The aim of the present study is to selectively extract antimony and arsenic from decopperization slime through alkaline sulfide hydrometallurgy with a view to recycle the obtained solid residue within the copper smelter, and also regenerate the sulfide lixiviant during the process. Rechtschaffner experimental design was used to evaluate the joint influence of several experimental parameters such as leaching temperature, Na₂S concentration, solid concentration, and reaction time on the extraction of antimony and arsenic from the material. The most active parameters influencing the extraction of the metals are solid concentration and reaction period. In addition, the results show that solid concentration interacted strongly with the leaching time and slightly with reaction temperature, which is an indication that solid concentration is the predominant influencing factor in removing antimony and arsenic from the material. It is also indicated from the results that about 95% Sb and 89% As were extracted when 50 g/L of the decopperization slime was dissolved in alkaline sulfide lixiviant containing 200 g/L Na₂S + 20 g/L NaOH at 60 °C for 24 h. Moreover, analysis of the leach residue reveals that copper sulfide and lead sulfide remain as the main constituents of the residue. The bismuth-containing mineral phase was not observed in the residue because of its low concentration, and also the Sb/As-bearing mineral phases were not detected due to the selectivity of the leaching reagent to the metals. Based on the experimental results from this investigation, a process flowsheet for the alkaline sulfide treatment of a decopperization slime was proposed with a view to eliminating its antimony and arsenic contents in a sustainable manner.

In copper metallurgy, antimony impurity usually forms alloys and compounds with the transition metals to make up the basic building blocks of a speiss phase. This speiss phase is generally rich in copper and precious metals which are desirable to recycle and recover at the smelter. The presence of this impurity unfortunately creates a build-up of this metal in the copper circuit, leading to problems during copper refining processes. Therefore, a removal or reduction of the antimony impurity to an acceptable level is a necessary step before the speiss can be recycled at the smelter for the recovery of its valuable metals. A lead oxide slag, which was obtained after speiss had gone through a special pyrometallurgical process, was leached in alkaline sulphide solution to selectively dissolve its antimony content. Furthermore, the pregnant sulphide leachate was purified by precipitation and crystallization techniques to recover antimony as sodium thioantimonate and sodium hydroxyl antimonate using synthetic Na2S-NaOH-Sb2S3 solution. The leaching results indicate that the highest amount of antimony and arsenic extracted from the material after 24 h at 100oC and reagent concentration of 30 g/L NaOH + 30 g/L S2- was 83% and 90%, respectively. In the precipitation process, addition of hydrogen peroxide to the alkaline sulphide leachate prompts the precipitation of antimony as NaSb(OH)6. The result also implies that less than 100% of stoichiometric hydrogen peroxide is required to completely oxidize the total amounts of both Sb3+ and S2- in the solution to quantitatively precipitate more than 90% of the antimony in solution. The influence of catalytic agents and temperature on the process was not clearly reflected in this investigation due to the exothermic reaction with hydrogen peroxide. Moreover, addition of elemental sulphur to the sulphide leachate also influences the precipitation of antimony as sodium thioantimonate.

Elimination of antimony and arsenic impurities is one of the major difficulties encountered in copper metallurgy. 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. It is however imperative to selectively eliminate and recover the antimony impurity of the copper concentrates in an environmentally friendly process with a view of upgrading the concentrates for pyrometallurgical processing.This communication discusses (i) alkaline sulphide hydrometallurgy of antimony removal from a complex copper concentrate; and (ii) antimony recovery as a marketable product from synthetic alkaline sulphide pregnant leach liquors by electrowinning in a nondiaphragm cell. Also, the various experimental parameters that influence these processes are discussed.

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. 

The influence of initial antimony concentration, cathode current density, the concentrations of Na2S and NaOH, gas sparging and electrolyte temperature on average cell voltage, specific energy and current efficiency of antimony deposition has been studied. The experiments were conducted in a nondiaphragm electrolytic cell. 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 result showed that increase in NaOH concentration considerably promotes the formation of sulphate ions as the main anodic product. Anodic current efficiencies of 98% and 99% based on the amount of sulphate formed were obtained at sodium hydroxide concentrations of 8.75 M and 10 M, respectively. Average cell potential increased with increasing NaOH concentration and cathode current density. The preferred crystallographic orientations of the antimony deposit obtained at 2.5 M NaOH concentration are in the orders (012) (202) (110) (104), but the order becomes (012) (110) (104) (202) when NaOH concentration is increased further. The order of crystal orientations for antimony electrodeposition at 50 A/m2 cathodic current density is (012) (110) (104) (202), which does not change with increasing cathode current density but the peaks at (110) (104) (202) crystal planes become more broadened and suppressed as current density increased.

In a response to the recent growth in the global demand for copper products, mining industries have intensified in their mining operations. Unfortunately, the grade of copper ore concentrates mined today is declining due to the intensive mining of the relatively high grade copper resources. Therefore, future copper ore deposits to be mined are likely to be richer in impurity elements like antimony and arsenic which attract smelter’s penalty if the content of these impurities is too high. It is however imperative to selectively eliminate and recover the antimony impurity of the copper concentrates in an environmentally friendly process with a view of upgrading the concentrates for a pyrometallurgical processing.This paper discusses the alkaline sulphide hydrometallurgical technology to selectively solubilize antimony impurity from a copper concentrate. The effect of sodium sulphide and sodium hydroxide concentrations, leaching time and leaching temperature on antimony dissolution will be examined. Furthermore, antimony recovery as a marketable product from simulated pregnant leach liquor through electrodeposition will be discussed. Various experimental factors that influence antimony deposition from alkaline sulphide electrolyte are reported.

