Method for recovering and exploiting waste of the chromic anhydride production

Claim 1

A process for treating the waste residue, resulting from the reaction of sodium bichromate and sulfuric acid in the production of chromic anhydride, to recover chromium values and other products therefrom, such waste residue consisting essentially of sodium bisulfate, sulfuric acid, and a hexavalent chromium compound consisting essentially of chromium trioxide, all of which are water-soluble, and a trivalent chromium compound consisting essentially of chromium bisulfate which is water-insoluble, said process comprising: a. mixing said waste residue with water to form a first aqueous slurry comprising a solution containing sodium bisulfate, sulfuric acid, and chromic acid with the water-insoluble chromium bisulfate suspended therein; b. forming a reaction mixture of the first aqueous slurry obtained in step (a) with an aqueous solution of sodium chromate, said reaction mixture having a specific gravity of about from 1.2 to 1.4 and a pH of about from 2 to 8, and heating said reaction mixture at a temperature of about from 70° to 250° C for about from 10 to 600 minutes, whereby part of the water-insoluble chromium bisulfate is converted into water-insoluble chromium chromate which precipitates out, thereby to obtain a second aqueous slurry comprising a solution containing sodium sulphate and sodium bichromate with the precipitated water-insoluble chromium chromate and the remainder of the unconverted, water-insoluble chromium bisulfate suspended therein; c. curing the second aqueous slurry obtained from step (b), having a pH of about from 2 to 8, at a temperature of about from 70° to 250° C for about from 10 to 600 minutes to convert the remainder of the water-insoluble chromium bisulfate into water-insoluble chromium chromate thereby to obtain a third aqueous slurry comprising a solution containing sodium sulfate and sodium bichromate with the water-insoluble chromium chromate suspended therein; and d. subjecting the third aqueous slurry obtained from step (c) to solid-liquid separation to recover a solid consisting essentially of chromium chromate and an aqueous solution containing sodium sulfate and sodium bichromate.