JPS6054886B2 - Method for purifying aqueous alkali chloride solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for electrolyzing an aqueous alkali chloride solution, and more specifically, uses a chelate resin to stably remove calcium and/or magnesium in an aqueous alkali chloride solution to extremely trace amounts over a long period of time. The present invention relates to a method for purifying an aqueous alkali chloride solution.

It is well known that in alkali salt electrolysis for producing alkali metal hydroxide and chlorine, an aqueous alkali chloride solution used as a raw material must be sufficiently removed and purified, particularly for magnesium, calcium, etc. therein.

In particular, in the case of ion exchange electrolysis that has been proposed in recent years, an aqueous alkali chloride solution and an aqueous alkali metal hydroxide solution come into contact with each other through an ion exchange membrane, so calcium and magnesium ions in the aqueous alkali chloride solution are ,
When it comes into contact with hydroxide ions inside the ion exchange membrane, it forms a poorly soluble salt and generates fine precipitates within the membrane, leading to the fatal phenomenon of an increase in electrolytic voltage over time and destruction of the ion exchange membrane. . For this reason, in the case of ion-exchange membrane electrolysis, strict purification of the raw material aqueous alkali chloride solution is required compared to the conventional mercury method or asbestos diaphragm electrolysis. Conventionally, a widely used method for purifying aqueous alkali chloride solutions has been to add caustic alkali or alkali carbonate to precipitate and remove magnesium, calcium, etc. as magnesium hydroxide and calcium carbonate, respectively. Although this method has the drawback of requiring a long time, the extent of calcium and magnesium removal by this method is not sufficient, especially for ion exchange electrolysis, because there is an economic limit to the amount of purifying agent added.

Instead of or after the sedimentation separation method, impurities such as calcium and magnesium in the aqueous alkali chloride solution can be removed through a chelate resin ion exchange layer that forms an intramolecular complex with calcium and/or magnesium. This method has been proposed and is attracting attention not only because of its high efficiency but also because it can remove calcium and magnesium almost completely. However, when using such a chelate resin ion exchange method to purify an aqueous alkali chloride solution, according to studies by the present inventors, there may be major problems in the purification depending on the type of the aqueous alkali chloride solution to be purified and the purification conditions. It was discovered that something could happen.

That is, in a certain kind of aqueous alkali chloride solution, it is preferable to use it as a chelate resin, for example, at a pH of 6 to 6.
When processing by contacting with a chelate resin under the acidic conditions described in 7, there is an abnormal increase in pressure loss when the aqueous solution is passed through the chelate resin layer, and the adsorption capacity of the resin used may also decrease. Found out. Although the increase in pressure loss and decrease in adsorption capacity in the chelate resin layer will return to their original state when the resin is treated with an alkaline aqueous solution, the need for regeneration treatment within a short period of time is industrially difficult. This poses a significant problem when used. The inventor of the present invention conducted various studies regarding such a short-term decrease in the ability of chelate resins, and found that the cause was a problem with the aqueous alkali chloride solution that was purified.
It has been found that it is often found in the humic acids and humic substances contained therein, such as hummus. If it is found that the above cause is due to humic substances in the aqueous alkali chloride solution, it is possible to adopt a known means of removing such humic substances before purification, but this requires adding a new process. Not only this, but it is also extremely difficult to completely remove such humic substances.

Taking the above circumstances into consideration, the present inventors investigated the problems caused by humic substances in aqueous alkali chloride solutions. As a result of investigating methods that can advantageously carry out purification using a chelate resin without causing any oxidation, it was found that an aqueous solution of alkali chloride contained in a caustic alkali aqueous solution obtained by electrolyzing asbestos, etc. Even when an aqueous alkali chloride solution containing a considerable amount of humic substances is used as a raw material, it is shown in the examples below that the humic substances are not contained to the extent that they cause problems during purification with the chelate resin. It was discovered that

Thus, in the present invention, an aqueous alkali chloride solution containing humic substances is electrolyzed in an asbestos or the like overnight diaphragm electrolytic cell to obtain a caustic alkali aqueous solution containing an alkali chloride, and an aqueous alkali chloride solution separated from the aqueous solution is obtained. This is a method for purifying an aqueous alkali chloride solution using a chelate resin, which comprises contacting the aqueous solution with a chelate resin to remove calcium and/or magnesium from the aqueous solution.

