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.     

Electro-recovery of gold and silver from a cyanide leaching solution using a three-dimensional reactor

The selective electro-recovery of gold and silver values from cyanide leaching solutions containing copper was accomplished in a three-dimensional (3D) electrochemical reactor. This case let to contrast three different points of view when dealing with a composed metallic solution: First, the thermodynamic predictions; second, the microelectrolysis approach and finally, the macroelectrolysis experiments. Standard electrode potentials for the study solution would indicate a tendency for gold to deposit first. However, microelectrolysis studies of the three-metallic solution indicated that gold and silver are co-deposited onto a Vitreous carbon (VC) electrode without copper interference in a narrow potential range. Mass balances during the macroelectrolysis experiments (batch model assuming mass transfer control) indicated a preferential deposition of silver during the first ten minutes, even if gold deposition also occurred. On the other hand, values of Stanton (St) for different linear flow velocity corroborated that metals concentration gradients may establish a limit to make profitable the fluid velocity increase in an electrochemical flow cell. Electrolysis experiments were carried out under potentiostatic (at −1400 mV versus SCE) and galvanostatic (at −3.9 Am⁻²) conditions in the FM-01 LC flow cell.     

Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH^–] = 0.25 M, the Tafel slope was about 0.12 V decade^–1. Cu(CN)₃ ^2– was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ^2– is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ^3–. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade^–1 and the second one 0.17–0.20 V decade^–1. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.     

Electrochemical treatment of dilute cyanide solutions containing zinc complexes by oxidation at carbon felt (Sigratherm)

The electrochemical oxidation of dilute solutions containing cyanide complexes of zinc using a carbon felt anode is described. The composition of the solution was determined using stability constants for particular complexes. The application of a GFA5 electrode enables the oxidation of cyanides (10 mmol dm^–3) in 99.2% at 0.7 V and 30 C (1.5 Q _t). Under these conditions, the electrical energy consumption is about 24 kWh (kg CN^–)^–1. Modification of the anode surface with copper oxides allows a cyanide conversion degree of 99.6% at 0.7 V and 17.5 C (0.875 Q _t with electrical energy consumption of 17.6 kWh (kg CN^–)^–1. However, modification of the anode resulted in 75% destruction.     

An electrochemical investigation of the suppression of silver dissolution in aqueous cyanide by 2-mercaptobenzothiazole

Triangular potential sweep voltammetry, potentiokinetic generation of polarization curves, and coupon corrosion tests have been carried out to determine the influence of 2-mercaptobenzothiazole (MBT) on the dissolution of silver in cyanide solutions at pH 11. MBT has been shown to be an effective inhibitor for silver dissolution at concentrations similar to those used when MBT is applied as a flotation collector. The inhibition efficiency (i.e., [1 – the ratio of the corrosion rates in the presence and absence of MBT], expressed as a percentage) in 10^–2 and 10^–3 mol dm^–3 CN was found to increase with increase in MBT concentration in the range 10^–6 to 10^–4 mol dm^–3, and with increase in time of exposure of the silver to the MBT solution. The inhibition efficiency found for 10^–4 mol dm^–3 MBT in quiescent 10^–2 mol dm^–3 CN^– solution at 23 °C was 98.9%, 99.4% and 99.99% for exposure times of 10 min, 2 h and 5 days, respectively. Surface enhanced Raman spectroscopy showed that inhibition was associated with adsorption of MBT displacing cyanide from the silver surface.     

An electrochemical study of gold cementation with zinc powder at low cyanide concentration in alkaline solutions

The half-reactions involved in gold cementation on zinc powder from low concentration alkaline cyanide solutions were studied in a steady state regime, employing electrodes of glassy carbon and graphite paste with zinc powder. The effects of pH, cyanide and initial gold concentrations were investigated using various electrochemical techniques. The results were used to determine the controlling step of the cementation at low cyanide concentration and to interpret the influence of the variables on this process. Mixed potentials and associated currents were determined from the Evans' diagrams constructed using sampled current–potentials curves from chronoamperometric results for half redox reactions. These values do not adequately describe the global cementation reaction because gold reduction in low cyanide concentration solutions is greatly influenced by a strong contribution from adsorptive processes. This behaviour is different from that found by other authors for concentrated gold and cyanide solutions, where the process is controlled by the complex ion diffusion (Au(CN)₂ ^–). The strong component of adsorption found in the present work does not permit the determination, using the Evans' diagrams, of the cementation velocity. Direct monitoring of the mixed potential was proposed, employing an electrode made of zinc powder in carbon paste submerged in a gold cyanide solution. The results of these experiments at low cyanide concentrations show that zinc oxidation is controlled by the formation of different passivating layers, the nature of which depended on the pH of the solution, and that the gold reduction reaction is strongly influenced by adsorption phenomena.     

