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DOUGLAS 0. RALEIGH Atomics International, Division of North American Aviation, Inc., Canoga Park, Calif.
Prepare Thorium-Aluminum Alloys +
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Direct Reduction Breeder reactor fuel can be prepared more economically by reducing thoria with aluminum. Sodium metal could be a valuable by-product
CURRENT
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INTEREST in breeder reactors employing the thorium-uranium-233 conversion has led to the consideration of appropriate alloy systems for use as breeder blankets and fuel materials in such reactors. The suitable characteristics of the aluminum-thorium-uranium system have been pointed out by Bobeck and Wilhelm (7). Alloys in this system can, of course, be prepared by comelting their metallic components. Metallic uranium and thorium, however, are considerably more expensive than their common compounds, so it would be of great advantage to prepare the alloys by aluminum reduction of such compounds. The aluminum reduction of UO2 has been reported (74). I t was of interest to see if the appropriate conditions could be achieved for the efficient aluminum reduction of thoria. A number of early workers (4,6,9, 70) tried to reduce ThOz with aluminum, all with indifferent success. The general result was a particulate product of questionable composition, and low yields were generally involved. Modern reviewers (2) have concluded that good reduction is not possible because of unfavorable thermodynamics. However, in the course of some fluoride drossing experiments in this laboratory, aluminum was found to reduce ThFa with nearquantitative yields, a result reported earlier by Leber (8). Since the thermodynamics of this reduction were also believed to be unfavorable, the whole situation was felt to have been inadequately studied, and some possibility was seen for an efficient T h o 2 reduction as well. Preliminary trials with ThOz-fluoride melts in contact with molten aluminum showed considerable promise. An experimental program was set up to determine the optimum conditions of the reduction and investigate its thermodynamics. As a result, a process was developed for preparing thorium-aluminum alloys by direct T h o z reduction into an aluminum melt. Near-quantitative reduction yields can be obtained in preparing alloys up to at least 20% (weight) thorium. Alloys up to 40% (weight) thorium can be prepared with a cryolite
salt bath, if some loss in reduction yield can be tolerated. The optimum reduction conditions were with 25y0 (weight) T h o z and 75% (weight) cryolite as the initial salt phase, using purest grade cryolite. Under these conditions 9770 reduction occurs in 0.5 hour at 1050 C. The reaction rate is independent of the thoria particle size, at least below 2 microns, but is apparently increased by some inductive stirring. The predominant driving force for the reduction is the considerable free energy of formation of ThA13. The sodium metal produced may be a useful by-product. Economics and feasibility of scale-up are promising. General Features
of the Reduction
From Glassner's free energy tables ( 3 ) , the standard free energy of the reaction ThOz(s) 4/3 Al(1) 2/3 A1203(s) Th(s) (1) is highly endoenergic, being +38 kcal. at 1050' C. Standard state thermodynamics, however, can be quite misleading. If ThOz is present in a fluoride melt and an excess of aluminum is used, the actual reaction to consider is ThOs(fluoride soln.) 4/3 Al(A1-Th soln.) + 2/3 AlzO&fluoridesoln.) Th(A1-Th s o h ) ( 2 ) I n this case, the activities of the oxides and of aluminum are lowered by their presence in a solution. Further, the activity of thorium may be lowered considerably by the formation of aluminides and their presence in a dilute solution of aluminum. The lowered thorium activity was, in fact, found to be the major driving force for the observed reduction.
