Energy Reduction In Ore Ion

ENERGY REDUCTION IN ORE COMMINUTION THROUGH MICROWAVE Ashish Kumar1, V. V. Ramaro1, Balachandran P. Kamath2, K. P. Ray3, K. R. Kini3 1

Central Research & Development Laboratory, Hindustan Zinc Limited, Zinc Smelter, Debari – 313024, Udaipur (Rajasthan), India 2 Jagadia Copper Limited, 747, GIDC Industrial State, Jagadia – 393110, Bharuch (Gujrat), India 3 Society for Applied Microwave Electronics Engineering & Research, IIT Campus, P.B. No. 8448, Powai - 400076, Mumbai (Maharashtra), India Keywords: Microwave, Comminution, Energy Saving Abstract Comminution is an essential mineral processing operation to liberate the minerals from the ore. This however, is an energy intensive step in mineral processing industries. This work highlights the possibility of saving in grinding energy for ore comminution reducing work index of the ore by effective utilization of microwave energy. This reduced work index results in increased throughput of the grinding circuit in mineral beneficiation plant in order not to shift the product size. The fundamental principle behind this application remains the ability of microwave to heat individual phases within the ore matrix. The constituents of the ore typically having different thermal and mechanical properties develop stress of sufficient magnitude to create intergranular and transgranular fractures during heating and subsequent quenching of the ore. The experiments conducted with the ore samples of Zawar Mines and Rampura Agucha Mine of Hindustan Zinc Limited, India (A member of Vedanta Resources Plc) reflect a substantial up-shift in cumulative weight percentage passing in finer sieve fractions under quenched conditions. Further, the experiments carried out on Rampura Agucha ore reveal 20-30% reduction in work index, which result in decreased milling time or saving in grinding energy. The simulation studies estimates a 2-4% increase in plant throughput. Moreover, preliminary flotation studies indicated a significant increment in total metal recovery and concentrate grade for the desired grade and recovery values respectively. This paper is a technical note on the laboratory investigations carried out at Central Research & Development Laboratory of Hindustan Zinc Limited. Results have been encouraging to progress the work further. It is also proposed to carry out modeling work for simulation so as to predict the changes in minerals.

* Corresponding author: Ashish Kumar, Ph. No. +91-294-2655618, Email: [email protected]

Introduction Comminution of ore is an energy hungry process. A huge slab of energy is being consumed during size reduction of big chunks of mined ore to liberate the interlocked minerals. Thermally assisted liberation (T.A.L.) has been studied since the early 1900’s to increase grindability of the ore by developing intergranular and transgranular fractures during heating of the ore. The major disadvantage of the conventional T.A.L. has always been the economics of the process, as conventional heating requires vast amount of energy to heat the entire system. New researches in the field of microwave and radiofrequency pioneered microwave energy as an effective solution for heating source. The absorption of microwaves by various minerals is a characteristic phenomenon of respective mineral. These electromagnetic waves of frequency range 3000 MHz – 3000 GHz cause friction to generate heat inside the mineral at molecular level and transform it inside the lattice itself based on its magnitude. This intramolecular energy / heat transfer avoids the conduction and radiation modes of heat transfer thus fastening the heating process exponentially. The ore constitutes of various minerals. The differential absorption capacities of different mineral constituents cause differential heating of the ore and results in development of cracks and/ or micro-fractures in the ore matrix. For example, presence of sulphides, oxides and graphitic carbon enhances the fracture formation. Vedanta Resources Public Limited Company (plc) is a London stock exchange listed metals and mining major in Aluminum, Zinc, Copper, Silver and Lead, having operations in India, Australia and Zambia. Hindustan Zinc Limited, India (A member of Vedanta Resources Plc) has a Research & Technology Development center as Central Research and Development Laboratory (CRDL) located at Zinc Smelter, Debari, Udaipur (Rajasthan). The research group as part of its mineral processing road map is working along with Society for Applied Microwave Electronics Engineering & Research (SAMEER), Mumbai on utilization of microwave technology in beneficiation plants. Under the project, it is proposed to set up plant scale facility, on trial basis first, to utilize microwave energy in the grinding operations at Rampura Agucha (R A) beneficiation plant. Details of work presented here are related to lab-scale studies before implementing the technology on site. Brief Review on Microwave Applications A variety of applications for microwave radiation in the mineral processing and extractive metallurgical industries have been proposed. Some of the early applications include communication, navigation, vulcanization of rubber, medical therapy, drying of food items etc. In the recent past two decades, the remarkable success of microwave was observed in ore comminution, drying, carbon reactivation, flotation, pressure leaching, roasting and sintering. Walkiewicz et al (1988) have reported the microwave heating characteristic of selected natural occurring minerals and reagent grade compounds and concluded stress fracturing at mineral grain boundaries in gangue matrix significantly affecting grinding energy requirements and liberation properties. Hwang et al (2002) studied microwave assisted chalcocite leaching with a microwave hydrothermal reactor. The leachability was much better in comparison to conventional one. Agrawal (1999) has worked on microwave sintering of metals. Kinectrics Inc. (Canadian company formerly Ontario Hydro Technologies) has found microwave as an attractive alternative to conventional heating methods. With calcinations of Alumina and Barium Titanate about 15% electricity at 60% time and 70% electricity, 85% time respectively were saved, while in sintering of Alumina and Zinc Oxide 50% electricity, 60% time and 45% electricity, 50% time

