Cuprous hydrometallurgy : Part II. The electrowinning of copper from cuprous sulphate solutions containing organic nitriles

Copper can be won with low power consumption from an acidified cuprous sulphate solution containing organic nitriles in a one electron process to give smooth non-dendritic copper cathodes and cupric sulphate at the inert anode. A diaphragm is necessary if cupric sulphate is allowed to concentrate in the cell. In the presence of an organic nitrile in water, cuprous sulphate is stable and cupric sulphate oxidises copper to cuprous sulphate. A scheme for refining crude copper to cathode copper without casting anodes is proposed. Various inert anode materials are compared.     

Cuprous hydrometallurgy: Part I. Electrorefining copper via acidic solutions of cuprous sulphate containing organic nitriles

Cuprous sulphate is stable in acid solutions containing organic nitriles. Such solutions may be used as electrolytes in the electrorefining of copper much like the aqueous cupric sulphate electrolytes. High quality fine-grained copper is deposited from cuprous sulphate electrolytes in a one-electron process offering savings in energy or time compared with the conventional two-electron process. The effect of nitriles on the cell voltage, optimum current density and role of additives is considered and optimum electrolyte compositions and cell arrangements are proposed.     

Cuprous hydrometallurgy Party VI. Activation of chalcopyrite by reduction with copper and solutions of copper(I) salts

Chalcopyrite is reduced by solutions of copper(I) sulfate and copper(I) chloride to chalcocite (Cu₂S) and bornite (Cu₅FeS₄) whilst the iron reports to the solution. Factors which affect the rate and efficiency of reduction are examined. The reaction is rapid on fresh surfaces of chalcopyrite but slows markedly as a film of chalcocite or bornite forms. The reduction in the presence of copper metal goes to completion and gives a material which is more readily leached by oxidising agents than is chalcopyrite. Thus 99% of the copper in the reduced chalcopyrite is leached when copper(II) sulfate in aqueous acetonitrile is the oxidising agent, whereas only 30% of the copper is leached from pure chalcopyrite under similar conditions. Concentrated solutions of copper(I) salts are less effective in reducing CuFeS₂ in a heterogeneous solid-liquid reaction than is copper metal in a “galvanic” solid-solid reaction. Solutions of copper(II) sulfate plus concentrated copper(I) sulfate in dilute acetonitrile (4 M) containing copper sheets are an effective reductant for chalcopyrite.     

Leaching of silver with copper(ii) ions in aqueous acetonitrile solutions: solubility of salts and equilibrium constant measurements in sulphate media

Silver metal may be oxidised by copper(II) ions in acidified aqueous acetonitrile. The extent of reaction was shown to be directly related to the proportion of acetonitrile in solution and to the sulphuric acid concentration up to about 5% v/v. The solubility of silver sulphate was measured in a variety of acetonitrile—water compositions containing varying amounts of sulphuric acid. Increasing temperature had little effect on either equilibrium constant values or solubility of silver sulphate in the range 25–50°C.Suitable leaching conditions were defined from the data collected and a process flowsheet in proposed for the recovery and refining of silver. Silver powder can be obtained from the leach solutions via distillation to remove the acetonitrile.     

The effect of Fe(II) and Fe(III) on the efficiency of copper electrowinning from dilute acid Cu(II) sulphate solutions with the chemelec cell. Part II. The efficiency of copper electrowinning from dilute liquors

This programme of work forms part of a study into the application of the Chemelec cell for direct electrowinning of copper from dilute leach liquors. A typical liquor is defined as containing less than 2 g l^?1 Cu(II), with an equivalent or greater concentration of iron as Fe(II) and Fe(III). Electrowinning experiments using simulated liquors showed that the current efficiency decreased in proportion to the increase in the Fe(III) concentration. A current efficiency of less than 30% was obtained during the electrowinning of a copper solution (1–2 g l^?1 Cu) with an Fe(III) concentration of 0.5–3.0 g l^?1. Electrowinning from leach liquors (1–2 g l^?1 Cu, 1–2 g l^?1 Fe(III) and 1–5 g l^?1 Fe(II)) showed improved efficiencies. A current efficiency of 47% was obtained for copper removal to less than 50 mg l^?1, with an electrolytic energy cost of about £198 per tonne at 3.3p per kWh. The impurity content of the copper was low, the most significant impurity being lead at 0.012%. This work has shown that the Chemelec cell can achieve reasonable efficiencies for direct electrowinning from dilute leach liquors. Further work is required in order to confirm the efficiency of electrowinning and to assess the operating and capital costs of a commercial cell.     

