US3620942A

US3620942A - Natural circulation of cathode metal of electrolytic cell

Info

Publication number: US3620942A
Application number: US808404A
Authority: US
Inventors: George G Day; Charles F Bonilla
Original assignee: HASKETT BARRY F
Current assignee: HASKETT BARRY F
Priority date: 1969-03-19
Filing date: 1969-03-19
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16

Abstract

A natural circulation process for the removal of the product of electrolysis from a flowing molten metal cathode wherein such removal of the product of electrolysis is achieved through the flash vaporization of the metal of the electrolyte salt in a vacuum still with recirculation of the cathode metal to the electrolytic cell by gravity due to increased density associated with the vaporization of the metal of the electrolyte salt and the cooling of the cathode metal from the flash vaporization. The process is particularly applicable to the separation of sodium from a lead-sodium system, the sodium being separated containing approximately 0.1-1 percent lead impurity.

Description

United States Patent 72] Inventors George G. Day

Maderia Beach, Fla.; Charles F. Bonilla, Tenafly, NJ. [21] Appl. No. 808,404 [22] Filed Mar. 19, 1969 [45] Patented Nov. 16, 1971 [73] Assignee F. Barry Haskett New York, N.Y.

[54] NATURAL CIRCULATION 0F CATIIODE METAL 0F ELECTROLYTIC CELL 7 Claims, 2 Drawing Figs.

[52] US. Cl 204/68, 204/220, 204/237, 204/245, 204/250 [51] Int. Cl C22d 3/06, C22d 3/02 [50] Field of Search 204/68, 219, 220, 250, 66, 245, 237

[56] References Cited UNITED STATES PATENTS 1,597,231 8/1926 Haynes 204/247 X 2,862,863 12/1958 Griffith 204/68 X 3,167,492 1/1965 Szechtrnan 204/68 3,265,606 8/1966 Marullo et al. 204/220 X 3,502,553 3/1970 Gruber 204/245 X Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Attorney-Sherman and Shalloway ABSTRACT: A natural circulation process for the removal of the product of electrolysis from a flowing molten metal cathode wherein such removal of the product of electrolysis is achieved through the flash vaporization of the metal of the electrolyte salt in a vacuum still with recirculation of the cathode metal to the electrolytic cell by gravity due to increased density associated with the vaporization of the metal of the electrolyte salt and the cooling of the cathode metal from the flash vaporization. The process is particularly applicable to the separation of sodium from a lead-sodium system, the sodium being separated containing approximately 0.1-1 percent lead impurity.

PATENTEDunv 15 Ian 3, 620. 942

a! H FIG. 2

q T M I 1 INVENTORS W CHARLES F. BONILLA 3a 7 GEORGE 6. DAY

*"' BY K 6m gala/W7 ATTORNEYS NATURAL CIRCULATION OF CATHODE METAL OF ELECTROLYTIC CELL The present invention is directed to a natural circulation system for the recovery of the metal of the electrolyte salt employed in the electrolysis process and for the recirculation of the flowing metal cathode employed therein; more particularly, the process of the present invention comprises a natural circulation system for the separation of a substantially leadfree sodium vapor from a recirculating lead-sodium cathode metal, i.e. lead containing about l percent sodium generated in the electrolytic cell. This process also serves to control the composition and temperature of the cathode metal, and also to some extent the purity of the recovered sodium, or other metal of the electrolyte salt.

In electrolytic processes employing a molten metal cathode, e.g. a heavy metal such as tin or lead and a fused electrolyte salt, such as sodium or potassium chloride, a mixture is formed by the dissolving of the electrolyte metal in the molten cathode, generally forming a liquid alloy containing from about 5-20 percent by weight of the electrolyte metal, with the remainder being the cathode metal. Thus, for example, in the case employing a molten lead cathode in the electrolysis of sodium chloride, during the electrolysis process sodium metal deposited at the cathode, which is the flowing molten lead,

becomes dissolved in the lead.

