I
WILLIAM
B. TARPLEY, MARVIN L. BROMBERG,
and JOHN
G. THOMAS
Aeroprojects Inc., West Chester, Pa.
UItrasonic Energy in Producing Finely Dispersed Thorium Bismuthide Nucleation and fracture of thorium bismuthide platelets have been accomplished through use of high-intensity ultrasonic equipment designed to operate under severe conditions of temperature, corrosion, and radioactivity. The intermetallic compound particles so produced are of a size to permit rapid extraction of bred products and minimize slurry pumping difficulties.
A axed
of essentially insoluble equithorium bismuthide particles (ThBiJ in bismuth has been proposed as a breeder blanket for the liquid metal fuel reactor (LMFR). As the slurry is circulated around the reactor core (79), uranium and protoactinium are bred primarily in the solid thorium bismuthide. To recover the bred products i t is proposed to transfer them to the liquid bismuth phase for subsequent extraction (77, 78). At least two methods are currently considered for this transfer, both improved through the use of ultrasonic energy. I n the first method, the bred 5 weight slurry is completely dissolved by a temperature increase to above 350' C. (78); cooling to 450" C. precipitates more than 99% of the thorium bismuthide as large platelets, the bred products being retained in the liquid bismuth phase. The precipitate is filtered, mixed with liquid bismuth, and fed back into the breeder loop. The size of the thorium bismuthide platelets varies with thorium content and rate of cooling. Cooling 5 to 10 weight % ' thorium alloys at a moderate rate of 30" C. per minute produces platelets u p to 1 cm. in diameter; cooling rates as high as 100" C. per second are necessary to precipitate platelets less than 50 microns in diameter. I t is anticipated that large platelets when fed back into the process loop will increase pumping loads and cause plugging (6). The second method involves thermal pursing to speed equilibration and bred product removal from the slurry ( 5 ) . If the slurry is heated to 875' C., a significant percentage of the solids will dissolve and upon cooling reprecipitate esSLURRY
sentially uranium-free thorium bismuthide on existing equiaxed crystals. Repetition for several cycles produces adequate transfer to the liquid phase but eventually leads to formation of large thorium bismuthide platelets and the process must be supplemented by a treatment to reduce particle size. The use of vibratory energy to initiate formation of small particles or to fracture the preformed platelets appeared advantageous, as it would permit transfer of the bred products by an isothermal treatment akin to leaching. Bulk nucleation by ultrasonic application has been repeatedly demonstrated on a laboratory basis in grain refinement of aluminum alloys (3, 9, 7 7 , 75) and other metallic systems (7, 70, 73, 7 4 , as well as in aqueous precipitation of silver halides ( 8 ) and calcium oxalate ( 7 ) . Effective ultrasonic size reduction of agglomerates in aqueous systems ( 8 ) ,of uranium dioxide in sodium-potassium alloy ( 4 ) ,and of dis-
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An ultrasonic resistance-heated, controlled-atmosphere furnace was used to treat slurries
INDUSTRIAL AND ENGINEERING CHEMISTRY
A. B. C. D.
E.
F. G. H.
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K. I.
Resistance furnoce Furnace tube Molten metal container Thermocouple junction location Ultrasonic coupler Cooling water inlets and outlets Thermocouple gland and port Vacuum and back-fill inlet Bellows assembly Force-insensitive mount Magnetostrictive transducer
Furnace and tube are withdrawn to show treatment vessel
persed phases in molten metal systems including zinc, magnesium, aluminum, and beryllium (2, 9, 70, 72, 73) has been reported. These are a few of the many experimental efforts to apply ultrasonic energy to molten metal systems, carried out generally on a laboratory scale. Full scale application in continuous processing in the LMFR blanket system depends on integration of furnace design with appropriate means for introducing substantial amounts of vibratory energy into a melt under vacuum or in controlled atmosphere. The development of hermetically sealable pressure- and temperature-insensitive mounts for ultrasonic systems, with experimental background in the design of transducer-couplings for use in unusual environments (9, YO), has indicated the possibility of largescale controlled-atmosphere treatment. Critical appraisal a t Aeroprojects of the ultrasonic, metallurgical, and chemical engineering requirements indicated sufficient promise, and a program was undertaken to investigate prevention of thorium bismuthide platelet formation during cooling from solution temperature to 450' C.; fracture of preformed thorium bismuthide platelets at 400' to 500' C. to reduce particle size below 50 to 100 microns; and reduction in size of equiaxed thorium bismuthide crystals by isothermal treatment at 450" to 500' C.
