US2424399A - Apparatus for chemical fractionation - Google Patents

Description

July 22, 1947.

c. A. KRAUS ET AL APPARATUS FOR CHEMICAL FRACTIONATION original Filed oct. 7,5 1945 2 Sheets-Sheet l CHARLES A. K 4 ALDO' SPENCER LEHMANN CHARLES R. WITSCHONKE CHARLES L. MORRIS July 22, 1947. Q A KRAUS ET AL 2,424,399

APPARATUS FOR CHEMICAL FRACTIONATION original Filed oct.' 7, 1945 2 sheets-sheet 2 .CHARLES A. KRAUS ALDO SPENCER LEHMANN CHARLES R. WITSCHONKE CHARLES L. MORRIS Patented July 22, 1947 ICE APPARATUS FOR CHEMICAL FRACTIONATION Charles A. Kraus and Aldo Providence, R.

Spencer Lehmann,

I., and Charles L. Morris, Cambridge, Mass., and Charles R. Witschonke, Chicago, Ill.

Original application October 7, 1943, Serial No. 505,376. Divided and this application March 15, 1945, Serial No. 582,956

s claims. l

This invention relates to apparatus used for the production of metallic potassium by the method disclosed in United States patent application Serial No. 505,376, iiled October 7, 1943, of which this application is a division.

Heretofore potassium has been produced by decomposition of certain fused or molten potassium compounds, either electrolytically or by reduction with such materials as calcium carbide or sodium. The electrolytic method is difficult to operate because the potassium tends to form a metal fog or dispersion in the molten electrolyte, which is difficult to recover and sometimes causes fires, and reduction of molten potassium compounds involves the disadvantage of requiring relatively expensive materials for construc tion of the apparatus (e. g. stainless steel). An additional disadvantage encountered where the reducing agent is sodium has been the fact that the potassium was always obtained as a sodium alloy.

The principal object of this invention is to provide an apparatus for the production of ptassium and potassium-sodium alloys in which means are `provided for the extraction of potassium from its compounds and for the recovery of the potassium so extracted.

A secondary object of the invention is to provide effective means for bringing into contact the potassium compound and the sodium used to remove the potassium therefrom.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.

The method of this invention differs fundamentally from those used heretofore in that the sodium is reacted with the potassium compound (hereinafter often referred to as the salt) while the latter is in the solid phase. The invention is based on the discovery that sodium will replace potassium from its solid compounds at elevated temperatures, the sodium itself being in either liquid or vapor form (i. e. at a temperature above its melting point). The simplest method of carrying out` the reaction is to bring liquid sodium into contact with the potassium compound desired, in the absence of air or oxygen, and after equilibrium between the compound and the sodium has been established, to remove the resulting alloy. If potassium compounds having fairly high melting points are used, it is feasible to remove the alloy from the compound by distillation under reduced pressure. As the boiling points of the two metals are well separated, they may be easily separated by fractionation, and in fact, an appreciable enrichment of potassium in the condensate over the original alloy is obtained by distilling only part of the alloy from the compound. Where pure potassium is desired it is suiiicient to distill sodium-potassium alloys high in sodium content through a short length of a standard packed column having a cooling coil at the top to provide a controlled amount of reflux.

A still better method of producing substantially pure potassium with a maximum amount of potassium displacement from the compound involves passing sodium vapor through a column of the compound maintained at the desired temperature and provided'with evacuating and condensing means for removal of potassium vapor at the opposite end, As the sodium vapor proceeds through the salt column, it becomes enriched with potassium by displacement, and as the enriched vapor then comes into contact with freshv compound, still furthe;` enrichment takes place,

thus resulting in what may be called a chenilcal fractionation. That this takes place is shown by the fact that sodium vapor passed through a potassium salt column maintained at a temperature well in excess of the condensation point of the vapor (under the pressure used), leaves the alloy considerably richer in potassium than the alloy obtained by a simple distillation (i. e. without fractionation) of a mixture of liquid sodium and potassium compound in equilibrium.

