US3520787A - Method for electrolytic production of chlorine - Google Patents

Abstract

1,128, 804. Electrolytic production of chlorine and hypochlorite. E. H. B. NYSTROEM. Nov. 5, 1965 [Dec.3, 1964; Dec.4, 1964], Nos.49300/64 and 49364/64. Heading C7B. Chlorine is produced by electrolysis of an aqueous chloride solution at a pressure not exceeding about 200 mm. Hg. Typically sodium or potassium chloride solution is continuously introduced into the cell and chlorine is continuously removed. Chlorine so removed or an aqueous solution of active chlorine (hypochlorite) produced by such electrolysis is added to water to be chlorinated at a rate depending on a determined property of water which has been previously chlorinated, the amount of chlorine or active chlorine solution being controlled by varying the electrolysis current. Typically in this case the electrolysis is carried out at a pressure of 1 to 10 mm. Hg. obtained by use of a jet pump employing as the jet a stream of the water to be chlorinated, the pump also removing electrolysis products from the cell. The electrolysis cell may be a tubular cathode surrounding an anode or may be of a rectangular vertical stack construction held together by I-bolts, the stack consisting in order upwardly of a plastic plate, an apertured packing piece, a graphite anode, a further apertured packing piece, a plastic frame, another apertured packing piece and a further plastic plate, the cathode of perforated iron (or an iron-containing compound) being disposed close to the anode in a holder depending from the second plastic plate. The cell temperature may be held below a value at which side-reactions occur by allowing boiling, this occurring at a temperature dependent on the pressure, and cooling tubes may also be provided.

Description

United States Patent 3,520,787 METHOD FOR ELECTROLYTIC PRODUCTION OF CHLORINE Ernst Holger Berti] Nystroem, 11 Vasavaegen Djursholm 2, Stockholm, Sweden No Drawing. Filed Oct. 19, 1965, Ser. No. 498,152 Claims priority, application Great Britain, Dec. 3, 1964, 49,300/ 64; Dec. v4, 1964, 49,364/64 Int. Cl. Blllk l /00; C01b 7/02 US. Cl. 204-94 4"Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of electrolysis and to an electrolytic cell for carrying out the method of the invention.

In the electrolytic production of chlorine and hypochlorite, from which also chlorates are produced, very many different constructions of cells have been made. The fundamental idea has been to keep the electrolytically produced products apart and for that reason some constructions have a membrane between the electrodes. Others have mercury as cathode so that the mercury will maintain the produced alkali metal. The reasons for these different constructions has mostly been to avoid the formed chlorine, which is to be converted to hypochlorite, being reduced to chloride by the action of the atomic hydrogen produced at the cathode.

However, when it is desired to produce hypochlorite and chlorate, cells are used in which the products from the anode and the cathode are mixed with each other. In these cells it is not possible to convert more than a small part of the salt in the electrolyte to chlorine, hypochlorite or chlorates because of undesired side-reactions. These side-reactions include reactions of the abovementioned type where reduction of the formed chlorine to chloride takes place and other reactions where, for example, oxygen is formed, e.g. from further electrolysis of the produced hypochlorite. This phenomenon is well known and is described in the literature.

In the above cells it has also been necessary to keep the temperature down since otherwise the above sidereactions are assisted. Temperature control has been carried out by using, for example, cooling tubes but this has not always proved satisfactory.

The present invention provides a method for the electrolytic production of chlorine, hypochlorite and chlorates where the above disadvantages have been substantially eliminated. The invention also includes an apparatus for carrying out the above method.

According to the present invention there is provided a method of electrolysis in which the electrolysis is carried out at reduced pressure.

More particularly there is provided a method for the electrolytic production of chlorine in which the electrolysis is carried out at lower pressure.

The reduced pressure may be chosen so that the temperature of the electrolyte is maintained at or below a certain value. The pressure is controlled so that if the temperature exceeds a desired level the electrolyte starts to boil. The evaporation then cools the electrolyte down 3,520,77 Patented July 14, 1970 to below the required temperature; until it does this the temperature is maintained at boiling.

The present invention also includes cells adapted to operate at reduced pressure. It should be emphasized that these cells may be provided with conventional cooling means if desired.

Operating the cell at low pressure means that the chlorine produced at the anode only dissolves in the electrolyte to a very small amount, the rest evaporates. This amount is still smaller where the electrolyte is kept at the boiling point and especially when the electrolyte is under boiling. This low concentration of chlorine will form a low amount of hypochlorite and chlorate, that is, very little side-reaction will take place. In addition the electrode-material will last longer as there will be only a small concentration of chlorine and consequently but little corrosion. This is especially the case regarding porous material such as graphite or ferrite.

