US2787536A - Process for melting and refining uranium - Google Patents

Description

April 2, 1957 F. H. SPEDDING ETAL Y 2,787,536

PROCESS FOR MELTING AND REFINING URANIUM 4 Filed Dec. 3, 1946 IAii Vnt@ rnoonss non MEL'rlNG AND unrININGV y URANIUM Frank H. Speddingaand Harley; Wilhelm-,-.Am'es,' Iowa,

assignorsto the United. States gofz America asfreprfsented by the United-States AtoprncfEnergy-Commission Application December 3, 1946, Seriall\lo:"713,706'` This invention relates` to the.melting.and casting. of uranium, `and more particularly toa novel method and means for economically melting rand casting this metal without materially impairing the neutronic purity thereof.

This problem arisesfrorn the fact that molten metals such as uranium react with most containers and thus tend to become impure. As will` be understood by those skilled in the arts related to nuclear physics, neutronically pure uranium has found wide use, as for example, in so`-called thermal-neutron reactors wherein nuclear fission chain reactions are'sustained ata low average neutron energy of the order of about 0.3 e. `v. In` such re-Y actors, the uranium is preferably of `high neutronic purity in order to maintain neutron losses by impurity absorption thereof at a relatively lowl value. In this connection, it may be noted that neutronic purity does not n necessarily imply chemical purity, but does imply the absence of foreign materials having a relatively high neutron-capture cross section.

Many attempts have been made to successfully. melt and cast uranium in. graphite crucibles Vand molds, inasmuch as graphite has a relatively low neutron-capture cross section and may be economically purified from a neutronic standpoint. However, in the past, such attempts have been generally unsuccessful inasmuch as the moltenuranium reacted with the graphite to form uranium carbide which dissolved in the molten uranium. Thus, the uranium castings or ingotsproduced by the prior art methods contained vlarge quantities of. uranium carbide. For this reason, those skilled in the art .turned to beryllia and other relatively expensive and. less desirable materials for constructingcrucibles and molds utilized in the melting and casting of uranium andother metals `of relatively great atomic number.

According to the presentinvention, it hasbeen diso covered that by maintaining the temperature of molten uranium at a relatively low value of the order of 1650 C. or less the uraniumcarbide is not dissolved inthe molten uranium but formsa skin or-liner at .the interfacer between the molten uranium..and the .graphite re-A ceptacle. This skin or liner under someconditions ad-V heres to the receptacle, Iand under other conditions clings to thel exterior of the uranium Veastingor ingot from v which it may be readily removed in any convenient manner. Thus, by preventing ,thef uranium carbide 1 from dissolving in the molten uranium', the n ovelprocess en ables the economical production-ofneutronically pure uranium castings with the utilization of-graphite crucibles, molds, and other portions,k .tfthe. apparatus contacting the molten uranium.

The foregoingV and otherA objectsand,advantages o f the invention will become apparent from a consideration ofthe following specilcationand' the accompanying drawings, wherein:

Fig; fis'ayertical sectional view qt'aniapparatus'for carrying` out theinventionj Prg; zafis across sectional' viewj'takehfiir'fthcplane"in dicated-by4 the line 2,-2 of Fig. l, the heatingY coil being. omitted; and

Fig. 3 is an enlarged fragmentary sectional view of a portion of the apparatus shown inlFig. 1.

The apparatus shown in Fig. l comprises'a melting chamber 2 formed by a graphite Crucible 3 having a lid 1i and pe forate bottom 5, said chamber containing uranium blocks or ingots 1. The melting chamber isiof sufiicient interior dimensions to contain one or more ura-v nium ingots generally having both skin and interior impurities, land is of sutiicient wall thickness to resist the' high temperatures and corrosion products of the casting operation. Lid 4 is designcdto engage'the upper end of crucible 3 and preferably has an orifice 7 to permit; the escape of various volatile products given off during the casting operation. To prevent deleterious eifects of such volatile products, protectivel capl S may be placed Bottom S includes one or more perforations i2. asl

best seen in Fig. 3 to accommodate flow of the molten uranium during the casting operation. The size of the openings i2 are dependent somewhat upon bthe amount and area of'skin impuritiesthat are associated with the uranium ingot 1. ln practice, it has been found that openings of a diameter-ranging from im, to l are suitable for this purpose.

