Separation of zinc and iron from secondary manganese sulfate leachate by solvent extraction

Purification of concentrated manganese sulfate solution by solvent extraction is discussed in this paper. The use of bis(2-ethylhexyl) hydrogen phosphate (D2EHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (BTMPPA) was studied in removal of impurities from zinc electrowinning anode sludge leachates. Over 99 % of zinc and iron were removed by both extractants at around pH 3 in two mixer-settlers operating in continuous countercurrent mode at a solvent-to-feed (S/F) ratio of 0.43 and T = 22 +/- 1 degrees C. BTMPPA had higher selectivity for zinc and iron over manganese than D2EHPA under all experimental conditions. Extraction of manganese was typically below 10 % and can be limited by crowding the extractants, since the fraction of manganese in both loaded extractants was decreased by decreasing organic to aqueous volumetric ratio (O/A). A significant amount of the co-extracted Mn was recovered by selective stripping with 0.5 M sulfuric acid. Extraction by BTMPPA was more sensitive to pH adjustment than extraction by D2EHPA. Increasing the mean residence time in mixer from 3.6 min to 6.0 min improved the removal of zinc and iron with BTMPPA but the change in residence time had little or no effect on zinc and iron removal with D2EHPA.