size distributionIsaMills’™ superior grinding productThe IsaMill™ operates by stirring fine media at high speed (over 20m/s tip speed). The fine media and high disc speed increases the probability of media and particle collisions, as well as the energy of these collisions. The IsaMill™ breaks particles using both attrition and abrasion breakage modes. Particles are chipped repeatedly to produce a fine sized product at relatively low power consumptions.
The excellent size reduction performance of the IsaMills™ is a result of using small media in the order of 1 to 4mm. Smaller media means a lot more grinding events. For example, 730 spheres of 1mm media occupy the same volume as one 12mm ball. Therefore, an IsaMill™ using 1mm media will have 730 times more media particles per cubic metre than a tower mill using 12mm balls. This explains the high efficiency of the IsaMill™. Another feature of the IsaMill™ is
very high power intensity – it applies 300kw/m
3 of grinding volume, compared with a maximum of 20kw/m
3 in a Tower Mill or ball mill. This high power intensity has big implications for circuit layout and capital costs – a 3MW IsaMill™ has a grinding volume of only 10 cubic metres, low head height, simple crane needs, and doesn’t need closed circuit cyclones.
Grinding performance of an IsaMill™ is fundamentally different from a ball mill. The impact mechanism and long residence times in ball mills causes indiscriminate grinding and poor size distributions. This leads to slimes generation, higher reagent consumption and energy wastage.
In contrast, the IsaMill™ produces a different size distribution. Sequential grinding in the IsaMill™ doesn’t flatten the size distribution, it
sharpens it, even in open circuit. This remarkable characteristic has crucial implications for energy efficiency and downstream processing – the grinding enegy is focussed on the coarse particles that need it, not wasted creating ultrafines. There are four reasons for this:
- The small media size which favours attrition grinding.
- Each ore particle has to pass through 8 consective grinding chambers (the grinding discs), ensuring no short circuiting.
- Particles have to escape the high centripetal forces of the product separator (20m/s tip speed) before leaving the mill (this is why IsaMills™ do not need to be closed circuited with cyclones).
- The high power intensity means that all of the above happens in a short residence time. Typical average residence time in an IsaMill™ is only 90 seconds, leaving little opportunity for overgrinding.
The combination of tight size distribution, small footprint, and inert media has profound implications for circuit design. Mills can be easily distributed throughout a flotation circuit, grinding only those particles that need it and producing a tight size distribution (without cyclones) ideal for subsequent flotation. The use of inert media means that rather than harm flotation chemistry, it is often assisted by the grinding step.
Similarly the tight size distribution is crucial in leaching applications, where recovery is usually determined by the coarser end of the size distribution (best measured by P98 or P95, not P80).
The product separator of the IsaMill™ is one of its key advantages. The 8 consecutive grinding chambers, and high tip speed of the separator means that the mill constantly delivers sharp sizing performance. There are no screens or trommels to fail or block, any particle that leaves the mill must escape the same centrifugal forces. A further advantage of this mechanism is that it allows the IsaMill™ to use a wide range of natural media. Even if a portion of the media degrades early, it can exit the mill without blocking screens, with the remaining competent media retained for grinding. In contrast, mills closed by fine screens are limited to using only the most competent media.
