EP0571273A1 - Process for the production of alkali metal chlorate and apparatus therefor - Google Patents

Abstract

• électrolyse dans une cellule de type "chlore-soude" (1) d'une saumure de chlorure de métal alcalin pour former d'une part du chlore gazeux, et d'autre part une solution concentrée d'hydroxyde de métal alcalin,
• transfert du chlore gazeux et de la solution d'hydroxyde de métal alcalin produits, dans une colonne d'abattage (2), pour les faire réagir l'un sur l'autre, et
• récupération de la solution saline ainsi obtenue, pour son emploi comme anolyte dans la cellule à membrane (3).

The present invention relates to a process for the preparation of alkali metal chlorate by electrolysis in a membrane cell (3) of an anolyte comprising a solution of alkali metal chloride and of a catholyte comprising a solution of alkali metal hydroxide, the alkali metal chloride solution being previously purified by the following sequence of steps:

• electrolysis in a "chlorine-soda" type cell (1) of an alkali metal chloride brine to form on the one hand chlorine gas, and on the other hand a concentrated solution of alkali metal hydroxide,
• transfer of the chlorine gas and of the solution of alkali metal hydroxide produced, into a slaughter column (2), to make them react one on the other, and
• recovery of the saline solution thus obtained, for its use as an anolyte in the membrane cell (3).

The present invention also relates to a device for implementing this method and a device for purifying an alkali metal chloride brine.

Description

The present invention relates to a process for the manufacture of alkali metal chlorate by electrolysis in a membrane cell without the addition of chromium.

The preparation of alkali metal chlorate by electrolysis in a membrane cell is described in particular in patent applications FR-A-2 638 766 and FR-A-2 655 061.

Membrane cells generally consist of two compartments, one anodic, the other cathodic, separated by a membrane which allows the selective transfer of ions from one compartment to another, under the action of an electric field. .

For the known preparation of alkali metal chlorate, the anolyte consists of a chloride salt brine of said alkali metal, to which may be added, if necessary, a determined quantity of chlorate of the same alkali metal, the catholyte being for its part consists of an alkali metal hydroxide solution.

This process for the preparation of alkali metal chlorate has many advantages over the prior art which required the use of costly and dangerous additives for the environment, in particular hexavalant chromium, sodium chromate or dichromate, to limit the negative influence of the cathodic reduction of hypochlorite and / or chlorate ions.

Nevertheless, and despite this clear progress, membrane cells require the use of electrolytes which are particularly free of impurities.

Indeed, the brine of alkali metal chloride salt which feeds the anode compartment of the cell, contains small amounts of metal salts, particularly salts of alkaline earth metals, metals such as aluminum, copper, manganese or zinc, or impurities such as silica, sulfate salts, bromine or iodine that may damage or clog the membrane during electrolysis.

It is therefore necessary to purify the brine before it is introduced into the anode compartment of the cell, so as to lower the content of impurities to acceptable levels.

If the usual techniques for the purification of brines of chloride salts, by precipitation and / or absorption on resin, make it possible to lower the contents of certain impurities, in particular calcium and magnesium salts, there is no industrial process allowing to reduce the content of elements, such as silicon, aluminum or other metals to a few ppm or even a few ppb.

The present invention therefore relates to a process for the preparation of alkali metal chlorate by electrolysis in a membrane cell, of an anolyte comprising a solution of alkali metal chloride and of a catholyte comprising a solution of alkali metal hydroxide, the alkali metal chloride solution being obtained from a brine previously purified so as to remove almost all of the impurities which would risk damaging or clogging the membrane during electrolysis.

• électrolyse dans une cellule de type "chlore-soude" d'une saumure de chlorure de métal alcalin pour former d'une part du chlore gazeux, et d'autre part une solution concentrée d'hydroxyde de métal alcalin,
• transfert du chlore gazeux et de la solution d'hydroxyde de métal alcalin produits, dans une colonne d'abattage, pour les faire réagir l'un sur l'autre, et
• récupération de la solution saline ainsi obtenue, pour son emploi comme anolyte dans la cellule à membrane.

According to the present invention, the purification of the brine is obtained by the following sequence of steps:

• electrolysis in a "chlorine-soda" type cell of an alkali metal chloride brine to form on the one hand chlorine gas, and on the other hand a concentrated solution of alkali metal hydroxide,
• transfer of the chlorine gas and of the solution of alkali metal hydroxide produced, into a slaughter column, to make them react one on the other, and
• recovery of the saline solution thus obtained, for its use as an anolyte in the membrane cell.

