The targeted yield is determined by the distribution of specific gravities (SGs) in the ore and the SG of the heavy minerals to be recovered. From these a cut point, which represents the center of the separation, is chosen. This point is expressed as a density (specific gravity) or typically as the effective density of separation. For instance, the cut point may be set at a SG of 3.2. Allowance for imperfection in the liberation of the mineral and unit recovery of the concentrator means that it is generally necessary to produce higher than optimum yields to force higher recoveries.
In order to force higher yields to concentrate the cut point is reduced, resulting in a lower grade concentrate but at a higher recovery. In the case of gold, with an effective SG between 16 and 19, the density differential is high, which should in theory result in a simple separation. However, the key factor in gold’s response to gravity is its aspect ratio or shape factor. Whether comminution is carried out in tumbling mills or in the natural environment in which alluvial gold is found, malleable gold particles eventually assume a shape with a high aspect ratio, variously identified as flakes, platelets or lamellae. This shape, combined with the natural hydrophobic nature of gold, impacts severely on its potential for gravity recovery. On the other hand, sulfides, inclusive of gold carriers, tend to have a considerably lower SG but grind in a far more equi-axed fashion, which in turn aids in gravity recovery. This relationship of factors adds to the relative density or in other words the way in which a particle acts when settling in a fluid after all characteristics of density, aspect ratio and surface conditions are considered.
Fig. 1 shows a typical yield/recovery curve that represents the response of the ore to a shaking-table separation. The left-hand side shows the gold recovery relative to the yield or mass recovered to concentrate, and the right hand chart shows the reduction of gold grade in the concentrate as the yield is increased. The sink/float result with a SG of 2.9, which is indicative of a ‘perfect’ separation, is also shown. Results of Fig. 1 are below those normally obtained by heavy liquids, not only reflecting the limitations of single g separation, but also the size, shape and liberation issues previously discussed.
The better an ore responds to gravity the further the curve will bend to the top left corner of the chart. The initial ‘low-yield’ points tend to represent the GRG component of the ore while the initial curve represents the lower density particles and the near-density material, i.e. the gold carriers.