Aspects of the development of a copper electrowinning cell based on reactive electrodialysis

This paper reports work aimed at developing a new copper electrowinning cell based on reactive electrodialysis (RED) which uses Fe²⁺→Fe³⁺+e as anodic reaction. In this lab-scale cell, the anolyte (aqueous FeSO₄+H₂SO₄) and the catholyte (aqueous CuSO₄+H₂SO₄) are kept separate by an anion membrane which prevents cation and water transport between the electrolytes. Both solutions are agitated by recirculation. The kinetics of the anodic reaction have been studied via potentiodynamic experiments on various anode materials (lead, platinum, ruthenium oxide, iridium oxide and graphite). The highest oxidation rate was obtained on platinum and the lowest one on lead, whereas the remaining materials showed satisfactory performance. Results in the lab-scale RED cell show that, depending on experimental conditions, for a cell current density of , the cell voltage ranges from 1.81 to , the cathodic current efficiency from 97.2% to 98.3% and the specific energy consumption, from 1.53 to of deposited copper.     

Modelling the effect of temperature and time on the performance of a copper electrowinning cell based on reactive electrodialysis

A temperature- and time-dependent mathematical model for the operation of a laboratory-scale copper electrowinning cell based on reactive electrodialysis (RED) has been developed. The model is zero-dimensional. The cathodic reaction was copper electrodeposition and the anodic reaction was ferrous to ferric ion oxidation. The catholyte was aqueous cupric sulphate and the anolyte was aqueous ferrous sulphate, both in sulphuric acid. Catholyte and anolyte were separated by an electrodialytic anion membrane. The model predicts the effect of temperature and time on: (a) cathodic and anodic kinetics, (b) speciation of catholyte and anolyte, (c) transport phenomena in the electrolytes and (d) ion transport through the membrane. Model calibration and validation were carried out. Its predictions are in good agreement with experiments for: amount of deposited copper, amount of produced Fe(III) species, cell voltage and specific energy consumption.     

Photocatalytic intrinsic reaction kinetics. II: Effects of oxygen concentration on the kinetics of the photocatalytic degradation of dichloroacetic acid

The effect of oxygen on the reaction kinetics of the photocatalytic oxidation of dichloroacetic acid at pH below the titanium dioxide isoelectric point (pH from 5 to 4) was studied. A kinetic scheme based on the direct hole attack to the dichloracetate ion in conjunction with the classical role of oxygen acting as an electron acceptor is proposed. Oxygen also intervenes in a direct addition reaction to one of the intermediate reaction products. Experiments were conducted in a well-mixed, small, flat plate reactor employing Aldrich titanium dioxide as the photocatalyst in the suspension mode and using polychromatic irradiation with energy in the range from 275 to 385 nm in wavelength. A complete mathematical model, including the effect of the absorbed radiation intensities and catalyst concentration was developed. Experimental data agree well with theoretical predictions employing just two kinetic parameters derived from the proposed reaction mechanism.     

Electrowinning of copper in two- and three-compartment reactive electrodialysis cells

Two- and three-compartment copper electrowinning (EW) cells based on reactive electrodialysis (RED) have been studied. The catholyte was cupric sulphate and the anolyte was ferrous sulphate, both dissolved in sulphuric acid. Copper mesh cathodes and graphite bar anodes have been used. The effects of cell current density, temperature, electrolyte recirculation flowrate and nitrogen sparging flowrate on cell performance (cathodic current efficiency, cell voltage and specific energy consumption (SEC)) have been determined. The cell voltage increased with cell current and it decreased with temperature and nitrogen sparging flowrate. The effect of nitrogen sparging flowrate on the cell voltage is stronger than the effect of electrolyte recirculation flowrate, whereas its enhancing effect on mass transfer is stronger than its deleterious effect on electrolyte conductivity. The SEC ranged from 0.94 to 1.39 kW h/kg at cell current densities between 200 and 600 A/m². These values are considerably better than those for conventional copper EW (about 2 kW h/kg at 350 A/m²). The morphology of the electrodeposits has been observed and a comparison between a three-compartment cell and a previously studied squirrel-cage cell (both based on RED) has been drawn.     

Temperature oscillation calorimetry for the determination of the heat capacity in a small-scale reactor

This contribution describes a method for heat capacity determination in a small-scale reaction calorimeter under quasi-isothermal conditions ( to ). The heat capacity of the reactor content is calculated from the amplitudes and the phase shifts of the reactor temperature, of the electrical heater and of the cooling rate when forced temperature oscillations are applied. The heat capacities of eight solvents (water, ethanol, methanol, acetone, 1-octanol, diethylenglycol, toluene, and 1-butanol) covering a wide range of viscosity were calculated for various experimental conditions, including reactor volume and stirrer speed. Systematic deviations were detected when compared to the corresponding literature values. Straight line calibration with the total heat transfer coefficient and two modern multivariate calibration techniques (partial least squares and neural network) were applied to correct for these deviations. The different calibrations show similar precision and allow for an online determination of the heat capacity with an accuracy comparable to other published methods. Successful applications include the determination of the heat capacities for n-heptane, for various homogenous ethanol/water mixtures, and during the course of the hydrolysis of concentrated sulfuric acid.     

