Treatment of molten salt reprocessing wastes

Details Treatment of molten salt reprocessing wastes Treatment of molten salt reprocessing wastes

2000-10-20

2002-10-22

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Halogen or compound thereof

Binary compound containing metal

C423S499300, C423S499500, C423S021100, C423S155000, C423S179000, C423S253000, C376S311000, C376S360000

Reexamination Certificate

active

06468495

ABSTRACT:

This invention relates to the treatment of salts and, more particularly, the treatment of salts previously used in molten form in the reprocessing and treatment of nuclear materials.
Molten salts are known for their use as solvents and they have in fact been proposed for use in the reprocessing or treatment of irradiated fuels from light water reactors (LWRs) and fast reactors. In such a method, an irradiated fuel is dissolved into molten alkali metal chlorides. The dissolved uranium and plutonium species are extracted from the salt and, optionally, processed into fuel, and the molten salt is re-used. Eventually the molten salt becomes significantly contaminated with fission products which must be removed before the salt can be re-used. One process which uses molten salts for reprocessing irradiated fuel uses lithium chloride/potassium chloride eutectic [{LiCl+KCl} eutectic] and another uses sodium chloride/potassium chloride eutectic [{NaCl+KCl} eutectic]. When used herein, the term “molten salt” refers to a salt, or a mixture of salts, having inorganic cationic and anionic species, and having a melting point in excess of 100° C., usually at least 300° C. The term “metal salt”, when used herein, refers to a material which gives rise to a molten salt when melted. In addition to chloride salts such as those mentioned above, other molten salts with which this invention may be concerned include fluorides and carbonates.
Recently, salts, mixtures of salts, or mixtures of components which produce salts, which melt below or just above room temperature have become known. (In the terms of this invention, a salt consists entirely of cationic and anionic species). Such liquids are known as “ionic liquids” although this term can be used for salts which melt at relatively high temperatures, including for example temperatures of up to 100° C. They normally include at least an organic cation. Common features of ionic liquids include a zero vapour pressure at room temperature, a high solvation capacity and a large liquid range (for instance, of the order of 300° C.).
When used herein, the term “ionic liquid” refers to a salt, a mixture of salts, or a mixture of components which produce salts and which melts at a temperature up to 100° C. and/or includes an organic cation.
Other ionic liquids are, for example, nitrates, fluoroborates, ethanoates or hexafluorophosphates, of which nitrates and fluoroborates are discussed in PCT/GB97/02057,. Mixtures of any of the previously described ionic liquids may likewise be used.
Known ionic liquids include halides such as an imidazolium halide, a pyridinium halide or a phosphonium halide as well as such materials in combination with, for instance, a metal halide such as aluminium chloride. Examples of ionic liquids include 1-ethyl-3-methylimidazolium chloride, N-butylpyridinium chloride, tetrabutylphosphonium chloride and a mixture of 1-ethyl-3-methylimidazolium chloride and aluminium(III) chloride.
E. S. Lane,
J. Chem. Soc.
(1953), 1172-1175 describes the preparation of certain alkylpyridinium nitrate ionic liquids, including sec-butylpyridinium nitrate. No use of the liquids is mentioned but reference is made to the pharmacological activity of decamethylenebis(pyridinium nitrate).
Ionic liquids based on various anions, including nitrate, fluoroborate and ethanoate, are disclosed by J S Wilkes et al.,
J. Chem. Soc. Chem. Commun.,
965-967 (1992). The use as solvents for catalysis of ionic liquids based on non-nucleophilic ions such as tetrafluoroborate and hexafluorophosphate is described by Y. Chauvin et al.,
Angew. Chem. Int. Edit. Engl.,
34, 2698-2700 (1995).
L. Heerman et al.,
J. Electroanal. Chem.,
193, 289 (1985) describe the dissolution of UO
3
in a system comprising N-butylpyridinium chloride and aluminium(III) chloride.
WO 95/21871, WO 95/21872 and WO 95/21806 relate to ionic liquids and their use to catalyse hydrocarbon conversion reactions (e.g. polymerisation or oligomerisation of olefins) and alkylation reactions. The ionic liquids are preferably 1-(C
1
-C
4
alkyl)-3-(C
6
-C
30
alkyl) imidazolium chlorides and especially 1-methyl-3-C
10
alkyl-imidazolium chloride, or 1-hydrocarbyl pyridinium halides, where the hydrocarbyl group is for example ethyl, butyl or other alkyl.
The present invention provides a method of removing from a metal salt ionic species therein, which method comprises contacting the metal salt with an ionic liquid to dissolve the metal salt, the ionic species or both, thereby to form a resultant ionic liquid composition and, at least in the case where both the molten salt and the ionic species are dissolved, said ionic liquid composition is treated to separate the ionic species therefrom and subsequently processed to recover the metal salt.
The method of the invention has the advantage that it can be performed at relatively low temperatures (e.g. of about 50° C. or less), for example at temperatures at or close to room temperature.
The ionic species with which this invention is concerned include ions of fission products, minor actinides, activation products, corrosion products, fuel additives and process additives.
The metal salt is preferably an alkali metal halide or a mixture of alkali metal halides, for example a mixture of lithium chloride and potassium chloride or a mixture of sodium chloride and potassium chloride. Such alkali metal halide mixtures are suitably eutectics.
The species removed from the metal salt may comprise fission product ions, for example as their chlorides, fluorides or nitrates. Exemplary fission products include Cs, Sr, Ba and those of the actinides and lanthanides. The invention is particularly concerned with methods in which the ionic species comprise cations of the lanthanides (for example Sm, Gd and Ce) and the metals Cs, Sr and Ba.
In preferred processes, the metal salt and the ionic species are dissolved in the ionic liquid and known techniques may be used to separate the dissolved products. Suitable separation techniques include salting out, electrochemical methods, precipitation and ion exchange. In one class of methods, the solution is treated to separate the ionic species, typically fission product chlorides, therefrom and subsequently processed to recover the metal salt.
Alternatively, a component from the metal salt and contaminant fission product mixture is dissolved in the ionic liquid. The presence of an insoluble component leads to the first step in the separation sequence.
In one preferred class of processes, LiCl+KCl or NaCl+KCl, in either case containing dissolved fission products, is contacted at, for example, room temperature with an ionic liquid system in which the alkali metal halides as well as fission products to be separated therefrom are soluble. The invention contemplates in particular ionic liquids containing an organic halide, optionally in combination with a metal halide such as aluminium(III) chloride; such ionic liquids include in combination with an imidazolium halide, a pyridinium halide or a phosphonium halide as well as these materials in combination with aluminium (III) chloride. Examples of organic halide ionic liquids include 1-ethyl-3-methylimidazolium chloride, N-butylpyridinium chloride and tetrabutylphosphonium chloride. Preferred ionic liquids include 1-ethyl-3-methylimidazolium chloride and a mixture of basic (that is, Franklin basic) 1-ethyl-3-methyl-imidazolium chloride and aluminium(III) chloride (“[emim]Cl—ACl
3
”). Of course, the aforegoing ionic liquids may be used to dissolve salt compositions other than LiCl+KCl or NaCl+KCl.
A further preferred option is combining the ionic liquids to form a mixture, such as 1-ethyl-3-methyl-imidazolium chloride and 1-octyl-3-methyl-imidazolium chloride.
The composition of the [emim]Cl—AlCl
3
mixture determines whether the liquid has Franklin acidic, basic or neutral properties. A basic melt has an AlCl
3
:[emim]Cl ratio<1.0, whilst an acidic melt has an AlCl
3
:[emim]Cl ratio>1.0.

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