respectively was saved. Shuey (2002) have reported the application of microwaves in various fields of mining like; comminution, drying, roasting, flotation, Carbon reactivation and concluded microwave as an efficient tool for the mining industries in sight of energy conservation with the better product quality. Vorster et al (2001) observed the effect of microwave radiation upon the processing of Neves Corvo Copper ore. A maximum reduction in work bond index of 70% within exposure of 90 seconds was observed while in the quenched samples it was 15% more than the unexposed one. They have also simulated the process using USIMPAC. Kingman et al (2000) worked on influence of mineralogy on microwave assisted grinding and concluded regarding the economic implementation of this technology. Kingman et al (2003) studied the effect of microwave on Copper carbonatite. It has been shown that very short exposure time can lead to significant reduction in ore strength (determined by point load test). Salsman et al (1996) studied the feasibility of using short pulse microwave energy as a pretreatment step in comminution. Experimentation Experiments with Zawar Ore Zawar Mines ore samples of minus 10-mesh size were exposed to microwaves at various powers for different time intervals, using the microwave facilities at SAMEER. The -10 mesh feed sample to ball mill in the laboratory grinding tests was analyzed for various sieve fractions (+1651, 1180, 500, 30, 60, 100, 200, 300, 400 & -400 µm. Further, the unexposed and each exposed sample were ground using laboratory wet grinding mill (12” i.d. x 12”) at 80% solids (by wt) pulp density for 15 min during individual grinding test. The ball mill was washed thoroughly with water and the slurry was homogenized in a repulper separately. Representative slurry samples were drawn for the sieve analysis. These samples were dried and sieve analysis was carried out for various sieve fractions (48, 60, 100, 150, 200, 300, 400, -400 mesh) using the similar procedure as mentioned above. The sieve analysis data are given in Fig 1 and interpreted in the Results and Discussion paragraph. Experiment with Rampura Agucha Ore (R A Ore) Preparation Of Sample: A few lots of representative rod mill feed sample have been obtained from R A mine. Particle size distribution was ascertained through sieve analysis using 19 mm, 12.5 mm, 6.3 mm, and 2.0 mm sieves. Particles passing 2 mm were discarded. Above sieve fractions were then mixed synthetically in a way for desired amount of samples, so that the proportion of each size fraction in -19 mm,-19mm +12.5 mm, -12.5 mm +6.3 mm and -6.3 mm +2 mm fractions was similar to the original sieve fraction available in the rod mill feed. Removal of particles passing 2 mm size were removed from feed in accordance with certain observations from previous work on Zawar ore. Grindability Studies: To observe the effect of water quenching just after microwave exposure, samples of 3-kg weight each were prepared as above. Samples were exposed to various intensities of power of microwave source available at SAMEER. Microwave power intensities varied between 0.5 and 5.0 kW and the exposure was for different time intervals (30, 60, 90 seconds). Some of the exposed samples were quenched in water immediately after microwave exposure. The samples (unexposed, unquenched exposed and quenched exposed) were ground in batch type laboratory ball mill (12” dia x 12” long) at a pulp density of 66 % solids (by wt). The ground slurry was analyzed for sieve analysis for various sieve fractions (+8, +30, +60, +100, +200, +300, +400, -400 mesh) through wet sieving. Sieve analysis results for 30, 60 and 90 minutes microwave exposure are given in Fig 2, 3 and 4 respectively.

