Investigating the reasons for the improvement in flotation grade and recovery of an altered PGE ore when using sodium silicate

Abstract

Certain South African platinum-group element (PGE) ores contain abundant phyllosilicate minerals which result in ultrafine coatings on valuable minerals, increase pulp viscosity, and dilute the grade through the recovery of talc composites. The highlighted problems usually lead to low recoveries and grades within the flotation process. Sodium silicate has been identified to have properties that can address these challenges and improve flotation performance. This study was conducted on a PGE-bearing ore with high quantities of serpentine and talc. The ore had poor PGE flotation recoveries and grades under baseline conditions. Sodium silicate dosages of >1000 g/t were found to significantly improve both PGE recoveries and grades. Additional targeted experiments were performed to decouple the multiple effects of sodium silicate on the flotation of the ore. Microflotation tests highlighted that chalcopyrite (used as a platinum group mineral proxy) recovery was decreased in the presence of finely ground serpentine. Zeta potential experiments indicated that electrostatic attraction of the oppositely charged serpentine and chalcopyrite minerals resulted in a serpentine surface coating of the chalcopyrite, which inhibited collector adsorption. The addition of sodium silicate restored the chalcopyrite recovery, mainly due to the reversal in serpentine surface charge. Secondly, the addition of sodium silicate decreased pulp viscosity as indicated by rheology tests. The ability of sodium silicate to disperse the slurry results in increased recovery due to better gas dispersion and bubble-particle contact within a flotation cell. Lastly, high sodium silicate dosages were shown to act as a depressant as confirmed by microflotation tests of talc and detailed mineralogical analyses. This study elucidated and decoupled the mechanisms whereby sodium silicate can mitigate the problems associated with altered ores, which are becoming increasingly problematic worldwide due to the necessity to mine mineralogically complex ore bodies.