ELECTROLYTIC PRODUCTION OF URANIUM METAL FROM URANIUM OXIDES R . D . P I P E R A N D R . F . LEIFIE,LD1 .liallinckrodt Chemical T'Vorks, Cranium Diimision, St. Charles. .\lo.
Massive uranium metal which exceeds AEC specifications for purity has been produced with nearly 100% recovery from uranium oxides. Electrolysis at 1150" C. in a fused fluoride electrolyte produces molten metal. Oxides are excluded from the metal product to obtain well coalesced metal, by incorporating the oxide to b e reduced in a consumable uranium oxide-carbon anode. Means of preparing uranium oxidecarbon anodes with electrical resistivities of less than 0.03 ohm.-cm. have been found. Cell designs amenable to continuous operation have been developed on a laboratory scale.
ELECTROLYSIS is potentially a cheap method of producing reactive metals. Earlier processes (3, 7, 9, 71)) however, have not been economically competitive, largely because of the costs of preparing the halide feed and of processing the powdered or dendritic metal product into massive shapes. Some attempts to produce molten metal directly from uranium oxides were made in the wartime work a t Westinghouse ( 8 ) and further developed at the Knolls Atomic Power Laboratory (70). Poor coalescence of the metal and low yields proved to be major unsolved problems. Both result from the loiv solubility of uranium oxides in fluoride electrolytes. Excess oxides become mixed with the metal; this inhibits coalescence of the metal and leads to poor quality metal and low yields. To avoid the oxide contamination-poor coalescence problem, the oxide feed is incorporated in a consumable oxide-carbon anode. Several references to metal oxide-carbon anodes have been found in the literature (4: 6. 72). However, this approach has not been extensively developed in electrolytic metal production techniques. Use of the oxide-carbon anode has permitted laboratory-scale demonstration of a process for producing well coalesced, high purity uranium directly from uranium dioxide. Reduction of the oxide directly to massive metal is inherently cheaper than the present method of uranium production in which UO. is converted to UF4 and then reduced by magnesium (.5).
T h e Mallinckrodt electrolytic process is analogous to the Hall process for aluminum production, in that the oxide reacts \vith carbon at the anode to form a n oxide of carbon. T h e higher operating temperature required to form molten uranium leads to the formation of CO rather than CO? as in aluminum reduction. T h e reactions may be represented as ;\node reaction: LO,
+ 2C
-f
Cathode reaction: C+' Over-all reaction: UO2 EoA,,,' c ,
=
U4 '
4- 2 C O
+ 4e-
+ 2C
-f
+
U
+ 4e-
U
+ 2CO
(1)
I
1
I
I I
Present address, Mallinckrodt Chemical Works, Mallinckrodt
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
I I
INTO ANODE
I
I I
CARBONIZE r*,
I I O O ~c.
!
BoFp-LiF - UF4
MOLD SCRAP
(3)
kcal./mole.
St., St. Louis 7 , Mo.
-Ipi 1 I
PRESS
(2)
