METHOD FOR ION-EXCHANGE OR ADSORPTION

Abstract: A process for exchanging or adsorbing ions and a process for producing pure water, which comprises processing an untreated solution with an ion-exchange res­ in and then with ion-exchange fibers. This process makes it possible to exchange or absorb ions of valuables or non­ valuables in an untreated solution to a high extent and produce super-pure water.

Description: This invention relates to a method of ion-exchanging and/or adsorption, and to a method of producing pure water. Ion-exchange resins are conventionally widely used for treating liquid for ion-exchanging and/or adsorption. However, since most of the ion-exchange groups are located in the inner portion of the resin rather than in the surface area thereof, this treatment has drawbacks in that the reaction rate is low and so the treatment is time-consuming, and it is also difficult to conduct ion-exchanging and/or adsorption with high efficiency. If the ion-exchange resins are powdered or finely grained, other drawbacks are brought about that their handling becomes troublesome and the liquid-flow resistance becomes very high. It was proposed recently to treat a liquid with an ion-exchange fiber having a large surface area, so as to attain high reaction rate and high freedom in the selection of the form of the ion-exchanger in use. However, this method has an insurmountable drawback in its low treatment capacity due to the voluminousness of the fiber. The present inventors intensively researched to eliminate the above-mentioned drawbacks to complete the present invention. The technique of treating water first with a mixed-bed ion exchange resin and then with ion-exchange fiber is not broadly speaking new. Thus GB-A-1 123 908 describes such a technique for trearting water destined to be fed to a high-pressure boiler. The present invention is based on the surprising discovery that by treating water first with an ion-exchange resin and then with an ion-exchange fiber, not only are the useless and/or the useful product to be separated ion-exchanged or adsorbed with high sensitivity, but the treatment capacity is drastically increased, provided that the ratio of the ion-exchange capacities of the ion-exchange fiber and the ion-exchange resin are maintained within certain limits. The inventors have found that in practice the ion-exchange capacity of the ion-exchange fiber to that of the ion-exchange resin should be between 0.05 and 30%. If this ratio is too low, highly efficient ion-exchange and/or adsorption in a short time becomes difficult, while if it is too large, the treatment capacity in a fixed bed is decreased. The preferred ratio lies betwen 0.1 and 20%. According to the present invention, therefore, pure water is produced by a method which comprises treating the water with an ion-exchange resin and then with an ion-exchange fiber, the ion-exchange capacity of the ion-exchange fiber relative to that of the ion-exchange resin being within the range of from 0.05 to 30%. The ion-exchange resins used in the present invention include known and/or commercially available ion-exchange resins of usually 100 to 1000 micrometers diameter. More specifically, examples of the resins used in the present invention include gel-type and MR-type ion-exchange resins produced by introducing ion-exchange groups to a styrene-divinyl benzene copolymer with a high chemical-resistance and heat-resistance. The ion-exchange groups include cation-exchange groups such as sulfo group, phospho group and carboxyl group; anion-exchange groups such as primary, secondary, and tertiary amino groups and quaternary ammonium group; and chelate groups such as aminocarboxylic group, amidoxime group, aminophosphoryl group, polyamine group, pyridyl group, dithiocarbamyl group. The ion-exchange fiber used in the present invention includes known ion-exchange fibers of usually 0.1 to 100 µm diameter and preferably 1 to 100 µm diameter. The examples of the fiber include insoluble synthetic organic ion-exchange fibers produced by introducing an ion-exchange groups to a synthetic organic polymer (ion-exchange polymer) of, for example, polystyrene-based, polyphenol-based, polyvinyl alcohol-based, polyacrylic-based, polyethylene-based or polyamide-based. In view of the mechanical strength and the retainability of its form, it is preferred that the fiber comprise a reinforcing polymer in addition to the ion-exchange polymer. Preferably, the fiber is a composite fiber comprising a reinforcing polymer as its core(s) and an ion-exchange polymer as its sheath which surrounds the core. The content of the reinforcing polymer in the fiber is usually 10 to 90% by weight, but preferably 20 to 80% by weight since if