Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-11-02
2003-05-20
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C570S127000, C423S181000, C423S182000, C210S634000, C210S681000, C568S842000
Reexamination Certificate
active
06566561
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a composition and process for extracting metals from liquids. More particularly, this invention relates to a class of phenoxy fluoro-alcohols, their preparation, and their use as phase modifiers and solvating agents in a solvent composition for the extraction of cesium from alkaline solutions.
BACKGROUND OF THE INVENTION
Many nuclear energy complexes and treatment sites have environmental problems where cesium removal is needed. Alkaline wastes containing radioactive Cesium-137, such as those stored at the Department of Energy's Hanford, Wash., Oak Ridge, Tenn., and Savannah River, S.C. sites are examples
Among the technologies that are currently being investigated for cesium removal from alkaline nuclear waste are solid-phase sorbent methods, such as crystalline silicotitanate, and precipitation methods, such as addition of sodium tetraphenylborate to form insoluble cesium tetraphenylborate. While generally adequate, both crystalline silicotitanate and in-tank-precipitation using sodium tetraphenylborate possess certain disadvantages that prevent them from being completely satisfactory treatment technologies. As the concentration of radioactive cesium-137 that is sorbed onto a column of crystalline silicotitanate (CST) increases, the radiation and thermal heat generated produce changes in the sorption properties of the CST material leading to desorption of cesium. Flammable hydrogen gas has also been observed to evolve from CST columns that are loaded with radioactive cesium-137. Disadvantages in using sodium tetraphenylborate for in-tank precipitation include breakdown of the inherently unstable tetraphenylborate to liberate benzene, potentially at unexpected rates and places in the treatment process.
Liquid-liquid handling methods, such as solvent-extraction, on the other hand, can be designed and used in a manner to minimize chemical and radiolytic breakdown of the reagents involved in the separation process. The reagents used in solvent extraction can be designed to have high chemical and radiolytic stability, and use of contacting equipment such as centrifugal contactors can minimize the exposure time of the solvent to high radiation fields, thus extending the useful life of the solvent. Solvent extraction processes can contribute great flexibility in being able to treat large volumes of highly radioactive waste in a safe manner, decontaminate the waste to a high level, and enable the extracted cesium to be delivered to a suitable waste form such as vitrified glass. However, there are currently no practical, developed solvent extraction processes for the removal of cesium directly from the tanks with the waste in high-salt alkaline form that the Applicants are aware of; only acid-side extraction has been addressed as described in Dietz et al. U.S. Pat. No. 5,888,398, Mar. 30, 1999, and the use of acid-side solvent-extraction technology to treat the alkaline tanks would require acidification of the waste—a costly option. In addition, the selectivity for cesium over sodium for the process described in U.S. Pat. No. 5,888,398 is generally less than 100, making separation of cesium from waste solutions that are highly concentrated in sodium relative to cesium inefficient. The ratios of the sodium to cesium concentrations in alkaline wastes such as those stored at the Department of Energy's Hanford, Wash., Oak Ridge, Tenn., and Savannah River, S.C. sites are generally in excess of 10,000.
Accordingly, the present invention alleviates the necessity of adding acid or other substances to the waste since cesium extraction may be effected directly from the waste matrix. Also, the present invention is highly selective for cesium over sodium (selectivity generally greater than 10,000). As such, the present invention could play a key role in a grand treatment scheme for alkaline nuclear wastes, especially wastes with a high concentration of competing alkali metal cations.
A further problem to be solved is the need for a method which regenerates the extractant by utilizing a safe and cost-effective stripping procedure, and which avoids further generation of waste. Such methods should also release cesium from the extractant solvent without employing highly concentrated mineral acids, solvent evaporation, or distillation, or contacting of the solvent with cation exchangers. Thus, the present invention comprises a solvent extraction and stripping process cycle for the removal of cesium from alkaline tank waste. After the solvent is stripped of cesium, the solvent can be recycled in a continuous extraction and stripping process cycle.
Previously reported extractants have generally possessed insufficient selectivity or extraction power to remove cesium from a matrix concentrated in competing alkali metal cations. In addition, earlier extraction solvents involved difficulties with stability, stripping, or phase disengagement. Thus, no other candidate solvent system has emerged as a serious contender for the targeted application.
In U.S. patent application Ser. No. 09/146,800 filed Sep. 3, 1998 and hereby incorporated by reference, a solvent composition and process for extracting cesium from alkaline solutions, in particular alkaline nuclear waste solutions containing high concentrations of sodium nitrate, was described. The solvents described are composed of a calix[4]arene-crown ether extractant (most preferably calix[4]arene-bis-(tert-octylbenzo-crown-6) ether), an alkylaryl ether alcohol phase modifier, and a diluent. The calixarene-crown extractant complexes cesium cation (Cs
+
), and extracts it into the solvent phase as an ion-pair with, an anion such as nitrate to balance the positive charge. The alkylaryl ether alcohol phase modifiers described were a class of alkylphenoxy alcohols designed to improve the cesium extraction strength, prevent third-phase formation, and provide good phase-coalescence behavior. It is desirable to have as potent a modifier as possible, so that high cesium distribution ratios can be obtained at low extractant concentrations (as the calixarene extractants are expensive). Of the modifiers listed and tested in U.S. patent application Ser. No. 09/146,800, 1-(1,1,2,2-tetrafluoroethoxy)-3-(4-tert-octylphenoxy)-2-propanol (shown in Formula (I) below) was found to be especially effective with respect to affording a good balance of high cesium extractive strength and good phase-coalescence behavior. This modifier, a phenoxy fluoro-alcohol, code named “Cs-3”, was recently found to be especially effective for cesium extraction from high sodium/low potassium alkaline nuclear waste such as that stored at the U.S. Department of Energy's (DOE's) Savannah River Site (SRS), giving rise to good (e.g., >10) cesium distribution ratios at low (0.01 Molar) concentrations of the calixarene-crown ether extractant. The magnitude of the cesium distribution ratio was also dependent upon such factors as temperature and the volume ratio of the aqueous and organic phases.
However, as shown in
FIG. 9
, the Cs-3 modifier was found to slowly degrade following prolonged contact with an SRS alkaline waste simulant (1.75 molar in free hydroxide), with a concomitant erosion in cesium extraction efficiency as measured by the cesium distribution ratio, D
Cs
, making this modifier less than optimum for the sought solvent extraction process. Unlike the non-fluorinated modifiers described in U.S. patent application Ser. No. 09/146,800, Cs-3 contains a base-sensitive functional group (the tetrafluoroethoxy moiety, —OCF
2
CF
2
H). Following prolonged contact with strongly alkaline (>1 molar in free hydroxide) solutions, the Cs-3 modifier present in the solvent appears to react with itself in a condensation reaction, with concomitant loss of one tetrafluoroethoxy moiety to form a larger molecule which is ineffective as a modifier (a proposed mechanism is shown in FIG.
10
). The solvent thus loses potency with respect to cesium extraction efficiency as a function of exposure time t
Bonnesen Peter V.
Moyer Bruce A.
Sachleben Richard A.
Gottlieb Paul A.
Padmanabhan Sreeni
Schneider Emily G.
The United States of America as represented by the Department of
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