JPH0337967B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for desalting an aqueous electrolyte solution containing calcium ions and phosphate ions. When an electrolyte aqueous solution containing calcium ions and phosphate ions as impurities is desalted using an ion exchange membrane electrodialysis device (hereinafter also referred to as ED), scale is generated within and on the surface of the anion exchange membrane. This results in the phenomenon of damaging the film.
For example, in the process of using ED to remove electrolyte substances that accumulate in a system that circulates process water,
The above phenomenon occurs when the process water contains calcium ions and phosphate ions. In particular, when the electrolyte solution is alkaline, even a very small amount of these ions will cause damage to the anion exchange membrane in a very short period of time. In such a case, it is necessary to stop the operation of the ED and replace the membrane, which is not only economically disadvantageous, but also
It also has a negative impact on processes related to ED. The present inventors have conducted extensive research to establish a method for preventing damage to anion exchange membranes due to scale formation and for stable operation in desalting alkaline electrolyte aqueous solutions containing calcium ions and phosphate ions. layered. As a result, they discovered that by adjusting the PH of the concentration chamber solution to a specific value relative to the PH of the electrolyte solution in the demineralization chamber, it was possible to completely prevent the formation of scale within the anion exchange membrane, and completed the present invention. I came to the conclusion. That is, the present invention provides a method for desalting an electrolyte aqueous solution containing calcium ions and phosphate ions as impurities using an ion exchange membrane electrodialysis device when the pH of the desalination chamber solution is in the range of 7 to 12. A method for desalting an electrolyte aqueous solution, which is characterized by adjusting the PH (y) of a concentrated chamber solution to a value calculated by the formula y = -0.8x + 10.4 with respect to the PH (x) of a room fluid. . In the present invention, the ED is particularly limited to those having a known structure in which a plurality of cation exchange membranes and anion exchange membranes are arranged alternately between an anode chamber and a cathode chamber to constitute a concentration chamber and a desalination chamber. It can be used without any problem. Also,
The electrolyte aqueous solution containing calcium ions and phosphate ions is not particularly limited. For example, an electrolyte aqueous solution in which an inorganic salt such as common salt, mirabilite, potassium chloride, ammonium sulfate, etc. is dissolved as an electrolyte substance is generally used. Further, in the method of the present invention, the pH of the electrolyte aqueous solution, that is, the desalination chamber solution is 7 to 12, preferably 8.
It is particularly effective when it is in the range of ~10. The most important requirement in the present invention is the demineralization chamber fluid.
The PH (y) of the concentration chamber liquid is calculated using the formula y=
The purpose is to adjust the pH so that it is below the PH calculated by -0.8x + 10.4. The present inventors have conducted repeated research into the causes of scale that forms within the anion exchange membrane when desalting an electrolyte aqueous solution containing calcium ions and phosphate ions as impurities using ED. I learned that the PH relationship with the saline solution has a subtle effect. As a result of further research, they discovered the above relationship. Therefore, by adjusting the pH of the concentration chamber solution to a value below the value calculated by the above formula, it is possible to completely prevent the formation of scale within the anion exchange membrane. In the present invention, the means for adjusting the pH of the concentration chamber liquid to the above range is not particularly limited. Generally hydrochloric acid,
The pH may be appropriately adjusted to a suitable pH using an inorganic acid such as sulfuric acid or nitric acid, or an organic acid such as acetic acid. As can be understood from the above explanation, the desalination method of the present invention can completely prevent the formation of scale within the anion exchange membrane due to calcium ions and phosphate ions, and can therefore achieve stable demineralization for a long period of time. You can do salt. EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. Example 1 Neocepta ACH-45T as a cation exchange membrane
(Product name: Tokuyama Soda Co., Ltd.) as an anion exchange membrane.
Experiments were conducted using a clamp-type ED with an energizing area of 2 dm ² , 10 demineralization chambers, and 11 concentration chambers. Calcium ions are used as the desalination chamber fluid.
Sodium hydroxide was added to a 0.15N aqueous sodium sulfate solution containing 20ppm and 4ppm when converted to phosphoric acid ions to obtain the pH shown in Table 1, and a 0.4N aqueous sodium sulfate solution was used as a concentration chamber solution. A solution adjusted to the pH shown in Table 1 by adding sulfuric acid was used. In addition, the desalination chamber liquid is a continuous one-pass type, and the linear velocity inside the desalination chamber is 6cm/
sec, the concentration chamber liquid is partially circulated, water and sulfuric acid are replenished appropriately, and the current density is supplied so that the linear velocity in the concentration chamber is 3 cm/sec.
Electrodialysis was performed at 2 A/dm ² . The stack voltage is higher than the stack voltage at the start of electrodialysis.
Observe the time it takes for the scale to rise to 1.5 times and the state of scale adhesion on the anion exchange membrane at that time.
Shown in the table.

[Table] Example 2 In Example 1, the aqueous sodium sulfate solution was replaced with a saline solution of the same concentration, the sulfuric acid was replaced with hydrochloric acid, and when the PH of the desalting chamber solution was 10.5, the PH of the concentration chamber solution was adjusted to 1.8. Electrodialysis was performed under the same conditions except for the following. As a result, there was almost no increase in stack voltage even after 30 days had passed. Furthermore, there was no scale adhesion to the anion exchange membrane. Example 3 Using the same equipment as in Example 1, 10 ppm of calcium ions and 10 ppm of phosphate ions were added as a desalination room solution.
The same operation as in Example 1 was carried out except that a 0.15N aqueous solution of Glauber's salt containing 6 ppm in terms of Example) An experiment was conducted using two types of liquids prepared respectively. In the former case, there was almost no increase in stack voltage even after 30 days had passed. On the other hand, in the latter case (comparative example), after 10 days the stack voltage was approximately
It increased by 1.5 times. At this time, scale was attached to the entire surface of the anion exchange membrane on the concentration chamber side. Comparative Example 1 Electrodialysis was carried out in the same manner as in Example 2 except that the pH of the concentration chamber was adjusted to 3.0. As a result, the stack voltage increased to 1.5 times the stack voltage at the start of electrodialysis within 7 days. In addition, scale adhesion was observed inside and on the entire surface of the anion exchange membrane. Comparative Example 2 Same as Example 1, but with calcium ion
A desalting room solution with a concentration of 20 ppm and no phosphate ions was used. The pH of the desalination chamber solution/concentration chamber solution is 9/3, 9/5 ^* , 10/
3 ^* , 11/1 and 11/2.5 ^* (marked with * is outside the scope of the present invention), electrodialysis was performed for 20 days, but there was almost no increase in voltage, and after the operation was stopped, internal and anion exchange No scale was observed on the membrane surface.

Claims (1)

[Claims] 1 When desalting an electrolyte aqueous solution containing calcium ions and phosphate ions as impurities using an ion-exchange membrane electrodialysis device when the pH of the desalting chamber solution is in the range of 7 to 12, PH
The pH (y) of the concentration chamber liquid for (x) is expressed by the formula y=-
A method for desalting an electrolyte aqueous solution, characterized by adjusting the amount to a value equal to or less than a value calculated by 0.8x+10.4.