JPS5952711B2 - Electrolysis method of aqueous alkali chloride solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for electrolyzing alkali chloride, and particularly to a method for obtaining caustic soda and chlorine by stably electrolyzing a saline solution over a long period of time using a cation exchange membrane as a diaphragm.

In recent years, as a method for producing chlorine by electrolyzing an aqueous alkali chloride solution, the diaphragm method has been adopted in place of the mercury method from the viewpoint of pollution prevention.

In the normal diaphragm method, an asbestos diaphragm is used to prevent hydroxide ions generated in the cathode chamber from migrating to the anode chamber by partially flowing an aqueous solution of alkali chloride from the anode chamber to the cathode chamber through the diaphragm. ing.

Therefore, the caustic alkali generated in the cathode chamber contains approximately the same amount of alkali chloride as an impurity.

In response to this, a method for electrolyzing alkali chloride using a fluorine-containing cation exchange membrane having chlorine resistance and oxidation resistance has recently been proposed.

When a method using a cation exchange membrane as a diaphragm is adopted,
This has the advantage that the content of alkali chloride in the caustic alkali formed in the cathode chamber can be significantly reduced. However, in the case of ion-exchange membrane electrolysis, the aqueous alkali chloride solution and the aqueous caustic alkali solution come into contact with each other through the ion-exchange membrane, so calcium ions in the aqueous alkali chloride solution,
Magnesium ions come into contact with hydroxide ions inside the ion-exchange membrane, forming poorly soluble salts and producing fine precipitates within the membrane, which can lead to fatal increases in electrolytic voltage over time and destruction of the ion-exchange membrane. For this reason, ion-exchange membrane electrolysis requires more precise purification of the raw alkali chloride aqueous solution for calcium and magnesium than in conventional mercury or asbestos diaphragm electrolysis. It is said that

Therefore, as a purification method for an aqueous alkali chloride solution, instead of or in addition to the conventional method of adding caustic alkali and alkali carbonate to precipitate and remove calcium and magnesium as calcium carbonate and magnesium hydroxide, Alternatively, it has been proposed to remove calcium and magnesium by using a chelate resin that forms an intramolecular complex with magnesium and bringing it into contact with the chelate resin.

The purification method using a chelate resin can almost completely remove calcium and magnesium, and is an excellent method due to its high efficiency. However, even when an aqueous alkali chloride solution purified almost completely for canolecium and magnesium is electrolyzed by the ion exchange membrane method, the type of aqueous alkali chloride solution and its electrolytic conditions are still the same, according to the inventor's study. It has been found that in some cases, it causes major problems in ion-exchange membrane electrolysis.

In other words, in ion-exchange membrane electrolysis, certain aqueous alkaline chloride solutions are electrolyzed under acidic conditions with a pH of 5 or less, which is considered preferable in order to increase the purity of chlorine generated at the anode. It has been found that even if the electrolysis conditions are kept constant, the electrolysis voltage gradually increases as the electrolysis time increases. For example, in the case of electrolysis of a certain saline solution, the sliding voltage, which was 3.5V at the beginning of electrolysis, is
After 3400 hours (approximately 140 days), a phenomenon was observed in which the voltage rose to 3.75V. The present inventor further investigated the increase in electrolysis voltage caused by an increase in electrolysis time, and found that the surface or surface layer on the anode side of the ion exchange membrane used in the electrolytic cell with increased electrolysis voltage. A thin brown resinous layer was observed. Analysis of the thin layer of the ion exchange membrane revealed that it consists of various organic substances, including humic substances such as humic acid and humus contained in the aqueous alkali chloride solution and the anode chamber of the electrolytic cell. It was determined that this is a modified product under oxidizing conditions. If it is found that humic substances in the aqueous alkali chloride solution are the cause of the above, please
Although it is possible to employ known means for removing humic substances, it not only adds a new process, but also
It is extremely difficult to sufficiently remove such humic substances without interfering with electrolysis. Taking the above circumstances into consideration, the present inventor pursued a means to advantageously carry out ion exchange membrane electrolysis without causing troubles due to humic substances in aqueous alkali chloride solutions, and found that it is possible to conduct ion exchange membrane electrolysis in an electrolytic cell with a filter diaphragm containing asbestos, etc. The aqueous solution of alkali chloride separated from the aqueous caustic alkali solution obtained by electrolysis with a filtration diaphragm can be used as a raw material for diaphragm electrolysis, as will be clear from the examples described later. Furthermore, it has been found that ion exchange membrane electrolysis does not cause problems such as the increase in electrolytic voltage as described above.

