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8.11 Chlor-Alkali 8.11.1 General1-2 The chlor-alkali electrolysis process is used in the manufacture of chlorine, hydrogen, and sodium hydroxide (caustic) solution. Of these 3, the primary product is chlorine. Chlorine is 1 of the more abundant chemicals produced by industry and has a wide variety of industrial uses. Chlorine was first used to produce bleaching agents for the textile and paper industries and for general cleaning and disinfecting. Since 1950, chlorine has become increasingly important as a raw material for synthetic organic chemistry. Chlorine is an essential component of construction materials, solvents, and insecticides. Annual production from U. S. facilities was 9.9 million megagrams (Mg) (10.9 million tons) in 1990 after peaking at 10.4 million Mg (11.4 million tons) in 1989. 8.11.2 Process Description1-3 There are 3 types of electrolytic processes used in the production of chlorine: (1) the diaphragm cell process, (2) the mercury cell process, and (3) the membrane cell process. In each process, a salt solution is electrolyzed by the action of direct electric current that converts chloride ions to elemental chlorine. The overall process reaction is: 2NaCl 2H2O → Cl2 H2 2NaOH In all 3 methods, the chlorine (Cl2) is produced at the positive electrode (anode) and the caustic soda (NaOH) and hydrogen (H2) are produced, directly or indirectly, at the negative electrode (cathode). The 3 processes differ in the method by which the anode products are kept separate from the cathode products. Of the chlorine produced in the U. S. in 1989, 94 percent was produced either by the diaphragm cell or mercury cell process. Therefore, these will be the only 2 processes discussed in this section. 8.11.2.1 Diaphragm Cell - Figure 8.11-1 shows a simplified block diagram of the diaphragm cell process. Water (H2O) and sodium chloride (NaCl) are combined to create the starting brine solution. The brine undergoes precipitation and filtration to remove impurities. Heat is applied and more salt is added. Then the nearly saturated, purified brine is heated again before direct electric current is applied. The anode is separated from the cathode by a permeable asbestos-based diaphragm to prevent the caustic soda from reacting with the chlorine. The chlorine produced at the anode is removed, and the saturated brine flows through the diaphragm to the cathode chamber. The chlorine is then purified by liquefaction and evaporation to yield a pure liquified product. The caustic brine produced at the cathode is separated from salt and concentrated in an elaborate evaporative process to produce commercial caustic soda. The salt is recycled to saturate the 7/93 (Reformatted 1/95) Inorganic Chemical Industry 8.11-1PDF Image | chlor-alkali electrolysis process
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