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It is somewhat ironic that while rock crushing and grinding professionals continually seek new comminution techniques to gain greater control over their milling processes, they must ultimately relinquish this command and share their developed technologies with sector colleagues in order to move forward.

Perhaps one of the best examples of this seen in Australia over the past few years has been with the IsaMill process, which was inherited by the publicly-listed Xstrata after its takeover of Australian mining house MIM Holdings Ltd back in 2003. A high-speed stirred mill used for the fine and ultrafine grinding ore at an industrial stage, the IsaMill can be applied to base metals operations such as lead and zinc mines as well as used for the liberation of gold particles from sulphide minerals for cyanidation. It is more energy-efficient than conventional grinding technologies (such as ball and tower milling) and has the ability to utilise cheaper non-metallic grinding media like river sand and furnace slag to grind to far finer sizes than traditional comminution technologies. However, being a relatively new process, the IsaMillís optimum control and scale-up rely on empirical information, industry experience as well as trial and error testing. It is perhaps for these reasons that the so-called shared approach is preferred by Xstrata, which has actively sought to ensure the technology would not remain 'in house'.

Speaking at the recent Minerals Engineering Internationalís Comminution 2006 Conference in Western Australia, Lindsay Clarke from the mining houseís Brisbane office said the company expected that most of its IsaMill installations over the next few years would be for other national and international customers.

"Weíre not in the business of just supplying mills - we are in the business of providing our technology to customers and the technology is more than just the mill," he explained. "We hold user meetings every two years - we get the IsaMill users together and have a conference were we talk to them about their operations and try to come up with improvements to the design. "So we also provide extensive customer technical support to their sites."

The first 1.2 megawatt IsaMill was built to treat MIMís fine grain lead-zinc ores at Mt Isa and McArthur River in the mid 1990s. Then, in 2000, it installed a facility at Kalgoorlie Consolidated Gold Mine Ltdís Gidgee operation in WA. Since then the resources house has built mills in South Africa, the US and Kazakhstan, with another soon to be commissioned in Chile.

When pressed why the exchange of technology was so important to Xstrata, Clarke said the company would not have been able to develop its 3 Mw M10, 000 IsaMill without some kind of collaboration with client Anglo American Platinum Corporation Ltd."It couldíve have happened if we didnít go external," he explained. "If it had just been left within Mt Isa-Xstrata, it would have reached a plateau of development." The IsaMill consists of eight grinding discs which provide stage-by-stage grinding. It is a very high intensity process and uses clean, reactive surfaces. "And, because the media used is generally inert, it also helps in downstream flotation and leaching," Clarke added. "Itís very efficient - you are reducing the top size, which is really what you are aiming to do, and you are not over-grinding, so you are putting power into reducing the size of the particles you wish to operate on."

During the opening of the three day conference, which was attended by around 50 international delegates, and sponsored by Metso Minerals, Xstrata Technology, and the Gold & Minerals Gazette, MEIís Dr Barry Wills said while it seemed there had been no big developments in comminution during the 20th Century, crushing and grinding machines had in fact gained a greater level of control over the process during the past 20-30 years. "In recent years we have had great innovations, particularly in fine grinding," he remarked.

And two leading Australian groups that have made significant progress in this area is the CSIRO and Queensland Universityís Julius Kruttschnitt Research Centre, which have conducted 3D discrete element method (DEM) simulations of the flow of grinding media in a laboratory-scale 1.5 kilowatt Kubota tower mill and a 7.5 kW Sala agitated mill to investigate the relative performance of stirred mills.

According to the CSIROís Matt Sinnott, while stirred mills are becoming increasingly used for the fine and ultra-fine grinding of ores, this technology is still poorly understood when utilised in a minerals processing context. Sinnott told MEI delegates that the basic stirred mill consisted of a grinding chamber filled with some grinding media (such as steel balls).Although the mill shell remains stationary, the media is set into motion by the action of a motor driven internal impeller (or stirred agitator). He also said while most mills could typically operate with wet or dry feeds, wet grinding was generally more efficient as the liquid could be used to transport the fines out of the plant.

DEM, Sinnott explained, was a computational technique which allowed particle flows in various types of equipment to be simulated. Its simulation involves following the motion of every particle in the flow and modeling each collision between the particles and between the particles and their environment (such as the mill liner, the grate or pulp lifters). This had the advantage of being able to simulate equipment under very controlled conditions and make detailed predictions of specific outputs while providing insight into the flow packages and breakage processes.

"DEM has now reached a level of sophistication that allows millions of particles to be simulated, providing detailed information on the local breakage environments experienced by particles of different sizes in different parts of the mill," he said. The grinding chamber for each of the mills was similar, with the tower facility employing a double helical steel screw agitator (which stirs the media while simultaneously lifting and circulating it throughout the mill). The screw design places an upper limit on rotational speed and, when it does reach excessively high speeds, the media experiences too much lift and ball-ball grinding motion is reduced. Meanwhile the pin agitator, consisting of a central shaft with rows of pins (offset to each other), is attached. This typically runs at higher speeds than the tower mill and the pins are believed to push media around the mill in circular paths.

Sinnott said DEM simulations were run using the geometrics for each chamber and agitator as boundary conditions. Dry media (ceramic balls) were modeled with the assumption of no breakage/attrition of the balls. A coefficient of restitution of 0.3 and a fraction co-efficient of 0.5 were used for ball-ball and ball-liner collisions. Preliminary DEM models of the mills, Sinnott noted, revealed that their dynamics were very different.
The tower mill, he said, was relatively gentle with gravity being its key driving force, while for the pin mill the main force was centrifugal. This led to very different flow patterns in the mills, different distributions of energy absorption and, ultimately, different grinding behaviour. Specifically, the rate of energy absorption by the media from normal impact collisions decreased with depth in the tower mill, but rose with depth in the pin mill.

"It is shown that the structure of the media flow patterns and of density variations in the packed beds of these mills can be very well explored using isosurfaces of the key variables arising from collecting stress and motion data collected on an Eulerian grid co-moving with the agitators," Sinnott added. "This isosurface technique is expected to be a powerful tool for understanding the internal dynamics of media and rock motion on stirred and other high intensity mills."

Papers presented at the conference are available on a Proceedings CD-ROM from MEI (www.min-eng.com/comminution06/paps.html), and selected papers will be published in a special issue of Minerals Engineering later in the year.

Mark Fraser, Gold & Minerals Gazette, Australia. Email: mfraser@riu.com.au

More on the Comminutioní06 Conference can be found in the April2006 edition of Gold & Minerals Gazette.



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