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.     

Transport and conductivity properties of an advanced cation exchange membrane in concentrated sodium hydroxide

An electrochemical concentrator for application to the chlorine-caustic industry is currently under development. In it 30 to 35 wt % NaOH enters the anolyte and catholyte chambers and exits at 20 and 50 wt %, respectively. Consequently, in support of the electrochemical concentrator development, the conductance and transport properties of advanced cation exchange membranes in concentrated sodium hydroxide, are being investigated. The membrane voltage drop, sodium ion transport and water flux of these membranes in 20 to 35 wt % sodium hydroxide anolyte and 30 to 50 wt % sodium hydroxide catholyte at 75°C are presented. To better understand the behaviour of these membranes, electrolyte sorption measurements were conducted in the anolyte/catholyte environment appropriate for the electrochemical concentrator. The water uptake data appear to correlate well with the conductance data and the combined NaOH and water sorption data are consistent with the sodium ion transport data.     

离子交换膜的维护

介绍离子交换膜的工况。分析盐水中的杂质，阴、阳极液浓度变化，温度。压力对离子交换膜性能和寿命的影响原理。提出维护离子膜的方法有：平稳运行，减少开、停车次数和增加预防控制措施。修复离子膜的方法有停车时循环阳极液、用纯水清洗离子膜或根据实际情况改变运行状态，如离子交换膜受Ca^2 、Mg^2 轻微污染时，先停车再启动；受中度污染时可将阴极液质量分数从32％调整到33％，或将阳极液质量浓度从190g／L升至200g／L，或同时调整阴极液、阳极液浓度。     

Recovery of chlorine from dilute hydrochloric acid by electrolysis using a chlorine resistant anion exchange membrane

A new chlorine resistant anion exchange membrane enables innovative possibilities for hydrochloric acid electrolysis for recovery of chlorine. This is of interest for hydrochloric acid that is neutralized in the chemical industry because purity and concentration are not sufficiently high for recycling. In the common electrolysis process hydrochloric acid is fed into the anode compartment and needs a satisfactory HCl concentration for supplying the anode with chloride ions. Using an anion exchange membrane as a cell separator the feed flows into the cathode chamber where a low HCl concentration is acceptable because Cl⁻ ions at the anode can be supplied by addition of a salt which is not consumed. Experimental data of the membrane and the process are presented: membrane permselectivity improved up to above 97% using CaCl₂ as added salt, chlorine current efficiency up to 98% and oxygen content as low as 0.5 vol%, cell voltage at 4 kA m⁻² 2.3 V, equivalent to 1740 kWh per t produced chlorine, even at low HCl concentrations. Thus, the power consumption is comparable with the common process. A problem of the new process is the high water transport through the membrane. Therefore, experiments for two process alternatives were carried out. Disadvantages of water transport can be avoided by using a high concentrated CaCl₂ solution as anolyte and catholyte and as absorption medium for diluted HCl gas streams. Additionally, a cell design was investigated where the anode is directly connected to the membrane in an empty (gas filled) anode compartment.     

Development and evaluation of a Pt-Ru bimetallic catalyst based alkali concentrator in NaOH solutions

An alkaline H₂-O₂ fuel cell based electrochemical alkali concentrator using Nafion® 961 cation exchange membrane has been constructed. It is found that the concentrator can operate at a cell voltage around 0.6 V and the catholyte can be simultaneously concentrated to a level of 40 wt %, provided the outlet anolyte concentration is maintained at a level not below 23 wt %. Some possible directions for further improvement are indicated.

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Dual solution-type HCHO fuel cell systems using an anion exchange resin membrane with electroless-plated Cu or Pd anode

Loading characteristics of a prototype HCHO fuel cell systems with an anion exchange membrane which separates the anolyte from the catholyte were investigated. Electrodes of Cu or Pd, deposited by an electroless-plating technique onto the membrane, showed high electrocatalytic activity to the anodic oxidation of HCHO in 1 M NaOH solution. The system with Cu anode and 1 M NaOH for both anolyte and catholyte showed high loading characteristics but poor durability, whereas that with 1 M K₂CO₃ showed low characteristics because of lowered pH of the anolyte. It was shown that a dual solution-type cell with 1 M K₂CO₃ anolyte and 1 M NaOH catholyte yielded improved characteristics as compared with the simple K₂CO₃ system. The output level was, however, at an unsatisfactory level owing to poor membrane conductance. The temperature dependence of the output performance was studied in the range 7–55°C.     

The Application of Electrodialysis to Extend the Lifetime of Commercial Electroplating Baths

Abstract

Electrodialysis has been investigated as a method to extend the lifetime of industrial electroplating solutions via the selective removal of inert electrolyte salts that build up during electroplating operations.

The electrodialysis measurements were made using a commercially available plate-and frame-type cell and various combinations of Nafion cation exchange and either Tosflex or Neosepta anion exchange membranes.

