Method for selectively removing fluorine and fluorine-containing contaminants from gaseous UF.sub.6

Abstract

This invention is a method for effecting preferential removal and immobilization of certain gaseous contaminants from gaseous UF.sub.6. The contaminants include fluorine and fluorides which are more reactive with CaCO.sub.3 than is UF.sub.6. The method comprises contacting the contaminant-carrying UF.sub.6 with particulate CaCO.sub.3 at a temperature effecting reaction of the contaminant and the CaCO.sub.3.

Description

FIELD OF THE INVENTION

This invention relates to a method for the removal of fluorine and reactive fluorides from gas streams containing the same. More particularly, it relates to a process for removing gaseous fluorine and/or certain fluorides from a stream of gaseous UF.sub.6 by passing the stream through a selected particulate trapping agent.

Problem

For many years, there has been a need for a practical method for the removal of gaseous CIF.sub.3 and decomposition products thereof from gaseous UF.sub.6 containing the same. This need has existed in facilities for the production of UF.sub.6 feed for gaseous-diffusion operations, as well as in various research activities associated with UF.sub.6. This need would be met satisfactorily by a relatively simple, dry process for selectively removing ClF.sub.3, ClF, ClO.sub.2 F, and the like from streams of gaseous UF.sub.6, but hitherto no such method has been available.

Related Art

U.S. Pat. No. 3,125,409 (S. H. Jury; Mar. 17, 1964) discloses the removal of UF.sub.6 from gas mixtures containing the same by contacting the mixture with anhydrous calcium sulfate. U.S. Pat. No. 3,625,661 (L. W. Anderson et al; Dec. 7, 1971) discloses selectively removing titanium or niobium values from a gas mixture of UF.sub.6, niobium pentafluoride, and titanium tetrafluoride by passing the mixture through a complex fluoride comprising an alkali metal cation and a complex fluoro anion. U.S. Pat. No. 3,708,568 (W. R. Golliher et al; Jan. 2, 1973) discloses selectively removing plutonium values from a fluid mixture containing PuF.sub.6 and UF.sub.6 by passing the mixture through a bed of pelletized cobaltous fluoride. The use of a limestone-packed tower to absorb HF from gases is described in the following article: N. Gilbert et al, Chem. Engr. Progress, 49, No. 3, pp. 120-127, March, 1953. The recovery of fluorine from stack gases by passing the gases through lump limestone is described by T. Hignett et al, Ind. and Engr. Chem., 42, No. 11, pp. 2493-2498, November, 1979. The use of a limestone bed to remove halogen gases from air is described by R. Liimatainen et al, in Argonne National Laboratory Report ANL-5015, Apr. 1, 1953. The removal of various volatile metal fluorides from UF.sub.6 by selective sorption on particulate MgF.sub.2 and NaF is described by S. Smiley et al in Trans. Am. Nucl. Soc., Vol. 10, No. 2, 1967, p. 507. Pelletized NaF commonly is used for preferentially trapping UF.sub.6 from gas streams also containing other fluorides.

Objects

Accordingly, it is an object of this invention to provide a novel method for selectively removing fluorine and fluorine-containing compounds from gas streams also containing UF.sub.6.

It is another object to provide a dry method for preferentially removing fluorine and fluorine-containing compounds from gas streams mainly comprising UF.sub.6.

It is another object to provide a method for selectively removing contaminants such as fluorine-containing compounds from a gas stream also containing UF.sub.6, the removed contaminant(s) being immobilized in non-volatile form.

SUMMARY

This invention is directed to a method for removing a gaseous contaminant such as fluorine and fluorides more reactive than UF.sub.6 from a gas mixture containing the contaminants and UF.sub.6. Removal is effected by contacting the mixture with particulate CaCO.sub.3 at a temperature effecting reaction of the contaminant. The resulting purified gaseous UF.sub.6 is recovered. The advantages of this method include the following: (1) the method utilizes dry chemistry; (2) the reaction zone can be defined by determining temperature profiles; (3) removal efficiency is relatively high; and (4) the UF.sub.6 remains in the gas phase, with the exception that about 2-3 wt.% is retained as uranium on the bed material.

