Conventional Ball and Tower Milling were not economic to grind below 25 microns, due to low energy efficiency, low power intensity and high media consumption. Further, the high steel media consumption in these mills was severely detrimental to flotation performance.
The birth of a technology
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 they demonstrated the principle that stirring fine media at high speed was highly efficient. Perhaps the concepts could be developed to treat large tonnages of lower value materials?
The challenge was transferring this concept to continuous, high tonnage and lower-value streams in the minerals industry.
In 1991 the introduction of a Netzsch laboratory stirred mill to the Mount Isa site was a turning point in fine-grinding 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.
After 7 years of development and testing of prototypes in the Mount Isa operations, the IsaMill™ evolved. It was large scale, continuous, and most importantly robust because it was developed by operators. The crucial breakthrough was the perfection of the internal product separator – this allowed the mill to use cheap natural media (sand, smelter slag, ore particles) and to operate in open circuit. These are significant advantages for operating cost and circuit simplicity. 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 Glencore 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.
Moving into Mainstream Grinding
Moving stirred milling "from ink to zinc"was the hard part. The next challenge was to apply the same energy and processing advantages to mainstream grinding. This needed two further developments – an even bigger mill and low-cost, competent ceramic media. The first M10,000 IsaMill™ was installed in South Africa in 2003, and Magotteaux developed KERAMAX® – MT1™ ceramic media specifically for IsaMilling in 2005.
Since then, the 3MW M10,000 has become the most commonly installed IsaMill™. The power installed in regrind and coarse grinding applications has quickly outstripped that used in ultra fine grinding. This remarkably rapid adoption in coarse grinding reflects the robustness and simplicity of IsaMill™ technology.
In just 11 years since the first IsaMill™ was installed at the Hilton Concentrator, IsaMills™ 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 2010 there will be 81 IsaMills™ operating with over 130MW of installed power. Feed sizes are as coarse as F80 of 250µm. 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 grinding to achieve metal recovery.
||Flotation benefit of inert grinding demonstrated with a Netzsch 075 mill
||Prof. Bill Johnson at MIM worked with Netzsch on large horizontal bead mill development
||Successful test of 100 litre Netzsch mill at McArthur River Mining
||Scale up to 3,000 litre mill at Mount Isa Mines
||Proven, and now essential at MRM and Mount Isa Mines (14 x M3000 IsaMills installed across the 2 sites)
||Commercialisation of the IsaMill™ Technology
||Scale up to M10,000 IsaMill™ installed at Anglo Platinum's WLTRP
||First industrial application of Magotteaux's Keramax MT1™ ceramic media
||Development of 8MW M50,000 IsaMill™
||100 IsaMill™ sale
||In operation/installed 121 IsaMills™, with over 206MW installed power