JPS59112890A - Desalination by reverse osmosis membrane device - Google Patents

Abstract

PURPOSE:To efficiently remove silicic acid from raw water containing the large amount of silicates, by desalinating raw water having a pH adjusted above 8 with a reverse osmosis membrane device using an alkali-resisting reverse osmosis membrane such as a polyether amide composite film. CONSTITUTION:Raw water A is sent to a means 1 for softening hard water to remove hard components from said raw water. Thereafter, an aqueous caustic soda solution is injected from a caustic soda tank 3 into the soft water by a pump 2 to adjust the pH of said soft water above 8, pref. above 9. Said soft water having its pH adjusted is forcibly poured in a reverse osmosis membrane device 8 by a high pressure pump 7 to separately collect a desalinated filtrate B and unpermeating water C in which salts are concentrated. As a reverse osmosis membrane to be used in said reverse osmosis membrane device, an alkali-resisting reverse osmosis membrane such as a polyether amide composite film, a polyvinyl alcohol composite film, an aromatic polyamide film or a polybenzimidazole film is used.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for desalinating raw water containing a relatively large amount of silicic acid using a reverse osmosis membrane device.

In the electronics industry, which manufactures LSIs and super LSIs,
The amount of colloidal substances and ions used in cleaning the product
It was reduced to the pb order. It requires so-called ultrapure water. When producing such ultrapure water, reverse osmosis membrane devices are often used in recent years. In other words, raw water that has been subjected to appropriate pretreatment such as coagulation-sedimentation clarification, activated carbon filtration, etc. is desalted using a reverse osmosis membrane device, and then residual salts, impurities, etc., are removed from the desalted water using a pure water production device. , precision filtration equipment.

It is common to process with a polisher.

A reverse osmosis membrane device supplies raw water to a reverse osmosis membrane under pressure higher than the osmotic pressure of dissolved salts, and the reverse osmosis membrane blocks most of the salts to obtain permeated water with reduced salts as treated water. During this treatment, the colloidal substances contained in the raw water can be blocked by the reverse osmosis membrane, which is convenient for producing ultrapure water. . Since reverse osmosis membrane equipment processes raw water through the operations described above, the higher the concentration ratio of raw water, the more permeated water can be obtained from a fixed supply of raw water, which is advantageous in terms of cost. Become.

However, if the concentration ratio is increased too much, silicic acid, which has a relatively low solubility, will be concentrated.

In particular, it precipitates on the membrane surface of reverse osmosis membranes, reducing their performance.

For example, the silicic acid content of raw water is approximately 20■assi02. /
If the silicic acid content of the raw water is less than 40 mf aBS i02/l, there is not much of a problem, but if the silicic acid content of the raw water exceeds 40 mf aB Si02/l, the solubility of silicic acid becomes an obstacle and the recovery rate of permeated water has to be lowered, resulting in severe problems. It becomes uneconomical.

Therefore, if raw water with a relatively high silicic acid content is treated with a reverse osmosis membrane device and the recovery rate of permeated water is to be increased, it is necessary to perform the desilication treatment before the reverse osmosis membrane device. .

There is no way to reliably remove silicic acid at low cost.

This point has been an unresolved technical issue in reverse osmosis membrane devices.

The present invention has been made in view of this point.

The purpose is to process raw water containing a relatively large amount of silicic acid using a reverse osmosis membrane device without desilicating it, and to obtain permeated water with a high yield. When desalting raw water using a reverse osmosis membrane device, the pH of the raw water is kept at 8 or higher, and polyetheramide composite membranes and polyvinyl alcohol composite membranes are used.

This invention relates to a desalination method using a reverse osmosis membrane device that uses an alkali-resistant reverse osmosis membrane such as an aromatic polyamide membrane or a polybenzimidacylon membrane, and whose fi-characteristic is desalination using a reverse osmosis membrane device. .

The present invention will be explained in detail below.

The solubility of silicic acid in water varies greatly depending on pHK.
For example, the solubility of silicic acid at 25° C. is as shown in the curve shown in FIG. In other words, when the pH is below 7, it is about 100
'If assio, /l, but the pH is 8.0
The more alkaline it becomes, the greater its solubility; at pH 9, it is approximately 1'70'ml as 5i
02/l, approximately 320 Mf as 5i at pH 10
02/l.

The present invention was made based on the above finding that the solubility of silicic acid increases as it becomes more alkaline.
The pH of the raw water is 8 or higher.

Preferably, the concentration is increased to 9 or higher and desalted using a reverse osmosis membrane device.

