The gold absorption properties of shaly material from Beatrix Gold Mine

Abstract

Shaly material from Beatrix Gold Mine 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. Mineralogically the shales comprise muscovite, chlorite, pyrophyllite, and chloritoid which suggest that the shale is a low grade metamorphic rock. Two types of carbonaceous material are present in low concentrations and resemble kerogen. The one type can be described as filamentous and the other as roundish. Many workers gave convincing evidence that kerogen from Witwatersrand rocks are of biogenetic origin and that it resembles coal in many respects. Gold adsorption tests were done at Gencor Process Research Laboratories and also at the University of Pretoria. The aim of these experiments was to find a correlation between the propensity of a shale to adsorb gold and other properties such as its mineralogy and carbon content. 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 were detected in the solutions which suggests that any dissolved Mg and Fe precipitate immediately on the surface of the solids so that gold could 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 + Al203 (corrected for muscovite) in the shales. Rank measurements done on the carbonaceous particles in the shales showed that they fall in the anthracite range. To relate gold adsorption to the rank of carbonaceous material, gold adsorption tests were done on coal of varying rank. These tests showed that anthracite has the highest propensity to adsorb gold. This suggests that the carbonaceous particles would have similar gold adsorption properties as anthracite. The lack of an obvious correlation between the propensity of a shale to adsorb gold and its carbon content is probably due to the fact that not all the carbonaceous particles are liberated during milling so that they can be brought into contact with the gold cyanide solution.