The present invention will be described 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
30-300mg/e1 magnesium as aO
It contains 30 to 7,300 mg/' of O, and according to the analysis of the present inventors, it contains humic substances such as humic acid and humus. is 10m9/e or more and even 300
Alkaline chloride aqueous solutions containing up to m9/e can be applied well. Of course, an aqueous solution containing less humic substances than this may be used. Such an aqueous alkali chloride solution is first prepared using soda ash and caustic soda to convert the calcium content into calcium carbonate and the magnesium content into magnesium hydroxide, preferably each as CaO, in a concentration of 1 to 10 m9/E. ．． As MgO, 0.5-5m9
Refined to /e. In the present invention, after adjusting the pH of such aqueous alkali chloride solution to 3 to 11 as necessary,
Next, it is supplied to the anode chamber of the Fuchi diaphragm method electrolysis using asbestos or the like as a filtration membrane, and electrolyzed.

Various types of diaphragms can be used here, but it has been found that asbestos or asbestos-containing membranes are particularly preferred in order to satisfactorily achieve the above-mentioned objects of the present invention. Asbestos, chrysotile-based asbestos is preferably used. The asbestos may optionally be reinforced by using a tetrafluoroethylene resin, an ethylene-tetrafluoroethylene resin, etc., preferably at a weight of 5 to 2%.The diaphragm electrolysis is carried out under known conditions. For example, the current density is 5 to 30 A/Dm2, and the voltage is 3.3 to 4.3 V1.
Electrolysis is carried out at a temperature of 70-10°C. In this manner, an aqueous solution containing usually 8 to 15% by weight of caustic alkali and 4 to 2% by weight of alkali chloride is obtained from the cathode chamber by the diaphragm electrolysis. Such a catholyte is usually concentrated by evaporation to 48 to 5% by volume of caustic alkali, and during this time, the salt contained therein is precipitated as crystals. Such electrolysis by diaphragm electrolysis can yield, for example, about 1.66 t while obtaining 4% 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 approximately 10 m9/e or less,
It was found that the viscosity of the raw material for diaphragm electrolysis was reduced to about 1110 to 11300. In addition, the amount of calcium and magnesium in the saturated aqueous solution of alkali chloride is C
2-10ppm as aO 0.5-2p as Mg0
Most of them are pm. According to the present invention, the alkali chloride aqueous solution is brought into contact with a chelate resin,
Calcium and magnesium in it are adsorbed and removed.

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 them, the general formula in the molecule,
>N-CH2COO- functional groups, such as ethylenediaminetetraacetic acid, trimethylenediaminetetraacetic acid, iminodiacetic acid, aminomethylphosphonic diacetic acid, or their oligomers, or alkali salts of these carboxylic acids are 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.
A feature of the present invention is that the ability of these chelate resins to adsorb and remove calcium and magnesium in aqueous alkali chloride solutions can be maintained for a long period of time.

This is particularly noticeable when purifying an acidic alkali chloride aqueous solution with a pH of 6 to 7, where the adsorption ability of the chelate resin can be maximized. That is, in such a case, the adsorption capacity of the chelate resin is higher than that when purifying an aqueous alkali chloride solution having the same calcium and magnesium content by the same amount, compared to when purifying a conventional aqueous alkali chloride solution containing humic substances. It is extremely advantageous because about 10 to 20% more can be retained, there is no sudden increase in pressure loss due to the passing aqueous solution, and the time and number of times the chelate resin is regenerated can be reduced accordingly. Any known means can be used to adsorb calcium and/or impurities by contacting the aqueous alkali chloride solution with the chelate resin.

That is, an aqueous alkali chloride solution adjusted to a pH of 6 to 7, which is usually preferable in terms of the performance of a chelate resin, is supplied to a fixed bed or a moving bed filled with a chelate resin and brought into contact therewith. The contact time is preferably set to a space velocity (S) of 5 to 50.
Chelate resin with saturated adsorption performance is purified using a caustic alkaline aqueous solution after releasing adsorbed impurity metal ions using a releasing agent such as hydrochloric acid, sulfuric acid, or phosphoric acid.Alkali metal salt of alkali chloride It is regenerated into the form and subjected to adsorption operation again. In this way, calcium in an aqueous alkali chloride solution can be removed by adsorption using a chelate resin.
Magnesium is stably refined over an extremely long period of time to 0.2 m9/f or less as CaO and 0.2 m9/f or less as MgO, respectively. Examples are given below to more specifically illustrate the present invention, but the present invention is not limited by the above description or the following examples, and various modifications may be made within the scope of the present invention. It is possible.