The method, and the cells, according to the present invention may be used continuously or intermittently. In the first case the low pressure in the cell can be used to draw the electrolyte into the cell. As this happens a gas phase and a liquid phase are drawn out of the cell. It is possible either to keep these phases apart or to mix them, as desired. In the gas phase the chlorine and the hydrogen are mixed and can be separated from each other in known manner, or if hydrochloric acid is to be produced, then the mixture can be directly burned to that. Where the gas phase and the liquid phase from the cell are mixed with each other then the chlorine and the produced alkali are in stoichiometric balance with each other and form hypochlorite which can be used directly or can be converted to chlorate or perchlorate.

The present invention also concerns a method for the continuous treatment of water with chlorine and/or an aqueous solution of active chlorine, preferably in free and/or hypochlorite-bound form, the chlorine or the aqueous solution being supplied to the water in a predetermined amount. The amount of chlorine to be added to an amount of water may be assessed by determining a convenient property of a further amount of water chlorinate dsubstantially immediately before. The invention also encompasses an apparatus for carrying out the method.

Processes for the chlorination of water are known. The best known of them is the chlorination of water to obtain drinking-water. In this connection, it has been common practice to buy liquid chlorine from a chlorinealkali plant and to transport it, under pressure in pressure resistant containers, to the place of use. The gaseous chlorine is then continuously dosed from the container into a stream of raw water at a sufiicient rate to give the water a content of active chlorine sufiicient, in view of its content of bacteria and viruses, for disinfection of the water. As however this method often gives fault in the dosing it has been necessary to maintain the concentration of chlorine higher than what is necessary for attaining disinfection. This, however, results in an increased corrosion in water conduits, impaired taste of the drinking-water and risk of poisoning plants by watering them therewith. Further, a very slight excess of chlorine in water results in considerable additional costs for the water purification.

One known method of chlorinating water comprises producing the chlorine or the aqueous solution of active chlorine by electrolysis at the place of treating the water, the chlorine being added in a continuous process in an amount depending on a determined property of a bath of water previously chlorinated. However, in the above method, apparatus with a separating membrane between the two electrodes is used, and the construction and also the handling is very complicated. On the other hand, if the electrolytic cell is used without any membrane between the electrodes then the formed chlorine and hypochlorite will partly be reduced at the cathode by the atomic hydrogen formed there. It is possible to cut down this reduction but only to a degree where the low chlorine production, in comparison with the theoretical production, is still very low.

The object of the present invention is to avoid these disadvantages.

According to the present invention there is provided a method of chlorinating Water which comprises producing the chlorine gas or the aqueous solution of active chlorine by electrolysis of a chloride solution at the place of treating the water, the chlorine being added continuously in an amount depending on a determined property of 'water previously chlorinated, said amount being controlled by varying the current for electrolysis, and said electrolytic cell being kept at reduced pressure during the electrolysis.

The vacuum pumps used to evacuate the cells according to the present invention may be of any type. In many cases it may be useful to protect the electrolyte with a scrubber or wash placed between the cell and the pump. Between the cell and the scrubber or wash may then be placed one or two separators to take away the gases and/or the liquid. Very often it has been useful to have as vacuum pump a jet pump where the jet stream consists of products from the cell pumped in circulation. This is especially the case when the chlorine is to be dissolved into the departing electrolyte. In other cases it has been useful to use a liquid-piston-type rotating blower using the departing electrolyte as liquid.

In the embodiment of the invention in which the chlorine produced is used to treat water, a preferred pump for producing the low pressure in the cell is a jet pump. Such a pump may use a stream of water to be chlorinated as the jet. The gaseous contents of the cell are extracted by this jet and the resulting solution may be fed into a further amount of water to be chlorinated. The vacuum can also be created by a pump through which the liquid from the electrolytic cell as well as the gases are drawn. Then for instance a liquid-piston-type rotary blower can be used where the liquid at least partly consists of the products from the electrolytic cell. Products from the pump are mixed with the water which is to be chlorinated.

It is of importance to have a low pressure throughout the method according to the present invention. The pressure used is determined depending on the desired temperature. It may be useful to have a high degree of vacuum such as under 1 mm. Hg. Preferred pressures are however between 1 mm. Hg and mm. Hg, although pressures between 10 mm. Hg and 200 mm. Hg (e.g. 100 mm. Hg) or even higher gives advantage compared with electrolysis under ordinary pressures. In particular, the pressure from a jet pump as described above, run with water of a pressure, for example, in the range 1 to 10 kg. per cm. and preferably not under 3 kg. per cm. is suitable.

A typical procedure according to the method of the present invention is as follows:

A solution of chlorides of sodium or potassium, preferably saturated, is continuously brought into an electric cell in controlled amounts. The cell is under vacuum to keep the amount of chlorine in the salt solution low. By working with the electrolytic cell under reduced pressure only very little of chlorine produced at the anode will go in solution. The majority will be removed from the cell as a gas. This means that the amount of hypochlorite which is formed in the cell from the chlorine in solution and the alkali from the cathode is small. In the normal cell the formation of hypochlorite and the reduction of chlorine, at the anode by atomic hydrogen reduce the yield of chlorine. Again, with little chlorine in solution there is clearly very little reduction at the anode. An-

Other advantage of working with the cell at low pressure is the ability to keep the temperature down. The pressure determines the boiling point of the electrolyte and hence its maximum temperature, while heat produced by the electricity in the cell will be taken up by the evaporation of the electrolyte.

