CH406166A - electrolytic cell - Google Patents

Description

Electrolytic cell The subject of the present invention is an electrolytic cell for the production of an alkali metal hydroxide, comprising an anode, a cathode and, between these, a diaphragm, preferably made of asbestos.

It is known that solutions of alkali metal chlorides can be electrolyzed in so-called diaphragm cells to produce chlorine and an alkali metal hydroxide such as sodium hydroxide. In diaphragm cells, there are normally one or more anodes arranged in front of one or more cathodes, with a brine solution between them. Cathodes normally have a perforated structure and are usually in the form of a strong wire mesh. The anodes are separated from the cathodes by a diaphragm, generally made of asbestos in the form of paper or felt, which is in contact with the face of the cathode lattice which faces the anode. Thus, the diaphragm separates the electrolytic cell into an anode compartment and a cathode compartment.

Typically, the brine solution leaving the cell contains 120 to 135 g/L of alkali metal hydroxide. Further, it is substantially saturated with Alkali Metal Chloride and is almost inevitably laden with a substantial Proportion of Chlorate which must eventually be removed for many uses, for example for the use of caustic soda or other alkali metal in the rayon industry.

The present invention provides a new cell producing a solution of sodium hydroxide or other alkali metal hydroxide in which the alkali metal chloride content is substantially below saturation. In many cases, the alkali metal hydroxide thus obtained contains little or no chlorate.

The electrolytic cell which is the subject of the invention is characterized in that its diaphragm is coated or impregnated with a polymer resulting from the polymerization of at least one ethylenically unsaturated monomer, for example a polymer maleic anhydride and divinylbenzene.

This can conveniently be accomplished by forming a mixture of maleic anhydride and divinylbenzene or similar polymerizable composition, and impregnating the asbestos with this mixture. Then, the mixture is polymerized by heat and/or ultraviolet light, thus forming in the asbestos matrix, a polymer containing RTI ID="0001.0270" WI="20" HE="4" LX="1106" LY ="1850"> generally carboxylic groups - COO - due to the presence of maleic anhydride or another similar acid. Following this polymer formation, the impregnated asbestos can then be coated with halogenated drying oil to further improve the diaphragm.

Other polymerizable olefinic acid-forming monomers such as maleic anhydride, acrylic acid, crotonic acid, methacrylic acid, acrylic esters, (methyl acrylate or corresponding ethyl, propyl esters and the like ), methyl methacrylate or homologous methacrylic esters, anhydride and chloride of acrylic acid, styrene-p-sulfonic acid or its salts or esters and other similar monomers preferably containing carboxyl groups or of sulfonic acid present in the form of free acid, salt or anhydride, can be incorporated into asbestos and copolymerized with polyvinyl aromatic compounds such as 1e divinylbenzene, divinyltoluenes, divinylnaphthalenes, & hers divinyl -phenyl-vinyls, divinyl-diphenyls, their alkyl derivatives such as dimethyldivinyl-benzenes, and similar polymerizable aromatic compounds which are polyfunctional in vinyl groups, or diallyl phthalate (or analogous diallyl esters), dimdthaerylate ethylene glycol or like substances containing at least two polymerizable -C=C- groups and which polymerize to form a polymer which is insoluble in organic solvents such as acetone.

Copolymers of polyvinyl aromatic compounds, acid-forming monomers and monovinyl aromatic compounds can also be used to impregnate asbestos. By way of example of monovinyl aromatic compounds which can be used in copolymerizations of this type, mention may be made of vinyltoluenes, vinylnaphthalenes, vinylethylbenzenes, vinylchlorobenzenes, vinylxylenes, the a-alkyl derivatives thereof, such as as α-methylvinylbenzene and other like polymerizable materials.

In addition, other resins, preferably containing free acid groups, can be used to impregnate asbestos and lead to diaphragms with improved properties. Thus, the resins of: styrene and maldic anhydride copolymerized; copolymerized styrene, maleic anhydride and linseed oil; styrene copolymerized with acrylic acid; styrene and methacrylic acid copolymeris6s; mamic anhydride, copolymerized divinylbenzene and styrene; dialkylaminostyrene and acrylic acid dialkyl-p-vinyl-benzylamine and acrylic acid; copolymerized polyvinyl- and monovinyl-aromatic compounds, including styrene and divinyltoluenes, vinylnaphthalenes and dimethyldivinylbenzene, ethylene glycol dimethacrylate, vinylethylbenzenes and divinylnaphthalenes and other similar substances containing at least least two polymerizable groups -C=C- and which polymerize to form polymers which are insoluble in organic solvents such as acetone.