Accordingly, in order that the flowing molten lead cathode can be recirculated and reused in the electrolysis process, it is necessary to remove the product of the electrolysis, that is the sodium metal, from the lead as fast as it is produced.

Sodium-lead mixtures predominating in lead have high boiling points. Thus, for example, a mixture containing percent sodium and 90 percent lead by weight has a boiling point of about 970 C., whereas pure sodium boils at 883 C. at atmospheric pressure. Accordingly, the high operating temperatures necessary for separating sodium and lead favor the use of compact apparatus in order to minimize heat losses. Similarly, it has been found desirable to use special materials for construction, for example, stainless steel, refractories or even refractory alloys or ceramics, to resist internal corrosion by sodium-lead alloy and external oxidation by air at the high operating temperatures required.

Two primary processes have been developed for the circulation of the lead cathode alloy from the electrolytic cell to the sodium vaporizer and back to the cell. One of these involves the use of a mechanical pump, usually of the type known as a centrifugal pump. A number of precautions, however, should be taken in selecting a pump of this type. For example the bearings should generally be above and outside of the main casing, with inert gas under pressure keeping the liquid metal below the bearings. In addition, the shaft must be long, and generally is tapered, to minimize weight yet achieve stiffness to avoid vibration, and the pump should presumably be run at a low speed, to avoid cavitation. Furthermore, the materials of construction must be carefully selected for the various components of the pump and system. All of these factors and criteria combine to make such a system for the circulation of the cathode metal, e.g. lead, a fairly complicated and expensive one.

A second method which has been proposed for handling the liquid metals involved in the electrolysis process involves the use of an electromagnetic pump of an alternating current or direct current type. This type of pump appears to work quite satisfactorily providing the temperature is not too high and the electrical conductivity of the liquid metal is good. However, this is not the case in the distillation of sodium and similar electrolyte metals from an alloy of such electrolyte metal and a molten cathode metal such as lead, since its electrical conductivity is low and thus the performance and efiiciency of the pump are poor. In addition, electrical connections, and usually windings, are necessary in this type of pump which generally involves special insulation to stand the temperatures, with air-cooling and possibly water-cooling required for some of the parts. Again, this results in a complicated and expensive system.

Even though these two methods of pumping the cathode metal alloy are fairly well known, they both require pumping equipment, driving power, and substantial piping external to the electrolytic cell, and thus are moderately expensive, and require considerable maintenance, and have an inherent possibility of failure. Accordingly, a simpler, and thus more foolproof and economical method of transporting the cathode metal alloy into a vaporization chamber and back to the cell is desirable. Such a simple, yet efficient process for the recirculation of a flowing cathode metal alloy has been provided with the process of the present invention.

The basic feature of the present invention is that the objective of the recirculation of the cathode metal alloy, which is to accomplish vaporization and separate recovery of the sodium produced by electrolysis, is employed to cause the recirculation. This is accomplished by the fact that when the cathode metal alloy enters the vaporization chamber, in which the absolute pressure is maintained below the vapor pressure of the sodium in the cathode metal alloy by means of a vacuum, some of the sodium flashes" out of the cathode metal alloy until the cooling down of the alloy and its loss of sodium have caused its vapor pressure to fall to the pressure being maintained in the vaporization chamber by a sodium vapor conidenser and a subsequent inert gas vacuum pump. In this vaporization of sodium and cooling of the cathode metal alloy its density rises appreciably for both reasons. Therefore, the molten alloy leaving the vaporization chamber will have a substantially higher density than the molten alloy entering it. By making the piping through which the molten alloy flows large enough, the rate of recirculation can be made as large as needed. Furthermore, this piping can be shorter than when pumps are used. This method of circulation is evidently simple and economical, and represents a considerable advance over conventional pumping technology for the unusual and high temperature liquid metals herein involved.