However a greater enrichment of the vapor passing through the column can be obtained if the column is mounted vertically and operated so that some condensation takes place as the vapor passes upward. This is most easily accomplished by providing a small cooling coil at the top of the column to return a part of the vapor as reflux to the salt column. Of course, the column may be maintained at a temperature slightly below the condensation point of the vapor instead of using the cooling coil described, but this method is not as easy to control. It is believed that the mechanism of the reaction under these conditions involves the formation of a film of liquid metal around the salt particles, sodium atoms diffusing from the film into the solid salt and potassium atoms diffusing from the salt into the liquid film. From the film the potassium passes up the column and the sodium downward in the manner of ordinary fractionation. For the purest potassium the alloy vapors from the salt column described may be passed directly into a packed `fractionating column, mounted above the salt column, the reiiux being provided by a cooling coil or other means at the top of the packed column instead of the top of the salt column.

In order that the invention may be clearly understood, it is described in detail with reference to the accompanying drawing, in which:

Fig 1 is a schematic diagram of an apparatus for carrying out the reaction wherein the alloy is fractionally distilled from th e compound through a standard type of packed iractionating column;

Fig. 2 is a diagram of an apparatus for carrying out the reaction by passing sodium vapor through a salt column;

Fig. 3 is a diagram of an apparatus wherein the sodium vapor passes first through a salt column and then the alloy vapor passes through a packed fractionating column; and

Fig. 4 is a diagram of an apparatus for carrying out a continuous, counter-current process.

Figs. 1, 2 and 3 also show the novel closure device of this invention, and Figs. 2, 3 and 4 addi tionally show the novel supporting means for suspending the salt in the column.

Referring to Fig. 1, the potassium containing compound Ia is placed in a boiler I through a suitable conduit II which is closed by a valve I2 after introduction of the salt. 'Ihen the apparatus is evacuated through a conduit I3 and sodium is placed in a reservoir I4 which is heated sumciently to melt the metal. The liquid metal is allowed to iiow into the boiler IIJ through a conduit I5 controlled by a valve I6 and to mix with the salt. If desired, the sodium level in the reservoir I4 may be determined by means of a system of staggered electrical contacts arranged to close by contact with the sodium. The apparatus is maintained under vacuum by means of any standard evacuating system, and therefore the system is not shown. The reaction is started by heating the boiler II) to the desired temperature by means of a heating coil I1 (or other type of electric heater, or the boiler I8 may be direct fired) and the sodium-potassium alloy distills through a packed column I8. Due to the high temperatures involved, it is generally necessary to heat the entire column to compensate for normal heat losses, and so the coil I1 is shown extending from the boiler nearly to the top of the column. In practice a series of coils with separate electrical control to avoid local overheating may be employed. The column I8 may be packed with small iron Raschig rings I8a held between two perforated plates, or other types of standard packing materials which will withstand the temperature employed and the action of the metal vapor. Iron rings or other resistant metallic packing are far superior to non-metallic materials because their high heat conduction results in a greater separation of the potassium and sodium at high rates of flow, or greater column efllciency. After passing through the column I8 part of the vapor is condensed by a cooling coil I9 and drips back onto the packing in the column I8 as reflux. The coil I9 is most conveniently cooled by blowing a stream of air through it.

As the vapors leave the column I8 they pass into a conduit where they are condensed and 'then pass into a receiver 2 I. Due to the high condensation temperature of potassium, it is unnecessary to take special means for cooling the receiver, and condensation is conveniently obtained by leaving the receiver 2l and conduit 28 uninsulated, whereas the boiler I0 and column I8 are carefully insulated to avoid loss of heat.

f The end of the conduit 20 extends within the receiver 2I past the conduit I3 and is beveled so as to direct the condensate downward and against the bottom of the receiver 2| to minimize vapor entrainment into the evacuation system. The condensed metal may be withdrawn from the receiver 2| through a pipe 22 and valve 23, into an evacuated receptacle, or the system may be filled with nitrogen, which can be done through the conduit I3. At the end of the run, the sodium remaining in the boiler III can be distilled and, after breaking the vacuum with nitrogen, the spent salt is dumped through a tapered plug 24.

