NZ247644A

NZ247644A - Process for preparing alkali metal chlorates, apparatus therefor

Info

Publication number: NZ247644A
Application number: NZ247644A
Authority: NZ
Inventors: Francois Delmas; Dominique Ravier
Original assignee: Atochem Elf Sa
Priority date: 1992-05-20
Filing date: 1993-05-18
Publication date: 1994-09-27
Also published as: FR2691479B1; SK49793A3; MX9302943A; PL299000A1; EP0571273A1; IL105718A0; NO931798D0; CZ95293A3; CA2096588A1; FI932299A; BR9301932A; FR2691479A1; AU3868193A; CN1084584A; YU34493A; ZA933454B; TW230848B; KR930023492A; JPH06158373A; NO931798L

Description

<div class="application article clearfix" id="description"> 24 7 6 4 4 PfiOii.v C. COM' faf. ,CPI»s/oey f ;W/#! z 7 Sep to iRub!ic-:v:ci iT-?lc: F.O. Jc-ut.'I : '2 o JUL W9(< Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION PROCESS FOR THE MANUFACTURE OF AN ALKALI METAL CHLORATE AND DEVICE FOR THE IMPLEMENTATION THEREOF WE, ELF ATOCHEM S.A, a French company, of La Defense 10, 4 et 8 cours Michelet, 92800 Puteaux, France hereby declare the invention, for which We pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - 1 - (followed by page la) 24 7 6 4 - io- The present invention relates to a process for the manufacture of an alkali metal chlorate by electrolysis in a membrane cell without the addition of chromium. 5 The preparation of an alkali metal chlorate by electrolysis in a membrane cell is described in particular in the Patent Applications FR-A-2,638,766 and FR-A-2,655,061. Membrane cells generally consist of two compart-10 ments, one anodic and the other cathodic, separated by a membrane which allows the selective transfer of ions from one compartment to another under the action of an electrical field. For the known preparation of an alkali metal 15 chlorate, the anolyte consists of a chloride salt brine of the said alkali metal to which may be added, if appropriate, a predetermined amount of the chlorate of the same alkali metal, the catholyte for its part consisting of an alkali metal hydroxide solution. 20 This process for the preparation of an alkali metal chlorate has many advantages with respect to the prior art which required the use of expensive and environmentally dangerous additives, in particular hexavalent chromium, sodium chromate or sodium dichrom-25 ate, in order to restrict the harmful effects of the cathodic reduction of the hypochlorite and/or chlorate ions. Nevertheless, and despite this clear progress, membrane cells require the use of electrolytes which are 30 particularly free from impurities. Indeed, the alkali metal chloride salt brine which supplies the anode compartment of the cell contains small amounts of metal salts, particularly alkaline-earth metal salts, salts of metals such as aluminium, copper, 35 manganese or zinc, or impurities such as silica, sulphate salts, bromine or iodine which risk damaging or blinding the membrane during electrolysis. (followed by page 2) - 2 - It is thus necessary to purify the brine before introducing it into the anode compartment of the cell, so as to lower the impurity content to acceptable levels. Although the usual techniques for purifying 5 chloride salt brines by precipitation and/or absorption on a resin, make it possible to lower the content of certain impurities, especially calcium and magnesium salts, no industrial process exists which makes it possible to reduce the content of elements, such as 10 silicon, aluminium or other metals, to a few ppm or even a few ppb. The present invention thus relates to a process for the preparation of an alkali metal chlorate by electrolysis, in a membrane cell, of an anolyte compris-15 ing an alkali metal chloride solution and of a catholyte comprising an alkali metal hydroxide solution, the alkali metal chloride solution being obtained from a brine purified beforehand so as to remove virtually all the impurities which would risk damaging or blinding the 20 membrane during electrolysis. According to the present invention, purification of the brine is obtained by the following sequence of stages: - electrolysis in a cell of "chlor-alkali" type of 25 an alkali metal chloride brine to form, on the one hand, gaseous chlorine and, on the other hand, a concentrated alkali metal hydroxide solution, - transfer of the gaseous chlorine and of the alkali metal hydroxide solution produced into a knockout 30 column in order to make them react with each other, and - recovery of the saline solution thus obtained in order to use it as anolyte in the membrane cell. The cell of "chlor-alkali" type used in the 35 process according to the invention is preferably a membrane cell. 24 7 - 3 - This type of cell is well known in the prior art, as described especially in the Patent US-A-4,285,795 or in "Ullmann's Encyclopedia of Industrial Chemistry" (5th Edition, Vol. A6, p. 399-481). 