HYDROGEN REDUCTION OF URANIUM OXIDES A Phase Study by Means of a Controlled-Atmosjhere Dzfractometer Hat Stage K . J.
NOTZ,' C. W.
HUNTINGTON, AND WINSTON BURKHARDT
National Lcod Co. o/ Ohio, Cincinnati 39, Ohio
The successive phase changes which occur during the hydrogen reduction of uranium oxides were studied in situ b y means of o controlled-atmosphere x-ray diffractometer hat stage a t 480' and 750' C. The phase sequence which occurs during reduction of y-UO,, the most stable allotrope of UOs, is: y-UOa (orthorhombic) U30s(hexagonal) -+2 U30s- (orthorhombic) 5 U02+ (cubic) UOz (cubic). The upper compositional limit of the nonstoichiometric phase, UOz+, is greater a t the higher reduction temperature, in agreement with the equilibrium phase diagram. When starting with U3OS,reactions 2, 3, and 4, as given above, were observed. In conjunction with this work, p-UJOswas prepared and found to revert endothermically to conventional UzOs at about 130' C.
5
of the phases which occur during the reduction higher uranium oxides to UO? is of primary importance i n the interpretation of reaction kinetics, sintering phenomena, and surface area changes. I n previous studies (7, 2, 9), the intermediate phases which occurred during the hydrogen reduction of UOs, U308, and U 4 0 a to UOz were identified by x-ray diffraction of quenched, partially reduced samples. Conceivably, the quench technique might yield misleading results due to the occurrence of phase changes during the cooling period. Thereforc, it was desirable to perform the phase identifications in situ at the temperatures of reduction, during the course of reaction. YOWLEDGE
K-of
Materials
Gamma-UOs, the most stable allotrope of UOa, was prepared by the denitration of uranyl nitrate hexahydrate at 230' C. in a 3-liter agitated vessel. 'The analyses on this sample were 83.027, U, 0.26% NOS-, 0.10% HzO, and 0.09% UzOo,. T h e surface area was 2.6 sq. meters per gram, as determined by the BET (Brunauer, Emmett, Teller) method, using nitrogen as the adsorbate. Four UaOs samples were used. Sample 1 was prepared from the above y U O 8 by calcining for 1 hour a t 950' C. and cooling slowly in air. Sample 2 was prepared from commercial, reactor-grade y-UOa by calcining at 900- C. for 0.5 hour and cooling slowly in air. Sample 3 was obtained by oxidizing UOs prepared by hydrogen reduction of the above laboratoryprepared r-UOn; the oxidation was performed by heating the sample from 150' to 510' C . in air over an 8-hour period, holding it a t 510' C. for 4 days, and then cooling it slowly to room temperacure. Sample 4 was prepared by passing a portion of sample 1 through three reduction-oxidation cycler at 500' C. The BET surface areas of these samples were 0.42, 1.8, 3.4, and 4.1 sq. meters pergram, respectively.
A thin layer of powdered sample is rubbed onto a slightly roughened quartz plate, which is heated from below by Kanthal A resistance wire coiled through a Lavite furnace block. The temperature of the sample plate is measured by means of a thermocouple mounted against the underside of the sample place. This thermocouple is calibrated relative to the upper surface of the sample plate. The estimated accuracy of the true sample temperature is + Z O O C. Heating (or cooling) rates are programmed by means of a gear-driven Variac. Isothermal control is maintained with a proportional time controller which switches the power soucce between two Variacs, one of which provides slightly more, and the other slightly less, power than is required far the desired temperature The vertical position of the heater block and sample plate assembly is adjusted by an Invar screw which passes through the housing. Angular alignment is by means of the standard Norelco shaft clamp and fine adjustment cam. Simultaneous "zero setting" and "2-to-1" alignment are facilitated by use of a steel block which has a 0.002-inch-deep channel milled into its bottom surface. This block is placed on the quartz sample plate, thus farming a 0.002-inch slit 1 cm wide and 3 cm. long. At room temperature, alignment accurate to ~t0.02~ 28 is obtainable. With the present design, at temperatures of 450" to 800°C., thermal expansion of the various components causes an angular shift of +0.3" 28 in the sample plate alignment.
