US2782475A - Apparatus for vacuum casting of uranium - Google Patents

Description

Feb. 26, 1957 Filed Dec. 8, 1944 H. A. WILHELM EI'AL APPARATUS FOR VACUUM CASTING `0F' URANIUM 2 'sheets-sheet 1 Feb. 26, 1957 H. A. WILHELM ETAL 2,782,475

APPARATUS FOR VACUUM CASTING oF URANIUM Filed Deo. 8, 1944 2 sheets-sheet 2 APPARATUS FOR VACUUM CASTING OF URANIUM Harley A. Wilhelm and C. I". Gray, Ames, low-a, assignors to the United States of America as represented by the United States Atomic Energy Commission Application December 3, 1944, Serial No. 567,283

2 Claims. (Cl. 22-73) The invention relates to a process and apparatus for treating uranium metal.

Where uranium has been obtained in the form of substantially pure metal, the reactivity of the uranium causes impurities in the form of uranium compounds, such as uranium oxide, to be associated with the metal immediately upon exposure of the metal to ordinary atmospheric conditions. The formation of uranium oxide, for example, givesthe uranium metal a hard, dense skin or tilm which tends to increase in thickness as.the metal is continued to be exposed to air.

Uranium metal in the form of an ingot or biscuit is produced by the reaction of a uranium halide such as Ulit and a reducing metal such as calcium or magnesium at a temperature sufficiently high to form molten uranium and a molten slag and under conditions such that the uranium remains in the liquid state for atime suliicient to permit layer separation of a molten uranium and a slag layer. The resulting uranium upon cooling is relatively pure but may also include impurities such` as uranium carbide, calcium oxide, or the like, that are solid at the melting and casting temperature of uranium. Such impurities are usually on the surface of the ingot and are associated with the skin of uranium oxide.

lt is found that uranium metal may be separated from the solid impurities such as the skin portion of the ingot by heating a uraniumV mass containing such impurities to at least the melting point of the uranium metal and then withdrawing the molten uranium away from the solid impurities. Thus it has been found that the skin portion of a uranimn body has a higher melting point than the interior thereof and that it is possible to heat the body at a temperature such that the interior of the body becomes molten while the exterior skin remains essentially solid. The separation may be carried outV by supporting the uranium mass in a melting chamber over a closed opening or aperture in the bottom of the chamber. After the uranium metal mass has become molten the aperture is opened, thereby Causing the molten uranium to ilow through the aperture and away from the solid impurities. Where the solid impurities are in the form of a skin around the uranium metal mass, the skin may be punctured as the aperture is opened and the molten uranium released from the interior of the mass.

In the reaction in which calcium, magnesium, orA other reducing metal is reacted witha uraniumV halide, a portion ofthe reducing metal may be associated with the uranium metal mass. Such reducing metal, and particularly magnesium, is volatileV atthemelting and casting temperatures of uranium and may be removed for the most part during the melting of the uranium ingot. This invention relates to improvements in the apparatus disclosed in the copending applicationY of'Frank` I-I. Spedding et al., Serial No. 554,920, tiled September 20, 1944,` and also nto improvements in the processes disclosed inthe copending application of George Meister, Serial No. 533,112, led April 28, 1944, now PatentNo. 2,756,138, granted Iulyf2`4, 1956. y

States aten-t It is an object of the invention to provide a process and apparatus for purifying uranium metal; for separating uranium metal from various impurities associated vwith it that are in solid form at the casting temperature of uranium; and for removing from the uranium impurities that are volatile 'at the melting and casting temperature of uranium. A further object of the invention is the provision of a process and apparatus for casting molten uranium metal into a carbon mold without excessive formation of uranium carbide. It is a still further object of the invention to obtain a shaped product of exceptionally pure uranium metal by casting molten uranium metal into a carbon mold. Other objects and advantages will appear from the following description and drawings appended thereto.

