( 6 ) Holland, L., “The Properties of Glass Surfaces,” pp. 128-38, Wiley, New York, 1964. (7) Lagally, P. (to Brockway Glass Co.), U. S. Patent 3,801,920 (Oct. 29. 1963). ( E ) Lagally, P., T a f f i 39, 747 (1956). ( 9 ) McGregor, R. R., “Silicones and Their Uses,” p. 260, McGraw-Hill, New York, 1954. ( 1 0 ) Platzer, N., Mod. Plastics 28, 95 (1951). ( 1 1 ) Poole, J. P., Snyder, H. C., Glass Znd. 44, 175 ( 1 962). ( 1 2 ) Schafer, H., Z . Anorg. Chem. 247, 96 (1941). ( 1 3 ) Sharf, J. M. (to Armstrong Cork Co.), U. S. Patent 2,965,596 (Dec. 20, 1960). ( 1 4 ) Sharf, J. M., Mod. Packaging 32, 113 (February 1959).
( 1 5 ) Sommer, L. H., Bailey, D. L., Whitmore, F. C., J . Am. Chem. SOC. 70, 2869 (1948). ( 1 6 ) Sprung, M. M., Aduan. Polvmer Sci. 2, 442 (1961). ( 1 7 ) Wagner, G. H., Bailey, D. L., Pines, A. N., Dunham, M. L., McIntire,-D. B., Znd. Eng. Chem. 45, 367 (1953). (18) Wagner-Jauregg, T., Ber. 63, 3213 (1930). (19) Whitmore, F. C., Sommer, L. H., Gold, J., Van Strien, R.E., J . Am. ChemSoc. 69,1551 (1947).
RECEIVED for review November 1, 1965 ACCEPTEDJune 7, 1966 Work sponsored by the Brockway Glass Co., Brockway, Pa.
PREPARATION OF THORIA AND MIXED=OXIDE MICROSPHERES PAUL A. HAAS AND S. D. C L I N T O N Oak Ridge National Laboratory, Oak Ridge, Tenn.
A process was developed for preparing thoria or thoria-urania spheres of 50- to 1000-micron diameter, high strength, and nearly theoretical density. Colloidal oxide sols were dispersed in an organic liquid and converted to gel spheres by extraction of water. To fluidize the so1 drops and to avoid coalescence, clustering, and deposition on the wall, the use of surfactants in the solvent and of special vessel configurations was necessary. The gel spheres were separated from the solvent, dried, and calcined to 1150” C. Microspheres were prepared using 2-ethyl-1 -hexanol as the drying solvent in a continuous column system, including solvent recovery. Demonstrated capacities of the system were 5 kg. of 240-micron thoria spheres in one day or 700 grams per hour for short periods. development of the sol-gel process at the Oak Ridge National Laboratory and its application to the preparation of the theoretically dense fragments of thoria-urania for vibratory compaction have been reported ( I , 2, 3, 6). Small, spherical particles of high density oxide or dicarbide are the preferred fuel materials for many high temperature gas-cooled reactor designs. The spherical shape is preferred when pyrolytic carbon coatings are used, since the coating is a small “pressure vessel” for fission products. T h e greater mechanical strength, the controlled void volume, the bulk flow properties, and/or the over-all uniformity possible with spheres are important advantages for many applications. Therefore, we started to modify the sol-gel process to prepare small, high density spheres while retaining simplicity of production and relatively low sintering temperatures. T h e basic flowsheet (Figure 1) for the sol-gel process was developed for a thoria sol.
This report is limited to the development of the sphereforming process without discussions of the procedures for sol preparation. Most of our studies were with pure thoria sols, but the same equipment and procedures were used with other sols mentioned above. However, different surfactants and calcination conditions were necessary for some of them.
Thorium nitrate is steam-denitrated to form a thorium oxide powder that can be dispersed into a stable sol by the addition of very dilute nitric acid or uranyl nitrate solution. The thoria or thoria-urania sol is evaporated to form a gel and then calcined to achieve density. A particle density of 9.9 grams per cc. or greater is achieved in 1 hour at 1150’ C. ; however, to increase the rate of uranium reduction, the furnace is blanketed with Ar-4y0 H2 for 4 hours at 1150’ C. and then cooled down in argon. After grinding and sizing, the oxide particles are suitable for vibratory compaction into fuel tubes.
