Reduction of by-product formation in alkali chloride membrane electrolysis

To obtain higher chlorine purity hydrochloric acid can be added to the feed brine of membrane cells in alkali chloride electrolysis. During the electrolytic process hydroxide ions migrate from the cathode compartment into the anode compartment. Hydrochloric acid neutralizes these hydroxide ions and, hence, formation of the by-products (oxygen in the anode gas and sodium chlorate in the anolyte) is reduced. With laboratory membrane cells the effects of varied amounts of hydrochloric acid on concentrations and current efficiencies of these by-products have been studied. Under normal operating conditions (with pH of feed brine between 2 and 11) the formation of by-products is not influenced by the addition of acid. Effects can only be observed at brine pH values<1. Maximum effects occur if the brine pH is 0.1 and the anolyte pH is 2. The latter value is the limiting pH given by the membrane suppliers. At this point 6.3 dm³ hydrochloric acid (37% HCl) per 1 m³ of the feed brine have to be added in order to obtain an anode gas with 0.4% oxygen by volume. The formation of sodium chlorate is completely suppressed. Problems connected with this process and its application to industrial electrolysis are discussed.     

Electrochemical oxidation of an acid dye by active chlorine generated using Ti/Sn<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Ir<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> electrodes

The generation of active chlorine on Ti/Sn_(1−x)Ir _x O₂ anodes, with different compositions of Ir (x = 0.01, 0.05, 0.10 and 0.30 ), was investigated by controlled current density electrolysis. Using a low concentration of chloride ions (0.05 mol L⁻¹) and a low current density (5 mA cm⁻²) it was possible to produce up to 60 mg L⁻¹ of active chlorine on a Ti/Sn_0.99Ir_0.01O₂ anode. The feasibility of the discoloration of a textile acid azo dye, acid red 29 dye (C.I. 16570), was also investigated with in situ electrogenerated active chlorine on Ti/Sn_(1−x)Ir _x O₂ anodes. The best conditions for 100% discoloration and maximum degradation (70% TOC reduction) were found to be: NaCl pH 4, 25 mA cm⁻² and 6 h of electrolysis. It is suggested that active chlorine generation and/or powerful oxidants such as chlorine radicals and hydroxyl radicals are responsible for promoting faster dye degradation. Rate constants calculated from color decay versus time reveal a zero order reaction at dye concentrations up to 1.0 × 10⁻⁴ mol L⁻¹. Effects of other electrolytes, dye concentration and applied density currents also have been investigated and are discussed.     

ELECTROCHEMICAL RECOVERY OF CHLORINE AND HYDROGEN FROM HYDROGEN CHLORIDE BY-PRODUCT PRODUCED IN THE PETROCHEMICAL INDUSTRY

Simultaneous production of hydrogen as an energy carrier and chlorine as a valuable chemical from recycled hydrogen chloride was investigated employing a lab-scale membrane electrolysis setup. The effects of various process parameters including current density (1–4 kA m^?2), cell temperature (45°–75°C), flow rate of hydrochloric acid feed (200–500 mL min^?1), and concentration of acid (18–21 wt.%) on the cell voltage and chlorine current efficiency (ChCE) were studied. The Taguchi design of experiments (L₁₆ array) was employed to design the minimum number of experiments necessary to fully study the process. A filter press type cell of 10 cm² surface area comprising a DSA anode, an alloy of predominantly nickel cathode and Nafion 115 membrane, was used. It was observed that increasing anolyte flow rate, anolyte concentration, or cell temperature caused a decrease in cell voltage and an increase in ChCE, while increasing current density linearly increased cell voltage and decreased ChCE.     

Competitive transport of hydrochloric acid and zinc chloride through polymeric anion‐exchange membrane

Competitive transport of hydrochloric acid and zinc chloride has been investigated in a two‐compartment mixed cell with an anion‐exchange membrane Neosepta‐AFN developed and produced by Tokuyama Soda Co. These experiments have proved that hydrochloric acid permeates well through the membrane used but, on the other hand, zinc chloride is not effectively rejected. The flux of zinc chloride has been found to be increasing with increasing acid and salt concentrations. Furthermore, it has been found that it is approximately one order of magnitude higher than that found in the case of simultaneous transport of sulfuric acid and zinc sulfate through the same membrane. The further calculations concerning the ionic equilibria with sorption isotherms for the HCl? ZnCl₂ system, which have been measured experimentally, have revealed that high flux of ZnCl₂ is due to the fact that a considerable amount of zinc chloride in the membrane is in the form of ZnCl₃^? complex, which is relatively small and passes well through this membrane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1391–1397, 2006     

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl₂ salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl₂ or CaCl₂–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO_2?x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.     

