Morphological analysis
of alluvial gold recovered from the same segment of a placer by four common
commercial methods indicates that gold shape and the presence of quartz
intergrowths or sulphide overgrowths substantially influence the recovery
efficiency of each method. The amount and types of gold saved by each method
varies according to the different hydrodynamic conditions under which each of
the methods operates. Reverse circulation drilling has the lowest efficiency and
preferentially loses moderate-high flatness gold and intergrown or overgrown
gold particles with relatively low density. Gravity riffle, hydraulic riffle,
and jigcentrifugal concentrator systems respectively save progressively more of
these problematic gold morphotypes, but still lose significant and sometimes
substantial amounts (of profit) to the tailings. Because of the differences in
recovery efficiency, reverse circulation drilling probably underestimates the
grade of most placers and may have resulted in bypassing several economic
deposits. Losses to tailings by commercial recovery plants arise from
inadequacies in fundamental plant design features. These inadequacies compromise
plant efficiency by limiting the ability of plants to simulate the wide range of
hydrodynamic conditions necessary to operate effectively throughout placer
systems. Incorporation of sand pumps and multiple recovery methods into the more
sophisticated plants only partly overcomes these inadequacies. At best,
efficiency will be maximised in a few segments of the placer determined by
individual plant design, and will decrease progressively upstream and downstream
of those segments. Without prior knowledge of the morphological characteristics
of the gold to be mined, it is difficult to choose the most appropriate plant
design, or to estimate what proportion of the gold will be recoverable by any
given plant. If the relevant relationships are not considered beforehand, the
recovery plant is likely to lose a significant proportion of the gold, and
sacrifice maximum profit.