Sodium sulfate activation mechanism on co-sulfating roasting to nickel-copper sulfide concentrate in metal extractions, microtopography and kinetics

Abstract

Due to the complexity of occurrence of valuable metals and high content of alkali gangue in nickel-copper sulfide concentrate, traditional pyrometallurgical processing is not suitable. In this work, a two-stage co-sulfating roasting process was used to selectively extract valuable metals from Chinese nickel-copper sulfide concentrate. A mixture of (NH4)2SO4 and Na2SO4 was selected as the co-sulfating agents. The effect of the dosage of (NH4)2SO4 and Na2SO4, first stage roasting temperature, roasting time and second roasting temperature on metal’s extractions were investigated. XRD, SEM, DTA-TG, and DSC-TG were used to reveal the mechanism of action of Na2SO4 and the mineral phase transformations in the two-stage sulfating roasting process. The results show that nearly all of Ni and Co, Cu, and 1.12% of Fe were extracted at the mass ratio of ammonium sulfate to ore of 3.5, first roasting at 500 °C for 2 h, dosage of sodium sulfate of 4 g (10 g ore), followed by second roasting at 680 °C for 2 h, then water leaching at 95 °C for 2 h. Na2S2O7 generated from Na2SO4 in the roasting process, shrinks the particle size of the ore, decrease the surface area of the roasted product, significantly promotes extent of the sulfating of metals, in particular for Ni. Thermal analysis kinetics indicates that Na2SO4 significantly reduces the apparent activation energy of the sulfating roasting process. The metal bearing mineral phases were transformed into sulfates or sodium double or ternary sulfates in the first roasting process, then the sodium double sulfates of Na3Fe(SO4)3 were decomposed.