Effluent purificn from chemical treatment of metal surfaces - using hydrosulphite ions to eliminate hexavalent chromium ions

摘要

The purificn. of waste effluent from a multi-stage chemical treatment for surfaces of metal componens comprises degreasing, chemical conversion and passivation with interstage and final rinsing. Effluents from the treatment stages are all mixed together with rinsing effluents and the mixt. is dosed with a quantity of hydrosulphite ions to reduce the chromium ions in the mixt, the pH being adjusted to 5-8. 5, after which the mixt. is submitted to flocculation and decantation treatments. When the chemical conversion stage produces a crystalline phosphatation an alkalien hydrosulphite should be used, p ref. Na2S2O4, and when the phosphatation is amorphous, zinc hydrosulphite is used instead. Used for purifying waste effluent from a multi stage, chemical treatment plant used to modify metal surfaces, partic. iron, steel, zinc and aluminium in order to improve corrosion resistance and paint adhesion. Pretreatment of the effluent from the passivation stage before mixing with the other effluents is avoided.