FR2691479A1 - Process for the production of alkali metal chlorate and device for its implementation - Google Patents

Abstract

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 an alkali metal chloride solution and a catholyte comprising an alkali metal hydroxide solution, the alkali metal chloride solution being purified beforehand by the following succession 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 gaseous chlorine and the solution of alkali metal hydroxide produced, in a knockdown column (2), to make them react the 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 production of chlorate

  alkali metal by electrolysis in a membrane cell without

addition of chromium.

  The preparation of alkali metal chlorate by electrolysis in a membrane cell is described in particular in the

  FR-A-2,638,776 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 a compartment to a compartment.

  other, under the action of an electric field.

  For the known preparation of alkali metal chlorate, the anolyte is constituted by a salt brine salt of said alkali metal, to which may be added, if appropriate, a determined amount of chlorate of the same alkali metal, the catholyte being for her

  part consisting 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 expensive and environmentally hazardous additives, in particular hexavalant chromium, chromate or sodium dichromate, to limit the harmful effect of cathodic reduction of hypochlorite ions

and / or chlorates.

  Nevertheless, despite this clear progress, membrane cells

  require the use of electrolytes particularly free of impurities.

  In fact, the salt brine of alkali metal chloride which feeds the anode compartment of the cell, contains small amounts of metal salts, particularly alkaline earth metal salts, metals such as aluminum, copper, nickel, manganese or zinc, or impurities such as silica, sulphate salts, bromine or iodine which may damage or clog the membrane

electrolysis.

  It is therefore necessary to purify the brine before its introduction into the anode compartment of the cell, so as to

  to lower the levels of impurities to acceptable levels.

  While the usual techniques for purifying chloride salt brines, by precipitation and / or absorption on resin, make it possible to lower the levels of certain impurities, in particular the calcium and magnesium salts, there is no industrial process that makes it possible to reduce to reduce the content of elements, such as silicon, aluminum or

  other metals at a few ppm or even a few ppb.

  The present invention thus relates to a process for the preparation of alkali metal chlorate by electrolysis in a membrane cell, an anolyte comprising an alkali metal chloride solution and a catholyte comprising an alkali metal hydroxide solution, the alkali metal chloride solution being obtained from a brine previously purified so as to eliminate almost all the impurities which could deteriorate or clog the membrane when

electrolysis.

  According to the present invention, the purification of the brine is obtained by the following succession 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 chlorine gas and alkali metal hydroxide solution produced, in a slaughter column, to react with one another, and recovering the saline solution thus obtained, for its

  use as anolyte in the membrane cell.

  The "chlorine-soda" type cell used in the process

  according to the invention is preferably a membrane cell.

  This type of cell is well known in the prior art, as described in particular in US Pat. No. 4,285,795 or in Ullmann's

  Encyclopedia of Industrial Chemistry "(5th ed., Vol A 6, p 399-481).

  The membranes are synthetic ion exchange membranes, preferably fluorocarbon polymers capable of withstanding severe operating conditions, in particular

  strong alkaline solutions, at high temperatures.

  These flurocarbon polymers are associated with carboxylic and / or sulphonic acid functions, preferably in the form of a salt.

  Preferably, the fluorinated polymers are

  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 the degradations and the clogging due to the impurities present in

the electrolyte.

  In order to preserve the life of the membranes, in the process according to the invention, the alkali metal chloride brine is generally pre-purified by conventional precipitation methods.

and / or adsorption on resins.

  The alkali metal brine used as anolyte in the "chlorine-soda" type cell preferably comprises between 170 and 315 g / l.

  alkali metal chloride, preferably between 290 and 310 g / l.

  Moreover, this brine is used in preference to a p H

  between 2 and 7, advantageously between 2.5 and 4.5.

  The overall reaction carried out in the "chlorine-soda" type electrolysis cell can be summarized by the following equation A: (A) 2 Me Ci + 2 H 2 2 2 Me OH + C 12 + H 2

  with Me representing an alkali metal.

