US3136627A - Process and apparatus for selective condensation of metals - Google Patents

Description

J1me 1954 H. s. CALDWELL, JR., ETAL 3,136,627

PROCESS AND APPARATUS FOR SELECTIVE CONDENSATION OF METALS Filed June 11, 1959 2 Sheets-Sheet l I WEIGHT or CO/VDENSATE r- WEIGHT 0F CONDENSATE TEMP COOLA/VT FLU/D 1 OUT t TO VACUUM INVENTORS. HERBERT S C4LDWELL JR MAX J SPENDLOl/E ATTORNEY CRUC/BLE June 9, 1964 H. s. CALDWELL, JR., ETAL 3,136,527

PROCESS AND APPARATUS FOR SELECTIVE CONDENSATION OF METALS 2 Sheets-Sheet 2 Filed June 11, 1959 70 VA C UUM COOL/N6 FLU/D COOL/N6 FLU/D INVENTORS. HERBERT s CALDWELL,JR

MAX J SPENDLOI/E %MA% ATTORNEY United States Patent 3,136,627 PROCESS AND APPARATUS FGR SELECTIVE v QGNDENSATION 0F METALS Herbert S. Caldwell, Jr., Hyattsville, and Max J. Spendlove, Takoma Park, Md., assignors to the United States of America as represented by the Secretary of the Interior Filed June 11, 1959, Ser. No. 819,737 Claims. (Cl. 75-63) (Granted under Title 35, US. Code (1952), see. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties therein or therefor.

This invention relates to the selective separation of components from a mixture. More specifically, it relates to the separation of components of such a mixture by distillation and selective condensation of the constituent compounds or elements in a high state of purity. In the most specific form it pertains to the separation of individual metals in a high state of purity from a mixture of metals or an alloy.

When distilling a charge of a military incendiary alloy, at 600 0, assuming that the partial pressures obey Raoults law, the vapor can be shown to be initially nearly all cadmium with only a small percent magnesium and a fraction of one percent of zinc, and no aluminum. However, as the distillation progresses, the vapor composition changes so that it can be shown by calculation from the Rayleigh distillation equation that the vapor contains equal proportions of magnesium and cadmium after about 10 percent of the alloy is evaporated and that when the alloy is 30 percent evaporated the vapor contains roughly about 75 percent Mg and 25 percent Cd. Cadmium is known to have a negative deviation from Raoults law, thus indicating that cadmium cannot be separated from magnesium by simple distillation.

This was borne out in actual tests wherein a binary alloy of 80 Mg- Cd was placed in a crucible having a condenser connected thereto as shown in FIG. 1, de scribed below. Runs were made under vacuum at temperatures of 550C. and 700 C. and condensate was removed from the wall of the condenser at intervals. The following table gives the composition, and the percent of evaporated charge.

Table 1.Simple Distillation Tests on Prepared 80-20 Magnesium-Cadmium Alloys at 550 and 700 C.

These results indicate that separation by a fractional distillation method is not feasible.

It is interesting to note that, if the partial pressures followed Raoults law, vapor having a composition of 80 Avei-age composition of incendiary alloy:

Percent by? wt.

Elements Cd 19.1 A1 4.13 Z11 .45

Other .74:

3,136,627 Patented June 9, 1964:

Mg-2O Cd would have to come from an alloy containing 99 percent Mg and only 1 percent Cd. This indicates that the composition of the surface is enriched in magnesium because of the relative slowness with which cadmium diffuses from the interior of the melt to re place that which has been evaporated. Cadmium, having an atomic weight much greater than magnesium, will not diffuse to the surface as rapidly as it is initially evaporated. Furthermore, as the surface is no longer enriched in cadmium the liquidus temperature rises so at any temperature below the melting point of magnesium the surface may be covered with a film of solid magnesium-rich alloy, which also inhibits the evaporation of cadmium.

Briefly, this invention consists of a method and apparatus separating mixtures, which are diflicult to separate by fractional distillation because of nearly equal partial vapor pressures, by selective condensation. The apparatus consists of a retort having a lower crucible portion and an upper cooled condenser portion. The mixture is placed in the crucible, heated under reduced pressure and the evolved vapors pass from the crucible to the condenser along a tortuous path provided by a bafile within the lower portion of the condenser.

less volatile constituents condense near the entrance to the baffle. The vapors impinge on a plate cooled to condense some of the less volatile constituents admixed with some of the more volatile components, and the uncondensed vapors are reheated by heat exchange within the baflle. After leaving the baflle the more volatile constituents condense on the condenser walls.