-0.9 volt
A H l ~ j o c" . = +207
208
1
D. C.
Process Description
1
T h e first steps of the process (Figure 1) involve preparing a mixture of CO. and pitch, pressing into a n anode. and heating to carbonize the pitch. These steps are analogous to the preparation of baked carbon anodes such as are used in aluminum reduction cells. T h e UOa-carbon anodes formed in this-!say
MACHlN E TO CORES Figure 1. Proposed flowsheet for electrolytic reduction of UOsto metal
1
consist of oxide particles held in a carbon matrix; a typical stsucture is shown in Figure 2. I n the electrolytic cell, the UOz and carbon react a t the anode. Carbon monoxide is evolved and some nonoxygencontaining uranium ions (as yet unidentified) are formed in the electrolyte and reduced to metal at the cathode. Oxides are present only a t the anode, so contamination of the metal with oxides is avoided. Since the temperature is above the melting paint of uranium (1133’ C , ) ,molten metal is formed. Tapping molten metal into molds has not yet been attempted. I n the laboratory cell, the metal is allowed to freeze in place at the end of a run, and is removed by breaking the crucible. Stopper rod, freeze plug, or tilt-pour mechanisms presumably could be used to cast the metal into molds. Scrap metal has been recycled by burning the metal to UsOs (which is reduced to UOz in the carbonization step). If a special basket anode arrangement is used, metallic scrap uranium can be recycled directly to the cell. Satisfactory results have been obtained by bath of these techniques. Experimental
Anode Fabrication. Satisfactory, hut not necessarily optimum, anodes are prepared by mixing UO, with commercial electrode binder pitch. Carbon black has also been used on occasion to obtain the desired carbon content. Trichloroethylene has been used in the laboratory to facilitate blending, but blending without the solvent is possible. A laboratory kneader fitted with “fish-tail” blades and heated to 100” C. has been satisfactory for use with low melting point pitches. Pellets of the mixture are prepared by pressing in 2 ’/z-inch diameter steel molds at about 100’ C. and 2000 to 7000 p s i . The pellets are packed in graphite powder and carbonized by heating slowly to 1100O C. and holding a t temperature for 3 to 6 hours. Rcsulting anode pellets have a density of about
6 grams per cc., crushing strengths u p to 18,000 p.s.i., and a resistivity of approximately 0.03 ohm.-cm. T h e UOz-carbon anodes should contain close to the stoichiometric 8.2y0carbon for optimum performance. T h e amount of pitch required to produce this much carbon depends on the “coking value” (the amount of fixed carbon farmed on carbonization) of the pitch. Coking values have averaged around 65% far a commercial, 100’ C. softening point pitch. Anode pressing pressure, oxide feed size, pitch type, and baking cycle all affect caking values. Since electrical conductivity improves with increasing pitch content, it is desirable to obtain as much of the required carbon content as possible from carhonization of pitch. However, there is a n upper limit on the amount of pitch that can be used without cracking and bloating of the pellet during carbonization. This is primarily dependent on the surface area of the oxide. For low surface area oxide, only 10% pitch can be used and carbon black must be added to obtain the desired carbon content. High surface area oxides, however, can accommodate enough pitch to supply all of the required carbon. Cell Design. Two cell arrangements, differing in the type of anode used, have been used for most of the work to date. T h e “basket anode” cell, shown in Figure 3, essentially consists of a cylindrical graphite crucible thai serves as the container lor the molten salt and molten metal and is the cathode of the cell. T h e crucible is enclosed in a quartz tube and heated by an induction coil outside the tube. (Heat supplied by the direct current would make external beating unnecessary in a larger cell.) Electrode connections are made by steel rods extending from the cell through the top of the enclosure. T h e anode consists of a perforated graphite basket containing crushed lumps of the UOz-carbon mixture. Addition of more of the U02-carbon mixture and of UF, (which is sometimes
-q-
Vycor Filling T-*Steel Cathode Steel Anode RI Rubber Stoppe Stqd,Lid and 0 Ring Sea Cooling Coils Tmnsite L i d Anode Basket ‘Foamsil‘ Ring
..^
- - .. .
Calhode-Cruclble R. F. Coils Elecfrolyte Metal Pool
Quartz Tube
Figure 2. UOz particles in carbon matrix form the consumable anode moteriol (500X. reflected light)
~
~
~
4 Figure 3.
Basket anode-container I:athode cell VOL. 1
NO. 3
JULY 1 9 6 2
209
Table I.