5 The membranes are synthetic ion-exchange mem branes, preferentially made of fluorocarbon polymers capable of resisting drastic operating conditions, in particular highly alkaline solutions, at high temperatures . 10 Carboxylic and/or sulphonic acid functional groups, preferentially in the form of an alkali metal salt, are associated with these fluorocarbon polymers. In a preferential way, the fluorocarbon polymers are poly-tetrafluoroethylenes (PTFES). The membranes used are 15 obtained by extruding or rolling the polymer and may be reinforced by woven pieces of PTFE fibres. Membranes, developed since 1970, have a selectivity at least equal to that of diaphragms but are much more sensitive to damage and blinding due to impurities 2 0 present in the electrolyte. So as to maintain the lifetime of the membranes, in the process according to the invention, the alkali metal chloride brine is generally pre-purified by standard methods of precipitation and/or of adsorption on 25 resins. The alkali metal brine used as anolyte in the cell of "chlor-alkali" type preferably comprises between 170 and 315 g/1 of alkali metal chloride, preferably between 290 and 310 g/1. 30 Moreover, this brine is preferably used at a pH of between 2 and 7, advantageously of between 2.5 and 4.5. The overall reaction carried out in the electrolysis cell of "chlor-alkali" type may be summarised by 35 the following Equation A: (A) 2 MeCl + 2 H20 -> 2 MeOH + Cl2+H2 24 7 6 - 4 - with Me representing an alkali metal. This reaction involves the transfer of two electrons per two molecules of alkali metal chlorides involved. 5 During electrolysis, gaseous chlorine is produced in the anode compartment (2C1~ -+ Cl2 + 2e") and gaseous hydrogen in the cathode compartment (2^0 + 2e" -► 20H" + H2). At the same time, under the action of the electrical field, the two alkali metal ions corresponding to 10 the chlorine generated are transferred through the membrane from the anode compartment towards the cathode compartment of the cell of "chlor-alkali" type in order to balance the electrical charge due to the simultaneous production of two hydroxyl anions. 15 The formation of chlorine in the anode compart ment is thus accompanied by a lowering in the concentration of alkali metal chloride in the anolyte simultaneously with an enrichment in alkali metal hydroxide in the cathode compartment. 20 After its electrolysis, the brine, which is impoverished in alkali metal chloride, is discharged from the cell of "chlor-alkali" type. The recycling of this impoverished brine by the addition of alkali metal chloride can thus be envisaged. 25 Advantageously, the alkali metal hydroxide solution obtained by electrolysis has a concentration of between 10 and 55 % by weight, preferably of between 30 and 50 % by weight. By the process according to the invention, the 30 gaseous chlorine and the hydroxide solution produced are free from detectable impurities. They are then transferred into a knockout column in order to make them react with each other. The reaction in the knockout column may be 35 summarised by the following Equation B: (B) 6 MeOH + 3C12 - 3 MeCl + 3 MeCIO + 3 H20, Me being defined above. The hypochlorite obtained will then disproportionate, on the one hand, to alkali metal chloride and, on the other hand, to alkali metal chlorate according to 5 Equation C below: (C) 3 MeCIO - 2 MeCl + MeCl03, Me being defined above. The saline solution obtained at the outlet of the knockout column comprises between 50 and 200 g/1 of 10 alkali metal chloride and between 30 and 700 g/1 of alkali metal chlorate. Advantageously, this saline solution comprises between 70 and 170 g/1 of alkali metal chloride and between 400 and 650 g/1 of alkali metal chlorate. 