US2690421A - Electrolytic production of uranium powder - Google Patents

Description

be- 28, 1954 w. c. LILLIENDAHL ET Al., 2,690,42

ELECTROLYTIC PRODUCTION OF' URANIUM POWDER Filed March 6, 1943 2 Sheets-Sheet 1 5em- 23, 1954 vv. c. LILI IENDAHL ETAL 2,690,42

ELECTROLYTIC PRODUCTION OF' URANIUM POWDER 2 Sheets-Sheet 2 Filed March 6, 1943 f M ATTORNEY Patented Sept. 28, 1954 ELEo'rnoLr'rrc PnoDUoTIoN 0F URANM POWDER William C. Lilliendahl, Mountain Lakes, Donald Wrouglito'n, East Orange, RudolphV Nagy, Bloomfield, and John W. Marden, East Grange, N. J., assignors to the United States of Americaas reprcs entegl` by the United States Atomic Energy Commission Application March 6, 1943, Serial No. 478,270

3 Claims.

This invention relates to the manufacture of uranium, and more particularly to the com-mercial p-roduction of said material in powdered form.

The principal object of our invention, generally considered, is to produce uranium of exceedingly high purity on a commercial scale.

Another object of our invention is the large scale production of an oxygen-free' halide `salt". of uranium containing no undesirable impurities,

A further object of our invention is the electrolysis of a bath composed of a pure' halide of uranium fused in a mixture of chlorides held at such a temperature that the metal uranium' is deposited as a coarse powder on a cathode of refractory material such as molybdenum.

A still further object of our invention is the removal of the uranium-containing deposits from the cathode and treatment thereof' to con'- serve the uranium powder produced.

Other objects and advantages of the invention', relating to the particular arrangement and construction of the various parts, will become apparent as the. description proceeds'.

In the drawing illustrating ourV invention:

Fig. 1 is a flow diagram showing how potassiurn uranous fluoride is formed on a commercial scale and then washed, dried, yand electrolyzed to produce. uranium powderA embeddedA in other' electrolyte material; and' how said electrolyz'ed material is removed from the cathode, crushed, ground, washed in a tumbling barrel, ground, again treated in a tumbling barrel, filtered, and

dried in vacuum, after which the metal powder may be pressed, and sintered' and/or fused to produce' the finishedw product.

Fig. 2 is a vertical sectional View illustrating apparatus for introducing green salt or other' uranium compound to be electrolyzed, into the molten bath below" the surface thereof.

Fig. 3 is a View corresponding to Fig; 2 butV showing an alternative apparatus for introducing pressed blocks of green salt beneath thev surface ofl the' bath.

Fig; 4 is a View correspondingv to Fig. 3 but showing a further alternative apparatus, Where-f by green saltl may be introduced as a steady stream beneath the surface.

Fig.' 5 illustrates another embodiment for introducing green salt into the bath,. while prdshowing anordinary' type of central electrodetrode, alternative to that shown in Fig. 1, in a fused bath held in its associated Crucible.

Fig. 8 is a View corresponding to Fig. '7, but

functioning as a cathode, with a modified form of crucible associated therewith;

Fig. 9 is a View corresponding to Fig. 8 but showing another modification for the same 91,1.1131056- Fis.'- 10 is a' transverse Sectional View' Qn the line X-:X of Fig. 9', in the direction of the atr-4 rows; and Y Fig. l1 is a' vertical sectional View of a water cooled' container for smothering coatedcathodes in salt. t

Making green Salt In accordance' with our invention we rst preparer potassium uranous fluoride, KUFs (called green $3145, and Wlflfh uranium iS .tetra- Valerlt.) by a photoghanigal; reduction, prefer-- ably using sunlight, of a solution containing a uranium salt such asthe nitrate (in which the uranium has avalency of six), potassium fluoride, hydrouoric acid, and a mougins agent Such as formic acid or sucrose The potassium uranf ous'iiuoride precipitatedras a heavy green salt and collects at the bottom of the vessel from which the liquor may be .dcanted and the desired product removed, filtered, washed, and

dried to yield a salt suitable for the yc electrolysis operation.

The reducing agentV used for the production of the KUF5 may be formic acid, sucrose, oxalic acidI or other suitable, preferably organic, reducing. agen-t. Thel equation when using form-ic acid is `as follows:

agents. the production of potassium uranous fluoride;

When oxalic acid is used as a reducing agent,

A potassium uranous fluoride is produced in accord?- ance with the following. equation:

It will, of course be u ndersGoodthat the `above equations are merely illustrative of reducing actions for the production of potassium uranous iluoride.

Sucrcse is preferred as the reducing agent, as the rate of precipitation of potassium uranous fluoride is increased by more than over that obtained with formic acid. However, there is more discoloration when using sucrose, which discoloration is of course undesirable, as excluding some of the light needed for reduction.

