JPS61181502A - Reverse osmosis membrane and its production - Google Patents

Abstract

PURPOSE:To obtain the title reverse osmosis membrane capable of separating a soln. of salts in remarkably high degree by allowing a metal to exist locally in the vicinity of the surface of a cellulose derivative membrane in high concn. CONSTITUTION:A polymer consisting of a cellulose derivative and org. additives such as maleic acid and butane tetracarboxylate are dissolved in a solvent consisting essentially of dioxane, and a film is formed from the soln. When the obtained reverse osmosis membrane is then heat-treated, metals such as Fe, Ti, Pb and Cr having >=4sp.gr. are added to a heat treating bath in the form of ions, chelates and elements. Concretely, inorg. salts such as ferric sulfate, ferric chloride and ferric nitrate are preferably used. The heat treating temp. is preferably controlled in the temp. range 30-95 deg.C. Consequently, a reverse osmosis membrane on the surface layer part of which metals are distributed in high concn. can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a reverse osmosis membrane made of a cellulose derivative having excellent liquid separation ability and a method for manufacturing the same.

More specifically, the present invention provides a separation membrane with a novel structure in which metal is present near the surface of the membrane, and also provides an improved technique for heat treatment.

[Prior art]

Reverse osmosis membranes are being used not only for desalination of seawater and brackish water, but also for the production of ultra-pure water and wastewater treatment, and their applications are expanding more and more.

Among these, cellulose-based reverse osmosis membranes have been steadily gaining success in various fields since the asymmetric membrane forming method was invented by Mr. Robb et al. in 1961 and put into practical use by Mr. Manjikian et al. in 1966. Improvements in membrane performance are also being actively pursued.
Many improved cellulose-based reverse osmosis membranes have been proposed to date.

Many of these are improvements to film-forming solutions, such as using cellulose derivatives such as cellulose acetate with organic acids (Japanese Patent Publication No. 45-20483), maleic acid monomethyl ester (Japanese Patent Publication No. 50-35074). For example, a film formed from a film forming solution prepared by dissolving a film forming solution in a solvent together with additives such as those disclosed in Japanese Patent Publication (Kokai).

Generally, reverse osmosis membranes made of cellulose derivatives are formed by wet membrane forming, in which the membrane forming solution is cast onto a support to an appropriate thickness, an appropriate amount of solvent is evaporated (casting step), This is carried out by placing the cast film in a coagulation bath, eluting the remaining solvent and additives, and coagulating the polymer (coagulation step). The membrane with the porous structure obtained in this way has an active layer with a separation function on the surface,
Although it has a sponge layer that serves as a support layer inside, it is usually loose and does not exhibit a sufficient salt rejection rate if it is left as is, so it cannot become a practical reverse osmosis membrane until it undergoes a rough heat treatment process.

However, although this heat treatment process improves salt rejection properties, it generally reduces water permeability, making it difficult to obtain a reverse osmosis membrane with a high salt rejection rate and a high magnolia excess. The current situation is difficult.

In the conventional technology, the improvement in the heat treatment process is &
At present, very little research has been done on this, and therefore a membrane with truly excellent separation ability has not yet been realized.

(Problems to be Solved by the Invention) In view of these circumstances, the present inventors aimed to improve the performance of reverse osmosis membranes made of cellulose derivatives by
The present invention was arrived at as a result of various intensive studies regarding the manufacturing process. That is, an object of the present invention is to provide a separation membrane with a novel structure in which metal is present near the surface of the membrane, and also to provide a method of treatment in which metal ions are present during heat treatment.

[Means for solving problems]

In order to achieve the above object, the present invention has the following configuration.

[(1) In a reverse osmosis membrane having a liquid separation ability made of a cellulose derivative, the average value of the membrane is 10 pp
A reverse osmosis membrane, characterized in that it contains a metal of m or more, and the metal is distributed at a high concentration on the surface of the membrane.

(2) In the production of a reverse osmosis membrane having a liquid separation ability made of a cellulose derivative, when the casting liquid is evaporated, then solidified, and then heat treated, metal ions are not included in the heat treatment bath. Characteristic method for producing reverse osmosis membranes. A reverse osmosis membrane made of a cellulose derivative as used in the present invention refers to a membrane formed using cellulose acetate, cellulose acetate butyrate, methyl cellulose, ethyl cellulose, or the like as a main polymer component. Among these, cellulose acetate is particularly preferred. Cellulose acetate herein refers to cellulose diacetate, cellulose triacetate, acetate with an intermediate degree of substitution, or a blend of cellulose acetate and cellulose triacetate. The practical acetyl group concentration in cellulose acetate is usually 39.8 to 43.2%, but in order to obtain a membrane with a balanced salt rejection rate and water passing rate, it is necessary to
Those having an acetyl group concentration of ˜42.5% are preferred.