Additional objects, advantages, and novel features of the invention will be made evident hereinafter. The objects and advantages of the invention may be realized by practicing the method defined in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

This invention is based on a discovery made in the course of experiments which we conducted on the assumption that particulate CaCO.sub.3 would prove to be an effective trapping agent for gaseous UF.sub.6. Chemical-thermodynamic calculations indicate that these two compounds will react. Contrary to expectations, the experiments demonstrated that little reaction takes place between CaCO.sub.3 and gaseous UF.sub.6, even at temperatures as high as 1000.degree. F. On the basis of this finding, we have developed a novel method for removing gaseous fluorine and certain gaseous fluorine-containing compounds from gaseous UF.sub.6.

The following is an example of our method as conducted with particulate oolitic CaCO.sub.3 as the reactant for effecting the removal of gaseous ClF.sub.3 from a gas stream including gaseous UF.sub.6. The CaCO.sub.3 was limestone obtained from a Kentucky supplier (referred to herein as A).

EXAMPLE 1

In this run the gaseous feed stream consisted of UF.sub.6, ClF.sub.3, and inert-gas diluents, such as N.sub.2. The gas stream was passed upwardly through a bed of particulate oolitic CaCO.sub.3, confined in the central section of a vertically disposed Monel pipe. The inlet section of the pipe and the section containing the bed were provided with tube heaters. The bed had a weight of 1871 g, a diameter of 3" and a length of 12". The typical CaCO.sub.3 particle was of irregular shape and measured 1/2"-1/4" screen fraction (USS, coarse series). Thermocouples were positioned in the bed at depths of 1", 6", and 11", as well as in the inlet section of the pipe. Conventional infrared analysis was used to determine the inlet and outlet concentrations of ClF.sub.3. For about 15 hours before the run, the pipe heaters were energized to maintain a temperature of about 600.degree. F., during which period dry air was passed through the bed. At the beginning of the run, the bed heaters were deenergized because of the exothermic nature of the reaction. The run was conducted until 100% breakthrough of ClF.sub.3 occurred, to ensure that the entire bed would be reacted. The run is summarized in Table 1.

An analogous run was conducted in the same system with a fresh bed of the kind described in Example 1. In this additional run, the gaseous mixture fed to the above-described trap contained concentrations of ClF.sub.3 as high as 9 mol%. UF.sub.6 concentrations in the feed were typically between 70 and 80 mol% while the total mass flow to the trap was maintained at 0.13 lb/min. Maximum temperatures in the bed were below 825.degree. F. and fluoride removal efficiencies remained at approximately 100% until the loading factor was approximately 22%. Analyses of the spent bed material indicated a total fluoride loading factor of 31%. The amount of uranium retained by the spent bed material was 2.6%. These runs confirmed that CaCO.sub.3 beds are efficient means for trapping ClF.sub.3 in the presence of UF.sub.6, with minimal retention of the UF.sub.6.

Additional runs were conducted to compare the performance of the CaCO.sub.3 from Source A with the following: oolitic CaCO.sub.3 from a Tennessee supplier (B) and an Illinois supplier (C); and non-oolitic CaCO.sub.3 from another Kentucky supplier (D). Physical property measurements and analyses showed only minor differences in the four materials. Slight variations in impurity levels were noted; however, these do not appear to have any significant effect.

Table 2 summarizes the results of comparative runs conducted with the three oolitic materials and one non-oolitic material. The runs were made using a 1-in. OD by 14-in. long Monel reactor filled to a bed depth of 12 in. The total gas pressure in the system was maintained at 4 psia for the runs, with a superficial gas velocity of 2.4 ft/sec at 600.degree. F. Each run was continued for about one hour after reaching approximately 100% ClF.sub.3 breakthrough, to ensure complete reaction of the entire bed. Fluoride analyses of the bed materials were used to calculate total fluoride loading factors. Plots of efficiency with time were used to obtain breakthrough times and to calculate fluoride loading factors at breakthrough.

Maximum ClF.sub.3 removal efficiencies prior to breakthrough, uranium retention on the bed material, and total fluoride loading factors were approximately the same for all three oolitic materials. For reasons which are not clear, the fluoride loadiing factor at breakthrough with the Source-B CaCO.sub.3 is significantly lower than with the other two oolitic materials. In some applications, however, total fluoride loading would be a more important consideration.

