EXTRACTING METALS DIRECTLY FROM METAL OXIDES

Abstract: A method of extracting metals directly from metal oxides by exposing the oxide to a supercritical fluid solvent containing a chelating agent is described. Preferably, the metal is an actinide or a lanthanide. More preferably, the metal is uranium, thorium or plutonium. The chelating agent forms chelates that are soluble in the supercritical fluid, thereby allowing direct removal of the metal from the metal oxide. In preferred embodiments, the extraction solvent is supercritical carbon dioxide and the chelating agent is selected from the group consisting of beta -diketones, halogenated beta -diketones, phosphinic acids, halogenated phosphinic acids, carboxylic acids, halogenated carboxylic acids, and mixtures thereof. In especially preferred embodiments, at least one of the chelating agents is fluorinated. The method provides an environmentally benign process for removing metals from metal oxides without using acids or biologically harmful solvents. The chelate and supercritical fluid can be regenerated, and the metal recovered, to provide an economic, efficient process. Also disclosed is a method to separate a desired metal from impurities formed during a supercritical extraction process based on solubility differences between the components, and the ability to vary the solvent power of the supercritical fluid. The use of adduct forming agents to separate metal in such processes is further disclosed.

Description: EXTRACTING METALS DIRECTLY FROM METAL OXIDES FIELD OF THE INVENTION This invention concerns extracting metals from metal oxides. ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT This invention was funded in part by the United States Department of Energy under Grant No. DE-FG02-91ER75679. The United States Government may have certain rights in this invention as a result of this grant. BACKGROUND OF THE INVENTION In order to make use of metals, they first must be extracted from raw materials, such as metal oxides, or from waste streams. One particular problem is how to extract metals from metal oxides, while minimizing environmental contamination. Metals currently are liberated from metal oxides by first crushing the oxide and then treating the crushed material with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very nonspecific, and often produces many by-products, including aqueous and organic wastes, that create serious environmental problems in their own right. It also is known to extract metal ions from materials using fluid extraction, such as supercritical fluid extraction. A supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and pressure above its critical temperature and pressure. Supercritical solvents can be used to extract organic materials, such as caffeine from coffee beans. U.S. Patent No. 4,911,941 provides an example of supercritical carbon dioxide extraction of caffeine in which green coffee beans are moved periodically through an extraction vessel and contacted with continuously flowing supercritical carbon dioxide. U.S. Patent No. 4,898,673 shows a similar system in which soluble materials are continuously extracted from solids using supercritical carbon dioxide. The soluble solids are circulated in a closed-loop pipeline with the supercritical fluid. Supercritical extraction of environmental wastes is disclosed in Wai et al.'s United States Patent No. 5,356,538, as well as Wai et al.'s United States Patent Application entitled FLUID EXTRACTION, filed June 2, 1995, which is a continuation-in-part of co-pending United States Patent Application, Serial No. 08/253,000, filed June 2, 1994 (Wai's patent documents). Wai's patent documents, which are incorporated herein by reference, disclose the extraction of metalloid and metal ions from materials by exposing the material to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent. Directly extracting metals from metal oxides apparently is not known. As stated above, known methods involve dissolving crushed metal oxides in a strong acid, such as concentrated nitric acid, before extraction to separate the metal from the metal oxide. This is done to disrupt the metal oxide matrices which possess high structural stability. This characteristic of metal oxides has discouraged efforts at direct extraction of metals from metal oxides in the past. Moreover, the purification or separation of certain metals from other metals or impurities is critical in many instances. For instance, in the commercial processing of uranium, iron is one common impurity and must be separated out. Further, uranium and plutonium are separated from fission products of spent nuclear fuel by using a solvent extraction reagent. The uranium is subsequently separated from the plutonium. Major disadvantages of some traditional purification processes for metals, such as uranium, include: (a) low purified quantities of metal; (b) time consuming purification steps; and (c) the creation of undesirable waste streams. A need exists, therefore, for an environmentally safe method for directly extracting metals from metal oxides while avoiding intermediate processing steps. A need also exists for a method to separate or purify metals from impurities which is environmentally safer and quicker, and which does not require the use of traditional solvents. The need further exists for such a method which is efficient and which provides greater yields of the extracted and purified metals. SUMMARY OF THE INVENTION The present invention provides a method for extracting metals from metal oxides by exposing the oxides to a chelating agent and a fluid solvent, particularly a supercritical fluid solvent such as supercritical carbon dioxide. The metal oxide is exposed to the fluid or supercritical fluid solvent and chelating agent for a sufficient period of time so that the metal forms chelates with the chelating agent, which chelates are soluble in the fluid. The fluid or supercritical fluid containing the dissolved chelates is then separated from the metal oxide. The chelates subsequently can be precipitated from the fluid. For example, if the fluid is supercritical, then the chelates can be precipitated by reducing the pressure of the supercritical fluid. The chelating agent can be regenerated for reuse after separation from the metal. The resulting process is an efficient, cost- effective method for removing metals, especially actinides and lanthanides, from metal oxides without using environmentally harmful extraction methods and solvents. The metal oxides of the present invention can be virtually any metal oxide, particularly metal oxides that can react with protic (i.e., capable of donating a proton) chelating agents. Preferably, the metal of the metal oxides is selected from the group of metals consisting of the actinides and the lanthanides. The lanthanide metals are rare-earth elements, having atomic numbers 57 through 71. Examples of lanthanide metals include Cerium (atomic number 58) and Terbium (atomic number 65). The actinide metals extend from actinium (atomic number 89) or thorium (atomic number 90) through lawrencium (atomic number 103) on the periodic table. The chelating agents can be any agent that (1) forms a chelate with the metal being extracted, and (2) forms chelates which are soluble in the fluid or supercritical fluid solvent. The chelating agents generally should be sufficiently acidic to donate a proton to the metal oxide, thereby rendering the metal available to form chelates with the chelating agent. Without limiting the present invention, examples of suitable chelating agents include β-diketones, halogenated 0-diketones, phosphinic acids, halogenated phosphinic acids, carboxylic acids and halogenated carboxylic acids, particularly lower alkyl carboxylic acids. As used herein, the term "lower alkyl" refers to compounds having ten or fewer carbon atoms, and includes both straight-chain and branched-chain compounds. In one embodiment of the method, the chelating agent is selected from the group consisting of J-diketones, halogenated 0-diketones, phosphinic acids, halogenated phosphinic acids, carboxylic acids, halogenated carboxylic acids, and mixtures thereof. In another embodiment, the chelating agent is selected from the group consisting of 3-diketones, halogenated 3-diketones, phosphinic acids, halogenated phosphinic acids, and mixtures thereof. In another embodiment, the chelating agent is selected from the group consisting of 0-diketones, halogenated 3-diketones, carboxylic acids, halogenated carboxylic acids, and mixtures thereof. In a preferred embodiment, the chelating agent is selected from the group consisting of /S-diketones, halogenated 3-diketones, and mixtures thereof. The β-diketones generally satisfy the following Formula 1 FORMULA 1 wherein R, and R 2 typically are independently selected from the group consisting of lower alkyl groups, halogenated lower alkyl groups, and thenoyl groups. As used herein, a "halogenated, lower alkyl group," such as a fluorinated ethyl group, means that at least one of the hydrogen atoms present on the alkyl group is replaced with a halogen atom, preferably a fluorine atom. A "halogenated lower alkyl group" also can refer to compounds wherein all, or any number, of the hydrogen atoms have been replaced with halogens, preferably fluorine atoms. More specifically, R, may be selected from the group consisting of methyl, fluorinated methyl, trifluoromethyl, ethyl, fluorinated ethyl, pentafluoromethyl, propyl, fluorinated propyl, heptafluoropropyl, butyl, fluorinated butyl, and nonafluorobutyl, and R 2 may be selected from the group consisting of methyl, fluorinated methyl, trifluoromethyl, ethyl, fluorinated ethyl, pentafluoromethyl, propyl, fluorinated propyl, heptafluoropropyl, butyl, fluorinated butyl, and nonafluorobutyl. Specific examples of suitable 0-diketones include, without limitation, acetylacetone, trifluoroacetylacetone, hexa-fluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluorobutanoylpivaroylmethane. Especially preferred |8-diketones include trifluoroacetylacetone, hexafluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluoro- butanoylpivaroylmethane. In preferred embodiments, the chelating agent is halogenated to enhance the solubility of the chelate in supercritical carbon dioxide. In especially preferred embodiments, the chelating agent is fluorinated. The solubilities of some halogenated metal chelates in supercritical carbon dioxide, and in particular the fluorinated metal chelates, are enhanced by two to three orders of magnitude relative to the corresponding non-halogenated metal chelates. Fluorinated chelating agents have been found to enhance the efficiency of metal extraction in supercritical carbon dioxide. As a result, fluorinated chelating agents currently are preferred chelating agents useful for practicing the present invention. For illustrative purposes only, and without limiting the present invention, a suitable fluorinated chelating agent is shown below. 