US2873184A - Thermal decomposition of uranium compounds - Google Patents

Description

Feb. l0, 1959 T. T. MAGEL ErAL v 2,873,184

THERMAL OECOMPOSITION OF URANIUM COMPOUNDS Filed March 25, 1947 FIV-5.1-

nite Sites THERMAL nEcoMPosrrroN or URANIUM coMPoUNns,

Theodore T. Magel, Cambridge, Mass., and Leo Brewer,

' Berkeley,v Calif., assignors to the United States of America as, represented byv the. United States Atomic Energy Commission y Application March 25, 19147, sean No. 137,152 .7 claims. .(ci. 'is-s4) This invention relatesto ai. method;` and apparatus for ,telit preparing metals, and particularly toI a novel method of preparation of high-purity uranium, plutonium, neptunium, and other metals` by thermal decomposition of the respectivemetal halides in a static system or in a dynamic flow system.

2f .Y For the purpose of illustration, thereare described. herein astatic method anda continuous-flow lmethQd.rIIlployingsimilar apparatus for the preparation of uranium metal of extremely high purity from an impure mass of uranium metal. The broader aspects -of the` inventionhand its api. plication to other metals are apparent fromthe illustrative 1 examples and are coveredin the appended claims.r

' formed,V so that the metalI dripsr` into and is collected in a The prior hot-lilament method-of producing pure metals I Aconsists broadly ofvolatilizing `a metal compound, such as a metal halide, in the presence of a wire lament which is maintained at a temperature above the decompof siti-on temperature of the-metal halide whereby themetal halide is thermally decomposed and the metal is deposited on the hot surface. This prior system is limited to small yields and has notbeen found satisfactory for producing massive spectroscopic pure metals having an atomic number of 90l or greater. Metals of' high atomic'number are highly reactive att the higher temperatures, especially above their melting points,A and will readily `alloy with the hot filament and the associated metallic structure.

Hence, broadly an object of` the present invention is to produce a metal, particularly uranium,rof extremely high. purityin massiveform by thermal decomposition` of a: volatile halide of the metal at a temperature'substantial# 1yV above the decomposition temperature of' the metal halide. v f

Another object of the invention is to provide a novel static hot-surface,decomposition method wherein a reducing gas is employed andan apparatus in which the heated surface may be operated* at a temperature substantially above the thermal decomposition temperature of Vthe particular metal halide "and preferablyabove the'melting point of the particular' metal, a'ndthe heatedv surface is substantially unreactive with the metal formed at temperatures above' the thermal decomposition Atemperature of themetal halide.` t

` Another' object is alinethod to produce metal of extremely high spectroscopic -purity'pfrom a source or mass suitable refractory container whicliis positioned so, that the metal, during itsfall,V cools below a temperaturelat which it may 'react appreciably ywith the container, and collects iniriassiveforrn thereinf ,l

In application to uranium 'metal and employinga dynamic flow-system, a halogen such asy iodine vapor,

preferably in the rpresence of a reducing gas,'such as hydrogen, is.V passed continuously'over an impureY mass ol uraniummetal at very low pressure and in the presence l coated tungsten Wirey -or special carboniilament which is heated to a temperature at least' above the decomposition temperature of the metalv halide,andpreferablylabove the melting point of uraniumand isl substantially unreactive with respect to the uranium atsaid. temperature. The pure uranium metal is precipitated` on the refractory or unreactive surface and continuously drips offi'falling into a suitable refractory'containerwhich is locatedY so that the dripping uranium metal coolsV suflicientlyduring its fall to be substantially unreactive with the refractory containerwhenit isreceived therein. .The wastegases are lswept out of the system so that equilibrium conditions are momentarily established an'd subsequently-the excess gases 'are removed, thereby making the` operation` continuous. j l j i` It is desirable to maintain thepressurein'the'system very low so as to drive the reactionin thedirectiori ,ofi defcomposition and. precipitation of the metal. Bysup'p'lyr ing optimum and controlled amounts of iodinexvapor l and hydrogen and maintaining the system at-very lowpressuresLthe decomposition reaction will proceed jin vthe direction of metal formation.Offthesefvery low presof` the impure metal by a novel continuous-flow hotfilament method 'wherebya reducing gas is employed and also ari apparatus in which a heated surface maybe operated at a temperature at least above the decomposition temperature of the metal halide andpreferably above Y `continuous-How hot-surface method which may be accurately and eliiciently controlled.

