Sodium chloride electrolysis of solutions

Manufacture. Most chlorate is manufactured by the electrolysis of sodium chloride solution in electrochemical cells without diaphragms. Potassium chloride can be electroly2ed for the direct production of potassium chlorate (35,36), but because sodium chlorate is so much more soluble (see Fig. 2), the production of the sodium salt is generally preferred. Potassium chlorate may be obtained from the sodium chlorate by a metathesis reaction with potassium chloride (37). 

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... 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Sodium hydroxide is manufactured together with chlorine by electrolysis of sodium chloride solution. Various types of electrolytic cells are used commercially. They include the mercury cell, the diaphragm cell, and the membrane cell. 

Chlorate. Sodium chlorate, chlorate of soda, [CAS 7775-09-9], NaC103. white solid, soluble, mp 260 C, powerful oxidizing agent and consequently a fire hazard with dry organic materials, such as clothes, and with sulfur upon heating oxygen is liberated and the residue is sodium chloride formed by electrolysis of sodium chloride solution under proper conditions. Used (1) as a weedkiller (above hazard), (2) in matches, and explosives, (3) in the textile and leather indusUies. 

Hypochlorite. Sodium hypochlorite, [CAS 7681-52-9], NaOCl, commonly in solution by (1) electrolysis of sodium chloride solution under proper conditions. (2) reaction of calcium hypochlorite suspension in water and sodium carbonate solution, and then filtering. Used (1) as a bleaching agent fa textiles and paper pulp. (2) as a disinfectant, especially fa water. (3) as an oxidizing reagent. 

Another somewhat more complicated cell for the production of chlorine and sodium hydroxide by the electrolysis of sodium chloride solution is the Castner-Kellner cell, which employs a liquid mercury cathode. 

Sodium hypochlorite. If, in the electrolysis of sodium chloride solution, provision is made for the intimate mixing of the chlorine and sodium hydroxide at low temperatures, the following reaction occurs ... 

Sodium carbonate. When sodium hydroxide is produced by the electrolysis of sodium chloride solution saturated with carbon dioxide, sodium carbonate is produced as follows ... 

Another widely used commercial method for the production of sodium hydroxide lies in the electrolysis of sodium chloride solution — a method which is economical when the chlorine simultaneously produced can also be utilized. 

A clear picture of the course of electrolysis of sodium chloride solution with a concentration of 5.1 moles of NaCl per litre at 12 °C and a current density of 6.7 A/sq. em can be seen in the graphical representation in Fig. 125. 

Sodium hypochlorite is also manufactured by the electrolysis of sodium-chloride solution without a diaphragm (p. 97), the solution being less concentrated than that prepared by the chlorine process from sodium hydroxide, but free from the excess of alkali characteristic of that prepared by the older method.2 The process is carried out either in the apparatus designed by Kellner,3 or in that of Haas-Oettel,4 sodium chlorate being a by-product (v. infra). It is noteworthy that electrolysis of sodium-chloride solution with an alternating current also produces sodium hypochlorite.5... 

Electrolytic Process.—The production of sodium carbonate by the electrolysis of sodium-chloride solution is gradually supplanting the older methods. In the.operation a diaphragm is employed, and the details are described on pp. 97 and 98. The solution of sodium hydroxide formed is converted into carbonate by the action of carbon dioxide, the sodium hydrogen carbonate formed being decomposed by heat. 

Fig. 5.—E. Miiller a Curves showing the Electrolysis of Sodium Chloride Solutions.

Small scale consumers produce hypochlorite solutions directly by electrolysis of sodium chloride solutions in a diaphragmless cell... 

Electrolytic cells are used in industry for metal plating, and for purifying metals. For instance, pure sodium can be collected through electrolysis of sodium chloride solution in a Downs cell. The half reactions are as follows ... 

Industrial applications of perfluorinated ionomer membranes such as the electrolysis of sodium chloride solution to produce chlorine and sodium hydroxide often involve the use of highly concentrated solutions at elevated temperatures. The optimization of these systems depends upon a sound characterization of membrane transport processes under such conditions. Sodium ion is the major current-carrying species through the membrane in a chlor-alkali cell, and... 

Using the treated membrane electrolysis of sodium chloride solution was carried out under the same electrolysis conditions as 1). The treated surface of the membrane was faced to the cathode side in the electrolyzer. When 6.5 N sodium hydroxide solution was obtained as catholyte, the current efficiency was 93% and the cell voltage was 3,85v. On the other hand, the ion exchange membrane not treated by phosphorous pentachloride and triethylamine showed the current efficiency of 52% and the cell voltage of 3.68v when 6.5 N sodium hydroxide was obtained as catholyte. 

In electrolysis of sodium chloride solution under the same conditions as mentioned before, 8,0 N sodium hydroxide was obtained as catholyte at the current efficiency of 95 % and the cell voltage of 4.1 V. 

The NEOSEPTA-F membrane properties examined are mainly of those relating to the electrolysis of sodium chloride solution. Table III shows characteristics of typical grades of NEOSEPTA-F, These membranes are chemically stable, i.e., against acid, base, oxidants and reductants because the membranes have perfluorocarbon backbone. And also the membranes have strong mechanical strength because of reinforcement with the fabric of polytetrafluoroethy-lene. Used in the electrolysis of sodium chloride solution, no deterioration of performance or mechanical strength was observed in continuous service for 2 years under appropriate electrolysis conditions. NEOSEPTA-F membranes are always improved to get better performance in the electrolysis and various grades which show better performance are developed. 