Leaching of silver with various copper(II) salts in aqueous acetonitrile: Solubility measurements and recovery of silver powders via distillation

Copper(II) ions in acidified aqueous acetonitrile solutions are capable of leaching metallic silver. Silver powder may be recovered by distilling off the volatile acetonitrile. The type of acid used in the process is important in determining the solubility of the copper(II) salt and the ultimate solubility of silver. Sulphuric or p-toluenesulphonic acids are preferred. Other acids tested, which proved to be unsatisfactory, were sulphamic, trichloroacetic and phosphoric. The nature of the silver powder produced by distillation of filtered leach solutions was shown to depend on the solution composition and the presence of additives such as gelatin, thiourea and polyacrylamide. Particle sizes ranged from below 5 μm up to 100 μm. The powders were deposited as sheets or fluffy aggregates made up of microcrystals. These silver microcrystals were either flat platelets or nodular in shape with well-defined crystal faces.     

Competitive solvation and complexation of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) in aqueous ethanol,acetonitrile, and dimethylsulfoxide solutions containing chloride ion with applications to hydrometallurgy

The changes in reduction potential and single ion activity of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) have been measured in a range of aqueous ethanol (EtOH), acetonitrile (AN), and dimethylsulfoxide (DMSO) compositions containing excess chloride ion. The results are compared with changes in such solutions in the absence of chloride ion and with the changes in strong brines and rationalized in terms of the various competitive ion-solvent and ion-chloride interactions. Organic solvents are shown to generally enhance chloride ion activity and promote complex ion formation. But AN is a stronger ligand for Cu(I) and DMSO is a stronger ligand for Cu(II) and Zn(II) than is Cl⁻ or the other solvents. The decrease in metal ion activity in mixed aqueous solvents containing Cl⁻ is greater than that in concentrated aqueous chloride salt solutions, according to the strength of the chloro- or solvo-complex. These fundamental changes lead to applications in the extraction of metal ion complexes and promote the dissolution of AgCl, PbCl₂, and CuCl in aqueous DMSO containing Cl⁻. Formerly Doctoral Student, Murdoch University     

Zinc,nickel and lead in copper deposited from copper(II) ammine sulphate electrolytes

The influence of electrolytic variables on the content of Zn, Ni and Pb in copper deposited cathodically from ammine solution has been investigated. Solutions spiked with zinc or nickel sulphate were studied using a rotating cylindrical titanium cathode and a lead anode.The nickel and zinc contents in the cathodic deposit increase with increasing current density and their concentration in the electrolyte. A rise in the rotation speed of the cathode increases the lead and nickel contents but diminishes the zinc content in the deposit.To explain these results an attempt has been made to schematically reconstruct polarisation diagrams for each metal. Direct potential measurements are subject to considerable experimental error in the system studied. Polarisation measurements performed on a simplified system were in good agreement with the hypothetical diagram.     

The effect of Fe(II) and Fe(III) on the efficiency of copper electrowinning from dilute acid Cu(II) sulphate solutions with the chemelec cell: Part I. Cathodic and anodic polarisation studies

The results are presented from an investigation into the effect of the Fe(III) and Fe(II) concentration on the current efficiency for copper electrowinning using the Chemelec cell. The results are from polarisation studies of acidic sulphate electrolytes containing Cu(II), Fe(II) and Fe(III). The mass transfer coefficients for these cationic species are determined, and the effect of using different anode materials on the overvoltage for the oxidation of Fe(II) and oxygen evolution is also examined.     

