LU81828A1 - PROCESS FOR PRODUCING ALKALINE METAL CARBONATES IN A MEMBRANE TANK WHERE THE MEMBRANE AND THE CATHODE ARE IN CONTIGUOUS CONFIGURATION - Google Patents

Description

D. So.332

~ '' ~ ........... ~~~ GRAND-DUCHY OF LUXEMBOURG

4) 1 8 Y 8..

of .29 ", îJ; OîtVK.S ..„. X.97.9 Minister

Title issued: from National Economy and Middle Classes ___Service of Industrial Property

/>,, mi LUXEMBOURG

C / Y ^ ilL 'σ n Invention Patent Application ---- I. Request ..La ... company known as: ...... DIÂM03D SHAMROCK CORPORATION, ..... lloo Superior (1) ... Avenue .. "...... â ..... C.LEVEL.AHD ..... Ohio, ..... United States of America" ...... represented - .by-Konslenr Jacques ..... <3rd Kuysery ..... acting as (2) .. agent ...................... .................................................. .................................................. .................................................. .................................................. ................

deposit ........ this twenty-ninth October ..... 19oo seventy-nine (3i ► to ........ 1.5 ............ ... hours, at the Ministry of National Economy and the Middle Classes, in Luxembourg: 1. this request for obtaining a patent for invention concerning: - "Process .of ..production of ... ..carbonates of alkali metals in (4) ........ a tank ..... in membrand where the msnsbrane ..... and the cathode ..... are in ..... ... contiguous configuration "............................................ .................................................. .................................................. .........................

declares, assuming responsibility for this declaration, that the inventor (s) is (are): .... 1 .., -..... Donald L ....... DERÏÏSPIRIS.,. ..... 8So4 Pipe Î7ood Court, ..... â MElîTOR, .................... (5) ......... ............ Ohio r United States of America ................................ .................................................. .................................................. .....

.... 2 .., -..... Thomas J ....... GILLIG & N, ..... III./.....1174o Jason Avenue, at COIICQRD, ..... ................. Ohio "...... United States of America 2. the delegation of power, dated. .2. October X979 .....

3. description in French ............ language ............................. of the invention in two copies; 4 ............ U ............. drawing boards, in two copies; 5. the receipt of the taxes paid to the Luxembourg Registration Office on ..... October 29 ..... 1979 .................... .................................................. .................................................. .................................................. ..........................

claims for the above patent application the priority of one (s) application (s) of (6) ...................... patent .... .......................................... filed (s / Vn (7) in the United States of America ....................

le ...... 3.Q ..... QC.tobr.e ..... 19.7.8 ........... (Ne. * ...... 9.55 .,. 7.05.) ............................................ .................................................. ............

7 ................................................. .................................................. ...................... v ....................-...... ........................................-......... .................................................. ..................................

, in the name of S. iUv'iSK'fcÆUr S ..................................... .................................................. .................................................. .................................................. ..... (9) take up residence for him / her and, if designated, for their proxy, in Luxembourg ........................ .............. „..... 35, ..... bld, ...... Rayai .............. .................................................. .................................................. .................................................. ........................................... (io> requests the delivery of '' a patent of invention for the subject described and represented in the abovementioned appendices, - with deferment of this delivery to ................... 5 ..... .................... months.

tj r \ t

... pan.d ".ta.ire. v ............ l ........ G-r. AT

II. Deposit Minutes

The above application for a patent for invention has been filed with the Ministry of National Economy and the Middle Classes, Industrial Property Service in Luxembourg, on: October 29, 1979 .. · ......... . · .. Pr. The Minister at ......... 15 ............ hours / \ of the National Economy and the Middle Classes, /; P · d ·

Cl S B Aezm__ S D. 5ο.832

r1 'CLAIM OF PRIORITY

V μλϊ.ι ^ '·' 'i' - - ~~~ 1 ^ · - nn n— Tinrr · ι mini 1 1 m ~ "mnn" iiini ι · ιι · of the patent application / ΜΜΦ & ΪΖ /], whiwwiii i »m τι -f riiri-i ~ fiifïrnTM ~ iTrniwÿiiimfni ^ iai | iiiwn« '- tr î1 m- mu fiinniii wiiiiiii pihimii mw. ^ iaiiimwn i'miu1 »—i—

ί ‘Æ eye IN THE UNITED STATES OF AMERICA

I, OCTOBER 30, 1978 * t i 1 \ \ \ \ \ • Brief submitted in support of a request for

PATENT

i at

Luxembourg

in the name of: DIAMOND SHAMROCK CORPORATION

i; for: "Process for the production of alkali metal carbonates in;‘ a membrane tank where the membrane and the cathode are in ^ contiguous configuration ".

i-> I · *!

