Abstract: China is the world's largest producer of stainless steel. The production process of stainless steel generates a large amount of pickling wastewater, which contains high concentrations of strong acids and heavy metal salts. If discharged directly, it will seriously pollute the environment and cause resource waste. Therefore, this study designs a combined method of "diffusion dialysis + three-step precipitation" for the recycling treatment of stainless steel pickling wastewater. Firstly, the diffusion dialysis technology was adopted to separate the high-concentration mixed acids (including HNO3 and HF) from the heavy metal salts (including iron, chromium, and nickel heavy metal salts) in the wastewater to recover the mixed acids. Subsequently, a three-step precipitation method was used to recover heavy metals (including iron, chromium, and nickel) from the residual liquid of diffusion dialysis. The influence of ion-exchange membrane type, feed flow (the wastewater) and its flow ratio (diffusion side to dialysis side) were investigated on both the mixed acid recovery rate and heavy metal retention rate in the diffusion dialysis process. Meanwhile, the three-step precipitation technics was optimized including the precipitant composition, precipitation temperature and pH value, etc. The research results showed that the optimal process for diffusion dialysis is: TWDDA3S anion exchange membrane from Shandong Tianwei Company was adopted, the feed flow was 6 ml/min (wastewater), and the flow ratio of the diffusion side to the dialysis side was 1.5:1. Under these conditions, the total recovery rate of nitric acid and hydrofluoric acid could reach up to 69.5%. Meanwhile, the retention rates of the three heavy metal ions including iron, chromium and nickel reached 97.0%, 98.0% and 96.4% respectively. The optimal process for the first-step precipitation is as follows: the mole concentration ratio of the composited precipitants of KOH and KF is 7:1, and the precipitation temperature is 15℃. Under these conditions, the recovery rates of heavy metals of iron and chromium reach 96.2% and 90.7% respectively. The optimal process for the second-step precipitation is as follows: 0.02mol/L HNO3 and 0.3mol/L KF are used to conduct advanced treatment on the filtrate from the first-step precipitation. Under these conditions, the retention rate and purity of nickel ions reach 98.7% and 89.6%, respectively. The optimal process for the third-step sedimentation is to control the pH of the filtrate at around 11 through KOH, under which all nickel can be recovered.

Key words: High-salinity unsymmetrical dimethyl wastewater, Diffusion dialysis, Three-step precipitation, Recovery of acids, Recovery of heavy metals