Further, investigations were carried out with a batch mode industrial type set up of 1.5 kW microwave power supplied by SAMEER to CRDL. The samples were exposed to microwave for various time intervals (1-60 minutes) and relative work index of the R A ore as feed to the rod mill was calculated using Berry & Bruce Method (Berry and Bruce, 1966). The results are shown in Fig 5. Relationship for estimating work index for estimating work index for test sample from known work index of reference is given below;

In the relationship, Wi is the work index kWh/t; r and t refer for reference and test samples respectively; P and F refer to 80% passing of the product and feed stream respectively. Supplementary experiments with the same microwave power and a definite exposure time (observed from previous experiments) were accomplished and the effect of microwave on grinding time of the ore is plotted in Fig 6. Simulation Studies: Simulation studies were carried out using JKSimMet comminution software. The increase in the rod mill throughput was predicted through simulations for observed reductions in work index of R A ore against the present cumulative 80% passing values of beneficiation plant. The reductions in work index were calculated using Berry and Bruce Method from the above experiments. Results observed are shown in Fig 7. Flotation Studies: The batch flotation tests were conducted in a view to observe the effect of microwave on mineralogy of the ore. The exposed and unexposed ore samples were ground for cumulative 80% passing of 200 mesh at a pulp density of 66 % solids (by wt). Using the laboratory bench scale flotation set-up a series of flotation tests have been carried out with unexposed samples and microwave exposed samples for different exposure intensities under similar parameters like, pulp density, reagents dosage, agitation, pH conditions etc. Inference from the test work observations is shown in Fig 8. Results and Discussion Grindability Studies of Zawar Ore: Fig 1 indicates that the fine-particle fraction generated after microwave exposure is more or less similar to the unexposed ore. This supports the observations mentioned in literature by Kingman, 2004 that poorest response could be expected from ores containing highly disseminated and fine-grained material through microwave irradiation. This partial effect of microwave for fine-grained material may not be so economic at commercial scale. Further experiments were carried out with the coarser size fraction of the R A Mine ore for commercial applicability of this project. Grindability Studies of RA Ore: Fig 2, 3, 4 indicate that the extra fine-particle fraction generated after microwave exposure under unquenched condition is limited, while it is substantially higher in case of water quenching after microwave exposure. This reflects that microwave power will be more effective under quenched conditions with respect to unexposed and exposed ore without water quenching. This was due to the extensive intergranular fractures caused by differential heating and subsequent thermal expansion of the microwave responsive grains within the mineral lattice. The reverse holds true for the quenching process. The minerals at higher temperature cool

quicker because they have a higher temperature gradient. The experiments were continued with microwave exposure followed by water quenching.

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Figure 1. Average Particle Size Distribution of Zawar Mines Samples Under Microwave Exposed & Unexposed Conditions

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Figure 2. Effect of Microwave on Particle Size Distribution After 30 Seconds of Exposure Under Quenched, Unquenched & Unexposed Conditions (R A Ore)

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Figure 3. Effect of Microwave on Particle Size Distribution After 60 Seconds of Exposure Under Quenched, Unquenched & Unexposed Conditions (R A Ore)

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Figure 4. Effect of Microwave on Particle Size Distribution After 90 Seconds of Exposure Under Quenched, Unquenched & Unexposed Conditions (R A Ore) Fig 5 shows variation in the percentage reduction in relative work index according to time of microwave exposure. It reveals 20-30% reduction in work index for RA ore after microwave treatment. The percentage reduction in work index was calculated from the relative work index values of the microwave exposed ore obtained from the Berry & Bruce method. Fig 6 shows the effect of microwave on the grinding time for ore. The results have been plotted between 80% passing of ground product for 200, 150, 100 & 60 mesh individually and percentage saving in grinding time. The graphs reflect 11 – 16 % saving in grinding time of the ore for above sieve sizes. The observation reveals that less grinding time (which is directly proportional to savings in grinding energy) is desired to generate same cumulative percentage

passing value (P-80) for every mesh. This signifies that the exposure of ore to microwave results in its decreased hardness or work index.

