Aspects of the development of a copper electrowinning cell based on reactive electrodialysis

This paper reports work aimed at developing a new copper electrowinning cell based on reactive electrodialysis (RED) which uses Fe²⁺→Fe³⁺+e as anodic reaction. In this lab-scale cell, the anolyte (aqueous FeSO₄+H₂SO₄) and the catholyte (aqueous CuSO₄+H₂SO₄) are kept separate by an anion membrane which prevents cation and water transport between the electrolytes. Both solutions are agitated by recirculation. The kinetics of the anodic reaction have been studied via potentiodynamic experiments on various anode materials (lead, platinum, ruthenium oxide, iridium oxide and graphite). The highest oxidation rate was obtained on platinum and the lowest one on lead, whereas the remaining materials showed satisfactory performance. Results in the lab-scale RED cell show that, depending on experimental conditions, for a cell current density of , the cell voltage ranges from 1.81 to , the cathodic current efficiency from 97.2% to 98.3% and the specific energy consumption, from 1.53 to of deposited copper.     

Influence of ionic equilibrium in the CuSO4-H2SO4-H2O system on the formation of irregular electrodeposits of copper

The relative concentration of hydrogen ion (H⁺) as a function of sulfuric acid (H₂SO₄) concentration (calculated using Pitzer's model) and the electrochemical processes by which irregular copper deposits are formed were correlated. Irregular deposits are formed by potentiostatic electrodeposition at a high overpotential where the hydrogen evolution reaction occurs parallel to copper electrodeposition. Two sets of acid sulfate solutions were analyzed. In one set of experiments, the concentration of CuSO₄ was constant while the concentration of H₂SO₄ was varied. The other set of experiments was performed with a constant concentration of H₂SO₄ and different concentrations of CuSO₄. Then, the volumes of the evolved hydrogen (calculated as the average current efficiencies of hydrogen evolution) and the morphologies of copper deposits, characterized by the SEM technique, obtained for the same ratio of CuSO₄/H₂SO₄ were mutually compared and discussed in terms of the relative concentrations of hydrogen ions (H⁺) as a function of the H₂SO₄ concentration. Good agreement between the ionic equilibrium in the CuSO₄-H₂SO₄-H₂O system and the results of the electrochemical processes was obtained. In this way, it was shown how this ionic equilibrium can be applied to predict and analyze the solution composition in electrolytic copper deposition processes.     

The anodic dissolution of copper single crystals

The electrochemical dissolution of copper single crystal spheres in 0.5 M H₂SO₄ with CuSO₄ (0.02 or 0.1 M) results in the Dissolution-forms [111] (Octahedron) at low (< 130 mV) and spheres at high (> 130 mV) anodic overpotentials. The formation of low indexed steps can be observed. The charge-transfer reaction given by the Butler—Volmer equation is the rate determining step of the dissolution at high overpotentials. In the case of low overpotential surface diffusion depending on the crystal-orientation affects the dissolution process also. The dissolution rate at low overpotential depends strongly on the dislocation density of the copper single crystals. Two dimensional models for the construction of dissolution forms are presented.     

High-rate electrochemical copper deposition on bars

The industrial electrodeposition of copper from cupric acid sulphate baths is typically carried out at approximately 3 kA m^?2. A much higher rate of copper deposition is necessary to improve this electroplating process significantly. To achieve this higher rate for the deposition of copper on a round bar, the solution flow is directed normal to the axis of a round bar. The current efficiencyη _Cu for copper deposition on a round bar, 9 mm in diameter, has been determined from 1 M H₂SO₄+1 M CuSO₄ bath as a function of current density, solution flow rate and temperature. A set of relations has been proposed for calculating the current efficiencyη _Cu for a broad range of parameters.     

