlier. It should be noted that in order to obtain the ‘‘site specific’’ model constants, detailed surveys of the milling circuit are required: the size distribution of the material in each stream as well as from different points inside the mill. In this study the model constants published in the literature (Benzer et al., 2001, 2003) were used. The base case flowsheet generated in the JKSimMet grinding simulation software is shown in Fig. 6. The first compartment was modeled as two ball mills in series, diaphragm between the first and second compartments was represented as a screen and the second compartment is represented as one ball mill. Information given in the flowsheet is solids throughput (t/h), 80% passing size and % passing 0.01 mm. Using the above base case model, simulations with different feed size distributions (raw clinker, and precrushed clinker using the Barmac crusher) were carried out, keeping the product size constant at P80 = 0.038 mm. The resulting increase in throughput is shown in Table 2. Table 3 summarises the potential increase in throughput from using a Barmac crusher to pre-crush the cement clinker. It should be noted that this result does not consider any circuit physical limitations such as conveying, aeration and air classifier capacity. It can be observed that Bond calculations gave less throughput increase than PBM simulations. This could be because the PBM simulation results may have been overoptimistic as the separator performance was kept the same. A model capable of simulating changes in performance with different air separator loads was not available.
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Fig. 6. Greens Island base case JKSimMet simulation flowsheet.
Table 2 Simulated increase in throughput F80 (mm)
Simulated throughput (t/h)
Increase (%)
15.5 4.5 3.0 1.8
110 125 135 140
0.0 13.6 22.7 27.3
Table 3 Predicted increase in throughput using the Bond and grinding modelling method—base case throughput 110 t/h Feed F80 (mm)
minus 75 l in Barmac product (%)
% throughput increase, Bond method
% throughput increase, PBM
15.5 4.5 3.0 1.8
0 12.1 15.4 20.5
0 9.1 14.3 18.2
0 13.6 22.7 27.3
partment is usually double that of the first compartment. Smaller grinding balls are also used in the second compartment. The efficiency of fine grinding in the second compartment is largely controlled by the size of grinding balls. Due to limitation of the millÕs rotational speed, the smallest ball size is usually restricted to about 15 mm. Cement grinding using stirred mills (Pilevneli and Azizli, 1999) indicates that using smaller media (5–8 mm range) improves grinding energy efficiency up to 50% using stirred mills. For specialized types of cement, which are finer than Portland, this figure would be even higher. Significant benefits of using Tower mill were also reported (Shibayama et al., 2000) as well as the industrial applications for production of fine 12 000 Blane cement. The stirred mills can be used in different roles and it is expected that their application in the future will be significant.
6. Conclusion 5. Stirred milling potential As the product size decreases the energy required for particle breakage increases rapidly. The pre-crushing stage increases the milling efficiency in the first compartment of the two-compartment mill where coarse milling takes place. Pre-crushing however does not affect milling in the second compartment apart from producing significant amount of final product size (10–15%). In order to produce the final product size, the length of second com-
There is a scope for significant optimisation of the traditional cement grinding circuits. Grinding process modelling and simulation methods can be used for optimisation. A case study conducted using the data from an industrial cement plant indicates that: • Pre-crushing of cement clinker using a Barmac crusher offers realistic benefits to a cement plant in terms of process efficiency.
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• The introduction of the Barmac crusher can increase the cement circuit throughput in order of 10–20%, providing that there is no capacity limitation in other parts of the circuit. This is an attractive option due to relatively low capital investment of the Barmac crusher. The overall energy efficiency of the circuit can also be improved in order of 5–10%. The stirred milling technology could further improve energy efficiency of cement grinding. Initial work indicates great potential and significant development in this direction should be expected.
References Austin, L.G., Weymont, N.P., Knobloch, O., 1980. The simulation of air-swept cement mill. In: Proceeding of European symposium. Particle Technology, Amsterdam. Benzer, H., Ergun, L., Oner, M., Lynch, A.J., 2001. Simulation of Open Circuit Clinker Grinding. Minerals Engineering 14 (7), 701– 710. Benzer, H., Ergun, L., Oner, M., Lynch, A.J., 2003.Case studies of models of tube mill and air separator grinding circuits. In: Lorenze, V., D.J. Bradshaw, D.J. (Eds.), Proceedings: XXII International Mineral Processing Congress, pp. 1524–1533. Bond, F.C., 1961. Crushing and Grinding Calculations Parts I and II. British Chemical Engineering 6 (6&8). Lynch, A.J., 1977. Mineral Crushing and Grinding Circuit – Their Simulation, Optimisation, Design and Control. Elsevier Scientific, Amsterdam.
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Magdalinovic, N., 1990. Mathematical Model for Determination of an Optimal Crusher Product size. Aufbereitungs-Technik 31, 5. Morrell, S., Shi, F., Tondo, l997. Modelling and Scale-up of High Pressure Grinding Rolls. In: Proceedings of the XX International Mineral Processing Congress(IMPC). Aachen, Germany, September 1997. Pilevneli, C.C., Khairun Azizi Mohd Azizli, 1999. Semi-Batch Dry Grinding Tests of a Pilot Scale Vertical Stirred Mill. In: Proceedings of VIII Balkan Mineral Processing Conference. Belgrade, Yugoslavia. Rowland, C.A., 1975. The tools of power: How to evaluate grinding mill performance using Bond Work Index to measure grinding efficiency AIME Annual Meeting. Tucson, Arizona. Shibayama, A., Mori, S., Bissombolo, A., 2000. Studies on Comminution Mechanism of the Dry Tower Mill KD-3. In: Proceedings of the XXI International Mineral Processing Congress. Rome. Zhang, Y.M., Napier-Munn, T.J., 1995. Effects of particle size distribution, surface area and chemical composition on Portland cement strength. Powder Technology 83, 245–252. Zhang, Y.M., Napier-Munn, T.J., Kavetsky, A., 1988. Application of comminution and classification modelling to grinding of cement clinker. Transaction Institution of Mining and Metallurgy (Section C: Mineral Processing and Extractive Metallurgy). C207–C213.
Further reading Bond, F.C., 1985. Testing and Calculations. SME Mineral Processing Handbook, Norman L. Weiss, Editor in Chief. Bond, F.C., 1962. Crushing and Grinding Calculations—April 1962 Additions and Revisions. Allis-Chalmers Manufacturing Co., Milwaukee, Wisconsin. Brugan, M.J., 1991. Mills at grinding edge. Pit and Quarry, 34–41.