Experimental
Ultrasonic High-Temperature, Controlled-Atmosphere Furnace. T h e high chemical reactivity of thorium bismuthide and the necessity for operating at temperatures up to about 1050" C. dictated the construction of
a high-power ultrasonic furnace array in which the ultrasonically active zone could be isolated from the atmosphere. Vibratory energy was introduced into the molten metal container through a transducer-coupling system and an allmetal mount, which permitted delivery of high-intensity vibration through the controlled-atmosphere furnace tube wall without significant energy loss. The attached metal bellows permitted precise positioning of the assembly within the furnace tube. The melt container was a recess l 3'4 inches in diameter and 1 inch deep hollowed out of the free end of the Type 410 stainless steel coupler bar, selected because of its low nickel content. T h e magnetostrictive nickel transducer was operated at a nominal frequency of 20 kc. per second and driven by a 2-kw. electronic generator from which the transducer would accept approximately 1450 watts. All power measurements were made in terms of electrical watts delivered to the transducer. A flanged ceramic or stainless steel furnace tube could be placed over the coupler assembly and sealed to the bellows flange ring, using an appropriate watercooled gasket. Provisions were incorporated for vacuum and inert-gas lines, and temperature was controlled with a Chromel-Alumel thermocouple and controller. Heat was supplied to the upper end of the assembly by an electric resistance furnace insulated \vith refractory brick. A rack and pinion permitted the furnace to be positioned around the treatment array. Experimental Materials. The thorium was electrolytic material supplied by Brookhaven National Laboratory. The bismuth was obtained as C.P. lump material. Argon for inert blanketing of the melts was used directly- from cylinders without purification. Except for a master dispersion of equiaxed thorium bismuthide crystals prepared by the technique of Bryner and others ( 6 ) , each melt consisted of 158 grams of a mixture of thorium and bismuth containing 570 thorium by weight. Control melts were prepared by heating the mixture to approximately 1050" C. for complete solution, soaking for 1 hour, then cooling to solidification of the bismuth in approximately 1 hour to permit precipitation of large thorium bismuthide platelets. Test Procedures. T o explore the effect of ultrasonic application on bulk nucleation, the charge of thorium and bismuth was placed in the molten metal container of the furnace array and the system evacuated and heated. At a temperature between 400" and 500' C., the system was back-filled with argon and retained under a slight positive argon pressure for the remainder of the run. After a 1-hour soak at 1050' C., heating was stopped and ultrasonic energy applied in pulses of 30 seconds on and 30
seconds off for a total of approximately 1 hour. Experiments to investigate the ultrasonic fracture of preformed platelets were initiated -by soaking the thoriumbismuth mixture at 1050" C. for 1 hour as described above and allowing the system to cool over approximately 1 hour to 450' or 500" C. The temperature was maintained a t this level while ultrasonic energy was applied in pulses of 30 seconds on and 30 seconds off for varying lengths of time. To determine the ultrasonic effect on equiaxed thorium bismuthide crystals, the requisite amount of the equiaxed master melt was heated to approximately 500" C. in the ultrasonic treatment vessel
under a n argon atmosphere, and pulsed ultrasonic energy was applied for varying lengths of time. I n all tests, as soon as ultrasonic trealment had been discontinued, the molten charge was chill-cast into a graphite mold approximately 1 inch in inside diameter and 1 inch deep. The solidified ingots were sectioned axially and were ground and polished by the usual techniques. Exposure to air, resulting in oxidation and darkening of the thorium bismuthide particles, produced adequate contrast for photography. Each specimen was examined microscopically, representative areas were photographed, and average particle sizes were estimated. No effort was made to
Figure 1. Ultrasonics have a marked effect on bulk nucleation o f thorium bismuthide ( 2 8 X ) left. Control, cooled normally from 1 0 5 0 " C. Platelets have o maior dimension up to 1 cm. Right. Ultrasonically treated during cooling from 1050' C. Dispersion i s uniform and particle size greatly reduced
Figure 2. Thorium bismuthide ultrasonically treated during cooling from approximately
1050" C. (500x1 Rounded edges suggest bulk nucleation rather than particle fracture
Figure 3.
