US3043757A - Electrolytic production of sodium chlorate - Google Patents

Description

July 10, 1962 A. J. HOLMES 3,0 3,

ELECTROLYTIC PRODUCTION OF SODIUM CHLORATE Filed July 8. 1959 CHLORATE CRYSTALS l3 2 l 4 I BRINE L/ PLANT RECYCLE LIQUOR ACID INVENTOR. ARTHUR J. HOLMES AGENT steam directly from the cells.

Unite 3,043,757 Patented July 10, 1962 tree 3,043,757 ELECTROLYTIC PRODUCTION OF SODIUM CHLORATE Arthur J. Holmes, Tonawanda, N.Y., assiguor to Olin Mathieson Chemical Corporation, a corporation of Virginia Filed July 8, 1959, Ser. No. 825,735 3 Claims. (Cl. 204-95) 'lized directly as a first crop, advantageously simplifying the process of recovery of the solid chlorate.

Conventional sodium chlorate cells using steel cathodes and graphite anodes can be operated in the range of'about or lower up to about 70 C. At temperatures of 60 to 70 C. however, losses of graphite by oxidation become intolerably high and the usual temperature of operation is about to 45 C. Were it not for the graphite losses, higher temperatures would be desirable because of the decreasing voltage requirements as the temperature increases.

Conventional sodium chlorate cells are usually equipped with cooling-coils to maintain the temperature within the limits appropriate for use with graphite anodes. Thecooling coils are an expensive part of the original cells and are expensive to maintain. Because of the heat produced as a by-product of the electrolysis, they are essential in conventional cells.

According to the process of the present invention, the electrolysis of sodium chloride to sodium chlorate is carried out at temperatures of about 105 to 123 C., the

boiling range of the brine and preferably at about 105 to 115 C. Provision is made for removal of water as The eflluent liquor from the cell is cooled, advantageously by flashing off a further minor proportion of water from the liquor in the crystallizer. The crystals of sodium chlorate are separated and dried and the liquor is recycled.

The process of the present invention is carried out in simple cells with no provision for cooling. An outlet is arranged, suitably in the cell covers for removal of steam or water vapor. The electrodes are fabricated of titanium bearing superficially thereon a coating of platinum. Un-

coated titanium used anodically in chloride brines is rapidly inactivated by the formation of a non-conductiing, passive film thereon. Electrodes of titanium, however, do not form this passive film where they bear a superficial .coating of platinum. The titanium electrodes bearing of 1958 by Imperial Chemical Industries Ltd., opened to public inspection July 23, 1958 or by other suitable methods.

Using the electrodes as described above, the cells are operated at temperatures up to the boiling point of the brine saturated with both sodium chlorate and sodium chloride (123 C), suitably at about to C. Voltage requirement is reduced from about 4.2 volts to about 3.9 volts due to the increase in brine temperature from about 35 to 45 C. up to about 105 to 115 C. Current density is one ampere per square inch or higher compared to 0.13 to 0.35 ampere per square inch in the operation of conventional cells with graphite anodes.

By operating the electrolytic cells at temperatures of 105 to 115 C., it is possible to maintain the cell liquor at higher concentrations of chloride and considerably higher concentrations of chlorate than in cells operated at 35 to 45 C. The recycle cell liquor contains about 75 to grams per liter of sodium chloride and 550 to 650 grams perliter of sodium chlorate. By the addition of about one volume of saturated sodium chloride brine feed to one volume of recycle liquor, the concentration of sodium chloride is brought up to about to 225 grams per liter in the cell feed. The sodium chloride content of the efliuent liquor from the cells is reduced by electrolysis to about 75 to 125 grams per liter and the chlorate content is raised by electrolysis and evaporation of water to about 675 to 800 grams per liter of sodium chlorate. By evaporative cooling of the efiluent cell liquor of this concentration and removal of a portion of the sodium chlorate content which separates as the first solid phase, the recycle liquor contains about 80 to 125 grams per liter of sodium chloride and 550 to 650 grams per liter of sodium chlorate. Preferred concentrations lie within these ranges. For example, the recycle liquor suitably contains about 100 grams per liter of sodium chloride and 600 grams per liter of sodium chlorate when the crystals are separated at a temperature of about 45 C. The fortified liquor fed to the cells suitably contains about 200 grams per liter of sodium chloride and the effluent contains about 80 grams per liter of sodium chloride and 800 grams per liter of sodium chlorate.

In the more dilute liquors obtained by graphite anode cell operations containing about grams per liter of sodium chloride and 40-0 grams per liter of sodium chlorate, sodium chlorate is recovered by cooling the liquor from 45 to 0 to -10 C. using expensive refrigeration. By an alternative method the cell liquor is evaporated to remove part of the water. As the solution becomes more concentrated, sodiumchloride crystallizes and is separated md returned to the process. The residual solution is cooled to crystallize sodium chlorate which is removed from the solution. In contrast with these conventional processes, the present process-obtains sodium chlorate in the first crystallization and unconverted salt remains in the liquor and is recycled to the cells. Only cooling water is required and the necessity of expensive refrigeration to lower temperaturesis avoided.

especially suitable feed. saturated or substantially saturated, having a concentration of about 9-0 to 100 percent of saturation.