the reinforcing polymer content is too low, the mechanical strength of the fiber is lowered while if the reinforcing polymer content is too high, the ion-exchange and/or adsorption capacity is lowered. Poly(monovinyl aromatic compounds), especially polystyrene-based compounds are preferred as the ion-exchange polymer since they have excellent chemical-resistant and heat-resistant properties, so that operation can be repeated many times. Poly-α-olefins are preferred as the reinforcing polymer since they have excellent chemical-resistant properties. The "water content degree" of the fiber used in the present invention is usually 0.5 to 10, but preferably 1 to 5 since if it is too low, it becomes difficult to conduct ion-exchange and/or adsorption with high efficiency, while it is too high, the liquid-flow resistance becomes high. The "water content degree" herein used is defined by the following equation: wherein W is the weight of a Na type (Cl type) cation (anion)-exchange fiber which, after being immersed in distilled water, is dehydrated of its surface water by being subjected to hydroextraction with a domestic centrifugal hydroextractor for 5 minutes, and W o is the weight of the fiber after completely dried. The ion-exchange fiber used in the present invention includes cation-exchange fibers, anion-exchange fibers and chelate fibers each having the above-mentioned cation-exchange groups, anion-exchange groups or chelate groups, respectively. The fiber may be used in a conventional form such as short fiber, filament, felt, fabric, unwoven fabric, fiber bundle, strand and paper, as well as their cut pieces. Among these, short fibers having a length of 0.1 to 3 mm, preferably 0.2 to 2 mm are preferred since they are easy to pack and different kinds of fibers can be readily mixed. Although the method of the present invention may be conducted batchwise, it is preferred to conduct the method continuously using fixed beds such that the liquid to be treated is first passed through a layer of an ion-exchange resin, and then passed through a layer of an ion-exchange fiber, since the operation is easier and the efficiency of the ion-exchange and/or adsorption may be promoted. In particular, this invention is most advantageously applied to the production of pure water by treating a raw water with a cation-exchanger and an anion-exchanger. A method of producing pure water will now be described in detail. A cation-exchanger containing cation-exchange groups, preferably sulfo groups is used after being activated with an acid, and an anion-exchanger contaning anion-exchange groups, preferably quaternary ammonium groups is used after being activated with an alkali. Ordinary industrial water, city water, well water, tap water, underground water, water passed through a reverse-osmosis (RO) membrane, and recycled water in nuclear power plants and thermoelectric power plants are advantageously used as the raw water. It is preferred that the method comprise the step of treating the raw water with an ion-exchange resin and then treating the water with the same kind of ion-exchange fiber. The examples of the order of the treatment includes, but not limited to, K R → K F → A R , K R → A R → K F , K R → K F → A R → A F , K R → A R → K F → A F , K R → A R → K F A F , K R A R → K F A F , and K R → A R → K R A R → K F A F , wherein K R and A R means a cation-exchange resin and an anion-exchange resin, respectively, K F and A F means a cation-exchange fiber and an anion exchange fiber, respectively, K R A R means a mixture of cation-exchange resin and an anion-exchange resin, and K F A F means a mixture of cation-exchange fiber and an anion-exchange fiber. Instead of the mixture of a cation-exchange fiber and an anion-exchange fiber, a mixture of a cation-exchange fiber and a powdered anion-exchange resin, or a mixture of an anion-exchange fiber and a powdered cation-exchange resin may be used. To prepare highly pure water, it is preferred to treat the raw water with at least a mixture of a cation-exchange fiber and an anion-exchange fiber after treating the water with an ion-exchange resin, and the preferred order of treatment includes the above-mentioned K R → A R → K F A F , K R A R → K F A F , and K R → A R → K R A R → K F A F . The mixing ratio (in equivalence) of the cation-exchanger and the anion-exchanger, especially the cation-exchange fiber and the anion-exchange fiber is usually 10:1 to 1:10, preferably 5:1 to 1:5, and more preferably 3:1 to 1:3. By applying the present invention to a known superpure water production system comprisinng Pretreatment → RO membrane → Ion-exchange → Sterilization with UV light → MT (membrane filter), UF (ultrafilter), or RO membrane, a superpure