Metals recovery from industrial wastewaters and metallic hydroxide sludges through microbial sulphate-reduction.

This paper describes how metals originate in industrial wastewaters from the mining and metal finishing industries. Treatment of wastewaters containing dissolved metals produces a metallic hydroxide sludge which has to be disposed of in an environmentally acceptable way. It is estimated that the metal finishing industry annually disposes of 2,500,000 pounds and 49,000,000 pounds of non-ferrous metals in Canada and the United States respectively. These non-ferrous metals are either discharged to municipal sewers as untreated wastewater where they can interfere with the sewage treatment process and can also pose serious problems for sewage sludge disposal, or they can be disposed of as metallic hydroxide sludges in landfills, deepwells, or holding lagoons. The value of the metals contained in these sludges has long been recognized, and technology has been developed for their recovery. This recovery technology is based on well known chemical unit processes such as leaching, selective precipitation, ion exchange, and electrowinning. These processes are capable of recovering metals in a pure form, however, this technology does not appear to be economical at this time due to high energy costs, high chemical cost, and undue competition of this pure product from virgin metals. The paper describes a process whereby these non-ferrous metals can be recovered via microbial sulphate-reduction as metallic sulphides suitable as basemetal smelter feed. The bacterium Desulfovibrio desulfuricans is capable of reducing sulphate sulphur to sulphide sulphur while simultaneously oxidizing organic carbon to carbon dioxide. The hydrogen sulphide thus produced reacts with metal hydroxide sludge leachate to produce a high grade metallic sulphide concentrate suitable for basemetal smelter feed. The process described utilizes three industrial waste streams, namely: metallic hydroxide sludge, waste sulphuric acid, and biodegradable waste organics, as feedstocks for the reaction. Bench scale experiments are described which produced concentrates containing in excess of 28 per cent copper and zinc as sulphides. The paper includes a flow sheet and process design for a pilot plant recovering 1000 pounds per year of non-ferrous metal as metallic sulphides