The cell of the "chlor-sodium hydroxide" type used in the process according to the invention is preferably a membrane cell.

This type of cell is well known in the prior art, since it is described in particular in US-A-4,285,795 or in "Ullmann's Encyclopedia of Industrial Chemistry" (5th ed., Vol. A6, p. 399-481) .

The membranes are synthetic ion-exchange membranes, preferably made of fluorocarbon polymers capable of withstanding drastic operating conditions, in particular strong alkaline solutions, at high temperatures.

These flurocarbon polymers are associated with carboxylic and / or sulfonic acid functions, preferably in the form of an alkali metal salt. Preferably, the fluorocarbon polymers are polytetrafluoroethylenes (PTFE). The membranes used are obtained by extrusion or rolling of the polymer, and can be reinforced with woven pieces of PTFE fibers.

The membranes, developed since 1970, have a selectivity at least equal to that of the diaphragms but are much more sensitive to degradation and clogging due to impurities present in the electrolyte.

In order to preserve the lifetime of the membranes, in the process according to the invention, the alkali metal chloride brine is generally prepurified by conventional methods of precipitation and / or adsorption on resins.

The alkali metal brine used as an anolyte in the "chlor-sodium hydroxide" type cell preferably comprises between 170 and 315 g / l of alkali metal chloride, preferably between 290 and 310 g / l.

Furthermore, this brine is preferably used at a pH of between 2 and 7, advantageously between 2.5 and 4.5.

avec Me représentant un métal alcalin.The overall reaction carried out in the "chlorine-soda" type electrolysis cell can be summarized by the following equation A: $$\mathbf{\text{(AT)}}\text{2 MeCl + 2 H₂O → 2 MeOH + Cl₂ + H₂}$$

with Me representing an alkali metal.

This reaction involves the transfer of two electrons for two molecules of alkali metal chlorides involved.

During electrolysis, chlorine gas is produced in the anode compartment (2Cl⁻ → Cl₂ + 2e⁻), and hydrogen gas in the cathode compartment (2H₂O + 2e → 2OH⁻ + H₂).

At the same time, under the action of the electric field, the two alkali metal ions corresponding to the chlorine generated are transferred through the membrane, from the anode compartment to the compartment cathode of the cell type "chlorine-soda" to balance the electric charge due to the simultaneous production of two hydroxyl anions.

The formation of chlorine in the anode compartment is therefore accompanied by a drop in concentration of alkali metal chloride in the anolyte simultaneously with an enrichment in alkali metal hydroxide in the cathode compartment.

After its electrolysis, the brine depleted in alkali metal chloride is removed from the "chlorine-soda" cell. We can therefore consider recycling this depleted brine by adding alkali metal chloride.

Advantageously, the alkali metal hydroxide solution obtained by electrolysis has a concentration of between 10 and 55% by weight, preferably between 30 and 50% by weight.

By the process according to the invention, the chlorine gas and the hydroxide solution produced are free of detectable impurity.

They are then transferred to a slaughter column to make them react one on the other.

Me étant défini précédemment.The reaction in the slaughter column can be summarized by the following equation B : $$\mathbf{\text{(B)}}\text{6 MeOH + 3Cl₂ → 3 MeCl + 3 MeClO + 3 H₂O,}$$

Having defined myself previously.

Me étant défini précédemment.The hypochlorite obtained will then disproportionate on the one hand into chloride, and on the other hand into alkali metal chlorate according to equation C below $$\mathbf{\text{(VS)}}\text{3 MeClO → 2 MeCl + MeClO₃,}$$

Having defined myself previously.

The saline solution obtained at the outlet of the slaughter column comprises between 50 and 200 g / l of alkali metal chloride and between 30 and 700 g / l of alkali metal chlorate. Advantageously, this solution saline comprises between 70 and 170 g / l of chloride and between 400 and 650 g / l of alkali metal chlorate.

In order to promote the disproportionation of hypochlorite, the saline solution before its use as an anolyte in the membrane cell can be advantageously transferred to an evolution tank for a prolonged residence time, at a pH of between 6 and 8, of preferably between 6.5 and 7.

It then comprises less than 5 g / l of alkali metal hypochlorite, preferably less than 1 g / l.

