The effects of nickel and cobalt and their interaction with antimony on zinc electrowinning from industrial acid sulphate electrolyte

The effects of nickel and cobalt and their interaction with antimony on the electrowinning of zinc from industrial acid sulphate electrolyte were studied using X-ray diffraction, scanning electron microscopy and cyclic voltammetry. Concentrations of cobalt as high as 20 mgl^–1 had no effect on the zinc deposition current efficiency. The current efficiency decreased rapidly when the electrolyte contained >5 mgl^–1 nickel. Neither cobalt or nickel had an effect on the morphology of the 1-h zinc deposits. Nickel and cobalt caused characteristic changes in the cyclic voltammograms for zinc deposition. As a result this technique might provide a rapid means for evaluating the electrolyte prior to zinc electrowinning. The combined presence of cobalt and antimony in the zinc electrolyte was more deleterious to zinc electrowinning than was the combined presence of nickel and antimony. The presence of 0.08 mgl^–1 antimony in the electrolyte counteracted the effect of nickel both on the current efficiency for 1-h deposits and on the zinc deposition polarization curves.     

The effect of tin on zinc electrowinning from industrial acid sulphate electrolyte

The effect of tin on the electrowinning of zinc from industrial acid sulphate electrolyte was studied using cyclic voltammetry, X-ray diffraction and scanning electron microscopy techniques. Tin concentrations < 2 mg l^–1 resulted in a rapid decrease in the current efficiency for l-h zinc deposits electrowon at 430 A m^–2; for tin concentration > 2 mg l^–1, the current efficiency remained constant at about 75%. The presence of tin in the electrolyte depolarized the zinc deposition reaction and resulted in a zinc deposit having a preferred [002] orientation. The addition of glue appeared to aggravate rather than counteract the adverse effects of tin.     

The effects of saponin,antimony and glue on zinc electrowinning from Kidd Creek electrolyte

The effects of Saponin alone and in combination with antimony and glue on zinc deposition current efficiency and polarization and on the morphology and orientation of 6h and 24h zinc deposits electrowon at 500 A m^–2 and 38°C from Kidd Creek zinc electrolyte were determined. Saponin, at concentrations to 100mgl^–1, was weakly polarizing, changed the preferred deposit orientation from basal to intermediate and decreased the current efficiency. At optimum glue + antimony levels, Saponin concentrations 5mgl^–1 resulted in an increase in zinc deposition current efficiency and consistently gave either a <114><112><103><102><101> or a <101><112> preferred deposit orientation depending on the antimony + glue combination used. Tests run for 24h for selected combinations of Saponin, antimony and glue confirmed the results obtained for the 6h studies.     

Evaluation of zinc sulphate electrolytes containing certain impurities and additives by cyclic voltammetry

The polarization characteristics of acid zinc sulphate electrolytes containing various amounts of germanium and cobalt were examined by cyclic voltammetry. The effects of zinc and acid concentration, temperature, and surface preparation were also investigated. Small concentrations of impurities are shown to cause measurable changes in polarization behaviour. Levels as low as 0.02 mgl^–1 Ge and 0.1 mgl^–1 Co can be detected using this technique. The actual mechanism of impurity behaviour is more clearly delineated using this technique and evaluation of the data from these tests indicate that germanium and cobalt form local galvanic cells. The results of these short-term tests are shown to correlate with classical long-term efficiency tests. The deposit morphologies obtained for short-time cathodic cycles were also studied using scanning electron microscopy.     

Effect of nickel in zinc electrowinning under periodical reverse current

The effects of nickel impurities were investigated during the electrowinning of zinc in acidic media using periodical reverse current (PRC) along with anodic potential current response and cathodic current efficiency. The chemical and structural characteristics of the cathode deposit were also determined using nickel concentrations of 3, 5 and 10 mg l^–1 in sulphuric acid (200 gl^–1 H₂SO₄) containing 60 gl^–1 of zinc. The cathodic current efficiency drop caused by nickel was less detrimental in PRC than DC. It was found that the morphology of the deposit plays an important role in the redissolution process caused by impurities. The levelling effect caused by PRC decreases the rate of zinc redissolution, which results in an increase of current efficiency. However for concentrations equal to or greater than 10 mgl^–1, the levelling effect caused by PRC is no longer dominant. On the anode nickel did not affect the overpotential of the lead-silver electrode in PRC or DC modes.     

