Chlorine-alkaline electrolysis

Rare, shiny, and lightest metal of the platinum group. Hardens platinum and palladium. The presence of 0.1 % of ruthenium in titanium improves its resistance to corrosion 100-fold. The spectacular catalytic properties of ruthenium are used on industrial scales (hydrogenations, sometimes enan-tioselective, and metathesis). Titanium electrodes coated with ruthenium oxide are applied in chlorine-alkaline electrolysis. Suitable for corrosion-resistant contacts and surgical instruments. 

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

The electrodialytic processes have experienced a steady growth since they made their appearance as industrial scale separation processes about 20 years ago. Currently the desalination of brackish water, the chlorine-alkaline electrolysis and the production of table salt are still the dominant applications, but new areas of application in the food and chemical process industry and the use of hybrid processes are gaining interest rapidly. 

The electrochemical reactor must be divided because hydrogen evolution is the main cathodic reaction (Eq. 4, avoiding explosive mixtures). As in industrial chlorine-alkaline electrolysis, chlorine can be directly contacted with the water to be disinfected, or it can be used for preparation of sodium hypochlorite by reacting with NaOH (Eq. 11) for subsequent addition ... 

The fundamental principle of SPE reactors is the coupling of the transport of electrical charges, i.e. an electrical current with a transport of ions (cations or anions), through a SPE membrane due to an externally applied (e.g. electrolysis) or internally generated (e.g. fuel cells) electrical potential gradient. For example, in a chlorine/alkaline SPE reactor (Fig. 13.3), the anode and cathode were separated by a cation-SPE membrane (e.g. Nafion 117) forming two compartments, containing the anolyte (e.g. 25 wt% NaCl solution) and the catholyte (e.g. dilute sodium hydroxide), respectively. 

Chlorine gas is usually used, but electrolysis of alkaline salt solutions in which chlorine is generated in situ is also possible and may become more important in the future. The final pH of solutions to be sold or stored is always adjusted above 11 to maximize stabiUty. The salt is usually not removed. However, when the starting solution contains more than 20.5% sodium hydroxide some salt precipitates as it is formed. This precipitate is removed by filtration to make 12—15% NaOCl solutions with about one-half of the normal amount of salt. Small amounts of such solutions are sold for special purposes. Solutions with practically no salt can be made by reaction of high purity hypochlorous acid with metal hydroxides. 

The electrolysis Of fused alkali salts.—Many attempts have been made to prepare sodium directly by the electrolysis of the fused chloride, since that salt is by far the most abundant and the cheapest source of the metal. The high fusion temp. the strongly corrosive action of the molten chloride and the difficulty of separating the anodic and cathodic products, are the main difficulties which have been encountered in the production of sodium by the electrolysis of fused sodium chloride. Attention has been previously directed to C. E. Acker s process for the preparation of sodium, or rather a sodium-lead alloy, by the electrolysis of fused sodium chloride whereby sodium is produced at one electrode, and chlorine at the other but the process does not appear to have been commercially successful. In E. A. Ashcroft s abandoned process the fused chloride is electrolyzed in a double cell with a carbon anode, and a molten lead cathode. The molten lead-sodium alloy was transported to a second chamber, where it was made the anode in a bath of molten sodium hydroxide whereby sodium was deposited at the cathode. A. Matthiessen 12 electrolyzed a mixture of sodium chloride with half its weight of calcium chloride the addition of the chloride of the alkaline earth, said L. Grabau, hinders the formation of a subchloride. J. Stoerck recommended the addition of... 

For anodic processes the choice of materials for the electrode is much more limited than for cathodic ones, as the anode could bo easily attacked by the products of the electrolysis (chlorine, oxygon etc.), or electrochemioally dissolved. In alkaline solutions the selection will be restricted to the application of platinum (or alloys of platinum with irridium or rhodium), palladium, carbon (or rather graphite) iron and nickel, while for acid solutions only metals of the platinum group and graphite will be suitable in a special case of the electrolysis in sulphuric acid solutions lead has found wide use, it getting coated with a conductive film of lead dioxide. 

While the catholyte becomes alkaline during electrolysis as a result of the discharge of hydrogen ions the solution at the anode, originally neutral, acquires an acid reaction due to the mentioned hydrolysis of dissolved chlorine. 

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