Removal of uranium from aqueous HF solutions

This invention is a simple and effective method for removing uranium from aqueous HF solutions containing trace quantities of the same. The method comprises contacting the solution with particulate calcium fluoride to form uranium-bearing particulates, permitting the particulates to settle, and separting the solution from the settled particulates. The CaF.sub.2 is selected to have a nitrogen surface area in a selected range and is employed in an amount providing a calcium fluoride/uranium weight ratio in a selected range. As applied to dilute HF solutions containing 120 ppm uranium, the method removes at least 92% of the uranium, without introducing contaminants to the product solution.

This invention was developed in response to a need for a simple but 
effective method for decreasing the uranium content of a large volume of 
by-product aqueous hydrofluoric acid contaning uranium in the 
parts-per-million range. The acid solution comprised about 20 wt-% HF and 
76 ppm uranium (uranium species in solution not known). Sale of the 
solution on the open market required that its uranium concentration be 
decreased below 10 ppm by some technique which would not introduce 
additional impurities into the product solution. 
An attempt was made to decrease the uranium content of the solution by 
passing it over amorphous carbon maintained at 350.degree.-450.degree. C. 
This decreased the uranium content by only 64%. In another approach, the 
solution was passed through an anion exchange column of the hydroxide 
type. This decreased the uranium concentration by only 32%. Various other 
removal techniques were considered but rejected as impractical. 
Various publications describe the adsorption of uranium from solutions by 
selective adsorption. The following is an example of such a publication: 
Minhai Dai and Shaw-Chii Wu, "Adsorption of Uranium from Dilute Aqueous 
Solution on Inorganic Adsorbents," Separation Science, 10(5), pp. 633-638 
(1975). That paper described recommends adsorbing uranium with a mixture 
of aluminum hydroxide, ferric hydroxide, and activated carbon. It also 
describes experiments conducted with various alkaline earth oxides, 
hydroxides, and sulfates as adsorbents. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of this invention to provide a novel method 
for decreasing the uranium content of aqueous hydrofluoric acid containing 
trace amounts of uranium in solution. 
It is another object to provide a simple and effective method for 
decreasing the uranium content of a uranium-bearing aqueous HF solution 
without introducing a contaminant into the product solution. 
Other objects will be made evident hereinafter. 
SUMMARY OF THE INVENTION 
This invention may be summarized as being a method of decreasing the 
uranium content of an aqueous HF solution containing uranium, said method 
comprising mixing particulate calcium fluoride with said solution to form 
uranium-bearing particulates; permitting said particulates to sediment 
from said solution; and separating the resulting solution from the 
sedimented particulates. 
BACKGROUND OF THE INVENTION 
This invention is generally applicable to the recovery of uranium from 
aqueous HF solutions containing the same. It is based on our finding that 
intimately contacting such solutions with particulate CaF.sub.2 and then 
sedimenting the particulates effects removal of much of the uranium. That 
is, uranium is carried down out of solution by the sedimented CaF.sub.2 
particles. The resulting solution then is separated from the sedimented, 
uranium-bearing particulates by any suitable technique. The mechanism by 
which the uranium is carried down is not yet well understood. 
So far as is known, this process has not been reported previously. It is 
clear from the very low solutility of CaF.sub.2 in aqueous solutions that 
the well-known common-ion effect is not involved here. If the uranium is 
in solution as UF.sub.4 and no ionization has occurred, then the 
solubility-product principle would not be governing the carry-down of 
uranium. If the uranium is present as UO.sub.2 F.sub.2, the solubility of 
this species is too high to account for the carry-down of uranium when 
present in trace quantities.

