The recovery of chlorine from hydrogen chloride Part 1: new method using a molten salt as the electrolyte

A new method of recovering chlorine from by-product hydrogen chloride is proposed and developed. Hydrogen chloride gas is led through a carbon pipe to a gas diffusion-type porous carbon cathode, which is immersed in a molten salt of lithium chloride (58 mol%)-potassium chloride (42 mol%) at 400°C. A graphite anode is immersed in the same electrolyte. By the direct electrolysis of gaseous hydrogen chloride, hydrogen is obtained from the cathode and chlorine is obtained from the anode. Bench scale tests were also carried out.The current capacity of the cell is 20 A. The cell voltage is 4.9 V at 20 A (500°C, electrode distance 3.8 cm) and in this case, more than 65% of it is the ohmic loss. Current efficiency is more than 90%. It can be concluded that this method is very promising.     

The recovery of chlorine from by-product hydrogen chloride Part 2: Molten metal cathode method

A new method of recovering chlorine from by-product hydrogen chloride is proposed and developed. According to the reaction Me+2HC1 = MeCl₂+H_o (Me = Metal) hydrogen chloride is reduced to give hydrogen and metal chloride. Gaseous hydrogen was drawn out from the reaction system and the metal chloride dissolved in the electrolyte, where it was electrolysed to give chlorine and metal using molten metal as a cathode. The metal was recovered on the cathode in a molten state and reused for the former reaction. Bench scale tests were also carried out, where zinc was used as a molten metal cathode and the cell capacity was about 50 A. The cell voltage was 6.5 V at 50 A (working temperature 560°C, distance between anode and cathode 5 mm) and in this case, the ohmic loss was about 70%. The current efficiency was about 90% (anodic current density 200 A/dm²) when the working temperature was 500°C and electrode distance between anode and cathode was 18 mm.This method seems very promising on the basis of the above-mentioned data.     

Role of gas bubbles adsorbed on a carbon electrode: electroreduction of chlorine gas in fused salts

The reaction Cl₂+2e 2Cl^– has been studied by transient electrochemical techniques on a vitreous carbon electrode in fused alkali chlorides. Two regimes have been detected: for small overpotentials the electrode response obeys the classical laws of linear diffusion, for larger overpotentials the electrode behaviour involves the gas coming from the bubbles. A model is presented to describe these mechanisms. Four steps are considered: dissolution of chlorine around a gas bubble, diffusion in the electrolyte, charge transfer and chloride ion migration. The first step is always rapid. The influence of the three other steps is examined. It is indicated that the largest part of the reaction occurs near the triple contact interface. At the very extremity of the tip, charge transfer is the primary limiting factor: it is considered that over a distance of a few atomic diameters the mass transport is very rapid. At the largest distances the diffusion of dissolved chlorine constitutes the main limiting factor. The ohmic drop due to the electrical resistance of the electrolyte remains small. The influence of the interfacial tension and of the shape of the bubbles are predicted. The non-stationary situation is also examined in order to take into account the size variation of the gas bubbles during transient conditions.Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Estimation of optimum anode geometry in chlor-alkali membrane cells

In order to explore the dependence of cell voltage on electrode geometries, precious metal oxide anodes (DSA^®) with the following three kinds of geometries were constructed: an assembled strip with a parallel array (louvre type), a circularly perforated plate and a flattened mesh. Voltages in the membrane cell of a laboratory scale were measured with chlorine gas evolution in NaCl solution. The cell voltages exhibited a linear relation to a unit cell characteristic dimension as well as to the square of the per cent open area of the anode, as predicted from the theoretical analysis of the primary current distribution in the two-dimensional rectangular model cell. Therefore, small sizes of the iterative pattern unit reduced cell voltages. Slopes of the linear relation gave the increase in solution resistivity caused by the gas bubbles, indicating the presence of a bubble curtain around the anode.Nomenclature d ₁ distance between the anode and the membrane - d ₂ thickness of the membrane - j averaged current density, i.e. the current divided by (width of the anode)x(height of the anode) - o _p per cent open area defined by Equation 2 - p pitch of the unit region - p _cp pitch of the unit region for the circularly perforated anode - p _L pitch of the unit region for the louvered anode - p _LW,p _SW pitch of the unit region for the mesh anode (see Fig. 2c) - r radius of the circular open part - V cell voltage of the whole cell - V _rs residual voltage, i.e. the sum of the voltages such as overpotential due to electrode kineties both at the anode and the cathode, ohmic drop in the catholyte, membrane potential drop and ohmic drop within both electrodes - _bc apparent resistivity of the anolyte containing gas bubbles - ₂ resistivity of the membrane - arctan (p _SW/p _LW)     