Each drop of sol from the dispersion contains the amount of thorium which will be in a microsphere. Any drops that contain too much or too little thorium will form oversize or undersize spheres; these will represent waste or have to be recycled. I n the second step, the extraction of water causes gelation and thus converts the drop of sol into a solid sphere. This is the key process step. The interfacial tension holds the drop in a spherical shape. This limits the maximum microsphere size, since very large drops will distort. The slow extraction of water is essential to obtaining a microsphere with a high density and high strength. If the water i s extracted too fast, the drop breaks into fragments or forms a hollow particle. Most of the process development work was on the first two operations with 2-ethyl-1-hexanol (2EH), an eight-carbon alcohol, as the organic liquid. T h e calcination and sintering
HE
The gel sphere-forming procedure replaces the evaporation step in the basic sol-gel process flowsheet. The gel sphereforming operation has been carried out with sols containing oxides of Th, U, Zr, Pu, rare earths, and other actinides. Dicarbide spheres have also been prepared by the sol-gel process modification, in which carbon black is added to thoria-urania sols (4,6). 236
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Process and Equipment
The complete process for converting a sol into calcined microspheres involves six operations. 1. Dispersion of sol into drops. 2. Suspension in solvent and extraction of water to cause gelation. 3. Separation of gel microspheres from solvent. 4. Recovery of solvent for re-use. 5. Drying of gel microspheres. 6. Calcination and sintering.
c
T
THORIUM NITRATE SOLUTION (-2 E)
SOL PREPARATION
STEAM DENITRATION
STEAM 350-45OeC, DENSE U02-Th02
~
TO SIZING AND VIBRATORY COMPACTION
BLENDING, 8 0 T , NH r = O . O 7 7 -$= 0.017
I 1
uo,-Tho
AIR, 3 W D 4 h rTO 1150% I I5ODC, I hr
------
REDUCTION ARGON-4% HYDROGEN I IM'C, 4 hr lo 'oo'c.
H20
SOL
GELATION
EvZFE~~~GN
l., UO,-ThO,
-
(DENSITY: -7 kg/lifer) I
I
1
Figure 1 .
Flowsheet of sol-gel process
were done using the procedures and equipment used for the original sol-gel process development studies. The initial studies of gelation by extraction of water were for preparation of dicarbides with CCl4-isopropyl alcohol as the organic liquid
(4. Equipment. Most of the development studies were made with a continuous column system in which the first four process steps were accomplished (Figure 2). To start the process, the sol is dispersrd into drops which are released into the enlarged top of a tapered column. T h e drops are suspended or fluidized by a circulating upflowing stream of an organic liquid. T h e organic liquid suspends the drops individually and extracts water to bring about gelation. As the water is extracted and the drops gel into solid microspheres, their settling velocity increases. T h e column configuration and the fluidizing flow rates are selected to permit the gelled particles to drop out continuously while sol drops are added to the top of the column. Then the separation of the gel spheres from the organic liquid is completed by discharging the product collector onto a fritted-glass filter and draining the solvent off through the fritted glass. T h e gel spheres are dried by gently heating the filter vessel and passing heated gases or superheated steam up through the fritted glass. T h e calcining and sintering which provide the final, high density, high strength product are done in controlled atmosphere and controlled-temperature-program furnaces with final tempera-
tures of 1150' C. for oxide microspheres and 1700' C. for dicarbide microspheres. As to the path taken by the solvent, fresh or purified solvent is continuously added to the column and displaces a stream of wet solvent to a recovery system. Water is removed from the wet solvent by distillation. Other treatments may be necessary to remove impurities that accumulate as solvent degradation products or are extracted from the sol. Other types of equipment have been used for the same process steps. The column has been operated batchwise without removal of product or recovery of solvent. Agitation in baffled vessels has also been used for dispersion and suspension of the sol drops when microspheres of less than 100-micron diameter were prepared.