In conventional chlorate cells the operating pH is usually approximately 6.7 and generally is within the range of 6.5 to 7.0. Small proportions of chromates or bichromates are added to the electrolyte to prevent re duction at the cathodes. The addition of chromates in the present process using platinized titanium electrodes is also advantageous for the same reason.

In conventional chlorate cells with graphite anodes the current density can be up to about one ampere per square inch. Practically, the wear on the graphite anodes at such current density is intolerably high and limits actual operations to about 0.13 to 0.35 ampere per square inch. Decreasing the current density decreases the voltage drop across the cell and increases the size of the electrodes, cells and buildings required for a given tonnage per day of product. The present process advantageously makes possible high current densities without deleterious effect on platinum surfaced titanium anodes. The voltage drop. across the cells is reduced from about 4.2 to about 3.9 volts and the current efliciency is significantly raised from about 80 toabove 90 percent. This improvement in current efiiciency is due primarily to the discharge of less hypochlorite ion at the anodes. at the elevated temperatures. More hypochlorite is converted to chlorate and less decomposes to liberate oxygen. The oxygen liberation represents an inefficiency which is materially reduced in the present process. A further advantage of the present process is that a holding time and the tanks conventionally required for conversion of hypochlorite to' chlorate are entirely avoided. The resultant savings in size of electrodes, cells, aux- V iliary equipment and buildings permits a material reduction in the cost per ton of chlorate.

In the flow sheet of FIGURE 1, sodium chloride brine introduced via line 11 is mixed with recycle liquor returning via line 32 and treated in brine plant 12 including particularly acidification by means of hydrochloric acid introduced via line 13. The treated brine flows via line 14 to cells 15 where a portion of the chloride content is converted to chlorate. Water is vaporized directly from the cells and removed via line 16. The cell efliuent flows via line 17 to a crystallizer 18, removing additional water overhead via line 19. Liquor is recycled from the bottom. of crystallizer 18 via line 20 using pump 21 to return the liquor via line 22 to crystallizer 18. A slurry Operating the process as described in the flow sheet of the attached figure, fresh brine is charged to the brine plant in the form of a 25 percent aqueous sodium chloride solution at the rate of 1840 pounds per hour together with a recycle liquor stream comprising 1080 pounds per hour of sodium chlorate, 181 pounds per hour of sodium chloride and 1260 pounds per hour of water. The mixed brine is treated with '10 percent bydrochloric acid to produce a feed which will maintain a pH of about 6.5 to 7 in the cells. Usually a pH of about in the feed is satisfactory. The feed to the cell comprises 1080 pounds per hour of sodium chlorate, 640

poundsper'hour of sodium chloride and 2640 pounds per hour of water. During the electrolysis water is vaporized from the cells at the rate of 1340 pounds per hour. The efiiuent from the cells at a temperature of 112 C. comprising 1910 pounds per hour of sodium chlorate, 181 pounds per hour of sodium chloride and 1290 pounds per hour of water is charged to the crystal lizer from which water is removed at the rate of about 200 pounds per hour. The slurry removed from the crystallizer at a temperature of 45 C. comprises 1080 pounds per hour of sodium chlorate, 181 pounds per hour of sodium chloride and 1090 pounds per hour of water. Wash water is used at the rate of 178 pounds per hour. The liquor is recycled to the 'brine feed tank and the crystals transferred from the centrifuge to the dryer comprise 840 pounds per hour of sodium chlorate, one pound per hour of sodium chloride and 13 pounds per hour of water. About 13 pounds per hour of water are removed in the dryer and the crystalline chlorate product comprises the 840 pounds chlorate containing only one pound of sodium chloride and about 0.25 pound of water.

Example II A glass chlorate cell was constructed having a capacity of 4 liters. It carried a tight one inch thick Lucite cover carrying in turn leads for a pH controller, leads for a liquid level controller, leads for the principal electrodes and lines for the introduction of water, for .the addition of dilute hydrochloric acid and for the removal of cell gas and water vapor. The line for the latter was connected to a condenser for the separation of the aqueous condensate from by-product gases. The liquid level controller was actuated by leads in the cell operating the valve on a water feed tank which replaced condensate and maintained constant level in the cell. The pH controller feeds one percent hydrochloric acid to the cell to maintain the pH of the cell contents at 7. The cell also contained a thermometer and pro.- vision for heating and stirring the cell contents. The principal electrodes suspended throughthe top cover comprised two titanium sheets each three inches square and onesixteenth inch thick welded to titanium rods and bearing a dull gray platinum coating 0.0001 inch thick. Teflon spacers maintained the electrode distance at one-eighth inch. Direct current was supplied to the electrodes at about 3.5 to 4.0 volts and 9 amperes giving a current density of about 1 ampere per square inch. The polarity of the cell was reversed periodically to remove any deposition of impurities on the cathodes tending to increase cell voltage. The cell liquor comprised an aqueous solution originally containing approximately 750 grams per liter of sodium chlorate, grams per liter of sodium chloride and 2 grams per liter of sodium dichromate. The volume of the electrolyte was maintained at 2.5 liters. heated to the boiling point which varied from about to C. A sample of 200 milliliters of the cell liquor was removed at 24 hour intervals and replaced with approximately 80 grams of sodium chloride with sufficient water to bring the cell volume back to 2.5 liters. Sodium chlorate crystallized from the sample on cooling and was processed as described in Example I.