This method of recirculation of the cathode metal alloy could be employed in conjunction with any method of recovery of the sodium. For instance, a recent process for separating sodium and similar electrolyte metals from cathode metal alloys, improving the disadvantageous processes of the prior art discussed above, can be found in a copending application Ser. No. 586,859 filed Oct. 14, 1966, now US. Pat. No. 3,484,233, in the name of Charles F. Bonilla. Such a process is one which allows for the recovery of pure sodium metal, that is, sodium metal having less than one part per million lead. The process and apparatus disclosed in such application involves a two-stage or double-efiect" process by which through two (or more) successive evaporations with intermediate total condensation it is possible to reduce the lead content of a sodium vapor to a point of high purity.

However, it is another objective of the present invention to combine the simple gravity circulation system for the cathode metal alloy of a single cell with a simple sodium recovery system so as to produce sodium acceptable for some uses in the very simplest and most economical manner. Thus, the requirements of the vaporization phase of the process would only be, according to this consideration, to obtain the necessary density change in the cathode metal alloy to maintain the recirculation, while allowing the sodium produced to contain moderate amounts of impurities, and in particular lead. This is achieved most simply by providing only a single vaporization stage or effect." Further purification of the sodium could then be obtained by a later vaporization operation, probably in a single piece of equipment for the whole plant.

The proper elevation of the level of the inlet pipe in the vaporization chamber, or of the liquid level of the main pool,

(whichever is higher) over the level in the electrolytic cell is the difference between those two desired pressures divided by the density of the desired composition of cathode metal alloy. Thus, the cell may be at atmospheric pressure (760 torr) and the vaporization chamber vapor may be at torr absolute pressure (corresponding to approximately l0 percent sodium in lead at 810 C.). The difference in pressure is 690 torr, to be counteracted by the lead of the 10 percent sodium alloy plus negligible friction. The density of this alloy at approximately 10.2 percent sodium and 830 C. is 5.5 grams/cc. Therefore, to maintain these conditions the level in the vaporization chamber should be approximately 69 l3.6/5.5=I 70 centimeters (5.6 feet) above that in the electrolytic cell, or several centimeters less to allow for some friction in the pipe.

Thus, in accordance with the present invention, a process has been found which simply, efiiciently and economically causes the circulation of cathode metalelectrolyte metal alloy, through a single vaporization chamber without a pump, to produce an electrolyte metal product by vaporization containing from about 0.1 to about 1 percent by weight of the cathode metal impurity, with the cathode metal being returned to the desired composition for recirculation to the electrolytic cell. Such a process developed in accordance with the present invention takes advantage of the increase of density of the cathode metal associated with the flash evaporation of the electrolyte metal caused to occur in the vaporization chamber and the cooling therewith of the alloy system, to achieve the natural circulation of the present invention.

Accordingly, it is a principal object of the present invention to provide a process for the natural circulation of the cathode metal through an electrolytic cell, including separation of electrolyte metal therefrom in a manner which eliminates the normally required pump and other inherent deficiencies and disadvantages of prior known processes.

It is a further object of the present invention to provide such a process for the natural circulation of the cathode metal in an electrolytic cell which in a simple, efficient and economical manner allows for the production of an electrolyte metal containing from about 0.1 to about 1 percent by weight of the cathode metal, which in the case of the production of sodium produces a material which can be effectively employed, for example, in the preparation of tetraethyl lead and similar products.

It is yet a further object of the present invention to provide a process for the circulation of molten alloys from an electrolytic cell to a vaporizer and back to the cell, the recirculation of the cathode metal being effected by the increase in density due to the vaporization of the lighter electrolyte metal 'and cooling of the cathode metal composition.

Still further objects and advantages of the process of the present invention will become more apparent from the following more detailed description thereof.

