Product Sizing of HPGR
As noted earlier, product fineness increases with operating pressure (and therefore power), generally up to a point of diminishing returns. It has been observed elsewhere that it is more energy efficient to operate an HPGR at low pressures and in closed circuit with a screen, so that less energy is wasted on compacting the product. However, this generally would require more or larger HPGRs to handle the increased circulating load. Also, it is not clear whether the analysis included the cost of conveying the increased cir- culating load of screen oversize.
Product fineness generally decreases with increasing “texture” of the roll surface; so smooth rolls give the finest product, with profiled surfaces in the mid-range and studded surfaces the coarsest. This is due to the reduced slip between the rolls and the ore, giving a higher throughput for a given power draw. For the same product fineness, therefore, a studded or profiled roll machine would have to be operated at higher pressures than a smooth roll unit. However, the effect is relatively small, and the benefits of profiled or studded rolls usually outweigh the reduced product fineness. Furthermore, the effect appears to be ore specific, and some operations (e.g., Jwaneng) have recorded an increase in fineness with studded rolls compared to smooth rolls.
Increasing roll speed leads to a reduced product top size and improved F50/P50 reduction ratio, without significantly changing the fine end of the sizing spectrum.
A slight mismatch or differential in roll speeds has been found to enhance grinding performance, and though this could be considered intuitively plausible, it might also be expected that adopting this as a deliberate control strategy could lead to increased roll surface wear rates due to this imposed speed differential. This effect is therefore regarded as being of academic interest rather than practical significance.
Product sizing is largely independent of feed moisture. Product sizing is a function of roll aspect ratio. A high aspect ratio gives an inherently coarser product for the follow- ing reasons:
The proportion of edge material in the product is greater.
The pressure peak in the compression zone is lower (for a given specific pressure). However, the overall effect is generally fairly modest.
The shape of the HPGR product sizing curve is dissimilar to that of conventional crushers, so that for products with nominally the same P80, the HPGR product contains considerably more fines below this size than from a conventional crusher. The implica- tions of this are that, where the product is delivered to, for example, a ball milling oper- ation, mill capacity will be greater when treating HPGR product than predicted by the standard Bond equation. Milling power requirements are thus reduced by both the sizing of the HPGR product and the microcracking of the product particles, and are therefore best determined by pilot testing.
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