T h e rate of accumulation of surface material in the Coalinga run appeared to increase with the air content of the feed brine. .\ plausible explanation for the accumulation of the surface coating is the precipitation of ferric hydroxide and mucilaginous secretions bv means of iron bacteria. Acknowledgment
R. Phelps, Superintendent of Public il'orks in the city of Coalinga. provided every possible aid. The test program \vas carried out by H. Baldwin of the Coalinga group, whose conscientious efforts were a key factor in the success of the program. literature Cited
(1) Blunk, R. \V.,"Study of the Criteria for the Prediction of the Semipermeability of Cellulose Acetate Membranes to Aqueous Solutions." M.S. thesis. Department of Engineering, Vniversity of California, Los Angeles, June 1964.
(2) Breton, E. J., Jr.: "\Vatu and Ion Flow through Imperfect Osmotic Membranes," University of Florida for U.S. Dept. Interior, Office of Saline LVater. Res. Develop. Progr. Rept. 16 (Xpril 1957). (3) Clark. F. M., "Iron Bacteria in Quality Aspects of Water Distribution Systems,'' Univ. Illinois Eng. Expt. Sta., Circ. 81 (Jan. 29, 1963). (4) Loeb. S., Manjikian, S., "Brackish LVater Desalination by an Osmotic Membrane," Univ. of California, Dept. of Eng., Los Angeles, Progr. Rept. 63-22 (May 1963). (5) Ibid.. 63-37 (July 1963). (6) Loeb. S., Sourirajan, S.. Aduon. Chem. Ser., No. 38, 117-32 (1962). (7) Merten. U., IND.ENG. CHEM. FUNDAMENTALS 2, 229-32 (1963). (8) Merten, U.. "1963 Saline IVater Conversion Report." U. S. Department of Interior, Office of Saline FVater. Washington, D. C. (9) Reid, C. E., Breton, E. J.. J . Appl. Polymer Sci. 1, 133-43 (1 959). (10) Sourirajan, S.. IND. ENG. CHEM.FUNDAMENTALS 3, 206-10 (1964). for review June 25, 1964 RECEIVED ACCEPTED October 19, 1964
SOL-GEL PROCESS FOR PREPARING SPHEROIDAL PARTICLES OF T H E DICARBIDES OF THORIUM AND THORIUM-URANIUM MIXTURES JAMES L. K E L L Y , ' A. T O D D AND ORLEN C. DEAN
KLEINSTEUBER, SAM D . C L I N T O N ,
Oak Ridgr .Volional Laboratory, Oak Rzdce, Tenn.
Spheroids of dicarbides of thorium and thorium-uranium mixtures have been prepared from uranyl and thorium nitrate solutions b y a simple sol-gel process. Thorium dioxide, prepared by the steam denitration of thorium nitrate, i s dispersed to a sol in an aqueous nitrate solution. High-surface area carbon i s mixed into this sol. The resultant Tho*-C i s formed into spheroids b y dispersing the sol into carbon tetrachloride. The spheroids are set to gel b y adding isopropyl alcohol to the mixture to extract some water. The resultant 150to 450-micron spheroids are recovered, washed with an organic solvent, dried, and fired to produce the dicarbide. In a typical firing of the Th02-C gels, greater than 98% conversion to ThC2 i s obtained in 1 '/z hours at 1600" C. in vacuum. Kinetic studies indicate that the carburization reaction i s pseudo-first order, with an activation energy of about 1 12 kcal. per mole of ThO2. Thorium-uranium dicarbide spheroids are produced similarly b y adding uranyl nitrate solution of an appropriate concentration at the sol preparation step, Coated spheroids of these dicarbides are of interest as fuel particles for graphite-matrix fuel elements, which are favored for high-temperature nuclear reactor fuel. PHEROIDAL
particles of thorium dicarbide and thorium-
S uranium dicarbide having diameters of 100 to 300 microns have been prepared on a laboratory scale by a sol-gel process. Such particles. \\.hen coated with pyrolytic carbon. silicon carbide. or other coatings. are of interest as fuel materials for advanced gas-cooled reactors. T h e process is an adaptation of the sol-gel process for the preparation of high-density thorium oxide and thorium--uranium oxide (2. .?). T h e steps are few and relatively simple. High-temperature (>2100" C.) furnaces and arc-casting (>2450° C.) techniques are circumvented. as \vel1 as the need for extensive inert gas-blanketed o p ~ r a r i o n soften associated Ivith other dicarbide processes.
