NO145566B - COLORFUL PROTECTIVE FOR THREE. - Google Patents

Description

Process by electrolysis of aqueous alkali metal chloride solutions for the production of chlorine and alkali metal carbonates.

The present invention relates to

preparation of carbonates of alkali metals. The invention relates more precisely

a method by electrolysis of alkali metal chloride for the production of chlorine and

carbonates of alkali metals.

In Norwegian patent no. 109 760 is described

a procedure by overdrawing a

asbestos membrane' for use in an electrolytic chlor-alkali cell with a copolymer of an unsaturated organic compound

with which an acid group or acid-forming group is attached, and a polymerisable, ethylenically unsaturated compound, by

that the membrane is brought into contact1 with a

solution containing the unsatd

organic connection with the connected

acid group, so that the membrane thereby

impregnated with this and then removed

from the contact with the solution, and at

the method according to the patent is evaporated

the solvent from the membrane thus

that it becomes essentially solvent-free before it is brought into contact with it

polymerizable, ethylenically unsaturated compound, and is copolymerized with this

essentially in the absence of solvent.

The electrolyte present in that room

which contains the anode is denoted as

anolyte. The electrode in the space containing the cathode is called catholyte or cell fluid.

When cells equipped with this type of asbestos membranes containing polymer or a shaped plate of said polymer are used for the electrolysis of alkali metal chloride solutions, a migration of alkali metal ions from the anolyte chamber to the catholyte chamber takes place, resulting in the formation of a alkali metal hydroxide solution in the catholyte chamber, while gaseous chlorine is developed in the anolyte chamber. The alkali metal hydroxide thus formed in the catholyte chamber contains negligible or no impurities of alkali metal chloride. The production of alkali metal hydroxide solution which contains negligible or no such alkali metal chloride contamination is highly desirable, as the process steps normally required to remove chloride contamination contained in electrolytically produced alkali metal hydroxides in electrolysis cells of the usual membrane type, then get a significantly smaller scope.

The method which is shown in the above-mentioned Norwegian patent and which is mentioned here only as a reference, serves in a very desirable way to substantially reduce the amount of contamination from alkali metal chloride.

It has been shown that an improved efficiency, e.g. over 90 per cent and in many cases greater than 95 per cent, can be obtained in cells of the type described in the aforementioned patent by maintaining in the catholyte an ion ratio of carbonate (C03--^ to alkali metal of from 0.03 to 0.49, preferably between 0.08 and 0.42.Furthermore, the recovery of carbonate values present in the catholyte is improved by withdrawing at least a portion of the catholyte as an aqueous solution containing alkali metal hydroxide and carbonate, and having an alkali metal concentration of at least 105 g/liter of catholyte, preferably at least 165 g/liter, usually not greater than 175 g/liter, and carbonization of the thus extracted catholyte at a temperature of between 38 and 100°C; preferably between 54 and 93°C, in a carbonization zone outside the anolyte and catholyte chambers, whereby alkali metal carbonate, alkali metal bicarbonate and alkali metal sesquicarbonate are produced. As the cell fluid or catholyte is withdrawn from the cell for carbonization, a co continuous flow of carbonates of said alkali metal number from the carbonization zone outside the cell to the catholyte space in the cell, whereby the said ionic ratio of carbonate to alkali metal is maintained in the catholyte.

The carbonate ion in the sense used above is understood to mean the carbonate radical (COs—) in the ionized state or ionically bound to alkali metal in aqueous solution and as alkali metal carbonate in the anhydrous state. Alkali metal ions in the sense used above are to be understood as alkali metal ions in the ionic state or ionically bound to hydroxyl or carbonate ions in aqueous solution and as alkali metal hydroxide or alkali metal carbonate in the anhydrous state.

In a preferred embodiment according to the invention which leads to the production of alkali metal carbonate, in particular sodium carbonate (Na2COs), cell liquid with the above-mentioned carbonate values is used and this is extracted from the catholyte compartment and divided into two portions. A portion of cell fluid is fed to a carbonization zone which is maintained at a temperature of between 38 and 100°C, where the fluid is carbonized to produce a mixture of sodium carbonate and sodium bicarbonate. Appropriate setting of the carbonization will result in the production of sodium sesquicarbonate. The amount of sodium bicarbonate thus produced is preferably at least equimolar with the amount of sodium hydroxide in the second portion of cell fluid. The feed liquid for the carbonization is converted into a slurry in the carbonization zone. The resulting slurry is fed to a precipitation zone where the solids (primarily alkali metal carbonates) are separated from the precipitation liquid. This liquid, which contains significant amounts of sodium carbonate and bicarbonate, is recycled to the catholyte compartment for

to obtain the desired carbonate and hydroxide values in the cell. The precipitated solids are combined with the other portion of the cell fluid in a reactor. The temperature in the reactor is maintained so that all sodium hydroxide and bicarbonate are converted to sodium carbonate. The desired temperature lies

between 35.5 and 110°C. The resulting slurry of sodium carbonate monohydrate

then filtered and dried to give anhydrous sodium carbonate. The filtrate is recycled to the carbonization zone.

