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).     

In situ scanning tunneling microscopy study of selective dissolution of Au3Cu and Cu3Au (0 0 1)

We present an electrochemical study of Au₃Cu (0 0 1) single crystal surfaces in 0.1 mol dm⁻³ H₂SO₄ and 0.1 mol dm⁻³ H₂SO₄ + 0.1 mmol dm⁻³ HCl, and of Cu₃Au (0 0 1) in 0.1 mol dm⁻³ H₂SO₄. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential.     

High speed zinc electrowinning system using a hydrogen anode and a rotating aluminium disc cathode

The performance of a novel high speed zinc electrowinning system using a hydrogen anode and an aluminium rotating disc cathode (1 m diam.) was investigated under various experimental conditions. This new type of zinc electrowinning system was continuously operated at a current density of 70 A dm^–2, which is twelve times higher than that usually employed. Current efficiency is 90% at 50 A dm^–2 in an electrolyte containing 60 g dm^–3 Zn + 160 g dm^–3 H₂SO₄, the zinc purity being at least 99.999%. The energy usage of the system is 1650 kWh per tonne of zinc, 380 m³ of H₂ gas being required.     

The Faradaic impedance of the lithium-sulphur dioxide system. A kinetic interpretation

Impedance measurements have been made on Li/SO₂(C) cells containing an acetonitrile-based electrolyte in a range of states from newly assembled to completely discharged. The cell behaviour can be explained if it is assumed that the lithium is an irreversible electrode and that the SO₂ electrode is reversible. The nominal exchange current density on the lithium is 0.7 mA cm^–2 and 0.37 for the cell Li/LiBr(2.35 mol dm^–3), CH₃CN, S₂O ₄ ^2– ¦SO₂(6.25 mol dm^–3)C     

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.     

Adsorption of chlorophyll-a from acetone solution on natural and activated bentonite

The adsorption of chlorophyll-a on bentonite desiccated at 110°C, untreated and acid-treated with H₂SO₄ solutions over a concentration range between 0·25 and 2·50 mol dm^?3, from acetone solution at 25°C has been studied. The adsorption isotherms may be classified as using Giles' classification, as type S (untreated sample and 0·25 mol dm^?3 H₂SO₄-treated sample), type H (0·50 mol dm^?3 H₂SO₄-treated sample) and type L (1·00 and 2·50 mol dm^?3 H₂SO₄-treated samples). This fact suggests that the bentonite surfaces (low, high and medium affinity, respectively) behave in differently relation to the adsorption of the chlorophyll-a molecules. The experimental data points have been fitted to the Freundlich equation in order to calculate the adsorption capacities (K_f) of the samples; K_f values range from 0·43 mg kg^?1 for the untreated bentonite up to 108·89 mg kg^?1 for the 0·50 M H₂SO₄-treated bentonite. The removal efficiencies (R) have also been calculated and range from 5·71% for the untreated bentonite up to 85·18% for the 0·50 M H₂SO₄-treated bentonite.     

Anode-support system for the direct electrorefining of cement copper Part II: Process conditions using a circular cell with vertical rotary cathode

This paper describes the second part of a study on the development of a modified anode-support system for the direct electrorefining of cement copper. The proposed system is an alternative process for small mines that produce cement copper after the leaching of copper oxide minerals. It is feasible to utilize a circular cell provided with an annular AISI-316 stainless steel mesh supporting a mass of cement copper as the anode system. A vertical rotary cylinder of AISI-316 stainless steel can be used as the cathode. The rotary cylinder, totally immersed in acidic copper sulphate electrolyte must have two isolated slots at the surface to permit the stripping of the copper deposits in the form of thin sheets. As operating conditions, a solution of CUSO₄ · 5H₂0: 150 g dm^–3 and H₂SO₄: 50 g dm^–3, j _e: 5.0 A dm^–2, T: 40 °C, u: 60 rpm, can be used. The cement copper must be maintained wet to avoid rapid oxidation. At this stage of the study cement copper of 90–95% purity was utilized.     

The breakdown of PbO2-Ti anodes

Lead dioxide-titanium electrodes have been tested as oxygen evolving anodes in 1 mol dm^–3 H₂SO₄, at high current densities. Three modes of failure, shedding, thinning and deactivation have been identified. The first two are the result of stresses produced during electrodeposition, compounded by the thermal effects of polarization. Some improvements in these aspects of performance have been made by manipulation of the plating conditions. Deactivation has been modelled by considering the diffusion of special sites from the bulk material. There is continuous redeposition of PbO₂ from the solution.     

