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.     

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.     

In situ electrochemical Scanning Kelvin Probe Blister-Test studies of the de-adhesion kinetics at polymer/zinc oxide/zinc interfaces

Fundamental investigations of the polymer/zinc oxide/zinc interface corrosion stability were performed in situ by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister-Test (HR-SKP-BT) under controlled atmospheric conditions. A hole under an adhesive layer film served as electrolyte reservoir to initiate cathodic de-adhesion processes. Then a combinatorial approach was undertaken to simultaneously study the influence of electrolyte pressure at constant defect polarisation and of relative atmospheric humidity on the de-adhesion rate. The time resolved blister growth and the propagation of the three phase boundary polymer/oxide covered zinc/interfacial electrolyte layer could be detected. It could be proven that the oxygen reduction induced electrochemical damage of the interface precedes the subsequent mechanical de-adhesion process. By variation of the relative atmospheric humidity the water concentration within the bulk adhesive and its interphase adjacent to the metal substrate could be adjusted. These processes were further analysed by peel-tests and in situ Attenuated-Total-Reflection Infrared Spectroscopy (ATR-IR) studies of water diffusion. A decrease of the interphasial water concentration led to a deceleration of the de-adhesion kinetics for constant defect conditions and to smaller interfacial ion transport rates. This could be assigned to an inhibition of the electron transfer reactions at the front of de-adhesion and an increased adhesion force between polymer film and oxide covered metal preventing the formation of an extended interfacial electrolyte layer.     

Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al–F–H₂O, Zn–F–H₂O and Fe–F–H₂O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al–Fe alloy accelerates the corrosion of aluminium in H₂SO₄ and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.     

Use of experimental design to empirically model atmospheric corrosion of galvanised steel

Electrochemical impedance spectroscopy in 500–1100 μm large liquid droplets containing chloride and sulphate ions has been used to investigate the local corrosion rate of galvanised steel. The objective was the empirical modelling of the corrosion rate as a function of temperature, electrolyte thickness, and chloride and sulphate concentrations. The first experiments showed that the corrosion time was one of the critical parameters; it was then included in the investigation. An empirical numerical model was obtained for steel and galvanised steel. The statistical model highlighted the respective importance of the five parameters on atmospheric corrosion and gave qualitative results in accordance with the literature. The statistical study also suggested that the corrosion potential was easier to model than corrosion currents.     

Enhanced methods for experimental investigation of single droplet drying kinetics and application to lactose/water

In this work, a suspension device is presented that can be used for the investigation of single droplet drying kinetics at temperatures from ambient to 200°C. Evaluation is hybrid, using both camera data and a water mass balance. In this way, drying kinetics can be captured for shrinking droplets and also after a constant diameter of the wet particle has been reached and during inflation/deflation periods. Results can be further enhanced by X-ray μ-CT investigation of the internal morphology of dried particles. It is proposed to read critical conditions for crust formation from a plot of evaporation flux over water mass fraction. Repeatability and accuracy of the experimental and evaluation methods were checked with pure water droplets. Moreover, the entire evaluation was conducted for aqueous solutions of lactose with various initial concentrations.     

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.     

High throughput electrochemical screening and dissolution monitoring of Mg–Zn material libraries

A combinatorial study on Mg–Zn material libraries obtained by thermal evaporation is performed in order to investigate the effect of alloying magnesium on the electrochemical behaviour and dissolution rate of zinc in borate buffer of pH 7.4. The surface morphology of the graded samples is complex and subject to a detailed discussion, whereas the crystal composition revealed Mg, MgZn₂ and Zn exclusively.Open circuit potential measurements and potential sweeps along the graded samples are combined with downstream zinc detection and revealed several strongly non linear dependencies between electrochemical features and magnesium content. While the chemical dissolution rate of zinc by the electrolyte was found to reflect the film stoichiometry except in the regions of high surface roughness, the open circuit potential revealed a local minimum around 20 at.% magnesium accompanied by a maximum in the current plateau during the anodic sweep and a high thickness of the native oxides present prior to electrochemical experiments. All compositions showed passive like behaviour during anodic sweeps with high plateau current densities (200–350 μA cm⁻²) originating from slow but constant oxide dissolution as supported by XPS analysis of the surface before and after contact with the electrolyte.     