Equipmenl The c ~ n t ~ o l l ~ d - ~ t m hot~ stage s p h ~used ~ ~was designed and constructed a t Armour Research Foundation under the direction of F. V. Schassberger. T h e entire assembly was mounted on a Norelco diffractometer (Figure 1). Either hydrogen or oxygen, as well as inert gases, may be circulated through the chamber a t temperatures u p to 900" C. T h e housing and cover are of chrome-plated brass with copper coils for circulating cooling water. A 180' beryllium window permits the passage of x-rays. Present address, Lithium Corp. of America, Inc., Bessemer City, Pi. C .
Figure 1.
Controlled-atmosphere diffractometer hot stage VOL. 1
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The data reported in this paper have been corrected for this shift and have an estimated accuracy to hO.1 " 20. T h e hot stage was designed so that the standard Norelco collimating system can be utilized. One-degree divergence and scatter slits and a 0.003-inch receiving slit were used. Nickel-filtered copper radiation was employed. T h e diffrartometer scanning speed was 1 " 20 per minute. Differential thermal analyses (DTA) were performed with a Deltatherm (Technical Equipment Corp., Denver, Colo.). using undiluted samples us. a n alumina standard. A heatins rate of 10" C. per minute and a sensitivity of 2.5' C. per inch were used.
occurs gradually, because of the gradual approach of the orthorhombic a / b axial ratio to a value of 2 / 3 / 3 , at which point the structure may be indexed on a hexagonal basis. I n the present study, this transition was observed on a continuous basis by oscillating over the 26" (20, copper Ka radiation) region while increasing the temperature at the rate of about 10" C. per minute. The transition was followed by the merging of the (110) and (200). orthorhombic peaks into the (1 10) hexagonal peak. Complete mergence was indicated by the attainment of maximum intensity. A typical diffractometer trace is shown in Figure 2. For clarity, only the forward oscillations are shown. All four Us08 samples tested gave similar results. O n cooling, reverse behavior was observed. The composition of the products was unchanged, remaining at stoichiometric u308 (determined by calcination at 850" (2,). The transition appeared to be complete at 250" C., although the previous workers (7> 70) reported 365" to 400" C. as the transition temperature. This difference is probably due to a slight loss of oxygen by the U308 in the earlier work, which \vas performed with the sample in a n evacuated capillary. Loss of oxygen favors the orthorhombic structure. Hexagonal U 3 0 8 apparently possesses enhanced structural stability, since the hexagonal structure was retained up to 750" C., the maximum temperature employed in the present work. There has been some question, however, whether it should be considered a distinctive phase. As Hoekstra, Siegel, Fuchs, and Katz (7) point out, the orthorhombic-to-hexagonal transition may be a n ordering or disordering process. DT.4 was performed on all four c3os samples with the instrument set at maximum sensitivity. There was no detectable heat of transition for any of the samples. From this work, it would seem reasonable to interpret the phase change as a secondorder transition to hexagonal U 3 0 8 . Beta-U30s. While attempting to prepare a very lowsurface-area Us08 by extreme sintering, P - U 3 0 8 was obtained. Since relatively little is known of this phase, it was studied further. Hoekstra, Siegel, Fuchs, and Katz (7) prepared p-U30s by oxidizing u5013a t 750' C., but this treatment did not always yield the fl form. TVilson (72) obtained a mixture of a-U3O8and p-LT30sby heating Teflon containing U 3 0 8 to 870" C. and cooling rapidly to 200' C. I n the present Lvork, C ~ - U samples ~ O ~ 1> 2: and 3 yielded P-U308 when fired at 1200" C. in air for one week, and then cooled very slowly to room temperature. Altering this procedure, either by cooling too rapidly or by firing for less than a week, resulted in an a - L 3 0 sproduct.
