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in the crushing zone. Since the NCRC is not designed to act as a "'high pressure grinding roll" a larger
number of oversize particles would pass between the rolls under these circumstances.
Roll gap
As with a traditional roll crusher, the roll gap setting on the NCRC has a direct influence on the product
size distribution and throughput of the crusher. Figure 7 shows the resulting product size distribution
obtained when the Aurora Gold ore (mill scats) was crushed at three different roll gaps. Plotting the PSO
values taken from this graph against the roll gap yields the linear relationship shown in Figure 8. As
explained previously, the actual roll gap on the NCRC will vary over one revolution. This variation
accounts for the difference between the specified gap setting and product Ps0 obtained from the crushing
trials. Figure 8 also shows the effect of roll gap on throughput of the crusher and gives an indication of the
crushing rates that can be obtained with the laboratory scale model NCRC.
Roll force
The NCRC is designed to operate with minimal interaction between particles, such that comminution is
primarily achieved by fracture of particles directly between the rolls. As a consequence, the roll force only
needs to bc large enough to overcome the combined compressive strengths of the particles between the roll
surlaces. If the roll force is not large enough then the ore particles will separate the rolls allowing oversized
particles to lall through. Increasing the roll force reduces the tendency of the rolls to separate and therefore
provides better control over product size. However, once a limiting roll force has been reached (which is
dependent on the size and type of material being crushed) any further increase in roll force adds nothing to
the 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 bc 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 Fs0 of 34mm to a
Ps0 of 3.0mm (reduction ratio of 11:1), while the finer ore was reduced from an Fs0 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