In another preferred embodiment according to the invention, sodium bicarbonate is produced by adding all of the above

cell fluid to the carbonization zone which is maintained at a temperature of between 38 and

100°C. The carbonation is set so that a slurry is produced which contains predominantly amounts of sodium bicarbonate in solid form. The resulting slurry is fed to a precipitation zone where the solid bicarbonate is concentrated. A portion of the remaining liquid after precipitation is fed to the carbonization zone and a portion is fed to the catholyte compartment, where it, in conjunction with the filtrates mentioned below, provides the desired carbonate and hydroxide values. The precipitated bicarbonate is then filtered in solid form, washed and dried. The filtrate mixed with the washing solution is recycled to the catholyte compartment as mentioned above, to help obtain the desired carbonate and hydroxide values.

In the previous description of the production of sodium carbonate, it is mentioned that regulation of the carbonisation leads to the production of sodium sesquicarbonate in solid form. Instead of combining the entire amount of sodium sesquicarbonate in solid form with the other portion of the cell fluid in the reactor, it is possible to extract part of this to a drying device and thus recover sodium sesquicarbonate.

Carbonization preferably takes place at a temperature of between 38 and 100°C, as a significant amount of water is removed from the cell fluid. The temperature is usually achieved by carbonizing hot liquid with a gas containing CO 2 , preferably flame tube gas obtained from a natural gas, coal or fuel oil combustion boiler. Flare tube gas contains large amounts of C02. The gas's other constituents are demonstrably inert to the alkali metal materials to be treated. The flame tube gas usually has a temperature of between 38 and 71 °C when introduced into the carbonization zone. The cell liquid is usually introduced into the carbonisation apparatus at a temperature of between 82 and 93°C. In the preferred operation, the temperature of the cell liquid at the introduction into the carbonization zone is greater than the temperature of the C02 gas stream. In this way, it is possible to remove significant amounts of water together with the exhaust gases during carbonisation.

In most cases, it is desirable to remove catholyte from the cell in the form of a saturated solution. This means that the amount of alkali metal ions, hydroxyl ions and carbonate ions in the solution is at a point where any additional amount will cause precipitation of either alkali metal carbonate or/and alkali metal hydroxide. Naturally, this does not preclude the production of a saturated catholyte liquid containing suspended alkali metal carbonate in solid form and/or hydroxide. The amount of solid matter that can be produced in the catholyte compartment depends on the degree of permitted contamination of the catholyte compartment as a result of sedimentation during the electrolysis. Good practice excludes precipitation of solids at the bottom of the catholyte compartment. If, on the other hand, excessive precipitation were to occur, this precipitate can be removed by redissolving the solids. This can be achieved by arranging a diluted aqueous solution in the catholyte compartment and temporarily feeding the recycled carbonate solution to the diluted liquid after the liquid has been taken out of the catholyte compartment. In some cases, it will be necessary to arrange agitators in the catholyte space to cause rapid dissolution of the solid alkali metal carbonates and/or solid alkali metal hydroxides. The hydrogen gas developed at the cathode should provide sufficient stirring.

In some cases, it may be desirable to produce a dilute aqueous cell fluid, namely with an ionic ratio of carbonate to alkali metal of between 0.03 and 0.1. Although it is possible to carry out carbonisation of such a liquid to effect the production of alkali metal carbonates in solid form, in practice it is better to combine a solution of carbonate used for recycling with this cell liquid before the carbonisation of the latter solution. In this embodiment, on the other hand, approximately half or more of the solution of carbonate used for recycling is led to the cathode space for maintenance of the aforementioned ion ratio carbonate to alkali metal, to the carbonization zone or to the cell liquid prior to the carbonization.