Typical Operating Data for Electrolytic Production of Uranium from UOs
Cell size Anode type
Electrolyte Current, amp. Voltage, volts Anod; current density, amp./sq. cm. Cathode current density, amp./sq. cm. Rate of formation of metal, kg./hr. Off-gas composition, % Current efficiencv, 47, Metal recovery, 90 Maximum ingot size, kg. . I
Basket Anode 4 inch I.D. 23/a inch O.D. Graphite basket with 150 threesixteenth inch holes containinc chunks of Ub-carbon 65% BaFz-lO% LiF-25yo UF, 300 5 5
Bare Anode 4 inch I.D. UOz-carbon pellet 21/2 inch diameter by 2 inches long
1. 2
0.5
0.21
0.07
65% BaFz-lO% LiF-25% UFI 140 5 1.4
-99% 80% C O , 5% Con, 15% CF4 35 22 >99 99 0.4 6.8
co
depleted from the electrolyte via CF? formation) can be made through the Vycor feed tube. The second type of cell, the “bare anode” cell. is essentially similar to the basket anode arrangement except that in place of the basket. a block of the UOz-carbon mixture attached to a graphite rod via a threaded connection is used. A sliding steel connector at the top of the enclosure permits lowering the anode into the cell as it is consumed by the reaction. A wet-test gas meter and a gas chromatograph connected in the off-gas line from the cell permit measurement of the volume and composition of gases generated. These measurements provide a means of evaluatinq experimental variables during the course of a run. Materials of Construction. Graphite has proved to be a very satisfactory material of construction for the laboratory electrolytic cells. It i s resistant to the molten fluorides and only slightly attacked by uranium at 1150’ C. ( 73). However. for some purposes, particularly small experimental cells, electrical insulation of parts of the cell is desirable. A brief evaluation of several possible materials revealed no satisfactory material for this purpose, Small crucibles of aluminum oxide. beryllium oxide. magnesium oxide. thorium oxide, uranium oxide, calcium zirconate, and spinel were all severely attacked or completely disintegrated by molten BaF2-MgFz-UFa electrolyte at 1150’ C. Boron nitride and aluminum nitride were stable in the electrolyte over short times, but both formed a n electrically conducting film when used at the cathode of a cell. This destroyed their usefulness for many purposes in possible cell arrangements. Procedure, In making a run, the enclosure is evacuated and the cell is heated to 700 ’ C. to outgas the electrolyte and graphite. The system is then filled with helium and heated to the operating temperature. A pre-electrolysis with reversed polarity is used to remove oxides initially present in the electrolyte. I n thjs step, the graphite crucible serves as the anode. Oxides are removed by the same over-all reaction that occurs in normal electrolysis. T h e pre-electrolysis is carried out a t 1050’ C., so that any uranium produced would remain solid and not drip down to the anodic surface of the container. T h e pre-electrolysis is continued until the ap210
l & E C PROCESS D E S I G N A N D DEVELOPMENT
pearance of appreciable quantities of CF4 in the off-gases. At this point, polarity is returned to normal, and the temperature is increased to the operating level of about 1150’ C. Runs are usually 4 to 6 hours in duration, but continued operation for as long as 39 hours has been maintained. At the end of a run, the metal and electrolyte are allowed to freeze in the cell. A potential of 3 volts is generally maintained on the cell during the cooling period to minimize back-reaction of the metal with the electrolyte.
Results
Some typical operating conditions and results for cells using basket and bare anodes are shown in Table I. These data are representative of laboratory-scale operation attained to date. Individual runs have varied considerably depending on experimental conditions. Variables studied include electrolyte composition, current density, composition of the UOz-carbon anode, and anode geometry. The electrolyte consists of UFd plus diluent fluorides. ,4 fluoride, rather than chloride, electrolyte is necessary to avoid the volatility of UCla and the discharge of chlorine at the anode. The diluent fluorides should have decomposition potentials higher than UF,. This limits the choice to combip2tions of the alkaline earth, lithium, and possibly rare earth fluorides. Results obtained with several possible combinations of alkaline earth and lithium fluorides are shown in Table 11. Since the cell arrangement used for these tests resulted in the electrolytic reduction of considerable UFd via Reaction 4, calculated values UFI
+C
+
U f CF,
(4)
for reduction of UOZalone are included for comparison. The equimolar mixture of BaFz and LiF has provided the best production rates and current efficiencies of salts tested to date. I n most of the work, this composition has been used. Volatility of the LiF from this electrolyte might be a problem in continued operation, but has not been troublesome in laboratory work. The UF, content of the electrolyte has a marked effect on rates and current efficiency. Quantitative data on this effect show considerable scatter; qualitatively there is an increase in maximum possible current and a decrease in current efficiency with increasing U F 4 content in the electrolyte. The optimum concentration will depend on a compromise between these two factors. In most of the work a n electrolyte containing 25% UF, has been used.
Table II.