Procedure
T h e general procedure followed for x-ray identification was to scan repeatedly over a critical 20 region while the sample was slowly heated or while hydrogen was introduced a t the desired partial pressure. I n some instances, where the only necessary observation was to monitor the appearance or disappearance of a diffraction peak, the diffractometer was automatically oscillated over the appropriate region. For other cases, where the position of a diffraction peak, as well as its intensity, was of importance, the diffractometer was repeatedly reset manually. Since a range of about 2' 20 was usualIy covered in each scan, observations of a particular peak were repeated about every 2 minutes. Reductions were carried out at two temperatures, 480' and 750" C. At 480" C., hydrogen was used undiluted a t 1 arm., which brought about complete reaction in 20 to 30 minutes. At the higher temperature, the hydrogen was diluted with nitrogen, so that the reaction still required about 15 to 20 minutes for completion. A hydrogen partial pressure of 0.03 atm. was used lvith samples 1 and 2 ; 0.015 atm. was used with sample 3. Results and Discussion Consideration of the uranium-oxygen phase diagram ( 3 , 6, 7) indicates that cos-, U 3 0 8 + , r~o13=t, U409, and UOS+ might occur during the reduction of L-03 to C O P . Conventional L T 3 0 s is orthorhombic cy-L-308; in addition, two hightemperature modifications, hexagonal L-308 and orthorhombic p-LT80s,can also occur (7, 70). Therefore, prior to investigating the phase changes M hich result from chemical reduction a t elevated temperatures, the effects of increased temperature alone were considered. Hexagonal UBOs. Siegel (70) and Hoekstra, Siegel. Fuchs. and Katz ( 7 ) have reported the transition of orthorhombic r30s to a hexagonal structure above 400' C. This transition
I
1
I
I
I
50
100
150
20 0
250
TEMPERATURE,
Figure 2.
Transition of orthorhombic
'C. u1-U308
Repeated scans of 26' 28 region 214
I & E C PROCESS D E S I G N A N D DEVELOPMENT
to hexagonal U 3 3 ~
T h e O j U ratio of p-UIOg, determined by grinding and calcining a t 850°C. in air for 1 hour, was 2.676 f. 0.008. Although some loss of oxygen would be expected during the preparation of p-U308 because of the prolonged firing a t 1200" C., reoxidation Lould, and obviously did, occur during the slow cooling period. Thus, it is possible that U5013 was Cormed as a n intermediate. Hoekstra, Siegel, Fuchs, and Katz (7) found that p-LT308 rrverted to a-U30a when the temperature was recycled to 750" C. D T A of p-LT30gprepared in this study showed that a nonreversible endothermic transition occurs at about 130 ' C . I n Figure 3 are shoirn both a conventional D T A trace and a differential cooling curve obtained by shutting off the power to the heater immediately after the 130" C. endotherm. The absence of a n exothernl during the cooling c) cle demonstrates the nonreversibility of the transition. X-ray diffraction data taken a t 75", 120°, and 180" C. identify the transition as conversion to C Y - U ~ OThe ~ . diffraction peaks in the 25" to 2'' region a t these three temperatures are shown in Figure 4. Reduction of U308. All four U3OSsamples followed the same phase sequence during reduction. During the early stages of reaction, the same results were obtained a t 480 ' and 'io" C. However. during the latter part of the reaction, after the appearance of a cubic phase, somewhat different results were obtained at the two temperatures. T h e first transition to occur during the reduction of O 1 - L T 3 0 8 , which was in the hexagonal form a t reduction temperatures, was reversion to a n orthorhombic structure. Typical data taken a t 480" C. are shown in Figure 5. Again, the transition was monitored by observing the 26' region. A cubic phase did not appear until after the conversion to the orthorhombic form was complete. Obviously, the formation of a n orthorhombic phase was due to a decrease in the 0,'U ratio. T o determine whether this orthorhombic structure was U5013 or L T 3 0 8 - , a special run was conducted wherein only a very small amount of hydrogen was introduced, so that only partial reduction to a cubic phase could take place. Diffraction patterns were then obtained a t 480' C. and at room temperature. T h e two patterns were identical, except for a slight shift in peak positions indicative of a small amount of contraction due to cooling. T h e room temperature pat, than tern was essentially identical to that of U ~ O Brather U,013. Thus, the first reaction to occur during the hydrogen reduction of L T 3 0 s is the conversion of hexagonal U308 to orthorhombic U308- which could be considered to be in a semitransitional state. The next reaction observed was the appearance of a cubic phase which was first detected by observance of the strong (111) line in the 28" region. U409, UOz+, and UOZ are all cubic structures whose lattice parameters differ only slightly. I n Table I are given the calculated lattice parameters for LIOn and U409 at 480" and 750' C., according to several sources. T h e values for UOz+ are intermediate to those for UOz and U 4 0 , . T h e cubic phase was therefore
Table 1.