In accordance with the process of the present invention it has been discovered that it is possible to efficiently, economically and simply provide a recirculating system in an electrolytic process for the purification and recirculation of the molten metal cathode. As indicated previously, the process of the present invention is particularly applicable when employing molten lead as the flowing cathode metal. More particularly, the process of the present invention is exceptionally suitable for the electrolysis of a fused or molten salt, e.g. sodiurn chloride in a horizontal electrolytic cell wherein the flowing lead cathode flows along the bottom with the electrolyte salt floating on top of the same. Such a typical horizontal electrolytic cell, for example can be found in U.S. Pat. No. 3,l04,2 l 3. In this regard, the process of the present invention directed to the natural recirculation of the cathode metal is applicable to all and any of those electrolytic cells which conventionally employ a molten metal cathode.

In accordance with the process of the present invention, the driving force involved in the separation of the electrolyte metal, sodium, from the alloy of cathode metal, lead, and sodium is in actuality a double one. Thus, for instance, the natural recirculation of the alloy having the decreased sodium content is effected through gravity by the increase in density of the sodium-lead alloy when the lighter metal sodium vaporizes out of it. A further increase in the density of the sodium-lead alloy assisting the same to be effectively recirculated to the electrolytic cell, occurs due to the cooling of the alloy, from the flash vaporization of the sodium, which extracts sensible heat from the alloy. It is this combined driving force increasing the density of the sodium-lead alloy for recirculation to the electrolytic cell that allows the same to be easily and naturally circulated through gravitational forces only. Thus, the necessity for a pump is completely eliminated in accordance with the process of the present invention.

Thus, in accordance with the process of the present invention, the circulation of a cathode metal is caused to take place in a closed loop through changes in the density of the liquid alloy resulting from changes in composition and changes in temperature. Such a process is carried out under vacuum, (i.e. low pressure) conditions, the vacuum conditions permitting that the cathode metal-electrolyte metal alloy can be pushed up from the discharge point of the electrolytic cell into a vaporizer of a flash still provided for effecting the flash vaporization of the electrolyte metal from the alloy of electrolyte metal and cathode metal by the atmospheric pressure of the chlorine in the electrolytic cell. In the vaporizer operated under vacuum conditions, a portion of the electrolyte metal, usually sodium is flashed off, the vapors then passing through a condenser where the electrolyte metal vapor is condensed to form the liquid electrolyte metal. In such a process wherein a single flash vaporization or flash distillation of the electrolyte metal and cathode metal is employed, the condensed electrolyte metal will generally contain from about 0.1 percent to about l percent by weight lead as a directly produced product of the process of the present invention which can be effectively employed, for example in the production of tetraethyl lead.

The heat required for the vaporization of the electrolyte metal is supplied by the sensible heat of the main body of the alloy of electrolyte metal and cathode metal drawn into the vacuum still or evaporator. Accordingly, upon the evaporation of the electrolyte metal from the main body of alloy, the temperature of the main body of alloy will decrease. This drop in temperature may be up to 20 C., or more depending upon the temperature of the alloy and the electrolyte metal content thereof.

Similarly, in view of the vaporization of the electrolyte metal containing only a very minor amount of the cathode metal, the composition of the main body of alloy will change due to the flash vaporization. This change, of course, will bring about an alloy composition which will contain a lower percentage of the lighter electrolyte metal, sodium, and a higher percentage of the heavier metal, lead. Accordingly, for this reason, also, the density of the main body of alloy of electrolyte metal and cathode metal will change due to the flash vaporization of the electrolyte metal from the main body of alloy. This combined driving force of increase in density due to both a lowering of the temperature of the main body of alloy and a change in the composition of such main body, will result in a return by gravity of the alloy to the electrolytic cell. Thus, the more dense alloy resulting from the vaporization of the electrolyte metal in the flash evaporator will flow back to the electrolytic cell at an entrance point thereof due to the increase in density of the alloy composition.

