NO743912L - - Google Patents

Description

The present invention relates to - production of alkali metal hydroxides in concentrated solution by electrolysis of water gas; alkali metal halide solutions. in two different types .of electrolysis cells:,- each of which uses one or more cation-active perm^ cell-selective membranes of a special type.

Chlorine and alkali metal hydroxides are essential products which are necessary basic chemicals in all industrial societies and they are produced commercially by electrolysis of aqueous salt solutions and a significant proportion of this production takes place by means of "diaphragm cells". These cells have been improved by using

of dimensionally stable anodes, including precious metals, alloys or oxides thereof or mixtures thereof, on so-called "valve" metals

The idea of using permselective diaphragms to separate anolyte from cations during electrolysis is not new, and electrolysis cells with several compartments and which use one or more such membranes have previously been proposed. Improved membranes consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and one fluorosulfonated perfluorovinyl ether.In some

, experiments, these membranes are used between the anolyte and buffer zones

in chioral alkali cells or have been used to separate the anolyte and catholyte zones in such cells. Nevertheless, although el ek. the faith of '

Aqueous 'salt solutions' are a technologically advanced area with a lot

great commercial interest and where intense research is being carried out, and although the importance of improved production methods is obvious, prior to the present invention no improved processes have been described in which concentrated hydroxide solutions with a low chloride content could be produced with a reasonably high hydroxide stream yield.

At the same time as the present invention, a method has been developed for the simultaneous electrolytic production of concentrated and diluted aqueous hydroxide solution, where electrolysis cells with at least three compartments are described for the production of sodium hydroxide; in that the aforementioned type of cation-active permselective membranes does not separate the buffer department from the other departments. With such a method, however, no dilute sodium hydroxide solution is produced in the buffer compartment, and even though this material is valuable, it will

of the presence of a large amount of water therein often entail prohibitive

• transport costs.' The present' method allows utilization

of the diluted hydroxide solution when producing a more concentrated sodium hydroxide solution, so that even if there is no factory or facility nearby that can use the diluted hydroxide solution, so much water will be "removed" from the solution as that it can be marketed as a commercial product because it can be transported profitably over long distances.

The present invention thus relates to a method for. electrolytic production of concentrated hydroxide solutions containing more than 250 g/l sodium hydroxide or an equivalent hydroxide, and the distinctive feature of the method according to the invention is that concentrated and diluted aqueous hydroxide solutions are produced simultaneously by electrolysis of an aqueous solution containing halide ions in an electrolytic cell with at least three compartments, an anode, a cathode, at least two cation-active permselective membranes of a polymeric material selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluoroyinyl ether and a sulphostyrenated perfluorinated ethylene propylene polymer, which delimit anode and cathode side walls in a buffer compartment and "compartments between anode and cathode compartments, as these walls, with surrounding walls, delimit anode and cathode compartments, with water being added to the buffer compartment at a rate - such that a dilute hydroxide solution is produced therein at the same time as a more concentrated hydroxide solution, containing over 250 g/l sodium hydroxide - or equivalent thereof is produced in the cathode compartment, while maintaining, a high hydroxide - current yield, the dilute hydroxide solution is removed from the buffer compartment and to the before és-cathode compartment in a two-compartment electrolysis cell with anode and cathode compartments separated by means of a cation-active permselective J membrane of a polymeric material selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluoro-sulfonated perfluorovinyl ether, and a sulfostyrene-perfluorinated ethylene propylene polymer, whereby an aqueous solution containing halogen ions is electrolysed in the anode compartment of the cell, and from the catholyte compartment in the cell with two compartments, a concentrated hydroxide solution containing more than 250 g/l sodium hydrogen ksyd or equivalent thereof, so that the cattery department current yield in the combined processes is over 70%.

In preferred embodiments of the invention, they consist of perm-. selective membranes of a hydrolyzed copolymer of tetrafluoroethylene and one-fluorosulphonated perfluorovinyl ether with the formula <FS0>2<CF>2<CF>2<OCF>(<CF>3)<CF>2<OCF>=CF2,. hereinafter called PSEPVE, the polymer having an equivalent weight of approximately 900 to ; 16(50), as only two such membranes are used and the membranes are placed on networks of supporting materials such as tetrafluoroethylene, perfluorinated ethylene propylene polymer, polypropylene, asbestos, titanium, tantalum, niobium or precious metals.

The invention will be more easily understood by reference to the following description of exemplary and preferred embodiments and the attached: drawing of apparatus suitable for carrying out the process.

The figure is a schematic drawing of an electrolysis cell with three compartments for the production of alkali metal hydroxide by electrolysis of brine, connected to a cell with two compartments for increasing. the hydroxide concentration in the diluted hydroxide solution that is removed from the buffer compartment. in there first cell. The cells include membranes of it. described preferred hydrolyzed copolymer

In the drawing--the points of supply and withdrawal of typical and preferred reaction components and products are illustrated. Although.. the making of; sodium hydroxide solutions are illustrated, other halide-forming cations can also be used, and in some cases caribromine can completely or partially replace chlorine in the halide.