In more detail, our process is carried out in the trays H, I2, i3, lll, l5, i6 and Il.

The trays illustrated in the drawing are in tended to be 10 X 12 feet in plan and lled to a depth of 4". This would make approximately 1080 liters per tray. The reagents required per tray are as follows:

lar operation) and the desired quantities of potassium nuoride, hydrofluoric acid, and a reducing agent such as sucrose dissolved therein. Uranium nitrate is dissolved in its own weight of Water and the concentrated solution added to the liquor in the trays. It is undesirable to add the concentrated reagents to the nitrate solu tion or to dissolve the solid nitrate in the solution of the other materials.

On cool days a yellow precipitate or scale is obtained, which is, however, soluble in hot water, with conversion to nitrate solution and KUF5. It is necessary, therefore, in case this salt separates, to dissolve it by agitating it in hot water and then filtering and proceeding as directed for potassium uranous fluoride.

When the reaction has reached the desired The amount of potassium uranous.` fluoride, which will be called for convenience green salt, which is produced per day is dependent upon the amount of sunlight.

Two days of sunlight in the spring and summer months at New Jersey latitudes are required for conversion of 90% of the nitrate to potassium uranous fluoride. A good sunny day, using the equipment disclosed, should produce about 150 grams of green salt per sq. ft. of tray area. This makes it possible to produce from 200 to 400 lbs. of green salt per day, depending on the ultra-violet intensity of the sunlight. It has been determined that all of the ultra-violet component of the sunlight is utilized in the precipitation of the green salt as well as visible components including up to approximately 4300 Angstrom units.

The actual quantities and ratios of the various reagents used for the production of KUFS may vary, depending upon the desired method of handling, and weather conditions. For example. in cold weather, it may be desirable to reduce the charge shown in the above table by as much as 50% in order to reduce the amount of the yellow scale formed.

While other halides of tetravalent uranium such as "GF4 or UCl4 may be used in the electrolysis process described below, unless they are very pure the preparation of KUF5 in the manner described has certain advantages when uranium metal of high purity is desired. The precipitation of KURS in the manner described results in the removal of certain impurities which are present in the normal commercial grade of uranyl nitrate, resulting in a potassium uranous fluoride of high purity which in turn may be electrolyzed as hereinafter described to a very pure metal. The process is thus of great value when halides of tetravalent uranium of the desired chemical purity are not available commercially.

Other uranous fluorides such as lNaUF5 may be produced by a process similar to that described by changing the proper reagents, as substituting sodium fluoride for potassium uoride.

The trays are first partially lled with water (or with supernatant liquor from a previous simidegree of completion, the supernatant liquid is decanted into the next lower tray and the precipitated potassium uranous fluoride washed from the trays. The suspension is passed through a filter i8 and washed with distilled water and alcohol. In order to increase the rate of iiltration a vacuum connection 2i is desirably made to the container 22 which holds the ilter i8. The salt is then dried in air, as in an oven 23, at about C. Water I9 from filtration of green salt is discarded.

In accordance with the drawing it will be seen that we propose to start with full charges of the green-salt-producing bath in trays Il to ld, inclusive, while allowing the uranium content to gradually deplete in trays l5, I0 and Il by failure to add any more uranium nitrate to these trays. After recharging the sugar-containing solutions four times, it is considered necessary to discard the entire batch due to discoloration of the oxidized reducing agent.

The solution, depleted by precipitation of potassium uranous Iiuoride, contains some potassium nitrate and considerable hydrofluoric acid and potassium fluoride. To the trays containing uranium depleted solutions, that is, those numbered l2 to ld, inclusive, which are to be maintained up to strength and which successively receive the solutions from the preceding ones of lower numbers, one mol of formic acid, cr 1/6 mol of sucrose, together with 3 mols of potassium fluoride, and 2 mols of hydroiiuoric acid are added for each mol of uranyl nitrate. This formula has proven satisfactory but we do not wish to be limited to these proportions.

As an alternative, di-potassium uranous fluoride, KzUFe, produced by a process described in the copending application, Serial No. 478,271 filed March 6, 1943, by John W. Marden and Rudolf Nagy, may be used in place of potassium uranous fluoride. The metal produced from this salt is in every way comparable with that produced from KUFs and UF4.

Electrolysis of "green salt The electrolysis of green salt to produce uranium powder may be carried out in furnace lll consisting ofy a graphite crucble 25 and a. molybdenum electro-de 2B. Thecrucible and electrode are connected toa source of direct current 2-1 so that the crucible is the anode and the` electrode 26' theA cathode. Means may be provided for heating the contained electrolyte, other than by the electrolysis currentl between the electrodes, such as resistance windings about the Crucible (not shown). I-Iowever, we have found that the desired results may be obtained by heating the electrolytel with the electrolysis current only.

