Selective extraction of Ni and Co from a pyrrhotite-rich flotation slime using an alkaline glycine-based leach system

Ni and Co, mostly produced from sulfide resources, are critical energy metals required for the green energy transition. A significant portion of the Ni and Co in the processed ore, particularly for disseminated mafic/ultramafic ores, is not recovered due to the deportment to fine slimes which has been dumped as tailings during the flotation process. This study applied an alkaline glycine-based leaching method to selectively recover Ni and Co from a pyrrhotite-rich flotation slime and investigated the deportment of impurities. Leaching variables, including dissolved oxygen (DO) level (5-25 ppm), pH modifier (NaOH, KOH and ammonia), particle size and pre-oxidation, were investigated. The highest extractions of 86.2 % for Ni and 79.7 % for Co were achieved by leaching at glycine/(Ni + Cu) molar ratio of 4, DO of 15 ppm, pH of 10.2 (adjusted with ammonia), 10 % solids and 30 degrees C using a sample size at a grind size P-80 of 17 mu m within 72 h. Ni that is mineralised in pentlandite can be easily leached by glycine, and oxygen addition is essential for the extraction of Ni and Co when mineralised as sulfides. The study has shown a higher overall metals extraction and faster leaching kinetics, shortening the leaching from 72 to 24-48 h, when ammonia was used as a pH modifier. The alkaline glycine-based leaching achieved both higher Ni and Co extractions and higher rejections of impurities compared with conventional sulfuric acid leaching, with the concentrations of Fe being <250 ppm and Ca and Mg <15 ppm in the leachate. Residue analysis revealed a mechanism of preferable dissolution of pentlandite over pyrrhotite. The Fe and Mg deportment before and after leaching remained similar, which confirmed the high selectivity of the alkaline glycine-based leaching.

Automated summary

This study successfully recovered Ni and Co from flotation slime using an alkaline glycine-based leaching method, achieving higher extraction rates and faster leaching kinetics. Using ammonia as a pH modifier improved the leaching efficiency and resulted in higher impurity rejection.