Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1999-08-27
2001-07-24
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S913000, C502S401000, C560S068000
Reexamination Certificate
active
06264840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing an insoluble tannin which can adsorb actinides such as uranium, thorium, transuranium elements, etc.; heavy metal elements such as cadmium, lead, chromium, mercury and iron, etc.; or metal elements such as cobalt, cesium, strontium, etc.
Also, it relates to a method for adsorbing hexavalent chromium by using the insoluble tannin.
2. Background Art
Nuclear fuel elements such as uranium, thorium, etc. are contained in the waste liquid effluent from nuclear fuel processing facilities. As a process for producing the adsorbing agent for adsorbing the nuclear fuel elements, the prior art discloses a process for producing an insoluble tannin by dissolving a condensed tannin powder in an alkaline solution, mixing the resulting solution with an aqueous aldehyde solution to form a gel composition, and aging the gel composition at room temperature or under heating to stabilize it (U.S. Pat. No. 5,320,664).
Also the prior art discloses a process for producing an insoluble hydrolysable tannin which comprises dissolving a hydrolysable tannin powder in an aqueous ammonia, mixing the resulting solution with an aqueous aldehyde solution to form a precipitate, heating the precipitate, soaking the heated precipitate in a mineral acid such as nitric acid, filtering the resulting mixture to leave a residue on a filter and drying it (U.S. Pat. No. 5,274,169).
The insoluble tannins obtained by the above-mentioned two methods have characteristics that they have a high ability of adsorbing metal elements and low flow resistivity when used in a packed column of the waste liquid effluent treating apparatus. When the water content of the insoluble tannin which is an adsorbent is controlled, the network structure of the gel changes so that its molecular space suitable for the size of the metal element ion to be adsorbed can be retained whereby there is a possibility that an adsorption capacity and adsorption rate of the metal element can be changed. Particularly, the insoluble tannin disclosed in U.S. Pat. No. 5,274,169 has a merit that it is resistant to crush when packed in a column and the adsorption rate of the metal element is more improved by contacting with a mineral acid to harden the tannin surface. Also, it has a merit of preventing occurrence of mold at the insoluble tannin by maintaining the gel to acidic conditions.
However, the ranges of water contents of the insoluble tannins controlled by the above-mentioned two methods are limited only to about 70% to about 85% in practical use.
Also, in the above-mentioned two methods, the insoluble tannins obtained by aging the gel composition at room temperature or under heating for stabilization are crushed into particle size by a mechanical means such as a mixer, etc. and sifted out a desired size so as to be packed in a column, etc. However, the particle size and shape of the insoluble tannin are not constant and when a fine particle insoluble tannin is desired, it must be finely pulverized specifically.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for producing an insoluble tannin which can change an adsorption capacity and adsorption rate of a metal by changing a network structure of the gel and its molecular space wider as compared with the above-mentioned conventional adsorbents
Another object of the present invention is to provide a process for producing an insoluble tannin having a desired particle size and a desired water content without pulverizing the tannin into particle size when it is used as an adsorbent.
The first invention relates to a process for producing an insoluble tannin which comprises (a) a step of dissolving a condensed tannin powder in an aqueous alkaline solution, (b) a step of admixing an aqueous aldehyde solution with the aqueous solution obtained in the step (a), (c) a step of adding the aqueous solution obtained by the above step (b) to a hydrophobic solvent containing a polyether type nonionic surfactant under heating and stirring to disperse the aqueous solution in the form of droplets in said hydrophobic solvent and (d) evaporating water components from the above-mentioned droplets to form spherical and gelled insoluble tannins.
Incidentally, in the present specification, the term “insoluble tannin” means a tannin which is insoluble in water or an acid or an alkali.
Through the steps (a) and (b), preliminary crosslinking of tannin is carried out to form a highly viscose solution. When the hydrophobic solvent is stirred in the step (c), the liquid surface of the solvent becomes conical shape to increase an area of gas-liquid interface. Here, the highly viscose solution dispersed in the hydrophobic solvent forms droplets, which flow along the interface. And the water components are evaporated from the droplets in the step (d) whereby the droplets become particles to form gelled insoluble tannins having various kinds of the network structure and the molecular space. The particle size and shape of the gelled insoluble tannin can be controlled depending on the flow conditions, etc. of the hydrophobic solvent such as the degree of its hydrophobicity, the viscosity, stirring rate, etc. Also, the water content of the gelled insoluble tannin can be controlled by changing the network structure and the molecular space thereof depending on the temperature of the hydrophobic solvent, the water evaporation time from the droplets, etc.
When the resulting insoluble tannin is contacted with a solution containing metal elements such as uranium, thorium, transuranium elements, cadmium, lead, chromium, mercury, iron, cobalt, cesium, strontium, etc., the gelled insoluble tannin adsorbs the metal elements with extremely efficiently. This is because the polyphenolic hydroxyl group possessed by the gelled tannin becomes a functional group and adsorbs a metal element by a hydrogen ion exchange reaction more effectively. Also, by making the gel, the degree of freedom of the molecular chain increases whereby the functional group of the molecular chain becomes a steric structure which is easily coordinated with a metal element so that the gelled insoluble tannin shows extremely excellent adsorption ability of the metal element.
The second invention relates to a process for adsorbing a hexavalent chromium to the insoluble tannin by adding the spherical and gelled insoluble tannin with a water content of 70 to 90% prepared by the process of the first invention to an aqueous solution containing the hexavalent chromium.
The gelled insoluble tannin the water content of which is adjusted to 70 to 90% can adsorb the hexavalent chromium with high efficiency.
REFERENCES:
patent: 5158711 (1992-10-01), Shirato et al.
patent: 5274169 (1993-12-01), Shirato et al.
patent: 5296629 (1994-03-01), Shirato et al.
patent: 5300677 (1994-04-01), Shirato et al.
patent: 5320664 (1994-06-01), Shirato
patent: 5460791 (1995-10-01), Shirato et al.
patent: 5986166 (1999-11-01), Mukaida et al.
Nakamura Yasuo
Nakano Yoshio
Cintins Ivars
Mitsubishi Nuclear Fuel Co. Ltd.
Reed Smith LLP
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