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A number of preliminary experiments established the general features of the reduction. When a fluoride mix containing more than enough T h o 2 to saturate it is melted above aluminum and brought to 1050' C., two liquid phase regions result-a salt layer and a metal phase layer. Some of the T h o 2 dissolves in the salt mix, but most of it settles to the phase interface. The molten metal phase does not wet the thoria, so it remains at the interface. The situation provides a short reaction path and, if enough aluminum is present, all of the settled T h o 2 reacts with it quite rapidly. A slower reduction step follows, in which the salt phase is depleted of dissolved ThOz. The extent of reduction in a given period of time depends on the salt mix composition. The setup of an experimental program to determine the optimum reduction conditions involved a good deal of arbitrariness in selecting the nature and sequence of the variables to be studied. Runs were made using constant quantities of T h o 2 and aluminum under various conditions of interest, and ThOz reduction yield (percentage ThOz reduced) was determined in each case. The conditions of interest were: salt bath composition, weight ratio of ThOz to salt bath, reusability of salt bath, and the effects of various experimental conditions on the yield. Enough ThOz and aluminum were used to result in an aluminum-20~o (weight) thorium alloy if 100% reduction occurred. An operating temperature of 1050' C. was selected to exceed the melting points of all salt bath compositions of interest. Fluoride mixes were selected for the salt baths because of their solu-
Energetics for metallothermic reductions are considerably improved if the product metal forms a stable intermetallic compound with an excess of the reactant. The nominally unfavorable reduction of Tho2 by aluminum can be carried out with high yields by taking advantage of this phenomenon
VOL. 53, NO. 6
JUNE 1961
445
ARGON OUTLET, TO BUfifiLER ' 1
O-RING
BRASS FLANGES BAFFLE P L A T E S
High-temperature reduction of Tho2 b y aluminum from fused fluoride melts was studied in this experimental assembly
COOLING COILS
QUARTZ ROD THERMOCOUPLE
u
v*-
BRASS SPlD PEDESTAL
BRASS FLAIYGES
ARGON I N L E T
HERMETICALLY SEALED TERMINALS
bility for metallic oxides. Following these runs, a series was carried out to determine the attainable thorium buildup in the metal phase; a number of runs were made to examine the mechanism and thermodynamics of the process. Experimental
Equipment. All of the experimental runs were carried out in a vacuum furnace with provision for argon flow through the system (above). Heating was provided by an induction coil connected to a 2.5 kv.-amp. Lepel induction unit. All runs were made in 1-inch outer diameter, 2-inch-high cylindrical Graphitite (lampblack - impregnated graphite) crucibles to minimize high temperature evaporation loss.
-Economics
Temperature was measured with a nickel-sheathed Chromel-Alumel thermocouple set into the crucible as shown and connected, through sealed copper leads, to a Type 8662 Leeds and Xorthrup millivolt indicator. The thermocouple sheath was carefully grounded to insulate the thermocouple from induction currents. The temperature measured with this setup checked that of a test aluminum slug in the crucible, as measured with a calibrated optical pyrometer, to within several degrees a t 1050' C. The temperature was first controlled manually on the Lepel unit and, in later runs, by a Wheelco Capacitrol controller, generally to 1 5 ' C. The argon used was purified by passing it over uranium turnings a t 500" C. The vacuum line was connected to a
and Scale-up
Based on current price quotations for 100-pound lots, reactor grade thorium is $25 per pound; a comparable grade of Tho2 is $7.00 per pound. Thus, the advantage o f Tho2 reduction over comelting of the alloy components i s obvious. Thorium fluoride reduction from a fluoride mix has been used for preparing thorium-aluminum alloys, but the purest grade of ThF4 costs $7.30 per pound of contained thorium. The comparable grade of ThOz i s $6.70 per pound of thorium. From the current price of cryolite and the optimum Tho2 to cryolite ratio, the cost of the Tho2 plus cryolite comes to less than that of the ThF4 alone. The nature of the Thoz reduction, as described, appears to lend itself well to scale-up. Since the bulk of the material to be reduced i s found at the phase interface, process scale-up does not introduce any greatly enlarged effective reaction paths, so reduction should occur in a comparable length of time. Reduction yields on scale-up may well be much better than 9770 with lower recovery losses. Further, since UOa is reducible from cryolite baths with aluminum ( 1 4 ) , the processes could be combined, if desired, to produce thorium-uranium-aluminum alloys of a wide range of composition
