US2390114A - Electrolysis of fused baths - Google Patents

Description

D.4,1945. www 2,390,114

ELECTROLYSIS OF FUSED BATHS Filed Nov. v18, 1939 4 Sheets-Sheet l INVENTOR De@ 4 1945..- R. J. McNlT-r 390,114

ELECTROLYSIS OF FUSED BATHS Filed Nov. 18, 1939 4 Sheets-Sheet 2 INVENTOR Novum/1 {fwd ATTORNEYS Dec. 4, 1945. R. J. McNn-T 2,390,114

ELECTROLYSIS 0F FUSED BATHS Filed Nov. 1a, 19:59 4 sheets-sheet s 5705er/ Vl/ygzf BMA-@Dmffa/MHMEd-m ATTORNEYS A Dec. 4, 19.45. RJ, McNrrT 2,390,114

ELECTROLYSIS 0F FUSED BATHS Filed Nov. 18,1939 4 Sheets-Sheet 4 Patented Dec. "1, :19x45 UNITED STATES PATENT OFFICE 2,390,114V l .I ELEcTRoLYsrs oF FUsEnBATHs .A Robert J. Monia, Perth Amboy, N. J. Application November-18,1939, serial No. 305,211

1`o Claims.

This invention relates to the electrolysis of fused baths and in particular to the production of light metals by the electrolysis of fused mixed baths, wherein the compound of the metal sought is fused with one or more uxing substances to lower the melting point of the bath.

In the electrolysis of fused mixed baths, it is important that there shall be adequate circulation of the electrolyte to insure that in all parts of the zone of electrolytic action, the various constituents of the electrolyte shall always be present in suitable proportions.

In the past, convection currents in thel bath have a'orded a certain amount of circulation of more or less indefinite character. Heat liberated by the flow of electric current between the electrodes raised the temperature and lowered the density of the electrolyte in the zonel of electrolytic action. In other parts of the cell, heat departed from the bath causing a drop in temf perature and an increase in density of the electrolyte in those parts. 'I'he convection currents resulting from these differences in density in various parts of the cell produced more or less localized circulation, such as eddy currents, which enabled relatively small cells with relatively widely spaced electrodes to be suiliciently provided with replenished electrolyte at the zone of electrolytic action. Heretofore, there has been no adequate means of effecting a strong positive circulation of electrolyte, by controlledgravity, in large cells with cathodes of extensive area, especially where the active surfaces of anode and cathode are'spaced suiciently close together to give maximum economy in the consumption of electric power.

.My invention aims to meet these modern requirements by providing avigorous cathode stream of electrolyte, owing strong and continuously upward past the active cathodesurface, and downward in other parts of the cell.

I accomplish this by maintaining a greater difference in temperature and density between the hot ascending portion and the cooler descending portion ofthe cathode stream; raising the temperature and lowering the density of the former, and lowering the temperature and raising the density of the latter.

Owing to the electrical resistance offered by the fused bath to the passage of the electrolytic current, there is a loss of energy which appears in the bath as heat. I force a large part of this heat into the hot ascending portion of the cathode stream by repressing its iiow* to the outside of the cell in other ways, and I repress the nowA of the heat from the ascending portion of the stream into the descending portion of the stream. In these ways I am able to maintain the bath in the ascending portion of the stream at a higher mean temperature and a lower-mean density.

By preventing the intermingling of the hot bath in the ascending portion of the stream with the cooler bath in the descending portion of the stream, and by representing the flow of heat from the ascending portion of the stream .to the descending portion of the stream, furthermore, by removing heat from the cathode stream at a point which is high up in the cell, I am able to maintain the bath in the descending portion of the stream at a lower mean temperature and higher mean density.

By thus increasing the difference in density between the ascending and descending portions of the cathode stream, I am able to establish and maintain a more positive, vigorous, and rapid circulation of electrolyte upward past'the active cathode surface.

The heat which is added to the cathode stream while it passes through the zone of electrolytic action, or thereafter, must be withdrawn from the stream before it re-enters the zone, in ordern to maintain the electrolyte in balanced thermal equilibrium at the desired temperature. We shall call this increment of heat which must be removed f rom the circulating stream the excess heat.

In removing this excessheat from the cathode stream, I may utilize the radiation and conduction from the upper surface of the bath and from the walls of the cell, or I may employ chilled surfaces placed in contact with the bath. Often it will be advantageous to employ both of these methods of cooling.