In the chlorate preparation process according to the present invention, the saline solution obtained by the purification process described above, is then used as an anolyte in the membrane cell at a pH between 1 and 8, preferably between 2 and 5, and at a temperature between 50 and 100 ° C, advantageously between 70 and 90 ° C.

Preferably, after its electrolysis, part of the anolyte is recycled to the slaughter column.

Advantageously, the alkali metal hydroxide solution, obtained by electrolysis in the membrane cell, has a concentration of between 10 and 55% by weight and preferably between 30 and 50% by weight. It is also transferred to the slaughter column.

During electrolysis in the membrane cell, chlorine gas is also produced in the anode compartment.

This chlorine is then transferred to the slaughter column, advantageously in admixture with the chlorine gas produced during the electrolysis of the "chlorine-soda" type.

It is therefore possible to define an anode loop constituted by the anode compartment of the membrane cell and the slaughter column, the products in solution of the electrolysis of the membrane cell being transferred to the slaughter column and vice versa the solution obtained. at the outlet of the slaughter column being used as an anolyte in the membrane cell.

During the process according to the present invention, a stationary state is quickly reached where the different solutions, at the outlet of the slaughter column or at the outlet from the anode compartment of the membrane cell have a constant composition.

The anolyte contains between 50 and 200 g / l of alkali metal chloride and preferably between 70 and 170 g / l. The concentration of chlorate leaving the membrane cell necessary for it to be isolable directly by crystallization, is easily determined from known crystallization diagrams of water-chloride-chlorate systems (thesis by A. NALLET, Faculty of Sciences of the University of Lyon, Order number 209, defended on January 19, 1955). It is for example between 400 and 650 g / l of anolyte.

Thus, according to the present invention, part of the anolyte after its electrolysis is transferred to a crystallizer or the chlorate is left to crystallize, the mother liquors being recovered and recycled in the anode loop of the membrane cell.

One can also, optionally, send the anolyte to a progress tank before its transfer to the crystallizer.

The alkali metal used in the process according to the invention is chosen from lithium, sodium and potassium, preferably sodium.

The present invention also relates to a device for the preparation of an alkali metal chlorate, implementing the method described above, comprising the combination of a "chlorine-soda" type cell for the preparation of chlorine gas and hydroxide of an alkali metal, of a column for the removal of chlorine by an alkali metal hydroxide, and a membrane cell for the electrolysis of an anolyte comprising a solution of alkali metal chloride and a catholyte comprising a solution of alkali metal hydroxide.

• la figure 1 représente un diagramme général de dispositif de mise en oeuvre du procédé selon la présente invention,
• la figure 2 représente une réalisation préférentielle du dispositif de purification de la saumure de sel de métal alcalin.

Other characteristics of the device according to the present invention will appear on reading the detailed description given below, with reference to the accompanying drawings in which:

• FIG. 1 represents a general diagram of the device for implementing the method according to the present invention,
• FIG. 2 represents a preferred embodiment of the device for purifying the brine of an alkali metal salt.

FIG. 1 shows a device, used in a preferential manner, and in which the "chlorine-soda" type cell (1) is a membrane cell, comprising one or more anode compartments (11) separated from the cathode compartment (s) (12) corresponding by a membrane (13), the anode compartment or compartments each comprising a suitable device for admitting (111) and recovering (113) the anolyte and a suitable device for recovering chlorine gas (112), the cathode compartment (s) (12) each comprising a suitable device for admitting (121) and recovering (122) the catolyte and a suitable device for removing hydrogen gas (123).

The slaughter column (2) comprises at least one suitable device for admitting an alkali metal hydroxide solution (21) from the cell of the "chlor-sodium hydroxide" type, a suitable device for admitting the chlorine gas (22) and preferably a device (24) for admitting a saline solution poor in chloride and a suitable device (23) for recovering the saline solution obtained.

The membrane cell (3) comprises, like the "chlorine-soda" type cell (1) one or more anode compartments (31) separated from the cathode compartment (s) (32) corresponding by a membrane (33), the compartment (s) anodic each comprising a suitable device for admitting saline solution (312), a suitable device for recovering chlorine gas (314) and a device for recovering saline solution after its electrolysis (311).

The cathode compartment (s) (32) of this membrane cell (3) include a suitable device for admitting water (321), a suitable device (322) for recovering the catholyte after its electrolysis and a suitable device hydrogen extraction (323).

