Discussion of a Treatment Technique for Spent Picking Solution Recycling

Abstract

Pickling is a necessary process in many metal processing industries, such as wire rod, iron and steel sheet plate, and tubing processing industries, screw and nut manufacturing and processing industries, corrosion resistant plate processing industries, and paint industry. Pickling is used to remove black rust, oxides, and other contaminants from metal surfaces in order to improve processing quality. Hydrochloric acid is often used in the pickling process. As the acid concentration decreases gradually during the process, while metal (Fe and Zn) concentrations increase gradually, when the residual concentration of hydrochloric acid in the spent pickle liquor decreases to about 3%, or the specific gravity of spent pickle liquor is greater than 1.3 (iron ion concentration is greater than 100g/L), the aged pickle liquor must be discharged and fresh pickling solution is prepared. In the wire rod industry in Taiwan, the pickling process results in highly corrosive spent pickle liquor. Although there have been multiple resource processing schemes developed, including spray roasting process, fluidized bed roasting process, and sulfuric acid displacement process, it is limited to various factors, such as the treatment capacity, initial cost, operating cost, selling prices, purity of recycled products, and market scale. This study reviewed related patented processing modes in literature, and considered the reclamation cost of waste pickling solution (SPS). The proposed technique adopts wet-process metallurgy, combined with diffusion dialysis, for reclamation of SPS and mineralization crystallization of heavy metal of residue iron. The mineralization crystallization treatment produces high-purity recycled products of ferrite and goethite processes. SPS is reclaimed, and the ferric heavy metal recycling meets the economic benefit of an actual plant. The research site of this study produces 12 tons of SPS per day, which is about 300 tons monthly, and about 3,600 tons annually, with the removal cost of 7.2 million NTD annually. If the proposed treatment system is adopted, the costs of fresh hydrochloric acid, removal, and the traditional neutral agent treatment and subsequent landfill could be greatly reduced, thus effectively reducing the environmental load. In comparison to other recycling treatment techniques, the main advantage of the proposed technique is that the treatment capacity depends on the amount of SPS, the energy consumption is relatively low, the treatment scheme is simple, the operating and maintenance costs are low, and the mineralization can produce magnetic ferrite and iron oxide recycled products with high purity.