Recovery of acid and iron values from the spent pickle liquor by solvent extraction

Abstract

Hot dip galvanization in molten zinc at 450-500 0C is the common practice to improve the corrosion resistance of various types of steel. To get effective coating of zinc, oxides and scales must be removed from the metal surface prior to galvanization. In order to clean the surface, the steel goods are pickled in hydrochloric acid. When the concentration of HCl becomes ~10% and the concentration of metallic impurities such as Fe, Zn, Cr, Mo, etc. builds up to a certain level due to repeated pickling in the same liquor, the pickle liquor becomes unacceptable for further use and taken out as waste pickle liquor. The waste pickle liquor is environmentally hazardous, and conventionally the pickle liquor is neutralized with lime and the sludge generated is stored/dumped as landfill. This process leads to loss of acid as well as metal values. At NML a process has been developed to recover acid and iron values from the waste pickle liquor as valuable material by solvothermal synthesis route. A typical waste pickle liquor generated at a steel plant contained 90 g/L HCl, 120 g/L Zn, 30 g/L Fe(II), 1 g/L Fe(III) and 230 g/L Cl-. A model waste pickle liquor solution containing 115 g/L HCl, 28 g/L Fe(II), 2 g/L Fe(III) was used for extraction of hydrochloric acid using Tri-iso-octyl amine (TiOA) and iron by Versatic acid as extractants. The extraction of acid increased from 9 to 68% with the increase in concentration of TiOA from 6 to 80% in kerosene as diluent. The plot of logD vs. log[TiOA] gave a straight line with a slope of 1.2 indicating involvement of 1 mole of TiOA for the extraction of one mole of hydrochloric acid from the aqueous to organic phase. The loading capacity of 50% TiOA was found to be 71 g/L for HCl. From the McCabe Thiele plot number of counter current extraction stages required for complete extraction of acid was determined. At a phase ratio of O:A= 3:1 in single stage acid was completely (99%) extracted from the pickle liquor leaving iron in the raffinate. The pH of final raffinate was found to be 3.0. From the acid free pickle liquor, of iron was extracted to the tune of 80% with 33% Versatic acid in kerosene, from which high purity magnetic oxide could be prepared by hydrolytic precipitation stripping process.