Electrolytic purification of uranium

2,739,934

March 27, 1956 r. kunin

ELECTROLYTIC PURIFICATION OF URANIUM

Filed April 15, 1954

Anionic

Permselective

Membrane

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INVENTOR.

ROBERT KUNIN

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United States Patent Office .. .. _M^2&gt;72⁹;’^

Patented Mar. 27, 1956 . 1

2,739,934

ELECTROLYTIC PURIFICATION OF URANIUM

Robert Kunin, Trenton, N. J., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission

Application April 15,1954, Serial No. 423,401

Claims. (Cl. 204—96)

This invention relates to the recovery of uranium. It relates to a method of concentrating and purifying uranium values. More particularly, it relates to an electrolytic process wherein uranium, in the form of the uranyl sulfate anionic complex, is separated from other metallic ions present in sulfuric acid leach liquors.

The process of this invention comprises passing a direct electric current through an electrolysis cell which is divided by means of two anionic permselective membranes into three compartments; viz., a cathode compartment, an anode compartment, and a center compartment. At the beginning of the operation of the cell, the cathode compartment contains an aqueous solution of a soluble sulfate salt, such as sodium, potassium or ammonium sulfate—preferably, sodium sulfate—and the anode compartment contains an aqueous mineral acid—preferably sulfuric acid—and the center compartment contains a sulfuric acid leach liquor in which are dissolved the uranium values together with ions of other metals such as iron, magnesium, or aluminum. The leach liquor can be the unfiltered acid slurry (ore pulp) obtained from the acid digestion of uranium-bearing ore as well as the filtered leach liquor. During electrolysis the uranyl sulfate anionic complex migrates into the anode compartment while the cations of the other metals, present in the leach liquor, remain in the center compartment, as is described in greater detail below. In this way there is effected a separation of the uranium values from the metallic contaminants in leach liquors.

The electrolysis cell which is employed can best be understood by a reference to the attached drawing wherein the single figure is a diagrammatical representation of a typical cell which is divided into three compartments by means of two anionic permselective membranes. In said figure, 1 represents a container which is divided into three compartments, 5, 6, and 7, by two anionic permselective membranes, 2α and 2c, which are described in greater detail below. Compartment 5 is an anode compartment by virtue of the presence there of anode 3, while compartment 6 is a cathode compartment because it contains the cathode 4. When the cell is in operation, the electrodes are connected to a source of electric power not shown. Between the anode compartment 5 and the cathode compartment 6 is the so-called center compartment 7.

The cell which is employed in this invention can be varied as to size, shape, volumes of the individual compartments, vents, ports, exits, construction materials, controls, embellishments, means for admitting and removing the contents of the compartments, et cetera, without departing from the spirit of this invention. What is essential is that the cell have three compartments, one a cathode compartment, another an anode compartment, and a center compartment, the anode and cathode compartments being separated from the center compartment by means of the two anionic permselective membranes or diaphragms which contain an anion exchange resin.

The permselective membranes which divide the elec2 trolysis cell into the three compartments are essential to the success of this process. They function by allowing anions to pass through them while at the same time preventing the passage through them of cations. The mem5 branes contain anion exchange resins; and it is the presence of the anion exchange resin which imparts the property of permselectivity to the membrane. While the composition of the permselective membranes can vary within reasonable limits, it is important that they contain enough 1° anion exchange resin as to have suitably high conductance when employed in the electrolysis cell. The permselective diaphragms, membranes, or films which have proven to be most suitable for use in this process are those tough and relatively flexible products made by incorporating ¹⁵ particles of an anion exchange resin in a film-forming matrix such as polyethylene or a vinyl resin. Such membranes are available commercially and are described in U. S. patent application Serial No. 205,413, now U. S. Patent No. 2,681,319, and the corresponding Canadian patent, No. 493,563, of June 9, 1953.

In the process of this invention a uranium-bearing, sulfuric acid leach liquor which contains metallic ions in addition to the complex uranium anions is placed in the center compartment of an electrolysis cell such as that one described above. An aqueous solution of a mineral acid— preferably sulfuric acid—is employed as the anolyte while an aqueous solution of a water-soluble sulfate salt— preferably a sulfate of an alkali metal—is used as the catholyte.