The effects of Dowfroth,antimony and Saponin on zinc electrowinning from Kidd Creek electrolyte

The effects of Dowfroth alone and in combination with antimony and Saponin on zinc deposition current efficiency and polarization and on the morphology and orientation of 6h and 24h zinc deposits electrowon at 500 Am^–2 and 38°C from Kidd Creek zinc electrolyte were determined. Dowfroth, at concentrations as low as 7 mg dm^–3, was strongly polarizing, changed the preferred deposit orientation from basal to intermediate and decreased the current efficiency and zinc deposit quality. Dowfroth had a positive interaction with antimony such that certain combinations of these reagents maximized current efficiency and improved the deposit quality. Saponin combined with various concentrations of Dowfroth resulted in a <1% change in current efficiency and did not modify the preferred deposit orientation. At optimum Dowfroth + antimony combinations, the addition of Saponin again did not affect significantly the current efficiency. The preferred deposit orientation was either [1 0 1][1 1 2] or [1 0 1] depending on the combination of these reagents employed. Tests run for 24h for selected combinations of Dowfroth, antimony and Saponin confirmed the results obtained for the 6h studies.     

The effects of chloride ion and organic extradants on electrowon zinc deposits

The effects of chloride ion and the organic extractants: Kelex 100, Versatic 911, di-2-ethylhexyl phosphoric acid, tri-n-butyl phosphate, LIX65N and Alamine 336 on the structure of 1h zinc deposits electrowon from synthetic and industrial acid sulphate electrolytes are presented. Under simulated tankhouse conditions the effect of chloride ion concentration on the zinc deposit morphology and orientation becomes significant only at the 500 mgl^–1 level. The tolerance limit of the zinc deposits to organic extractants such as Kelex 100 is extremely low. In some cases the adverse effect of these organic impurities is offset by the presence of chloride ion in the electrolyte. Purification of electrolyte contaminated with organic extractants by activated charcoal results in a vastly improved zinc deposit structure. Organic extractants such as Kelex 100 and TBP which have an adverse effect on the zinc deposits, also have a pronounced effect on the zinc deposition polarization curve.     

Electrochemical refining of tin through thin-layers of molten electrolytes

A new principle of tin refining in molten electrolytes is proposed. Its main feature is mass transfer between electrodes through a thin porous diaphragm of (0.5–1.0) × 10^–3 m thickness, which is impregnated with molten electrolyte. The distance between the electrodes is equal to the diaphragm thickness. A mixture of zinc chlorides, alkali chlorides and tin chlorides is proposed as an electrolyte. It is shown that thin layer electrolysis makes it possible to produce tin with an impurity content no more than 1 × 10^–3%. The thin layer electrolysis in melts reduces the specific electrical energy by a factor of 1.5–2.0 and the electrolyte consumption by a factor of 2.5–6.0 per ton of purified tin in comparison with current practice. The results show the proposed method of thin layer electrolysis in molten electrolytes to be promising for tin refining.     

The effect of thiourea on zinc electrowinning from industrial acid sulphate electrolyte

The effect of thiourea, with and without glue and antimony additions, on the current efficiency (CE) and polarization behaviour of zinc deposition and on the morphology and preferred orientation of the zinc deposits electrowon (at 430 A m^–2 and 35°C) from industrial acid sulphate electrolyte (55 g l^–1 Zn and 150 g l^–1 H₂SO₄) has been determined. Increasing concentrations of thiourea in the electrolyte decreased the CE for zinc deposition; the additional presence of antimony did not significantly alter the decrease in CE but the presence of glue resulted in a further substantial decrease in CE. Thiourea changed the zinc deposit morphology and orientation, and also altered the shape of the zinc deposition cyclic voltammogram.     