A spectroelectrochemical investigation of the influence of sodium diisobutyldithiophosphinate on silver dissolution in aqueous cyanide

Polarization studies have been carried out to determine the influence of diisobutyldithiophosphinate (DIBDTPI) on the dissolution of silver in cyanide solutions at pH 11. DIBDTPI was found to inhibit dissolution at concentrations similar to those used when this compound is applied as a flotation collector. The inhibition efficiency in 10^–2 mol dm^–3 CN^– was found to increase with increase in DIBDTPI concentration in the range 10^–6–10^–4 mol dm^–3, and with increase in time of exposure of the silver to the DIBDTPI solution. The inhibition efficiency found for 10^–4 mol dm^–3 DIBDTPI in quiescent 10^–2 mol dm^–3 CN^– solution at 23 °C was 64.6% and 95.0% for exposure times of 10 min and 2 h, respectively. These values are significantly less than those found previously for 2-mercaptobenzothiazole under the same conditions. Surface enhanced Raman spectroscopy showed that inhibition was associated with adsorption of DIBDTPI displacing cyanide from the silver surface. Voltammetry at 0.5 mV s^–1 indicated that adsorption of DIBDTPI involves charge transfer.     

Mechanistic and kinetic aspects of silver dissolution in cyanide solutions

The factors influencing the dissolution kinetics of pure silver in cyanide solution have been analysed in terms of an electrochemical mechanism. A kinetic model is presented which incorporates coupled diffusion and charge transfer for the anodic branch, and combined diffusion, adsorption and charge transfer for the cathodic branch. The anodic oxidation of silver has been investigated using a silver rotating-disc electrode for concentrations between 10^–3 and 10^–1 m NaCN. Oxygen reduction on silver has been studied at oxygen partial pressures between 0.104 and 1.00 atm. Mechanistic aspects of the oxygen discharge reaction are considered in explaining the kinetic differences between gold and silver dissolution in cyanide solution. It is shown that under conditions typical of conventional cyanidation gold dissolves measurably faster than silver.     

Electrochemical studies of gold ore processing wastewater containing cyanide, copper, and sulfur compounds

The electrochemistry of real gold ore processing wastewater solutions from copper sulfide containing gold ore has been investigated. Analysis shows that the wastewater contains a range of sulfur compounds in various oxidation states from sulfide to sulfate. The electrochemical characteristics of the gold ore processing wastewater were evaluated using rotating disk, cyclic voltammetric, polarization and preparative electrolysis studies. The solutions show clear differences versus synthetic alkaline copper cyanide solutions. The copper cyanide/copper oxide catalysis normally seen in synthetic alkaline copper cyanide solutions is strongly inhibited. Two components of the wastewater solution identified as inhibiting the copper cyanide/copper oxide catalysis are copper sulfide complexes and thiocyanate. The inhibition of the copper cyanide/copper oxide catalysis appears to have an initiation time possibly related to the accumulation of copper-sulfur compounds at the electrode surface. The passivated surface is still able to oxidize cyanide, though at a maximum rate that corresponds to the limiting current for free cyanide assuming 1 electron per cyanide. The lack of the copper oxide coating that typically forms during oxidation of synthetic alkaline copper cyanide solutions, plus possibly the presence of various sulfur compounds, results in corrosion at higher anodic potentials when stainless steel is used as an electrode. However, stainless steel can be successfully used as an electrode material to treat the solutions if the potential is carefully controlled.     

Reduction of hexavalent chromium by copper

The galvanic reaction of metallic copper in Cr(VI)-laden aqueous solutions of varying pH was examined by in situ u.v.–visible spectrophotometry, rotating disc electrode chronopotentiometry and cyclic voltammetry. The galvanic reaction in 0.2 M H₂SO₄ solutions was pseudo first order in Cr(VI) concentration. Experiments with both magnetically stirred solutions and a copper mesh or a copper film in a rotating disc electrode configuration revealed the reaction to be diffusion-controlled with respect to Cr(VI) transport to the copper surface. Finally, cyclic voltammetry data in Cr(VI)-laden media of varying pH underline the important role of protons in the galvanic reaction.     

Electrooxidation of Cyanide on Cobalt Oxide Anodes

Oxidation of cyanide ions at a Ti/Co₃O₄ electrode in aqueous base solution has been investigated. The cyclic voltammetric curve for the oxidation of cyanide at Ti/Co₃O₄ shows a well formed wave prior to oxygen evolution at a potential where the spinel surface itself undergoes oxidation. Using a flow cell it is confirmed that the conversion of cyanide (CN^–) to cyanate (CNO^–) can be achieved galvanostatically with a reasonable current efficiency. As an example, at a current density of 100 A m^–2, CN^– concentration can be lowered from 10 to 0.2 mM with an electric energy consumption of about 18 kWh kg^–1 of CN^– oxidized and a global current efficiency of 28.5%. The oxide coating appears to be quite stable during repeated electrolyses.     