I
I
HE A l l N 0
COOLINO
I
Figure
3. Differential thermal analysis of P-U308 Downward displacement i s endothermic
120.
75.C.
c.
160' C.
BETA ALPHA
t
ALPHA
EETA
-
24
26
28
DEGREES
Figure 4.
TWO
THETA
Diffraction data showing transition of p-U308 to
Cr-Usos
lattice Parameters (A.) of Cubic Phases 480' C. 750' C.
CO* Gronvold (6) Burdick and Parker (4) Kempter and Elliot (8) Average
5.497 5.497 5.492 5.495
5.513 5.507 5.508 5.509
I 0
5.482
Figure 5. Transformation of hexagonal UaOs to orthorhombic U308- during reduction a t 480" C.
u409
Gronvold (6) Weissbart et al. ( 77)
5.465 5.465
...
1
I
5
I5
10
ELAPSED
TIME
Repeated scans of
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26' 28
region
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identified by x-ray data from the back-reflection region. T h e (531) and (600) lines were selected for this purpose. The positions of these two lines, in degrees 26 for CuKa radiation, for CO2 and U409 are given in Table 11. The results obtained with the four U 3 0 8 samples are summarized in Tables I11 and I\'. The approximate degree of
Table II.
location (' 26) of (531) and (600) Cubic linesa RT 480' C. 750' C. For UOn (531) line 113.0 112.2 111.8 (600) line 114.8 114.3 115.6 For UIOg (531) line 114.2 113.4 112.9 (600) line 116.7 115.9 115.4 a For CuKa radiation. Table 111.
Cubic Phase Data at 480' C.
(Hydrogen pressure o f 1 atm.)
Conversion,
%
Sample 1
2
(53;) Line, 28
40 45 60 65 85 90 95 100 40 65 75 85 90 95 100 100
112.3 112.5 112.5 112.3 112.4 112.4 112.5 112.3 112.0 112.3
85 90 95 100 100 30 40 65
112.3 112.3 112 2 112.2 112.1 112.2 112.3 112.3 112.3 112.3 112.3
(600) Linp. O 28
ii4:7 114.7 114.7 114.6 114.8 114.9 114.7
- - -
1 1 1 112.2 112.1
3
4
80
100 100
Table IV.