Besides the fact that degrees of effectiveness of over 90 and up to 95 per cent are achieved by adding alkali metal carbonate and bicarbonate to the catholyte to produce a mixture of alkali metal hydroxide and alkali metal carbonate, it has been shown that this latter mixture offers a significant saving in the production of high purity alkali metal hydroxide. For example, a portion of the liquid which already has a high content of alkali metal hydroxide can be treated with slaked lime to convert the carbonate to hydroxide by using i and

method known per se. Less slaked lime is needed as a result of the large size

content of alkali metal hydroxide already present in the liquid. More specifically, the cell fluid which is rich in alkali metal hydroxide can be fed together with a slurry of slaked lime and the reaction product fed to an evaporator for recovery of the resulting hydroxide. Slaked lime is thus converted into calcium carbonate by reaction with alkali metal carbonate. The calcium carbonate can then be split to form

carbon dioxide which is then fed to the carbonisation zone and calcium hydroxide which is used for further conversion of alkali metal carbonate into hydroxide of the corresponding alkali metal. It is thus possible to produce one, two or three commercial products at the same time by processing the same cell fluid solution.

The attached drawing of a special embodiment according to the invention must not be understood as a limitation of the invention to the characteristic features shown there. Fig. 1 schematically shows different methods for carrying out the processes according to the invention and Fig. 2 is an isometric elevation of a cell with an ion-permeable membrane, in which the above-mentioned electrolytic process can be carried out, the parts being shown separated from each other.

With reference to fig. 1, electrolysis of an aqueous saturated salt solution of sodium chloride takes place in a cell 10 provided with separate compartments, which has anolyte compartment X and catholyte compartment Y mutually separated by means of an ion-permeable membrane Z. The salt solution is introduced through lines 43 and 24 and unsplit salt solution is recycled through line 24 and saturated again by the addition of further salt solution from line 43. Catholyte liquid is formed by introducing water through line 23 and solutions consisting of carbonates of sodium through lines 22 and 28 which will be explained in more detail below.

■ To carry out an electrolysis with a saturated salt solution, any cell equipped with an ion-impermeable membrane can be used. Cells that

contain these ion-impermeable membranes comprise an anode and a cathode with an ion-impermeable membrane

which separates the two, whereby an anolyte and a catholyte space are formed in connection with the cell's structure. Fig. 2 shows a typical example of such a cell consisting of an anode 100 comprising a graphite sheet, a rectangular spacer 103 which is hollow inside, a rectangular ion-impermeable membrane 104 and another spacer 106 followed by a cathode grid 101, all of which are arranged in series and attached to each other. Spacer gaskets have been used to separate the spacers from the anode and the cathode spacer from the membrane. In fig. 2, the anode 100 is thus separated from the spacer 103 by the gasket 102, which has exactly the same shape as the spacer 103. The membrane 104 lies close to the spacer 103. This membrane is a rectangular sheet with the same rectangular surface as the graphite sheet 100. The membrane 104 is separated from the cathode grid 101 using the gasket 105 and the spacer 106 in the order shown in fig. 2. Rubber gaskets 113 are used as a substrate for the graphite anode and the shielded cathode. The electrical connections at 107 and on the side of the cathode grid (not shown) are arranged in the usual way. When the different sections are clamped together into a unit, the structure has a hollow interior space that is characterized by the spacers' rectangular cavities. This internal cavity would extend from the anode to the cathode except for the presence of the barrier membrane. Thus, the membrane forms an anode space and a cathode space in which liquids can flow.

Through the spacer 103 extends

a drainage pipe for salt solution and an introduction line for salt solution (not shown in Fig. 2) placed diagonally 1 relative to the overflow pipe for the salt solution on the opposite side of the spacer near its bottom. On the spacer 103, outflow lines for chlorine are arranged in this part in such a way that the developed

chlorine is led through the spacer and the lines to the chlorine collector pipe 108. In the spacer 106 there is a line 110 for

removal of cell fluid, a line 111 for removing hydrogen and a line 112 for introducing water. Each of these pipelines is arranged in such a way that they receive products from or introduce materials into the space that makes up the catholyte chamber. All parts in fig. 2 are bolted together to form a unitary structure capable of continuously producing chlorine gas and cell fluid by electrolysis of sodium chloride as described above.

A number of different membranes can be used. A preferred membrane material consists of maleic anhydride divinylbenzene-styrene terpolymer as prepared in example 1 in Norwegian patent no. 109 760. Other known polymers containing cross-linked carboxyl can also be used. The resin is preferably coated on an asbestos membrane by polymerization in situ on its surface.

Other applicable membrane materials include inorganic exchangers such as zeolite and synthetic organic polymers such as sulfonated styrene-divinylbenzene copolymers.

The cell described above can be used as a unit in a bipolar cell with several compartments as described in Norwegian patent no. 110 146. A number of these units inserted next to each other in a cell are preferably used in large-scale production of chlorine and catholyte products according to the present invention. Reference is made to fig. 1 in the following example when describing the continuous operation of a specific process which includes the features according to the invention.

Example.