The extraction of copper(II) and iron(III) from chloride and sulphate solutions with LIX 64N in kerosene

A comparison has been made of the extraction of Cu(II) and Fe(III) from chloride and sulphate solutions with LIX 64N in kerosene. The effects on extraction of pH, anion concentration, and temperature were examined, and attention was paid to the ionic strength of the aqueous media, some of which contained aluminium and magnesium; extraction was carried out under ‘practical’ rather than ‘ideal’ conditions. Extraction of both Cu and Fe was enhanced from chloride solutions compared with sulphate; although separation of Cu from Fe was slightly reduced, extraction of Cu from chloride liquors appears to be applicable to commercial leach solutions.     

Particulates in hydrometallurgy: Part II. Dewatering behavior of unflocculated laterite acid leach residues

The effects of ore type and leaching conditions (e.g., temperature and solid/liquid ratio) on the settling behavior of laterite acid leach residues were investigated. Settling rate, supernatant turbidity, and filtrability of the slurry were used in evaluating the settling behavior. The slurries showed three different regimes of sedimentation, involving free, hindered, and compression settling behavior. An improvement in the dewatering characteristics of the leach residues was observed as the leaching temperature was increased. Furthermore, the leach residues showed markedly different settling behaviors depending on the temperature. Below 175°C, the settling rate for both 2.5 and 25 pct solids increased steadily with temperature. However, the results indicated clearly that at higher temperatures (>175°C), there is a steep increase in settling rate. These pronounced differences in settling behavior with temperature increase are believed to be due to the sharp increase in particle size above 175°C. The mineral content in the ore has a great effect on the settling behavior of the leach residues. It was found that the oxide-silicate material (Ore No. 2 leach residues) showed a lower settling rate than the iron-rich material (Ore No. 1 leach residues). This behavior is attributable to the presence of a gelatinous, siliceous product in the Ore No. 2 leached pulp.     

Kinetics of leaching of copper metal in copper(II)-ammonium sulphate solutions as determined by the rotating disc method

The influence of the concentration of copper(II)-ammine-sulphate complexes, temperature and the concentration of ammonia on the velocity of the leaching of copper metal in sulphate solutions was investigated by application of the rotating disc method.The results show that the following factors determine the velocity of the process: transport of copper(II) complexes to the reaction surface and the rate of the surface reaction (mixed control).An attempt was made to derive a semi-empirical equation describing the kinetics of the process. The value of the diffusion coefficient, calculated on the basis of the derived formula and the experimental data, is equal to (1.2 ± 0.1) × 10^?9 m² s^?1 (for solutions containing from 0.0012 to about 1 mol/l copper, 6.5 mol/l ammonia and 1.2 mol/l sulphates; temperature 25°C).The diffusion activation energy, determined in the range of temperatures from 15 to 50°C equals 30.1 ± 1.7 kJ/mole. This is surprisingly high especially in comparison with the activation energy of viscous flow determined at the same conditions, which is 15.9 ± 0.4 kJ/mole.Discussion of the results leads to the conclusion that free ammonia (the stoichiometric excess in relation to the amount necessary for a complete bonding of copper in a complex) does not participate in the electrode reaction. However, it diminishes the velocity of transport of copper complexes to the reaction surface.     

A study of equilibria in the extraction of uranium (VI) from aqueous sulphate solution by tri-n-octylamine in benzene or petroleum spirit

An account of a fundamental equilibrium study on the distribution of uranium(VI) between an acidic sulphate solution of ionic strength 1.0 and non-aqueous tri-n-octylammonium sulphate solutions of practical significance in the processing of uranium ores is given. Most previous work has concerned only the stoichiometry of U (VI) species. Experimental conditions were chosen to avoid aggregation effects. The study required aqueous phase pH between 2.6 and 3.6. A detailed model developed to describe the system and the quantitative data derived from its study are presented. For the reactions:

