The IsaMill is a type of efficient stirred grinding mill for fine and coarse griding used in mineral industry. It was jointly developed in the 1990s by Mount Isa Mines Limited ("MIM", a subsidiary of MIM Holdings Limited and now part of the Glencore Xstrata group of companies, Australia) and Netzsch Feinmahltechnik ("Netzsch"), a German manufacturer of bead mills.C R Fountain, ‘Isasmelt and IsaMills—models of successful R&D,’ in: Young Leaders’ Conference, Kalgoorlie, Western Australia, 19–21 March 2002 (The Australasian Institute of Mining and Metallurgy: Melbourne, 2002), 1–12. The IsaMill is primarily known for its ultrafine grinding applications in the mining industry, but is also being used as a more efficient means of coarse grinding.G S Anderson and B D Burford, "IsaMill—the crossover from ultrafine to coarse grinding," in: Metallurgical Plant Design and Operating Strategies (MetPlant 2006), 18–19 September 2006, Perth, Western Australia (The Australasian Institute of Mining and Metallurgy: Melbourne, 2006), 10–32.M Larson, G Anderson, K Barns and V Villadolid, "IsaMill—1:1 direct scaleup from ultrafine to coarse grinding," in: Comminution 12, 17–20 April 2012, Cape Town, South Africa (Minerals Engineering International). Accessed 24 May 2013. By the end of 2008, over 70% of the IsaMill's installed capacity was for conventional regrinding or mainstream grinding applications (as opposed to ultrafine grinding), with target product sizes ranging from 25 to 60 μm.J Pease, "Case Study – Coarse IsaMilling at McArthur River," Presentation at: Crushing and Grinding, Brisbane, September 2007. Accessed 24 May 2013.
Introduction
While most grinding in the mineral industry is achieved using devices containing a steel grinding medium, the IsaMill uses inert grinding media such as silica sand, waste smelter slag or ceramic balls. The use of steel grinding media can cause problems in the subsequent flotation processes that are used to separate the various minerals in an ore, because the iron from the grinding medium can affect the surface properties of the minerals and reduce the effectiveness of the separation.N W Johnson, M Gao, M F Young and B Cronin, "Application of the ISAMILL (a horizontal stirred mill) to the lead–zinc concentrator (Mount Isa Mines Limited) and the mining cycle," in: AusIMM ’98 – The Mining Cycle, Mount Isa, 19–23 April 1998 (The Australasian Institute of Mining and Metallurgy: Melbourne, 1998), 291–297. The IsaMill avoids these contamination-related performance issues through the use of an inert grinding medium.
First used in the Mount Isa lead–zinc concentrator in 1994, by May 2013 there were 121 IsaMill installations listed in 20 countries, where they were used by 40 different companies.IsaMillTM installation list. Accessed 25 May 2013.
IsaMill Operating Principles
The IsaMill is a stirred-medium grinding mill, in which the grinding medium and the ore being ground are stirred rather than being subjected to the tumbling action of older high-throughput mills (such as ball mills and rod mills). Stirred mills often consist of stirrers mounted on a rotating shaft located along the central axis of the mill.C Jayasundara, R Yang, B Guo, A Yu and J Rubenstein, "Discrete element method – computational fluid dynamics modelling of flow behaviour in a stirred mill – effect of operating conditions," in: XXV International Mineral Processing Congress (IMPC) 2010 Proceedings, Brisbane, Queensland, Australia, 6–10 September 2010 (The Australasian Institute of Mining and Metallurgy: Melbourne, 2010), 3247–3256. . The mixing chamber is filled with the grinding medium (normally sand, smelter slag, or ceramic or steel beads) and a suspension of water and ore particles, referred to in the minerals industry as a slurry. In contrast, ball mills, rod mills and other tumbling mills are only partially filled by the grinding medium and the ore.
In stirred-medium mills, the stirrers set the contents of the mixing chamber in motion, causing intensive collisions between the grinding medium and the ore particles and between the ore particles themselves. The grinding action is by attrition and abrasion, in which very fine particles are chipped from the surfaces of larger particles,M Gao, M Young and P Allum, "IsaMill fine grinding technology and its industrial applications at Mount Isa Mines," in: Proceedings of the 35th Canadian Mineral Processors Conference, Vancouver, 28 April – 1 May 2002 (Canadian Institute of Mining, Metallurgy and Petroleum: Ottawa, 2002), 171–188. Available from: Accessed 24 May 2013. rather than impact breakage. This results in the generation of fine particles at greater energy efficiency than tumbling mills. For example, grinding a pyrite concentrate so that 80% of the particles are less than 12 μm (0.012 mm) consumes over 120 kilowatt-hours per tonne (kWh/t) of ore in a ball mill using 9 mm balls, but only 40 kWh/t in an IsaMill using a 2 mm grinding medium.J D Pease, D C Curry, K E Barns, M F Young and C Rule, "Transforming flowsheet design with inert grinding – the IsaMill," in: Proceedings of the 38th Meeting of the Canadian Mineral Processors (CMP 2006) (Canadian Institute of Mining, Metallurgy and Petroleum: Ottawa, 2006), 231–249. Accessed 24 May 2013.
