Sodium from electrolysis

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. 

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. 

Magnesium is reduced from a mixture of magnesium, calcium, and sodium chlorides. Electrolysis from aqueous solution is also possible zinc, copper, and manganese dissolved as sulfates in water can be reduced electrolytically from aqueous solution. 

Some metals are extracted in electrolytic cells. In section 11.3, you saw the extraction of sodium from molten sodium chloride in a Downs cell. Other reactive metals, including lithium, beryllium, magnesium, calcium, and radium, are also extracted industrially by the electrolysis of their molten chlorides. 

Titanium metal also can be produced by electrolytic methods. In electrolysis, fused mixtures of titanium tetrachloride or lower chlorides with alkaline earth metal chlorides are electrolyzed to produce metal. Also, pure titanium can be prepared from electrolysis of titanium dioxide in a fused bath of calcium-, magnesium- or alkali metal fluorides. Other alkali or alkaline metal salts can be substituted for halides in these fused baths. Other titanium com-pouds that have been employed successfully in electrolytic titanium production include sodium fluotitanate and potassium fluotitanate. 

G. von Hevesy obtained greater yields of potassium by the electrolysis of its hydroxide than of sodium with its hydroxide at the same temp. For example, between 320° and 340°, a 27 per cent, yield of sodium, and a 55 per cent, yield of potassium, were obtained. This was traced to the increased rate of diffusion of sodium from cathode to anode where the metal is lost by secondary reactions and this the more, the higher the temp. F. C. Wickel and W. Lobel obtained potassium from its hydroxide and metallic sodium, by melting the two together in the absence of air sodium oxide is formed and potassium distils from the mixture hydrogen gas also escapes. 

Reactive metals are usually difficult to extract. The preferred method is by electrolysis of the molten ore (electrolytic reduction) for example, sodium from molten sodium chloride. 

The only reasonably successful advance in this sense is the Alcoa process, based on the electrolysis of aluminium trichloride in a 2-15 per cent concentration at 700°C in a 3 2 mixture of molten sodium chloride and potassium chloride using carbon electrodes. Aluminium oxide is previously converted into aluminium chloride using chlorine from electrolysis. The reactions are thus... 

The density of the melt is 1.89-1.94 gem-3. During electrolysis, along with sodium metal, small quantities of calcium ( 4 wt%) are deposited at the cathode. Calcium has a melting point (804°C) far higher than sodium and a low solubility in sodium (see Table 13) [302], Because of that at the cathode a solid alloy phase Na-Ca is accumulated and this blocks the circulation of the electrolyte in the electrolysis cell and the removal of sodium from the cell. Also this solid alloy sometimes causes short-circuiting in the electrolysis cell. Sodium obtained by electrolysis is cooled to 110-120°C and filtered for the removal of calcium. The effect of temperature on the solubility of calcium in sodium [302] is shown in Table 13. At 110°C the calcium content in sodium is reduced to <0.04%. 

Amalgam process Cooled aqueous solutions of sodium hydrogen sulfite are reduced with sodium amalgam (from sodium chloride electrolysis). The sodium dithionite formed is recovered by crystallization. 

Use Production of sodium by electrolysis, heat transfer agents, reaction medium in chemical synthesis, heat-treatment of metals (from 350 to 2400F), solvents for the metals corresponding to their cations, nuclear power reactors. 

Occasionally, the market will swing the other way leaving chlorine in short supply. Fused sodium chloride is commercially electrolyzed in Down s cells to give chlorine and metallic sodium [11]. Sodium production in the U.S.A. has averaged 135,000-150,000 metric tonnes annually since 1968, which represents about a 2-3% contribution to the chlorine supply from this source. In the U.K., it is estimated that as much as 10% of the available chlorine arises from Down s cell technology. Potassium chloride solutions are also electrolyzed for commercial potassium hydroxide, but the contribution to the chlorine supply from this source is even less than from fused sodium chloride electrolysis. 

How is electrolysis of a sodium chloride solution different from electrolysis of molten sodium chloride In molten NaCl, the only ions present are Na+ and CU. What ions are present in an aqueous solution of rock salt Recall that water dissociates slightly to form H+ and OH ions. 

The use of electrochemistry to convert propylene to the oxide was researched in particular by Bayer and Kellogg. In this method, propylene is injected in the neighborhood of the anode of a sodium chloride electrolysis cell with a mercury athode (see Section 11.2.5.ZB). The hypochlorous add formed with the chlorine liberated at the anode is added to the propylene. The chlorohydrin obtained is hydrolysed at the cathode by the caustic produced by the action of water on the amalgam. Propylene oxide is separated from the mixture by stripping, while the. sodium chloride is returned to the electrolyser. The overall reaction is as follows ... 

EXAMPLE 8 Calculate the mass of sodium produced from electrolysis of 10.0 g of sodium chloride dissolved in water by passage of 1.00 A for 40.0 minutes. 

This can be exemplified by the solution of complex-forming equilibria in wastewaters from sodium chloride electrolysis which contains a number of chloro complexes of mercury (HgCl , HgCl2, HgCl, HgCl ). In Table 3.12 partial and total stability constants are presented. A non-... 

Nearly all applications of caustic soda require separation of the chloride from the hydroxide. In sodium-brine electrolysis, fortunately, the phase equilibrium allows a rather effective separation by evaporation of the liquor. If water is removed until the concentration of NaOH approaches 50%, nearly all of the NaCl falls out of solution. After cooling, the residual concentration is about 1.0-1.1%. This removal of salt causes the concentration of NaOH to increase. The solution produced by evaporation therefore can contain somewhat less than 50% NaOH. This is discussed in some detail in Section 9.S.3.3. Dissolved salt is not acceptable in some uses of NaOH, and so there has always been a split market. Part has been reserved to a purified version of the diaphragm-cell product and to mercury-cell, and now membrane-cell, NaOH. 

With a porous diaphragm, it is difficult to achieve efficient separation of the acidic anolyte from the alkaline catholyte. The result is a significant loss of current efficiency. Also, the choice of a diaphragm resistant to both sulfuric acid and caustic soda and the selection of an anode stable in sulfuric acid solution are limited. These difficulties have retarded the development of sodium sulfate electrolysis. 

Another chemical coulometer depends on the production of a gas. Classically, this was the combined H2 and O2 from electrolysis of water containing an indifferent electrolyte such as sodium sulfate. This is fairly sensitive, but involves the inconvenient measurement of a gas volume, which must be corrected to standard conditions of temperature and pressure. The apparatus for collecting and measuring the mixed gases was called a "voltameter," but T. W. Richards (16), noting the possible confusion with "voltmeter," coined the name "coulometer."... 

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