Ignition waves in a stirred PEM fuel cell

A mathematical model of slow transient behavior in an autohumidified stirred tank reactor (STR) polymer electrolyte membrane (PEM) fuel cell is developed. The key feature of the model is the positive feedback between current, water production, and membrane resistance which leads to two stable “ignited” states, corresponding to either a uniform current distribution or a partially ignited cell with localized current production. The switching between the two regimes is accompanied by hysteresis and transient behavior on the order of 2-4 h in a small cell. We compare the numerical results to experimental data gathered by [Benziger et al. 2005. Chemical Engineering Science 60 (6), 1743-1759] and show that the lateral diffusion of water within the ionomer membrane is a possible mechanism behind the hysteresis and slow transient behavior they observed.     

Modeling the liquid-phase oxidation of hydrocarbons over a range of temperatures and dissolved oxygen concentrations with pseudo-detailed chemical kinetics

The ability of pseudo-detailed chemical kinetic modeling to simulate the oxidation behavior of Exxsol D-80, a paraffin blend whose oxidative characteristics are representative of severely hydrotreated jet fuels, is assessed. The effects of temperature and initial dissolved O₂ concentration on oxidation are considered. A 17-step pseudo-detailed mechanism is shown to provide reasonable simulations of Exxsol D-80 oxidation over a range of temperatures, but not over a range of initial dissolved O₂ concentrations. The addition of alkyl-peroxy radical isomerization to the pseudo-detailed mechanism did not reconcile the initial dissolved O₂ limitation. With the addition of a peroxy radical decomposition reaction to the original 17-step pseudo-detailed mechanism, reasonable simulations of Exxsol D-80 oxidation over a range of temperatures and initial dissolved O₂ concentrations were obtained. Analysis of the rate parameters associated with peroxy radical decomposition suggests that aromatic hydrocarbons play a significant role in the oxidation of fuels, even at low (<1% by weight) aromatic levels.     

The continuous self stirred tank reactor: measurement of the cracking kinetics of biomass pyrolysis vapours

The continuous self stirred tank reactor is a well known suitable device for studying the kinetics of high-temperature gas phase reactions. Temperature and composition are uniform at any point of the reactor, and for a given residence time, it is easy to calculate the values of kinetic constants from a very simple mathematical model. The aim of the present paper is to report the first experimental results obtained on the thermal cracking of vapours produced by the pyrolysis of biomass. The experiments are carried out between 836 and 1303 K and under mean residence times ranging from 0.3 to 0.5 s. The calculated activation energy and preexponential factor are in good agreement with those obtained by other authors operating in more usual devices, but needing the solving of more sophisticated models.     

The effect of Cl(I)− ions on kinetics and mechanism of anodic dissolution and cathodic deposition of copper

The influence of Cl(I)^? ions on kinetics and mechanism of anodic dissolution and cathodic deposition of copper in acidic sulfate was investigated. For this investigation the galvanostatic single-pulse method has been used. The results indicate that Cl(I)^? ions change the exchange current density and transfer coefficients as determined from tafel analyses of the anodic and cathodic reactions.     

Photopolymerization kinetics of bis-aromatic based urethane acrylate macromonomers in the presence of reactive diluent

The present work deals with the photopolymerization of bis-aromatic based urethane acrylate macromonomers in the presence of excess end capping agent as reactive diluent and estimation of their kinetic parameters. Formulations were made by independently homogenizing the macromonomers with photoinitiators of three different classes. Three different compositions of photoinitiators were used to study the effect of concentration of photoinitiator on cure kinetics. These compositions obtained were tested for photo curing performance using photo DSC under polychromatic radiation. The heat flows against time were recorded for all formulations under isothermal condition and the rates of polymerization, peak maximum times as well as the percentage conversions were estimated. It was observed that due to a longer timescale for reaction diffusion, formulations with macromonomer containing propoxylated backbone showed higher conversions than the corresponding ethoxylated analogue. The photopolymerization and kinetic estimations of the formulations including evaluation of kinetic model are discussed.     

