TECHNOLOGY IN THE CHEMICAL AND PETROLEUM INDUSTRIES EM0 CHlOTTl and H. E. SHOEMAKER tute for Atomic Research and Department of Chemistry, Iowa State College, Ames, loswa
Pyrometallurgical Separation of Uranium from Thorium Perhaps simple separation can be accomplished by extraction with liquid magnesium
VARIOUS
METHODS have been developed (2, 5 ) for separating uranium-233 from thorium or the blanket material, which require aqueous dissolution of the metal and conversion of the separated components back to the metallic state. However, it would be advantageous if the components could be separated in their metallic state. Previous work (3) suggested that thorium could be separated from uranium by extraction with liquid magnesium. Therefore, equilibrium phases in the magnesium-rich end of the ternary system, magnesium-thorium-uranium, were studied along with pyrometallurgical methods for separating uranium and thorium. All percentages mentioned are weight per cent.
Experimental Commercially pure magnesium, obtained from the Dow Chemical Co., contained 0.031% of iron and only traces of beryllium, aluminum, calcium, copper, and silicon. Thorium and uranium were production grade. Principal impurities in the thorium were (in per cent) 0.12 oxygen, 0.035 carbon, and less than 0.01 each of nitrogen, aluminum, silicon, iron, and beryllium. Those in the uranium were 0.0035 iron, 0.02 to 0.04 carbon, 0.01 manganese, and 0.0005 magnesium. Two types of apparatus were used. For solubility studies a t temperatures up to roughly 1000' C., a n apparatus was designed so that solutions could be
mixed and sampled under an inert atmosphere (Figure 1). The reaction chamber, 27 inches long by 3l/2-inch inside diameter, was made from iron pipe and enclosed in a lightweight No. 304 stainless steel jacket to protect the iron from excessive oxidation a t elevated temperatures. The inner walls were protected by either a graphite or iron liner and the charge was contained in a crucible made of magnesia containing 15% magnesium fluoride. Thus, the reaction chamber did not come into contact with molten alloy if the crucible failed. ' I n the actual experimental setup, the crucible did not fit tightly against the liner as indicated in Figure 1. At high temperatures, magnesium condensed on the upper part of the reaction chamber or liner and was trapped between the liner and crucible; thus, !/4# VACUUM COUPLING
I I m S A M P L l N G CHAMBER
Up0
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. 15%
MlFt CRUCIBLE
Figure 1. For solubility studies up to about 1000' C., this apparatus was designed for mixing solutions in an inert atmosphere
the charge was not contaminated with either iron or carbon. The top of the reaction chamber and sampling chamber were water cooled. Clean magnesium, 800 to 900 grams, was placed in the reaction chamber. Sufficient thorium to form the desired magnesium-thorium solution was placed in the hollow, perforated, rectangular tantalum stirrer (Figure 1). The system was then evacuated to a pressure of less than 1 micron of mercury, and with the stirrer near the top of the reaction chamber, the system was heated to 600' C. and then filled with purified helium or argon to slightly greater than atmospheric pressure. The inert gas, initially 99.99% pure, was passed over uranium shavings a t 600' C. t D remove traces of impurities before it was introduced into the reaction chamber. When the temperature was raised to above the melting point of magnesium, the stirrer containing the thorium was lowered into the melt and stirring began. The stirrer, mechanically driven a t 86 r.p.m., was connected to a S/s-inch steel shaft extending through an O-ring seal a t the top of the reaction chamber. After stirring for 1 to 2 hours a t between 700' and 800' C., the thorium dissolved, and the stirrer was removed, the charge cooled to room temperature, and the uranium added to the stirrer. The system was again evacuated and the procedure described for thorium was repeated, except that several hours' stirring was required before taking a sample. This two-step procedure was the most satisfactory-when uranium was included with thorium in the first step, a refractory film, formed on the uranium surface, did not readily dissolve and retarded or prevented attainment of equilibrium. This was probably caused by reaction of the uranium with reactive gases given off during initial outgassing of the apparatus. VOL. 50, NO. 2
FEBRUARY 1958
137
WELD # 309
Figure 2. Alloy constituents were sealed in tantalum crucibles enclosed in stainless steel jacket
The sampling chamber can be isolated from the reaction chamber and separately evacuated and filled with inert gas. I t is also equipped with a stopcock having 3/4-inch diameter opening. With the sampling chamber filled with argon the stopcock was opened to the reaction chamber. A '/?-inch diameter magnesia crucible, held in a tantalum holder fastened to a steel rod, was lowered into the melt and a sample taken. With the crucible drawn back into the sampling chamber, the stopcock was closed. When the sample
Charge, G.