It is an object of this invention to provide an improved method and apparatus for separating the components of a mixture of substances whose partial vapor pressures are nearly equal to reasonable distillation temperatures, by selective condensation under reduced pressure.

It is a further object of this invention to provide an apparatus forseparation by selective condensation, consisting of a retort having a crucible and a connecting condenser, wherein the mixed material is heated under reduced pressure in the crucible and the evolved vapors pass from the hot crucible region to the cool condenser, and wherein baffie means are provided within the condenser for reversing the vapor path and causing a reheating of the vapors whereby a more clearcut separation of the various constituents in condensation is obtained. Said baffie means provide tortuous path for the vapors and act by virtue of the restrictive effect, to increase the pressure in the crucible portion of the retort, whereby the release of the vapors from the baflle passages into the rest of the condenser area results in an expansion and resultant cooling of said vapors.

The various figures in the drawing are as follows:

FIG. 1 shows, diagrammatically, a simple retort having a lower crucible portion and an upper condenser portion.

FIGS. 2 and 3 show a group of graphs wherein the ordinate in each case is the distance along the distillation zone of FIG. 2 and the abscissas are as indicated.

FIG. 4 shows a View in cross section of one embodiment of the retort. FIG. 5 is a fragmentary view of a portion of FIG. 1 without the condensed metal.

Without being bound to any theory, the failure of separation by fractional distillation can be explained as follows:

Consider a simple binary vapor composed of constituents A and B whose partial vapor pressures are nearly equal at the temperature and composition range considered, A being the more volatile component. Although there is a degree of selectivity in condensing the constituents, the zones in which A and B condense overlap. The composition of the condensate at any point depends on the temperature, the relative concentrations of the constituents in the vapor, and the relative rates of diffusion of the two species of vapor molecules. The separation is not sharp because as the B molecules they leave a preponderance of A molecules in the vicinity near the condenser wall. The large number of A molecules near the condenser wall increases the probability that some of them will condense prematurely with the B molecules. Under the circumstances, some of B molecules diffuse much farther than they would otherwise, and many of them condense belatedly with the A species.

This is illustrtaed in FIG. 2, portion (a) showing the temperature distribution along the length of the vapor path through the crucible and condenser of FIG. 1. FIG- URE 2(b) shows the relative distribution of the constituents Within the condenser. It is evident that constitutent B is more abundant at the entrance zones of the condenser, while constituent A, being more volatile is condensed in greatest abundance at a more remote location in the condenser. However, the B condensate contains an appreciable amount of A and the A condensate contains an appreciable amount of B. V In other words, the respective constituents are not selectively condensed so that they can be separated into two relatively pure fractions.

By increasing the temperature of the vapors at that location in their path after the bulk of element B has condensed, the separation of A from B may be enhanced. This is shown in FIG. 3 where the graphs show the eifects of heating this vapor.

We have found the best mode of heating the vapor is to insert a bathe of suitable design in the retort, so as to re erse vapor flow for heat exchange relationship within the condenser. This is best shown in FIG. 4.

The apparatus, generally denoted as retort, consists of a crucible member 1 having mounted atthe top in any suitable manner a plate 2 having a concentric opening 3 therein. A tube 4 mounted over the opening 3 extends vertically upward. Covering the tube 4 is 'a tubular cap 5 having apertures 6, at the bottom, and a cover 7 at the top.

Fastened to the upper part of the crucible by any suit- Because the baffle reduces the diameter of the vapor path, the pressure buildup in the crucible 1 is greater than would normally exist without said bathe. Near the junction of plate 2 and crucible 1, there is a sharp drop in temperature due to the removal of heat by conduction through the outer wall. Condensation of at least some of the less volatile components occur at location 15 as solid deposits 18. The more volatile components remain in vapor state along with some of the less volatile constituents, and pass through tube 4. Only slight cooling occurs during the passage of the vapors through 4, and the heat that is lost is absorbed by the walls of tube 4. In the downward passage of the vapors between 4 and 5, most of the heat lost in transit is reabsorbed from the outer surface of tube 4. Some heat is lost from vapors condensing on cover 7, which is much cooler than the temperature of the impinging vapors because of its proximity to water cooling jacket 10. By pro-per spacing and control of heat exchange conditions, cover '7 may be maintained at a temperature below the condensing point of the less volatile constituents, and above the condensing point of the more volatile constituents. Solids 17 condense on cap 7 as shown. The more volatile constituents remain in the vapor state and pass downwardly through the relatively restricted annular space 4a between tube 4 and cap 5, during which the heat is absorbed from the walls of 4. The pressure and temperature of the more volatile constituent vapor is thus maintm'ned above the condensation conditions until it emerges from the annular space 5a between cap 5 and sleeve 8. At this point there is a sudden expansion of relatively compressed vapors as they enter the large cool condenser space at 16. The resulting sharp drop in temperature causes the more volatile constituents to condense immediately on the walls of the condenser as deposit 19.

