IsaMill

The IsaMill is an energy-efficient mineral industry grinding mill that 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) and Netzsch Feinmahltechnik ("Netzsch"), a German manufacturer of bead mills.

[7] The mixing chamber is filled with the grinding medium (normally sand,[2] smelter slag,[2] or ceramic[7] or steel beads[7]) and a suspension of water and ore particles,[7] referred to in the minerals industry as a slurry.

[10] 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.

[10] 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).

[5] Consequently, Mount Isa's head of mineral processing research, Dr Bill Johnson, began looking at grinding practices outside the mining industry.

[4] He found that fine grinding was well established for such high-value manufactured products as printer inks, pharmaceuticals, paint pigments and chocolate.

[1] They used expensive grinding media that frequently needed to be removed, screened and replaced so that the mills would continue to operate properly.

[1] 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.

[1] Such a high-cost and short-lived grinding medium would be uneconomic in an industry processing hundreds of tonnes of ore an hour.

[1] Subsequent test work focussed on finding a cheaper grinding medium that might make the bead mill viable for mineral processing.

[1] MIM's development efforts focussed on finding a lining that could withstand the wear and on designing a separator that would retain the oversize grinding medium within the mill while allowing the fine ore slurry to exit.

[14] The development of the IsaMill gave the MIM Holdings Board of Directors the confidence to authorise construction of the McArthur River mine and concentrator.

A slight decrease in gold recovery was more than offset by increased availability, with the operation of the Gidgi plant no longer being constrained by air quality control requirements.

The next major leap was driven by problems experienced by South Africa's platinum producers, driving the development of larger mills and initiating the global penetration of the technology.

[14] Anglo Platinum, which had at the time 20 operating concentrators around the Bushveld complex,[17] followed in 2003 with the purchase of a smaller M250 IsaMill for testing in its Rustenburg pilot plant.

[14] After doing the test work, Anglo Platinum decided to use a scaled-up version of the IsaMill in its Western Limb Tailings Retreatment ("WLTR") project.

[14] It worked with Xstrata Technology, by then the holders of the marketing rights, and Netzsch to develop the M10000 IsaMill, which has a volume of 10,000 L and, at that time, a 2600 kW drive.

[14] The grinding medium selected was a newly available and low-cost zirconia-toughened alumina ceramic material,[14] which was developed by Magotteaux International.

[6] IsaMills are now used in lead–zinc, copper, platinum group metal, gold, nickel, molybdenum and magnetite iron ore applications.

[6] Xstrata Technology has recently been developing a larger M50000 model IsaMill, with an internal volume of 50,000 L, with a drive up to 8 MW.

[20] The advantages of the IsaMill include: The development of an economic ultrafine grinding technology has made possible atmospheric leaching of minerals for which this was previously impossible.

By using IsaMills to grind the particles of refractory minerals to ultrafine sizes, the Albion Process increases the activity of sulfide concentrates to the point where they can be readily oxidised in conventional open tanks.

Figure 1. The growth of both the number of IsaMill installations and their total installed power can be seen in this graph. The gap between the two lines has closed as more powerful mills have been developed and installed.
Figure 2. Schematic view of an IsaMill showing the principles of its operation.
Figure 3. Photograph of an IsaMill disk being pushed into place on the mill's shaft. The slots in the disks are clearly shown. The orange device behind the second disk is the product separator.
Figure 4. Schematic diagram showing the flow patterns of the grinding medium inside an IsaMill.
Figure 5. Photograph on an IsaMill with the split-shell design to allow easier replacement of the shell liner.
Figure 6. Schematic diagram of an IsaMill showing how the shell slides away from the shaft and grinding disks to allow easy access to the mill's internal components.
Figure 7. Photograph of IsaMills with their shells pulled back, exposing their internal components.
Figure 8. Photograph of the patented IsaMill product separator.