The present invention relates, in general, to a process for the electrolytic production of a rail "" - ’·’. Λ nate of an alkali metal. More particularly, the invention -éÿS ijrjl relates to an improved process for the electrolv-tic production of a carbonate of an alkali metal using a membrane tank in which the membrane and the cathode are in a particular configuration.

It is known that one can carry out the electrolytic production of an alkali metal carbonate from alkali metal chlorides in diaphragm and membrane vessels by introducing carbon dioxide into the cathode compartment or into the catholyte recycled outside the tank, as described, for example, in United States Patent Nos. 552,895, 2,967,807, 3 · 179 · 579, 3 · 374 · ΐ64 and 4.OSO.27O . In all of these patents, with the exception of patent No. 552,895, the diaphragm or membrane separating the electrolytic tank into an anode compartment and a cathode compartment is spaced from the cathode to allow CO 2 gas or ions HCÛ ~ introduced (formed by the reaction of v CO2 with OH ions ~) to reach and react more easily with the OH- ions formed on the membrane side of the cathode, thus reducing the backward migration of OH ions in the compartment -; anodic through the separator (main cause of the low current efficiency in the electrolysis process). Specifically, in processes for producing carbonate salts in which the cathode is separated from the membrane, current efficiencies (measured on catholyte products) are obtained of at least 95 ^ and very often approaching the theoretical value. In addition, in US Patent No. 3,374.I64, it is stipulated that the process in which a membrane tank is used, is clearly improved when: ¾¾¾¾¾ this membrane is separated from the cathode: the quantity of ions · "- ·

Xa + flowing through the membrane and transformed into Na ^ O ^ ",. /, ·.

increases to SOp against b0% sfulencnt when "the ^ 'the cathode are contiguous as described in the patent des î; t nt-_. ^

United States of America n ° 552.895 · i / lî // ·: / ;?

It is known to reduce the electrolysis voltage by juxtaposing the membrane and the cathode or by laminating them as described, for example, in the patents of the United States of America Nos. 2.967-807, 3-057-794 and 4,101,395, as well as in patent applications published in the Federal Republic of Germany 2,704,213 and 2,74I,956. However, these systems have a drawback in that the OH ions formed at or near *% &&& $ & the membrane / cathode interface can escape more easily in the membrane and reach, by migration, in the anode compartment, causing thus a loss of efficiency of the treatment current as described above. Because of this characteristic and of the efficiencies close to the theoretical value which is reached when the cathode is separated from the membrane, in the prior art, a configuration has been adopted in which the cathode and the membrane are separated for the production of alkali metal carbonates in membrane tanks.

In view of this state of the art, an object of the present invention is to provide a process taking place in a membrane tank for the production of alkali metal carbonates by working at a low electrolysis voltage and at a high current efficiency, thereby achieving a more efficient process than those currently known.

These various objects and advantages, as well as others which will appear more clearly on reading the description and the claims below, are achieved by carrying out the electrolysis of an alkali metal chloride in an electrolytic cell in which the anolyte compartment ..

and the catholyte compartment are separated by a cation exchange membrane, hydraulically impermeable and permea- v "" "h rr · -. * bility si'lrct.ive which ι-st laminated with eathucie or juxt ap.v .ée '; in continuity with it, as well as by introducing carbon dioxide into the catholyte in the tank or recycled outside thereof, and this in an amount sufficient to transforming practically the entire quantity of alkali metal hydroxide forming in the cathode compartment into an alkali metal carbonate. Surprisingly, current efficiencies of 95-100 ^ are reached even when the catholyte has a high total content solid and also with a low electrolysis voltage.

The electrolytic cell which is used in the method of the invention and in which the membrane and the cathode; .

are in a contiguous configuration, can be a single cell or a group of cells combined together in a single electrolysis unit, either in series using bipolar electrodes, or in parallel using unipolar electrodes.