The change in the composition of the alloy of cathode metal and electrolyte metal need not be great for operation of the process of the present invention. Thus, for example, on an industrial scale, the electrolytic cell is generally operated with only a small increase of the electrolyte metal content of the alloy and accordingly, only a small amount of such metal need be evaporated prior to recirculation of the cathode metal to the electrolytic cell. In this respect, for example, an industrially operated electrolytic cell c'an preferably be operated with a cathode having a content of approximately percent lead and 10 percent sodium. The increase in the sodium content of the cathode metal upon exiting from the electrolytic cell may be only a matter of 2/10 of 1 percent or less and it is only this amount which must be vaporized in accordance with the present invention, so that the cathode metal returned to the electrolytic cell has the same composition as the cathode metal utilized under normal operating conditions at the inlet. In this way, by eliminating all of the product of electrolysis,

that is, that additional amount of sodium metal present in the cathode metal from the electrolysis of the fused sodium chloride, it is possible to have a continuously running operation without any appreciable variation of the electrolyte metal in the operating cathode. This, of course, is a desirable feature of the process of the present invention which allows the electrolytic cell to be run continuously with the employment of a cathode metal of a constant composition.

The amount of sodium metal evaporated from the alloy of cathode metal and electrolyte metal can be easily controlled through a simple control of the degree of vacuum employed in the flash vaporizer in accordance with the process of the present invention, as well as the temperature of the alloy entering the flash vaporizer from the electrolytic cell. For this purpose, an optional heating element can be employed to heat the alloy leaving the electrolytic cell to that temperature desired for the necessary flash vaporization.

The present invention has the further feature that it serves as an inherent stable control on the operation of the process, assuring that it will be smooth without requiring manual control or mechanical control instruments of usual types. For instance, if the electrolytic current should be increased, generating sodium at a greater rate than previously, the concentration of sodium in the lead entering the vaporization chamber would rise, increasing its vapor pressure, thus increasing the rate of vaporization, thus increasing the density rise of the alloy in the vaporization chamber, thus increasing the rate of natural circulation, thus after a moderate interval reestablishing sodium concentrations in the alloy streams entering and leaving the vaporization chamber and thus the electrolytic cell, close to the concentrations in the alloy streams before the rise in current. The final result of the rise in current thus will be a substantially proportional increase in alloy flow rate, to yield substantially the same sodium concentrations in and out of the cell as before, which presumably are desired operating concentrations. Such an increase in alloy flow rate would not occur automatically with any pumping system, but would require a control instrument sensitive to sodium concentration controlling the pump or a valve in the piping.

The process of the present invention will now be described with reference to the drawings wherein:

FIG. 1 is a diagrammatic representation of the process of the present invention; and

FIG. 2 is an enlarged view of a modified flash vaporizer or still employed in accordance with the process of the present invention.

In the figures, like numerals represent like elements.

The process of the present invention is diagrammatically illustrated in FIG. 1. In FIG. 1, the electrolytic cell is simply represented as 1, such cell containing a flowing cathode 5 on which rests the fused electrolyte salt 3. For purposes of the discussion of the drawings, it will be assumed that the cathode metal is lead and the electrolyte salt is sodium chloride. For simplicity in this drawing, anodes spaced above the flowing molten electrolyte, and the inlet or outlet of the cell from which the initial cathode metal and electrolyte as well as subsequent electrolyte are sent into the electrolytic cell and where the chlorine gas evolved is given off and collected are not shown.

As shown in FIG. 1, the molten alloy of cathode metal and the electrolyte metal, that is, of lead and sodium, is withdrawn from the electrolytic cell 1 through line 7 by the vacuum pump operating on the system, the vacuum pump not being illustrated in FIG. 1. The alloy then enters an evaporator or flash still 9 operated without the addition of heat and at an absolute pressure of about 40 to about lOO torr (mm. Hg), such reduced pressure being obtained by the vacuum pump. Due to the high temperature of the alloy and the vacuum conditions within the evaporator or flash still 9, a portion of the sodium within the alloy will flash off and exit from the evaporator or flash still 9 through line 13.

