Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Patent
1993-12-08
1997-01-21
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
205508, 205510, 205555, 205554, 204258, 204263, 204257, 204265, C25B 100
Patent
active
055956416
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/EP92/01442 filed Jun. 26, 1992.
BACKGROUND OF THE INVENTION
The electrolytic production of chlor-alkali the most widespread process in the electrochemical field. This process utilizes sodium chloride which is converted into sodium hydroxide and chlorine by applying electric current.
Also known, even if not so common, is the process based on the use of potassium chloride as starting material, to obtain potassium hydroxide and chlorine as final products. Chlorine and caustic soda may be also produced respectively according to the methods schematically resumed as follows: large amounts as a by-product of the chlorination of organics. Hydrochloric acid may be further obtained by a reaction between sodium chloride and sulphuric acid, with the side-formation of sodium sulphate; filtration of the by-produced solid calcium carbonate and concentration of the diluted solution of sodium hydroxide containing various impurities deriving from the lime and from the sodium carbonate solution. based on the conversion of sodium chloride brine into sodium bicarbonate, which is scarcely soluble, by means of a chemical reaction with ammonia, which is then recycled, and carbon dioxide. Bicarbonate is then converted into sodium carbonate by roasting. dioxide, both obtained from calcium carbonate, and the ammonia necessary to make up for the unavoidable losses. mineral ores which contain sodium carbonate and bicarbonate and minor percentages of other compounds, such as sodium chloride.
It is evident that the above alternatives are based on complex processes which involve high operation costs. For these reasons, these processes were gradually abandoned in the past and the market become more and more oriented towards the chlor-alkali electrolysis process which is intrinsically simpler and energy-effective due to the development of the technology based on mercury cathode cells progressively evolved to diaphragm cells and now to membrane cells. However, chlor-alkali electrolysis is today experiencing a decline, which is connected to the rigid stoichiometric balance between the produced quantities of sodium hydroxide and chlorine. This rigid link was no problem when the two markets of chlorine polyvinyl chloride or (PVC, chlorinated solvents, bleaching in paper industry, various chemical reactions) and of sodium hydroxide (glass industry, paper industry, various chemical uses) were substantially balanced. Recently, a persistent downtrend in the chlorine market (reduced use of PVC and chlorinated solvents, decreasing use in the paper industry) combined with a robust demand of caustic soda, seemingly bound to increase in the near future, pushed the industry towards alternative routes for producing sodium hydroxide without the concurrent production of chlorine, in some cases even considered an undesirable by-product. This explains the revival of the sodium carbonate causticization process, notwithstanding its complexity and high costs.
In this scenery, the electrochemical industry is ready to propose alternative processes evolving from the existing ones (see C. L. Mantell, Industrial Electrochemistry, McGraw-Hill) and made more competitive by the availability of new materials and of highly selective ion exchange membranes. The most interesting proposal is represented by the electrolysis of solutions of sodium sulfate, either mined or as the by-product of various chemical processes. Electrolysis is carried out in electrolyzers made of elementary cells having two electrolyte compartments separated by cation-exchange membranes or in a more sophisticated design, electrolyzers made of three electrolyte compartment elementary cells containing anion- and cation-exchange membranes. This process, also known as sodium sulfate splitting, generates sodium hydroxide (15-25%), hydrogen, oxygen and, in the simplest design, diluted sodium sulphate containing sulphuric acid, or in the more sophisticated design, diluted sodium sulphate and pure sulphuric acid. While sodium hydroxide is a desirable product, pure sulfuric
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Faita Giuseppe
Traini Carlo
Denora Permelec S.p.A.
Gorgos Kathryn
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