In actually gold ore plant operations, all gravity concentrates are further upgraded using shaking tables. These tables typically yield gold recoveries between 75 and 90%. The gold losses are due to incomplete liberation only in part, and in fact, result mostly from fines and flakes. Fines often float to the tails, whereas flakes tend to report with the heavy minerals in the middlings. When a table processes the concentrate from a Knelson Concentrator, recoveries can be particularly low, because the table, with a gravity acceleration of only 1 ‘g’(9.8 m.s-2), cannot recover all the gold originally recovered by the Knelson Concentrator, which operates with a theoretical acceleration of 60 ‘g’ (588 m.s-2). Furthermore, table operation is labour intensive. which creates a security risk because of the high gold content of the material being processed. Hence, there is a strong incentive to supplement or even replace tables by a unit which requires little or no supervision, such as an automated Knelson CD.
One logical candidate for this duty is the Knelson Concentrator itself, whose efficiency for primary recovery is well documented. However. before the Knelson Concentrator can be proposed for cleaning applications, it is necessary, ideally, to develop a better understanding of how it works -i.e. what are the important mechanisms that are responsible for selectivity. Two additional challenges must be met. First, the gangue is not as easily rejected, since it generally has a much higher specific density. Second, much higher concentrate grades are targeted, 40-70% Au rather than the 0.5 to 3% of most primary concentrates. It follows that the effect of both the specific density and size distribution of the Knelson Concentrator feed should be well characterized and understood in order to mess the impact of the coarser, denser feed. Further, factors capable of lowering concentrate grade, such as short circuiting of feed into the concentrate at the beginning of the recovery stage, require investigation.
For the further upgrading of high grade table concentrates, the traditional approach is smelting. The high grade of the furnace feed, obtained at the cost of recovery during tabling, minimizes smelting costs. However, smelting still remains expensive, environmentally costly and incapable of separating gold from silver, whose losses can be significant. Alternatively. the production of very pure gold from gravity concentrates by hydrometallurgy rather than pyrometallurgy could be attractive. First, smelting and traditional refining would be avoided, and second, gold could be separated from silver.