The IsaMill usually consists of a series of eight disks mounted on a rotating shaft inside a cylindrical shell (see Figure 2).B D Burford and L W Clark, "IsaMill technology used in efficient grinding circuits," in: VIII International Conference on Non-ferrous Ore Processing, Poland. Accessed 24 May 2013. The mill is 70–80% filled with the grinding medium, and is operated under a pressure of 100 to 200 kilopascals. The disks contain slots to allow the ore slurry to pass from the feed end to the discharge end (see Figure 3). The area between each disk is effectively an individual grinding chamber, and the grinding medium is set in motion by the rotation of the disks, which accelerate the medium toward the shell. This action is most pronounced close to the disks. The medium flows back toward the shaft in the zone near the midpoint between the disks, creating a circulation of the grinding medium between each pair of disks, as shown in Figure 4.
The average residence time of the ore in the mill is 30–60 seconds. There is negligible short-circuiting of the grinding zone by the feed, as a result of having multiple grinding chambers in series.
The ground product is separated from the grinding medium at the discharge end of the mill. This is achieved without using screens by using a patented product separator that consists of a rotor and a displacement body (see Figure 2 and Figure 4). The relatively short distance between the last disk results in a centrifugal action that forces coarse particles towards the mill shell, from where they flow back towards the feed end. This action retains the grinding medium within the mill.
The product separator is a very important part of the IsaMill design. It avoids the need to use screens to separate the grinding medium from the ground particles. Using screens would make the mills high-maintenance, as they would be prone to blocking, necessitating frequent stoppages for cleaning.
Fine particles are not as susceptible to the centrifugal forces and stay closer to the center of the mill, where they are discharged through the displacement body at a rate equal to the mill's feed rate.
The design of the IsaMill results in a sharp product size distribution, meaning that the IsaMill can operate in open circuit (i.e. without the need for an external separation of the discharged particles in screens or hydrocyclones to allow coarse over-size product to be returned to the mill for a second pass). It also means that there is less overgrinding at the finer end of the size distribution, such as occurs during the operation of tower mills.
History
The development of the IsaMill was driven by MIM Holdings’ desire to develop its McArthur River lead–zinc deposit in Australia's Northern Territory, and by the need for finer grinding at its Mount Isa lead–zinc concentrator.
The mineral grains in the McArthur River deposit were much finer than those of operating mines. Test work had shown that it would be necessary to grind part of the ore so that 80% of the ground particles were less than 7 μm (0.007 mm) if a saleable concentrate of mixed lead and zinc minerals (referred to as a "bulk concentrate") were to be produced.
At the same time, the mineral grain size of the lead–zinc ore mined and processed at Mount Isa was decreasing, making it harder to separate the lead and zinc minerals.M F Young, J D Pease, N W Johnson and PD Munro, "Developments in milling practice at the lead–zinc concentrator of Mount Isa Mines Limited from 1990," in: 6th Mill Operators’ Conference, Madang, 6–8 October 1997 (The Australasian Institute of Mining and Metallurgy: Melbourne, 1997), 3–12. The liberation of sphalerite (zinc sulfide) grains dropped from over 70% to just over 50% between 1984 and 1991. As a result, the Mount Isa lead–zinc concentrator was forced to produce a bulk concentrate from the beginning of 1986 until late 1996. Bulk concentrates cannot be treated in electrolytic zinc smelters, due to their lead content, and are typically treated in blast furnaces using the Imperial Smelting Process. The Imperial Smelting Process has higher operating costs than the more common electrolytic zinc process, and therefore the payment received by producers of bulk concentrate is lower than that received for separate lead and zinc concentrates. The zinc in the Mount Isa bulk concentrate was eventually worth less than half that of the zinc in the zinc concentrate.
These issues provided a great incentive for MIM to grind its ores finer. MIM metallurgists had undertaken fine grinding test work on samples from both deposits using conventional grinding technologies between 1975 and 1985. However, it was found that conventional grinding had a very high power consumption and that contamination of the mineral surface by iron from the steel grinding media adversely affected flotation performance. It was concluded in 1990 that there was no suitable existing technology for grinding to the fine sizes in the base metals industry. Consequently, Mount Isa's head of mineral processing research, Dr Bill Johnson, began looking at grinding practices outside the mining industry. He found that fine grinding was well established for such high-value manufactured products as printer inks, pharmaceuticals, paint pigments and chocolate.
Early IsaMill development work
MIM decided to work with Netzsch, which was a pioneer in the fine grinding field, and still a leader. Test work was undertaken using one of Netzsch's horizontal bead mills. It showed that such a mill could achieve the required grind size. However, the mills used in these industries were used on a small scale and were often batch operations. They used expensive grinding media that frequently needed to be removed, screened and replaced so that the mills would continue to operate properly. The traditional grinding medium consisted of silica-alumina-zirconium beads that, in those days, cost about US$25 per kilogram ("kg") and lasted for only a few hundred hours. Such a high-cost and short-lived grinding medium would be uneconomic in an industry processing hundreds of tonnes of ore an hour.
Subsequent test work focussed on finding a cheaper grinding medium that might make the bead mill viable for mineral processing. This work included using glass beads (about US$4/kg) and screened river sand (about US$0.10/kg) before it was found that the rounded beads produced by granulating reverberatory furnace slag from the Mount Isa copper smelter constituted an ideal grinding medium.M Gao, M F Young, B Cronin and G Harbort, "IsaMill medium competency and its effect on milling performance," Minerals & Metallurgical Processing, 18(2), May 2001, 117–120.