Thus, the present invention provides an aqueous alkali chloride solution obtained by separating an aqueous alkali chloride solution containing humic substances from a caustic alkali aqueous solution obtained by electrolyzing an aqueous alkali chloride solution containing humic substances in an electrolytic cell with a filtration membrane made of asbestos, etc., into the aqueous solution as necessary. Electrolysis of an aqueous alkali chloride solution, characterized in that after removing as much calcium and/or magnesium as possible, the aqueous solution is supplied to an anode chamber of an ion exchange membrane electrolytic cell and electrolyzed under acidic conditions with a pH of 5 or less. It's in the method.

To explain the present invention in more detail below, the aqueous alkali chloride solution containing humic substances used in the present invention includes aqueous solutions of many industrial solid salts imported from overseas, especially solar salts.

For example, in a saturated aqueous solution of such a salt, calcium is
30 to 300 ppm as CaO, Mg as magnesium
It contains 30 to 300 ppm of O, and according to the inventor's analysis, it also contains humic substances such as humic acid and humus.
Humic substance content is 10mg or more, and even 300mg
An aqueous alkali chloride solution containing up to C can be applied successfully. Of course, an aqueous solution containing less humic substances than this may be used. The chloride/alkali aqueous solution is first prepared by using soda ash and caustic soda to convert the calcium content into calcium carbonate and the magnesium content into magnesium hydroxide, preferably in an amount of 1 to 1 as CaO.
0mg/1. It is purified to 0.5 to 5 mg as MgO. The pH of the alkaline chloride aqueous solution is adjusted as necessary.
After adjusting the value to 3 to 11, in the present invention, the material is then supplied to the anode chamber of the filtration diaphragm electrolysis using asbestos or the like as the filtration diaphragm for electrolysis. Various types of filter membranes can be used, but it has been found that asbestos or asbestos-containing membranes are particularly preferred in order to satisfactorily achieve the above objects of the present invention. The asbestos used is chrysotile. The asbestos may optionally be reinforced by impregnation with preferably 10 to 20% of tetrafluoroethylene resin, ethylene monotetrafluoroethylene resin, or the like. Filtration membrane electrolysis is carried out under known conditions. For example, the current density is 10 to 30 A/Dm2, and the voltage is 3.3 to 4.3 V.
, carried out at an electrolysis temperature of 70 to 100°C. In this way, by carrying out electrolysis using a filter diaphragm, an aqueous solution containing usually 8 to 15% by weight of caustic alkali and 14 to 22% by weight of alkali chloride is obtained from the cathode chamber. The catholyte is usually concentrated by evaporation to 48 to 50% by weight of caustic alkali, during which the salt contained therein is precipitated as crystals. Electrolysis performed by filter diaphragm electrolysis can yield, for example, about 1.66 tons while yielding 48% by weight of NaOHlt. The alkali chloride thus obtained no longer contains almost any humic substances such as humic acid or humus, as described above.

According to the inventor's analysis, the content is less than 10 mg, which is about 1/10 to 1/3 of the raw material for the above-mentioned diaphragm electrolysis.
It was found that the number had decreased to 00. The amount of calcium and magnesium in the saturated aqueous solution of alkali chloride is 2 to 10 ppm as CaO. 0 as Mg0
．． In most cases, it is between 5 and 2 ppm. The aqueous alkali chloride solution is preferably, but not necessarily, brought into contact with a chelate resin to further adsorb and remove calcium and magnesium therein.

As the chelate resin to be used, any resin that forms an intramolecular complex with calcium and/or magnesium can be used without any problem. Among these, those having a functional group of the general formula, 〉NCH2COO-, in the molecule, such as ethylenediaminetetraacetic acid, trimethylenediaminetetraacetic acid, iminodiacetic acid, aminomethylphosphonic diacetic acid, or a polygomer thereof, or an alkali of these carboxylic acids. Salt is preferred.

Two or more types of these chelate resins can be used if necessary, and they can be supported on activated carbon, coal, silica gel, zeolite, or the like. The contact between the aqueous alkali chloride solution and the chelate resin is preferably carried out at a pH of
The process is carried out at a temperature of 50 to 70° C. under alkaline conditions of less than 11 liters, particularly less than PH10.

