Catalysed air oxidation of thiosulfate and tetrathionate for pollution abatement

Abstract

Thiosulfate and tetrathionate are oxidized to sulfate by air at atmospheric pressure in the presence of CuS, Cu 2S or chalcopyrite (CuFeS 2) concentrate under mild temperature conditions (30–60°C). In the presence of CuS and Cu 2S, sulfate is produced from thiosulfate by series-parallel reaction paths involving tetrathionate as an intermediate product. However, in the presence of chalcopyrite which contains ferric iron, thiosulfate is rapidly and quantitatively oxidized to tetrathionate which then undergoes slow oxidation to sulfate. Of the three chalcopyrite concentrates used in this work; viz., Opemiska, Bethlehem and Lornex, best results were obtained with the Opemiska concentrate. In the presence of this material used as a 15% slurry in water, 450 ppm thiosulfate (S 2O 3 2−) is completely oxidized to sulfate by air in 24 hours at 30°C and in 9 hours at 40°C. Use in slurry reactors in continuous operation over several weeks shows that the catalytic activity of the chalcopyrite concentrate remains constant, obviating the need for any regeneration treatment. The rate data obtained with CuS, Cu 2S and chalcopyrite concentrates agree well with a pseudo-homogeneous first order kinetic scheme, yielding values of rate constants for series-parallel reaction paths which have been used in modelling, successfully, the catalyzed air oxidation of thiosulfate.