Leaching free gold with Cyanide

The introduction of carbon-in-pulp technology to the gold industry has rekindled the dormant research and development interests in gold metallurgy. Over the past decade many new developments have emerged which have capital’ :ed on the advances in science and technology in other disciplines. Five years ago, Australian producers were catching-up with the new C.I.P. technology and learning of its idiosyncracies. They quickly realised that Australian conditions did not match those of South Africa and America and that each plant or ore body needed its own refinement Since 1982 several workshop courses on C.I.P. Technology have been presented in South Africa and advances have been made both overseas and in Australia which broaden the scope of investigations as producers struggle to cope with the decreasing grades and increasing complexity of gold ores [4]. Through the Australian Minerals Industry Research Association (AHIRA) and the Western Australian Mining and Petroleum Research Institute (WAMFRI), the industry has funded various applied research projects into various areas of processing – including carbon fouling and regeneration, and gold elution.

Gold metallurgy covers many facets including leaching, concentration on carbon, elution and recovery. This paper attempts to briefly cover each aspect in turn with particular regard to Australian contributions and future directions and developments.

Although leaching with cyanide is not a recent advance, it illustrates the importance of linking fundamental research with plant practice which is essential to all facets of the process. Leaching with cyanide is one aspect which metallurgists have successfully applied for over 100 years yet it is only in the last 20 years that scientists have been able to explain why leaching with CN- is so slow and varies from ore to ore. It is now established that the gold leaching rate is usually diffusion controlled, but depending on conditions, different factors such as CN~ and Oz diffusion, or Oz reduction on the mineral may limit the rate. It is not possible to increase the rate significantly because gold is passivated by cyanide or oxide films.

gold_metallurgy

The mechanism established by Cathro and Koch at CSIRO over 20 years ago involves separate anodic and cathodic processes where gold dissolves via passivating films at one site, and oxygen is reduced at other sites (Scheme 1). Of particular interest was their observation that certain heavy metal impurities can enhance the rate by disrupting the film. This mechanism has been further refined by Nicol in recent years.

The electrochemists can show this heavy metal effect by looking at the corrosion currents of gold at particular potentials. Only trace concentrations of Pb and Hg {~10-6 M) are required to enhance the rate of gold dissolution when typical operating potentials are applied. (Figure 1).

The metallurgist now knows that the use of more CN~, H2O2, O2 or pressure may not give much improvement in rate due to the passive film on gold and that heavy metal additives may help. However many pulps already contain trace heavy metals or may scavange them from solution; thus each ore has to be tested. At the Kambalda plant, for example, Pb(NO3)2 is added to the leach tanks, but it is not clear how effective it is because the presence of sulphate in the water leads to precipitation of PbSO4.

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