Hourly readings were taken of the amperage, voltage, temperature, pH, condensate volume and acid volume which were averaged over each 24 hour period. A sample I of the gas was analyzed each 24 hours for chlorine, hydro- The cell contents were stirred and failure or other interruptions are shown in the following table:

electrodes selected from the group consisting of steel and composite electrodes, at least the anodes being composite Average Readings Cell Gas Analysis Hours, Aver- Current Current aged Cell Conden- Acid Chlorine, Oxygen, Effi- Con- Voltage pH Temp, sate, Added B Percent Percent ciency, sumed b C. mL/hr. Percent e Pounds of 36 percent hydrogen chloride per ton of sodium chlorate. b Kilowatt hours per ton of sodium chlorate.

graphite consumption at higher temperatures and low production rates and refrigeration requirements at lower temperatures. With conventional cell-s, current efficiencies vary from about 70 to 85 percent as contrasted with the above values of about 90 percent or better. Conventional current densities are in the range of about 0.13 to 0.35 amperes per square inch in contrast with the completely satisfactory current density of one ampere per square inch used above. The above-described run was continued for over 3000 hours with no visible change in the electrodes.

The process of the present invention is useful in batch or continuous operations or in any desired combination thereof. One cell may be charged with electrolyte and electrolyzed to suitable concentrations and the brine Worked for sodium chlorate as described above The mother liquor from the crystallization is charged with fresh brine to the next batch. In continuous operation, the electrolyte flows through a series of cells in which the concentration of chlorate in the electrolyte increases to the desired concentration. The electrolyte flows out of the last cell and is worked with recycle as described above.

What is claimed is:

1. In an electrolytic process for the production of sodium chlorate in which a chlorate brine containing sodium chloride and sodium chlorate is electrolyzed between electrodes, said composite electrodes consisting of a core of titanium bearing platinum superficially thereon, the improvement which comprises electrolyzing at a temperature of about to 123 C., simultaneously vaporizing and removing water from the chlorate brine to maintain a chlorate concentration in the chlorate brine of about 675 to 800 grams per liter, removing the chlorate brine from the zone of electrolysis, cooling to crystallize sodium chlorate from the electrolyzed chlorate brine and separating the crystals from the residual liquor, fortifying the residual liquor by adding sodium chloride brine and charging the fortified chlorate brine to the electrolysis 2. The process of claim 1 in which the residual liquor is fortified by the addition of sodium chloride brine to a concentration of about to 225 grams per liter of sodium chloride.

3. The improvement of claim 2 in which the electrolyzed brine is cooled from a temperature in the range of 105 to 123 C. to a temperature above about 45 C. to crystallize sodium chlorate.

References Cited in the file of this patent UNITED STATES PATENTS 1,143,586 Laib June 15, 1915 1,173,346 Gibbs Feb. 29, 1916 2,511,516 Schumacher June 13, 1950 OTHER REFERENCES Chemical and Metallurgical Engineering, vol. 45, No. 12, December 1938, pages 692-696.

Chemical Age, January 3, 1959, page 9.

Corrosion Technology, February 1959, pages 49-62.

Claims (1)

1. IN AN ELECTROLYTIC PROCESS FOR THE PRODUCTION OF SODIUM CHLORATE IN WHICH A CHLORATE BRINE CONTAINING SODIUM CHLORIDE AND SODIUM CHLORATED IS ELECTROLYZED BETWEEN ELECTRODES SELECTED FROM THE GROUP CONSISTING OF STEEL AND COMPOSITE ELECTRODES, AT LEAST THE ANODES BEING COMPOSITE ELECTRODES, SAID COMPOSITE ELECTRODES CONSISTING OF A CORE OF TITANIUM BEARING PLATINUM SUPERFICIALLY THEREON, THE IMPROVEMENT WHICH COMPRISES ELECTROLYZING AT A TEMPERATURE OF ABOUT 105 TO 123*C., SIMULTANEOUSLY VAPORIZING AND REMOVING WATER FROM THE CHLORATE BRINE TO MAINTAIN A CHLORATE CONCENTRATION IN THE CHLORATE BRINE OF ABOUT 675 TO 800 GRAMS PER LITER, REMOVING THE CHLORATE BRINE FROM THE ZONE OF ELECTROLYSIS, COOLING TO CRYSTALLIZE SODIUM CHLORATE FROM THE ELECTROLYZED CHLORATE BRINE AND SEPARATING THE CRYSTALS FROM THE RESIDUAL LIQUOR, FORTIFYING THE RESIDUAL LIQUOR BY ADDING SODIUM CHLORIDE BRINE AND CHARGING THE FORTIFIED CHLORATE BRINE TO THE ELECTROLYSIS.

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