Mass transfer to and copper deposition on a round bar in a new type of electrolytic cell: the helix cell

High-rate electrodeposition of copper from CuSO₄-H₂SO₄ baths can be achieved by using crossflow of solution. To obtain copper layers of uniform thickness and quality, a new type of electrolytic cell, the helix cell, has been proposed. An experimental dimensionless relation has been given to describe the mass transfer to a round bar, in crossflow, in a helix cell. Moreover, the current efficiency of copper deposition has been obtained as a function of current density, flow rate of solution, temperature and weight per cent CuSO₄ in the CuSO₄-H₂SO₄ solution.Nomenclature A _e working-electrode surface area (m²) - c concentration of Cu²⁺ (mol m^-3) - c _e c at electrode surface (mol m^-3) - c _b c in bulk of solution (mol m^-3) - C constant factor (-) - d _c inner diameter of central cylinder of helix cell (mm) - d _c volumetric hydraulic diameter of helix cell (mm) - d _h width of helical slots in central cylinder of helix cell (mm) - d _s diameter of working electrode (round bar) (mm) - D _i diffusion coefficient of species i (m² s^-1) - F Faraday constant (C mol^-1) - I current (A) - k _i mass-transfer coefficient for a species i (m s^-1) - i current density (kA m^-2, A m^-2) - L _c length of working-electrode compartment of helix cell (m) - n number of electrons involved in electrode reaction (-) - n₁, n₂, n₃ constants (-) - R gas constant: R=8.31 J K^-1 mol^-1 - Re Reynolds number: Re=v _c d _h/v (-) - Sc Schmidt number: Sc=v/D (-) - Sh Sherwood number: Sh=kd _h/D - t time (s) - T temperature (K) - U _s volumetric rate of solution through the helix cell (m³ s^-1) - v _c flow rate of solution through working-electrode compartment of the helix cell (vc = U/(d _c –dw)Lc) (ms–1) - density of solution (kg m ^-3) - u dynamic viscosityofsolution (kg m-1 s-1) - v kinematic viscosity of solution (m2 s-1) - _i current efficiency for formation of a species i (-)     

Studies on the effects of copper salts on the separation performance of chitosan membranes

The effects of copper salts on the pervaporation performance of chitosan membranes for water/ethanol mixtures were studied. The results show that the selectivity to water can be improved by addition of Cu²⁺ in the feed, and that CuSO₄ was more effective than CuCl₂. The reason for the marked increase in selectivity and the difference in performance between the copper salts is explained as follows. With CuSO₄, the multivalent metal ion forms a complex with four  NH₂ ligands of the chitosan molecules and the new conformation is simultaneously immobilized by the ionic crosslinking with SO₄². This is different from the Cu²⁺ of CuCl₂, which mainly coordinates with  OH in the C‐6 position of chitosan, and Cl⁻ cannot crosslink the complexes. Therefore, chitosan membranes have a stronger absorption capability for the Cu²⁺ of CuSO₄ than for Cu²⁺ from CuCl₂. In addition, ion cross‐linking formed by CuSO₄ more strongly decreases the permeation of ethanol. Studies of the effects of acid on pervaporation performance of the Cu²⁺‐treated chitosan membrane show that separation properties of the membrane change with addition of acid to the feed. This is attributed to Cu²⁺ being eluted from the chitosan membrane followed by the reaction of acid molecules with the  NH₂ functional groups of the chitosan membrane. © 2000 Society of Chemical Industry     

Electrodeposition of Co–Cu alloy coatings from glycinate baths

The cathodic polarization, cathodic current efficiency of codeposition, composition and structure of Co–Cu alloy as a function of bath composition, current density and temperature were studied. Electrodeposition was carried out from solutions containing CuSO₄ · 5H₂O, CoSO₄ · 7H₂O, Na₂SO₄ and NH₂CH₂COOH. The cathodic current efficiency of codeposition (CCE) was high and it increased with increasing temperature and Cu²⁺ content in the bath, but it decreased with current density. The codeposition of Co–Cu alloys from these baths can be classified as regular. The Co content of the deposit increased with Co²⁺ content and current density and decreased with glycine concentration and temperature. The structure of the deposited alloys was characterized by anodic stripping and X-ray diffraction techniques. The data showed that the deposited alloys consisted of a single solid solution phase with a face-centred cubic (f.c.c.) structure.     

