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.     

Transpassive dissolution of iron in sulphuric acid solution

Anodic oxidation characteristics of iron in 3, 10 and 12 mol dm^–3 sulphuric acid solutions have been studied in the transpassive region. Dissolution current efficiency measurements have been carried out using potentiostatic current-voltage curves and solution analysis techniques. The current-voltage curves were split into metal dissolution and oxygen evolution curves assuming that the iron goes into solution as Fe³⁺. The current density value in the passive region increased whereas the current density in the transpassive region decreased with the increase of sulphuric acid solution concentration. In order to obtain information about the nature of the films present on the surface, potential decay curves from different anodic potentials in the transpassive region have been recorded. It seems that there is no passive film present on the specimen surface in 12 mol dm^–3 sulphuric acid solution and a better surface finish is obtained after the dissolution. Depth profile analysis of oxide films by the AES technique in 3 mol dm^–3 sulphuric solution reveals that the sulphur concentration is maximum at the metal/oxide interface rather than at the oxide/electrolyte solution interface as required by an ion exchange mechanism for film dissolution.     

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.     

Concentration effects of hydrochloric acid on the anodic behaviour of lead sulphide

The anodic dissolution of lead sulphide is studied at various chloride concentrations and at different pH values. At 25° C it is found that in hydrochloric acid the dissolution rate reaches a maximum around 3.0 mol dm^–3. It has also been observed that at concentrations between 0.7 and 1.2 mol dm^–3, a crystalline sulphur deposit formed during the dissolution process leads to an independent peak on theI-E curve whereas at higher concentrations it merges with the PbCl₂ peak formation. pH has no significant effect on the dissolution rate. The results of a systematic study on the kinetics of the dissolution process as a function of concentration, temperature and pH are discussed.     

Electrochemical and surface analysis study of copper corrosion protection by 1-propanethiol and propyltrimethoxysilane: A comparison with 3-mercaptopropyltrimethoxysilane

The inhibition of Cu corrosion by 1-propanethiol (1-PT) and propyltrimethoxysilane (PTS) molecules, in 0.100 mol L^–1 KCl solution, was investigated and compared to 3-mercaptopropyltrimethoxysilane (MPS). Corrosion inhibition was studied as a function of the 1-PT and PTS concentration in ethanol, between 1.0 × 10^–7 mol L^–1 and 1.0 × 10^–2 mol L^–1. Inhibition efficiency was calculated from Tafel plots in 0.100 mol L^–1 KCl solution. It improved with an increase in 1-PT or PTS concentration. The maximum efficiency was obtained at a 1-PT or PTS concentration of 1.0 × 10^–3 mol L^–1 or 1.0 × 10^–5 mol L^–1, respectively. Adsorption of 1-PT and PTS on copper followed a Langmuir behaviour. Potentiostatic polarization measurements indicated that 1-PT and PTS are mixed anodic/cathodic inhibitors, in the presence of dissolved oxygen. When the inhibitor exposure time of the pretreated Cu surface in 0.100 mol L^–1 KCl solution was varied, a loss on the corrosion inhibition efficiency was observed for the three (MPS, PTS and 1-PT) compounds. However, the 1-PT compound maintained excellent protection in the first 12 h of exposure to a 0.100 mol L^–1 KCl solution; afterwards, there was a significant loss in the inhibition efficiency. Surface analysis studies with Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) showed that the inhibitors modified the Cu surface.     

The preparation and behaviour of Ti/Au/PbO2 anodes

Ti/Au/PbO₂ electrodes have been prepared and their stability in H₂SO₄ (2–12 mol dm^–3) has been studied. It has been found that incorporation of a gold layer between the Ti substrate and the PbO₂ decreases the resistance of the electrode. The corrosion of an electrode polarized anodically increases with H₂SO₄ concentration especially above 8 mol dm^–3 H₂SO₄.     

