Gold losses from cyanide solutions Part I: The influence of the silicate minerals

Shale material from Beatrix Gold Mine in South Africa has been found to be capable of adsorbing gold from cyanide solutions. Black shale bands occur in the reef zone at Beatrix Mine, but because of the mode of occurrence of the shale bands selective mining cannot be practised and ore delivered to the plant is contaminated by shale. Petrographical investigations and gold adsorption experiments were undertaken on samples of these shales, in an attempt to quantify their gold adsorption properties. logically the shales comprise muscovite, chlorite, pyrophyllite, and chloritoid which suggest that the shale is a low grade metamorphic rock. Gold adsorption experiments have shown that over time there is a drop in pH of the gold cyanide solutions. This is caused by decomposition of the phyllosilicates in the strongly alkaline solutions. Analysis of these solutions showed that high amounts of Si and Al are present. No Mg and Fe have been detected in the solutions which suggests that any dissolved Mg and Fe precipitate immediately on the surface of the solids so that gold can co-precipitate with colloidal Mg(OH)2 and Fe(OH)3. It was found that a tendency exists for high gold adsorption values to be preferentially related to high percentages of FeO (corrected for pyrite) + MgO + Al2O3 (corrected for muscovite) in the shales.