In the figure, the electrolysis cell 11 comprises an outer wall 13, anode 15, cathode 17 and conductive devices 19 and 21 to connect the anode and cathode -to respectively. sources.for positive and negative electric potential. Within the cell walls-parts per.msel lek ti ve membranes 23 and. 25 the volume in the anode and anolyte compartment 27, the cathode or cathode compartment...29 and the buffer compartment 31. An aqueous solution of alkali metal halide, preferably acidic, is supplied to the anolyte compartment through the line 33 from the saturation device 35. During the electrolysis, chlorine gas is removed from the top of the anode compartment through line 37 and hydrogen gas is similarly removed from the top of the cathode compartment through line 39. More concentrated hydroxide solution is extracted from the cathode compartment 29 through line 41, while the corresponding solution, with a lower concentration, is extracted from the buffer compartment through the line 43 and is taken to the cathode compartment 45' in the cell 47

with two departments. Water can be added to the buffer compartment 31 of the three-compartment cell 11 through line 49 to maintain the desired concentration of hydroxide in this compartment to maintain a high current yield, cathode current yield, cathode division current yield, hydroxide current yield, or sodium ioH current yield ( all these designations can be used interchangeably) by limiting the migration

of hydroxide ions. to anolyte g through the membrane 23.. ; Solid sodium chloride or another source of chloride ions can be supplied to the saturation device 35 through line 51 to raise the chloride concentration in the supply to the cells. The anolytes can be recycled back to the saturation device for supplying salt to maintain the desired concentration of this in the anolyte. A control valve 53 controls the flow of refreshed electrolyte to the anolyte compartment 27 and 55 through the lines 33' and 75.

In the cell 47, the anode 57 is connected to a source for positive

electric potential across conductor 59 and cathode 61 is equivalent

■connected with eri corresponding conductor 63. The cation-active permselective membrane 65 separates the departments 45 dg.55. Low concentration sodium hydroxide solution from buffer compartment 31 passes through conduit 67 to cathode compartment 45 to add its hydroxyl ion content to the contents produced in the two compartment cell by electrolysis of the contents. A sodium hydroxide solution with a high concentration is thus produced in the cathode compartment 45 and this is extracted through the conduit 69. Chlorine and hydrogen are extracted from the respective the anode and cathode compartments through wires 71 and 73.

The highly concentrated sodium hydroxide solution produced can be: sold, used • for chemical reactions, e.g. for the production of chlorate, or can be evaporated to even higher concentrations. <. •Chlorine can be converted with the hydroxide to form hypochlorite or , chlorate or.kan. is reacted with hydrogen to produce hydrochloric acid... The product is otherwise salable, usually in a liquid state, as . any oxygen present is usually removed. The chlorine, the hydrogen - and the highly concentrated hydroxide solution from the cells with resp. three departments and two departments can be kept separate or can be 'combined. - In preferred operation, anolyte is recycled back to the saturation device • because if the chlorate concentration in it becomes too high, due to the reaction between the chlorine and the hydroxide in the part in the cell with two compartments, some of .the anolyte is sent to a separator or crystallizer for removal: of the chlorate, preferably in crystalline form, or it can be supplied as a chlorate cell.

electrolyte...

By it. process described above, highly concentrated chloride-free hydroxide solutions can be produced with a high hydroxide current yield, e.g. above 80 or 90%, in a cell with three compartments, due to the intermediate buffer, which reduces the migration of hydroxyl ions to the anolyte. The produced chlorine will thus contain less oxygen and the electrical energy is used more for the production of hydroxide than in cells with only two compartments. In cells with two compartments, hydroxide flow yields of 50 or 60% (or more) can only be achieved, but there is no dilute hydroxide by-product, since all hydroxide solution is produced in form of a salt-free highly concentrated hydroxide solution." The overall effect of the process is generally over. '70% and often'over 75%, which is'a startling increase over "the 50 or 60% power yields that can be achieved by the application of only the two-compartment cell for the production of strong ' hydroxide solutions..

In the drawing, the supply from the cell with three compartments is shown

a simple cell with two compartments. This is shown for easier drawing, but in . In practice, in order to balance the different currents and achieve the best properties in the product and the highest current yield, a majority of cells with three compartments are operated together, both with regard to

-supplies and products, and the products can be supplied to the majority of cells

with two. departments In practice, it has been found preferable to use from 1 to 2 cells with three departments per cell at two compartments, the ratio preferably being from -1.3' to 1.7 and most preferably about 1'. 5 ..

The highly concentrated catholyte hydroxide which is withdrawn has a concentration of over 250 g/l and preferably over 350 g/l, with a maximum of approximately 450 g/l. Preferably the concentration is 3.75 to 425 g/l, e.g. 400 g/l. The diluted hydroxide solution taken from the buffer department has a concentration of over 20 g/l and. is usual in the range 50 to 200 g/l, preferably above 75 g/l, e.g.

100 to 150 g/l and hdst approx..125 g/l. ".

The selective ion-permeable effects of cationic membranes are known from the past, but the membranes used in the present invention are. not used previously in processes of

the present' type and their .■unexpected beneficial effects et-not previously achieved or proposed. ISed use of one relatively thin membrane, preferably supported. as described herein, thus several years of operation under normal commercial conditions can be achieved without the need for removal or replacement of the membrane, while effectively preventing unwanted migration of chloride ions from the anolyte to the catholyte. • Correspondingly, it prevents hydrogen formed on the cathode side from entering the halogen formed on the anode side and prevents hydrogen from mixing with the chlorine and forming an explosive mixture. In this respect, the present membranes are better than previously known membranes because they are more impermeable to the passage of hydrogen, even in relatively thin films, than

the various other known polymeric materials.

