1. Field of the Invention
This invention relates to a semipermeable membrane for separating ingredients of liquid mixture by selective permeation, and more specifically, it relates to a semipermeable composite membrane for the desalination of sea water, brackish water and selectively removing or recovering pollutant or useful materials from waste water causing public pollutions, for example, from dyeing process or from electrodepositing process, and thereby capable of contributing to the completeness of treatment of such waste water under a closed system.
2. Description of the Prior Art
Asymmetric membranes prepared from cellullose acetate are known as the semipermeable membranes utilized industrially, for example, so-called "Loeb" type of those membranes as disclosed in U.S. Pat. Nos. 3,133,132 and 1,133,137. However, these membranes are poor in the resistivity to hydrolysis, bacterial attack and chemicals. Particularly, if tried to improve their permeability, their pressure resistance and durability are sacrificed, and accordingly they have not yet been put into practical use for a wide range of application although employed in a restricted field.
Extensive studies have been made for investigating a new material free from the drawbacks of the asymmetric cellulose acetate membranes mainly in U.S.A. and Japan, and those materials partially improved the foregoing drawbacks have been obtained, such as aromatic polyamide, polyamide hydrazide (U.S. Pat. No. 3,567,632), polyamide acid (Japanese Patent Publication No. 55-37282, U.S. Pat. No. 3,878,109), cross-linked polyamide acid (Japanese patent Publication No. 56-3769), polyimidazopyrrolone, polysulfonamide, polybenzimidazole, polybenzimidazolone and polyarylene oxide. However, none of them is superior to the cellulose acetate membrane with respect to the selective separation characteristic or permeability.
On the other hand, composite membranes comprising ultra-thin membrane which is substantially responsible to membrane performances and a microporous substrate for supporting the ultra-thin membrane have been developed as different type of semipermeable membrane from the "Loeb" type. In such composite membrane, it is possible to select the most optimum materials to each of the ultra-thin membrane and microporous substrate according to the purposes, and this can increase the degree of freedom for membrane fabrications. Another advantage of those composite membranes is said that they can be stored under the dry state different from the "Loeb" type membranes which must be stored always in a wet condition.
These composite membranes are classified into two types, that is, those comprising an ultrathin membrane and a microporous substrate coated with the ultrathin membrane directly and those comprising an ultrathin membrane, a microporous substrate and a gel layer intermediating between them. Example of the former are described in U.S. Pat. Nos. 3,744,642 and 3,926,798, Japanese Patent Application Laid-Open No. 55-147106 (U.S. Pat. No. 4,227,344), U.S. Pat. Nos. 4,557,949 and 4,366,062 and Japanese Patent Application Laid-Open No. 58-24303, and examples of the latter are described in Japanese Patent Application Laid-Open No. 49-133282 (U.S. Pat. No. 4,039,440), Japanese Patent Publication Nos. 55-49524 (U.S. Pat. No. 3,951,815 and 55-38164 (U.S. Pat. No. 4,005,012), PB Report No. 80-182090 and U.S. Pat. Nos. 4,387,024 and 4,559,139.
Those prior semipermeable composite membranes mentioned above, however, can not satisfy all of the desired properties required to the membranes, such as high solute rejection against both organic and inorganic materials dissolved in water, high water flux rate, durability such as heat resistance, chemical resistance, pressure resistance, chlorine resistance and hydrogen peroxide resistance.
The reverse osmosis is generally expected to be applied to various fields, such as, desalination represented by the conversion of sea water or brackish water into portable water, concentration or recovery of valuable materials, production of ultrapure water and separation of organic liquid mixture. Accordingly, the desired specific properties required to the reverse osmosis membrane would vary multifariously depending on its use and application. The important properties among them are selective permeability (solute rejection: Rej (%), water flux: Flux (m.sup.3 /m.sup.2.day)), durability (m-value relating to the change of flux with the lapse of time), pressure resistance, heat resistance, oxidation resistance, workable pH range, resistance to microorganism, durability for chemicals and solvent, storage stability, etc. The m-value mentioned above is represented by the formula log (Qt.sub.2 /Qt.sub.1)/log (t.sub.2 /t.sub.1) where t.sub.1 is an initial time and t.sub.2 is a time after a predetermined period and Qt.sub.1 and QT.sub.2 represent the flux at the time t.sub.1 and the time t.sub.2, respectively.
However, no asymmetric nor semipermeable composite membranes fulfilling all of these requirements has been developed at present. For example, the prior composite reverse osmosis membrane having an ultrathin membrane of cross-linked polyamide is said to show significant reduction of the flux with the lapse of operation period. While the desirable m-value for practical use of the reverse osmosis membrane is usually around -0.02, which corresponds to about 20% reduction of the flux during the operation of three years, any reverse osmosis membrane which has the m-value far from the above can not be considered useful from the viewpoint of efficiency and economization, even though its initial characteristics and properties are excellent and acceptable.
While on the other hand, the practical level of the solute rejection against organic material is usually required to be higher than 90%, representatively, against neutral, basic and acidic molecules such as isopropyl alcohol, ethylenediamine and acetic acid, no reverse osmosis membrane capable of fulfilling this requirement has been developed.
In addition, although some of the prior reverse osmosis membranes made of polyamide or cross-linked polyamide had been considered as chlorine resistant, their water flux is, in fact, usually reduced by about 20 to 30% or more after they are brought into contact with chlorine (refer to Journal of Applied Polymer Science, Vol. 29, 3369 (1984)), and the loss of the rejection and increase in the flux rate occur in an unrecoverable manner possibly due to the decomposition of polyamide upon contact with chlorine for a long period of operation or at high chlorine concentration.
A practically attractive reverese osmosis membrane should have the salt rejection of 99% or higher and water flux of 0.8 m.sup.3 /m.sup.2.day or greater under the condition of the applied pressure of 56 kg/cm.sup.2 and the feed saline water of 35,000 ppm in the case of sea water desalination, or the salt rejection of 99% or higher and water flux of 0.7 to 1.3 m.sup.3 /m.sup.2.day or greater under the conditions of the applied pressure of 15 to 30 kg/cm.sup.2 and the feed saline water of 1500 ppm in the case of brackish water desalination, in addition to a small reduction of water flux with the lapse of operation period, an improved rejection against organic materials and an oxidation resistance. The reverse osmosis membranes having been developed so far can only partially satisfy these requirements, and accordingly are subjected to various restrictions when put into practical use.
The object of the invention is to provide a semipermeable composite membrane of which ultra-thin membrane is made of specific cross-linked aromatic polyamide, having superior hydrophilicity, and high cross-linking density to improve especially the solute rejection against both inorganic and organic materials and water flux as well as the mechanical durability of the ultra-thin membrane.
Other objects and advantages of the invention will be apparent from the descriptions hereinafter.