US2994650A - Preparation of pure metals from their compounds - Google Patents

Description

nited States Patent Ofiice 2,994,650 Patented Aug. 1, 1961 2,994,650 PREPARATION OF PURE METALS FROM THEIR COMPOUNDS Harvey L. Slatin, New York, N.Y., assignor t the United States of America as represented by the United States Atomic Energy Commission N0 Drawing. Filed Oct. 24, 1951, Ser. No. 252,987 2 Claims. (Cl. 204-64) This invention relates to the preparation of .certain pure metals from compounds thereof and more particularly to the preparation of certain pure metals of the group known as the transition heavy metals, particularly cerium, thorium, titanium and uranium, by the electrolytic treatment of certain compounds of these metals.

Prior art methods used to obtain pure'metals of the foregoing class by electrolytic treatment of their ores are cumbersome and ineificient. Such prior art methods are unsatisfactory in that they are unable to provide metals having the desired purity and are unsuited for the extraction of pure metal from low grade ore at a reasonable cost, usually because they require considerable pre-treatment and chemical conversion of the metalliferous material prior to purification.

It is therefore an object of this invention to provide a simple, efiicient, and inexpensive method for obtaining pure metals of the aforementioned class from compounds of said metals. Other objects and advantages of the invention will become apparent from the description and examples given hereinafter.

In essence, the process of this invention comprises the electrolytic conversion to metal of a carbide or sulfideof a metal of the class consisting of cerium, thorium, tita nium and uranium in a fused halide bath containing '11 cation less electropositive than the aforesaid metals. Ref

erence is made to the Handbook of Chemistry and Phys ics, 31st edition, 1949, page 336, published by the Chemical Rubber Publishing Company, Cleveland, Ohio, wherein the periodic table taken from Demings General Chemistry is given and to Chapter I of Chapters in the Chemistry of the Less Familiar Elements, by B. Smith Hopkins, Stipes Publishing Company, Champaign, Illinois, 1939, both showing the community of characteristics which establishes this class of metals.

It should be noted that the metals of this class possess the following characteristics:

(1) The metal ores or oxides are reducible by carbon or sulfur to the respective carbide or sulfide;

(2) The metal carbide or sulfide can conduct an electric current;

(3) The carbide or sulfide reacts anodically to form metal ions and/or reacts with halogens to form corresponding halides;

(4) Ahalide of the metal is ionized in a fused alkali and/or alkaline earth halide bath; and

(5) The metal ion is discharged at a lower potential than alkali or alkaline earth metals.

metal compound such as the metal oxide. The use of graphite in the electrodes not only supplies a quantity of carbon to insure the formation of carbide, but also increases the conductivity of the electrodes of metal oxide, sulfide and/ or carbide.

The metalliferous anodes used in the present invention may be formed by any of several methods. For example, the powdered metal oxide may be admixed with an excess of graphite and compacted or briquetted without heat into rods or sticks. The carbide is formed in the rods while being used as anodes without further treatment. However, it is preferable to convert the oxide to the carbide prior to the electrolytic treatment.

A preferred embodiment is set forth in the following example to illustrate but not to limit the present invention:

Example I Equal parts by weight of uranium oxide (U 0 and Wood charcoal are mixed under non-oxidizing conditions and briquetted at 20 tons per square inch to rods or sticks of the desired size. The briquetted pieces are placed in a resistance furnace which is fitted with an open chimney and heated slowly to about 1300 C. at which time the uranium oxide reacts quantitatively with the carbon to form the carbide. At a temperature of about 1500 C. a luminous flame appears at the chimney. The temperature is further elevated to about 1950 C. in 30 minutes and the flame subsides. The temperature is then raised'to about 2150 C. and held there for 10 minutes, at which time the current is turned off and the furnace permitted to cool. The contents of the furnace are found to have been converted into sticks of about one .half inch in diameter and four inches long comprising crystals of uranium carbide.

