Powder Technology 217 (2012) 100–106

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Converting AG to SAG mills: The Gol-E-Gohar Iron Ore Company case

M. Maleki-Moghaddam, M. Yahyaei, S. Banisi ⁎Shahid Bahonar University of Kerman, Mining Engineering Group, Engineering Faculty, Islamic Republic Blvd., Kerman, P.O. Box 761175-133, I.R. Iran

⁎ Corresponding author. Tel.: +98 341 2515901; fax:E-mail address: banisi@uk.ac.ir (S. Banisi).

0032-5910/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.powtec.2011.10.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 June 2011Received in revised form 11 September 2011Accepted 10 October 2011Available online 14 October 2011

Keywords:SAG millMill fillingLifter face angleBall trajectory

At the concentration plant of the Gol-E-Gohar Iron Ore Company (Iran) three 9 m×2.05 m AG (autogenous)mills (fixed speed) are used in parallel in a dry operation. The performance of AG mills has been lower thanthe target value indicated by an increase in the product size (P80; 80% passing size) from 450 μm (target) to515 μm and low power draw (50% of nominal). By simulation it was found that increasing the liner lifter faceangle from 7 to 30° while keeping the original lifter height (i.e., 22.5 cm) could provide an appropriate chargetrajectory. When the proposed liners installed in one of the AG mills, the throughput increased by 27%; when5% (v/v balls were added the throughput increased by 40%. The product size also decreased from 515 to501 μm.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The use of large AG (autogenous)/SAG (semiautogenous) mills inthe mineral industry has been on rise due to a significant reductionin capital and operating costs and increase in the plants throughputs.The availability of these mills plays an important role in the econom-ics of the operation. One of the main reasons for mills down time isthe time required to change worn or broken liners [1,27,28]. A nota-ble advantage of SAG over AG mills is their flexibility in accommodat-ing feed hardness variations.

The use of liners with large face angle lifters along with wider lift-er spacing has grown in recent years, especially in large SAG mills.The claimed benefits were reduced ball on liner impacts, leading toincreased liner lives and less ball breakage. These reduced packingand improved ball-on-charge impacts, leading to improved mill per-formance [23,25]. Furthermore, there has been the parallel develop-ment towards the use of larger diameter steel grinding balls inorder to increase grinding efficiency. A traditional top ball size wasaround 104 mm. Currently, 125 mm balls are in common use withsome mills using 140 and 152 mm balls (165 mm balls are nowbeing proposed). These developments have required a new approachto liner design through consideration of charge motion, especially themotion of the steel media in the mill. Aims of new designs are to im-prove the effective grinding through changes in shell lifter face anglesand the spacing of the lifters [13,20,23,26].

It has been known for many years that a change in the face angleon SAG mill shell lifters results in a change in the trajectory of the

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“thrown” grinding balls. With the development of trajectory generat-ing computer programs, the effects of face angle, packing and lifterheight have been incorporated in shell lifter design. Increasing shelllifter face angles (for the same mill speed) does reduce the impactpoint of thrown balls and can reduce shell liner damage [19,23,29].Charge trajectory modelling has been used to present and interpretaggregate motion of a number of balls critical to the performance ofthe mill [24,25].

Lifters are essential elements in the tumbling action of a grindingmill, whose purpose is to prevent slippage of the mill charge. Suchslippage consumes energy without substantial breakage, and themill may not lift the charge sufficiently high to promote impactbreakage. Hence, the size, shape and number of lifters have a strongimpact on the tumbling action of the media [4,18].

The availability of large-scale cost-effective computing power inrecent years has allowed the development of Discrete ElementModel-ling (DEM) computer programs that model charge flow in mills. DEMhelps to understand chargemotion in SAGmills for given liner designsand lifters, ball and rock properties and mill operating conditions.Based on the success of the 2-D model of the SAG mill the 3-D modelin DEM was developed. The model could help to study charge motionand energy draft more accurately and also could provide informationregarding the effect of shell liner spacing as well as lifter face angleon charge motion. [3,8,9,11,15,21,22].