EXAMPLE I 
The invention was tested by mixing selected quantities of particulate 
CaF.sub.2 in six 0.200-liter samples of an aqueous HF solution containing 
120 ppm uranium. The solution comprised 20 wt-% HF. The CaF.sub.2 powder 
was acid-grade fluorspar, manufactured by Allied Chemical Corporation. 
This powder had a nitrogen surface area of 1.6 m.sup.2 /g (based on the 
well-known B.E.T. measurement). Tyler-sieve data for the powder were as 
follows: 45% of the powder passed through a 325-mesh screen and 15% was 
retained by the screen; 27% was retained on a 200-mesh screen; 9% was 
retained on a 100-mesh screen; and 4% was retained on a 65-mesh screen. 
Each of the samples containing particulate CaF.sub.2 was stirred at room 
temperature for an hour. Following stirring, the resulting slurries were 
either filtered promptly or the supernate was separated by decanting. The 
resulting solutions were analyzed for uranium by gamma-spectrometry and 
for calcium by atomic absorption. 
The accompanying table shows the CaF.sub.2 -to-uranium weight ratios 
employed in each of the six tests, together with the results obtained. It 
will be noted that removal of 50% of the uranium was accomplished at a 
CaF.sub.2 /U ratio of 8 and that removal of 92% of the uranium was 
accomplished at ratios exceeding 37. As shown, the product solutions 
contained very little calcium--only 9 ppm if the solution was not 
filtered, and less than 0.2 ppm if it was filtered. Thus, the process was 
found to remove uranium effectively while avoiding contamination of the 
product solution. 
__________________________________________________________________________ 
Lbs CaF.sub.2 
Initial 
Final Final 
Sample 
per 13,500 
CaF.sub.2 /U 
U Conc., 
U Conc., 
U Removal, 
Ca.sup.++ Conc., 
No. Gal. Solution 
by Weight 
ppm ppm % ppm 
__________________________________________________________________________ 
1 56 4 120 72 40 &lt;0.2 
2 84 6 120 71 41 &lt;0.2 
3 112 8 120 60 50 &lt;0.2 
4* 112 8 120 59 51 9 
5 500 37 120 10 92 &lt;0.2 
6 1000 74 120 10 92 &lt;0.2 
__________________________________________________________________________ 
*Sample 4 was allowed to stand overnight before decanting. All other 
samples were filtered before analysis. 
EXAMPLE II 
A 67,000-pound batch of aqueous 20%-HF solution containing 74 ppm uranium 
was admitted to a railroad tank car which previously had been used to 
recover uranium in accordance with this invention. The rubber-linked tank 
contained a heel of CaF.sub.2 /U slurry remaining from the previous 
recovery operation. The tank car was provided with means for sparging with 
air. 
With the sparging means energized, approximately 400 pounds of particulate 
CaF.sub.2 (identified in Example I) was added to the solution to provide a 
total CaF.sub.2 /U weight ratio of 159 to 1. After three days of air 
agitation and then four hours of sedimentation (both conducted at room 
temperature), the solution was analyzed. The analysis was as follows: HF, 
.about.20 wt-%, uranium, 7 ppm; calcium, &gt;0.2 ppm. Following analysis, the 
solution was decanted for sale. 
As indicated above, this method for recovery of uranium has significant 
advantages. For example, it entails comparatively simple process 
operations and requires only readily available equipment. Again, uranium 
removal is effected without introducing a contaminant into the product 
solution. Furthermore, a wide variety of CaF.sub.2 powders may be 
employed, such as powders having nitrogen surface areas in the range of 
from about 1 to 200 m.sup.2 /g. The method is believed effective for 
reducing the uranium content of both dilute and concentrated aqueous HF 
solutions containing a trace amount of uranium. By "trace amount" is meant 
in the range of from a few ppm to thousands of ppm. In general, 
appreciable uranium carry-down may be obtained if the CaF.sub.2 /U weight 
ratio is in the range of from about 8 to 75. 
It will be understood that the process parameters cited in Examples I and 
II, above, are not necessarily the optimum. For example, even higher 
percentages of uranium might have been removed if the runs had been 
conducted with (a) higher-surface-area CaF.sub.2, (b) a tank having a more 
suitable geometry with respect to mixing, or (c) more efficient mixing 
means--e.g., a propeller-type mixer. Given the teachings herein, one 
versed in the art will be able to determinne the preferred process 
parameters (e.g., CaF.sub.2 powder surface areas, CaF.sub.2 /U weight 
ratios) for a given application of this invention by merely routine 
experimentation, as by testing on a laboratory scale. 
The foregoing examples are provided for the purpose of illustration only, 
and it will be understood that the scope of the invention is to be 
interpreted in terms of the following claims.