Effect of sodium and potassium sulphates on zinc electrowinning

In order to evaluate the intrinsic effect of high concentrations of sodium and potassium sulphates in zinc electrowinning solutions, measurements of coulombic efficiency were carried out under mass transfer-controlled conditions in synthetic solutions of very high purity. A solution composition of 1 mol dm^–3 ZnSO₄+1.5 mol dm^–3 H₂SO₄ was employed with and without additions of 0.5 mol dm^–3 Na₂SO₄ and/or 0.25 mol dm^–3 K₂SO₄. With temperature and current density similar to plant practice (37° C, 650 A m^–2) and electrode rotation rates of 10 and 45 s^–1, the coulombic efficiency for three successive batch tests (200 mg zinc) increased by an average of 1.2% (from an average of 96.0%) for additions of 0.5 mol dm^–3 Na₂SO₄+0.25 mol dm^–3 K₂SO₄. The results were evaluated in terms of available theories, solution purity and predicted changes in solution composition (zinc and hydrogen ion activities) and physical properties following additions of Na₂SO₄/K₂SO₄. It was concluded that in the plant situation the increase in coulombic efficiency would probably be offset by an increase in cell voltage of about 2%, the net effect on power efficiency being a decrease of about 1%. The zinc deposit morphology and preferred orientation were also studied. The addition of sodium and/or potassium sulphate to the solution resulted in rougher, darker zinc deposits, a slight grain refining effect, and a change from random to predominantly basal (002), (004) crystal orientation (at 45 s^–1).     

Adsorption of Microamounts of Silver on Manganese Dioxide from Acid Solutions

Abstract

Adsorption of microamounts of silver on manganese dioxide from nitric and perchloric acid solutions has been studied and optimized with respect to shaking time, concentrations of electrolyte, adsorbent, and adsorbate. Maximum adsorp- tion (>99.5%) has been achieved from 0.01 mol/dm³ acid solution using 50 mg oxide at 10^?5 mol/dm³ silver concentration after 30 min shaking. The adsorption decreases with increasing concentration of acid and adsorbate from both the acids. The presence of a 10⁴-fold greater concentration of cyanide, thiocyanate, thiosulfate, and Pb(II) than silver reduces the adsorption drastically. The adsorption of silver follows the Freundlich adsorption isotherm over the entire concentration range investigated from 9.27 × 10^?6 to 2.92 × 10^?3 mol/dm³ with a value of A = 49 mmol/g and 1/n = 0.93. Moreover, the Langmuir adsorption isotherm is also valid except at the lowest and highest concentrations. The values of the limiting adsorption concentrtion (C_m ) have been found to be 1 mmol/g and of the equilibrium constant for adsorption 87 dm³/mol at 23 ± 2°C.     

Inhibitory mechanism of low-carbon steel corrosion by mimosa tannin in sulphuric acid solutions

Mimosa tannin was investigated as inhibitor of low-carbon steel sulphuric acid corrosion in concentrations from 10^–5 to 10^–1 mol L^–1, at the temperature of 298 K in the solutions of pH 1, 2 and 3. The inhibitor effectiveness increases with increase in concentration. The adsorptive behaviour of mimosa tannin in solutions of pH 1 and 2 may be approximated, both by Temkin and Frumkin type isotherms, probably due to the chemisorption of tannin molecules on the metal surface. The free energies of adsorption are in the range from –35.1 to –39.5 kJ mol^–1. At pH 3, a Freundlich type isotherm is obeyed, probably due to the physisorption of ferric-tannate that forms at this pH, both on the metal surface and in the bulk electrolyte. The free energy of adsorption at pH 3 is –11.8 kJ mol^–1. The activation energy of the iron dissolution process at pH 1 was found to be 51.4 kJ mol^–1 and decreased to 48.0 kJ mol^–1 on the addition of 1.25 × 10^–2 mol L^–1 mimosa tannin. The addition of the same amount of mimosa tannin into solutions of pH 2 and 3, increased the activation energy of iron dissolution from 15.6 to 34.3 kJ mol^–1 and from 12.0 to 19.2 kJ mol^–1, respectively.     

