Chemistry: electrical and wave energy – Processes and products
Patent
1984-10-30
1986-04-22
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
204128, 210683, C25B 116, C25B 126
Patent
active
045840717
DESCRIPTION:
BRIEF SUMMARY
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to the electrolysis of aqueous alkali metal chloride solutions, using a fluorinated membrane between the anode and cathode. In particular, it relates to such a process in which the ionic iodide or iodate content of the anolyte feed is maintained below 1 ppm.
Iodine, when present in brine, is generally in the form of sodium iodide. Many sources of brine contain less than 1 ppm iodine, but higher levels are found in brine associated with oil and gas fields and in salt deposited from sea water. Sea water contains about 35,000 ppm total salts and 0.05 ppm iodine which is equivalent to approximately 0.5 ppm iodine in concentrated brine solutions. Further, iodine can be concentrated in certain seaweeds, which may explain the observation that some salt deposits from prehistoric seas contain up to 100-200 ppm iodine. High iodine content has been reported for brines in Michigan, Oklahoma, Louisiana, California and Japan.
There has now been discovered an improved electrolytic process for the production of chlorine and an alkali metal hydroxide containing soluble, iodine-containing salts and employing an electrolytic cell comprising an anode compartment containing an anode and aqueous alkali metal chloride and a cathode compartment containing a cathode and aqueous alkali metal hydroxide separated by a fluorocarbon cation-exchange membrane containing at least one of the types of cation exchange groups selected from carboxyl cation-exchange groups and sulfonyl cation-exchange groups and through which an electrical current is passed at a current density in the range of 20 to 80 A/dm.sup.2, preferably 30 to 50 A/dm.sup.2 while continuously adding concentrated alkali metal chloride free from harmful levels of alkaline earth salts and iron or other heavy metal salts to the anode compartment and continuously removing chlorine and depleted anolyte solution from the anode compartment and continuously adding water or dilute alkali metal hydroxide to the cathode compartment and continuously removing alkali metal hydroxide having a concentration maintained at a value between 20% and 45% by weight, preferably between 25% and 35% by weight from the cathode compartment, in which the improvement comprises maintaining the concentration of soluble iodine-containing salts in the concentrated alkali metal chloride added to the anode compartment at a concentration not higher than 1 ppm, preferably not higher than 0.4 ppm.
Particularly preferred for use in the above process are perfluorinated membranes, especially bimembranes having a layer containing carboxyl groups facing the catholyte and a layer containing sulfonyl groups facing the anolyte. Current efficiencies above 90% can readily be obtained and maintained for long periods of time, that is for two or more years, by operating according to this process. The process is particularly useful commercially for preparing concentrated caustic soda (NaOH) from sodium chloride brine but can also be applied to the commercial production of potassium hydroxide from potassium chloride solution.
In the event that the raw brines available naturally contain more than 1 ppm of soluble, iodine-containing salts, such as sodium iodide or sodium iodate, methods are known in the art for removing these iodine-containing salts. A useful reference is Kirk-Othmer, "Encyclopedia of Chemical Technology", 3rd Edition, volume 13, pp. 649-677, John Wiley & Sons, N.Y. 1981. Other methods for removing iodide from brine are taught in "Ultrapurity-Methods and Techniques" edited by Morris Zief and Robert Speights, Marcel Dekker, Inc. New York, N.Y., 1972. In the event that very pure brine free from soluble iodine-containing salts is also available, a brine solution containing less than 1 ppm soluble, iodine-containing salts can be prepared by mixing the brine containing more than 1 ppm of the iodine-containing impurity with sufficient of the very pure brine to reduce the level of iodine-containing salts to 1 ppm or below in the mixed brine.
In laboratory ele
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Chapman Terryence
E. I. Du Pont de Nemours and Company
Niebling John F.
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