Polymers and copolymers of other ethylenically unsaturated compounds may also be used as impregnants, and may include butadiene-styrene copolymers, natural rubber, butadiene-acrylonitrile copolymers, isobutylene polymers, and copolymers of isobutylene with other polymerizable compounds such as vinyl acetate, vinyl chloride, styrene, methyl methacrylate, and the like.

Although, generally speaking, polymers and copolymers which RTI ID="0002.0273" WI="7" HE="4"LX="565" LY="2408"> are substantially insoluble in water and relatively inert in acid and alkaline solutions give the asbestos diaphragm improved properties, it is preferable that the polymer impregnating the asbestos contains free acid groups. Accordingly, polymers and copolymers impregnating an asbestos diaphragm which do not contain acid groups are preferably treated by sulfonation or carboxylation to introduce these groups into the finished impregnating material. Thus, an asbestos impregnated with a polymer containing no acid group can be treated by contacting the asbestos containing the polymer with chlorosulfonic acid or a solution of sulfur trioxide in an organic solvent such as ethylene chloride or carbon tetrachloride.

For the impregnation of asbestos, the monomers are advantageously present in a suitable solvent. Solvents suitable for this use are dioxane and other ethers, chlorinated hydrocarbons such as ethylene chloride or ketones such as acetone. The solvent is suitable as long as it allows the formation of a solution containing the desired concentration of polymer and does not interfere with the polymerization.

Catalysts can be incorporated into solutions of polymerizable compounds prior to polymerization. Among the catalysts which can be used, mention may be made of benzoyl peroxide, 2-azo-bis-(isobutyronitrile), boron trifluoride, isopropyl peroxydicarbonates and other peroxide catalysts.

The polymer loading of asbestos diaphragms prepared as described above is generally 6300% by weight polymer, based on dry asbestos. Preferably, the polymer is present in a proportion of 82 to 122% by weight. Dry asbestos is asbestos dried at 100°C to constant weight. The polymer proportions given above include polymers in which halogenated oils are also present.

The term asbestos as used herein refers to the mineral varieties amphibole and serpentine and includes chrysotile, crocidolite, anthophillite, amosite and similar minerals. The preferred asbestos for forming new diaphragms are the chrysotile and crocidolite varieties.

The diaphragm can also be impregnated with a halogenated drying oil or a solidified halogenated drying oil, in particular a solidified or unsolidified oil, chloride or fluoride. By the expression RTI ID="0002.0544" WI="6" HE="4" LX="1118" LY="2077"> drying properties which are normally used for obtaining films of paint, etc., the iodine value of which is generally greater than about 130. This term therefore includes drying and semi-drying oils. . As representative examples of these oils, there may be mentioned linseed oil, tung oil, soybean oil, dehydrated castor oil and sunflower oil.

Halogenated oil can be prepared by halogenating the above oils. In current practice, an exposes the solidified oil to the action of chlorine or other halogen in the liquid phase, ie in liquid chlorine or in an organic chlorine solution. Normally, the oil is solidified by oxidation and/or polymerization by normal drying (hardening) processes, then chlorinated or otherwise halogenated. Halogenation is normally continued until the oil or solidified oil contains at least 20%, typically 35-70% by weight chlorine or a molecularly equivalent amount of chlorine. another halogen. Best results are obtained with a chlorine content of about 30 to 45 percent.

The oil may be prehalogenated before being brought into contact with an asbestos sheet, or the asbestos may be impregnated with oil, after which the oil is reacted in situ with the chlorine before or after allowing it to oil to dry out and/or solidify. In any case, precautions are necessary, since liquid chlorine tends to react with certain drying oils, notably linseed oil, with explosive violence.

In general, it is desirable to operate in the presence of a diluent, such as a halogenated hydrocarbon, in order to moderate the intensity of the reaction. Examples of halogenated hydrocarbons suitable for this purpose include methylene chloride, chloroform, carbon tetrachloride, perchlorethylene, trichlorethylene and similar halogenated hydrocarbons boiling at a lower temperature. at about 1250 C.

Following the impregnation of the asbestos with the halogenated oil, the elimination of the solvent is conveniently carried out by drying the asbestos at a temperature of 110 to 1500 C until the solvent evaporates. The halogenated oil content of the asbestos is generally 5 to 75% by weight of the impregnated asbestos.

If desired, the asbestos can be deposited on the cathode before carrying out the impregnation. A convenient method of depositing asbestos on a cathode involves immersing the cathode in an asbestos slurry and sucking the slurry through the perforations in the cathode so as to form an asbestos deposit. on the surface of the cathode.