Yahyaei and Banisi [29] used 3-D liner wear modeling to showthat the liner wear along the mill length is not uniform and providedindustrial data from a SAG mill used at the Sarcheshmeh copper com-plex [28,29]. They developed a spreadsheet-based software to modelcharge trajectory in tumbling mills using Powell's [17] analytical ap-proach and investigated charge trajectory change during the lifetime of a liner set. According to Powell if the grinding media is posi-tioned at the tip of lifter it will start its free flight after the point of

101M. Maleki-Moghaddam et al. / Powder Technology 217 (2012) 100–106

equilibrium. Otherwise, it will start to role or to slide on the lifter sur-face until it reaches the edge of the lifter. Then it will fall into freeflight. When the grinding media reaches the edge of liner, the refer-ence frame should change to a Cartesian coordination which its originis positioned at the mill center to simplify the calculations. The speedof the particle composed of its linear velocity parallel with the lifterface and angular speed resulted from mill rotation. These two typesof speeds should resolve into components parallel with Cartesianaxes for calculating the charge trajectory. The free flight of chargewill end when it encounter the mill shell. Yahyaei and Banisi [29]showed that the largest difference in the angles of charge impactpoints along the mill length due to non uniform wear was 34°which occurred after 3000 h of operation.

As the charge volume increases, the greater fraction of the netpower draw is consumed in the form of charge abrasion and lowenergy impacts, while the fraction of net power that leads to highenergy impacts gradually decreases [5].

A correct balance between the low and high energy impacts is es-sential for successful comminution practices. This could be achievedby introduction of wide-sized (mature) balls into the mill which inturn provides a broad spectrum of forces to be applied to the particles.This research was carried out to convert existing AG mills to opti-mized SAG mills at the Gol-E-Gohar concentration plant to improveboth grinding efficiency and to decrease the sulphur content of con-centrate by increasing the degree of liberation of sulphur bearingminerals. There had previously been at least one report of such aconversion on a smaller scale, which resulted in higher grinding per-formance [12]. A methodology is proposed beginning with finding anappropriate charge trajectory by mathematical modelling consideringvarious liner profiles, testing physical models of the selected linerprofiles in a laboratory mill and finally arriving at a liner design forlarge-scale industrial mills. It is believed for the first time the trajecto-ry obtained for a single ball is corrected with the introduction of asafe operating window concept. In fact, the effects of charge andliner profile which impose extra forces on balls are taken indirectlyinto account on the charge trajectory calculation through using thesafe operating window.

Fig. 1. The polyurethane ring machined to scale down the liners arrangement of theplant mill.

2. Materials and methods

2.1. Charge trajectory predication software

In order to determine charge trajectory, a software called GMT(Grinding Media Trajectory) was used [29]. The approach taken byPowell [17] was used as a base to determine charge trajectory. GMTin comparison with other software packages has the advantages ofusing all MS Excel© readily available functions and capabilities. Themain feature of GMT is the ability to show the kidney-type shape ofthe charge along with the trajectory which has not been incorporatedin similar software packages. Morrell's [16] approach was applied todetermine shape of the charge in which he used a laboratory millwith one transparent end and photographed the load under variousoperating conditions. The variation of the toe and shoulder positionsfor three different liner profiles at various mill speeds and fillingswere studied in his work. Then he proposed various empirical equa-tions to relate the positions of toe and shoulder and inner load radiusto mill speed and filling. Because of the differences in the calculationof charge trajectory for rods and balls an option in GMT has been pro-vided which enables the user to choose the media type. The dataentry section of the software includes five parts to be completed bythe user: general information, mill, grinding media and liner data,and parameters. In the mill section, the values indicating the effectivediameter, speed and filling are entered. In the grinding media section,the media type (i.e., ball or rod), diameter and density are recorded.In the liner section, lifter face angle, liner height, lifter width, and

number of lifters are entered. In parameter section, static and dynamicfrictions and mill speed for each liner design are entered.

2.2. The model mill

In order to determine ball trajectories at different operating condi-tions and liner profiles, a 100 cm×3.6 cm mill was designed and con-structed. The transparent end of the mill made possible accuratetrajectory determination by image analysis. A 2.5 kW motor with avariable speed drive was used to provide sufficient flexibility to testvarious operating speed conditions. A diameter scale down ratio of9 to 1 was used to construct the model mill using the plant mill di-mensions. The mill served as a 2-D model, but there is the possibilityof changing it to a 3-D model mill.

