Influence of the nutrient medium composition during the bioleaching of polymetallic sulfidic mining residues

Although bioleaching, i.e. the extraction of metals using microorganisms, remains a niche application, it has garnered growing interest as an ecologically acceptable and economic alternative to conventional processes such as pyrometallurgy or conventional hydrometallurgy. In particular, bioleaching is envisaged as a viable option to treat materials that in the past would have been considered as waste. The objectives of reprocessing mining wastes are both the recovery of remaining elements as well as preventing their potential environmental impacts. During such process, the requirements in carbon and nutrients for microbial growth and activity should be carefully determined, as they may be involved in side reactions or have non-negligible economic and environmental costs. This study aimed at determining the ammonium and potassium requirements to optimise the bioleaching performances of mining residues from Sotkamo (Terrafame, Finland). Co, Cu, Ni and Zn dissolution kinetics were evaluated in 2 L stirred tank reactors at 20% (w/w) solid with various nutrient media. By analysing the pregnant leach solutions and the residues, ammonium and potassium consumptions for microbial growth and activity were estimated. This study showed that using high concentration of potassium in the nutrient medium was not necessary, as there were some K-bearing minerals in the mining waste that could release potassium to enhance the microbial activity. On the contrary, amending the nutrient medium with high concentration of ammonium was required to ensure high bioleaching kinetics.