10 Hexafluoroacetylacetone The phosphinic acid chelating agents generally satisfy the following Formula 2 FORMULA 2 0 II P, R OH R wherein R 3 and R 4 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, aryl, halogenated aryl, thenoyl, and mixtures thereof. The carboxylic acids generally satisfy the following Formula 3 FORMULA 3 O R OH wherein R 5 generally is selected from the group consisting of lower alkyl, halogenated lower alkyl, aryls, halogenated aryls, halogenated aryls, and thenoyl groups. In yet other embodiments of the invention, a modifier is added to the supercritical fluid to further enhance the efficiency of the extraction method by increasing the solubility of the metal chelate in the supercritical fluid. Supercritical carbon dioxide, for example, is a relatively non-polar solvent. Its polarity can be increased by adding a more polar solvent to the carbon dioxide. Disclosed examples of more polar solvents include water and low to medium boiling point alcohols or esters, such as lower-alkyl alcohols and esters, methanol being one example. The alcohol or ester increases the polarity of the supercritical fluid, enhances the solubility of the metal chelate in the fluid, and further improves the extraction efficiency of the method. A preferred method according to the present invention comprises exposing a metal oxide to carbon dioxide, particularly supercritical carbon dioxide, containing a ligand selected from the group consisting of fluorinated β-diketones, fluorinated phosphinic acids, fluorinated carboxylic acids, and mixtures thereof. At least one of the fluorinated 0-diketones, fluorinated phosphinic acids, or fluorinated carboxylic acids, forms chelates with the metal. The chelated metals are soluble in the supercritical carbon dioxide. In more particular embodiments of the invention, a system is provided for treating materials comprising metal oxides, preferably actinide oxides and lanthanide oxides. Most preferably, the metal oxides are actinide oxides, such as, for example, uranium oxide, thorium oxide or plutonium oxide. The material is placed in a container through which the fluid or supercritical fluid and chelating agents are passed to solubilize the metal. The fluid or supercritical fluid containing the solubilized metal is removed from the container to separate the metal. In preferred embodiments, the chelates are fluorinated to further increase their solubility and enhance the extraction efficiency of the separation method. Polar solvents, such as alcohols or esters, can be added to the supercritical fluid to further enhance solubility of the metal chelate in the supercritical fluid. Fluid or supercritical fluid can be flowed continuously through the waste material, or exposed to the material in a discontinuous batch process. In one embodiment, a supercritical fluid is flowed through a chelating agent to dissolve the chelating agent in the fluid before exposing the waste material to the fluid. After the supercritical fluid containing the dissolved metal chelates has been removed from the waste material, the pressure on the supercritical fluid can be reduced to below supercritical levels such that the fluid becomes a gas and the metal chelates are precipitated from the system. The pure metal can then be collected, and the chelating agent recycled. The chelating agent can be separated from the metal, for example, by 0.1 M or more concentrated nitric acid with a pH less than or equal to 1. The present invention also provides for a process to separate and purify a metal from other metals, impurities, and unreacted reagents. The method of the present invention is environmentally safer in that it reduces undesirable waste streams. The method can be carried out efficiently in a relatively short time period, while providing greater yields of the extracted and purified metal without the necessity of using traditional solvents. More specifically, the present invention provides for the separation of metal chelate complexes based on the solubility differences of the complexes in supercritical fluids (preferably supercritical carbon dioxide). In one illustrative embodiment, uranyl chelate complex, which is less soluble in supercritical carbon dioxide, is separated from an iron chelate complex, which is more soluble in supercritical carbon dioxide. In further accordance with the present invention, unreacted reagents, such as chelating agents or ligands, can be separated from desired metal chelates based on the solubility differential of the unreacted reagents from the metal chelate complexes. In one embodiment, unreacted chelating agent, thenoyltrifluoroacetone (or "TTA"), is separated from uranyl/TTA complex since it is more soluble in the supercritical carbon dioxide solvent used in the extraction process. In one manner of doing so, the reaction cell can be flushed with supercritical fluid (e.g., CO 2 ) subsequent to the reaction phase in order to dissolve and separate any unreacted chelating agent (e.g., TTA) from the metal chelate complex formed (e.g. uranyl/TTA complex). (It should be appreciated that the metal can then be separated from the extracted and purified metal chelate complex by methods discussed herein). In accordance with a further aspect of the present invention, by varying the physical conditions of the supercritical fluid, the degree of separation of the components or metal chelate complexes formed during the supercritical fluid extraction process is increased or enhanced. In this manner, the solvating power of the supercritical fluid can be varied to increase or enhanced the solubility of certain impurities or metal chelates with respect to other such components, thereby leading to their separation. In particular, the physical conditions of the supercritical fluid can be varied to achieve a solvation region that is favorable to one particular component over at least one other component. For instance, in one embodiment of the present invention, increasing the pressure will increase the separation of a desired metal chelate from impurities or other undesirable metal chelates. More specifically, in one illustrative embodiment, increasing the pressure of supercritical carbon dioxide solvent increases the solubility of iron chelate complex in the solvent, thereby resulting in its desired separation from less soluble uranyl chelate complex. In a further aspect of the present invention, the increased solubility of one component can be used to inhibit the uptake or solubility of other components in the supercritical fluid. More specifically, in one embodiment, the increased solubility of iron chelate complex in supercritical carbon dioxide inhibits the uptake of other compounds or metal chelate complexes (such as uranium chelate complex) in the supercritical carbon dioxide. Hence, in this way, iron can be further selectively purified from metals, such as uranium, thorium, or plutonium, in supercritical fluid extraction processes ("SFE"). In yet a further aspect of the present invention, the solubility of a metal chelate complex in supercritical fluid is increased by forming an "adduct" of the complex, thereby allowing better control of the transport and separation of the desired metal complex in the supercritical fluid during the extraction process. In one illustrative embodiment, uranium is directly extracted from metal oxide by exposing it to TTA dissolved in supercritical carbon dioxide. Tributyl phosphate (or "TBP"), an adduct forming agent, is added to the system to form adducts of -l i¬ the uranium chelate complex in order to increase its transport in supercritical carbon dioxide. The present invention can be used in connection with the separation of metals extracted directly from metal oxides by supercritical fluid extraction. In one embodiment, uranium is separated by the method of the present invention after being extracted from UO 3 . The metal oxide, UO 2 (NO 3 ) 2 , can also be used. The present invention, however, is also suitable for use with the extraction of thorium from thorium oxide, plutonium from plutonium oxide, or the extraction of other metals from metal oxides as discussed herein. Preferably, a protic chelating agent and supercritical carbon dioxide are used in a direct extraction of the metal from metal oxide. The separation and purification techniques can also be used with other supercritical fluid extraction processes. Accordingly, it is an object of this invention to provide a method for directly (i.e. , without taking any initial steps to disrupt the metal matrices) extracting metals from metal oxides. A further object of the invention is to provide a method for extracting actinides and lanthanides from oxides thereof. Another object of the invention is to provide a method for extracting metals from their respective oxides using supercritical fluid extraction. Another object of this invention to provide a process for the efficient and direct extraction of metals from metal oxides using a protic chelating agent, such as a / S-diketone, particularly a halogenated 3-diketone. Another object of the invention is to provide a method for extracting uranium from uranium ore and uranium fuel without generating high volumes of aqueous acidic waste streams. A further object of the invention is to provide an improved method for extracting metals from metal oxides efficiently using environmentally benign extraction agents and methods. Yet another object of this invention is to provide such an improved method that is efficient and economical compared to many other extraction processes. These and other objects of the invention will be understood more clearly by reference to the following drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a phase diagram for carbon dioxide. FIG. 2 is a schematic drawing of a system for extracting metal from metal oxides in accordance with the present invention. FIG. 3 is an Arrhenius plot showing the energy of activation for uranium oxide dissolution at varying hexafluoroacetylacetone (HFA) concentrations