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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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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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。     

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.     

A new method for the electrolysis of sodium chloride using a β-alumina-molten salt system

This paper is concerned with a new process for NaCl electrolysis. The catholyte is molten sodium hydroxide, containing a small quantity of water. The anolyte is the molten mixture of sodium chloride and zinc chloride. As a diaphragm, sodium ion conducting -alumina is used. By the electrolysis, chlorine, sodium hydroxide and hydrogen are obtained almost quantitatively from sodium chloride and water. The operating temperature is about 330° C. A terminal voltage of 5 V at 40 A dm^–2 is estimated from the experimental data and by numerical calculation. This process is very promising on the basis of the current efficiency and estimated voltage balance data.     

Corrosion behaviour of tantalum in sodium hydroxide solutions

The corrosion behaviour of tantalum has been investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential measurements, potentiodynamic polarization, polarization resistance method and electrochemical impedance spectroscopy. Tantalum showed a passive behaviour in 5 and 10 wt % NaOH at 25, 50 and 75 °C, and in 15 wt % NaOH at 25 and 50 °C. In 15 wt % NaOH at 75 °C and in 30 wt % NaOH at all temperatures, tantalum presented a passive–active transition (self-activation) due to the spontaneous dissolution of its superficial air-formed oxide, and afterwards remained in the active state for long times, forming a polytantalate compound. In all cases, the corrosion rates increase with increasing NaOH concentration and temperature.     

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.     

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 treatment of industrial effluents containing sodium hydroxide to enable the reuse of chemicals and water

An economically viable treatment sequence has been developed and piloted at two textile factories for the recovery and reuse of water, chemicals and heat energy from sodium hydroxide effluent produced during the scouring of cotton fibre.The treatment sequence involves pretreatment of the scour effluent by neutralisation, using an acidic gas, cross-flow microfiltration and charged membrane ultrafiltration (also called nanofiltration). The sodium hydroxide is then recovered in an electrochemical membrane cell with the simultaneous evolution of acidic gas which is recycled within the treatment process.Two possible configurations of the treatment process, where the acidic gas is either chlorine or carbon dioxide, have been discussed. Pilot plant results have been presented for both systems. The carbon dioxide system was the preferred route and is discussed in detail.The pretreatment sequence neutralised the scour effluent, lowered its chemical oxygen demand by 86% and removed 65% of both the calcium and the organics and 50% of the magnesium. The sodium hydroxide (100 to 200 g/l) and depleted brine solution (total solids 500 mg/l) from the electrochemical membrane cell were of suitable quality for reuse in the factory process.The electrochemical membrane cell produced sodium hydroxide at 62% current efficiency at an electrical power consumption of 4 000 kWh/ton 100% NaOH. The effect of electrolyte, in particular, anolyte flow rate, temperature and concentration on the limiting current density and power consumption has been investigated. Some design data for a full scale treatment plant has been presented.The operation of an acceptable background concentration closed-loop recycle wash system in the scour process was found to reduce the required membrane area by 82%.Minimum dissolution of the precious metal oxide anode coating occurred and long anode life was predicted. Serious electromembrane fouling, with increased resistance, was not apparent.     

Dehydrochlorination of propylene chlorohydrin with sodium hydroxide and with catholyte

Dehydrochlorination of an aqueous solution of propylene chlorohydrin with sodium hydroxide (12·5 and 30% by weight), with catholyte (5 or 10% by weight NaOH + 14% by weight NaCl) and with milk of lime (15% by weight) were compared. The use of catholyte (10% by weight NaOH + 14% by weight NaCl) enables propylene oxide to be obtained in a yield of 94·3%, with 100% conversion of chlorohydrin. The concentration of 1,2-propylene glycol in the waste is 0·07% by weight.     

Corrosion Behaviour of Niobium in Sodium Hydroxide Solutions

The electrochemical behaviour of niobium was investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that Nb is spontaneously active in 10, 15 and 30 wt % NaOH at 25, 50 and 75 °C. The anodic polarization curves in all cases show a dissolution/passivation peak followed by a current plateau, corresponding to Nb₂O₅ formation. The spontaneous active corrosion of Nb leads to the formation of soluble niobates that precipitate to sodium niobates. The evaluation of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicates that the corrosion rates of niobium increase with increasing NaOH concentration and temperature.     

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.     

Nitric acid and sodium hydroxide generation by electrodialysis using bipolar membranes

An electrodialysis process with bipolar membranes was used to generate HNO3 and NaOH from NaNO3 which can be found in industrial waste waters. The current efficiency of this process is limited by proton leakage through the anion exchange membrane (AEM), co-ion leakage through bipolar membranes (BPM) and water transport through the ion exchange membranes. Three cell configurations using three or two compartment cells with different anion or cation exchange membranes (CEM) in stack series were used and compared. Electrodialysis with three compartments gives the best current efficiencies for nitric acid and sodium hydroxide production from sodium nitrate.     