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Kinney KC-5 vacuum pump, and the vacuum was measured with a standard thermocouple gage. The furnace tube top was equipped with a bame and filter setup to entrain reaction fumes. Procedure. The T h o z and fluoride mix were weighed, thoroughly mixed in the crucible, and set in the furnace. The mix was outgassed under vacuum to 5 microns of mercury-at room temperature and then a t gradually increasing temperatures u p to 700' C. When degassing was completed, argon was admitted to 1 atm. pressure, a slight argon flow was maintained, and the mix was heated to and held a t 1050" C. for 13 minutes and cooled. -4solid salt plug resulted, which was then placed on top of the aluminum. 'The system was degassed as before, argon was admitted to 1 atm. pressure, and the system was brought to and held a t the run temperature for the required amount of time. The procedure of prefusing the salt phase was done to help reproduce the initial conditions and prevent any oxidation of the aluminum by material outgassing from the salt mix. I t probably need not be done in routine alloy preparations. The T h o 2 reduction yield was determined from the thorium content of the metal phase. This was first determined by chemical analysis but later by measuring the ingot density and relating it to a density-composition graph derived from chemical and density measurements on a number of ingots. Densities were measured on d Becker specific gravity balance. The alloy density was within experimental error, a linear function of the atomic percentage of thorium u p to 4070 (weight) thorium, obeying the equation: d = 0.1648 atom
70 T h
+ 2.69
Res uI ts Optimization Runs. The runs in this series used 1.15 grams of T h o , in the fluoride mix and 4.46 grams of aluminum as the metal phase. The first runs were to determine the fluoride composition for
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C " i 3 - TF THOP A
t LIG
IT ?A'
LO
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Figure 1. Thos reduction yield declines sharply with small amounts of cryolite in the initial salt mix and decreases less rapidly with large excesses
THORIUM-ALUMINUM A L L O Y S
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the greatest reduction yield in the shortest time. T h e fluorides considered were LiF, LiF-AlF, mixes, cryolite (Na3AlF6),and cryolite-NaF eutectic. A series of runs with LiF and LiF-AlFs mixes for 4 hours a t 1050' C. gave a maximum reduction yield of 82% with LiF-15 mole 7 0 AlFs (the eutectic composition). Runs under the same conditions with cryolite and cryoliteNaF eutectic both gave yields of 91%. Four runs with a reduced amount of new, purer grade cryolite gave 97 =t 1% T h o 2 reduction in 0.5 hour. Two comparable runs with cryolite-NaF eutectic, using the same grade of cryolite, gave 73 =t 2y0 yield in the same time. I t was concluded that cryolite was the most promising fluoride melt. Since cryolite is one of the least expensive inorganic fluorides, no further work was carried out on fluoride compositions. To determine the optimum cryolite to T h o 2 ratio, batches of T h o 2 and aluminum were contacted at 1050' C. for 0.5 hour with various amounts of cryolite. The optimum ratio found was about 3.0 (Figure l ) , with a sharp dropoff at lower cryolite contents and a more gradual one with large amounts of cryolite. The latter is probably due to the fact that, with larger volumes, a greater fraction of the ThOz will dissolve in the cryolite rather than settle to the phase interface. Reduction of T h o 2 a t the interface is expected to be rapid, compared with subsequent reduction of the dissolved ThOz, because of its proximity to the aluminum and its greater activity. Hence, over-all yields in a given time would be lowered when large amounts of cryolite are used. T h e sharp drop-off with small amounts of cryolite appears to be related to the formation of dense, semisolid salt phase slurries because of the large excess of solid oxide. This was indicated in the appearance of the recovered salt phase and the fact that its A1203 concentration was actually lower than in higher yield runs with more cryolite. Apparently the optimum ratio is with enough molten cryolite to wet the ThOz and form a slight layer of supernatant. The reduction yield with cryoliteN a F eutectic was 73y0 for an 0.5 hour run at 1050' C., so it was of interest to see if this reduction could be hastened by using ThOz of a smaller particle size. The average particle sizes of the T h o 2 used in all the above runs and of a new batch were found to be 2.10 and 1.40 microns, respectively. Although the surface to volume ratio of the new ThOz was about 2.25 times greater, no difference in reduction yield was noted. I t was concluded that reduction rate is not controlled by rate of solution of the solid ThOz at the interface, at least with particle size below 2 microns.