470" C.
Ultrasonic breakup of preformed thorium bismuthide platelets a t Irregular shapes indicate breakup of larger particles
Increased treatment time evidently improves the action. left. 1 0-minute ultrasonic treatment. Middle. 15-minute ultrasonic treatment. Right. 20-minute ultrasonic treatment
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establish particle-size distributions, as the primary intent was to determine whether size reduction had been produced by the ultrasonic treatment. Results and Discussion
Prevention of Platelet Formation. Figure 1 illustrates the effect of vibratory treatment applied during cooling from solution temperature. After treatment with pulsed ultrasonic energy a t 1000 watts during cooling from 1050” to 500” C., the largest particles were estimated to have a maximum dimension of approximately 100 microns, and most of the particles are no larger than 25 to 50 microns. The ultrasonic effect on particle size reduction is apparent. Figure 2, a t higher magnification (500X), reveals the regular shape of the small particles, suggesting a bulk nucleation effect rather than fracture of large particles. Fracture of Preformed Platelets. Figure 3 illustrates a typical thoriumbismuth mixture subjected to a timetemperature treatment for the precipitation of large platelets and then treated isothermally at 470” C. a t an ultrasonic power level of 1450 watts for varying periods. At a magnification of 15OX, these specimens show significant platelet fracture with only 10 minutes of ultrasonic treatment. The progressive size reduction, with treatment time increased to 15 and 20 minutes, is evident. These fractured particles have a n irregular shape, in contrast to the uniform shape of the particles obtained with ultrasonic treatment during cooling from solution temperature. Size Reduction of Equiaxed Crystals. Figure 4 (15OX) contrasts the master melt containing equiaxed particles with a portion of this melt ultrasonically treated for 5 minutes a t 1200 watts and 450” C. The control melt shows agglomerates of small thorium bismuthide particles as well as larger equiaxed particles. The short ultrasonic treatment dispersed many of the agglomerates and produced a melt
with more clearly defined equiaxed particles and fewer large agglomerates. Ultrasonic Coupler Erosion. Certain construction materials that are normally unaffected or only mildly attacked by molten metals are rapidly eroded under the same ambient conditions when activated by ultrasonic energy (9, 73, 76). Selection of suitable refractory metals can alleviate such problems. Examination of the ultrasonically treated melts exhibited inclusions suggestive of such erosion from the ultrasonically activated stainless steel coupler face in contact with the liquid bismuth. The appearance of these inclusions was sporadic, and the coupler itself exhibited little evidence of pitting or erosion after several hours of intermittent ultrasonic exposure. It is therefore believed that the particles were probably introduced during ultrasonic treatment or formed during air exposure in the chill-casting procedure. Conclusions