The percentage of conversion is very good and of the order of about percent. The uranium carbide sticks formed by this'method are then used as the anode in a fused halide bath comprising about 31 percent uranium trichloride dissolved in 69 percent of a mixture comprising 43 mole percent strontium chloride and 57 mole percent potassium chloride. The salts are melted in a quartz electrolytic cell equipped with a Wolfram cathode, the uranium carbide anode and leads for connecting the electrodes to a source of potential. A graphite sleeve surrounds the cell and is heated by induction means to an elevated temperature sufiicient to maintain the salt bath in a molten state. A current of about 5 amperes at a potential of 7 volts is passed through the solution causing chlorine to be liberated at the anode Where it reacts with the uranium carbide and converts it to soluble chloride. Uranium chloride goes into the solution and uranium ions are reduced at the cathode to the pure metal which falls and collects in the bottom of the cell and may be withdrawn as desired. There is a recovery of uranium considerably in excess of that introduced in the seed charge of uranium chloride in the fused bath.

The reactions involved in the transformation of the uranium oxide to the metal are probably represented in the following equations:

In Equation 2 it will be noted that the carbide usually reacts with the excess chlorine to form CCl but other chlorocarbon compounds such as tetraohlorethylene and the like may be formed.

Sulfide compounds may be similarly employed as an anode. Furthermore, the process may be carried out continuously. For example, cerium and other metals of the first rare earth series may be obtained from their sulfides as well as from the carbides by an embodiment of the invention which provides for the removing of the reduced metal throughout the process. The following example illustrates a method of the continuous production of cerium from the cerium sulfide.

Example II Cerium sulfide powder milled to a diameter range of from 50 to 200 mesh particles is compacted to half inch diameter rods about 4 inches long, using as a binder a small amount of paraffin and naphtha. The rods are sintered at 650 C. for minutes to consolidate them and to remove the volatile binder materials. These cerium sulfide rods are then successively inserted as anodes in a molten salt bath of 55 mole percent potassium chloride and 45 mole percent sodium chloride having dissolved therein 20 percent cerium trichloride. The bath is contained in a quartz vessel which in turn is placed within an Alundum crucible. The vessel is heated by induction to a temperature of about 1250 C., a Wolfram cathode is inserted in the salt bath to complete the circuit, and a current of 4 amperes is passed theret-hrough at a potential of 8 volts. In about 30 minutes under these conditions the cerium sulfide in one rod has been sub stantially quantitatively reduced to the metal. Being heavier, said metal in molten form collects under the molten salt at the bottom of the quartz vessel, where it may be withdrawn by means of a suitable conduit or left to soliilify and later separated from the fused bath when coo Certain advantages of using metal sulfide anodes are apparent. Sulfides are somewhat better electrical conductors than carbides. It is not necessary to convert the metal containing compound to the carbide as directed in Example I. It is possible and more efficient in many cases to briquet a mixture of the compound with carbon whereby the conversion to carbide and the subsequent chloride formation are accomplished almost simultaneously. The binder materials may comprise any material which does not adversely affect the primary reactants.

As another embodiment of the invention, the following example sets forth a method for the preparation of a second rare earth series metal, thorium, from its oxide.

Example III Two hundred fifty grams of thorium dioxide were mixed with 150 grams of lamp black and pressed into 6-inch bars /z-inch square under 20 tons per square inch pressure. These are placed into a heat-resistant electrolytic cell containing a mixture of 45 mole percent sodium chloride and 55 mole percent potassium chloride. Heat is supplied by induction heating means to bring the temperature to 950 C. to melt the salt mixture. An inert cathode 1s inserted in the melt, the thorium dioxide-carbon rods are connected to anode leads and a current of about 10 amperes is permitted to flow through the solution. At a potential of volts, the thorium dioxide in the rods is converted by a suitable mechanism to the chloride which is then reduced to the metallic thorium at the cathode. The thorium collects as a fine powder at the cathode which is vibrated occasionally to dislodge the metal and cause it to fall to the bottom of the cell.

Substantially pure thorium metal is obtained by this method. Although the mechanism by which the carbon reacts is not absolutely certain, it is believed that there is substantially complete conversion of the oxide to carbide at the temperatures of operation of about 1000 C.

followed by the chlorination and reduction steps. In any event, the process is extremely useful and effective in obtaining essentially pure thorium metal from the oxide, which is not the case if the carbon is omitted from the anode mixture.

