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 xe2x80x9cmolten saltxe2x80x9d refers to a salt, or a mixture of salts, having inorganic cationic and anionic species, and having a melting point in excess of 100xc2x0 C., usually at least 300xc2x0 C. The term xe2x80x9cmetal saltxe2x80x9d, 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 xe2x80x9cionic liquidsxe2x80x9d although this term can be used for salts which melt at relatively high temperatures, including for example temperatures of up to 100xc2x0 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 300xc2x0 C.).
When used herein, the term xe2x80x9cionic liquidxe2x80x9d refers to a salt, a mixture of salts, or a mixture of components which produce salts and which melts at a temperature up to 100xc2x0 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 UO3 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-(C1-C4 alkyl)-3-(C6-C30 alkyl) imidazolium chlorides and especially 1-methyl-3-C10 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 50xc2x0 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 (xe2x80x9c[emim]Clxe2x80x94ACl3xe2x80x9d). 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]Clxe2x80x94AlCl3 mixture determines whether the liquid has Franklin acidic, basic or neutral properties. A basic melt has an AlCl3:[emim]Cl ratio less than 1.0, whilst an acidic melt has an AlCl3:[emim]Cl ratio greater than 1.0. A neutral melt has an AlCl3:[emim]Cl ratio=1.0.
The invention will now be described in more detail primarily in relation to alkali metal halide salts previously used in reprocessing and contaminated with fission products. It will be understood, however, that the invention may equally be applied to other metal salts, especially to remove contaminants.
Thus, in the treatment of salt compositions resulting from reprocessing and treatment of nuclear materials, the ionic liquid (e.g. basic [emim]Clxe2x80x94AlCl3) may be used to dissolve one or more of the alkali metal halide[s] and/or (usually by complexing) the bulk of the fission product halides (normally chlorides). If the alkali metal halide component of the composition is relatively insoluble in the ionic liquid, as in the case of NaCl in basic [emim]Clxe2x80x94AlCl3, the fission products may be leached into solution. Such leaching is desirably aided by reducing the particle size by, for instance, mechanically dividing (e.g. crushing or grinding) the salt.
The ionic liquid mixture is then treated to separate the alkali metal halides from the fission products, for example by using one or more techniques known per se for separating solutes. Suitable separation procedures include salting out, other precipitation methods, ion exchange, and electrochemical separation. The fission products may not have been completely dissolved in the ionic solvent because, for instance, they may be xe2x80x9clockedxe2x80x9d within the particles of an insoluble component. A further washing step may be necessary in these circumstances. In such cases, the invention normally includes the decontaminating of such insoluble residues remaining after the dissolution of the alkali metal halide composition. Each of the foregoing procedures will now be considered in turn with specific reference to the treatment of reprocessing salt compositions.
Salting Out of Chlorides
A majority of radioactive fission product halides (but not Cs halides) have lattice energies, which are considerably greater than those of the alkali metal halides. Thus, when the salts have been dissolved in an ionic liquid the fission product chlorides may be precipitated by the addition of a species (for example toluene, benzene or other organic solvent) which makes the ionic liquid more like an organic solvent, thereby destabilising the fission product complexes: as the ionicity of the solution decreases a fission product
Decontamination of Halides Insoluble in the Initial Ionic Liquid Solvent
Halides not dissolved in the chosen ionic liquid may be treated to remove radioactive ions by contacting the undissolved residue with another ionic liquid composition. For example, halides not dissolved in the basic ionic liquid used in the initial dissolution may be treated to remove radioactive ions by contacting the undissolved residue with acidic [emim]Clxe2x80x94AlCl3, in which it may be dissolved. The radioactive ions (fission products) may be removed from the resulting solution by, for example, any of the separation techniques mentioned above, especially ion exchange. Acidic [emim]Clxe2x80x94AlCl3 is not suitable for dissolving mixtures containing significant NaCl, because this ionic liquid reacts with NaCl to yield neutral, buffered [emim]Clxe2x80x94AlCl3 composition which will not dissolve fission product chlorides.
Acidic [emim]Clxe2x80x94AlCl3 may be used as solvent in other contexts. Moreover, irrespective is of the context, other acidic ionic liquids (e.g. those based on substituted imidazolium and/or containing AlCl3) may be used in place of [emim]Clxe2x80x94AlCl3. Some preferred methods of the invention therefore involve dissolving a metal halide in an acidic ionic liquid.
The cleaned salt composition is then recovered from the ionic liquid and, if required, has its composition adjusted to the eutectic. In one variant, the cleaned salt composition is dissolved in water or another aqueous medium and then extracted with a water-insoluble ionic liquid (e.g. [bmim][PF6], where [bmim]=1-butyl-3-methylimidazolium cation). The salt composition may then be re-used.
It will be appreciated, therefore, that the invention includes a method of removing radioactive contaminants from an alkali metal halide composition used in the reprocessing of nuclear fuel, comprising:
(i) dissolving in an ionic liquid said contaminants to obtain an ionic liquid solution;
(ii) separating said contaminants from the solution by precipitation, solvent extraction or electrochemical reduction;
(iii) then optionally removing contaminants remaining in the solution by ion exchange; and
(iv) recovering the alkali metal halide composition from the solution and, if necessary, adjusting its composition to the eutectic.
In some of these methods a residue comprising alkali and/or alkaline earth metal halides remains undissolved in the ionic liquid. The residue is dissolved in an acidic ionic liquid, especially acidic [emim]Clxe2x80x94AlCl3, to obtain a mixture from which contaminants are removed especially by precipitation or electrochemical reduction and/or by ion exchange.
Also included in the invention is the use of an ionic liquid as a solvent for an ionic composition in the selective removal from the dissolved composition of one or more species contained in the composition.