Removal of Thiosalts from Mill Effluents

摘要

During the grinding and flotation of complex sulphide ores in alkaline media, oxidation of sulphide minerals produces a series of sulphur oxyanions such as thiosulphates (S_2O_2~2), trithionate (S_3O_6~(2-)) and tetrathionate (S_2O_3~(2-)), and finally sulphite and sulphate, which are collectively called "Thiosalts". Thiosalts represent a delayed acid generating capacity in mill effluents that results in a drop in pH and, subsequently, an increase in metal and solids concentrations. The current technology has not been able to prevent the production of thiosalts. Therefore, when required, treatment of the mill water has to be considered to reduce the concentration of thiosalts. Based on the degradation principles of thiosalts, several methods have been developed and investigated for their potential to reduce/remove thiosalt concentrations from mill effluents. Many of the methods have been evaluated only in laboratory scale tests and a few of them has been pilot tested. The treatment options include: Chemical oxidation including hydrogen peroxide (H_2O_2), chlorine (Cl_2) and ozone (O_3); Membrane and electrochemical processes including reverse osmosis (RO), electrodialysis (ED) and electro-oxidation; Air oxidation including alkaline oxidation, Cu-catalyzed air oxidation and SO_2-air oxidation; Biological oxidation including the use of biological contactors, activated sludge and packed columns; and Other methods such as reduction by metals and sea disposal. This communication summarizes the formation of thiosalts in mill effluents and the associated environmental issues and the studies carried out at the Boliden Apirsa s.l. (Spain) site. It discusses the treatment alternatives tested and the method selected to remove/reduce thiosalts from the mill effluent along with their technical and economical feasibility for the given site.