Crystallization and purification of nickel sulfate hexahydrate from agromining leach solution

The growing demand for lithium-ion batteries has intensified the need for high-purity nickel sulfate hexahydrate (NSH₆), a key precursor for nickel-rich cathode materials. In this context, agromining represents a promising alternative nickel source, although the resulting leachates contain various impurities that challenge downstream purification. This study investigates the evaporative crystallization of NSH₆ from a synthetic nickel-rich sulfate solution formulated to reproduce the composition of post-solvent extraction (SX) feed obtained from a bio-ore leach solution (BLS), with a particular focus on impurity partitioning, incorporation mechanisms, and post-crystallization purification. Evaporative crystallization enabled efficient nickel recovery while rejecting most impurities into the mother liquor. However, selective incorporation of Co(ii) and Fe(ii) into the crystal phase was observed, whereas K, Mg(ii), Ca(ii), and Mn(ii) were largely excluded. Controlled dissolution experiments revealed distinct impurity uptake mechanisms, including surface adsorption for Ca(ii), growth-induced inclusions for Co(ii), Fe(ii), and Mg(ii), and mixed inclusion/solution entrapment for K, while Mn(ii) was completely eliminated by crystallization. These mechanisms were corroborated by impurity distribution coefficients, incorporation ratios, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. A simple displacement washing step effectively removed surface-associated and interstitial impurities, increasing the purity of nickel sulfate hexahydrate to 99.99 wt%, with a single washing stage sufficient to meet battery-grade specifications. Overall, these findings demonstrate that the relatively low impurity-to-nickel ratio of the post-SX agromining-derived feed favors efficient impurity rejection during crystallization, highlighting the potential of agromining as an attractive alternative nickel source for the sustainable production of battery-grade nickel sulfate hexahydrate.