Electrolysis for the production

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. 

Figure 3.5. Process diagram of alkaline electrolysis for the production of H2 Polymer Electrolyte Membrane (PEM) Electrolysis...

Bergmann, M.E.H. (2007a) On the electrochemical flow-through electrolysis for the production of waters with disinfecting ability (in German). In Suchentrunk, R. (ed.) Jahrbuch fuer Ober-flaechentechnik, Leuze, Saulgau, pp. 315-330. 

During electrolysis for the production of a metal, the metal is produced ... 

Wong, M M and Haver, F P, 1977. Fused salt electrolysis for the production of lead and zinc metals, in Proceedings international Symposium on Molten Salt EJectrolysis in Metal Production, pp 21-29 (Institution of Mining and Metallurgy (IMM) London). 

By the electrolysis of concentrated sodium chloride solution this process was initially used primarily for the production of sodium hydroxide but the demand for chlorine is now so great that the chlorine is a primary and not a by-product. 

Water-Splitting Techniques. Only one water-sphtting method, electrolysis, is practiced industrially for the production of hydrogen, and that only to a limited extent. 

Electrolytic Precipitation. In 1800, 31 years before Faraday s fundamental laws of electrolysis, Cmikshank observed that copper metal could be precipitated from its solutions by the current generated from Volta s pile (18). This technique forms the basis for the production of most of the copper and 2inc metal worldwide. 

Electrolysis. For reasons not fiiUy understood (76), the isotope separation factor commonly observed in the electrolysis of water is between 7 and 8. Because of the high separation factor and the ease with which it can be operated on the small scale, electrolysis has been the method of choice for the further enrichment of moderately enriched H2O—D2O mixtures. Its usefiilness for the production of heavy water from natural water is limited by the large amounts of water that must be handled, the relatively high unit costs of electrolysis, and the low recovery. 

The electrolysis of NaCl brine for the production of chlorine and caustic soda is one of the oldest and certainly one of the most important industrial electrochemical processes (22—26). The overall reaction is... 

Heavy water [11105-15-0] 1 2 produced by a combination of electrolysis and catalytic exchange reactions. Some nuclear reactors (qv) require heavy water as a moderator of neutrons. Plants for the production of heavy water were built by the U.S. government during World War II. These plants, located at Trad, British Columbia, Morgantown, West Virginia, and Savaimah River, South Carolina, have been shut down except for a portion of the Savaimah River plant, which produces heavy water by a three-stage process (see Deuterium and tritium) an H2S/H2O exchange process produces 15% D2O a vacuum distillation increases the concentration to 90% D2O an electrolysis system produces 99.75% D2O (58). 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

The first production of aluminum was by the chemical reduction of aluminum chloride with sodium. The electrolytic process, based on the fused salt electrolysis of alumina dissolved in cryolite, was independently developed in 1886 by C. M. Hall in America and P. L. Heroult in France. Soon afterwards a chemical process for producing pure alumina from bauxite, the commercial source of aluminum, was developed by Bayer and this led to the commercial production of aluminum by a combination of the Bayer and the Hall-Heroult processes. At present this is the main method which supplies all the world s needs in primary aluminum. However, a few other processes also have been developed for the production of the metal. On account of problems still waiting to be solved none of these alternative methods has seen commercial exploitation. 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Preparation. Li is currently produced by electrolysis of molten LiCl, the melting point of which (614°C) is lowered by addition of KC1. The salt mixtures used (not very different from the eutectic one) contain about 45-55 mass% LiCl the electrolysis is carried out at 400-460°C. The cells commonly used resemble the cell used for the production of sodium (Downs cells). 

The theory of electrolysis having been considered, it remains to describe some of the more important applications of this phenomenon for the production of hyi-ogen and oxygen. 

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