Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-03-02
2003-05-20
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S525000, C204S529000, C204S533000
Reexamination Certificate
active
06565725
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing deionized water by using electro-regenerating deionization (hereinafter referred to as EDI). More particularly, it relates to a method 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 a pharmaceutical-manufacturing industry, a semiconductor-manufacturing industry and a food industry, or boiler water and other laboratory facilities.
BACKGROUND OF THE INVENTION
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 ion exchange resins so that impurity ions are adsorbed on the ion exchange resin and removed. In this method, the ion exchange resins having its ion-exchanging and adsorbing abilities lowered have 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 resins, 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 wherein anion exchange membranes and cation exchange membranes are alternately arranged to form demineralizing compartments and concentrating compartments alternately, and a mixture of anion exchange resins and cation exchange resins is accommodated in the demineralizing compartments. Voltage is applied while supplying water to be treated to the demineralizing compartments and supplying a concentrated water to the 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 resins. Accordingly, with said method, no additional regeneration of the ion exchange resins has to be carried out.
A conventional EDI method employs a deionized water producing apparatus comprising an electrodialyzer wherein 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 to form demineralizing compartments partitioned by the anion exchange membranes on the anode side and partitioned by the cation exchange membranes on the cathode side, and concentrating compartments partitioned by the cation exchange membranes on the anode side and partitioned by the anion exchange membranes on the cathode side, and anion exchange resins and cation exchange resins are accommodated in the demineralizing compartments. Impurity ions in water to be treated are removed by applying a voltage to the deionized water producing apparatus, 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 the concentrated 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 are problems 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 to decrease in electric current, and further, the resistivity of the treated water 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 acidic water-producing electrodialyzer which is prepared separately, and passing the acidic water produced in an anode compartment through the concentrating compartments of the EDI apparatus (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 has been proposed in which a liquid having an electro-conductance of from 100 to 800 &mgr;S/cm by adding an aqueous solution of a hydrochloride or sulfate of an alkali metal added thereto is supplied to the concentrating compartments of the EDI apparatus to stabilize the electric current in the EDI method to obtain a treated water having a high purity (JP-A-9-24374), but the long-term stability in the performance is not clarified.
The present invention relates to a method for overcoming 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. Thus, it is an object of the present invention to provide an expedient and inexpensive EDI demineralization method which can prevent and overcome decrease in performance due to impurities such as hardness components contained in the water to be treated, to be supplied in the EDI apparatus.
DISCLOSURE OF THE INVENTION
The present invention provides a method for producing deionized water to attain the above object, which comprises employing a deionized water producing apparatus comprising an electrodialyzer, wherein cation exchange membranes and anion exchange membranes are arranged alternately between an anode compartment provided with an anode and a cathode compartment provided with a cathode so as to form demineralizing compartments partitioned by the anion exchange membranes on the anode side and partitioned by the cation exchange membranes on the cathode side and concentrating compartments partitioned by the cation exchange membranes on the anode side and partitioned by the anion exchange membranes on the cathode side, and ion exchangers being accommodated in the demineralizing compartments, supplying water to be treated in the demineralizing compartments and supplying a concentrated water which is an electrolyte solution to the concentrating compartments under applying a voltage to transfer and remove impurity ions contained in the water to be treated, wherein the concentrated water at the outlet of the concentrating compartments has a S value of 7 or more as defined by the following formula (1) and a pH of 2.5 or more:
S
value=(&ggr;−420000×
A
)/(
B
×(1−(
A/
0.004))
3
) Formula(1),
wherein &ggr; is electro-conductivity (&mgr;S/cm), A is hydrogen ion concentration (mol/l), and B is magnesium ion concentration (ppb).
In accordance with the present invention, the S value of the concentrated water at the outlet of the concentrating compartments (hereafter, referred to as outlet concentrated water) is maintained 7 or more, whereby can be suppressed such problems that hardness components such as calcium ions, magnesium ions and the like are bonded to OH ions and carbonate ions to form slightly soluble salts in the vicinity of anion exchange membranes on the concentrating compartment side.
In the present invention, there are generally two types of means for maintaining the above-mentioned S value of the outlet concentrated water at 7 or more.
The first means is the one that water to be treated having the S value of 7 or more, preferably 10 or mo
Matsumura Yukio
Sugaya Yoshio
Toda Hiroshi
Asahi Glass Company Limited
Phasge Arun S,.
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