In order to exactly duplicate the plant liners arrangement in themill, a polyurethane ring was accurately machined to create 36 liftersin a row (Fig. 1). By this method any number of liners in a row withdifferent designs could be tested. Between the lifters 36 plates werealso included consistent with the plant liner design. Only one of thethree sections of the liners along the mill length was built andinstalled in the 2D model mill.

2.3. The Gol-E-Gohar iron ore concentration plant

The Gol-E-Gohar iron ore concentration plant located in southeastIran. Three 9 m×2.05 m AG mills are used in parallel in a dry opera-tion to grind a feed nominally passing 32 cm, which is the productof a gyratory crusher. The SAG mills is driven by a 4023 hp motorand runs at a constant (12 rpm rotational) speed (i.e., 85% of criticalspeed) in one direction. The mill shell is lined with three series(57 cm each) of 36 row liners. Specifications of liners used areshown in Table 1. According to the liner manufacturer recommenda-tion, the liners should be changed when the lifter height reaches100 mmwhich is one third of its initial height. Due to large variationsin feed characteristics and inadequate blending, the performance ofAG mills has been lower than the target value. An increase in P80 ofthe product from 450 μm (nominal) to 515 μm indicates the extentof the change. Another indication was low power draw of the millwhich was only 50% of the nominal value. Furthermore, the coarser

Table 1Specifications of liners at the Gol-E-Gohar concentration plant used in GMT software.

Platethickness(mm)

Lifter Liftersin row

Numberof rows

Face angle (°) Height (mm) Width (mm)

75 7–30 225 125 3 36

Fig. 2. Simulation of 125 mm ball trajectory for old liner (18% filling) by the GMTsoftware.

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product resulted in higher sulphur content of concentrate due tolower liberation of pyrite, which is the main sulphur bearing mineral.

To convert AG to SAG mills 5% (v/v) balls with the sizes andamounts shown in Table 2 were used.

3. Results and discussion

3.1. Charge trajectory prediction

The charge trajectory of 125 mm balls with 18% total filling, whichwas predicted by the GMT software, is shown in Fig. 2.

The balls impact points clearly indicates that with the originalliner design ball addition could results in direct impact of balls onthe liners causing severe damage. The low throughput of the millcould also be attributed to inappropriate trajectory where the toe ofthe charge does not receive any appreciable direct impacts from thefalling load. The effect of different fillings on the positions of theballs impacts was also studied (Table 3). This part of the study wasperformed with the objective of determining whether the ballscould fall on the toe. The distance between impact points and thetoe was measured in degrees. Even at 24% filling which was consid-ered high for the Gol-E-Gohar operation there was 55° difference be-tween the balls impact points and the position of toe.

3.2. Safe operating windows

The analysis performed by the software does not take into accountthe interaction between balls and particles and also the forces actingon the balls due to charge and liner profile. It is also a 2-D analysis anddoes not consider the effect of charge interaction along the milllength. In addition to some simplified assumptions in arriving at thetrajectory, the assumed shape for the charge also is not very realistic.In order to find safe operating windows where no direct impacts ofballs on liners occur, the charge trajectories of some mills with noliner breakage were determined by the GMT software. The name ofthe plants, and all input data along with the balls impact points mea-sured as the distance between the impact points and the toe areshown in Table 4.

Considering the standard deviation of the distances (σ=4.8°), as-suming a normal distribution, the range of distance between the im-pact points and the toe with 95% confidence level was found to bebetween 17.4 and 36.2° with the average of 26.8°. This value wasused to design the new liners for the conversion from AG to SAG mill-ing in order to be certain that the damage to the liners due to the di-rect impact of balls will be avoided. The wide observed range (7.7 to52.5°) difference between impacts points of a single ball with that ofthe charge indicates the effect of differing operating parameters such

Table 2Size and amount of balls added for 5% (v/v) filling.

Ball size (mm) Amount (%) Weight (t)

125 27.1 5.7115 21.4 4.5100 16.7 3.590 11.9 2.580 22.9 4.8Total 100 21

as amount of charge and mill rotational speed in the studied plants.Since the average difference (26.8°) was used in the design of thenew liner, the difference in rotational speed of the mill (85%) withthe surveyed plants (73–80%) was not deemed to introduce any sig-nificant error.

