Ionic liquids as solvents

Details Ionic liquids as solvents Ionic liquids as solvents

1999-07-02

2002-04-30

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Radioactive metal

C423S008000, C423S009000, C423S010000, C423S251000, C423S260000, C423S261000

Reexamination Certificate

active

06379634

ABSTRACT:

This invention relates to the reprocessing of irradiated fuel, as well as to a method for dissolving metal oxides in ionic liquids and to novel products or compositions of matter comprising ionic liquids.
By way of example, the irradiated fuel may be that resulting from the use of a fuel assembly in a light water reactor (LWR). Reference will be made below to such fuel but it should be understood that the invention is not restricted to the reprocessing of any particular type of irradiated fuel.
The irradiated fuel from an LWR is located within a Zircaloy cladding which has become oxidised as a result of the irradiation. In the known PUREX process for reprocessing irradiated fuel, the first stage involves the shearing and chopping of the fuel rods so that the irradiated fuel itself can be dissolved in nitric acid.
Molten salts are known for their use as solvents and they have in fact been proposed for use in the reprocessing of irradiated fuels from LWRs. These molten salts are typically mixtures of salts which are liquid only at high temperatures and offer little advantage as solvents over aqueous or organic media.
Recently, a salt, 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. 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.).
Known ionic liquids include aluminium(III) chloride in combination with an imidazolium halide, a pyridinium halide or a phosphonium halide. Examples include 1-ethyl-3-methylimidazolium chloride, N-butylpyridinium chloride and tetrabutylphosphonium chloride. An example of a known ionic liquid system is 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).
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 96/32729 teaches that oxide nuclear fuels may be dissolved in a fused alkali metal carbonate to produce a compound which may be further processed so as to extract uranium therefrom.
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 in a first aspect the use of an ionic liquid containing an oxidant to dissolve a metal, optionally in the form of a compound thereof. The oxidant oxidises the metal to a higher oxidation state, which is normally more soluble in the ionic liquid than is the metal in its original oxidation state. More particularly there is provided a method of dissolving in an ionic liquid a metal in an initial oxidation state below its maximum oxidation state, characterised in that the ionic liquid reacts with the metal and oxidises it to a higher oxidation state.
As used herein, the term “metal” includes not only metallic elements in the (0) oxidation state but also metals in an oxidation state greater than zero, bonded to other elements, for example U(IV) and U(VI). Thus, the metal in its original oxidation state may comprise a metal compound, for example a metal oxide.
The metal preferably comprises uranium (typically as UO
2
and/or U
3
O
8
) or plutonium (typically as PuO
2
), or both, and usually fission products. The UO
2
or PuO
2
is not directly dissolved in the oxidising ionic liquid but, rather, the oxide reacts with the ionic liquid to form an oxidised species which dissolves in the ionic liquid. Such preferred dissolution processes may be used in the reprocessing of an irradiated nuclear fuel. The invention also includes the use of the oxidising ionic liquid to dissolve other metal species, for example a zirconium alloy, which may be in the form of cladding of a nuclear fuel rod.
In another aspect, therefore, the invention provides a method of dissolving a metal in less than its maximum oxidation state in an ionic liquid, wherein one component of the ionic liquid is an oxidant to oxidise the metal to a higher oxidation state. The metal is typically in the form of an oxide thereof.
The invention further provides an ionic liquid comprising an agent to increase the oxidising power of the liquid, for example to make a non-oxidising liquid to be oxidising. Thus, the liquid will contain not just one anion and one cation but in addition another component which enhances the ability of the liquid to react to oxidise a substrate. In preferred embodiments, the liquid contains both the mildly oxidising anion [NO
3
] and an acid, which may be a Bronsted or Franklin acid such as HNO
3
, H
2
SO
4
or [NO
+
], e.g. from [NO][BF
4
]. The acid makes the liquid more oxidisingly reactive towards various substrates, such as UO
2
and PuO
2
, for example. Thus, the invention includes an ionic liquid capable of oxidising UO
2
to convert the uranium to U(VI), especially an ionic liquid which comprises nitrate anions and nitronium cations in concentrations sufficient to enable the liquid to react with UO
2
and oxidise the uranium to U(VI), the ionic liquid also containing tetrafluoro-borate(III) and an organic cation. A preferred product is an ionic liquid containing [NO
+
], typically having been added to the liquid as [NO][BF
4
].
The products described in the preceding paragraph may often be viewed in a notional sense as comprising an ionic liquid base to which has been added an agent to increase the oxidising reactivity of the liquid. The nature of the notional “base” ionic liquid is not critical to the invention but preferred liquids comprise nitrate anion and an organic cation, especially nitrogen heterocycles containing a quaternary nitrogen, such as pyridinium or substituted imidazolium ions, for example. Exemplary ionic liquids include 1-butylpyridinium nitrate, 1-octylpyridinium nitrate, 1-butyl-3-methylimidazolium nitrate, 1-hexyl-3-methylimidazolium nitrate and 1-octyl-3-methylimidazolium nitrate.
The invention provides in addition novel nitrate-based ionic liquids, including those containing imidazolium and phosphonium cations as well as those containing pyridinium cations other than such cations disclosed by E. S. Lane.
Particularly preferred and novel ionic liquids are 1-butylpyridinium nitrate and 1-octylpyridinium nitrate. These products themselves, free from any oxidation enhancer, forms one aspect of the invention. To avoid ambiguity it should be stated that systematic names are used herein for individual compounds or moieties, i.e. “butyl” here refers to the group sometimes called n-butyl (CH
3
—CH
2
—CH
2
—CH
2
—).
The invention also includes ionic liquids which are compound mixtures of ionic liquids, for example ternary liquids, the constituent liquids of which can in combination achieve dissolution (typically by oxidising reaction) of inter alia selected substrates or substrate concentrations which the individual liquids cannot so achieve.
The invention includes further the use in a method for reprocessing an irradiated fuel of an ionic liquid to dissolve the fuel, as well as reprocessing methods which includ

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