As another embodiment of the invention, titanium oxide may be converted to the pure metal by electrolytic conversion as set forth in the following example.

Example IV Two moles of titanium dioxide are ground with 8 moles of bone charcoal in a porcelain mortar until a fine powder mixture is obtained. About grams of the mixture are pressed into a bar /2-inch square and about 5 inches long and heated in an induction coil to about 1200 C., at which temperature the dioxide is converted by an exothermic reaction to the carbide. The titanium carbide is made the anode in a fused salt bath comprising 52 mole percent lithium chloride, 10 mole percent sodium chloride and 38 mole percent potassium chloride. Ten percent by weight of titanium trichloride is dissolved in the solvent, and a Wolfram cathode provided. The system is electrolyzed at 6 volts, a copious deposit of grey metal is obtained at a current of about 5 amperes. The metal analyzed 99.8 percent titanium. The loss in weight of the titanium carbide anode corresponds to an amount of titanium about one half of the total metal recovered.

Many other embodiments of the invention are possible. The carbon for the carbide may be supplied by any carbonaceous material such as graphite, wood charcoal, bone charcoal, lamp black or the like. The fused electrolyte may comprise any of the alkali or alkaline earth metals or any combination thereof although it is preferred to use mixed alkali metal and alkaline earth metal halides such as strontium, potassium, barium, lithium, and sodium chlorides, bromides, and iodides. The halides may be of any metal which is more electropositive than the metal which it is desired to recover. Of course, the bath must be capable of dissolving the halide of the metal which is desired to be recovered.

It should also be noted that the process of this invention involves among other things the novel preparation of halides of thorium, titanium and uranium from the oxide through the carbides thereby avoiding the formation of the undesirable oxyhalides. Although the preferred method of performing this process involves electrolysis, the invention is not limited to this method of operation inasmuch as straight chemical conversion may be used employing free halogens.

Since many of the embodiments of this invention can be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited except as indicated in the appended claims.

What is claimed'is:

1. A method of preparing uranium from uranium tritaoctoxide that comprises forming said oxide and carbon into an electrode, reacting the oxide and carbon at an elevated temperature to convert the oxide to uranium carbide, making said electrode the anode in an electrically conductive molten bath consisting essentially of uranium trichloride and at least one of the alkali and alkaline earth metal chlorides, passing an electrolyzing current through said bath between said electrode and a cathode and collecting uranium at the cathode.

2. A method of preparing uranium from uranium tritaoctoxide comprising forming the oxide into an electrically conductive electrode by reacting the uranium oxide with carbon at an elevated temperature to convert the oxide to uranium carbide, making said electrode the anode in an electrically conductive molten bath consisting essentially of strontium chloride, potassium chloride and uranium trichloride, passing an electrolyzing current through said bath between said electrode and a cathode and collecting uranium at the cathode.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Transactions (1948), page '1. Transactions Kolodney Jan. 15, 1957 Wilhelm et a1 Feb. 12, 1957 FOREIGN PATENTS Great Britain Apr. 15, 1950 OTHER REFERENCES of the Electrochemical Society, vol. 94

of the Electrochemical Society, vol. 87

(1945), pages 551-569. An article by Kroll.

Claims (1)

1. A METHOD OF PREPARING URANIUM FROM URANIUM TRITAOCTOXIDE THAT COMPRISES FORMING SAID OXIDE AND CARBON INTO AN ELECTRODE, REACTING THE OXIDE AND CARBON AT AN ELEVATED TEMPERATURE TO CONVERT THE OXIDE TO URANIUM CARBIDE, MAKING SAID ELECTRODE THE ANODE IN AN ELECTRICALLY CONDUCTIVE MOLTEN BATH CONSISTING ESSENTIALLY OF URANIUM TRICHLORIDE AND AT LEAST ONE OF THE ALKALI AND ALKALINE EARTH METAL CHLORIDES, PASSING AN ELECTROLYZING CURRENT THROUGH SAID BATH BETWEEN SAID ELECTRODE AND A CATHODE AND COLLECTING URANIUM AT THE CATHODE.