Environmentalists have targeted chlorine and its downstream products for many years based on concerns about pollution related to mercury-based chlorine production, the persistence of many chlorinated products in the environment, and the safe disposal of chlorinated wastes. A counterview is presented by chlorine industry advocates who point out that chlorine is a vital building block in a sustainable world. Currently, a staggering 55-60 percent of all industrial chemicals rely on chlorine in some way in their production. About 35 percent of chlorine is obtained by recycling. Chlorine advocates further point out that chlor-alkali production, with its primary inputs of salt and electricity, has a number of environmental benefits. Salt is an almost inexhaustible resource since it is so abundantly available from the sea. Electrochemical use of electricity is intrinsically efficient, with efficiencies presently at about 96 percent. Power consumption and manufacturing emissions have been reduced substantially. Nonetheless, considerable discussion and negotiation continues regarding the gradual phase-out of mercury cell use in Europe and the disposition of the mercury no longer needed in chlor-alkali manufacture. Three commercial processes are used in the industry for producing chlorine, caustic soda, and hydrogen: the mercury cell process, the diaphragm cell process, and the membrane cell process. Advantages and disadvantages of the three chlor-alkali processes are shown in the table below.
| Process | Advantages | Disadvantages |
|---|
| Diaphragm process | Use of well brine, low electrical energy consumption | Use of asbestos, high steam consumption for caustic concentration in expensive multistage evaporators, low purity caustic, low chlorine quality, cell sensitivity to pressure variations | | Mercury process | 50% caustic direct from cell, high purity chlorine and hydrogen, simple brine purification | Use of mercury, use of solid salt, expensive cell operation, costly environmental protection, large floor space | | Membrane process | Low total energy consumption, low capital investment, inexpensive cell operation, high purity caustic, insensitivity to cell load variations and shutdowns, further improvements expected | Use of solid salt, high purity brine, high oxygen content in chlorine, cost of membranes |
In the chlor-alkali electrolysis process, an aqueous solution of sodium chloride is decomposed electrolytically by direct current, producing chlorine, hydrogen, and sodium hydroxide solution. The overall reaction is as follows:
Regardless of cell type, the evolution of chlorine takes place at the anode (positive electrode) of the cell: 
Based on cell type, hydrogen and the hydroxide ions to form sodium hydroxide are generated, directly or indirectly, at the cathode (negative electrode) of the cell: 
In each of the three basic electrolytic processes for chlorine, the nature of the cathode reaction and the means of keeping the chlorine produced at the anode separate from the hydrogen and caustic soda produced at the cathode vary. Electrolytic hydrogen is very pure, > 99.9 percent. The mercury cell and diaphragm cell processes have been in use for over 100 years, while the membrane cell process is newer and has been in commercial use for about 25 years. Since 1970, graphite anodes have been largely superseded by activated titanium anodes in the diaphragm and mercury cell processes. The membrane cell process uses only activated titanium anodes. Even though mercury cell use is slowly decreasing in Western Europe, it still makes up over 50 percent of their capacity with the balance of capacity split evenly between membrane and diaphragm cells. Mercury cell use in Japan was outlawed by the Japanese government in 1986. Japan's chlor-alkali capacity is based exclusively on membrane cell technology. About 67 percent of U.S. capacity is in diaphragm cell plants, and the corresponding percentage is about 53 percent in Canada. Globally, the diaphragm cell process is expected to continue a slow decline, while the mercury cell process experiences a more rapid decline. These declines will be offset by an increase in the membrane cell process. In 2002 the mercury cell process had about one fourth of world capacity and the membrane and diaphragm cell processes shared the remaining capacity about equally. Membrane cell use has increased at a mostly linear rate since 1980. During the 1980s and 1990s, the greatest improvements in chlor-alkali technology have occurred with membrane cell technology. Recent improvements in commercial membrane electrolyzers have been directed at reducing cell voltage, increasing current density, and increasing membrane life. Use of a multilayer membrane produced by chemical conversion of the surface of a perfluorosulfonic acid membrane to form a carboxylic acid layer minimized voltage loss across the membrane and consequent power consumption, while avoiding the problems of layer separation possible with laminated membranes. Operation at the optimum caustic concentration (usually 25-35 percent) will give the lowest cell voltage and power consumption. Operating the membrane cell under modest pressure decreases the volume of gas in the cell, resulting in lower cell voltage and more uniform current density. Pressurization also makes possible operating the cell at higher temperature which lowers cell voltage by maximizing sodium chloride diffusion rates. Close spacing of the anode to the membrane serves to lower cell voltage by disrupting mass transfer boundary layers with turbulence generated by the evolving chlorine gas. New plant construction has favored construction of membrane cell plants because of low capital and operating costs relative to diaphragm and mercury cell processes. The diaphragm process produces a lower-quality sodium hydroxide and involves the use of asbestos in forming the cell diaphragms. The mercury cell process produces high-quality sodium hydroxide with simple brine purification, but the use of mercury leads to the cost disadvantages associated with environmental controls. While the vast majority of chlorine is produced by one of the three electrolytic methods, relatively smaller quantities of chlorine are produced by other processes. These include electrolytic decomposition of hydrogen chloride, coproduct of the electrolytic production of potassium hydroxide, coproduct of the electrolytic production of sodium metal and magnesium metal. Other known processes produce chlorine by chemical means, including the Kel-Chlor process and the sodium chloride/sulfuric acid process, among others. This report describes the characteristics of the three types of electrolytic cells for chlor-alkali manufacture and the variations in the other process elements that follow from the cell characteristics. Economics are developed for greenfield plants of typical size and cell type in five world regions: United States, Western Canada, Western Europe, Japan, and the Middle East. Regional variations in electricity and salt costs, as well as plant size, have the largest effects on relative economics. Physical and chemical properties of chlorine and caustic soda are given as a prelude to description of their widely varied uses. Overall supply and demand estimates are made for both chlorine and caustic soda for the United States, Western Europe, and Japan. Most chlor-alkali capacity expansion is expected to occur in Asia, followed by North America. The Middle East is developing its potential, based on low-cost electricity that is, in turn, derived from low-cost natural gas. |
