Plotting of isolines of equal degree of precipitation in the FeCl2-Na2SO3-H2O system as the basis for isolation of ferrous sulfite from solution

Abstract

Ferrous ions are removed from acidic solutions of nonferrous metal compounds by precipitation of Fe(OH) 3 after oxidation of Fe 2+ in solutions while increasing their pH. The mother solution of this product continues to contain valuable components, in particular, nonferrous metal ions irremovable by repeated repulping of the solution by acidified water. The utilization of ferric hydroxide is an important problem for various industries, first of all, nonferrous metallurgy [1]. We solved the problem of utilizing ferric hydroxide, which is now a production waste, by developing a technology based on sulfite conversion, the theoretical basis for which was a study of the pseudoquaternary system Fe(OH) 3 (H 2 SO 4 )‐Na 2 SO 3 ‐H 2 O [2]. For the first time, information was visualized by plotting on a plane of the multicomponent system a combination of two surfaces using two types of isolines: isolines of equal Fe(III) reduction state FeO 1.5 (final)/FeO 1.5 (initial) (%) and isolines of equal molar ratio Na 2 SO 3 /FeO 1.5 . This related the redox process with the reactant ratio and, thereby, clarified the mechanism of interaction in the system that is necessary for optimization and practical application of the process [3]. In the system Fe(OH) 3 (H 2 SO 4 )‐Na 2 SO 3 ‐H 2 O, three moles of Na 2 SO 3 react with one mole of Fe(OH) 3 to lead to virtually complete precipitation of ferrous sulfite. In this reaction, the reduction of Fe(III) to Fe(II) consumes two moles of Na 2 SO 3 , which is oxidized to dithionate Na 2 S 2 O 6 , and only one mole of Na 2 SO 3 is used to precipitate Fe(II) as sulfite FeSO 3 · 2.5H 2 O. In the technological process of sulfite conversion of ferrous cake, cake is initially separated by oxidizing the solution while increasing pH and then sulfitized to convert Fe(III) to Fe(II); ferrous sulfite precipitates; and sodium sulfite is recovered by thermal hydrolysis of the suspension, with SO 2 being distilled off and trapped by a Na 2 CO 3 solution. It seemed reasonable to develop a process of direct removal of ferrous sulfite from solution without a complex step of ferric hydroxide separation. For this purpose, it is necessary to investigate the FeCl 2 ‐Na 2 SO 3 ‐ H 2 O system at room temperature. This system has not yet been studied. The FeCl 2 ‐Na 2 SO 3 ‐H 2 O system was produced from a 0.18 M FeCl 2 solution (pH 2.6) and a 0.18 M Na 2 SO 3 solution (pH 8.4). The FeCl 2 solution was added to the H 2 SO 3 solution, and the mixture was supplemented with a 1 M H 2 SO 4 solution or a 1 M NaOH solution until a certain pH value from 4.0 to 1.5 at an interval of 0.5 was reached. H 2 SO 4 or NaOH was not a component of the pseudoternary system FeCl 2 ‐Na 2 SO 3 ‐(H + or OH ‐ )‐ H 2 O since either of the two substances was introduced only for reaching a certain pH value and was not involved in phase formation.