Zinc electrowinning from aqueous chloride electrolytes

Smooth, compact, dendrite-free zinc deposits can be electrowon from acid zinc chloride electrolyte using a diaphragm cell featuring catholyte agitation by air sparging. A correlation exists between the degree of zinc deposition polarization and the zinc deposit structure and orientation that can be used to evaluate the effectiveness of addition agents and, ultimately, to control deposit quality. The effect of impurities on the current efficiency for zinc deposition and on the morphology and orientation of 24h zinc deposits is discussed.     

Effects of organic additives on zinc electrodeposition from alkaline electrolytes

This work reports the effects of four organic compounds (referred to as levelers) on the electrodeposition of Zn on steel from alkaline free-cyanide electrolytes. The additives tested included polyvinylalcohol (PVA) and the condensation products of epichlorhydrin with amines, called polyamines (PAs), that were synthesized using an aliphatic amine (PA-I, from diethylamine and PA-II from diethylamine-triethylamine), and a heterocyclic quaternary imidazolium molecule (PA-Imid, from imidazole). These compounds were evaluated in the absence and in addition to a quaternary ammonium brightener, N-benzyl-3-carboxylpyridinium chloride (3NCP). The imidazole derivative-based polyamine (PA-Imid) causes greater inhibition of the zinc reduction process than the aliphatic polyamine, and more cathodic overpotential is necessary to promote massive metal deposition. The morphology of the deposits is modified when polyamines are added to the bath; more compact and smaller crystals are obtained with PVA as well as with polyamine PA-I. The addition of PA-II as well as PA-Imid yields crystals growing perpendicular to the substrate. The addition of 3NCP with PVA, PA, or PA-Imid increased the deposition overpotential and modified the morphology by diminishing the grain size. In the absence of additives, crystallographic orientation favored the basal Zn(002) with high atomic packing. The addition of the levelers favored the high-angle pyramidal Zn(101) with low atomic packing. The combination of the levelers with (3NCP) favored the prismatic Zn(100) crystallographic orientation. Additives decrease the size of zinc crystals and tend to increase the energy of lattice favoring the growth of pyramidal and prismatic planes.     

Investigation of optimal conditions for zinc electrowinning from aqueous sulfuric acid electrolytes

Results are reported of an experimental investigation of the effects of aqueous zinc(II) and sulfuric acid concentrations on current efficiencies and deposit morphologies of metallic zinc, aimed at designing a process for zinc recovery from solid industrial wastes by leaching and electrodeposition. Voltammetry and chronopotentiometry of additive-free solutions of zinc(II) sulfate and sulfuric acid were used to determine the zinc(II) reduction kinetics, prior to investigating the deposition in a Hull cell to observe the effects of the current density and the bath composition on current efficiencies and deposit morphologies. For a current density of 45 mA cm⁻², best performance was obtained with zinc(II) concentrations of the same order of magnitude as the H⁺ concentration; too acidic solutions resulted in lower current yields and pronounced 3-D growth of the deposit.     

Nickel and cobalt synergism effect in zinc electrowinning from sulphate electrolytes

An investigation has been made on the mechanism and kinetics of the electrochemical processes occurring during zinc electrowinning from sulphuric acid electrolytes containing Ni²⁺, Co²⁺ and mixtures of the two impurities. The electrochemical nature of the synergism effect manifested by the common action of these two metal impurities has been established. It has been demonstrated that as a result of specific adsorption of (Co(SO₄))^(2n-2)– anions onto the zinc electrode, the partial discharge rate of Ni²⁺ is catalyzed, leading to an increase of its bulk content in the zinc deposits.     

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.     

The coulombic efficiency of zinc electrowinning in high-purity synthetic electrolytes

Measurements of coulombic efficiency (QE) for zinc electrodeposition were carried out under mass transfer-controlled conditions using a rotating disc electrode in synthetic acidic zinc sulphate electrolytes. At 25°C in 0.8 M ZnSO₄+1.07 M H₂SO₄ prepared from reagent grade chemicals, the QE at an aluminium cathode was 95.7–97.6%. In order to study the influence of electrolyte purity on QE several preparation and purification techniques were employed. While different sources of chemicals produced different QEs, the main source of impurities seemed to be the zinc-containing reagent rather than the sulphuric acid. Improvements in purity either had a negligible effect or lowered the QE, indicating that some impurities are beneficial to electrolyte performance. In the purest solutions prepared, an effect due to residual impurities still seemed to be present. The maximum QE obtainable through variation of the three parameters, i.e. temperature, current density and electrode rotation rate, was determined for two electrolytes of different purities; the values of QE obtained were 98.4 and 98.8%, with temperature as the dominant factor. Wark's Rule (the dependence of QE on zinc/acid ratio) was obeyed approximately in the purest electrolyte prepared, over a limited range of composition.     

