Purification of concentrated nickel sulfuric liquors via synergistic solvent extraction of calcium and magnesium using mixtures of D2EHPA and Cyanex 272

Abstract

A synergistic solvent extraction circuit consisting of D2EHPA and Cyanex 272 was applied to purify a concentrated Ni sulfuric solution ([Ni] = 88 g L−1, [Mg] = 3.1 g L−1, [Ca] = 0.5 g L−1) similar to the MHP industrial liquors that feed the electrowinning step. The temperature was identified as a key parameter to optimize Mg extraction as well as to promote the selective recovery of co-extracted Ni in the extraction stages and make the stripping of Ca feasible using HCl as a stripping agent. Ca was crowded out by Ni from the loaded D2EHPA + Cyanex 272 solution and limited the operating pH range between 2 and 4. Under optimum conditions, 98.0% Ca and 98.4% Mg were simultaneously extracted in two and three theoretical stages by D2EHPA (0.30 M) + Cyanex 272 (0.32 M) mixture (pH 4, A/O ratio = 0.7 and 50 °C). Co-extracted Ni was selectively recovered (99.7%) from the loaded D2EHPA + Cyanex 272 using [H2SO4] = 0.063 M in three stages (O/A ratio = 1 and 50 °C). Ca and Mg can be completely stripped out from the loaded D2EHPA + Cyanex 272 at 50 °C in only 1 ([HCl] = 3 M, O/A ratio = 5) or two theoretical stages ([H2SO4] = 1 M, O/A ratio = 2). The purified liquor containing 80 g L−1 Ni (99.9% purity) can be directed for the electrowinning step to produce Ni cathodes. Extraction and stripping kinetics are rapid (equilibrium reached within 5 min). Thermodynamic analysis was done to support the synergism/antagonism trends observed experimentally.