Thiosalt Stability - Impact on AMD

Thiosalts are sulphur oxides and include thiosulphate (S₂O₃^2-) and polythionates (S_xO₆^2- where 3≤x≥10). During the milling, grinding, and floatation of sulphide ore partial oxidation occurs forming thiosalts. Thiosalts can pass through traditional effluent treatment systems relatively unaffected and oxidize to sulphuric acid in water bodies. Studies indicate that thiosalts themselves are not toxic but, rather, the problem is the resulting pH depression. It is, therefore, important to accurately determine the reaction rates of the formation of thiosalts, degradation pathways/rates, and speciation as a function of the pH, temperature and other parameters in the receiving water body. There was a lack of comprehensive data over the range of temperatures and pHs of interest, particularly typical ambient conditions. The thiosalt degradation/formation in an aqueous environment independent of other factors in the treatment ponds which could accelerate/retard thiosalt reactivity was studied initially.

We have studied the behaviour of the most common thiosalt species found under pond conditions both thermodynamically and experimentally. In these experiments the thiosalts have been studied under aqueous conditions individually and in mixtures at pH from 2-9 and temperatures from 4 to 30^oC. The pH and temperature ranges were chosen to represent conditions associated with the pond. We also performed experiments to test the stability of our samples during the freeze/thaw process and storage prior to analysis and also to initiate a series of experiments we plan to do to mimic freeze/thaw cycles in the ponds. As part of this research we wanted to be able to quickly and accurately measure thiosalt species and also other species formed in reactions. A capillary electrophloris (CE) combined with a UV detector was employed. The experiments were successful in confirming already identified trends for some species and qualifying behaviour of thiosalts under pH and temperature conditions encountered in the pond