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.     

Electrolytic manganese metal from chloride electrolytes. II. Effect of additives

Results of beaker scale and large laboratory scale experiments on the deposition of manganese from chloride electrolytes using selenium based additives are reported. High cathode efficiencies (85–90%) were obtained at moderate selenium levels (0.03–0.06 gl^–1). Selenium pick-up in the deposit was lower when selenium was added as the selenate [Se(VI)].A cost/benefit analysis of selenium additions is presented.     

New boron compounds as additives for lithium polymer electrolytes

A new class of difluoroalkoxyborane compounds ([R_nOBF₂]₂) containing oligooxyethylene groups of various molecular weight in the form of a methyl monoether (R_n = CH₃(OCH₂CH₂)_n, n = 1, 2, 3 and 7) has been obtained in the reaction of BF₃ etherate with appropriate glycols. ¹H, ¹¹B and ¹⁹F NMR spectral analysis of the derivatives obtained was carried out and the properties as Lewis acids of these derivatives have been compared with that of corresponding trialkoxyboranes and boron trifluoride in reaction with pyridine. The strength of the interaction of [R₂OBF₂]₂ with the differing in “hardness” anions of various lithium salts has been analyzed on the basis of NMR spectra. The [R_nOBF₂]₂ obtained were used as additives for polymer electrolytes containing PEO as polymer matrix and various lithium salts at an equimolar ratio of the boron compound to salt. The highest ionic conductivities, in the order 10⁻⁵ to 10⁻⁴ S cm⁻¹ at 20-70 °C, were achieved for systems containing LiI and LiN(CF₃SO₂)₂. The lithium transference number (t₊) values, determined by the electrochemical method by steady-state technique for LiF and LiCF₃SO₃ are in the 0.6-0.8 range.     

The effects of metallic impurities and 2-butyne-1,4-diol on zinc electrowinning from chloride solutions

Zinc electrowinning tests from acidic chloride solutions showed that metallic impurities can be classified in the following order of detrimental effect on current efficiency: Sb>Co, Ni>Cu. This effect increased with higher HCl concentrations. Analysis of the impurity content of the zinc deposits showed the regular codeposition of Cu and the anomalous codeposition of Ni and Co.The use of 2-butyne-1,4-diol (0.86 g l^–1) was proven to be beneficial in the presence of Sb and particularly Ni. Both current efficiency and zinc deposit morphology were improved. The additive acted by selectively slowing the codeposition of the impurities with zinc.     

On the nature of the ‘induction period’ during the electrowinning of zinc from nickel containing sulphate electrolytes

As a result of detailed voltammetric, impedance, electron microscopic and opticalin situ investigations of the peculiarities encountered in zinc electrowinning from nickel-containing acid electrolytes, a model for the induction period is proposed and its dependence on the process conditions is elucidated. The model is based on the screening effect of hydrogen bubbles formed on the nickel-rich regions of the cathode which give rise to local galvanic cells.Nomenclature C electrode capacitance - C _dl double layer capacitance - C _Ni volume concentration of Ni²⁺ ions - d diameter of the circle along which the hydrogen bubble is attached to the surface - D _Ni diffusion coefficient of Ni²⁺ ion - E _ze potential of zero charge - f ^* frequency at the apex of the capacitance loop - F Faraday constant - F _c capillary force - F _h hydrostatic force - g acceleration due to gravity - q specific mass of the liquid - h height of the hydrogen bubble - R electrolyte resistance - R _t charge transfer resistance - V volume of the gas phase - thickness of the diffusion layer - wetting angle at the metal-solution-gas interface - ₁₂, ₂₃, ₁₃ surface tensions between: solid-liquid, liquid-gas and solid-gas phases, respectively - kinematic viscosity - rotation speed of the cathode The first results were presented at the International Conference on Base Metals Technology, 8–10 February, 1989, Jamshedpur, India.     