Of course, the pH may be outside the above range, but if carried out under acidic conditions, humic substances will partially precipitate in the chelate resin, which will shorten the performance retention of the chelate resin, which is not preferable. The contacting is carried out in a fixed or moving bed, and the contacting time is preferably carried out at a space velocity (S) of 5 to 50.
The chelate resin that has reached saturation is regenerated with an acid such as hydrochloric acid according to known means and reused. In this way, preferably calcium and magnesium are each
0.2 mg or less as aO, 0.2 m as MgO
An aqueous alkali chloride solution purified to an alkali chloride concentration of 290 to 325 g, preferably 290 to 325 g, is subjected to ion exchange membrane electrolysis.

As the ion exchange membrane electrolytic cell, preferably a two-chamber type cell is used, which has a cation exchange membrane and partitions a cathode and an anode. The cation exchange membrane is preferably one having chlorine resistance and oxidation resistance; for example, a fluorine-containing cation exchange membrane is preferably used. As a fluorine-containing cation exchange membrane,
For example, tetrafluoroethylene and sulfonated perfluoroethylene
[Cation exchange membrane made of a copolymer with G-ether,
It is preferable to use a cation exchange membrane having appropriate chlorine resistance and oxidation resistance, such as a copolymer of perfluorovinyl ether and tetrafluoroethylene having an appropriate cation exchange group such as a carboxyl base or phosphate group. Note that an ion exchange membrane electrolytic cell may be formed by having a plurality of cation exchange membranes.
In this case, the ion-exchange membrane on the anode side should be chlorine-resistant and oxidation-resistant as mentioned above, and the other ion-exchange membranes should be hydrocarbon-based, such as styrene-divinylbenzene-based. be done. Graphite can also be used7 as the anode used in the electrolytic cell, but dimensional stability is achieved by coating a base material such as titanium or tantalum with a platinum group metal, an alloy thereof, or an oxide of a platinum group metal as appropriate. It is preferable to use a corrosion-resistant electrode.

Further, known materials such as iron, nickel, and stainless steel can be used as the cathode. 5. As for the type of electrolytic cell, any suitable type of cell such as a vertical cell or a horizontal cell may be adopted. When performing electrolysis in an ion exchange membrane electrolytic cell, an aqueous sodium chloride solution is supplied to the anode chamber, and water or a dilute aqueous caustic alkaline solution is supplied to the cathode chamber.

Known conditions can be adopted as the electrolytic conditions θ, but in order to prevent oxygen from being mixed into the chlorine generated at the anode and maintain its purity, it is preferable to adjust the pH of the anode chamber during electrolysis. It is preferable to maintain acidic conditions of 5 or less, particularly 3 or less. In order to maintain the anolyte in such an acidic state, an acid such as hydrochloric acid may be added directly to the anode chamber, but it is preferable to adjust the supplied aqueous alkali chloride solution to such acidity. The concentration of the caustic aqueous solution as the catholyte is preferably selected from an arbitrary value of 20 to 40% depending on the cation exchange membrane used and the electrolytic conditions. The current density in electrolysis is, for example, 5 to 30 Nd.
m! , voltage is 3.3 to 4.3, temperature is 70
Any value between ~100°C is adopted. In this way,
According to the present invention, when an aqueous alkali chloride solution is electrolyzed, even if the electrolysis is continued for a long time, the resistance of the ion exchange membrane due to the humic substances contained in the aqueous alkali chloride solution and even the sliding without causing a voltage increase.
It is extremely advantageous industrially. Examples are shown below to more specifically illustrate the present invention, but the present invention is not limited to the above description and the following examples, and various changes can be made within the scope of the present invention. .

Example 1 An aqueous solution of imported solid salt from Mexico was diluted with NaOH.
A saline solution purified by adding Na, ω, etc. (NaCl
Concentration 320g/1. Calcium content (CaO equivalent) 5.3
ppm, magnesium (MgO equivalent) 1.4 ppm, humic substance 100 ppm) were added to the anode chamber of a granol bipolar diaphragm electrolytic cell with two asbestos membranes at 10.0 ppm per tank.
While introducing at a rate of 3m3/hour, the temperature is 90℃ and the sliding voltage is 3.
．． 55, electrolysis was performed at a current density of 19.8 A/Dmj, and the generated catholyte was continuously taken out.