  This reaction involves the transfer of two electrons to

  two molecules of alkali metal chlorides involved.

  During the electrolysis, the chlorine gas is produced in the anode compartment (2 C 1 C 12 + 2 e-), and the hydrogen gas in the

  cathode compartment (2 H 20 + 2 e 20 H- + H 2).

  At the same time, under the action of the electric field, the two alkali metal ions corresponding to the generated chlorine are transferred through the membrane, from the anode compartment to the cathode compartment of the "chlorine-soda" cell to balance load

  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 a hydroxide enrichment

  alkaline metal of the cathode compartment.

  After its electrolysis, the brine depleted in alkali metal chloride is removed from the cell type "chlorine-soda" can therefore be considered to recycle this brine depleted by the addition of chloride

of alkali metal.

  Advantageously, the alkali metal hydroxide solution obtained by the 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 solution

  of hydroxide products are free of detectable impurity.

  They are then transferred to a slaughter column for

to react one on the other.

  The reaction in the slaughter column can be summarized by the following equation B (B) 6 Me OH + 3 C 12 3 Me CI + 3 Me CIO + 3 H 20,

Me being defined previously.

  The hypochlorite obtained will then be disproportionated on the one hand into chloride, and on the other hand into alkali metal chlorate according to equation C below: (C) 3 Me CIO 2 Me Cl + Me Cl O 3,

Me being defined previously.

  The saline solution obtained at the outlet of the slaughtering 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 saline solution comprises between 70 and 170 g / l. 1 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 anolyte in the membrane cell can be advantageously transferred to an evolution tray for a time of

  extended stay, at a pH between 6 and 8, preferably between 6.5 and 7.

  It then comprises less than 5 g / l of metal hypochlorite

  alkaline, preferably less than 1 g / l.

  In the chlorate preparation process according to the present invention, the saline solution obtained by the purification method described above is then used as anolyte in the membrane cell at a pH between 1 and 8, preferably between 2 and 5, and at a temperature between 50 and 1000 C, advantageously between 70 and 901 C. Preferably, after its electrolysis, a part of

  the anolyte is recycled to the felling 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.

  weight It is also transferred to the felling 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 mixed with the chlorine gas produced during

  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 into the slaughter column and conversely the solution obtained. at the outlet of the slaughter column being used as anolyte in the

membrane cell.

  During the process according to the present invention, a stationary state is rapidly reached where the different solutions, at the outlet of the slaughter column or at the outlet of the anode compartment of the cell

  with membrane 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 taken out of the membrane cell necessary for it to be isolatable directly by crystallization, is easily determined from the known crystallization diagrams of water-chloride-chlorate systems (thesis of A NALLET, Faculty of Sciences of the University of Lyon, NI of order 209, defended on January 19, 1955) It is for example understood

between 400 and 650 g / I of anolyte.

  Thus, according to the present invention, part of the anolyte after electrolysis is transferred to a crystallizer or the chlorate is allowed to crystallize, the mother liquors being recovered and recycled in the loop

anodic membrane cell.

  It is also possible, optionally, to send the anolyte

  in a tray evolution before its transfer to the crystallizer.

  The alkali metal used in the process according to the invention is

  selected from lithium, sodium and potassium, preferably sodium.

  The present invention also relates to a device for the preparation of an alkali metal chlorate, using 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, a chlorine removal column with 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 an alkali metal hydroxide solution.

  Other features of the device according to this

  invention will appear on reading the detailed description made

  Hereinafter, with reference to the accompanying drawings, in which: FIG. 1 represents a general diagram of a device for implementing the method according to the present invention, FIG. 2 represents a preferred embodiment of FIG.

  device for purifying the alkali metal salt brine.

  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 an appropriate device (111) for admission and recovery (113) of the anolyte and an appropriate device for recovering the chlorine gas (112), the cathode compartment or compartments (12) each comprising an appropriate device (121) for receiving and recovering (122) the catolyte and a

  suitable device for evacuating gaseous hydrogen (123).

