Reduction of chloride emission by thickening of metallurgical wastewater

Purification and thickening tests of metallurgical industrial wastewater from sintering dust dechlorination unit were carried out. Raw wastewater of pH 6.95 consisted primarily of Zn2+, Pb2+, Cd2+, Tl+, Cl-, SO4-2, at concentrations of approx. 3,200; 60; 11,500; 100; 40,100 and 5,300 mg/dm3 respectively. The heavy metals removal consisted in chemical precipitation and sedimentation, firstly by sodium hydroxide, and secondly by sodium sulphide solutions resulting in average removal to levels below 1mg/dm3. Moreover, the experimental studies of evaporation process confirmed the possibility of chloride ions reducing, the distillate yield was approx. 80% of the initial effluent stream. The solid product obtained by two-stage evaporation was characterized by satisfactory purity. The results indicate maturity of the applied processes for wastewater treatment, allowing conditions close to zero-liquid-discharge to be achieved.