垂直惰性阳极铝电解槽内的析气行为

垂直惰性阳极铝电解槽内,析气行为会影响氧化铝浓度分布和电流效率。利用新设计的透明电解槽进行了电解试验,观察了大尺寸惰性阳极气泡的析出及逸出过程。试验结果表明：在阳极底掌下,气泡进行周期性的生长、长大、并聚和脱离,但大尺寸阳极上气泡的滑动和并聚过程与小尺寸阳极上的不同。阳极工作面上则形成了气泡群,新形成的气泡迅速脱离。紧贴着阳极的气泡运行速度慢,外层的气泡运动速度快。所有气泡最终都经液面逸出,大部分气泡到达液面时立即逸出,少部分未及时逸出的气泡随着电解质做一段水平运动后才逐步逸出。测量到的惰性阳极的气泡运动速度为0.006~0.445 m·s^-1,底部的气泡运动速度分布范围宽,然而,受电解质的限制,中上部的范围窄。     

电解法制备活性氯的研究

以石墨为阴极和 Ir Ru Ti贵金属氧化物电极为阳极 ,采用离子隔膜电解装置 ,直接电解低浓度的氯化钠溶液制备活性氯。对氯化钠浓度、表观电流密度、温度、p H值、电解时间和流速在电化学反应过程中对活性氯浓度及电流效率的影响进行了研究。由实验结果得出最佳的试验条件为 :电解温度 2 2°C,电解液浓度 2 0 .0 g/ L,电流密度 4.46m A/ cm2 ,时间 2 h,流速 1 .40 m/ s,p H值 4.2 0～ 5.0 0。另外 ,利用活性氯分别对生活污水和河水进行杀菌消毒 ,测定了杀菌消毒后的生活污水和河水中的 COD值、细菌和大肠杆菌。由于活性氯具有很强的杀菌消毒功能 ,此方法在水处理中将会得到广泛应用     

Design, construction and operation of a laboratory scale electrolytic cell for sodium production using a β″-alumina based low-temperature process

Sodium metal can be produced at low temperatures (523 K) by electrolysis of sodium tetrachloroaluminate (NaAlCl₄) in a cell, which employs sodium ion conducting beta-alumina as diaphragm. A laboratory-scale electrolytic cell and associated systems were designed and constructed to study the various aspects of the energy efficient process. Graphite/reticulated vitreous carbon (RVC) was used as the anode and molten sodium as the cathode. Electrolysis was carried out at 523 K with currents in the range 1–10 A (10–125 mA cm^–2). The cathodic current efficiency was close to 100%, but the anodic current efficiency was very low (20–30%), probably due to the consumption of chlorine in the intercalation reaction of graphite and aluminium chloride. The sodium metal was analysed by AAS and found to have 5N purity. On prolonged electrolysis, the graphite anode disintegrated due to the formation of graphite intercalation compounds. RVC behaved as a better chlorine-evolving anode in the initial period of electrolysis, but its ability for chlorine evolution decreased on continuous electrolysis. The study indicated the need for effective stirring of the electrolyte with excess NaCl to avoid build up of aluminium chloride and the resultant complications in the cell.     