Characteristics of Product. Examples of four sizes of thoria spheres that have been formed in 2-ethyl-1-hexanol (2EH) and calcined at 1150' C. are shown in Figure 3. The first sample was formed in an agitated beaker a t a stirrer speed of 1200 r.p.m. All of the particles are less t h a n 40 microns in diameter and a high percentage of the spheroids are smaller than 10 microns. The next three samples in Figure 3 were formed in the tapered column system and, compared to the first sample, are shown with one eighth the photographic enlargement. The average diameters of these three samples are 250,500, and 750 microns.
COOLING WATER L
Figure 2.
Sol-gel microsphere column and solvent-recovery system VOL. 5
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Gelled and calcined spheres of thoria-8 weight 7, urania (fully enriched) are shown in Figure 4. T h e gel spheres are 350 to 400 microns in diameter and the calcined spheres are 250 to 300 microns. About 2200 grams of these spheres were prepared in the tapered column for irradiation testing, T h e physical properties of the calcined spheres are: particle density, 99.4% of theoretical; krypton surface area, 0.003 sq. meter per gram, which is Less than twice the geometric surface area; and particle crushing strength (25 particles), greater than 5.7 kg. for each. Over 50 kg. of thoria microspheres were prepared for waluation tests, pyrolytic carbon coating studies, and other uses. Some of the amounts and size distributions of products are listed below. Dispersion of Sols
A major part of the studies of microsphere preparation has been the development of sol dispersers which give the necessary control of mean drop diameter (100 to 2000 microns), uniformity of drop diameter, and capacity. None of the dispersers tested were optimum in all respects, but the important requirements can be met. The sol flow rates for the process development studies were 1 to 5 cc. per minute,
Lefl. Right.
238
but scale-up by factors of 10 in the immediate future and by 100 eventually will be necessary. The need for uniform drops to minimize the amount of waste or recycle material was previously mentioned. Uniform sol drops are also necessary for continuous operation of the microsphere-forming columnfor example, oversize drops would fall out immediately without drying and would resol and thus spoil part of the product. Undersize drops would never settle fast enough to fall out and would thus accumulate in the column. Two-Fluid Nozzle Dispersers. Sol droplets of controlled diameters were initially formed in a tapered column system by discharging the sol through a small orifice. To eliminate the undesirably small orifice sizes required for free-fall drop formation, a two-fluid nozzle was used to form the droplets (Figure 5A). The sol is introduced in the center of a flowing organic stream, which acts as the drive fluid. The continuous flow of sol is accelerated to the velocity of the drive fluid and then breaks up by a varicose mechanism to give sol droplets with a diameter 2 to 2.5 times the minimum sol strceam diametes. This type of breakup has been predicted theoretically and observed experimentally ( 5 ) . Theoretically, the diameter of the sol droplet should be independent of the diameter of the sol entry tube. A nozzle length of 10 inches has been used to en-
Thoria-8 weight 70 Ul(lni0 gel spheres Thoria-8 weight % wmia colcined spheres
l&EC P R O D U C T RESEARCH A N D DEVELOPMENT
sure droplet formation before discharge into the tapered column. The breakup of a 3M thoria sol in a two-fluid nozzle (glass) was photographed (Figure 6). The organic flow was adjusted to form droplets of the same size in each case. About inch of the sol entry tube is visible a t the extreme left of the figure. From this photograph it is evident that sol drops are gradually formed in a relatively short distance from the outlet; however, the photograph also indicates the presence of some satellite droplets, which are undesirable. T h e sol drop diameter can be predicted by the following equation:
where
d f
= sol droplet diameter, cm. = sol flow rate, cc./min.
