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th performance of the roll crusher. This is demonstrated in Figure 9，which shows that for granite feed of 25-3 Imm size, a roll force of approximately 16 to 18 kN is required to control the product size. Using a larger roll force has little effect on the product size, although there is a rapid increase in product P80 if the roll force is reduced bek>w this level.
As mentioned previously, the feed size distribution has a significant effect on the pressure generated in the crushing chamber. Ore that has a finer feed size distribution tends to "choke" the NCRC more, reducing the effectiveness of the crusher. However, as long as the pressure generated in not excessive the NCRC maintains a relatively constant operating gap irrespective of the feed size. The product size distribution will, therefore, also be independent of the feed size distribution. This is illustrated in Figure 10，which shows the results of two crushing trials using identical equipment settings but with feed ore having different size distributions. In this example, the NCRC reduced the courser ore from an FsO of 34mm to a PsO of 3.0mm (reduction ratio of 11:1), while the finer ore was reduced from an FsO of 18mm to a Pso of 3.4mm (reduction ratio of 5:1). These results suggest that the advantages of using profiled rolls diminish as the ratio of the feed size to roll size is reduced. In other words, to achieve higher reduction ratios the feed particles must be large enough to take advantage of the improved nip angles generated in the NCRC.
Mill scats
Some grinding circuits employ a recycle or pebble crusher (such as a cone crusher) to process material which builds up in a mill and which the mill finds hard to break (mill scats). The mill scats often contain worn or broken grinding media, which can find its way into the recycle crusher. A tolerance to uncrushable material is therefore a desirable characteristic for a pebble crusher to have. The NCRC seems ideally suited to such an application, since one of the rolls has the ability to yield allowing the uncrushable material to pass through.
The product size distributions shown in Figure 1 1 were obtained from the processing of mill scats in the NCRC. Identical equipment settings and feed size distributions were used for both results, however one of the trials was conducted using feed ore in which the grinding media had been removed. As expected, the NCRC was able to process the feed ore containing grinding media without incident. However, since one roll was often moving in order to allow the grinding media to pass, a number of oversized particles were able to fall through the gap without being broken. Consequently, the product size distribution for this feed ore shows a shift towards the larger particle sizes, and the PsO value increases from 4ram to 4.7mm. In spite of this, the NCRC was still able to achieve a reduction ratio of almost 4:1.
Wear
Although no specific tesls were conducted to determine the wear rates on the rolls of the NCRC, a number of the crushing trials were recorded using a high-speed video camera in order to try and understand the comminution mechanism. By observing particles being broken between the rolls it is possible to identify portions of the rolls which are likely to suffer from high wear and to make some subjective conclusions as to the effect that this wear will have on the perlbrmance of the NCRC. Not surprisingly, the region that shows up as being the prime candidate for high wear is the transition between the flat and concave surfaces. What is surprising is that this edge does not play a significant role in generating the improved nip angles. The performance of the NCRC should not be adversely effected by wear to this edge because it is actually the transition between the fiat and convex surfaces (on the opposing roll) that results in the reduced nip angles.
The videO also shows that tor part of each cycle particles are comminuted between the flat surfaces of the rolls, in much the same way as they would be in a jaw crusher. This can be clearly seen on the sequence of images in Figure 12. The wear on the rolls during this part of the cycle is likely' to be minimal since there is little or no relative motion between the particles and the surface of the rolls.