In this way, when the pH of the raw water is made alkaline, the solubility of silicic acid increases, so even if the concentration ratio is increased, silicic acid does not precipitate on the membrane surface.However, conventional methods for desalting water have generally been the reverse of cellulose acetate. A permeable membrane is used.

The cellulose acetate membrane deteriorates significantly under alkaline conditions,
Therefore, it cannot be used in the present invention.

On the other hand, recently developed polyetheramide composite membranes, polyvinyl alcohol composite membranes, aromatic polyamide membranes, polybenzimidacylon membranes, and the like have alkali resistance.

Therefore, in the present invention, an alkali-resistant reverse osmosis membrane as described above is used.

In the present invention, the pH of the raw water is 8 or higher.

The main point is to use a reverse osmosis membrane that is preferably 9 or higher and is resistant to alkali.Therefore, reverse osmosis membranes other than the above four types may be used as long as they are resistant to alkali. do not have.

In the present invention, based on the silicic acid content of the raw water and the desired concentration ratio, an alkali such as hydric soda is added to the water supplied to the reverse osmosis membrane device based on the silicic acid content of the raw water and the desired concentration ratio, and the pH of the water is increased.
However, if the pH of the feed water is made alkaline to 8.0 or more, in addition to adding an alkaline agent, all or part of the raw water should be treated with strong basic anions in the OH form. Treatment with exchange resin or weakly basic anion exchange resin is also acceptable.

In this way, by making the pH of the raw water alkaline to 8.0 or more, the solubility of silicic acid increases.

Thereby, the conventional problem of silicic acid depositing on the membrane surface of the reverse osmosis membrane can be solved. However, if large amounts of hardness components such as magnesium ions or calcium ions coexist in the raw water.

Magnesium hydroxide or calcium carbonate may then precipitate on the membrane surface on the concentration side.

Therefore, if a large amount of hardness components coexist in the supplied water, it is desirable to remove the hardness components in advance.

Hardness components can be removed by water softening equipment or dealkalization softening equipment that uses Na-type or H-type strongly acidic cation exchange resins or weakly acidic cation exchange resins, or by adding lime extract or lime and soda carbonate to water. Lime method, lime soda method, etc. are used, in which the components are precipitated and removed as solids such as magnesium hydroxide and calcium carbonate. In addition, in the lime method, lime method, and soda method, the pH of the treated water is 8.
Since it is alkaline as described above, it also has the advantage of saving the trouble of adding alkali to the treated water again.

Next, embodiments of the present invention will be explained based on the drawings.

FIG. 1 is an explanatory diagram of a flow that is an example of an embodiment of the present invention, where J is a water softener filled with Na-type strongly acidic cation exchange resin, 2 is a hydrocarbon soda injection pump, and 3 is a hydrocarbon soda tank. .

4 is a line mixer, 5 is a pH detection unit, 6 is a pH controller, 7 is a high-pressure pump, and 8 is a reverse osmosis membrane device using an alkali-resistant reverse osmosis membrane.

In the present invention, raw water A is first passed through a water softening device 1 to remove hardness components from the raw water.

Next, an aqueous solution of sodium hydroxide from the sodium hydroxide tank 3 is added to the soft water through the injection pump 2 to bring the pH of the soft water to 8 or higher, preferably 9 or higher. In addition, during this injection, the pH is detected by a pH detection section attached after the line mixer 4, and the amount of the sodium hydroxide solution injected from the injection pump 2 is instrumented using a pH controller so that the pH value reaches a predetermined pH value. Adjust by 6. Through these operations, the hardness components of the raw water A have been removed and the soft water, which has been made alkaline in pH, is pressurized into the reverse osmosis membrane device 8 by the high-pressure pump 7, and the desalted permeated water B and the non-permeated water with concentrated salts are Collect water C. Although not shown, it is preferable to remove suspended matter using a precision filter or the like before forcing the soft water into the reverse osmosis membrane device 8.

In the present invention, since the pH of the raw water is made alkaline in advance, the solubility of silicic acid increases, and therefore, even if the concentration ratio is increased, silicic acid precipitates, which were previously contained, do not adhere to the membrane surface, and vice versa. Since the permeable membrane is made of alkali-resistant material, the performance of the membrane does not deteriorate due to high pH. Even if the raw water still contains a large amount of hardness components, by removing the hardness components in advance,
Precipitates such as magnesium hydroxide and carbonate do not adhere to the membrane surface.

Examples will be shown below to clarify the effects of the present invention.