Electrolytes for Electrolytic Reduction of U 0 2 Evaluated in basket anode-type cell Electrolytes initially contained 25% UF4 0ut.r-all (U0g -f-
UF,) Reductton Current Diluent Fluorides ( M o l e Ratio) LiF : BaFz LiF: 2BaF2 LiF : 2MgF 2 LiF: 2CaF2 MgFl :BaFz 2MgFz:BaFz MgFz: CaF2 CaFz :BaFz
e$$-
Rate, czency, g . Ulmin. % 3.3 30 1.5 15 2.4 27 2.7 43 2.2 27 1.1 16
Average
U O ZReduciion Current
CFa i n
Of-Gas, R a t e , ,
@-
czency,
92
g. U l m z n .
Yo
17 6
2.2 1.3
20 13
1.2
19
1.6 0.9 0.8 1.4
20
..
28 13 7
1.6
30
24
2.4
34
20
...
..
18 16 20
during the carbonization step of the anode preparation proce7dure and the resulting UOe-carbon anodes are similar to anodes
7
35
made from UOS. 3 0 1
251
Metal Quality
20
L
w
0
50
I 150
I 100
I 200
1 250
I
I
300
350
I 400
CURRENT, AMP.
Figure 4. The improvement in current efficiency with increasing current Values for basket anode cell
Pieces of sound uranium metal up to 15 pounds in weight have been produced in the laboratory cell. Typical analyses of electrolytically produced metal and the UOz-carbon mixture from which it was made are shown in Table 111. The impurity levels are well within AEC specifications for uranium metal. The low oxygen and carbon levels are noteworthy inasmuch as the material w’as produced from oxide and formed in a graphite cell. Also noteworthy is the apparent decontamination obtained for several elements: boron, chromium. iron, lithium. magnesium, nickel, and silicon. Part of this apparent decontamination may have resulted from an accumulation of impurities in the electrolyte so that periodic replacement of the electrolyte would be required to maintain these decontamination levels. However, continued removal of elements that form volatile fluorides might be expected. This feature might lead to application of this process to the reprocessing of reactor fuel elements. Discussion
Current efficiency improves with increasing current. This relationship, as measured in a basket anode cell, is showm in Figure 4. Parasitic cyclic reactions (5 and 6) are believed to be responsible for this effect. UL4 LT-3
+ e-
+
+
u74
U-3
(cathode)
+ e - (anode)
(5) (6)
T h e ner effect of these reactions is unproductive consumption of current. T h e carbon content of the UOZ-carbon mixture is also important. Reduction rates improve with decreasing carbon contents. I n the basket anode, feed stock with carbon contents less than the stoichiometric 8.2% has produced good metal a t rapid rates, but rapid erosion of the basket occurred simultaneously, because the extra carbon, needed for the reaction in this case, is supplied by the graphite basket. If a bare anode is used, carbon contents less than stoichiometric lead to release of oxides into the electrolyte. This excess oxide settles and interferes with coalescence of the metal; “shot” rather than massive metal is then formed. If bare anodes containing excess carbon are used, the excess carbon forms a layer of soft, soot-like carbon over the surface of the UOZ-carbon anode. This results in a decrease in reaction rate with time as more of this material accumulates. If the carbon content of the bare anode is very high (9 to 1Oyo total carbon), the residual carbon forms “skeletons” that retain the shape of the original anode. Accumulation of carbon skeletons in the anode basket or over the surface of a bare anode causes a severe drop in reaction rates. Recycle of scrap metal to the basket anode is possible. Oxides contained on the scrap surface react with the graphite basket to form CO. This avoids the oxide contaminationpoor coalescence problem that is encountered upon attempting simple melting of oxide-coated scrap uranium. Turnings obtained from machining of uranium have been recovered as massive metal with >987c yield. The amount of C O formed indicated that the turnings were contaminated with about 15% oxide. I t is also possible to burn the metal to U30g and use this as the feed to the process. The U30, is reduced to UOz
Although insufficient data are available for the formulation of any detailed theories as to mechanism or ionic species involved in the reaction, one point of considerable importance is apparent from the work to date: the cyclic reaction (Reactions 5 and 6) that results from the formation of trivalent uranium a t the cathode. Trivalent uranium then migrates to the anode where it is reoxidized to quadrivalent uranium. The formation of trivalent uranium at the cathode is shown by the
Table 111. Analysis of Uranium Metal and the U02-Carbon Anode Material from Which It Was Produced
P.P.M. Elrrnent
.Metal product 320 10 0.2