Although in preferred embodiments of the invention a pair of the described membranes are used to form the "three compartments in

the present cells with three. departments, it is clear that a larger number of departments, e.g....four to six, comprising several buffer zones, can also be used. Similarly, when the cell divisions

in both types of cells will usually be separated by means of planar membranes with essentially rectilinear "or parallel-epipedic construction, other shapes may also be present, e.g. curved shapes, e.g. an ellipsoid and irregular surfaces, e.g. e.g. sawtooth .or walls with

a large number of outstanding points are used. In a modification of the invention, the buffer zone or zones, formed by the majority ■ of membranes, are arranged between bipolar electrodes - and not the monopolar electrodes which - are described herein. Bipolar electrodes

can also be used for the cells with two departments. The person skilled in the art will easily find out the variations in the structure that are made to adapt bipolar - instead of monopolar - electrodes, and; these should therefore not be described in detail. Of course, a majority of can

the individual cells are used in a known manner in multi-cell units.,

often- with joint leads. for importation and. products and arranged in uniform constructions. These constructions will also be well known to the person skilled in the art and need not be discussed further here.

For most satisfactory and efficient operation, the volume of the buffer delirigen or compartments is usually from 1 to 100%, preferably from 5 to 70% of the sum of the volumes of the anode and cathode compartments.

The aqueous solution containing -chloride ions is usually an aqueous solution of sodium chloride, although potassium and other soluble chlorides,' e.g. magnesium chloride, sometimes wholly or partly can be used. However, it is preferred to use the alkali metal chlorides and of these, sodium chloride is best. Sodium and potassium chlorides comprise cations which do not form insoluble salts or precipitates and which produce stable hydroxides. The concentration of sodium chloride in the brine that is supplied will usually be as high as possible, normally from 200 to 320 g/l for the sodium chloride and from 200 to 360 g/l

. for potassium chloride, with intermediate numbers for mixtures of sodium and potassium chloride in there. Electrolytes can be neutral or acidified to a pH in the range from about 1 to 6, the acidification usually

carried out with one suitable acid such as e.g. hydrochloric acid. The supply of the brine takes place to the anolyte department, usually at a concentration of 200 to 320 g/l, preferably 250 to 300 g/l. - Intra-departmental recycling of the contents in them. different departments are often desirable so that the concentrations are kept the same everywhere.

The preferred cation-permselective membranes consist of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be used, of which the monoolefin hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and preferably those of t 2 to 3 carbon atoms, such as, for example, tetrafluoroethylene or hexafluoropropylene." Where sulfonated perfluorovinyl ethers are best used, they have the formula FS02CF2CF2OCF(CF3)CF2OCF=GF2. Such a material, named per fluoro o-(2-(2-fluorosulfonylethoxy)-propyl vinyl ether/, hereafter named PSEPVE), can be modified into corresponding monomers, e.g. by modifying the inner |5erfluorosulfonylethoxy component to the corresponding propoxy component and by changing propyl to ethyl or butyl, plus by rearranging the positions for the "substitution of the sulfonyl group thereon and respectively by using isomers of perfluoro-lower alkyl groups. However, it is preferred to use PSEPVE.

The method for producing the hydrolyzed copolymer is described in example XVIt in US patent 3,282-865 and an alternative method is Canadian patent 849,670, which also honors the use of the finished membrane in fuel cells, or as mentioned therein electrochemical cells. In short, curry copolymer. is produced by reacting PSEPVE or an equivalent thereof with tetrafluoroethylene or an equivalent thereof in desired quantities in water at elevated temperatures and. press -for more than l hour, after which

the mixture is cooled. It is separated into a lower perfluoroether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular weight is undetermined, but the equivalent weight is about 900 to 1600, preferably 1100 to 1400, and the percentage of PSEPVE or similar compound is about 10 to 30%, preferably 15 to 20%, and preferably about 17%. The unhydrolysed copolymer can be compression molded at high temperature and pressure and gives sheets or membranes, which can vary in thickness from 0.02 to 0.5 mm. These are further treated to hydrolyze protruding -S02F groups to SO^H groups. e.g. by treatment with 10% sulfuric acid or by the methods in the above-mentioned patents. The presence of the -SO^H groups can be demonstrated by titration,. as. described in the Canadian patent specification.

Further details of the various treatment steps are described

■in Canadian patent document 752,427 and US patent document 3,041,317.'

Because it has been found that the hydrolysis of the copolymer is followed

of some expansion, it is preferred to place the copolymer membrane after the hydrolysis on a frame or other support which will hold it in place in the electrolysis cell. It can be clamped or glued in place and will be dimensionally stable without sagging. The membrane is preferably joined to a substrate of tetrafluoroethylene or other suitable filaments before the hydrolysis, while it is still thermoplastic, and the film of copolymer covers each filament, penetrates

in the space between the filaments and even around and behind them, as the films are somewhat diluted in the process where they cover the filaments.

The described membrane is far superior, by the present method, to all other previously proposed membrane materials. It is more stable at elevated temperatures, e.g. above 75°C. •-

It lasts for much longer periods of time in the electrolyte mixture and hydroxide product and does not become brittle when exposed to chlorine at high cell temperatures. Considering the savings in time and manufacturing costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become unduly high, unlike many other membrane materials,

hair hydroxide concentration in the cathode compartment increases to above

200 g/l hydroxide. The selectivity of the membrane and its tolerance

towards the electrolyte is not reduced in a harmful direction when

.hydroxyl concentration in the catholyte liquid increases, in contrast to other membrane materials. Viaere the hydroxide-s.stream yield is at"

electrolysis is not as noticeably reduced as with other membranes when the hydroxyl ion concentration in the catholyte increases. - These differences in the existing process make it practicable, while. previously described processes-have not achieved commercial exploitation. While the more preferred copolymers have equivalent weights of 900 to 1600, 1100 to. 1400 is preferred, some usable membranes of the mentioned type can have equivalent weights from 500 to 4000. The