The electrolysis process consists of fusing a. mixture of sodium and calcium chlorides' in acrucible 25, adding toA said fused mixture the desired proportion of green salt, andelectrolyzing the bath until the uranium is depositedonr the'A cathode. Using a batteryl` of five furnaces withv crucibles 9l inside diameter and 21" deep, with an effective volume of; approximately 1300 cu; on three eight-hour shifts, approximately 250= lbs. oi-v metal powder per day may be pro-- duced. For each furnace; 18001amperes of' our rent with electrode changes every 60? minutes are required.

The crucbles are' chargedV with metal halidesalts; desirably initially charged with a mixture consisting of about 80% calcium chloride by weight and about 20% sodium chloride. A totalof 80 to 1'00 pounds isj required.

If the salts are not fused by external heating', an auxiliary or "dummy` electrode may be used. The use of a dummy electrode during charging serves not only for the hea-ting but also for the removal of deleterious impurities in the bathsuch as iron and boron prior to electrolysis. Such; impurities are deposited on the dummy during the charging operation orvolatilized from the bath by the heating.

The' bathis heated to the temperature of4 approximately 950 C. by the passage of current and the cruciblelled towithin 6" of the top. When all the salt is melted about one poundof green salt is added to the crucible. About liveminutes after the addition of green salt the original electrode, now called the dummyf is removed.

About seven pounds of green salt are then added to the'bath and the melt allowed tostand: until the temperature of thel bathfalls to 900 C. A cleanV electrode is then introduced' into-L the bath and the electrolysis carried to completion. Allowing the; meltl to stand Without electrolysis serves to eliminate any volatile dele-- terious materials, such asboron which passesoff as a halide or mixtureof halides, from the bathprior to electrolysis.

The amount of uraniumi salts initiallyv added to@ the fused chlorides shouldbe of the order of: to 3% of theztotal weightof the chlorides; in the crucible. The amount. of uranium saltsl added after the dummy cathode is removed from the.: crucible should be of the order of 5% '60,15% of the total weight of the chlorides in the crucible.

One batch of.V metal. powder made from several iirst electrodes containedmore than parts perV million of' boron. Powder made from corresponding second electrodes. averaged about part per million. Powder made by similar operations, except that one electrodeV was'used` for both thev charging and the electrolysisaveraged I tor 3* parts permillionofv boron. Forcertain pur'- poses a minimum boron content is desirable;

It external heat has initially been applied to-y f-use the bath; the intensity thereof: mustbe:

gas.

reduced while the: cell? is in. operation because of Ithe energyv dissipated byV the electrolysis current, asthetemperaturer mustbe hel-dv at 900LT Cl. i25` C. throughout aA run. Low temperatures result in large deposits of salts.- on the electrodes but. low yields because of the fineness of deposited powder.

The ideadisclosed in- Fig. l. is to introducethe green salt into the fused bath by throwing it on top of the surfaces of said bath.Y It" the salt. is potassium uranous` iiuoride'- or uranous-fluoridef, it may oxidize. tol` some extent before becoming completely dissolved. Once.` dissolvedit ispro.- tected by the fusedbath and no oxidation seems. to take place. When the salt can beY plunged directly belowv the surface it is heavier than the calcium chloride or sodiuml chloride bath andi sinks to the bottom where it is: dissolved withoutoxidation.

In accordance with- Fig. 2': we propose. lio-introduce the dry' salt below the surface ofthe bath by weighing the desired amount into.v a-u tube d'4 with a hinged cover' l5` at its` lower end and. initially closed, as shownl infull lines. A plunger 46 is inserted into the tubewith the green salt. or other compound to be electrol-yzedf, placed in. the' bath 28a in the crucible 25e. and said salt expelled beneaththe surface after dropping theA cover 45, as by operating the rod 41 connected' to a crank 481 thereon. We have simplified the showing by omitting the electrode.

Fig. 3 shows alternative apparatus for introducing "green salt in the. form of a pressed'. mass 49 beneath the surface of the electrolyte. bath 20h in crucible-25b, as by holding it inf tongs: 51 and forcing it duicl'ilyY below the surface.V

In L we have shown furtherv alternative apparatus, for introducing"green salt by means. of a small screw type conveyor 52 operating in a cylindrical extension 5-3L on the bottom of a hopper 54contai'ning' the desired quantity of' salt whichY is introduced into the bath- 28 froml the. lower open end- 55 of sai'df extension 53. The. screw conveyor member 52 may be operated through suitable gearing' by means of' anelectric motor or other' power means 56; In all'. forms. of' our invention illustrated in- Figs. 2, 3f. andl 4 the parts of the apparatus which. come in con tact with the fused bath are desirably made of molybedenumv or some such material in order to avoid contaminating said bath.