These tanks are generally of conventional type: they comprise an electrolyte resistant housing and they are subdivided, by a membrane, into an anolyte compartment and a catholyte compartment, the anolyte compartment comprising an inlet and an outlet for brine of alkali metal chloride, as well as an outlet for chlorine gas, while the catholyte compartment has one or more inlets for water and / or recycled catholyte, outlets for the form and * · χ catholyte · '· · >> ··: ·· hydrogen, as well as an inlet for the CC ^, preferably at the bottom or near the bottom of the tank if the CO2 must be introduced into the catholyte contained in that -this.

The membrane subdividing the tank housing into an anolyte compartment and a catholyte compartment can, in general, be any cation exchange membrane, hydraulically impermeable and becoming 'electrolytically conductive when hydrated in the conditions' ...

operating conditions of the tank, this membrane being equal to useful c-mcrit. . . '.Y. ! i for 11 electrolysis of alkali metal chloride brines.

These membranes consist of a film of a polymer: i chemically resistant to the anolyte and to the catholyte and containing hydrophilic ion-exchange groups such as sulfonic groups, carboxylic groups and / or sulfo-namido groups. It has been found that membranes formed from polymers containing sulfonic and / or carboxylic groups have good selectivity (i.e., they carry practically only alkali metal ions), as well as good low voltage characteristics for the production of sodium and potassium carbonates, while membranes containing sulfonamido groups seem to be useful for the production of sodium carbonate, however they require a slightly higher electrolysis voltage. Specifically, these polymers involved in the formation of membranes have an equivalent weight of ion exchange groups of approximately 800-1,500 and, on a dry basis, they can absorb more than 5% by weight of gelling water.

The cation of the ion exchange group (-C0oH, n -S0,, H, -ΞΟ, ΚΓ 'and the like) of the membrane will most often be the same alkali metal as that present in the chloride salt to be electrolyzed into salt carbonate. Although, at the outset, the alkali metal salt can be used in the acid form or in another form, it will be understood that the membrane will exchange practically all these cations for the cation of the salt to be electrolyzed during the operation of the tank during a relatively short period. In order to obtain maximum chemical resistance to the anolyte, it is particularly preferable to use polymers in which all the hydrogen atoms attached to the carbon atoms are replaced by fluorine atoms or polymers in which the major part /: ¾ of these hydrogen atoms are replaced by atoj; "-" of% lu .r - and the others, by chlorine atoms, while the · ". -Y ion exchange groups are attached to a * carbon atom to which is; connected at least one fluorine atom .? vS;? qj; £;

In order to reduce. the electrolysis voltage at minimum '· * · the membrane preferably has a thickness of between approximately 0.0762 and 0.254 mm, the membranes the thickness of which lies in the upper values of this range, being used for ensure better durability and better selectivity. Due to the large cross sections of electrolytic cells used on an industrial scale, when the membrane is not supported on its two faces by contiguous electrodes, it is specifically laminated with; or impregnated in an inert, electrolytically non-conductive and hydraulically permeable reinforcement member such as a woven or non-woven fabric made from asbestos fibers, glass, TEFLON and the like. In composite membranes made of a film and a fabric, it is preferable that the laminate has an uninterrupted surface of a resin film on both sides of the fabric in order to prevent leakage caused through the membrane by seepage. along the threads of the fabric. Such composite structures, as well as their manufacturing processes, are described in US Patent No. 3,770 * 567 · Alternatively, on each side of the fabric, can laminate films of the polymer constituting the membrane.

Appropriate membranes are sold by the firm "EI duPont de Nemours & Co." under the trademark "NAFIOX". The manufacture and description of appropriate membranes of the "KAFION" type and others are given, among others, in the patent. British 1.1δ4 · 321, in the patent application published in the Federal Republic of Germany 1,941,847, in the patents of the United States of America n ° 3.O4I.3I7, 3.282.875, --- 3 * 624.053 , 3.784.399, 3.849.243, 3.909.378, 4.025.405, 4.080.270>.

ϊ · ν ··.

f · · -.

4j.101.395j as well as in the patent application of El at s - Vu i s! · - "·“. · Of America No. Si / .007.