medium equivalent weight polymers are preferred because they have satisfactory strength and stability, enable better selective ion exchange to take place and have lower internal resistance, all of which are important in the electrochemical cells used. : Improved versions of the above-described copolymers can be produced by chemical treatment of their surfaces, e.g. by treatment to modify the -SO^H groups thereon. E.g. the sulfonic acid group can be changed on the membrane to produce a concentration gradient or can be partially replaced with a phosphoric acid or phosphonic/re moiety. • These changes; can be carried out during the production process or after the production of the membrane. When the change - is carried out as a subsequent surface treatment of the meinbrane, the depth of the treatment will usually be from 0.001. to 0.01 mm., The hydroxide flow yields in the process where such modified versions of the mentioned membranes are used can increase from approximately 3 to "20%, most often 5 to 15%. Examples of such treatments are • described in - French, patent publication 2.152.-194 where one side of the membrane is treated with NH^- to form SC^NI-^ groups. In addition to the copolymers previously discussed, including modifications thereof, it has been found that an 'other' type of membrane material is also superior to previously known films for use in the present process. Although it appears that tetrafluoroethylene Tm (TFE) polymers which are sequentially styrenated and suffused are not usable for the production of satisfactory cation-active permselective membranes for use in the present electrolysis process, it has been determined that perfluorinated ethylene propylene polymer (FEP) which is styrened and sulfonated constitutes a usable membrane. While service lives of as much as 3 years or more (use of. the" preferred copolymers) cannot be obtained, they are sulfostyrene FEP polymers are surprisingly resistant to hardening and other in-use failure under the present process conditions. For the production of de'sulphostyrenated FEP membranes, a standard FEP, e.g. a FEP prepared by E.I. DuPont de Nemours & Co., Inc., and the styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene of a suitable concentration - in the range of about 10 to "20% is prepared, and a sheet of, for example, FEP polymer with a thickness of about 0.02 to 0.5 mm , preferably 0.05 to 0.15 mm, is dipped into the solution. After being taken up, it is subjected to irradiation treatment using a cobalt 60

.. radiation source. The irradiation can be approximately 8000 rad/hour and

a total amount of irradiation is approximately 0.9 megarad. After purification with water, the phenyl groups in the styrene part of the polymer are monosulfonated, preferably in the para position, by treatment with chlorosulfonic acid, fuming sulfuric acid or SO 3 . Chlorosulfonic acid in chloroform is preferably used, and the sulfonation is carried out within 1/2 hour. '

'Examples of usable membranes prepared by the method described above are products from RAI Research Corporation, Hauppauge, New York, designated as "18ST12S"' respectively. "16ST13S", the first being 18%' styrenated and having 2/3 of the phenyl groups monosulfonated and the latter ; is 16%, styrene and 13/16 of the phenyl groups are monosulfonated. To obtain 18% styrene, a solution of 17.5% styrene in methylene chloride is used, and to obtain 1.6% styrene, a solution is used of 16% styrene in methylene chloride.

The products obtained can be compared favorably with the previously described preferred copolymers, and give a voltage drop of approximately 0.2 volt t each in the previously used cells with a current density of 0.32 amp/cm,: the same as is achieved using the aforementioned' copolymer.

. The membrane walls are normally from 0.02 to 0.5 mm thick, preferably from' - 0.1 to 0.'5 mm and preferably 0.1 to 0.3 mm...When mounted on a support of polytetrafluoroethylene, asbestos, titanium or other suitable network:, network- the filaments or fibers usually have a thickness of 0.01 to 0.5 mm, preferably 0.05 to 0.15 mm, and correspondingly up to the thickness. of the membrane. It is often preferred that the fibers are less than half as thick as the film, but filament thicknesses greater than the film thickness can also be used successfully, e.g. 1.1 to 5 times the film thickness. The network, grid or fabric can have a percentage opening area of from 8 to 80%, preferably 10 to 70% and preferably 30 to 70%. In general, the cross-section of the filaments is circular, but other shapes: such as, for example, ellipses, squares, or rectangles can also be used. The supporting network is preferably a knot or a tissue and - although. this can be glued to the membrane, it is preferred..to melt it firmly by compression at high temperature and-high pressure before the hydrolysis of the copolymer. Then the composite membrane network structure can

clamped or otherwise fixed in place - in one. holder or support

The construction material for the cell body can be a common material such as concrete or prestressed concrete lined with mastic compound, rubber, e.g. neopre.n, polyvinylidene chloride, FEP," polyester

with chlorine-containing phthalic acid component, poly propylene,, polyvinyl chloride,

TFE or other suitable plastic material or can consist of corresponding lined containers or other construction materials.' Essentially cable-stayed constructions, if.e.g. of rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins can be used, preferably reinforced with embedded fibres, textiles or weaves of glass filaments, steel, nylon, etc.

The electrodes: i. the cell can be made of any electrically conductive material, which will resist attack by the various cell contents. In general, the cathodes are made of graphite, iron, lead dioxide on graphite or.titanium, steel or precious metals sdmff.eg. platinum, iridium, ruthenium or rhodium". Of course, when noble metals are used, these can be deposited on the surfaces of conductive substrates, e.g. copper, silver, aluminium, steel, iron. The anodes also consist of" materials or have surfaces of materials that. e.g. noble metals, noble metal'1-1 egers, noble metal oxides, noble metal oxides-mixed with valve metal oxides, e.g. ruthenium oxide plus titanium dioxide, or mixtures thereof, on a substrate which is conductive. Preferably, these surfaces are on or with a valve metal and connected to a conductive material, e.g. those described. Especially useful are platinum, platinum on titanium, platinum oxide' on titanium., mixtures of ruthenium and. platinum and their oxides on titanium and similar surfaces on other valve metals, e.g. tantalum. The conductors for these materials can be aluminium, copper, silver, steel or iron, copper being rarely preferred. A preferred dimensionally stable anode is a ruthenium oxy-titanium dioxide mixture on a titanium substrate, connected to a copper conductor.