conversion was estimated from the magnitude of the (111) line. A conversion of 25%; or greater was required before the back-reflection lines could be adequately located. There is no evidence of U10, formation in detectable quantities at either temperature. At 750' C., UOz- is obviously formed as an intermediate ; the approximate composition is estimated as UOz.15,based on the observed line positions. This agrees with the compositional limit of 2.14 at 750' C., as given in the phase diagram (6). At 480' C., the line positions indicate that the initial cubic phase does not deviate very much from stoichiometric UO2. If it is assumed that these line positions may be in error up to 0.2', then the calculated upper phase limit is UO2.04, slightly lower than the equilibrium value of UO?.oc at that temperature. 'Thus, the second reaction to occur during the reduction of r 3 0 8 is the conversion of orthorhombic u 3 0 8 - to cubic Con+. Finally. the third reaction which occurs is the homogeneous transition from UOZ- to COZ, as clearly indicated by the gradual shift in line positions at 750" C. In a kinetic and x-ray study of U4O9reduction, Aronson and Clayton ( 2 ) found that at 470' C. the first reaction was the heterogeneous formation of LOz, ~, followed by homogeneous conversion to UOS, in agreement with the present work. Aronson ( 7 ) also obtained limited x-ray data which suggested that L-409was formed during the reduction of U3O8, contrary to the present results. However, Aronson's data could also be interpreted in terms of a L-02 + phase, rather than U,09, since the lattice parameters had an intermediate value and a cubic tivo-phase (U40, plus UOp-) region was not observed. Reduction of yUO,. .4 kinetic and x-ray study by Notz and Mendel ( 9 ) of -y-UOa reduction suggested that the reLaosaction sequence followed was: -f-UOs U308t cubic phase(s). hlthough the first and last reactions were verified, there was some doubt as to the validity of the middle reaction, which was postulated as the homogeneous conversion of orthorhombic UO2.; to orthorhombic UO2.6. These phases Lvere identified on the basis of x-ray diffraction of quenched samples. I n viekv of the reduction results described above, if a UaOg phase is formed during the reduction of UOS, i t should be the hexagonal modification of cu-UaOg at reduction temperatures. Further reaction should then result in the formation of orthorhombic Uc:sOs-. Such a sequence was verified by the present technique.
114.7 114 5 114.6 114.7 114.7 114.9 114.6 114.7 114.8 114.6 114.8
Cubic Phase Data at 750' C.
(Hydrogen pressure o f 0.03 otm. for samples 1 ond 2, 0.015 a h . for sample 3)
Conu rsion, Sample 1
07 /C
25
(537) Line, 28
112.6
(600) Line, o 2e 114 9
I
2
3
25 40 75 100 25 45 85 100
112.7 112.7 111.9 111.9 112.1 112.0 111.8 111.8
Figure 6.
I & E C PR0:CESS
DESIGN AND DEVELOPMENT
20
IS TIME
,
mlnulra
Phase changes during reduction of T-UOa Diffraction lines
1. 2. 3.
4. 216
IO
ELAPSED
114.5 114.3 114.2 114.2
I
I
5
0
T-UOs
25.80 28
Hexagonal U s 0 ( 1 10) Orthorhombic UaOs- (200) Orthorhombic U 3 0 8 - ( 1 1 0 )
2 6 . 3 0 28 2 6 . 5 0 ze 26.1'28
25
T h e 26 " region was scanned semicontinuously while Y - L - 0 3 \cas being reduced with hydrogen a t 450" C. Disappearance of the ? - C o s was followed by the diminution of the intensity of the strongest U 0 3 line, a t 25.8". Hexagonal c 3 0 8 was observed, as before, by means of the (110) line at 26.4'. The transition to orthorhombic L T 3 0 8 - was monitored by the (1 10) and (200) lines a t 26.1 " and 26.5". Typical data are shown in Figure 6. T h e UOZline disappears prior to the observation of the orthorhombic doublet. Thus. the correct reaction sequence for reduction of y-cO3 is: 7-L-03 (orthorhombic) U 3 0 8 (hexagonal) 4 L-308(orthorhombic) UOS+ (cubic) A UOs (cubic). This sequence provides a physical basis for the observed three-step kinetics of y U 0 3 reduction (9). Each time a structural change occurs, as at steps 1>2, and 3, there is also a change in the reaction rate. .An additional source of differentiation between these reactions is that l and 3 are heterogeneous in the solid phase, involving two different solid phases simultaneously, \vhile reactions 2 and 4 are homogeneous transitions \vithin monophasic fields. This reaction sequence provides a plausible explanation for the two-step reaction postulated by DeMarco and Mendel (5) for high-surface-area uo3 on the basis of kinetic data. They observed the reduction to proceed as UOa + LTO?.j6, followed by LT02,56L-02. They did not observe a rate change at the L T 3 0 s composition corresponding to the intermediate formation of hexagonal U 3 0 8 . Since their high sur-