Determination of Diffusivity From Steady‐state Mass Transfer Measurements in a Continuous Dialyzer Under Conditions of Non‐zero Solution Flux Through a Membrane

The transport of selected inorganic acids (HCl, H₃PO₄) through an anion‐exchange membrane, Neosepta‐AFN, was investigated in a two‐compartment counter‐current continuous dialyzer with single passes at steady state. The basic data obtained were completed by the measurement of sorption isotherms. The mass transfer rate of the individual acids was quantified by the diffusivity of acid in the membrane, which was determined from the acid concentrations in the streams entering and leaving the dialyzer. For this purpose, two ordinary differential equations describing the steady‐state concentration profiles of acid in both the compartments of the dialyzer were numerically solved in connection with an optimizing procedure. In the mathematical model used, the mass transfer resistances in the liquid films on both sides of the membrane and the flux of solution through the membrane were taken into account. All the experiments, carried out at a constant temperature of 25 °C with subsequent data treatment, revealed that the diffusivity of hydrochloric acid is about one order of magnitude higher than that of phosphoric acid. Moreover, regarding the dependence of the diffusivity of hydrochloric acid upon its concentration in the membrane, a maximum at an acid concentration of 0.622 kmol/m³ can be identified, while the diffusivity of phosphoric acid remains practically constant.     

Study of Proton Leakage at Interface of Anion-Exchange Membrane in Solutions of Acids,Salts, and Solvents Using Current/Voltage Characteristics

Water dissociation and proton leakage using the anionic exchange membrane (AMH) are studied by means of current/voltage characteristics and confirmed by simulation of transport number using Hittorf's method. The acids used are HCl, HNO₃, and H₂SO₄ and the salts are NaCl, ZnCl₂, and NaNO₃. Concentration polarization of such membrane is accompanied by a change in the electrolyte concentration/solution interface due hydrolysis reactions. The results show that when the concentration of the electrolyte increases, the limiting current density increases linearly and the transmembrane resistance decreases systematically. The thickness of the diffusion layer is always higher in presence of acid than salt, making in evidence the proton leakage through the membrane. Besides, when the membrane is selectively permeable to chloride anion in the case of ZnCl₂, the thickness of the double layer is rather bigger and far exceeds that of the membrane. The voltamperometry method seems reliable and offers some advantages over that of Hittorf because it shows the effects of some parameters on the value of limiting current: concentration, counter-ion types (Cl^?, NO₃^? SO₄^2?), and the gradient of concentration in the anode and cathode compartments. It can, therefore, allow to optimize the value of the current which should be used in electrodialysis in any form and without a great consumption of energy. Moreover, the simulation carried out for transport number of proton, shows its sensitivity toward the variation in concentration in the receiving compartment. In effect, a small decrease in concentration implies an enormous decrease in its value.     

Preparation of high-purity antimony by electrodeposition

Detailed studies were carried out on the electrowinning of antimony from SbCl₃–HCl–H₂SO₄ and SbCl₃–HCl baths using a graphite anode and a tantalum cathode, the anode being enclosed in a cylindrical glass compartment provided with a sintered disc. Disintegration of the graphite anode increased with increase in anode current density and duration of electrolysis. Analysis of gas evolved at the anode indicated that the presence of sulphuric acid increased the production of CO₂. The anodic disintegration was reduced to a negligibly low value by circulation of 6m HCl through the anode compartment. Optimum conditions were determined for electrowinning of antimony from a SbCl₃–HCl bath. The tantalum content in the metal was 0.1–0.2 p.p.m.     

Fundamental and applied aspects of the electrochemistry of chlorine

Data so far published on the thermodynamics of sodium amalgams and chloride solutions allow us to calculate the reversible potential for the electrolysis of brines at concentrations and temperatures corresponding to industrial processing. Data are also reported for the electrolysis of hydrochloric acid solutions. Kinetics of chlorine discharge and passivation phenomena on Pt, Pt-Ir and oxide-based electrodes are reviewed, in relation to the use of dimensionally stable electrodes, on titanium supports. The electronic conduction of RuO₂ and other oxides is also discussed.     

