Downs process sodium produced

Fig. 10.1 A schematic representation of the electrolysis cell used in the Downs process to produce sodium commercially from NaCl. The products (Na and CI2) must be kept separate from each other to prevent recombination to form NaCl.

K.18 The industrial production of sodium metal and chlorine gas makes use of the Downs process, in which molten sodium chloride is electrolyzed (Chapter 12). Write a balanced equation for the production of the two elements from molten sodium chloride. Which element is produced by oxidation and which by reduction ... 

Sodium metal is produced from the electrolysis of molten sodium chloride in the Downs process (Section 12.13). Determine (a) the standard Gibbs free energy of the reaction... 

In a thermite process to produce chromium metal, the mechanically-mixed ingredients were ignited in a large crucible and the reaction proceeded smoothly. When the mixer broke down, manual mixing was used but gave poorer dispersion of the constituents. An explosion after ignition is attributed to a high local concentration of sodium chlorate and aluminium powder in the mixture. 

In industry, chemical reduction is preferred over electrolytic processes for potassium production. Application of the Down s electrolytic sodium process to produce potassium has not been successful. Potassium—sodium alloy is easily prepared by the reaction of sodium with molten KC1, KOH, or solid KjCC powder (see Sodiumand SODIUMALLOYS). 

Sodium chloride is plentiful as rock salt, but the solid does not conduct electricity, because the ions are locked into place. Sodium chloride must be molten for electrolysis to occur. The electrodes in the cell are made of inert materials like carbon, and the cell is designed to keep the sodium and chlorine produced by the electrolysis out of contact with each other and away from air. In a modification of the Downs process, the electrolyte is an aqueous solution of sodium chloride. The products of this chloralkali process are chlorine and aqueous sodium hydroxide. 

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 ... 

In the Downs process, molten sodium chloride is electrolyzed to produce sodium. A valuable byproduct is chlorine. Write equations representing the processes taking place at the anode and at the cathode in the Downs process. 

In the renal buffering process, sodium (Na+) is exchanged for hydrogen ions (H+) and binds with some of the bicarbonate (NaHCOj), which later breaks down again as Na" is actively removed through a Na - K+ mechanism (discussed in more detail in Chapter 5). The H" ions are bound with carbonic anhydrase on the border of the proximal tubules of the kidneys, which convert the H first to H COj and then to H O and CO. Some H+ ions also bind with the ammonia (NHj) produced in the kidneys as a result of amino acid catabolism and an abundant anion found in the glomerular filtrate, chloride (CT), to form ammonium chloride (NH Cl), a weak acid that is excreted in the urine. Thus it is clear that other electrolytes are involved in the acid-base balancing process and can be affected by acid-base imbalances. These impacts will be discussed with each electrolyte. 6... 

Sodium occurs widely as NaCl in seawater and as deposits of halite in dried-up lakes etc. (2.6% of the Earth s crust). The element is obtained commercially via the Downs process by electrolysis of NaCl melts in which the melting point is reduced by the addition of calcium chloride sodium is produced at the steel cathode. The metal is extremely reactive, vigorously so with the halogens and also with water, in the latter case to give hydrogen and sodium hydroxide. It is used as a coolant in fast-breeder nuclear reactors. The chemistry of sodium is very similar to that of the other members of group 1. 

The first, and now obsolete, industrial processes for producing raw sodium metal were based on the carbon reduction of sodium carbonate or sodium hydroxide. The first industrial production of pure sodium metal was performed by molten-salt electrolysis of the pure sodium hydroxide, NaOH, in so-caUed Castner cells. Most modern processes for the production of sodium now involve molten-salt electrolysis of highly pure sodium chloride. Actually, since 1921, when the process was invented by J.C. Downs, the electrolysis has been performed in Downs electrolytic cells at the DuPont de Nemours Canadian facilities at Niagara Falls, Ontario, Canada. The electrolytic cell consists of four cylindrical anodes made of graphite surrounded at the bottom of the cell by steel cathodes, and a fine steel mesh acts as a separator between anodic and cathodic compartments. Each cell contains a batch of 8 tonnes of a molten-salt mixture with the following chemical composition NaCl (28 wt.%), CaCl (26 wL%), and BaClj (46 wt.%). 

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). 

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. 

Benzene SuIfona.tion. In the benzene sulfonation process, benzene reacts with concentrated sulfuric acid to form benzenesulfonic acid at about 150°C. The benzenesulfonic acid is neutralized with sodium sulfate to produce sodium benzenesulfonate, which is then fused with caustic soda to yield sodium phenate. The sodium phenate is acidified with sulfur dioxide and a small amount of sulfuric acid to release the phenol from the sodium salt. The phenol yield by this process can be as high as 88 mol % to that of the theoretical value based on benzene. Plants employing this technology have been shut down for environmental and economic reasons. 

Lithium. Several processes for lithium [7439-93-2], Li, metal production have been developed. The Downs cell with LiCl—KCl electrolyte produces lithium ia much the same manner as sodium is produced. Lithium metal or lithium—aluminum alloy can be produced from a mixture of fused chloride salts (108). Granular Li metal has been produced electrochemically from lithium salts ia organic solvents (109) (see LiTHlUM AND LITHIUM compounds). 

You have already seen that chlorine gas can be made by the electrolysis of molten sodium chloride. In industry, some chlorine is produced in this way using the Downs cell described earlier. However, more chlorine is produced in Canada using a different method, called the chlor-alkali process. In this process, brine is electrolyzed in a cell like the one shown in Figure 11.32. Brine is a saturated solution of sodium chloride. 

Sodium is produced by an electrolytic process, similar to the other alkali earth metals. (See figure 4.1). The difference is the electrolyte, which is molten sodium chloride (NaCl, common table salt). A high temperature is required to melt the salt, allowing the sodium cations to collect at the cathode as liquid metallic sodium, while the chlorine anions are liberated as chlorine gas at the anode 2NaCl (salt) + electrolysis —> Cl T (gas) + 2Na (sodium metal). The commercial electrolytic process is referred to as a Downs cell, and at temperatures over 800°C, the liquid sodium metal is drained off as it is produced at the cathode. After chlorine, sodium is the most abundant element found in solution in seawater. 

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