Voltammetric and galvanostatic studies of hydrous and anhydrous iridium oxide films

Electrolytically grown hydrous oxide films on iridium wire electrodes have been thermally treated from 473 to 773 K. Anhydrous oxide films formed by this treatment have been subjected to cathodic polarization at the potential of the hydrogen evolution reaction, square-wave pulsing of potential from –0.25 to +1.25 V with respoect to SCE and to anodic galvanostatic polarization in 0.5 mol dm^–3 H₂SO₄. Cathodic pretreatment caused an increase of the voltammetric charge in the oxide formation region while the square-wave pulsing formed a hydrous oxide film whose voltammetric charge was superimposed on the charge of the anhydrous oxide film. Both procedures restored the hydrophilic nature of the electrode/solution interface. Potential-time curves during anodic galvanostatic polarization served as a diagnostic criterion for the stability and the state of the oxide film.     

Effect of surface modification using various acids on electrodeposition of lithium

Sulface modification of lithium was carried out using the chemical reaction of the native film with acids (HF, H₃PO₄, HI, HCl) dissolved in propylene carbonate (PC). The chemical composition change of the lithium surface was detected using X-ray photoelectron spectroscopy. The electrodeposition of lithium on the as-received lithium or the modified lithium was conducted in PC containing 1.0 mol dm^–3 LiClO₄ or LiPF₆ under galvanostatic conditions. The morphology of electrodeposited lithium particles was observed with scanning electron microscopy. The lithium dendrites were observed when lithium was deposited on the as-received lithium in both electrolytes. Moreover the dendrites were also formed on the lithium surface modified with H₃PO₄, HI, or HCl. On the other hand, spherical lithium particles were produced, when lithium was electrodeposited in PC containing 1.0 mol dm^–3 LiPF₆ on the lithium surface modified with HE However spherical lithium particles were not obtained, when PC containing 1.0 mol dm^–3 LiClO₄ was used as the electrolyte. The lithium surface modified by H₃PO₄, HI, or HCl was covered with a thick film consisting of Li₃PO₄, Li₂CO₃, LiOH, or Li₂O. The lithium surface modified with HF was covered with a thin bilayer structure film consisting of LiF and Li₂O. These results clearly show that the surface film having the thin bilayer structure (LiF and Li₂O) and the use of PC containing 1.0 mol dm^–3 LiPF₆ enhance the suppression of dendrite formation of lithium.     

Morphology of PbSO4 formed on solid Pb in concentrated sulphuric acid: a note pertaining to a recent paper

A scanning electron microscopy (SEM) study of the formation of PbSO₄ on Pb in H₂SO₄ at concentrations greater than 5 mol dm^?3 is presented. The production of a tight and mechanically sound PbSO₄ film in concentrated H₂SO₄ postulated in an earlier communication is confirmed.     

Electrochemical formation of green rusts in deaerated seawater-like solutions

Carbon steel electrodes were polarised at a potential ∼150 mV higher than the open circuit potential, in a deaerated seawater-like electrolyte (0.5 mol dm⁻³ NaCl, 0.03 mol dm⁻³ Na₂SO₄, 0.003 mol dm⁻³ NaHCO₃). X-ray diffraction and μ-Raman analysis demonstrated that a layer mainly composed of GR(SO₄²⁻) had grown on the steel surface. GR(SO₄²⁻) was accompanied by traces of GR(CO₃²⁻). Similar experiments performed in a solution composed of 0.3 mol dm⁻³ of Na₂SO₄ and 0.03 mol dm⁻³ of NaHCO₃ led to the same result. The nature of the GR forming on steel is thus mainly linked to the sulphate to carbonate concentration ratio. Finally, carbon steel coupons immersed for 11 years in the harbour of La Rochelle (Atlantic coast) were removed from seawater for analysis. The inner part of the rust layer proved to be mainly composed of magnetite, GR(SO₄²⁻) and iron sulphide FeS. This definitively confirms that GR(SO₄²⁻), as Fe₃O₄ and FeS, can form from steel in O₂-depleted environments.     