Galvanostatic cycling of graphite intercalation electrodes with anions in aqueous acids

Natural graphite flakes (80 wt%), with polypropylene (20 wt%) as a binder, constitute a practical and non-expensive graphite electrode of high crystallinity CPP. Galvanostatic cycling of these electrodes with current densities in the range 0.3–30 mA cm^–2 (charging time 5–120 min) has been investigated in aqueous acids (12, 20 and 36 mol dm^–3 HF, 6 and 12 mol dm^–3 H₂SO₄, 4 mol dm^–3 HClO₄). The anion of the acid is anodically intercalated and cathodically de-intercalated. In spite of the high water concentrations, quantitative current efficiencies have been obtained. From variation of the rest times after charging, a corrosion current density of less than 0.03 mA cm^–2 (j _ch=3 mA cm^–2) has been derived. The overvoltage during charge and discharge is typically about 0.1 V. The potential at the start of the charging process coincides with the intercalation potential defined previously. A strong electrode formation effect is observed upon cycling. The electrode is initially smooth and non-porous; it acquires a high surface roughness after a few cycles, which is then stable. The initial charging curves increase witht ^1/2, while the charging curve after electrode formation is linear. Both clearly indicate a linear relationship between surface concentration of intercalated anions and potential. This agrees with our previous finding of linear dependence with respect to acid concentration in the solution.     

3-Mercaptopropyltrimethoxysilane as a Cu corrosion inhibitor in KCl solution

3-mercaptopropyltrimethoxysilane (MPS) has been used as a copper corrosion inhibitor in 0.100 mol L^–1 KCl solution. The inhibition was studied as a function of the MPS pretreatment concentration in ethanol. The MPS concentration used was between 1.0 × 10^–8 mol L^–1 and 1.0 × 10^–1 mol L^–1. A freshly-cleaned Cu electrode was inserted in an ethanolic solution of MPS for 30 min for pretreatment, and was then exposed to a 0.100 mol L^–1 KCl aqueous solution for 1 h. From the polarization resistance, the inhibition efficiency improved with increase in MPS concentration during the pretreatment. The MPS adsorption on Cu followed a Langmuir adsorption behaviour. However, at MPS concentrations larger than 1.0 × 10^–4 mol L^–1 the inhibition decreased. Moreover, the inhibition efficiency decreased with increase in the exposure time of the MPS modified Cu electrode in the KCl aqueous solution. Polarization studies suggest that MPS is an anodic as well as a cathodic inhibitor, in the presence of dissolved oxygen. X-ray photoelectron spectroscopy (XPS) analysis of the Cu samples showed that the organic compound modifies the Cu surface and scanning electron microscopy (SEM) studies indicated that MPS protects the Cu surface when exposed for 350 h to laboratory environment. Polarized grazing angle Fourier transform-infrared (FTIR) microscopy analysis determined the presence of a polymer on the Cu surface.     

Electrochemical production of cuprous oxide using the anode-support system

The preferred process for the production of cuprous oxide powder is by the anodic dissolution of copper in an alkaline solution of sodium chloride. The purpose of the present investigation was to develop a cuprous oxide process suitable for use on an industrial scale usiing the anode-support system, i.e. an anode comprising a titanium mesh basket loaded with small pieces of high-grade copper scrap. Laboratory investigations with this type of anode together with a titanium mesh cathode were conducted using cells having capacities up to 400 dm³. The recommended operating conditions based on 120 h runs using the 400 dm³ cell are as follows: NaCl: 250 g dm^–3; c.d.: 6 A dm^–2; CI: 0.37 A dm^–3; temperature: 80°C; pH 10. Of particular importance, especially as regards the quality of the product and cell scale-up, is the relationship between the current and the volume of the electrolyte, denoted as Cl and expressed as A dm^–3. The use of anode and cathode diaphragms of polypropylene obviated the need for additives to counteract copper redox reactions in the cell. The power yield was 0.8–0.9 kWh kg^–1. The product was well within ASTM specification D912-65 for Cu₂O for use in antifouling paints.     