5 This catholyte contains NaOHl 28g/1. It contained 6 g of NaCl2O, 3.9 ppm of calcium (in terms of CaO), and 0.6 ppm of magnesium (in terms of MgO). The catholyte was heated at 9.2 g/l until the NaOH concentration was 737 g/l.
When concentrated at a rate of m$/hour, NaCl was formed as crystals (1
．． It was produced at a rate of 9 tons/hour. This NaCl crystal was separated, dissolved in water at a temperature of 460°C, and the NaCl concentration was 302g.
When added to an aqueous solution of Ha, the calcium content (CaO equivalent) was 2.08 ppm, and the magnesium content (MgO equivalent) was 0.
．． Contains 43ppm, and humic substances are 1pprn.
The amount was extremely small. By adding hydrochloric acid to electrolyze the salt solution in an ion exchange membrane electrolytic cell,
After adjusting to PHLO, the aqueous solution was brought into contact with a chelate resin to further remove calcium and magnesium from the aqueous solution.

As the chelate resin, 297-11 having an iminodiacetic acid group based on a styrene-divinylbenzene polymer is used.
Both types of contact were carried out by using a 90μ granular ion exchanger "Diaion CR-10" (manufactured by Mitsubishi Kasei Corporation) and supplying the above saline solution at a rate of 45001/hour to a packed tower filled with 150111 of the resin. By contact with the chelate resin, the calcium and magnesium contents in the saline solution were each purified to 0.2 mg or less.

On the other hand, as a cation exchange membrane, a 100 micron thick membrane obtained by copolymerizing tetrafluoroethylene and CF2=CFO(CF2)3C00CH3 was hydrolyzed with an alcoholic aqueous solution of caustic soda to form an ion exchange membrane. Ion exchange capacity of 1.40 meq
A two-chamber ion-exchange membrane electrolytic cell was constructed by using a fluorine-containing cation exchange membrane consisting of /g (dry resin) and partitioning a cathode and an anode.

A rhodium-coated titanium electrode was used as the anode, and stainless steel was used as the cathode. The distance between the electrodes was 2.2 cm, and the effective area of the membrane was 2 m2. The anode chamber was filled with 302 g of Na prepared above.
A Cl aqueous solution was charged, a 253 g/1 caustic soda aqueous solution was charged into the cathode chamber, and a 0.9 g/l caustic soda aqueous solution was charged into the anode chamber.
4 m3/hour, while supplying a predetermined amount of water to the cathode chamber so that the concentration of caustic soda obtained therefrom would be 576 g/l, the current density was 17.5 A/Dm2, the sliding voltage was 3.7 V,
Electrolysis was performed at a liquid temperature of 86°C and an anolyte pH of 3.

While the NaCl aqueous solution overflowed from the anode chamber, the caustic soda aqueous solution overflowed from the cathode chamber.The current efficiency was determined from the generated caustic soda and was 93.7%. Although the test was carried out for several hours, the sliding voltage remained at 3.71 V, and its rise was extremely slow compared to the comparative example. Furthermore, when the electrolytic cell at this point was disassembled and the anode surface of the ion exchange membrane was observed, almost no deposits were observed. For comparison, a saline solution with the same composition as that supplied to the granol electrolytic cell was purified with a chelate resin in the same manner as above without electrolyzing, and directly fed to the cation exchange membrane electrolytic cell under the above supply conditions. When electrolysis was carried out using the same supply and the same operating conditions, the sliding voltage was 3.70 V at the beginning, which was almost the same as above.
After 125 hours of testing, the sliding voltage significantly increased to 3.90V.

When the tank was dismantled and examined, significant non-uniform resin-like adhesion was observed on the side of the cation exchange membrane facing the anode.
When the deposit was analyzed by infrared absorption analysis and NMR, it was found to be humic. In addition, as the electrolytic diaphragm in the above-mentioned granol diaphragm electrolytic cell, an asbestos membrane (resin content: 8% by weight) reinforced with ethylene tetrafluoroethylene copolymer was used instead of the asbestos membrane. Almost the same results were obtained.

Claims (1)

[Scope of Claims] 1. An aqueous alkali chloride solution obtained by separating an aqueous alkali chloride solution containing humic substances from a caustic alkali aqueous solution obtained by electrolyzing an aqueous alkali chloride solution containing asbestos or the like in an electrolytic cell with a filtration diaphragm made of asbestos, if necessary. After removing as much calcium and/or magnesium as possible, the alkali chloride aqueous solution is supplied to the anode chamber of an ion exchange membrane electrolytic cell and electrolyzed under acidic conditions with a pH of 5 or less. Electrolysis method. 2. The method according to claim 1, characterized in that a chelate resin is used as a means for removing calcium and/or magnesium from the aqueous alkali chloride solution obtained by filtration membrane electrolysis. 3. The method of claim 2, wherein the chelate resin has the structural formula >NCH_2COO-.