The operation of this invention is shown by the following specific example.

A military incendiary alloy of the composition given in the footnote was placed in crucible 1 and heated by electrical induction coil 13. Vacuum was applied through tube 12. The conditions'and results of two runs are summarized in the following Table 2 as tests C and D.

Table 2.Segregati0n of Constitutents in Condensates Produced by Selective Condensation With a Vapor System Chemical analysis, percent Percent Test Temp, Time, pressure, Material Weight, of total No. 0. min. microns gm. conden- Mg Cd A1 Zn sate ghafigaug 72. 19.3g .4. 3 O. 5, 588 on ensa e17" 8 17. .01 581 18.2 0 Condensate 19 0.1 98.4 .01 .87 47s 14.s Condensate 18.. 98. 8 .1 .01 Trace 2,134 66. 9 ghalgitgenu1E 7;. $9.; 4. 3 .45 4, 920 on ensa c 17 6 5. .01 369 17.1 D 550 300 50 Condensate 19- .3 99.4 .01 .30 303 14.3 Condensate '18 99. 9 Trace race Trace 1, 478 68. 6

able manner is a sleeve member 8. In the embodiment shown, see FIG. 5, this is done by means of a shoulder at the upper end of crucible ll.- Plate 2 and sleeve 8 fit into the shoulder 9 in the manner shown, and the structure can therefore be readily disassembled. Sleeve 8 is cooled by a heat exchanger it), attached around the outside of the sleeve to cover substantially all the outer surface of the sleeve between its upper edge and that circumferential part of the sleeve in close proximity to the cover 7 of the cap. .The upper part of the sleeve is covered by a plate 11. Tube 12 passing through plate 11 is connected to a vacuum pump. A l joints shown are close fitting and/ or gasketed to prevent leakage.

In operation, the vapor passes through tube 4 and is cooled slightly. Heat is absorbed by the walls of 4 and is transferred to the vapor passing outside of the tube, which produces the retrograde temperature distribution shown in FIG. 3.

As shown in the table, condensates 18 and 19 are relatively pure metal. Condensate 17 is a mixture of cadmium and magnesium which can be recycled in the process.

Analogous results are obtained by employing a zinc cadmium and a zinc-magnesium alloy in the above method.

it is not necessary that in the case of metal mixtures that the components be alloyed together. Under certain circumstances it is feasible to separate particulate mixtures of metals in this manner. While in the example the alloy was heated to melting temperature, this is not essential. The-process may be operated where feasible under sublimation conditions, Where the vapor pressures are high enough to cause evaporation at a relatively rapid rate. Materials other than metals may be separated in this manner, such as mixtures of organic compounds or inorgnic salts.

The materials of construction employed in the retort are governed by properties of the substances treated. In

the case of the alloy shown, carbon lined mild steel is suitable. With organic or inorganic compounds, glass may be necessary.

It is obvious various other arrangements and substitutions may be made in the method and apparatus without changing the esential inventive concept. Thus, instead of having the cooling means surrounding the upper zone of the condensing sleeve, a cooling well may be mounted in said zone. Any of the common means known to the art may be employed for joining the various sections of the retort.

We claim:

l. A method for separating a mixture of metals having different melting points and volatilities by vacuum distillation and selective condensation which comprises; placing the mixture into an evaporation zone, heating the mixture to a temperature at which at least some of the metals have vapor pressures sufiicient to cause evaporation at a relatively rapid rate, passing the evolved vapors to a condensation zone, said vapor tracing a path having an initial, intermediate and final section, each of said sections having an entrance and a rearward portion, said evaporating and condensation zones being under a reduced pressure, cooling the entrance of the initial section sutficiently to condense at least one of the less volatile metals, passing the vapors through the initial section of the vapor path with only slight cooling, impinging said vapors at the exit of said initial zone onto a surface cooled to just below the condensing point of said less volatile metal, passing the uncondensed vapors through the intermediate section, increasing the temperature gradient along the vapor path of the intermediate section sufficiently for heating the vapors to prevent the condensation of the more volatile metals, passing the vapors through the final path section, and decreasing the temperature gradient along the vapor path of said final section for cooling the vapors whereby at least one of the more volatile substances is condensed.

2. A method for separating a mixture of metals having different melting points and volatilities by vacuum distillation and selective condensation, which comprises; placing the mixture into an evaporation zone, heating the mixture to a temperature at which at least some of the metals have vapor pressures sufiicient to cause evaporation at a relatively rapid rate, passing the evolved vapors upwardly from said evaportion zone, cooling the vapors sufficiently to condense at least one of the less volatile metals, passing the remaining vapors through a first restricted, elongated zone, impinging said vapors at the end of said restricted zone onto a surface cooled to just below the condensing point of said less volatile metal, whereby an additional less volatile metal condenses thereupon, passing said vapors downwardly through a first annular space surrounding said elongated zone and in heat exchange relationship therewith, whereby the vapors in the first annular space are heated, passing the vapors from said first annular space upwardly into a second annular space surrounding the first annular space and communicating therewith, cooling and expanding the vapors from said second annular space into a relatively cooler condensation zone, whereby cooling, and condensation of at least one of the more volatile metals takes place.

3. A method for separating magnesium and cadmium from mixtures consisting essentially of these two metals by selective condensation which comprises; placing said mixture into an evaporating zone, heating said mixture to a temperature at which melting takes place and at which the magnesium and cadmium have vapor pressures sufiicient to cause evaporation at a relatively rapid rate, passing the evolved vapors to a condensation zone, said vapors tracing a path having an initial, intermediate and final section through said condensation zone, said evaporating and condensing zones being under reduced pressure, each of said sections having an entrance and rearward portion, cooling the entrance of the initial section sulficiently to condense at least some magnesium, said condensed magnesium being relatively pure, passing the vapor with only slight cooling through the initial section, impinging the vapors on a surface cooled just below the condensing point of magnesium, whereby additional magnesium present in the vapors is condensed, together with some cadmium, increasing the temperature gradient along the vapor path of said intermediate section sufiiciently for heating the vapors to prevent condensation of cadmium, passing the uncondensed vapors through the intermediate section, passing the metal vapors consisting essentially of cadmium into the final section, and cooling the final section sufficiently to condense said cadmium vapors, at the rearward portion thereof.

4. The method of claim 2, where the mixture is an alloy consisting essentially of a magnesium and cadmium.

S. The method of claim 2, wherein the mixture is a. binary alloyof magnesium and zinc.

6. The method of claim 2,wherein the mixture is a binary alloy of zinc and cadmium.

7. The method of claim 4, wherein the mixture is an alloy having the composition Mg, 75.4%, Cd, 19.1%, Al, 4.31%, Zn, .4S%, other .74%.

8. A method of separating magnesium from cadmium in an alloy containing Mg, 75.4%; Cd, 19.1%; Al, 4.31%; Zn, .45%, which comprises placing said alloy in an evaporation zone, heating said alloy to a temperature of about 550 C. under a pressure of about 50 microns of mercury, passing the evolved vapors upwardly from said evaporation zone, cooling the vapors sufliciently to condense a portion of the magnesium in relatively pure form, passing the remaining vapors through a first restricted elongated zone, impinging said vapors at the end of said restricted zone onto a surface cooled to just below the condensing point of magnesium, whereby additional magnesium together with some cadmium condenses, passing the remaining vapors through a first annular space surrounding said elongated zone and in heat exchange relationship therewith, whereby the vapors in the first annular space are heated, passing the vapors from said first annular space upwardly into a second annular space surrounding the first annular space and communicating therewith, expanding and cooling the vapors from said second annular space into a relatively cooler condensation zone, whereby cooling and condensation of cadmium takes place.

9. An apparatus for separating the components of a mixture by selective condensation which comprises, an evaporator, a condenser in communication with said evaporator, said condenser, having an upper and lower portion and an inner wall providing a condensing surface for the most volatile of the components to be separated, apertured separating means dividing said evaporator from said condenser, a condensing surface for the least volatile of said components being provided by that part of the separating means inside the evaporator, bafile means within said condenser, said baffie means comprising a first inner conduit member generally parallel to the condenser and connected to said separating means, said conduit member communicating with the evaporator via said aperture, a cap member having side wall means surrounding said first conduit member and spaced therefrom to define a first annular space, said cap member being spaced from the condensers inner surface to define a second annular space, the cap member having a closed top portion and apertured at the lower portion of the side wall means, said top portion providing on its internal side an additional condensing surface, said cap member extending up into the lower portion of the condenser, and cooling means adapted to cool the upper portion of the condenser.

10. An apparatus for separating the components of a mixture by selective condensation which comprises, an evaporator, a tubular condenser in communication with said evaporator, said condenser having an upper and lower portion and an inner surface, apertured separating means dividing said evaporator from said condenser, a first condensing surface for one of the components to be 5 separated being provided by that part of the separating means inside the evaporator, baffle means within said condenser, said bafilemeans comprising a first tubular member having its axis along the axis of the condenser, said tubular member communicating with the evaporator via said aperture, a second tubular member having upper and lower ends surrounding said first tubular member and concentric therewith, said second member being longer and greater in diameter than said first member, and extending into the lower portion of the condenser, cap means closing the upper end of said second member, said cap means providing on its internalside a second condensing surface, the space between said tubular members being designated a first annular space, said second tubular member being References Cited in the file of this patent UNITEDSTATES PATENTS 875,381 Rice Dec. 31, 1907 2,236,234 Hanak Mar. 25, 1941 2,458,253 Chisholm et a1. Jan. 4, 1949 2,508,234 DuiTey May 16, 1950 2,782,023 Weiss Feb. 19, 1957 2,814,561 De Wet Erasmus Nov. 26, 1957 OTHER REFERENCES Badger and McCabe: Elements of Chemical Engineering, 2nd Ed, McGraw-Hill Book Co., New York.

Claims (1)

1. A METHOD FOR SEPARATING A MIXTURE OF METALS HAVING DIFFERENT MELTING POINTS AND VOLATILITIES BY VACUUM DISTILLATION AND SELECTIVE CONDENSATION WHICH COMPRISES; PLACING THE MIXTURE INTO AN EAPORATION ZONE, HEATING THE MIXTURE TO A TEMPERATURE AT WHICH AT LEAST SOME OF THE METALS HAVE VAPOR PRESSURES SUFFICIENT TO CAUSE EVAPORATION AT A RELATIVELY RAPID RATE, PASSING THE EVOLVED VAPORS TO A CONDENSATION ZONE, SAID VAPOR TRACING A PATH HAVING AN INITIAL, INTERMEDIATE AND FINAL SECTION, EACH OF SAID SECTIONS HAVING AN ENTRANCE AND A REARWARD PORTION, SAID EVAPORATING AND CONDENSATION ZONES BEING UNDER A REDUCED PRESSURE, COOLING THE ENTRANCE OF THE INITIAL SECTION SUFFICIENTLY TO CONDENSE AT LEAST ONE OF THE LESS VOLATILE METALS, PASSING THE VAPORS THROUGH THE INITIAL SECTION OF THE VAPOR PATH WITH ONLY SLIGHT COOLING, IMPINGING SAID VAPORS AT THE EXIT OF SAID INITIAL ZONE ONTO A SURFACE COOLED TO JUST BELOW THE CONDENSING POINT OF SAID LESS VOLATILE METAL, PASSING THE UNCONDENSED VAPORS THROUGH THE INTERMEDIATE SECTION, INCREASING THE TEMPERATURE GRADIENT ALONG THE VAPOR PATH OF THE INTERMEDIATE SECTION SUFFICIENTLY FOR HEATING THE VAPORS TO PREVENT THE CONDENSATION OF THE MORE VOLATILE METALS, PASSING THE VAPORS THROUGHT THE FINAL PATH SECTION, AND DECREASING THE TEMPERATURE GRADIENT ALONG THE VAPOR PATH OF SAID FINAL SECTION FOR COOLING THE VAPORS WHEREBY AT LEAST ONE OF THE MORE VOLATILE SUBSTANCES IS CONDENSED.

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