Electrochemical synthesis of N2O5 by oxidation of N2O4 in nitric acid with PTFE membrane

Electrochemical synthesis of dinitrogen pentoxide (N₂O₅) by oxidation of dinitrogen tetroxide (N₂O₄) in a plate-and-frame electrolyzer was investigated. As the separator, different porous polytetrafluoroethylene (PTFE) membranes were tested in this process and the effects of hydrophilicity and of hydrophobicity on the electrolysis were discussed. The transport of N₂O₄ and water from catholyte to anolyte through membrane occurred in the electrolysis, especially at the end of the electrolysis. The water transport had a much more effect on the electrolysis than that of the N₂O₄ diffusion. The hydrophobic PTFE membranes had better performance on control of water transport from catholyte to anolyte than that of the hydrophilic ones. Hydrophobicity can increase the chemical yield of N₂O₅. The membranes with a low hydrophobic surface were preferred. All the hydrophobic PTFE membranes with low resistance have the specific energy of 1.1-1.5 kWh kg⁻¹ N₂O₅. The current efficiency of 67.3-80.2% and chemical yield of 58.9-60.9% were achieved in production of N₂O₅. The technique of replacing the catholyte with fresh nitric acid can minimize the transport of N₂O₄ and water to a great extent, it can further improve the chemical yield and reduce the specific energy.     

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.     

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.     

The characterisation of the polyethylene-polystyrene-divinylbenzene interpolymer cation-exchange membranes in the electrolysis of borax solutions

The synthesis and the characterization of the PE-PS-DVB interpolymer cationexchange membranes in the electrolysis of borax solutions to produce boric acid and sodium hydroxide simultaneously was carried out. The characterization of the teflon based “Nafion 324” and polystyrene-DVB based “Permaplex C-20” membranes was also performed in the same system for comparison. The DVB contents were varied between 3 – 12% (by wt. in total monomers) and the increase of the cross-links resulted in the decrease of the water contents and ionexchange capacities of the membranes. The PE present in the membrane acted as a barrier for electroosmotic water transport. The membranes with high DVB contents showed better electrolysis performance due to their low water contents and low electroosmotic water transport properties. The use of the interpolymer membrane containing 11.6% DVB resulted in high current efficiency and high sodium cation dynamic transport number and worked satisfactorily at the process conditions of borax electrolysis.     

Ion transport in a two membrane electrowinning cell for the production of hydrochloric acid

A three-compartment electrowinning cell has been evaluated for the regeneration of HCl from metal chloride catholyte. By segregating the catholyte with an anion exchange membrane, and the anolyte with a cation exchange membrane from the middle electrolyte(ampholyte) compartment, HCl could be produced in the ampholyte electrochemically up to one molality. The anolyte consisted of sulphuric acid. Successful operation of such a double membrane cell depends on controlling ion transport through the membrane, especially chloride migration into the anolyte. Results of electrowinning and self-diffusion experiments for three types of cation exchange membrane are presented and discussed.     

2-氯-5-三氯甲基吡啶电化学氢化脱氯合成2-氯-5-甲基吡啶

2-氯-5-三氯甲基吡啶（TCMP）选择性氢化脱氯制备2-氯-5-氯甲基吡啶（CCMP）或2-氯-5-甲基吡啶（CMP）在农药“吡虫啉”合成中具有重要应用价值。首先在弱酸性的甲醇/乙酸/水混合溶剂中研究了TCMP电化学脱氯合成CCMP或CMP的可行性;其次,研究了阴极材料和电解液组成对TCMP选择性脱氯反应的影响;最后,采用膜厚度极距的板框式电解槽分别研究了阴、阳极支持电解质对电解槽压和电流密度与底物浓度对脱氯反应效率的影响。实验结果表明,在弱酸性的甲醇/乙酸/水混合溶剂中,TCMP能在银网阴极上高选择性的氢化脱氯成CMP;CMP收率从高到低的阴极依次为：银> 铜> 锌> 铅> 钛> 石墨> 镍。阴阳极支持电解质从四丁基高氯酸铵分别换成乙酸锂和硫酸,电解槽压大幅度下降。降低电流密度和提高底物浓度有利于TCMP电化学氢化脱氯效率。在优化条件下（阴极液：含10%乙酸+5%水+0.2 mol·L^-1乙酸锂的甲醇溶液;阴极：银网;电流密度：333 A·m^-2;温度：30℃）,0.2 mol·L^-1 TCMP 能高效地转化为CMP（收率：91%）,电流效率可达54%,电解槽压大约为3.0 V。     

电解电渗析制取硼酸和烧碱

研究了硼砂电解电渗析制取硼酸和烧碱（ＥＢＳ法）新工艺中阳极、阳离子膜及阴极碱浓度等因素的影响。结果表明，离子膜的选择对槽电压、电流效率以及电耗有着决定性的影响。较适宜于ＥＢＳ体系的阳离子膜为Ｎａｆｉｏｎ９０１膜；ＰｂＯ２／Ｔｉ阳极在ＥＢＳ体系中性能稳定，消耗低，是一种实用的阳极。