Behavior of Uranium and Other Selected Materials in Fluorinating Reagents L-kTRENCE STEIS ~ N DR. C. 1-OGEL Chemical E n g i n e e r i n g Diuision, Argonne .YutiorwI L n b o r u t o r > . L ~ i r i o rti . 111
B I W N I S E trifluoride, bromine pentafluoride, and chlorine trifluoride are vigorous fluorinating agents, which can be used in place of fluorine for the production of a numher of inorganic fluorides ( I , 5-8). As these halogen fluorides can be stored as liquids a t room temperature, they provide more compact sources of fluorine than fluorine gas. 1Ietals or osides can frequentl?- be converted to fluorides a t lower temperatures n-ith the liquid halogen fiuorides than n-ith fluorine gas. The present esperiments had the main objective of qualitatively characterizing the behavior of uranium in the halogen fluorides and fluorine at elevated temperatures and qiiditatively investigating the dability of less reactive materials in these reagents. Uranium metal is converted to uranium hexafluoride by bromine trifluoride. bromine peiitafluoride, and chlorine triflnoride as shown in Equations 1, 2, and 3.
U (solid)
+ 2BrF4i1iq.j
U (solid) + 3BrF5 (liq.)
50-125"
c
----+
UF, (liq.)
--c. t50-75°
c.
\lETIIODS .AS11 41'1'11~4TUS
Tilc ~ p p : ~ i x t iused is in these expci.iinents is shown in Figure 1, Tlic p u t s \rcrcs constructed of niclic~l or AIonel, except for valves, which w r e of brahs nith triple bronze and steel diaphragms (Hoke 1Iodel 322). .I11 parts in contact. with liquid or vapor \\-ere> lie:it,ecl clertrically 13)- ushestos-c.overed Xichrome wire, Therinocouples n-ere externally attached to the vessels anti lines at numerous points for measuring temperatures. The entire :i1)paratus was huilt behind a concret,e Iilorli will for t,he prot,ection of t h e operat,or; valves \\-ere opened and closed by ext,ension handles: and pressure gages \\-ere ohserved a i t h mirror$. The two free-volume vessels shown in the figure \\--ere present to increase the vapor space and thus prevent the pressure from rising too r:rpiclly during the reaction. The 400 pounds per square inch rupture disk and 10-liter safety tank provided further protection in the event of a rapid, uncontrolled pressure increase. T h e reaction cell n-as constructed so that the sample being investigated could lie n-atched during the fluorination. It, is shown in det,ail in Figure 2. Quartz windows 311, inch thick and 13/16 inch in diameter were seat,ed against Teflon gaskets a t the front and rear of this cell. -4light hehind the rear window silhouetted the samplr. and the operator observed an enlarged image of the cell from the other side of the proteerive wail by a mirror anti monocular system. The sample was generally in the form of a r.ul)e, slightly less than '4 inch on each side, with a circular hole i n one face into which an'Incoiie1-ronsta.ntanneedle thermocouple f . j i fitted. The cube \vas supported in the, field of view by t.hc thermocouple and isolated from the n-nlla of the cell: heat loss from t h e cube 13)- conduction was minimized. A high-speed rerording potentiomet,er vas used with the needle thermocouple. The sample was usually he:tted t,o t,he desired tempernt,urP in the evacuated ceil, and the liquid halogen fluoride was he:it,ed separately in the prehenter. The liquid was then flashed into the cell to a level completely covering the metal, using pressure to complete the transfer if necessary. The r rould I)e carried out in the liquid phase, or the liquid level could lie lomred t o expose tlie sample to vapor hy opening the drainagr valve. In the latter rase, sonie liquid remained in t,he bottom of the cell and maintained its vapor pressure in the system. Because of the very high vapor pressures of chlorine t,rifluoride and liromine pent,afluoride. in some experiments residual liquids were vaporized completely before very high temperatures were reached.
(1)
UFc (lis.) - 3BrF8 jliq.) (2)
---25-;5"
C (solid) -t 3C1F1 (liq.)
+ Br2 (liq.)
aelec~tidmntrrials rewalitl qu:ilitativ E heatrd from room temperature i n the vapor.
Needle Thermocouple
1400
( Inconel tubing,
Constantan center w i r e )
m
noae Vessel
3/8" C o u p l i n g N u t and P l u g
z
I n c o n e l Wire, Silver- Soldered t o Tubing ( Electrical heating
Figure 2.
