Beneficiation of Nickel from Ultramafic Ores: Using Sodium Citrate as a Green Processing Reagent

Abstract

The processing of ultramafic ores requires fine grinding, which can result in slime-coating of the nickel-bearing mineral, pentlandite, by the phyllosilicate gangue mineral, serpentine. This lowers nickel recovery and concentrate grade during froth flotation operations. The presence of divalent cations in process water exacerbates the detrimental effects of phyllosilicates on nickel recovery by enhancing the positive surface charge of serpentine and causing charge reversal of pentlandite. To mitigate these issues, an environmentally benign, nontoxic, and biodegradable reagent, sodium citrate, was used in this study to chelate divalent cations on the pentlandite surface and desorb them from the mineral surface. The metal-citrate complexes adsorb onto the positively charged serpentine surface, reversing its charge. Sodium citrate was able to restore the surface charge and hydrophobicity of pentlandite, reverse the charge of serpentine, desorb the divalent cations from the mineral surface, and prevent MgO gangue recovery to the froth phase. Microflotation studies demonstrated that upon the addition of sodium citrate, nickel recovery increased from 74 to 96% and nickel grade was improved from 11 to 15.1% . The hydrophobicity of pentlandite was restored upon conditioning with citrate as evident by increase in water static contact angle from 50° to 70° , DLVO interactions showed that addition of citrate prevents slime coating of pentlandite by serpentine. The results demonstrated the depressive and dispersive action of sodium citrate.