V,, k
= =
maximum drive-fluid velocity, cm./min. constant for a particular system, 2.0 to 2.5
T o produce uniformly sized droplets with a two-fluid nozzle, the drive-fluid flow must be laminar, and therefore the sol should be injected in such a manner as to minimize turbulence. By use of two nozzles simultaneously, a 3.1.44 thoria sol was fed
through each two-fluid nozzle a t a rate of 2.5 cc. per minute for 4 hours. With a value of 2.4 for constant k in Equation 1, the drive-fluid flow rate was set to produce spheres with a calcined diameter of 230 microns. About 950 grams of thoria spheres were formed with the following size distribution after calcination to 1150' C . : 3.9 weight % > 250 microns; 72.3 weight TO,210 to 250 microns; 13.5 weight 70,150 to 210 microns; and10.3weightYo< 150micrans. Dispersion by Use of Shearing Forces. T h e most promising method we have found for uniform dispersion of a sol on a large scale is to shear the sol stream emerging from an orifice by maintaining a velocity gradient in the 2EH a t the orifice. This method was tested with rotary feeders (Figure 5B) immersed in the tap of the column. The sol stream leaving small orifices (0.010 or 0.016 inch in diameter) in the feeders is sheared off, with most of the force from the velocity gradient in the relatively stagnant 2EH. I n other devices (Figure 5 C ) ,the orifices are stationary and the 2EH is pumped past the orifice through a confined flow channel. Nine rotary dispersers of three principal types were made and tested. I n all types, the sol flowed through eight holes, 0.010 or 0.016 inch in diameter. T h e level of sol inside the disperser varied, so that the centrifugal plus gravity head equaled the hydrostatic head of solvent outside the disperser plus the pressure drop required for the sol flow through the orifices. I n the
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rotary disperser of the first type (Type I), the orifices are in the surface of maximum outside diameter, as shown in Figure 5B. The centrifugal force on the sol stream emerging from the orifice is perpendicular to the plane of the orifice, while the shear force from the solvent is approximately parallel to the orifice plane. I n the second type (Type 11), the orifices are recessed in a groove to shield the sol stream partially from the shear in the solvent. I n the third type, the holes are in the bottom plane surface of the feeder, so both the centrifugal force and the shear force are parallel to the plane of the orifice. All three types of rotary dispersers can give the mean sol drop sizes required for the microsphere-forming column. Types I and I11 can give product uniformity equal to that of the twofluid nozzle. The size of the drops was independent of the sol flow rates tested (0.05 to 2.0 cc. per minute per hole); apparently a drop of the same size is sheared off independently of whether the drop is formed slowly or quickly. This means that a large number of orifices can be supplied from a single sol feed without causing size variations due to unequal flows or plugging of some of the holes. I n some cases partially plugged holes produced smaller drops than unplugged holes, but this is not a serious problem. This ability to feed a number of orifices from one sol feed stream without changes in drop size due to flow variations is the principal advantage of the rotary dispersers over the two-fluid nozzles. Also the rotary dispersers do not require a nonpulsating sol feed and can thus use metering pumps which give a pulsating flow. The problem of entrainment is reduced by the absence of an organic liquid flow, but is increased by the agitation from the rotation. The shear that occurs in the solvent at the surface of the rotary dispersers is a complex function of a number ofvariables. Therefore, the results for the drop size distribution from these dispersers are not easily correlated. The configuration of the sol stream is controlled by the diameter of the orifices. But the shear may be functions of the rotational speed, the configuration of the disperser, the configuration of the solvent container, the properties of the solvent, and the presence of eccentricity or vibrations in the disperser. The centrifugal and gravitational forces may also be significant. The mean diameters of the products ranged from 165 to 670 microns; the product size decreased as the rotational speed was increased or the orifice size was decreased. The results, exclusive of several results with unbalanced or partially plugged dispersers, show consistent variations with rotation speed for ' of each type (Figures 7 and 8). Twenty, 50, and 80 weight % the products were smaller than 0'20, d60, and dso, respectively. Location of the orifices a t the maximum outer diameter of the disperser gave smaller drops than other locations. Type I dispersers (A and E) had such exposures and produced smaller drops than the other dispersers a t the same tangential speed. T h e Type I1 dispersers (B, C, and F), where the orifices were partially shielded from shear by being recessed in grooves, produced drops of appreciably larger diameters. Disperser F had a narrower groove and more effective shielding from shear, and produced larger drops than disperser B. Disperser D had an enlargement of 11/4- to 13/4-inch 0.d. a half inch above the orifice location. This configuration could be considered to be between those for Type I and Type I1 and it gave an intermediate size distribution. The most uniform product diameters were obtained at tangential speeds of 1 to 11/* feet per second for the orifices (Figure 9). The Type I dispersers seemed to produce a more uniform product than Types I1 or 111, but this difference is not certain. Most of the product size distribution showed a uniform large diameter fraction and a smaller fraction of fines. 240
