Comparative study of iron(III) separation from zinc sulphate–sulphuric acid solutions using the organophosphorus extractants, OPAP and D2EHPA: Part I: Extraction

Iron(III) removal from strong zinc sulphate–sulphuric acid solutions is investigated by comparing the performance of octylphenyl acid phosphate (OPAP) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). Two extraction temperatures were tested: 20 and 50 °C. The aqueous feed composition utilised (i.e. 25 g/L iron(III) as Fe2(SO4)3, 90 g/L zinc as ZnSO4, 12.5–50 g/L H2SO4) approximates a hot-acid leach solution composition. Ambient extraction (20 °C) yielded acceptable kinetics and loading efficiency for 0.95 F OPAP, but less favourable results for 1.25 F D2EHPA. D2EHPA extraction at 50 °C avoided the slow extraction kinetics exhibited at 20 °C, reducing the required contact time from >30 to 10 min. OPAP exhibited fast extraction kinetics (10 min contact time) at either temperature. At 50 °C, 1.25 F D2EHPA yielded a comparable iron(III) loading capacity (∼17 g/L) as that of 0.95 F OPAP at 20 °C. No aqueous feed neutralization was performed prior to or during extraction. A major advantage with OPAP was the low associated sulphate co-extraction. D2EHPA co-extracted approximately 20 times the sulphate amount extracted by OPAP. Fe/Zn selectivity with OPAP at 20 °C was close to that of D2EHPA obtained at 50 °C (separation factors: 3000 and 3700, respectively). However, loaded 1.25 F D2EHPA showed stability problems as it was found to precipitate over time. Preliminary scrubbing tests determined that 30 g/L (0.82 N) HCl effectively scrubs both co-extracted zinc and sulphate from either of the loaded organic phases.