I
G. J. VOGEL, OSCAR SANDUS,
R. K.
STEUNENBERG, and W. J. MECHAM
Argonne National Laboratory, Lemont, 111.
Fluidized-Bed Techniques in Producing Uranium Hexafluoride from Ore Concentrates Uranium hexafluoride has been produced by fluorination of crude uranium tetrafluoride, using batch and continuous fluidized-bed reactors, with reasonable fluorine efficiency A L m o u m large quantities of uranium ore are processed yearly, few operational analyses have been made on either reactors or schemes for converting the ore concentrate to hexafluoride. A scheme proposed by Argonne National Laboratory differs from the conventional method for producing uranium hexafluoride. Instead of removing metallic impurities from the ore concentrate by solvent extraction before reduction, the concentrate is reduced directly and impurities are removed at different points in the processing, mainly from the fluorinator. Distillation of volatile impurities from the crude uranium hexafluoride completes the purification. Because of their known efficiency in gas-solid-type reactions, fluidized-bed reactors are used. Reduction and hydrofluorination of refined oxides and ore concentrate have been studied in a fluidized-bed pilot plant (7). That study has been extended to production of uranium hexafluoride from the tetrafluoride using elemental fluorine. Chemical Aspects of the Reaction
The fluorination reaction is UF,
+ F, + UFG
Rate data on this reaction ( Z ) , obtained with a thermobalance, indicate that it does not proceed a t an appreciable rate below 200' C. At higher temperatures however, the rate is rapid and temperature dependent. The temperature dependence obeys the Arrhenius relationship, yielding an activation energy of 19 to 20 kcal. per mole. The rate is not affected by gas velocity but is proportional to the fluorine concentration. I n addition to the uranium tetrafluoride, there are present small quantities of uranyl fluoride and uranium dioxide, which are fluorinated readily to uranium hexafluoride and oxygen. In addition to the uranium, impurities are present as fluorides and oxyfluorides (Table I). Of these, the only ones
1744
present in sufficient concentration and having a boiling point close to uranium hexafluoride are the molybdenum and vanadium compounds. Materials
The properties of the crude uranium tetrafluoride (green salt) used in either laboratory or pilot-plant tests are given in Table 11. Two types of uranium tetrafluoride materials have been processed-crude and refined. The crude uranium tetrafluoride, a material that contains considerable metallic impurities, has been prepared by reducing and hydrofluorinating one of three ore concentrates:
Table 1. Of These Impurities Found in Processing Crude Uranium Tetrafluoride, Only Molybdenum and Vanadium ComDounds Have a Boiling Point Close to That of Uranium Hexafluoride
B.P.,
B.P., Fluoride
Ups VOF3 VF3 VFs PF3 PFj POFI MoFs SF 6 S02F2 FeF2 FeF3
c.
Fluoride
56. 4a llOb
1400 48 -101 -85 -39
PbF4 PbFi
-64 -55 1800'
c.
500b
BiFB
550b
SbFs
150
AsF5 BF3
35
O
1290
-53 -100
CaFz
2500
SiF, AlF8
-95b
Loboratory Experiments
1257h
CrCF3 CrFi MgFz
14OOc 300b
Equipment and Procedure. The laboratory equipment consisted of the fluorine and nitrogen metering system, a nitrogen preheater, the fluidized-bed reactor, the uranium hexafluoride condensing units, and a caustic gas scrubber. The reactor was constructed from a section of 3/4-inch Monel pipe topped by a 2-inch diameter disengaging section
i i n o_ c _. _
N aF
1704
CuF CuFz NiF2
13OOc
'2227
. . I
16OOc
Triple point. Sublimation temperApproximate temperature. ature at 1 atm. a
INDUSTRIAL AND ENGINEERING CHEMISTRY
South African acid-leached, Anaconda acid-leached, or Anaconda carbonateleached. The uranium in South African ore is extracted by acid leaching and is concentrated by ion exchange followed by ammonia precipitation to form ammonium diuranate, which is calcined to give the uranium oxide ore concentrate. The Anaconda acid-leach concentrate, a domestic product, is obtained in a similar manner. The Anaconda carbonate-leach concentrate is obtained by solubilizing the uranium with sodium carbonate and precipitating the uranium as a sodium diuranate with sodium hydroxide. Both the uranium oxide and sodium diuranate ore concentrate are reduced at approximately 575" C., and hydrofluorinated at 350" to 600' C. in a fluidized-bed reactor to produce the crude uranium tetrafluoride (7). T o obtain the refined material, the ore concentrate is dissolved and passed through a solvent-extraction step to remove most of the metallic impurities. The uranyl nitrate product is reconverted to oxide and processed to the tetrafluoride in fluidized-bed reactors ( 3 ) . The fluoride for the fluorination was of 98.5% minimum purity, the remainder being nitrogen, oxygen, and hydrogen fluoride. The nitrogen had a minimum purity of 99.6%, the remainder being mostly oxygen. The calcium fluoride used as a bed diluent in the laboratory runs was a commercial grade crystalline materia1 screened to +lo0 mesh and having a bulk density of 1.7 gram per cc.
FlLT
P
DISENGAGING SECTION FLUID BED c-REACTOR SCREW FEED HOPPER
PRODUCT CONTAINER
t
1
TO SCRUB TOWER
@ @
Tb SCRUB TOWER
PRESSURE MEASUREMENT TEMPERATURE MEASUREMENT
W ELECTRIC HEATING ELEMENT
This pilot-plant fluid-bed fluorinator consists of three systems: gas-metering, reactor, and condensing containing a disk filter, 10-micron pore size. The laboratory reactor was loaded with a weighed amount of either uranium tetrafluoride or uranium tetrafluoride plus calcium fluoride. The bed was fluidized with nitrogen as it was heated. When temperature equilibrium had been attained, one stoichiometric equivalent of fluorine was passed through the bed, and the product uranium hexafluoride was condensed and weighed. The bed residues were analyzed for uranium to determine the fluorine efficiency, which is defined as the grams of uranium i n the product divided by the grams of uranium originally in the fluidized bed. Samples of product and bed residues were also analyzed spectrographically. Using crude uranium tetrafluoride derived from South African ore plus diluent, the effect of temperature on efficiency was determined by operating the reactor in a semicontinuous fashion, adding enough uranium tetrafluoride to the bed to replace that which has been fluorinated, and then repeating the fluorination. The effect of fluorine concentration on fluorine efficiency for undiluted crude uranium tetrafluoride was determined. With both South African and Anaconda acid-leach green salts, the locations and amounts of the gangue metallic impurities were determined. Coupons of Monel, Inconel, and L and A nickel were placed in the reactor bed to obtain corrosion data.
Results and Discussion. Data in Table 111 show that with uranium tetrafluoride from South African ore diluted with calcium fluoride to improve the
Table IV. Some Nonvolatile as Well as Volatile Metal Fluorides in Crude Green Salt Move Out of Reactor with Uranium Hexafluoride Product" South African Anaconda Ore Acid Leach In In In In green product green product Element salt UF6 salt UFB
CONSIDERED MORE VOLATILETHAN UFI
B P As S Si
0.5
0.04
0.0001 1% 1% to 0.1% 0.1% to 0.01%