Electrochemical recovery of silver from cyanide leaching solutions

In the hydrometallurgy industry cyanide solutions are the most common leaching baths used during the extraction of metals such as silver and gold. After extraction, the solution containing various cyanide species, and usually copper cyanide, has a higher concentration than the gold and silver complexes. Higher concentration of copper species may interfere during the selective recovery of precious metals. This work presents a study of the selective recovery of silver from leaching solutions mimicking those used in industry. Chemical speciation and cyclic voltammetry studies showed that copper reduction occurs at more negative potentials than silver in the cyanide leaching solutions. The cyclic voltammetry of cyanide solutions on a vitreous carbon electrode showed that copper cyanide ions modify the interface properties, lowering the overpotential required for silver reduction. A macroelectrolysis study of a simulated leaching solution (10^–4 M Ag(I), 0.1 M Cu(I) in 0.5 M CN^– at pH 10), was carried out in a filter press electrochemical reactor (ElectroCell AB) with a reticulated vitreous carbon electrode (RVC), nominally having 45 pores per inch and a flow rate of 5 cm³ s^–1 at 25 °C. The study showed that the high copper concentration does not interfere in the selective deposition of silver.     

The behaviour of PbO2 in concentrated sulphuric acid

Linear sweep experiments on Pb in H₂SO₄ at concentrations in excess of 5 mol dm^–3 have been conducted using computer controlled techniques. Measurements have indicated that the maximum charge in the PbO₂ reduction peak occurs at 5 mol dm^–3, the available capacity decreasing with the concentration of H₂SO₄.     

Electrodeposition of nickel-cobalt-zinc alloys from a borate bath

The electrodeposition has been studied of nickel-cobalt-zinc alloys from a borate bath containing nickel sulphate (120–140 g dm^–3), cobalt sulphate (30–46 g dm^–3), zinc sulphate (144–168 g dm^–3), boric acid (30 g dm^–3) and ammonium chloride (2 g dm^–3). The operating conditions were: current density, 2.0–5.0 A dm^–2; temperature, 30–40°C and pH, 2.4 to 5.4. Light grey, semibright, stressed films have been obtained. However, the deposits consist partially of black powder when the concentration of the various components is increased. The brightness is found to increase with decreasing temperature and pH of the solution. The total cathode efficiency increases when the pH and temperature of the solution decrease, whereas at any particular pH and temperature it first decreases, reaches a minimum and then increases with increasing current density.     

Electroplating of Fe-Ni alloys: a sulphate-amine bath

Thin films of Fe-Ni alloys have been electroplated from acidic sulphate bath (0.06 mol dm^–3 NiSO₄, O.O15 mol dm^–3 FeSO₄, 0.005 mol dm^–3 ascorbic acid, 20 g dm^–3 boric acid and 1 g dm^–3 saccharin) containing aliphatic amines. The percentage Ni in the alloy varied with bath composition (Fe/Ni), current density, stirring of the medium, nature and concentration of the amine. Increase of temperature and pH of the medium increased the percentage Ni in the deposit. The composition of the alloy remained constant with thickness of the film. The cathodic current efficiency depends on the plating variables. The plating potential in acidic sulphate bath was shifted in the less noble direction by the presence of amines. Smooth and bright films are obtained with small grain size when the Ni in the film is 75% or above. Electroplating conditions are optimized to get thin, magnetic 2080 Fe-Ni films.Presented at the International Symposium on Recent Aspects of Electroanalytical Chemistry and Electrochemical Technology at Chanigarh, India.     

Corrosion rates of lead based anodes for zinc electrowinning at high current densities

The corrosion rates of anodes made from various lead/silver alloys have been determined during electrolysis in sulphuric acid solution, pure and containing additives, using current densities in the range 2500 to 10 000 A m^–2. An increase in acid concentration, and in some cases temperature, caused an increase in the corrosion rate. In the absence of manganese in the bath, the corrosion rate was effectively independent of current density in the range studied, whereas in the presence of manganese, the corrosion rate decreased with decreasing current density. The corrosion rates of various calcium, tin and thallium alloys of lead were also determined. The presence of chloride ions in the electrolyte increases the corrosion rate, whereas potassium ions and strontium carbonate have a negligible effect. Pre-treating silver/lead anodes with a solution of acidic potassium fluoride at 500 A m^–2 prior to testing markedly decreased the corrosion rate in the presence of manganese, but increased the corrosion rate with manganese absent. The effect of zinc on the corrosion rate in synthetic electrolyte solutions, with and without manganese present, has also been determined for silver/lead alloys at 10 000 A m^–2. At zinc levels over 1 M, the corrosion rate increased with and without added manganese. As the work has been undertaken in an attempt to improve the electrowinning of zinc, an electrolyte based on acidified industrial solution has also been tested. The rates observed were similar to those obtained for synthetic zinc-containing solutions.     

