Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1998-12-15
2001-04-10
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S688000
Reexamination Certificate
active
06214234
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of prior PCT International Application No. PCT/FI97/00413 which has an International filing date of Jun. 26, 1997 which designated the United States of America.
The present invention relates to a method for cesium removal from aqueous solutions, particularly radioactive waste liquids.
According to the method, a cesium-containing aqueous solution is contacted in an ion-exchange column with a solid-state hexacyanoferrate compound of a transition series element for the purpose of binding cesium in the hexacyanoferrate, after which step the aqueous solution of reduced cesium content is separated from the hexacyanoferrate.
The present invention also concerns a method for industrial-scale production of granular hexacyanoferrate compounds of a transition metal suitable for use in a column. In the method, an aqueous solution of the soluble hexacyanoferrate is gradually added in a solution of a salt of the transition element with simultaneous stirring in order to produce a precipitate of the transition element hexacyanoferrate compound, after which the hexacyanoferrate precipitate thus formed is separated and recovered.
In the low- and medium-level radioactive waste solutions released by nuclear power plants and fissionable nuclear fuel reprocessing plants,
137
Cs is generally the most important radioactive material owing to its relatively long half-life (30 a) and the large amounts of this isotope formed in a fission reaction. Removal of cesium from radioactive waste liquids makes the treatment of remaining solutions much easier. Particularly, if a major portion of the radioactivity of high-level solutions can be separated from the bulk of the solution into a small-volume batch and thereafter the remaining solution treated as a medium- or low-level waste, extremely large cost savings are possible. In some cases, the waste liquids of nuclear plants may after cesium removal even be discharged to the environment owing to their very low residual radioactivity after this separation stage. The primary goal of separating radionuclides from radioactive waste liquids is to reduce the volume of such wastes, whereby the handling and permanent repository costs of the wastes are lowered. The more selective, i.e., the more specific the separation method with respect to given radioactive material as compared to other substances in the solution, the higher the volume reduction achievable by the method. Another goal of separation of radionuclides is to reduce the amounts of effluents to the environment, which can be rendered economically viable by selective separation methods.
For the removal of radioactive cesium from radioactive waste liquids, either precipitation or ion-exchange methods have been used. Precipitation does not provide as high a degree of separation as ion exchangers and often it is difficult to effectively separate the precipitated solids from the solution. Most generally, organic ion-exchange resins have been used as ion exchangers adapted for cesium removal, and since the late 1980's, also zeolite minerals have been used to an increasing degree (cf. Lehto 1993a).
Already for decades, hexacyanoferrates of transition elements have been known to be superior to organic resins and zeolites in cesium-binding ion exchangers (cf. Loewenschuss 1982, Hendrickson 1975). Table 1 shows the higher efficiency of, e.g., potassium-cobalt hexacyanoferrate in comparison to zeolites and organic resins or ammonium phosphomolybdate, the latter being recognized as an efficient ion exchanger of cesium (cf. Lehto 1993b). No practical usability have been found for zeolites and organic ion-exchange resins in the removal of cesium from, e.g., concentrated NaNO
3
solutions of nuclear fuel reprocessing plants and waste liquid evaporation residues of nuclear power plants.
TABLE 1
Distribution coefficients of radioactive cesium for different ion
exchanger materials in 1M KCl and NaCl solutions. The value of
the distribution coefficient indicates the waste-handling capacity
of a column exchanger, i.e., how many liters of solution can be
processed by a 1 kg batch of the ion-exchange material.
Distribution coefficient
Exchanger
1M KCl
1M NaCl
K—Co hexacyanoferrate
50,000
ml/g
200,000
ml/g
Ammonium phosphomolybdate
2,500
100,000
Zeolite (mordenite)
60
570
Sulfonic acid resin
0
10
The composition of hexacyanoferrates of transition elements is represented by the formula
A
x
M
y
[MFe (CN)
6
]·zH
2
O
where A is either an alkali metal ion or ammonium ion and M is a transition element with valence 2, such as Ni or Co. The ions written within the square brackets form a crystalline structure with a negative total charge −2, and the ions of positive total charge written outside the square brackets are situated in the channels of the crystalline structure, wherefrom they can be replaced by the ions, such as Cs, of the external solution.
Ion exchange of cesium using hexacyanoferrates is represented by the formula:
A
x
M
y
[MFe(CN)
6
]+2Cs
+
≈Cs
2
[MFe(CN)
6
]+xA
+
+yM
2+
where x+y/2=2.
Up to date, hexacyanoferrates have been used as precipitation reagents for removal of cesium from the waste liquids of reprocessing plants. Columns are known, however, to provide a much higher degree of separation and simpler process than direct use of precipitation reagents. This is also verified in the literature: cesium removal in a column packed with granular hexacyanoferrate is around hundred times more efficient than precipitation with a corresponding amount of hexacyanoferrate when separation efficiency is indicated as a product of the decontamination factor (ratio of pretreatment radioactivity to posttreatment radioactivity) and the volume reduction factor (Harjula 1994). Yet, use of hexacyanoferrates in columns has been ignored in the nuclear industry. The most important obstacle has been the poor quality of ion-exchange materials; the granular particles of tested hexacyanoferrate grades have undergone disintegration, thereby plugging the column. In fact, the inventors of the present method found in the late 1980's that the potassium-cobalt hexacyanoferrate grade which at that time was commercially available from a single manufacturer alone disintegrated into useless sludge in water.
While a number of different methods have been developed for the production of hexacyanoferrate grades suitable for column use through binding the ion-exchange compound on, e.g., organic ion-exchange resins (cf. Narbutt, Stejskal 1974, Watari 1965, Lehto 1993c), none of these products has, however, been adopted in industrial use. Hexacyanoferrates bound on organic ion-exchange resins exhibit an extremely reduced resistance to high radiation doses, which limits their binding capacity to relatively small amounts of separated
137
Cs (cf. Lehto 1993d).
U.S. Pat. No. 3,296,123 (Prout 1967) discloses a method of producing potassium-cobalt hexacyanoferrate, in which method an aqueous solution of potassium ferrocyanide trihydrate is buffered at pH 5.3 with a mixture of acetic acid and sodium acetate, after which the solution is slowly added at a temperature below 15° C. into an aqueous solution of a cobalt salt under continuous stirring. The precipitate thus obtained is recovered and dried at an elevated temperature to make crystalline potassium-cobalt hexacyanoferrate. According to cited publication, the cobalt salt is used by an excess amount of 40-50% with respect to said ferrocyanide compound in order to prevent the formation of colloidal or difficult-to-centrifuge products.
The prior-art method makes it possible to produce an ion-exchange material that functions satisfactorily also in a column at least in laboratory equipment scale. However, the distribution coefficient of the product remains rather low due to the relatively high cobalt content of the product; this is because the fraction of cobalt in the amount of exchangeable ions have been found to bear a direct detrimental effect on the cesium removal capa
Harjula Risto
Lehto Jukka
Birch. Stewart, Kolasch & Birch, LLP
Cintins Ivars
Ivo Power Engineering Oy
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