Alkali Process

In the chlor-alkali electrolysis process, an aqueous solution of sodium chloride is decomposed electrolytically by direct current, producing chlorine, hydrogen, and sodium hydroxide solution. The overall reaction of the process 

There are three basic processes for the electrolytic production of chlorine, the nature of the cathode reaction depending on the specific process. These three processes are (1) the diaphragm cell process (Griesheim cell, 1885), (2) the mercury cell process (Cast-ner-Kellner cell, 1892), and (3) the membrane cell process (1970). 

Each process represents a different method of keeping the chlorine produced at the anode separate from the caustic soda and hydrogen produced, directly or indirectly, at the cathode. 

In the mercury cell process, sodium amalgam is produced at the cathode. The amalgam is reacted with water in a separate reactor, called the decomposer, to produce hydrogen gas and caustic soda solution. 

Because the brine is recirculated, solid salt is required for resaturation. The brine, which must be quite pure, is first dechlorinated and then purified by a straightforward precipitation - filtration process. 

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Chlorine Plant Auxiliaries. Flow diagrams for the three electrolytic chlor—alkali processes are given in Figures 28 and 29. Although they differ somewhat in operation, auxiUary processes such as brine purification and chlorine recovery are common to each. 

Asahi Chemical Membrane Chlor—Alkali Process, Asahi Chemical Industry Co., Ltd., Tokyo, Japan, 1987. 

Y. Sajima, K. Sato, and H. Ukihashi, Recent Progress of Asahi Glass Membrane Chlor—Alkali Process, AICHE Symp. Ser. 82(248), 108 (1985). 

Membrane Chlor—Alkali Process, Oronzio De Nora Technologies, BV Milan, Italy and Houston, Tex., 1988. 

Cellulose produds There is little scope for the use of austenitic steels in the alkali processes for the digestion of wood pulp, but both molybdenum-free and molybdenum-bearing steels are extensively used for sulphite pulp digestion, choice depending on concentration, temperature and working experience. 

Pouradier Venet (1952), showed that an equation of the type [tf = 0.166M o. ss exists between the intrinsic viscosity and molecular weight of gelatin. Whereas k and a were each the same for two alkali-processed gelatins, they were different for an acid-processed gelatin (Zhao 1999). 

Gelatins are classified according to whether an acid or an alkali is used in the final preextraction step. If an acid solution is used as the final solvent, type-A gelatin (acid process) is obtained. In case of alkali as the final solvent, type-B gelatin (alkali process) is obtained. Type-A gelatin s isoelectric point is higher compared to that of type-B gelatin, as a... 

The starting material for all industrial chlorine chemistry is sodium chloride, obtained primarily by evaporation of seawater. The chloride ion is highly stable and must be oxidized electrolytically to produce chlorine gas. This is carried out on an industrial scale using the chlor-alkali process, which is shown schematically in Figure 21-15. The electrochemistry involved in the chlor-alkali process is discussed in Section 19-. As with all electrolytic processes, the energy costs are very high, but the process is economically feasible because it generates three commercially valuable products H2 gas, aqueous NaOH, and CI2 gas. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Johannes Richard Lischka. The Leblanc Alkali Process in Britain, 1815-1890 From Handicraft to Corporate Organization in the Chemical Industry. Duke Univer-... 

Celdecor A process for making paper from straw or bagasse. The fiber is digested in aqueous sodium hydroxide and bleached with chlorine. The essential feature is that the alkali and chlorine are used in the proportions in which they are made by the Chlor-Alkali process. 

Diaphragm cell A family of electrochemical chlor-alkali processes using cells with semi-permiable membranes which minimize diffusion between the electrodes. The overall reaction is 2NaCl + 2H20 = 2NaOH + H2 +C12... 

Glanor A Chlor-Alkali process using a bipolar diaphragm cell. Developed by PPG Industries and Oronzio de Nora Impianti Elettrochimic in the early 1970s. 

SAMEX A process for removing traces of mercury from the waste brine from the chlor-alkali process. 

Tekkosha An electrolytic process for obtaining sodium from the sodium amalgam formed in the chlor-alkali process. The electrolyte is a fused mixture of sodium hydroxide, sodium iodide, and sodium cyanide. The sodium deposits at the iron cathode. Developed by Tekkosha Company, Japan, in the 1960s and commercialized in 1971. 

PCDFs are also found in residual waste from the production of vinyl chloride and the chlor-alkali process for chlorine production. Factors favourable for the formation of PCDD/PCDFs are high temperatures, alkaline media, the presence of ultraviolet light, and the presence of radicals in the reaction mixture/chemical process (Fiedler, 1999 Hutzinger and Fiedler 1993). 

Chlor-alkali cell gas effluent, gas purification, l 618t Chlor-alkali electrolytic process, 13 809 Chlor-alkali processes, 13 775... 

The substitution of conventional hydrogen-evolving cathodes with oxygen-consuming gas-diffusion electrodes (GDE), often referred to as oxygen-depolarised cathodes (ODC), also allows a substantial reduction in the energy consumption of the chlor-alkali process. 

Many other goals lie ahead for the further advancement of the membrane chlor-alkali process and technology. Asahi is dedicated to achieving these goals and to providing new standards of efficiency and performance in the future. 

Where most utility installations are the lime or limestone processes, it can be seen from Table IV that a very small percentage of industrial installations are of this type. Most of these installations are the once-through sodium carbonate, sodium hydroxide, and double alkali processes. Where the utility installations have been plagued with corrosion, erosion, scaling and fouling problems, the industrial installations, have to date performed much better. A number of systems showed a process reliability of greater than 85%. 

Several power plants have been equipped with dual alkali processes. These are throw-away processes with two liquid loops. In one common process, the scrubbing liquid is a clear solution of sodium sulfite. The absorption of sulfur dioxide converts the sodium sulfite to sodium bisulfite. In the regeneration loop, an alkali such as lime slurry is added the sodium sulfite solution is regenerated and a mixture of calcium sulfite and calcium sulfate is precipitated. The slurry is... 

You have already seen that chlorine gas can be made by the electrolysis of molten sodium chloride. In industry, some chlorine is produced in this way using the Downs cell described earlier. However, more chlorine is produced in Canada using a different method, called the chlor-alkali process. In this process, brine is electrolyzed in a cell like the one shown in Figure 11.32. Brine is a saturated solution of sodium chloride. 

The half-reactions and the overall cell reaction in the chlor-alkali process are as follows. 

The products of the chlor-alkali process are all useful. Sodium hydroxide is used to make soaps and detergents. It is widely used as a base in many other industrial chemical reactions, as well. The hydrogen produced by the chlor-alkali process is used as a fuel. Chlorine has many uses besides water treatment. For example, chlorine is used as a bleach in the pulp and paper industry. Chlorine is also used in the manufacture of chlorinated organic compounds, such as the common plastic polyvinyl chloride (PVC). 

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