15 In order to promote the disproportionation of the hypochlorite, the saline solution, before it is used as anolyte in the membrane cell, may advantageously be transferred to a development tank for a prolonged residence time, at a pH of between 6 and 8, preferably of 2 0 between 6.5 and 7. It then comprises less than 5 g/1 of alkali metal hypochlorite, preferably less than 1 g/1. In the process for the preparation of chlorate according to the present invention, the saline solution 25 obtained by the purification process described above is then used as anolyte in the membrane cell at a pH of between 1 and 8, preferentially of between 2 and 5, and at a temperature of between 50 and 100° C, advantageously of between 70 and 90° C. 30 Preferably, after it has been electrolysed, a portion of the anolyte is recycled to the knockout column. Advantageously, the alkali metal hydroxide solution obtained by electrolysis in the membrane cell 35 has a concentration of between 10 and 55 % by weight and preferably of between 30 and 50 % by weight. It is also - 6 - transferred to the knockout column. During electrolysis in the membrane cell, gaseous chlorine is also produced in the anode compartment. This chlorine is then transferred to the knockout 5 column, advantageously as a mixture with the gaseous chlorine produced during electrolysis of "chlor-alkali" type. It is thus possible to define an anode loop consisting of the anode compartment of the membrane cell 10 and the knockout column, the products in solution from the electrolysis in the membrane cell being transferred to the knockout column and, conversely, the solution obtained at the outlet of the knockout column being used as anolyte in the membrane cell. 15 During the process according to the present invention, a stationary state is rapidly reached in which the various solutions, at the outlet of the knockout column or at the outlet of the anode compartment of the membrane cell, have a constant composition. 20 The anolyte contains between 50 and 200 g/1 of alkali metal chloride and preferably between 70 and 170 g/1. The concentration of the chlorate exiting from the membrane cell required for the chlorate to be directly isolated by crystallisation is easily determined 25 from known crystallisation diagrams of the water/ chloride/chlorate systems (thesis by A. Nallet, Faculty of Sciences of the University of Lyons, Order No. 209, viva on 19 January 1955). It is, for example, between 400 and 650 g/1 of anolyte. 30 Thus, according to the present invention, a portion of the anolyte, after it has been electrolysed, is transferred to a crystalliser where the chlorate is left to crystallise, the mother liquors being recovered and recycled to the anode loop of the membrane cell. 35 It is optionally possible to convey the anolyte to a development tank before it is transferred to the crystalliser. 9 - (L ^ - 7 - 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 5 for the preparation of an alkali metal chlorate, using the process described above, comprising the combination of a cell of "chlor-alkali" type for the preparation of gaseous chlorine and alkali metal hydroxide, a column for knocking out the chlorine using an alkali metal 10 hydroxide, and a membrane cell for the electrolysis of an anolyte comprising an alkali metal chloride solution and of a catholyte comprising an alkali metal hydroxide solution. Other characteristics of the device according to 15 the present invention will become apparent on reading the detailed description given below, with reference to the appended drawings in which: - Figure 1 represents a general diagram of the device for implementing the process according to the 20 present invention, - Figure 2 represents a preferential embodiment of the device for purifying the alkali metal salt brine. Figure 1 shows a device, of preferential use and in which the cell of "chlor-alkali" type (1) is a mem-25 brane cell, comprising one or a number of anode compartments (11) separated from the corresponding cathode compartment (s) (12) by a membrane (13), the anode compartments) each comprising a suitable device for entry (111) and recovery (113) of the anolyte and a suitable 30 device for recovery of the gaseous chlorine (112), the cathode compartment(s) (12) each comprising a suitable device for entry (121) and recovery (122) of the catholyte and a suitable device for discharge of the gaseous hydrogen (123). 