Still anothermeans of introducing green salt" Without oxidation is to placca cover` 51 of asbestos, or other halide-resistive insulating material, over the Crucible 215idw and flow inert or protective: gasY such as argon or hydrogen, onto the surface vof the bath as by means of a valve-controlled pipe58; to prevent oxidation of theuranium salts or metal during the process of introducing the saltv from the bottom of hopper 59. Holes Pr'l` may be provided in thel cover 5'!Y to= allow for the escape of gasgenerated during the-electrolysis operation and aslv ventsvfor the protectivey A valve-controlled? branch` 62 ofthe pipe` 581-mig-ht servey as ameans forI introducing gasv to control or expedite the flow ofthe salt from. the hopperr 59. Another valveecontrolledf branch. 63 might be employed to keep a protective at-l mosphere over thev top of the salt, a coverV 64 with a valve-controlled vent' piper 65being in. suchl instance provided for the hopper 591.

The; cathodeI 2B is? lowered into thea bath tm within 2 or the Crucible bottom and the; elec trolysis: currentv of about. 180,05 amper-es; started and maintained'- for. aboutl 60 inmates..

The furnaces or crucibles previously described for fused salt electrolysis, in accordance with our invention, are of the single unit type, that is, the same Crucible is set in insulation and a single electrode placed in the center thereof. Any number of these furnaces may be operated together in series or in parallel, but each is an individual unit. When an electrode is changed in a single unit type, the furnace must be shut olf or shunted out. In very large installations such construction is not suitable 1because the electrode becomes very large and the cathode to Crucible spacing is too great. Handling the electrode then becomes a problem and the voltage required to operate the furnace is increased.

For operations on a large scale Where the metallic deposit is removed on the cathode, a furnace with several cells built together, as illustrated in plan in Fig. 6, is desirable. The crucible 25e, which also serves as the anode, in this instance is elongated and provided with baffles or partitions 66, said baiiles projecting alternately from the opposite long sides 73 and 76 of the Crucible. As in the previous instance, a suitable conductive material, such as carbon or graphite, must be used. An electrode 26e to function as a cathode is suspended in each compartment, except the first or melting compartment 6l, and all connected together, as illustrated, so that the entire unit behaves as several cells connected in parallel.

The operation of such apparatus may be to charge a substantial quantity of the salts to make up the fused bath each time the uranium salts to be electrolyzed are added. These salts may be charged into the end compartment 6l, where they are melted, as by alternating current between dummy electrodes S8 and 69. The molten salt will then flow into section 'l l, through space l2 between partition @t and the long wall T3 of the assembly toward which it projects. From section ll it would flow on to section 1d, through space 'l5 between the next partition 66 and the long wall 'it toward which it projects. From thence it would ilow on to the remaining sections or compartmentsI in a similar manner.

Uranium salt to be electrolyzed is added to each section except the rst and last, from time to time. In the last section no uranium salt is added, this being used to take out the last trace of uranium before the salts run to the overflow pipe ll. The electrodes may be removed one at a time whenever sufficient deposit has been collected. Moinentary interruption of the current in one section does not materially effect the operation of the battery as a whole, as the other electrodes would carry the eXtra current for a short while. Such a furnace is particularly useful for operations where a large amount of makeup bath salts must be used. It avoids loss of uranium in the discharge and makes the process substantially continuous. Current limiting devices may be incorporated for use with the arrangement. Especially where it is not necessary to recharge the bath, the rst section designated 6l may also be used for electrolysis like one of the single unit type furnaces previously described.

Referring now to the embodiment of our invention illustrated in Fig. 7, there is shown a crucible 25f, which may be like a single unit crucible 25 or a multiple unit crucible 25e, containing a fused salt electrolyte bath 28f and in which is suspended a special type of electrode 26f to function as a cathode. The reason for the special type illustrated is that while rnolybv` denum is at present the most suitable metal forthe cathode, it is heavy and diilicult to fabricate. For best results such a cathode should have a large diameter and surface area, Without eX- cessive cross sectional mass. A hollow cylinder satisfies these conditions, but the fabrication of such a cylinder of sufficiently large dimensions out of molybdenum is difficult.

It is. therefore, desirable to construct a cathode with a surface of molybdenum on a base metal which may be readily shaped as desired. Thin cylinders of molybdenum, such as might be constructed of sheet metal, do not have sufficient mass to conduct the heavy currents used in electrolysis furnaces. In accordance with our invention we, therefore, propose to make the electrode 26f as a hollow cylinder 13 of steel or other suitable metal, closed at its lower end as indicated at 19, and coated or sheathed from the bottom to above the top surface of the electrolyte bath With molybdenum 8l. The molybdenum is desirably applied as a continuous sheet or cup over the core '18, or as a ribbon or wire Wound helically on the tube 18, so that the adjacent edges thereof abut and are thereafter desirably welded together so as to form, in effect, a continuous molybdenum surface. Such an electrode has the advantage of a large molybdenum surface, light weight, and a sufiicient but not excessive cross sectional area. Covered electrodes of the type mentioned above are disclosed and claimed in the copending application of William C. Lilliendahl and Donald Wroughton, Serial No. 517,016, led January 5, 1944, now abandoned.

rl'he assembled electrode 26f may be supported in a head forming a flange 82 from which a pipe 83 extends and connects with a port 8d to the interior of the cylinder 73. The upper end of the cylinder is closed by a cap 35 carrying a pipe 86 of steel or other suitable material, which extends down to near the bottom of the cylinder 1S, and through which air or other fluid may be introduced to said cylinder for cooling the electrode when desired, as during withdrawal from the bath, in order to minimize oxidation of the deposit. The cooling fluid would pass down the pipe E6 and return between the pipe and the cylinder 18 through the port tl and out of the pipe 83, thereby effecting the desired cooling action.

