Communition 2012 Abstract Fine grind attritional mills; can they or should they go coarser D Capstick and B Currie Since the introduction of fine grind attritional mills a number of years ago they have predominately been used for ultra fine and fine grind milling activities. The technology has been successfully applied extensively on regrind circuits, tailings and dump retreatment operations. Recently more interest has been shown on moving this technology further up the milling circuit to operate as a tertiary and secondary milling application downstream from the traditional ball mills. The move towards manufacturing much larger stirred mills has enabled the technology to be considered for run of mine processing. Is this a threat to traditional tumbling ball mills? Or can it been seen as complimentary? This paper investigates the experience of using the Deswik mills in coarse feed applications, the costs benefits in terms of OPEX and the implications of using what was thought to be a ultra and fine grind technology in a different milling environment.
Introduction Since the introduction of fine grind milling technology in the early eighties the application has constantly been evolving and seeking to find its niche in the minerals processing industry. Currently a number of manufacturers offer fine grind mills and they are used throughout the world in different commodities and conducting various milling duties. Many of the mills are now used in much coarser applications than the technology was previously intended. Speaking from our own experiences more process engineers are examining the potential of using the ceramic fine grind mills in coarser milling applications. Background The Deswik mill was initially designed as an ultra fine grind milling technology for the pigments industry. Over the last seven years the technology has been applied to a number of mining applications targeting ultra fine grind products. Clients in the precious metals commodities wish to mill refractory sulphide gold ores to between 10 and 25 microns from a fairly standard ball mill feed of 75 microns. The platinum and chrome industry has generally preferred to mill the end product to between 25 and 38 microns in their regrind circuits. Results Test results are available from a number of materials with a coarse feed size with our lab and pilot size mills and we have production mills that operate specifically on a coarse feed. One of the challenges of using 2 and 10 liter mills is that the coarser size feed material has a tendency to block the relatively small inlet apertures of lab size mills that are a ¾ inch diameter compared to that of a 2 inch to 6 inch inlet for a production mill. An opportunity to perform a grind tests on a magnetite material using a Deswik 10 mill was presented. The requirements for this material were for a product size of 45µm. As can be seen from the profiles below (fig 1) the magnetite ore was reduced from a feed of D90 372µm to a product of D90 41µm. The test work indicated that a very homogenous product can be produced. This is a critical requirement for the product specification as it is used in coal processing as a dewatering agent.
Figure 1 Test work here indicated that a production mill would be capable of producing 40 dry tonnes per hr operating at an estimated power consumption of 12.7 kwh/t. Similarly pilot test work on a gold mine where the testing examines the grind performance on both the final tailings and the feed from the primary cyclone. Figure 2
The Deswik 50 mill milled the 300µm feed to the target size of 69µm at a production rate of approximately 800 dry kgs/hr. A Deswik 250 mill has been operating for over nine months on a tertiary grind on a gold mine in Zimbabwe. The owners were convinced that an attritional mill was the best way to ensure a specific product size for a CIL circuit rather than having a larger recirculation load with the ball milling circuit. The Deswik 250 mills the ore in a cyanide solution passing the milled product to a leach circuit that has a leach time of 12 hours and maintains a tailings grade of 0.22 g/t. The mill produces 3.5 dry tonnes per hour milling a feed from a P80 350µm to a product of P80 110µm. Grinding media consumption does not seem to have been affected by the coarser nature of the feed material, though impellor wear of the third and fourth disc from the bottom is higher than with the finer feed. The mill lining is not exhibiting any additional wear from the coarser nature of the feed material. We continue to investigate other materials for the wear part components to reduce overall OPEX costs and maintenance time. Without doubt ceramic mills are being used on coarser feed material and their performance is comparable to ultilising a secondary or even tertiary ball mill in the circuit. As presented previously by Anglo Platinum, the M10, 000 IsaMill TM have been applied specifically to conduct coarser grind duties as tertiary grinding applications, producing pre-scavenging flotation products at an approximate P80 of 53 microns. The grind curves below (fig 3) from a chromite ore demonstrate the initial particle size reduction of coarse feed material is extremely rapid milling the D90 feed of 354µm to 44µm in five minutes of grinding, representing an eight fold reduction in particle size. It takes a further 10 minutes to reduce the 44um by half and then a further 15 minutes to mill the 22µm to 12µm. This is a considered to be a normal grind curve, and not particular to chromite ores. The rapid reduction of feed material to the standard P80 75µm ball mills products leads one to believe that these mills can offer a viable alternative to tumbling ball mills.