Two commercial plating solutions were studied: a zinc-tin bath in which there is a buildup of excess potassium hydroxide and a nickel-tungsten bath characterized by a buildup of excess sodium sulfate. Potassium hydroxide was effectively removed from the zinc-tin bath with very little loss of the heavy metals. Two configurations were investigated: a three compartment configuration with potassium hydroxide in the anolyte strip and sulfuric acid in the catholyte strip, and a two compartment configuration with sulfuric acid in the catholyte strip and the anode placed directly in the plating solution. In both cases potassium hydroxide was stripped from the plating solution at greater than 94% current efficiency, but at a slightly greater voltage in the three compartment cell due to increased resistance caused by the extra membrane.

A three compartment configuration was used to remove sodium sulfate from the nickel-tungsten bath, with acid solution in the catholyte and alkaline solution in the anolyte. Current efficiencies for salt removal were high but with appreciable loss of tungsten and nickel to the strip solutions.     

The electrolytic reduction of carbon dioxide and monoxide for the production of carboxylic acids

This work examines the possible production of carboxylic acids from inorganic feedstocks such as carbon dioxide and monoxide. The technique consists of dissolving the gas in a catholyte consisting of an organic solvent containing a tetraalkyl ammonium halide, separated by a cationic membrane from the aqueous sodium chloride anolyte. The performance of a number of cathode materials, catholyte formulations and cell designs have been investigated both at atmospheric and elevated pressure. It was found that the sole product was oxalic acid which was obtained with current efficiencies of over 50%. Current densities and hence space time yields were, however, disappointingly low.     

Effect of fluid density on flow in bagged electrowinning cells

In bagged electrowinning cells where there is gas evolution at the anode, such as the NiCl₂ system, flow across a porous diaphragm bag is observed to occur predominantly from the bottom of the cell. A model based on a hydrostatic argument is proposed which can account for this phenomenon on the basis of bulk density difference between the catholyte and anolyte. Experimental evidence by means of visualization through acid-base reactions shows good agreement with the model.     

PAIRED ELECTRO-GENERATION OF GLYOXYLIC ACID USING BIPOLAR MEMBRANE MADE FROM SODIUM ALGINATE AND CHITOSAN

Sodium alginate (SA) and chitosan (CS) were modified with Ca²⁺ and glutaraldehyde linking reagents to prepare the mSA/mCS bipolar membrane (BPM). The morphology of the membrane was characterized by SEM. The membrane was used as a separator in an electrolysis cell for the production of glyoxylic acid simultaneously at both the cathode and the anode. The catholyte consisted of a mixture of saturated oxalic acid and 0.1 mol/L HCl, and the anolyte was a mixture of glyoxal (10 wt.%) and KBr (10 wt.%). A nickel mesh was placed on the surface of the mSA cation exchange layer to act as the cathode, and the anode was PbO₂. The electrolysis voltage was as low as 2.7 V during operation at room temperature with a current density of 20 mA · cm^?2. Current efficiencies reached 82.9% in the cathode chamber and 75.7% in the anode chamber.     

Recovery of metals from complexed solutions by electrodeposition

Electrolytic recovery of metals from aqueous solutions containing complexing chelating agents such as EDTA, NTA, and citrate was studied in a two-chamber cell separating with a commercial cation-exchange membrane (CEM). Equimolar solutions of metal and a chelating agent as a catholyte and NaNO₃ as an anolyte were used; the effect of current densities, initial catholyte and anolyte pH, metal concentration and the type of the CEM, chelating agent and metal on the recovery of metals was determined. The recovery of metal increased with higher initial anolyte pH, concentration and current density, whereas it decreased with lower initial catholyte pH. The results show that electrodeposition seems to be an applicable method for the recovery of metals under appropriate conditions.     

影响离子膜电解槽槽电压的因素

阐述了离子膜电解槽操作温度、电流密度、电解液流量、阴极液质量分数、阳极液质量分数、阳极液ｐＨ值、阳极液中金属离子以及阴极材料等九方面对电解槽电压的影响。     

Analysis of molar flux and current density in the electrodialytic separation of sulfuric acid from spent liquor using an anion exchange membrane

Separation of sulfuric acid from a dilute solution involved a plate and frame type electrodialysis unit using a commercial anion exchange membrane. Experiments were conducted in batch with catholyte concentrations ranging from 1 to 5 wt%. Effect of applied current density, initial catholyte concentration and initial concentration difference of catholyte and anolyte on the molar flux was studied extensively. The maximum molar flux was estimated to be 10.52×10^-8 mol cm^-2s^-1 at 4.45 wt% catholyte concentration and applied current density of 30 mA cm^-2. Current efficiencies were observed to be 75 to 85% at lower current density, which rose to more than 100% at 20 and 30mA cm^-2, at equal initial concentration of catholyte and anolyte. Diffusive flux and flux due to membrane potential contributed very less compared to total flux in presence of applied electric current. An equation was developed to predict the practical molar fluxes, which fitted satisfactorily with minor standard deviation. Pristine and used membrane specimens were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).     

Evaluation of transport properties of cation exchange membranes for application in alkali concentrators

Conductivity and transport properties of commercially available cation exchange membranes were investigated for their application in an electrochemical alkali concentrator. Among the three membranes studied, Nafion 961 was found to have high conductivity, high sodium ion transport and lower water transport characteristics which are the desired characteristics for fabrication of an alkali concentrator. The effect of concentration gradient between anolyte and catholyte on the overall transport properties is also reported.

