Removal Of Cr⁶⁺ And Mn²⁺ From Electrolytic Manganese Wastewater

Abstract:The electrolytic manganese wastewater contains high amounts of manganese and chromium. When treated by conventional technologies, effluent concentrations of heavy metals were difficult to comply the discharge standard, and the sludges were instability, easy to create secondary pollution. This study chose three new technologies, reduction precipitation combined with MnO2aeration, MnO2aeration combined with ambient temperature ferrite process, and ambient temperature ferrite process, to remove manganese and chromium from the electrolytic manganese wastewater. The effects of different factors on heavy metals removal from simulated wastewater and sludge stability were investigated. The results are as follows:(1) Treatment of electrolytic manganese wastewater using reduction precipitation combined with MnO2aeration:Treatment has three-staged procedures. In the first step, Cr6+was reduced to Cr3+by chemical reduction reaction using Na2SO3as reductant when the simulated wastewater was treated at reduction pH4for6min in the presence of0.5g/L Na2SO3. In the second step, Cr3+was removed in the form of Cr(OH)3by chemical precipitation reaction when the simulated wastewater was treated at precipitation pH8for10min. Then the last step, Mn2+was removed by aeration using MnO2as removal agent when the simulated wastewater was treated at aeration pH9, for10min in the presence of25g MnO2.(2) Treatment of electrolytic manganese wastewater using MnO2aeration combined with ambient temperature ferrite process:Treatment has two-staged procedures. In the first step, Mn2+was removed by aeration using MnO2as removal agent when the simulated wastewater was treated at aeration pH9, for10min in the presence of20g MnO2. In the second step, Cr6+was removed by ambient temperature ferrite process when the simulated wastewater was treated at reaction pH8, water bath25℃for10min in the presence of300mg/L Fe+ (3) Treatment of electrolytic manganese wastewater using ambient temperature ferrite process:Treatment has two-staged procedures. In the first step, Cr6+and Mn2+were removed by ambient temperature ferrite process when the simulated wastewater was treated at reaction pH9, for40min in the presence of300mg/L Fe2+. In the second step, more purer manganese ferrite could be obtained by sludge aging process when the sludge generated from the first step was treated at sludge aging pH12for30min stirring. Then the sludge generated from the second step, as crystal seed, was returned to the reactor to induce the formation of more manganese ferrite.Under these conditions, effluent concentration of total chromium was less than0.187mg/L from100mg/L, total chromium removal rate below99.81%, effluent concentration of Mn2+was less than1.253mg/L from1000mg/L, and manganese removal rate below99.875%, reaching the national sewage effluent discharge standard (GB8978-1996). MnO2produced from the aeration process could be reused as catalyst and absorbent. The sludge produced from ambient temperature ferrite process, safe and nontoxic, could be reused as magnetic materials. It is a reproducible and available approach to completely recycle manganese and chromium.