Electrochemical study of a hydrogen diffusion anode-membrane assembly for membrane electrolysis

The membrane electrode assembly (MEA) studied was constituted with a gas diffusion electrode (E-TEK) impregnated with Nafion^® solution which was assembled with a Nafion^® 117 cation exchange membrane under heat and pressure. The MEA was used as anode in a membrane electrolysis (ME) cell with the objective to regenerate HCl and NaOH from NaCl. Current efficiency for hydrogen oxidation was determined and its value is 100%, which indicates that the only reaction occurring at the anode is the oxidation of hydrogen. Current-potential curves, recorded in different conditions, showed a linear variation in the range 0-3000 A m⁻² when hydrochloric acid concentration is below 2 mol dm⁻³. In this case, the overvoltage was shown to be mainly due to the ohmic drop in the membrane and in the layer where Nafion^® impregnation was performed. MEA overvoltage necessary to reach 3000 A m⁻² current density was about 0.12 V. For high HCl concentration (6-8 mol dm⁻³), the MEA overvoltage increased sharply with current density due to the adsorption of chloride anions on platinum catalyst.     

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.     

Experimental and modeling of the electrodialytic and dialytic treatment of a fly ash containing Cd,Cu and Pb

A one-dimensional model is developed for simulating the electrodialytic and dialytic treatment of a fly ash containing cadmium, copper and lead. Two experimental systems have been used, a column of ash and a stirred ash suspension. The movement of Cd, Cu and Pb has been modeled taking into account the diffusion transport resulting from the concentration gradients of their compounds through the membranes and boundary layers and the electromigration of their ionic, simple and complex species during the operation. The model also includes the electromigration of the non-contaminant most important principal ionic species in the system, H⁺ and OH⁻, proceeding of the electrolysis at the electrodes, Ca²⁺, CO₃ ⁼, SO₄ ⁼, etc. proceeding from the ash and Na⁺ and NO₃ ⁻, or citrate and ammonium ions incorporated as electrolyte solutions and/or as agent solution during the ash treatment. The simulation also takes into account that OH⁻ generated on the cathode, during the electrodialytic remediation, is periodically neutralized by the addition of nitric acid in the cathode compartment. The anion and cation-exchange membranes are simply represented as ionic filters that preclude the transport of the cations and anions, respectively, with the exception of H⁺ which is retarded but pass through the anion-exchange membrane.     

Evaluation of concepts for hydrogen – chlorine fuel cells

Three different concepts for H₂–Cl₂ fuel cells have been evaluated. An ordinary PEM fuel cell based on a Nafion membrane, a fuel cell based on a combination of circulating hydrochloric acid and a Nafion membrane and a system based on a phosphoric acid doped Polybenzimidazole (PBI) membrane. None of the investigated systems were able to demonstrate stable operation under the conditions used in this study, due to electrocatalyst corrosion, membrane dehydration and/or electrode flooding. All systems studied achieved open circuit voltages close to the reversible thermodynamic value for production of aqueous hydrochloric acid, suggesting formation of dissolved HCl in the electrolyte and fast electrode kinetics.     

In situ active chlorine generation for the treatment of dye-containing effluents

This study examined the possibility to remove colour causing-compounds from synthetic effluent by indirect electrochemical oxidation using iridium oxide anode electrodes. Using a high concentration of chloride ions (17.1 mM) and various current densities, it was possible to produce high concentration of active chlorine with a specific production rate of 2.8 mg min⁻¹ A⁻¹. The best performance for acid methyl violet 2B dye (MV2B) decomposition was obtained using Ti/IrO₂ anodes operated at a current density of 15 mA cm⁻² during 40 min of treatment in the presence of 3.42 mM of chloride ions. Under these conditions, more than 99% of MV2B was removed (with a reaction rate apparent constant of 0.20 min⁻¹), whereas COD and TOC removal were 51% and 75%, respectively. The electrolytic cell was then used for the degradation of three other synthetic dye solutions: Eosin yellowish (EOY), Trypan Blue (TRB), Acridine Orange (ACO). TRB was the most difficult dye to remove from solution with a value reaction rate constant of 0.12 min⁻¹, compared to 0.19 min⁻¹ and 0.24 min⁻¹ recorded for ACO and EOY dyes, respectively. More than 99% of these dyes were removed by electrochemical oxidation.     

Coproduction of sulphuric acid and hydrogen by sulphur-assisted water electrolysis process

The addition of sulphur powder to the anode compartment of the sulphuric acid-water electrolysis cell resulted in the suppression of oxygen evolution rate; at a cell voltage of 2 V, there was an acceleration of the hydrogen production rate, and the anode reaction led to the production of sulphate ion (SO ₄ ^2? ) by the oxidation of the sulphur powder. The net equation for this sulphur-assisted water electrolysis system may be written as S + 4H₂O → H₂SO₄ + 3H₂ whose stoichiometry was proved experimentally using phosphoric acid as the electrolyte. The whole process can be rationalized if the anodic oxidation of sulphur occurs at a lower potential than oxygen evolution and hence the energy consumption is lower. Thus the addition of sulphur to the anolyte helps to improve the energy efficiency of the acidic water electrolysis system and generates, simultaneously, sulphuric acid and hydrogen.     