Example-1 Raw water having the composition shown in Table 1 was first softened in a water softening device filled with Na-type strongly acidic cation exchange resin according to the flow of the present invention shown in Figure 1, and then treated with hard soda. In addition, the pH was adjusted to 9.0, and the permeated water recovery rate was 75% and the supply pressure was 30 ky/c using a reverse osmosis membrane device using a polyether amide composite membrane, 5PIJo membrane (trade name) made by Tsukushishi ■.
, l conditions.

On the other hand, for comparison, as a conventional method, the raw water shown in Table 1 was
Add acid to prevent precipitation of calcium carbonate.
H was adjusted to 6.0, and then treated under the same conditions using the same reverse osmosis membrane apparatus as used in the method of the present invention.

The results are shown in FIGS. 3 and 4.

In addition, Fig. 3 shows the amount of permeated water Qt as the operating time t elapses.
Figure 4 shows the change in the permeated water amount ratio (Q, t/Qo) based on the permeated water amount Qo at operating time t=o, and Fig. 4 shows the following equation as the operating time elapses. It shows the change in desalination rate (1) defined.

Water Quality of Raw Water 9 Table 1 shows the water supplied to the reverse osmosis membrane device in the method of the present invention and the water supplied to the reverse osmosis membrane device in the conventional method.

As is clear from Figures 3 and 4, in the conventional method, the performance of the reverse osmosis membrane device deteriorates significantly as the operating time passes, whereas in the method of the present invention, the performance of the reverse osmosis membrane device decreases significantly. There is no decrease in

It was shown that it is sufficiently practical.

Table 1 Example-2 The raw water shown in Table 2 was adjusted to pH 8.5 by adding sodium hydroxide, and then an aromatic polyamide membrane (trade name) made by DuPont, USA (trade name) was used. 70% yield of permeated water using reverse osmosis membrane equipment, supply pressure 25 ky/c
It was treated under fL conditions.

On the other hand, for comparison, as a conventional method, the same raw water was treated with cellulose acetate membrane manufactured by UOP Co., Ltd., without pH adjustment.
It was treated under the same conditions using a reverse osmosis membrane device using GA (trade name).

The results are shown in FIGS. 5 and 6.

Figure 5 shows the ratio of permeated water amount to operating time (
Similarly to FIG. 4, FIG. 6 shows the change in desalination rate (R) with respect to operating time.

This example also shows that the method of the present invention provides much more stable performance than the conventional method.

Table 2

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a flow showing an example of an embodiment of the present invention, and FIG. 2 is a graph showing the solubility of silicic acid at a water temperature of 25° C. The vertical axis shows solubility and the horizontal axis shows pH. Figures 3 to 6 show the ratio of permeated water amount or desalination rate to the operating time in the examples. In FIGS. 3 and 5, the vertical axis shows the VCC permeate water ratio. The horizontal axis shows the operating time, and in FIGS. 4 and 6, the vertical axis shows the desalination rate, and the horizontal axis shows the operating time. Figure 1 3 4 5 6 7 8 9 IQ H
+2 13 PH Figure 3 1 Transfer time (hr) Procedural amendment (spontaneous) March 19, 1980 Kazuo Wakasugi, Commissioner of the Patent Office 1, Indication of the case, 1988 Patent Application No. 222155 2, Invention Name of Desalination Method 3 Using Reverse Osmosis Membrane Device, Relationship with the Amendment Case Patent Applicant Representative Kunio Nagai
-4, Agent Address: 5-5-16 Hongo, Bunkyo-ku, Tokyo
￥￥8 1 2-5 1 5 1 5. Drawings subject to correction (Fig. 3 and Fig. 5) 6. Contents of correction As shown in the attached sheet, Fig. 3 Operating time (hr)

Claims (1)

[Claims] (↑) When raw water containing a relatively large amount of silicic acid is desalted using a reverse osmosis membrane device, the pH of the raw water is maintained at 8 or higher, and polyetheramide composite membrane, polyvinyl alcohol composite membrane, aromatic Desalination method using a reverse osmosis membrane device (2) Coexisting in raw water Desalination method using a reverse osmosis membrane device according to claim 1, in which the hardness component is removed in advance by a hardness component removal means (3) The hardness component removal means is a water softening device using a cation exchange resin or a de-delcalization device. Desalination method using a reverse osmosis membrane device according to claim 2, which is a water softening device (4) The hardness component removing means adds lime or lime and soda carbonate to remove hardness components from calcium carbonate, magnesium hydroxide, etc. A desalination method using a reverse osmosis membrane device according to claim 2, which is a device for removing precipitation as a solid substance.