To increase the temperature and decrease the density of the ascending portion of the cathode stream, I may repress the flow of heat to the outside of the cell from the electrolyte which flows up from the anode, and which I may call the anode stream, thus facilitating the absorp tion by the cathode stream of heat generated ai: the anode, as well as vthe heat generated at the cathode.

To decrease the temperature and increase the mean density of the descending portion lof the cathode stream, I may repress the flow of heatfrom the lower part of the descending .portion of the stream and remove as much of the excess heat as practicable from the upper part of the descending'portion of the stream. In this way I get the lowest mean temperature in the descend--4 ing stream without risking the possibility of congealing the bath at the bottom of the cathode.

To gain the maximum advantage in circulation of the electrolyte, due to the increased mean difference in density between the ascending and descending portions of the stream, I may extend the cell upward and cause the cathode stream to flow higher up before withdrawing the excess heat therefrom As an example, I shall describe my invention as applied to cells for the electrolytic production of sodium from a fused bath consisting of sodium chloride and one or more iiuxing agents such as calcium chloride.

The accompanying drawings illustrate, more or less diagrammatically, apparatus embodying the invention, in which:

Fig. 1 is a vertical sectional view of a cell in which the cathode stream circulates below the submerged collector of the sodium metal;

Fig. 2 is a plan view of the coils and chilled surfaces employed in cooling, that is, removing excess heat from the cathode stream shown in Fig. 1;

Fig. 3 is a side view of these coils and chilled surfaces taken along the line 3--3 of Fig. 2;

Fig. 4 is a diagrammatic illustration of the circulation of the medium employed in transferring the heat removed from the cathode stream to the outside of the cell.

Fig. 5 is a vertical sectional view of a cell in which the cathode stream circulates above as well as below the sodium collecting hood;

Fig. 6 is a vertical sectional view of a cell in which the cathode stream circulates upward past the active surface of a cathode which is surrounded by an anode, and the return now passes down through the interior of the cathode body, and

Fig. '7 is a vertical sectional fragmentary view of a cathode provided with a thermal barrier.

Referring to Fig. 1, the cell is a type particularly suitable for the electrolysis of fused sodium chloride and comprises a steel shell or containing vessel 2, lined with a refractory material 3. A cylindrical graphite anode 4 passes up through the bottom of the cell and is preferably concentric with the shell. An annular steel cathode 5 is placed concentric with the active surface A of the anode and is supported by two arms 8 which pass through the cell wall and serve to conduct the electric current from the cathode to the outside of the cell. A cylindrical perforated diaphragm 1 is hung concentric with the active surface A of the anode and active surface B of the cathode and is supported by the steel collector 8 suspended from beams (not shown) which rest on the top of the shell 2. The collector 8 serves to collect and deliver from the cel the products of electrolysis,

namely, chlorine gas by way of the dome 9 and duct I0, and liquid sodium by way of hood I5 and duct I6. Y The cell is provided with a heat transfer system, as best shown in Figs. 2, 3 and 4, comprising a tube system 20 having a heat absorbing device in the form of a plurality of coils 2| and 22 which may be placed in the bath adjacent the top of the cathode and outside thereof, preferably concentric with the active cathode surface. Each of the coils 2| and 22 has welded thereto a number of heat conducting elements or ns 45, preferably in the form of fiat steel plates and in sufficient size and number to transfer the required amount of heat from the bath into the coils. The tubes of which the coils are formed pass out through, thi? lnects the heat exchanger with the coil 2|. Above the heat exchangers 25 and 28, the tubes 24, 21, 26 and 29 are connected with expansion chambers 3b.

The heat exchangers 25 and 28 are preferably in the form of steam boilers, each comprising a steel tube 35 cast into a metal jacket 31 which surrounds the tube and is provided with metal nns 36. The jacket is provided with a vaportight housing 38. From a reservoir (not shown) at the bottom of the condenser 40, water is admitted to the boiler through tube 4|. As the water comes in contact with the hot fins, it is converted into steam/which flows through tube 39 to condenser 40 where it is again changed to water and stored in the said reservoir.

As shown in Fig. 4, the arrows indicate the direction of flow 0f the fluid medium circulated in the tube system. The heat exchange system is so arranged that the iiuid medium circulates in opposite directions in the coils and the heat conbath will cover the coils 2| and 22 most of the time, and the heat conducting elements or fins 45 on the lower coil are placed approximately midway between the fins on the upper coil, thus equalizing the removal of heat throughout the cooling zone.

