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Grinding process within vertical roller mills: experiment and simulation

1 1 WANG Jian-huai , CHEN Qing-ru , KUANG Ya-li1 , LYNCHAP, ZHUO Jin-wu 1Sc hool 01 Chemical Engineering and Technology, China University 01 Mining & Technology, Xuzhou, Jiangsu 221116, China 2Juius Kruttschnitt Mineral Research Centre, Queensland Universi秽 ， Brisbane, Australia

Abstract: Basedon screening analysis, laser size analysis, grindability and rigidity tests of samples collected on line from a cement and a power plant, a simulation of the grinding process in vertical roller mills was carried out. The simulation calculation used a breakage function, B. The results indicate that the breakage function, B, and the selection function, S, in the form of a matrix, can be used to express the probability of the material breaking during the grinding process. This allows the size distribution of 也e product to be numerically estimated. The simulation results also show that the simulated size distribution curves fit the actual ex­ perimental product curves quite well. The model provides a good starting point for simulation of the grinding process. Further re­ search is needed to determine the proper breakage function and the matrix value of the selection function. Keywords: vertical roller mill; grinding; simulation

1 Introduction fired power plants and cement plants used 6.16% of all nationwide industrial power-consumption (see Grinding is a highly energy consuming process. Table 1). Statistics also show 也at the grinding energy Considerable coal, cement raw materials and clinkers, consumption in China was 50% higher than what metal and non-metal minerals, and the like, are would be expected at the world class, advanced 2 crushed every year in China for power generation, level[I- l. Therefore, optimizing grinding process pa­ cement manufacturing, industrial combustion and rameters is a very important field of study related to other industrial pu甲oses. According to statistics, the energy-saving and the reduction of power consump­ energy consumption of the coal and cement grinding tion in China. It is estimated that power consumption processes in power and cement plants accounted for could be lowered by 30%-50% after optimization of 7.26% of all nationwide industrial power consump­ the grinding process. tion in 2003. In 2004 grinding associated with coal- Table 1 Power-consumption Statistics

Amountof Power cOqYKnWsmhn)pt10n Account for Posshle opyokwme斤r。四 savmg grinding (Gt) industrial power-consumption 2003 2004 2003 2004 2003 2004 2003 2004 0[3-5] Cement plant 0.82 . 1. 82.0 100.0 6.27 5.18 24.6-41.0 30-50 Power plant 0.826[1] 0.9[1] 17.43 18.90 1.3 0.98 5.23-8.72 5.67-9.45 Tota1 99.43 118.90 7.57 6.16 29.83-49.72 35.67-9.45 Note: not including other industries involving grinding. Simulation of grinding processes started nearly a was also making progress. The approach by Lynch A century ago. These early researchers found that the J was to use industrial scale experiments, which grinding processes cost huge amounts of energy. studied not only power consumption but also the en­ Therefore, most of their studies were aimed at energy tire grinding process, and to investigate other factors consumption of the mills. In the meantime the mod­ affecting grinding such as the type of mills, the en­ eling of the hydrocyclone used in grinding circuits ergy costs, the prope仕y of the feed materials and the Received 16 July 2008; accepted 30 October 2008 Project PBK2005022 supported by the Natural Science Foundation of Jiangsu Province Corresponding 扭曲or. Tel: +86-516-83883530; E-mail address: [email protected] 98 Mining Science and Technology Vo1.l9 No.l