The influence of bubble shape and the thickness of the wetting film on the incremental electrical resistance caused by the presence of a single bubble in Hall-Héroult cells

Bubbles play an important role in the transport phenomena existing in an electrolysis cell. They increase the total ohmic resistance of the electrolyte but their contribution is still not well quantified. During their movement under the anode, the bubbles are separated from the solid by the so-called wetting film, that is by a thin liquid layer. In order to develop a mathematical model to compute the increment of the electrical resistance of the electrolyte due to the presence of several bubbles under the anode, the effects of the bubble shape and the thickness of the wetting film for a single bubble must be quantified a priori. In this first paper, these effects are computed using the finite element method (FEM). The results have shown that the influence of the bubble shape and that of the wetting film is small, about 5% and 2%, respectively.     

An investigation into the recovery of zinc from zinc chloride-sodium chloride mixtures by electrolysis

The electrolysis of zinc chloride-sodium chloride mixtures to yield zinc and chlorine has been examined. The optimum electrolyte composition was found to extend from 75% to 60% zinc chloride. In order to obtain the maximum current and energy efficiencies, it was found necessary to use horizontal electrodes with perforations to allow easy escape of the electrolysis products.     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

The conductivity of aqueous zinc chloride solutions

The conductivity of aqueous zinc chloride reaches a maximum of 10.7 ^–1 m^–1 at 3.7 M ZnCl₂. Measurements on chlorinated ZnCl₂ showed that at low chlorine concentrations, the conductivity increased linearly with the square root of the chlorine concentration. The increase was due to the three species: dissolved chlorine, Cl ₃ ^– and HClO. Ammonium chloride additions increased the conductivity of aqueous zinc chloride substantially.     

Hypochlorite electro-generation. I. A parametric study of a parallel plate electrode cell

A parametric study is described of a parallel plate Ti/PbO₂/x mol dm^–3 NaCl/Ti hypochlorite cell, for which the cell voltage, current efficiency, and energy yield (mol ClO^– kWh^–1) were examined as functions of current density, chloride concentration, and electrolyte flow rate, inlet temperature and pH.The cell was found to behave ohmically, with current efficiencies of 85–99% for 0.5 mol dm^–3 NaCl electrolyte, a typical chloride concentration for sea water. However, the hypochlorite energy decreased substantially with increased current density, reflecting the large contribution of the electrolyte ohmic potential drop to the cell voltage.The behaviour of the Ti/PbO₂ anode was found to be irreproducible, and low temperature (say 278K)/high current density operation was irreversibly detrimental both in terms of the anode potential/cell voltage and current efficiency.Nomenclature b polarization resistance (ohm m²) - d _min interelectrode spacing to minimize the cell voltage (m) - f(x) volume fraction of gas at levelx f - _av average volume fraction of gas - F Faraday constant (96487 C mol^–1) - h electrode length/height (m) - i(x) current density at positionx (A m^–2) - i _av average current density (A m^–2) - I cell current (A) - P pressure of gas evolved at electrodes (N m^–2) - R universal gas constant (8.314 J mol^–1K^–1 ) - R _eff total ohmic resistance of electrolyte and gas in cell (ohm) - s bubble rise rate (m s^–1) - chloride ion transport number - T electrolyte temperature (K) - w electrode width (m) - x distance from bottom of electrodes (m) - z number of Faradays per mole of gas evolved - (x) overpotential at positionx (V) - resistivity of gas free electrolyte (ohm m) - (x) resistivity at levelx of electrolyte containing bubbles (ohm m)     

Bubble characteristics and aerosol formation in electrowinning cells

The nature of the aerosol emitted from a scale model zinc electrowinning system, operated under industrial conditions, has been established as a function of bubble formation rate, electrode surface characteristics, coalescence of bubbles, and control strategies. The emitted aerosol was collected and characterized using an Andersen Ambient Impactor. The effect of the relative humidity of the ambient air on the composition and density of the aerosol droplets was determined, permitting comparison of emissions on a standard basis. The size distribution of the aerosol was found to be of a bimodal nature indicating the presence of both film and jet droplets of geometric mean size 2 and 30m respectively. The amount and distribution of the aerosol were found to depend on the bubble size distribution in the cell which in turn depends on the anode surface, the amount and nature of the MnO₂ scale and surface active substances in the electrolyte. The gross emission rate was about 2–3 mgm^–2 s^–1 without any control. A new method of control of the acidic emissions is proposed based on observations made and results obtained from the study of the aerosol characteristics. Providing means of enhancing bubble coalescence below the electrolyte surface results in fewer and larger bubbles which yield less aerosol. The method has been evaluated in both zinc and copper electrowinning and leads to reduction of aerosol emission by over 90%.Paper presented at the International Meeting on Electrolytic Bubbles organized by the Electrochemical Technology Group of the Society of Chemical Industry, and held at Imperial College, London, 13–14 September 1984.     

Dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte Part II: Investigation of elementary phenomena and electrode modelling

The dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte was investigated. In a previous paper some results were obtained with high particle concentrations by means of spectral analysis of the electrolyte resistance fluctuations and of the potential fluctuations. Here low bead concentrations were used. This allows potential and electrolyte resistance transients to be well separated and to be studied in the time domain. This analysis gives a detailed view of the approach and residence of the beads near the collector, and a quantitative estimation of the ohmic drop effect for insulating and conductive beads. Ohmic drop fluctuations account for the potential fluctuations for insulating glass beads. Zinc beads behave as insulating particles in the low frequency range and generate a similar ohmic drop effect. They behave as conducting particles in the high frequency range and the fast charge exchange induces depolarizing pulses which favour the formation of compact zinc deposits on the current collector. From the analysis of both transients and PSDs of the electrolyte resistance and potential fluctuations, the mean percentage of the particles colliding with the collector has been estimated even for high bead concentrations.     

Formation of hypochlorite,chlorate and oxygen during NaCl electrolysis from alkaline solutions at an RuO2/TiO2 anode

On-site electrolysis of a weak alkaline solution of NaCl has been applied to an increasing extent for disinfection. To optimize the electrolytic cell and the electrolysis conditions, the current efficiency for hypochlorite, chlorate and oxygen formation at a commercial RuO₂/TiO₂ anode were determined under various conditions. It was found that for solution with low NaCl concentrations, (lower than 200 mol m^?3), and at 298 K, solution flow velocity of 0.075 ms^?1 and high current density, (higher than 2 kA m^?2), hypochlorite formation is determined by mass transfer of chloride. The formation of chlorate in weakly alkaline media at a chlorine and oxygen-evolving anode is ascribed to two reactions, namely, the direct oxidation of chloride to chlorate and the conversion of hypochlorite. This is suggested to split the well-known electrochemical Foerster reaction into a chemical reaction for the conversion of hypochlorite in chlorate and the electrochemical oxidation reaction of water. It is proposed that in an acidic reaction layer at the anode the mechanism of chlorate formation may be given by the following:      

Study of the effect of electrode resistance on current density distribution in cylindrical electrochemical reactors

The distribution of current density in a cylindrical electrochemical reactor was determined experimentally, a thin Pt wire, 0.2 mm diameter and 500–600 mm long, being employed as central electrode. In these investigations two methods are used: (i) a reactor with a segmented counterelectrode; in this case, measurements of the current in each ring of the counterelectrode were made; (ii) a reactor with a bi-electrode probe; in this case, the distribution was obtained by measuring the ohmic drop in the solution phase, with the probe being positioned at different heights. A mathematical model to represent such reactors was developed assuming an axially constant electrolyte potential. The experimental and theoretical values are compared in order to determine the predictive suitability of the proposed model. Both the error in predicting the feeder overvoltage and a statistical parameter denote the agreement between the computed and the measured current density distributions. The parameters acting upon the current distribution were lumped in a single dimensionless variable, the so-called modified Wagner number, used to determine the applicability range of the proposed model. It was concluded that when this number exceeds 15×10^–3, for concentrated solutions, the model can be used to design this type of reaction.     

