Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1999-09-17
2002-11-12
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S682000, C210S694000
Reexamination Certificate
active
06478970
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the removal of radioactive thorium from an aqueous solution. The invention particularly relates to a process to reduce the amount of radioactive thorium discharged with the effluent from a solvent extraction uranium purification process.
BACKGROUND
A small amount of radioactive thorium (Th-231 and Th-234) is produced from the radiolytic decay of uranium. The thorium is normally in equilibrium with the uranium and it stays with the uranium throughout normal uranium processing operations. Because of its low concentration and the shielding provided by the uranium, the radioactivity from the thorium associated with the uranium is normally not a problem. However, when uranium is purified by solvent extraction the uranium is extracted into the organic phase and the thorium remains in the aqueous effluent (raffinate) solution. Although the amount of thorium associated with the uranium is very small, it has a high specific activity and therefore significantly increases the radioactivity of the liquid effluent.
Thorium is normally removed from the solvent extraction liquid effluent by chemical separation followed by aging of the separated thorium-containing solids. Alternately, the entire liquid effluent can be held until the thorium decays to an acceptable level for residual radiation. Because Th-234 has a half-life of 24 days, very large tanks or surface impoundments are normally needed to hold the entire liquid effluent stream for the required decay time. Since Th-231 has a half-life of only 25 hours, it is not usually a factor in determining the required decay time of the thorium-containing solids or liquid effluent.
Elimination of the need to store and age the thorium-containing solids or liquids would greatly simplify the design of thorium treatment systems and eliminate the need for potentially dangerous handling and treatment of the thorium.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a much simpler thorium removal process that is cheaper to build and operate than traditional chemical thorium removal processes. It is a further object of the present invention to provide a process for removing thorium from aqueous solutions which does not require an additional aging step for the separated thorium to allow its decay to a safe level of radioactivity. It is yet another object of the instant invention to provide a process for removing thorium from aqueous solutions which does not require large tanks or surface impoundments to age the liquid effluent solution. Other objects and features of the present invention will become apparent when considered in combination with the following summary and detailed descriptions of certain preferred embodiments of the invention.
The foregoing and related objects of the invention are achieved by the following continuous process for removing radioactive thorium from an aqueous solution:
providing a quantity of ion exchange resin which is selective for binding thorium, and
passing the aqueous solution through the quantity of ion exchange resin at a substantially constant flow rate,
wherein, the quantity of the ion exchange resin and the substantially constant flow rate combine to provide an average residence time for a thorium ion in the quantity of ion exchange resin which is greater than the average time required for the radioactive decay of the thorium ion.
As the aqueous solution enters the ion exchange resin, the thorium ions in the aqueous solution are selectively bound by the ion exchange resin. As is well known in the art of ion exchange resins, an equilibrium is formed between thorium bound to the ion exchange resin and thorium in the aqueous solution flowing through the ion exchange resin. Therefore, the thorium from the aqueous solution passes through the ion exchange resin at a slower rate than the flow rate of the aqueous solution, because the thorium spends only a fraction of its time free in the aqueous solution and the remainder of its time bound to the ion exchange resin. In the instant invention, since the average residence time of the thorium in the ion exchange resin is equal to or longer than the average time required for the radioactive decay of the thorium, each ion of thorium that is held by the ion exchange resin which decays and is washed off of the resin by the constant flow of aqueous solution is replaced by a fresh ion of thorium. The aqueous solution which emerges from the ion exchange resin contains the products of the radioactive decay of thorium, uranium and protactinium, but is substantially free of thorium.
In a preferred embodiment of the present invention, the aqueous solution undergoes a pretreatment process to enhance its ability to release thorium to the ion exchange resin.
Although removal of thorium and uranium from a solution by ion exchange is known in the field, an ion exchange process wherein the thorium is both removed from the effluent and allowed to decay on the same ion exchange column under continuous operation is new. This makes a much simpler and more efficient process than traditional ion exchange which must be accompanied by elution of the thorium from the column and subsequent treatment and handling of thorium removed from the column or disposal of thorium-loaded ion exchange resin.
DESCRIPTION OF THE INVENTION
The present invention consists of an ion exchange process in which a continuous flow of an aqueous solution contaminated with thorium is passed through an ion exchange resin which selectively retards the thorium for a sufficient period of time to allow the thorium to undergo, radioactive decay, thereby removing the thorium from the aqueous solution. The ion exchange system runs continuously without requiring regeneration of the ion exchange resin or separate storage of the radioactive thorium solids while they decay.
The ion exchange resin type, amount of ion exchange resin and flow rate of the aqueous solution through the ion exchange resin are selected so that the thorium residence time on the ion exchange column is longer that its radiolytic decay time. This insures that the thorium never reaches break-through and the column therefore does not require regeneration.
The particular ion exchange resin to be used is selected according to its thorium ion exchange capacity, its selectivity of thorium over other metal ions present in the aqueous solution to be used treated and the chemical compatibility of the ion exchange resin with the aqueous solution. For instance, in the preferred embodiment in which raffinate is the aqueous solution to be treated, the ion exchange resin should be selective for thorium over iron, gadolinium, protactinium and uranium, which are also present in raffinate, and the ion exchange resin must be compatible with the nitric acid which is also present in raffinate. For use with raffinate, particularly preferred ion exchange resins include, but are not limited to, Purolite S-950 resin (manufactured by Purolite company, Bala Cynwyd, Pa.), Amberlyst A-15 resin (manufactured by Rohm and Haas Company, West Philadelphia, Pa.) and the inorganic ion exchanger antimony pentoxide.
One of skill in the art of ion exchange resins would well be able to choose an effective ion exchange resin or inorganic ion exchanger for any particular thorium-containing aqueous solution based on a known chemical analysis of that solution.
The ion exchange resin is preferably housed in a column, with the aqueous solution applied at the top of the column flowing out the bottom of the column, but any other configuration may be used which allows the aqueous solution a flow path through substantially the entire quantity of ion exchange resin.
The amount of ion exchange resin used and the flow rate of the aqueous solution through the ion exchange resin are determined based on the amount of residual thorium that is desired in the aqueous solution which exits the ion exchange resin. Although the amount of ion exchange resin and flow rate of the aqueous solution can be calculated from the known affinity of
Cintins Ivars
Framatome ANP Inc.
Kenyon & Kenyon
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