operating parameters[l-21. Grinding process models Sampling and experiments were also carried out in for a ball mill were established where a selection a power plant that has four ball-mill circuits used for function and a breakage function were used to de­ coal grinding so that different equipment could be scribe the breaking rate and the particle size distribu­ compared. tion. Other models that have been developed can be 2.2 Sampling and experiments used to optimize production flow design, process pa­ rameters, equipment performance, raw material and All the samples used in the lab experiments were operating conditions for ball mills, classifying cy­ taken 仕om the production line of the plant. The feed, clones, screen classifiers and other equipment intermediate materials, products of the limestone and through on-line simulations. These results were pub­ clinker systems and the coal feed were each sampled lished early in the 1980's and were 位ansformed into 企om conveyor belts or chutes. The ground coal and the commercial so仕ware JKSIMMET, which has final cement products were sampled with a CY40 been used in Australia, Turkey and other countries[3-41. automatic powder sampler. Each sample was taken Current research abroad into the grinding process of over a period of at least two hours to ensure that a ball mills, rod mills, cone crushers and autogenous representative sample had been collected. Sampling mills is comparatively mature. time intervals, and the weight of the subsamples, There have been some reports on the mathematical conformed to the national and coal industry modeling of grinding equipment by Chinese re­ regulations and standards. searchers. For example: The study of general grindÏ!!g Laboratory grindability tests, screening analyses dynamic equations - of ultra-critical- speedmi1ls[ST; (岛r +0.5 mm particles), laser size analyses (岛r -0.5 multi-factor experiments to determine the variation of mm particles) and comminution rate testing were grinding fineness and net energy consumption as a carried out using the samples obtained from the three function of ball-mill rotation speed, grinding medium types of mills and the four grinding circuits. filling-ratio, feed quanti肌 slurry concentration and Sufficient reliable data was collected so that a other-processing v~riable~[61; anrl research on classi­ mathematical model for a vertical roller mill could be fier overflow con位01 systems using a new prediction set up. Grindability tests were completed on a L/J con仕01 algorithm and computer simulation • non-standard ball mill.τberefore ， these results are There are few studies both at home and abroad only indirectly comparable to the other data. The goal concerning process simulation of recently developed of the experiments was to optimize the grinding new equipment such as vertical roller mi1ls and stir­ process, to improve comminution efficiency and to ring mills. In China almost no work has been done on reduce energy consumption. the simulation of multi-factor industrial manufactur­ mg systems. 3 Results and discussion This paper describes the simulation of the grinding process in vertical roller mills. It is based on actual 3.1 Comparison of grinding results using differ­ experimental data obtained on a production line at the ent kinds of coals plant and from lab experiments. The following data were obtained 企om the experiments on the coal samples 企om both the 2 Experimental cement plant and the power plant (Table 3). Table 3 Grinding results: different types of coals 2.1 Equipment >74 m Gr~dability Power The main experimental equipment was a large­ Coals缸nple yield (%) factor Mill type consurnptlOn (g/r) (kWhJt) scale cement plant with a capacity of 4000 tld. The Powerplant plant is equipped with advanced technologies and has coal 20.8 0.74 Ballmills 21 different types of grinding mills. Four grinding Cement plant Vertical roller circuits are used with different materials: verticàl coal 6.0 0.45 mills 12 mills (Loesche) for coal and cement feed; vertical roller mills for cement clinkers; and ball mills for The experimental results in Table 3 show that the cement products. Table 2 shows the main technical 仕actions of particles sized greater than 74μm are parameters ofthe vertical roller mill (CKP) studied in 20.8% and 6.0% in the screening of the power plant this pape汇 coal and the cement plant coal, respectively. The data indicate that the product size 企om the vertical roller Table 2 Main p缸ameters ofthe vertical roIler miII (CKP) mill in the cement plant is far smaller than that of the Rated Measured Roller Rotating table Rotation power plant. The grindabi1ity result is the grams of Capacity power 时一叩diameter diameter speed (的1) fine particles, which pass a 0.125 mm screen aperture (kW) (mm) (mm) (r/min) size, produced per revolution of the mill. A standard 99 950 600 1700 39.8 procedure is used to determine this value. The lower WANG Jian-huai et al Grinding P而 cess within vertical roller mills: experiment and simulation 99 the value the harder the material is. The difference in , Table 4 Screening data of samples grindability between power and cement plant coal is Cyc10ne Cyc10ne Size C1inker C1inker mill Ball mi11 separation separation also quite notable. Moreover, the feed coal of the miI1 feed product output (mm) fmal cement plant is coarser than that of the power plant yield (%)yield (%)YIeld (%)YICeOl町d (s%e ) yield (%) (Fig. 1). 25 0.8 120.0 13 14.6 2.4 :? 100.0 F 6 16.5 5.9 ) 18.6 10.2 号-ωEL 800 3 21.2 13.9 0.5 12.2 11.6 -• Powerp1ant (-0.5) (16.1) roi//20.0 --- Cement p1ant 0.25 8.2 0.9 。。 +25 25-13 13-6 6-3 3-1 1-0.5 -0.5 0.125 6.6 1.9 2 Size(mm) 0.074 6.5 7.1 11.6 Fig. 1 Cumulative size curves of 也e feed coal 0.045 6.3 19.9 30.8 1.5 of the two plants (-0.045) 28.4 71.1 54.7 98.5 The above study shows that the hardness' of the coal samples taken 企om the cement plant is greater 4 Modeling a vertical roller mill than the hardness of power plant coal. The feed sizes are also larger in the cement plant, compared to the A mathematical model for the grinding process in power plant, but the size of the product 企om the vertical roller mi11s has been made. The cement vertical roller mi11 in the cement plant is smaller than plant's vertical roller m i11s (CKP) are used to crush the size of product from the power plant ball mi11. It cement c1inkers and prepare for the next step, which can be conc1uded that the process of grinding coal are ball mi11s. The grinding mechanism of the CKP into fine partic1es for cement or power plant imposes pressure on the partic1es instead of impact consumption uses less energy when grinding is done forces. It has higher energy efficiency than do the ball with a vertical roller mi11 rather than a ball mi11. mi1ls. Although more actual operational data are Obviously, this conc1usion is based on the crushing required to set up an industrially practical process only. The energy consumption of the fans mathematical model, the data obtained 企om the used in the c1assification process has not been taken open-circuit vertical roller mi1ls (CKP) are sti11 into account. The example shows that optimum valuable. selection of the equipment could reduce power consumption enormously. 4.1 Breakage probability function 3.2 Analysis of the vertical roller mill Observation of the size range of the input and output materials indicates that a simple breakage By analyzing the size distribution of the samples probabi1ity function might be a suitable starting step (Table 4) it can be seen that some improvements are for setting up the model, i.e.; needed in the performance of the cement plant r'··A、‘ EJ equipment. The product size 企om the clinker m i11s is P = B. f 飞 fine enough for the following 阳be mi11s: +6 mm where P and f are partic1e size distributions of the accounts for only 8.3% (+ 13 rnmI1.4%) and 97% of output and input materials, respectively, and B is a that is recyc1ed by the mi11 's separator. So there are breakage function. only a few coarse partic1es in the feed material going 4.2 Breakage function to the ball mi11s. The diameter of the steel ball is much too large and much energy is wasted lifting The breakage function, B , represents the these large diameter balls rather than grinding probabi1ity that ith sized partic1es proceed to the size efficient1y. Therefore, the c1inker grinding rate and fractions from (i+ 1) to (i+n). The function depends on the overall production capacity of the ball mi11 circuit the property of the material and can be expressed as a could be improved, without more energy matrix: consumption, by adjusting the ratio of chamber length to steel ball diameter in the following tube BIl 0 ..' 0 0 m i11s. This example shows that grinding circuit (2) simulation can be used to analyze the production B=\ Bil … Bij circuit, to optimize the operating parameters and to O reduce energy consumption. Bnl …B叨... B 100 Mining Science and Techno1ogy Vo1.l9 NO.1