Due to the flashing off of the sodium vapor, the liquid alloy 11 within the evaporator or flash still 9 will cool, and become more dense also due to the lower concentrations of the lighter sodium metal. The more dense alloy, corresponding substantially to the composition of the liquid cathode employed during the normal operation of the electrolytic cell, continually passes back to the electrolytic cell through gravitational forces alone through line 21, which may be mostly within the cell. This withdrawal of the alloy rich in sodium and return of the cathode metal having a composition for use in normal operation of the cell, without the need of special pumps or special equipment to effect the same, comprises the essential features of the natural circulation system of the present invention.

The sodium vapor passing out of the evaporator or flash still 9 through line 13 passes under condenser 15 and subsequently receiving vessel 17, both of which are maintained under vacuum conditions. Thus, the vacuum pump not illustrated in Figure 1 pulls a vacuum through line 19, the receiver 17, condenser 15, vapor line 13 and evaporator or flash still 9, so that the entire system is maintained under vacuum conditions. As pointed out previously, the sodium metal produced in accordance with this process and collected in receiver 17 is of a purity which can be directly used in many industrial processes, wherein exceptionally high-purity sodium is not required. Thus, the sodium metal produced having from about 0.1 to about 1 percent by weight lead can be conveniently employed, for example, in the industrial process utilized in the manufacturing of valuable tetraethyl lead. Furthermore, if higher purity is required, an additional vaporization stage can be added, as described in copending application, Ser. No. 586,859, now US. Pat. No. 3,484,233, or the sodium produced can be subsequently purified by an independent distillation operation, as previously pointed out.

As also pointed out previously, it is sometimes necessary to heat the molten alloy exiting from the electrolytic cell so that the same, when introduced into the evaporator or flash still, will be at a temperature appropriate for the flash vaporization of the required amount of sodium. For this purpose line 7 may be equipped with a heater 25 which can effect the heating of the alloy composition. This heat will hardly affect the natural circulation forces. Similarly, due to the cooling of the cathode metal upon the evaporation of the sodium it may be desirable to heat the returning cathode metal prior to introduction into the electrolytic cell. For this purpose, therefore, line 21 may be optionally equipped with a small heating element 27 to heat the cathode metal returning to the electrolytic cell to the proper temperature. Since, however, the amount of cathode metal returning to the electrolytic cell will be quite small in comparison to the body of liquid cathode metal present in the cell, it generally is not necessary to effect the heating of the recirculating cathode metal to stabilize cell temperatures.

- Furthermore, heating of return line 21 slightly diminishes the natural circulation forces, so would need to be minimized.

Convenient, rapid and reliable control of the level of the cathode metal in the electrolytic cell is important in the operation of this process. It is possible to alter the level in the electrolytic cell by mechanisms within the cell, or by receivers connected to

pipes

7 or 21, etc. However, it would be more desirable to operate on the cathode metal alloy in the vaporizer itself by varying its volume while maintaining its composition constant, or by changing its composition while keeping its volume in the vaporizer constant, or by simultaneous changes in both composition and vaporizer volume.

Variations in the level in the electrolytic cell can be carried out with the arrangement of FIG. 1 by changing the flow of air to the sodium condenser, which changes the vacuum in the vapor space in the same direction. An increase in condenser airflow, which increases the vacuum (or decreases the absolute pressure) will cause the level in the vaporizer to rise (and simultaneously will gradually cause the cathode metal composition to decrease in sodium content to a new lower equilibrium composition). There may thus be an immediate, and possibly undesirably rapid, change in cathode metal level, unsuitable for a continuing smooth control of the process.

In FIG. 2 a modification of the vacuum still or vaporizer is shown which permits a somewhat simpler and more gradual control of the level of the cathode metal in the electrolytic cell. In this design the level of the pool will remain substantially constant at the top of the outlet pipe 21. Accordingly, when the condenser air flow is adjusted in the desired direction a gradual change in the sodium content of the cathode metal will begin to occur, and the level of the cathode metal in the electrolytic cell will begin to change and can be permitted to continue as long as desired. The level of the cathode metal in the exit pipe 21 is shown at 33 and will vary somewhat in operation. However, the volume of the pipe is small, so such variation will have little effect on the level of cathode metal in the electrolytic cell.