The voltage drop from anode to cathode is usually in the range of about 2.3 to 5 volts, although sometimes they can be a little over 5 volts, e.g., up to 6 volts. Preferred are in the range 3.5 to 4.5 volts.' The current densities, although they may range from 0.08 to 0.64 amp/cnf electrode area, are preferably from 0.16 to 0.48 amp/cm and ideally about 0.32 amp/cm 2 ... The voltage ranges, are for fully aligned, electrodes and it is understood that when this alignment is not exact, eg as in laboratory units, the voltage drop can be up to 0.5 volts higher..

Among the important advantages of the present invention is the production of concentrated hydroxide solution with low chloride

content, without the diluted hydroxide solution being an end product of the method. This is nevertheless done at a relatively high current yield due to the use of the cell with three compartments and the preparation to begin with of diluted hydroxide solution in its buffer compartment. The use of the buffer compartment and the presence of dilute hydroxide solution in it reduces the "pressure" on the hydroxyl ions to penetrate into the anode compartment of the cell and thereby improves the current yield - due to less oxygen or other reactively useless products being produced - in the anolyte than ' in the case where more hydroxide was allowed to penetrate the anolyte. The concentration of the hydroxyl ions can also be reduced by adding additional water to the compartment and preparing a weaker hydroxide solution therein, e.g. with a concentration as low as 25. to 5.0 g/1.- The improved current yield must, however, be weighed against the use of a so. weak hydroxide solution- as supply to the catholyte in the cell with•two compartments...

It is desirable that the anolyte 1 in both cells should be acidic so that it can react with any hydroxyl that enters the anode compartment from the buffer zone, so that oxygen formation is prevented. pH in the range 1+6 can be used, with the range i - 5 being preferred and 2 - 4

is best. Of course, the pH of the buffer solution and catholyte is 14. The temperature in the electrolyte is kept below 105°C, preferably at

20 to 95°C, particularly at 50 to 95°C and preferably at 65 to 95°C. Electrolyte temperatures can be controlled by recycling parts

of the electrolyte and by regulating the supplied amounts to the different zones and the temperatures in these supplies. When the temperatures cannot be lowered sufficiently by recirculation or supply control, cooling or other cooling devices or liquids can also. is applied.' E.g. can supplies of f or dilution water

to the buffer compartment and diluted hydroxide solution to the catholyte compartment in the cell by two compartments and possibly used 'i-o^ir;:.v.l-..."-c-^-

recirculating anolyte is cooled to about 10 to 20°C, preferably to about 10°C prior to introduction into the compartment and recirculating electrolyte which moves mainly "intra-compartmentally" may also be cooled. Alternatively, the cooling can take place only by exposing the fluids to the ambient conditions before they enter the cells or re-enter the cells.

The greatly improved current yields in the present process are largely due to the 90 - 97% chlorine current yield and over 80%, often "85% hydroxide current yield", which can be achieved

in the cell with three compartments on .due to the application of the buffer compartment. It has been found that the -hydroxide current yield (Faraday) decreases as the hydroxide concentration in the buffer outflow increases,

■ as the ratio is essentially a straight-line function of the concentration from 90% at 73 g/l. to 82% at 150 g/l - and then drops more strongly - to 72% at 180 g/l.

The highly concentrated hydroxide solution prepared is free of chloride or is essentially free of this, often containing as little as 0.01 to 16 g/l and usually about 1 g/l, with hydroxide concentration in the range of 250 to 400 or 250 to 450 g/l. Such hydroxide concentrations can be further increased, by absorption and relatively little heat energy is required to increase = concentration to 50%. A possible disadvantage of the present method, namely the formation of chlorate in the anolyte in the cell .with two. departments, can be turned to an advantage when this-anoiytt outflow • is supplied to chlorate cells.

■Alternatively, the chlorate can be separated from the recirculating anolyte

and can be exploited commercially.: -.

The cells used can be used in large or small plants for -fro.. c ' cLllx production of usable hydroxide solutions during production

of 2(3 to 1000^ tons per day of chlorine or equivalent and in all cases the power yields obtained can be such that the process is economically advantageous. However, it is strongly preferred that the plant should be located... near and used in connection with a cellulose bleaching plant so that chlorine and chlorate, if some of the latter salt is formed, can be used together with the concentrated hydroxide solution in bleaching of cellulose pulp or in the manufacture

of bleaching agents.<,>f .e.g. chlorine dioxide. - Of course, the produced chlorine and hydroxide solution can also be used or sold in another way. The following examples illustrate the invention and, unless otherwise specifically stated, all parts are by weight and all temperatures are in °C.

EXAMPLE 1.

The cells with three compartments and with two compartments as illustrated are used, with the changes described, for the production of chlorine, hydrogen and concentrated sodium hydroxide solution from an aqueous sodium chloride solution. As illustrated, it is used for simplification; manufacturing a cell with three compartments and a simple cell with two compartments, but in current practice the ratio between cells with three compartments and cells with two compartments can advantageously be approximately 1.5, as the supplies to the different cells and the outlet from these often occur through wires common to a number of cells. The cell walls are made of asbestos-filled polypropylene or polypropylene or may consist of steel lined with un-softened polyvinyl chloride and in some cases with polypropylene. Polyesters, such as polyesters with a chlorine-containing phthalic acid component, e.g. "hetron" manufactured by Hooker Chemical Corp., can also be used as a wall lining. Rubber or other synthetic plastic gaskets can serve as seals between the cell walls, lids and other parts. The electrodes are in contact with membranes that separate the buffer compartment from the electrode compartments in the cell with three compartments and these membranes are cation-active permselective membranes, produced by E.I. DuPont de Nemours & Company, Inc. and sold as their "XR" type membranes. The membranes are

about. 0.2 mm thick and is united with a substrate or supporting network of polytetrafluoroethylene ("Teflon") filaments of approximately 0.1 mm diameter, woven into a textile having a percentage opening area of approximately 22%.- The filaments were originally plane, and was melted onto the mesh or textile of "teflon" by compression under high temperature and high pressure, whereby some parts of the membrane actually floated around; the filaments during the melting process and thus attached to the textile, without the membrane becoming thicker between

the textile filaments.

