GB1567274A

GB1567274A - Electrolytic production of hypochloites

Info

Publication number: GB1567274A
Application number: GB31593/77A
Authority: GB
Original assignee: Diamond Shamrock Corp
Current assignee: Diamond Shamrock Corp
Priority date: 1976-07-28
Filing date: 1977-07-27
Publication date: 1980-05-14
Also published as: JPS5315294A; DE2733731A1; SE7708610L

Description

(54) IMPROVEMENTS IN OR RELATING TO THE ELECTROLYTIC PRODUCTION OF HYPOCHLORITES (71) We, DIAMOND SHAMROCK CORPORATION, of 1100 Superior Avenue, Cleveland, Ohio 44114, United States of America, a corporation organised and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement::- This invention concerns an improved apparatus and process for the electrolytic production of an alkali metal hypochlorite, such as sodium hypohlorite. More particularly, the invention concerns such an improved apparatus and process, wherein a gas-depolarized cathode is utilized to avoid the generation of undesirable amounts of hydrogen gds.

Alkali metal hypochlorite is commonly produced by the electrolysis of an aqueous alkali metal chloride solution under conditions such that alkali metal hydroxide and chlorine are generated and then allowed to react to form the desired hyprochlorite. Apparatus utilized for the production of alkali metal hypochlorite therefore generally includes an electrolysis chamber which contains an aqueous alkali halide electrolyte, an anode and a cathode positioned in the chamber, a system for removing the resultant products from the electrolysis chamber and means for passing electrical current between the anode and the cathode.

In such an apparatus, one of the functions of the cathode is to produce hydroxyl ions which react with other components in the electrolyte to produce alkali meta hypochlorite. Although this type of apparatus is used commercially, nevertheless there are certain problems associated with it. For example, the cathode normally also evolves gaseous hydrogen. Obviously, in many situations, the presence of hydrogen gas is highly undesirable from a safety standpoint. In addition, and perhaps even more important from a commercial point of view, operation at voltages necessary to produce hydrogen has an adverse effect upon the hypochlorite concentration of the final product. The reason for this is that, if gaseous hydrogen is generated at the cathode, a highly concentrated product cannot be obtained, because of hypochlorite reduction at the higher voltage.

It has been discovered, in accordance with this invention, that an improved apparatus and a process for its operation can be provided which enable the electrolytic production of alkali metal hypochlorite to be effected without the generation of undesirable amounts of hydrogen gas.

In accordance with a first aspect of this invention, an apparatus for the production of an alkali metal hypochlorite is provided, which comprises walls defining a chamber for containing a body of liquid electrolyte, an inlet in the chamber walls for supplying liquid to the chamber, at least one outlet in the chamber walls for withdrawing gas from the chamber above the body of electrolyte when contained therein and for withdrawing liquid from the chamber, an anode and a cathode disposed so as to be in electrolytic communication with the electrolyte when contained in the chamber and also disposed so that the products of electrolysis are formed within the chamber and can thus react to form hypochlorite, the anode being disposed within the chamber in spaced relation to the cathode and the cathode comprising a gas-permeable part of the walls of the chamber arranged so as to prevent the electrolyte from passing through it, the cathode containing a catalyst to catalyze the reaction of water to form hydroxyl ions, whereby in operation the cathode can be contacted from outside the chamber with an oxygen-containing gas while electrolyte supplied to the chamber undergoes electrolysis effected by passing direct electrical current between the cathode and the anode and while the products of electrolysis are removed from the chamber.

The apparatus of the present invention can be utilized to electrolyze various materials.

However, for brevity, the discussion below is primarily concerned with the electrolysis of an aqueous sodium chloride solution to produce sodium hypochlorite.

In accordance with a second aspect of this invention, a process for the electrolytic production of an alkali metal hypochlorite com- prises electrolyzing an aqueous solution of an alkali metal halide in an apparatus as just defined and contacting the permeable cathode with an oxygen-containing gas while passing current between the anode and cathode, whereby oxygen gas is caused to react with water to produce hydroxyl ions.

The anode can be fashioned from any conductive material which is compatible with the electrolyte under anodic conditions. It can be of any convenient size and geometric form.

Generally, it is made from graphite or from one of the valve metals coated with precious metals or precious metal oxides and is in the shape of a square or rectangle.

