Comprehensive Treatment For Acid Wastewater Containing Heavy Metals

The wastewater, after recycling acid from stainless steel pickling, usually contains high concentration of sulfuric acid, iron, chromium, nickel and manganese, which makes it poisonous and hard to treat. The traditional method of neutralization precipitation has several defects such as high cost and second pollution. Meanwhile, valuable metal resources cannot be well retrieved and reused which in a certain extent causes hazardous risk. Neutralization precipitation process related improvements were not effectively solve the problem of the separation of heavy metals recycling and comprehensive utilization.Aiming at the high acidity and high heavy metal of the acid wastewater, the step-by-step precipitation method was used to recycling heavy metal ions in waste liquid. The sodium jarosite process and neutralization precipitation process ware applied in recycling iron and chromium. In the sodium jarosite process, sodium carbonate was used to adjust p H to 1.5 ~ 1.9, 7 ~ 8% sodium carbonate solution was used to keep the reaction p H=1.5 ~ 1.9,and the temperature of reaction was 95℃. When the p H of reaction end point was 1.9, the recovery rate of iron and chromium was 97.0%, 67.4. %,respectively. Moreover,the XRD reveals that the iron recovery mechanism, the iron was recycled as sodium jarosite, but the mechanism of precipitation of chromium still need further studies. After recycling the most iron and chromium, neutralizing hydrolysis precipitation was applied in recycling the rest iron and chromium by controlling p H = 6, and the recovery rate of iron and chrome was 99.96% and 99.1%, respectively.After recovery of iron and chromium, sodium dimethyldithiocarbamate was used to precipitate nickel. When the molar ratio of sodium dimethyldithiocarbamate with nickel is 3:1, nickel recovery rate can reach 99.5%. And the rest of the manganese in the solution ware precipitated with sodium carbonate to adjust p H to 10. And the XRD analysis showed that manganese was recycled through manganese carbonate directly.The acid wastewater was treated by step-by-step precipitation method, and the amount of iron, chromium, nickel and manganese content in the solution are: Fe3+ < 0.1 mg/L, Cr3+ < 1mg/L, Ni2+ < 1mg/L, Mn2+ < 5mg/L,which meets wastewater discharge standard. Recovery of iron and chromium slag can be used for steelmaking, nickel and manganese slag can be purified and then sold as heavy metal products. Not only to achieve the effluent discharge standard, but also can make full use of the resources of heavy metals in waste liquid, obtaining good economic benefit.