For vertical roller mills B is a pressure breakage 4.3 Selection function function and is determined from test data. It is All feed particles have certain breakage probabili­ different 企om an impact force breakage function ties. These probabilities are affected by technical appropriate for a jaw crusher, cone crusher or ball conditions and will va可 with particle size and the mill. Fig. 2 shows equipment to obtain B. Pressure is grinding stage. Call the breakage pr由ability of size applied to the hammer and the sizes of the resu1ting fraction i, Si. This is the selection function, which can particles allow a breakage function, in the form of be expressed as a diagonal matrix: Eq.(2), to be obtained. This is a thought experiment and, therefore, it may not fully simulate vertical roller S = (S1, ...，乱， ..., Sn) (3) mills. Experimental data and research experience of A The selection function, S, for the materials in J Lynch has allowed the breakage function, B, for the question has been established as: materials in question to be established (see Table 5). S = (0.3, 0.3, 0.3, 0.2, 0.2, ..., 0.2) 4.4 Results of simulation Pressure applier Table 6 shows how the breakage function P=B-J Slideway can be used to model the grinding process. In this example all materials are comminuted according to the breakage function, B, to gain the final products. Broken Hammer The size of the first grinding cycle product material Tray calculated from Eq.(1) is larger than that of the experimental product. This is because the experimen­ tal materials are crushed repeatedly in the ro11er mill. Therefore, multiple cycles must be taken into account Fig.2 Experimental set for obtaining the breakage function in the model. Suppose that the breakage probability of the second grinding cycle depends on the size of Table 5 Breakage function obtained 企om experiments input material. and experience The model of the second grinding cycle can be D町 h-0000 B , B2 B3 B4 Bs B6 -00000B9 considered as an expansion of the first one. It is 0.227 0 0 0 0 0 0 expressed as: 0.213 0.227 0 0 0 0 0 P2 = (B-S + 1 - s) P1 (4) 0.180 0.213 0.227 0 0 0 0 where S is the selection function. 0.146 0.180 0.213 0.227 0 0 0 Table 6 shows the results of calculations using this 0.109 0.146 0.1 80 0.213 0.227 0 。 0 model function. The first and the second grinding 0.074 0.109 0.1 46 0.180 0.213 0.227 0 。 0 cycles have the same breakage function, B , because 0.046 0.074 0.109 0.1 46 0.180 0.213 0.227 0 0 they have the same breaking mechanism. The final 0.031 0.046 0.074 0.1 09 0.146 0.1 80 0.213 0.227 0 products are the sum of a11 broken and unbroken 0.011 0.031 0.046 0.074 0.109 0.1 46 0.180 0.213 0.227 materials.