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Figure 5. Effect of Microwave Exposure on Work Index of Ore

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Figure 6. Effect of Microwave Exposure on Grinding Time of Ore Simulation studies of RA Ore: The results obtained from the previous experiments were used to simulate the effect of reduced work index on the throughput rate of the milling area. As microwave energy aided grinding helps generation of more fines during grinding, in order not to shift product particle size distribution (for not disturbing downstream flotation), throughput can be increased for an operating mill. This will result in reduction of grinding energy per metric ton of ore. Fig 7 shows simulated values of percent increase in throughput for various values of percent reduction in the work index.

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Figure 7. Effect of Reduced Work Index on Throughput Rate of Ore Milling Plant Flotation studies of RA Ore: Observations shown in Fig 8 infer that improvement in flotation performance could be possible as microwave exposure renders improved liberation conditions, like more liberated minerals at coarser size. This is because of the fractures created by microwave at grains boundaries, which result in liberation of interlock particles within the ore matrix and under such conditions, the ore may be assumed having different characteristics.

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Figure 8. Effect of Microwave on Flotation Studies Conclusion From the grinding test work, it is concluded that galena-sphalerite (lead-zinc) ore at coarser size is responsive to microwave radiation. It was shown that the microwave radiation followed by water quenching causes substantial reduction in work index of the ore. This phenomenon results in finer grinding. In an operating plant, mill capacity could be increased when the shift in the plant designated particle size is not desired and thus power consumption per ton of ore treated will be reduced. The preliminary batch flotation tests undertaken with the test samples indicate an improvement in flotation performance with no adverse affect.

Acknowledgements Authors would like to thank SAMEER (Society for Applied Microwave Electronics Engineering & Research), Mumbai, a Govt. of India Laboratory, for supplying batch mode industrial type microwave source to CRDL designing of applicator cavity and personnel involved from Central Research & Development Laboratory and Rampura Agucha Mine for their assistance during project work. References 1. Agrawal, D., 1999, “Microwave sintering of metals,” Materials World, 7 (11), pp. 672-673. 2. Berry, T.F. and Bruce, R.W., 1966, “A simple method for determining the grindability of ores,” Canadian Mining Journal, 6(6), pp. 385-387. 3. Hwang, J.Y., Shi, S., Xu, Z. and Huang, X., 2002, “Oxygenated leaching of copper sulfide mineral under microwave hydrothermal conditions,” J. Minerals & Material Characterization & Engineering, 1 (2), pp. 111-119. 4. Kingman, S.W., Vorster, W. and Rowson, N.A., 2000, “The influence of mineralogy on microwave assisted grinding,” Minerals Engineering, 13 (3), pp. 313-327. 5. Kingman, S.W., Jackson, K., Cumbane, A., Bradshaw, S.M., Rowson, N.A. and Greenwood, R., 2003, “Recent developments in microwave assisted comminution.” International Journal of Mineral Processing, “unpublished.” 6. Kingman, S.W., Jackson, K., Bradshaw, S.M., Rowson, N.A. Greenwood, R., 2004, “An investigation in to the influence of microwave treatment on mineral ore comminution.” Powder Technology, 146, pp. 176-184. 7. Salsman, J.B., Williamson, R.L., Tolley, W.K. and Rice, D.A., 1996, “Short-pulse microwave treatment of disseminated sulfide ore,” Mineral Engineering, 9 (1), pp. 43-54. 8. Shuey, S.A., 2002, “Microwaves in mining,” Engineering and Mining Journal, Feb. 1, pp. 22-28. 9. Vorster, W., Rowson, N.A. and Kingman, S.W., 2001, “The effect of microwave radiation upon the processing of Neves Corvo copper ore,” International Journal of Mineral Processing, 63, pp. 29-44. 10. Walkiewicz, J.W., Kazonich, G. and McGill, S.L., 1988, “Microwave heating characteristics of selected minerals and compounds,” Minerals and Metallurgical Processing, Feb., pp. 3942.