Sodium electrolytic production

Sodium Hydroxide. Before World War 1, nearly all sodium hydroxide [1310-93-2], NaOH, was produced by the reaction of soda ash and lime. The subsequent rapid development of electrolytic production processes, resulting from growing demand for chlorine, effectively shut down the old lime—soda plants except in Eastern Europe, the USSR, India, and China. Recent changes in chlorine consumption have reduced demand, putting pressure on the price and availabiHty of caustic soda (NaOH). Because this trend is expected to continue, there is renewed interest in the lime—soda production process. EMC operates a 50,000 t/yr caustic soda plant that uses this technology at Green River it came onstream in mid-1990. Other U.S. soda ash producers have aimounced plans to constmct similar plants (1,5). 

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

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. 

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. 

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. 

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). 

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. 

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. 

C. i1. Burgess1 and C. Hambuechen, in 1903, investigated the various conditions requisite for the electrolytic production of a good white lead. They found that a two-compart-ment cell is necessary to obtain a pure product. When lead anodes and sodium nitrate solution are employed a certain quantity of basic lead salt is produced, and there is not therefore a 100 per cent, formation of pure lead nitrate. The reduction of sodium nitrate at copper cathodes cannot be prevented so that a certain amount of ammonia is formed, and the solution being alkaline after a time, plumbates are formed and a layer of spongy lead is deposited on the cathode. If, therefore, the cathode compartment be not separated from the anode, the loosely-deposited cathodic lead will fall into the white lead which is collecting at the bottom of the cell. 

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 ... 

Capodaglio, E., G.Pezzagno, G.C.Bobbio, and F.Cazzoli. 1969. Respiratory function test in workers employed in electrolytic production of chlorine and sodium, [in Italian], Med. Lav. 60 (3) 192—201. 

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. 

Most metals can be electrolytically deposited from water-free melts of the corresponding metal salts. It is well known that aluminum, lithium, sodium, magnesium, and potassium are mass produced by electrolytic deposition from melts. Industrial processes for the melt-electrolytic production of beryllium, rare earth metals, titanium, zirconium, and thorium are also already in use. Pertinent publications [74, 137, 163] describe the electrolytic deposition of chromium, silicon, and titanium from melts. Cyanidic melts are used for the deposition of thick layers of platinum group metals. It is with this technique that, for instance, adhesion of platinum layers on titanium materials is obtained. Reports concerning the deposition of electrolytic aluminum layers [17, 71-73, 94, 96, 102, 164, 179] and aluminum refinement from fused salts [161] have been published. For these processes, fused salt... 

Sodium chloride Sodium chloride is, as a starting material for the electrolytic production of chlorine and sodium hydroxide, available in unlimited quantities. It is either extracted from natural deposits (up to 70%) or from seawater. In the USA, the economically workable deposits of sodium chloride are estimated to be greater than 55 10 t and in the Federal Republic of Germany there is estimated to be 100 10 km of deposits. Extraction is either carried out by mining or leaching (i.e. dissolution of... 

Mercury can be used for the extraction of gold. In hospitals and homes, it is still used in thermometers and blood-pressure cuffs, can be found in batteries, switches, and fluorescent light bulbs. Large amounts of metallic mercury are employed as electrodes in the electrolytic production of chlorine and sodium hydroxide from saline. Today, exposure of the general population comes from three major sources fish consumption, dental amalgams, and vaccines. 

Consider the sources of some of the common chemical raw materials and relate these to products that are accessible via one or two chemical transformations in a typical chemical complex. Starting with just a few simple components—air, water, salt (NaCl), and ethane—together with an external source of energy, quite a range of finished products is possible (Fig. 1.1). While it is unlikely that all of these will be produced at any one location, many will be, and all are based on commercially feasible processes [1]. Thus, a company which focuses on the electrolytic production of chlorine and sodium hydroxide from salt will often be sited on or near natural salt beds in order to provide a secure source of this raw material. A large source of freshwater, such as a river or a lake will generally be used for feedstock and cooling water... 

Three main methods are used to keep these electrolytic products apart. One involves the separation of the electrochemical cell into two compartments by a porous vertical diaphragm, which permits the passage of brine and ions, but keeps the products separated. Another employs a flowing mercury cathode to continuously carry sodium, in the form of an amalgam, away from the brine and... 

FIGURE 8.4 Mercury cell process for the electrolytic production of chlorine and sodium hydroxide. Packing in the vertical decomposer is graphite lumps. For treatment of cell products, see Fig. 8.2. 

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. 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

Electrocatalysis can modify the composition of the electrode surfaces and the nature of the electrolytic products. The perchlorate decomposition (cathodic production of chloride) on platinum catalysts is one of the examples [57] and the IrCT decomposition during the sodium chlorate production [58]. The electropolymerization of the organic substances is critically dependent on the type of the electronic/ionic conductors, electrolyte characteristics, and the electrolysis resident time of the monomer [59]. 

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