Low energy chlorate electrolytic cell and process

Abstract

Alkali metal chlorates are produced by electrolyzing an anolyte contained in an anode compartment of an electrolytic cell, the anode and cathode compartments separated by means of a permselective membrane having low alkali metal ion transport efficiency. The final chlorate product can be directly crystallized from the electrolyzed anolyte or fed directly to a chlorine dioxide generator. Alternatively, a microporous, hydrophilic diaphragm can be substituted for the permselective membrane provided that the catholyte compartment is maintained at a higher hydraulic pressure than the hydraulic pressure in the anolyte compartment.

Description

BACKGROUND OF THE INVENTION [0001] (1) Field of the Invention [0002] This invention relates to an electrolytic cell, a low transport efficiency alkali metal ion permselective membrane, and a cyclic electrolysis process for the preparation of an alkali metal chlorate in an aqueous medium from the corresponding alkali metal chloride. [0003] (2) Description of Related Art [0004] It is known in this art to form sodium chlorate by the electrolysis of sodium chloride using a sodium chlorate electrolyte which is a mixture of sodium chloride and sodium chlorate at concentrations close to the saturation point. In the prior art commercial process, an electrolytic cell is used in which the anode and cathode are exposed to the same electrolyte; the cell not being divided into cathode and anode compartments by a porous barrier, diaphragm, or membrane. At the anode in such a cell, chlorine is evolved and at the cathode, hydrogen is evolved. Hydrogen produced at the cathode and chlorine produced a...

AI Technical Summary

Benefits of technology

[0010] In the electrolysis process of the invention, electrolytic cells having as a cell separator a low transport efficiency alkali metal ion permselective membrane or an hydrophilic, microporous diaphragm are used for the production of an alkali metal chlorate. In order to provide a process with greater electrical efficiency, the uncatalyzed mild steel or titanium cathodes utilized in commercial, prior art cells for the production of an alkali metal chlorate are replaced in the cells of the invention with either catalytic, metal cathodes or gas-diffusion cathodes, which are depolarized by feeding air or oxygen to the cathode. To utilize such cathodes without cathode corrosion, the anolyte and catholyte compartments must be separated by a microporous diaphragm or a low transport efficiency alkali metal ion permselective cationic membrane. The electrolysis process of the invention can be carried out with essentially no losses of chlorine, produced in the process, and, in the absence of hexavalent chromium values, derived from the addition of sodium chromate or sodium bichromate to the electrolyte, thus providing economic as well as environmental advantages. The use of a hydrophilic, microporous diaphragm in the electrolysis process of the invention requires that an higher hydraulic pressure be maintained in the catholyte compartment of the cell than in the anolyte compartment of the cell.

[0011] The electrolysis process of the invention is advantageous in providing energy savings by (1) reducing the voltage consumption in the electrolysis process, (2) eliminating the addition of chromium ions in the form of hexavalent chromium in the electrolyte with attendant economical and environmental advantages, and (3) eliminating the need for external sources of hydrochloric acid and sodium hydroxide. Added advantages of the process of the invention include (1) eliminating the possibility of explosive mixtures of oxygen and hydrogen gases and (2) providing an electrolysis process for the production of an alkali metal chlorate in which there are essentially no losses of chlorine. The hydrogen produced in the cathode compartment of the cell is confined therein by use of a low transport efficiency alkali metal ion permselective membrane or a hydrophilic, microporous diaphragm thus eliminating contact of hydrogen with the cell anolyte where it can strip away the chlorine produced therein. The anolyte is electrolyzed to a desired solution of an alkali metal halate from which said halate can be directly crystallized.

Problems solved by technology

The use of hexavalent chromium in the electrolyte is disadvantageous economically as well as environmentally, as set forth in U.S. Pat. No. 5,104,499 to Millet and U.S. Pat. No. 4,295,951 to Bommaraju et al.

Because of the presence of the highly oxidizing hypochlorite ions in the electrolyte solution, activated cathodes such as the precious metal oxide coated cathodes disclosed in the prior art for use in the production of alkali metal halatas cannot be used.

However, such cathodes have not been useful in an electrolytic cell for the production of alkali metal chlorate because of the presence of hypochlorite ions in the electrolyte.

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