Hydrometallurgical process for the recycling of copper using anodic oxidation of cuprous ammine complexes and flow-through electrolysis

Flow-through electrolysis for copper electrowinning from cuprous ammine complex was studied in order to develop a hydrometallurgical copper recycling process using an ammoniacal chloride solution, focusing on the anodic oxidation of cuprous to cupric ammine complexes. The current efficiency of this anodic oxidation was 96% at a current density of 200 A m⁻² under a batch condition. In a flow-through electrolysis using a sub-liter cell and a carbon felt anode, the anodic current efficiency increased with the flow rate and was typically higher than 97%. This tendency was explained by the backward flow of the cupric ammine complex, which was formed on the anode, through the diaphragm. The anodic overpotential was lower than 0.3 V even at an apparent current density of 1500 A m⁻². A similar current efficiency and overpotential were also achieved in a liter scale cell, which indicates the scale flexibility of this electrolysis. The power consumption requirements for copper electrowinning in this cell were 460 and 770 kWh t⁻¹ at the current densities of 250 and 500 A m⁻², respectively, which were much lower than that of the conventional copper electrowinning despite the longer interpolar distance.     

Influence of magnesium and aluminium ions on the copper a.c. deposition into aluminium anodic oxide film nanotubes

The influence of Mg²⁺ and Al³⁺ ions on a.c. deposition of copper nanowires into aluminium anodic oxide film (AOF) nanotubes has been studied using cyclic voltammetry and d.c. plasma emission spectrometry. From the analysis of copper quantities deposited into the Al AOF nanotubes (m _Cu), 0.02 M MgSO₄ concentration was found to be optimal for Cu(II) solutions. Moreover, it was shown that Mg²⁺ and Al³⁺ ions not only prevent the breakdown of the barrier layer of AOF, but change the rate of copper deposition and modify the shape of the m _Cu against pH plots depending on the a.c. voltage applied. From the analysis of the quantities of magnesium (m _Mg) incorporated into the Al AOF nanotubes, presumably in the form of Mg(OH)₂, the m _Mg against pH dependences were determined in MgSO₄ and MgSO₄ + CuSO₄ solutions. An increase in m _Mg from 30 g dm^–2 to 1 mg dm^–2 at pH 1.5 and from 6 g dm^–2 to 16 g dm^–2 at pH 7.0 was found under the same a.c. treatment conditions from MgSO₄ solutions without and with Cu²⁺ ions, respectively, indicating the incorporation of Mg(OH)₂ into the Al AOF nanotubes to be lower up to about one hundred times in the case of Cu deposition. Based on the experimental results, it was suggested that incorporation of the Mg(OH)₂ particles into the Al AOF nanotubes occurred simultaneously with growing copper nanowires under a.c. bias is insignificant, if the pH of the CuSO₄ + MgSO₄ solution is 2.5.     

Anodic behaviour of alkaline solutions containing copper cyanide and sulfite on the graphite anode

Sulfite may be added to copper cyanide solutions to reduce cyanide oxidation at the anode during copper electrowinning. Anodic sulfite oxidation is enhanced in the presence of copper cyanide. Sulfite also suppresses the oxidation of copper cyanide. The effect of sulfite on the oxidation of copper cyanide decreases with increasing mole ratio of cyanide to copper. This is related to the shift in the discharged species from Cu(CN)₃ ^2– to Cu(CN)₄ ^3– with increasing mole ratio of cyanide to copper. Sulfite is oxidized to sulfate. At [Cu⁺] = around 1 M, CN:Cu = 3.0–3.2, [OH^–] = 0.05–0.25 M, [SO₃ ^2–] = 0.4–0.6 M and the temperature = 50–60 °C, the anodic current efficiency of sulfite reached 80–90%. With further increase in sulfite concentration beyond 0.6 M, the current efficiency of sulfite oxidation will not be increased significantly. Further increase in CN:Cu mole ratio will result in decrease in the anodic current efficiency.     

A study on acid reclamation and copper recovery using low pressure nanofiltration membrane

In this study, nanofiltration membrane is used to separate proton (H⁺) and copper ions from a ternary ions mixture (H⁺, Cu²⁺, SO₄^2?). The performance of membrane in separating Cu²⁺ and H⁺ was tested under the effect of pressure, concentration and different acid strength (pH). It was found that the H⁺ rejection is independent of the applied pressure. Permeability of solution decreased linearly with the increase of CuSO₄ concentration. In terms of H⁺ rejection, there is a continuous drop in rejection from 0.1 mM CuSO₄ to 10 mM CuSO₄ solution. H⁺ was poorly retained and concentrated in the permeate stream in corresponding to the electro-neutrality requirements, on the other hand, the rejection of copper ion was almost constant with pH. In overall, optimum acid reclamation and copper recovery can be achieved at higher volume flux. A Three Parameters-Combined Film-Extended Nernst-Planck Equation (CF-ENP) model is successfully applied to predict the performance of nanofiltration membrane in separating the ternary ions.     