Circular fixed bafile 29 surrounding pipe 21 is a desirable additional feature. Baffle 29 rises above the highest possible level in chamber 9, and descends possibly halfGway to the floor of the chamber 9. Bafflc 29 prevents the floating matter or lighter alloy entering through pipe 21 from floating and channeling over to pipe 21 and running quickly down pipe 21 without releasing sodium to the same extent as the rest of the stream. Bafile 29 assures that if there is stratification of the alloy in vaporizer 9 only dense alloy, collecting just below baffle 29, will flow down pipe 21. The rest of the volume below baffle 29 is available for any accumulation of dense dross, which can be removed therefrom during any needed periodic cleaning of the system. As another possible feature, a flash vaporization surface 31 is added around the top of pipe 7, as shown in FIG. 2.

It may be noted that a variation in the constant cathode metal volume retained in the vaporizer as per the design of FIG. 2 can be incorporated by employing an externally adjustable or changeable length for pipe 21, as by a fairly closefitting sleeve which can be raised or lowered in or around the top of pipe 21 by a handle extending through a stufiing box in the vaporizer roof, not shown in FIG. 2 but readily visualized. Alternatively, pipe 21 can be manifold rising alongside vessel 9 and connected to it through several horizontal valves at different levels, the valve at the best level being open at any one time.

The process of the present invention will now be described by reference to the following specific example.

Using a system such as illustrated in FIG. 1, an electrolytic cell was operated by the introduction initially of a cathode composition comprising an alloy of 90 percent by weight molten lead and percent by weight sodium. The electrolyte employed in a horizontal electrolytic cell was molten sodium chloride which was continuously introduced into the cell as a layer flowing on the molten cathode composition between the same and the anodes.

By pulling a vacuum through the receiver, sodium vapor condenser, and vaporizer and applying cooling air to the sodium condenser so as to maintain a pressure in the vaporizer of 70 mm. Hg., the cathode composition rich in the electrolyte metal, i.e. a composition containing about l0. 1 3 percent sodium, was withdrawn into the vaporizer in which flash vaporization of the sodium metal takes place. Due to the increase in density of the cathode composition based upon the vaporization of the lighter sodium metal and the decrease in temperature due to the flash vaporization, a cathode composition approximating that initially introduced into the electrolytic cell, i.e. 90 percent by weight lead and 10 percent by weight sodium, was recirculated from the vaporizer. The sodium vapor withdrawn from the flash vaporizer was condensed and collected in a receiver to yield a sodium product containing approximately 0. l-l percent by weight lead.

By this process, it is possible to continuously operate the electrolytic cell through the continuous recirculation of a cathode composition, the same as initially introduced into the cell. Accordingly, by the natural recirculation of the cathode composition based upon increase in density associated with a cooling of the same and a vaporization of the lighter electrolyte metal, it is possible to operate the electrolytic cell in a simple and efficient manner.

While the present invention has been described primarily with respect to a system wherein the cathode metal comprises lead or a mixture of lead and sodium and the electrolyte comprises sodium chloride, it is of course possible to utilize other cathode metals and electrolytes in accordance with the process of the present invention. Thus, for example, the electrolyte metal employed in accordance with the process of the present invention can conveniently be other alkali metals, e.g. lithium or potassium or alkaline earth metals such as calcium; in addition, mixtures can be employed such as a mixture of sodium and potassium chlorides, in which case the sodiumpotassium alloy, known as NaK, can be directly produced. Furthermore, the heavy metal molten cathode can include other metals such as zinc or tin instead of the conventionally employed lead.

