The recycling of pebbles back to the SAG mill, be they crushed or uncrushed, displaces new feed. The extent to which new feed is displaced has been observed to increase with the size distribution of the recycled pebbles, the extreme case being the return of uncrushed pebbles to the SAG mill. An example of the impact that the return of uncrushed pebbles can have on plant throughput is shown in Figure 16. These results were obtained during the course of a 12 hour shift. These results have not been calibrated to account for the effect of any change in new feed ore characteristics on plant throughput during this period. Regular control tests were not conducted throughout the shift where by the plant throughput was measured without the recycle of pebbles. The plant throughput rate when uncrushed pebbles were recycled to the SAG mill however was well below the normal operating rate
A 12 day on-off trial involving the rejection of pebbles was also undertaken. Table 4 shows the overall results from Train 2 for this period. When pebbles were not recycled, the new feed rate increased by 15.4 per cent, however plant feed decreased by 6.7 per cent. This is because the increase in the new feed rate is not sufficient to compensate for the loss of recycled pebbles, despite the pebble production rate decreasing when pebbles are not recycled.
The new feed displacement ratio averaged 0.57, ie for every tonne of crushed pebbles recycled to the SAG mill, 0.57 tonne of new feed was displaced. This ratio is less than 1.0 because despite the pebbles having a greater resistance to breakage than the new feed, they are significantly finer. The new feed displacement ratio can be used to estimate the increase in plant throughput that would occur if the crushed pebbles were sufficiently fine to enable open circuiting of the SAG mill. This could be achieved by close circuiting the recycle pebble crushers with a screen, the undersize of which reported to the mill discharge hopper.
As an example, if 290 t/h pebbles were not returned to the SAG mill, the plant throughput rate would increase by 165 t/h, provided the downstream processes could cope with such an increase. Such a number should be used with caution however, because the ratio is not only dependent on the size and breakage characteristics of the pebbles being recycled, but also on the size and breakage characteristics of the new feed。
When pebbles were not recycled, the percentage of new feed that discharged from the SAG mill as a pebble was observed tobe 19.1 per cent. If an assumption is made that this percentage of new feed that reports as a pebble remains constant when pebble sare recycled, the source for the remainder of the pebbles can be attributed to the pebble recycle stream. Given this assumption,28.6 per cent of the crushed pebbles remained too coarse to passthrough the SAG trommel/pebble screen when they again discharged from the SAG mill.
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