Solvent extraction of calcium and magnesium from concentrate nickel sulfate solutions using D2HEPA and Cyanex 272 extractants

Selective solvent extraction of Ca and Mg was applied to purify Ni from a concentrate sulfate solution ([Ni] = 88 g/L, [Mg] = 3.1 g/L, [Ca] = 0.5 g/L). In the first extraction circuit using D2EHPA as an extractant, 98.6% Ca were extracted (2 stages, [D2EHPA] = 0.3 M, pH 3, 50 °C, A/O ratio = 1). The co-extracted Ni (~ 2.8%) was selectively stripped out (99%) from the loaded organic phase with only 2.6% Ca using a dilute H2SO4 solution (2 stages, [H2SO4] = 0.038 M, 50 °C, A/O ratio = 1). Over 99% Ca were stripped out from the loaded D2EHPA in one theoretical stage with 2 M HCl (O/A ratio = 15). The aqueous solution depleted in Ca was subjected to the second extraction circuit using Cyanex 272 as an extractant, where 99% Mg were extracted (3 stages, [Cyanex 272] = 0.32 M, pH 5.7, 50 °C, A/O ratio = 1). The co-extracted Ni (~ 6.9%) was recovered (99%) from the loaded organic phase with only 2.1% Mg using a dilute H2SO4 solution (3 stages, [H2SO4] = 0.031 M, 50 °C, A/O ratio = 1). Magnesium was stripped out (> 99%, 50 °C) from the loaded Cyanex 272 in one theoretical stage using 2 M H2SO4 or 3 M HCl (O/A ratio = 20 or 11, respectively). The purified liquor with approximately 80 g/L Ni is suitable for electrowinning. The apparent equilibrium constants using real concentrations of metals were obtained.