The effect of copper on zinc electrowinning from industrial acid sulphate electrolyte

The effect of copper on the electrowinning of zinc from industrial acid sulphate electrolyte was studied using X-ray diffraction, scanning electron microscopy and cyclic voltammetry techniques. Concentrations of copper as high as 50 mg 1^–1 had no effect on the zinc deposition current efficiency for 1-h deposits. Copper co-deposited with zinc and reduced the deposit grain size. The copper content of the zinc deposits increased with increasing copper concentration in the electrolyte and with decreasing current density. The cyclic voltammogram for copper-containing electrolyte was characterized by an appreciable cathodic current in the reverse scan after zinc dissolution indicating the presence of previously deposited copper on the cathode.     

Effect of chromate ion on zinc electrowinning from acid sulphate electrolyte

The effect of chromate ion and its interaction with antimony and glue on zinc electrowinning from acid sulphate electrolyte were studied. During electrolysis at 430 A m^–2 and 45°C, the chromate ion was found to polarize the cathodic reaction resulting in a refined zinc deposit having a morphology featuring a vertical orientation of the zinc platelets. Although chromium did not codeposit, the current efficiency for zinc deposition was dramatically decreased in the presence of > 1000 mg dm^–3. This effect became more pronounced when the electrolyte also contained additives such as antimony and glue.     

The role of zinc and sulfuric acid concentrations on zinc electrowinning from industrial sulfate based electrolyte

The effects of varying simultaneously the zinc/acid concentrations at a fixed total sulfate, on the current efficiency, energy requirements, and deposit physical characteristics for the zinc electrowinning, using Kidd Creek zinc electrolyte, were investigated. The electrowinning experiments were conducted using a laboratory scale apparatus, at plating cycles of 24 and 30 h, a current density of 500 A m^–2 and a temperature of 38 °C. These conditions are typical of those applied at the Kidd Creek zinc tankhouse. The reagents presently used at Kidd Creek, namely strontium carbonate, Saponin, Dowfroth 250, antimony and sodium silicate, were also continuously added to the cell electrolyte at levels similar to Kidd Creek practice. Scanning electron microscopy and X-ray diffraction techniques were used to characterize the deposits with respect to morphology and preferred orientation, respectively. Cyclic voltammetry was used to study the effect of the zinc/acid concentrations on the polarization behaviour of the electrolyte. In addition, the electrical conductivity of the Kidd Creek zinc electrolyte was measured and compared with other industrial sulfate-based zinc electrolytes.     

Electrodeposition of chromium from Cr (III) electrolytes in the presence of formic acid

Chromium coatings deposited from sulphate, chloride and perchlorate electrolytes based on the [Cr(H₂O)₄(HCOO)]²⁺ complex ion were investigated. The current efficiency reached 30% in the case of chloride electrolyte for various current densities in the range 4–10 A dm^-2 depending on pH. Such a large current efficiency is due to the catalytic effect of the chloride ions on the electroreduction of the chromium complex to metallic chromium. Deposition of chromium from sulphate electrolyte took place with a current efficiency of 16% which rose for higher pH and lower current densities. Semi-bright and bright coatings with thickness of approximately 10m of good adhesion to a copper electrode were deposited from chloride and sulphate electrolytes.     

The effects of antimony and glue on zinc electrowinning from Kidd Creek electrolyte

The individual and combined effects of antimony and glue on zinc deposition current efficiency and polarization and on the morphology and orientation of 6h zinc deposits electrowon at 500 A m^–2 and 38° C from Kidd Creek zinc electrolyte were determined. Glue increased zinc deposition polarization, reduced the deposit grain size, changed the preferred deposit orientation from basal to intermediate, but decreased the current efficiency. At concentrations above 0.02 mgl^–1, antimony had a strong depolarizing effect on zinc deposition resulting in a basal deposit orientation and very low current efficiency. Certain combinations of antimony and glue, however, optimized zinc deposition current efficiency and consistently gave an intermediate 114 112 103 102 101 preferred deposit orientation. A correlation was observed between the current efficiency (CE) and the nucleation overpotential (NOP) for zinc deposition such that the CE was a maximum when the NOP of the initial cell electrolyte was 120–130 mV and when the NOP of the final cell electrolyte was 100–110 mV.     