Other objectstandvl advantageswill become apparent from theffollowing description Wherein'reference is made to thel accompanying drawings in` which:

Fig. l isla diagrammatic side elevation, parts being in' section,'of an apparatuslsuitablefor.practicing the present invention; and

- FigLZis a` cross section-of a4 modication of the reac- `tionA receptacle' of thefapparatus' illustrated in Fig; 1.

jpurity from an impures'ource of the metal by a' novel sures, which are `usually less thaiilmm, of mercury, a pressure in the system of' about 0.10 to as Vlow as 8X10-f5 mm. of mercury has proven most satisfactory. n

In anotherapplication to uranium metal and) employing. a static system, a 'quantity ofimpure metal is placed in the reaction chamber on a quartz cylinder so that the uranium metal is close to the heated surface.;l Thesystem isv subsequently evacuated to a pressure of approximately lll5 mm. of mercury` The furnacearea and the refractory filament are .graduallyY then .brought up to a temperature of approximately 500 Clior theffurnace area and 1350 C. for the heated g surface in 'orderv to suiiicientlyV degas the refractorylilament as well jas the cury is then introduced into the system, and finally iodine vapor ispassed over the impure uranium "mfet'all at a pressure of about .9 mrrnof mercury thereby forming the? metal halide.= Thesystem is Vthen closed; mostf'ot the metalfhalideis thermally decornposeclvvitlqlinl a few patented. Feb. 1o, resar 3 hours'. The apparatus is then cooled and the high-purity metal is removed from the system.

Referring to Fig. 1, an apparatus which has been found suitable for practicing the above methods as a flow or staticmethod is illustrated; it comprises a furnace 1 in which is installed a reaction chamber 'or receptacle 2, preferably 'of quartz, near one 'end of which is an exhausttube 3 Awhich leads to a vacuum pump of the mercury diffusion type (not shown). Intermediate be-l tween thereaction chamber 2 and the vacuum pump, a stopcock 3a is placed in the exhaust tube 3 which affords a means fo'r closing Aoff the system when employing static batch method. Extending into the opposite end of the reaction chamber 2 is an inlet tube 4 having a restricted orifice Sfor producing a jet, and an extension or support lt'which extends therefrom and is 'positioned to support a 'piece lor' mass of impure uranium metal in alignment with the orifice 5. A filament 7, preferably a Wire, is adaptedto conduct, and be heated by, electricity, and `itis connected to lead wires which extend through the end of chamber 2 that is opposite inlet duct 4. The filament 7 is positioned in alignment with the orifice 5. Io'facilitate the removal of'the filament, its lead wires 'are' mounted in a ground gl'ass'stopper capable of a sealed fit with the end of the chamber 2.

The chamber 2 has a dependent welly portion 8 arrangedto accommodate a refractory receiving container 9' directly beneath the filament 7' and attached to the chamber 2 by a ground quartz joint. The spacing of the receiving container 9 beneath the filament 7 is sutiiciently vdistant to permit molten uranium metal which drips off "fr'lom the filament to cool sufficiently so as to bel subs'ta'ntiallyunreactive with Vthe receiving container 9.

` For supplying controlled amounts of the halogen and the reducing gas into the chamber 2, the inlet 4 is conynect'ed by oney branch to a receptacle 10 from which a halogemsuch as iodine vapor, may be introduced under the control of a' stopcock 11, and by a second branch connected to receptacle 12 Whichcontains hydrogen. In preparinga purer uranium, thel halogen is introduced into 'the 'inlet duct 4 with hydrogen which is preferably in a' very pure and dry condition and the flow rate of which is' controllable. of'pure dry hydrogen,` said second branch of the tube f4 is connected to the outlet of the receptacle 12 in which 'isfa palladium thirnble 13. The receptacle 12 is located within a'furnace 14 and its inlet is connected to` a drying receptacle 15 into which hydrogen may be introduced from a suitable source; a purifying trap 16, preferably a'lfliguid' air trap, may be interposed between the drying receptacle 15 Vand the receptacle 12. The receptacle 15 `contains 'an' agent for drying the hydrogen used. For hydrogen a suitable agent is P205. The trap 16 is for removal' of other impurities and in the apparatus illustrated is a conventional liquid 'air trap. Control of the iiowo'f "hydrogen is effected by heating and cooling of the palladium thmble'in the receptacle 12; .additional :gemist iodide vapor thus formed, along with any excess hydrogen and iodine vapor, contacts the filament 7 which may be heated to a temperature at least above the decomposition temperature of the uranium halide and preferably above the melting point of the uranium. In order to continuously establish the melting point of uranium metal, the temperature of the filament 7, should be kept at from 1350 C. to about 1500 C. Uranium of a very high degree of purity will form in small globules on the filament 7 as a result of this decomposition; these coalesce into droplets of sufficient size to drip from the filament 7 into the receiving container 9 which is positioned therebeneath so that when the droplets reach the container 9, they have become cooled below a temperature at which the metal would react appreciably with the container 9. Waste gases and decomposition products are withdrawn from the system through the exhaust tube 3 by the high-capacity vacuum pump when utilizing the dynamic flow method.