I200
l i n e s not s h o w n )
Cell for observing uranium ignition in fluorinating media
1000
w 2
2
800
W
a
zc z
C
600
-
3
Some uranium was recovered aftcr reactivii, either alloycd with the thermocouple metals as a shiny: spherical button or siirrounding the thermocouple stem as a black deposit coated with fluoride. Figure 3 is a sample curve of "temperature" 2's. "time" obtained from the chart of the recording potentiometer. ;it point .4 the liquid level !vas lowered, exposing the cube to vapor. A l t B the temperature began to rise suddenly; the approximate starting point of this sharp ascent {vas taken as the ignition temperature. Incandescence of the uranium began a t about point C'. The maximum temperature \vas above 1100" C. but was not accurately known. Although the needle thermocouple n-as often destroyed, the metal junctions in the solidified melt usually continued to generate a potential. A sequence of photographs taken from a color motion picture of uranium igniting in bromine trifluoride vapor is shon-n in Figure 4. A shows the uranium cube on the needle thermocouple immediately before ignition. B , one corner of the cube is incandescent; in C the entire cube is incandescent and beginning to melt. I n D and E the molten glohule is decreasing in size as metal runs down the thermocouple. Only the thermocouple tip remains in view in F after a total elapsed time of 3.9 seconds. The data for ignition of uranium in bromine trifluoride vapor after immersion in liquid :ire given in the first part of Table 11.
Z
4
5
400
200
J \
0 I
VAPOR
Figure
3.
2
3
CONTACT ( m i n u t 8s)
TIME
Representative
4
temperature
curve for uranium ignition in bromine
trifluoride vapor
The ignition temperatures did riot v w y systematically with pressure in any single reagent, as indicated by the scatter of the data shown in Table I11 for each reagent. As the experiments, except runs 38 and 43, were done in a static system, reaction products could accumulate in the vicinity of the uranium and hinder further fluorination. It seems probable that in a flow system lower ignition temperatures might be found.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
420
Ignition of Uranium Metal in Vapor-Phase Fluorination
'rahle 11.
Table IV.
Approxiinately 3 g r a m s of alpha-rolled uranium used in each experiment. a t 145O, 10J3,a n d I'ressure: BrF3 a t vapor pressure of liquid reagent-Le., 201' C., vapor pressures are 1364, 2384, a n d 5840 m m . , respectively. Run No.
A. 13 14 15 1t j 17 18
19 20 21
.>., -23 24 '5
'6 30 31 "8 32 49 .j0
pl
22
H. I05 106
U
Reagent (Vapor)
Initial Temp.,
C.
Ignition Temp.,
C.
Tqs t
".
Specimen Th
Ys,,nr . -*
('ontact Time. .\Iin.
t-ranium Exposed to Vapoi a f t e r Iiiiinersion in Liquid 205 203 205 205 I55 185 8 135 150 20 2054 145 48 150 2 150 210 210 1 185 185 1 I 190 190 183 185 210 210 175 175 205 1 205 260 200 1 170 170 1 1BO 2 160 160 170 17 145 18 175 135 11 170 1:i 150 160 135 1FO 14 145 160 7
.4 1
CU
Fe
llg
Uraniuiii Exposed t o I-apor without Prior Liquid Immersion BrF, BrF3
23 25
190 68 190 io t r a p ; ignition produced by addition
Zr
f Initial solution, 16 mole 7c Brs.
I' t
Table 111.
Run No. 34 33 :i 3 H8 43 36 39 37 40 42 41 44
Vol. 48, No. 3
Qualitative Behaviors of Selected .\faterials in Keageri ts (1)uratiuri of exposures. 10 t o 30 ininutes) .\laximuin Maximum Trrnp., Pressure, Reagent c c. XIm. BIFI liquid 1tj.5 BrFa r a p o r :363 BrFb vapor 340 ClF8 vapor 3.50 F? gas :340 HrFs liquid UrF3 vapor I3rFb liquid BrFb vapor ClFa vapor Fz pas BrFa liquid 13rFa vapor H r F j llquid HrFa vapor ClFa r a p n r I:? gas HrFs liquid BrF3 vapor HrFs liquid BrFa vapor ClFa vapor Fz gas RrFJ liquid HrFa v a p o r BrFa liquid BrFa vapor ClFt vapor Fz gas BrFa liquid BrFt vapor BrFa vapor BrFa liquid HrFs vapor C1F3 r a p o r Fz gas BrF3 liquid BrFa liquid HrF3 vapor
Re>ults
1
> 2000 "100 3700 4700 22,j 410 100 /, ~
ht,J
310 380
170 410 I :3n 350 340 800
.>',j L ii i
130 350 840 ,340
2:i00
"700 3700 7000 4 600
3000 4900 6000
> 2000 2700 3400 4000
A A -4 A
-1 .1 A A h A A A .4 ..I 4 A 4 A
> 2000 120
:iiO 340 340
1.500
SO00
9200 2000
I50 20.j
..
400
4060
D E A
Ignition of Uranium .\Ictal in Vapor-Phase Fluorination
Reagent
Approx. Pressure a t Ignit,ion, llni.
5300 5.500 >I5000 750 750 1250 roo0 2530 2800 3800 5750 000
Initial
U
Temp., O C. 170 170 1BO 23
'5 25 25 25 25 25 70 25 23 25 180
Ignition Tpp.. C. 225 240 225 Xo ignitionas 275O S o ignitionc 255 270 203 270 235 1.0 ignitiond 260 355
Vapor Contact Time, llin.