The influence of temperature on the anodic oxidation/dissolution of uranium dioxide

The anodic dissolution of U^IVO₂ has been studied at 60 °C in 0.1 mol dm⁻³ KCl using a range of electrochemical methods and X-ray photoelectron spectroscopy (XPS). The results were compared to previous results obtained at 22 °C. This comparison shows that the threshold for the onset of anodic dissolution (−400 mV versus SCE) is not noticeably changed by this increase in temperature. However, both the oxidation of the surface (to U^IV/VO_2+x) and the rate of anodic dissolution (as U^VIO₂²⁺) leading to the formation of a U^VIO₃·yH₂O deposit are accelerated at the higher temperature. The XPS analysis shows that the conversion of U^V-U^VI occurs at lower potentials at 60 °C. Consequently, once the surface becomes blocked by the presence of a U^VIO₃·yH₂O deposit, rapid dissolution coupled to uranyl ion hydrolysis causes the development of locally acidified sites within the fuel surface at lower potentials at the higher temperature.     

Experimental and modeling study of the kinetics of oxidation of ethanol-n-heptane mixtures in a jet-stirred reactor

The kinetics of oxidation of ethanol-n-heptane mixtures (20-80 and 50-50 mol.%) was studied experimentally using a fused-silica jet-stirred reactor. The experiments were performed in the temperature range 530-1070 K, at 10 atm, at two equivalence ratios (0.5 and 1), and with an initial fuel concentration of 750 ppm. A kinetic modeling was performed using schemes resulting from the merging of validated kinetic schemes for the oxidation of the components of the present mixtures (n-heptane and ethanol). Good agreement between the experimental results and the computations was observed under the present conditions when using detailed chemistry whereas the used of semi-detailed chemistry yielded acceptable but less accurate prediction of the fuel oxidation kinetics.     

Temperature dependence of kinetics and diffusion coefficients for ferrocene/ferricenium in ammonium-imide ionic liquids

The electrochemical behavior of ferrocene (Fc) and ferricenium (Fc⁺) was investigated by cyclic voltammetry with convolutional and semi-differential electroanalyses in the temperature range 298-373 K in the hydrophobic room temperature ionic liquid systems consisting of bis(trifluoromethanesulfone)imide anion with quaternary ammonium cation. The experimental results indicated that the redox reaction of Fc/Fc⁺ was reversible in this ionic liquid and the charge-transfer rate constant (k⁰) has been obtained. Mass transport towards the electrode is a simple diffusion process and the diffusion coefficient (D) for Fc/Fc⁺ has been also calculated. These results indicated that the k⁰ and D increased with increasing temperature in ionic liquids. The validity of the Arrhenius law was verified by investigating the temperature dependences of k⁰ and D.     

Influence of cobalt ions on the anodic oxidation of a lead alloy under conditions typical of copper electrowinning

The influence of cobalt ions in the solution on the anodic oxidation of a commercial Pb–Ca–Sn alloy under conditions typical of copper electrowinning was studied using cyclic voltammograms and potential decay transients. The cobalt ions changed the structure, morphology and chemical composition of the surface film from a loose porous film to a thin dense film. This change of surface film is the cause for the decreased rate of oxidation for the lead anode in the presence of cobalt ions. The steady state potential of the alloy during anodic oxidation decreased with (i) increasing cobalt ion concentration, (ii) increasing rotation speed, (iii) increasing temperature, (iv) decreasing acid concentration and (v) decreasing current density. The steady state potentials were lower in the presence of cobalt ions. This decrease in the steady state potential may indicate that oxygen evolution is more rapid on the more compact film formed in the presence of cobalt ions. Alternatively, an additional pathway of oxidation may be provided by the cobalt ions.     

Mass transfer and kinetics of the three-phase hydrogenation of a dinitrile over a Raney-type nickel catalyst

The hydrogenation kinetics of a dinitrile over a Raney-type nickel catalyst was evaluated from experiments performed in a fed-batch operating autoclave at 320- and 2- hydrogen pressure. This complex catalytic reaction consists of two main parts: almost 100% selective hydrogenation of the dinitrile to the corresponding aminonitrile and consecutive hydrogenation to either the desired primary diamine or to pyrrolidine via ring formation. An extensive study has been made on the effects of mass transfer in the applied slurry-type reactor for this reaction. The gas-liquid mass transfer is enhanced by the presence of catalyst particles, and at typical hydrogenation conditions, k_La values up to can be reached. A Sherwood correlation for the three-phase reactor showed that important parameters in the gas-liquid mass transfer are stirrer speed and the density and viscosity of the solvent. The kinetic experiments were performed in absence of mass and heat transfer limitations. The kinetic data were modeled using two rate models based on Langmuir-Hinshelwood kinetics, assuming the reaction of dissociatively adsorbed hydrogen and nitrile compound as rate-limiting step. The first model involved competitive adsorption between hydrogen and organic compound and the second model was based on non-competitive adsorption. Both models successfully described both reaction parts. The reaction of dinitrile to aminonitrile is nearly 100% selective due to the relatively strong adsorption of the dinitriles as compared to the aminonitriles. By increasing the hydrogen partial pressure, higher yields of primary amine can be obtained. The models predict that operating in the mass-transfer regime at relatively high temperatures reduces the formation of the primary diamine.     

