Electrolysis of sodium chloride

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

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). 

Chlora.tes. Sodium chlorate is produced by the electrolysis of sodium chloride at pH 6.5—7.5 in a one-compartment cell. DSA anodes and steel cathodes are generally used in chlorate cells. The electrolysis products, hypochlorous acid, and hypochlorite ions, react chemically to produce chlorate (eq. 

Later, Du Pont in America developed its own ionically conducting membrane, mainly for large-scale electrolysis of sodium chloride to manufacture chlorine, Nafion , (the US Navy also used it on board submarines to generate oxygen by electrolysis of water), while Dow Chemical, also in America, developed its own even more efficient version in the 1980s, while another version will be described below in connection with fuel cells. Meanwhile, Fenton et al. (1973) discovered the first of a... 

The development of electrical power made possible the electrochemical industry. Electrolysis of sodium chloride produces chlorine and either sodium hydroxide (from NaCl in solution) or metallic sodium (from NaCl fused). Sodium hydroxide has applications similar to sodium carbonate. The ad vantage of the electrolytic process is the production of chlorine which has many uses such as production of polyvinyl chloride. PVC, for plumbing, is produced in the largest quantity of any plastic. 

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... 

Electrolysis of sodium chloride in presence of nitrogenous compounds to produce chlorine may lead to formation of explosive nitrogen trichloride. Precautions are detailed. 

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). 

The large cell used for the electrolysis of sodium chloride in industry is known as a Downs cell. To decrease heating costs, calcium chloride is added to lower the melting point of sodium chloride from about 800°C to about 600°C. The reaction produces sodium and calcium by reduction at the cathode, and chlorine by oxidation at the anode. 

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. 

The indirect hydrocyanation of butadiene as practiced by du Pont (/) involved the electrolysis of sodium chloride, formation of sodium cyanide from HCN using the NaOH, chlorination of butadiene to give 1,4-dichloro-but-2-ene, chloride displacement with sodium cyanide, and subsequent hydrogenation, as indicated in Eqs. (l)-(5), with the net result of Eq. 6. 

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. 

Chlorine is the most abundant of the halogens, especially in sea water, a ton of which contains in grams chlorine 15,000, bromine 97, iodine 0-17 (ratio 106 6,000 1). Its preparation depends on the discharge of its ion, either directly (i.e., electrolytically) or by oxidation. The older methods of oxidation (by manganese dioxide or by air in presence of certain catalysts) have now been replaced for technical purposes by the electrolysis of sodium chloride, which is primarily for the production of caustic soda, the chlorine being a by-product the chloroparaffins which are now so much used as solvents were developed to utilize this chlorine. 

Practically all of the sodium produced in the United States involves the use of the Downs electrolytic cell, which consists of a massive graphite anode surrounded by two or more iron cathodes. The electrolyte is an aqueous solution of sodium choloride. The NaCl salt is continuously added. As electric current is passed between the electrodes, chlorine gas is collected in a hood over the graphite anode and piped off to further processing for marketing. The electrolysis of sodium chloride proceeds as follows ... 

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. 

This is one of the largest electrochemical industries in the world. It consists in the electrolysis of sodium chloride as brine to give chlorine and caustic soda. Chlorine is used in the preparation of vinyl chloride for PVC, as a bleaching agent for paper and paper pulp, as a disinfectant, besides other chloration applications. Caustic soda is important in mineral processing, and in the paper, textile, and glass industries. Table 15.2 shows recent data for industrial consumption of chlorine and caustic soda in the USA. 

For example, during electrolysis of sodium chloride (with unit activity of sodium ions) the reversible deposition potential of sodium 7Tn,i+/niv — —2.714 V during deposition on circulating mercury, in which the concentration of sodium... 

Therefore, the main processes occurring during the electrolysis of sodium chloride in an electrolyzer with a mercury cathode are represented by the following equations ... 

If we combine equations (XIII-11) and (XIII-1) we obtain a formula which characterizes the whole process of electrolysis of sodium chloride from the moment when the hypochlorite concentration attains a constant value and the whole current is consumed in forming chlorate and oxygen ... 

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. 

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