Chlor-alkali electrolysis

The electrolysis of sodium chloride solutions (brine) producing chlorine at the anode and sodium hydroxide (NaOH, caustic soda) in the catholyte via the overall reaction 

In the past, graphite had been used as the anode in chlorine cells. During electrolysis, it was gradually consumed by oxidation so that the gap between the electrodes increased, producing an important increase in ohmic resistance, cell voltage, and power consumption. With the general introduction of dimensionally stable anodes, DSA working with layers of mixed titanium-ruthenium oxides, this problem was overcome completely. 

Chlorine is a very active oxidizing agent and readily undergoes electrochemical reactions. The question arises, then, whether chlorine could not be used as an oxidizing agent in fuel cells. Some attempts undertaken in this direction are discussed in Section 10.2.2. 

In existing chlorine cells, the cathodic process is hydrogen evolution  

As this reaction proceeds, the layer of electrolyte solution (brine) next to the cathode becomes strongly alkaline, so that for the reaction formula and for the thermodynamic potential, the versions applicable to alkaline solutions must be used. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction 

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

The third reaction product, hydrogen, is usually not utilized in chlor-aUcali electrolysis. Current annual world production of chlorine by electrolysis is over 30 that of alkali is 35 megatons, and it increases by 2 to 3% per year. This industry consumes about 100 billion kilowatthours of electrical energy per year. 

This yields a solution of highly pure alkali (free of chloride ions), which can be used in the manufacture of synthetic fibers. The mercury, which has been stripped of sodium, is returned to the electrolyzer. The cost of chlorine is higher in the mercurycell than in the diaphragm-cell process. In addition, the mercury-cell process is ecologically dangerous, owing to the possible escape of mercury into the environment hence, it has increasingly been discontinued in all countries. 

In the diaphragm-cell process, a solid cathode (iron) is used where hydrogen is evolved [reaction (15.4)]. Porous asbestos diaphragms are used to prevent mixing of the catholyte and anolyte, but owing to the finite permeability of these diaphragms, the alkaline solution that is produced near the cathode stiU contains important levels of chloride ions as an impurity. 

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There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

EEC Directive on mercury (chlor-alkali electrolysis industry)... 

When chlor-alkali electrolysis is conducted in an undivided cell with mild-steel cathode, the chlorine generated anodically will react with the alkali produced cathodically, and a solution of sodium hypochlorite NaClO is formed. Hypochlorite ions are readily oxidized at the anode to chlorate ions this is the basis for electrolytic chlorate production. Perchlorates can also be obtained electrochemically. 

Membranes in chlor-alkali electrolysis require highly pure brine feed the water used in membrane water electrolyzers must also be rather pure. 

Their availability has greatly expanded the potential for electrolytic processes in synthesis and fuel cells as well as in environmental control. Perfluorinated cation exchange membranes such as Nafion outlast the material that preceeded them by up to four and a half years [48], Unfortunately very little has been published on their behaviour outside their use in chlor-alkali electrolysis. 

Fig. 4.2 The effect of pressure compensation in chlor-alkali electrolysis.

The flow-through cathode is the result of a tailored-to-the-process evolution of the GDE structure, which is available also in two additional configurations double-sided (originally developed for fuel cell servicing) and single-sided (see Fig. 9.7). The double-sided type is particularly suited for the electrochemical process where the product should not be released on to the back surface of the cathode, as in the case of oxygen-depolarised chlor-alkali electrolysis, discussed in Section 9.3. 

The first practical example of electrodes able to satisfy many of the characteristics required in the application of chlor-alkali electrolysis is a particular family of doublesided gas-diffusion electrodes introduced some years ago under the trade name of ESNS , by E-TEK Inc. (now a Division of DeNora North America). The dual function (electrode and separator) of this electrode structure was achieved with an accurate choice of the basic components. 

This chapter has introduced the RNDS application in the removal of impurities from brine destined for chlor-alkali electrolysis. On top of this, however, RNDS has potential use in other markets, including water treatment. Chlorine Engineers will continue its innovative work to meet the various requests coming from the chlor-alkali industry. 

The GDE for chlor-alkali electrolysis plants is still a relatively new concept compared with other chlorine technologies. It may be assumed that there is still considerable development potential in this newer technology. The above cost and Rol figures are based on optimistic values and should be regarded as provisional. In... 

All the examples quoted show how costs can be lowered, profit for products increased and the turnover enlarged by selecting KU know-how and technology for chlor-alkali electrolysis plants. 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

Wolff, J.J. (1985) Ion exchange purification of feed brine for chlor-alkali electrolysis cells the role of Duolite C-467. Rohm c Haas Bulletin IE-D-285, March. 

Development of chlorine electrode materials has benefited from the experience of chlor-alkali electrolysis cell technology. The main problem is to find the best compromise between cycle life and cost. Porous graphite, subjected to certain proprietary treatments, has been considered a preferable alternative to ruthenium-treated titanium substrates. The graphite electrode may undergo slow oxidative degradation, but this does not seem to be a significant process. 