Indole and 5-chloroindole as inhibitors of anodic dissolution and cathodic deposition of copper in acidic chloride solutions

The effect of indole, IN, and 5-chloroindole, Cl-IN, on the anodic dissolution of copper in acidic sodium chloride solutions was studied using voltammetry on a rotating disc electrode (RDE). Both compounds used at 10^–3 M concentration act as strong inhibitors on the copper dissolution, but indole exhibits better inhibiting properties. The inhibitory action substantially increases with decreasing solution pH. The influence of these organic additives on the electrodeposition of copper on platinum was also investigated using RDE and electrochemical quartz crystal microbalance (EQCM) techniques. The EQCM measurements show that a sparingly soluble layer of the inhibitor is responsible for the protective effects observed in chloride solutions.     

Kinetics and mechanism of the formation of doped skeletal copper catalysts: the effect of zincate compared to undoped and chromate-doped systems

Addition of zincate to the leach liquor for the preparation of skeletal copper increases the copper surface area; however it does not stabilize the structure against rearrangement. The leaching kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0.0005–0.1 M Na₂ZnO₂. Zincate ions precipitate as zinc oxide, due to the local consumption of hydroxide ions near the leach front as the aluminium dissolves. This oxide hinders the aluminium dissolution, slowing the leaching rate. It also hinders copper dissolution/redeposition and prevents copper diffusion, thus reducing the structural rearrangement significantly, and causing the formation of a much finer copper structure with increased surface area. The zinc oxide redissolves as the leach front passes, releasing the copper to rearrange once more, thereby allowing the surface area to decrease with time. The activation energy for leaching was found to be 84 ± 6 kJ mol^–1.     

Copper corrosion at various pH values with and without the inhibitor

The inhibitory action of BTAH on copper was investigated in 1 M sodium acetate solution in the pH range 4–10, using cyclic voltammetry and impedance spectroscopy. Cyclic voltammetry showed that the rearrangement of the surface oxide layer in the presence of BTAH is very fast in slightly alkaline solutions, while it is time- and concentration-dependent in neutral and slightly acidic solutions. The adsorption behaviour of BTAH on the electrode surface at c(BTAH) 0.5 mM followed a Flory–Huggins adsorption isotherm with G ranging from –30.0 to –39.0 kJ mol^–1, depending on the pH. Impedance spectra were characterized by two time constants relating to the charge transfer and transport of copper ions through the oxide layer, the latter being the rate determining step. These enabled the determination of important properties of the adsorbed layer and the passivated film. The results indicate that the surface layer is of dielectric nature, and its protection increases with increasing inhibitor concentration and solution pH. The finite diffusion impedance was analysed using a diffusion factor B, and the values of the diffusion coefficient and concentration of copper species in the film were estimated.     

Effect of chromate additive on the kinetics and mechanism of skeletal copper formation

The addition of chromate to the leach liquor for the preparation of skeletal copper increases and stabilizes the copper surface area and slows the leaching rate. The kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0–0.1 M Na₂CrO₄. The rate of leaching was found to be constant with time, with an activation energy of 74 ± 7 kJ mol^–1. By monitoring the kinetics and free (mixed) corrosion potential, it was possible to elucidate the mechanistic effect of chromate causing the increased surface area. Chromate was found to deposit on the copper surface as chromium(III) oxide, hindering the leaching reaction as well as the dissolution/redeposition of copper, the main mechanism of structural formation/rearrangement for skeletal copper. This blocking of the surface resulted in a finer structure, with a corresponding larger surface area. It also stabilised the surface area by minimizing the rearrangement. The effect of chromate was found to reach a limit at around 0.03 M Na₂CrO₄.     

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

Electrolytic oxidation of cuprocyanide electroplating waste waters under different pH conditions