Example 1 An aqueous solution of imported solid salt from Mexico was diluted with NaO according to a conventional method.
H. ．． A saline solution purified by adding Na2CO3 etc.
NaCl concentration 320g/', calcium content (CaO equivalent) 5.3ppm1 Magnesium. (MgO conversion)
1.4ppm1 humic substance 100ppm) was placed in the anode chamber of a granol bipolar diaphragm electrolytic cell with asbestos as the diaphragm.
While introducing at a rate of 10.3 i/hour per tank, the temperature was 90
℃, a sliding voltage of 3.55 V, and a current density of 19.8 A/Dm2, electrolysis was carried out, and the produced catholyte was continuously taken out.

This catholyte was NaOHl28g/E. ．． NaCe2O
6g/e1 Calcium content (CaO equivalent) 0.6ppm1
It contained 0.6 ppm of magnesium (calculated as MgQ).

When the catholyte was concentrated at a rate of 9.2 d/hour until the concentration of NaOH reached 737 g/e, 1
．． It was produced at a rate of 9 tons/hour. This NaC' crystal was separated and dissolved in water at a temperature of 46°C, and the NaCe concentration was 302g/
’, calcium content (CaO
Conversion) 2.08ppm1 Magnesium (MgO conversion) 0
．． Contains 43 ppm, and humic substances are 1.0 ppm.
The amount was extremely small. The saline solution was adjusted to pH 7 by adding hydrochloric acid for electrolysis in an ion-exchange membrane electrolytic cell, and then brought into contact with a chelate resin to further remove calcium and magnesium from the aqueous solution.

As the chelate resin, 297-11 having iminodiacetic acid group using styrene-divinylbenzene polymer as the base material is used.
Using a 90μ granular ion exchanger "Diaion CR-1σ" (manufactured by Mitsubishi Kasei Corporation), contact between the two was carried out by supplying the above saline solution at a rate of 4500'/hour to a packed tower filled with 150e of the resin. By contact with the chelate resin, the calcium and magnesium contents in the saline solution are purified to 0.2 m9/e or less, respectively, and this purification continues until approximately 20% of the theoretical capacity of the chelate resin is reached. At this point, the pressure drop increase due to the aqueous solution passing through from the initial stage was 0.2 g/d.

Which chelate resin has reached saturation? The regeneration treatment was carried out by using hydrochloric acid. After such regeneration, the saline aqueous solution was purified in the same manner as above, and a saline aqueous solution with approximately the same purification degree as above for calcium and magnesium was obtained, and the performance of the chelate resin was maintained for approximately the same amount of time as above. Ta. In addition, as the electrolytic diaphragm,
In place of the asbestos membrane, an asbestos membrane reinforced with ethylene-tetrafluoroethylene resin (resin content: 8% by weight) was used, but almost the same results as above were obtained. Comparative Example Similar to Example 1, imported solid salt aqueous solution (NaCe concentration, calcium content, magnesium content,
Humic substance is the same as in Example 1) is supplied to the diaphragm electrolytic cell,
After adjusting the pH to 7 by adding hydrochloric acid without electrolyzing, it was directly purified using a chelate resin in the same manner as in Example 1.

Purification using a chelate resin was carried out under exactly the same conditions as in Example 1, and a purified saline solution containing both calcium and magnesium contents of 0.2 m9/e or less was obtained.

Claims (1)

[Scope of Claims] 1. An aqueous alkali chloride solution containing humic substances is electrolyzed in an electrolytic cell with a filtration membrane made of asbestos or the like to obtain an aqueous caustic alkali chloride solution, and an aqueous solution of alkali chloride separated from the aqueous solution, 1. A method for purifying an aqueous alkali chloride solution using a chelate resin, the method comprising contacting the aqueous solution with a chelate resin to remove calcium and/or magnesium from the aqueous solution. 2. The method according to claim 1, wherein the contact is carried out with the chelate resin under conditions of pH 6 to 10. 3. The method according to claim 1 or 2, wherein the chelate resin has a structural formula ▲a mathematical formula, a chemical formula, a table, etc.▼. 4. The method according to claim 1, 2 or 3, wherein the aqueous alkali chloride solution is a saline solution.