A marked decline in the gold recovery factor of ore from the Moab Khotsong mine, treated through the Great Noligwa recovery plant, prompted a detailed geometallurgical characterisation of the Vaal Reef A-facies conglomerate at the Moab Khotsong mine. This paper serves to document the results of the investigation.

Six samples of the Vaal Reef A-facies were collected in-situ from the underground operations at Moab Khotsong mine for mineralogical study and metallurgical tests. Laboratory scale leaching tests for gold (cyanide) and uranium (sulphuric acid) were carried out using dissolution conditions similar to that in use at the Great Noligwa treatment plant, followed by further diagnostic leaching in the case of gold.

The gold in the ore is readily leachable with plant recoveries ranging between 89 and 93%. Additional recovery can be achieved through leaching at excess cyanide conditions, for a recovery of 92 to 97%. In addition to this, the recovery of gold can be further enhanced through the pre-treatment of the milled ore with acid prior to cyanidation i.e. the ‘reverse gold leach’ in which uranium is recovered first through acid leaching, followed by cyanidation for gold.

The leach-based recovery of uranium varies between 30 and 64%. This is well correlated with the abundance of brannerite, which is known to be relatively refractory compared to uraninite. However a substantial amount of uranium is also contained in coffinite which leaches faster than brannerite (but more slowly than uraninite). Dissolution of coffinite and thus enhanced uranium recovery may be accelerated by increasing the oxidation-reduction potential of the leach, as well as raising the temperature at which the leaching is conducted.

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