US3008881A - Production of uranium - Google Patents

Description

Nov. 14, 1961 R. ALLEN ET Al.

PRODUCTION OF URANIUM Filed June 50, 1958 u ma INVENTORS. Dona/0'2. Hl/en flnne/re J. fl/Ien HTTORNE'Y ving point of the electrolyte.

3,008,881 PRODUCTION OF URANIUM Donald R. Allen and Anneke S. Allen, Lake Jackson,

Tein, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed June 30, 1958, Ser. No. 745,716 3 Claims. (Cl. 204-1.5)

The invention is directed toward a method of producing uranium metal electrolytically and toward an improved cell and electrolyte employed in such method.

The increased demand for uranium due to its importance in the study of nuclear principles and in the application of these principles has accentuated the need for more eflicient and more economical methods of and means for producing uranium.

Methods currently employed in the production of uranium include the preparation of UF from uranium oxide, extracted from ore containing commercial quantities of such oxides, and reducing the UP, by the action of calcium or magnesium metal to metallic uranium wherein one mole of the uranium metal to two moles of the CaE or MgF is produced. Methods currently employed also include the production of uranium metal electrolytically inwhich either KUF or UF is dissolved in a fused salt bath consisting essentially of a major proportion of CaCl and a minor proportion of NaCl to comprise the electrolyte which is electrolyzed in a graphite crucible at a temperature of about 900 C. The crucible serves as the anode and a high melting metal, e.g., Mo, somewhat centrally located, serves as the cathode upon which the uranium metal deposits during the electrolytic process.

These methods do not meet all the requirements for large-scale low-cost production of uranium metal. UP, is an undesirable intermediate since it is corrosive and presents certain health hazards. The UF salt must meet rigid specifications: it should be substantially anhydrous and free of contaminants and of a particle size such that about 86 to 99 percent passes through a number 60 US. Standard sieve and 52 to 62 percent of it passes through a number 325 sieve. Furthermore, the employment of known salt baths in electrolytic cells of known structure has not resulted in a satisfactory electrolysis of the uranium salt. I

A need, therefore, exists for better ways and means for producing uranium metal. 'Accordingly, the fundamental objects of the invention are to provide an improved method of producing uranium metal and to provide a novel electrolyte and electrolytic cell for use in such method.

The accomplishment of the foregoing and related objects by the practice of the invention, as hereinafter fully described in reference to the annexed drawing and particularly pointed out in the appended claims, consists of an improved electrolyte, electrolytic cell, and method of producing uranium metal by recovering it at the cathode of an electrolytic cell provided with an alkali metal anode suspended or floated on an electrolyte consisting of a mixture of an alkali metal chloride and an alkali metal u-r'anous fluoride at a temperature above the melt- The temperature of the electrolyte should not be substantially above its melting point. because an unnecessarily high temperature tends to raise the temperature of the exposed portion of the alkali metal anode above its vaporization point.

The anode can be molten potassium, sodium or lithium, but is usually sodium. It is contained in an elongated substantially vertical holder having an open lower end having any desired cross-sectional shape, cylindrical being the most practical. The vertical holder can be of any material which has a sufliciently high melting point att of the invention.

3,008,881 Pat'e'n'te-d Nov. 14, 1961 and is unreactive with both the anode material and the electrolyte. It is preferably an electrical conductor such as carbon, graphite, molybdenum, tantalum, or tungsten, or uranium or molybdenum-clad steelso that the electrical connection'may be made to it. However, the electrical connection may be made to the alkali metal directly in which case the holder may be either a conductor or non-conductor of electricity. I

Some of the alkali metal in the vertical holder tends to volatilize when the height of the column of metal in the holder is low. This condition can be overcome either by providing a cover for the upper end of the anode holder or for the entire top of the electrolyte chamber and maintaining an inert gas in the zone above the alkali metal and/ or by maintaining the alkali metal column of sufiicient height to result in a temperature at the top of the column which is substantially lower than the vaporiza-tion temperature of the alkali metal and to provide a hydrostatic head to discourage volatilization of the alkali metal in contact with the electrolyte. The holder for the alkali metal should be suificiently long to provide a zone above the metal for condensation of any alkali metal vapors formed and to extend suificiently far beneath the surface of the electrolyte to permit a sufficiently high column of alkali metal for the desired hydrostatic head. The metal at the upper part of a column is at a desired height when it is below the vaporization temperature and thereby serves as a sort of cover. 7

L'iCl, KCl, or-NaCl may be used as the chloride in the electrolyte of the invention; however, NaCl is preferred.

