Selection of a synergistic solvent extraction system to remove calcium and magnesium from concentrated nickel sulfate solutions

Synergistic solvent extraction of Ca and Mg from concentrated Ni sulfuric solutions was assessed using extractants Cyanex 272, D2EHPA, and naphthenic acid. D2EHPA has a strong preference for extracting Ca; however, Ca was crowded out by Ni at pH > 3.5. Cyanex 272 has a higher affinity for Mg at pH > 5, while both earth alkaline metals are rejected by naphthenic acid at pH = 2-6 as well as by the system Cyanex 272 + naphthenic acid. The system D2EHPA + naphthenic acid has affinity only for Ca. The system D2EHPA + Cyanex 272 can extract Ca and Mg simultaneously; however, selectivities Ca/Ni and Mg/Ni are dependent on the liquor pH and D2EHPA concentration in organic mixture solution. In this system, a strong synergistic effect was found for Ca and Mg, whereas contradictory behavior was observed for Ni. To avoid crud formation, the selective extraction of Ca and Mg should be performed at pH < 6. Ni can be selectively stripped from the loaded D2EHPA + Cyanex 272 using diluted H2SO4 solution, whereas Ca and Mg can be completely stripped with concentrated H2SO4 solution.