35 The knockout column (2) comprises at least one suitable device for entry of an alkali metal hydroxide 24 7 6 4 4 - 8 - solution (21) originating from the cell of "chlor-alkali" type, a suitable device for entry of the gaseous chlorine (22) and preferably a device for entry (24) of a chloride-poor saline solution and a suitable device (23) for 5 recovery of the saline solution obtained. The membrane cell (3) comprises, similarly to the cell of "chlor-alkali" type (1), one or a number of anode compartments (31) separated from the corresponding cathode compartment(s) (32) by a membrane (33), the anode 10 compartment(s) each comprising a suitable device for entry (312) of the saline solution, a suitable device for recovery of the gaseous chlorine (314) and a device for recovery of the saline solution after it has been electrolysed (311). 15 The cathode compartment(s) (32) of this membrane cell (3) comprise, for their part, a suitable device for entry (321) of water, a suitable device for recovery (322) of the catholyte after it has been electrolysed and a suitable device for extraction of the hydrogen (323). 20 According to the invention, the suitable device for recovery (313) of the anolyte from the membrane cell (3) is connected to a suitable device for introducing this anolyte (24) into the knockout column (2). Likewise, the device for extraction of (314) the 25 gaseous chlorine is also connected to a knockout column via the suitable device for entry (22) of the gaseous chlorine. Finally, the anode compartment (31) of the membrane cell (3) is connected, either directly by a 30 suitable means or by the device for recovery of the saline solution after it has been electrolysed (311), to a crystalliser (4) . Advantageously, the crystalliser comprises a suitable device for recovery of the mother liquors (43), 35 which device is connected to the anode compartment (31) of the membrane cell (3). In a variant of the device according to the invention, the mother liquors may be conveyed to the anode loop defined above. - 9 - 4 7 6 4 4 When the overall balance of the reactions of the "chlor-alkali" cell (A) and of the knockout column (B + C) is taken into account, the following general equation D is obtained: 3xA 6 MeCl + 6H20 6 MeOH + 3 Cl2 * 3 H2 B 6 MeOH + 3 Cl2 —* 3 MeCl + 3 MeCIO + 3 H20 C 3 MeCIO —* 2 MeCl + MeClOj 6F D 6 MeCl - 3 H20 —> 5 MeCl + MeClOj + 3 H2 5 Me being defined above and F representing one faraday. As the alkali metal chlorate obtained forms part of the final balance of the preparation of chlorate according to the invention, it may thus be regarded not 10 as an impurity. Consequently, the combination of an electrolysis of "chlor-alkali" type with a column for knocking out the chlorine using sodium hydroxide may be regarded as a purification stage of an alkali metal brine. 15 In fact, the chloride solution obtained is virtu ally free from any impurities. Such an electrolysis/ knockout column combination appears unexpectedly as the only industrial process which makes it possible to avoid all the impurities which are harmful to the reliable 20 operation of the membranes, these being calcium, magnesium, strontium, barium, iodine, bromine, aluminium, silica, sulphate, iron, manganese, copper and the like. Consequently, the present invention also relates to a device for the purification of an alkali metal 25 chloride brine, comprising the combination of an electrolysis cell of "chlor-alkali" type (1) and a column for knocking out (2) chlorine using sodium hydroxide. Figure 2 shows a membrane cell of "chlor-alkali" type (1), consisting of one or a number of anode compart-30 ments (11) separated from the corresponding cathode compartment(s) (12) by a membrane (13), the anode ^ 4 /' g - 10 - compartment(s) each comprising a suitable device for entry (111) and recovery (113) of the anolyte and a suitable device for recovery of the gaseous chloride (112), and the cathode compartment(s) (12) each 5 comprising a suitable device for entry (121) and recovery (122) of the catholyte and a suitable device for discharge of the gaseous hydrogen (123) . The knockout column (2) comprises at least one suitable device for entry of an alkali metal hydroxide solution (21), one suitable 10 device for entry (22) of gaseous chlorine and one suitable device for recovery (23) of a purified solution of alkali metal chloride, the devices for entry of hydroxide (21) and of chlorine (22) being connected directly to the devices for recovery of the catholyte (122) and of 15 gaseous chlorine (112) respectively of the cell of "chlor-alkali" type (1). Advantageously, there may be added, to the