When using a cathode in the form of thin strips, a cylinder, or rectangular rod of molybdenum, it is found that When operated in a Crucible of the design shown in Figs. 1 to 7, inclusive, the deposit is non-uniform. That is, the rate of deposition at the top portion of the electrode, or near the surface of the bath, is faster than at the lower portion. This results in a cone-shaped deposit, as shown in Fig. 1, on the electrode 2t. This condition of unequal distribution is most pronounced in electrodes of small cross sectional area.

Such a deposition is objectionable because it has a tendency to peel due to the Weight of the deposit formed and sometimes results in loss of deposit. The formation of such deposits also reduces the electrolyzed metal weight per unit area of cathode surface, which in turn limits the charge per run of material electrolyzed, thus reducing the yield per man hour of operation. The condition appears to be due to the relatively large voltage drop in the electrode. In operation, this may be of an order of magnitude of one to two volts.

As electrolysis starts, the voltage drop across the bath to the lower portion of the cathode is less than at the top, which results in increased deposit at the top of the electrode. This may also be due to convection currents of the electrolyte. As electrolysis proceeds, the deposit builds up at the top of the bath so that the resistance at that point decreases, resulting in further increase in the rate of deposition at the top of the electrode at the expense of the lower portion.

This condition of unequal deposition may be prevented by tapering or forming the crucible 25g with an inverted frusto-conical interior, so that the current along the cathode 26g is uniform throughout the submerged length, as shown in Fig. 8, or by having the cathode 25h flare downwardly, or with its lowerportion truste-conical, in a crucible 25h of uniform cross section throughout its height, as shown in Figs. 9 and 10, or by tapering the crucible to some extent and aring the cathode to some extent, whereby the distance between the cathode surface and the crucble surface steadily decreases from the top of the bath to the bottom of the cathode, the rate of decrease being sufficient to balance the drop along the cathode, in order to equalize the current between cathode and crucible and result in uniform thickness of deposit, as illustrated in Figs. 8, 9 and 10.

The cathode 26h, illustrated in Figs. 9 and 10, may be formed of two molybdenum plates, trapezoidal in shape, with their widened portions bulged or formed as hollow anged half-cones 8'! and 88, and then connected together as by molybdenum bolts 89. Current leads 9i are provided for introducing the electrolysis current to the electrode. When the electrolysis operation is completed the electrode may be withdrawn from the bath as usual, and after cooling the plates separated by removing the bolts, thereby facilitating the removal of the deposit from both inside and outside. The upper bolts 80 (Fig. 9) are not immersed in the bath and are easily removed. However the lower bolts acquire a coating during electrolysis that must be removed before disassembly of the electrode plates. The shape illustrated is not only conducive to a uniform deposit, but is more eifective for preventing any slippage of the deposit by virtue of the vertical component of the pressure between the deposit and the cathode, acting to support said deposit. Tapered electrodes of the type mentioned above are disclosed and claimed in the copending application of Towson Price, Serial No. 510,557, filed November 16. 1943, now abandoned.

Operation of the electrolyte bath at about 900 C. is an improvement over prior electrolysis processes in which operation was effected at lower temperatures, in that the proportion of metal iines is decreased and a coarser uranium powder produced, which is less pyrophoric.

If a hollow electrode, as disclosed in connection with Fig. 7, is employed, it is also desirable in accomplishing the above results to inject a rapid stream of air through it during the operation of raising said electrode from the bath, thus assisting in the solidication of salts on the cathode to form a protective layer on the removed 10 approximately 100 C., it has been found that said metal does not have the pyrophoric properties which are present in powders when baths are operated at 800 C.

The melting point of uranium is relatively low, that is approximately 1150 C. With baths operating at 875 C. to 950 C., the actual temperature at the cathode is materially aifected by current density and cathode potential, which is in turn affected by bath composition. It has been discovered that, by a proper control of the variables, uranium metal is produced on the cathode in such a compact form that the deposit can be removed, washed with water, and the coherent metalized, sintered, mass fused in vacuo, thereby eliminating the necessity of the washing and grinding operations later fully disclosed.