Electrodeposition of catalytically active Co-Ni-Tl alloy powders from dilute sulphate baths

Cobalt-nickel-thallium alloy powders were electrodeposited from dilute metal sulphate baths of composition: 0.007–0.0245 mol l^–1 CoSO₄·7H₂O, 0.0245–0.007 mol l^–1 NiSO₄·6H₂O, 0.001 mol l^–1 TlCl, 0.5 mol l^–1 (NH₄)₂SO₄, 0.07 mol l^–1 Na₂SO₄·10H₂O and 0.4 mol l^–1 H₃BO₃. The cathodic polarization curves were traced during electrodeposition and utilized in the discussion of a reaction mechanism for the electrolytic powder deposition. The alloy composition and the cathodic current efficiency were influenced to a great extent by the bath composition (I) and slightly by the deposition current density (II). Irrespective of variables (I) and (II), the electrodeposition of the alloy belonged to the anomalous type. The surface morphology and the catalytic activity, towards the decomposition of 0.4% H₂O₂ solution, of the as-deposited alloy powders were affected predominantly by the percentage of cobalt in the alloy. X-ray diffraction studies showed that the alloys consisted mainly of the face-centred cubic nickel phase either alone or with minor proportions of face-centred cubic cobalt phase and hexagonal close-packed -cobalt phase. The occurrence of the latter phases was observed only in the alloys with a higher cobalt percentage than nickel.     

Anode-support system for the direct electrorefining of cement copper Part I: Process conditions using horizontal rotary cathodes

This paper describes a preliminary study towards a modified anode-support system for the direct electrorefining of cement copper. The proposed system is an alternative process for small mines that produce cement after leaching of copper oxide minerals. It is feasible to utilize a cell provided with a horizontal AISI-316 stainless steel mesh covered with a layer of cement copper as anode system. As cathode system, several horizontal rotary cylinders of AISI-316 stainless steel can be used. The rotary cylinders, partially immersed in acidic copper sulphate electrolyte, must have an internal electrical contact with a copper shaft and two or more isolated slots at the surface to permit the stripping of the copper deposits in the form of thin sheets. The cement copper must be maintained wet to avoid rapid oxidation. The use of pulsed current permits control of the formation of a non-conductive copper sulphate layer on the cement surface, avoiding a continuous increase in cell voltage. As operating conditions a solution of CUSO₄.5H₂O (160 g dm^–3) and H₂SO₄ (50 g dm^–3) with j _c = 5.0 A dm^–2, T = 40 °C, u = 200 rpm; and pulsed current with t _d = 3 min and t _d/t ₀ = 20 can be used. At this stage of the study cement copper (90–95% purity) was utilized.     

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.     

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.     

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.     

Self-Assembly of Metalloporphyrin-L-Cysteine Modified Gold Electrode

A novel composite film containing metalloporphyrins was fabricated by in situ electrochemical scanning on an L-cysteine self-assembled monolayer modified gold electrode. SEM and ATR-FTIR were used to characterize the structure of the film. The electrochemical properties were investigated through techniques such as a.c. impedance, cyclic voltammetry and chronocoulometry. The porphyrin-L-cysteine film showed no peak in the first cycle, while each of the composite films derived from three different metalloporphyrin-L-cysteines presented a pair of reversible redox peaks in 1.0 mol L^–1 H₂SO₄. These peaks correspond to the rapid redox process of the metal. The supporting electrolyte and its pH value influenced the stability and sensitivity of the composite film. Cupric-porphyrin-L-cysteine film showed good catalytic activity for the reduction of H₂O₂. The catalytic current was linear to H₂O₂ concentration in the range 1.0 × 10^–6 to 3.0 × 10^–5 mol L^–1, with a correlation coefficient of 0.9995. The detection limit was 1.0 × 10^–7 mol L^–1 at a signal to noise ratio of 3. The relative standard deviation was calculated as 2.4% for solutions containing 1.0 × 10^–5 mol L^–1 H₂O₂(n= 11).     

Electrochemical production of ceric sulphate in concentrated H2SO4

The electrochemical preparation of ceric sulphate has been studied in concentrated H₂SO₄(10 mol dm^?3) and we have found that the current efficiency of the reaction increases considerably if a mixed catalyst is used (Ag₂SO₄ + MnSO₄).