A In the drawings, in which like reference characters refer to like parts: t

Fig. l is a sectional elevation of one embodiment of the invention;

Fig. 2 is a sectional elevation of a modified form of the invention;

Fig. 3 is a sectional View taken on line 3-3 of Fig. l;

Fig. 4 is a sectional view taken on line 4-4 of Fig. 1;

Fig. 5 is a sectional view taken on line 5-5 of Fig. 2; and

Fig. 6 is a sectional view of a modified form of the casting chamber shown in Fig. 5.

.in one embodiment of the invention, an apparatus 1 is provided as shown in Fig. 2 which includes a crucible 3 having a melting chamber 2 formed within and adapted to contain one or more uranium ingots or biscuits, 4. The Crucible is preferably of cylindrical form and comprises crucible walls 6 and sloping bottom 7. A circular lid E is adapted to engage the upper end of the Crucible. Preferably, the lid has an opening 9 which facilitates the escape of volatile impurities vaporized during the melting and casting operation. To prevent direct impingement of the volatile irnpurties upon outer parts of the apparatus, cap 11 is provided which includes sideopenings 12 to permit the escape of vaporized impurities from the cap. If it is desired to enlarge melting chamber 2, suitable extensions may be placed on the upper end of crucible walls 6 with the lid 8 engaging the topof the uppermost extension.

The crucible 3 has an aperture 16 extending vertically through the bottom of the Crucible, the aperture being sufficiently large for the passage of molten uranium therethrough. Although the size of aperture 16 isso'mewhat dependent upon the character of the solid skin irnpurities, it has been found that circular apertures of a diameter ranging from 1A; inch to two inches are'suitable, for this purpose. The aperture 16 may be closed by suitable means such as a plug 18 which seats on vsloping bottom 7 or, preferably, into rabbrets providedV for thatV purpose. A recess 52 is provided in the lower portion of the Crucible 3 to receive temperature measuring means used during the process. '1

The Crucible 3 rests upon and is supported by a mold 13 which includes one or more casting chambers 1,4. Chambers 14 are of the shape of the desired casting and should be of a size sucient to receive all'of the uranium metal contained in the ingots 4. An opening or bore 49A is Vformed in mold 13 to permit the insertion of a temperature-measuring device such as 4a thermocouple. An

opening or bore 32 extends vertically through the center of mold 13. The upper edgesof the interior portion of mold 13 are preferably bevelled to direct the flow of molten uranium into casting chambers 14 from aperture 16'. If desired, one-or more casting chambers 14 may include a lower and smaller castingV chamber 15l as shown in Fig. l. In this way uranium castings of different-sizes*- may. be-producedin the one castingloperation; One for more vents 19 are formed in the upper wall of the mold to facilitate evacuation of the casting chambers.

The elements of the apparatus above described may be formed of any refractory material suitable for the treatment of uranium. Beryllia, thoria, alundum, magnesia, sillimanite, or the like may be used although there is a tendency for such refractory materials to contaminate the cast uranium. Preferably, all of the elements of the apapratus in direct contact with the uranium are formed of a hard, dense form of carbon such as graphite that may be machined to the proper size and shape. Although there 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; and the uranium carbide formation may be minimized by proper casting procedure.

The mold 13 rests upon block 21, which may be formed of a material such as rebrick, a metal or graphite, containing channels 22 and 23 for the purpose of evacuating the system or for the introduction of an inert atmosphere. This block is supported by a base 24 which is preferably a water-cooled brass casting including a water chamber 26 and inlet and outlet connections 27. Channel 28 extends through base 24 in alignment with channel 23 of the block 21.

To dislodge plug 18, a trip rod 29 is inserted in openings provided in the block 21 and mold 13 for that purpose. Trip rod 29 is a two piece unit; the upper part 55, preferably formed of graphite, engages the under surface of the plug and is a-dapted to lift the plug free from bottom 7 of the crucible for a sufficient distance to permit the molten uranium to tlow beneath the plug and through aperture 16. The lower section 60 of trip rod 29 is made of metal preferably of steel and it engages the upper section 55 at point X within the provided opening in the mold. The diameter of this provided opening in the mold is such with respect to the sizes of the two sections of the trip rod that the upper section rests freely but firmly and in alignment on the lower section.

