The influence of humic substances on the weathering of PGE in chromitite of the Bushveld Complex: An experimental simulation of the weathering environment

Organic acids, including humic acid, play a significant role in the weathering of minerals containing metals such as Pt and Pd. They are also among the reactants which are under consideration for new hydrometallurgical methods of liberating unconventional PGE ores (such as the oxidised ores of the Great Dyke, Zimbabwe, and at locations in the Bushveld Complex of South Africa where PGE-bearing layers have been exposed to weathering). In order to better understand the processes operating during weathering of PGE-bearing chromitite seams of the Bushveld Complex, chromite concentrate originating from a South African chromium mine was subjected to reaction with different concentrations of synthetic humic acid. The results confirm the greater mobility of palladium in the environment compared to platinum. Crushed chromite concentrate showed greater mobility of Cr, but not of Pd or Pt, compared to uncrushed concentrate. Increasing the concentration of humic acid increased the amount of Pd and Pt in solution. These experiments give insight into the processes that govern the weathering of chromitite in the Bushveld Complex. The main Pd– and Pt-bearing minerals are not enclosed within chromite but occur at grain boundaries. Thus, they can be liberated by disaggregation of chromite and infiltration of water along chromite grain boundaries. Once in solution, Pd is more mobile than Pt and is dispersed further. Organic acids play an important role during the weathering process as they are capable of enhancing the mobility of the PGE, particularly Pd.