Process For Selenium Recovery From Copper Anode Slime By Alkali Pressure Leaching

Selenium, which is a 6A group element of the periodic table, constitutes about 7x10-5 % of the earth?s crust. Even though selenium is a dispersed element with respect to its commercial exploitation, it is capable of forming number of minerals and nearly forty selenium minerals have been identified, out of which the copper ores are the richest sources of selenium. Electronics industries and glass industries are the major consumers of selenium, while pharma grade selenium used as micronutrient for humans and animals. Various methods have been developed to recover selenium as a by-product from copper anode slime during copper refining. Selenium being amphoteric in nature, it is soluble in both acid and alkali. While acid roast process is widely used, considerable work has been carried out in Russia for recovering selenium through alkali route. This paper enlightens the process developed at Central Research and Development Laboratory, for recovering selenium from decopperised slime through alkali pressure leaching. The decopperised slime is pressure leached with alkali under oxidizing conditions by which almost all the selenium present is converted to alkali selenate, which is easily separable from the rest of the decopperised slime. The solution so obtained is neutralized, during which, most of the impurities are precipitated out. Then selenium is precipitated in its elemental form upon purging sulphur-di-oxide under warm acidic medium. The purity of the selenium obtained by this process is higher than that of the selenium from acid roast process. Moreover selenium can be crystallized as sodium selenate and it can be marketed. The acidic content of the effluent is recovered and it is recycled back to the process. The sodium content of the effluent is converted into high purity sodium sulphate, which is marketable. The hazardous waste containing arsenic is precipitated as its sulphide and can be disposed off as landfill.