The Crucible rests upon and is supported by mold i3 which contains one or more casting chambers 14. Chamber 14 is of the shape of the desired casting `and' should be of Aa size sufficient to receive all of the pure uranium contained in ingots Crucible 3 may be secured on mold i3 in alignment by a conventional shouldered joint.

Preferably, all of the above-described elements ofthe-apparatus are formed of a hard, dense formof car-V bon such as graphite which may be. machined to the proper size and shape. Althoughthere is a tendency for the molten uranium to react with-the carbon to form uranium carbide, for most purposes such carbide formation is preferred to contamination resulting from the use of other refractory materials for the apparatus. thermore, by the novel casting procedure hereinafter described, the formation of uranium carbide and dissolv-V ing thereof in the molten uranium are greatly minimized.

The mold 13 rests upon insulating block 15 which may suitably be formed of a refractory material such as'rebrick. For the purpose of obtaining a vacuum in the apparatus, channels i6 may be for-med in the insulating block 15. The insulator 'block i5 is preferably supported by a water-cooled brass head `17, which includes a water chamberilS and inlet and outlet means 19. Channel 2&3 extends through yth'ehead in communication with channels16l of the insulating block. Projection 2l may lbe used for attaching-suitable evacuating means to the head. To permit the use of temperature-measuring means such as thermocouples or the like, an opening or passage ZZ is provided, saidpassagerextending through the head 17', the block 15, and into mold13. During the casting operation, thermocouples may be inserted through opening'ZZr Patented Apr. 2, 1957 Furafsfise couples where it is desired to ascertain the temperature in melting chamber 2.

To minimize the loss of heat 'from `the melting chamber 2 and mold 13, insulati-on may be provided around the apparatus as shown in Fig. l, the insulation preferably being in the form `of sleeve or sleeves 23 formed of thermo-setting sillimanite and sawdust which is shaped and then fired at a high temperature. Sleeves 23 may be laid up around the apparatus and a flat circular sheet 35 of the same material placed over the top, the sheet being protected by cap 8 from the deleterious effects of the volatile products given off during the casting operation. The insulating sleeves rest upon an extension of head 17 provided for that purpose.

Inasmuch as the casting operation is preferably per formed in a Vacuum or in an inert atmosphere, shell 24 is used to envelop the entire apparatus, said shell being sealed in any convenient manner to flange 25 of head 17. The shell is formed of a non-conducting heat-resistant material such as quartz.

To secure high temperatures in melting chamber 2 and casting chamber 14, induction coil 28 is provided which is suitably arranged to surround shield 24 and to be raised and lowered into any desired position with respect to the apparatus.

The apparatus is assembled by placing insulating block 15 on head 17. Mold 13 is then placed on block 15 so that `openings 22 are in alignment to receive the thermocouple which is sealed to head 17. Crucible 3, with perforate bottom in place, is positioned over the casting chamber 14, and the crude uranium ingots 1 placed inside the crucible. Lid 4 and cap S are placed in position and insulation members 23 and 3S are assembled around and over the apparatus. Shell 24 is then properly positioned over the insulation and securely sealed to ange 2S. The assembled unit is evacuated by evacuating means attached to projection 21, and the apparatus is tested for leakage of air into the same.

In carrying out the process, the induction coil 28 is lowered around the shield 24 with the bottom portion of the coil slightly below the bottom of crucible 3 as shown in solid lines in Fig. 1. The coil is operated at sufficient power to bring the temperature of the uranium to approximately 1300 C. which temperature is maintained until the molten uranium flows out of the melted ingot 1, through orifice 12, and into casting chamber 14. The coil is then lowered to the position indicated by dotted lines in Fig. l in which the lower end or" the coil is approximately level with the bottom of casting chamber 14. A

temperature of approximately 1300 C. is maintained for r from five to fifteen minutes in the casting chamber to vaporize the volatile impurities remaining within the cast uranium. Where the ingots 1 used are formed by the reduction of a uranium halide with magnesium, the -above step is particularly important in that it removes substantially all of the magnesium associated with the cast uranium. The power to the coil 2S is t-hen shut off -and the apparatus permitted to cool. After cooling, the vacuum is released and the apparatus disassembled for removal of the purified uranium.

