GB187636A - Improvements in the manufacture of chromium compounds - Google Patents

Abstract

Chromite is decomposed by treatment with sulphuric acid and an oxidizing agent, such as chromic acid, at a temperature above 150 DEG C., the oxidizing agent, whether added directly to the reaction-mixture or generated in situ by electrolysis, being present in amount insufficient to oxidize the iron completely. The chromite may first be treated with carbonaceous matter or a gaseous reducing agent at a red heat and part of the iron then extracted with dilute acid. Under the conditions set out above, the chromium and part of the iron are converted into insoluble sulphates, from which the chromium is separated by adding a reducing agent to the diluted reaction-mixture when the ferric sulphate is reduced leaving a residue of insoluble chromium sulphate. This compound may be decomposed by boiling in aqueous suspension with caustic alkalis or alkaline-earths, alkali sulphides or carbonates, alkaline-earth, carbonates or ammonia to yield chromium hydroxide and/or basic carbonate; it may be heated with caustic soda or potash in presence of air and with the optional addition of other oxidizing agents to yield sodium or potassium chromate; or it may be boiled with strong barium-chloride or -acetate solution to yield the corresponding chromium salt and barium sulphate. Alternatively it may be dissolved in dilute sulphuric acid in presence of ferric oxide and a small amount of oxidizing agent and the solution, after reduction of any ferric salt formed, treated with potassium sulphate for the production of chrome alum. Instead of first separating the insoluble chromium sulphate, the mixture of insoluble chromium and iron sulphates obtained in the initial decomposition may be treated directly for the production of chromium oxide. In one process the mixed sulphates are washed and heated with excess of coal in a reducing atmosphere, a mixture of iron sulphide and chromium oxide being obtained. In another process, the mixed sulphates are boiled with a slight excess of alkali-sulphide solution and the black precipitate obtained ignited in a reducing atmosphere at red heat. In a third process the mixed sulphates are calcined and the resulting oxides mixed with coal and heated in a reducing atmosphere. In each case the product is treated with acids to dissolve the iron, and the residual chromium oxide may be dissolved in sulphuric acid in presence of a small amount of oxidizing agent and used for the preparation of other chromium compounds.