The production of chlorine and sodium hydroxide

1 Energetics of the electrolytic process Chlorine and sodium hydroxide are prepared by the following series of reactions  

The free energy of the reaction is positive and is provided in the form of electricity. 

The minimum voltage required for reaction to take place at a working temperature of 95 °C is determined as 2.23 V. However, this does not take into consideration the anodic and cathodic overvoltages nor the voltage required to overcome the internal resistance of the cell. A more realistic value is about 

2 Types of cell There are three main types of cell the diaphragm and membrane cells in which the anode and cathode compartments are separate, and the mercury cell. The mercury cell is being phased out because of the hazards involved with use of mercury. 

Anode OH ions are present in the water and some pass across the diaphragm from the cathode compartment. 

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Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

Kvaerner Chemetics have developed a novel, patented process [1] for the removal of multivalent anions from concentrated brine solutions. The prime market for this process is the removal of sodium sulphate from chlor-alkali and sodium chlorate brine systems. The sulphate ion in a brine solution can have a detrimental effect on ion-exchange membranes used in the production of chlorine and sodium hydroxide consequently tight limits are imposed on the concentration of sulphate ions in brine. As brine is continuously recycled from the electrolysers back to the saturation area, progressively more and more sulphate ions are dissolved and build up quickly in concentration to exceed the allowable process limits. A number of processes have been designed to remove sulphate ions from brine. Most of these methods are either high in capital or operating cost [2] or have large effluent flows. 

Hydrogen is also obtained as a by-product of brine electrolysis for the production of chlorine and sodium hydroxide. Small electrolytic plants (hydrogen generators) are often used for in situ applications when small quantities of hydrogen are required at remote locations. 

Another somewhat more complicated cell for the production of chlorine and sodium hydroxide by the electrolysis of sodium chloride solution is the Castner-Kellner cell, which employs a liquid mercury cathode. 

Electrolysis of aqueous sodium chloride (brine) is an important industrial process for the production of chlorine and sodium hydroxide. In fact, this process is second only to the production of aluminum as a consumer cf electricity in the United States. Sodium is not produced in this process under normal circumstances because H20 is more easily reduced than Na+, as the standard reduction potentials show ... 

The most important commercial application of perfluorinated ionomer membranes is currently in the chlor-alkali industry. These materials are used as permselective separators in brine electrolysis cells for the production of chlorine and sodium hydroxide. This... 

The electrolysis of aqueous sodium chloride (brine) is an important industrial process for the production of chlorine and sodium hydroxide. In fact, this process is the... 

For example, the production of chlorine and sodium hydroxide alone accounts for approximately 2% of the total consumption of electricity in the United States and, together with that for Al, much more. This apparent lack of acceptance of organic electrode processes as viable means for (certain types) of syntheses (see later) is perhaps all the more surprising when it is noted that electrolytic reactions are simply very direct oxidation and reduction processes which do not produce in the solution other conjugate reduced or oxidized species, respectively, such as would occur with homogeneous redox reagents. 

Electrolysis has been used for the production of chlorine and sodium hydroxide for over a hundred years. Even so, the technology has developed very rapidly during the last decade and the trends Indicate well the application of the principles of electrochemical engineering. Of course, the driving force for these developments Is the massive scale of the process, approximately 3.3 x 10 ton Cl2/year and the consequent energy consumption, some 10 kWh/year. In addition, concern over mercury pollution has led to a need to develop cells which do not use the amalgam system. 

Because there are few major natural sources of mercury and since most inorganic compounds of this element are relatively insoluble, it was assumed for some time that mercury was not a serious water pollutant. However, in 1970, alarming mercury levels were discovered in fish in Lake Saint Clair located between Michigan and Ontario, Canada. A subsequent survey by the U.S. Federal Water Quality Administration revealed a number of other waters contaminated with mercury. It was found that several chemical plants, particularly caustic chemical manufacturing operations for the production of chlorine and sodium hydroxide, were each releasing up to 14 or more kilograms of mercury in wastewaters each day. 

Electrolyzers for the production of chlorine and sodium hydroxide, including both diaphragm and membrane cells, are classified as either monopolar or bipolar. The designation does not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyzer construction or assembly. There are many more chlor-alkali production facilities with monopolar cells than with bipolar cells. 

An important sector of heavy inorganic chemical manufacturing is the production of chlorine and sodium hydroxide — the chlor-alkali industry. The manufacture of these chemicals has a long history. Today they are produced simultaneously by the electrolysis of sodium chloride solutions, but this was not always the case. The two chemicals were originally manufactured by different routes. In the 19th century chlorine was made by the oxidation of hydrogen chloride (itself made by reaction of salt with sulfuric acid) using the Deacon process. Sodium hydroxide was prepared by the reaction of calcium hydroxide with sodium carbonate — the lime-soda process. 

Since the production of chlorine and sodium hydroxide is inextricably linked, a surge in demand for one creates problems with sales of the other. For example, increased demand for chlorine for vinyl chloride production meant that there was an abundance of sodium hydroxide. As a result it replaced sodium carbonate in some of its applications. 

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