Electrolytic Production of Sodium

Electrolysis of Fused Sodium Hydroxide. The first successful electrolytic production of sodium was achieved with the Castner cell (2) ... 

FIGURE 14.17 A diaphragm cell tor the electrolytic production of sodium hydroxide from brine (aqueous sodium chloride solution), represented by the blue color. The diaphragm (gold color) prevents the chlorine produced at the titanium anodes from mixing with the hydrogen and the sodium hydroxide formed at the steel cathodes. The liquid (cell liquor) is drawn off and the water is partly evaporated. The unconverted sodium chloride crystallizes, leaving the sodium hydroxide dissolved in the cell liquor. 

Only a small number of compounds are produced directly by electrolysis. To illustrate this type of process, the electrolytic production of sodium hydroxide is described in detail. Then it is shown how this process may be modified to permit the formation of two other valuable commercial chemicals. 

There have been severe criticisms about the extended use of chlorine gas in industry, owing to concern primarily derived from its ability to form toxic chlorinated organic compounds. In order to avoid its co-production during the electrolytic production of sodium hydroxide, a process has been developed in which a sodium carbonate (soda ash) solution is used as the anolyte in an electrochemical reactor divided by an ion-exchange membrane. Hydrogen gas is produced at the cathode and sent to a gas diffusion anode. Assuming no by-products in the liquid phase and only one by-product in the gas phase ... 

However, in many situations, water is hardly the ideal solvent. Take the electrolytic production of sodium metal, for exanple. If an aqueous solution of a sodium salt is taken in an electrolytic cell and a current is passed between two electrodes, then all that will happen at the cathode is the liberation of hydrogen gas there will be no electrodeposition of sodium (see Chapter 7). Hence, sodium cannot be electrowon from aqueous solutions. This is why the electrolytic extraction of sodium has taken place from molten sodium hydroxide, i.e., from a medium free of hydrogen. This ... 

In the electrolytic production of sodium at the cathode of an electrolytic cell we may say that the cathode, with its excess of electrons, is the reducing agent which reduces sodium ion to metallic sodium. Similarly we may say that the anode with its deficiency of electrons is the oxidizing agent which oxidizes chloride ion to free chlorine. 

Electrolytic production of sodium hydroxide and chlorine from sodium chloride solutions so heavily dominates the supply of these chemicals now that the production of chlorine can be quite closely approximated by multiplying the sodium hydroxide figures by the theoretical ratio of chlorine to sodium hydroxide (70.906 79.996) of 0.886 1.000. For the U.S.A., for example, the actual production ratio of chlorine to sodium hydroxide is 0.953 to 1.000, quite close to the theoretical ratio. 

J.E. Colman. Electrolytic Production of Sodium Chlorate. In R. Alkire and T. Beck (eds.). Tutorial Lectures in Electrochemical Engineering cmd Technology, AlChE Symposium Series 204, vol. 77, American Institute of Chemical Engineers, New York (1981), p. 244. 

In 1888, the Castner process was industrialized for the production of sodium by electrolysis in molten NaOH at 330 °C [1]. The Downs cell for the electrolytic production of sodium was patented in 1922 [2], and soon after, it replaced the Castner process for Na production, and a modified Downs cell is still being used industrially. The original Downs cell is shown in Fig. 1. A modified version of the Downs cell is shown in Fig. 2. The modified Downs cell is equipped with four anodes and four cathodes. The steel gauze diaphragm prevents direct contact between the products chlorine bubbles evolved on graphite anodes and liquid sodium deposited on steel cathodes. 

Castner-Kellner cell An electrolytic cell for the production of sodium hydroxide. ... 

Sir Humphry Davy first isolated metallic sodium ia 1807 by the electrolytic decomposition of sodium hydroxide. Later, the metal was produced experimentally by thermal reduction of the hydroxide with iron. In 1855, commercial production was started usiag the DeviUe process, ia which sodium carbonate was reduced with carbon at 1100°C. In 1886 a process for the thermal reduction of sodium hydroxide with carbon was developed. Later sodium was made on a commercial scale by the electrolysis of sodium hydroxide (1,2). The process for the electrolytic decomposition of fused sodium chloride, patented ia 1924 (2,3), has been the preferred process siace iastallation of the first electrolysis cells at Niagara Falls ia 1925. Sodium chloride decomposition is widely used throughout the world (see Sodium compounds). 

Chemical Production. Electrolytic production of chemicals is conducted either by solution (water) electrolysis or fused-salt electrolysis. Fluorine, chlorine, chlorate, and manganese dioxide are Hberated from water solutions magnesium and sodium are generated from molten salt solutions. 