Considerations on the steady-state modeling of methanol synthesis fixed-bed reactor

This paper proposes and compares different steady-state models for the methanol synthesis fixed-bed reactor. The main goal is to indicate the pros and cons of these alternative models, to highlight their peculiarities in providing accurate and reliable estimations, and to warn the reader about possible risks involved in certain modeling assumptions. The Lurgi-type shell and tube boiler-reactor mathematical modeling is also laid out, before more complex issues and research, such as dynamic simulation or PDE-based predictive control, are broached.     

The anodic formation and cathodic reduction of cuprous oxide films on copper in sodium hydroxide solutions

The potentiodynamic response of the Cu/alkaline solution interface in the potential range where Cu₂O is formed and reduced has been investigated using a broad range of experimental conditions, including different electrode treatments. The reproducibility of the results is also critically considered. The cathodic reaction revealed that at least three different Cu(I) species are formed during the Cu₂O formation. One of them has been attributed to a soluble Cu(I) species. One out of the two remaining Cu(I) surface species undergoes potentiostatic ageing effects. The results are discussed in terms of previously proposed reaction mechanisms related to the Cu₂O formation and reduction.     

Modeling of monolithic and trickle-bed reactors for the hydrogenation of styrene

A kinetic study into the styrene hydrogenation over a palladium on alumina catalyst has been made. Styrene was used as a model component for pyrolysis gasoline. A kinetic rate expression has been derived and the inhibiting effect of sulfur components has been included. Using this kinetics and mass-transfer models compiled from literature, the performance of two types of reactors for the styrene (pyrolysis gasoline) hydrogenation has been evaluated. A structured reactor such as a monolith has large advantages over a conventional trickle-bed reactor. For the monolithic reactor a more than 3 times higher volumetric productivity is obtained with much less catalyst. The modeling results indicate that deactivation by gum formation should be significantly less due to much better hydrogen mass transfer in the reactor.     

Effects of Temperature and Catalyst Type on Chemical Degradation of Radiation Grafted Membranes in PEFCs

Proton exchange membranes prepared by radiation grafting are a promising alternative material to perfluoroalkyl sulfonic acid (PFSA) for fuel cell application. The temperature effect on the chemical degradation of radiation grafted membranes is evaluated by testing styrene grafted and sulfonated membranes at elevated temperatures (90 to 110 °C) and open circuit voltage (OCV) conditions. The results show that the increased temperature can markedly enhance membrane degradation, as expected, which is similar to the case of PFSA membranes. Moreover, elevated operating temperature leads to brown discoloration of the tested membrane, which may be attributed to the formation of a large conjugated π‐electron system in the membrane chemical structure. The OCV tests in which the styrene grafted membranes were tested at 80 °C with different kinds of catalyst (Pt/C and Pt black, respectively) in the gas diffusion electrodes (GDE) show that the membrane combined with Pt black GDEs exhibited a significantly lower degradation rate than that combined with Pt/C GDEs. The influence of differences in the amount of contaminants and the possibility of H₂O₂ formation between these two types of catalyst are put forward to explain this phenomenon.     

Influence of lead dioxide surface films on anodic oxidation of a lead alloy under conditions typical of copper electrowinning

Cyclic voltammograms, current transients at constant potential and potential decay transients have been used to study the formation of lead dioxide surface films in the presence of cobalt ions and their role in decreasing the oxidation rate of a lead alloy under steady state conditions typical of copper electrowinning. The observations in the present work indicate, consistent with the surface film model, that the formation of a continuous PbSO₄ + α-PbO₂ film on the surface of the lead alloy in the presence of cobalt ions hinders further oxidation of the metal. The protectiveness of the film is dynamic in the steady state; the film is continuously forming and dissolving. Also studied was the potential of the oxygen evolution reaction on α-PbO₂ and β-PbO₂ in 170 g L⁻¹ H₂SO₄ with and without cobalt ions. The steady state potential for oxygen evolution on β-PbO₂ in 170 g L⁻¹ H₂SO₄ at 285 A m⁻² decreased in the presence of cobalt ions and the steady state potential of β-PbO₂ was essentially the same as that of (i) the Pb–Ca–Sn alloy and (ii) α-PbO₂. The implication is that the potential of the Pb–Ca–Sn alloy is determined by the α-PbO₂ and/or β-PbO₂ on its surface.