according to the embodiment described in Example IV. FIG. 4 is a schematic drawing of a waste treatment system in accordance with the present invention. FIG. 5 is a schematic drawing of a reaction system utilized in connection with the present invention. FIG. 6 is a comparison of the solubility parameter of SF CO 2 with those of conventional solvents. FIG. 7 shows the effect of pressure on the reaction rate of UO 3 to UO 2 (TTA) 2 at 60° C and after 20 minutes of static reaction time followed by 20 minutes of dynamic flushing of the cell. FIG. 8 shows the effect of temperature on the reaction rate of UO 3 to UO 2 (TTA) 2 at 150 atm and after 20 minutes static reaction time followed by 20 minutes of dynamic flushing of the cell. FIG. 9 shows the solubility of TTA in SF COj at 40°C, 60°C and 90°C. FIG. 10 shows the solubility of Fe(TTA) 3 in SF CO 2 at 60°C (data points shown as squares), and 90 °C (data points shown as circles). FIG. 11 illustrates data on the separation of uranium and iron into a supercritical fluid (CO 2 ) based on differential solubilities of FefTTA^ and UO 2 (TTA) 2 » H 2 O complexes (at 60° C). FIG. 12 illustrates data on the separation of U^OTA^ from Th(TTA) 4 at 60° C. FIG. 13 illustrates data on the separation of four metal/TTA complexes at 60° C. FIG. 14 shows the extraction (at 60°C and 150 atm) of UOzfTTA^ in SF CO 2 by adduct formation with tributyl phosphate (or "TBP"). DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS I. General Discussion of the Invention The present invention concerns a method for extracting metals from oxides thereof by exposing the metal oxide to a fluid solvent or a supercritical fluid solvent. The fluid or supercritical fluid contains a chelating agent that forms a metal chelate with the extracted metal. The fluid may also include a solubility-enhancing agent. The present invention is suitable for removing many different types of metals from metal oxides. A metal is an element that forms positive ions in solutions, and produces oxides that form hydroxides rather than acids with water. Metals include alkali metals, alkali-earth metals, transition metals, noble metals (including the precious metals gold, platinum and silver), rare metals, rare-earth metals (lanthanides), actinides (including the transuranic metals), light metals, heavy metals, synthetic metals and radioactive metals. Specific examples are provided herein of extraction methods for separating actinides, lanthanides and transition metals from the corresponding metal oxides. A preferred method of the present invention comprises extracting actinides and lanthanides from actinide oxides and lanthanide oxides. A more preferred method is used to extract actinides from actinide oxides. Suitable fluids and/or supercritical fluids for use in the present invention include carbon dioxide, nitrogen, nitrous oxide, methane, ethylene, propane and propylene. Carbon dioxide is a particularly preferred fluid for both subcritical and supercritical fluid extractions because of its moderate chemical constants (T C =31 °C, P c 73 atm) and its inertness (i.e., it is non-explosive and thoroughly safe for extractions, even extractions performed at supercritical conditions). Carbon dioxide also is a preferred solvent because it is abundantly available and relatively inexpensive. FIG. 1 is a phase diagram for carbon dioxide. FIG. 1 shows the conditions necessary to produce either subcritical liquid carbon dioxide or supercritical carbon dioxide. All conditions above the triple point (T P ) produce a supercritical carbon dioxide fluid solvent effective for practicing the present invention. However, virtually any conditions that are above the critical point are acceptable for producing a supercritical carbon dioxide fluid solvent useful for practicing the extraction process of the present invention. Alternatively, liquid CO 2 may be suitable for use in the present invention. At room temperature carbon dioxide becomes a liquid above 5.1 atm. Depending on the pressure, liquid carbon dioxide has a density comparable or slightly greater than supercritical carbon dioxide, thus the solvation power of liquid carbon dioxide is comparable to that of supercritical carbon dioxide. This means liquid carbon dioxide should also be able to dissolve the metal complexes described above. However, liquid carbon dioxide does not have the "gas-like" properties of the supercritical carbon dioxide. This means liquid carbon dioxide has a large viscosity, small diffusivity, and consequently poor penetration power compared with the supercritical carbon dioxide. The extraction efficiency of liquid carbon dioxide is expected to depend on the applied pressure. It also is expected that the extraction efficiency with liquid CO 2 can be improved with agitation. The fluid solvents of the present invention may be used either individually or in combinations, as mixed fluids or supercritical fluid solvents. Examples of suitable fluids, and their critical temperature and pressure, are shown in the following Table I: TABLE I PHYSICAL PARAMETERS OF SELECTED SUPERCRITICAL FLUIDS * Molecular Water H,0 374.1 218.3 * data from Matheson Gas Data Book (1980) and CRC Handbook of Chemistry and Phvsics (CRC Press, Boca Raton, Florida 1984). •* T, = 1.03 In addition, a modifier may be added to the fluid, including supercritical fluids, to improve the solvent characteristics thereof. The most useful modifiers are water and the low to medium boiling point alcohols and esters, such as the lower alkyl alcohols and esters. Typical modifiers may be selected from the group consisting of water, methanol, ethanol, ethyl acetate and the like. The modifiers typically are added to the fluids at proportions of between about 0.1 % and 20.0% by weight. The modifiers contemplated for use herein are most typically not supercritical fluids at the disclosed operating conditions. Rather, the modifiers are simply dissolved in the fluid solvents, including the supercritical fluid solvents, to improve their solvent properties. In one embodiment the chosen enhancer is combined with a supercritical fluid at the described proportions prior to feeding the supercritical fluid to the extraction vessel. Alternatively, the supercritical fluid is fed to the extraction vessel without the enhancer. The enhancer is then introduced into the extraction vessel and thereby combined with the supercriticalffluid. Preferably, the chelating agents of the present invention are protic. The donation of a proton to the metal oxide apparently sufficiently frees the metal from the metal-oxide lattice so that the metal can form chelates with the chelating agent. Without limiting the invention, chelating agents that appear useful for forming chelates with the metal of metal oxides include the 0-diketones and related halogenated reagents, the phosphinic acids and related halogenated reagents, and the carboxylic acids and related halogenated reagents. A. Extraction of Metal Ions with /3-diketones Metals can be extracted from metal oxides by fluid CC^, particularly supercritical CO 2 , containing /3-diketones. The carbon atoms of a ketone are assigned greek letters to designate their positions relative to the carbonyl carbon. The first carbon adjacent the carbonyl carbon is designated a, the second such carbon being designated β, and so on. A 0-diketone has at least two ketone carbonyls wherein one ketone carbonyl is located on a carbon β to the other ketone functionality. A generic formula for (8-diketones useful for chelating agents according to the present invention is provided below as Formula 4. FORMULA 4 1. Extraction with Halogenated /3-diketones The extraction efficiency appears to be enhanced when the 3-diketone is halogenated. As a result, particularly preferred /3-diketone chelating agents are fluorinated. A number of fluorinated 0-diketones are commercially available from such companies as Aldrich Chemical Company of Milwaukee, Wisconsin. These 0-diketones form stable complexes with metals, including lanthanides and actinides, and hence are useful ligands for extracting metals from metal oxides. Extraction and separation of actinides by supercritical fluids are of particular interest because of the potential applications for nuclear waste analysis and management. Several fluorinated 0-diketones have been used for the extraction of uranyl and Th(IV) ions using carbon dioxide fluid solvents as shown below in Table II. All /S-diketones tested, except TTA, are liquids at room temperature and at atmospheric pressure. TABLE H Fluorinated jS-diketones Used for the Extraction of Uranyl and Th(IN) Ions Using Supercritical Carbon Dioxide -diketone Abbr. R, R 2 Mol.Wt.B.P. °C Acetylacetone AA CH 3 CH 3 100.12 139° (760 Torr) Trifluoro¬ acetylacetone TAA CH 3 CF 3 154.09 107° Hexafluoro¬ acetylacetone HFA CF 3 CF 3 208.06 70-71° Thenoyltri- fluoroacetone TTA Thenoyl CF 3 222.18 103-104 c (9 Torr) Heptafluorobutanoyl- pivaroylmethane FOD C(CH 3 ) 3 C 3 F 7 296.18 33° (2.7 Torr) The fluorinated beta-diketones were purchased from the Aldrich Chemical Company of Milwaukee, WI, and were used without further purification. j8-diketones exist in at least two tautomeric forms, the "keto" tautomer and the "enol" tautomer. Tautomerism is a type of isomerism in which migration of a hydrogen atom results in two or more structures called tautomers. 