The electrochemical equivalent of' tetravalent uranium is 2.22 grams per ampere hour. The 7 pounds of green salt contain 2020 grams of uranium, which would require 910 ampere hours at 100% eiciency. In practice, however, approximately 1800 ampere hours are required for 7 pounds of green salt. The current efflciency does not appear to be appreciably affected by current density at the cathode. The most convenient combination of current and time which will yield 1800 ampere hours for that size charge can be used, but the current strength must be that required for keeping the furnace at the desired temperature.

At the completion of the run, the cathode is desirably taken quickly from the bath for removal of the deposit. It is then preferably immediately placed in a tall cylindrical vessel 92 with a funnel shaped top 93, as shown in Fig. 11, which is then at once filled with dry salt, such as dry sodium chloride, anhydrous calcium chloride, or similar material 94. The salt is partly melted on the surface of the electrode, sealing the metal particles and preventing oxidation. If a hollow air-cooled electrode 2Ef, as shown in Fig. 7, is used, then it may not be necessary to use the enclosing receptacles 95 containing cooling means, such as water 96, which is introduced by pipe 97 and overflows from pipe 98.

However, for the ordinary type of electrode 26j shown in Fig. 1l, the external cooling means is desirable as it facilitates the operation by effecting a quicker cooling. The purpose of this treatment is to prevent oxidation while the deposit cools. Other methods of accomplishing the result, such as quickly redipping in the bath after partial cooling, or pouring some of the electrolyte over the withdrawn cathode after partial cooling, have also been used successfully. Use of this improved technique of salt smothering has enabled us to substantially increase the average yield.

At the end of an electrolysis run, enough salt should be removed from the bath so that 12 to 15 pounds (that is about 15%, of the total charge of fresh salt mix), of calcium chloride and 10% sodium chloride, may be added to the crucible. Some salt is always removed on the electrode and the rest may be dipped from the molten bath, as by means of a suitable cup. The furnace is then charged, preferably using the dummy electrode as described, and the operation repeated.`

We do not wish our invention to be limited by the specific dimensions of the furnaces as given above. Other sizes of furnaces have been used for carrying out the same process with good results. Also bath-temperatures between 750 C.

and 950 C. will yield uranium metal, although 900 C. is the preferred temperature since, other factors being equal, operation at 900 C. has been shown to result in an increase in yield of usable metal powder.

The bath composition may vary considerably from the original charge, i. e. 80% CaCl2-20% NaCl, with good results. The bath described is desirable because it has a sufficiently low melting point and requires a minimum amount of recharging to preserve fluidity.

Furthermore, halides of tetravalent uranium other than KUFs, such as UE; and UCli, may be electrolyzed in the same bath. Uranium tetrailuoride, (UFi) for example, has the advantage that it gives a better material yield than KDF-5, based upon the uranium content. Whenever it can be obtained in the required degree of purity it then becomes the preferred uranium salt. When using UF4, it is desirable to recharge the furnaces with an 80% Camz-20% NaCl mixture rather than with the 90% CaCl2-10% NaCl mixture used with KUFs. This compensates for the potassium content of the KUF5.

In all cases where the fluoride salts of uranium are used for electrolysis, it is necessary to discard a certain fraction of the bath at frequent intervals to remove part of the nuorides from the bath. For best results, the calcium fluoride content should not exceed 30%, although good yields of metal have been obtained from baths containing as much as 40% CaFz. A bath which gave very good results, for example, was composed of 18.7% CaFz, 60.8% CaClz, and 10.5% NaOH-% KCl after all the uranium salts had been electrolyzed.

However, when operating the electrolyte bath at about 900 C., the electrolyte is very fluid even with high percentages of calcium fluoride. We have, therefore, been able to use the same bath continuously for more than times with no removal of electrolyte, as by dipping. Salts, generally calcium chloride and sodium chloride, as in the ratio of 4 to 1, are added but only to compensate for what is lost by evaporation and removal on the electrode.

Baths operated continuously, as described, are considered superior to periodically replaced baths in the following respects:

( 1) Contamination by iron is minimized.

(2) The amount of calcium chloride and sodium chloride used in the process is substantially reduced.

(3) The dangers atending dipping are eliminated.

(4) The power necessary for heating the recharged bath is saved.

(5) The oxides produced in the bath, due to heating by electrolysis current are eliminated. Such oxides increase the melting point, and induce slipping and poor yields of metal.

(6) More runs per day can be made.

(7) Larger charges of uranium salts can be used, thus increasing the electrical efliciency.

(8) More than one electrode can be used in a bath with alternate removals.

As an alternative, to the use of the dummy electrode, the salt charge may be melted in an auxiliary furnace and poured into the electrolysis cell when molten. In such cases, a dummy electrode need not be used if the salts are of satisfactory chemical purity.

Conversion of electrode deposit The deposit on the electrode is converted to pure metal powder by a process which consists essentially of crushing and grinding said deposits, and washing and rinsing the crushed and ground material until the residue is of the desired fineness and substantially free of deleterious substances, such as salts from the electrolysis bath and finely divided uranium metal and oxide.