$\text{L}{}_2\text{SO}{}_4\text{+ UO}{}_2\text{SO}{}_4\text{?}\text{L}{}_2\text{UO}{}_2\text{(SO}{}_4\text{)}{}_2\text{and}\text{2}\text{L}{}_2\text{SO}{}_4\text{+ UO}{}_2\text{SO}{}_4\text{?}\text{L}{}_4\text{UO}{}_2\text{(SO}{}_4\text{)}{}_3$

equilibrium constants in the ranges 10³–10⁴ and 10⁶–10⁸ are obtained respectively, depending on the composition of the non-aqueous phase. (L₂SO₄ is amine sulphate and a bar indicates a non-aqueous species.) The effect of the composition of the non-aqueous phase (benzene or petroleum spirit, with or without 2-octanol) on the actual constants is discussed. The dissociative reaction

$\text{L}{}_4\text{(SO}{}_4\text{)}\text{? UO}{}_2\text{SO}{}_4\text{+}\text{L}{}_2\text{SO}{}_4$

with an equilibrium constant in the region of 10^?3 is obviously of little consequence.     

Studies on the application of copper(I)-ammune sulphate electrolytes in copper electrorefining

The electrolysis of a copper(I)-ammine sulphate electrolyte between copper electrodes has been investigated. A sealed cell and a solution containing 30 g/kg Cu, 95 g/kg NH₃ and 95 g/kg SO₄ were used.Current efficiencies (cathodic and anodic), at current densities over 100 A/m², were higher than 95% and were practically independent of the electrolysis variables (25–55°, 100–500 A/m² and 70–130 g/kg of NH₃ solution).It was shown that at temperatures over 40°C the energy consumption for electrorefining is lower in the copper(I)-ammine electrolyte than in a typical acidic electrolyte.Contributions to the energy consumption from the electrode processes and the resistance of the electrolyte have been determined from the experimental data.     

Phase equilibria and thermodynamics of binary copper systems with 3d-metals. II. The copper-vanadium system

Thermodynamic assessment of the Cu-V system was carried out using the CALPHAD method. The excess heat capacity of the liquid phase was taken into account in the model of its excess Gibbs free energy. Excess thermodynamic properties of limiting solid solutions were represented by regular solution models. A self-consistent set of thermodynamic parameters was obtained using data on the mixing enthalpy and information on the phase equilibria. The thermodynamic properties of the phases and the phase diagram along with its metastable extension were calculated using this set of parameters. The thermodynamic model of the system was used in order to predict the composition limits of formation of supersaturated solid solutions prepared by highly nonequilibrium methods of synthesis. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 71–79, May–June, 2006.     

The kinetics of the reaction between copper(II) and some ortho-hydroxyoximes

The kinetics of the reaction between copper(II) and salicylaldoxime were studied in various ethanol-water mixtures. The rate of reaction was found to be first order in both copper(II) and the neutral ligand. Deviations from this behaviour at low pH can be accounted for if allowance is made for the reverse reaction and for protonation of the ligand. A mechanism that involves the rate-determining formation of the mono-complex has been found to describe the kinetic data with the rate constant in 80 per cent ethanol being 3.7 × 10⁴ mol^?4 ls^?1 at 11°C. The kinetic data have been used in a preliminary assessment of the location of the reaction in the extraction of copper(II) by hydroxime solvents.     

Anodic dissolution of copper sulphides in sulphuric acid solution: II. The anodic decomposition of CuS

The anodic decomposition of CuS in sulphuric acid solutions was investigated by means of cyclic voltammetry. The pH of the electrolyte and the scan rate were varied. Our results are interpreted using the experience of semiconductor electrochemistry and of passivation of pure metals. At potentials E < E_np (E_np = potential of pitting nucleation), the current/voltage curves were interpreted by the formation of metastable nonstoichiometric copper oxide or hydroxide. After the dissolution of these layers at the potential E_np, total decomposition of CuS starts. These results were confirmed by rest potential measurements and by scanning electron microscopy.