In melting the uranium in chamber 2, it is preferred that the heating be carried out at such a rate that the temperature of the uranium throughout the ingots does not appreciably lag behind the temperatureV of the uranium adjacent the walls of crucible 3, or the temperature of the carbon walls. Where an induction coil is used to heat the crucible, the temperature of the uranium in the crucible is controlled by increasing or decreasing the power supply to the induction coil. In this way, uranium carbide formation in chamber 3 is reduced and violent ebullition of the volatile impurities is prevented during the melting operation.

Such uniform heating is.

particularly desirablein the initial stage of melting the.`

uranium. A slight lag between the temperature of the inner and outer ingots is not'objectionable although the temperature of the outer ingots should not be permitted to rise to a point where there is excessive formation of uranium carbide.

It has been found that pipes or hollows in the upper surface of the final casting may be prevented and better castings generally obtained by regulating the cooling of the cast metal in such a manner that the solidification begins at the bottom of the casting chamber and proceeds upwardly. This may be done either by having the upper portion of mold 13 at a higher temperature than the lower portion when the casting is made, or by gradually raising coil 28 from the dotted-line position of Fig. l after the uranium has iiowed into chamber 14 to maintain the upper portion ofthe casting chamber at a higher temperature than the lower portion. Preferably, some heating is supplied to the upper portion of the casting chamber while the cast uranium is cooling.

The withdrawal of the molten uranium from the skin impurities through orifices 12 and into chamber 14 during the casting operation results from the weight of the molten uranium 30 bearing upon the portions of film 29 between supporting points 31, as shown in Fig. 3. As the uranium is converted to a molten state with film 29 being flexible but relatively inelastic, the molten uranium tends to stress the film within orifices 12. Eventually, the sl-:in bursts and the molten uranium ows through orifices 12 and into chamber 14, leaving film 29 in melting chamber 2.' It is contemplated that means of various shapes and sizes may be used to provide support for the lm.

Preferably, the inner surface of melting chamber 2 and particularly the conical surface 10 have adherable characteristics which aid in maintaining the skin in chamber 2. It has been found that the skin 29 tends to adhere to machined graphite as the molten uranium flows through orifices 12. Where a plurality of ingots are treated, the side walls of chamber 2 retain portions of the skin of the top ingots.

The casting process may be carried out in the presence of air at atmospheric pressure, although there is a greater tendency for the formation of uranium oxide on the cast uranium than where the process is carried out under vacuum or in an inert atmosphere. In a specific example of casting the uranium in the presence of air, a uranium ingot weighing 8.37 pounds was placed in the melting chamber and the apparatus assembled as shown in Fig. l. The induction coils were used to raise the temperature of the ingot to l400 C. in ten minutes. This temperature was held for ve minutes and then lowered to approximately 1165 C..and the latter temperature maintained for fifteen minutes, giving a total heating period of thirty minutes. The molten uranium broke through the film of impurities and flowed into the casting chamber. The apparatus was allowed to cool for several hours and then disassembled. The impurities which remained in the melting chamber were flaky and comprised approximately l0 percent of the total weight of the ingot. The cast uranium Was removed from the casting chamber and was comparable in appearance to the recasting carried out in vacuo. After being sandblasted, the ingot was found to weigh 7.25 pounds, thus giving a casting yield of approximately 87 percent.

In heating ingots resulting from the reaction of a uranium halide and a substantial excess of reducing metal such as calcium or magnesium, there is a rapid vaporization of the reducing metal before the uranium metal ows into the casting chamber. This sudden vaporization, resembling an explosion, usually takes place at approximately 1050" C. where magnesium is used, and at between l300 C. and l400 C.,where calcium is used. It has been found that such violent vaporization may be prevented byv decreasing the amount of reducing vmetal used during the reaction to a point where there is only suflicien't excess 'to obtain a complete reduction of the uranium halide.