Reaction Engineering Approach (REA) to model the drying kinetics of droplets with different initial sizes—experiments and analyses

Droplets with different initial sizes, which are typical in conventional liquid atomization for spray drying applications, will result in varying drying and crust formation histories. It is essential for any droplet drying model to accurately capture such fundamental phenomena. This study used a newly constructed glass-filament single droplet rig to evaluate the applicability of the Reaction Engineering Approach (REA) in describing such effect. For the three initial sizes (1, 2 and 3 μL) tested, the glass filament gravimetric method clearly distinguished the different drying kinetics and the crust formation phenomenon, delineated by the drying behavior. Analysis from the drying kinetics revealed that the main premise of the REA, which utilizes a material-specific master activation energy curve, is applicable to droplets of different initial sizes at all the three air temperatures tested. This allowed the REA to accurately predict the different temperature and moisture histories given by droplets with different initial sizes. The result supports the REA as a good modeling approach for a wide range of initial droplet conditions. A new master curve approach was proposed to predict the diameter change of droplets with different initial concentrations. Validation with the current and past experimental data revealed that this approach has strong potential to account for the different feed concentrations typically found in spray drying applications.     

All solid-state nickel/metal hydride battery with a proton-conductive phosphoric acid-doped silica gel electrolyte

All solid-state nickel/metal hydride (Ni/MH) battery was fabricated by using a proton-conductive phosphoric acid-doped silica gel as an electrolyte. The H₃PO₄-doped silica gel was dried in vacuum at various temperatures for 1 h before fabricating the battery in order to reduce the possibility of corrosion by water on the surface and in the micropores of the gel. The influence of drying temperature on the crystallinity, water content and conductivity of the gel was investigated. The conductivity depended on the drying temperature of the gel. The fabricated all solid-state Ni/MH battery was able to operate several tens of charge-discharge cycles at relatively high current density although the utilization of the battery was low.     

Effect of zinc on the electrocrystallization of cobalt

The effects of zinc on the current efficiency, power consumption, deposit quality and contamination of cathode deposit during electrocrystallization of cobalt were studied. The presence of zinc affected current efficiency at all temperatures and the effect was intensified at lower temperature. Increase in power consumption was significant at lower temperature. The quantity of zinc in the electrodeposited cobalt invariably increased with increase in zinc concentration in the electrolyte, however, the zinc content of the deposit was significantly low at higher temperature. The tolerance limit of zinc in the electrolyte with respect to deposit quality was also greater at higher temperature. Increase in current density decreased the current efficiency, increased the power consumption and lowered the cathode contamination.     

Improvement of the anticorrosion properties of polypyrrole by zinc phosphate pigment incorporation

A commercial zinc phosphate pigment was incorporated into polypyrrole (PPy) matrix during its electrochemical synthesis in order to improve the corrosion protection of polypyrrole on AISI 1010 steel. PPy/zinc phosphate composite films were synthesised in sodium salicylate medium with high current efficiency and containing 10% by weight of zinc and 4% by weight of phosphate. The influence of stirring and concentration of the electrolyte on the degree of pigment incorporation were investigated, as well as polymerisation time and applied current density. The morphology of the films was determined by scanning electron microscopy (SEM) and the distribution of pigment in the polymeric matrix was carried out by X-ray photoelectron spectroscopy (XPS). The PPy and PPy/zinc phosphate films were submitted to salt spray corrosion test, weight loss test and to electrochemical measurements like corrosion potential with time. In all tests, the composite films showed an enhancement in its protective action in comparison with PPy films.     

Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.     

Corrosion and polarization characteristics of zinc in battery electrolyte analogues and the effect of amalgamation

The corrosion and polarization characteristics of zinc have been investigated in concentrated NH₄Cl and NaCl solutions which represent suitable electrolyte analogues for Leclanché and zinc chloride cells, respectively. The effects of zinc amalgamation in the range 0–1 mg Hg cm^?2 have also been examined and the results compared and contrasted with those obtained in concentrated KOH solution which is a useful electrolyte analogue for alkaline cells. It is shown that the structure of the zinc-solution interphase and the electrochemical reactions which occur are critically dependent upon the particular electrolyte considered. Despite this, the absolute corrosion rates of unamalgamated zinc are of the same order. This is an unexpected result. The effectiveness with which both dissolved zinc salts and mercury reduce the corrosion rate decreases in the electrolyte order KOH ? NH₄Cl > NaCl This is explained by the increasing participation of diffusion as the factor which controls the rate of the cathodic hydrogen evolution process. Maximum effectiveness for corrosion inhibition is approached when the mercury level exceeds 100μg cm^?2. At this concentration, coverage of the surface by a zinc rich amalgam of almost constant composition is complete.     

Comparison of the efficiency of inorganic nonmetal pigments with zinc powder in anticorrosion paints

This paper deals with the study of properties of anticorrosion pigments of varying chemical composition in epoxyester paints. Two type lines of paints were prepared. The first line comprised an anticorrosion pigment with a PVC concentration of 10% while the other line comprised an anticorrosion pigment with a PVC concentration = CPVC. The following nontoxic anticorrosion pigments were observed: zinc phosphate, zinc phosphomolybdate, calcium hydrogen phosphate, zinc phosphate modified with an organic corrosion inhibitor, strontium–aluminum polyphosphosilicate, zinc–aluminum polyphosphate, calcium metaborate, calcium ferrite, calcium borosolicate, and strontium chromate. The epoxyester primers were observed for the effect of the type of pigment on the anticorrosion properties. Anticorrosion efficiency was derived from tests in a condenser chamber and in a salt spray cabinet as well as from a test of chemical resistance of pigmented coatings. The evaluation of anticorrosion efficiency of inorganic nonmetal pigments was carried out by means of comparison with anticorrosion efficiency of metal dust.     

