Efficient Recycling and Utilization Strategy for Steel Spent Pickling Solution

Before steel can be utilized, pickling is necessary to remove surface oxidation products. However, as the ferrous ion concentration in the pickling solution increases, the pickling rate significantly diminishes, necessitating the treatment of spent pickling solution (SPS) to mitigate its hazardous effects prior to disposal. Current industrial methods predominantly rely on neutralization and precipitation techniques, which are cost-prohibitive and generate substantial by-products, thus failing to meet environmental protection standards. In this study, a new method, which is based on the formation of FeC2O4·2H2O precipitate in a strong acid solution, is proposed to treat the SPS. Initially, the SPS undergoes a two-step impurity removal process, followed by the controlled addition of oxalic acid dihydrate (H2C2O4·2H2O) to precipitate iron. The resulting precipitate is filtered, washed, and vacuum-dried, and the regenerated acid is recycled back into the pickling tank. When 1 g/10 mL of H2C2O4·2H2O is used, the iron removal rate achieves 60%, and the acidity of the regenerated acid increases by 11.3%. X-ray diffraction pattern (XRD) and thermogravimetric–differential scanning calorimetry (TG-DSC) characterization showed that the precipitate was α-FeC2O4·2H2O, with an average particle size of about 3.19 μm and a purity of 95.24%. This process innovatively achieves efficient recycling of acid and iron resources, offering a potential solution to the industrial challenge of difficult SPS treatment in the steel industry and meeting the urgent need for sustainable development.