According to the invention, the appropriate device for recovering (313) the anolyte from the membrane cell (3) is connected to a suitable device for introducing this anolyte (24) into the slaughter column (2).

Likewise, the chlorine gas extraction device (314) is also connected to the slaughter column by means of the appropriate chlorine gas intake device (22).

Finally, the anode compartment (31) of the membrane cell (3) is connected either directly by an appropriate means, or by the device for recovering the saline solution after its electrolysis (311) to a crystallizer (4).

Advantageously, the crystallizer comprises an appropriate device for recovering mother liquors (43) connected to the anode compartment (31) of the membrane cell (3). In a variant of the device according to the invention, the mother liquors can be returned to the level of the anode loop defined above.

When we take the overall balance of reactions of the "chlorine-soda" cell (A) and the slaughter column (B + C), we obtain the following general equation D:

Having defined myself previously, and F representing a Faraday.

Since the alkali metal chlorate obtained enters the final balance of the chlorate preparation according to the invention, it can therefore be considered that it is not an impurity.

Consequently, the combination of a chlorine-soda type electrolysis with a column for the removal of chlorine by soda can be considered as a step for purifying an alkali metal brine.

In fact, the chloride solution obtained is practically free from any impurity. Such an electrolysis-slaughter column combination unexpectedly appears to be the only industrial process making it possible to avoid all of the impurities, harmful to the proper functioning of the membranes, such as calcium, magnesium, strontium, barium, iodine, bromine, aluminum, silica, sulfate, iron, manganese, copper, etc.

Consequently, the present invention also relates to a device for purifying an alkali metal chloride brine, comprising the combination of an electrolysis cell of the "chlor-soda" type (1) and a slaughter column ( 2) chlorine by soda.

FIG. 2 shows a cell of the "chlorine-soda" type (1) with a membrane, consisting of one or more anode compartments (11) separated from the cathode compartment (s) (12) corresponding by a membrane (13), the anode compartments each comprising a suitable device for admitting (111) and recovering (113) the anolyte and a suitable device for recovering chlorine gas (112), and the cathode compartment (s) (12) each comprising a suitable device inlet (121) and recovery (122) of the catholyte, and a suitable device for evacuating hydrogen gas (123). The slaughter column (2) comprises at least one suitable device for admitting an alkali metal hydroxide solution (21), a suitable device for admitting chlorine gas, and a suitable device for recovering (23) of a purified solution of alkali metal chloride, the hydroxide inlet devices (21) and chlorine (22) being respectively connected directly to the devices for recovering the catholyte (122) and chlorine gas (112 ) of the "chlorine-soda" type cell (1).

Advantageously, one can add to the purification device according to the invention, an evolution tank directly connected to the recovery device (23) of the purified solution of alkali metal.

The following examples will illustrate the different steps of the process according to the invention.

EXAMPLE 1: Purification of brine

A cell, "chlorine-soda" (1) equipped with a membrane (13) N 90209 (marketed under the NAFION brand by the company DU PONT) produced under 30 A / dm², 19 g / h of Cl₂ and 32% sodium hydroxide.

During 4h the chlorine is recovered at the foot of a slaughter column (2) mounted above a thermostatically controlled tank at 50 ° C which contains 0.5 l of water. A pH measurement makes it possible to regulate the addition of sodium hydroxide to 32% to reduce the chlorine and maintain the pH between 6.5 and 7.

About 269 g of 32% sodium hydroxide was required to remove all of the chlorine.

Finally, a solution containing 12.3% by weight of NaCl and 4.5% by weight of NaClO₃ at 50 ° C. is recovered in the reactor. The content of the various impurities is below the detection limits (Ca, Mg, Sr, Ba, Si, Al, Mn, Fe, Cu, Zn, Pb ≦ 50 ppb and SO₄ ≦ 1 ppm)

EXAMPLE 2 : Influence of the development bin

We use the operating device described in Example 1, associated with a membrane cell (3).

The membrane electrolyzer anolyte (3) contains 120 to 150 g / l of NaCl and 450 to 500 g / l of NaClO₃.

The sodium hydroxide of the catholyte is 32% by weight and the temperature at 90 ° C. The voltage across the electrolyzer is between 3.7 and 3.8 V at 30 A / dm².

The chlorine produced by the membrane cell (3) and the "chlorine-soda" cell (1) is killed in column (2).