The electrode deposit as removed from the bath consists of metal powder and solidified salts which serve to protect the metal from oxidation. The ratio of salt to metal powder depends on the temperature and composition of the bath, high temperature and low viscosity favoring a low ratio of salt to metal powder. Operating at 875 C. to 900 C., the ratio of salt to metal is approximately 2 to l.

The handling of the cathode and its deposit after removal from the bath may, for example,

be divided into several unit operations, which comprise:

l. Removal of deposit from the cathode, as by means of an air hammer 29 or a cold chisel.

2. Coarse crushing to i mesh or nner, as by means of a jaw Crusher 3|.

3. Fine crushing, as by means of a roller crusher 32.

4. Water washing to remove soluble salts, as in a tumbling barrel 33.

5. Wet grinding to break up aggregates and assist in the removal of insoluble salts, as by means of disk grinder 34.

6. Further washing, which may include the use of acetic acid, to remove salts retained from previous operations, as in a tumbling barrel 35, followed by repeated decantation.

7. Filtration of metal powder followed by Washing with alcohol and ethyl ether, as in vacuum lters 3G.

8. Drying in vacuo and. preserving in an atmosphere of inert gas, as in vacuum ovens 31.

For producing uranium of extreme purity, all operations should be conducted in equipment composed of non-contaminating material, such as wood or hard rubber, where practical, and the grinding and crushing equipment made of steel, to reduce the pick-up of foreign elements. Hard glass which might contain borates is to be particularly avoided and where the use of glass equipment is necessary, silica or lime glass should be used. The steps previously noted will now be discussed in detail with respect to the practice for large scale production.

For handling deposits comprising 250-300 lbs.

of metal powder per day, a jaw crusher capable of handling 900 lbs. of material per day is required. The crusher should have an efectve jaw opening of approximately 1A".

After passing through the coarse jaw crusher 3i the material is passed through the roller crusher 32 set at approximately 0.100 and then transferred to a wooden tumbling barrel 33, rotating at from 60 to 80 R. P. M., and provided with suitable mechanism for tipping to obtain rapid washing by decantation. Sufiicient distilled water is added to dissolve the soluble salts and the barrel rotated for a period of 20 minutes. Considerable heat is evolved during this operation, due to the solution of the calcium chloride contained in the deposit, and it is necessary to cool, as by the addition of ice. The temperature of the solution should be maintained below 30 C.

during this operation to avoid oxidation of the uranium powder. Approximately 8 liters of distilled Water are required per pound of metal pow- 13 der in the deposit, `to which-21m 3 lbs. of ice are added.

After completion of the tumbling, .the barrel is tipped and the metal sludge washed three times -by decantation, rotating two to three minutes between each operation. The wash vliquors should be recovered and settled, as `the uranium metal nes and oxides should be reconverted to nitrate for subsequent precipitation as green salt in order to avoid loss of valuable material.

The metal sludge after removal from the tumbling vbarrel is run Wet through a .disk grinder 34 to break up aggregates ,and assist in further removal of insoluble oxides and fluorides. A grinder capable of yielding material between 50 and 100 mesh is required for satisfactory removal of the insoluble materials. Particles coarser than 50 mesh interfere with successful removal of impurities in the washing to follow.

The ground material is transferred to the rtating barrel 35 and washed with water. After rotating for minutes, the barrel is tilted and the light impurities poured off. This process, tumbling for 2 to 3 minutes and pouring off, is repeated with distilled water until the supernatant liquid is free from light material, such as CaF-z. In some cases a 2% solution of acetic acid may be used, in one of the early washes to assist in the process.

The metal powder is then transferred to a suction filter of the Buchner type and the filtration facilitated by a connection to vacuum line 39. The metal on the iilter is desirably washed twice with 95% ethyl alcohol, using suflicient reagent to cover the sludge and draining between additions. The powder is then desirably washed twice with ethyl ether, acetone or other volatile solvent, and after partial drying is immediately transferred to tared cans and placed in Vacuum ovens 3l. After drying in vacuum for eight hours, the ovens are lled with an inert gas such as nitrogen, carbon dioxide or argon, but preferably with the latter, and the metal powder preserved therein until ready for the pressing operation. However, the long vacuum oven drying may be omitted, if desired, as the powder can be sufficiently dried on a suction filter after washing with a volatile solvent. To reduce re hazard, the metal powder should never be handled in lots larger than about 10 lbs., especially after washing with ether or acetone.

Further operations may involve consolidating the metal powder in press il and nally sintering and fusing, or otherwise heat treating, in furnaces 42, as in accordance with Patent No. 1,814,719.

From the foregoing it will be seen that we have devised an improved method comprising rst purifying the raw material by transforming to an oxygen-free iiuori-de, while simultaneously avoiding contamination with deleterious materials, particularly boron, and after electrolysis preventing subsequent contamination and oxidation, using only water which is distilled, so that the uranium metal powder when nished is suitable for subsequent processing to form pure metal articles of the desired shapes.