The electrolyte may consist of a Weight ratio of between 1 and 2 parts of the alkali metal uranous fluoride to one part of the alkali metal chloride but it is preferable that it be about 2 pants of the alkali metal uranous fluoride to 1 part of the alkali metal chloride. A percentage composition of about 65 percent of KUF or NaUF and about 35 percent NaCl is recommended. The temperature of the bath can be from 650 to 850 C. but it is preferred that it be from about 750 to 800 C. Only the alkali metal uranous fluoride enters into the electrolytic action. This i's in contrast to known methods and cells for the production of uranium which employ mixtures containing an alkaline earth halide,

usually a major proportion of such mixture being CaCl which is a reactant during the electrolysis, forming CaF FIGURES 1 and 2 of the drawing illustrate the cell FIGURE 1 is an elevational view largely in section taken at line 1+1 of FIGURE 2. FIG- URE 2 is a horizontal section taken at line 22 of FIG- URE 1.

Referring to the drawing in more detail there is shown electrolyte chamber 10 of electrically conducting mate rial usually steel and consisting essentially of bottom 12 and cylindrical sidewall 14 about the upper edge of which is flange 16. Protective liner 17 covers the inside of bottom 12 and sidewall 14. It must 'lJBDlE an electrically conductive material substantially unreactive with uranium. Carbon or uranium is usually used. Liner 17 together with b'ottorn 12 and sidewall 14 also serves as the cathode of the cell. Electrical heating coil 18 is positioned about sidewall 14 to heat chamber 10. Electrical lead-in lines 19 and 20 passing through electrically insul'ating ring-shaped gaskets 21 and 22, respectively, lead to a som'ce of AC. or DC. to provide energy to the heating coil. Protective s'te'el shell '23 incloses heating coil 18 and the bottom 12 of "chamber 10. Electrically insulating spacers 24 help to support chamber 10 and arovide free movement of heated air under bottom 12. Molten electrolyte 25 is shown partially filling chamber 10.

Electrically insulating gasket 26 is positioned on flange posite the lower face of the anode.

16 which supports annular top plate 27. Cover 28 havcess to the interior of the apparatus. I

Upwardly extending neck 38 'is shown centrally looated in top plate 27. Supported substantially centrally in neck 38 .by outwardly radiating arms 40 open-end graphite anode tube 42. Tube 42 extends"v downwardly below the level of electrolyte 25 but does not reach bottom 12. Molten alkali metal 44' is shown floating on electrolyte 25 within tube 42. I

Positive electrical lead-in line 46 passing through electrical insulating tightly fitting ring-shaped gasket 48 carries current to anode tube 42 from the positive terminal of a source of DC. and line 50passing through insulating tightly fitting ring-shaped gasket 52 carries current to sidewall 14 of chamberlO from the negative terminal of the source of DC.

Inlet line 54 having valve 56 therein provides a means 7 for introducing an inertgas into the top of the'celln Line 58 having valve 60 therein provides a means forescape of gases from the top ofchamberylfl of the cell.