1
212
Prrsrnt addrrss. Uni\.ersity of Virginia, Charlottesville, Va. I&EC PROCESS D E S I G N A N D DEVELOPMEN1
Grinding and mechanical shaping of the extremely hard dicarbides are also unnecessary. Dusting problems are minimized. Description of Process
A chemical flowsheet for the preparation of the T h C z and (ThzU1-,)C? spheroids is presented in Figure 1. The process consists of four relatively simple steps: preparation of the oxide sol; incorporation of carbon in the sol; formation of gel spheroids; and firing of the spheroids. T h e first three steps are performed in air. the last in vacuum or an inert atmosphere. The spheroids are sized and shaped before the gel is fired, thus minimizing the reoxidation problems associated with handling the reactive dicarbides. T h e carbide formation reaction occurs a t a practical rate a t relatively low temperatures, 1500'
b
,APERED GLASS SlSi (1 IN.)
81 "OL % CCI l i VOL % 1so$@OWL ALCOHOL
with alcohol, the spheroids were dried in an oven at 100' C. for 12 hours. Figure 3 (right) i s a photomicrograph of ( T h o . n U ooJ) oxide-carbon eel mheraids prepared in this manner. Figure 3 (left) i: i a photomicrogra ph of product (Tha.erUo,os)C1 partides oht ained by reaction of spheroids from the population represent ed a t right. The carbide spheroids shown at left are _. I L L L 81.znrc~U C L ~uelow. discussed :Far larger-scale operations, a spray column technique for "
1
,:..._ I....^
r
PRODUCT COLLECTOR
Figure 4.
Spray column for forming spheroids
Figure 5. Thoria gel spheroids formed b y spraying sol into isopropyl alcohol-carbon tetrachloride
spray column equipment i s shown in Figure 4. In initial studies with this system, a ThOz sol was successfully converted to the gel spheroids seen in Figure 5. The sol was sprayed downward through an immerscd 0.006-inch diameter orifice into a 15 volume % &propyl alcohol-CC1~ solution. The organic solution was circulated upward a t a controlled rate to regulate the settling velocity of the droplets and, hence, the contact time between the water-extracting organic solution and the sal. T h e partially gelled spheroids were withdrawn from the bottom of the column and dried to solid T h O s gel particles. Problems related to control of sizes are now being investigated. Spheroidizing of oxide-carbon sols will be studied in this type of equipment when the oxide system studies are completed. Oxide-carbon spheroids which do not meet specifications may he recycled by two possible procedures: ( 1 ) addition of nitric acid and water to the dried oxide-carbon gel, which redisperses the solids hut sometimes results in an unstable sol; (2) air-oxidation to remove carbon at a temperature of 200' to 350' C . If care is taken during the oxidation to avoid heating above 500" C., the oxide can again be dispersed to sol by heating with dilute HNOI. Formation of Dicarhide Spheroids. The dried oxidecarbon spheroids are converted to the corresponding dicarbides by heating in either vacuum or a current of inert gas, such as argon. Essentially complete (>99'%) conversion of the oxide to the carbide may be attained in 1 to 6 hours in the temperature range 1500' to 1750' C . Thermodynamic data (Figure 6) reported for the reaction ( 5 4 , 70) ThOq.)
+ 4C{q
-+
ThCq.)
+ 2CO(,)
indicate that very low conversions should he expected for temperatures less than 2000" C . However, if the reaction occurs in vacuum or in a stream of inert gas, the gaseous
Figure 6. Thermodynamic data for oxide-carbon reaction, Tho2 4C ---f ThCz 2CO
+
214
+
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Figure 7. Schematic diagram of kinetic studies
apparatus used in
+(
c
-\A'
GEL
-.I L:E:
C l T h ATOM RATIO
*G-7
-2
METAL ATOM X U
0% u 0%U 5%u
68, 4 3, 4 3,
-~ L-.-_1-_1-1-1___-~~_1.~~_____
50
52
54 T - ' X l O 4 (*K:')
56
58
Figure 8. Kinetic data obtained in firing of Tho*-C gel a t 1445" C.
Figure 9. Arrhenius plot showing variation of rate constants with temperature
product, CO: is continually removed, thereby maintaining the activity of that product belo).; its equilibrium value. Therefore, the reaction proceeds until one of the reactants is consumed. '1 he kinetics of the reaction to form thorium dicarbide and thorium-uranium dicarbide \cas investigated. A schematic dralving of the equipment used in the kinetics study is sholcn in Figure 7 , The reaction took place in an inductively heated graphite cylinder suspended in an evacuated quartz envelope. 'I'he vacuurn varied from about 1000 microns at the start of the rraction to about 10 microns at the finish. Nominal reaction temprratures \\priI 10, 1952). (10) 'l'ripp, €1. P., Kiiig, LV., J . Am. Chem. .roc. 38 (12), 432 (1955). RECEIVEn for review January 17, 1964 ACCEPTED July 8, 1964 Division of Industrial and Engineering Chemistry, 145th Meeting, ACS, New York, N. Y . , September 1964. Research sponsored hy the U. S. Atomic Energy Coinrnission under contract with the Union Carbide Corp.
