The use of heap bioleaching as a pre-treatment for platinum group metal leaching

The majority of platinum group metals (PGMs) are produced from Ni-Cu smelter mattes produced from PGM bearing ores after concentration (a small quantity is recovered from spent catalysts and copper anode slimes). A critical step in the process is a pressure oxidation leach (in either sulfuric acid or ammonia solution) to leach base metals from the matte, prior to a more aggressive chloride leach to leach the residual PGMs. This approach is not feasible for leaching lower grade and secondary materials. The aim of this study is to investigate the use of heap bioleaching as a pre-treatment to PGM leaching from low grade materials to solubilise base metals and liberate PGM bearing minerals for recovery in a secondary leach step.
Two samples, a Platreef flotation concentrate and coarse ore (<6 mm), were column bioleached at 65°C using a culture dominated by Metallosphera. hakonensis. Based on solution assays, extractions in excess of 90% Cu and Ni were achieved from the flotation concentrate, while from the coarse ore 96% Cu and 67% Ni extractions were achieved. The difference in extraction levels and leaching patterns despite identical conditions used for both samples is discussed, as is the performance of the samples during a follow-up leach step using cyanide to extract the PGMs in a separate column leach experiment. While the recovery of Pt and Au was excellent during these steps, recovery of Pt was limited to 35% after 45 days for the concentrate and 56% after 60 days for the whole ore material, primarily due to the presence of a refractory Pt mineral. Recovery from a concentrate without pre-treatment was substantially lower.