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

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. During smelting of copper concentrates, arsenic is easily removed into the offgas while antimony is not readily removed due to its lower partial pressure and high affinity for liquid copper. Therefore, 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) depositing antimony as a marketable product from synthetic alkaline sulphide pregnant leach liquors by electrowinning. The mineralogical investigations conducted on the concentrates studied revealed that tetrahedrite, chalcopyrite, galena, sphalerite and pyrite were the common mineralogical phases present in the concentrates. Silver and arsenic were found as solid solution in the tetrahedrite crystal structure. Alkaline sulphide solution was used to dissolve antimony from the concentrates. Antimony recovery from tetrahedrite dissolution was increased by approximately 280% when the reaction temperature was increased from 84⁰C to 105⁰C. By raising the concentration of Na2S from 60 g/L to 100 g/L, the extraction of Sb was raised by a factor of 3 while increase in NaOH concentration from 30 g/L to 60 g/L enhanced the recovery by 140%. It was found that the leaching yield decreased by about 37% when the mineral particle size of the concentrate was increased from -53+38 µm to -106+75 µm. Under the selected leaching conditions, the estimated activation energy of tetrahedrite dissolution in the leaching reagent was 81 kJ/mol, which is indicative of a chemically controlled leach process. Characterisation of the leach residue by XRD and QEMSCAN proves that the alkaline sulphide lixiviant is selective and effective to dissolve the antimony and arsenic from the complex concentrate. The average crystal chemical formulae of the solid residue determined by QEMSCAN indicate the conversion of tetrahedrite into a new copper sulphide having stoichiometry of Cu1.64S. Tetrahedrite in the concentrate was reduced from 30.2% to 1.1% in the purified leach residue.Moreover, the results of electrowinning tests showed that the initial Na2S concentration had a significant influence on Sb deposition from this specific system. Current efficiency decreased remarkably when Na2S concentration was increased to 150 g/L. The test results indicated that the desired Na2S concentration should be less than 100 g/L. Faraday efficiency increased with increase in current density provided that the residual Sb concentration in the electrolyte remained above 20 g/L. Increase in NaOH concentration from 100 to 400 g/L raised the current efficiency by a factor of almost 1.5 while the specific energy requirement was reduced from 2.3 to 1.9 kWh/kg. Experimental results demonstrated that the specific energy decreased by almost 38% as the electrolyte temperature increased from 45 to 90⁰C and the optimum temperature should be between 50 and 75⁰C to reduce the heating cost. It was noted that polysulphide and thiosulphate had an adverse effect on Sb deposition. Current efficiency of the process decreased sharply from 83% to 32% when the polysulphide concentration was increased from 0 to 30 g/L; and at this polysulphide concentration, the specific energy was raised from 1.7 to 4.9 kWh/kg. Sparging of the electrolyte facilitates a smooth and adherent antimony deposit with an improved purity. The results from these experiments demonstrated that the anodic reactions were influenced by anodic current density and NaOH concentration. The molar concentration ratio between hydroxide and free sulphide ions must be ≥ 7.3 to produce appreciable amounts of sulphate in the electrolytic process. The amount of sulphate formed increased from 0.5 to 16.9 g/L when the anodic current density was increased from 500 to 2500 A/m2. By raising NaOH concentration from 100 to 400 g/L, the production of sulphate at the anode was enhanced by 6.2 g/L increment. However, the concentration of thiosulphate formed during the electrolysis decreased with increasing anode current density and NaOH concentration. The main factors influencing the purity of the antimony deposits were current density and NaOH concentration. Antimony purity was lowered from 99.9% to 99.2% when the current density was increased from 50 to 250 A/m2. Sparging of the electrolyte during the electrodeposition enhanced antimony purity by 0.4%. Finally, a simplified integrated hydro-/electro-metallurgical process flowsheet for antimony removal and recovery from Rockliden sulphide copper concentrate was developed. The experimental results from this investigation confirmed that different concentrations of Na2S and NaOH were needed at leaching and electrowinning stages to achieve an efficient process.