Caustic Soda The Chloralkali Industry

Annual production of caustic soda (sodium hydroxide) in the United States is about 10 million metric tons, of which 50% is consumed by the chemical industry and a further 20% by pulp and paper plants. Sodium hydroxide is made by electrolyzing strong brine, that is, decomposing it by passing an electric current. Saturated brine contains about 360 g or 6.2 mol NaCl per kilogram water. 

Passage of 1.0 mol of electrons (one faraday, 96,485 A s) will produce 1.0 mol of oxidation or reduction—in this case, 1.0 mol of Cl converted to 0.5 mol of CI2, and 1.0 mol of water reduced to 1.0 mol of OH plus 0.5 mol of H2. Thermodynamically, the electrical potential required to do this is given by the difference in standard electrode potentials (Chapter 15 and Appendix D) for the anode and cathode processes, but there is also an additional voltage or overpotential that originates in kinetic barriers within these multistep gas-evolving electrode processes. The overpotential can be minimized by catalyzing the electrode reactions in the case of chlorine evolution, this can be done by coating the anode with ruthenium dioxide. 

Since the products of the electrolysis of aqueous NaCl will react if they come in contact with each other, an essential feature of any chloralkali cell is separation of the anode reaction (where chloride ion is oxidized to chlorine) from the cathode reaction (in which OH and H2 are the end products). The principal types of chloralkali cells currently in use are the diaphragm (or membrane) cell and the mercury cell. 

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Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

Of the industrial activities under the heading of process engineering the chloralkali industry is probably the most important. Chlorine world capacity in 1985 was estimated as 43 million metric tons and total world production in 1983 as 31 million tons. Over 80% of the chlorine produced is used to produce other chemicals. Large-tonnage products associated with chlorine include caustic soda and soda ash (anhydrous sodium carbonate). 

The electrolytic cell shown in Figure 2 is the industrial chloralkali cell in which brine (an aqueous sodium chloride solution) is electrolytically converted to chlorine and caustic soda (sodium hydroxide, NaOH). The external power source supplies electric energy to drive the overall reaction. 

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