Synthesis of 7-ketolithocholic acid via indirect electrooxidation of chenodeoxycholic acid

7-Ketolithocholic acid was synthesized by indirect electrooxidation of chenodeoxycholic acid using the medium of Br⁻/Br₂. Some important factors such as anode material, solvent, initial concentration of chenodeoxycholic acid, current density, electrolysis time have been investigated in detail. By comparing the experiment results, it can be concluded that productivity and current efficiency in the divided electrolytic cell are higher than that in the undivided electrolytic cell. The optimized process conditions have been found to be: the anode material PbO₂/Ti, the solvent acetonitrile, and initial concentration of chenodeoxycholic acid 23.8–31.8 mg/mL, current density 95.2–142.9 A/m². The current efficiency could reach 85% and the productivity of 7-ketolithocholic acid could reach 83%. The melting point, IR spectrum, ¹H NMR, ¹³C NMR and EI-MS were used to characterize the product. The analytical results confirm that the product is 7-ketolithocholic acid.     

两期离子膜电解氯气压缩机前并网的实施

针对两期电解阳极室操作压力差别极大的问题,实施了氯气压缩机前氯气并网设想。经过科学分析,选择适宜的工艺控制方案,从而使两期电解氯气并网后,2套不同的电解槽阳极室压力及阴、阳极压差均达到了良好的控制效果。     

Comparative study of chemical and electrochemical Fenton treatment of organic pollutants in wastewater

The technological and economic aspects of using the Fenton process to treat industrial wastewater containing morpholyne and diethylethanolamine, as well as sodium salts of naphthalene sulfonic acid and of ethylenediaminetetraacetic acid based on data obtained in pilot tests are discussed. Chemical Fenton technology was tested using commercial 30–35% solutions of H₂O₂ and iron (II) salts, which was followed by the additional electrochemical destruction of organic pollutants in an undivided reactor with catalytic stable anodes (CSA) and 1 g L⁻¹ NaCl as a supporting electrolyte and a source of active chlorine. An alternative electrochemical method involving the electrogeneration of hydrogen peroxide in polluted water at the gas -diffusion cathode was studied both with the addition of ferrous salt to the electrolyte prior to electrolysis (in-cell electro-Fenton) as well as with the post-electrolysis addition of Fe²⁺ in another reactor (ex-cell electro-Fenton). The accumulation of hydrogen peroxide in concentrations sufficient for the mineralization of organic pollutants was achieved in both cases with near 100% current efficiency. In comparison with wastewater treatment processes which use a purchased hydrogen peroxide reagent, the Fenton-like processes achieved an economic savings of as much as 64.5% in running costs due to the on-site electrochemical generation of H₂O₂. Preparative electrolysis in the membrane reactor showed higher current efficiencies and lower specific energy consumptions for H₂O₂ electrogeneration in comparison with the results of tests carried out in an undivided cell.     

Electrochemical behavior of a platinum anode for reduction of uranium oxide in a LiCl molten salt

The electrochemical behavior of a platinum anode has been investigated during the electrolysis of uranium oxide in a LiCl molten salt. Pt is oxidized to Pt²⁺ at 2.6 V (vs. Li–Pb reference electrode) in the absence of O²⁻ ion. The platinum dissolution takes place at a more anodic potential with an increase of O²⁻ ion. Although the main anodic process in the electrolysis is the oxygen evolution by oxidation of O²⁻ ion at a higher concentration of Li₂O, a thin film due to the formation of Li₂PtO₃ was coated on the anode surface. The platinum dissolution proceeds with an intergranular corrosion-like behavior at a low concentration of Li₂O.     

Chlorate and oxygen formation in alkali chloride membrane electrolysis

During the operation of an industrial-scale membrane electrolysis plant over a number of years, a record was kept of the formation of the byproduct oxygen in the anode gas and chlorate in the anolyte parallel to the decline of the current efficiencies of the main products. It was found that the current efficiencies of the byproducts increase linearly with the declining current efficiencies of the main products, chlorine and caustic soda. Of the two types of anode used, one exhibited considerably more oxygen formation than the other. The high-oxygen anode was associated with distinctly lower chlorate formation than the low-oxygen anode. The increasing oxygen contents and chlorate formation rates associated with falling caustic current efficiency are reported for both types of anode. If hydrochloric acid is used to destroy the chlorate, the amount of acid must be increased as the caustic current efficiency falls. The amounts of hydrochloric acid required for the two types of anode are calculated as examples for 96% and 93% caustic current efficiency.