Process for electrolytically preparing uranium metal

The invention was developed pursuant to a contract with the U.S. Department of Energy.

This invention relates to a process for making uranium metal from uranium oxide using a fluorination by CF 4 and then by electrolytically converting UF 4 to U. Since CF 4 is the fluorinating agent for making the UF 4 as well as the by-product of the electrolysis it is regenerated and reused in the process.

Electrolytic reduction has been studied for the preparation of uranium metal from uranium oxides. The desired overall reaction is UO 2 + 2C→2CO+U, and the process has many similarities to the Hall-Heroult processes for preparation of aluminum from aluminum oxide using molten cryolite. At temperatures above the melting point of uranium, uranium metal product of good purity was achieved but yields and current efficiencies were very low. Satisfactory feed of UO 2 to electrolytic cells is much more difficult than the feed of Al 2 O 3 to Hall cells. The solubility of UO 2 is about one-tenth that of Al 2 O 3 in Hall cells and the dense UO 2 settles to the bottom where it fouls the metal surface.

Yields and current efficiencies were improved by using compartmental cells to confine the oxide feed and prevent fouling of the metal surfaces. The current efficiencies were 31% as a maximum, and the cell design was not suited to scale-up to large size.

An alternative reduction reaction, UF 4 +C→CF 4 +U, is possible. However, this reaction requires a makeup of UF 4 which is made from the reaction of UO 2 and large amounts of HF, a very costly chemical. Furthermore, CF 4 is a more troublesome gaseous waste than is CO.

Preparation of uranium in electrolytic cells has been successfully demonstrated. One study showed good cell operation using UF 6 as the feed. In another case, a large electrolytic cell was operated with 30 to 50% of the U metal from reduction of UO 2 , 50 to 70% from reduction of UF 4 . The objective of these studies was to maximize the amount of UO 2 feed; the higher the percentages of UF 4 were easier to use, but a large fraction of UF 4 feed with little UO 2 was not tested. A problem revealed by these studies was that UO 2 settled out in the electrolysis cell due to its low solubility and high density.

In view of the above needs, it is an object of this invention to provide a process for electrolytically preparing uranium metal from UF 4 with a recycling of the fluoride.

It is another object of this invention to provide a process for electrolytically preparing uranium metal wherein fouling of the electrolytic cell is minimized.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the process of this invention may comprise: a first reaction wherein a fluoride and UO 2 are reacted to form UF 4 ; a second reaction wherein the UF 4 formed in the first reaction is electrolytically converted to U.sup.° by using a carbon anode resulting in the formation of CF 4 as by-product., and subsequent to the second reaction, the CF 4 formed is recycled into the first reaction for reuse as fluorinating agent. The process allows the recycling of a troublesome fluorocarbon, CF 4 , and also minimizes the feed of UO 2 to the electrolytic cell thus avoiding the problem of fouling the metal surface. Also the consumption of the carbon anode is reduced since the waste product is CO 2 instead of CO.

FIGURE

The FIGURE is a schematic chemical flowsheet representing the electrolytic reduction process of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is a combination of an electrolytic cell where the principal overall reaction is UF 4 +C→CF 4 +U.sup.° with a feed reactor where the primary reaction is CF 4 +UO 2 →UF 4 +CO 2 . The two reactions together give the preferred overall reaction UO 2 +C→CO 2 +U.sup.°. Moderate amounts of UO 2 that remain in the UF 4 feed to the cell will dissolve and produce metal by the overall reaction UO 2 +2C→2CO+U.sup.°. The utilization of two separate reactors allows separate optimization of the operating conditions of each reaction. The electrolytic cell is optimized for reduction to metal without concern about the amounts of CO and CF 4 products. The feed rector is optimized for recycle of fluorides as UF 4 to give an efficient conversion of reactants to UF 4 , thus eliminating the mixing and settling of UO 2 solids as a cell operating problem. The consumption of anode carbon is reduced because the final by-product is mostly CO 2 instead of CO.

The reactions of CF 4 with uranium oxides are thermodynamically favorable. A number of reactions can be written with UO 2 and CF 4 as reactants and UF 4 as one product. Some of interest are:

UO.sub.2 (s)+CF.sub.4 (g)→UF.sub.4 (s)+CO.sub.2 (g)

UO.sub.2 (s)+CF.sub.4 (g)+H.sub.2 (g)→UF.sub.4 (s)+CO(g)+H.sub.2 O(g)

Both these reactions are favored with Σ∇ f G.sup.° of -310. KJ mol -1 and equilibrium constants at 1100° K. of about 5×10 14 . This result agrees with the behavior in-cell where UF 4 reacts to form CF 4 only when there is a deficiency of UO 2 . The second reaction is probably more favorable as an excess of H 2 and the two gaseous products should allow a more complete utilization of the CF 4 . The H 2 can also provide a complete reduction of U(VI) to U(IV).

The modified electrolytic reduction process can be shown as a schematic chemical flowsheet as shown in the FIGURE. Fluorination 1 takes place in the fluorination and reduction reactor where UF 4 is produced to be sent to the electrolytic cell accompanied by a small amount of UO 2 . Also produced in the fluorination and reduction reactor are byproducts CO 2 , H 2 O, H 2 , CO, HF and a small amount of CF 4 that are expelled as offgases 5. The electrolytic reaction 3 takes place in the cell containing a carbon anode and produces uranium metal product as well as CF 4 by-product that is recycled to the fluorination reaction 1 along with a small amount of CO.

In prior studies of electrolytic reduction, the use of UO 2 was considered preferable to the use of UF 4 to avoid evolution of the undesirable CF 4 by-product. Therefore, even when UO 2 and UF 4 were used together as starting materials, which improved efficiency, the use of UF 4 was minimized to minimize CF 4 offgas. This invention solves that problem by reusing instead of disposing of waste CF 4 .