Reverse pulse plating of copper from acid electrolyte: A rotating ring disc electrode study

Coulombic or cathode efficiencies (CE) were determined for the reverse pulse plating of copper from CuSO₄/H₂SO₄ electrolyte for a variety of pulse conditions. The CE was seen to decrease as the magnitude of the current on the anodic pulse increased. This may be explained by an increase in Cu⁺ intermediates near the electrode surface and was verified by polarization data obtained from a rotating ring disc electrode (RRDE). The influence of certain additives on the CE during reverse pulse plating and on the polarization curves was also examined. When polyethylene glycol and Cl^– (0.86mm) were added to the electrolyte, the CE was observed to drop significantly for a particular set of pulse parameters. The polarization curves at the RRDE suggested that the copper-electrolyte interface was blocked by an adsorbed layer over a wide potential range. The results are explained in terms of a model in which Cl^– ions are concentrated near the electrode surface within the adsorbed polyethylene glycol layer and this is supported by observed rotational dependencies for the RRDE.     

Inactivation of Escherichia coli in Na2SO4 electrolyte using boron-doped diamond anode

Electrochemical disinfection in chloride-free electrolyte has attracted more and more attention due to advantages of no production of disinfection byproducts (DBPs), and boron-doped diamond (BDD) anode with several unique properties has shown great potential in this field. In this study, inactivation of Escherichia coli (E. coli) was investigated in Na₂SO₄ electrolyte using BDD anode. Firstly, disinfection tests were carried on at different current density. The inactivation rate of E. coli and also the concentration of hydroxyl radical (OH) increased with the current density, which indicated the major role of OH in the disinfection process. At 20 mA cm⁻² the energy consumption was the lowest to reach an equal inactivation. Moreover, it was found that inactivation rate of E. coli rose with the increasing Na₂SO₄ concentration and they were inactivated more faster in Na₂SO₄ than in NaH₂PO₄ or NaNO₃ electrolyte even in the presence of OH scavenger, which could be attributed to the oxidants produced in the electrolysis of SO₄²⁻, such as peroxodisulfate (S₂O₈²⁻). And the role of S₂O₈²⁻ was proved in the disinfection experiments. These results demonstrated that, besides hydroxyl radical and its consecutive products, oxidants produced in SO₄²⁻ electrolysis at BDD anode played a role in electrochemical disinfection in Na₂SO₄ electrolyte.     

Studies on the velocity profile in natural convection during copper deposition at vertical cathodes

The velocity profile of natural convective flow near a plane vertical cathode in aqueous 0.6 mole/l CuSO₄ and 0.6 mole/l CuSO₄—1.5 mole/l H₂SO₄ solutions was measured under the experimental condition of uniform cathodic cd. The maximal velocity, u_m, and the horizontal distance between cathode surface and the position of maximal velocity, τ, increased with the height from the lower edge of the cathode. When the cd was raised, u_m increased and τ decreased at any height of cathode. Furthermore, both u_m and τ were lower in the electrolyte containing CuSO₄ and H₂SO₄ compared with the aqueous CuSO₄ solution. Theoretical expressions for u_m and τ in the electrolyte containing CuSO₄ and H₂SO₄ were derived by applying the von Kármán—Pohlhausen integral method. The density distributions near the cathode surface in the above two electrolytes were calculated. From the comparison of both density distributions, it was suggested that the upward natural convection is depressed in the electrolyte containing CuSO₄ and H₂SO₄ by the gravitational force due to the enrichment of H⁺ ion in the outer portion of the diffusion layer.     

Anodic substitutions in emulsions under phase transfer catalysis conditions. I. Cyanation of dimethoxybenzenes

An emulsion electrolysis technique in the two-phase system water-dichloromethane containing NaCN and a phase transfer agent (PTA) has been examined with 1,2- and 1,3-dimethoxybenzenes as a function of various parameters (nature of Q⁺, X^–, anodic potential, cyanide ion concentration in the organic phase, preparative current potential curves). The anodic cyanation results indicate that the anode wetting phenomena, the extraction of cyanide ion and the competitive oxidation of X^– are the determining factors. It is shown that the best criterion for a successful anodic cyanation is to operate under conditions of maximum coverage of the anode by the organic layer. Among all the PTA studied (cetyltrimethylammonium bromide,nBu4N⁺HSO ₄ ^– ,nBuP⁺₃Br^–, benzethonium chloride and A 336), A 336, a very hydrophobic PTA, affords the best chemical (81%) and current (77%) yields with 1,2,-dimethoxybenzene.     