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

We have already described the refining of copper and the electrolytic extraction of aluminum, magnesium, and fluorine. Another important industrial application of electrolysis is the production of sodium metal by the Downs process, the electrolysis of molten rock salt (Fig. 12.15) ... 

This type of electrolytic cell consists of anodes and cathodes that are separated by a water impermeable ion-conducting membrane. Brine is fed through the anode where chlorine gas is generated and sodium hydroxide solution collects at the cathode. Chloride ions are prevented from migrating from the anode compartment to the cathode compartment by the membrane and this, consequently, leads to the production of sodium hydroxide, free of contaminants like salts. The condition of the membrane during operation requires more care. They must remain stable while being exposed to chlorine and strong caustic solution on either side they must allow, also, the transport of sodium ions and not chloride ions. 

Figure 11.8 shows the Kvaemer Chemetics electrolytic process for the production of sodium chlorate (in a concentrated solution of strong liquor ) and hydrogen from sodium chloride solution ( brine ). The overall reaction is NaCl + 3H20 - NaC103 + H,(g). The part of the system shown consists primarily of brine electrolyzers, a degasi-fier, a chlorate reactor, and an electrolyte cooler. 

Figure 11.8 Kvaemer Chemetics electrolytic system for production of sodium chlorate flow through the system is by natural convection (Courtesy of Kvaemer Chemetics Inc.)...

Preparation. Oxidation of the chromite ore by air in molten alkali gives sodium chromate, Na2Cr04 that is then converted to Cr203. The oxide is further reduced with aluminium or silicon to form chromium metal. Solutions suitable for electrolytic production of chromium (for plating) can be obtained from ore by oxidative roasting in alkali or by dissolution of chromite in H2S04 and especially by dissolving ferro-chromium in sulphuric acid. 

The electrolysis of molten sodium chloride is an important industrial reaction. Figure 11.15 shows the large electrolytic cell used in the industrial production of sodium and chlorine. You will meet other industrial electrolytic processes later in this chapter. 

Potassium chloride (KCl) is used in drug preparations and as a food additive and chemical reagent. It is possible to reduce the sodium in your diet by substituting potassium chloride for table salt (sodium chloride), which may be healthier. Molten potassium chloride is also used in the electrolytic production of metaUic potassium. KCl is also found in seawater brine and can be extracted from the mineral carnalhte. 

Other Sources of Fluorine. M. H. Klaproth discovered that cryolite, the mineral which later came to be used as a flux in the industrial electrolytic production of aluminum, is a fluoride of sodium and aluminum (76). In 1878 S. L. Penfield, in a research consisting of eight analyses of amblygonite, proved that, contrary to the views of Carl Friedrich Rammelsberg, fluorine and hydroxyl can replace each other in the same mineral (155). Traces of fluorine are found in all types of natural water in oceans, lakes, rivers, and springs (156). 

To summarize the production of sodium hypochlorite is favoured by (i) Neutral cone. soln. of sodium chloride (ii) Low temp. (iii) High anodic current density (iv) The presence of potassium bichromate and (v) An adequate circulation of the electrolyte. G. E. Cullen and R. S. Hubbard have studied the best... 

Electrolysis ol the fused alkali hydroxide.—At the time of Davy s discovery the dynamo had not been invented, and the electric current derived from batteries was far too costly for the production of sodium on a manufacturing scale. In modern works, where cheap electrical energy is available, modifications of Davy s original process—electrolysis of fused sodium hydroxide—are used for preparing sodium industrially —e.g. H. Y. Castner s electrolytic process (1890).a Potassium can also be made by H. Y. Castner s process. 

M.Babor, BullSectSciAcadRoumaine 19, 213-18(1937-38) CA 34, 5661(1940) 9)A.L.Pitman et al, ChemMerEngrg 45, 692-96 (1938)(Production of sodium chlorate by electrolytic method) 10)Davis(l943), 357-66 10a)J.Phillips, PATR 1277(1943)(Study of NaC103 explosive developed by Kiernan ... 

Sodium compounds are important largely because they are inexpensive and soluble in water. Sodium chloride is readily mined as rock salt, which is a deposit of sodium chloride left as ancient oceans evaporated and it is also obtained from the evaporation of brine from present-day seas and salt lakes (Fig. 14.19). Sodium chloride is used in large quantities in the electrolytic production of chlorine and sodium hydroxide from brine. 

FIGURE 18.17 A membrane cell for electrolytic production of CI2 and NaOH. Chloride ion is oxidized to CI2 gas at the anode, and water is converted to H2 gas and OH-ions at the cathode. Sodium ions move from the anode compartment to the cathode compartment through a cation-permeable membrane. Reactants (brine and water) enter the cell, and products (CI2 gas, H2 gas, aqueous NaOH, and depleted brine) leave through appropriately placed pipes. 

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