USH659H - Process for electrolytically preparing uranium metal - Google Patents

Abstract

A process for making uranium metal from uranium oxide by first fluorinating uranium oxide to form uranium tetrafluoride and next electrolytically reducing the uranium tetrafluoride with a carbon anode to form uranium metal and CF₄. The CF₄ is reused in the fluorination reaction rather than being disposed of as a hazardous waste.

Description

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 CF₄ and then by electrolytically converting UF₄ to U. Since CF₄ is the fluorinating agent for making the UF₄ as well as the by-product of the electrolysis it is regenerated and reused in the process.

BACKGROUND OF THE INVENTION

Electrolytic reduction has been studied for the preparation of uranium metal from uranium oxides. The desired overall reaction is UO₂ + 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 UO₂ to electrolytic cells is much more difficult than the feed of Al₂ O₃ to Hall cells. The solubility of UO₂ is about one-tenth that of Al₂ O₃ in Hall cells and the dense UO₂ 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, UF₄ +C→CF₄ +U, is possible. However, this reaction requires a makeup of UF₄ which is made from the reaction of UO₂ and large amounts of HF, a very costly chemical. Furthermore, CF₄ 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 UF₆ as the feed. In another case, a large electrolytic cell was operated with 30 to 50% of the U metal from reduction of UO₂, 50 to 70% from reduction of UF₄. The objective of these studies was to maximize the amount of UO₂ feed; the higher the percentages of UF₄ were easier to use, but a large fraction of UF₄ feed with little UO₂ was not tested. A problem revealed by these studies was that UO₂ settled out in the electrolysis cell due to its low solubility and high density.

SUMMARY OF THE INVENTION

In view of the above needs, it is an object of this invention to provide a process for electrolytically preparing uranium metal from UF₄ 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 UO₂ are reacted to form UF₄ ; a second reaction wherein the UF₄ formed in the first reaction is electrolytically converted to U.sup.° by using a carbon anode resulting in the formation of CF₄ as by-product., and subsequent to the second reaction, the CF₄ formed is recycled into the first reaction for reuse as fluorinating agent. The process allows the recycling of a troublesome fluorocarbon, CF₄, and also minimizes the feed of UO₂ 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 CO₂ 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 UF₄ +C→CF₄ +U.sup.° with a feed reactor where the primary reaction is CF₄ +UO₂ →UF₄ +CO₂. The two reactions together give the preferred overall reaction UO₂ +C→CO₂ +U.sup.°. Moderate amounts of UO₂ that remain in the UF₄ feed to the cell will dissolve and produce metal by the overall reaction UO₂ +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 CF₄ products. The feed rector is optimized for recycle of fluorides as UF₄ to give an efficient conversion of reactants to UF₄, thus eliminating the mixing and settling of UO₂ solids as a cell operating problem. The consumption of anode carbon is reduced because the final by-product is mostly CO₂ instead of CO.

The reactions of CF₄ with uranium oxides are thermodynamically favorable. A number of reactions can be written with UO₂ and CF₄ as reactants and UF₄ 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 5×10¹⁴. This result agrees with the behavior in-cell where UF₄ reacts to form CF₄ only when there is a deficiency of UO₂. The second reaction is probably more favorable as an excess of H₂ and the two gaseous products should allow a more complete utilization of the CF₄. The H₂ 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 UF₄ is produced to be sent to the electrolytic cell accompanied by a small amount of UO₂. Also produced in the fluorination and reduction reactor are byproducts CO₂, H₂ O, H₂, CO, HF and a small amount of CF₄ 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 CF₄ 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 UO₂ was considered preferable to the use of UF₄ to avoid evolution of the undesirable CF₄ by-product. Therefore, even when UO₂ and UF₄ were used together as starting materials, which improved efficiency, the use of UF₄ was minimized to minimize CF₄ offgas. This invention solves that problem by reusing instead of disposing of waste CF₄.

Claims (2)

I claim:

1. A process for converting UO₂ to uranium metal comprising:

a first reaction wherein a fluoride compound and UO₂ are reacted in a first reaction chamber to form UF₄ ;

a second reaction wherein said UF₄ and unreacted UO₂ are electrolytically converted in a second reaction chamber, said second reaction chamber being an electrolytic cell, to U° by using a carbon anode resulting in the formation of CF₄ as by-product;

and subsequent to said second reaction, recycling said CF₄ into said first reaction for reuse of said fluoride.

2. The process of claim 1 wherein said first reaction further comprises adding H₂ in said first reaction chamber.

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