3-diketones react with metal ions to form chelates either through the enol tautomer or through an enolate anion (a negatively charged "enol" form of the molecule) illustrated by the following equilibria: The presence of a small amount of water has been found to significantly increase the CO 2 extraction efficiency of metal ions from a solid support using fluorinated 3-diketones as an extractant. Without limiting the invention to one theory of operation, water molecules likely form adducts with the metal//3-diketone complex. One skilled in the art will realize that the amount of water used during the extraction process may vary. However, where about 10 μg of metal ion is to be extracted, at least about 10 μL of water should be added to the system prior to the extraction process. In other words, it appears sufficient to use about 1 μL of water per 1 μg of metal ion. B. Phosphinic Acids as Chelating Agents As stated above, preferred chelating agents are capable of donating a proton to metal oxides, β-diketones are a preferred group of protic chelating agents useful for the invention, but there are also other examples. For example, and without limitation, phosphinic acids also can be used as chelating agents for directly extracting metals for metal oxides. As used herein, "phosphinic acid" refers to an organic derivative of hypophosphorous acid [HP(OH) 2 ]. Phosphinic acid chelating agents generally satisfy the following Formula 5 FORMULA 5 0 II P, OH R wherein R 3 and R 4 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, aryl, halogenated aryl, thenoyl, and mixtures thereof. R 3 and R 4 preferably are selected from the group consisting of lower alkyl and fluorinated lower alkyl groups. One example, without limitation, of a suitable phosphinic acid chelating agent is bis(2,4,4- trimethylpentyl)phosphinic acid as shown below. Bis(2,4,4-trimethylpentyl)phosphinic acid C. Carboxylic Acids as Chelating Agents Besides the jS-diketones and phosphinic acid chelating agents discussed above, the carboxylic acids also appear to be suitable for use as chelating agents for the direct separation and fluid extraction of metals from metal oxides. There are a number of carboxylic acids that potentially may be useful as chelating agents for practicing the direct separation and extraction of metals from metal oxides. The carboxylic acids generally satisfy the following Formula 6 FORMULA 6 wherein R 5 is selected from the group consisting of lower alkyl groups, halogenated lower alkyl groups, aryl groups, and halogenated aryl groups. Examples, without limitation, of carboxylic acids that satisfy this formula include methanoic acid, also referred to as formic acid (shown below), ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonionic acid, decanoic acid, and branched analogs of these compounds. Unsaturated carboxylic acids, such as acrylic or methacrylic acid, as well as cyclic carboxylic acids, such as cyclohexanecarboxylic acid, also can be used as suitable chelating agents, as long as such acids form chelates with the metal of the metal oxide, and thereafter are capable of solubilizing the metal as a metal-chelate in the fluid or supercritical fluid. Halogenated forms of the alkyl, unsaturated alkyl and cyclic carboxylic acids also can be used as chelating agents. One example, without limitation, of a suitable carboxylic acid chelating agent, is formic acid. Formic acid Examples, without limitation, of aryl carboxylic acids suitable as chelating agents include benzoic acid and phenylacetic acid, as well as halogenated derivatives thereof. D. Examples of Extracting Metal Directly from Metal Oxides The following examples are provided solely to illustrate certain embodiments of the invention. The invention should not be limited to the particular embodiments discussed in the examples. For instance, the examples illustrate the use of fluorinated ligands, but the invention should not be limited to just fluorinated compounds. Non-fluorinated ligands, including other halogenated ligands, also perform satisfactorily. EXAMPLE I This example describes extracting a transition metal from a transition metal oxide. More specifically, this example describes the extraction of iron from iron oxide using thenoyltrifluoroacetone (TTA) as the chelating agent and supercritical CO 2 modified with methanol as the extraction solvent. One gram of ferric oxide (Fe 2 O 3 ) and 100 mg TTA were added to a 20 ml extraction cell of an ISCO oven system along with 50 μ\ H 2 O. The Fe^ was obtained from Fisher Chemicals. TTA was obtained from Aldrich Chemical Company of Milwaukee, Wisconsin. The vessel was heated to 60°C, pressurized with supercritical CO 2 supplied at 3000 psi by an ISCO 2600 syringe pump and sealed for five minutes to allow time for Fe(III)/TTA chelates to form. The CO 2 had been modified by addition of methanol to form a 5% (v/v) methanol:CO 2 solution before injection into the extraction vessel. After five minutes, a dark red extract was collected and the procedure repeated until the extract became clear. The extract solution was then analyzed spectrophotometrically to determine whether chelated iron had been obtained by direct treatment of the iron oxide with TTA and supercritical CO 2 . The UV spectra gave a large UV peak from 230 nm to 400 nm and a visible peak at 490 nm, which indicated the presence of Fe(III)/j3-diketone complexes. The following examples describe the extraction of actinides from actinide oxides, and in particular the extraction of uranium from uranium oxides, using fluorinated / 8-diketones. Examples II-IV describe a static extraction method in which an extraction cell containing uranium oxide and a /3-diketone is heated, pressurized with CO 2 and sealed. After allowing a period of time for the uranium to form chelates with the 3-diketones, the extraction cell is dynamically flushed with supercritical CO 2 , sweeping the uranium chelates dissolved in the CC^ into a collection vessel. Example V describes a more preferred dynamic extraction method in which pressurized CO 2 containing a known concentration of jS-diketone is continuously flowed over a uranium oxide sample. 1. Static Extraction EXAMPLE II This example describes the extraction of uranium from U 3 O g using only CO 2 and TTA. The U 3 O g and TTA were obtained from Aldrich Chemical Company of Milwaukee, Wisconsin. Liquid CO 2 , SFC grade, was obtained from Scott Specialty Gases. U 3 O g and TTA were added directly to the 20 ml extraction cell of an ISCO oven system in the amounts shown in Table III below. The cell was heated to 60°C and pressurized with CO 2 supplied at 2250 psi by an ISCO 2600 syringe pump. A period of ten minutes was allowed for chelates to form and solubilize in the CO 2 The extraction cell was then dynamically flushed with CO 2 for ten minutes and the extracted complexes were collected in chloroform. Samples and standards were analyzed by nuclear activation analysis (NAA). This procedure comprises irradiating both samples and standards for 1 hour in a 1 MW Triga nuclear reactor at a steady flux of 6 x 10 12 ncm "2 S"'. After irradiation, the samples were cooled for 48 hours before counting. Each sample was counted for 60 seconds in a large volume ORTEC Ge (Li) detector with a resolution (FWHM) of approximately 2.3 KeV at the 1332 KeV^CO peak. Uranium was determined by the 228 KeV 7-ray emitted during the decay of its daughter 239 Np (t 1 2 = 2.36 d). Uranium samples were back extracted from the chloroform collection solvent into 50% M HNO 3 solution and made up to 3 ml. 250 μL of this was taken for NAA. Analysis was carried out by comparing count ratios against standard solutions. As shown in Table III below, significant amounts of uranium were obtained from the 10 mg U 3 O g samples. The remaining U 3 O g had changed color to a deep orange, indicating that substantially more uranyl/TTA complex had formed than had been extracted under the relatively mild temperature and pressure conditions used. EXAMPLE in This example describes the extraction of U 3 O g using CO 2 , TTA and H 2 O. Reaction conditions and procedures were substantially as described above in Example II, except that about 1 μl of water was added for every 1 μg of metal. The water was added to the extraction cell before the cell was heated and pressurized with CC Samples and standards were analyzed as described in Example II above. The results, in terms of amounts of uranium extracted, are shown in Table III below. EXAMPLE IV This example describes the extraction of U 3 O„ using TTA, CO 2 , H 2 O, and HPLC-grade methanol. Reaction conditions and procedures were substantially the same as in Examples II and III above. Methanol was added to water to form a 5% (v/v) water: methanol solution before the extraction cell was heated and pressurized with CO 2 . Samples and standards were analyzed as in Examples II and III above. The results, in terms of the amount of uranium extracted, are shown in Table III below. TABLE HI Table III shows the amounts of uranium extracted from several U 3 O g samples under the experimental conditions described in Examples II-IV. In the four columns of Table III from left to right are listed: (1) a summary of the reaction conditions, (2) the U 3 O g sample sizes in milligrams, (3) the number of milligrams of uranium calculated to be present in each U 3 O g sample, and (4) the amount of uranium extracted from each sample in milligrams. In each case, the remaining U 3 O 8 had changed color to a deep orange, indicating that substantially more uranyl/ TA chelates had formed than had been extracted under the relatively mild temperature and pressure conditions used. It is believed that substantially more metal would be extracted under conditions of higher temperature and/or pressure. 2. Dynamic Extraction EXAMPLE V This example describes a method for extracting uranium from uranium oxide using dynamic extraction, i.e., continuously flowing HFA dissolved in supercritical CO 2 over a UO 2 sample, according to the system depicted in FIG. 2. An ISCO 2600 syringe pump 11 was used to supply liquid CO 2 at 2250 psi. A Kratos HPLC pump 12 supplied methanol containing dissolved HFA. The methanol mixed with the liquid CO 2 under pressure via a "T" junction 13. The mixture was flowed into the ISCO oven system 14 containing the UO 2 sample. In some experiments, the UO 2 was ground to increase the surface area available for reaction, as indicated in Table V below. The methanol was added to the CO 2 as 16% (v/v) methanol .