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statistical tests, which indicated that at least 2000g of sample needed to be crushed in order to estimate the
product P80 to within +0.1ram with 95% confidence. The product was collected and riffled into ten subsamples,
and a standard wet/dry sieving method was then used to determine the product size distribution.
For each trial, two of the sub-samples were initially sieved. Additional sub-samples were sieved if there
were any significant differences in the resulting product size distributions.
A number of comminution tests were conducted using the NCRC to determine the effects of various
parameters including roll gap, roll force, feed size, and the effect of single and multi-particle feed. The roll
speed was set at maximum and was not varied between trials as previous experiments had concluded that
there was little effect of roll speed on product size distribution. It should be noted that the roll gap settings
quoted refer to the minimum roll gap. Due to the non-cylindrical shape of the rolls, the actual roll gap will
vary up to 1.7 mm above the minimum setting (ie: a roll gap selling of l mm actually means 1-2.7mm roll
gap).
RESULTS
Feed material
The performance of all comminution equipment is dependent on the type of material being crushed. In this
respect, the NCRC is no different. Softer materials crushed in the NCRC yield a lower P80 than harder
materials. Figure 4 shows the product size distribution obtained when several different materials were
crushed under similar conditions in the NCRC. It is interesting to note that apart from the prepared concrete
samples, the P80 values obtained from the various materials were fairly consistent. These results reflect the
degree of control over product size distribution that can be obtained with the NCRC.
Multiple feed particles
Previous trials with the NCRC were conducted using only single feed particles where there was little or no
interaction between particles. Although very effective, the low throughput rates associated with this mode
of comminution makes it unsuitable for practical applications. Therefore it was necessary to determine the
effect that a continuous feed would have to the resulting product size distribution. In these tests, the NCRC
was continuously supplied with feed to maintain a bed of material level with the top of the rolls. Figure 5
shows the effect that continuous feed to the NCRC had on the product size distribution for the Normandy
Ore. These results seem to show a slight increase in P80 with continuous (multi-particle) feed, however the
shift is so small as to make it statistically insignificant. Similarly, the product size distributions would seem
to indicate a larger proportion of fines for the continuously fed trial, but the actual difference is negligible.
Similar trials were also conducted with the granite samples using two different roll gaps, as shown in
Figure 6. Once again there was little variation between the single and multi-particle tests. Not surprisingly,
the difference was even less significant at the larger roll gap, where the degree of comminution (and hence
interaction between particles) is smaller.
All of these tests would seem to indicate that continuous feeding has minimal effect on the performance of
the NCRC. However, it is important to realise that the feed particles used in these trials were spread over a
very small size range, as evident by the feed size distribution shown in Figure 6 (the feed particles in the
Normandy trials were even more uniform). The unilormity in feed particle size results in a large amount of
free space, which allow:s for swelling of the broken ore in the crushing chamber, thereby limiting the
amount of interaction between particles. True "choke" feeding of the NCRC with ore having a wide
distribution of particle sizes (especially in the smaller size range) is likely to generate much larger pressures