Sulfide capture of Co and Ni after peroxymonosulfate-assisted autocatalysis of EDTA complexes

EDTA-enhanced Co and Ni extraction from low-grade ultramafic ore deposits can be promising if coupled to a concentration process for those elements. This study evaluates the efficiency of autocatalysis and decomplexation of metal-EDTA chelates using peroxymonosulfate (PMS) followed by recovery as sulfide minerals. At a relatively low PMS:EDTA ratio of 4 and low pH (1.9–2.1), both Co and Ni autocatalyzed the decarboxylation of 46% of EDTA, but at different rates (1 h for Co; 7 days for Ni). At these conditions, changes in the UV–Vis spectra of Co solutions are consistent with formation of a stable Co(III) complex, while approximately 10% of Ni remains a stable Ni-EDTA complex, preventing complete precipitation. Sulfide addition doubles the efficiency of Ni recovery (73%) but does not improve the Co recovery (35%) relative to the traditional NaOH method. Recovery yields are increased to 100% for Ni and 55% for Co at a PMS:EDTA ratio of 16 using sulfide addition. Addition of 10–25% Co to either Ni or Fe did not significantly increase the rate of decarboxylation of EDTA, however recovery of Co during sulfidation is more strongly and positively affected by metal mixtures than either Ni or Fe, resulting in higher yield. Further, it seems the presence of Ni has a much larger effect on Co precipitation than expected based on the amount of decarboxylation.