Hydrogen production during zinc deposition from alkaline zincate solutions

The determination of the amount of hydrogen produced during the electrodeposition of zinc from alkaline zincate solutions was carried out using the rotating ring-disc electrode (RRDE) technique. The experimental conditions for which the RRDE technique offers reliable results are discussed. Hydrogen production during zinc deposition was studied for a range of cathodic (disc) current densities (20–500 A m^–2) and electrolyte compositions (1–7 M KOH, 0.01–0.2 M zincate). It was found that an increasing amount of hydrogen was formed with increasing (disc) current density and decreasing KOH and zincate concentration. The impact of hydrogen formation during the charging process on nickel oxide/zinc secondary battery performance is expected to be small. It is concluded that in battery electrolytes (8 M KOH, 1 M zincate) hydrogen is formed chiefly by corrosion of the zinc electrode rather than by electrochemical formation during the electrochemical reduction of zinc.     

Effects of cobalt, temperature and certain impurities upon cobalt electrowinning from sulfate solutions

The effects of cobalt concentration, temperature and the presence of zinc, copper and iron ions in the electrolyte on current efficiency and cathodic quality were investigated by cyclic voltammetry and galvanostatic methods during cobalt electrowinning. The results showed that high cathodic efficiency of cobalt deposition was obtained from solutions containing cobalt concentration in the range 30–60 g l^–1. Current efficiency increased from 94% to 97% with increase in cobalt concentration to 60 g l^–1 at 20 °C. It was also found that increase in temperature to 50 °C enhanced the cobalt deposition reaction, along with the rate of hydrogen evolution, resulting in little change in current efficiency. The presence of foreign cations in the electrolyte not only adversely affects current efficiency but also promotes cracking and peeling.     

Identification of optimum conditions for zinc electrowinning in high-purity synthetic electrolytes

The optimum conditions for zinc electrowinning in synthetic acidic zinc sulphate electrolytes (0.8 M ZnSO₄+1.07 M H₂SO₄) were determined using response surface statistical analysis. The coulombic efficiency (QE) was optimized with respect to temperature (T), current density (J) and electrode rotation rate (n). For an electrolyte prepared from AR zinc sulphate and Aristar sulphuric acid, containing trace lead and nickel, QE reached a maximum of 98.8% on a zinc substrate under the following conditions:T=50°C,J=500 A m^–2,n=35s^–1. For a very-high-purity electrolyte, prepared by dissolution of 99.9999% zinc in Aristar sulphuric acid, a maximum QE of 98.4% was predicted and obtained at:T61°C,J890 A m^–2,n38s^–1. Using a statistical response surface model calculated during the optimization process, QE contours giving an overall view of electrolyte performance were constructed. The QE responses were determined principally byT andJ, with significant interaction betweenn andJ orT andJ, depending on the impurity composition of the electrolyte. The model was also used to predict the QE response of the above electrolytes under conditions similar to industrial practice.     

High speed zinc electrowinning using a hydrogen gas-diffusion electrode

The feasibility of a high speed zinc electrowinning cell using a Pt catalyzed hydrogen gas-diffusion electrode as an anode is investigated. This new type of zinc-winning cell is operated at a current density of 1.0 A cm^–2, which is 20 times higher than usually employed in conventional methods. Current efficiency is 86% at 0.5 A cm^–2 in an electrolyte containing 60 gl^–1 Zn+270 gl^–1 H₂SO₄, the zinc purity being at least 99.999%. The energy usage of the system is 1400 kWh and 380 m³ H₂ gas per ton of zinc.     

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.     

Electrolytic manganese metal from chloride electrolytes. I. Study of deposition conditions

Results of beaker scale and large laboratory scale experiments on the deposition of manganese from chloride electrolytes are reported for conditions where chlorine is the secondary anode product.Manganese concentration in the catholyte was found to be the most significant variable, with higher manganese levels (to 33 g Mn l^–1) resulting in higher current efficiency (65–70%) and lower power consumption (5.5 kWh kg^–1). Temperature (30–60°C) and current density (1.0–2.0 kAm^–2) had opposing effects on current efficiency and power consumption. In the ranges studied, lower temperatures and higher current densities gave rise to increases in current efficiency and power consumption.The literature on the electrowinning of manganese from chloride electrolytes is reviewed and the anomalies in previous reports discussed.