Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1998-10-27
2000-03-21
Bell, Bruce F.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204268, 204280, 204294, C25B 900
Patent
active
060398523
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Membrane electrolysis processes of industrial interest such as chlorine and caustic soda production from sodium chloride solutions and even more for the production of chlorine from hydrochloric acid solutions or directly from gaseous hydrochloric acid as described in U.S. Pat. No. 5,411,641, J. A. Trainham III, C. G. Law Jr, J. S. Newman, K. B. Keating, D. J. Eames, E. I. Du Pont de Nemours and Co. (USA), May 2, 1995, undergo extremely aggressive conditions.
In the process for the production of caustic soda and chlorine, the anodic reaction produces chlorine gas which, as is well known, is a strongly corrosive agent. For this reason, in industrial practice usually titanium is used for the anodic elements of the elementary cells forming the electrolyzers. The use of titanium, in this case, is permitted by the relatively modest acidity of the sodium chloride brine in contact with said anodic parts. The acidity is kept at low levels for process reasons and mainly not to damage the delicate ion-exchange membranes separating with a high efficiency the produced caustic soda from the acid brine Suppliers of this kind of membranes specify in fact that the minimum pH for continuous operation must be kept around 2.
Titanium cannot be used for the construction of the cathodic parts of the elementary cells forming the electrolyzer, as the hydrogen evolution, which is the only cathodic reaction, would cause a dramatic embrittlement. In most cases the cathodic parts of the elementary cells are made of high-alloy stainless steels or even better nickel. As a consequence, in bipolar electrolyzers, the bipolar elements which coupled together in a filter-press arrangement form the elementary cells, are made of two layers made of nickel and titanium connected either mechanically (U.S. Pat. No. 4,664,770, H. Schmitt, H. Schurig, D. Bergner, K. Hannesen, Uhde GmbH, May 12, 1987) or by welding (U.S. Pat. No. 4,488,946, G. J. E. Morris, R. N. Beaver, S. Grosshandler, H. D. Dang, J. R. Pimlott, The Dow Chemical Co., Dec. 18, 1984), optionally with an internal layer directed to ensure the electrical conductivity and necessary rigidity. These bipolar elements obviously entail a complicated construction and therefore high costs.
In the production of chlorine by electrolysis of hydrochloric acid, the aggressivity is much greater due to the concurrent presence of chlorine and high acidity. Under particular conditions (temperature below 60.degree. C., acid concentration below 20%, addition of passivating agents) a titanium--0.2% palladium alloy (ASTM B265, Grade 7) may be used with the interstice areas suitably protected by a proper ceramic coating. With temperatures and acid concentrations higher than the above mentioned ones and in the absence of passivating agents, the only suitable material for the construction of the anodic parts of the electrolyzer is tantalum, an extremely expensive material which poses a lot of problems for its working.
Anyway, tantalum, just as titanium, is not compatible with hydrogen and therefore cannot be used for the cathodic parts. A possible solution is given by the nickel alloys of Hastelloy B.RTM. type, but they are very expensive and undergo corrosion during the shut-downs of the electrolyzers. To avoid this severe inconvenience, it would be necessary providing the electrolysis plants with polarization systems, which would make scarcely practical the whole construction.
A possible alternative is offered by graphite, which is sufficiently stable at the process conditions, both the anodic (chlorine evolution with minor quantities of oxygen, in the presence of chlorides and acidity), and the cathodic ones (hydrogen in the presence of caustic soda--chlor-alkali electrolysis--or in the presence of acidity electrolysis of hydrochloric acid). Therefore graphite may be used in the form of plates directly forming the elements which are then assembled in a filter press-arrangement to form the elementary cells of electrolyzers. In the case of bipolar electrolyzers the two faces of th
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Bell Bruce F.
De Nora S.p.A.
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