Solvent Extraction of Magnesium as an Unwanted Impurity from Sulfate-Based Nickel Processing Streams

Residual Magnesium (Mg) persists in the feed solution of a Nickel (Ni) production facility, even after preceding solvent extraction treatments. The accumulation of Mg during the Ni electrowinning process can detrimentally impact the mechanical properties of the deposited Ni. This study investigates the selective separation of Ni and Mg from a sulfate solution containing 46.04 g/L Ni and 28.02 g/L Mg, utilizing three extractant systems: Cyanex272, D2HPA, and a synergistic mixture of Cyanex272 and D2HPA. Initially, the influence of operational parameters on extraction efficiency was assessed, identifying critical parameters and their effective ranges. Subsequently, statistical modeling and optimization were employed to quantify the impact of each parameter on metal extraction behavior. Optimization results revealed that the most favorable conditions involve a solution pH of 5.12, an organic-to-aqueous phase volume ratio of 3:1, a contact time of 25 min at ambient temperature, and the use of 20% (v/v) Cyanex272 dissolved in kerosene as the extractant. A three-stage extraction process successfully achieved approximately 97% Mg recovery, while limiting Ni co-extraction to about 7%. These findings underscore that meticulous control and optimization of key process parameters are crucial for the efficient selective separation of Mg from Ni in sulfate solutions.