Removal of Cr⁶⁺and Mn²⁺from electrolytic manganese wastewater

A new technology of reduction precipitation combined with crystal seed aeration is proposed for completely removing Cr⁶⁺and Mn²⁺ from electrolytic manganese wastewater by studying the optimum treatment conditions.In the first step, Cr⁶⁺was removed by chemical precipitation by transforming into Cr³⁺using Na₂SO₃ as reductant.Then Mn²⁺ is removed by aeration using MnO₂ as crystal seed.The results are as follows: Cr⁶⁺can be transformed into Cr³⁺completely with Na₂SO₃ dosage of 0.5 g/L, reduction pH value of 4, reduction time of 6 min.Precipitation of Cr³⁺is the most complete when pH value is 8 and the effluent concentration of total chromium is less than 0.5 mg/L from 100 mg/L.After chromium removal, the effluent concentration of Mn²⁺ is less than 0.4 mg/L from 1 000 mg/L when MnO2 dosage is 25 g/L, wastewater pH value is 9 and aeration time is 10 min which meets the first grade national sewage effluent discharge standard (GB8978-1996).