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
'rD e
3
I
0
1.0
2.0
3.0
TANGENTIAL SPEED AT ORIFICE (FPS)
Figure 7. Microsphere product diameters for dispersers with holes at maximum outside diameter
rotary
Type I
These fines usually represented 5 to 20 weight yo of the total product and usually had a much less uniform distribution than the material of large diameter. The uniformity was expressed in terms of (dso - d20)/db0 (Figure 9), since giving a value of might be interpreted as indicating a simple distribution. A shear dispersion nozzle (Figure 5C) was tested, in which sol flowing through 0.010-inch diameter holes in a center tube was sheared off as drops by solvent flowing in an annulus. This device has fewer variables than the rotary disperser and is therefore simpler to operate. The drop diameter decreases as the solvent flow rate is increased. Multiple orifices give it a higher capacity than the two-fluid nozzle. I n preliminary tests, the diametral uniformity was good at a low solvent rate, but poor a t a higher flow rate. The uniformity of the products can be seen from test results which include most of the larger samples prepared (Table I). The results for the various sol dispersers may be summarized as follows : Two-fluid nozzles with sol and solvent flows metered to each nozzle give the most uniform and predictable product diameters. For multiple two-fluid nozzles, the sol flows must be metered separately. Uniform division of the sol flows cannot be maintained if the nozzles are in parallel. T h e uniformity of the product from Type I rotary dispersers or from a shear nozzle can be almost as good as that of a twofluid nozzle, but the uniformity from the two-fluid nozzle is good for a wider range of mean diameters. T h e two-fluid nozzle and the shear nozzle have fewer variables controlling the product size than the rotary dispersers. However, both require a solvent flow in addition to the fluidizing flow of solvent and thus increase the entrainment from or size of the enlarged top section of the column. Either the rotary disperser or the shear nozzle can use
Figure 8.
Microsphere product diameters for rotary dispersers Types II and 111
70
I
I
I
1 0
60 0 0
500
F
t
-8
0
40.
A
0
A
?
%
7" 30
0
s 20
.
\ o
I 0
TYPE SYMBOL I .
0
10.
0
A
I
I 1.0
I1 111 2.0
1 3.0
d
TANGENTIAL SPEED AT ORIFICE (FPS)
Figure 9.
Relation of rotational speed to uniformity of rotary disperser product
multiple orifices and thus give large capacities; the two-fluid nozzle is limited to about 5 cc. per minute maximum sol flow for each nozzle.
Solvents and Surfactants T h e slow extraction of water from a sol drop requires an organic liquid which has a low solubility in water and in which water is moderately soluble. Stability during recycle, high flash point, and low toxicity are also desirable properties. Finally, the density, viscosity, and ease of water removal are important to convenient operation of a column system. Several organic solvents were tried in tapered column systems to obtain a suitable water-removal medium. I n general, the
long-chain alcohols are the most satisfactory solvents for forming spherical gelled particles. 2-Ethylhexanol (2EH) and 2-methylpentanol were superior to the other alcohols tried. T h e halogenated solvents were avoided because of the health hazard and possible halogen contamination of the product. Almost all the engineering scale-up studies were done with 2EH. Considering its many excellent properties for this use, we are unlikely to find any solvent which is significantly better. I n our column system, a surfactant must be dissolved in the organic solvent to prevent coalescence of the sol drops with each other or on the column walls, or clustering of partially dried drops. Surfactants also lower the interfacial tension VOL 5
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Table 1.
Product Sizes of Calcined Microspheres from Several Sol Dispersers
Sol feed. Thoria sols of 3.05M T h ; sol drop diameters 2.4 times diameters of theoretically dense T h o a product Other Designation or Conditions
Optimum condiiions
Two-Fluid Nozzle Dispersers Four nozzles 94-hour run in parallel
Sol feed rate, cc./min. 1.2 2.4 Weight of product, g. 540 20,200 Product distribution, wt. yo 35/40“ or 420-500 p 40/45 or 350-420 p 2.6 45/50 or 297-350 g 50/60 or 250-297 p 22.0 60/70 or 210-250 p 50.8 70/80 or 177-210 p 8 O j l O O or 149-177 1I 1 7 . 3I1 - / l o 0 or
