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CANMET's thiosalt program, as well as the relevant open literature on thiosalts, is reviewed in a non-critical manner. Prior to this program, very little was known about thiosalts or their behaviour in milling effluents. Analytical techniques previously available usually had been tested only on pure synthetic solutions often with only one thiosalt species present. Adequate sampling and storage procedures were not available. Virtually nothing was known about thiosalt generation in milling circuits. Some studies had been made on the chemistry of thiosalts, especially thiosulphate, and although many of the early studies suffered from analytical uncertainties, they indicated that thiosalt destruction might be achieved by a variety of unevaluated techniques. During CANMET's thiosalt program it was found that thiosalt solutions were somewhat unstable at room temperature; freezing of solutions appeared to be the best method of sample preservation. Colorimetric analytical methods based on selective cyanolysis of thiosulphate, trithionate and tetrathionate contained in mill effluents were developed and evaluated. Titrimetric methods, based on EDTA back titration of barium remaining after selective oxidation of the individual thiosalts to SO4 , were developed. The Noranda mercuric chloride total thiosalt titration method was perfected. Methods were tried to permit the determination of low (<50 ppm) thiosalt levels, but these were subject to large uncertainties. As part of the general analytical work, the infrared and ultraviolet spectra of the dissolved thiosalts were measured; the crystal structures of trithionate and tetrathionate were determined; the crystal structures of trithionate and tetrathionate were accurately measured. The factors affecting thiosalt generation in milling circuits were determined, and the principal ones were grinding pH, sulphide content of the ore, residence time and temperature. Mass balances were conducted at two operating mills to define the importance of the various unit operations. Fourteen methods of thiosalt destruction were evaluated on technical and economic grounds. Although some methods such as electrodialysis were the subject of paper studies only, most of the techniques were extensively examined at the laboratory level and those such as biological oxidation, were tested at pilot scale. When all aspects of the treatment are considered, thiosalt destruction costs become very high; furthermore, none of the methods has yet been proved to be continuously effective for the large-scale treatment of actual thiosalt effluents.

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