Figure 3 Table 1 below is generated from a grind test on a chromite ore with a feed of D80 200µm and milling to product of D90 106µm. Density water-kg/l 1.00 Density of solids-kg/l DRY 3.70 Throughput l/h 65,000 Density of slurry-kg/l 1.38 Slurry mass-kg/h / lph 89,700 % solids 38 Mass solids-kg 33,848 Amperage drawn-amps 400 Voltage-Volt 525 Root 3 1.73 Motor efficiency-% 80 Power factor 0.95 kw-h consumed-kw-h 276 kw-h/ton 8.17 Table 1 Test work indicated that a 2500 mill would produce 33.8 dry tonnes per hour with an estimated power consumption 8.17kW-h/t. When comparing the performance of tumbling ball mills and attritional mills in secondary and tertiary milling circuits the OPEX costs are comparable, though this obviously depends upon the ore type, milling duties and operational performance.
Generally speaking the steel media consumption of the ball mills is much higher at over 1 kg/t rather than 50 100 grams of ceramic grinding media per tonne processed. This is counter balanced by the higher cost of the ceramic media per tonne. Historical data suggests that the attritional mills are more efficient in terms of power consumption milling ore from 150um to 45um with power consumptions in the range of 10-18 kwh/t. Whereas ball mills are estimated to operate within 15 20 kwh/t range at similar milling duties. The operational advantages are increased when one considers that the attritional mills can be used in open circuit and generally produce a more normalized product than the ball mills. Furthermore in remote mining locations the costs of shipping and transporting many tonnes of steel for grinding purposes can be costly and add an element of risk in terms of guaranteed continuous grinding media supply to the operation. Developments Over the last few years the attritional mill manufacturers have started to design larger mills. This has been a direct response from the equipment industry to provide attritional mills/machines that can operate within the run of mine material. FLSmidth has a KD-VGM5000 under development and the product is in the detailed design phase. Three of these mills are committed to be manufacturer later this year, for a copper project in Mongolia. They will have an installed power of 1300kW and a design speed of 175 rpm, delivering an operating torque of 65, 000Nm and peak torque on start-up of up to 150%. Xstrata Technology on the back of their success with the M10, 000 mills have also announced that the M50, 0000 IsaMill TM is available to the industry. The limits to attritional mill design will probably be dictated by efficiencies of the drive systems. But if anything is to be learnt from the development of ball mills over the last few years then it seems inevitable that attritional mills are set to become larger and larger over time. Media manufactures are investigating the possibility of producing larger ceramic grinding media to accommodation the coarser grind requirements. Commonly the largest size beads available are 3.5mm we expect this to increase in future as some manufactures are looking to develop larger beads.
Conclusion Without doubt there is growing interest in using attritional grinding mills in coarser milling applications rather than merely as fine grinding technology on concentrates or tailings material. The technology can be seen as being complimentary to traditional ball mills whereby attritional mills act as secondary or tertiary milling equipment post ball milling, thus enabling the tumbling mills to concentrate on their areas of high efficiency. The mining industry will come under greater pressure both environmentally and commercially to explore every avenue to reduce energy consumption and costs. As noted in the Mining Industry Energy bandwidth study in 2007, the largest energy savings opportunity (70%) lies in improving the energy efficiency of the two most energy intensive processes grinding and materials handling. The study goes on to state that energy savings can be realised through best practices and research and development that develops more energy efficient technologies. I believe the application of attritional mills in more secondary and tertiary milling applications could form part of this initiative. Key words Ultra fine grinding Stirred Mills Attritional milling Acknowledgments Staff and management at ZMI Tannahill mine and New Dawn s Turk mine in Zimbabwe. FLSmidth colleagues References RULE CM Stirred milling new communition technology in the PGM industry Journal of the South African Institute of Mining and Metallurgy pages 101-107 RULE CM and De Waal H IsaMill Design Improvements and Operational Performance at Anglo Platinum US Dept Energy - Mining Industry Energy bandwidth study