Where the system is operated in vacuo, a T-itting 33 having a leg 34 eonnectible with vacuum and an end opening 36 is attached ot the exit end of channel 28. A sleeve 37 is inserted into opening 36 to receive rod 29. A resilient seal 38 of rubber or the like is placed over the sleeve and rod to prevent the access of air to the system. The interior of seal 33 may be lubricated to secure a tight engagement of the seal to the rod without unduly restricting the movement of the rod. Suitable evacuating means (not shown) is attached to vacuum connection 34.

To minimize heat loss from melting chamber 2 and upper portion of casting chambers 14, insulation 39 may be provided around the mold and crucible. Preferably, the insulation is in the form of curved sheets fabricated from thermal-setting sillimanite yand lsawdust which is shaped and then red at a high temperature. The insulation is laid up around the apparatus, resting upon water cooled base 24, and `a flat circular sheet 40 of a similar material is placed over the top, sheet 40 being protected by cap 11 from the deleterious effects of the volatile impuritiles vaporized in the melting chamber.

As the casting operation is preferably performed in a vacuum, a shield 41 is used to envelop the apparatus. A water cooled extension 42, resting on ilanges of the base 24, supportsythe shield. The extension 42, of brass or the like, includes water chambers 43 and inlet and outlet connections 44. An airtight seal is provided by rubber gaskets 46 interposed between the lower end of the shield and the extension 42 and also between the extension and the base 24. Sealing material 47, which may be a high temperature melting point wax, pitch or the like may be by an induction coil 4S which is ararnged to surround shield 41 and to be raised and lowered vertically into any desired position with respect to the apparatus. To permit the use of temperature measuring means such as thermocouples or the like, an opening 49 is provided through water chamber 26 of the base, block 21 and the wall of mold 13. A thermocouple 51 is inserted in the opening 49 with its upper end resting in recess 52 of the crucible, thus coming into close proximity with the melting chamber 3. The thermocouple may be sealed to open- 49 by means of sealing material.

The base 24 is permanently mounted on a cart (not shown) and the thermocouple S1, T-tting 33, sleeve 37, seal 3S, steel portion 60 of rod 29, water hose (not shown) on connections 27, block 21 and water cooled extension 4Z are more or less permanently mounted in position as shown. This assembly is used for a number of runs without dismantling. Only occasional repair or cleaning makes it necessary to remove any of these parts from the set-up. With these parts in position the apparatus is then assembled for each run by placing mold 13 on block 21 with opening 49 in alignment. The graphite section 55 of rod 29 is put in place and crucible 3 is positioned on mold 13 with recess 52 directly over opening 49. Plug 18 is seated over orice 16 and the uranium ingots or biscuits 4 are placed within melting chamber 2. It has been found that in the evacuation of the apparatus there is a tendency for the plug 13 to becc-me unseated with the result that orifice 16 is not completely closed when the ingots are being melted. To prevent this, it is desirable to place `a small amount of a finely divided uranium compound, such as uranium oxide 53, over the plug followed by resilient turnings 54 of uranium metal. The weight of the uranium ingots upon the turnings causes the plug to securely engage the bottom of the crucible. After assembling the crucible, insulation 39 and sheet 4t) are laid up around the apparatus and shield 41 is positioned over the insulation. Evacuating means is then attached to conection 34. Induction coil 48 is lowered around the shield 41 until the bottom end of the coil is approximately one inch below the bottom of crucible 3.

In carrying out the process, the induction coil is operated until the temperature within recess 52 reaches l0O0 C., this temperature being maintained for about eight to twelve minutes. The temperature of the uranium is then raised to approximately 1360 C. and this temperature maintained for approximately l() minutes. The uranium is then cooled to approximately l200 C., and plug 18 is unseated by trip rod 29 thereby permitting the molten uranium to ow through aperture 16 and into casting chambers 14. The coil 48 is then lowered approximately 3 inches and operated for ten minutes. The current is then shut off, and the apparatus is allowed to cool. After the cooling, the Vacuum is released and the apparatus disassembled for the removal of cast uranium.