The cathode is electrically-conductive and porous to gases such as oxygen. Its interstices are catalyzed with a material, e.g. platinum, which is suitable for causing oxygen to react with water at low temperatures to form hydroxyl ions. Such catalysts are well known in the art and will not be discussed herein in detail. This reaction of oxygen and water is generally described by the formula: O, + 2 H20 + 4e'= 4 OW This reaction causes the electrolysis cell or apparatus to be operable (that is, to produce hypochlorite) at a voltage below that at which hydrogen gas is produced at the cathode.

The cathode can be formed in any desirable configuration. It can be fabricated from any of a number of different materials. For example, it can be produced from porous graphitic carbon or from commercially-available porous hydrophobic material of the type used as an electrode in fuel cells. In whatever form the cathode is produced and incorporated in the apparatus of the invention, it should be provided with a means for preventing it from being flooded with electrolyte, i.e. so that while being gas-permeable the electrolyte is prevented from passing through the cathode.

The cathode can be rendered fluid-impervious, for example, by forming it from or incorporating into it a porous hydrophobic material.

Alternatively, the surface of the cathode which would be in contact with the electrolyte is provided with a diaphragm, such as asbestos, or a cation permselective membrane. Basically, in an apparatus according to the invention, a cathode is used which has one surface adapted to be in contact with an oxygen-containing gas and another surface adapted to be at least in ionic contact with the electrolyte.

In addition, it is not necessary for the cathode actually to be part of an exterior wall of the electrolysis chamber. For example, it is possible to use a hollow cathode fabricated from the typical materials herein described in putting the invention into practice. In such a case, the cathode is suspended in the electrolysis chamber. Oxygen gas is then forced into the hollow cavity during the electrolysis.

Any conventional means for passing direct electrical current between the anode and cathode can be utilized. In a preferred embodiment of the invention, the electrolysis apparatus is provided with a voltage limit switch which terminates cell operation in the event that cell voltage has increased to a point where hydrogen is generated.

Electrolyte can be transported to the cell or apparatus by any convenient means. Likewise, the products of electrolysis can be removed therefrom by any suitable technique and/or means.

The porous cathode can be contacted with an oxygen-containing depolarizing gas in a myriad of ways. The preferred apparatus includes a depolarizing chamber, having at least one inlet port and at least one outlet port, and provided in juxtaposition with the cathode. A baffle plate can be positioned in the chamber to achieve better contact of oxygen with the cathode. An oxygen-containing gas, such as air, is forced into the chamber through the inlet port. The air, and therefore the oxygen therein, impinges on the cathode during cell operation to form hydroxyl ions.

The spent air is then exhausted from the chamber by at least one suitable outlet port.

In certain situations, it is not necessary for a depolarizing chamber to be utilized. For example, in some situations it may be sufficient simply to blow air against the exposed surface of the cathode or to let air impinge on the cathode by forced or natural convection.

The herein described improved apparatus is used in a new and improved process for the production of alkali metal hypochlorite. In this improved process a suitable alkali metal halide is electrolyzed in a manner such that hydrogen gas is not generated at the cathode.

In addition, a product is obtained which is more highly concentrated than that obtained when conventional apparatus is utilized.

Other features of the invention will be apparent from the following detaiIed description and claims.

The invention can be better understood from the following drawings wherein: FIGURE 1 is a cross-sectional view of one embodiment of the invention; FIGURE 2 is a cross-sectional view of another embodiment of the invention.

In the drawings, like components are identi fied by like numerals with a prime (') suffix being utilized in connection with FIGURE 2.

With regard to FIGURE 1, the electrolysis apparatus 10 of the invention includes an electrolysis chamber 12 having an anode 14 positioned therein and a cathode 16 spaced from the anode 14. The anode 14 and the cathode 16 are so positioned that the products of electrolysis are allowed to react to form the desired hypochlorite.

The anode 14 can be fabricated from any suitable material which is not adversely affected by the electrolyte under anodic conditions.

In practice, the anode is commonly fabricated from graphite or from titanium coated with precious metals or precious metal oxides.