purification device according to the invention, a development tank connected directly to the device for 20 recovery (23) of the purified solution of alkali metal. The following examples will enable the various stages of the process according to the invention to be illustrated. rxampt.k 1 : Purification of the brine 25 A "chlor-alkali" cell (1) equipped with an N 90209 membrane (marketed under the tradename Nafion by the company Du Pont) produces, at 30 A/dm2, 19 g/h of Cl2 and 32 % sodium hydroxide solution. The chlorine is recovered for 4 h at the foot of 30 a knockout column (2) mounted above a reservoir, thermostatically controlled at 50°C, which contains 0.5 1 of water. A pH measurement makes it possible for the addition of 32 % sodium hydroxide solution to be adjusted in order to knock out the chlorine and maintain the pH 35 between 6.5 and 7. <L - 11 - Approximately 269 g of 32% sodium hydroxide solution were required to knock out all the chlorine. Finally, a solution containing 12.3 % by weight of NaCl and 4.5 % by weight of NaC103 is recovered in the 5 reactor at 50°C. The contents of the various impurities are below the detection limits (Ca, Mg, Sr, Ba, Si, Al, Mn, Fe, Cu, Zn, Pb < 50 ppb and SO<, < 1 ppm) . EXAMPLE 2: Effect of the development tank The operating device described in Example 1 is 10 used in combination with a membrane cell (3). The anolyte of the membrane electrolyser (3) contains 120 to 150 g/1 of NaCl and 450 to 500 g/1 of NaC103. The catholyte is 32 % sodium hydroxide by weight 15 and the temperature is 90°C. The voltage at the terminals of the electrolyser is between 3.7 and 3.8 V at 30 A/dm2. The chlorine produced by the membrane cell (3) and the "chlor-alkali" cell (1) is knocked out in the column (2) . 20 The sodium hypochlorite content of the solution recovered at the outlet of the knockout column (2) is from 7.5 to 8 g/1. After transferring it into a development tank maintained at 70°C, the sodium hypochlorite content is from 1 to 2 g/1. The pH is adjusted to 6.5 by 25 addition of sodium hydroxide solution. The reaction balance in the membrane cell (3) may be summarised by the following general equation E: E 5 MeCl + MeC103 + 15 H20 - 6 MeCl03 + 15 H2 Me being defined above and involving the transfer 30 of 30 electrons. The overall balance D+E of the process for the preparation of the alkali metal chlorate according to the invention may thus be summarised by the following equation F: - 12 - 6F D 6 MeCl + 3H20 —» 5 MeCl - MeClOj - 3 'H2 30F E 5 MeCl - MeCIO3 - 15 H20 —> 6 MeClOj - 15 H? 36F F 6 MeCl + 18 H20 —> 6 MeCIOj + IS H2 Me and F being defined above. It will be noted that only 1/6 of the overall electron transfer is carried out in the cell of "chlor-alkali" type (I), supplied with crude brine or brine pre- 5 purified using conventional techniques, and 5/6 of this transfer is carried out in the membrane cell (3). The products (Cl2 and alkali metal hydroxide solution) passing from the "chlor-alkali" cell (1) to the following stage are very pure and thus form, at the 10 outlet of the knockout column (2), a very pure brine which enters the membrane cell (3). This cell (3) and its membrane (33) will thus operate under very good condi tions which will prolong the lifetime (greatly influenced by the impurities content of the electrolyte) of the 15 membrane. Thus, 5/6 of the chlorate produced are produced under very good conditions for the lifetime of the membranes, which are expensive. Moreover, from the water balance viewpoint, the usual process for the production of sodium chlorate 20 requires the introduction of 1,563 kg of water per tonne of NaCl03, in combination with the sodium chloride sup plied in the form of brine containing 26 % by weight of NaCl. In the process described, the cell (3) is sup- 25 plied with a flow resulting from the reaction between the chlorine and the 33 % by weight aqueous sodium hydroxide solution. Overall, this leads to the introduction of only 719 kg of water per tonne of NaC103 produced. There is thus a saving of 844 kg of water which it would be 2 4 7 6 4 4 - 13 - necessary to evaporate in a plant in which the sodium chlorate emerges in the solid form, that is to say in which the entire amount of water entering must be evaporated. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> f* a a ' i / D A * (h > r y,y 1 qQ, I v I; i / • i iJv 1? / ^ *f - (i WHAT-)(/WE CLAIM IS:- ;N\>' 1.