A
face U 0 3 \vas probably hexagonal ~u-L-03,the actual reaction could have been CY-UO~ (hexagonal) L-308 (hexagonal) A C308(orthorhombic) cubic phase(s). Thus, a rate change Ivould not be expected at the end of reaction 1: since both structures are hexagonal. literature Cited
(1) Aronson, S.,U. S. At. Energy Comm. WAPD-TM-44, 15 (March 15: 1957). (2) J. C.. J . Znorq. 'Vuclear Chem. 7, 384 . , Aronson,. S.,. Clavton, . (1958). (3) Blackburn, P. E., J . Phys. Chem. 62, 897 (1958). (4) Burdick, M. D., Parker. H. S., J . Am. Ceram. SOL. 39, 181 (1956). (5) DeMarco, R. E., Mendel, M. G., J . Phys. Chem. 64, 132 (1960). (6) Gronvold, F., J . Inorg. .Yuclear Chem. 1, 370 (1955). (7) Hoekstra, H. R., Siegel, S., Fuchs, L. H., Katz, J. J., J . Phys. Chem. 59, 136 (1955). (8) Kempter, C. P., Elliot, R. O . , J . Chem. Phys. 30, 1524 (1959). (9) Notz, K. J., Mendel, M. G., J . Znorg. .Vuclear Chem. 14, 55 fl9hO\.
(lo'i !&el, S.,.4cta Cryst. 8, 617 (1955). (11) \Yeissbart. J.. Blackburn, P. E., Gulbransen. E. A., C. s. At Energy Comm. AECU-3729 (June 6, 1957). 112) LVilson. \Y. B.. U. S.At. Energy Comm. APEX-187 (Jan. 31, ' lb55) ; APEX-202 (June 1955) (uclassified)
RECEIVED for review June 9, 1961 .ACCEPTED November 29, 1961
--f.
Division of Industrial and Engineering Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. \Vork supported by the U. s. Atomic Energy Commission under contract .AT (30-1)-1156.
LOW TEMPERATURE GASEOUS REDUCTION OF IRON ORE IN T H E PRESENCE OF ALKALI A R T H U R M C G E O R G E , J R . , l A .
N O R M A N H I X S O N , A N D
K . A.
K R I E G E R
The School of Chemical Engineering, C-ninioersitj of Pennsylvania. Philadelphia, Pa.
Rates of reduction by hydrogen of pure Fe203 and selected iron ores were measured from 250" to 500" C., with and without the addition of N a 2 0 as Na2C03, to test the reported beneficial effect of alkali metal compounds. The reduction rate, a t 50% reduction, was greater for pure Fen03 than for either ore in the absence of added alkali. Although the high surface area lateritic ore reduced rapidly a t first, its rate was slower than that of the low surface area magnetic ore a t reduction. Addition of N a 2 0 decreased the reduction rate with pure FelOs but increased it with the ores, indicating that N a 2 0 functions b y freeing iron oxides from impurities rather than by promotion in the catalytic sense. The suggested reaction mechanism involves reduction by weakly adsorbed hydrogen, inhibited by the strongly adsorbed product, water.
Soy0
HE PRODUCTIOS O F SPONGE IRON by low temperature reTduction of iron oxides with hydrogen or carbon monoxide has been studied by numerous investigators. The extensive literature has been surveyed in three articles ( 7 , 2. 22). Among other factors, it has been reported that the reduction rate is increased by adding to iron ores small quantities of alkali metal compounds. LVilliams and Ragatz (25) found that
Present address, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
potassium and sodium salts increased the reduction rate of a magnetite ore, the potassium salts being superior. Most effective were K2C03: KHC03. K?C?H302, KZCr?O,, and KOH. KC1 and K 3 P 0 4were not very effective. Barrett (2) also noticed a promotional effect of alkali on the reduction of a magnetite concentrate in a fluidized bed. One of the authors of this article had observed pilot plant tests on the reduction of a lateritic hematite ore in which small amounts of alkali had a pronounced beneficial effect. The mechanism of this promotional effpct is discussed here. VOL. 1
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