In practicing the invention, a mixture of an alkali metal uranous fluoride and anyalkali chloride, preferably NaUF and NaCl, in a weight ratio of about2NaUF to 1NaCl, is placed in the electrolyte chamber and melted by means of a source of heat such as. a gas [furnace or electric heating coil shown in the drawing. An alkali metal, usually sodium, is'placed'in the anode holder where it becomes molten, as shown, and forms electrical contact with the electrolyte. The height of sodium metal in the holder is such that the sodium column, due to; the

hydrostatic head of the electrolyte, isa short distance above the open bottom end of the holder. Nitrogen gas is introduced through line 54 to blanket and protect the sodium from contamination or reaction with oxygen The temperature of the electrolyte is maintained'usua-lly at firo'rn'780 to 850 C. In carrying out the electrolysis,

between'land 2 volts is preferred. The electrode spacing is preferably close, on the order of about 1 inch. The

current density based on thearea of the alkalirmetal.

anode in contact with the electrolyte can be betweenllO and 30 amperes/square inch, about 30 amperes/square inch usually being used.

anode is gradually converted tozalkali ,metal ions which enter the electrolyte, the alkali metal thereby gradually being consumed. ,When sodium is the alkali metal, the

sodium ions are thought to react in the electrolyte acl cording to the equation: 1

Sodiummetal' is, therefore, added periodicallyto tube 42 to replace the sodium thus used and NaUF is added to the electrolyte as required.

It might be thought that some chlorine or fluorine ions would be drawn to the positively charged alkali metal or holder therefor. However, there-.appears..to belittle evidence of this .ocourringto any a"ppi'ciable' extent.

Such occurrence would be readily detected because the chlorine or fluorine ions thus drawn would reactiminediately with the sodium to form the sodium halide and The alkali metal a 'only a relative low. voltage is required, A voltage .bei tween 0.5 and about '10, volts can be used but a voltage As desired, the uranium metal thus can be 7 removed fromthe eleetrolytejby any convement manner.

It may be raked out during operation or the operation discontinued and while the eleetrolyte'is molten, the

electrolyte may be decanted olf leaving the uranium metal and thereafter manually recovering the uranium metal; or the uranium can be removed-byfcool-ing-the electrolyte to room temperature, removing theeontents of the. cell chamber, andthereafter separating the solid uranium from the electrolyte. as by dissolving theelectrolyte and recovering the uranium metal, e .g.,'by zone melting as described in AINE Transactions, volume 194, page 747 (1952), by W. G. .Pfann.

,Since,as stated, the uranium metal largely collects on that part ofthe cathode opposite the anode, a removable cathode plate or, grid of unreactive conducting material,-

e.g., of uranium, may beplaced opposite to the anode,

periodically removed, and the accumulated uranium metal scraped therefrom.

Since the feed composition added to theroell before according to the iollowing equation V W t Uo,'+4Hc1+sNaF+ NaUF +2mo+4nac1 following example illustrates the inventioni V V I Example 7 An electrolytic cell of the type shown in the drawing was constructed and employed in the example.-- Covers 36 and 34 were removed and 300 grarns of a salt mixture consisting by weightof percent NaUF and 35 percent NaCl were placed in electrolyte chamber' lo oithe cell through openings 35 and 33. Electric current waspassed through heating coil 18 to heat the salt mixture. The salt; mixture was ,melted'thereby to form electrolyte "25 and: the temperature thereafter iwas maintainedat between, 780 and 800 C. .One hundred grams vof metallic. so-' dium were :placed tube 42 whereuponthe sodium rnelted. Covers 34 and 36 were-then replaced. :Valves 56 and 58 were opened; Nitrogen gas was introduced through line 54 which flushed the air out vent 58. Valve I 60 wasthen closed thereby blanketing the surface of the electrolyte and the sodium anode. 'Electric lines 46 and 50 were then connectedto' the positive andnegative terminals, respectively, of a source'jof DC. of between land 2 volts which provided a flow of electricity through the ;electrolyte at between .45 and 50 amper'esito effectelectrolysis of the NaUF The current density measured according to the areaat the sodium anodeexposed to the electrolyte was 30 amperes per square inch. As electrolysis proceeded, uranium metal accumulated at the bottom -12 of chamber 10. ,Because. sodium metal 44 would be measurable in the weight of sodium consumed 'per unit weight of uranium produced.