U, 1.3b; 4Th, 26.0;
No.
U, 126.7; Th, 10.3; Mg, 19.5
U, 145.5; Th, 10.5; Mg,
19.6 (Th 53.5 U)d,44.7; Mg, 41.7 U, 158.1; Th, 1.1; Mg, 19.7 (U 5.5 Cr)d, 44.7; Th, 15.2; Mg, 43.7 (U 5.5 Cr)d, 178.5; Th, 1.0; Mg, 19.0
Alloys in Table 1 were prepared in the apparatus shown in Figures 2 and 3. Each sample contained more than enough uranium to saturate the magnesium-rich phase. The indicated limits for the analytical results represent average deviations; when only one sample was analyzed, no limits are given. Thirteen samples containing 35% tho-
T h in Erich phase
0.003
...
+ 0.007
...
13
1000
60-120
800C
60-120
35.0 & 1.0
0.006
5 6
1000 1000
60-120 60-240
800C 1000e
120-180 60-240
34.3 i 1.2 34.0 & 1.0
0.009 0.10
3
1000 1200
90 10
12006
10
32.0
0.20
1
1200
10
12ooc
10
32.0
0.85
1
1200
15
12006
15
34.0
0.68
4
1200
10
12ooc
10
34.0
1
1200
15
12ooc
10
3.0
3
900
60-120
900e
1
900
60
900e
Processed in tantalum cruciebls. Shavinga. nium, 93.5 i 0.08; chromium, 6.5 f 0.5. a
1 38
Experimental Results
Table 1. Analyses of Thorium-Magnesium-Uranium Alloys" Mixing Settling -- Av. Compn., Wt. % Temp., Temp., Mg-Rich -Phase O C. Min. C. Min. Th U
Mg,
46.0 (Th, 2.25 U)d 24.0; Mg, 44.0 (Th 2.0 U),d 22.0; Mg, 40.0 (Th 53.5 U)d,43.4; Mg, 43.0
time was increased to 2 hours as the processing temperature was lowered to 800' C. After mixing, the furnace was left erect, for about the same time required for mixing, to permit separation of the phases. The samples were then either furnace-cooled or removed from the furnace and quenched (Table I). Exterior of the samples were machined off in a lathe and a central portion of each phase was removed. The magnesium-rich samples were carefully cleaned and handled with tongs to avoid contamination by uranium. Uranium content in magnesium-rich alloys was determined by an ammonium thiocyanate spectrophotometric method (7) which is considered good for analyzing small quantities. However, results obtained here were not accurate beyond iO.OOS%. Fresh ammonium thiocyanate solution of precise concentration is needed for accurate analyses and absorbance must be read 4 to 8 minutes after adding thiocyanate Thorium analyses of both the uranium- and magnesium-rich phases were performed by gravimetric oxalate precipitation ( 4 ) . The thorium oxalate was filtered, ignited at 1000' C.: and weighed as thoria. Results are considered accurate to 5 0 . 2 % .