  The felling column (2) comprises at least one suitable device for admitting an alkali metal hydroxide solution (21) from the "chlorine-sodium hydroxide" type cell, an appropriate device for admitting chlorine gas (22) and preferably an inlet device (24) for a chloride-poor salt solution and a suitable device (23)

  recovering the saline solution obtained.

  The membrane cell (3) comprises, as the "chlororeside" type cell (1), one or more anode compartments (31) separated from the corresponding cathode compartment (s) (32) by a membrane (33), the at least one anode compartment comprising each an appropriate saline intake device (312), a suitable chlorine gas recovery device (314) and a recovery device for the saline solution;

  saline solution after electrolysis (311).

  The cathode compartment or compartments (32) of this membrane cell (3) comprise, for their part, an appropriate device for admitting (321) water, a suitable device for recovering (322) the catholyte after

  its electrolysis and a suitable device for extracting hydrogen (323).

  According to the invention, the appropriate device (313) for recovering the anolyte from the membrane cell (3) is connected to a suitable device

  introducing this anolyte (24) into the slaughter column (2).

  Similarly, the extraction device (314) for the chlorine gas is also connected to the slaughter column via the device

  suitable inlet (22) for chlorine gas.

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

crystallizer (4).

  Advantageously, the crystallizer comprises an appropriate device for recovering the 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 may be

  returned at the level of the anode loop defined previously.

  When taking into account the overall results of the "chlororesoude" cell (A) and the slaughter column (B t C) reactions, we obtain the following general equation D 6 F 3 x A 6 Me CI + 6 ## STR2 ## Me CI + 3 H 20 5 Me CI + Me Cl O 3 + 3 H 2

  Me being defined previously, and F representing a Faraday.

  Since the alkali metal chlorate obtained is part of the final balance of the chlorate preparation according to the invention, it may be considered

  that it is not an impurity.

  Consequently, the combination of an electrolysis of the "chlorine-soda" type with a chlorine slagging column by sodium hydroxide can be considered as a purification step of an alkali metal brine. In fact, the chloride solution obtained is virtually free of any impurity. Such an electrolysis-slaughter column combination unexpectedly appears as the only industrial process.

  to avoid all the impurities, harmful to the good function-

  membranes, such as calcium, magnesium, strontium, barium, iodine, bromine, aluminum, silica, sulfate, iron, manganese, copper, etc. Accordingly, the present invention also relates to a device for purifying an alkali metal chloride brine, comprising the combination of a chlorine-type electrolysis cell.

  soda "(1) and a slaughter column (2) of chlorine with sodium hydroxide.

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

  chlorine gas inlet (22), and an appropriate recovery device

  (23) a purified solution of alkali metal chloride, the hydroxide (21) and chlorine (22) inlet devices being respectively connected directly to the catholyte (122) and chlorine recovery devices;

  gaseous (112) of the "chlorine-soda" type cell (1).

  Advantageously, it is possible to add to the purification device according to the invention, an evolution tank connected directly to the

  recovery device (23) of the purified alkali metal solution.

  The following examples will illustrate the different

  steps of the method according to the invention.

  EXAMPLE 1 Purification of the Brine A cell, "chlorine-soda" (1) equipped with a membrane (13) N 90209 (sold under the trademark NAFION by the company DU PONT)

  produced at 30 A / dm 2, 19 g / h of CI 2 and soda at 32%.

  For 4 hours the chlorine is recovered at the bottom of a felling column (2) mounted above a tank thermostated at 50 C which contains 0.5 l of water. A measurement of p H, allows to regulate the addition of

  soda at 3296 to break down the chlorine and maintain the pH between 6.5 and 7.

  About 269 g of 32% soda was needed to slaughter

all the chlorine.

  Finally, a solution containing 12.3% by weight of NaCl and 4.5% by weight of Na CIO 3 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 SO 4 1 ppm) EXAMPLE 2 Influence of the Evolution Tray The operating device described in Example 1 is repeated.

  associated with a membrane cell (3).