The cathode 16 is preferably fabricated from a catalyzed porous (gas-permeable) hydrophobic material. A non-wetting material of this type will prevent electrolyte from permeating the cathode 16. Well suited for this purpose are conventional carbon film electrodes of the type used in fuel cells. These electrodes generally are produced from a mixture of graphitic carbon, a suitable catalyst, e.g., a metal oxide, and polytetrafluoro- ethylene, which mixture is, in turn, moulded or formed into the desired configuration around a conductive substrate or support.

One commercially-available electrode material which has been used as a cathode in the practice of the invention with excellent results is type ESE fuel cell electrode stock, produced by American Cyanamid Corporation. Typically, such electrodes consist of about 95 percent graphitic carbon and 5 percent poly tetrafluoroethylene plus the desired amount of selected catalyst. The interior of the electrode is reinforced with a conductive wire mesh or screen. Such a structure is microscopically hydrophobic and will withstand a fluid head of aqueous electrolyte of 10 to 15 psig for an extensive period of time without leakage.

Catalysts for causing oxygen to react with water to form hydroxyl ions are well known in the art, and include oxides of silver, gold, platinum, cobalt, copper and others. These can be incorporated in the electrically-conductive hydrophobic cathode in a myriad of ways. For instance, they can be incorporated in the cathode during its fabrication, formed in situ or deposited after formation of the basic structure.

Direct electrical current is supplied to the anode 14 and cathode 16 by a conventional D.C. power source 18. In practice, the D.C.

electrical current is passed between the anode 14 and cathode 16 to electrolyze lli alkali metal halide present in the electrolyte 20.

When sodium chloride is being electrolyzed to produce sodium hypochlorite in he apparatus of the present invention, i.e., tbe apparatus having an oxygen depolarized cathode, the decomposition voltage is about 1.10 volts. In contrast, a normal decomposition voltage is about 2.30 volts. Therefore, to prevent the cell or apparatus from operating in a non-depolarized mode due to, e.g., a loss of oxygen supply, a voltage limit switch 22 is provided which prevents the cell or apparatus 10 from operating above a given voltage, viz., 2.30 volts, thus avoiding the chance of any undesirable hydrogen being generated.

A feeding means 24 is provided for delivering electrolyte to the cell. Likewise, a removal means 26 is provided for withdrawing hypochlorite from the apparatus. A vent 28 is provided for removing gases from the apparatus.

In order to ensure that sufficient oxygen is brought into contact with the cathode 16, the electrolysis apparatus of the invention is preferably provided with a depolarizing chamber 30 having a gas inlet port 32 and a gas outlet port 34. The depolarizing chamber 30 is in juxtaposition with the cathode 16. A baffle plate 36 is positioned in the depolarizing chamber to cause gases introduced therein to become turbulent and impinge on the cathode 16.

Apparatus of the foregoing type can be used to produce hypochlorite on site where the presence of free hydrogen gas cannot be tolerated. For example, it can be used to produce hypochlorite in the home, for swimming pools and at other places where hydrogen evolution cannot be tolerated.

In FIGURE 2 is shown another embodiment of the invention. The electrolysis cell or apparatus 10' comprises an electrolysis chamber 12' having an anode 14' positioned therein and a cathode 16' in à spaced apart relationship with the anode 14', so as to permit the products of the electrolysis to react to form hypochlorite. The cathode utilized is of the conventional type, e.g., catalyzed porous carbon. To prevent it from being flooded with electrolyte, the surface of the cathode 16' facing the electrolyte is covered with a conventional diaphragm material 40, e.g., asbestos, which is substantially impermeable to fluids but permits the passage of cations. The cell 10' is provided with a means 24' for supplying electrolyte to the chamber 12'. It is also provided with a vent 28' and a means 26' for removing electrolysis products from the chamber 12'.In addition1 it has associated therewith a power source 18' and a voltage limit switch 22'. Depolarizing oxygen is brought into contact with the cathode 16' from an exterior gas (oxygen-containing) source 42. Obviously, this source of oxygen can take many different forms. For example, it can be forced air or air supplied by normal convection.

The foregoing specific embodiments of the invention are set forth as exemplary only and are not intended to limit the scope of the invention. Obviously, various modifications in the apparatus and process of the invention may be utilized.

Example 1 An electrolysis cell or apparatus of the type shown in FIGURE 1 was employed to pro duce sodium hypochlorite. The cathode was fashioned from a catalyzed porous hydrophobic material commonly known as American Cyanamid Fuel Cell Electrode Type ESE.

