Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-12-08
2002-10-29
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C204S524000, C204S533000, C204S632000, C204S634000, C210S669000, C210S748080, C210S243000, C210S266000, C210S264000, C210S283000
Reexamination Certificate
active
06471867
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for producing deionized water by electro-regenerating deionization (hereinafter referred to as EDI) method. More particularly, it relates to an apparatus for efficiently producing pure water or highly deionized water which is called e.g. ultra-pure water, which is used for manufacturing industries such as pharmaceutical-manufacturing industries, semiconductor-manufacturing industries and food industries, or boiler water and other laboratory facilities.
Heretofore, as a method for producing deionized water, it is common to obtain deionized water by passing water to be treated through a bed packed with an ion exchange resin so that impurity ions are removed as adsorbed on the ion exchange resin. Here, the ion exchange resin having its ion-exchanging and adsorbing abilities lowered has to be regenerated, and the regeneration is carried out usually by using an acid or an alkali. However, this method has problems such as troublesome operation for regenerating the ion exchange resin, and discharge of a waste liquid due to the acid or alkali used for the regeneration.
Therefore, a method for producing deionized water which requires no such regeneration is desired. From such a viewpoint, an EDI method which requires no regeneration operation by a chemical reagent such as an acid or an alkali has been recently developed and practically used. This method employs an electrodialyzer having anion exchange membranes and cation exchange membranes alternately arranged to form demineralizing compartments and having a mixture of an anion exchange resin and a cation exchange resin accommodated in the demineralizing compartments, and is designed to apply a voltage while supplying water to be treated to the demineralizing compartments and supplying concentrating water to concentrating compartments arranged alternately to the demineralizing compartments to carry out electrodialysis to produce deionized water and at the same time, to carry out regeneration of the ion exchange resin. Accordingly, with said method, no additional regeneration of the ion exchange resin has to be carried out.
Thus, a conventional EDI method employs an apparatus for producing deionized water which comprises an electrodialyzer having cation exchange membranes and anion exchange membranes alternately arranged between an anode compartment provided with an anode and a cathode compartment provided with a cathode, demineralizing compartments compartmentalized with the anion exchange membranes on the anode side and compartmentalized with the cation exchange membranes on the cathode side, and concentrating compartments compartmentalized with the cation exchange membranes on the anode side and compartmentalized with the anion exchange membranes on the cathode side, the electrodialyzer having an anion exchange resin and a cation exchange resin accommodated in the demineralizing compartments, and impurity ions in water to be treated are removed by applying a voltage while supplying the water to be treated to the demineralizing compartments and supplying a part of the water to be treated or already treated water as a concentrating water to the concentrating compartments.
According to this method, as mentioned above, the ion exchange resins are continuously regenerated simultaneously, and it therefore has an advantage that regeneration by a chemical reagent such as an acid or an alkali is not necessary, and a treatment of a waste liquid to be generated by the regeneration is not necessary. However, there is a problem with the EDI apparatus that the electric resistance gradually increases due to hardness components in the water to be treated such as calcium ions, magnesium ions and the like, thus leading to increase in the applying voltage or decrease in electric current, and further, the resistivity of the treated water to be produced tends to decrease due to decrease in the demineralization performances.
Many methods have already been proposed to overcome such problems, and examples of which include a method of preliminarily subjecting water to be supplied for an EDI apparatus to a reverse osmosis membrane treatment in two steps to remove hardness components as much as possible and then supplying said water as, the water to be treated by the EDI method (JP-A-2-40220) and a method of subjecting water to electrolysis in an acid water-forming electrolyzer which is prepared separately, and passing the acid water produced in an anode compartment through the concentrating compartments of the EDI method (JP-A-10-128338). By employing such methods, long-term performance of the EDI method may be stabilized, but the investment cost tends to increase, and as a result, the advantages of the EDI system as compared with other deionization methods tend to diminish.
Further, a method of intermittently making the water to be treated, to be supplied to the demineralizing compartments, acescent in order that the ion components strongly adsorbed on an ion exchange resin in the demineralizing compartments are intermittently dissolved, has been proposed (JP-A-3-26390), but there is a problem such that the resistivity of the treated water tends to decrease during the intermittent treatment. Further, a method in which a liquid having an aqueous solution of a hydrochloride or sulfate of an alkali metal added thereto so that the electrical conductance becomes from 100 to 800 &mgr;S/cm, is supplied to the concentrating compartments in the EDI method to stabilize the electric current in the EDI method to obtain a treated water having a high purity, has been proposed (JP-A-9-24374), but the long-term stability in the performance is not clarified.
The present invention relates to an apparatus to overcome the above problems of the conventional system for producing deionized water by EDI and the improved method for producing deionized water having long-term stability which has been proposed later. Namely, it is an object of the present invention to provide a novel EDI demineralization system which prevents and overcomes decrease in performance due to impurities such as hardness components in the water to be treated, to be supplied in the EDI method.
SUMMARY OF THE INVENTION
The above object will be explained in further detail. In the conventional apparatus for producing deionized water by EDI method, as the concentrating water is recycled, the hardness components are concentrated in the concentrating compartments as operation time passes, and the hardness components (calcium and magnesium ions) are bonded to OH ions or carbonate ions transferred from the demineralizing compartments through the anion exchange membranes and are deposited and accumulated as hydroxides or carbonates to increase the electric resistance, and a good ion exchange state may thereby be impaired.
Studies by the present inventors clarified the following. Cation such as Na ions, Ca ions and Mg ions in a concentrating compartment is distributed in large part in the vicinity of the anion exchange membrane due to potential gradient. On the other hand, OH ions generated by dissociation of water in a demineralizing compartment are transmitted through the anion exchange membrane and reach the concentrating compartment, and accordingly, OH ions of high concentration are present in the vicinity of the anion exchange membrane. As a result, on the surface and in the vicinity of the anion exchange membrane on the concentrating compartment side, hardness components such as Ca ions and Mg ions, and OH ions or carbonate ions are bonded, and hydroxides or carbonates are deposited.
The present invention has been made on the basis of the above-mentioned results of studies and facts clarified, and it is an object of the present invention to provide an apparatus for producing deionized water by EDI method which is stabilized for a long period of time, wherein deposition and accumulation of hardness components on the surface of the anion exchange membranes on the concentrating compartment side are prevented to suppress increase in the elect
Matsumura Yukio
Sugaya Yoshio
Toda Hiroshi
LandOfFree
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