  The anolyte of the membrane electrolyser (3) contains 120 to 150 g / l of NaCl and 450 to 500 g / l of Na CIO 3. The soda of the catholyte is 32% by weight and the temperature at C. at the terminals of the electrolyser is between 3.7 and 3.8 The chlorine produced by the membrane cell (3) and the cell

  "chlorine-soda" (1) is slaughtered 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 / 1. After its transfer to a storage tank maintained at 70 ° C., the hypochlorite content of

  sodium 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: E 5 Me Cl + Me CIO 3 + 15 H 2 O 6 Me Cl O 3 + 15 H 2 where Me is defined above and involves the transfer of 30 electrons. The overall D + E balance of the process for preparing the alkali metal chlorate according to the invention can therefore be summarized by the following equation F 6 FD 6 Me CI + 3 H 20 5 Me Cl + Me CIO 3 + 3 H 2 FE MeCl + Me CIO 3 + 15H 20 6Me CIO 3 + 15H 2 36FF 6MuCl + 18H 20 - * 6Me CIO 3 + 18H 2

  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), fed with raw brine or prepurified by usual techniques, and 5/6 of this type.

  transfer in the membrane cell (3).

  The products (C 12 and alkali metal hydroxide solution) passing from the "chlorine-soda" cell (1) to the next step are very pure and thus constitute at the outlet of the slaughter column (2) a brine very pure entering the membrane cell (3) This cell (3) and its membrane (33) will therefore operate in very good conditions that will prolong the life (strongly influenced by the impurity content of the electrolyte) of the membrane Thus 5/6 of the chlorate produced are in very good conditions for the lifetime of the membranes

whose cost is high.

  On the other hand, from the point of view of water balance, the usual process for the production of sodium chlorate requires the introduction of 1563 kg of water per tonne of Na CIO 3, combined with the sodium chloride supplied in the form of sodium chloride.

  of brine containing 26% by weight of NaCl.

  In the process described, the cell (3) is fed with a flux resulting from the reaction between the chlorine and the aqueous solution at 33% by weight of sodium hydroxide. This generally induces the introduction of only 719 kg of water per tonne. Thus, there is a saving of 844 kg of water that would have to be evaporated in an installation where sodium chlorate comes out in the solid state, that is to say, in any quantity. incoming water must be evaporated.

Claims (17)