Where the melting chamber 2 and casting chamber 14 are formed of carbon such as graphite, it is desirable to keep the temperature of the uranium in such containers below from 1600 C. to 1700" C. to prevent excessive formation of uranium carbide. Uranium carbide is not formed to any great extent where the molten uranium is cast below 1600 C., particularly if the time of contact between the molten uranium and the carbon is kept to a minimum, for example, to to 20 minutes. It has been found that the formation of uranium carbide increases rapidly for the reasons above discussed where the temperature of molten uranium is above about 1650 to 1700 C. when cast. Where it is desired to obtain the cast uranium metal with a minimum of uranium carbide, the step of heating mold 13 with coil 28 in its lower position to vaporize the volatile impurities after the uranium has owed into chamber 14 may be omitted.

The term uranium ingot, as used in the description and claims includes any metallic uranium mass containing impurities of appreciable amount such as skin fragments which are solid at the melting point of uranium. Thus, it has been used to define a crude uranium ingot which contains impurities resulting from the production of the uranium, as well as any mass of uranium metal which has been exposed to air, thereby having uranium oxide formation on the outer surface of the mass.

The above detailed description is for purposes of illustration and the invention is to be limited only by the scope of the following claims.

What is claimed is:

1. A process for casting uranium, comprising introducing the uranium into a melting container and restricting the contact of the uranium to that with graphite surfaces; heating the uranium to at least its melting point but to a. maximum of 1650 C. whereby uranium carbide formed is prevented from dissolving in the uranium and retained on its surface in the form of a skin; and separating the uranium from said skin.

2. The process of claim 1 carried out in an oxygenpoor atmosphere.

3. The process of claim 1 wherein the melting is cartied out at a temperature below 1600 C.

4. The process of claim 3 wherein the melting is carried out at approximately 1300 C. Y

5. A process for casting uranium, comprising introducing the uranium into a melting container and restricting the contact of the uranium to that with graphite surfaces; heating the uranium to at least its melting point but to a maximum of 1650 C. whereby uranium carbide formed is prevented from dissolving in the uranium and retained on its surface in the form of a skin, and introducing it into a graphite mold; and cooling the uranium in the mold starting with the bottom layer and gradually proceeding upwardly with the cooling step until all of the uranium has solidied.

6. A process for casting uranium, comprising introducing the uranium into a melting container and restricting its contact to that with graphite surfaces; heating the uranium to a temperature of about 1400 C. within ten minutes and maintaining this temperature for about ve minutes; reducing the temperature to 1165 C. and maintaining this temperature for about fteen minutes whereby uranium carbide formed is prevented from dissolving in the uranium and retained on the surface in the form of a skin; separating the uranium from said skin; and allowing the separated uranium to solidify.

References Cited in the le of this patent UNITED STATES PATENTS 1,172,506 Vickers Feb. 22, 1916 1,477,508 Lohmann Dec. 11, 1923 1,518,083 Lohmann Dec. 2, 1924 1,568,685 Moore Ian. 5, 1926 OTHER REFERENCES Moore: Transactions of the American Electrochemical Society, volume 43, Preparation of Metallic Uranium" (1923), pages 317-328, page 323 especially relied upon. (Copy in Scientific Library.)

Claims (1)

1. A PROCESS FOR CASTING URANIUM, COMPRISING INTRODUCING THE URANIUM INTO A MELTING CONTAINER AND RESTRICTING THE CONTACT OF THE URANIUM TO THAT WITH GRAPHITE SURFACE; HEATING THE URANIUM TO AT LEAST ITS MELTING POINT BUT TO A MAXIMUM OF 1650*C. WHEREBY URANIUM CARBIDE FORMED IS PREVENTED FROM DISSOLVING IN THE URANIUM AND RETAINED ON ITS SURFACE IN THE FORM OF A SKIN; AND SEPARATING THE URANIUM FROM SAID SKIN.

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