Still referring to Table 2, two distinct differences in performance are apparent when comparing the non-oolitic and oolitic types. ClF.sub.3 breakthrough occurred as soon as the run was started with the non-oolitic type. A maximum temperature rise of 64.degree. F., due to the heat of reaction, was observed in the non-oolitic bed, whereas 138.degree. F. to 193.degree. F. was experienced with the oolitic beds. These runs show the non-oolitic CaCO.sub.3 to be less reactive than the oolitic types; this can be an advantage in those selective-removal applications where very high removal efficiencies are not a necessity.

In some applications of our method, it is preferable to conduct the removal reaction in a moisture-free system in order to minimize loss of gaseous UF.sub.6 by conversion to UO.sub.2 F.sub.2 and to avoid deposition of the UO.sub.2 F.sub.2 on the CaCO.sub.3. The elimination of moisture may not be necessary when the UF.sub.6 contains ClF.sub.3, since the latter converts UO.sub.2 F.sub.2 to gaseous UF.sub.6.

For brevity, our method has been illustrated in terms of the selective removal of ClF.sub.3 from gaseous mixtures containing UF.sub.6, but it is also generally applicable to the removal of gaseous fluorine and gaseous fluorides which are more reactive with CaCO.sub.3 than is UF.sub.6. The following are additional examples of fluorides which we believe would be so removed: ClF, ClOF.sub.2, PuF.sub.6, NpF.sub.6, TcF.sub.6, VOF.sub.3, RuF.sub.5, MoF.sub.6, and chromium fluorides. Given the teaching herein, one versed in the art can determine the applicability of our method with respect to other fluoride contaminants by means of only routine experimentation. The method also is applicable to the removal of HF from gaseous UF.sub.6 if the gas mixture also includes an agent (e.g., ClF.sub.3) for decomposing the water produced by the reaction of HF and CaCO.sub.3.

Our method is effective over a wide range of elevated temperatures. For instance, preferential removal of ClF.sub.3 from gaseous mixtures of ClF.sub.3 and UF.sub.6 will take place to some degree over a temperature range of from about 300.degree. F. to at least 1200.degree. F. (At temperatures below about 300.degree. F., the extent of contaminant removal is undesirably low, whereas at about 1200.degree. F. the amount of uranium retained on the bed approaches 25%). Preferably, the gas mixture to be purified is contacted with the reactant at a temperature in the range of from about 400.degree. to 1100.degree. F. More preferably, the range is from about 500.degree. to 1000.degree. F. In general, we prefer to conduct our method with superficial gas velocities of from about 0.2 to 2.5 ft/sec; in the system referred to in Examples 1 and 2, acceptable results were obtained at superficial velocites of from 0.4 to 5 ft/sec. It was noted that the progress of the reaction zone through the bed could be followed by measuring the temperature profile of the bed.

Our method has been illustrated above in terms of particular varieties of CaCO.sub.3, but these are not necessarily the optimum. Given the teachings herein, one versed in the art can determine the most suitable varieties, particle sizes, shapes, etc., for a given application with only routine experimentation. Although the method has been illustrated in terms of fixed beds of CaCO.sub.3, it is within the scope of the invention to use fluidized beds, if desired.

It will be apparent from the foregoing that our method meets the need discussed above under "Background of the Invention" The method is simple, reliable, and avoids the shortcomings of wet-chemistry processes. The selectively removed fluorine or fluorine-containing contaminants are immobilized in non-volatile and easily stored form. The sorbent is a readily available and relatively inexpensive material. Furthermore, the method can be conducted in compact apparatus.

We do not wish to be bound by any theory as to the mechanism by which our method effects selective trapping of the above-mentioned contaminants while removing only minimal amounts of UF.sub.6. X-ray analysis of CaCO.sub.3 granules which have been used in accordance with this invention shows that they typically include the following: (a) an outer white layer of CaF.sub.2 ; (b) an intermediate orange layer, rich in uranium; and (c) an internal gray core of unreacted CaCO.sub.3. The gas-solid, displacement-type reaction probably is limited kinetically by penetration of the particle or diffusion through the CaF.sub.2 reaction-product coating.

The foregoing description of a preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.