The following examples show how an asbestos diaphragm can be impregnated for the cell which is the subject of the invention.

Example <I>1</I> Ort prepared a solution by mixing 54 g of 20-25% divinylbenzene, 19.6 g of maleic anhydride and 0.5 g of benzoyl peroxide in 16 ml of dioxane . The solution was poured onto 15.25 cm squares of North American crocidolite asbestos, type 41. The sheets were then placed RTI ID="0003.0277" WI="15" HE="4" LX="392" LY="2384"> wet asbestos between two plates of glass and heated in an oven at 70-900 C for 4 hours. One of the membranes thus prepared was placed in an electrolytic cell as a cathode diaphragm, and the cell was operated. The liquor withdrawn from the cathode compartment after 40 hours of electrolysis contained 150 g/l sodium hydroxide and only 0.08% by weight sodium chloride, based on anhydrous sodium hydroxide. Example <I>2</I> Ort prepared a solution by mixing <B>54g</B> of divinylbenzene at 20-25%, 19.6 g of maleic anhydride and 0.5 g of benzoyl Jans 16 ml of dioxane. To this solution was added 5 g of chlorinated linseed oil. The solution was poured onto 15.25 cm squares of North American crocidolite asbestos, type 41. The wetted asbestos sheets were then placed between two glass plates and heated in a oven at 70-90 C for 4 hours.

The amsi prepared membrane was placed in an electrolytic cell as a cathode diaphragm, and the cell was operated. The liquor withdrawn from the cathode compartment after 40 hours of electrolysis contained 180 g/l sodium hydroxide and less than 0.2% by weight sodium chloride, based on anhydrous sodium hydroxide. Example <I>3</I> Ort prepares a solution by dissolving 12.7 g of 26.4% divinylbenzene in 54.5 ml of toluene containing 0.123 g of benzoyl peroxide. The solution is poured over two squares of 15.25 cm sides and 1.27 mm thick of North American crocidolite type 41 asbestos. heat them in an oven at 70-9011 C for 4 h. You place the finished membranes in electrolytic cells as a cathode diaphragm and operate the cell. The liquor withdrawn from the cathode compartment after 40 hours of electrolysis contains 60 g/l of sodium hydroxide and only 0.08% by weight of sodium chloride, based on anhydrous sodium hydroxide.

Example <I>4</I> A solution is prepared by mixing <B>22g</B> of maldic anhydride, 5.8 g of divinylbenzene 50 to 60%, 1.2 g of styrene and 1.8 g of Dichlorobenzoyl peroxide in 14.5 g of dioxane. 0n add 22 ml of this solution on a sheet of 15.25 cm side and 1.2 mm thick crocidolite asbestos. 0n then places the wet sheet between two pieces of glass. The edges of the glass are sealed with cellophane tape and the glass is placed in an oven at 5511 C for 4 hours. 0n repeats this treatment again RTI ID="0003.0545" WI="10" HE="4" LX="1231" LY="2294"> four times after evaporation of the solvent and the unconverted monomer. The sheet is removed and placed in an electrolytic cell as a cathode diaphragm and the cell operated. The liquor withdrawn from the cathode compartment after 40 hours of electrolysis contains 240 g/l of sodium hydroxide and less than 0.2% by weight of sodium chloride, based on anhydrous sodium hydroxide. The expression acid group such as used in the claims below includes carboxyl, sulphonic and similar groups, as well as their anhydrides and their salts, in particular their salts of sodium, potassium or other alkali metals.

Claims (6)

CLAIM Electrolytic cell for the production of an alkali metal hydroxide, comprising an anode, a cathode and a diaphragm therebetween, characterized in that the diaphragm is coated or impregnated with a polymer resulting from the polymerization of at least one ethylenically unsaturated monomer. SUB-CLAIMS 1. Cell according to claim, characterized in that the first diaphragm is made of asbestos. 2. Cell according to sub-claim 1, characterized in that said polymer comprises free acid groups. 3. Cell according to sub-claim 1, characterized in that the diaphragm is coated with a copolymer of divinylbenzene and maleic anhydride. 4. Cell according to claim or sub-claim 2 or 3, characterized in that the cathode has perforations, and in that the first diaphragm is in contact with the cathode. 5. Cell according to sub-claim 1 or 2, characterized in that the diaphragm is additionally impregnated with a halogenated drying oil. 6. Cell according to sub-claim 4, characterized in that the diaphragm is additionally impregnated with a chlorinated drying oil.