3.3. New liner design

The new liner not only should provide an appropriate load trajec-tory, but it should also be as similar as possible to the original linerdesign for easy installation and removal. By simulation of charge tra-jectories at various liner shapes it was found that lifter face angle wasthe most effective parameter influencing the charge trajectory; as aresult for the new design the lifter face angle increased while lifterheight was kept constant.

To obtain a lifter face angle which provides an appropriate chargetrajectory, the face angle increased from 7 to 30° and the trajectoriesobtained by the GMT software were compared (Fig. 3). The results in-dicated that when the face angle increased from 7 to 30°, the distancebetween the impact point and the toe decreased from 55 to 14° for24% filling and decreased from 63 to 22° for 18% filling.

Since balls impact points were within the safe operating windows(i.e., 17.4 to 36.2°), the lifter face angle of 30° was selected forthe new design. To ensure the appropriate load trajectory, thelaboratory-scaled models of the original and new liners were con-structed and tested in the 1-m diameter laboratory mill. The transpar-ent end of the mill made possible to monitor the charge trajectoryinside the mill. Steel balls within the size range of 4–12 mm wereadded to provide the desired fillings. This size range was selectedbased on diameter scale down ratio of 9 to 1 considering the AGmill feed size distribution. In other words, based on the size distribu-tion of the industrial mill feed the steel balls were chosen to simulatethe feed in the laboratory mill. For both the original and new liners,the tests were performed at 14, 16, 18, 20 and 24% fillings. Since therotation speed of Gol-E-Gohar concentration plant mills was fixed at85% of critical speed, this rotation speed was selected for all tests.Fig. 4 illustrates the charge shape and trajectory for the original andnew liners at 20% filling.

Table 3The distance between impact points and the toe measured in degrees for various millfillings.

Total filling (%) Distance between impact points and toe (°)

14 6916 6618 6320 6124 55

Table 4Operating data of the plants along with simulation results of ball trajectories obtained by GMT software.

Mine Total filling (%) Ball filling (%) Maximum ballsize (mm)

Speed(% of critical speed)

Distance between the impactpoints and the toe (°)

David Bell [14] 25 6–10 100 75 52.5Kennecott, liner 1 [2] 24 12 120 75 48.2Kennecott, liner 2 [2] 24 12 120 75 25.6Kennecott, liner 3 [2] 24 12 120 75 10Candelaria, liner 8° [10] 29 12 127 73 25.9Candelaria, liner 20° [10] 29 12 127 73 10.5Northparks [6] 20 10 125 78 20.1Anglo Chile, Mill 1 [19] 24 13 125 78 17.6Anglo Chile, Mill 2 [19,20] 18 14 125 74 7.7Newcrest Mining Ltd. [7] 15 - 125 75 12.5Sarcheshmeh [1] 18 12 120 80 47.2Sarcheshmeh [1] 20 12 120 80 44.5

Fig. 3. Simulation of 125 mm balls trajectories with different lifter face angles (24%filling).

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Considering the charge shape and impact points it is evident thatmill fillings should be more than 20% to prevent direct impact ofballs to liners. Given the promising results obtained in the laboratorymill, the new liners with 30° lifter face angle and unchanged lifterheight (i.e., 22.5 cm) was constructed and installed in one of thethree parallel dry AG mills. Fig. 5 shows the original and new liners.Because of the higher lifter face angle of the new liner the lifting lugused to move the liner was placed at the top. The weight of the linerswas kept equal to the original design, so as not to increase the totalweight of the mill.

Fig. 4. The charge shape and trajectory of the original

3.4. Effect of liner change on the mill performance

To evaluate the effects of new liner design, the average through-put of the mill (No. 2) for a period of 18 months before the linerchange was compared with the average throughput of the mill1.5 months after the change. The throughput increased from 419 to532 t/h after the new liners were installed. This significant increaseof 27% in throughput was attributed to the change in the charge tra-jectory and ball impacts on the toe which led to improved grinding ef-ficiency. For further analysis the tonnage range spectrum for the millwas obtained before and after installing new liners for the aforemen-tioned period (Fig. 6). Variation in throughput occurred because ofthe change in the ore hardness, feed size distribution, insufficientore blending and downstream capacity limiting issues. Before linerchange, at 50% of operation time the throughput was within400–500 t/h range while after installing the new liners at 74% of theoperation the throughput was within 500–600 t/h. The share of600–700 t/h range increased from 1 to 5.3% indicating a significantimprovement in the throughput.

