Process for the recovery of chromic anhydride from spent aqueous solutions from chromium-plating baths with value enhancement of the recovered chromium and application to tanning baths

摘要

The chromic anhydride contained in the soluble form (chromic acid) in spent and contaminated chromium-plating liquids is recovered by basifying the spent chromium-plating liquid with a 45 % commercial sodium hydroxide solution and at 70 DEG C until a pH in the range ranging from 3 to 7 is obtained, by keeping the mass in reaction at this temperature for one hour, by then separating by filtration the precipitate obtained from a solution, at the density 1.25 kg/l, containing CrO3 (22 % by weight); the hexavalent chromium in solution is then reduced with sulphur dioxide, a solution containing basic chromium sulphate and sodium sulphate being obtained, which solution may be used as a tanning bath; pollution is thus avoided and the reject chromium is enhanced in value. The solution containing CrO3 may also be fitted into a cycle for the manufacture of hexavalent chromium salts.