The cathode used in the electrolytic cell: of the process of the invention can be made of any conventional material which is electrically conductive and resistant to the catholyte, for example, iron, mild steel, stainless steel, nickel and the like. The cathode is perforated and gas permeable; preferably, at least 25 $ of its surface consist of openings facilitating the formation, the flow and the elimination of hydrogen gas in the compartment of> s: catholyte, as well as the circulation of carbon dioxide anhydrides and / or bicarbonate towards the cathode / membrane interface. In order to reduce the electrolysis voltage, all or part of the surface of the cathode may have a coating or a layer of a material reducing the hydrogen overvoltage of the cathode, for example, as described in the patents No. 4 »024.044 (coating of nickel and aluminum particles, applied by melt spraying and leaching), 4 * 104.133 (coating of an electrolytically deposited nickel / zinc alloy ) and 3 * 350.294 (coating of molybdenum, tungsten, cobalt, nickel or iron).

If a certain part of the surface of the cathode does not have this coating or this layer, this part will then specifically constitute the zone of the cathode which is juxtaposed with the membrane or laminated with it. Suitable cathodes can be made, for example, of an expanded fabric, a woven wire fabric or perforated plates.

Particularly preferred are cathodes having an opening area (voids) of at least about $ 50 with good gas evolution characteristics, for example, the parallel plate electrodes described in South African Patent No. 73 / S433 (and in United States Patent Application No. 303,052), the description of which is given here '.

/ ixi 'i.-xpl i citc: r more widely had types <11 clcrtrodes, Ce. *; cathodes are, particularly effective for concentrated solutions of catholytes (eg saturated solutions at S0-100a), in particular, solutions of K ^ CO ^, in which higher viscosities and other hydrodynamic effects in solution prevent the formation, flow and escape of hydrogen gas. The retained hydrogen forms an "obstruction or blanket of gas" at the cathode, thereby raising the electrolysis voltage. Since the parallel and vertical arrangement of the electrode elements reduces the backlash of the gas and hence the formation of a gas obstruction at the electrode, the electrolysis voltage is reduced. Although parallel plates are described and illustrated in the aforementioned South African patent, it is obvious that, for these preferred cathodes, it is also possible to use other elongated electrodes having other cross sections, for example circular cross sections , elliptical, triangular, diamond-shaped and square, provided that they are arranged in a practically vertical alignment and with sufficient spacing between adjacent elements to ensure good circulation of the electrolyte without hampering the flow and release of the gas in the catholyte compartment.

In the method of the invention, the cathode and the membrane are juxtaposed so that at least most of the cathode / membrane interface is in contiguous configuration. Although it is advantageous to have a contact at a value as low as 50a, the lowest voltage drops are obtained when 90 to 100a of the interface of this cathode and of this membrane are in contiguous configuration . The means for achieving this characteristic /; · are varied and well known: for example, a higher hydrostatic anolyte pressure can be used to support the

1 "'" V

- 9 - ··. · Membrane against the cathode, as described in the patent of the United States of America n ° 3 »057» 794 ï one can arrange the ner.-: Λ brane between the anode and the cathode without no free space at their 5

* V

interface $ the membrane between the cathode and the anode can be compressed with suitable elastic compression elements, as described in US Patent 3,573,437 the membrane can be formed in situ on the surface of the cathode (for example, by coating, by immersion, by spraying, by polymerization or by fusion of polymeric precursors · y ·· :.: 'suitable, solutions, dispersions, powders or fibers), as described in the patents of United States of America No. 4.Ο36.728 and 4.101.395 i or alternatively, a film membrane can be laminated with the cathode using heat and pressure, as described in the United States patent. America n ° 4.101.395 · In one or the other of these methods, one can first of all apply, to the cathode, (by spraying, by brush, by immersion and the like) an intermediate layer of a suitable inert nonconductive polymeric material or a polymer precursor (not hydrophilic and practical completely free of ion exchange groups) in order to render inactive the cathode surfaces on which the membrane is to bear and / or in order to improve the interfacial adhesion when the membrane is formed in situ on the cathode or when is laminated with it. In view of the foregoing considerations, it is evident that the expressions "juxtaposition" "contact" and "contiguity", used in the present specification and the claims below, include, unless otherwise indicated, not only configurations in which the membrane and the cathode touch each other at their interface, but also <configurations in which the membrane is formed on the cathode or is laminated therewith.