was gradually consumed during the electrolytic process,

cover 3 6 was removed during ,electrolysisand addi-. 7

tional grams of sodium metal added to tube 42. The cell .was operated for sixty-two. minutes. .The electric current was' then shut 0E and the cell cooled to roomtempera ture.

produce-52 grams'of 98 percent uranium. a It is: to be understood that the above example is an advancement in the 'art of uranium production. Here 7 Covers 36 and 34 were taken ofi and the uranium metal which, had collected on bottom 12 of chamber 10 removed. 400,950 .coulomb's of electricity wereused to tofore, a consumable alkali metal anode has not been successfully employed in an electrolytic bath containing a compound of uranium. By practicing the invention, uranium metal may be produced at atmospheric pressure directly from a novel bath consisting of an alkali metal uranous fluoride and an alkali metal chloride which is readily prepared. Additional alkali metal uranous fluoride may be added to the electrolyte periodically, and the alkali metal chloride may be maintained more-or-less constant, during the electrolysis by merely adding additional alkali metal, e.g., sodium, to the anode. The need for further addition is determined by calculating the NaUF and Na metal used according to the equation set out hereinabove. NaF formed during electrolysis builds up in the electrolyte. By drawing off the excess electrolyte periodically, the Na therein can be separated and thereafter conveniently employed to produce additional NaUF for use in the electrolyte of the invention.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The method of producing uranium metal comprising electrolyzing a molten salt bath consisting essentially of at least 1 part by weight of an alkali metal uranous fluoride and not over 1 part by weight of an alkali metal chloride by passing 13.0 between a cathode and an alkali metal anode in contact with the bath, said anode being substantially protected from contact with the air by a substantially unreactive gas, and recovering uranium metal at the cathode.

2. The method of producing uranium metal comprising passing D.C. between a negative electrode and a positive sodium metal electrode, substantially protected from contact with the atmosphere by a substantially unreactive gas, said electrodes being in contact with a fused salt electrolyte consisting essentially of from about to percent NaUF and from about 40 to 30 percent NaCl by weight, recovering uranium metal at the cathode, and thereafter separating the uranium metal from the electrolyte.

3. A continuous method of producing uranium metal by floating a molten alkali metal on a molten salt electrolyte consisting essentially by weight of between 1 and 2 parts of an alkali metal uranous fluoride and 1 part of an alkali metal chloride confined in a conducting nonreactive container, said alkaline metal and molten salt electrolyte having a protective blanket of an inert gas thereover, making the alkali metal the anode and the conducting container the cathode, depositing said uranium metal on the floor of the container, adding additional alkali metal and alkali metal uranous fluoride, and removing the uranium metal thus deposited.

References Cited in the file of this patent UNITED STATES PATENTS 801,199 Ashcroft Oct. 10, 1905 816,928 Lyons et al. Apr. 3, 1906 2,512,206 Holden et a1 June 20, 1950 2,598,833 Reuman June 3, 1952 2,717,234 Nagy et al. Sept. 6, 1955 2,752,303 Cooper June 26, 1956 2,773,825 Newcombe Dec. 11, 1956 2,773,826 Beese et al. Dec. 11, 1956 2,774,729 Meister Dec. 18, 1956

Claims (1)

1. THE METHOD OF PRODUCING URANIUM METAL COMPRISING ELECTROLYZING A MOLTEN SALT BATH CONSISTING ESSENTIALLY OF AT LEAST 1 PART BY WEIGHT OF AN ALKALI METAL URANOUS FLUORIDE AND NOT OVER 1 PART BY WEIGHT OF AN ALKALI METAL CHLORIDE BY PASSING D.C. BETWEEN A CATHODE AND AN ALKALI METAL ANODE IN CONTACT WITH THE BATH, SAID ANODE BEING SUBSTANTIALLY PROTECTED FROM CONTACT WITH THE AIR BY A SUBSTANTIALLY UNREACTIVE GAS, AND RECOVERING URANIUM METAL AT THE CATHODE.

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