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Electrochemical membrane reactor: In situ separation and recovery of chromic acid and metal ions

An electrochemical membrane reactor with three compartments (anolyte, catholyte and central compartment) based on in-house-prepared cation- and anion-exchange membrane was developed to achieve in situ separation and recovery of chromic acid and metal ions. The physicochemical and electrochemical properties of the ion-exchange membrane under standard operating conditions reveal its suitability for the proposed reactor. Experiments using synthetic solutions of chromate and dichromate of different concentrations were carried out to study the feasibility of the process. Electrochemical reactions occurring at the cathode and anode under operating conditions are proposed. It was observed that metal ion migrated through the cation-exchange membrane from central compartment to catholyte and OH⁻ formation at the cathode leads to the formation of metal hydroxide. Simultaneously, chromate ion migrated through the anion-exchange membrane from central compartment to the anolyte and formed chromic acid by combining H⁺ produced their by oxidative water splitting. Thus a continuous decay in the concentration of chromate and metal ion was observed in the central compartment, which was recovered separately in the anolyte and catholyte, respectively, from their mixed solution. This process was completely optimized in terms of operating conditions such as initial concentration of chromate and metal ions in the central compartment, the applied cell voltage, chromate and metal ion flux, recovery percentage, energy consumption, and current efficiency. It was concluded that chromic acid and metal ions can be recovered efficiently from their mixed solution leaving behind the uncharged organics and can be reused as their corresponding acid and base apart from the purifying water for further applications.     

Processes for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen

The long history of the synthesis of hydrogen peroxide via the cathodic reduction of oxygen in caustic soda catholyte is reviewed. Recent progress is analysed on the electrochemical syntheses of mixtures of caustic soda and hydrogen peroxide in various by-weight ratios from 2.3: 1 NaOH to H₂O₂ to about 1 : 1. The analysis presented focuses primarily on published work concerning planar fuel cell type electrodes in membrane-divided cells and particulate bed electrodes in cells employing microporous separators with well-defined anolyte-to-catholyte flows. Potential ancillary technology for changing the ratios of products is also discussed. One configuration of the processes described encompasses the simultaneous near 50/50 use of two variations of generation technology. A highly desired product, for instance 1.2: 1 NaOH to H₂O₂, may be formed using the catholyte product of a membrane or diaphragm cell with a caustic anolyte as the catholyte feed stream for a membrane cell with an acidic anolyte. Although the particulate bed cathode approach has reached commercial trials, the planar cathode membrane cell approach may prove a difficult process to develop as the performance of electrodes optimized for realistic hydraulic depths may prove very different to that of electrodes used in small scale laboratory development.     

Boron removal by electrodialysis with anion-exchange membranes

The removal of boron from an aqueous solution was studied in a two-chamber cell separated with a commercial anion- exchange membrane as a function of current density, pH, type of the membrane, concentration and different type of salt solutions. At the end of these studies, the maximum value of boron removal was obtained under the conditions where the maximum current was applied; 0.1 M H₃BO₃ (pH = 9) and 0.001 M NaCl solutions were used as a catholyte and an anolyte solution, respectively. The AHA membrane was used for separating the two cells used in the electrodialysis experiments. All experiments were carried out at room temperature, and the concentration of boron at the anode cell was determined by ICP–AES. It was concluded that electrodialysis is an appropriate method for boron removal from aqueous solutions under suitable conditions.     

影响离子膜电解液质量因素分析

从离子交换膜的特性,阴极液浓度,阳极液浓度,电流密度,槽温,阳极室压力等方面分析了影响离子膜电解液质量的因素。     

Recovery of Nickel and Cobalt from Spent NiMH Batteries by Electrowinning

For nickel and cobalt recovery from spent NiMH batteries by electrowinning, the effect of different electrowinning parameters as boric acid concentration, temperature, current density, and pH were studied using different synthetic solutions. The optimized operational parameters were applied in an electrowinning test with a solution achieved by leaching the electrodes of NiMH batteries. The electrowinning tests were performed galvanostatically in a two‐compartment cell separated by an anionic membrane. A platinum/iridium‐coated titanium anode and a stainless‐steel cathode were used. A sodium sulfate solution served as anolyte. The results demonstrate the technical viability of nickel and cobalt recovery. The chemical composition of the obtained deposit presented high nickel and cobalt concentrations.     

Chlorate Transport through Ion-Exchange Membranes

A laboratory scale chlor-alkali membrane cell was used to measure the chlorate concentration in the outlet NaOH as a function of current density, temperature, film thickness, brine strength and various membrane properties. The chlorate concentration in the NaOH increased with increasing anolyte chlorate spiking level and temperature and decreasing current density and carboxylate film thickness and was strongly dependent on the type of ion-exchange membrane used. In addition, the presence or absence of sacrificial fibers in the membrane did not measurably influence the resultant chlorate concentration. Chlorate ions were transported to the catholyte side by diffusion and electroosmotic convection and transported toward the anolyte side by migration. This balance between the three modes of transport dictates the chlorate concentration present in the NaOH product.