TABLE 1______________________________________SELECTIVE REMOVAL OF ClF.sub.3 FROM UF.sub.6 WITHOOLITIC CACO.sub.3 (SUPPLIER A)Pilot Test Run Number 1______________________________________Total Gas Flow, lbs/min 0.13 to 0.17Average Inlet ClF.sub.3 Conc., Mol % 2 to 3.7Average Inlet UF.sub.6 Conc., Mol % 70 to 80Initial Bed Temperature, .degree.F. 550 to 650Maximum Temperature Rise, .degree.F. 170Maximum ClF.sub.3 Removal Efficiency*prior to Breakthrough, % approx. 100F.sup.- Loading Factor** @ Breakthrough, % approx. 13Total F.sup.- Loading Factor,** % 22Uranium Retention, % 3______________________________________ ##STR1##- - ##STR2## TABLE 2__________________________________________________________________________SELECTIVE REMOVAL OF ClF.sub.3 FROM UF.sub.6 WITH CaCO.sub.3 MATERIALS Supplier: Oolitic CaCO.sub.3 Non-Oolitic CaCO.sub.3 A B C DRun Number 57 75 66 74__________________________________________________________________________Total Gas Flow, Sccm 2,300 2,300 2,300 2,300Average Inlet ClF.sub.3 Conc., Mol % 9.0 7.3 8.9 6.8Average Inlet UF.sub.6 Conc., Mol % 6.5 3.1 5.8 9.0Bed Temperature, .degree.F. 600 600 600 600Maximum Temperature Rise, .degree.F. 173 138 193 64Maximum ClF.sub.3 Removal Efficiency*prior to Breakthrough, % >99.4 >98.6 >99.6 Instant BreakthroughF.sup.- Loading Factor** @ Breakthrough, % 22 12 19 Instant BreakthroughTotal F.sup.- Loading Factor,** % 32 36 35 28Uranium Retention, % 1.8 1.6 1.5 2.0__________________________________________________________________________ ##STR3##- - ##STR4##

Claims

1. The method of treating a gaseous mixture of UF.sub.6 and a contaminant selected from the group consisting of fluorine and gaseous fluorides which are more reactive with CaCO.sub.3 than is UF.sub.6, to effect selective removal of said contaminant from said mixture, comprising:

contacting said mixture with particulate CaCO.sub.3 at a temperature effecting reaction of said contaminant and said CaCO.sub.3, and

recovering the resulting purified gaseous UF.sub.6.

2. The method of claim 1 wherein said CaCO.sub.3 is at a temperature in the range of from about 300.degree. to 1200.degree. F.

3. The method of selectively removing a gaseous contaminant selected from the group consisting of F.sub.2, ClF.sub.3, ClF, ClO.sub.2 F, PuF.sub.6, NpF.sub.6, TcF.sub.6, VF.sub.5, VOF.sub.3, RuF.sub.5, MoF.sub.6, and chromium fluorides from a stream of gaseous UF.sub.6 containing said contaminant, said method comprising:

passing said stream through a bed of particulate CaCO.sub.3 at a temperature effecting reaction of said contaminant and said bed.

4. The method of claim 3 wherein said bed is substantially free from moisture.

5. The method of claim 3 wherein said bed is at a temperature in the range of from about 300.degree. to 1200.degree. F.

6. The method of claim 5 wherein said bed is at a temperature in the range of from about 400.degree. to 1000.degree. F.

7. The method of claim 3 wherein the superficial velocity of said stream is in the range of from about 0.4 to 5 sec.

8. The method of selectively removing a gaseous contaminant selected from the group consisting of F.sub.2, metal fluorides, ClF.sub.3, ClF, ClO.sub.2 F, and VOF.sub.3 from a stream of gaseous UF.sub.6 containing said contaminant, comprising:

providing a substantially moisture-free bed of particulate CaCO.sub.3, heating said bed to a temperature in the range of from about 300.degree. to 1200.degree. F., and

passing said stream through said bed at a flow rate providing a residence-time in said bed of from about 0.4 to 5 ft/sec.

9. The method of selectively removing HF from a gaseous stream containing HF and UF.sub.6, comprising:

passing said stream through a bed of CaCO.sub.3 at a temperature effecting reaction of said HF and said CaCO.sub.3, said stream also having incorporated therein a gaseous agent for decomposing water formed as a byproduct of said reaction.

10. The method of claim 9 wherein said agent is ClF.sub.3.