High performance copper oxide cathodes for high temperature solid-oxide fuel cells

The cathodic polarization characteristics of CuO and YBa₂Cu₃O_7- electrodes were studied in the temperature range 600 to 800°C and at oxygen partial pressures ranging from 10^–4 to 0.21 atm. The activity of oxygen reduction on a CuO electrode is closely related to the electronic conductivity and the oxygen ion vacancy density in the surface layer of the electrode. The oxygen ion vacancies created in CuO by doping with Li and the modification of the electronic conductivity by adding Ag provide a new way of enhancing the activity of an oxide electrode for oxygen reduction. It is demonstrated that the rate limiting steps for oxygen reduction at high overpotential and low overpotential are oxygen adsorption and charge transfer on CuO, respectively.List of symbols F Faraday constant - f F/RT - i current - i₀ exchange current - k ⁰ intrinsic rate constant of charge transfer - N() electron density with an energy level E - n number of electrons - R gas constant T temperature Greek letters transfer coefficient - conductivity - overpotential - energy level     

Application of an experimental design method to study the performance of electrochlorination cells

A study of the effects of some parameters on the active chlorine production from aqueous sodium chloride solutions in the hypochlorite electrochemical production cells was undertaken. These varying parameters included the anodic surface area (S_a), the ratio of anodic and cathodic surface areas () the inter-electrode gap, and the type of the cathode used. In addition, a study of the performance of some electrochemical cells that differ in the type of anodes (platinum-coated titanium, ruthenium-coated titanium, and graphite) was made. By means of the experimental design method employing a full factorial of2² the effects ofthe most influencing parameters, the set-up of optimum conditions, and the formation of optimal concentration of active chlorine were assessed. Under the following conditions, a concentration of as high as 65.67 g/L of active chlorine was gained: ruthenium-coated titanium anode (S_a = 24 cm²); titanium cathode, ; inter-electrode gap, 0.5 cm; current density, 35 A/dm² ; temperature, 20°C; concentration of NaCl aqueous solution, 3 M; time, 2 h.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.     

Modeling of the Current Distribution in Aluminum Anodization

The growth of porous anodic Al₂O₃ films, formed potentiostatically in continuously stirred 15 wt.% H₂SO₄ electrolyte was studied as a function of the anodization voltage (14–18 V), bath temperature (15–25 °C) and anodization time (15–35 min). The variation of the anodic surface overpotential with the current density was measured experimentally. The film thickness at the more accessible portions of the anode was observed to increase with the anodization voltage and the bath temperature. However, the film thickness on the less accessible portions of the anode did not significantly change with the voltage or the bath temperature. This indicates that the anodization process at the more accessible regions is more strongly influenced by the surface processes than by the electric migration within the electrolyte. Furthermore, analysis confirms that the major portion of the film resistivity resides within a thin sub-layer that does not vary with the anodization time, and the growing anodic layer contributes only marginally to the overall film resistance. Computer aided design software was employed to simulate the current density distribution. For the range of process parameters studied, the electrochemical CAD software predicts accurately the measured thickness distribution along the anode.     

A study of mass transfer in electropolishing of copper

Limiting current was measured for the anodic polishing of vertical copper electrodes in o-H₃PO₄ acid using: (a) a cell with diaphragm where the effect of H₂ evolved at the cathode on the rate of mass transfer at the anode is eliminated. A general correlation of the data may be represented by the equation Nu = 0·72 (ScGr)^0.23, where Nu is the Nusselt number, Sc the Schmidt number and Gr the Grasshof number. The experimental range used in the correlation was 6·49 × 10¹¹ < ScGr š 2·9 × 10¹⁴; (b) a cell without diaphragm, where the effect of H₂ evolved at the cathode on the anodic limiting current and the rate of mass transfer was studied. It was found that H₂ evolved at the cathode increases the rate of mass transfer and the anodic limiting current by a value ranging from 2·8–28·4% depending on the operating conditions.