Since the apparatus is operated under vacuum and the metal vapors are at relatively high temperature, it is difficult to obtain a tight seal around the large closure at the top of the column, if the healed surfaces are at the operating temperature of the apparatus. This diiculty has been overcome by design of the closure, shown in Figs. 1, 2 and 3. In Fig. 1 the sides of the column I8 extend a moderate distance beyond the conduit 20 and are uninsulated beyond this point. At the end they flare out into a ange 25 with a recessed or female annular ring 26 adapted to receive a corresponding raised, or male, ring 21 on a flange 28. The male and female rings are seated with a lead ring or packing and 4bolted together (clamp not shown) to form a vacuum seal. In order to maintain the flanges 25 and 28 cool enough to prevent the lead seal from failing, and at the same time to prevent condensation of alloy around the seall a tubular member 29 fastened to the flange 28 extends down into the column I8 almost to the conduit 20. The member 21 fits the sides of the column I8 fairly closely and it is filled with any insulating material 29a, such as diatomaceous earth or other well-known materials effective at high temperatures. As shown, the member 21 also serves as a support for the coil I9 and its associated piping. In this manner the seal formed by the rings 26 and 21 is maintained at a low temperature, while the heat loss from the top of the column and vapor condensation at the closure is reduced Ito a negligi-biy small amount.

In Fig. 2 the apparatus shown is similar to that of Fig. 1 and corresponding parts are similarly numbered. Fig. 2 differs from Fig. 1 in that the packed column I8 (Fig. 1) is replaced by a container 50 which holds the potassium salt 38a and rests on an annular seat 5I fastened to the sides of the column 38, and provision is made through a conduit 52 and valve 53 for bleeding a small amount of nitrogen into the space between the sides of the column 38 and the member 49 to prevent condensation. The bottom of the container 50 is perforated .to permit passage of sodium vapor through the salt 38a, and at the edges, where it rests on the seat 5 I, it is of spherical shape. The seat 5I is also spherical over the contact area with the container 5o so that a substantially vapor tight seal is formed which permits slight movement of the container 50, and thus remains tight during oper.r tion.

The operation of the apparatus shown in Fig. 2 is similar to that of Fig. l. However, instead of allowing the liquid sodium to flow around the salt, it is run into the boiler 30 where it is evaporated. The vapors pass upward through the salt 38a and become progressively enriched with potassium. A small reflux is maintained by the cooling coil 39, with the result that practically pure potassium is condensed in the conduit 40 and collected in the receiver 4I. The amount of nitrogen introduced through the conduit 52 should be kept very small, if any,` because an excess causes metal fog to form which carries over in large part into the evacuating system connected to the conduit 33. Long periods of operation have shown, however, that there is generally no great advantage in preventing the slight condensation which occurs between the column 38 and the member 49, and the nitrogen conduit 52 may usually,` be omitted.

When the salt 38a has been exhausted oi' potassium and the sodium evaporated from the boiler 30, the apparatus is filled with nitrogen and the receiver 4| emptied through the valve 48. Then the closure formed by the flange 48 and member 49 is opened and the container 58 lifted out, emptied and recharged for a second operation.

Where extremely pure potassium is desired a combination of the packed column` I8 (Fig. 1) and the apparatus of Fig. 2 is particularly suitable. This is shown in 1ig. 3.` The apparatus shown in Fig. 3 may be considered to be the same f as that of Fig. 2 with the addition oi' a small packed column 68 placed directly above the salt column 8|. A simpler method is to pack the top part of the salt column 6| with iron Raschig rings instead of potassium salt. .There are many obvious ways of'constructing the salt column 8| and packed column 6U, and the operation of the apparatus is identical with that described with reference to Fig. 2, and so is not repeated.

In Fig. 4 an apparatus is shown for operating the process of this invention in a continuous, countercurrent manner. The potassium compound is placed in a hopper 18 which is then closed by a vacuum-tight lid 1|. The appa s tus is ushed with nitrogen from conduits 12 an 18 and three- Way valves 14 and 15, and evacuated through conduits 16, 91 and 98. Sodium metal in a reservoir 11 is melted and run into a boiler 18 through a conduit 19l and valve 80, in the manner already described. f

The potassium salt in the hopper 18 is fed to a column 8| through a wide valve 82, feed mechanism 83 and conduit 84 which extends into the column 8| and has holes or slits 85. The feed mechanism 83 comprises a rotatable cylinder or wheel 86 which is rotated by means of a shaft 81 and gears 88 and 89. The cylinder 88 has one or more holes 90 which are positioned so as to come alternately, directly under the valve 82 and over the conduit 84 as the cylinder 85 is rotated. As a hole comes under the valve 82 it is filled with salt, and as the cylinder 86 rotates the salt is carried around to the conduit 84 where it drops into the column 8|. In this manner the potassium compound or salt is fed continuously in small increments to the column 8|, the rate depending on the speed at which the shaft 81 is rotated.