I prefer to use molten sodium as a fluid medium in the tube system because o f its high boiling point and its other favorable properties as a heat transfer agent.

The rate of heat removal from the cathode stream may be readily controlled by regulating the rate of flow of water into the boilers. Additional control may be obtained, however, by a valve 50 in each riser, arranged to control the iiow of sodium in the tube system.' These methods of control may be used in combination with the customary method of control which consists in shunting more or less electric current around the cell by means of electrical resistors connected across the cell terminals.

The heat absorbing apparatus including coils 2| and 22 may be supported from the cell wall as illustrated, or it may be supported from hood I5 and the tubes extending from the coils may pass upward through the upper surface of the bath. In the latter case, special care should be taken to insulate the tubes thermally at the place where they pass up through the bath above the hood I5.

At convenient points in the bottom of the coils submerged in the bath, catchalls 3| are connected as by welding, to drain from the coils sedimentary substances which might accumulate and interfere with the circulation of the heat transfer medium.

The course of flow of the cathode stream in this cell is indicated by the arrows. Electric current iiowing between the electrodes raises the temperature of the electrolyte whichl is between the cathode 5 and the diaphragm l|, resulting in a decrease in density so that this electrolyte rises up under hood I5, and gives up its excess heat to chilled surfaces of the fins 45 from which the heat flows to the iiuid medium of the tube system 20. The

resulting decrease in temperature causes the cathode stream to become more dense so that it falls to the bottom of the cell and passes under the cathode to replace the hotter -electrolyte which is rising between the cathode 5 and diaphragm 1,

By repressing the iiow of heat from the cell with thermal insulation 53 and 54 placed in the upper walls and top of the cell, a larger part of the heat generated at the anode ilows into the cathode stream ascending between the cathode and diaphragm, resulting in a higher mean temperature in that portion of the stream. rIhis temperature may be still further increased by repressing the iiow of heat through the cathode body into the descending portion of the stream by placing a thermal barrier around the outside of the cathode as illustrated in Fig. 7. The'thermal barrier may be formed, for example, of spaced steel plates |0| and |02 between which is placed a solid thermal insulating material |04. The spaces may contain gas which acts as a thermal insulation. A similar or related use of such a thermal barrier is described'and claimed in my copending application Seriall No. 305,212, led November 18, 1939, now Patent No. 2,315,443, granted March 30, 1943. The density of the bath in the ascending portion of the cathode stream becomes correspondingly decreased as the temperature is increased.

By repressing th'e flow of heat from the lower part of the celliby thermal insulation 5|, placed in the lower part of the side wall and bottom of the cell, and removing the excess heat from the cathode stream by a heat removing means such as that described at a place as high in the cell as may be practicable, the mean temperature of the bath in the descending portion ofthe cathode stream is made lower and its density correspondingly higher.

With the increased difference in density between its ascending and descendingportions, the cathode stream becomes a strong positive current aiiording ample circulation of the electrolyte upward past the active cathode surface.

' Referring to Fig. 5, the cell I, like the cell shown in Fig. 1, consists of a steel shell 2, lined with refractory 3; a graphite anode 4; cathode 5, supported by arms 6; a diaphragm 1, supported by steel collector 8, which serves to collect and deliver from the cell the products of electrolysis, namely, chlorine gas by way of the dome 9, and duct I0, and liquid sodium by way of hood |.5 and duct I6, respectively.

The excess heat is withdrawn at a higherpoint in this cell than in that of Fig. 1 and I provide a metal baille 60 concentric with collector 8 and dome 9 and supported thereby to separate the warmer portion 6|, of the cathode stream ascending through passage 62, from the cooler descending portion 63; and prevent these portions of the stream from intermingling. Baiile 60 may be made to repress more eiectively the iiow of heat from the ascending portion of the stream to the descending portion by forming it of spaced metal walls 65 and 66 closed at the top, thus providing a narrow chamber which may be filled with thermal insulators such as gases or combinations of gases and porous solids.

