Investigating the floatability of sperrylite and its interactions with selected standard and novel collectors

The Bushveld Complex in South Africa contains the largest reserves of platinum group elements (PGEs) in the world. It is composed of the Merensky reef, UG2 reef and the Platreef. Unlike the other reefs that have been largely exploited, the Platreef ores have proved to be difficult to process due to their low content of PGE sulphides and the low association of the PGM minerals with the base metal sulphides. Based on their high association with base metal sulphides in other ores in the Bushveld complex, thiol collectors are used to float PGM minerals. However, relatively high amounts of the PGE arsenide and telluride minerals have been found in the tailing streams of the Platreef concentrators despite being fully liberated. One of the major components in the Platreef ores is sperrylite (PtAs₂), which has been found to be slow floating compared to the other PGMs. This study aimed at determining the reasons for the poor floatability of sperrylite with a view to improving its recovery. Tests were conducted to determine the interactions of the mineral with a range of standard and novel collectors. These included microflotation tests to determine the hydrophobicity of the minerals before and after the adsorption of collectors, calorimetric experiments to determine the enthalpy and extent of collector adsorption onto sperrylite, and electrochemical tests to examine charge transfer reactions between sperrylite and the selected standard collectors.

The results indicated that poor collector adsorption appeared to be the major cause of the poor floatability of sperrylite. Sperrylite had both poor natural floatability (14.5%) and, when conditioned with collectors, the highest recovery of only 26% was observed when using PNBX collector at pH 9. Moreover, the rest potential tests showed that the interaction of sperrylite with standard collectors such as xanthates was poor. Only very minor changes in potential were observed after the addition of the standard thiol collectors under alkaline conditions and rest potentials remained essentially unchanged with a change from basic to acidic conditions, indicating its resistance to surface alteration. Surface alteration of the minerals is key for the chemisorption of collectors to take place on the mineral surface. This was further emphasised by flotation recoveries at acidic conditions, where only certain collectors responded positively to a decrease in pH. This work has highlighted the difficulties associated with sperrylite flotation and provides some direction for future research.