Electrochemical chlorine dioxide process

#The present invention relates to the production of chlorine dioxide. More particularly, the invention relates to the electrolytic production of chlorine dioxide gas. Chlorine dioxide is commercially employed as a bleaching agent and sanitizing agent.

#The electrochemical production of chlorine dioxide has been described previously, for example, by J. O. Logan in U.S. Pat. No. 2,163,793, issued June 27, 1939. The process electrolyzes solutions of an alkali metal chlorite such as sodium chlorite containing an alkali metal chloride or alkaline earth metal chloride as an additional electrolyte for improving the conductivity of the solution. The process preferably electrolyzes concentrated chlorite solutions to produce chlorine dioxide in the anode compartment of an electrolytic cell having a porous diaphragm between the anode and cathode compartments.

#British Pat. No. 714,828, published Sept. 1, 1954, by Farbenfabriken Bayer, teaches a process for electrolyzing an aqueous solution containing a chlorite and a water soluble salt of an inorganic oxy-acid other than sulfuric acid. Suitable salts include sodium nitrate, sodium nitride, sodium phosphate, sodium chlorate, sodium perchlorate, sodium carbonate, and sodium acetate.

#FIG. 1 is a flow sheet illustrating the novel electrolytic process of the present invention.

#FIG. 2 is a graph depicting the relationship between light transmission and the NaClO.sub.2 concentrations of aqueous sodium chlorite solutions containing dissolved ClO.sub.2.

#In the process of FIG. 1, sodium chlorite solution, contained in tank 10, is pumped to anode compartment 18 of electrolytic cell 16. Electrolytic cell 16 is comprised of anode compartment 18 containing anode 24, and cathode compartment 20 containing cathode 28, separated by membrane 22. Anode 24 receives electric current from D.C. power source 26 and passes current through the chlorite solution to cathode 28 to produce chlorine dioxide at anode 24 and hydrogen gas and sodium hydroxide solution at cathode 28. A sodium chlorite solution containing dissolved chlorine dioxide is removed from anode compartment 18 and passed through photoelectric cell 12. Photoelectric cell 12 measures the concentration of chlorite ion of the solution and records the measurement on photometer 14. When the concentration of chlorite ion in the sodium chlorite solution is above a predetermined upper limit, as determined by photometer 14, valve 30 is operated to return the chlorine dioxide-containing chlorite solution to tank 10. When the concentration of chlorite ion is below the predetermined upper limit, the chlorine-dioxide containing chlorite solution passes through valve 30 to resaturation column 32. Resaturation column 32 contains particles of sodium chlorite, a portion of which dissolve in the chlorine dioxide-containing chlorite solution to provide the desired chlorite ion concentration as determined by the photometer. The resaturated chlorite solution is returned to tank 10. The level of sodium chlorite solution in tank 10 may be controlled by a level controller (not shown) which also permits the addition of water to tank 10 when required. To recover chlorine dioxide gas from the chlorine dioxide-containing chlorite solution, an inert gas such as air is passed through the solution to produce a chlorine dioxide gaseous mixture which is removed from tank 10. The chlorine dioxide gaseous mixture is passed through monitor 36 which measures the concentration of ClO.sub.2.

#Sodium hydroxide solution and hydrogen gas produced during the electrolysis are removed from cathode compartment 20. The hydrogen gas may be vented or, where suitable volumes are produced, dried and used, for example, as a fuel. The concentrated sodium hydroxide solution is conveyed to storage vessel 34. If desired, water may be added to storage vessel 34 to dilute the concentrated sodium hydroxide solution. NaOH solution may be recycled to the cathode compartment 20 of electrolytic cell 16.

#More in detail, the novel process of the present invention electrolyzes an aqueous solution of sodium chlorite to produce a solution of chlorine dioxide. To prevent the formation of the complex ##STR3## as described above, the concentration of sodium chlorite in the aqueous solution is maintained within the range of from about 0.8 to about 5.0, preferably from about 1 to about 3 percent, and more preferably from about 1 to about 2 percent by weight. Control of the concentration is provided by passing the sodium chlorite solution containing dissolved chlorine dioxide through a concentration measuring device which measures the visible light transmission of the aqueous chlorite-chlorine dioxide solution. One suitable device is a photometer which records the visible light transmitted through the sodium chlorite solution containing dissolved ClO.sub.2 as it passes by a photometric cell. The sodium chlorite solution containing dissolved ClO.sub.2 having the desired concentration range of chlorite ion is normally yellow in color, but begins to darken as the chlorite ion concentration increases. The darkening of the solution is evidence of the formation of the dark mahogany complex discussed above.