These runs show that reductions in both cryolite and NaF-cryolite eutectic will go to near-completion, but that reduction is faster in cryolite. While this makes the cryolite reduction of greater practical interest, it also makes it more difficult to study in terms of the effects of various conditions on the rate. The slower reduction in eutectic serves as a more sensitive indicator of favorable conditions, so it was carried out with various changes of setup. I n general, those changes of conditions that lowered the degree of inductive stirring of the metal phase (which was present to some extent in all runs) lowered the reduction yield. This should be true in cryolite as well and would show u p in largerscale reductions. Salt-Phase Recovery. A number of runs were carried out to see if the salt phase from a reduction run under the established optimum conditions could be recovered and used in a subsequent run. Some reusability was indicated, but "mileage" or reusability of a salt bath is distinctly dependent on required thorium content in the alloy being prepared. Thorium Build-Up. A group of runs was carried out to determine the achievable thorium build-up in the metal phase. A ThOz reduction run carried out under the above-determined optimum reduction conditions (1.15 grams of ThOz; 3.0 cryolite to T h o 2 ratio; 4.46 grams of aluminum: 1050' C.; 0.5 hour; best grade cryolite) gave a 97% reduction yield. The metal phase ingot from this run was recovered and used as the initial metal phase in a second run with the same amounts of T h o 2 and cryolite. The process was repeated for a third and fourth run. The results of this series of runs are shown in Figures 2 and 3. Figure 2 shows the reduction yield as a function of initial thorium content in the metal phase. Figure 3 shows ingot thorium content as a function of the number of
0
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10 20 30 475 I N I T I A L WEIGHT PERCENT Th IN INGOT
throughputs-Le., the number of successive runs made on the ingot. The series shows a gradual decrease in reduction yield and rate of thorium build-up, with an apparent limit a t about 40% (weight) thorium. T o see if the thorium content could be further increased by another run with increases in reaction time, temperature, and amount of salt phase, the 4Oy0 (weight) thorium ingot was contacted with doubled quantities of cryolite and ThOz for 1 hour a t 1150' C . No significant T h o z reduction was noted. T h e 40% (weight) thorium is a t or near the practical limit of thorium build-up. By-product Sodium. An interesting side reaction was observed in all runs where cryolite or cryolite-NaF eutectic was the salt mix used. Starting at the onset of the operating temperature, fumes of metallic sodium were evolved throughout the run. From runs in which the ThOz was omitted, it was shown that the reactions involved were the reduction of cryolite and NaF by aluminum to release gaseous sodium. T h e reaction with cryolite is : A1 NaaAIFe + 3Na 2AlF3
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From recent calorimetric data on the reaction components (5, 7, 72), the standard free energy of this reaction a t 1050' C. was calculated and found to be +56 kcal., indicating a grossly disfavored process. T h e standard state of the sodium gas, however, is 1 atm. pressure, which need not be its pressure in the above reaction. The much lower pressure a t which the sodium can be evolved is limited only by its solubilities in the reacting phases, which are quite low. Sodium would be evolved spontaneously at pressures up to 1 mm. of Hg. Similar considerations apply to : A1 6NaF --t 3Na Na3AlFe
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in pure NaF, except that it is much less disfavored. For this reaction, a standard free energy of +0.9 kcal. a t 1050' C ,
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50
Figwe 2. Percentage of Tho2 reduced depends on initial thorium content in the aluminum-thorium metal phase
N U M B E R OF THROUGHPUTS
Figure 3. Thorium build-up rate in albminum ingot subjected to successive reductions decreases with increasing ingot thorium content VOL. 53, NO. 6
JUNE 1961
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was calculated, indicating a nearly spontaneous process for the evolution of sodium a t 1 atm. pressure. Of importance in this case, however, is that the free energy of solution of the product cryolite in the NaF adds several extra kilocalories to the standard state value, so that the actual reaction is spontaneous a t greater than 1 atm. of sodium pressure. This was confirmed in a run with aluminum and pure NaF, where the sodium was evolved with vigorous boiling. In view of the above reactions, especially in cr)-olite, sodium may he an economic by-product in the aluminum reduction of T h o 2 . Several runs were also carried out to investigate the mechanism and thermodynamics of ThOz reduction by aluminum. A detailed account of this work appears elsewhere (73), so it is not reported here. T h e results are cited as needed in the following discussion.