had solidified, the vacuum seal at the top was removed and the sample was raised from the chamber. A new crucible was placed in the sample holder which was then re-inserted into the sampling chamber and the vacuum seal replaced preparatory to taking another sample. Samples were taken at various temperatures u p to the practical maximum attainable with this apparatus. The stirrer was then removed and samples were taken from the top of the melt a t various temperatures as the charge was cooled. The charge was held a t constant temperature for 1 or more hours to permit any precipitated uranium to settle before a sample was taken. Alloys were also prepared by heating the constituents in sealed tantalum crucibles. Weighed amounts of clean magnesium, along with the desired amount of thorium and uranium or thorium uranium alloy, were placed in tantalum crucibles, a cap was pressed into the open end, and a welded closure made. This assembly in turn was enclosed in a welded No. 309 stainless steel jacket, 3 inches long by 1-inch inside diameter. Charging the crucible and all welding operations were done under an argon atmosphere (Figure 2). Assemblies of this type were heated in an oscillating resistance furnace supported on a shaft that was driven by a reversible motor automatically reversed by a toggle switch activated by a bar fastened to one end of the drive shaft (Figure 3). With this apparatus the furnace was rotated through an arc of approximately 180 degrees once every 50 seconds. The samples heated a t 1200' C. were mixed for 10 to 15 minutes, and rotating
INDUSTRIAL AND ENGINEERING CHEMISTRY
Furnace cooled.
60-120 60
&
0.01
i. 0.08
2.0
...
1.2
0.84
0.23
0.3
30 i 4'
0 . 0 5 3 i 0.008
3.6
Arc melted alloy.
I
0.052
Quenched.
f
0 . 4 rt 0.2s 0.2
Chromiuln, 0.13 i. 0.02.
Ura-
NUCLEAR TECHNOLOGY rium, processed in this manner and furnace cooled, gave an average residual uranium content in the magnesium-rich phase of 0 . 0 0 6 ~ 0with an average deviation of 0.003%. Five other samples processed similarly, but with thorium and uranium added as an arc-melted alloy instead of pure components, showed an average residual content of 0.009 0 . 0 0 7 ~ 0 . Considering the scatter in the analyses, there is no significant difference in the amount of uranium retained in the magnesium-rich phase. The time required to cool to 585' C., the freezing point of the magnesium-thorium eutectic, was approximately 4 hours. The average uranium content in the magnesium-rich phase in six samples equilibrated a t 1000" C. and quenched in water was 0.10 i 0.01 %. The scatter in uranium analyses of the magnesium-rich phase obtained from quenched alloys is considerably greater than that for furnace-cooled alloys, probably because of several factors-partial mixing of finely divided uranium-rich particles with the magnesium rich phase during the quenching operation would give high results. Also, if quenching is not sufficiently rapid, partial precipitation and segregation of the uranium would give low results. Access of air or carbonaceous gases to the charge is also believed to affect the results. T h e top, middlr, and in some cases the bottom sections, of each alloy were analyzed for uranium. An average of the top and middle sections was taken as the equilibrium uranium content for the magnesium-rich phase, and for furnace-cooled alloys, these were in agreement within the limits of accuracy. Thus, it may be concluded that excess uranium had precipitated and effectively settled to the bottom of the charge during the cooling process. Quenched alloys showed a much greater deviation in uranium content for the top and middle sections. The average deviation for six alloys quenched from 1000" C. was 0.01% uranium (Table I). Analysis of alloys quenched from above the melting point of uranium showed a much wider scatter. This is believed caused by difficulty in removing the sample from the furnace and quenching rapidly enough to prevent uranium from precipitating or avoiding agitation or mixing of the charge. Results obtained by this procedure for uranium in the magnesium-rich phase are generally higher than those obtained with the stirring apparatus. All samples heated above the melting point of uranium formed two liquid phases. Thorium in the uranium-rich phase existing in equilibrium with magnesium-32 to 34% thorium solution is 0.9 zt 0.3%. Thorium in this phase
*
-
