GB808637A - Improvements in or relating to the recovery of selenium - Google Patents

Abstract

Selenium is separated from selenium-containing materials, e.g copper refinery anode slimes, by treatment with an alkali metal hydroxide in either molten or dissolved form to obtain a soluble alkali metal selenate; precipitating the selenate in the form of a water-insoluble selenate, preferably the barium, strontium or radium selenate, and especially barium selenate, reducing the insoluble selenate to a soluble selenite, and reducing the selenite in solution to form selenium. When the initial stage is carried out by fusing the material with an alkali metal hydroxide, it is convenient to add an oxidation promoter, preferably selected from the following: hydrazine, cobaltimine ion, lead dioxide, chlorate ion, perchlorate ion, tellurate ion, peroxides, nickel dioxide, hypoiodite ion, manganate, bromate, chlorite, hypobromite, hypochlorite, ferrate, or an acid periodate (H3CO6­) ions, hydroxylamine, chlorine dioxide or ozone. In an example, unroasted copper refinery anode slimes are treated with an alkali-metal hydroxide, either in molten condition or in solution at elevated temperatures and pressures. In the first case, the product is leached, and then in both cases the solution is filtered. To this solution is added a soluble barium compound, preferably the chloride or oxide, thereby precipitating barium selenate, which is collected and washed. The precipitate is now reduced by the action of an acid containing an oxidizable ion, preferably hydrochloric acid, either alone or mixed with sulphuric acid. Hydrobromic and hydroiodic acids may also be employed. When hydrochloric acid alone is used, chlorine is liberated and a solution of barium selenite is obtained. When sulphuric acid is present, barium sulphate is precipitated and a solution of selenious acid is obtained. An alternative procedure is to treat the precipitate of barium selenate with sulphuric acid alone to obtain a solution of selenic acid, and then reduce this to selenious acid with hydrochloric acid. The solution containing the selenite ion is now reduced, preferably by sulphur dioxide, or by zinc powder or hydrogen, to give a precipitate of substantially pure selenium.