Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
1999-08-16
2001-05-29
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S770000
Reexamination Certificate
active
06238544
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of purification of salt solutions to be subjected to electrolysis, and more particularly to a method of removing iodine and silica ions from the salt solutions, which ions adversely affect electrolysis, using zirconium hydroxide.
2. Description of the Related Art
It has been known that when a salt solution (brine) is subjected to electrolysis by using an ion exchange membrane method, iodine ions of around 1 ppm (weight) and silica ion of 10 to 20 ppm (weight) contained in the raw salt solution adversely affect electrolysis performance, and in order to overcome such a problem, various methods have been used to remove iodine and silica ions contained in the salt solutions to be electrolyzed. For example, an ion exchange method as disclosed in Japanese Laid-open Patent Application No. Hei-7-237919, an activated carbon adsorption method as disclosed in Japanese Post-examined Patent Application No. Hei-7-91666, a sedimentation method as disclosed in Japanese Post-examined Patent Application No. Hei-6-88777, etc. have been proposed as methods of removing iodine ions from the salt solutions.
For silica ion removal from salt solutions, there have been proposed the magnesium ion addition method disclosed in Japanese Post-examined patent application No. Sho-55-3290, etc., the primary refined sedimentary slurry circulation method disclosed in Japanese Post-examined patent application No. Sho-59-43556, etc. and the chelating resin adsorption method disclosed in Japanese Laid-open patent application No. Sho-60-125388, etc.
However, it has been found that the above conventional methods have the following disadvantages.
According to the ion exchange method as disclosed in Japanese Laid-open Patent Application No. Hei-7-237919, iodine ions are oxidized into iodine complex ions, and then the iodine complex ions thus obtained are removed through ion exchange using an anion exchange resin. However, it is difficult to surely control the oxidation of iodine ions to iodine complex ions. For example, iodate ions are also produced as one type of oxidized products obtained by oxidizing iodine ions. However, iodate ions cannot be adsorbed by the anion exchange resin. Therefore, when iodate ions are produced, some of iodine ions remain in the salt solution even when the salt solution is passed through the anion exchange resin. Accordingly, this method has a low efficiency in removal of iodine ions, and it is difficult to reduce the concentration of the iodine ions down to 0.2 mg/l or less, which is the maximum for the ion exchange membrane electrolysis process.
In the activated carbon adsorption method as disclosed in Japanese Post-examined Patent Application No. Hei-7-91666, it is difficult to surely control the oxidation of iodine ions to iodine complex ions. In addition, the iodine complex ions thus produced are reduced into iodide ions by activated carbon, and the adsorption efficiently is thereby reduced.
According to the sedimentation method as disclosed in Japanese Post-examined Patent Application No. Hei-6-88777, iodine ions are oxidized into periodate ions to form and deposit periodate which is only sparingly soluble, thereby removing the iodine ions. However, this method has also a low removal efficiency for iodine ions.
Among the above references disclosing silica ion removal, the magnesium ion addition method disclosed in Japanese Post-examined patent application No. Sho-55-3290 involves silica ion adsorption by magnesium hydroxide ion produced by adding magnesium chloride solution to the raw salt solution, but needs further improvement because of the high cost of disposal of the salt solution mud generated by the addition of magnesium hydroxide.
The primary refined sedimentary slurry circulation method disclosed in Japanese Post-examined patent application No. Sho-59-43556 forms silica salts from the silica ions by circulation of a portion of the primary refined sedimentary slurry of calcium carbonate or magnesium hydroxide, etc., settled in a sedimentation vessel. However, this method cannot reduce silica ion concentration to less than 2 ppm (weight) as SiO
2
.
The chelating resin adsorption method disclosed in Japanese Laid-open patent application No. Sho-60-125388 treats salt solutions with a strong cationic micro-porous type chelating resin under a slightly acidic condition. But this method has a lower silica ion removal efficiency than the above two methods.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of removing iodine and silica ions from salt solutions which overcomes the disadvantages of the prior art noted above and which removes iodine and silica ions from the salt solution with high efficiency, and also which is stable, economical and industrially highly practical. It has now been found that iodine and silica ions are effectively adsorbed by zirconium hydroxide under certain conditions, and that the iodine and silica ions thus adsorbed are desorbed from the zirconium hydroxide under certain conditions.
The present invention provides a method of removing ions of impurities from salt solutions to be used for electrolysis, in which the salt solution containing the ions of impurities and zirconium hydroxide are contacted with each other under acidic conditions to adsorb and remove the impuritiy ions. The present invention is preferably applied to a salt solution containing iodine and/or silica ions as the impurities.
More specifically, according to the present invention, there is provided a method of removing iodine and/or silica ions from a salt solution for electrolysis including (1) an adsorbing step in which the salt solution containing iodine and/or silica ions is brought into contact with zirconium hydroxide under acidic conditions to adsorb and remove the iodine and/or silica ions, and (2) a desorbing step in which the zirconium hydroxide, containing the adsorbed iodine and/or silica ions, is brought into contact with an aqueous solution of a higher pH value than the pH value in the adsorbing step to desorb the adsorbed iodine and/or silica ions from the zirconium hydroxide, and (3) conducting the above adsorbing and desorbing steps alternately in a single vessel or both steps continuously in separate vessels.
In the above method of removing impurity ions, for example typically iodine and/or silica ions in salt solutions for electrolysis, it is preferable that the adsorbing step and the desorbing step are sequentially performed in different treatment vessels while zirconium hydroxide is circulated between these treatment vessels. The method of removing impurities from the salt solutions for electrolysis according to the present invention can be also applied for the treatment of salt solutions having neutral and/or colloidal silica through a pre-treatment wherein the neutral and/or colloidal silica is converted to silicic ion.
It has been known that non-dissociative neutral silica and colloidal silica, in addition to the above silicic acid ion, exist as soluble silica. The present invention is preferably applied to removal of the silicic acid ion. Where the salt solution contains a large amount of non-dissociative neutral silica, silica ion can be removed by means of conversion of the non-dissociative neutral silica or colloidal silica into the silicic acid ion through pretreatment of the salt solution to make it alkaline. Therefore, an alkaline processing step, used as a pre-treatment for the above method of removing impurity ions from the salt solutions for electrolysis is in the scope of the present invention.
Ionic impurities in salt solutions to be subjected to electrolysis which adversely affect the electrolytic processing include iodine, silica, sulfate, chlorate, calcium, magnesium, strontium, barium, aluminum, iron, nickel, etc. The present invention can be applied to the treatment of salt solutions containing all these impurities. However, since iodine and silica ions govern the detailed description of the present inv
Kishi Takamichi
Matsuoka Terumi
Okuno Takako
Oohara Chisako
Chlorine Engineers Corp. Ltd.
Gorgos Kathryn
Keehan Christopher
Lorusso & Loud
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