When applied to liquid bismuth-5yo thorium mixtures during cooling from complete solution temperature to 450’ or 500’ C., ultrasonic vibratory energy suppressed the normal platelet precipitation of thorium bismuthide and produced much smaller, more nearly equiaxed particles generally less than 50 microns in diameter. When applied to melts containing large platelets formed by normal cooling from solution temperature, ultrasonic treatment at 450’ to 500’ C. fractured the platelets into generally equiaxed particles smaller than 100 microns; increased treatment time appeared to increase the effectiveness of particle breakUP. Ultrasonic energy applied to slurries containing equiaxed thorium bismuthide particles disrupted particle agglomerates, produced a more uniform dispersion, and may have effected a minor reduction in particle size. The ultrasonic melts contained evidence of contamination from the Type 410 stainless steel coupler, but the
mechanism of the contamination was not established in this study. This investigation was exploratory, and insufficient data are available for valid extrapolation of the results to estimate power requirements or equipment scale-up for plant operation. Acknowledgment
The authors acknowledge the interest of R. J. Teitel, Dow Chemical Co., D. H. Gurinsky and J. S. Bryner, Brookhaven National Laboratory, who were responsible for initiating the project, and thank Florence R. Meyer, Aeroprojects, for assistance in preparing the manuscript. literature Cited
(1) Aeroprojects Inc., “Applications of Ultrasonic Energy,” NYO-7919 (August 1957). (2) Aeroprojects Inc., “Ultrasonics Applied to Solidification and Solid-state Transformation,” Air Force Tech. Rept. 6675 (1 \ - 9511 _ - -
(3) Bradfield, G., Proc. Phys. S O ~(London) . 63, 305-22 (1950). 14) Brombere. M. L.. Tarulev. W. B.. “Feasibility’Study o n Ultrasonic Disper: sion of Uranium Dioxide in Molten Metals,” NYO-7929 (May 1958). (5) Bryner, J. S., private communication. (6) Bryner, J. S., Teitel, R. J., Brodsky, M. B., U. S. Atomic Energy Commission, TID-7526 (Pt. 11,103-14 (February 1957). (7) Govorkov, V. M., Shabalin, K. N., Zhur. Tekh. Fiz. 24, 41-9 (1954). (8) Jones, J. B., Tarpley, W. B., Herold, R. C., “Photoemulsion Manufacture Using Ultrasonics,” .4eroprojects Inc., Research Rept. 55-68 (1955). (9) Jones, J. B., Thomas, J. G., DePrisco, C. F., “Effect of Ultrasonics on Molten Metals,” WADC Tech. Rept. 54-490 (1954). (10) Maropis, N., Jones, J. B., “Investigation of Ultrasonic Grain Refinement in Beryllium,” NYO-7788 (1956). (11) Rostoker, W., Berger, M. J., Foundry 81,100-5, 260-5 (1953). (12) Schmid, G., Ehret, L., 2. Elektrochem. 43, 869-74 (1937). (13) Schmid, G., Roll, A., Ibid., 46, 653-7 (1940). (14) Seemann, H. J., Staats, H., Metal/. 9, 868-77 (1955). (15) Southgate, P. D., J . Metals 9, 514-17 11957’1. (16) Strachan, J . F., Harris, N. L., J . Inst. Metals 85, 17-24 (1956). (17) Teitel, R. J., J . Metals 9, 131-6 (1957). (18) Wilhelm, H. A , , Rogers, B. A., “Nuclear Metallurev.” Am. Inst. Mining Met. Engrs. I M D Spec. Rept. Series NO. 1, 39-63 (1955). (19) Williams, C., Miles, F. T., Nucleonics 12, 11-13 (July 1954). ,
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RECEIVED for review May 6, 1958 ACCEPTED September 11, 1958
Figure 4. Ultrasonic treatment during low-temperature exfoliation reaction between thorium and bismuth to produce equiaxed thorium bismuthide dispersed many agglomerates Left. Control Right. Ultrasonically treated 5 minutes at 450’ C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Division of Industrial and Engineering Chemistry, Nuclear Technology Section, Symposium on Chemistry and Reprccessing of Circulating Xuclear Reactor Fuels, 133rd Meeting, ACS, San Francisco, Calif, April 1958. Project sponsored by Reactor Engineering Division, Atomic Energy Commission, under Contract AT(30-1’1836.
