US3486994A - Process for preparing chlorine by electrolysis of aqueous hydrochloric acid - Google Patents

Description

United States Patent 4 ,078 Int. Cl. C01b 7/ 06, B01k ]/.00

US. Cl. 204128 3 Claims ABSTRACT OF THE DISCLOSURE A process for preparing chlorine by electrolysis of aqueous hydrochloric acid in an electrolytic cell, the anode and cathode spaces of which are separated by a diaphragm. As anolyte and catholyte solutions are used which are 4 to 8 times molar with respect to the hydrochloric acid and contain 0.5 to 2 mols per liter of copper chloride or iron chloride. During electrolysis hydrogen chloride is introduced into the anolyte in the same measure as it is consumed and an oxidizing gas is introduced into the catholyte, the differential pressure between the anode space and the cathode space being maintained at zero.

The present invention relates to a process for preparing chlorine by electrolysis of aqueous hydrochloric acid in which compounds of metals occuring in several oxidation stages that are easy to convert into one another are added to the electrolyte to cut down the cell voltage.

It is known to produce chlorine and hydrogen by electrolysis from hydrochloric acid, especially waste acid as obtained in the chlorination of organic compounds. A great advantage of this process resides in the fact that the investment costs are relatively low. In order to perform the electrolysis of aqueous hydrochloric acid in a still more economic manner it is desirable, however, to cut down the required cell voltage.

In British specification No. 834,640 it has been proposed to add metals to the electrolyte, especially metals of the platinum group, whereby the hydrogen overvoltage is reduced. The gain in voltage obtained is useful but not sufficient. A drawback of this known process is that the metal which deposits at first on the cathodes falls off, above all with interruptions of working. A sludge thus collects on the bottom in the cathode spaces of the cells, which sludge is almost impossible to dissolve. It is, therefore, necessary to open and clean the cell at certain intervals, whereby considerable costs are involved.

According to the processes described in US. Patents 2,468,766 and 2,666,024 hydrochloric acid is not subjected directly to electrolysis but after having been transformed into the chloride of a metal occuring in several oxidation stages easy to convert into one another, the redox potential Me +SMe must be more positive than the redox potential HSH and the said metal chloride solutions are electrolyzed. Suitable metals are, in the first place, copper and iron. This process permits a considerable reduction of the cell voltage. From the economic point of view it is often unimportant that no hydrogen is produced.

A serious drawback of this process is, however, that the metal which is reduced at the cathode must be substantially oxidized in an additional and expensive process step in which hydrogen chloride is replenished. It is known that the oxidation to a higher valence stage with 3,486,994 Patented Dec. 30, 1969 ice air or oxygen proceeds only at the beginning with a tolerable speed and slows down with a diminishing content of the substance to be oxidized. When, however, solutions are returned to electrolysis still having a considerable content of metal in the lower oxidation stage, important amounts of chlorine are consumed in the cell involving a considerable reduction of the current efiiciency so that the gain in energy obtained by the voltage decrease is compensated to the greater part by the loss of current efiiciency. The large amount of air required for re-oxidation expels hydrogen chloride from the solution, which must be absorbed again in an additional process step.

Finally, in the above process it is necessary to pump the electrolyte in a rapid flow through the horizontal cell so that the metal compound reduced at the cathode is carried away and does not approach the anode by diffusion where it would again consume chlorine and thus lead to losses in the current efficiency.

The present invention provides a process for the preparation of chlorine by electrolysis of aqueous hydrochloric acid in the presence of metal compounds occuring in several oxidation stages that are easy to convert into one another, wherein with considerable gain in voltage the current efliciency is over and no additional and complicated process steps or installations are required.

According to the invention the electrolysis of aqueous hydrochloric acid is carried out in cells with vertical electrode arrangement, the anode and cathode spaces of which are separated by diaphragms. As anolyte and as catholyte solutions are used that are 4 to 8 times molar with respect to the hydrochloric acid and contain 0.5 to 2 mols of copper chloride or iron chloride per liter of solution. In the course of electrolysis hydrogen chloride is introduced into the anolyte in the same measure as it is consumed and an oxidizing gas is introduced into the catholyte and the differential pressure between the anode space and the cathode space is maintained at zero.

The limiting current density is determined by the speed of the re-oxidation of the metal compound reduced at the cathode. As soon as the cathodic reduction is more rapid than the reoxidation due to too high a current density, the cell voltage gradually rises to the separation voltage of hydrogen.

In order to increase the speed of the re-oxidation it is expedient to add to the electrolyte nonionic, nonfoaming wetting agents. Especially suitable are butyl diglycol (butyl ether of diethylene glycol) and particularly dibutyl diglycol (dibutyl ether of diethylene glycol). The wetting agent is added to the electrolyte in an amount of up to about 1 gram per liter.

Air can be used as oxidizing gas.

The re-oxidation can be considerably accelerated when oxygen is used as oxidizing gas. With the use of oxygen as oxidizing gas an addition of about 1 g./l. of dibutyl diglycol to the electrolyte, current densities are obtained as reached in a conventional mode of operation while simultaneously current efiiciencies of over 90% are possible with about one-half of the hitherto required cell voltage.

The oxidizing gas is introduced into the catholyte in an amount such that at least about 40% of the metal ions present are in the higher valence stage and that the water formed in the oxidation is blown out of the cell with the gas current. By maintaining the concentration of the hydrochloric acid approximately at that of the azeotropic mixture, a simultaneous discharge of hydrogen chloride can be largely or wholly suppressed.