/ - 10 -

Similarly, the anode used in the electrolytic tank w · :; The tick of the process of the invention can be any conventional material, electrically conductive, electrocatalytically active and resistant to the anolyte, for example, graphite or, better still, a valve metal such as titanium, tantalum or their alloys, this material comprising, on its surface, a noble metal, an oxide of a noble metal (individually or in combination with an oxide of valve metal) or other corrosion-resistant, electrocatalytically active material. Anodes of this preferred class are dimensionally stable anodes which are well known and widely used in industry (see, for example, for example, United States Patent Nos. 3 · ^^ 7 · ^^ 3d 3.032.498, 3.84Ο.443 and 3.S46.273). Although solid anodes can be used when they are spaced from the membrane, it is nevertheless preferable to use perforated anodes the surface of which has openings with spokes. about 25% or more, for example, an expanded fabric, a woven fabric or a perforated plate, since anodes of this type have a larger electrocatalytic surface and facilitate formation, flow and elimination of chlorine gas in the anolyte compartment. Ig-wm As described above, electrodes ensuring good release of gases and the surface of which has openings at a rate of 50% or more (as described in South African patent No. 0 73 / S433 and as indicated above), may be particularly preferred when the anode is also juxtaposed and contiguous to the membrane and / or when almost saturated brines are used. i - - 11 - · '· ’

Regarding the spacing between the anode and 'y. *' The membranej this distance is ideally the minimum distance: i'1 ': maintaining a high current efficiency with respect to the formation of chlorine , while reducing tension. Usually, a minimum voltage is obtained when the anode is contiguous to the membrane or when the latter is laminated with the anode.

The process of the invention can be adopted for the production of any alkali metal carbonate from the corresponding alkali metal chloride. This is how: / :: ¾¾¾ the sodium, potassium and carbonates are prepared.

lithium respectively from sodium, potassium and lithium chlorides.

f

As in the conventional electrolysis of alkali metal halides to form chlorine, alkali metal hydroxides and hydrogen, the alkali metal chloride is charged into the anode compartment which will become the anolyte of the tank in the form of an aqueous solution which is commonly called "brine". More specifically, this brine is acidified to a pH of about 4 or less with an acid such as acidv. hydrochloric acid in order to reduce the evolution of oxygen at the anode, as well as the formation of precipitates /'·.·. insoluble in or on the membrane from salts of polyvalent cations tels such as calcium chloride or magnesium chloride, present in the brine. As a variant or in addition to the abovementioned adjustment of the pH, the harmful effect exerted by the salts of polyvalent cations can be reduced by adding, to the brine, a compound capable of forming, with the salts of polyvalent cations and at a pH greater than 5.5j an insoluble gel at the anolyte / membrane interface, the gel being reversible at a pH of less than 3, as described in US Patent No. 3,793,163. By way of illustration, among the; Gel forming compounds which can be used. According to the present invention, there are, for example, phosphates,. -u ·. ' orthophosphates and metaphosphates of alkali metals (of V; /: preferably, of the same alkali metal as that of charged brine) or the free acid form of these phosphates.

Specifically, the brine is charged to the saturation state or to a state close to saturation in order to bring the concentration of the anolyte to its maximum and, consequently, reduce the tension that the tank requires. Likewise, the speed at which the brine is loaded, as well as the intensity of the tank current influence the concentration of the anolyte.

Faster brine loading increases the solids concentration of the anolyte while higher current intensities, on the other hand, deplete the solids content of the anolyte more quickly. Ideally, these three interrelated parameters are chosen and adjusted so that the anolyte has a solids concentration of about 75 ^ or more saturation in order to minimize the required voltage. Of course, anolyte concentrations of less than 75 / £ saturation are also suitable when higher voltages are tolerated in the tank.

The electrolyte is charged in the compartment. . ·.

cathodic at the start of the process to obtain an initial catholyte. Specifically, this electrolyte will contain the same alkali metal as that of the brine and it will consist of a carbonate salt in order to facilitate rapid obtaining of the state of equilibrium. After start-up, the catholyte is continuously renewed during electrolysis by the alkali metal ion of the charged brine migrating through the membrane, likewise, the solids content of the catholyte is adjusted to the concentration: - desired tration by adding water to this catholyte. If it is desired to minimize the energy required for drying the carbonate salt, the concentration of the catholyte will be maintained. . -.

