Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2000-11-22
2002-11-19
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C501S134000, C501S154000, C210S681000, C210S682000, C210S688000
Reexamination Certificate
active
06482380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a new silicotitanate molecular sieve ion exchange material for the capture and immobilization of divalent cations from aqueous and/or hydrocarbon solutions, including elements such as radioactive strontium or industrial RCRA metal cations. The invention further relates to the ability to either recycle the captured metal for future use or to encapsulate the cation through thermal treatment of the molecular sieve to a condensed phase.
2. Description of the Prior Art
It is known that crystalline silicotitanates (CST; commercially available through UOP LLP, as IE-911) can selectively remove 100 ppm Cs and Sr cations from 5M Na
+
solutions over a broad pH range (1 to 14) and, because of this, they have been found effective to clean up radioactive
137
Cs and
90
Sr from waste tanks at the US Department of Energy Hanford waste tanks. To immobilize the Cs-loaded CSTs, there is used a combination of high activity waste and melting with borosilicate glass to create a glass log waste form which can be stored indefinitely. However, since titania induces crystallization and separation in borosilicate glass, the Cs-CST must be diluted to a few weight percent, thus increasing the volume and the cost of waste form production. It has already been shown by Y. Su, [Y. Su, et. al, MRS Conference Proceedings (Boston) “Evaluation of Cesium Silicotitanates as an Alternative Waste Form” p. 457 (1997)]. that a waste form that is more chemically durable than borosilicate glass logs can be generated by direct thermal conversion of Cs-CST. However, an improvement in the isolation of divalent radioactive and industrial waste remains quite desirable.
Therefore, an object of this invention is to provide a new type of inorganic molecular sieve materials. Another object is to provide inorganic materials that are mechanically and chemically stable and are free from the traditional problems associated with organic-based molecular sieves. A further object is to provide microporous compounds that can be used for radionuclides and industrial metals sorption. A still further object is to provide a molecular sieve that can be back-exchanged by acid wash to recover/recycle the sequestered metal cation. Still another object is to provide molecular sieves that can be thermally condensed to form leach resistant phases for radionuclide storage.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description, or will be learned by practice of the invention.
BRIEF SUMMARY OF THE INVENTION
There is now disclosed that the radionuclides
137
cesium, and to a lesser extent
90
strontium, as well as divalent cations of several metals can be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger, namely a Cs—Si—Ti—O phase. However, an improved divalent ion exchanger and its condensed counterpart phase have now been hydrothermally synthesized, characterized, evaluated, and described in this specification. The viability of the new materials for divalent sequestration and subsequent encapsulation is based on chemical, mechanical, and thermal stability, leachability, and ion exchange capabilities. The two novel Cs—Si—Ti—O phases are Cs
3
TiSi
3
O
9.5
.3H
2
O (SNL-B), a porous phase which adsorbs the divalent cation metal, and Cs
2
TiSi
6
O
15
(SNL-A), a condensed stable form in which the metal is immobilized for storage purposes, if that is desired. The two phases are also identified by their crystallographic parameters: SNL-B, orthorhombic, unit cell parameters a=10.83 Å, b=7.43 Å and c=7.11 Å; SNL-A, monoclinic, Cc space group, unit cell parameters: a=12.998 (2) Å, b=7.5014 (3) Å, c=15.156 (3) Å, &bgr;=105.80 (3)°.
REFERENCES:
patent: 5591420 (1997-01-01), Balmer
patent: 5935552 (1999-08-01), Bedard
Nenoff Tina M.
Nyman May D.
Durkis James C.
Gottlieb Paul A.
Kehl Dickson G.
Sample David
The United States of America as represented by the Department of
LandOfFree
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