Next, the separation membrane of the present invention will be explained using the drawings.

FIG. 1 shows the metal concentration in the cross-sectional direction of the active layer portion of the film of the present invention. The vertical axis shows the metal concentration, and the horizontal axis shows the depth of the film from the surface.

As is clear from FIG. 1, the film is characterized by a high concentration of metal distributed in the surface layer.

The surface layer portion of the membrane refers to at least one of the upper surface or the lower surface when the membrane is placed horizontally, and may of course be both surfaces. Further, the surface layer portion refers to a thickness on the order of microns. The membrane may then be an asymmetric membrane or a composite membrane.

In the present invention, the metal may be any metal, but preferably has a specific gravity of 4.0 or more, and more preferably one or more selected from l''elTi, Pb, and Or.The reason for this is unclear. This has been confirmed by experimental facts.Metals can be in any form, including ions, chelates, and elemental forms.

In the present invention, the concentration of metal in the surface layer of the membrane is preferably 10 times or more as compared to the center of the membrane in order to improve the separation ability, particularly 100 times or more.

Further, the shape of the reverse osmosis membrane of the present invention is not limited to a flat membrane, a hollow fiber membrane, a tubular membrane, or the like.

Next, the method of the present invention will be explained.

As the film-forming solution in the method of the present invention, various film-forming solutions consisting of polymers, additives, and solvents are applicable, and the type and composition thereof are not limited. However, it is preferable to use a film-forming solution containing dioxane as a main component as a solvent and an organic acid such as maleic acid or butanetetracarboxylic acid as an additive.

There are also no particular limitations on the film forming method;
A film obtained by a conventional wet film forming method in which the film is cast onto a support, evaporated part of the solvent, and then introduced into a coagulation bath is preferred for the present invention.

The metal ions added to the heat treatment bath, which is a feature of the present invention, may be any of the metal ions mentioned above. That is, any material may be used as long as it can adsorb metal onto the film during the heat treatment process. Specific examples of trivalent iron ions include inorganic salts such as ferric WL acid, ferric chloride, and ferric nitrate, complex salts such as potassium ferricyanide and sodium ferricyanide, and ferric hydroxide. bases, organic metal salts such as ferric acetylacetone and ferric thiocyanate. Among them, ferric sulfate and ferric chloride
Inorganic salts such as iron and ferric nitrate are suitable for the present invention.

Further, the optimum temperature for the heat treatment according to the present invention varies depending on the type and concentration of the metal compound added. Usually 3
The heat treatment temperature is carried out in the range of 0 to 95°C, preferably in the range of 60 to 85°C, but the temperature of the heat treatment is ultimately determined depending on the required performance of the reverse osmosis membrane.

〔Example〕

The present invention will be explained below with reference to Examples and Comparative Examples.

Generally, the performance of reverse osmosis membranes is expressed by the salt rejection rate and the amount of permeated water, but if the salt rejection rate is different, the membrane performance cannot be compared based on the amount of permeated water. In this example and comparative example, the heat treatment temperature was changed to make the salt rejection rates of both the same, so the membrane performance can be compared based on the amount of permeated water.

The membrane performance is 30-/30% of saline solution containing isooppm salt
Measurements were made by applying pressure at - and flowing it onto the membrane surface at a speed of 10 m/min.

Example 1 To 20 parts of a mixture of cellulose triacetate with a degree of acetylation of 43.2% and cellulose diacetate with a degree of acetylation of 39.8% in a weight ratio of 2=3, 40 parts of disuxane and 27 parts of acetone were added. A membrane-forming solution was obtained by adding 3 parts of butanetetracarboxylic acid dissolved in 10 parts of methanol to the solution. To form a film, use an applicator to cast the solution onto a glass plate to a thickness of 0.21 parts and 1 m in an atmosphere of 30°C, and after drying for about 1 minute, about 2
The film was peeled off from the glass plate by immersing it in cold water at 10°C for minutes. The obtained membrane was mixed with 5% by weight of ferric sulfate.
The sample was heat-treated for 5 minutes in a hot water bath (temperature 75°C).

The membrane thus prepared had a salt rejection rate of 98% and a water throughput of 0.75./-m2.day.

Further, the amount of metal present in the cross-sectional direction of the film was analyzed by the following method.