-CC^ solution. Concentrations of HFA in the supercritical fluid were 10 μl/ml to 30 μl/ml. This equates to 1.67 x iσ 3 mg HFA (at 10 μl) per ml of liquid COz + 16% MeOH. However, since the density of the supercritical fluid phase varies with temperature and pressure, the actual concentrations of HFA under specific experimental conditions were calculated in order to determine the rates of reaction. The results of these calculations are shown in Tables IV and V below. The actual concentration of HFA under supercritical fluid conditions was estimated by using a density calculation for CO 2 /MeOH system at 10% MeOH. These were obtained from a "SF-solver program" available from the ISCO company. The density of the liquid CO 2 /MeOH system was taken as 1 mg.ml. Samples and standards were analyzed as described above in Example II. The amounts of uranium extracted from UO 2 are shown in Table V below. The interfacial kinetics of the uranium dissolution were calculated according to the equations given below, the results of which also are shown in Table V: EQUATION 1 R = KA [HFA] where K is the rate constant at temperature T, R is the rate of uranium dissolution, and A is the area of UO 2 (assumed to be 1 cm 2 ); and EQUATION 2 K = R/A [HFA] Rate constants calculated using equation (2) are shown in Table V. The rate constants are expressed in terms of grams of uranium dissolved per second per molar concentration of HFA per square centimeter of oxide surface. TABLE TV TABLE V Table IV shows actual concentrations of HFA in supercritical CC^ (modified with methanol to form a 16% (v/v) methanol :CO 2 solution) at varying temperature and pressure conditions. In the four columns of Table IV from left to right are listed: (1) HFA concentrations in 16% (v/v) methano CC^ solutions before the CO 2 is placed under heat and pressure conditions; (2) the temperatures and pressures applied to the HFA/CO^methanol solutions; (3) the density of the supercritical CO 2 under heat and pressure conditions; and (4) actual HFA concentrations under heat and pressure conditions. Table V shows the interfacial rate of uranium dissolution for ground uranium oxide samples using actual HFA concentrations. Dissolution rates were calculated according to Equations 1 and 2 above. The foregoing data is expressed as an Arrhenius plot in FIG. 3 for the data obtained at all HFA concentrations. E. Specific Waste-Treatment Embodiment EXAMPLE VI One proposed embodiment for a continuous selective-chelation supercritical-fluid-extraction process is illustrated in FIG. 4. This process is suitable for chelating metals that are contained in metal oxide held in a container 50. A supercritical fluid, such as carbon dioxide gas, is supplied from a C gas reservoir 52. Reservoir 52 is connected by a conduit 54 containing a valve 55 to a pressurization unit 56 that increases the pressure on the gas to greater than 73 atmospheres at a temperature greater than 32 °C to form supercritical carbon dioxide. The supercritical CO 2 then travels through a valve 57 and conduit 58 to a reservoir 60 that holds a chelating agent, such as any of the agents listed or described in this specification as suitable for the present invention. The CO 2 is there passed through a column containing a chelating reagent to dissolve the chelating agent into the supercritical fluid CO 2 stream. The supercritical fluid and chelating agent leave reservoir 60 through a conduit 62 and are introduced into container 50. The supercritical fluid/chelating agent is intimately mixed with the metal oxide in container 50 using either a batch or continuous process. In a batch process, simple mixing would occur through stirring or sonication. Alternatively, mixing could occur by allowing CO 2 to flow through a column of metal oxide waste. In a continuous mixing embodiment, CO 2 would flow through a column of metal oxide waste. After mixing, CO 2 containing the metal chelate is removed through a conduit 64. A depressurizer valve 66 is present in line 64 to reduce the pressure to below seventy-two atmospheres such that the metal chelate precipitates in container 67. The CO 2 gas is then recycled by pump 68 through line 70 to gas reservoir 52. Metal chelates can be removed from the bottom of container 67 through line 72 such that the chelating agent can be regenerated from the metal chelate. When regeneration of the chelating agent is desired, metal ions can be stripped from the chelate using a nitric acid solution having a pH less than one. The extraction system should be thermally controlled, either by known electrical means or immersion in a constant temperature bath. Thermal control allows the carbon dioxide or other supercritical fluid to be maintained above its supercritical temperature. F. Separation of Metal From Impurities The present invention further provides a method of purifying metal from unreacted chelating agents and from other metal chelates or impurities formed during supercritical fluid extraction (or "SFE"). In accordance with this method, certain properties of the supercritical fluid (such as pressure) are varied to achieve a solvating region which is favorable to the solubility of one particular component or metal chelate in a mixture of components or metal chelates. The increased or enhanced solubility of one component can also be utilized to inhibit the uptake or solubility of other components in order to further separate the components, i.e., to separate the impurities from the desired metal or metal chelate. Further, in addition to varying the solvent power or strength of the supercritical fluid, the volatility of metal complexes can be controlled to achieve separation from other metal complexes. In further accordance with the present invention, the solubility differential between the desired metal (or metal chelate) and an impurity is relied upon to separate and purify the desired components. In this manner, a less soluble desired metal chelate complex can be separated from a more soluble metal chelate impurity. In yet another aspect, adduct forming agents are added to form adducts of desired metal chelates to make them more soluble for transport in supercritical fluid, and thus, easier to separate and purify from impurities. The present invention can be used, for instance, in connection with the separation of metals extracted directly from metal oxides by supercritical fluid extraction. In one illustrative embodiment, uranium is separated by a method of the present invention after being directly extracted from UO 3 . The metal oxide, UO 2 (NO 3 ) 2 , can also be used. The present invention, however, is also suitable for use with the extraction of thorium from thorium oxide, or the extraction of other metals from metal oxides, such as plutonium from plutonium oxide. The metal oxides, chelating agents, supercritical fluids, and other reagents discussed above are likewise suitable for this aspect of the present invention. Preferably, however, for the extraction and separation of uranium, supercritical carbon dioxide is utilized along with a protic chelating agent. Semivolatile metal chelate complexes formed by SFE can be separated into different fractions based on differing solubilities in supercritical fluid, particularly supercritical carbon dioxide fluid (or SF CO 2 ). The ability to vary the solvating power of supercritical fluids by varying their physical conditions in order to extract and separate certain metals is especially beneficial in connection with extracting and separating nuclear materials without creating significant secondary waste streams. SF is most effective as a solvent for a large number of semivolatile compounds which would otherwise decompose if high temperatures were used as a means of their separation. Hence, the varying solvent power of SF can be utilized in combination with ligand selectivity as a means of facilitating the extraction and separation of metal complexes. One means of representing the solvent power of SF (where the solid solute is treated as a subcooled liquid) uses a solubility parameter, and in most cases the Hidlebrand parameter, δ, is used. The solubility behavior of analytes in SFs can be semiquantitively related to the δ (δ SF ) of the SF, δ, f = 1.25P c ,/2 (p f / Pl ) where P c = critical pressure of the solvent, p f = the reduced density of the SF, and Pi = the reduced density of the fluid in the liquid state, which is close to 2.66. In general, the larger theβ„ value, the more soluble a solute is at any given temperature. The δ, f of SF C0 2 under varying conditions is shown in FIG. 6. (FIG. 6 is a comparison of the solubility parameter of SF CO 2 with those of conventional solvents. The data for the conventional solvents is for 25 °C and 1 atm pressure, and the conditions for SF CO 2 are as shown.) It was observed that changing the physical conditions of the SF C0 2 greatly influenced the solvating ability of the fluid. As represented in FIG. 6, under certain conditions, SF C0 2 can approach or exceed the solvent power of these solvents. The magnitude of a solutes solubility in a SF is not only related to the solvating power of the solvent but also to the volatility of the solute. Therefore, compounds of high volatility tend to show higher solubility in SF CO 2 than those with low volatility. Hence, since the solvent power or strength of supercritical fluids, such as SF CO 2 , and the volatility of compounds, such as metal complexes, can be controlled, then very selective extraction and purification processes are feasible. As discussed herein, the properties of supercritical fluids, such as SF COj, can be varied such that separation of differing metals (or metal chelate complexes) can be achieved. In one embodiment, detailed below, a uranyl/TTA complex is first prepared by exposing UO 3 to SF CO 2 containing dissolved TTA, and then purified from unreacted TTA ligand and likely metal ion contaminants (such as iron) based on the differing solubilities of these various components in SF CC EXAMPLES (VII) The following experimental examples are provided solely to illustrate certain embodiments of the invention. The invention should not be limited to the particular embodiments discussed in the examples. Reagents The following reagents were used in the experiments detailed herein: Uranium trioxide or UO 3 (Alfa Aesar, UK), thorium oxide (Alfa Aesar, UK), TTA (Aldrich, Milwaukee, Wisconsin), iron (iii) chloride, copper nitrate, tributyl phosphate ("TBP") (Aldrich, Milwaukee, Wisconsin), sea sand, HNO 3 , and CHC1 3 (Fisher Certified ACS, Fairlawn, New Jersey), and methanol. These reagents were used as received. Carbon dioxide (Air Products, AUentown, Pennsylvania, SFE/SFC grade) was used in the extractions. Equipment As schematically illustrated in FIG. 5, the extraction system 100 utilized a 260D syringe pump 101 (ISCO, Lincoln, Nebraska) equipped with a cooling jacket 102 (controlled at 5°C) for liquefying carbon dioxide 103. A SX 2-10 extractor 104 (ISCO, Lincoln, Nebraska) was used with 9 ml volume PEEK sample cartridges (ISCO, Lincoln, Nebraska). For reaction kinetic studies, a complete PEEK system was utilized consisting of a IO ml PEEK extraction cell or reactor (Altech, Deerfield, Illinois), connected to PEEK 1/16th inch tubing 106. The exit stream 108 from the system (to a ligand collector 109) was controlled via a 0.05 mm (id) capillary restrictor 110 (JW Scientific, Folsom, California). A micro cutting torch was used in sealing the capillary restrictor s. This system was placed inside a heater system 112 consisting of a stainless steel tube surrounded by heat tape (Omegalux, Stanford, Connecticut). The temperature of the reactor was monitored using a thermocouple (Omegalux, Stanford, Connecticut) attached to the outside of the reaction cell. A UV-vis spectrometer was used for Fe(TTA) 3 analysis. Extracted samples were collected in 10 ml test tubes and placed into 25 ml sealed teflon vessels. Uranium analysis was performed using a Perkin Elmer Scienx ELAN 5000 ICP-MS (Norwalk, Connecticut) equipped with a Gilson 212 autosampler (Middleton, Wisconsin). Procedures Initially, glass wool was placed in the reactor, followed by a weighed amount (~0.1g) of U0 3 , 0.5 ml of methanol, and 0.01 ml of H 2 0. The top of the extraction cell was filled with glass wool. The system was placed in the heated zone and pressurized with SF C to the desired value. The temperature was controlled to ±. 