The construction of the cell for use in the method of the present invention is very simple and may utilise solid electrodes. One preferred cell is one in which the cathode is in the form of a tube around the anode.

To control the amount of salt solution which is fed into the cell an ordinary dosing pump can be used but the pressure difference between the atmosphere and the inside of the cell may be utilised. By means of the pressure difference salt solution can be sucked into the cell. The salt solution is, for example, sucked through an electric magnet valve and the amount of salt solution through the cell is controlled by the periods of time when the valve is open and closed. Under short periods of time in continuous series the valve is open, closed, open, closed etc. These periods are given to the valve by a time relay which can be of transistorized type with thyristor or of a cold cathode valve type. For instance the time of current How can be under a period of second and the period without current flow can be of one second. By changing these periods the amount of salt solution fed into the cell can be varied.

Included in this invention is also the arrangement with this magnetic valve with the time relay to control the amount of salt solution, the cell under vacuum and the pump which can be of either jet pump type or of liquidpiston-type rotary blower.

The electrolytic cell used should be of a type which will permit the gases produced to leave the cell without causing disturbance in the function of the cell. Due to the fact that the gases are under vacuum the volume of the gases is very large in comparison with cells working under ordinary pressure or under only slightly reduced pressure. This can cause a pumping action of the electrolyte and may cause the electrolyte to be present in the cell to a minor degree. The result of this is that a higher voltage must be used for the same electricity or else the effect of the cell is smaller.

To avoid the disadvantage mentioned above the cell can either for instance be equipped with means for circulation of the electrolyte or/and with electrodes which for instance because of perforation permit the gases to leave between the electrodes.

One form of the electrolytic cell of the invention may be constructed following:

A rectangular horizontally placed anode of graphite is located under a rectangular cathode of iron or an iron containing compound. The cathode is perforated or otherwise so constructed so that gases may pass through it. For instance the cathode may be made of thin bars between which there is a passage for the gases. Above the cathode is a space for the gases. The inlet and outlet of the cell are located on the short side of the rectangle.

In practice the cell is of sandwich construction: the first layer is a plastic plate, the second layer is a packing piece, the third layer is the graphite anode, the fourth layer is a packing piece, the fifth layer is a plastic frame, the sixth layer is a packing piece and the seventh layer is a plastic plate.

The layers are held together by means of I-bolts. A holder for the cathode extends through the seventh layer. This holder also forms the conductor between the cathode and the supply of electricity. The holder keeps the cathode, which is made of perforated iron, in position with its underside close to the topside of the anode. Above the cathode is space for the produced gases. The inlet to the cell, which for instance is controlled by a periodically working magnet valve, can be placed anywhere on one short side of the cell. On the opposite short side is the outlet placed just over the cathode, thus connecting with the space for the gases. The out et is connected with a water jet pump which sucks the complete amount of gases and liquid from the cell and brings it into the jet. When the cell is being used to produce chlorine for use in treating water the jet water is delivered to the water to be chlorinated.

The chlorination method and apparatus of the present invention applies not only to drinking water but to all water which it is wished to disinfect, for example, water discharged from waste water treatment plants, water in food-manufacturing plants and in other plants where for other reasons the infection of Water must be avoided. Furthermore, the invention includes processes in which materials dissolved, dispersed or otherwise present in water, for example, cellulose fibres, are chlorinated, the completion of the chlorination being possible to measure on the basis of the properties of the treated materials, but in which it is the chlorinated products that are to be recovered. In the latter case especially the chlorination can proceed to a certain degree of reaction and when that degree is reached, the chlorination may be stopped.

I claim:

1. In a method for the electrolytic production of chlorine which comprises introducing a solution of a chloride selected from the group consisting of sodium chloride and potassium chloride into an electrolytic cell, electrolysing said solution of said chloride and removing chlorine from said cell, the improvement comprising maintaining said cell at a pressure not exceeding about 200 mm. Hg during said electrolysis to thereby reduce the amount of side reactions occurring in the system and at the same time eliminate the need for auxiliary cooling.

,2. A method according to claim 1 wherein said cell is kept at a pressure of from about 5 mm. Hg to about 200 mm. Hg during said electrolysis.

{3. A method according to claim 1 wherein said cell is kept at a pressure of from about 1 mm. Hg to about 10 mm. Hg during said electrolysis.

4. A method according to claim 1 wherein said cell is kept at a pressure of less than 1 mm. Hg during said electrolysis.

References Cited UNITED STATES PATENTS 1,398,658

HOWARD s. WILLIAMS, Primary Examiner A. C. PRESCOTT, Assistant Examiner U.S. Cl. X.R. 2049 5, 149