The batch methodV employs the same reacting constituents and apparatus as in the flow method, except thatr the stopcocks 3a, 11. and A11a are closed after'tlle required amount of uranium halide is introduced into the system at a low pressure. The thermal decomposition reaction of the metal halide will proceed at a more rapid rate and will yield a greater `percentage of the metal upon the introduction of hydrogen at a low pressure.

For obtaining a controlled ow control may be provided by a suitable stopcock 11anterposed between the receptacle 12 and the conventional T-connection wherein the halogen is introduced into the system.

The halogen shown illustratively may be iodine vapor and is'supplied from a mass of iodine crystals in the receptacle 10, the rate of'fiow of iodine vapor therefrom being [controlled by the temperature and shape of the receptacle 10.

The mixture of iodine vapor and hydrogen in controlled amounts is passed through the orice 5 and impnges as a jet on the mass M of impure uranium metal vwhich is positioned on the support 6. At the point of impingement of the jet onto the metal, the receptacle 2 is maintained at a temperature at which uranium halide vapor is formed as a result of the reactionof the halogen with the uranium metal. A temperature of from 450 to `50"V Q issatisfactory for this purpose. The uranium Referringto Fig. 2, a reaction chamber 17 is illustrated, this chamber being in all respects the same as chamber 2 except that it is made of Pyrex glass instead vof quartz. The use 'of Pyrex glass instead of the more lexpensive quartz is made possible by the introduction of'a 'tantalum radiation shield 18 which surrounds the filament in spaced relation thereto and to the walls of the receptacle 17. The shield is provided with apertures 19 so arrangedto permit viewing of the filament and to permit the dripping of the metal from the filament into a receiving container 9. The function of the tantalum shield 18 is tol prevent aging and deterioration of the glass which would be occasioned Iby a direct exposure thereof to radiation.

. In operation, to provide a continuous flow, the apparatus, which is sealed 'between the source of hydrogen and the exhaust tube 3, is first flushed with hydrogen and then evacuated. These steps are repeated alternately until the system is thoroughly cleansed and until the furnace and filament have reached the desired temperatures; a satisfactory vacuum is finally established in the chamber 2. The iodine is brought to the desired temperature. The furnace is heated to produce a temperature of 515 to 520 C. in the reaction chamber 2 at the location of the metalM. The filament temperature may be raised to as high as 1350 to 1500 C. and in the case of urani- .um to 1450 C. A few experiments will be necessary for determining thetemperatures in the case of metals other than uranium.

For contact with uranium, because of its high degree of reactivity at elevated temperatures, a refractory material has to be used that is unreactive with the uranium above the melting point of the latter. A filament found satisfactory for use with uranium is a tungsten wire coated with thoria. A Nernst type filament is particularly good.

The present novel methods of producing pure metals in the presence of hydrogen, therefore, have certain definite advantages among which are that it permits -limits `but 'maintains itself at a rather constant is fiiietals other than the uraniumillustr'ativly described. Among these factors'are the temperature of the'rneta'l, filament tempera-ture,"pressures, rate of halogen `ow, rate of reducing gas or hydrogen iiow, and also the surface characteristics fof the impure metal'source.

The temperature in the vaporizing portion of the reaction chamber should be maintained high enough to cause rapid and complete reaction `of ythe iodine vapor and metal to produce metal iodide vapor, but at the ture, the greaterthe tendency ofthe -hot metal to react with any excess iodine to reform volatile or non-volatile `metal ioddes whichwould be swept out of thesystem in the dynamic method but not in the static method. For uranium, a filament temperature of 1350 C. appears to be about the minimum temperature for an vappreciable reaction rate and operation has been successful at as high when producing liquid metal.` The metal iodide decom- .poseson the heated surface ofthe coated filament and the purified uranium metal formedideposits thereon. The deposited uranium metal, in turn, provides a surface on which additional uranium Viodidelis decomposed. Since the surface area of the metal on the filament increases until the metal in globules formeddrip ofi, the effective decomposition surface varies periodically within certain average for any substantial period of operation, i