Castner, Hamilton Young — (Sep. 11, 1858, Brooklyn, New York, USA - Oct. 11,1899, Saranac Lake, New York, USA) Castner studied at the Brooklyn Polytechnic Institute and at the School of Mines of Columbia University. He started as an analytical chemist, however, later he devoted himself to the design and the improvement of industrial chemical processes. He worked on the production of charcoal, and it led him to investigate the Devilles aluminum process. He discovered an efficient way to produce sodium in 1886 which made also the production of aluminum much cheaper. He could make aluminum on a substantial industrial scale at the Oldbury plant of The Aluminium Company Limited founded in England. However, - Hall and - Heroult invented their electrochemical process which could manufacture aluminum at an even lower price, and the chemical process became obsolete. Castner also started to use electricity, which became available and cheap after the invention of the dynamo by - Siemens in 1866, and elaborated the - chlor-alkali electrolysis process by using a mercury cathode. Since Karl Kellner (1851-1905) also patented an almost identical procedure, the process became known as Castner-Kellner process. Cast-... 

In polyelectrolyte gels the variation of pH or salt concentration (cs) causes a swelling or shrinkage. Therefore, in this case chemical energy is transformed to mechanical work (artificial muscles). An increase of cs (or a decrease of temperature) makes the gel shrink. Usually, the shrinking process occurs smoothly, but under certain conditions a tiny addition of salt leads to the collapse of the gel [iii, iv]. Hydration of macroions also plays an important role, e.g., in the case of proton-conductive polymers, such as -> Nafion, which are applied in -rfuel cells, -> chlor-alkali electrolysis, effluent treatment, etc. [v]. Polyelectrolytes have to be distinguished from the solid polymer electrolytes [vi] (- polymer electrolytes) inasmuch as the latter usually contain an undissociable polymer and dissolved small electrolytes. 

Asahi Kasei develops membranes mainly for chlor-alkali electrolysis technology with Aciplex F PFSA membranes. The Aciplex F membrane is employed in plants with a total production capacity of over 5 million tons of sodium hydroxide... 

Council Directive of 22 March 1982 on limit values and quality objectives for mercury discharges by the chlor-alkali electrolysis industry (OJ No L81, 27.3.82, p.29). 

Fig. 7 Principle of the membrane process for chlor-alkali electrolysis [14, p. 441],...

Introducing the electrochemical reactions taking place during chlor-alkali electrolysis in Sect. 5.2.2, it was noted that some more detailed information would follow. For better understanding, it is necessary... 

Fig. 14 Schematic representation of the chlor-alkali electrolysis using an oxygen depolarized cathode.

H. L. Yeager and J. D. Mallnsky, "Permselectivity and Conductance of Perfluorinated Ionomer Membranes 1n Chlor-Alkali Electrolysis Process", presented at the Amer. Chem. Soc. Mtg., Philadelphia, Aug. 1984. 

Perfluorinated ionomer membranes have been developed for use as separators in chlor-alkali electrolysis cells. Using an automated test apparatus, the current efficiency and voltage drop of such a high performance membrane were evaluated as a function of several cell parameters. Results are plotted as three dimensional surfaces, and are discussed in terms of current theories of membrane permselectivity. 

Seko, M. "Membrane for Chlor-Alkali Electrolysis", presented at the 159th National Meeting, The Electrochemical Society, Minneapolis, Minn., May 10-15, 1981. 

The Nafion membranes are produced in this way and with a fabric backing such as PTFE or mixed PTFE - rayon fabrics These supporting materials improve the mechanical strength of the film and keep the dimensional changes in bounds In general, for chlor-alkali electrolysis, the side of the membrane with the highest resistance, selectivity and charge density is preferred toward the cathode side to limit the undesirable effects of the back flow of hydroxide ions into the anode chamber The anolyte side of the membrane polymer is thus less dense, less selective and more conductive than the catholyte side of the separator film ... 

This chapter summarizes the preparation and the fabrication of perfluorocarboxylate polymers and their fundamental properties including those of the ionized salt-type membranes. The application of Flemion in chlor-alkali electrolysis is also described. 

In order to understand the full extent of the mercury problem in these times, one has only to consider the enormous loss rates. From the total of 2865 tons of mercury purchased in the U.S. in 1968, 76% or 2160 tons were lost to the environment. According to calculations of Kemp et al. (1974), the Lake Ontario reservoir contained a mass of 500 to 600 metric tons of "excess" mercury, i.e. discharged from anthropogenic sources. With the improvements in the methods of chlor-alkali electrolysis and by subsequent purification of waste streams the mercury loss has been reduced from 100 g per metric ton of manufactured chlorine to approx. 2 g per ton or less (Anon., 1973). The effect of these measures can be seen from concentration profiles of mercury in sediment cores taken off the mouth of Niagara River by Mudroch (1983), where a very distinct decrease from formerly approx. 4-7 ug Hg/g to less than lug Hg/g in recent years has occurred (Figure 2-6). 

Seko, M., Yomiyama, A. and Ogawa, S., Chlor-alkali electrolysis using... 

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