water which is superior to conventional pure water in its specific resistance, TOC (total organic compounds), the number of particles contained, and the number of live bacteria contained may be obtained. As stated above, the present invention has the following advantages: Firstly, ion-exchange and/or adsorption can be conducted in a short time. Secondly, ion-exchange and/or adsorption can be conducted with high efficiency. Thirdly, the ion-exchange and/or adsorption capacity is large. Fourthly, a large amount of highly pure water can be produced in a short time. Examples of the present invention will now be described. It should be noted that this invention is not limited to thereto. Example 1 Forty milliliters (76 milli equivalence) of commercially available cation-exchange resin Amberlite IR-120B were packed in a column (1.7 cm diameter x 18 cm) and activated with an acid (column K R ). Forty milliliters (13 milli equivalence) of cation-exchange fibers in the form of cut fibers of 1 mm length were packed in a column (1.7 cm diameter x 17 cm) and were activated with an acid (column K F ). Eighty milliliters (108 milli equivalence) of commercially available anion-exchange resin Diaion SA20AP were packed in a column (1.7 cm diameter x 36 cm) and activated with an alkali (column A R ). Tap water was passed through, in the order mentioned, column K R → column K F → column A R at a rate of 3 liters/hr to obtain pure water. The specific resistance of the eluted water was measured to evaluate the performance of the system in removing cations. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 1. Comparative Example 1 The same procedure as in Example 1 was repeated except that the order of the column was column K R → column A R , and column K F → column A R , respectively, and the specific resistance of the eluted water was determined. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 1. As shown in Table 1, according to the example of the present invention, the amount of treated water, at a flow rate of 3 liters/hr, in a run in which a water having a specific resistance of 1 MΩ.cm or more was obtained was 78 liters. On the other hand, according to the comparative examples, it was 42 liters and 9 liters, respectively, totalling only 51 liters. Thus, it can be seen that the treatment capacity of the method of the present invention is very large. Further, the specific resistance of the water obtained by the method of the present invention was very high. Thus, it can be seen that the cations in the water were ion-exchanged or adsorbed with high efficiency, and highly pure water can be produced in a large amount by the method of the present invention. Example 2 Thirty milliliters of commercially available mixed resin Amberlite MB-1 (29 milli equivalence of cations, 21 milli equivalence of anions) were packed in the lower portion of a column (1.7 cm diameter x 54 cm), and 90 ml of mixed resins Amberlite MB-2 (57 milli equivalence of cations, 81 milli equivalence of anions) were packed thereon (column K R A R ). In a column (1.7 cm diameter x 54 cm), 120 ml of a mixture of cut fibers of 1 mm length (15 milli equivalence of cations, 10 milli equivalence of anions) were packed (column K F A F ). Tap water was passed through, in the order mentioned, column K R A R → column K F A F to prepare pure water. The relationship between the amount of water which flowed and the specific resistance is shown in Table 2. Comparative Example 2 The same procedure as in Example 1 was repeated except that one of column K R A R and column K F A F was used in each run, and the specific resistance of the eluted water was determined. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 2. As shown in Table 2, according to the example of the present invention, the amount of treated water, at a flow rate of 3 liters/hr, in a run in which a water having a specific resistance of 10 MΩ.cm or more was obtained was 47 liters. On the other hand, according to the comparative examples, it was 25 liters and 7 liters, respectively, totalling only 32 liters. Thus, it can be seen that the treatment capacity of the method of the present invention is very large. Further, the specific resistance of the water obtained by the method of the present invention was very high. Thus, it can be seen that the cations in the water were ion-exchanged or adsorbed with high efficiency, and highly pure water can be produced in a large amount by the method of the present invention. Example 3 In the lower portion of a column (1.7 cm diameter x 54 cm), 30 ml of a mixture of cut fibers of 1 mm length (3.8 milli equivalence of cations, 2.5 milli equivalence of anions)were packed, and 90 ml of commercially available mixed resins Amberlite MB-2 (57 milli equivalence of cations, 81 milli equivalence of anions) were packed thereon (column K R A R 90 ml → K F A F 30 ml). Tap water was passed through the column from its top to the bottom at a flow rate of 3 liters/hr to produce pure water. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 3. Example 4 In the lower portion of a column (1.7 cm diameter x 54 cm), 10 ml of a mixture of cut fibers of 1 mm length (cation 1.3 milli equivalence, anion 0.85 milli equivalence) were packed, and 110 ml of commercially available mixed resin Amberlite MB-2 (cation 70 milli equivalence, anion 99 milli equivalence) were packed thereon (column K R A R 110 ml → K F A F 10 ml). Pure water was produced using this column in the same manner as in Example 3. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 3. Comparative Example 3 In a column (1.7 cm diameter x 54 cm), 120 ml of commercially available mixed resins Amberlite MB-2(76 milli equivalence of cations, 108 milli equivalence of anions) were packed (column K R A R 120 ml). Pure water was produced using this column in the same manner as in Example 3. The relationship between the amount of the water which flowed and the specific resistance is shown in Table 3. As shown in Table 3, according to the examples of the present invention, the amount of treated water, at a flow rate of 3 liters/hr, in a run in which a water having a specific resistance of 10 MΩ.cm or more was obtained was 35 liters and 43 liters, respectively. On the other hand, according to the comparative example, it was 30 liters. Thus, it can be seen that the treatment capacity of the method of the present invention is very large. Further, the specific resistance of the water obtained by the method of the present invention was very high. Thus, it can be seen that the impurities in the water were ion-exchanged or adsorbed with high efficiency, and highly pure water can be produced in a large amount by the method of the present invention. Further, it can be seen that if the ion-exchange capacity of the fibers used is increased, the purity of the treated water is promoted but the treatment capacity is decreased, and conversely, if the ion-exchange capacity of the fibers used is decreased, the treatment capacity is increased but the purity of the treated water is decreased. Example 5, Comparative Example 4 Water which had flowed through RO membrane was passed through a column (7.0 cm diameter) containing 1 liter of commercially available mixture of resins Amberlite MB-2, and then through a column (7.0 cm diameter) containing 1.8 liters of mixed resins for superpure water production, Amberlite EG-290, at a flow rate of 30 liters/hr. The resulting water was divided into two portions. One of these was passed through a column (7.0 cm diameter) containing 1 liter of a mixture of cut fibers of 1 mm length (150 milli equivalence of cations, 10 milli equivalence of anions), while the other portion of the water was passed through a column (7.0 cm diameter) containing 1 liter of mixed resins for superpure water production, Amberlite EG-290. In both cases, the flow rate was 15 liters/hr. The quality of the water from the outlet was checked. According to the example of the present invention (the former), after passing 2400 liters of water, the specific resistance was 18 MΩ.cm or more, the total number of particles of 0.2 µm diameter or more was 6/ml. On the other hand, according to the comparative example (the latter), after passing 2400 liters of water, the specific resistance was 17.5 MΩ.cm, the total number of particles of 0.2 µm diameter or more was 24/ml. From these results, it can be seen that a water can be obtained by the method of the present invention, which is much superior to that obtained in the comparative example especially in terms of the total number of particles contained in water. Example 6 A system comprising, in the order mentioned, a filter of 10 µm pore size, a column (7.0 cm diameter) containing 1 liter of Amberlite MB-2, a column (3 cm diameter) containing 0.2 liter of a mixture of cut fibers of 1 mm length (30 milli equivalence of cations, 20 milli equivalence of anions), and a filter of 0.22 µm pore size was prepared and tap water was passed through this system at a flow rate of 10 liters/hr. After passing 2300 liters of water, the quality of the water from the outlet was checked. The specific resistance was 18 (0.01) MΩ.cm or more, the number of live bacteria was 0.00 (0.63)/ml, pyrogen content was 0.003 (1.998) ng/ml, the total number of particles of not smaller than 0.2 µm was 4 (1.5 x 10⁵)/ml, TOC was 0.15 (1.2)ppm, sodium content was less than 0.001 (7.6)ppm, and the silica content was less than 0.005 (0.46)ppm. The values in the