Analyzer: Secondary ion mass spectrometer SIMS (Seco
ndary Ion Mass 5pectrosco
py ), manufactured by CAMECA in France, type IM
S-3F, (Principle: A method in which an ion beam with an energy of about 5 to 15 KeV is applied to the sample surface, and secondary ions generated from the sample by sputtering are mass analyzed.) Measurement conditions: 1st Ion species: 02 Primary ion acceleration voltage: 10.5KV Primary ion acceleration current 2~5μA Analysis area 150μmφ/Raster 500μm Mouth vacuum:
2×10' Torr off-set voltage: Ov Since the material was an insulating material, Au was deposited on the surface, and charge-up was prevented by measuring from the Au surface.

The results are shown in FIG. Note that the vertical axis in FIG. 2 indicates the iron concentration distribution in the cross-sectional direction of the film in the vicinity of the active layer.

As a result of analysis from the chart in Figure 2, 1μ from the surface of the membrane.
Approximately 11,000 pp of Fe exists in m, and 1
The amount of Fe present in the ~10 μm region was approximately 40 ppm.

Comparative Example 1 The unheat-treated membrane obtained by immersing in cold water in Example 1 was heat-treated for 5 minutes in a hot water bath (temperature 80°C) to which no iron ions were added, and the salt rejection rate of the membrane was 98%. Water velocity excess is 0
．． 6Trl! /Tr12.day.

Note that the amount of Fe metal present in the obtained film was 5 ppm.

Example 2 A membrane obtained by using 3 parts of maleic acid in place of butanetetracarboxylic acid in Example 1 and keeping the other conditions the same had a salt rejection rate of 98.1% and a water velocity excess of 0.70 m! / m2
・It was day. Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 2 The unheated membrane obtained by immersing in cold water in Example 2 was placed in a hot water bath (temperature 80°C) to which iron ions were not added.
The salt rejection rate of the heat-treated membrane was 98.1%, and the water rate excess was 0.
．． It was 57 m'/m2・day.

Example 3 Using the same cellulose acetate as in Example 1, 2 parts of butanetetracarboxylic acid and 1 part of maleic acid were dissolved in the same solvent.
A membrane was formed in the same manner as in Example 1 using a membrane forming solution obtained by adding 10 parts of methanol.

The resulting unheated membrane was heat treated for 5 minutes in a hot water bath (temperature 70'C) containing 5% by weight of ferric sulfate.

The membrane thus obtained had a salt rejection rate of 97% and a water rate excess of 0.95 Tr157.2·day. Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 3 The immature treated membrane obtained by immersing in cold water in Example 3 was placed in a hot water bath (temperature 75°C) to which iron ions were not added.
The salt rejection rate of the membrane heat-treated for 5 minutes in the chamber was 97%, and the water velocity excess was 0.8 m'/m2·day.

Example 4 A membrane obtained by using 1 part of glycolic acid in place of maleic acid in Example 3 and keeping the other conditions the same had a salt rejection rate of 97.
%, water velocity excess was 0.82711'/m2·day.

Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 4 The immature treated membrane obtained by immersing in cold water in Example 4 was placed in a hot water bath (temperature 75°C) to which iron ions were not added.
The salt rejection rate of the membrane heat-treated for 5 minutes in the interior is 97%, and the water velocity excess is 0.75T11! /Tr12.day.

Example 5 Cellulose diacetate 20 with acetylation degree of 39.2%
40 parts of acetone, 15 parts of dioxane, and 12 parts of acetonitrile were added to the solution, and 3 parts of butanetetracarboxylic acid and 10 parts of methanol were added thereto. The film was formed using

The resulting unheated membrane was heat-treated in a hot water bath (75°C) containing 5% by weight of ferric sulfate, and the resulting membrane had a salt rejection rate of 93.
%, and the water velocity excess was 1.3 tn'/m2·day.

Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 5 The untreated membrane obtained in Example 5 was heat-treated for 5 minutes in a hot water bath (temperature 80°C) to which no iron ions were added.The membrane had a salt rejection rate of 93% and a water velocity excess of 1. 0m! /Tr12.day.

Example 6 To 8 parts of a mixture of cellulose triacetate with a degree of acetylation of 43.2% and cellulose diacetate with a degree of acetylation of 39.8% in a ratio of 1:1 by weight, 55 parts of dioxane and 15 parts of acetone were added. 10 parts of ethyl lactate and 12 parts of methanol were added to the dissolved solution to obtain a membrane forming solution. To form a film, the solution is cast onto a glass plate to a thickness of 0.2 mm using an applicator in an atmosphere of 20°C, and after drying for about 2 minutes, the film is immersed together with the glass plate in water at 15°C for about an hour to remove the film from the glass plate. This was done by peeling off the The obtained semipermeable membrane was heat-treated for about 5 minutes in a hot water bath (temperature 70° C.) containing 5% by weight of ferric acid.