As indicated previously, the process of the present inven tion uniquely provides for a simple and efficient natural recirculation system for continuous operation of an electrolytic cell employing a flowing molten metal cathode. Particularly, when sodium is employed as the electrolyte metal, as in the electrolysis of sodium chloride, the process of the present invention provides a simple and economical manner for the production of a grade of sodium, which is entirely satisfactory without further purification for a number of industrial applications. Thus, the sodium produced in accordance with the process of the present invention containing from about 0.1 percent to about 1 percent by weight lead can be conveniently employed without further purification directly in the production of tetraethyl lead.

We claim:

1. A process for the purification and recirculation of the cathode metal of an electrolytic cell utilizing a flowing molten metal cathode, which process comprises withdrawing under vacuum conditions a portion of the cathode metal containing, as a product of the electrolysis reaction, electrolyte metal; vaporizing the electrolyte metal under vacuum conditions and without the addition of heat so as to remove all of the products of electrolysis and provide a cathode metal of a composition for recirculation to the electrolytic cell; and recirculating the cathode metal by gravitational forces through the increase in density associated with the vaporizing of the electrolyte metal and decrease in the temperature of said recirculated cathode metal.

2. The process of claim 1 wherein said cathode metal comprises lead and said electrolyte metal, sodium.

3. The process of claim 2 wherein the condensation of the vaporized electrolyte metal produces a sodium product containing from 0.1 to 1 percent by weight lead.

4. A process of operating an electrolytic cell employing a flowing molten metal cathode and a molten electrolyte salt, which process comprises introducing into the electrolytic cell a cathode composition comprising a molten cathode metal and the metal of the electrolyte salt; withdrawing under vacuum conditions a portion of the cathode composition richer in the electrolyte metal as a result of the electrolysis of the electrolyte salt; vaporizing a portion of the electrolyte metal from the withdrawn cathode composition under vacuum conditions and without added heat so as to remove all of the electrolyte metal resulting from the electrolysis of the electrolyte salt and to provide a cathode composition for recirculation to the electrolytic cell of the same composition as that initially introduced; and recirculating the cathode metal composition by gravitational forces through an increase in density associated with the vaporizing to the electrolyte metal and decrease in temperature of said recirculated cathode metal.

5. The process of claim 4 wherein said cathode composition comprises lead containing a minor amount of sodium and said electrolyte metal comprises sodium.

6. The process of claim 5 wherein said cathode composition comprises molten lead containing approximately 10 percent by weight sodium.

7. The process of claim 5 wherein the condensation of the vaporized electrolyte metal produces a sodium product containing from 0.1 to 1 percent by weight lead.

Claims

2. The process of claim 1 wherein said cathode metal comprises lead and said electrolyte metal, sodium.
3. The process of claim 2 wherein the condensation of the vaporized electrolyte metal produces a sodium product containing from 0.1 to 1 percent by weight lead.
4. A process of operating an electrolytic cell employing a flowing molten metal cathode and a molten electrolyte salt, which process comprises introducing into the electrolytic cell a cathode composition comprising a molten cathode metal and the metal of the electrolyte salt; withdrawing under vacuum conditions a portion of the cathode composition richer in the electrolyte metal as a result of the electrolysis of the electrolyte salt; vaporizing a portion of the electrolyte metal from the withdrawn cathode composition under vacuum conditions and without added heat so as to remove all of the electrolyte metal resulting from the electrolysis of the electrolyte salt and to provide a cathode composition for recirculation to the electrolytic cell of the same composition as that initially introduced; and recirculating the cathode metal composition by gravitational forces through an increase in density associated with the vaporizing to the electrolyte metal and decrease in temperature of said recirculated cathode metal.
5. The process of claim 4 wherein said cathode composition comprises lead containing a minor amount of sodium and said electrolyte metal comprises sodium.
6. The process of claim 5 wherein said cathode composition comprises molten lead containing approximately 10 perceNt by weight sodium.
7. The process of claim 5 wherein the condensation of the vaporized electrolyte metal produces a sodium product containing from 0.1 to 1 percent by weight lead.