decreases somewhat below this when that in the magnesium-rich phase is decreased. The balance of the uraniumrich phase is essentially uranium. Iron impurity concentrates in the uranium. A separate uranium-rich liquid phase can be obtained a t a much lower temperature by adding chromium to the charge. A eutectic occurs a t 5.2% chromium, whichmelts a t 860" f 10" C. I n these experiments the chromium was added as a uranium-chromium alloy. When 5% of chromium, based on the total uranium in the alloy, is added, a uranium-rich liquid phase forms which dissolves only 0.4 f 0.2% thorium when equilibrated with a 25 to 34y0 solution of thorium in magnesium (Table I). Solubility of uranium in magnesium and in solutions of 35% thorium in magnesium are plotted in Figure 4. Magnesium containing 1695, thorium is not shown because it parallels those for pure magnesium except a t temperatures below 750' C. where the solubility of uranium in the 16% thorium solution is somewhat greater. Smoothed values for all three solutions, given in Table 11, are based primarily on samples prepared by the first procedure described, using the apparatus shown in Figure 1. Uranium content in the first, third, and fourth magnesium-thorium alloys (Table I) are also plotted in Figure 4 for comparison. The other points mostly represent averages from two to five samples taken both on heating and cooling. The average deviation in the analytical results is indicated by a vertical line through the points. This is not done for points representing single samples. The curve for solubility of uranium in magnesium differs considerably, particularly in the temperature range of 665' to 900" C., from that obtained in a previous investigation (3). However, the analytical results for the uranium solubility in either case are within the average deviations reported. Results obtained here show a much smaller
Figure 3. Oscillating reversible furnace used to heat alloys
average deviation and are considered to be nearer the true values. This difference is believed attributable to improved sampling techniques and analytical methods used in this work. Because the solid phase in equilibrium with these solutions is essentially pure uranium, slopes of these curves are related to the partial molal heat of solution of uranium by the relation ( d In N u l d l / T ) B s tX , [NU(a In Y U / ~ N U ) T 11 = - L / R
+
where L is heat absorbed when 1 gram atom of solid uranium is dissolved a t constant temperature in an infinitely large quantity of essentially saturated solution (9). If it is assumed that Henry's law applies up to the saturation composition, the change in activity coefficient with composition a t constant temperature is zero. L is the sum of the heat of fusion for uranium and the heat absorbed when 1 gram atom of pure liquid uranium (hypothetical state) is transferred to an infinitely large quantity of essentially saturated solution. I t is reasonable to assume that heat absorbed in the latter process is constant for dilute solutions represented by the solubility curves. Then the curves in
Table II. Solubility of Uranium in Magnesium and in Thorium-Magnesium (Smoothed values obtained from plots of log Nu Temp.,
' c. 585 650 700 750 800 850 900 950 1000 1050 1100 1132
35% T h in Mg
N
x
10'
50 100 160 240 360 500 680 900 1180 1500 1890 2150
wt. % x 40 70 110 160 240 340 460 610 790 1010 1270 1440
1/T)
3s.
16% T h i n Mg 104 N
x
107
40 100 180 270 400 550 730 930 1170 1460 1640
wt. % x
104 N
40 80 150 230 330 460 610 780 980 1230 1380
VOL. 50, NO. 2
x
Pure Mg 107 wt. %
20 70 140 240 350 500 660 890 1160 1480 1700
x 104
20 70 140 230 340 490 650 870 1140 1450 1670
FEBRUARY 1958
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Figure 4. Solubility of uranium in magnesium and in solutions of thorium and magnesium
Figure 4 should be essentially straight lines with small discontinuous changes in slope a t the transformation temperatures for uranium. The curve for pure magnesium as drawn shows a rapid increase in slope at temperatures below 775' C., the gamma to beta allotropic transformation temperatures of uranium. However, the change in slope with decreasing temperature is far too great to be accounted for by the known heats of transformation for uranium. Data for the solution of 1670 thorium in magnesium shows the same trend but is not as pronounced as that for the pure magnesium solution ; data for the solution of 3597, thorium in magnesium can be approximated by a straight line and is more consistent with the trend expected. Accuracy of the solubility data is estimated to be &0.003% a t temperatures near the freezing points of the solutions and 10.02% at about 1000' C. However, precision is believed better for runs where sampling and analytical procedures were the same for all samples. Assuming a straight-line relationship, a least-square treatment of the pure magnesium data, neglecting the lower two points, and of the 35Y0 thorium in magnesium data, neglecting data obtained by processing in tantalum crucibles gives log A- = -(0.424 i 0.002)(104/~) -
0.689 ==! 0.017 and log N = -(0.3624 i 0.0005) X (104/T) - 1.0786 0.0044 respectively. The limits indicated represent the probable error. Extrapolation of the equation for pure magnesium to 650' C. gives the mole fraction of uranium as within (51 f. 6) X 10-7.