1n 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 apprecably lag behind the temperature of the uranium adjacent to the walls of crucible 3. 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 the melting chamber is lessened and the violent vaporization of impurities is prevented. Such heating is particularly desirable in the initial stage of melting the uranium. A slight lag between the temperature of the inner and outer portions of the ingot is not objectionable, although the temperature of the outer portion should not be permitted to rise to a point at which there is excessive formation of uranium carbide.

As there is a tendency for uranium metal to react with air to` form an exterior layer or coating of dense4 uranium oxide it mayv be necessary to push plug 1 8 upwardly into thev ingot when tapping the molten uranium. In this way, the outer skin of impurities on the ingot may be broken or punctured and the molten uranium from insidethe skin is; allowed to ilow through aperture 16.

Preferably, the inner surface of melting chamber 2 and particularly the surfaceof sloping bottom 7 have adherable characteristics which aid in retaining the solid impurities in chamber 2. It has been found that such impurities tend to adhere to machined graphite as the molten uranium flows through aperture 16. Where a plurality of ingots are treated, the interior surface of walls 6 retain theportions ofthe skin of the top ingots.

Pipes or hollows in the upper surface of the casting may be prevented and better castings are generally obtained by regulatingv the cooling` of the cast metal in such a manner that the solidication begins at the bottom ofthe castingY 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 48, while still in operation, after the uranium has flowed into chambers 14. Preferably, moderate heat is applied to the upper portion of the casting chamber while the cast uranium is cooling.

In a specic example of a casting operation using the apparatus described, the lower half of the 25-inch induction coil 48, having the bottom of the coil one inch below the top of mold 13, was operated at 35 kilowatts until 1300 C. was recorded by the thermocouple 51. The entire coilY was then operated atV 35 kilowatts for three to eight minutes. The lower half of the coil was then operated at 35 kilowatts to obtain a temperature of 1310 C. which was held for seven minutes. The coil was cut. oft and when the uranium had cooled to l260 C., itwas cast immediately. The coil was then lowered approximately three inches and operated at 25 kilowatts for five minutes.

ln afurther specific example of the process, the lower half of the coil was operated at 30 kilowatts with the bottom of the coil one inch below the top of mold 13 until 13009 C. was recorded'by the thermocouple. The entire coil was then operated, at 35 kilowatts for three to eight minutes. The upper halfof the coil was then shorted out and the lower half` of the coil was operated at 35 kilowatts until a-temperature of l300 C. was obtained. The uranium.wasv cooled to l260 C. and poured immediately. VThe coil was then lowered three inches and the-lower half wa-s heated at 20 kilowatts for tiveV minutes.

In heating.- uranium ingotsV resulting from the reaction of a uranium halide and a substantial excess of a reducing metalY suchas. calcium` or magnesium, there is a rapid vaporization of the unreacted reducing metal before the molten uranium. is tapped. This sudden vaporization, at times resembling an explosion, takes place at approximately l050 C. where magnesium is used, and between 1300 C. and 1400 C. Where calcium is used. It has been found that such violent vaporization may be prevented by using an excess of reducing metal just sufficient to obtain a complete reduction of the uranium halide.

In =a modification of the apparatus as shown in Fig. 1, elements similar to those described in connection with Fig. 2, have the same reference numerals as in Fig. Z. In the modified construction, the Crucible 3 and mold 13 are separated by a channel plate 56 containing channel 57 leading from orifice 16 to casting chamber 1d. As shown in Fig. 4, channel 57 may widen directly under orifice 16 to prevent restriction of the ilow of molten uranium through the channel. The vertical portion 58 of the channel directs the stream of molten uranium downward into the center of casting chamber 14. A