Methodology for zinc phosphate pigment incorporation into polypyrrole matrix

In this work, the incorporation of a commercial zinc phosphate pigment into polypyrrole (PPy) matrix during its electrochemical synthesis on mild steel was studied in order to produce PPy/zinc phosphate composite films as a protective layer against corrosion. Potassium nitrate, oxalic acid, tosylic acid and sodium salicylate solutions were used as electrolytes in cyclic voltammetry and galvanostatic polarisation studies. The influence of synthesis parameters such as nature, concentration, pH and stirring of the electrolyte on the degree of incorporation of the pigment was investigated, as well as time and current densities of the electropolymerisation process. Their influence on current efficiency was also evaluated. Sodium salicylate was the only electrolyte to show a high current efficiency in the polymerisation reaction and to yield a composite film with a reasonable amount of zinc phosphate. In this electrolyte medium, X-ray photoelectron spectroscopy (XPS) analysis showed that zinc phosphate may be found in the polymeric matrix: (i) as a conductive ionic minority form and (ii) as a non-conductive non-ionic majority one for higher incorporation levels. Scanning electron microscopy (SEM) showed that zinc phosphate is heterogeneously distributed on the surface of the polymer.     

Electrochemical impedance study on brass corrosion in NaCl and (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> solutions during cyclic wet–dry conditions

Atmospheric corrosion of brass in sodium chloride (NaCl) and ammonium sulfate (NH₄)₂SO₄ solution during wet/dry cyclic conditions was investigated. The effects of various parameters on the corrosion rate were studied including temperature, pH and surface inclination. The polarization resistance of brass samples, subjected to 1 h immersion and 7 h drying at 60% RH was monitored using an AC impedance. The corrosion rate measured at low temperature was found to be significantly lower than that observed at high temperature. The average reciprocal polarization resistance (V_ARPR) was also calculated during exposure. It decreased with the number of exposure cycles during the initial stages and attained a steady state value during the last stage of exposure. An atmospheric corrosion mechanism for brass, describing the successive stages during exposure, is subsequently proposed.     

Unraveling the droplet drying characteristics of crystallization‐prone mannitol – experiments and modeling

Spray‐dried mannitol is a potential lactose replacement in pharmaceutical formulations, yet the drying behavior of individual mannitol droplets within the spray chamber has not been fully understood. This work explored the drying characteristics of mannitol by employing the reaction engineering approach (REA) in data analysis. A glass filament single droplet drying technique was used to monitor the changes in droplet temperature, mass, and diameter. The drying kinetics data obtained clearly demonstrated the droplet “wet‐bulb” period, the crust formation, and the crystallization phenomena. The master activation‐energy curves developed from REA modeling responded sensitively to varying drying temperatures, which could have led to different crystallization events. The deviation of these plots from the expected norms that do not encounter a phase change was used effectively to discern the physics involved. A REA kinetic model was proposed to assist in process optimization of large‐scale spray‐drying operations. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1839–1852, 2017     

甘露醇喷雾干燥过程中液滴粒度分布变化的群体粒数衡算模拟和实验研究

利用群体粒数衡算（population balance，PB）计算机模拟和实验研究了甘露醇水溶液的喷雾干燥过程中液滴的粒度分布的变化规律。液滴干燥过程中的颗粒粒度的萎缩速率，在群体粒数衡算模型中描述为液滴的逆（或负）生长项，通过单个液滴反应动力学方法（reaction engineering approach，REA）获得。基于单个液滴干燥的反应工程方法模型REA和群体粒数衡算模型PB集成建立了PBREA模型。PBREA 模型的求解是通过高分辨率数值方法。本文模拟研究了不同工况下，不同粒径液滴的干燥时间、液滴平均含湿量以及液滴粒度分布随时间的变化。结果显示，液滴粒径越大，干燥时间越长，模型预测的颗粒平均粒径为实验值的1.0~1.5倍，粒度分布跨度是实验值的0.61~0.89倍。模拟误差主要来源于液滴及颗粒粒径分布统计精度、单个静止液滴与群体运动液滴干燥的差异、热导率及扩散系数是经验值3个方面。在使用Buchi 290 小型喷雾干燥仪进行的实验中，使用了图像采集和分析方法得到了液滴及颗粒的数密度分布，并和模拟结果做了对比。结果表明该模型可以有效地预测喷雾干燥过程中干燥颗粒的平均粒度及分布跨度。