The exhausted potassium compound (now essentially a sodiumv compound) is removed from the column 8| through a conduit 9| having perforations 92, and is discharged into a container 93 through a discharge mechanism 4|14 and a wide valve 95. The discharge mechanism 94 is driven by a shaft 96 and is identical to the feedmechanism 8 3. The valves 82 and 95 are used only when opening the lid 1| or removing the container 93, which operations are done by closing the valves 82 and 95 andreleasing the vacuum by turning the three- way valves 14 and 15 to admit nitrogen through the conduits 12 and 13. To resume operation of the feed mechanism the hopper 18 and container 93 are flushed with nitrogen, closed and evacuated, and then the valves 82 and 95 are opened.

The conduits 1B, 91 and 98 may be connected to the same evacuating system, but it is better to employ an independent system for the conduit 18 because small changes in pressure cause erratic operation. Y

The boiler 18 is heated electrically by a heating coil 99, and the column 8| is maintained at the proper temperature by one or more heating coils |88. The boiler 18 and column 8| may be covered by insulation to minimize heat loss.

During operation, the potassium salt is passed downward through the column while the sodium vapor, which enters the column at the perforations 92l proceeds upward through the salt and replaces the potassium in the salt. The potassium vapor thus formed leaves the column through the perforations 85 and is condensed in the conduit |82 and collected in the receiver |83. A problem encountered in the continuous, countercurrent process is that of obtaining proper passage of the salt downward through the column, due to the tackiness of the salt at the optimum temperatures of operation. It is a Well known fact that most salts become tacky at temperatures from about 0.6 of their melting points up to their melting points, and that free flow is dlillcult under such circumstances. ness is not due to surface melting the particles together because the readily flowable again merely on reducing its temperature. This tackiness of the potassium compounds may be reduced to a practicable degree by mixing a tack-inhibiting agent with the salt prior to introduction into the reaction cclumn. Various alkaline earth oxides are useful or fusing of salt becomes for this purpose, and magnesium oxide is preferred. The oxide may be mixed with'the potassium salt in any desired amount without materially affecting the potassium (or potassium alloy) obtained. However, excessive amounts of the oxide reduce the capacity of the apparatus and raise the problem of recovery from exhausted salt, for economical operation. Generally, amounts in excess of about 50% by weight of tack-inhibiting agent in with the salt disproportionately increase cost of production.

Although any solid inorganic potassium compound can be used for the production of potassium by the method described, it is obvious that those compounds which are solid at relatively high temperatures and which contain a large proportion of potassium in the molecule are more suitable, particularly from the commercial standpoint. In addition certain potassium compounds lend themselves particularly well to the production of potassium by this method, and they are the preferred ones in this invention. They do not enter side reactions, and they have, in general, an equilibrium ratio of potassium alloy to solid phase which is quite favorable to the production of potassium, at elevated temperatures. These compounds are seven in number, and they are the carbonate, hydroxide, sulfide and the halides of potassium. Of these, potassium chloride has the least favorable ratio, and the fluoride, sulfide and carbonate have the most favorable.

The carbonate is the most preferred of these three because it has a rather high melting point, is very stable and is not toxic.

According to the process of this invention the reaction must, of course, be conducted at a temperature below the melting point of the potassium compound (or mixture thereof) employed, and of the salts which form in the column. Therefore those compounds which do not have very low melting points are commercially more attractive because the higher reaction temperatures available permit higher operating pressures and greater rates of mass vapor flow through the column, for an apparatus of given size. Where the melting point of the potassium compound, or

This tackimixture, is high enough, the most suitable reaction temperature range is between about 500 and about 700 C.. and this range is preferred.