2°C. The end of the restrictor was initially sealed, using a micro cutting torch. The reaction proceeded for 20 minutes in a static mode (no C0 2 flow). After 20 minutes, the sealed end of the capillary was broken and the C0 2 allowed to flush the reaction cell. The reaction cell was flushed for 20 minutes at the same pressure as the reaction, then depressurized slowly to atmospheric pressure. The reaction efficiency was determined by washing the product with CHC1 3 to dissolve the complex formed. The unreacted U0 3 was then dissolved in nitric acid solution. The samples were then analyzed by the ICP-MS, using the following conditions: rf power lkW; and plasma argon, 15 L/min. Uranium was analyzed using M U isotope since it has the largest abundance (99.27%) and is free from interferences from other elements. Analyses were performed using external calibration, which was repeated every 10 samples. Blanks and standards were prepared in 1 % HN0 3 , and samples were diluted with 1 % HN0 3 to bring the concentration within the range of 1 to 100 ppb. Accuracy of this method is < 1.5% determined by analysis of standard reference materials. Solubility of Fe(TTA) 3 was determined using a previously known system consisting of a dual pumping system, extraction cell, and solvent trapping system. (The ISCO syringe pump has an electronically controlled stepping motor which provides for a high accuracy flow rate monitor.) Fe(TTA) 3 ( — 0.5 g) was loaded into the extraction cell and mixed with glass beads (60/80 mesh) to prevent sample compaction. Pure C0 2 was flowed through the cell at a flow rate < 1 ml/min. The C0 2 flowed for 15 minutes to allow equilibrium to be established, followed by three 15 minute collections, during which time the amount of COj flowing was monitored at the ISCO pump. The concentration of the extracts was determined using a UV-vis spectrometer, measuring absorbance at 335 nm. The spectrometer was calibrated with a standard prepared from Fe(TTA) 3 dissolved in chloroform. Samples were diluted with chloroform, so that they were within the linear range of the calibration. The RSD for the 3 repeated determinations was found to be -5%. The separation of the various metal TTA complexes was carried out using the ISCO SX 2-10 extraction system and a single ISCO pump. Metal complex (0.5 g of each) was loaded into the extraction cell, in a mixture of sea sand, and heated and pressurized with COj under static conditions for a short time period (10 to 20 minutes). The extraction exit was then opened allowing the C0 2 and dissolved metal complexes to exit the cell. The extracts were collected in chloroform, for a given time period (usually IO to 20 minutes), with the procedure repeated at each condition. The collected samples were back extracted into concentrated nitric acid and then diluted for analysis, as above. Complex Collection and Back Extraction To evaluate the efficiency of the back extraction procedures used, a number of known samples were back extracted from chloroform and the efficiency of the various back extraction processes evaluated. These are illustrated in Table VI. TABLE VI Back Extraction into HNO, Concentrated nitric acid was found to be the most effective system for the collection of samples and was utilized in these experimental examples. Results and Discussion Varying Solvation Power of SFC0 2 The experiments between UO 3 and TTA in SF seemed to indicate the formation of a complex as evidenced by the formation of a bright orange colored compound on the surface of the U0 3 . This complex was soluble in CHC1 3 . Although the complex was formed in the SF C0 2 , the complex was not easily transported, indicating limited solubility. The following reaction between the TTA and U0 3 predicted: U0 3 + TTA = > U0 2 »TTA 2 -I- H 2 0 The TTA complex was found to be reactive with other metal oxides, including iron oxide, forming a deep red complex. Reaction Between TTA and Uranyl Nitrate The reaction of TTA with uranyl nitrate was also studied and found to occur very rapidly, forming an orange yellow complex similar to that seem above in the reaction with U0 3 . The complex was soluble in CHC1 3 . Reaction efficiencies were high. Effect of Temperature and Pressure on the Reaction Between U0 3 and TTA The effect on the reaction of varying the pressure of the SF COj is shown in FIG. 7. (More specifically, FIG. 7 shows the effect of pressure on the reaction rate of UO 3 to UO 2 (TTA) 2 at 60°C and after 20 minutes of static reaction time followed by 20 minutes of dynamic flushing of the cell. The data is the mean of three repeated determinations. 0.5ml of methanol and 0.01ml of H 2 O were added to the 10ml reaction cell prior to the reaction.) At low pressure, or 70 atm, which is close to the critical pressure for SF C0 2 , and a temperature of 60° C, the lowest reaction efficiency was observed. Specifically, a 77% conversion was observed. The conversion efficiencies were slightly increased by increasing the pressure. At 300 atm, the efficiency was increased to 83 % . The lack of significant sensitivity of the reaction to pressure indicates that the reaction was not limited by the solubility of TTA in SF C0 2 (i.e., all TTA added to the cell was dissolved in the SF CO 2 , under the conditions studied). Also, since the orange complex formed did not dissolve readily into the SF CO 2 , then the reaction was probably limited by kinetics of the solid phase reaction at the oxide and not the transport properties of the SF CO 2 . The nature of the complex formed was a powdery substance loose on the oxide surface and appeared not to inhibit further reaction of the remaining oxide. At all conditions studied, a significant proportion of the U0 3 was converted to complex. The effect of temperature on the reaction is illustrated in FIG. 8. (More specifically, FIG. 8 shows the effect of temperature on the reaction rate of UO, to UO 2 (TTA) 2 at 150 atm and after 20 minutes static reaction time followed by 20 minutes of dynamic flushing of the cell. The data is the mean of three repeated determinations. 0.5 ml of methanol and 0.1 ml of H 2 O were added to the 10 ml cell prior to reaction.) This indicates that the reaction was limited by the kinetics of oxide ligand reaction rather than transport properties of the SF CO 2 . At 90°C, the reaction was quantitative after 20 minutes reaction time. The crystal structure of the complex was determined by X-ray diffraction, and was in fact a U^/TTA complex containing one molecule of H 2 0. The high coordination number of actinides is well known, and presence of H 2 0 in uranyl acetylacetone complexes is also known. The complex was dissolved in an organic solvent and was observed to change color slightly when TBP was added. It was observed that the coordinated H 2 0 was replaced by TBP in the complex structure. TBP containing complexes are highly hydrophobic, resulting in enhanced extractions into non-aqueous phases. These type of complexes are referred to as "adducts. " The hydrophobic nature of uranyl adduct complexes is important with respect to enhanced solubility in many non-polar solvents and this was found to be the case with SF C0 2 , as discussed herein. The complex formed was found to be highly sensitive to the exchange of the coordinated molecule of H 2 0. With recrystallization of the Uθ 2 (TTA) 2 .H 2 0 from chloroform (containing ethanol as a modifier), a crystal containing an ethanol adduct was obtained. Purification from Unreacted Ligand The purification of the uranyl/TTA complex from the free ligand was easily accomplished based on the solubility differences of the two compounds in SF CCVj. The solubility of free ligand TTA was measured and the results obtained are shown in FIG. 9. (In particular, FIG. 9 shows the solubility of TTA in SF COj at 40°C, 60°C, and 90°C. Solubility and density (d) are given in gL "1 . Each data point is an average of four repeated determinations. Due to limitations of the cell used, the highest temperature of 120°C could not be studied.) The data is presented in a log solubility versus log density manner, since a linear relationship should be observed, enabling simple correlation of the experimental data. Overall, high solubility of TTA was observed. The solubility of the U0 2 (TTA) 2 .H 2 0 complex was low in SF COj. Thus, purification of the complex was achieved by flushing the reaction cell with SF CQj after completion of the reaction, where the very soluble TTA was removed, leaving the majority of the low solubility uranyl/TTA complex in the reactor. This method of synthesis has a number of potential advantages over conventional solution based procedures, including: (a) dissolving the TTA for reaction with the solid oxide; (b) after reaction, separating unreacted TTA from the complex formed; (c) producing a pure complex without the use of any traditional solvents; and (d) producing a purified product quickly (< 1 hr). The method also has the advantage that a purified product is obtained with near 100% conversion of the oxide, whereas traditional solvent based methods provide yields in the range of 30% for uranyl/S-diketone complex preparations. Purification From Other Metal TTA Complexes Purification of uranyl chelate complex from other metal complexes is of importance in nuclear waste treatment or in the processing of ore. Separation is facilitated by the relatively low SF solubility of the uranyl chelate complex. For example, the solubility of Fe(TTA) 3 in SF C0 2 is illustrated in FIG. 10 at varying temperatures and pressures. (FIG. 10 shows the solubility of Fe(TTA) 3 in SF CO 2 at 60°C (data points shown as squares), and 90°C (data points shown as circles). Solubility and density (D) are given in gL "1 ). This complex is shown to be highly soluble in SF C0 