ventSS!v facilitates evacuation of chamber 14 without dis-Y turbing plug 10,. The trip rod 29 is of three sections of rod, and one oifset arm 62. The two lower sections 63 and 64 of rod up to point X and the oiset arm 62 are of steel while the section 55 of rod above X and to the valve ismade of graphite. The graphite collar 61 gives a close but sliding fit around the graphite section of the push rod. Such a fit does not allow molten metal to leak down into the bore 32 in the wall of the mold. This embodiment of the invention is particularly suitable for a single chamber mold as shown in Figs. l and 3. The single casting chamber 14 is preferably olf center to provide greater wall thickness for the bore or openingV 32 containing trip rod 29. lf desired, a smaller casting chamber 15 may extend from the lower portion of mold 14. If desired a strut extending across -chamber 2S (not shown) may be provided to center the trip rod. Since the casting chamber 14 extends over the center of the mold, the lower portion of trip rod 29 may be centered by sleeve 36 and be connected to the upper portion by a horizontal rod 62 in channel 23. The upper portion 5S of rod 29 extends to plug 18 through a bore 32 in the side wall of the mold. Channel 23 is enlarged to accommodate rod 62.

. The volatile -impurities are removed for the most part by heating the uranium ingot in the melting chamber at a temperature of between 1200 C. and 1500 C. While temperatures higher than 1500 C. may be used for vaporizing the impurities, it is preferred that the temperature be kept below 1500 C. to minimize the formation of uranium carbide. In vaporizing the vaporizable impurities, the temperature should be kept elevated for suicient time to permit the vaporizable impurities to be removed from the uranium. The length of -tirne for such vaporization should be as short as possible to minimize the formation of uranium carbide which latter increases as the time of Contact between molten uranium and carbon is increased.

To minimize the formation of uranium carbide in casting chamber 14, it is preferred that the molten uranium in the melting chamber 3 be cooled before plug 18 is unseated and the molten uranium is permitted to Vflow through aperture 16 into the casting chamber. Better Castings are obtained and the formation of uranium carbide lessened where the molten uranium in 4the melting chamber is cooled to slightly above its melting point before it is cast. Temperatures between ll00 C. and l300 C. have been found suitable for casting the molten uranium without causing a substantial formation of uraniumrcarbide in the casting chamber. The cooling step is particularly important in connection with the step of vaporizing the vaporizable impurities, because the temperature of the molten uranium is then maintained in the melting chamber at a temperature of between l200 C..and 1500" C.

Hitherto, carbon receptacles could not be used for making substantially pure castings of uranium metal inasmuch as the melting point of pure uranium had previously been considered to be approximately 1800 C. Consequently, in casting such uranium, the metal was raised to 1860 C. before pouring it into the carbon mold with the result that a large amount of uranium carbide was formed on the exterior surface of the uranium and then dissolved in the liquid metal at such temperature. The carbide formation of the cast uranium was also increased by additional time required for the molten uranium poured at a temperature of 1800a C. to be cooled to a point at which no uranium carbide was for-med. A further disadvantage of this process was that the carbon molds were severely attacked by the molten uranium and soon had to be replaced or relined. The resulting metal by this treatment was high in carbon content.

Fol-lowing the present invention molten uranium can be melted in graphite crucibles and can then be cast into carbon molds with little or no carbide formation, this resulting from the discovery that the true melting point of uranium is not approximately l800 C. but rather, is in the neighborhood of 1100 C. At this lower temperature there is little tendency for the molten uranium to combine with the interior surface of the melting crucible and the final carbon'mold to form uranium carbide. The greatly decreased time of cooling the uranium from the temperature at which it is cast to the temperature at which no uranium carbide is formed also contributes to obtaining cast uranium metal exceptionally free from uranium carbide. Where pure molten uranium was placed -in contact with carbon at varying temperatures, it was found that -the amount of carbon that combined with the uranium increased rapidly with the temperature, as shown in the `following table:

In this respect, the requirements that the uranium be maintained at an elevated temperature to remove the vaporizable impurities and the uranium be in contact with the carbon mold for a minimum of time and at a lower temperature, are seemingly incompatible. However, the casting process of the present invention, by maintaining the uranium at a higher temperature in the melting chamber until the impurities are vaporized, in spite of carbide formation, and then pouring into the casting chamber at a lower temperature With relatively fast solidication, satisfies both requirements, as the major por-tion of the uranium carbide formed in the melting chamber remains in the melting chamber as part of the retained solid impurities.