Discussion I t has been shown experimentally that the reaction in Equation 2 is spontaneous for producing Th-A1 alloys of considerable thorium content, despite the fact that the corresponding standard state reaction (Equation 1) above is energetically disfavored by 38 kcal. at the temperature of interest. To be sure, the reaction components in the second process are diluted, but in the present work the energetics of ideal dilution could account for a free energy difference between the two reactions of no more than 1 or 2 kcal. One suspects, then, that one of the reaction products in Equation 2 is involved in compound formation with its solvent. I t was shown that the product in question is thorium, by experimentally achieving the following : Th02(s)
+ 4/3 A1 (Al-Th s o h ) + 2/3 A1203(s)+ T h (-41-Ths o h )
This was done in a aeries of runs where molten thorium-aluminum alloys were equilibrated at 1050’ C. Ivith a saturated solution of T h o 2 and A1203 in fused LiF that contained a solid excess of both oxides. Under these conditions, each dissolved oxide is in equilibrium with pure solid oxide, so both oxides are in their standard states. Thus, any significant thorium content in the metal phase must be attributed to a considerably reduced activity of the dissolved thorium. Since the above runs showed an equilibrium metal phase composition corresponding to aluminum-28% (weight) thorium, it was concluded that the dissolved thorium was involved in compound formation or strong association with the aluminum. Thorium and aluminum are known to form several intermetallic compounds. T h e compound of interest here is the most aluminum-rich compound at 1050’ C.. since the difference in activities of
448
dissolved and pure solid thorium can be considered in terms of the virtual formation of this compound and its subsequent solution in aluminum to form alurninum-28Yo (weight) thorium. The details of this treatment ( 7 3 ) show that only a small part of the free energy difference between the reactions in Equations l and 2 can be explained by dilution processes and that the bulk of the 38-kcal. difference must be attributed to the free energy of formation of the intermetallic compound involved. Murray ( 7 7 ) and the present author have found that the compound of interest is ThAl3. From the present treatment, its estimated standard free energy of formation a t 1050’ C. is about 36 kcal. I t is believed that similar situations may exist in a number of other conceivable metallothermic reductions, where use of an excess of the reductant metal provides more favorable energetics, not only by diluting the product metal but also by forming with it a fairly stable intermetallic compound. I t is indicated, then, that the high free energy of formation of ThAl3 provides the bulk of the driving force required for reaction. However, the thorium build-up in the equilibrium runs was 28% (weight) thorium, while build-ups of u p to 407, (weight) thorium xvere achieved in other reduction runs. The difference was that, in the equilibrium runs, the salt phase was continuously saturated with the product A1203, while in the other runs saturation never occurred. Thus, as would be expected, the lesser activity of the A1203, when it is present in a subsaturated solution, allows the reduction to proceed further and leads to higher thorium build-ups in the metal phase. Similarly, it would be advantageous to operate as much as possible with a salt phase that is saturated or nearly saturated with ThOz. The requirement for the salt phase, then, is that it have a good solubility for A1203 and a