The rate of iodine iiiow is proportioned tothe state of subdivision of the impure uranium source. `Assuming that the surface area` ofthe uranium source is constant,the maximum iodine flow isA limited only -by the 4amount of iodine vaporthat 'can react Withthe uranium. Any excess of iodine 4above'this maximum tends to reverse the reaction at or beyond the filament =by combining with metal already precipita-ted on the filament. Obviously, the iodine iiow can be adjusted, and it is increased or decreased in a relationshipl directly proportional to an increase or decrease, respectively, of the surface area of the metal source.

As mentioned, the rate of iiow of iodine vapor is readily controlled by the temperature at which the iodine is maintained, the shape of the receptacle in which the iodine crystals are contained, the degree of low pressure in the system, and the amount of other gas, such as hydrogen, which is introduced.

The flow o f hydrogen is controlled by the temperature ofthe palladium thimble 13 through which the hydrogen is passed before it enters tube 4 and by which it is purified. In the particular case, hydrogen will also react l with the halogen freed by decompositionof the halide so as to prevent the freed halogen from recombining with the precipitated metal, as in the case of uranium and iodine vapor. Furthermore, the hydrogen may catalyze the formation of uranium iodide at low temperatures by forming uranium hydride or by forming HI which, in turn, reacts with the uranium. Again, the hydrogen tends to drive the high temperature reaction in the direction of metal production by combining with excess iodine vapor originally introduced, or formed as a result of decomposition of uranium iodide. Another effect which appears to result from the introduction of hydrogen along with controlled amounts of iodine vapor into a low pressure system is that the amount of iodine vapor which would tend to reverse the reaction at the filament if inexcessive amounts can be more nearly maintained at the exact amount required.

'25 a temperature as 1500n C.; about 1450c C. is "preferred It shall be noted that there will be a pseudo equilibrium for the unstable reversible reaction where uranium will react with the halogen vapor, such as iodine, to form the uranium tetraiodide. Also, there are two possible side reactions, namely the formation of hydrogen iodide and the formation of the uranium tri-iodide from uranium tetraiodide by reaction with hydrogen. Since the Iheat necessary to decompose or reduce the tri-iodide is somewhat less than that needed for the tetraiodide,`the' presence of hydrogen in the system will tend to produce-greater yields and va more efiicient end reaction.

The surface ofthe filament may be a refractory which is substantially unreactive with the `particular metal being prepared at the temperatures above the melting point of the metal, Vpreferably arefractory inorganic compound, or a nonrnetallic filament, such as a filament of fthe Nernst type, should be used. Thoria-lanthanaand uraniacoated filaments of tungstan have proven satisfactory. The hot surface used may also be madeof a substantially 'unreacting material such as tantalum; and such surface `has proven satisfactory.

It is desirable that the surface of the filament be of as high a degree of purity as possible.

For other metals, such asplutonium, thorium, ceriurn, titanium, formedl by thermal "decomposition of their respective metal halides, it is advantageous to use other types of filaments, such as Nernst-type filaments, which are refractory, with percent thoria and 15 percent yttria, and filaments with no metal at all. The filament temperature which will give appreciable yields of the metal desired without reacting 'with the metal or alloys thereof determines the type preferred in a particular case; it should be borne in mind that the lower-the operating temperatures required, the Vless is -the chance for alloy formation so that uncoated metal filaments are useful for some purposes.` The receptacle 9 may be off-thoria, cerous sulfide, or even platinum. Y

In a preferred operation'inv the case of uranium,'iodine vapor and hydrogen, the 'temperature in the reaction chamber is maintained at .515 C. to 520 fC., the filament is maintained at a temperature of about l450 C.,

'the pressure in thefsystem, in mm. of mercury, is maintained at about 8X10-5 when first evacuated, at 0.9 mm. for the iodine vapor and for hydrogen at from 3X1()-3 to l; the ratio of hydrogen to iodine vapor should be about 5 to 1.

While the above invention has been described illustratively as applied to uranium, it is applicable to other metals, including tungsten, vanadium, titanium, thorium, halfnium, zirconium, plutonium, neptunium and the rare earths.

The word filament is used herein to denote any body or surface which is heated for effecting decomposi tion of the vaporized volatile compound ofy the metal, regardless of its shape or size.

