Ion exchange resin beads are used as an anion or cation exchanger in many applications, such as water treatment, production of pure water, catalysts, and purification of sugar solutions. Ion exchange resins for use in these applications and copolymers to be converted ion exchange resins generally include a gel type resin having a non-porous structure and a macroporous type resin having a porous or macro-network structure. Superiority of the ion exchange resins of gel type to those of porous type resides in higher physical strength as represented by friability and higher exchange capacity. Superiority of the porous type ion exchange resins to those of gel type resides in higher physical stability as represented by osmotic pressure resistance and higher rate of ion-exchange reaction owing to their greater specific surface area in contact with a substance to be treated. Having a porous or macro-network structure, the macroporous type ion exchange resins are also useful for water treatment or removal of impurities from stock water for the production of pure water.
However, structurally lacking in exchange capacity and physical strength as compared with gel type ion exchange resins, the macroporous type ion exchange resin beads are liable to destruction by various stresses exerted thereon under conditions of use.
In order to overcome the respective disadvantages of the conventional ion exchange resins, there has been proposed a two-stage polymerization process for producing an ion exchange resin possessing advantageous characteristics of both gel type and porous type resins or a copolymer capable of providing such an ion exchange resin, comprising swelling a crosslinked copolymer having a macro-network structure with a monofunctional monomer and a polyfunctional monomer by imbibing and then polymerizing the monomers, as disclosed, e.g., in JP-B-48-17988 (the term "JP-B" as used herein means an "examined Japanese patent publication") and U.S. Pat. No. 3,991,017. However, when a functional group is introduced into the thus produced crosslinked copolymer by known techniques, an ion exchange resin having as much strength as can reasonably be expected from the total degree of crosslinking cannot be obtained. This appears to be because the monofunctional monomer and the polyfunctional monomer imbibed and filled into pores of the crosslinked copolymer having a macro-network structure has a weak interaction with the crosslinked copolymer and also because the imbibing is non-uniform only to produce a non-uniform crosslinked copolymer.
Examples of the water treatments with an ion exchange resins include removal of impurities from a condensate of cooling water for the core of a nuclear reactor or a condensate of the steam generated in the reactor. For example, in boiling water reactors (hereinafter abbreviated as "BWR") where highly purified water must be used as a primary coolant, several devices for purifying the spent water are provided in the course of the circulation equipment so that the water should be repeatedly used.
For better understanding, a basic construction of BWR is explained below by referring to FIG. 1. The steam generated in reactor 1 is forwarded to turbine 3 through main steam pipe 2. After working, the steam is transferred to condenser 4 where it is condensed. The condensate is then forwarded to condensate purifying apparatus 5 by pump 6. The thus purified water is fed to feedwater heater 8 by pump 7 where it is heated. Most of the heated water is supplied to reactor 1 through water pipe 9, while a part of the heated water is sent to condensate purifying apparatus 11 provided on reactor recirculation line 10.
Nuclear-power generation of the above-described type requires a large quantity of water having extremely high purity for reducing radioactivity induction. Criteria of water quality for nuclear-power generation are as follows.
______________________________________ Item Criteria ______________________________________ Conductivity (at 25.degree. C.) &lt;0.1 .mu..OMEGA./cm Metallic impurities &lt;15 ppb Chloride ion &lt;1 ppb ______________________________________
Since general tap water contains about 10 ppm of metals, it must be purified about 1000-fold so as to reduce the metals below 15 ppb. The condensate additionally contains trace amounts of metallic impurities released from the nuclear power plant. Such metallic impurities include ionic impurities and noinionic impurities called clad mainly composed of amorphous iron hydroxide and iron oxyhydroxide. These impurities are generally removed in a condensate purifying apparatus (e.g., as shown by numerals 5 and 11 in FIG. 1) using a demineralizer with an inlet and an outlet which is filled with a basic ion exchange resin and an acidic ion exchange resin.
Resins conventionally employed in such a condensate purifying apparatus are gel type ion exchange resins having a particle size of from about 0.1 to 1 mm and a specific surface area of about 0.03 m.sup.2 /g. The conventional ion exchange resins have a sufficient amount of ion exchange groups for removing ionic impurities in the condensate, and the thus purified water fulfills the above-specified criteria in most cases. However, nonionic impurities such as clad are difficult to remove with ion exchange resins, and some of the conventional gel type ion exchange resins attain a very low rate of removal of the nonionic impurities. Should the nonionic impurities remain in the condensate, they are activated in the reactor or adhered to the reactor to increase the radiation dose, and thus, removal of the nonionic impurities is of great importance. In particular, a fresh ion exchange resin has a small specific surface area and has therefore a small capacity of removing clad and particularly a low rate of removal. An old resin having been used for 3 to 4 years has an increased capacity of removal of clad probably due to swelling of the surface and so increased specific surface area.
It is known, on the other hand, that an ion exchange resin having its surface roughened through oxidation treatment or mechanical abrasion and thereby having an increased specific surface area is suitable for removal of clad as taught in JP-A-2-273550 and JP-A-2-307534. This is because clad, which mainly composed of amorphous iron hydroxide or iron oxyhydroxide and are therefore difficult to remove with an ion exchange resin, is easily dissolved in an acidic atmosphere created on the surface of an ion exchange resin to become an iron ion which is ready to be trapped on the surface of the ion exchange resin. Since the resin once catches an iron ion on the surface thereof, it is important for the resin to have an ability of entrapping the iron into the inside thereof. However, the above-described resin having its surface roughened by oxidation treatment is disadvantageous in that the strength is lessened by oxidation treatment and that the oxidation treatment is no more than accelerated deterioration, i.e., making the surface of the ion exchange resin particles substantially old like after repeated use. On the other hand, a porous type resin, although having a large specific surface area, has too small strength to withstand use in place of the gel type resin.
U.S. Pat. No. 4,564,644 discloses a gel type resin having a core/shell structure, and U.S. Pat. No. 4,975,201 proposes to use this resin for removal of clad from a condensate. Such a resin includes, as illustrated in the working examples, a styrene-divinylbenzene copolymer skeleton having bonded thereto an ion exchange group, the skeleton resin being produced by imbibing a monomer mixture of styrene, divinylbenzene, and a free radical initiator into seed particles of a styrene-divinylbenzene copolymer, polymerizing from 40 to 95% of the absorbed monomers within the seed particles, and then continuously adding thereto a styrene monomer containing substantially no free radical initiator under such a condition that the styrene monomer be absorbed into the copolymer particles and polymerized therein. It is stated that such an ion exchange resin has high strength and stability against osmotic pressure and is therefore suited for use in treatment of a condensate from a BWR reactor. Nevertheless, production of the resin having such a core/shell structure in which the polymer composition gradually varies from the center to the outer surface involves about three different polymerization steps each requires proper control of the degree of polymerization. In particular, the third step of polymerization must be carried out by adding a monomer in small portions under a constant condition.