The sodium hypochlorite content of the solution recovered at the outlet of the slaughter column (2) is 7.5 to 8 g / l. After its transfer to an evolution tank maintained at 70 ° C, the sodium hypochlorite content is 1 to 2 g / l. The pH is regulated at 6.5 by addition of sodium hydroxide.

The reaction balance in the membrane cell (3) can be summarized by the following general equation E: $$\text{E 5 MeCl + MeClO₃ + 15H₂O → 6 MeClO₃ + 15H₂}$$

Me being defined previously and involves the transfer of 30 electrons.

The overall balance D + E of the process for preparing the alkali metal chlorate according to the invention can therefore be summarized by the following equation F:

Me and F being defined previously.

It will be noted that only 1/6 of the total electronic transfer is carried out in the "chlorine-soda" type cell (1), supplied with crude brine or prepurified by usual techniques, and 5/6 of this transfer in the membrane cell. (3).

The products (Cl₂ and alkali metal hydroxide solution) passing from the "chlor-soda" cell (1) to the next step are very pure and thus constitute very brine at the outlet of the slaughter column (2) pure entering the membrane cell (3). This cell (3) and its membrane (33) will therefore operate under very good conditions which will prolong the life (strongly influenced by the content of impurities in the electrolyte) of the membrane. Thus 5/6 of the chlorate produced are produced under very good conditions for the life of the membranes, the cost of which is high.

On the other hand, from a water balance point of view, the usual process for the production of sodium chlorate requires the introduction of 1,563 kg of water per tonne of NaClO₃, associated with sodium chloride supplied in the form of brine containing 26% by weight of NaCl.

In the process described, the cell (3) is supplied by a flow resulting from the reaction between chlorine and the aqueous solution at 33% by weight of soda. This generally leads to the introduction of only 719 kg of water per tonne of NaClO₃ produced. There is therefore a saving of 844 kg of water which it would be necessary to evaporate in an installation where the sodium chlorate leaves in the solid state, that is to say where any quantity of incoming water must be evaporated.

Claims (21)

