Comparative study of iron(III) separation from zinc sulphate-sulphuric acid solutions using organophosphorus extractants, OPAP and D2EHPA

Iron(III) solvent extraction removal from zinc process solutions by OPAP (octylphenyl acid phosphate) and D2EHPA (di-(2-ethylhexyl)phosphoric acid) was studied from the standpoint of stripping with HCl such that a saleable waste-free iron product could be generated downstream using existing pyrohydrolysis technology. OPAP (0.95 F) at 20 °C was found to be an excellent extractant for the preparation of concentrated iron chloride strip solutions. Its major advantages included very low sulphate carry-over, reasonable zinc carry-over, very low chloride back-extraction, and economic iron build-up and acid balance when a 'moderate strength' HCl-FeCl2 strip feed (3.38 N HCl-58.5 g/L Fe(II)-9.7 g/L Fe(III)) was used. Major drawbacks with D2EHPA were identified which included significant sulphate carry-over and ineffective stripping and hence, poor iron build-up (≪ 100 g/L) with the 'moderate strength' strip feed. OPAP exhibited lower chloride back-extraction than D2EHPA. High molar ratio values of back-extracted chloride and unstripped iron(III) suggest a predominance of Fe(III)-chloro complexes in D2EHPA. OPAP stripping with 4 N HNO3 at 50 °C was found to be ineffective.