ln addition to providing cast uranium substantially free from uranium carbide, the process has the further advantage of increasing the life of the carbon molds. Substantial formation of uranium carbide causes a pitting ot the interior surface of the mold thereby necessitating machining of the mold to provide a smooth and regular surface for the next casting. In many cases the mold will be of such irregular shape that the machining can be carried out only with diliiculty if at all. Where the mold is machined down to provide a smooth interior surface, the Wall thickness of the mold is considerably decreased and the casting chamber is enlarged thereby preventing uniform castings. Also, substantial carbide formation tends to cause the casting to adhere to the mold with the result that there is a high probability of breakage of the mold in removing the casting.

The terms uranium ingot and uranium biscuit, as used in the description and claims, include any metallic uranium mass containing impurities internally and/or on the surface of thevmass. Thus, it. has been used to define a crude uranium ingot or biscuit 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 a skin or layer of uranium oxide on the outer surface of the mass.

The above detailed description of preferred apparatus and processes is for the purpose of illustration, and the invention is to be limited only by the scope of the following claims.

We claim:

1. An apparatus for treating uranium in vacuo which comprises a heat resistant base, a refractory block supported on said base, a mold supported on said block and having a casting chamber therein, a crucible supported on said mold and having a melting chamber therein, said chambers connected by an aperture, means adapted to close and open said aperture, insulation around said mold and crucible, a Shield over said insulation and in cngagernent with said base, whereby the assembly may be hermetically sealed, and an induction heating coil surrounding said apparatus along part of its height and being adjustable along its vertical axis, the uranium-contacting surfaces of said crucible, said mold and said clos* ing means consisting of graphite.

2. An apparatus for treating uranium, which comprises a Crucible having a melting chamber therein, a perforate lid on said crucible, a cap over the perforate portion of the lid, the sidewalls of said cap having at least one opening therein, a mold below and contiguous with said chamber, said mold and melting chamber being connected by an aperture in the lower part of said chamber, means for closing said aperture, a shell enclosing the Crucible and mold-assembly, means for establishing a subatmospheric pressure within said shell, and heating means extending along part of the height of the'assembly arranged adjustably along the vertical axis of said apparatus, the uranium-contacting surfaces of said crucible, said mold and said closing means consisting of graphite.

References Cited in the tile of this patent UNITED STATES PATENTS 454,759 Oden June 23, 1891 1,022,910 Whitney Apr. 9, 1912 1,540,515 Cuenot June.2, 1925 1,568,685 Moore ]an.'5, 1926 1,812,172 Rohn June 30, 1931 1,839,106 Loth Dec. 29, 1931 2,009,489 Fritzsche July 30, 1935 2,093,627 Yockey Sept. 2l, 1937 2,133,634 Rohn Oct. 18, 1938 2,156,998 McCullough May 2, 1939 2,257,713 De Bats Sept. 30, 1941 ncar www...

Claims (1)

1. AN APPARATUS FOR TREATING URANIUM IN VACUO WHICH COMPRISES A HEAT RESISTANT BASE, A REFRACTORY BLOCK SUPPORTED ON SAID BASE, A MOLD SUPPORTED ON SAID BLOCK AND HAVING A CASTING CHAMBER THEREIN, A CRUCIBLE SUPPORTED ON SAID MOLD AND HAVING A MELTING CHAMBER THEREIN, SAID CHAMBERS CONNECTED BY AN APERTURE, MEANS ADAPTED TO CLOSE AND OPEN SAID APERTURE, INSULATION AROUND SAID MOLD AND CRUCIBLE, A SHIELD OVER SAID INSULATION AND IN ENGAGEMENT WITH SAID BASE, WHEREBY THE ASSEMBLY MAY BE HERMETICALLY SEALED, AND AN INDUCTION HEATING COIL SURROUNDING SAID APPARATUS ALONG PART OF ITS HEIGHT AND BEING ADJUSTABLE ALONG ITS VERTICAL AXIS, THE URANIUM-CON-

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