Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Patent
1985-05-09
1986-07-22
Bleutge, John C.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
521 31, 522 3, 522120, 522156, 525276, 204296, 427 36, 427 44, 428520, C08F25908, C08F21426, C08F 254, C08J 718
Patent
active
046020454
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to membranes comprising cation-exchange resins and to their use in electrolytic cells, particularly to their use as permselective membranes in chlor-alkali electrolytic cells, and to the improvement of the selectivity of such resins and membranes.
Chlor-alkali electrolytic cells are used to manufacture chlorine and alkali metal hydroxide solution by the electrolysis of an alkali metal chloride solution. Either mercury or diaphgragm cells are generally used for this purpose. Mercury cells have the advantage of producing concentrated alkali metal hydroxide solutions substantially free of alkali metal chloride but give rise to problems associated with the disposal of mercury-containing effluents. On the other hand, diaphragm cells, in which the anodes and cathodes are separated by porous diaphragms which permit the passage of both positive and negative ions and of alkali metal chloride electrolyte, avoid the aforesaid effluent problem but suffer from the disadvantages that (1) relatively weak alkali metal hydroxide solutions are produced which must be evaporated to increase the concentration of the solution, (2) there is a possibility of the product gases, namely hydrogen and chlorine, becoming mixed, and (3) the alkali metal hydroxide solution which is produced is contaminated with a high concentration of alkali metal chloride and the solution must be purified in order to remove this alkali metal chloride.
Attempts have been made to overcome the disadvantages of both mercury cells and diaphragm cells by the use of cells in which the anodes and cathodes are separated by cation permselective membranes. Such membranes are selectively permeable and allow the passage of only positively charged ions and inhibit the passage of bulk electrolyte and negatively charged ions. Cation permselective membranes which have been proposed for use in chlor-alkali electrolytic cells include, for example, those made of fluoropolymers containing cation-exchange groups, for example sulphonate, carboxylate or phosphonate groups and their derivatives.
Fluoropolymers which will withstand cell conditions for long periods of time are preferred, for example the perfluorosulphonic acid-type membranes manufactured and sold by E I DuPont de Nemours and Company under the registered trade mark `NAFION` and which are based upon cation-exchange resins which are hydrolysed copolymers of perfluoro-olefins, for example tetrafluoroethylene, and perfluorovinyl ethers containing sulphonic acid groups or derivatives thereof. Such membranes are described for example in U.S. Pat. Nos. 2,636,851; 3,017,338; 3,496,077; 3,560,568; 2,967,807; 3,282,875 and UK Pat. No. 1 184 321. Other fluoropolymer membranes which may be used in chlor-alkali electrolytic cells are those manufactured and sold by Asahi Glass Company Ltd under the trade mark `FLEMION`and which are described for example in UK Pat. Nos. 1 516 048, 1 522 877, 1 518 387 and 1 531 068.
Although such membranes have many desirable properties which make them attractive for use in the harsh chemical environment of a chlor-alkali cell, such as good long-term chemical stability, their current efficiencies are not as high as is desired, especially when the sodium hydroxide is produced at high concentrations. As the sodium hydroxide concentration in the catholyte is increased so the tendency for back-migration of the hydroxyl ions to the anolyte is increased. This causes a drop in the current efficiency of the cell. Larger amounts of oxygen impurity in the chlorine are thereby produced, and there is a greater buildup of chlorate and hypochlorite contaminants in the brine, which contaminants must be removed and discarded to maintain acceptable cell operation. Current efficiencies of at least 90% are highly desirable.
The degree of back-migration of hydroxyl ions is related to the number and nature of the active sites in the membrane. However there is a "trade-off" situation between the number and nature of active sites, the voltage drop across the membrane and the thickness of
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Partial Translation of Asawa et al., Kokai 52-23192.
Barnett George H.
Markus Michael V.
Bleutge John C.
ICI Australia Limited
Koeckert A. H.
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