Material et.in the XR type permselective membrane is a hydrolyzed copolymer of a perfluorinated hydrocarbon and one fluorosulfonated perfluorovinyl ether. The copolymer consists of tetrafluoroethylene and FS02CF2CF2OCF(CF3)CF2OCF=CF'2 and has an equivalent weight in the range of 900 to 1600, preferably about 1.2500. The electrodes are in contact with the buffer membranes, as the more "flat" side of the membranes, are-facing the electrodes with which.they are in contact. • In some experiments, a 0.01 to 5 mm distance between the electrodes and the membranes was used "and satisfactory results were also obtained, but. the present device' with the absence of a distance is preferred.. The same membranes are used both in the cells with three compartments and with • two departments.

In the cell with three compartments, the buffer compartment is approximately 6 mm wide and the electrodes are placed against the membranes, so that the distance between the electrodes is approximately the same... "buffer departmentcathode department" is approximately .10:1:10' and the anode and cathode department in the cell, with two ■ departments having approximately the same volume," as the permselective membrane is placed centrally and the electrode area (from anode to cathode) is ' one to ten millimeters.

The anodes used consist of a mixture of ruthenium and titanium oxides on titanium connected to a current source by means of titanium-coated copper elements. 'The titanium substrate for the anode is titanium mesh approximately 1 mm in diameter and with approximately 50%. open area ,■ coated with a 1:3 mixture of ruthenium oxide and tin oxide. about-/.1 rami yr, thick. The cathodes consist of wire mesh - of mild steel, approximately 1 nirn equivalent diameter, with approximately 35% open area, and are connected to negative current sources or mains by means of a copper conductor...

The anode compartments in the cells are filled. with an almost saturated salt solution. or brine of sodium chloride at about 25% concentration and the cathode and buffer compartments are filled with water, initially containing a small amount of salt or brine to improve conductivity. The current is switched on and the chlorine and hydrogen as

<.>" is formed in the cells is removed. Water is supplied to the buffer f is a compartment in the cell with three compartments to maintain the concentration of sodium

hydroxide therein at a low level and at the desired concentrations, dilute and more concentrated sodium hydroxide solution is removed from the buffer compartment or - from'cathode compartment none in the cell with':three compartments. The solution removed from the buffer compartment is supplied to the cathode compartment x the cell with two compartments. It. spent anolyte,

in the cell with three compartments is passed through a device for renewed saturation in which its sodium chloride content is increased and the

is then sent back to the anode division. In general, the sodium chloride content in the anolyte that is extracted is about 22% by weight and the concentration in the anolyte that is fed back from the device for renewed saturation is about 25%. Part of the recirculating electrolyte can be led outside the device for renewed saturation. As illustrated in the cube, a corresponding recirculation and renewed saturation of the anolyte in the cell with two compartments has been carried out, using the same device for renewed saturation and valve regulation of the flows to each of the anolyte compartments in the cells with three compartments, respectively with two departments* If desired, separate circulation systems can of course be used. As. illustrated. it t.ru£fet measures for. to take out a part of the anolyte,. especially from the cell with two departments, as desired Removal

of the anolyte can be carried out to enable crystallization of any excess chlorate content therein.

In a,, facility constructed for the manufacture of ay 100. tons. chlorine per day, comprising 20,150 kiloampere cells, of which 12 are cells with three compartments and 8 are cells with two compartments, 25% sodium chloride solution is supplied to the head compartment in the cell with three compartments. anolyte with 22% sodium chloride content is removed from there and recycled back to the anode compartment. after refreshing by dissolving "

solid salt in this solution in the device for renewed saturation.

Acidification is also provided at the same time, to a pH of 3. A current density of 0.32 amp/cm<2> is applied to the electrodes with a voltage drop of 4.2 volts. Chlorine is produced with a current yield of approximately 96% and contains almost no free oxygen. The hydroxide flow yield in the cell with three compartments is approximately 90% and it produces 36 tonnes of sodium hydroxide per day _at a concentration of 400 g/l

and 18 tonnes per day with a concentration of .125 g/l. The more concentrated hydroxide solution contains only 0.5 g/l Nad.

1-

This hydroxide solution is sent to an evaporator and increased to 50% concentration, although in some cases it can be sold directly as a eh400 g/l solution or used in such form for the treatment of cellulose. ■

In the two-compartment cell, the distance between the anode and the cathode is about 2 mm, the supply of chloride solution is the same as that of the three-compartment cell, and in addition to the supply of hydroxide solution to the cathode compartment and the feed solution to the anode compartment, sometimes a water supply is sent to the cathode compartment f or to regulate the concentration of the product from

- - - 2

this. It works at 3.6 volts and 0.32 amp/cm and the cell then produces a total of 42 tonnes of sodium hydroxide .cpr. day with a concentration ratio of 400 g/l and a sodium chloride concentration of only 0.8 g/l. It is noted that the total hydroxide production for the twenty cells is 78 tons of sodium hydroxide per day, corresponding to a hydroxide flow yield of the whole plant of1 approximately <:>78%. The cells-with compartments are operated with a hydroxide current yield of approximately 85-90% and the cells with two compartments operate at*hydroxide current yield of approximately 60%.- When the anolyte.is not added acid can the chlorine produced in the two-compartment cells is kept separate from the chlorine from the three-compartment cells because its content of oxygen or impurities is greater than if the anolyte had been acidified. However, it "contains less than 6% oxygen." When hydrochloric acid is added to the anolyte in cells with two compartments in a sufficient amount to neutralize incoming hydroxyl ions, the chlorine *from the cell- can be as pure as from a cell with three compartments.