2 . Thus, the separation of low solubility U0 2 (TTA) 2 »H 2 0 complex from high solubility Fe(TTA) 3 complex can be facilitated by the suitable choice of SF CO 2 conditions. The separation of iron from uranium based on the differential solubilities of their corresponding metal/TTA complexes is shown in FIG. 11. (More specifically, FIG. 11 illustrates data on the separation of uranium and iron into a supercritical fluid (CO 2 ) based on the different solubilities of FefTTA^ and UO 2 (TTA) 2 «H 2 O complexes (at 60°C). Fractions were collected for 5 minutes after 20 minutes of static saturation time. Each data point shown represents the mean of three repeated determinations.) More specifically, this was achieved by extracting a mixture of Fe(TTA) 3 and U0 2 (TTA) 3 complexes for given time periods, and measuring the degree of separation based on the quantity of each metal extracted. With increasing pressure, the degree of separation of Fe from U increased. Hence, by control of the physical properties of the extraction system or supercritical fluid, the degree of separation of given metal systems can be optimized. Further, the separation of solid U0 2 (TTA) 2 *H 2 0 from Th(TTA) 4 is shown in FIG. 12. (More specifically, FIG. 12 illustrates data on the separation of UO 2 (TTA) 2 from Th(TTA) 4 at 60°C. Extracts were collected for 10 minutes in 20 ml of CHC1 3 after 10 minutes of static saturation time. Each data point is the mean of two repeated determinations). In this case, the separation of the different metals was less significant, with a slightly higher extraction rate of U0 2 (TTA) 2 .H 2 0 complex observed at all pressures above 100 bar. However, although the separation appears lower than in the case of Fe and U, the amounts of U extracted were double those of Th. In FIG. 13, the separation of a number of metal TTA complexes is illustrated. (In particular, FIG. 13 illustrates data on the separation of four metal TTA complexes at 60°C. Extracts were collected for 10 minutes in 20 ml of CHC1 3 after 10 minutes of static saturation time. Each data point is the mean of two repeated determinations). As illustrated, Fe was extracted selectively over all the other metal complexes. In fact, the solvation of one compound (such as Fe) in a supercritical fluid (such as SF C0 2 ) can inhibit the uptake of other compounds in that supercritical fluid. Hence, in accordance with the present invention, Fe can be selectively purified from a range of metal TTA complexes. Indeed, the ability to take advantage of the varying solvent power of the SF to extract small quantities of impurity from the metal is useful in simplifying the extraction and separation process. In other words, small quantities of impurity can be separated from large quantities of metal, such as uranium, rather than vice versa. Transport Of Uranyl As An Adduct Complex The transport of a metal chelate complex in a supercritical fluid can be enhanced by forming an adduct of the complex with an adduct forming agent as discussed herein. The low solubility of uranyl/TTA complex prevents extraction of significant quantities of uranium in SF C0 2 . If after complex formation a suitable ligand capable of forming adducts to the uranyl chelate complex is added to the SF C0 2 in small amounts, then the uranyl chelate complex is extracted in significant amounts from the system. It has been established during SFE and SF chromatography (SFC) studies that such adduct complexes show enhanced stability in SF C0 2 compared to uranyl/jS- diketone complexes in the absence of adducts. This is represented in FIG. 14, where the effects of adding excess TTA and that of adding TBP are illustrated. (More specifically, FIG. 14 shows the extraction (at 60°C and 150 atm) of in SF CO 2 by adduct formation with TBP. Each data point represents an average of six repeated determinations). With excess TTA in the extraction cell, it was observed that a high rate of uranium transport can be attained. When the extraction is carried out in the absence of TTA, the rate of extraction is much lower. For 0-diketone complexes of uranyl, it has been established that the complex U0 2 (A) 2 ,(HA) 2 is formed in organic phases (where A = 3-diketone anion and HA= /3-diketone). Hence, it is likely that excess ligand is required to enhance solubility in the organic phase. This appears to be the case for achieving high SF C0 2 solubility. The presence of TBP also enhances solubility as shown in Figure 14. SF C0 2 modified with 10% TBP results in a lOx increase in the amount of uranium transported. The formation of TBP adducts in the form of U0 2 (TTA) 2 *TBP has been demonstrated above. In solvent extraction, the inclusion of TBP into the complex results in the increased hydrophobic nature of the complex and thus higher extraction efficiencies. The increase in the hydrophobic nature of the complex also indicates that solubility in SF C0 2 should be higher and this was observed experimentally. Accordingly, this aspect of the present invention provides for an environmentally safer and more efficient method for the extraction and separation of metals and impurities, without the use of conventional solvents and the creation of undesirable waste st reams. Having illustrated and described the principles of the invention in several preferred embodiments, it should be apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the following claims.

Claims: WE CLAIM: 1. A method for extracting a metal from a metal oxide, comprising exposing the metal oxide to a supercritical fluid solvent containing a chelating agent capable of donating a proton to the metal oxide for a period of time sufficient for the chelating agent to form chelates with the metal, the chelates being soluble in the supercritical fluid. 2. The method according to claim 1 further comprising the step of separating the supercritical fluid from the metal oxide after the chelates are solubilized in the supercritical fluid. 3. The method according to claim 1 wherein the chelating agent is selected from the group consisting of 0-diketones, halogenated 3-diketones, phosphinic acids, halogenated phosphinic acids, carboxylic acids, halogenated carboxylic acids, and mixtures thereof, at least one of the chelating agents forming chelates with the metal. 4. The method according to claim 1 wherein the chelating agent is selected from the group consisting of 0-diketones, halogenated j8-diketones, phosphinic acids, halogenated phosphinic acids, and mixtures thereof, at least one of the chelating agents forming chelates with the metal. 5. The method according to claim 1 wherein the chelating agent is selected from the group consisting of /3-diketones, halogenated /3-diketones, carboxylic acids, halogenated carboxylic acids, and mixtures thereof, at least one of the chelating agents forming chelates with the metal. 6. The method according to claim 3 wherein at least one of the chelating agents is halogenated. 7. The method according to claim 3 wherein at least one of the chelating agents is fluorinated. 8. The method according to claim 1 wherein the exposing step further comprises providing a secondary modifying solvent. 9. The method according to claim 8 wherein the modifying solvent is selected from the group consisting of water, medium-boiling-point alcohols and esters. 10. The method according to claim 9 wherein the modifying solvent is methanol. 11. The method according to claim 1 wherein the supercritical fluid solvent comprises supercritical carbon dioxide. 12. The method according to claim 1 wherein the exposing step further comprises exposing the metal oxide to supercritical carbon dioxide and a chelating agent selected from the group consisting of fluorinated /3-diketones, fluorinated phosphinic acids, fluorinated carboxylic acids, and mixtures thereof, at least one of the chelating agents forming chelates with the metal. 13. The method according to claim 1 wherein: (a) the /3-diketone is represented by the formula wherein R, and R 2 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, thenoyl and halogenated thenoyl groups; (b) the phosphinic acids are represented by the formula 0 wherein R 3 and R 4 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, aryls, halogenated aryls, thenoyl groups, and mixtures thereof; and (c) the carboxylic acids are represented by the formula wherein R s is selected from the group consisting of lower alkyl, halogenated lower alkyl, aryls, halogenated aryls, and thenoyl groups. 14. The method according to claim 13 wherein R,, R 2 , R 3 , R 4 and R 5 each is independently selected from the group consisting of methyl, fluorinated methyl, trifluoromethyl, ethyl, fluorinated ethyl, pentafluoromethyl, propyl, fluorinated propyl, heptafluoropropyl, butyl, fluorinated butyl, and nonafluorobutyl. 15. The method according to claim 3 wherein the 3-diketone is selected from the group consisting of acetylacetone, trifluoroacetylacetone, hexa¬ fluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluorobutanoylpivaroylmethane. 16. The method according to claim 3 wherein the 3-diketone is selected from the group consisting of trifluoroacetylacetone, hexa-fluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluoro-butanoylpivaroylmethane. 17. The method according to claim 1 wherein the metal is an actinide. 18. The method according to claim 17 wherein the actinide is selected from the group consisting of uranium, plutonium and thorium. 19. The method according to claim 1 wherein the metal is a lanthanide. 20. The method according to claim 1 wherein the exposing step comprises continuously flowing supercritical carbon dioxide over the oxide. 21. A method for extracting an actinide or a lanthanide from an oxide thereof comprising exposing the oxide to carbon dioxide and a chelating agent selected from the group consisting of /3-diketones, phosphinic acids, carboxylic acids, and mixtures thereof, at least one of the chelating agents forming chelates with the actinide or lanthanide, the chelates being soluble in the carbon dioxide. 22. The method according to claim 21 wherein the carbon dioxide is subcritical. 23. The method according to claim 21 wherein the carbon dioxide is supercritical. 24. The method according to claim 23 wherein at least one of the chelating agents is halogenated. 25. The method according to claim 23 wherein at least one of the chelating agents is fluorinated. 