at or near the point of saturation of the carbonate salt, for example,. £ · = - 'at $ 75-100 from the saturation concentration. On the other hand, if lower electrolysis voltages are an essential characteristic, lower concentrations of catholyte will be used, the optimal concentration being determined by the type of cell and cathode, the type of carbonate salt and the temperature of the catholyte. For example, when using a small mesh 316 stainless steel cathode (with diamond-shaped openings of 0.35 x 12.7 mm) to produce {K9C0 at 0.31 A / cm2, the voltage drops to 4 , 77 volts at 640 g / liter at 4.2 volts at 49Ο g / liter, i.e. a voltage drop of 0.375 volts by reduction of 100 g / liter in the solids content of the catholyte. When using a cathode made up of parallel nickel plates such as that described in the examples below and having better characteristics with regard to the evolution of gases, it is observed that this voltage drops from 3.95 volts to 640 g / 1 to 3.7 volts at 490 g / 1, ie a voltage drop of 0.167 volts by reduction of 100 g / l of the solids content of the catholyte.

In the process of the invention, carbon dioxide gas is introduced into the catholyte so that this carbon dioxide and / or the bicarbonate ions resulting from the reaction of this carbon dioxide with OH ions react with the hydroxide of alkali metal formed in the catholyte compartment from alkali metal ions migrating through the membrane, as well as from hydroxy ions formed at the cathode. For this purpose, carbon dioxide can be injected directly into the catholyte compartment, usually at the bottom or near the bottom of the latter, in order to ensure maximum mixing and contact time of the carbon dioxide with the catholyte and, preferably, with a sufficient exit speed to minimize the obstruction of the intake orifice (s) of carbon anhydride, while ensuring good mixing. As a variant, carbon dioxide can be passed through the recycled catholyte ::: inside a carbonation device outside '.

tank . This operating mode and specific carbonation devices are described in the patents of the United States of America n ° 552.S95, 3 · 179 · 579 and 3.S19.S13 · Preferably, the recj'-clage rate must be sufficient for the catholyte solids in the tank to contain at least $ 90 by weight of the carbonate salt. 1¾¾¾ -

The amount of carbon dioxide introduced into the catholyte must be sufficient to obtain, in this catholyte, solids containing at least about 90 $ by weight of the desired carbonate salt if one wants to achieve high current efficiencies, that is ie current efficiencies in the range of about $ 90 or more. However, it is far better to use carbon dioxide in an amount giving about $ 95 by weight or more of an alkali metal carbonate in the solids of the catholyte, since in this interval, maximum current efficiencies (vis-à-vis catholyte solids) generally exceeding $ 95. This is - ; .

why, it is far better and ideal to use the carbon dioxide in an amount which produces practically only a carbonate salt. When a lower amount is used, the carbonate formed will contain small amounts of the alkali metal hydroxide, while higher amounts will give a carbonate containing a small amount of the bicarbonate salt.

The carbon dioxide used in the process of the invention can be essentially at a purity of $ 100 or · ’···.

it can be mixed with other gases such as nitrogen and oxygen, for example, when, as the source of carbon dioxide, gases from the combustion of 1/1 are used :: 1 Λ - 15 - '/' coal, gas, oil and the like. However, we do not use Wv: · ^

Normally will not use carbon dioxide from a combustion gas if it is desirable to have high purity hydrogen gas.

In the process of the invention, the temperatures of the anolyte and the catholyte are not particularly critical with regard to obtaining a high current efficiency. However, since the voltage decreases as the temperature rises, it is preferable to adopt temperatures of about 90 ° C or more when it is desired to minimize energy consumption.

In the process of the invention, a current intensity greater than 0.155 A / cm 2 of membrane surface is specifically adopted in order to reduce the alkali metal chloride content of the solids of the catholyte to less than 400 parts per million, as described in U.S. Patent No. 4.Ο8Ο.27Ο. The intensity of current required to reach this low content of saline impurities will vary according to the thickness, the ion exchange groups and the equivalent weight of the membrane used; moreover, this current intensity can be easily determined. If higher contents; impurities consisting of chloride salts can be tolerated,:;: '· we can then adopt lower current intensities.

Specific current intensities that can be adopted are between 0.155 and 0.62 A / cm2.