The products obtained are used in the manufacture and bleaching of cellulose, especially in the treatment of cellulose pulp. In some cases, the hydroxide solution and chlorine react to form bleaching agents, e.g. hypochlorite and chlorate, the latter of which can then be treated for production of chlorine dioxide for cellulose bleaching. - In a modification of the described process, after a build-up of chlorate in the anolyte in the cell with two compartments (which does not mix with the anolytes from the cells with three compartments), this anolyte is supplied to a chlorate cell or cells where the chloride is converted

.to chlorate and the chlorate content constitutes a useful additional product.

In other modifications of the process, the thickness of the membranes is increased to 0.25 to 0.35 mm, where the hydroxide current yield increases, but the voltage drops are also greater. Although the membranes with a greater thickness can thus be used, it is preferred to use membranes with a thickness of approx. 0.2 mm in these reactions. When the membrane thickness is reduced to 0.1 mm, the process is satisfactory, but the hydroxide current yield is reduced somewhat.

EXAMPLE 2

The procedure in example 1 is repeated using 0.25 mm

thick membranes of:the membrane materials- which were identified as "18ST12S"'' respectively. "16ST13S",; manufactured by RAI Research Corporation, to replace the hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether. The first of the mentioned RAI products is a sulfostyrene-t FEP in which the FEP is 18% styrenated and has 2/3 of the phenyl groups monosulfonated, and the latter is 16% styrenated and has 13/16 of the phenyl groups monosulfonated. Under working conditions, these membranes will be more resistant than other available cation-active permselective membranes, but not so

good as the membranes in example 1." After such application, their properties ie.' physical appearance, uniformity, voltage drop better than for other cation-active permselective membrane materials that are available (with the exception of the hydrolyzed copolymers of perfluorinated hydrocarbons and fluorosulfonated perfluoroxrinyl ethers) but especially" in the anode compartment where the chlorine attacks the membrane, extension of the lifetime is desirable.<:> In the cathode department is■however ; .RAI diaphragms decidedly better, in operation than - comparable commercial products, with the exception of "the" mentioned of the "px" type.

In a modification of this method, the working temperature is changed to 80°C and, although the current yields are somewhat smaller, the reactions are satisfactory. Similar good results are also obtained when the surface of the cathode is changed to platinum'or'..graphite and the surface of the anode is changed to platinum or ruthenium mox<y>d (on titanium)» The concentrated hydroxide solution and the chlorine from this example are used for the preparation of sodium chlorate' and the concentrated 400 g/l hydroxide solution is used without further concentration for the absorption of wood pulp. ' ■

EXAMPLE 3 '

The experiment from example 1 is repeated using a single cell with three compartments - and a simple cell with two compartments, with 2/3 of the diluted hydroxide solution produced from the cell with

three compartments are fed to the cathode compartment of the two-compartment cell. Otherwise, the conditions achieved are the same and the results are the same, even if the production of concentrated hydroxide solution is correspondingly reduced and some diluted hydroxide solution remains

and must be used in one way or another (it is used for: binding of wood pulp). When further changes are made to the working conditions, e.g. by varying the operating temperatures, - concentrations of chloride in the supply, changing the volume ratio in the departments,

the mesh or textile substrate for the membranes, the current densities, voltages and/or anolyte pH values, within the previously described

limits, satisfactory products are obtained with total hydroxide "current yields" of more than 70%. The current yields"at. these methods are further enhanced by using a modified membrane of the hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether type, as previously described. In other experiments, the sizes of the plant were increased or decreased by adding more or fewer cells and/or changing the cell sizes... In these experiments, the produced highly concentrated hydroxide solution contained from 375 to 425 g/l of sodium hydroxide and the feed to the two-compartment cell from the three-compartment cell contained from .100 to 150 g/l sodium hydroxide.

In further variations of these examples, the continuous processes as described were changed to batch operation and even if the current yields drop, the processes can still be carried out and produce the same highly concentrated hydroxide solutions with high purity.. '.. _ ' '■

Claims (9)

