Antimony recovery from complex copper concentrates through hydro- and electrometallurgical processes

University dissertation from Luleå tekniska universitet

Abstract: Today, one of the major difficulties confronted during copper metallurgy is the elimination of antimony and arsenic impurities from the process. This is because the pure copper ore reserves are becoming exhausted and the resources of unexploited ores often contain relatively high amounts of antimony and arsenic. Removal of these impurities at an early stage of processing will be beneficial for the copper making process. The present research is aimed at (i) purifying impure complex copper sulphide concentrates by selectively dissolving the impurities, and consequently, upgrading the concentrates for pyrometallurgical processing, and (ii) electrowinning of antimony from an alkaline sulphide pregnant leach liquor as a marketable product. Mineralogical examination of the complex concentrate revealed that antimony and part of arsenic were present as tetrahedrite and bournonite. Silver was found bonded in the tetrahedrite crystal structure. Dissolution kinetics of tetrahedrite in aqueous alkaline sodium sulphide solutions was investigated. It was found that the rate of dissolving tetrahedrite by the lixiviant increases with increase in reaction temperature, sodium sulphide concentration, sodium hydroxide concentration, and with decrease in mineral particle size. The kinetic study indicates that the rate of tetrahedrite leaching under the selected conditions is chemically controlled through the particle surface reaction. The activation energy of antimony dissolution from tetrahedrite was estimated as 81 kJ/mol. The apparent activation energy was within the range reported for a chemically controlled reaction process. Besides, the alkaline sulphide lixiviant proves selective and effective to dissolve the antimony and arsenic impurity from the concentrate with high recoveries. Analysis of the leach residue indicates that copper content of the tetrahedrite transformed into copper sulphides with the average chemical formula Cu1.64S. The economic value of the concentrate were improved after sulphide leaching, and therefore, suitable as a feedstock for smelting. The impurities in the concentrate were found to have reduced to a level satisfactory for smelting operation. Moreover, metallic antimony was recovered from model alkaline sulphide pregnant leach solutions by electrolysis in a nondiaphragm electrolytic cell. Various parameters that could affect the electrolytic process were investigated. The results revealed that increase in initial antimony concentration, temperature of the electrolyte and NaOH concentration enhanced the current efficiency. Excessive sodium sulphide concentration in the electrolyte promotes the formation of unwanted polysulphide and thiosulphate ions which can significantly decrease the current efficiency of the process. Sparging of the electrolyte facilitates a smooth and adherent antimony deposit with an improved purity. About 99.6% antimony purity was achieved when the electrolyte was sparged at 10 mL/min. The molar concentration ratio between hydroxide and sulphide ions required to produce sulphate as the main anodic product during the electrolysis was determined. Increase in anodic current density as well as NaOH concentration enhances the cathodic and anodic current efficiencies with respect to the antimony deposited and sulphate produced, respectively. The concentration of thiosulphate formed during the electrolysis decreased with increasing anode current density and NaOH concentration. A build-up of polysulphide and thiosulphate in the electrolyte adversely affect the current efficiency of the electrodeposition by (i) redissolving the antimony cathode, (ii) oxidizing Sb3+ to Sb5+ and (iii) being reduced to sulphide at the cathode. Sulphite and sulphate build-up in the solution does not have any detrimental effect on the current efficiency of antimony deposition. Finally, an integrated hydro/electrometallurgical process flowsheet for antimony removal and recovery from a sulphide copper concentrate was developed.

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