Organic compounds -- part of the class 532-570 series – Organic compounds – Radioactive metal containing
Reexamination Certificate
1999-05-07
2001-02-13
Jones, Dameron (Department: 1613)
Organic compounds -- part of the class 532-570 series
Organic compounds
Radioactive metal containing
C210S634000, C210S638000, C423S003000
Reexamination Certificate
active
06187911
ABSTRACT:
FIELD
This invention concerns supercritical fluid extractions of metal chelates, and the separation of metal chelates from other materials based on the relative solubilities of the metal chelates and other materials in the supercritical fluid.
BACKGROUND
In order to make use of metals, they first must be extracted from raw materials, such as metal oxides, and thereafter separated from other materials either used for or generated by the extraction process. One particular problem is how to extract metals from materials while minimizing environmental contamination. For example, 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 and which must be separated from the metals.
The purification or separation of certain metals from other metals or impurities is critical in many instances. For example, for the commercial processing of uranium, iron is one common impurity that must be separated from the uranium. Further, uranium and plutonium are separated from fission products of spent nuclear fuel using a solvent extraction reagent. The uranium is subsequently separated from the plutonium.
Alternatively, metals may need to be separated from waste streams containing other metals, metalloids or impurities. For example, heavy metals may need to be separated from a waste stream for environmentally benign disposal of other waste in the stream.
It is known to extract metals from materials using 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. Pat. 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. Pat. 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.
Various features of supercritical fluid extraction of metal and metalloids are disclosed in Wai et al.'s U.S. Pat., Nos. 5,356,538, 5,606,724, and 5,730,874; Wai et al.'s U.S. patent application, Ser. No. 08/686,422, entitled Fluid Extraction, filed Jul. 26, 1996; and United States Provisional Patent Application, Serial No. 60/056,749, entitled Method for Dissociating Metal-Ligand Complexes, filed Jul. Aug. 20, 1997. Wai's patents and applications, collectively referred to herein as Wai's patent documents, are incorporated herein by reference. Wai's patent documents disclose various features of extraction of metalloid and metal ions from materials by exposing the material to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent.
Despite these prior known processes, there are still some disadvantages of traditional purification processes for metals, such as uranium. These disadvantages include: (a) low purified quantities of metal; (b) time consuming purification steps; and (c) the creation of undesirable waste streams.
A need therefore exists for an environmentally safe method for separating and/or purifying metals from other metals, metalloids and/or impurities. The need further exists for a method which is both efficient and which provides greater yields of the extracted and purified metals.
SUMMARY
The present invention provides a process for separating and purifying a metal from other metals, metalloids, impurities, and unreacted reagents. The method of the present invention is environmentally safer than known methods 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 from other materials based on the solubility differences of the complexes and other materials 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.
In accordance with a further aspect of the present invention, by varying the physical conditions of the supercritical fluid the ability to seperate the components or metal chelate complexes formed during the supercritical fluid extraction process is enhanced. In this manner, the solvating power of the supercritical fluid can be varied to increase or decrease 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 the uranium chelate complex in order to increase its tran
Phelps Cindy
Smart Neil G.
Wai Chien M.
Idaho Research Foundation Inc.
Jones Dameron
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
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