More specifically, the catholyte is discharged from the cathode compartment or it is taken from the recycled catholyte at a flow rate proportional to the speed of transport of the hydrated alkali metal ions through the membrane (proportional speed to the current intensity), as well that debit from '; any water added from the outside to the catholyte in order to maintain the latter at an essentially constant volume.

- 16 - .- ·, -. ··

After being discharged, the catholyte is specifically conveyed to a tank before undergoing additional treatment such as concentration, drying or packaging for transport. At this time, any residual hydroxide or bicarbonate remaining as a by-product can be chemically removed if it is found to be inappropriate in the final product.

The alkali metal carbonates and, in particular: *. To bind, sodium and potassium carbonates are well known chemicals which exist in large quantities in the industry. Like the products of the prior art, the alkali metal carbonate obtained by the process of the invention can be sold in the form of liquors or anhydrous or hydrated solid materials which are prepared from the catholyte discharged by conventional means. known in the industry such as concentration, drying and the like.

Likewise, they can be used for analogous end uses, for example, in the manufacture of glass, alumina, paper and detergents, as precursors of other alkali metal compounds and as regenerated absorbents. saddles for carbon dioxide and hydrogen sulfide.

Although, for reasons of clarity, the above description and the examples below are given with reference to individual tanks, it is obvious that, in an operation carried out on an industrial scale, several tanks will usually be combined this type in a single electrolysis unit, either in a series arrangement using bipolar electrodes, or in a parallel configuration $ -: ¾¾¾ using unipolar electrodes. ·,

V

- 17 - EXAMPLES 1-7 / ';,

In the examples below, a tank is used:; laboratory electrolysis of cylindrical shape subdivided, by a cation exchange membrane, into an anolyte compartment and a catholyte compartment, this tank having an internal diameter of 50.5 mm. The anode and the cathode, both of slightly smaller diameter, are placed adjacent to the membrane, except in Examples 5 to 7 in which the anode is spaced from the membrane by a distance of 3.2 mm . In all the examples, the anode consists of an expanded fabric of metallic titanium comprising a coating of 2Ti09: Ru09. In Examples 1 to 4, the cathode consists of a low carbon steel plate with a thickness of 1.6 mm having holes of 6.4 mm in diameter spaced from one another with a center-to-center distance of 15.5 mm (opening area: £ 58) while, in Examples 5 to 7, a group of vertical nickel plates with a thickness of 1.6 is used mm and a width of 12.7 mm arranged vertically and perpendicular to the membrane being spaced from each other by a distance of about 4.8 mm. In all the examples, the hydrostatic pressure of the anolyte on the membrane is higher than that of the catholyte. With the exception of the test of Example 6 which is given for comparison, carbon dioxide is introduced at the rear of the cathode compartment and at the bottom of the tank in an amount theoretically sufficient to transform, into carbonate salt. , the total amount of alkali metal hydroxide formed electrolytically.

In Examples 1 to 4, the membrane used is a film deposited on a support ("TEFLON" fabric woven in square mesh "T-12" J having an average thickness' - of about 0.09 mm in the examples 1-3 and 0.13 mm in - Example 4, this film consisting of a copolymer - 18 - comprising periodic units of formula: ώ- -yy-b- - cf2-cf2- and - CF0- CF- 2 \

0-CF2-CF-0-CF2-CF2-S03H

with an equivalent weight of -SO ^ H of 1.100. For the membrane of examples 5 and 6, the same copolymer is also used and this membrane has a thickness of 0.13 nun, however that it: ': Γ ·: is reinforced with a fabric of "TEFLON" woven in meshes more open square "T-900 G".

The membrane used in Example 7 ("NAFI0N-415") consists of a 0.18 mm film comprising two layers integral with different copolymers laminated with the fabric "T-900 G". The layer laminated with the fabric has a thickness of approximately 0.1 to 5 mm and is made up of a copolymer comprising periodic patterns of formula: -cf2-cf2- and -cf2-cf-

0-CF2-CF-0-CF2-CF2-S03H

ip3 yyw with an equivalent weight of -SOjH of 1.100. The second layer is approximately 0.023 mm thick and consists of a copolymer comprising periodic patterns of the formula: -cf2-cf2- and -cf2-cf- Ô-cf2-cf-o-cf2-cooh έΓ3 with an equivalent weight of -C00H of 1.014 · In the electrolytic cell, the layer containing carboxy groups is y '';;: facing the cathode. In all the examples, a brine of KCl or NaCl '- 19 - · V' containing about 350 parts per million of H 2 P0 is charged into the anolyte compartment. and acidified ·; -: - 3 4 'at a pH of about 2 with HCl, at a rate of 0.75-1 ml / ampere-minute, while an aqueous carbonate salt is loaded into the catholyte compartment to get the initial catholyte.

