Study On The Treatment Of Steel Pickling Wastewater By Diffusion Dialysis And Electrodialysis

With the rapid development of China’s economy, the market has an increasing demand for steel, while the industrial waste water produced during pickling, rolling, cooling process have also increased significantly. Pickling waste water is acidic waste water containing iron, chromium, nickel, chlorine, fluorine and other inorganic pollutants. If directly discharged without any treatment, it would not only pollute the water environment seriously, but also waste a lot of valuable resources. The membrane separation technology to achieve a zero discharge of pickling waste water in iron and steel industry becomes a start point of this paper, aimming at solving the common scientific problems in membrane separation process, focusing on choosing an anion exchange membrane and developing an certain kind of electrodialysis anode material for the hydrochloric acid pickling waste water treatment by diffusion dialysis and electrodialysis.From the technical and economic consideration, the traditional approaches cannot achieve a zero emission of pickling waste water. In this study, the diffusion dialysis and electrodialysis have been selected to recover the acid and iron in hydrochloric acid pickling waste water. Because the ion exchange membrane and the electrode are key equipments of diffusion dialysis and electrodialysis, the study has selected an anion-exchange membrane with a good separation performance and exchange capacity to improve the separation efficiency of acid and salt in pickling waste water and ensure the diffusion dialysis system a higher efficiency and a more stable condition. Then a certain kind of titanium-based electrode material coated with the non-precious metal oxide of Sn and Sb was developed to meet the requirement of dealing with hydrochloric acid pickling waste water.The DF120anion exchange membrane was selected by static diffusion dialysis. When DF120membrane were employed, about5.46×10-4m·h-1dialysis velocity of HCl, 2.67×10-4m·h-1dialysis velocity of FeCl2could be achieved respectively, and the separation factor between HCl and FeCl2were23.7. In the dynamic system, when the flow rate ratio of water to feed was around1, flow rate was0.35L·h-1,0.20mol·L-1of the acid concentration in diffusate could be reached, with a FeCl2concentration less than0.002mol·L-1,and67.5%of the hydrochloric acid recovery rate could be reached, with a FeCl2leakage rate of7.3%.It was found that the developed titanium-based anode coated with the non-precious metal oxide of Sn and Sb has a good conductivity and corrosion resistance in the acid medium. In the eclctrodialysis system, the stainless steel cathode, the Titanium-based anode with metal oxide coatings of Sn, Sb and Pb (Ti/SnO2-Sb2O3/PbO2), and the DF120anion-exchange membrane were employed. The results of the static eclctrodialysis experiments which were set with simulated wastewater demonstrated that the Fe recovery rate reaches90%, the pH of the effluent from the cathode chamber reaches5.13with a Fe2+concentration less than40mg/L, the pH of the effluent from the anode chamber reaches1.08with a Fe2+concentration less than10mg/L. The other optimal conditions obtained can be shown as follows:cell voltage=10V, reaction time=240min, pH of the cathode electrolyte solution=2.50-3.00, Fe2+concentration of the cathode electrolyte solution=1000～1300mg/L, pH of the anode electrolyte solution=3.00. In the dynamic eclctrodialysis system, the flow rate of cathode and anode chamber remained at0.06L/h. The results of the experiments which were set with actual HCl pickling wastewater demonstrated that the limiting current density was33.3A/m2corresponding to a limiting voltage of11V.,the Fe recovery rate reaches91.8%, the current efficiency reaches70.3%, the pH of the effluent from the cathode chamber reaches6.00with a Fe2+concentration less than60mg/L, the pH of the effluent from the anode chamber reaches1.00with a Fe2+concentration less than25mg/L.The combined dialysis-electrodialysis system was employed to treat the hydrochloric acid pickling waste water taken from a steel mill. The results of the experiments demonstrated that the concentration of recovered acid reaches0.2mol/L, the recovery rate of HCl reaches66.7%, the concentration of Fe2+in the effluent was99.5mg/L, the Fe recovery rate reaches88%, the concentration of Cl-in the effluent was5352mg/L, the removal rate of Cl-reaches73.4%, the energy consume was 2.81KWh/KgFe, the current efficiency reaches81.7%. The other optimal conditions obtained can be shown as follows:the flow rate of diffusion dialysis was0.35L/h, the flow rate ratio (Qw/Qf) of diffusion dialysis was1, the pH of the cathode electrolyte solution of electrodialysis was2.50, the flow rate of electrodialysis was0.06L/h, and the cell voltage of electrodialysis was10V.As proven, the diffusion dialysis and electrodialysis technology is not only scientifically novel but also economically feasible and attractive besides its environmental benefits. In view of those properties characteristic of industrial ecology; the diffusion dialysis and electrodialysis technology will become one of the mainstays for the zero emission of the hydrochloric acid pickling in iron and steel industry.