Especially favorable results are obtained by the process according to the invention when the concentration of hydrochloric acid in the electrolyte is maintained at 22 to 23% by repenishing hydrogen chloride, about 1 g./l. of dibutyl diglycol and 1 to 2 mols/liter of copper ions are added to the electrolyte and oxygen is introduced into the cathode space to re-oxidize the copper (I) formed at the acthode in such an amount that the gas in fine distribution bubbles closely along the cathode surface.

4 EXAMPLE 1 In Table 1 are compared the results of an electrolysis carried out in conventional manner (test 1) and according to the invention using oxygen and dibutyl digylcol 5 (test 2).

TABLE 1 Cliloriiie/ Addition of Current current dibutyl didensity, Voltage, Teinpcra- Oxygen, efllcieiicy, Composition of electrolyte glycol, g./l. A./dm. v. ture, C. l./li. percent 1 Anolyte HCl of 237 strength Catholyte HCl of strength 22 85 80 97 2 Anolyte HCl of 22% strength plus 1.5 moIs/l. Cu 1 Catholyte HCl of 23% strength plus 0.7 mol/l. l 22 0.06 80 92.5

Ca and 0.3 mol/l. Cu 1 Maximum. Nora-Duration of test 100 hours.

In carrying out the process of the invention the elec- 20 EXAMPLE 2 trode spaces can be closed for the liquid and only provided with installations for the supply of hydrogen chloride and oxygen or air and for the removal of chlorine, air or oxygen and steam.

Table 2 indicates the results of a test carried out under the conditions of test 2 of Example 1 but without the use of a wetting agent.

TABLE 2 Chlorine Current current density, Voltage, Tempera- Oxygen, efficiency, Composition of electrolyte A./dm. v. ture, C. l./h percent Anolyte HCl of 22% strength plus 1.5 mols/l Cu Cetholyte HCl of 23% strength plus 0.7 mol Cu and 0.3 l 8 0.77 80 20 85 mol Cu 1 Maximum. Nora-Duration of test 10 hours.

e If it IS necessary to dissipate the heat, the lectrolyte EXAMPLE 3 can be circulated by means of a pump in the cathode space or in the anode space or in both electrode spaces or it can be cycled through external coolers.

as oxidizing gas.

TABLE 3 Chlorine/ Addition of Current current dibutyl didensity, Voltage, Tempera- Air, efficiency, Composition of electrolyte glycol, g./l. A./(l.m. v. ture, C. l./h. percent 1 Aiolyte HCl 0t 22% strength plus 1.5 mols/l.

2+ 1 Catholyte HCl of 23% strength plus 0.7 mol/l. 11 81 25 85 Cu and 0.3 mol/l. Cu 1 2 Same as No. 1 3 0.70 10 ca 60 1 Maximum. N o'rE.-Duration of test 9 hours.

The following examples serve to illustrate the invention 55 EXAMPLE 4 but they are not intended to limit it thereto. To carry out the examples a test cell was used having an electrode surface of 2 dm. the electrode distance being 2 millimeters. Between the electrodes a diaphragm was placed. Hydrogen Table 4 shows the results of a test carried out under the conditions as set forth in Example 1, test 2, with the exception that butyl diglycol was used as wetting agent.

TABLE 4 Chlorine/ Addition of Current current dibutyl didensity, Voltage, Tempera- Oxygen, efiiciency, Composition of electrolyte glycol, g./l. AJdm. v. ture, C. l./h. percent Anolyte HCl of 22% strength plus 1.5 rnols/l. Cu 1 Catholyte HCl of 23% strength plus 0.7 mol/l. Cu 1 17 0. 89 40 84 and 0.3 mol/l. Ca l Maxiumm. No'rE.Du.ration of test 8 hours.

chloride was blown into the anol te throu h a frit at the Y g EXAMPLE 5 lower edge of the anode and air or oxygen was blown into the catholyte through a frit at the lower edge of the cathode,

Table 5 shows the results of a test in which iron chloride 7 was used intsead of a copper salt.

TABLE Chlorine/ Addition of Current current dibutyl didensity, Voltage, Tempera- Oxygen, efiiciency, Composition of electrolyte glycol, g./1. A./d m. v. ture, C 1./h. percent 1 Argolgte H01 of 24.8% strength plus 1.2 mols/l. 1]

8 2 Catholyte 1101 of 19.8% strength p 0.53 11 95 74 2 mol/l. Fe and 0.47 mol/l. Fe 1 1 Maximum. Norm-Duration of test 8 hours.

The examples reveal that especially with the use of copper salts, oxygen and dibutyl diglycol the current efficiency is aobve 90% while the current density obtained equals that of the conventional mode of operation. Without the addition of a witting agent the current density and therewith the current efficiency diminish. However, a considerable reduction of voltage can be obtained so that in this case, too, high amounts of energy can be saved.

What is claimed is:

1'. In a process wherein chlorine is prepared by electrolysis of aqueous hydrochloric acid in an electrolytic cell, the anode and cathode spaces of which are separated by diaphragms, in the presence of a metal compound occurring in several oxidation states easily converted into one another and wherein an oxidizing gas is introduced into the catholyte to reoxidize cathodically reduced metal compounds, the improvement which comprises: increasing the rate of said reoxidation of metal compound with said oxidizing gas by introducing into said catholyte, a nonionic, non-foaming, wetting agent.

2. The process of claim 1 wherein the wetting agent is butyl diglycol or dibutyl diglycol.

3. The process of claim 2 wherein butyl diglycol or dibutyl diglycol is added to the catholyte in approximately one gram per liter.

References Cited JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner