US3484233A - Process and apparatus for separating metals by distillation - Google Patents

Description

Dec. 16, 1969 c. F. BONILLA PROCESS AND APPARATUS FOR SEPARATING METALS BY DISTILLATION Filed 001. 14. 1966 2 Sheets-Shet 1 I Z n Z INVENTOR. C/As /1 Bof/luf:

Arrop/vfv l Ill D. 1s, 1969 c. F. EQNILLA 3,484,233

PROCESSVAND APPARATUS FOR SEPARATING METALS BY DISTILLATION Filed OCT.. 14. 1966 2 SheBtS`-Sheet 2 s1 i V f f 49 i 1111i i 3; 40-

1 lei 2 f1 111i INVENTOR.

United States Patent Ov U.S. Cl. 7S-63 6 Claims ABSTRACT F THE DISCLOSURE Process and apparatus are disclosed for separating a relatively volatile first metal from a liquid alloy thereof with a second metal of substantially lesser volatility. This invention involves maintaining in a first zone a first pool of a liquid alloy of said metals, maintaining in a second zone a second pool of liquid alloy of said metals, the concentration of said first metal being greater in said second pool than in said first pool, evaporating a portion of said first metal from said first pool, conducting the thus-formed vapors of said first metal into indirect heat exchange with second pool, condensing said vapors by said indirect heat exchange, passing the resulting condensed liquid phase first metal into said second pool at a point beneath the surface of said second pool, evaporating a portion of said first metal from said second pool, conducting said evaporated first metal away from said second zone, and maintaining the pressure in said second zone at an absolute level less than the pressure in said first zone and at a level sufficient to permit the heat transferred by the condensing vapors from said first Zone to be sufiicient to evaporate some of the first metal from said second pool.

This invention relates to separation processes and apparatus, and more particularly to processes and apparatus for obtaining a relatively volatile metal in substantially pure form from an alloy of said metal with a second metal of substantially lesser volatility.

Mixtures of light metals, such as sodium or potassium, with heavy metals, such as tin or lead, are formed in various industrial processes. One example of such process is the fused salt electrolysis of sodium chloride using a molten lead electrode as described, for example, in U.S. Patent 3,104,213. The metallic sodium formed is dissolved in the molten lead electrode, forming a liquid alloy containing about to 20% by weight of sodium, with the remainder being lead. It is sometimes desirable to recover the sodium from this alloy in highly purified form containing not more than about one part per million of lead.

The recovery from a sodium-lead alloy of exceptionally high purity sodium is very difiicult to achieve by currently known processes. Sodium-lead mixtures predominating in lead have high boiling points; for example, a mixture containing sodium and 90% lead by weight has a boiling point of about 900 C. The high operating temperatures necessary foi separating sodium and lead favor the use of compact apparatus in order to minimize heat losses. It is also desirable to use special materials of construction, for example stainless steel or even more refractory alloys, to resist internal corrosion by the sodium-lead alloy and external oxidation by air at the high operating temperatures required.

Theoretically, sodium and lead could be separated by distillation using only a small number of plates, eg. about 2 or 3 plates. Actually it has been found that conventional distillation procses and apparatus, such as those used in separating petroleum mixtures, will not give pure sodium containing a maximum of one part per million of lead. A considerable amount of entrainment of liquid 3,484,233 Patented Dec. 16, 1969 ice droplets in the vapor phase takes place in the conventional distillation column. The principal reason for this is that vapor from each plate is buhbled through the liquid on the next higher plate, agitating the liquid and entraining some of the liquid in the form of fine droplets as each vapor bubble rises to the surface and bursts. Some of these droplets settle out on higher plates, but are replaced by other newer ones. As a result, liquid droplets may be carried in the vapor stream up through the entire distillation column, by-passing a number of plates and passing into the overhead product. In the case of sodium-lead and similar alloys, this would result in the presence of relatively substantial and undesirable concentrations of lead in the sodium product unless an impractically large number of plates were used.

In addition, when the components differ greatly in volatility, as with sodium lead, vapor bubbling through liquid on a plate approaches equilibrium much less completely than when it is condensed and mixed in, and then revaporized.

The principal object of this invention is to provide a process and apparatus for the recovery of high purity sodium or other relatively volatile metal from an alloy of this metal with a second metal of substantially lesser volatility.

According to the process of this invention, a liquid alloy of a relatively volatile first metal and a second metal of substantially lesser volatility is fractionated in an apparatus having a plurality of zones, each zone containing a pool of liquid alloy of the two metals. A portion of the first metal in each zone is evaporated, withdrawn from such Zone, and the vapors are condensed by indirect heat exchange with the liquid metal in the next zone. The condensate thus produced is then introduced into and below the surface of the pool of molten metal in the next zone. The vapors produced in the last zone consist of highly pure first metal which are withdrawn as product, except for a small portion, as required, which is condensed and returned to the system as reflux. The Operating pressure maintained in each zone is not only less than the operating pressure maintained in the preceding zone but is generally less by an amount greater than that which normally exists in distillation columns in order that the correlativo relationships of temperature, pressure and alloy composition between the liquid pool in a zone and the vapors from the preceding zone permit the total indirect heat exchange condensation of such vapors as aforesaid The total condensation of the metal vapor from each zone before it is introduced into the next zone absolutely transfers any entrainment therein at once to the liquid phase, preventing its by-passing this next zone in a vapor stream.

Introduction and mixing of the condensed vapor into the pool of metal well below the surface of the pool minimizes or eliminates the new entrainrnent that `would be generated by the flashing or spontaneous` explosive boiling of said condensed vapor if it emerged near the surface in the next zone, which zone is operated :at a combination of temperature and pressure conditions which would otherwise permit such condensed vapor to boil.

In order to continue the purification of the sodium or other volatile metal it is necessary for vapor to continue to rise from each zone to the next higher one, countercurrent to a descending stream of liquid refiux, as in a normal distillation column. Accordingly, it is necessary for the liquidpool on the above next zone to generate new vapor by vaporization to replace the vapor from the `zone below as it condenses. The heat of vaporization required and employed in generating the new vapor on this next zone is of course that given up by the vapor rising from the zone below as it condenses.

An additional desirable feature in applying this invention is that the indirect heat transfer tubes in which the rising vapor condenses be so shaped and placed that the new vapor they generate not be generated below the surface of the pool of molten metal, where it would in turn generate new entrainment as it bubbles through the surface. Instead, the liquid should be assisted to circulate steadily to the surface of the pool and there to vaporize slowly and uniformly without forming bubbles. This can be accomplished in liquid metals boiling at low temperatures, as primarily contemplated herein, by distributing said heat transfer tubes consistently somewhat below the surface of the liquid pool, but well and uniformly throughout the pool, and by not exceeding a reasonable rate of vaporization, such that corresponding to about 100,000 B.t.u. of latent heat per hour per square foot of pool surface.

The second metal is recovered from the system as a bottoms product alloy containing a lower concentration of the first metal than in the feed.

According to a preferred embodiment of this invention, a feed alloy of sodium and lead containing about to by weight of sodium is fractionated in two zones to produce substantially pure sodium containing no more than about one part per million of lead. This highly pure sodium is recovered as the distillate product. The bottoms product is a lead-sodium alloy containing a greater concentration of lead than the feed alloy.

The apparatus of this invention comprises a plurality of compartments arranged in series, each compartment providing a separation zone. Each compartment includes means for retaining a pool of liquid alloy and a vapor space above the pool. Each compartment maintains itself at its individual pressure, the pressure decreasing progressively from the first compartment to the last compartment in the series, as in an ordinary distillation column. A feed conduit introduces a molten alloy into one of the compartments in the series and most frequently into the lowest compartment. At least one and generally a plurality of vapor conduits are provided for conducting metal vapor from the vapor space of each compartment in the series to the liquid pool of the next compartment. These conduits are disposed beneath the surface of liquid in the next compartment, so that the vapors passing therethrough are passed in indirect heat exchange relationship with the liquid alloy in the pool, and are thereby condensed before their introduction into the liquid pool. An overflow tube permits return of overflow liquid alloy from each pool to the preceding pool in the series. The apparatus also includes an overhead product conduit for recovering the first metal in highly pure form, and a bottoms product conduit for recovering the second metal. A condenser may be provided for condensing a portion of the overhead product and returning it to the system.

This invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a vertical elevational view, with parts shown in section, of the apparatus according to one embodiment of the invention;

FIG. 2 is a sectional View looking downwardly along line 2--2 of FIG. l;

FIG. 3 is a vertical sectional View of the apparatus according to a second embodiment of this invention; and

FIG. 4 is a sectional view looking downwardly along line 4-4 of FIG. 3.

Referring now to FIGS. l and 2 and especially to FIG. l, there is shown a plural zone fractionation vessel 10 having a tangential feed conduit 11 for introduction of a liquid alloy of a relatively Volatile first metal and a second metal of substantially lower volatility. The outlet end of the feed conduit 11 terminates beneath the surface of a pool 12 of liquid alloy in the bottom of vessel 10. This pool is richer in the second metal than is the feed alloy entering through feed conduit 11. Vessel 10 has a bottoms product conduit 13 for withdrawing liquid alloy from the bottom of pool 12. It is desirable to withdraw the bottoms from a lower point in pool 12, in order to obtain the densest liquid and to minimize stagnant hold-up. Thus, an open-ended sleeve 36 is disposed around conduit 13. The height of the inlet end of this conduit 13 determines the height of molten metal in pool 12. In a typical operation, nearly all of the less volatile metal and much of the more volatile metal is withdrawn as bottoms through conduit 13.

A fluid tight partition 14 divides vessel 10 into a pair of compartments 15 and 16, which constitute separation zones. Partition 14 includes a generally circular plate 17 which conforms in cross section to that of the vessel 10 except for a sector adjacent the inner wall of vessel 10 which is cut out, a vertical plate 18 which is `welded along its lower edge to plate 17 and along its sides to the inner wall of vessel 10, and a small horizontal plate 19 which conforms in cross-sectional shape to the cutout portion of plate 17 and which is welded to vertical plate 18 and to the inner wall of vessel 10. Plates 18 and 19 together with the inner wall of vessel 10 form a chimney 20 in the first compartment 15, and plates 17 and 18 together with vessel wall 10 Iform a well 21 for containing a pool 22 of molten alloy in second compartment 16. This pool 22 contains both the first and second metals, but is richer in the first (i.e., the more volatile) metal than either pool 12 or the alloy feed mixture entering through pipe 11.

An overflow pipe 24 permits return of alloy from second compartment 16 to first compartment 15. The inlet of pipe 24 is sufficiently above plate 14 so that pool 22 of molten alloy has an appreciable depth. The outlet end of pipe 24 is located beneath the surface of molten alloy pool 12, so that pipe 24 cannot constitute a by-pass through which vapor can ow from compartment 15 to compartment 16, avoiding condensation tubes 25, hereafter to be described.

This invention provides a novel structure for conveying metal vapor from the fir-st compartment 15 to the second compartment 16. This structure includes a plurality of serpentine condensation tubes 25 having their inlet ends 26 in chimney 20 of first compartment 15. Tubes 25 are disposed beneath the surface of molten alloy in the pool 22 in the second compartment 16. The tubes 25 as shown are S-shaped, comprising three straight lengths of tubing joined by U bends. The outlet ends 27 of tubes 25 are down-turned and are the lowest points of the tubes.

Vapor of the more volatile metal, with only small amounts of the less volatile metal, enters the inlet ends 26 of tubes 25, and is totally condensed during its passage through tubes 25 by indirect heat exchange with the alloy in pool 22. The only portion of the gases and vapors in tubes 25 which is not condensed is non-condensable gas, such as nitrogen, when present, which will be compressed into the exit ends of tubes 25 by the incoming vapor, to eventually exit through outlet 27 in greatly reduced volume compared to the total vapor stream. An amount of the rst metal in pool 22 is evaporated by the heat transferred on condensation. The amount of liquid evaporated from pool 22 is comparable to the amount of vapor condensed in tubes 25. However, it is evaporated uniformly enough from the surface of pool 22 so that undesirably vigorous agitation is not caused thereby, and so that vaporization is from the surface rather than by bubble formation or boiling below the surface. Excessive agitation and boiling are avoided to prevent entrainment of liquid droplets, which contain appreciable quantities of the second metal, in the vapor of highly pure first metal which is formed in `compartment 16. The entire heat requirements for the second compartment 16 may be furnished by the vapor condensed in tubes 2S.

Liquid from pool 22 backs up only a short distance into the outlet ends 27 of tubes 25, until the gas pressure inside tubes 25 and the static head of the liquid in pool 22 adjacent the outlet ends 27 are equalized. The pressure of vapor in tubes 25 is not allowed to exceed the static head of liquid at outlet ends 27, because this would result in vapor bubbles passing through liquid alloy pool 22 with resultant entrainment of liquid droplets in the vapor above pool 22.

Condensation of the rst metal in tubes 25 and simultaneous evaporation of the first metal from alloy pool 22 is facilitated by the fact that the pressure of vapor in tubes 25 is higher than the pressure existing in zone 16.

Non-condensable gases can be removed from pipe 25 by means of a withdrawal `conduit 28 which is connected to a vacuum pump (not shown). Tubes 25 are connected near their outlet ends to a horizontal manifold 29, so that a single vacuum pipe 28 will rernOve non-condensable gases from the entire bank of tubes 25. A condenser 34 surrounds withdrawal conduit 28 on the exterior of vessel 10. This condenser condenses any metal vapors which may be present in the gas stream flowing through with drawal conduit 28. The resulting condensate travels down Wardly through withdrawal conduit 28 and is discharged through the outlet end 27 of one or more of the tubes 25. Withdrawal conduit 28 also includes a manually operated Valve 31 for cutting the Vcommunication with the vacuum pump, to simplify control when the vacuum pump is not needed during operation.

Vapor formed in the second compartment 16 is removed from vessel through overhead product outlet 32. The vapor leaving the apparatus through outlet conduit 32 consists of the first metal in highly purified form, with only traces of the less volatile metal. For example, where sodium and lead are the two metals being separated, the vapor in outlet 32 is substantially pure sodium which may contain no more than about one part per million of lead. A reflux condenser 33 surrounds outlet conduit 32 above vessel 10. This reflux condenser condenses a portion of the vapors in outlet conduit 32. These vapors descend along the inner wall of conduit 32. and are collected in an annular trap 34, which is in the shape of a channel. A downcomer tube 35 returns condensate from trap 34 to the molten alloy pool 22. The outlet end of downcomer pipe 35 is located beneath the surface of metal in pool 22 so as to prevent hashing or explosive boiling of this metal when it comes into contact with the hotter metal of pool 22.

Referring now to FIGS. 3 and 4, a modified form of the apparatus of this invention is shown. This apparatus includes a vessel 40, which is divided into a first compartment 41 and a second compartment 42 by means of a horizontal plate 43, which is welded to the inner walls of vessel 40 in uid-tight engagement therewiLh. As in the embodiment of FIGS. l and 2, the first compartment 41 is maintained at a higher pressure than the second cornpartment 42. The first and second compartments 41 and 42 contain pools 44 and 45 respectively of molten alloy` A tangential alloy feed conduit 46 supplies molten alloy to the rst compartment 41 beneath the surface of alloy pool 44 therein. The alloy in pool 44 contains a higher concentration of the second metal than does the alloy feed entering through feed conduit 46. In some cases, the alloy in pool 44 maybe virtually pure second metal. The alloy in pool 45 is richer in the first metal than either the feed alloy or the alloy in pool 44. In this embodiment of the invention, as in the embodiment of FIGS. 1 and 2, the term first compartment is used to refer to the compartment which is operated at the highest pressure and which contains the liquid pool having the higest percentage of the less volatile metal. However, in this embodiment the first compartment is located above the sccond compartment, opposite to the embodiment of FIGS. 1 and 2. A

Vapors formed in the first-compartment 41, which consist essentially of the rst metal with only small amounts of the second metal, are conveyed from the vapor space of compartment 41 downwardly through tubes 47. These tubes 47 have their inlet ends in the vapor space of compartment 41. Their outlet ends are substantially below the surface of the alloy in pool 45. This affords an opportunity for condensation of all of the metal vapors in tubes 47 tbefore they are introduced into pool 45.

An annular plate 48, joined to the eXteriors of tubes 47 by welding, positions tubes 47 accurately and provides structural reinforcement for the tubes.

Liquid from pool 45 rises a short distance into tubes 47, the distance being determined by the pressure in tubes 47 and in compartment 41. The: pressure in compartment 41 is greater than the pressure in compartment 42, so as to afford an opportunity for indirect heat exchange between vapors in tubes 47 with the liquid in pool 45. This indirect heat exchange results in total condensation of the vapors in tubes 47, and vaporization of an equivalent portion of the rst metal in alloy pool 45. The rate of vaporization taking place as a result of such condensation is well distributed over pool 45, and is insufficient to cause undue agitation of the liquid in pool 45, and boiling below the surface of pool 45, so as to avoid entrainment of liquid from this pool in the vapors rising from it. At the same time, the pressure in compartment 41 and tubes 47 must not exceed the pressure of liquid in pool 45 at the outlet ends of tubes 47, for this would result in gas bubbles emerging from tubes 47 and traveling upwardly through outlet pool 45, entraining liquid from this pool as the vapor bubbles break the liquid surface in compartment 42. However, in this embodiment any non-condensable gas entering compartment 42 with the vapor from compartment 41 would only be able to leave compartment 42 by being compressed and concentrated in the bottom of tubes 47, and thus would produce a small amount of entrainment thereby.

An outlet conduit 49 is provided for removal of a small stream of molten alloy from pool 45 in second compartment 42. In the first embodiment, and in most distillation columns this would be designated reflux, and would liow to the rst compartment. In. this embodiment, however, a special pump would be required. And since in this application the amount of this stream is only a few percent of the main Streams, it is preferably added directly to the bottoms product leaving the first `compartment through tube 51, and thereby returns to the manufacturing process. The height of the inlet end of return conduit 49 determines the height of liquid alloy in pool 45.

The drain line 50 is provided at the bottom of vessel 40 for removing liquid from compartment 42 during shutdown as desired. This drain line 5t) is kept closed during normal operation.

The bottoms product is withdrawn from liquid pool 44 in first compartment 41 through a bottoms product outlet conduit 51, which is located near the top of vessel 40. The location of the bottoms product outlet conduit 51 determines the height of liquid alloy in pool 44. The term bottoms product is used herein to denote the liquid product containing a preponderant proportion of the less volatile component, in conformity to the terminology generally used in distillation. It will be noted that the compartment 41 containing the liquid alloy of least volatility is at the top of the apparatus, rather than on the bottom. Bottoms product outlet 51 is placed above feed conduit 46, so that the outlet end of feed conduit 46 is submerged beneath the surface of pool 44.

Vapors of highly pure rst metal are withdrawn from the second compartment 42 through vapor outlet conduit 52. A reflux condenser 53 surrounds vapor product conduit 52 above vessel 4G. This causes condensation of a portion of the vapors in conduit 52. This `condensate is collected in an annular trap 54 in tube 52 and returned to liquid pool 45 through return tube .55, the outlet end of which is beneath the surface of pool 45 so as to avoid agitation and flash or explosive boiling when it comes in contact With the hotter liquid of pool 45.

This invention will now be described further with reference to a specific embodiment thereof. For the sake of illustration, the process will be applied to FIG. 1, although this process can be Carried out equally well in either of the illustrated embodiments of the invention with minor modifications.

A molten alloy feed containing 90% by weight of lead and 10% by weight of sodium is introduced into vessel 10 through pipe 11. The flow rate will be taken as 100 parts by weight per hour, consisting of 90 parts by Weight of lead and 10 parts `by weight of sodium. This alloy feed enters the first compartment beneath the surface of alloy pool 12 therein. The alloy pool 12 has a composition of 9.78 percent by weight of sodium and 90.22 percent by weight of lead, and a temperature of 800 C. Evaporation takes place from the surface of this pool, and the pressure in the vapor space above this pool is 59 mm. of mercury.

The bottoms stream issuing through conduit 13 has the same composition and temperature as alloy pool 12, and has a flow rate of 99.76 parts by weight per hour consisting of 90 parts by weight per hour of lead and 9.76 parts by weight per hour of sodium.

The vapor stream rising from alloy pool 12 contains approximately 0.24 percent by weight of lead, and consists of 0.25 part by weight per hour of sodium and 0.0006 part by weight per` hour of lead. This vapor passes into tubes 25, and is condensed therein at about 50 mm. of mercury at 640 C. The condensate is introduced into alloy pool 22 at a depth substantially below the surface thereof.

Alloy pool 22 is at a temperature of 610 C. Evaporation takes place from the surface of this pool, generating sodium vapor at a pressure of 31 mm. of mercury and at a flow rate of 0.25 part by weight per hour. The vapor, which is highly pure sodium, is removed through vapor outlet conduit 32. The coolant flow rate through reflux condenser 33 is adjusted so that the vapor withdrawn from the system has a purity of 99.9999 percent by weight of sodium. The reflux rate is quite small, typically amounting to about 0.01 part by weight per hour, consisting almost entirely of sodium. In some instances it may be necessary to increase the reiiux rate slightly in order to compensate for minor entrainment or nonequilibrium in the generation of the final sodium vapor. The alloy pool 22 contains approximately 6 percent by weight of lead and 94 percent by weight of sodium. The composition of this pool is inliuenced by the reflux rate. The liquid reflux stream descending through tube 24 has the same composition as the alloy pool 22. The liquid rate through tube 24 is about 0.0106 part by weight per hour, comprising 0.01 part by weight of sodium and 0.0006 part by weight of lead.

In the event that the vapors from first compartment 15 contain any non-condensable gases, these are removed through withdrawal conduit 28.

Various modifications can be made by those skilled in the art. For example, more than two separation Zones may be used, although both embodiments illustrated show apparatus having two zones. It is seldom necessary to use more than two separation zones, because of the great difference in volatilities of the two metals, and :because the absence of agitation of liquid in either pool 12 or pool 22 (or in pools 44 and 45) prevents entrainment of liquid droplets in the vapor obtained from any separation zone. This is an important factor in obtaining high purity sodium or other like metal. Any number of separation zones may be used, and in each case the first zone has the greatest operating pressure and the alloy pool of lowest volatility, and each succeeding zone in the series is operated at a lower pressure and has an alloy pool richer in the first metal and consequently of greater volatility. All separation zones are connected in series, so that the vapor from each separation zone (except the last) passes to the next zone and the liquid from each separation zone (except the first) passes to the preceding zone. A bottoms richer than the alloy feed in the second metal is withdrawn from the first zone, and highly pure first metal is withdrawn from the last zone as a vapor.

For the sake of convenience and heat economy, all separation zones are preferably housed in a single vessel as is illustrated in both embodiments of the invention described herein. However, separate vessels housing the different separation zones may be used.

It is necessary to provide thermal insulation around all outer walls of the apparatus, to minimize heat losses and the ensuing operational difficulties. The provision of electrical resistances for other heaters embedded in the thermal insulation is desirable, for heating up the apparatus after a shutdown and for prevention of heat losses from the liquid metals and vapors. A Sheet metal double wall may be provided below the walls 14, 17 and 18, if desired, in order to minimize the condensation of vapors arising from alloy pool 12. The space between such double wall may be vented to the vapor space of the second compartment 15, or may be drained into alloy pool 12 by means of a submerged pipe, in order to prevent filling of this space with liquid metal.

An external heat source, such as an electric, fuel fired, or other heater can be provided if desired, either outside vessel 10 or beneath the surface of liquid pool 12 inside the vessel, in order to provide heat.

The withdrawal conduit 28 for removal of non-condensable gases, and the associated manifold 29 and condenser 30, may be omitted where there is no non-condensable gas or air leaks in the system.

While this invention has been described primarily with respect to the obtaining of pure sodi-um from mixtures of sodium and lead, it will be understood that other light and relatively volatile metals, such as potassium, lithium and magnesium, may be substituted for sodium as the first metal, and that other heavy metals, notably tin, which are of substantially lesser volatility than the first metal, may be substituted for lead as the second metal. Although the second metal is of appreciably lower volatility than the first metal, the second metal nevertheless is preferably not a metal having a very high boiling point, or difficulty may be encountered in finding suitable materials of construction.

The process may be operated over a wide range of pressures, provided that the absolute pressure in each zone is less than the pressure in the preceding zone by an amount sufficient to permit condensation of the vapors of the more volatile metal from one zone by indirect heat exchange with the alloy pool of the next-following zone. Throughout the system, the pressure may range from above atmospheric to less than atmospheric, such as down to l mm. of mercury. Advantageously, the entire system is operated at sub-atmospheric pressures to permit the use of lower temperatures than if operated at atmospheric pressure.

While this invention has been described with reference to specific embodiments thereof, it shall be understood that these are merely :by way of illustration and not by way of limitation of the scope of this invention.

What is claimed is:

1. A process for separating a relatively volatile first metal from a liquid alloy thereof with a second metal of substantially lesser volatility, which process comprises maintaining in a first zone a rst pool of a liquid alloy of said metals, maintaining in a second zone a second pool of liquid alloy of said metals, the concentration of said first metal being greater in said second pool than in said first pool, evaporating a portion of said first metal from said first pool, conducting the thus-formed vapors of said first metal into indirect heat exchange with said second pool, completely condenscing said vapors of said first metal by said indirect heat exchange, passing the resulting condensed liquid phase rst metal into said second pool at a point beneath the surface of said second pool, evaporating a portion of said first metal from said second pool, conducting said evaporated first metal away from said second zone, and maintaining the pressure in said second zone at an absolute level less than the pressure in said first zone and at a level sufficient to permit the heat transferred by the condensing vapors from said first zone to be sufiicient to evaporate some of the first metal from said second pool.

2. A process for separating a relatively volatile first metal from a liquid alloy thereof with a second metal of substantially lesser volatility, which process comprises maintaining in a first zone a first pool of a liquid alloy of said metals, maintaining in a second zone a second pool of liquid alloy of said metals, the concentration of said first metal being greater in said second pool than in said first pool, evaporating a portion of said first metal from said first pool, conducting the thus-formed vapors of said first metal into indirect heat exchange with said second pool, condensing said vapors by said indirect heat exchange, separating non-condensable gases from said condensed liquid phase first metal and withdrawing the non-condensable gases under vacuum, passing the resulting condensed liquid phase first metal into said second pool at a point beneath the surface of said second pool, evaporating a portion of said first metal away from said second zone, and maintaining the pressure in said second zone at an absolute level less than the pressure in said first zone and at a level sufficient to permit the heat transferred by the condensing vapors from said first Zone to be sufiicient to evaporate some of the first metal from said second pool.

3. A process according to claim 2 in which said first metal is sodium and said second metal is lead.

4. Apparatus for recovering a relatively volatile first metal from a liquid alloy thereof with a second metal of substantially lesser volatility, said apparatus comprising a plurality of compartments arranged in series and providing a plurality of separation zones, each compartment including means for retaining a pool of liquid alloy and a vapor space above said pool, means for maintaining said compartments at separate predetermined pressures, said pressures decreasing progressively from the first compartment to the last compartment in the series, a feed conduit for introducing a liquid alloy of said first and second metals into said apparatus, a bottom conduit for withdrawing a liquid alloy richer in the second metal than the feed alloy, an overhead conduit for withdrawing a vapor of substantially pure first metal from the last compartment, at least one condensation tu-be extending from the vapor space of each compartment preceding the last compartment for conducting vapors therefrom, said tube being positioned beneath the intended liquid alloy surface level in said next compartment and adapted for indirect heat exchange relationship with said alloy and terminating in an outlet opening beneath said liquid alloy surface level, whereby vapors of said first metal flowing from said vapor space are condensed prior to introduction into the liquid alloy in said next compartment, a withdrawal conduit for non-condensable gases in communication with said condensation tube, and means for placing said withdrawal conduit in communication with a source of high vacuum.

5. Apparatus according to claim 4 including means for condensing at least a portion of said substantially pure first metal vapor in said overhead conduit and means for returning at least a portion of the resulting condensate.

6. Apparatus according to claim 4 including overflow conduit means for conveying liquid alloy from each compartment to the preceding compartment.

References Cited UNITED STATES PATENTS 165,201 7/1875 Beam et al 202--158 870,747 11/ 1907 Planckh 202--237 X 1,418,885 6/1922 Schulze 202-158 X 1,748,855 2/1930 Teter 202-158 X 1,854,002 4/1932 Subkow 202158 X 1,967,718 7/1934 Moeller 202--158 2,312,811 3/1943 Gentil 75--67 X 2,416,255 2/ 1947 Griswold et al 75-67 X 2,685,505 8/1954 Deyrup 75-66 L. WEWAYNE RUTLEDGE, Primary Examiner H. W. TARRING II, Assistant Examiner U.S. Cl. X.R.

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