The initial step oriented to determine accurately the relationship of fineness of primary grind and gold recovery is the grind-recovery curve. It is of great importance to optimize the throughput and product quality. The number of samples on which this should be done varies from one gold project to another gold project. If we consider a flotation process, the tests are oriented to gold rougher flotation only, not only for simplicity, but also because for this purpose it may be reasonably be assumed that the gold rougher concentrate will be sent to the cleaner circuit and the final concentrate will be sold or leached. In the second case, the concentrate will be reground to a fineness which is independent of the primary grind and the gold losses in the leaching circuit are independently of the rougher grind.
Basically, the testing program may be complicated by the fact that there are interactions between fineness of primary grind and other variables in flotation such reagent dosages and flotation time. In this way the metallurgist must determine whether one carefully selected set of standardized flotation conditions may be employed for the entire range of fineness of primary grind or whether other factors must be included as in a factorial design approach. If the gold ore will be leached directly, the reasoning is similar. In order to analyze the results, it is suggested that the percentage of gold recovery must be plotted against the 80% passing size. Thanks to Fred Bond, the relationship between grinding energy and fineness of primary grind is straightforward engineering even if not yet an exact discipline.
A study of several gold projects suggests that the typical grind-recovery curve approximates a straight line over the range of fineness usually considered. The line will be curve, sharply downward starting somewhere finer than 80% passing 100 microns, because of the of the ultrafine liberation problem, may in some cases curve downward at the coarse end of the flotation or cyanidation range. The projection of the of the straight line portion apparently intersects the recovery base line at or near the point representing 100% recovery of gold in the ore. The slope of the straight line is a measure of the fineness of dissemination of the gold ores.
Effect of primary grind on gold recovery and grinding power