During the electrolysis, water is charged to the catholyte compartment at a rate sufficient to obtain the desired concentration of the catholyte. The temperature of the anolyte is set at about 90 ° C, while the temperature of the catholyte is varied between 70 and 90 ° C depending on the current intensity used. Other process parameters and the results obtained are shown in Table 1 below.

i Ά - 20 - O C · 'b 3. ··. ·; ·:. ·. : =: C Ο 0 - '>; : · '· Ü C. ···.

fc G · '= - *' c ^ c -

> 0 4 ^ ° H

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G --I 04 in ^ LT) NO <<S-o>. * ________ ________ —i are in · -. ' - 2i - - '· According to the indications in Table 1, it can be seen that the process of the invention provides a current efficiency close to the theoretical value (100 ^) vis-à-vis the catholyte formed, this process taking place at low electrolysis voltages. In this respect, the advantage of the contiguity between the membrane and the cathode is demonstrated by making the latter move back away from the membrane in increments.

at intervals of about 1 minute in Example 1 and observing the effect on the electrolysis voltage. The Γ ·· results of this test are shown in Table 2 below which shows the significant voltage savings that are achieved by adopting the method of the invention.

TABLE 2

Effect of the spacing between the cathode and the membrane in Example 1

Distance between the cathode Electrolysis voltage

and the membrane (.nunJ

0 (start) 3.1 S

0.762 3.20 3.048 3.34: 6.35 3.57 9.652 3.77 12.7 4.01 15.748 4.23 0 (return) 3.18 Example 5 demonstrates that at higher concentrations of catholyte (50-100 ^ of saturation, in particular, for potassium carbonate which can give a total solids content of 50 ^ in the catholyte), the lowest electrolysis voltages are obtained with a cathode with a larger surface area. opening ensuring good release of gases. Finally, Example 6 which illustrates the prior art, demonstrates the current efficiencies.

poor characterizing the electrolysis process performed in a membrane cell under caustic conditions at high concentrations of catholyte, the membrane being contiguous to the cathode.

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Claims (7)

1. A process for producing an alkali metal carbonate, process in which: (a) an chloride of an alkali metal is electrolyzed in an electrolytic cell comprising an anode and a cathode arranged in an anolyte compartment and a catholyte compartment separated by a cation exchange membrane with selective permeability and impermeability: ·. ble to hydraulic fluids m, (b) carbon dioxide is introduced into the catholyte contained in the tank or in the catholvte recycled outside of the latter in order to transform practically all of the alkali metal hydroxide formed in the alkali metal carbonate catholyte compartment, and (c) removing the alkali metal carbonate from the catholyte or recycled catholyte compartment; characterized in that it comprises the complementary step consisting in: (d) juxtaposing the cathode in contiguity with the membrane. 2, A method according to claim 1, characterized in that the alkali metal chloride subjected to electrolysis is sodium chloride. . . 3 · A method according to claim 1, character ΐ 0 'risé in that the alkali metal chloride subjected to electrolysis is potassium chloride. / - 24 - ·· '; -Λ ;; '.'. 4 · A method according to claim 1, caraeté-> 0V ': risé in that the membrane is laminated with the cathode. 5. Method according to claim 1, carré-risé in that the membrane is. formed on the cathode. 6. Method according to claim 1, characterized in that the cathode consists of several elements forming electrodes, arranged practically vertically, juxtaposed in contiguity with the membrane and spaced from each other by a sufficient distance to facilitate circulation from the catholyte to the interface of the cathode and the membrane, as well as to promote the evolution of gas out of the catholyte and reduce the formation of a gas blanket at the cathode. 7. Method according to claim 6, characterized in that the alkali metal chloride subjected to electro- lysis is potassium chloride, the solids of the catholyte contain 75 to 100 ^ of potassium carbonate and the concentration of carbonate of potassium in the catholyte is equal to -60-100 $ of its saturation concentration. 1. : · ·, \ V \ \ v \ · - - V. v V