The main purpose of this work was to investigate the electrolytic oxidation of cuprocyanide solution with various total cyanide to copper molar ratios ranging from 2.8 to 20 and under different pH conditions. In strong alkaline solution (pH12), cuprocyanide ions Cu(CN) _n ^/(n–1)– , wheren=2, 3 or 4, are directly electroxidized, and copper oxide precipitates on the anode. Cyanate ions, as well as nitrogen gas, were detected as the products and 0.30–0.43 g mol of total cyanide was destroyed per Faraday. For less alkaline solutions (pH<12), cuprocyanide ions first dissociated to free cyanide ions and then electroxidized. At a pH of 10.5–11.7, cyanate ion and brown azulmin polymer were produced in the anolyte. In the neutral solution (pH=7.0–8.6), carbonate and ammonium ions and azulmin were formed and 0.52–0.56 g mol of total cyanide was destroyed per Faraday. In weak acidic solution (pH=5.2–6.8), oxalate and ammonium ions and white oxamide were produced and 1.01–1.18 g mol of total cyanide were destroyed per Faraday.Nomenclature C _CN molar concentration of total cyanide (kmol m^–3) - C _Cu molar concentration of total copper (kmol m^–3) - C _d equivalent concentration of cyanide destroyed due to the formation of cupric oxide (kg m^–3) - C _f concentration of cyanide destroyed by dissociation of complex ion to free cyanide ion and then electroxidized (kg m^–3) - C _i initial concentration of total cyanide (kg m^–3) - C _t change of total cyanide concentration during electrolysis (kg m^–3) - F Faraday constant (96 487 C mol^–1) - K ₁,K ₂,K ₃ formation constant of dicyanocuprate, tricyanocuprate and tetracyanocuprate ions - R molar ratio of total cyanide concentration to total copper concentration (i.e.C _CN/C _Cu) - W weight of precipitates on electrodes or in anolyte (kg) - angle of incidence     

An electrochemical study of the dissolution of gold in thiosulfate solutions. Part II. Effect of Copper

Thiosulfate has been considered as one of the most promising of the non-toxic alternatives to cyanide for the leaching of gold and much work has been carried out with the aim of understanding and improving the ammoniacal thiosulfate leaching process. In particular the behaviour of gold in thiosulfate solutions containing copper in the absence of ammonia has received little attention. It has been shown in this study involving electrochemical and leaching tests that copper ions catalyze not only the oxidation of thiosulfate but also the dissolution of gold in alkaline thiosulfate solutions. Electrochemical studies have shown that copper has a positive effect on the anodic dissolution of gold with increasing concentrations of copper resulting in higher dissolution rates of gold at a potential of 0.3V. Studies on the dissolution of gold powder in alkaline oxygenated thiosulfate solutions containing low concentrations of copper have shown that the role of copper in enhancing the dissolution rate of gold is possibly associated with the formation of a copper–thiosulfate–oxygen intermediate which is more reactive in terms of cathodic reduction than dissolved oxygen. The electrochemical experiments have been complemented by a leaching study which has shown that milling of gold powder in the presence of copper (added as ions, metal, or oxide) assists with the dissolution of gold in thiosulfate solutions.     

Some aspects of removal of copper and cobalt from mixed ion dilute solutions

Some aspects of the electrodeposition of copper and cobalt from aqueous sulphate solutions containing low concentrations of their ions were studied with a view to heavy metal removal via an electrochemical process. Both metals were deposited on a vitreous carbon rotating disc electrode. Deposits formed under different conditions were studied employing linear sweep voltammetry, scanning electron microscopy and EDAX surface analysis. Constant potential electrolysis was used to simulate recovery in a laboratory batch reactor. Copper can be deposited without cobalt interference at potentials as cathodic as –1.0 V despite high Co concentrations. At more negative potentials, both metals are deposited simultaneously, although the copper proportion in the binary mixture is greater than that corresponding to the solution concentration ratio. Voltammetry studies effected under conditions in which codeposition occurs show only minor changes in copper behaviour. On the other hand, cobalt behaviour exhibits significant modifications. Even though formation of an intermetallic compound is possible, ASVL and microscopy tests indicate cobalt deposition in different crystalline forms as the more probable cause. In turn, cobalt deposition depends on the polarization conditions of the electrode and on the cobalt and copper concentrations.     

Highly sensitive, reusable electrochemical aptasensor for adenosine

A reusable electrochemical aptasensor for highly sensitive detection of small molecules had been developed using adenosine as a model molecule. The sensing interface was fabricated by self-assembling the part DNA duplex hybridized by 5′-thiolated part complementary strand (TPCS) and 3′-ferrocene(Fc)-labeled adenosine-binding aptamer strand (FABA) through S–Au bonding on a gold electrode surface. When the modified electrode was incubated in the adenosine solutions, the aptamer made structure switching to bind adenosine. As a result, Fc-labeled adenosine-binding aptamer strand was taken off from the sensing interface, resulting in a decrease of the redox current. The aptasensor was characterized electrochemically by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV of the resulting aptasensor showed a linear response to the increase of the adenosine concentration in the range of 5.0 × 10⁻⁸–1.5 × 10⁻⁵ M with a linear correlation of r = 0.9898 and a detection limit of 1.65 × 10⁻⁸ M. Moreover, the aptasensor exhibited several excellent characteristics, such as high sensitivity, selectivity, good stability, and reproducibility.