In general the concentration of potassium in the alloy in equilibrium with the solid compound, for a given starting ration of sodium and potassium compound, increases with decreasing temperatures, but as too low a temperature results in an unfavorable production rate an optimum temperature is usually not far below the melting point of the salt or eutectic mixture of the sodium and potassium salts or mixed crystals of same, which may occur in the column. and preferably not exceeding about 500 to 700 C. for the high melting compounds.

Potassium carbonate is an exception to the general rule, inasmuch as the potassium concentration in the alloy in equilibrium with the salt reaches a maximum at about 550 C., falling off both above and below this temperature. However, the optimum temperature range of about 500 to about 700 C. holds for this compound. Typical equilibrium data comparing the carbonate, sulfide and fluoride at about 600 C. is given in Table 1 in which the starting ratio of sodium to potassium compound is given in atoms of sodium per equivalent of the salt. The data were obtained by bringing the liquid sodium into contact with the salt and, after establishing equilibrium, distilling the alloy from the mixed sodium and potassium salts resulting from the reaction.v It is to be noted that the equilibrium ratios referred to herein are higher in potassium than the actual alloys in equilibrium with the salt because, the vapor being richer in potassium than is the alloy from which it is distilled, the condensate is richer in potassium than the original alloy. The loss of potassium by the alloy in contact with the salt is in a large measure compensated by readjustment of the equilibrium between alloy and salt. There is a small diminuation in the potassium content of the condensate as distillation proceeds.

Table 1 Atom per cent Potassium in Condensed Alloy sulfide Na/K (equiv.) ,R

carbonate fluoride In Table 2 a comparison of the temperature effects on the equilibrium of the sulfide and carbonate are given, the starting ratio of atoms of sodium per equivalent of potassium compound being 0.75.

Table 2 Atom per cent Potasslum in Condensed Alloy sulfide Temperature, C.

carbonate 'the salt, temperature and In Table 1 the constancy of sition where the sodium to sulfide ratio varied up to about 0.50 is believed to be due to a stepwise reaction, forming NaKS first and then converting this compound to NazS after the KzS has been consumed. No such definite break was found in the case of the carbonate, although an alteration in the rate of change of equilibrium appeared at a sodium to carbonate starting ratio of about 0.75.

In addition to the differences in equilibrium ratios between various potassium compounds, as shown above, some potassium compounds react readily with sodium at lower temperatures than do others. For example sodium vapor (or liquid) reacts readily with potassium sulfide at temperatures as low as 450 C. Reaction at useful velocities takes place with the halides and carbonate at somewhat higher temperatures.

The enrichment of the metal vapor as it passes upward through a potassium salt column, hereinbefore referred to as chemical fractionation, is generally quite high, although this also varies with the potassium compound employed. For instance, on introducing sodium vapor into the bottom of a column of potassium sulfide maintained at 600 C. (starting ratio of 0.5 atom of sodium per equivalent of sulfide) a process of fractionation occurred as the vapors passed upward which was so efficient that the condensate analyzed potassium with a salt column not more than three inches high, and this was in the absence of any reflux. An enrichment was also obtained using potassium fluoride, although the equilibrium for this salt is less favorable than that for potassium sulde. Using columns only about three inches high at 600 C. enrichment from 75% to 87% potassium in the condensed alloy was obtained, with a starting ratio of one atom of sodium per molecule of fluoride. Using a column of potassium fluoride seventeen inches high and distilling all of the alloy, the potassium content was 98.8%. In this case the starting ratio was about 0.9 atom of sodium per molecule of potassium fluoride, the temperature being also 600 C. If, however, the temperature is raised to the point where atmospheric distillation occurs (about 790 C.) the condensate alloy contains only about 78% potassium, showing that lower reaction temperatures favor the potassium enrichment of the alloy.

The reaction between sodium and potassium carbonate was studied in great detail inasmuch as this salt has a number of advantages over other potassium compounds from the viewpoint of the commercial production of potassium. For this purpose the salt column was 1.5 inches in diameter and approximately 40 inches high. Reux was not provided, so that the apparatus was essentially as described in Fig. 2 with the omission of the coil 39. The various factors investigated include: ratio of sodium to potassium carbonate in the column, method of preparing pressure of the sodium in the boiler, mean density of the salt and size of the salt particles.