poor solubility for ThOz, especially if high thorium content allovs are desired. While no solubility data exist for ThOn in cryolite and the data on alumina show wide variations, other work of the author indicates that A1203 is at least several times more soluble in cryolite than T h o 2 . From this and the empirical fact that good reductions occur in cryolite, it is assumed that molten cryolite functions satisfactorily in this respect. Another function of the salt phase is to provide favorable reaction kinetics. The presence of the reactant and product oxides in a liquid medium facilitates transport of the reacting components. Runs with practically any salt mix composition will, in theory, produce alloys u p to 28y0 (weight) thorium, but cryolite was experimentally found to provide the most favorable kinetics of the group of compositions tried. Comparable re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
duction rates with other salt mixes could probably be achieved if the melt were stirred, but this is undesirable. As expected, both the reusability of the salt bath and the required purity of the cryolite to be used depend on the thorium content of the alloy to be produced. TYhen alloys of relatively high thorium content are to be prepared, the permissible A L 0 3 content in the initial salt mix is more severely restricted. This imposes limitations on both the salt mix reusability and the purity of the cryolite, since cryolite usually contains A1203 as a n impurity. As stated, the practical limit of thorium build-up in the alloy is about 40% (weight) thorium. Enough work was done to make some general observations as to the mechanism of the reduction process. The rate of reaction was found to depend on the degree of inductive stirring in the melt and the composition of the fluoride mix, but not on the T h o 2 particle size. I n a run that was stopped after partial reduction, x-ray diffraction analysis showed the ThO2-cryolite salt phase to contain ThF4. I t is thus indicated that the T h o 2 is a t least partially ionized in the melt. The observed dependence of the rate indicates that the slow steps are probably diffusion of the reaction products away from the interface, rather than the rate of solution of the thoria.
literature Cited (1) Bobeck, G. E., LVilhelm, H. A , , U. S. Atomic Enerqy Commission Rept. ISC832,1956. (2) Dean, 0. C., “Advances in Nuclear Engineering,” Vol. I, pp. 66-73, Pergamon Press, New York, 1957. (3) Glassner, A , , U. S. Atomic Energy Commission Rept. ANL-5750, 1957. (4) Goldschmidt, H., Vautin, C., J . Soc. Chem. Ind. (London) 17, 543 (1898). (5) Gross, P., Hayman, C., Levi, D. L.,
2nd Intl. Symp. Phys. Chem. Extractive Metallurgy, Pittsburgh, Pa., 1958. (6) Honigschmidt, O., Monatsh. 27, 205 (1906); Compt. rend. 142, 157 (1906). (7) King, E. G., J . Am. Chern. Soc. 79, 2056 (1957). (8) Leber, A., Z. anorg. u. allgern. Chem. 166, 16 (1927). (9) Matignon, C. A., Delepine, M., Comfli. rend. 131, 838 (1900); 132, 37 (1901); Ann. chim. _ phys. . [SI 10, 136 (1907). (10) Moissan, H., Compt. rmd. 122, 573 (1896); Ann. chim. phys. [7] 12, 427 (1897). (11) Murray, J. R . , J . Inst. Meials 87, 349 (1959). (12) O’Brien, C. J., Kelley, K . K., J . Am. Chem. Sot. 79, 5616 (1957). (13) Raleigh, D. O., U. S. Atomic Energy Cornmission Rept. NAA-SR-5689, 1960. (14) Saller, H. A.: Ibzd., TID-2501, 1948 (Del.). RECEIVED for review Xovember 14, 1960 ACCEPTED March 16, 1961
Division of Industrial and Engineering Chemistry, 138th Meeting, ACS, New York, September 1960. Work conducted for Atomic Energy Commission under Contract AT(11-1)-GEN-8.