26. The method according to claim 25 wherein (a) the 3-diketone is represented by the formula wherein R, and R 2 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, thenoyl and halogenated thenoyl groups; (b) the phosphinic acids are represented by the formula 0 II p. R , OH R 4 wherein R 3 and R 4 are independently selected from the group consisting of lower alkyl, halogenated lower alkyl, aryls, halogenated aryls, thenoyl groups, and mixtures thereof; and (c) the carboxylic acids are represented by the formula O R OH wherein R s is selected from the group consisting of lower alkyl, halogenated lower alkyl, aryls, halogenated aryls, and thenoyl groups. 27. The method according to claim 23 wherein the supercritical carbon dioxide includes a modifying solvent selected from the group consisting of water and lower alkyl alcohols. 28. The method according to claim 27 wherein the modifying solvent is methanol. 29. The method according to claim 23 wherein the exposing step comprises continuously flowing the supercritical carbon dioxide over the oxide. 30. A method for extracting a metal from a metal oxide, comprising exposing the metal oxide to a supercritical fluid solvent containing a /3-diketone chelating agent, wherein the 3-diketone forms chelates with the metal, and wherein the chelates are soluble in the supercritical fluid. 31. The method according to claim 30 further comprising the step of separating the supercritical fluid from the metal oxide after the chelates are solubilized in the supercritical fluid. 32. The method according to claim 30 wherein the 0-diketone is halogenated. 33. The method according to claim 32 wherein the jS-diketone is fluorinated. 34. The method according to claim 30 wherein the exposing step further comprises providing a secondary modifying solvent. 35. The method according to claim 34 wherein the modifying solvent is selected from the group consisting of water, medium-boiling-point alcohols and esters. 36. The method according to claim 35 wherein the modifying solvent is methanol. 37. The method according to claim 30 wherein the supercritical fluid solvent comprises supercritical carbon dioxide. 38. The method according to claim 30 wherein the exposing step further comprises exposing the metal oxide to supercritical carbon dioxide and a 0-diketone chelating agent. 39. The method according to claim 37 wherein the /J-diketone is represented by the formula wherein R, and R 2 are independently selected from the group consisting of lower alkyl, fluorinated lower alkyl, thenoyl and fluorinated thenoyl groups. 40. The method according to claim 39 wherein R, and R 2 each is selected from the group consisting of methyl, methyl, fluorinated methyl, trifluoromethyl, ethyl, fluorinated ethyl, pentafluoromethyl, propyl, fluorinated propyl, heptafluoropropyl, butyl, fluorinated butyl, and nonafluorobutyl. 41. The method according to claim 38 wherein the /3-diketone is selected from the group consisting of acetylacetone, trifluoroacetylacetone, hexa- fluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluorobutanoylpivaroylmethane. 42. The method according to claim 38 wherein the β-diketone is selected from the group consisting of trifluoroacetylacetone, hexa-fluoroacetylacetone, thenoyltrifluoroacetylacetone and heptafluoro-butanoylpivaroyl methane. 43. The method according to claim 30 wherein the metal is an actinide. 44. The method according to claim 43 wherein the actinide is selected from the group consisting of uranium, plutonium and thorium. 45. The method according to claim 30 wherein the metal is a lanthanide. 46. The method according to claim 37 wherein the exposing step comprises continuously flowing the supercritical carbon dioxide over the oxide. 47. A method for extracting a metal from a metal oxide, comprising preparing a supercritical fluid having a dissolved chelating agent therein, and exposing the metal oxide to the supercritical fluid solvent and dissolved chelating agent for a period of time sufficient for the chelating agent to form chelates with the metal. 48. The method of claim 47, further comprising the step of varying the solvating power of the supercritical fluid so that the solubility of at least one metal chelate in the supercritical fluid is enhanced over the solubility of another metal chelate, thereby allowing for their subsequent separation from one another. 49. The method of claim 48, wherein the step of varying the solvating power of the supercritical fluid includes varying a physical property of the supercritical fluid. 50. The method of claim 49, wherein the step of varying a physical property of the supercritical fluid includes changing the pressure of the supercritical fluid. 51. The method of claim 47, further comprising adding an adduct forming agent to enhance the solubility of a desired metal chelate in the supercritical fluid. 52. The method of claim 51, wherein the adduct forming agent is tributyl phosphate. 53. The method of claim 47, further comprising the step of separating unreacted chelating agent from the metal chelates based on the solubility differentials between the unreacted chelating agent and the metal chelates in the supercritical fluid. 54. The method of claim 47, further comprising the step of separating a desired metal chelate from at least one impurity based on the solubility differential between the desired metal chelate and the impurity in the supercritical fluid. 55. A method for extracting an actinide or a lanthanide from an oxide thereof comprising preparing carbon dioxide having a dissolved chelating agent therein selected from the group consisting of 0-diketones, phosphinic acids, carboxylic acids, and mixtures thereof, and exposing the oxide to the carbon dioxide and dissolved chelating agent, at least one of the chelating agents forming chelates with the actinide or lanthanide. 56. The method of claim 55, further comprising the step of varying the solvating power of the carbon dioxide so that the solubility of at least one metal chelate in the carbon dioxide is enhanced over the solubility of another metal chelate, thereby allowing for their subsequent separation from one another. 57. The method of claim 56, wherein the step of varying the solvating power of the carbon dioxide includes varying a physical property of the carbon dioxide. 58. The method of claim 57, wherein the step of varying a physical property of the carbon dioxide includes changing the pressure of the carbon dioxide. 59. The method of claim 55, further comprising adding an adduct forming agent to enhance the solubility of a desired metal chelate in the carbon dioxide. 60. The method of claim 59, wherein the adduct forming agent is tributyl phosphate. 61. The method of claim 55, further comprising the step of separating unreacted chelating agent from the metal chelates based on the solubility differentials between the unreacted chelating agent and the metal chelates in the carbon dioxide. 62. The method of claim 55, further comprising the step of separating a desired metal chelate from at least one impurity based on the solubility differential between the desired metal chelate and the impurity in the carbon dioxide. 63. A method for extracting a metal from a metal oxide, comprising preparing a supercritical fluid having a dissolved /3-diketone chelating agent therein, and exposing the metal oxide to the supercritical fluid solvent and dissolved jS-diketone chelating agent, wherein the β-diketone forms chelates with the metal. 64. The method of claim 63, further comprising the step of varying the solvating power of the supercritical fluid so that the solubility of at least one metal chelate in the supercritical fluid is enhanced over the solubility of another metal chelate, thereby allowing for their subsequent separation from one another. 65. The method of claim 64, wherein the step of varying the solvating power of the supercritical fluid includes varying a physical property of the supercritical fluid. 66. The method of claim 65, wherein the step of varying a physical property of the supercritical fluid includes changing the pressure of the supercritical fluid. 67. The method of claim 63, further comprising adding an adduct forming agent to enhance the solubility of a desired metal chelate in the supercritical fluid. 68. The method of claim 67, wherein the adduct forming agent is tributyl phosphate. 69. The method of claim 63, further comprising the step of separating unreacted chelating agent from the metal chelates based on the solubility differentials between the unreacted chelating agent and the metal chelates in the supercritical fluid. 70. The method of claim 63, further comprising the step of separating a desired metal chelate from at least one impurity based on the solubility differential between the desired metal chelate and the impurity in the supercritical fluid. 71. A method of separating metal chelates formed during a supercritical fluid extraction process, comprising varying the solvating power of the supercritical fluid so that the solubility of at least one metal chelate in the supercritical fluid is enhanced over the solubility of another metal chelate in the supercritical fluid, and separating the metal chelates from one another. 72. The method of claim 71, wherein the step of varying the solvating power of the supercritical fluid includes varying a physical property of the supercritical fluid. 73. The method of claim 72, wherein the step of varying a physical property of the supercritical fluid includes changing the pressure of the supercritical fluid. 74. The method of claim 71 , further comprising adding an adduct forming agent to enhance the solubility of a desired metal chelate in the supercritical fluid. 75. The method of claim 74, wherein the adduct forming agent is tributyl phosphate. 76. The method of claim 71, further comprising the step of separating unreacted chelating agent utilized in the extraction process from at least one metal chelate based on the solubility differential between the unreacted chelating agent and the metal chelate in the supercritical fluid. 77. A method of separating a metal chelate from an impurity, comprising first forming a metal chelate complex by a supercritical extraction process, and separating the metal chelate from the impurity based on the solubility differential between the metal chelate and the impurity in the supercritical fluid.

Inventors: PHELPS CINDY, SMART NEIL G, WAI CHIEN M, PHELPS, CINDY, SMART, NEIL, G., WAI, CHIEN, M.
Applicants: IDAHO RES FOUND, IDAHO RESEARCH FOUNDATION, INC.
Application year: 1996
Main classification: C22B3
IPC codes: C22B3, C22B60