Anodic formation of thin CdS films. I. Kinetics and mechanisms under galvanostatic and potentiodynamic conditions

The formation of thin anodic films of CdS is studied in 1 mol dm^–3 NaHCO₃ + 0.1 mol dm^–3 Na₂S solutions under galvanostatic and potentiodynamic conditions. Under both experimental conditions, films up to about 50 Å are formed according to the high field model of growth. In this thickness range an insufficient space charge is developed to noticeably affect the kinetics. From galvanostatic experiments at different temperatures, the activation energy was determined to be 10.6 kcal mole^–1. The surface density,N of the species participating in the rate determining step is unusually low, about 5×10⁶ cm^–2. So low a value forN precludes a process at the Cd/CdS interface as the rate determining step. The significance of various parameters in the observed rate equation are compared with and discussed in relation to the respective parameters in the rate equations developed for the high field model of growth.     

An electrochemical hydrocyclone cell for the treatment of dilute solutions: approximate plug-flow model for electrodeposition kinetics

The mass transfer conditions in a hydrocyclone cell have been analysed and an approximate plug-flow model has been developed to describe metal ion depletion during batch recycle operation. The resulting concentration-time relationship and reaction rate equation has been shown to describe satisfactorily the experimental data obtained for the electrodeposition of copper and silver from dilute solutions. Moreover, these relationships have enabled the evaluation of mass transfer coefficients in the hydrocyclone cell.List of symbols a ₁,a ₂,b numerical exponents - C concentration (mol dm^–3) - C _o initial bath concentration (mol dm^–3) - C(0) cell inlet concentration (mol dm^–3) - C(L) cell outlet concentration (mol dm^–3) - k rate constant (h^–1) - K mass transfer coefficient (ms^–1) - K _L volumetric mass transfer coefficient = 2RLK (m³ s^–1) - L active length of the cylindrical cathode (m) - Q volumetric flow rate (m³ s^–1) - r inside radius of the conical part of the cell - r _A reaction rate of component A (mol dm^–3 h^–1) - R inside radius of the cylindrical part of the cell (m) - t time - u vertical (axial) velocity in the annulus - U cell voltage (V) - _t horizontal (tangential) velocity in the annulus - V _B volume of the reservoir/bath - V _R volume of the cell/reactor - _B residence of time of the reservoir     

Electrodeposition of polypyrrole–titanate nanotube composites coatings and their corrosion resistance

Polypyrrole (PPy) films (2 μm) containing titanate nanotubes (TiNT) were deposited from 0.5 mol dm⁻³ pyrrole (Py) and 1 g dm⁻³ of TiNT in 0.1 mol dm⁻³ aqueous oxalic acid on 904 L stainless steel (SS) 0.1 mm thickness at 298 K. Electron microscopy showed that the nanotubes were adsorbed on the PPy surface and uniformly dispersed in the polymer matrix. The PPy/TiNT composite contained <10 wt.% titanates which showed an increase of 53% hardness compared with polypyrrole alone. The TiNT provide nucleation centres to catalyze the polymerization of pyrrole and can adsorb up to 240 mg g⁻¹ of the monomer. The corrosion rates for SS, SS/PPy and SS/PPy/TiTN composites, evaluated by linear sweep voltammetry and open-circuit potential measurements in 3% w/v NaCl, were 1.61, 0.008 and 0.004 mg dm⁻² day⁻¹, respectively, indicating that corrosion rates of stainless steel decreased by up to three orders of magnitude in the presence of the composite films.