1'able 6 Tab呗lation of actual and simulated products Size Actual products Feed Simulated products Simulated products S.P1 P1--S.P1 B.S.P, i旦旦i P F P,=B.F P2= B.S.P,+p,--s.p1 立a 526 13.000 2.4 15.40 3.50 1.05 斗 0.48 2.93 41AY 6.000 5.9 16.50 7.03 2.11 1.1 6 6.08

句 3.000 10.2 18.60 10.51 3.1 5 31JAυ 1.69 9.04 川队少 1.000 13.9 21.20 13.99 2.80 1.99 13.18 78166 0.500 11.6 12.20 11.33 2.27 2.10 11.1 6 714.Atmη句 0.250 8.2 1.78 9.22 1.84 3Ayroro 2.04 9.42 0.125 6.6 1.32 7.38 1.48 1.88 7.78 0.074 6.5 2.44 5.83 1.1 7 1.71 6.37 0.045 6.3 2.39 5.83 1.1 7 2.44 7.10

， 门呻 --0.045 28.4 8.17 25.40 5.08 zm1 0.00 26.93

Total 100.0 100.00 100.00 27.90 』 18.72 100.00

F our size distribution curves are given in Fig. 3 distribution of final products derived by calculation. where F, P, P1, and P2 represent the feed to the mills, It can be seen in Fig. 3 that the two simulated the actual products, the first and the second grinding product cu凹es resemble each other and are similar to cycle products, respectively. P2 is also the size the experimental product curve. The simulated curve WANG Jian-huai et al Grinding process within vertical roller mills: experiment and simulation 101 of the second cycle, P2, fits the actual product curve breaking cycles before becoming the final product better. It indicates that the feed materials are broken this fact should be taken into account by the model. more than once during the grinding process and that 3) A mathematical simulation of a vertical roller the working state of vertical roller mills can be mill based on experimental data has been carried out. simulated with a proper model. Further research is The simulation results show that simulated size dis­ needed to reduce simulation errors and to establish tribution curves fit the actual experimental product more reliable breakage and selection functions. curves quite well. It is feasible to simulate the grind­ ing process of vertical roller mills by using a proper 100.0 mathematical model; further research is still needed.

岂 80.0 吃-ω主3 60.0 References

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