Extraction of cobalt from historic sulphide tailings using bioleaching

Mineralogical and geochemical characterisation of sulphide minerals in historic tailings can assist in determining whether there are economic and environmental benefits of reprocessing such materials using modern metallurgical practices. At the Old Tailings Dam (OTD), Savage River, Western Tasmania, 38 million tonnes of pyritic tailings were deposited (1967 to 1982), and have since been generating acid and metalliferous drainage (AMD). Long term management options considered for this site have included the establishment of either a water, an hard engineered or a vegetation cover. However, due to geotechnical challenges and the lack of success demonstrated by previous pilot studies, these strategies have not been implemented. A detailed mineral chemistry study indicated elevated cobalt in the pyrite (up to 3 wt. %), however, as this is refractory, recovery via biohydrometallurgical processing was tested.

Bulk tailings samples (n = 4) were collected across the OTD from 0.5 to 1.5 m depth specifically targeting three sulphide-bearing facies, across four physical zones (defined by hydrological regime and grain size). A bulk composite of these materials was subjected to bacterial oxidation using BIOX® bacteria to determine if Co could be effectively leached under standard operating conditions. This study focussed on optimising the recommended procedure for undertaking BIOX® experiments, with pH, temperature and iron concentration in the nutrient medium varied. Our findings show that the most efficient conditions for leaching Co were under low pH (1.3-1.6) at 40°C and using a well adapted 4K or 9K nutrient medium. Using these optimal conditions 100% Co leached from the tailings after 10 to 12 days. Our results suggest that reprocessing these deleterious materials might be a viable option for managing this historic site as part of an integrated tailings management strategy.