This paper briefly describes the studies conducted in Rakha  concentrator  for  improvements  in economy of operation. Studies have been conducted on flotation of ore, ground to a coarser size than the present milling practice. The objective of grinding to coarser sizes is 

to optimise the grind size and mix so that overgrinding may be reduced, which in turn, will help to improve the overall economy

Khare, R N

Saha, S K

Sanyal, P

English

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Grind optimisation studies at Rakha concentrator by laboratory tests * P. Sanyal, 	 ** S. K. Saha, 	 *** R. N. Khare INTRODUCTION This paper briefly describes the studies conducted in Rakha concentrator for improve-ments in economy of operation. Studies have been conducted on flotation of ore, ground to a coarser size than the present milling practice. The objective of grinding to coarser sizes is to optimise the grind size and mix so that overgrin-ding may be reduced, which, in turn, will help to improve the overall economy. PLANT DESCRIPTION Rakha concentrator is one of the three concentrators operating in the Singhbhum Copper belt and is owned and operated by M/s. Hindu-stan Copper Ltd. Its treatment capacity is 1000 tpd. The ore at Rakha is mainly quartz and chlorite with chalcopyrite as primary economic mineral. It is medium to medium hard ore with work index of around 10. In the crushing plant 300 mm ore is crushed to -12 mm in three stages. The crushed ore is led off to the mill house, where two identical streams each consis-ting of a ball mill ( 2.59 m dia x 2.85 m long ) in closed circuit with 600 mm hydrocyclone, the overflow of which is sent to the flotation section at 25- 30% solids, and the underflow at about 70 -75% solids, is circulated back to mill. The flotation section also has two streams each con-sisting of one 3 x 3 m conditioner, 12 nos. of 1.7 cu.m. Agitair type rougher cells and 4 nos. of 1.12 cu.m. cells of same type as cleaner cell. The final concentrate is thickened and filtered and despatched to Moubhandar for smelting, and the tailing is sent to mines for back filling pur-pose after desliming. The petrological studies on Rakha ore have shown the liberation size to be 100 mesh ( 150 microns ) and operating experience over the years has brought out that grind of 50 percent passing 200 mesh (74 microns) gives the best size distribution for flotation. Based on this, the mills are run at 22 MT/hr, to deliver the required product. The typical data are given in Table : 1 to show the size distribution of various streams at above mentioned feed rate : Table—I : Size analysis of cyclone overflow and underflow Weight percent retained Mesh Cyclone overflow Cyclone underflow +48 0.99 35.94 - 48 65 6.69 10.72 -65 100 12.52 18.36 -100 +- 150 16.84 14.18 -150 4- 200 12.44 6.87 - 200 50.52 13.93 It is evident that the amount of finer parti- cles i. e. -100 mesh fraction, in 	 cyclone underflow is about 35%. If recirculation of these particles to mill can be reduced then circulating loads on mill will reduce, resulting in reduction in overgrinding and hence increase in throughput to mill. In Rakha concentrator to recover finer particles from cyclone underflow and for reduc-tion of circulation load, the concept of secondary classification has been studied in detail (Ref : 1) * Indian School of Mines, Ohanbad. 	 ** Hindustan Copper Limited/Rakha Copper Project. *** Hindustan Copper Limited I Indian Copper Complex. and it is now being used successfully on one stream. A reduction in circulating load from 300-400 percent to 200-250 percent has been achieved, resulting net increase of about 8-10% in mill throughput. Table-2 shows the analysis of cyclone overflow of secondary classification system at feed rate of 25 MT/hr. Table-2 : Sieve analysis of cyclone over-flow of multiple classification system Weight percent retained Mesh Primary cyclone overflow Secondary cyclone overflow Combined cyclone overflow +48 0.33 0.69 0.50 -48 +65 1.45 4.38 2.81 -65 +100 14.38 23.07 18.41 -100+150 17.52 19.91 18.63 -150+200 11.59 10.83 11.24 - 200 54.73 41.12 48.42 It can be observed in Table-2 that with secondary classification system in circuit the amount of - 200 mesh fraction decreases and simultaneously -1 100 mesh fraction increases slightly. The original idea of this study was to see the effect of coarseness thus imparted to flo-tation feed on flotation characteristics of Rakha ore (Ref : 2). It has been proved theoritically and also from operating experience that, this much increase in coarseness does not effect the flotation system significantly. After that the study has been extended to find out the limit of coarseness of flotation feed which can be impar-ted without sacrificing recovery and grade, so that mill throughput can be increased. EXPERIMENT AND DATA ANALYSIS : Flotation tests were carried out in labora- tory for studying the possibility of flotation of coarse ground ore. The feed of various coarse-ness for batch flotation tests have been obtained from laboratory ball mill. The plant mill feed samples have been collected and they were crushed to 100 percent passing 14 mesh in labo-ratory roll crusher, and then ground in laboratory batch ball mill at 70% solids for different grind times. The time versus grind was standardised for this particular batch ball mill for subsequent use. Table-3 shows the grind obtained at var-ious grind times. Table-3 Sieve analysis of product