,
10
?I 33 23 50 29 45 24 28 111 .jP
24 4600 45 :3 3 4800 40 Fz 30 4900 375 47 Fz 4 Gas f r o m cylinder allowed t o flow o v e r heated U a t atrnosplieric pressure. b R u n terminated a t 205O C. e Temperature surged f r o m 330° t o 410° C. upon reducing pressure a t end of r u n . d R u n terminated a t 340' C:.
T h e heat of formation of uranium hexafluoride is -505 kcal. per mole (3),so i t is not surprising t h a t the temperature rise is abrupt once t h e reaction proceeds a t a rapid rate in the absence of a mode of heat dissipation. Approximately 155 calories are required t o raise 3 grams of uranium from room temperature t o the melting point and to supply the heat of fusion, and the consumption of only Y X gram-mole of fluorine in t h e formation of uranium hexafluoride will provide the required heat. Approximately 100 times this amount of fluorine ivas prcsent in runs 45, 46, and 47. Poor heat removal is probably t h e biggest factor causing uranium t o ignite in all of the vapor-phase reactions. In a dissolution of uranium in a liquid halogen fluoride considerable heat nisy be evolved, b u t the liquid serves as a heat transfer medium and prevents the temperature from rising abruptly. If the liquid is a t t h e boiling point, heat can be removed b y vaporization as well as conduction t o t h e walk. I n a vapor-phase reaction, ho\wver,
.4. L ~ t t l eor no reaction; tiist -pecim,,~iunciiariged escepr for sinali loss w i n in weight. H . Erratic reaction; uccasional tein:~eriiturC' 5urgrcj: iorrnntion of thick Hiioricle coating. C . Ixnition of test specimen n t btared iiinsiinuiir teniperatiiri. U. Tesr specimen partly dissolved. E . Tes? specimen cumpletely dis*olred. oi
the rate of dissipation of heat from the uranium is mucli lower, and the rising temperature Ltcceleratcs the reaction until the ig-. nition point is reached. BEHAWOR O F m o R r u \ i , 4 1 - ~O-II'HER \ I ~ T E R I . A L S
Thorium and other selected mzrterials were exposed t o t h e fluorinating agents with the results listed in Table IT.'. All t h e test specimens were '/,-inch cubee except the platinum, which consisted of 1-gram pieces of 10-mil foil wrapped around the t o p of the needle tliermocouple. Thorium, aluminum, copper, iron, and magnesium were attacked very little by any of the reagents. T h e 1,'4-incli cubes usually 1o;t or gained a few milligrams as the result of corrosion or surfwe fluorination. Platinum readily dissolved in hot bromine trifluoride and \vas not tested further. Zirconium was onlj- slightly attacked by liquid bromine trifluoride and bromine pentafluoride, but proved to lie unstable in the vapor-phase reactions. The test Ppecimens ignited in vaporphase reactions with chlorine trifluoridc and fluorine. I n one experiment with bromine trifluoride vapor, no noticeable changes occurred, but in all subsequent vapor exposures the zirconium became heavily coated with a t h k k white scale, and rapid temperature surges LTere recorded. Fluorothene ignited in vapor-phase reactions lvith chlorine trifluoride and fluorine. hltliough i t did not ignite in the vapors of bromine trifluoride and bromine pentafluoride, t,he number of
March 1956
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Figure 4 . Approvimetely 3 .4. Z e r o time D. 8 . 0.8 second E. C . 2.0 wxonds F.
Uranium igniting in bromine trifluoride vapor
X 2.7 seconds 3.0 seconds 3.9 beronds
espciiiiii,iit? \\ xs too sm:i!! t o c.ttnt~!ii;li this n-ith cert:iiiitJ., lt prohalil!. c:innot h i , considered safe for use with any of thcb rcagelit> ;ilmve 12.5" C. T h i Toflon g:lskcts used to seal the wintioivs of thc' ~ 1 d i1d l i o t rcactt riiiticeaIi!y txJlon. 200" C., but a t higlici tempcratrire:: tlic,y occa,*ionally caught fire in thf. Huorimting agent.;. Izluorotheiic. and Teflon, as n-?!l as other fliior(ocarbons, itre 1mo~v11 t o he thermodynamically less stable than carbon tetmfluoridc ( 2 ) , w h i c l ~ Fhorild I x one of thr iiltiiiinte products of vigorous fluorinntioil. The nickel, ione el: arid Incoilel parts of the apparatus m r c etable iii all the esperinients. The brass parts were slightl~.?orroded but remained serviceable. The quartz n-iridow were etched by the halogen fluoride?. Iiowcver, they remained tranPparent while n-etted w i t h liquid and could be used for a riumbcr of experiments brforc rcplacemerit became necessary.
421