parentheses show the values obtained for tap water. It can be seen from these results that according to the present invention, a superpure water can be easily obtained, and its water quality is very high. The cation- and anion-exchange fibers used in the above examples and comparative examples were produced by the following process: Multi-core islands-in-a-sea type composite fibers (non-elongated fibers) (sea component (polystyrene/polypropylene)/islands component (polypropylene) = (47/4)/49 (the number of islands:16; diameter of the fibers:34 µm) were cut in pieces to obtain cut fibers of 1 mm length. One part by weight of the cut fibers was added to a cross-linking/sulfonating solution consisting of 7.5 parts by volume of commercially available sulfuric acid of first class grade and 0.15 parts by weight of paraformaldehyde. The mixture was heated at 80°C for 4 hours and the fibers were washed with water. The fibers were then treated with an alkali followed by washing with water to obtain cation-exchange fibers containing sulfo groups (ion-exchange capacity: 2.8 milli equivalence/g-Na; water content degree:1.5). On the other hand, 1 part by weight of the above cut fibers was added to a cross-linking solution consisting of 5 parts by volume of commercially available sulfuric acid of first class grade, 0.5 parts by volume of water and 0.2 parts by weight of paraformaldehyde, and the mixture was heated at 80°C for 4 hours. The thus cross-linked fibers were then added to a solution consisting of 8.5 parts by volume of chloromethyl ether and 1.5 parts by volume of stannic chloride, and this mixture was allowed to react for 1 hour at 30° C. After the reaction, the fibers were washed with 10% hydrochloric acid, distilled water, and acetone. The thus chloromethylated fibers were then added to 10 parts by volume of 30% aqueous solution of trimethylamine, and this mixture was allowed to react for 1 hour at 30°C to aminate the fibers. After washing the fibers with water, the fibers were treated with hydrochloric acid, and washed with water to obtain anion-exchange fibers containing trimethylammonium methyl groups (ion-exchange capacity of 2.4 milli equivalence/g-Cl; water content degree of 1.8). The mixtures of the fibers were obtained by mixing the cation-exchange fibers and anion-exchange fibers at a prescribed ratio after activating the cation- and anion-exchange fibers with an acid or an alkali, respectively.

Claims: A method of purifying water with an ion-exchanger which comprises treating the water with an ion-exchange resin and thereafter with an ion-exchange fiber, the ion-exchange capacity of the ion-exchange fiber relative to that of the ion-exchange resin being within the range of from 0.05 to 30%. A method according to claim 1, wherein the ion-exchange capacity of the ion-exchange fiber relative to that of the ion-exchange resin is within the range of from 0.1 to 20%. A method according to claim 1 or claim 2, wherein the ion-exchange resin has an average particle diameter of between 100 and 1000 µm. A method according to any preceding claim, wherein the ion-exchange fiber is a composite fiber comprising a reinforcing polymer core and an ion-exchange polymer sheath which surrounds the core. A method according to claim 4, wherein said ion-exchange polymer is a poly(monovinylaromatic) compound. A method according to claim 4 or claim 5, wherein said reinforcing polymer is a poly-α-olefin. A method according to any preceding claim, wherein the water content degree of the ion-exchange fiber is within the range of from 0.5 to 10. A method according to claim 7, wherein the water content degree of the ion-exchange fiber is within the range of from 1 to 5. A method according to any preceding claim, wherein the ion-exchange fiber comprises short fibers having a diameter of 1 to 100 µm and a length of 0.2 to 2 mm. A method according to any preceding claim, wherein the ion-exchange resin comprises a a mixture of cation-exchange resin and anion-exchange resin. A method according to any preceding claim, wherein the ion-exchange fiber comprises a mixture of cation-exchange fibers and anion exchange fibers. A method according to any of claims 1 to 9, wherein the water is passed firstly through cation-exchange resin, then through cation-exchange fiber, and then through anion-exchange resin. A method according to any preceding claim, comprising cation-exchange material having a sulfonic group. A method according to any preceding claim, comprising anion-exchange material having a quaternary ammonium group.

Inventors: SHIMAMURA, MASAHARU, YOSHIKAWA, SEIICHI, YOSHIOKA, TOSHIO
Applicants: TORAY INDUSTRIES, INC.
Application year: 1984
Main classification: B01J47
IPC codes: B01J47, C02F1