140
The curve in Figure 4 indicates this mole fraction to be approximately 20 X 10-7. These two values lie within the estimated accuracy of the data, However, since the trend toward lower values in uranium content at temperatures below 700" C. was also observed previously for solutions of 16% thorium in magnesium as well as for pure magnesium solutions ( 3 ) .deviation from a linear relationship at temperatures below 700' C. is probably real. Separation
Howe (6) estimates that uranium-233 concentration in thorium in some proposed reactors might build u p to slightly less than 1% before the thorium is removed and processed. This work indicates that a simple pyrometallurgical separation of the uranium from thorium should be possible. If the thorium is dissolved in enough magnesium to form a 35% solution of thorium in magnesium, the major part of the uranium would settle out a t 650' C. and could be collected in a relatively small volume. The uranium concentrated in this manner could be removed and further separated from the matrix material by heating it to above 1132' C. to form a separate liquid uranium phase containing less than 1% thorium. Thorium in the uranium could be further reduced by re-extracting with pure magnesium. A uranium-rich liquid layer can also be separated a t a much lower temperature by adding enough chromium to the charge to form uranium-chromium eutectic which separates readily as a liquid phase a t 900' C. The uraniumrich phase is essentially uranium-chromium eutectic containing less than 1% thorium. Formation of a liquid ura-
INDUSTRIAL AND ENGINEERING CHEMISTRY
nium phase at 900' C. rather than 1200' C. is a decided advantage. Containing the thorium-magnesium-uranium alloys for more than a half hour at 1200" C. in tantalum crucibles is difficult, but little trouble was encountered with uranium-chromium alloys a t 900' C. for much longer periods of time. The advantage of lowering the melting point of the uranium phase may be offset to some extent by contamination of both liquid phases with chromium. For a thorium-magnesium eutectic blanket, the uranium could be separated by simply heating the alloy above its melting point to permit the uranium to settle out. Many of the fission products probably could he separated by slagging with fused salts. When free energies of formation of alkali, alkaline earth, and rare earth chlorides, are considered, magnesium chloride slagging may remove many of these impurities from the thorium-magnesium solution. If necessary, magnesium could be separated from the thorium by distillation. Slagging or electrolytic refining could also be used in separating fission products from the uranium-rich phase. Acknowledgmeni
The authors wish to thank K. J. Gill and F. D, Ellson for their assistance ill some phases of this work. Chemical analyses were conducted by the analytical chemistry group of the Ames Laboratory under the direction of C. V. Banks. M. J. Tschetter was responsible for the majority of theanalyses. The tantalum welding was performed by Ardis Johnson undrr the direction of David T. Peterson. References (1) Carlson, 0. N., U. S. Atomic Energy Commission, AECD 3206, 8-31, 1950. (2) Chesne, A,, Regnant, P., Proc. Znteiu. Conf. Peaceful Uses of Atomic Energy 9, 585 (1955). (3) Chiotti, P., Tracy, G. A., Wilhelm, H. A., J . Metals 8, 562-7 (1956). (4) Ewing, R. E., Banks, C. V., Anal. Chem. 20, 233 (1948). (5) Gresky, A. T., Proc. Znlcrn. Conf. Peaceful Uses of Atomic Energy 9, 505-10 (1955). (6) Howe, J. P., Metal Progr. 71, 2, 97-103 (1957). (7) Nelson, C. M., Hume, D. N., G. S. Atomic Energy Commission Rept. MonC. 28, OTS, U. S. Dept. of Commerce, Washington 25, D. C . , 1951. (8) Yamamoto, A. S., Klimek, E. J., Rostoker, W., W A D C 56-411,ASTIA 118107, OTS, U. S. Dept. of Commerce, Washington 25, D. C., 1957. (9) Williamson, .4.T., Trans. Faraday Soc. 4D, 421-36 (1944). RECEIVED for review April 1, 1957 ACCEPTEDAugust 9, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technolow in the Petroleum and Chemical InduYtries, 131st Meeting, ACS, Miami, Fla., April 1957.