The anode was made of expanded titanium metal coated with mixed TiO2-RuO2. The electrolyte feed contained 100 grams per litre of sodium chloride. The cell was operated at a current density of about 025 ASI and voltage of about 1.8 volts. The cell operation temperature was about 34"C. Air under pressure was forced into the depolarizing chamber at a high flow rate to produce hydroxyl ions in an amount sufficient to prevent the formation of hydrogen gas. The cell operated without any sign of hydrogen in the electrolyte. The final concentration of sodium hypochlorite was about 19.5 grams per litre. This contrasts with an optimum concentration of about 7 to 10 grams per litre obtained in satisfactorily operating conventional sodium hypochlorite cells.

Example 2 An electrolysis cell or apparatus of the general type shown in FIGURE 2 was utilized to produce sodium hypochlorite, the only difference in structure being that an oxygendepolarizing chamber of the type shown in FIGURE 1 was affixed thereto. The cathode was a gold-plated nickel metal screen having a platinum catalyst deposited thereon. A diaphragm about 20 mils thick was positioned on the surface of the cathode to prevent it from being in direct fluid contact with the electrolyte. The diaphragm was substantially fluid impermeable but permitted the passage of cations therethrough. The electrolyte feed contained about 300 grams per litre of sodium chloride. The cell was operated at a current density of about 0.5 amps/in2 and a voltage of about 2.20 volts.The cell operating temperature was about 30"C. Air was introduced into the oxygen-depolarizing chamber at a high flow rate to produce hydroxyl ions in an amount sufficient to prevent the formation of any hydrogen. The cell operated without any sign of hydrogen in the electrolyte. The final concentration of sodium hypochlorite was about 6.5 grams per litre.

From the above, it is clear that the invention has provided a new and improved apparatus and process for the production of alkali metal hypochlorites, particularly sodium hypochlorite. As can be seen from the foregoing description and examples, the present invention provides a process and apparatus which is considerably safer than that provided by the prior art in that essentially no hydrogen is evolved at the cathode. In addition, it is to be noted that the present invention provides a means of producing hypochlorite at a lower voltage than required when operating prior art devices. This feature presents certain obvious economical advantages such as lower power consumption.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined in the appended claims.

WHAT WE CLAIM IS: 1. An apparatus for the electrolytic produc tion of an alkali metal hypochlorite, which comprises walls defining a chamber for con taining a body of liquid electrolyte, an inlet in the chamber walls for supplying liquid to the chamber, at least one outlet in the chamber walls for withdrawing gas from the chamber above the body of electrolyte when contained therein and for withdrawing liquid from the chamber, an anode and a cathode disposed so as to be in electrolytic communication with the electrolyte when contained in the chamber and also disposed so that the products of electrolysis are formed within the chamber and can thus react to form hypochlorite, the anode being disposed within the chamber in spaced relation to the cathode and the cathode comprising a gas-permeable part of the walls of the chamber arranged so as to prevent the electrolyte from passing through it, the cathode containing a catalyst to catalyze the reaction of water to form hydroxyl ions, whereby in operation the cathode can be contacted from outside the chamber with an oxygen-containing gas while electrolyte supplied to the chamber undergoes electrolysis effected by passing direct electrical current between the cathode and the anode and while the products of electrolysis are removed from the chamber.

2. An apparatus according to claim 1, wherein a hydrophobic material is incorporated in the cathode, for preventing the passage of electrolyte therethrough.

3. An apparatus according to claim 1 or 2, wherein the cathode is catalyzed with platinum.

4. An apparatus according to claim 1, wherein a diaphragm essentially impervious to electrolyte while being permeable to cations is provided, for preventing electrolyte from passing through the cathode.

5. An apparatus according to claim 4, wherein the diaphragm is composed of asbestos.

6. An apparatus according to any preceding claim, wherein the chamber has a first outlet for withdrawing liquid from the chamber and a second outlet for withdrawing gas from the chamber above the body of electrolyte when contained therein.