Four cell batteries, of which each cell battery contains 48 units of the one in fig. 2 shown species arranged in the cell battery in the manner shown in Norwegian patent no. 110 146 and with anode plates measuring 1.8 x 1.2 m, are set up in series of four to produce the cell fluid described below. The current applied to each series of four of these cell batteries is 3,000 A and the voltage in each of the cells is approximately 4.1 V. The anode and cathode in each cell are separated from each other by means of an asbestos membrane which has a coating of a malic acid anhydride divinylbenzene -styrene terpolymer, produced in accordance with example 1 in Norwegian patent no. 109 760.

An aqueous saturated NaCl solution is continuously supplied to the anolyte space at a rate of 510 kg/h. When the cells are in operation, the production of sodium ions is 16.4 kg/h and the temperature in the cells is maintained at 90°C. Hydrogen is taken out through the line 32 as shown in the schematic representation in fig. 1. 358 kg/h of cell fluid with a temperature of 90 °C is extracted from the cells to a common collection pipeline shown in fig. 1 as the line 20. The cell liquid is divided into two portions, of which 246 kg/h is passed through line 26 and 113 kg/h through line 20 to the carbonization tower 11. The cell liquid contains 21.9 percent sodium carbonate, 5.7 percent sodium hydroxide and 72.4 percent water based on the weight of the solution. During this operation, the valves J and A are open. The carbonizer 11 is a cylindrical steel container with a ratio of height to width equal to 8. The cell fluid is led by means of the line 20 to the top of the carbonization tower. At the same time, flame tube gas with a temperature of 38°C is introduced into the bottom of the tower 11 at a rate of 133 kg/h. The flue gas obtained from burning natural gas has the following composition in percentage by weight: 14.5% C02, 72.65% nitrogen, 1.02% oxygen and 11.84% water. The exhaust gas contains a significant amount of evaporated water and is removed through line 39 at a rate of 18.2 kg/h. The temperature in the carbonization tower is 71°C.

Through the bottom of the tower 11, slurried sodium sesquicarbonate is extracted there at a rate of 390 kg/h. The slurry is collected in a settling tank 12, which is a stirred thickening tank with a sloped bottom. The solid content of sodium sesquicarbonate in the slurry is fed from the precipitation tank 12 to the reactor 13 through the line 35 at a rate of 78.5 kg/h solids. Valve C is closed.

The remaining liquid at the top of the precipitation tank 12 is recycled through the line 22 to the catholyte compartment Y by means of the pump 19 at a speed of 340 kg/h and contains, in parts of the solution's weight, 18.3 per cent sodium carbonate, 5.5 per cent sodium bicarbonate and 76.2 percent water. Additive water is introduced into the cells' catholyte compartment through line 23 at a rate of 30.8 kg/h. Lines 26 and 35 lead the portions of treated and untreated cell liquid to the reactor tank 13, which can accommodate 680 kg of said material. The reactor 13 is kept at a temperature at around 55° C. The average residence time of the material in the reactor is 60 minutes. No external heating is required due to the absorbed heat held by the different proportions.

The material extracted from the reactor 13 is a slurried monohydrate and this is fed to the filtration apparatus 14 at a rate of 325 kg/h through line 41. No water is fed to: the filtration apparatus 14 through line 31. The filtrate, which is a solution of sodium carbonate and contains 32% by weight of this carbonate, is recycled to the carboniser 11 by means of the line 34. The recirculation rate is 281 kg/h. Valve C is open and valve H is kept closed. Sodium carbonate monohydrate in solid form extracted from the filtering apparatus 14 is fed at a rate of 44.2 kg/h to the drying apparatus 15, which is a rotary oven. The water is evaporated in the drying apparatus 15 and removed by means of the line 38 at a rate of 6.5 kg/h. Condensed sodium carbonate is extracted from the drying device through line 37 at a rate of 37.8 kg/h.

Although the description above shows the invention in the form of a very special embodiment, this should not be understood as any limitation of the present invention apart from the limitations given by the claim.

Claims (1)

Process for electrolysis of an aqueous solution of alkali metal chloride in an electrolysis cell with an ion-permeable membrane that separates the anode and the cathode and thus forms an anolyte and a catholyte compartment, for the production of chlorine and alkali metal carbonates, characterized in that as catholyte an aqueous solution is maintained with a ratio of CO:1 ion to alkali metal ion of from 0.03 to 0.49, preferably from 0.08 to 0.42, so that catholyte containing at least 105 g/litre of alkali metal, that the solution thus withdrawn is carbonized outside the anolyte compartment for the production of carbonates of alkali metals and that an aqueous solution consisting of part of these carbonates is recycled to the catholyte compartment.