Thermodynamic studies on sulphate roasting for zinc electrowinning from carbonate ore

The bulk of the work consists of a theoretical study of the possibility of submitting Umm-Gheig carbonate ore to sulphate roasting. The use of the admixture with pyrites is to enable a carbonate ore to be treated in a similar way to a sulphide ore, and by doing so, to produce a roasted product capable of being treated by orthodox zinc electrowinning methods using sulphate solutions. Thermodynamic studies have been made to find the optimum conditions for sulphate roasting, in either normal air or enriched 36% oxygen air. The results obtained from the experimental work at different roasting temperatures in a tube furnace indicated that a maximum dissolution of 91.2% Zn with a 17.9% Fe could be obtained at a roasting temperature of 650°C for 4 h, followed by leaching in 4% H₂SO₄ (by vol.) at 60°C. The results of the electron microscopic investigation confirmed by metal value data given in the ASTM cards coincide well with results given by chemical analysis.     

锌电积用节能阳极的研究现状

综述了新型铅合金阳极、钛基尺寸稳定阳极(DSA)、铝基阳极、复合阳极等锌电积用节能阳极材料的研究现状,展望了其发展趋势。     

Utilization of gas-diffusion electrodes catalysed with tungsten carbide as anodes for zinc electrowinning

Gas-diffusion electrodes with varying amounts of tungsten carbide catalyst are studied, and their feasibility as suitable anodes for zinc electrowinning is demonstrated. The energy savings achievable with the use of such anodes are evaluated.     

Influence of Zn2+ ions on copper electrowinning from sulfate electrolytes

During electrowinning of copper from wastes, it was observed that Zn2+ impurities had some effects on the aspect of the deposits although the codeposition of these two metals is impossible under normal conditions. Cyclic voltammetry, impedance analysis and rotating disc electrode techniques were used to characterize the effect of the presence of small amounts of Zn2+ on the copper electrodeposition kinetics from acidic sulphate electrolytes. SEM and X-ray diffraction analysis were used to determine the morphology and the structure of the copper deposits. Their composition was determined using X-ray dispersive analysis. The addition of small amounts of Zn2+ had no effect, either on the crystal growth orientation of the copper deposits, or on their composition, but it modified slightly their morphology by increasing their microscopic roughness. An increase in the cathodic peak of the cyclic voltammograms observed in the presence of Zn2+ can be attributed to the higher roughness of the copper deposits. From impedance analysis, it can be deduced that Zn2+ ions are electrosorbed at the interface and have an effect on the copper electrodeposition kinetics. The adsorption capacitance, Ca, was calculated and correlated with the Zn2+ concentration. It was found that the nucleation time constant, parameter influenced by the growing mode of the deposit, decreased in the presence of Zn2+, especially at low concentrations.     

The effect of additives on the morphology of zinc electrodeposited from a zinc chloride electrolyte at high current densities

There are advantages if zinc can be deposited electrochemically at high current densities. In order to improve the deposit at current densities exceeding 2000 Am^–2, various additions were made to the electrolyte. It was found that proteinous additives produced planar deposits with a (10.2) (10.3) orientation and a current efficiency of around 85% in a small laboratory cell.     

A validated mathematical model for a zinc electrowinning cell

A set of (95) equations forming a dynamic, nonlinear model of an industrial pilot-plant scale zinc electrowinning cell fed with high purity electrolyte is presented. Only the solution of the steady-state model is considered in this paper. Values for unknown model parameters have either been obtained from the literature or else estimated using experimental data taken from the pilot-plant cell. Sensitivity studies showed that uncertainties in the temperature dependency of the zinc and hydrogen reaction exchange current densities and the exchange coefficient for the hydrogen reaction have a major effect on the model predictions. Excellent agreement between predicted and experimental results was obtained, provided that cathodic mass transfer effects were included in the model. Both parameter estimation and solution of the steady-state model were carried out using the SPEEDUP flowsheeting package.     

新型氯化钾镀锌三价铬钝化剂和膜层耐蚀性

通过测定Tafel极化曲线、交流阻抗谱和进行盐雾试验,研究了新型三价铬彩色钝化剂SF-571和蓝白钝化剂SF-572对镀锌层耐蚀性能的影响,并与进口钝化剂进行对比。氯化钾镀锌工艺为:KCl200g/L,ZnCl250g/L,H3BO335g/L,柔软剂30mL/L,光亮剂1.5mL/L,25°C,25min,电流密度2A/dm2。电化学分析表明,所有镀锌层钝化膜在1%NaCl溶液中均存在钝化现象,采用自制钝化剂SF-571和SF-572所得膜层的耐腐蚀性能达到或优于用进口钝化剂所得膜层。对比膜层的表面形貌可知,自制钝化剂钝化所得膜层表面更平整、致密。     

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.