Although preferred embodiments of our ideas have been disclosed, it will be understood that modiiications may be made within the broad spirit and scope of the invention.

We claim:

1. The method of producing pure uranium powder that comprises introducing a single uranous halide selected from the class consisting of potassium uranous fluoride, uranous fluoride and uranous chloride into a carbon crucible containing a fused bath consisting of approximately calcium chloride and 20% sodium chloride by weight and having a molybdenum element therein, the uranous halide being introduced in an amount vapproximating 5% to 15% by weight of the fused bath, electrolyzing at `a temperature of approximately 900 C. with the Crucible Vas anode andthe molybdenum element as cathode to deposit uranium on said cathode, removing the coated cathode, permitting the cathode to cool, removing the deposit therefrom, and crushing, washing Yand drying said deposit to produce pure uranium powder.

2. The method of producing pure uranium powder that comprises introducing potassium uranous fiu'oride into a carbon Crucible containing a fused bath consisting of approximately 80% calcuim chloride, and 20% sodium chloride :by weight and having a molybdenum element therein, the potassium uranous fluoride being introduced in an amount approximating 5% to 15% by weight of the fused bath, electrolyzing at a temperature of approximately 900 C., with the crucible as anode and the molybdenum element as cathode to deposit uranium on said cathode, removing the coated cathode, permitting the cathode to cool, removing the deposit therefrom, and crushing, washing and drying said deposit to produce pure uranium powder.

3. The method of producing pure uranium powder by electrolysis in a continuously operating bath that comprises introducing potassium uranous fluoride into a carbon crucible containing a fused bath consisting of approximately 80% calcium chloride and 20% sodium chloride by weight and having a molybdenum element therein, the potassium uranous fluoride being introduced in an amount approximately 5% to 15% by Weight of the fused bath, electrolyzing at a temperature of approximately 900 C. with the crucible as anode and the molybdenum element as cathode to deposit uranium on said cathode, removing the coated cathode from the Crucible, removing a portion of said bath and refilling said bath to its original level with a mixture consisting of approximately calcium chloride and 10% sodium chloride, the portion removed being such that the addition of the mixture in an amount approximating 15% by weight of the original fused bath will restore the bath to its original level, introducing a new charge o-f potassium uranous fluoride into the refilled bath and electrolyzing at a temperature of approximately. 900 C'.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 596,458 Inglis Dec. 28, 1897 618,575 Lyte lIran. 3l, 1899 808,066 Borchers et al Dec. 26, 1905 850,376 Kugelgen et al Apr. 16, 1907 1,077,894 Stevens Nov. 4, 1913 1,197,137 McNitt Sept. 5, 1916 1,255,197 Malm Feb. 5, 1918 1,534,319 Hoopes et al Apr. 21, 1925 1,646,784 Marden Oct. 25, 1927 1,814,719 Marden et al July 14, 1931 1,821,176 Driggs et al Sept. 1, 1931 1,826,806 Marden et al Oct. 13, 1931 (Other references on following page) Number Number UNITED STATES PATENTS Name Date Driggs Dec. 8, 1931 Driggs Jan. 19, 1932 Driggs et al June 7, 1932 Driggs Aug. 30, 1932 Eldridge June 23, 1936 Lyons Nov. 3, 1936 Dandt et al Jan. 19, 1937 FOREIGN PATENTS Country Date France July 30, 1906 "16 OTHER REFERENCES Driggs et al., article in Industrial and Engineering Chemistry, May 1930, pp. 516-519.

Smyth, Atomic Energy for Miliary Purposes, page 27, para. 2.27, August (1945).

Goodwin, Electrolytic Calcium in Proceedings of the American Philosophical Society, vol. 43, pages 383 and 384 (1904).

Croggins, Unit Processes in Organic Synthesis, published 1938 by McGraw-Hill Book Co. Inc., page 604.

Morrow, Biochemical Laboratory Methods, published 1927 by John Wiley and Sons Inc., page 191.

Claims (1)

1. THE METHOD OF PRODUCING PURE URANIIUM POWDER THAT COMPRISES INTRODUCING A SINGLE URANOUS HALIDE SELECTED FROM THE GROUP CONSISTING OF POTASSIUM URANOUS FLUORIDE, URANOUS FLUORIDE AND URANOUS CHLORIDE INTO A CARBON CRUCIBLE CONTAINING A FUSED BATH CONSISTING OF APPROXIMATELY 80% CALCIUM CHLORIDE AND 20% SODIUM CHLORIDE BY WEIGHT AND HAVING A MOLYBDENUM ELEMENT THEREIN, THE URANOUS HALIDE BEING INTRODUCED IN AN AMOUNT APPROXIMATELY 5% TO 15%

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