Comparative study on different industrial oxidic by-products as neutralising agent in bioleaching

Abstract: A comparative study on bioleaching of a pyrite concentrate using ten different industrial oxidic by-products as neutralising agent has been performed with a commercial grade slaked lime chemical serving as reference material. The acid produced during oxidation of pyrite was neutralised by regular additions of neutralising agent whenever needed to maintain a pH of 1.5. Bioleaching was conducted as batch experiments in 1-L scale reactors, with a mixed mesophilic culture at a temperature of 35º C. The different industrial oxidic by-products used were steel slag, ashes, dust and lime sludge. The aim of the study was to investigate the possibility to replace normally used lime or limestone with oxidic by-products, considering their neutralising capacities and possible negative impact on the bacterial activity. The bioleaching efficiency was found to be equally good or better, when by-products were used for neutralisation instead of slaked lime, and the bioleaching yields of pyrite were in the range 69-80%, except the Waste ash, which had a leaching yield of 59%. Some of the by-products used contained potentially toxic elements for the bacteria, like fluoride, chromium and vanadium, but no negative effect of these elements could be observed on the bacterial activity. The Waste ash contained a large number potentially toxic elements and a high chloride concentration of 11%, which had a negative effect as observed on the lower redox potential and leaching yield. Slags originating from stainless steel production should be avoided for environmental reasons, due to the presence of chromium. The electric arc furnace (EAF) dust has a good potential to be used as neutralising agent in bioleaching processes for zinc recovery from zinc sulphides, due to the high content of zinc, however the chlorides present should be removed prior to its use. The neutralising capacity, as determined by the amount needed for neutralisation during bioleaching, were rather high for all the steel slags, EAF dust, Bioash and Mesalime with a range of 16-37 g as compared with 22 g needed for slaked lime. However, Waste ash and Coal & Tyres ash had lower neutralising capacities with 81 g and 57 g needed, respectively. Hence, it is concluded that considerable savings in operational costs can be obtained by replacement of lime or limestone with steel slag, ash, dust or sludge without negative impact on bioleaching efficiency. Use of industrial oxidic by-products would provide opportunities to recycle elements present in them as for example zinc rendering an eco-friendly process and a means for sustainable use of natural resources.