Performance of NEOSEPTA-F in Sodium Chloride Solution Electrolysis. Figure 5 shows the relationship of the cell voltage and the current efficiency respectively with the concentration of sodium hydroxide in catholyte when electrolysis of sodium chloride solution was carried out at the current density of 30 A/cm. From the economical viewpoint, i.e, the electrolysis power cost, depreciation of equipment cost, membrane cost and so on, the optimum concentration of sodium hydroxide for NEOSEPTA-F C-1000 is about 20 % and that for NEOSEPTA-F C-2000 is about 27 %. 

The Flemion membrane was applied for the use in the electrolysis of sodium chloride solution. In Fig. 20, electrolytic performance of the membranes having different ion exchange capacity (AR, meq/g) of 1.44 and 1.23 are shown against the concentration of caustic soda produced in the cathod chamber. 

The mining of mercury has declined in recent decades, as major international concern over the health threat of mercury s extensive pollution of the environment has mounted. Much American freshwater fish is contaminated. The U.S. Environmental Protection Agency estimates 3,000 uses of mercury. Mercury usage is down in the chloroalkali industry, in which mercury is the cathode material used in the electrolysis of sodium chloride solutions, which produce sodium hydroxide and chlorine. An abundance of... 

Permselectivity of counter-ions through the ion exchange membrane depends on the fixed ion concentration of the membrane (Chapter 2.3). Many attempts have been made to increase the fixed ion concentration of the membrane to increase the ion exchange capacity and to decrease the water content of the membrane, namely, to increase the fixed ion concentration without increasing the electrical resistance of the membrane. Figure 4.8 shows an example of the relationship between current efficiency to produce sodium hydroxide and the fixed ion concentration of the membrane for the electrolysis of sodium chloride solution.17 It is apparent that the current efficiency increases with increasing fixed ion concentration of the membrane. 

Figure 6.20 Principle of electrolysis of sodium chloride solution (chlor-alkali process).

If the electrolysis of sodium chloride solution is carried out with the solution being stirred, sodium chlorate(I) and hydrogen are obtained. The chlorine produced at the anode disproportionates when in contact with the alkali from the cathode ... 

Between 1914 and 1918, solvent and general chemical production had been geared to the needs of war. At war s end, the chemical surpluses included, in addition to phenol and cellulose acetate, the acetone and butanol from the Weizmann fermentation process and chlorine from poison gas production. The latter, made by electrolysis of sodium chloride solution, became the basis of the British chlorinated solvents industry. Here the outstanding firm was Albright Wilson, which began manufacture of carbon tetrachloride in 1925. Cleaning and degreasing processes led to production of trichloroethylene, for which demand increased dramatically between 1928 and 1936. The principal manufacturer was Weston Co., later taken over by ICl. 

The sodium hypochlorite solution is diluted with water in a mixing/holding tank. The diluted solution is then injected by a chemical pump in to the water supply pipe at a controlled rate. Adequate mixing and contact time must be provided. Sodium hypochlorite can be generated on site by using electrolysis of sodium chloride solution. Hydrogen gas is given off as a by-product and must be safely dispersed. 

The chlor-alkali industry is one of the largest electrochemical operations in the world, the main products being chlorine and sodium hydroxide generated simultaneously by the electrolysis of sodium chloride solutions. The chlor-alkali industry serves the commodity chemical business, chlorine and sodium hydroxide (also called caustic soda) being indispensable intermediates in the chemical industry [1-10]. 

As stated in Chapter 2, chlorine and caustic soda are produced by the electrolysis of sodium chloride solutions. The electrical enei required to operate the electrolytic cells constitutes a major portion of the operating cost in producing chlorine. The current efficiency of the electrolyzers can be as high as 98% or as low as 85%, depending on the technology used, the operation of the electrolyzers, and the purity of the brine used in the process. Schematics of the cell technologies are shown in Fig. 3.1. 

Chlorine is produced not only by the electrolysis of sodium chloride solutions but also from HCl, KCl, and other metal chlorides, by both chemical and electrochemical methods. The amount of chlorine from alternative processes is about 5.9% of the total world production. In the United States, it was about 4.0% of the total in 2002 [1]. Most of this chlorine was from the electrolysis of KCl in mercury or membrane cells (Table 15.1) and from HCl. Only small amounts are produced by the electrolysis of other metal chlorides. 

The chlorine that is produced during the manufacture of liquid sodium is a valuable byproduct that is used to manufacture chlorinated solvents, pesticides, and in other applications. Far more chlorine is needed than sodium. More economical processes involving the electrolysis of sodium chloride solutions in water are used to manufacture chlorine along with sodium hydroxide. 

Over 95% of the world s chloralkall production is achieved by the electrolysis of sodium chloride solution. The products are chlorine, caustic soda, and some hydrogen. The main types of electrolytic cells currently available are mercury, membrane, and diaphragm. The most commonly used in North America is the diaphragm cell (CIS, 1999). In Western Europe, the mercury cell is most commonly used, and in Japan the membrane cell is used because it requires much less energy. 

Certain processes are carried out through the electrolytic route, e g. production of caustic soda by electrolysis of sodium chloride solution, production of aluminium by electrolysis of aluminium fluoride. 

Sodium hydroxide (caustic soda), NaOH, is a white hygroscopic (water-attracting) solid, which dissolves readily in water. Its solutions have a smooth, soapy feeling, and are very corrosive to the skin (this is the meaning of caustic in the name caustic soda). Sodium hydroxide is made either by the electrolysis of sodium chloride solution or by the action of calcium hydroxide, Ca(OH)2, on sodium carbonate, NaXOa ... 

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