As electrolysis proceeds, the ions in solution tend to migrate to the electrode of opposite sign, but only the anions have complete freedom of migration because of the presence in the cell of the cation-restraining permselec35 ^dve membranes. Thus, the negatively charged uranyl sulfate complex ions migrate from the center compartment through the anionic membrane and become concentrated in the anode compartment. Sulfate ions, which are free to pass through the two permselective membranes, migrate to the anode compartment also. Oxygen is liberated at the anode. At the same time, hydrogen is liberated at the cathode and hydroxyl ions are formed in the catholyte. While the cations of the other metals which are present as undesirable impurities in the leach liquor tend ₄₅ to migrate from the center compartment into the cathode compartment, they are constrained by the anionic permselective barrier. The net result is that the catholyte is progressively changed from a solution of an alkali metal sulfate to a solution of an alkali metal hydroxide while ₅₀ the anolyte becomes more concentrated as to sulfuric acid and as to uranyl sulfate ions. Oxygen and hydrogen are liberated and the uranium values are separated from the other contaminating metals and are concentrated in the anolyte.

₅₅ The uranium values in the resultant anolyte are then recovered. This is done by electrolytic precipitation according to the process described in another of my patent applications, Serial No. 421,667, filed April 7, 1954. Or, the alkaline catholyte is used to neutralize the uranium₆₀ bearing acidic anolyte and to precipitate the uranium as an oxide. Then, the precipitated uranium oxide is separated by conventional means such as filtration, decantation, or centrifuging and is ignited to U₃Os. The liquid from which the precipitated uranium oxide is removed ₆₅ is essentally a solution of alkali metal sulfate which is then transferred to a cathode compartment of an electrolysis cell and is used as the catholyte in subsequent electrolytic separations.

It is, therefore, apparent that by means of this elec70 trolytic process uranium values are separated from contaminated leach liquors in a very efficient manner

The method, as described above, is a batch process.

2,739,934

Actually, however, the instant invention can be run on l continuous scale. In the continuous process, leach liquor s passed through the center compartment and the uranium values are removed electrolytically therefrom in a coninuous manner and appear in the anolyte. At the same ime the uranium-bearing anolyte and the alkaline cathoyte are removed and are mixed. After the precipitated iranium oxide is removed from the mixture, the resultant iolution is fed continuously to the cathode compartment. Sulfuric acid is also fed to the anode compartment to eplace the uranium-bearing anolyte which is removed.

The following example further illustrates the process &gt;f this invention.

Example

An electrolysis cell was constructed of polymethyl methacrylate (Plexiglas). It had a cross-sectional area jf ten square inches (2 x 5). The cell had an anode lompariment and a cathode compartment separated from i center compartment by means of two anionic permselective membranes (Amberplex A-l) known to contain ibout 70% anion exchange resin in a matrix of polyethylene and known to have been made according to the process of U. S. patent application Serial No. 205,413, now U. S. Patent No. 2,681,319, and the corresponding Canadian Patent No. 493,563 of June 9, 1953. The membrane contained a quaternary ammonium anion exchange resin made by aminating with a tertiary amine an insoluble, cross-linked chloromethylated copolymer of styrene and divinylbenzene. The cell Was generally similar to that shown in the drawing with the exception that every one of the three compartments had means for admitting and removing the contents continuously. The electrodes were platinum. Dilute sulfuric acid was employed as the anolyte while a 1.0 N solution of ammonium sulfate was used as the catholyte. A filtered, low-grade Belgian Congo sulfuric acid leach liquor was passed through the center compartment while a direct current of 2-3 amperes was passed through the cell at a potential of 5.5 volts. The ratio of uranium to iron in the leach liquor was 0.85 gram to 1 gram at the outset. The electrolysis was continued for four hours after which the anolyte was analyzed. It was found that a small amount of iron, presumably as a complex anion, had migrated to the anode compartment together with the uranium values. But, whereas the leach liquor at the outset had contained more iron than uranium (ratio U:Fe=.85:l), the ratio Of the amount of uranium to iron in the anolyte at the end of the electrolysis was 296 grams to 1 gram. During the period of electrolysis 71% of the uranium was removed from the leach liquor and was concentrated in the anolyte.

The catholyte meanwhile had become very alkaline. This was added to some of the uranium-bearing anolyte and uranium was precipitated as an oxide which was then ignited to UsOs.

It is evident from the above that uranium values in leach liquors are readily separated and concentrated by this process. Essentially the same results are obtained when an unfiltered leach liquor is electrolyzed in the same way. As might be expected, the cost of carrying out the electrolytic process varies inversely with the concentration of the uranium in the leach liquors being electrolyzed. Thus, at an initial concentration of 10.4 grams of uranium per liter, 79% of the uranium is removed from the center compartment to the anode compartment at a cost of 1 kilowatt of electricity per pound of uranium. The cost of removing the next 19% rises to about 4 kilowatts per pound of transported uranium. Nevertheless, the instant process is very efficient and provides a particularly convenient method of separating uranium values from other contaminating metals and of concentrating the uranium y values. /