Process for the preparation of alkali metal chlorate by electrolysis in a membrane cell (3) of an anolyte comprising a solution of alkali metal chloride and a catholyte comprising a solution of alkali metal hydroxide, characterized in that the alkali metal chloride solution is previously purified by the following sequence of steps: - electrolysis in an "chlorine-soda" type cell (1) of an alkali metal chloride brine to form on the one hand chlorine gas, and on the other hand a concentrated solution of alkali metal hydroxide, transfer of the chlorine gas and of the solution of alkali metal hydroxide produced, into a slaughter column (2), to make them react one on the other, and - recovery of the saline solution thus obtained, for its use as an anolyte in the membrane cell (3). Method according to claim 1, characterized in that the "chlorine-soda" type cell (1) is a membrane cell. Method according to one of claims 1 or 2, characterized in that the alkali metal chloride brine is prepurified by conventional methods of precipitation and / or adsorption on resins. Method according to one of claims 1 to 3, characterized in that the brine comprises between 170 and 315 g / l of alkali metal chloride, preferably between 290 and 310 g / l. Method according to one of claims 1 to 4, characterized in that the alkali metal chloride brine has a pH between 2 and 7, preferably between 2.5 and 4.5. Method according to one of claims 1 to 5, characterized in that the alkali metal hydroxide solution obtained by electrolysis of the "chlorine soda" type has a concentration of between 10 and 55% by weight, preferably between 30 and 50% by weight. Process according to Claim 1, characterized in that the saline solution obtained at the outlet (23) of the slaughter column (2) comprising between 50 and 200 g / l of alkali metal chloride and between 30 and 700 g / l of alkali metal chlorate. Process according to claim 7, characterized in that the saline solution comprises between 70 and 170 g / l of alkali metal chloride and between 400 and 650 g / l of alkali metal chlorate. Method according to one of claims 7 or 8, characterized in that the saline solution, before its use as an anolyte in the membrane cell (3), is previously transferred to an evolution tank for an extended residence time, at a pH between 6 and 8, preferably between 6.5 and 7. Process according to claim 9, characterized in that the saline solution, after an extended residence time, comprises less than 5 g / l of alkali metal hypochlorite, preferably less than 1 g / l. Method according to one of claims 1 to 10, characterized in that the saline solution used as an anolyte in the membrane cell (3) has a pH between 1 and 8, preferably between 2 and 5, and a temperature between 50 and 100 ° C, preferably between 70 and 90 ° C. Method according to one of claims 1 to 11, characterized in that part of the anolyte, after its electrolysis, is recycled in the slaughter column (2). Process according to one of Claims 1 to 12, characterized in that the solution of alkali metal hydroxide, obtained by electrolysis in the membrane cell (3), has a concentration of between 10 and 55% by weight, preferably between 30 and 50% by weight. Method according to one of claims 1 to 13, characterized in that the catholyte, after its electrolysis, is transferred to the slaughter column (2). Method according to one of claims 1 to 14, characterized in that the chlorine gas produced in the anode compartment (31) of the membrane cell (3) is transferred to the slaughter column (2). Method according to one of claims 1 to 15, characterized in that part of the anolyte, after its electrolysis, is transferred to a crystallizer (4) where the chlorate is crystallized, the mother liquors being recovered and recycled to the anode loop of the membrane cell (3). Method according to one of claims 1 to 16, characterized in that the alkali metal is chosen from lithium, sodium and potassium, preferably sodium. Device for the preparation of an alkali metal chlorate, characterized in that it comprises the combination of a cell of the "chlorine-soda" type (1), of a column for the removal (2) of chlorine by a alkali metal hydroxide and a membrane cell (3). Device according to claim 18, characterized in that it comprises the combination of the following elements: - a "chlorine-soda" type cell (1) consisting of a membrane cell comprising one or more anode compartments (11) separated from the cathode compartment (s) (12) corresponding by a membrane (13), the compartment (s) anode each comprising a suitable device (111) for collecting and recovering (113) the anolyte and a suitable device for recovering chlorine gas (112), the cathode compartment (s) (12) each comprising a device suitable for admission (121) and recovery (122) of the catholyte, and a suitable device for evacuating hydrogen gas (123); - a slaughter column (2) comprising at least one suitable device for admitting an alkali metal hydroxide solution (21), a suitable device for admitting chlorine (22), and a suitable device for recovering (23) of an alkali metal chloride saline solution, the hydroxide inlet devices (21) and chlorine (22) being respectively connected directly to the devices for recovering the catholyte (122) and chlorine gas (112 ) of the "chlorine-soda" type cell (1); - a membrane cell (3) comprising one or more anode compartments (31) separated from the cathode compartment (s) (32) corresponding by a membrane (33), the anode compartment or compartments each comprising an appropriate intake device (312) of the saline solution, a suitable device for recovering chlorine gas (314) and a device for recovering the saline solution after its electrolysis (311), the cathode compartment (32) comprising a suitable device (321) for admitting water, a suitable device for recovering (322) the catholyte and a suitable device for extracting hydrogen gas (323), the appropriate device (recovering (313) for the anolyte of the membrane cell (3) being connected to a suitable device for introducing this anolyte (24) into the slaughter column (2) and the device (314) for extracting chlorine gas being also connected to the slaughter column by means of the di suitable chlorine gas intake device (22); and - the anode compartment (31) of the membrane cell (3) being connected either directly by an appropriate means, or by the saline solution recovery device (311) after its electrolysis to a crystallizer (4), which comprises d 'Advantageously a suitable device for recovering mother liquors (43) connected to the anode compartment (31) of the membrane cell (3). Device for the purification of an alkali metal chloride brine, characterized in that it comprises the combination of a "chlorine-soda" type cell (1), and a slaughter column (2), chlorine by soda. Device according to claim 20, characterized in that the "chlorine-soda" type cell (1) is a membrane cell, consisting of one or more anode compartments (11) separate from the corresponding cathode compartment (s) (12) by a membrane (13), the anode compartment (s) each comprising an appropriate device for admitting (111) and recovering (113) the anolyte and an appropriate device for recovering chlorine gas (112), and the one or more cathode compartments (12) each comprising a suitable device catholyte intake (121) and recovery (122), and a suitable device for evacuating hydrogen gas (123) and the slaughter column (2) comprises at least one suitable device for admission of an alkali metal hydroxide solution (21), a suitable inlet device (22), and a suitable recovery device (23) from a purified solution of alkali metal chloride, the hydroxide inlet devices (21) and chlorine (22) being respectively connected directly to the devices for recovering the catholyte (122) and chlorine gas (112) from the "chlorine-soda" type cell (1).

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