A series of runs was carried out at 625 C. (boiler temperature) in which the atomic ratio of sodium to salt was varied between 0.5 and 0.9. In one series the salt was of 4 to 10 mesh and in another series it was of 3ft to 6 mesh. The results are presented in Table 3, the rate of distillation of sodium in minutes required per 100 grams of sodium is given in the last column.

the alloy compo- As may be seen from the table, the alloy dis-- tilled is richer in potassium as the proportion of sodium added is reduced. The potassium content of the alloy is relatively unaffected by the method of preparing the salt, but very fine particles of the salt render it diirlcult to force the sodium vapor through the column, so that any method of aggregation of the commercially available fine material into large particles is an aid to satisfactory operation.

Higher boiler pressures (which are related to boiler temperatures) permit a greater'mass flow of sodium vapo-r through the column for the same pressure drop. As the composition of the alloy iS only slightly affected by the` vapor pressure, it is generally desirable t operate at fairly high temperatures for large scale production, generally not far below the melting point of the salts present in the column. For potassium carbonate the preferred range is from aboutI 500o to about 700 C.

Numerous runs with a potassium carbonate column '6" high and about 3" in diameter, and with larger columns, have been made to determine the effect of reflux on the potassium content of the alloy obtained.` The results have shown that as much as 90% of the potassium can be recoveredin the form of an alloy containing more than 90% potassium, with no reflux maintained on the column. With reflux, such as is described with reference to Fig. 2, the potassium content exceeded 98 atomper cent, and with the addition of a packedvfractionating column (Fig. 3) the distillate obtained analyzed substantially pure potassium.

There are numerous modifications which may be made in the invention. For example, the reflux can be obtained by maintaining the top of the salt column at a temperature low enough to cause condensationv of vapor, instead of using a separate cooling coil, or part of the condensate collected in the receivers may be returned 'o'the column as reflux, but the method described singf` cooling coils is preferable for obvious reasons of design.

It is obvious that the potassium compound used in the foregoing process must be substantially anhydrous, inasmuch as water vapor reacts rapidly With both potassium and sodium.

Other variations will be apparent to those skilled in the art, and the invention should not be limited other than as defined by the appended claims.

We claim:

1. Apparatus for producing metallic potassium by treatment of a solid inorganic potassium coml0 pound with sodium above the melting point of sodium which comprises, a column, means for introducing sodium vapor into the bottom of said column, supporting means for suspending said solid potassium compound within said column, means for removing metal` vapor from the top of said column, and a closure at the top of said column for hermetically sealing said column for operation at high temperature under sub-atmospheric pressure.

2. In an apparatus for producing metallic ptassium by treatment of a solid inorganic potas- Isium ompound with sodium above the melting point of sodium, the combination comprising, a column, means for charging a potassium compound into said column, means 'for introducing sodium yvapor into said column, supporting means for maintaining said solid potassium compound in said column, outlet means for removing metal vapor from the top of said column, and a closure for hermetically sealing said column for operation at elevated temperatures and reduced pressures, said closure providing for sealing of the potassium compound charging means, wherein said closure additionally has means for supplying a small amount of inert gas to the space between said member and said column.

3. Apparatus for producing metallic potassium by treatment of a solid inorganic potassium compound with sodium above the melting point of sodium which comprises, a column, means for introducing sodium into said column, supporting means for suspending said solid potassium compound within said column, means for removing metal vapor from the top of said column, said supporting means comprising a container having a. perforated bottom and adapted to fit within said column, an annular ledge fastened Within said column and adapted to hold said container on placement of said container within said co1- umn; said ledge and the part of said container in contact therewith being seated together in a spherical surface to provide a substantially vaportight seal without straining said container.

CHARLES A. KRAUS.

ALDO SPENCER LEHMANN. CHARLES L. MORRIS. CHARLES R. WITSCHONKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTSy Number Name Date 1,837,935 Ylla-Conte Dec. 22, 1931 1,872,611 Thrum Aug. 16, 1932 2,312,811 Gentil Mar. 2, 1943 1,227,240 Bie May 22, 1917 1,141,266 Raschig 1---- June 1, 1915 OTHER REFERENCES Elements of Chemical Engineering, by Walter L. Badger and Warren L. McCabe, McGraw-Hill Book Co., New York and London, 1936. Pages 378 and 379. (Copy in Division 3.)

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