7. An apparatus according to any preceding claim, wherein a depolarizing chamber, having an inlet port and an outlet port, is provided in juxtaposition with the cathode, for contacting the cathode with an oxygen-containing gas.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. duce sodium hypochlorite. The cathode was fashioned from a catalyzed porous hydrophobic material commonly known as American Cyanamid Fuel Cell Electrode Type ESE. The anode was made of expanded titanium metal coated with mixed TiO2-RuO2. The electrolyte feed contained 100 grams per litre of sodium chloride. The cell was operated at a current density of about 025 ASI and voltage of about 1.8 volts. The cell operation temperature was about 34"C. Air under pressure was forced into the depolarizing chamber at a high flow rate to produce hydroxyl ions in an amount sufficient to prevent the formation of hydrogen gas. The cell operated without any sign of hydrogen in the electrolyte. The final concentration of sodium hypochlorite was about 19.5 grams per litre. This contrasts with an optimum concentration of about 7 to 10 grams per litre obtained in satisfactorily operating conventional sodium hypochlorite cells. Example 2 An electrolysis cell or apparatus of the general type shown in FIGURE 2 was utilized to produce sodium hypochlorite, the only difference in structure being that an oxygendepolarizing chamber of the type shown in FIGURE 1 was affixed thereto. The cathode was a gold-plated nickel metal screen having a platinum catalyst deposited thereon. A diaphragm about 20 mils thick was positioned on the surface of the cathode to prevent it from being in direct fluid contact with the electrolyte. The diaphragm was substantially fluid impermeable but permitted the passage of cations therethrough. The electrolyte feed contained about 300 grams per litre of sodium chloride. The cell was operated at a current density of about 0.5 amps/in2 and a voltage of about 2.20 volts.The cell operating temperature was about 30"C. Air was introduced into the oxygen-depolarizing chamber at a high flow rate to produce hydroxyl ions in an amount sufficient to prevent the formation of any hydrogen. The cell operated without any sign of hydrogen in the electrolyte. The final concentration of sodium hypochlorite was about 6.5 grams per litre. From the above, it is clear that the invention has provided a new and improved apparatus and process for the production of alkali metal hypochlorites, particularly sodium hypochlorite. As can be seen from the foregoing description and examples, the present invention provides a process and apparatus which is considerably safer than that provided by the prior art in that essentially no hydrogen is evolved at the cathode. In addition, it is to be noted that the present invention provides a means of producing hypochlorite at a lower voltage than required when operating prior art devices. This feature presents certain obvious economical advantages such as lower power consumption. While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined in the appended claims. WHAT WE CLAIM IS:

1. An apparatus for the electrolytic produc tion of an alkali metal hypochlorite, which comprises walls defining a chamber for con taining a body of liquid electrolyte, an inlet in the chamber walls for supplying liquid to the chamber, at least one outlet in the chamber walls for withdrawing gas from the chamber above the body of electrolyte when contained therein and for withdrawing liquid from the chamber, an anode and a cathode disposed so as to be in electrolytic communication with the electrolyte when contained in the chamber and also disposed so that the products of electrolysis are formed within the chamber and can thus react to form hypochlorite, the anode being disposed within the chamber in spaced relation to the cathode and the cathode comprising a gas-permeable part of the walls of the chamber arranged so as to prevent the electrolyte from passing through it, the cathode containing a catalyst to catalyze the reaction of water to form hydroxyl ions, whereby in operation the cathode can be contacted from outside the chamber with an oxygen-containing gas while electrolyte supplied to the chamber undergoes electrolysis effected by passing direct electrical current between the cathode and the anode and while the products of electrolysis are removed from the chamber.

8. An apparatus according to any preceding

claim, wherein a variable voltage limit switch is provided, for limiting the maximum voltage at which the apparatus will operate.

9. An apparatus according to claim 1, substantially as described with reference to either Figure of the accompanying drawing.

10. A process for the electrolytic production of an alkali metal hypochlorite, which comprises: a) electrolyzing an aqueous solution of an alkali metal halide in an apparatus constructed as defined in any preceding claim; and b) contacting the permeable cathode with an oxygen-containing gas while passing current between the anode and cathode, whereby oxygen gas is caused to react with water to produce hydroxyl ions.

11. A process according to claim 10, wherein the oxygen-containing gas is air.

12. A process according to claim 11, substantially as herein described with reference to either Figure of the accompanying drawing.

13. A process of production of an alkali metal hypochlorite, substantially as described with reference to the foregoing Examples.

14. An alkali metal hypochlorite, when produced in an apparatus according to any of claims 1 to 9 or by a process according to any of claims 10 to 13.