Operating three identical AG mills in parallel provided a uniqueopportunity to separate ore characteristics effects from the influenceof the liner change. The average throughputs of mill No. 1 and No. 2over a period of 18 months before the liner change (in mill No. 2)were 414±72 t/h and 419±67 t/h, respectively. Given the standarddeviations the difference was not found to be significant at 95%confidence level. During a period of 1.5 months after installing thenew liners, the average throughput of the mills No. 1 and No. 2were 414±37 t/h and 532±48 t/h, respectively, which indicated asignificant increase.

and new liners in the laboratory mill (20% filling).

Fig. 5. The original and new liners used at the Gol-E-Gohar concentration plant mills.

104 M. Maleki-Moghaddam et al. / Powder Technology 217 (2012) 100–106

The average product size (P80; 80% passing size) of the mill beforeand after the liner change were 516 and 513 μm, respectively indicat-ing no significant change in the fineness of the grind.

3.5. Converting AG to SAG milling and grinding performance

An important prerequisite to convert AG to SAG mills was to as-sure that the mill fillings was above 20%. On two occasions the millwas crash stopped and the fillings were found to be 22 and 26%. Toprovide 5% (v/v) balls, 21 t balls with sizes and amounts shown inTable 2 were added. The power draw of the mill was used as the

Fig. 6. Throughput range spectrum for mill No

Fig. 7. Throughput of mill No. 2

main parameter to control the mill filling. Based on the previous ex-perience the power draw was set at 2300 kW. The feed rate was themain manipulated parameter to keep the power draw at the setpoint. The conditions of the liners were checked after 20 h of opera-tion by crash stopping the mill. No broken or cracked liners werefound which verified the appropriate ball trajectory with new liners.The mill filling was 21% which was sufficient to provide self protec-tion of the mill shell from direct impacts of balls. The first and themost important effect of conversion to SAG milling was the stabilityof the process judged by the stable power draw of the mill. Thiswas also observed in 34% reduction in feed rate standard deviation(decreasing from 67 t/h with the original liners to 45 t/h) when theconversion to SAG milling practiced.

In SAGmilling mode the throughput increased from 532 to 589 t/h(two-month average) with the only liner change. Overall increase of40% in the throughput (419 to 589 t/h) was realized by both changein the liner design and addition of 5% balls. The throughput of themill (No. 2) for a period of 3.6 months is shown in Fig. 7. The steadyincrease in the throughput is evident due to liner change and ballsaddition. The change in the ore characteristics are also seen in thevariation of the throughput in each period (i.e., before changes, afterliner change, and after ball addition).

The product size (P80) also decreased from 513 to 501 μm.Throughputs and product sizes of the mill along with their standarddeviations before and after changes are summarized in Table 5.Three months after conversion to SAG milling 40% increase inthroughput in addition to 3% decrease in the product size could beconsidered as themain outcome of this change. The balls consumption

. 2 before and after installing new liners.

for a period of 3.6 months.

Table 5Throughputs and product sizes of the mill before and after liner change and after ballsaddition.

Averaging period(months)

Throughput(t/h)

P80(μm)

Before change 18 419±67 516±44After liner change 1.5 532±48 513±37After balls addition 2 589±45 501±37

Fig. 9. Product size and feed rate relationship for the SAG mill.

105M. Maleki-Moghaddam et al. / Powder Technology 217 (2012) 100–106

was determined to be 37 g/t. Make up balls (125 mm) were addedevery 8-hour shift.