from laboratory batch ball mill Grind time (min) Mesh 10 	 8 6 	 4 retained Weight percent +48 0.23 0.52 1.05 2.68 - 48 +65 1.79 3.05 6.19 10.86 -65 +100 3.85 6.82 8.61 9.56 -1004 150 22.99 24.46 24.44 23.54 -150+200 16.34 13.62 13.95 13.09 - 200 54.80 51,53 45.76 40.27 Kinetic flotation tests were then conduc-ted with these samples of various grind size and mix in a laboratory batch cell with volume make up. Initial pulp density in the batch cell was kept at 21% solids. The collector used was Sodium isopropyl xanthate and frother was pine oil. The amount of collector used was 6 cc of one percent solution of xanthate and the amount of frother was 6 drops. The concentrate collected at different time intervals and the final tailings was then assayed for the copper content. The data thus obtained was used to cal-culate recoveries and grades obtained at different time intervals for different grind times is given in Table-4. 30 Table-4 : Cumulative recovery Et grade at different times Flotation Time (min) 0.5 1.5 3.5 7.5 Grind time Cumulative Recovery 85.62 93.17 95.34 96.78 4 mins Cumulative Grade 21.40 18.25 14.70 12.34 Grind time Cumulative Recovery 86.59 93.44 95.71 97.08 6 mins Cumulative Grade 22.98 20.49 17.97 15.49 Grind time Cumulative Recovery 81.29 91.98 94.95 96.76 8 mins Cumulative Grade 24.40 21.61 18.83 16.22 Grind time Cumulative Recovery 81.66 91.41 94.31 96.17 10 mins Cumulative Grade 23.43 20.92 18.42 15.63 DISCUSSION : The time VS recovery studies shows that for different grind times though the end re-coveries are in close range, there is a recovery improvement at short flotation time for lower grind time. Also it can be observed that the maximum overall recovery possible for different grind times are close and all are above 97 per-cent.Table 5 shows the different values of R max ( maximum overall recovery possible ) for different grind times Table-5 : Values of R max for different grind times Grind Time (Mins) R max 4 97.56 6 97.81 8 97.84 10 97.19 The above fact implies that a desirable re-covery, which is in case of Rakha concentrator is 95.5°,/0-96.5% is possible even with coarser grind. It can be observed from Table-4 that for a coarser grind, the grade falls slightly. The difference of grades at lower recovery range is more compared to higher recovery zone. If the above is due to more scavenging at shorter period, it can be taken care of in cleaning stage, but if it is due to more gangue recovery,  regrinding may be required. But no definite modification can be designed before studying the plant flotation circuit completely. A plan for studying the flotation circuit has already been chalked out, and the study is in progress. SUMMARY AND CONCLUSION : Present milling practice is to grind 50 per-cent passing 200 mesh for efficient flotation. 2. Grinding consumes about 30 percent of total power and its operational cost is aiso about 30 percent of total cost. Hence any improvement in grinding throughput will improve the overall economy to a great extent. It can be safely assumed that if further coarseness does not effect the flota-tion system a 10% increase in throughput can be achieved without any substantial capital expenditure and enhanced operating cost which means about Rs. 1.50 can be saved for each ton of ore milled. 3. Batch flotation study shows that desired recovery is possible with coarser grind. 4. The study is now being extended to the plant. Extension of this may give the scope of improving throughput by minor modifica-tion in grinding and flotation circuit. 5. To analyse large volume of data with suffi-cient speed and accuracy, aid of computer is desirable. 31 REFERENCES 1. Rao T. C., Bandyopadhyay A, and Khare R. N. "Improvement in mill-cyclone classifier circuit — a case study" 1Vth International Mineral Processing Congress, Toronto 1982, Paper-111-7. 2. Kudbi G. K., Sanyai P. and Saha S. K. "Studies on Rakha ore flotation at coarser grind with use of models". National symposium on Mineral Beneficiation, 28th and 28th April 1983, Khetrinagar (Rajasthan), organised by 11M, Paper - 7. DISCUSSION P. S. R. Reddy R. R. L., Bhubaneswar. Question 1 . Recoveries are almost same from 4 to 10 minutes of grinding. Have you carried out studies at 2 minutes grinding ? If so, what is the recovery ? Author : No test has been carried out at 2 minutes grind; however such studies have been planned for broader understanding of flota-tion behaviour. Question 2 : As the recoveries are same, how could you optimise the grinding time ? Author : Recovery is not the only para-meter for optimisation of grinding flotation operation, the grade of concentrate also plays an important role. The exact optimisation rela-tionship has not yet been established, but the study is in progress. P. K. Nilosey M. B. E., Kumardhuby Question 	: When we compare recoveries against time of grind that amounts to fineness of product only. Why 'time of grinding' is consi-dered more important parameter, than "product size" of mill ? Author : Here the grinding time is used as a measure of fineness and size distribution of ground product; in other words, the grind time was standardised to produce a particular product size and mix (Table-3). 32 

Grind Optimisation Studies at Rakha Concentrator by Laboratory Tests

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Grind Optimisation Studies at Rakha Concentrator by Laboratory Tests

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