Antimony electrowinning from synthetic alkaline sulphide electrolytes has been studied in a nondiaphragm electrolytic cell. The electrodes were constructed in such a way that the anode produces ten times higher current density than the cathodic current density to promote sulphide oxidation to sulphate at the anode; and simultaneously decreasing the tendency of hydrogen evolution at the cathode. The result revealed that at an anodic current density lower than 1500 A/m2, minute amounts of sulphate ions were formed but when the anode current density increased beyond 1500 A/m2, sulphate formation was promoted. The initial molar concentration ratio between hydroxide and free sulphide ions should be ≥ 10.3 to avoid thiosulphate formation at 2000 A/m2 anodic current density under the conditions used in these experiments. The highest anodic current efficiency obtained based on the amount of sulphate formed was 89%. An increase in the anode current density as well as NaOH concentration enhances the cathodic and anodic current efficiencies with respect to the antimony metal deposited and sulphate ions produced, respectively. Despite the high anodic current densities used, the specific energy of this process ranges from 0.6 to 2.3 kWh/kg which is significantly lower than values reported previously due to the prevention of undesirable sulphur species from being formed. The tests revealed that the concentration of thiosulphate formed during the electrolysis decreased with increasing anode current density and NaOH concentration. Addition of polysulphide from 0 to 30 g/L to the electrolyte decreases the current efficiency from 83% to 32% and correspondingly increases the specific energy from 1.7 to 4.8 kWh/kg. Results showed that a build-up of sulphite and sulphate ions in the solution does not have any detrimental effect on the current efficiency of antimony deposition.

The technical feasibility, on laboratory scale, of hydro- and electrometallurgical processes of recovering metallic antimony from an antimony-bearing copper sulphide concentrate has been investigated. The influence of Na2S concentration, temperature and solid concentration was studied during the leaching test while the effect of current density, Na2S concentration, electrolyte temperature and NaOH concentration on antimony electrowinning from alkaline sulphide solutions was investigated. The leaching results showed that antimony dissolution is strongly dependent on the concentration of the leaching reagent as well as the leaching temperature. The antimony content in the concentrate was reduced from 1.7% to less than 0.1% Sb which is desirable for copper metallurgy. Cathode current efficiency is one of the important parameters to evaluate the performance of an electrolytic process. It is revealed in this study that current efficiency of antimony deposition from sulphide electrolytes is highly dependent on the concentration of sodium hydroxide and the current density used. The results illustrate that the combined effect of increasing anode current density (which was 10 times higher than the cathode current density) and NaOH concentration enhanced the current efficiency of the electrolytic process. It was demonstrated that excess free sulphide ions impacts the current efficiency of the process detrimentally. An integrated hydro-/electrometallurgical process flowsheet for antimony removal and recovery from a sulphide copper concentrate was developed.

The modelling and optimization for the alkaline sulphide leaching of a complex copper concentrate containing 1.69% Sb and 0.14% Sn were studied. Response surface methodology, in combination with central composite face-centred design (RSM-CCF), was used to optimise the operating parameters. The leaching temperature, sulphide ion concentration and solid concentration werechosen as the variables, and the response parameters were antimony and tin recovery, and the time required to achieve 90% Sb dissolution. It was confirmed that the leaching process was strongly dependent on the reaction temperature as well as the sulphide ion concentration without any significant dependence on the solid concentration. Furthermore, a mathematical model was constructed tocharacterise the leaching behaviour. The results from the model allow identification of the most favourable leaching conditions. The model was validated experimentally, and the results show that the model is reliable and accurate in predicting the leaching process.

Copper ore grades are diminishing worldwide as the higher grade reserves are exploited and pro-gressively depleted. Simultaneously, the global demand for copper is increasing continuously. Con-sequently, processing of future copper ores and concentrates will most likely involve the treatment of more complex, fine-grained minerals containing increased levels of impurity elements (e.g. Sb, As and Hg), which are detrimental to the smelting process as well as affecting the physical and me-chanical properties of the copper product. Unfortunately, the prevalence of antimony containing minerals among the copper-bearing ores will reduce their economic value, and therefore, need to be eliminated. However, it is beneficial if antimony can be removed at the early stage of the process as a saleable product instead of ending-up as a waste material in copper metallurgy. This article aims at pre-treating a tetrahedrite-rich copper concentrate by selective dissolution of antimony in alkaline sulphide media, thereby, upgrading the concentrate for smelting operation. Furthermore, the kinetic mechanisms of the leach process as well as the factors affecting it were investigated. The selectivity of the lixiviant towards antimony is also discussed. The results show that the extraction of antimony from the concentrate depends strongly on the concentration of sul-phide and hydroxide ions, reaction temperature, particle size and the leaching time. Kinetic data from the study indicates that tetrahedrite dissolution from the concentrate under selected conditions is chemically controlled through the particle surface reaction with an estimated activation energy of 81 kJ/mol. Analysis of the leach residue proves that the lixiviant is selective and effective to solubi-lize this impurity element from the concentrate with high recovery. The impurity content of the concentrate was found to have reduced to low levels acceptable for smelting operation, and there-fore, lessen the processing problems faced during pyrometallurgical treatment of such impure copper concentrate.