Removal of Mg and Ca from Ni-rich sulphate solutions by solvent extraction using cationic and neutral extractants

Abstract

Selective solvent extraction of Mg and Ca aiming to purify Ni-rich sulfate solutions ([Ni] = 88 g/L, [Mg] = 3.1 g/L, [Ca] = 0.5 g/L) was investigated using the Cyanex 923 + Cyanex 272, and D2EHPA extractive systems (A/O ratio = 1, T = 50 °C). Although Cyanex 923 rejected metals at pH 3–6, a synergistic selective separation of Mg (69%) over Ca (1.2%, βMg/Ca = 179.2) and Ni (5.7%, βMg/Ni = 37.2) was attained at pH 5 with 0.13 M Cyanex 923 + 0.64 M Cyanex 272. Ni crowded out Mg from the loaded organic phase and limited the operation at pH < 5.5. Antagonism on Mg extractions was observed at [Cyanex 923] ≥ 0.25 M + 0.64 M Cyanex 272. Ni was selectively stripped out over Mg, using a dilute 0.031–0.063 M H2SO4, while Mg was stripped out with 0.25 M H2SO4. By contrast, 0.15 M D2EHPA selectively extracted Ca (50.8%) over Mg (2.1%, βCa/Mg = 47.6) and Ni (1.2%, βCa/Ni = 86.4) at pH 3, with Ca being crowded out by Ni at pH > 3. Selective Ni stripping was obtained from the loaded D2EHPA, using a dilute H2SO4 (0.034 M), while Ca and Mg were stripped by H2SO4 or HCl solutions (1 M). The real number of moles of extractants involved in the metal extractions and apparent equilibrium constants were determined for both extractive systems. Moreover, the FTIR spectra were accessed to contemplate the possible coordination compound structures.