1. Process for the electrolytic production of a concentrated hydroxide solution containing more than 250 g/l sodium hydroxide or equivalent hydroxide, kara k fer i s. e, r t by simultaneous production of concentrated and diluted aqueous hydroxide solution by electrolysis of an aqueous solution containing halide ions in an electrolysis cell with at least three compartments. an anode, a cathode, at least two cation-active permselective membranes of one polymeric material selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and one fluorosulfonated perfluorovinyl ether and sulfostyrene perfluorinated ethylene propylene polymer, bounding . anode- and cathode-sidewalls in one. bufije department or departments between the anode and cathode compartments, as these walls together with surrounding walls define the anode and cathode compartments, while water is added to the buffer at a rate so that a dilute hydroxide solution is produced in it at the same time as a more concentrated hydroxide solution, containing over 250 g/l sodium hydroxide or equivalent is prepared in the cathode compartment, while maintaining a high hydroxide current yield, the diluted hydroxide solution is removed from the buffer compartment and supplied to the cathode compartment in an electrolysis cell with two. waste divisions with anode and cathode divisions, waste waste by means of cation-active permselective membranes avet. polymeric material - selected from the group consisting of. a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated fluorovinyl ether, and a sulfostyrene-based perfluorinated ethylene propylene polymer, an aqueous solution containing halide ions being electrolysed in the anode compartment of this cell, and from the catholyte compartment in the two-compartment cell, concentrated hydroxide solution containing more than 250 g/l sodium hydroxide or equivalent, so that the cathode section current yield for the combined processes are over 70%. 2. Method as stated in claim 1, characterized in that the electrolysis cells are cells with three compartments or two compartments, the walls between the buffer compartment in the cell with three compartments and its anode and cathode compartments and the walls between the anode and cathode compartments in the two-compartment cell consists of a hydrolyzed copolymer of tetrafluoroethylene and a fluorine perfluorovinyl ether with the formula FS02CF2CF2OCF(CF3)CF2OCF=CF2, the copolymer having an equivalent weight of approximately 900 to ; 1600, and addition of water to buffer the compartment in the three-compartment cell takes place at such a rate that the dilute hydroxide solution which is removed from the buffer compartment and supplied to the cathode compartment in the two-compartment electrolysis cell has a concentration greater than 50 q/l.. . 3. Procedure as stated in claim 2, Its characteristics are that sodium hydroxide, chlorine and hydrogen are produced from an aqueous solution of sodium chloride, the concentrated sodium hydroxide solution being removed from the cathode compartment in the three-compartment cell and the two-compartment cell having a concentration of over 350 g /l and ate the concentration of the sodium hydroxide solution which is extracted from the buffer compartment in the three-compartment cell and fed to the cathode compartment in the two-compartment cell. compartments are from 50 to 200 g/l, the permselective membranes are about 0.2 to 0.5 ram thick, the concentration of sodium chloride in the anode compartment" in the three-compartment cell and the two-compartment cell is from about 200 to 320 g/l, pH i' the anolytes.in both cells is approximately -1:5 and the temperatures for anolytes, catholytes and buffer compartment solution are Below 105°C. 4. Method as stated in claim 3, characterized by . that the permselective membranes are mounted on networks of a material selected from the group consisting of polytetrafluoroethylene, asbestos,, perfluorinated ethiferipropyene polymer, polypropylene, titanium, tantalum, ■ niobium and precious metals, - the networks having percentage opening areas therein from approximately 8 to 80%,. the temperatures, for anolyte, catholyte and buffer compartment dissolution - are in the range of 20 to 95%, the surfaces of the cathodes are of a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel, the surfaces of the anodes consisting of a material selected from the group consisting of noble metals, noble metal- ■ alloys, noble metal oxides, mixtures of noble metal oxides with vantil metal oxides, or mixtures thereof, on a valve metal, the voltage being from 2.3 to 6 volts bg the current density is from about 0.08 to. 0..64 amp/cm electroae surface. 5o • Procedure as stated in claim 4, characterized by the fact that the networks are meshes or weaves of polytetrafluoroethylene filaments with thickness. 0.1 to 0.3 mm, the membrane walls are from 0.1 to 0.3 mm thick, the thickness of the polytetrafluoroethylene filaments is less than or equal to the membrane walls, equivalent weights) of the copolymer are from approximately 1100 to 1400, the cathodes are of steel, the anodes are of ruthenium oxide on titanium, the aqueous sodium chloride electrolytes have a ]:^;-:n ,.:t concentration of about 250. to 320 g/l,, pft. in the anolyte is from 2 to 4 and the temperatures in the anblytfc, catholyte and buffer compartment solution are from 65 to 95°C. 6. Method as stated in claim 3, characterized in that the aqueous solution of sodium hydroxide which is extracted from the buffer compartment in the cell with three compartments has a concentration of approximately 100 to 150 g/l, the concentrations in the catholytes which are extracted out of the four cells with three compartments and the cells with two compartments there are approximately 375 more. 425 g/l and the hydroxide stream yield in the combined process is over 75%. 7. Method as stated in claim 5, characterized in that the aqueous solution of sodium hydroxide is then extracted from the buffer compartment in the cell with three compartments have a concentration of about 100 to 150 g/l, the concentrations of the catholytes extracted from the three compartment and two compartment cells are about .375 to 425 g/l and • the hydroxide current yield in the combined process is over 75%. 8. Procedure as stated in claim-3, characterized in that a plurality of three-compartment and two-compartment cells are used, the three-compartment cells being operated at approximately 90% hydroxide current yield or more and producing a buffer compartment solution containing approximately 125 g/1 sodium hydroxide and the two-compartment cells , is operated at approximately 60% hydroxide current yield or more, the solution which • removed from the cathode compartments in both cells contains approximately 400 g/l sodium hydroxide, as the ratio between cells with three compartments and cells with two compartments is approximately 1.5 and that the buffer compartment solution extracted from the cells with three compartments is supplied to the catholytes in the cells with two departments. 9...Procedure as stated in claim 7, characterized in that a plurality of cells with three compartments and with two compartments are used, the cells with three compartments being operated at approximately 90% hydroxide current yield or more and ; f rems to ler buf'f erapart ing solution containing approx .125 g/l sodium hydroxide and that the cells with two compartments are operated at approximately 60% hydroxide current yield or more, the solution removed from the cathode compartments in both cells containing approximately 400 g/l sodium hydroxide, the number of cells.with three compartments in ratio to cells with two compartments is approximately 1.5 and that the buffer compartment solution extracted from the cells with three compartments is supplied to the catholytes in the cells with two compartments.