development of the IsaMill™

Investigations into fine grinding started at Mount Isa started in the 1970s using conventional grinding technology to increase mineral liberation by grinding to fine sizes. These technologies were not only found to have high power consumption but also proved to be detrimental to flotation performance as a result of pulp chemistry and iron contamination from steel media. These poor results were revisited during pilot plant and tower mill testwork in the 1980s which also showed an inability of tower mills to economically achieve the required sizes.

It was clear that the solution to efficient fine grinding did not exist in the minerals industry. So we looked for ideas to “crossover” from other industries that also ground fine particles – pigments, pharmaceuticals, foodstuffs (e.g. chocolate). While these mills operated at a much lower scale and treated high value products, perhaps the concepts could be developed to treat large tonnages of lower value materials?

In 1991 the introduction of a Netzsch laboratory stirred mill to the Mount Isa site was a turning point in fine and ultrafine grinding. The ½ litre bench scale mill resembled a milk shake maker and used fine copper smelter slag as grinding media. Testwork on McArthur River ore started in 1991, and by January 1992, a small pilot scale mill, LME100, had been designed and installed at the Mount Isa pilot plant. The testwork showed that high speed, inert, horizontal mills could efficiently grind to 7 microns at laboratory scale providing major improvements in metallurgical performance. To make ultrafine grinding applicable to full-scale production a program of development was undertaken between Mount Isa Mines Limited and NETZSCH-Feinmahltechnik GmbH.

Scale-up was tested using trial installations at the Hilton and Mount Isa lead/zinc concentrators. By the end of 1994, the first full scale IsaMill™ (1.1MW) was installed in the Mount Isa concentrator.

In 1998 the rights for commercialisation of the IsaMill™ where transferred from Mount Isa Mines Limited to MIM Process Technologies (now Xstrata Technology) and under an exclusive agreement with Netzsch. In December 1998, the IsaMill™ technology was launched to the metalliferous industry as a cost effective means of grinding down to and below 10 microns.

In just 11 years since the first IsaMill™ was installed at the Hilton Concentrator, IsaMills™ have increased their capacity 16 fold from 205kW to 3MW and their volume has increased 20 times. This rapid increase in capacity can be compared to autogenous milling technology which although available since 1907 took 19 years (from 1940 to 1959) to increase power draw by just 6.
(from Burford Fine Grinding and Project enhancement, Innovative Mineral Developments AUSIMM 2004)

By the end of 2009 there will be 71 IsaMills™ operating with 127MW of installed power. Feed sizes are as coarse as F80 of 180µm. The product sizes ore as low as P80 of 7µm for materials ranging from lead and zinc sulphides, platinum concentrates, industrial minerals, iron oxide and refractory gold concentrate. Such a new technology has been embraced by those operations that rely on finer grinding to achieve metal recovery.

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