In addition to the thoria coating on the tungsten wire, other refractory surfaces may be provided. These' include lanthana for tungsten, tantalum nitride for tantalum, and also a properly machined graphite rod is usable; all of these are unreactive with metals at temperatures as high as 1650" C.

For preparing a thoria-coated tungsten filament, a small piece of tungsten wire is spot-welded across the gap between two ordinary pieces of tungsten and then is covered with a thoria-containing slurry either by dipping or by means of a dropper. The slurry coating is dried in air and then the wire is placed in a chamber where it is subjected to a vacuum while the temperature is slowly brought up to 1650 C.; it is maintained at that temperature for a While, the entire procedure requiring from one to two hours. The slurry coating preferably is made as follows: 25 drops of a saturated thorium nitrate solution in water are added to 5 c. c. of a very thick thoria paste and sufficient water is added to give the proper consistency. In some instances, where the exclusion of water is desirable, anhydrous materials, e. g.

vsistance. -portion ofthe lament depends entirely upon the cross .amyl or other alcohoL-.areused as ,solvents instead of .Watel-,lff

TheY lanthana-coated tungsten 4filament is prepared in the same manner, except for thev ingredients. Lanthana itself is extremely hard to dehydrate but a slurry of anhydrous lanthana in ethyl alcohol provides a coating which is suitable after the baking procedure.

=Another `filament ,is one of tantalum `coated with tantalum Vnitride obtained by yheating a tantalum wire very, rapidly to an extremely high temperature in an atmosphere of nitrogen. None ,of thesefilaments thus `coated reacts with metals deposited on them keven at .temperatures as lhigh as 1650 C., but the base metals of the filaments would react,

Ordinary carbonfilamentsrhave been found unsatisfactory. However, a lament prepared'by machining an .ordinary preformed graphite rod to a very small cross section with a portion having a cross section of less than normal was ,found satisfactory. The lament is then taperedA toward the smallest-diameter portion from a short distance on either side thereof. Such a `filament ,has a section of gradually decreasing cross section toward the intersection of thetapers and thus of varying re- The heat `at any given point along the tapered section. v r

What is claimed is:y l

1. A method of preparing uranium metal of high purity, consisting in contacting impure uranium metal with halogen vapor at a temperature of between 450 and 550 C. whereby uranium halide vapor is formed; contacting said uranium halide vapor, in the presence of hydrogen, with a refractoryrsurface lof a temperature above the `melting point of uranium whereby said uraniumhalide is thermally decomposed and molten uranium deposited on said surface, said surface consisting of material nonreactive with molten uranium;` collecting the molten uranium dripping from said surface; and maintaining a --subatmospheric pressure of below 1 mm. mercury during the entire operation.

- .1 2. The method of claim 1 wherein the temperature of .therefractory surface is between 1350 and 1550 C.

`3. The .method of claim. 2` wherein the' halogen is *,iodine.

`vacuum impure uranium with a mixture of rhydrogenand iodine at a pressure of from 3x10-3 to 1 mm. mercury for the hydrogen and of about 0.9 mm. mercury v,for tthe viodine and at a temperature of between 515 and 520 C. whereby uranium iodide vapor is formed, said mixture having a ratio of hydrogen to iodine of about-511, contacting said uranium iodide vapor with aI refractory sur face of a temperature of about 1450 C.l whereby said `uranium iodide is thermally decomposed and molten uranium is deposited on said surface, said surface con Vsisting of material nonreactive with molten, uranium;

and collecting the molten uranium dripping from lsaid surface. Y Y 6. The method of claim 5 wherein the refractory surfac consists ofvtantalum. 7. The method of claim 5 wherein the refractory-,surface consists of thoria. r

References Cited in the ile of this patent i UNITED STATESv PATENTS 553,296 Aylsworth Ian. 21, 1896 873,958 Von Pirani Dec. 17, 1907 903,922 Van Brunt Nov. 17, 1908 1,306,568 Weintraub June 10, 1919 1,648,962 Rentschler et al. Nov. 15,1927

OTHER REFERENCES v p Chemical Abstracts, vol. 2 (1908), p.:632, Uranium Iodide. ,l

Mellor: Inorganic and Theoretical Chem.,l vol. 12 (1932), publ. by Longmans, Green & Co., London, p. 93. Alnutt et al.: Electrochemical Society, Pre-printvSS-SO,

Zirconium Metal, lts Manufacture, Fabrication and Properties (vol. 88 of the Transactions), October 17, 1945, pp. 357-366.

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