Methanol and formic acid oxidation in zinc electrowinning under process conditions

The possibility of using methanol or formic acid oxidation as the anode process in zinc electrowinning was examined. The activity for methanol and formic acid oxidation on Pt coated high surface area electrodes was investigated over 36 h, at a current density used in industry. The activity could be maintained at a constant potential level in a synthetic electrowinning electrolyte if the current was reversed for short periods. During the tests, the anode potential was, more than 1.2 V below the potential for the oxygen evolving lead anodes used in modern zinc electrowinning. The lowered anode potential would lead to a significant energy reduction. However, tests in industrial electrolyte resulted in a very low activity for both methanol and formic acid oxidation. The low activity was shown to be caused mainly by chloride impurities. A reduction of the chloride content below 10⁻⁵ M is needed in order to obtain sufficient activity for methanol oxidation on Pt for use in zinc electrowinning. Pt and PtRu electrodes were compared regarding their activity for methanol oxidation and the latter was shown to be more affected by chloride impurities. However, at a potential of 0.7 V vs NHE, with a chloride content of 10⁻⁴ M, formic acid oxidation on PtRu gives the highest current density.     

Impact of chlorine and acidification in the electrochemical treatment of tumours

Electrochemical treatment (EChT) of tumours offers considerable promise as a safe, simple and relatively noninvasive antitumour therapy. When platinum is used as electrode material, the major electrochemical reactions at the anode are chlorine and oxygen evolution. Conflicting opinions can be found in the literature over the role of chlorine in the underlying destruction mechanism behind EChT. In this present study, the impact of chlorine in EChT treatments is investigated by means of mathematical modelling. The analysis focuses on the tissue surrounding a spherical platinum anode when applying a constant current density. Tissue is modelled as an aqueous solution of sodium chloride containing a bicarbonate buffer system and organic constituents susceptible to reactions with chlorine. Except for the case of very low anode current densities, the simulations clearly show that it is the spreading of hydrogen ions – and not chlorine molecules – that determines the extent of tissue destruction around the anode. Moreover, it is found that the reactions of chlorine with tissue play important roles as generators of hydrogen ions. The contribution of these reactions to the acidification of tissue, surrounding the anode, is strongly dependent on the applied current density and increases with decreasing current density.     

Pressure drop behaviour in a three-dimensional packed bed cell during copper deposition and hydrogen evolution

Pressure drop in a three-dimensional packed bed electrode cell has been investigated during electrowinning of copper with simultaneous hydrogen evolution. The presence of gas bubbles in the packed bed electrode leads to an increase in the pressure drop in the electrolyte flowing through the cell. This pressure drop increase was in the range of 30 to 70% relative to the bubble-free case. Parameters such as current density, electrolyte velocity and particle shape greatly influence the pressure drop behaviour. Experimental results are correlated using the Ergun equation and show satisfactorily good agreement in the investigated range of modified Reynolds number.     

Effect of gas expansion on the velocity of individual Taylor bubbles rising in vertical columns with water: Experimental studies at atmospheric pressure and under vacuum

This study was designed to determine the effect of gas expansion on the velocity of Taylor bubbles rising individually in a vertical column of water. This experimental study was conducted at atmospheric pressure or under vacuum (33.3 and ) using three different acrylic columns with internal diameters of 0.022, 0.032, and 0.052 m, and more than 4.0 m high. A non-intrusive optical method was used to measure velocity and length of Taylor bubbles at five different locations along the columns. The operating conditions used correspond to inertial controlled regime.In experiments performed under vacuum, there is considerable gas expansion during the rise of Taylor bubbles, particularly when they approach the liquid free surface where the pressure drop (due to the hydrostatic pressure) is of the order of magnitude of the absolute pressure. The liquid ahead of the bubble is displaced upward by an amount proportional to the gas expansion resulting in increased bubble velocity. The calculated Reynolds number suggests a laminar regime in the liquid ahead of the bubble. However, the experimentally determined velocity coefficient C for each column was much smaller than 2, which would be expected for laminar flow. The value of C obtained ranges from 1.13±0.09, for the narrowest column, to 1.40±0.24, for the widest column. This suggests that a fully developed laminar flow in the liquid ahead of the bubble is never achieved due to continuous bubble expansion at a variable rate, regardless of column height.The velocity coefficient C can be used to calculate the contribution of liquid motion to bubble velocity. Subtracting this contribution from the measured bubble velocity defines a constant value which is nearly identical to the bubble rise velocity measured in the same column operated as a constant volume system (two ends closed) where gas expansion is absent.