3.6. Effect of operational parameters on the SAG mill performance

The main manipulated parameter to control the dry SAG mill wasthe exhaust fan power draw. The higher the power draw of the fanthe higher the amount of discharged particles. When the amount ofdischarged particles increased the feed rate to the mill was also in-creased to keep the power draw of the mill at the set point value. Inother words, the power draw of the exhaust fan and the feed rate tothe mill was interrelated. The power draw of the mill was set at2300 kW to provide mill filling of at least 20% depending on the orehardness. At this operating condition (i.e., mill power draw of2300 kW) the effect of power draw of exhaust fan on the productsize was investigated over a period of two months (Fig. 8). Everypoint is a composite sample of four sample cuts each taken at15 min intervals. To reach steady state after any change in the ex-haust fan power draw a period of 2 h was allowed.

A liner relationship (Eq. (1)) was found between the exhaust fanpower draw (PEx) and P80. The relationship was then used to findthe exhaust fan power draw which granted the desired product size.

P80 ¼ 0:28PEx−22 ð1Þ

To arrive at the product size of 450 μm, the exhaust fan powerdraw should be set at 1650 kW which in turn dictates a certain feedrate. In Fig. 8 for equal exhaust fan power draws (1500 and1750 kW) different values for P80 were found which indicated thechange in the ore hardness over the study period.

When the feed rate (F) was plotted vs. P80 for the same study a lin-ear relationship (Eq. (2)) was also obtained (Fig. 9).

P80 ¼ 0:48Fþ 166 ð2Þ

To obtain the product size of 450 μm the exhaust fan power drawwas proposed to be between 1650 and 1700 kW. At this operatingcondition the dictated feed rate was 580 t/h.

Fig. 8. The P80 of SAG mill No. 2 in different exhaust fan power draws.

4. Conclusions

– The charge trajectory in the AG mills of the Gol-E-Gohar iron oreconcentration plant was simulated by a spreadsheet-based soft-ware called GMT with a view to optimizing the liner profile.

– The safe operating windows in plants, defined here as milling con-ditions that do not result in liner breakage because of direct im-pacts of balls to the mill shell, corresponded to the ball impactpoints with a distance between 17 and 36° from the predicted toe.

– Increasing the lifter face angle of AGmill from 7 to 30° while keep-ing the original lifter height (i.e., 22.5 cm) provided an appropriatecharge trajectory.

– Testing the original and proposed liners in a 1-m diameter 2Dmodel mill indicated that total mill filling should be more than20% to prevent direct balls impacts to liners.

– Throughput of the AG mill increased by 27% due to the change inthe liner design, without any adverse effect on the product size.

– Conversion to SAG milling by adding 5% (v/v) balls resulted in anoverall increase of 40% in the mill throughput, and a relativereduction of 3% in the product size.

– More stable plant operation was obtained in the SAG operatingmode, as quantified by a reduction of 16% and 15% in the standarddeviations of mill power draw and product size, respectively.

– The target product size (P80) of 450 μmwas obtainedwhen the ex-haust fan power draw was 1650–1700 kW which in turn dictatedthe feed rate of 580 t/h.

Acknowledgements

The authors would like to thank Gol-E-Gohar Iron Ore Companyfor supporting and implementing the outcome of this research andalso permission to publish the article. Special appreciation is also ex-tended to the operating, maintenance, metallurgy and R&D personnelfor their continued help.

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Mostafa Maleki-Moghaddam obtained his B. Eng.(Exploitation) andM.ENG. (Mineral Processing) fromMin-ing Engineering Departemnt of Shahid Bahonar Universityof Kerman, Iran. He worked for two years in titanium con-centration plant and then returned to the university. He iscurrently a Ph.D. student in the Mineral Processing group.

Mohsen Yahyaei obtained his B. Eng. in mining explora-tion from Sahand University of Technology, IRAN. Thenhe joined mineral processing group at Shahid BahonarUniversity of Kerman where obtained he M.ENG. (2002)and Ph.D. (2011). For 3 years (2004–2007) he workedas superintendent for coal processing plant (Interkarbon).He is currently research fellow at Kashigar mineral pro-cessing research center at Shahid Bahonar University ofKerman.

Samad Banisi graduated from Isfahan University of Tech-nology, Iran, in 1985, with a B.Eng. in mining engineering.He obtained his M. Eng. in 1991 and Ph.D. in 1994, in min-eral processing from McGill University, Montreal. He ispresently professor of mineral processing at the MiningEngineering Department of Shahid Bahonar University ofKerman. His research interests include plant trouble shoot-ing and optimization, column flotation and modeling ofgrinding.