Biologically produced sulphide for purification of process streams, effluent treatment and recovery of metals in the metal and mining industry

One of the best available technologies for the removal of metals from water is in the form of metal sulphides. Metal removal by sulphide precipitation is a well-known process that is characterised by compact residues and very high removal efficiencies. Compared to neutralisation alone the sludge volume is 6 to 10 times lower and the toxic metals are removed to a 0.01–1 ppm level. Furthermore, selective metal precipitation is possible, allowing for separate recovery of valuable metals like copper, nickel, cobalt and zinc from nuisance metals like arsenic and antimony. However, the cost of reagent (NaHS or H2S gas) and safety aspects are often prohibitive.This paper describes a novel biological process for safe and cost effective production of sulphide from elemental sulphur, waste sulphuric acid or sulphate present in effluents. With this technology, gaseous or dissolved H2S is produced on-site and on-demand in an engineered, high rate bioreactor.Experience with industrial applications at metal processing plants will be presented. The technology can serve to selectively recover metals from e.g. bleed streams, leach liquor, effluent streams and acid mine drainage. Lower overall costs and increased safety (no transport or storage of sulphide, production on-demand and at ambient pressure) are the main advantages of this new process compared to its alternatives.