Process for the preparation of an alkali metal chlorate by electrolysis in a membrane cell of an anolyte comprising an alkali metal chloride solution and 5 of a catholyte comprising an alkali metal hydroxide solution, characterised in that the alkali metal chloride solution is purified beforehand by the following sequence of stages: - electrolysis in a cell of "chlor-alkali" type 10 of an alkali metal chloride brine to form, on the one hand, gaseous chlorine and, on the other hand, a concentrated alkali metal hydroxide solution, - transfer of the gaseous chlorine and of the alkali metal hydroxide solution produced into a knockout 15 column in order to make them react with each other, and - recovery of the saline solution thus obtained in order to use it as anolyte in the membrane cell 2.
Process according to Claim 1, characterised in 20 that the cell of "chlor-alkali" type is a membrane cell. 3.
Process according to either of Claims 1 or 2, characterised in that the alkali metal chloride brine is pre-purified by standard methods of precipitation and/or 25 of adsorption on resins. 4.
Process according to any one of Claims 1 to 3, characterised in that the brine comprises between 170 and 315 g/1 of alkali metal chloride. 5.
Process according to Claim 4 wherein the brine comprises between 290 and 310 g/1 of alkali metal chloride. 6.
Process according to any one of Claims 1 to 5, characterised in that the alkali metal chloride brine has a pH of between 2 and 7. 7.
Process according to Claim 6 wherein the pH is between 2.5 and 4.5. 8.
Process according to any one of Claims 1 to 7, characterised in that the alkali metal hydroxide solution obtained by electrolysis of "chlor-alkali" type has a concentration of between 10 and 55 % by weight. - 15 - % 9.
Process according to Claim 8 wherein the concentration of the alkali metal hydroxide solution is between 30 and 50 % by weight. 10.
Process according to Claim 1, characterised in that the saline solution obtained at the outlet of the knockout column comprises between 50 and 200 g/1 of alkali metal chloride and between 30 and 700 g/1 of alkali metal chlorate. 11.
Process according to Claim 10, characterised in that the saline solution comprises between 70 and 170 g/1 of alkali metal chloride and between 400 and 650 g/1 of alkali metal chlorate. 12.
Process according to either of Claims 10 or 11, characterised in that the saline solution, before it is used as anolyte in the membrane cell, is transferred beforehand into a development tank for a prolonged residence time, at a pH of between 6 and 8. 13.
Process according to Claim 12 wherein the pH is between 6.5 and 7. 14.
Process according to either Claim 12 or Claim 13, characterised in that the saline solution, after a prolonged residence time, comprises less than 5 g/1 of alkali metal hypochlorite. 15.
Process according to Claim 14 wherein the saline solution comprises less than 1 g/1 of alkali metal hypochlorite. 16.
Process according to any one of Claims 1 to 14, characterised in that the saline solution used as anolyte in the membrane cell has a pH of between 1 and 8, and a temperature of between 50 and 100°C. 17.
Process according to Claim 16 wherein the pH is between 2 and 5. 18.
Process according to either Claim 16 or Claim 17 wherein the temperature is between 70 and 90°C. 19.
Process according to any one of Claims 1 to 18, characterised in that a portion of the anolyte, after it has been electrolysed, is recycled to the knockout column. 20.
Process according to one of Claims 1 to 19, characterised in that the alkali metal hydroxide solution, obtained by electrolysis in the membrane cell, has a concentration of between 10 and 55 % by weight. 21.
Process according to Claim 20 wherein the concentration of alkali metal hydroxide solution is between 30 and 50 % by weight. 22.
Process according to any one of Claims 1 to 21, characterised in that the catholyte, after it has been electrolysed, is transferred to the knockout column. 23.
Process according to any one of Claims 1 to 22, characterised in that the gaseous chlorine produced in the anode compartment of the membrane cell is transferred to the knockout column. 24.
