Recycling Technology For Stainless Steel Pickling Wastewater Treatment

Stainless steel pickling wastewater (SSPW) usually contains highly-concentrated metal ions such as Fe, Mn, Ni, Cr, as well as nitric and hydrofluoric acids, which make it hard to treat. The traditional method of neutralization precipitation has several defects such as high cost and secondary pollution. Meanwhile, valuable acid and metal resources cannot be well retrieved and reused which in a certain extent causes hazardous risk and also limits the achievement in relevant industries. Acid recycling and efficient separation of iron and nickel ions are still difficult in recently modified neutralizing precipitation processes.Firstly, acid retadation technology was applied to recycle nitric and hydrofluoric acid. The domestic commerical201×7resin owned an excellent acid retardation property with the total acid adsorption capacity for real SSPWup to1.77mol/L. The separation factor for acid and metal ions was8.34. Both of the flow rate and high-diameter ratio had little influence on acid retardation.Secondly, as to the high concentration of metal ions and fluorine, goethite technology was introduced into the successful removal of iron, chromium and fluorine. Subsequently, high purity of nickle was obtained by neutralizing precipitation process. Sodium carbonate was used as neutralization agent in the decorated goethite process, at the end of which calcium chloride with a stoichiometric dose was added to enhance the impurity removal. Under pH-value of4.5and the other optimum conditions,99.98%,98.9%and99.8%of iron, chromium and fluorine were removed, respectively. The mechanism of goethite reaction was revealed by XRD which showed that Na3FeF6and Na3CrF6crystalls formed fist at the presence of high concentration fluoride ions. After adding calcium chloride, Na3FeF6and Na3CrF6react with calcium chloride to generate calcium fluoride and goethite sediment.Additionally, this technology became harmless after treatment of goethite sediment and tail wastewater. The residual nickel in the sediment could be further recovered by recycling washing three times which increased the total recovery ratio for Ni up to99.7%, and synchronously the amount of nickle in the sediment was far reduced. The sediment after washing was stabilified with2%of calcium oxide, meanwhile achieve the harmful sediment with little leaching of nickle, chromium and fluorin. The concentration of fluoride in tail water could be reduced to lower than10mg/L by calcium salt and coagulation sedimentation process which met the "State Standards for Comprehensive Discharge of Effluent"(GB8978-1996) requirements.Consequently, the patent recycling technology for SSPW treatment owned remarkable benefits. The amount of emission reduction for such poisonous pollutants as heavy metals and fluorine reached12.5kg/t and70.5kg/t, respectively. The profit of this novel technology was about1386RMB per ton wastewater, versus329RMB of direct cost for the traditional neutralizing sedimentation method. Therefore, the newly-invented technology could support the upgrading and transformation for stainless steel pickling industry.