The membrane thus obtained had a salt rejection rate of 90.4% and a water rate excess of 2.6 tn'/m2·day. Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 6 The unheated membrane obtained by immersing in water in Example 6 was heat-treated for 5 minutes in a hot water bath (temperature 75°C) not containing iron ions, and the salt rejection rate of the membrane was 90.4%. , the water velocity excess is 2
,4Trl! /Tr12.day.

Example 7 Cellulose diacetate 25 with degree of acetylation 40.3%
45 parts of acetone and 30 parts of formamide were added and dissolved to obtain a membrane forming solution.

To form a film, use an applicator to cast the solution onto a glass plate to a thickness of 0.2 mm in an atmosphere of 25°C, dry it for about 30 seconds, and then immerse it together with the glass plate in ice water for about 20 minutes to remove the film from the glass plate. This was done by peeling it off. The obtained membrane was heat-treated in a hot water bath (temperature: 70°C) containing 5% by weight of ferric sulfate.

The salt rejection rate of the membrane thus obtained was 92.5%.
Water speed excess is 1.5 m! / m2・day. Further, the metal distribution curve of the obtained film was similar to that of Example 1.

Comparative Example 7 The untreated membrane obtained by immersing in ice water in Example 3 was heat-treated for 5 minutes in a hot water bath (temperature 75°C) not containing iron ions, and the salt rejection rate of the membrane was 92.5/%. , water velocity excess is 1
．． It was 3m'/m2・day.

Example 8 In Example 1, the unheated membrane that had been immersed in cold water and subjected to jq was soaked in a hot water bath (temperature 7) containing 5% by weight of titanium sulfate.
A film was obtained by heat treatment at 5° C. for 5 minutes. The obtained membrane had a salt rejection rate of 98% and a water rate excess of 0.68 m3/m2·day.

Further, the titanium distribution curve of the obtained film was similar to that of Example 1.

〔Effect of the invention〕

In the present invention, since the metal is partially present in high concentration near the surface of the membrane, the liquid separation performance is significantly improved, and high-performance quality can be maintained with good durability. It can be used as a separation membrane. Further, the separation membrane of the present invention has the remarkable effect that it can be efficiently manufactured by treating it in the presence of metal ions in the heat treatment step.

[Brief explanation of drawings]

FIG. 1 is a distribution diagram showing the amount of metal present in the thickness direction of the separation membrane of the present invention. FIG. 2 shows an actual measurement chart of the separation membrane of Example 1 of the present invention using a secondary ion mass spectrometer. Patent Applicant Toshi Co., Ltd. Company Brother 1st, 2nd 1st, Case Description 1985 Patent Application No. 21969 2, Name of Invention Reverse Osmosis Membrane and Method for Manufacturing the Same 3, Person Making Amendment Relationship with the Case Patent applicant address: 2-2 Nihonbashi Muromachi, Chuo-ku, Tokyo Name (3
15) Toshi Co., Ltd. 5. Number of inventions increased by amendment None 6. In the specification subject to amendment (1) In the first line of page 9, "Determined." was replaced with "Determined." The present invention The reverse osmosis membrane can be obtained by incorporating metal ions by means such as adsorption in a heat treatment tank as described above, but it is also possible to obtain a reverse osmosis membrane by incorporating metal ions by adsorption or other means in a heat treatment tank. It can be obtained by processing.It can also be obtained by regenerating Zaraani membrane.''

Claims (6)

[Claims] (1) In a reverse osmosis membrane having a liquid separation ability made of a cellulose derivative, the membrane contains an average value of 10 ppm or more of metal, and the metal is distributed at a high concentration on the surface of the membrane. Characteristic reverse osmosis membrane. (2) The reverse osmosis membrane according to claim (1), wherein the metal has a specific gravity of 4.0 or more as a pure metal. (3) 1 where the metal is selected from Fe, Ti, Pb, and Cr
Claim No. (1) characterized in that it is more than one species.
Reverse osmosis membrane as described in section. (4) The reverse osmosis membrane according to claim (1), wherein the concentration of metal in the surface layer of the membrane is 10 times or more as compared to the center of the membrane. (5) In the production of a reverse osmosis membrane having a liquid separation ability made of a cellulose derivative, when the casting liquid is evaporated, then solidified, and then heat treated, metal ions are not included in the heat treatment bath. Characteristic method for producing reverse osmosis membranes. (6) The metal ion is sulfate, acetate, hydrate, nitrate,
The method for producing a reverse osmosis membrane according to claim (5), characterized in that the membrane is one or more selected from oxalates and ammonium salts.