Process according to any one of Claims 1 to 23, characterised in that a portion of the anolyte, after it has been electrolysed, is transferred to a crystalliser where the chlorate is crystallised, the mother liquors being recovered and recycled to the anode loop of the membrane cell. 25.
Process according to any one of Claims 1 to 24, characterised in that the alkali metal is chosen from lithium, sodium and potassium. 26.
Process according to Claim 25 wherein the alkali metal is sodium. 27.
Device for the preparation of an alkali metal chlorate according to the process of any one of claims 1 to 26, characterised in that it comprises the combination of a cell of "chlor-alkali" type, a column for knocking out the chlorine using an alkali metal hydroxide, and a membrane cell. ' A 24764 £ t"AVi331 NiVirpit^ 28.
Device according to Claim 27, characterised in that it comprises the combination of the following components: - a cell of "chlor-alkali" type consisting of a membrane cell comprising one or a number of anode compartments separated from the corresponding cathode compartment(s) by a membrane , the anode compartment(s) each comprising a suitable device for entry and recovery of the anolyte and a suitable device for recovery of the gaseous chlorine , the cathode compartment(s) each comprising a suitable device for entry and recovery of the catholyte and a suitable device for discharge of the gaseous hydrogen ; - a knockout column comprising at least one suitable device for entry of an alkali metal hydroxide solution , a suitable device for entry of the chlorine and a suitable device for recovery of a saline solution of alkali metal chloride, the devices for entry of hydroxide and of chlorine being connected directly to the devices for the recovery of the catholyte and of gaseous chlorine respectively of the cell of "chlor- alkali" type ; - a membrane cell comprising one or a number of anode compartments separated from the corresponding cathode compartment(s) by a membrane , the anode compartment(s) each comprising a suitable device for entry of the saline solu tion, a suitable device for recovery of the gaseous chlorine and a device for recovery of the saline solution after it has been electrolysed , the cathode compartment comprising a suitable device for entry of water, a suitable device for recovery of the catholyte and a suitable device for extraction of the gaseous hydrogen , the suitable device for recovery of the anolyte of the membrane cell being connected to a suitable device for introducing this anolyte into the knockout column and the device for extraction of the gaseous chlorine also being connected to the knockout column via the suitable device for entry of the gaseous chlorine; and - the anode compartment of the membrane cell being connected, either directly by a suitable means or by the device for the recovery of the saline solution after it has been electrolysed, to a crystalliser. 29.
Device according to claim 28 wherein the crystalliser comprises a suitable device for recovery of the mother liquors 30.
Device for the purification of an alkali metal chloride brine according to the process of any one of Claims 1 to 26, characterised in that it comprises the combination of a cell of "chlor-alkali" type and a column for knocking out chlorine using sodium hydroxide. 31.
Device according to Claim 30, characterised in that the cell of "chlor-alkali" type is a membrane cell consisting of one or a number of anode compartments separated from the corresponding cathode compartment (s) by a membrane , the anode compartment(s) each comprising a suitable device for entry and recovery of the anolyte and a suitable device for recovery of the gaseous chlorine , and the cathode compartment(s) each comprising a suitable device for entry and recovery of the catholyte and a suitable device for discharge of the gaseous hydrogen , and the knockout column comprises at least one suitable device for entry of an alkali metal hydroxide solution , one suitable device for entry of the chlorine and one suitable device for recovery of a purified solution of alkali metal chloride, the devices for entry of hydroxide and of chlorine being connected directly to the devices for recovery of the catholyte and of gaseous chlorine respec^"'^', tively of the cell of "chlor-alkali" type . // ' i': ■, i' ' V • 'v -\ • Process for the preparation of an alkali metal chlorate by electrolysis in a membrane cell of an anolyte substantially as herein described with reference to the Examples.
33. An alkali metal chlorate when obtained by a process as claimed in any one of claims 1 to 26 and 32. 34. a device for the preparation of an alkali metal chlorate substantially as herein described with reference to the Drawings. ELF ATOCHEM S.A by their attorneys </div>