  Process for the preparation of alkali metal chlorate by electrolysis in a membrane cell (3) of an anolyte comprising an alkali metal chloride solution and a catholyte comprising an alkali metal hydroxide solution, characterized in that that the alkali metal chloride solution is first purified by the following succession of steps: electrolysis in a chlorine-soda type cell (1) of an alkali metal chloride brine to form on the one hand chlorine gaseous, and on the other hand a concentrated solution of alkali metal hydroxide, transfer of the chlorine gas and alkali metal hydroxide solution produced, in a slaughter column (2), to make them react one on the other, and recovery of the saline solution thus obtained, for its   use as anolyte in the membrane cell (3).   Process according to Claim 1, characterized in that the   "chlorine-soda" type cell (1) is a membrane cell.   3 Method according to one of claims 1 or 2, characterized in   what the alkali metal chloride brine is pre-purified by   conventional methods of precipitation and / or adsorption on resins.   Process according to one of Claims 1 to 3, characterized in   what the brine comprises between 170 and 315 g / l of metal chloride   alkaline, preferably between 290 and 310 g / l.   Process according to one of Claims 1 to 4, characterized in   that the alkali metal chloride brine has a p H between 2 and   7, preferably between 2.5 and 4.5.   Process according to one of Claims 1 to 5, characterized in   that the alkali metal hydroxide solution obtained by the "sodium chlorine" type electrolysis 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) comprises 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.   Process according to one of Claims 7 or 8, characterized in   that the saline solution, before its use as 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   alkali metal hypochlorite, preferably less than 1 g / l.   Method according to one of claims 1 to 10, characterized in   the saline solution used as anolyte in the membrane cell (3) has a pH between 1 and 8, preferably between 2 and, and a temperature of between 50 and 1000 ° C, preferably between 70 and 900 ° C.   Process according to one of Claims 1 to 11, characterized in   what part of the anolyte, after its electrolysis, is recycled in the felling column (2).   Method according to one of Claims 1 to 12, characterized in   the alkali metal hydroxide solution obtained by electrolysis in the membrane cell (3) has a concentration of between 10 and   % by weight preferably between 30 and 50% by weight.   Process according to one of Claims 1 to 13, characterized in   what the catholyte, after its electrolysis, is transferred in the column slaughter (2).   Process according to one of Claims 1 to 14, characterized in   what the chlorine gas produced in the anode compartment (31) of the   membrane cell (3) is transferred to the felling column (2).   Method according to one of Claims 1 to 15, characterized in   that part of the anolyte, after its electrolysis, is transferred into a crystallizer (4) where the chlorate is crystallized, the mother liquors being recovered and recycled in the anode loop of the membrane cell (3).   Method according to one of Claims 1 to 16, characterized in   the alkali metal is selected from lithium, sodium and   potassium, preferably sodium.   Apparatus for the preparation of an alkali metal chlorate, characterized in that it comprises the combination of a "chlorine-soda" type cell (1), a chlorine slaughter column (2) a   alkali metal hydroxide and membrane cell (3).   19 Apparatus 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 or cathode compartments (12) corresponding by a membrane (13), the anode compartment or compartments each comprising an appropriate device (111) for admission and recovery (113) of the anolyte and an appropriate device for recovering chlorine gas (112), the one or more cathode compartments (12) each comprising an appropriate device (121) for admission and recovery (122) of the catholyte, and an appropriate device for the evacuation of gaseous hydrogen (123);   a slaughter column (2) comprising at least one suitable means for admitting an alkali metal hydroxide solution (21), a suitable chlorine inlet device (22), and a suitable recovery device ( 23) of a saline solution of alkali metal chloride, the hydroxide (21) and chlorine (22) intake devices being respectively connected directly to the recovery devices of the catholyte (122) and chlorine gas (112) the "chlorine-soda" type cell (1); a membrane cell (3) comprising one or more anode compartments (31) separated from the corresponding cathode compartment (s) (32) by a membrane (33), the anode compartment or compartments each comprising an appropriate intake device (312) for the saline solution, an appropriate device for recovering the chlorine gas (314) and a device for recovering the saline solution after its electrolysis (311), the cathode compartment (32) comprising an appropriate device for the admission (321) of water , a suitable device for recovering (322) the catholyte and an appropriate device for extracting hydrogen gas (323), the appropriate device (313) for recovering the anolyte from the membrane cell (3) being connected to an appropriate device for introducing this anolyte (24) into the slaughter column (2) and the extraction device (314) for the chlorine gas being also connected to the slaughter column by means of mediary of the appropriate device (22) for admitting chlorine gas; and the anode compartment (31) of the membrane cell (3) being connected either directly by suitable means or by the saline solution (311) after its electrolysis to a crystallizer (4), which comprises Advantageously, an appropriate device for recovering mother liquors (43) connected to the compartment   anode (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 the soda.   21 Device according to claim 20, characterized in that the cell type "chlorine-soda" (1), is a membrane cell, consisting of one or more anode compartments (11) separated from the cathode compartment or compartments (12) corresponding by a membrane (13), the anode compartment or compartments each comprising an appropriate device (111) for admission and recovery (113) of the anolyte and a device   chlorine gas recovery system (112), and the compartment or compartments   cathodic elements (12) each comprising an appropriate catholyte inlet (121) and recovery (122) device, and a suitable hydrogen gas evacuation device (123) and the slaughter column (2) comprises at least one suitable device for admitting an alkali metal hydroxide solution (21), a suitable admission device (22), and an appropriate device (23) for recovering a purified solution of metal chloride alkaline, the hydroxide (21) and chlorine (22) inlet devices being respectively connected directly to the catholyte (122) and chlorine gas recovery devices (112)   of the "chlorine-soda" type cell (1).