Delivery pipe |6 may be placed either inside or outside of the baille 60. In the latter case the baille is recessed towards the collector 8 at the point where the delivery pipe rises from collector hood I5. Y

I may remove heat from the cathode stream to increase its density by any suitable, means. I may advantageously use the heat transfer system of Figs. 1 to 4. I prefer to place the heat-absorbing coils 2| and. 22 around the upper portion of the baille 60. In this form of construction, I prefer to submerge auxiliary metal hood 61 over the baille 60 and in spaced relation thereto. This hood forms a tight connection with dome 9 and may be supported therefrom'. The-lower edge\12 of ilange 1| may be sealed asdescribed in my copending application Serial No. 258,161, now Patent No. 2,291,644, granted August 4, 1942, to prevent the cathode stream from 'flowing to thesurface of the bath above the hood, but this will not be necessary if submerged cooling coils 2 |22 are used and the chilled surfaces are properly placed up under the hood since the chilled bathiowing down from the under side of the hood will not displace the warmer bath lying above the hood. The cooling coils 2|-'22 are preferably placed outside of bailie 60 with the chilled surfaces extending up under the hood.

In operating the cell of Fig. 5, the heat may be suiiiciently removed from the cathode stream by radiation and Vconduction from the upper part of the bath to'increase the density to the amount required Without resorting to any special heat transfer system. The heat may, under certain conditions of operation, adequately ilow from the upper portion of the cathode stream through the top or upper side wallto increase the density of the electrolyte. The rate of heat dissipation through the top or upper side walls may be controlled by manipulating the covering (varying the kind and amount of covering etc.) or by varying the circulation of air over the' surfaces as by means of a fan or the like. It is advantageousy in most large cells to provide a heat transfer system located to remove heat from the most desirable place in the stream. The coils 2| and 22 are, accordingly, located in the stream where they have the most eiIect upon the flow of thecathode stream. In either of the means just described for increasirg th density of the electrolyte, I eilect a positive control over the density and accordingly a control over the circulation of the cathode stream. I may operate this cell without the auxiliary hood 61, permitting the cathode stream to rise to the upper surface of the bath. In this case I may rely upon the radiation and conduction of heat from the upper surface of the bath and the conduction of heat through the upper cell wall to remove the excess heat from the cathode stream, or I may ksubmerge a cooling system similar to coils 2|-22 with chilled surfaces, 'just outside of the top of baille 60, and

remove heat in that manner also.

Although in describing the application of my invention I have shown the cooler portion ofA the cathode stream as descending within the conilnes of 'the cell walls, it will be apparent that my invention may also be applied in the case Where the cathode stream is withdrawn from the upper part of the cell and descending outside of the cell is returned to the cell at some lower point.

`The modied form oi cell illustrated in Fig. 6 comprises a steel shell 15 lined with refractory 16 and an exterior thermal insulating medium 11. The graphitic anode 18 is hollow, preferably cylindrical, and is supported in the electrolytic bath by the cover 19. The upper Wall-portion of the anode is thinner than the lower portion and is provided with a number of openings at the upper level 8| of the bath. A steel cylindrical drical wall 88 to which is attached a diaphragm l 89 of the usual perforated construction located concentric with the active surfaces A and B of the anode and cathode respectively. The upper portion of the hood wall 88 is provided with a refractory jacket 90 which protects the steel wall 88 from corrosion. In order to prevent the ascending portion of the cathode stream of electrolyte from intermingling with the descending portion of that stream, I mount a cylindrical baille 9| inside the metal-collecting hood. I find it convenient to attach a channel to the inner upper portion of the cathode and to insert the lower edge of the baille therein and separated therefrom by refractory insulators 86. I prefer to form the baille with a double wall in order that I may suppress the ilow of heat therethrough.

The cathode stream of electrolyte 92 flows upward between the active surface B of the cathode and the diaphragm 1, also between the ing in the descending and ascending portions of the cathode stream.

Under conditions not requiring maximum circulation of the cathode stream, I may use a shorter cell, omitting baffle 8| and placing cooling coils 93--94 inside of the cathode and near its top.

I claim:

1. In the electrolysis of fused baths in a cell having a cathode, an anode, a diaphragm and a cathode stream of electrolyte circulating in the bath in the form of a loop, ascending between the cathode and diaphragm and descending on the opposite side of the cathode, the crest of the loop being above the cathode, the improved method which comprises removing by means of a heat exchange medium a very substantial quantity of heat from the upper portions of the descending baille 9| and the inner surface of the wall 88,

then loops over the top of the baffle 9| and flows downward through the center thereof to the bottom of the cathode where it turns upward and flows between the active surface of the cathode and the diaphragm. p

In vorder to increase the density of the cathode stream descending through the central opening in the baflle and the cathode, I provide a heat transfer means in the upper central portion of the baille. I prefer to employ coils 93 and 94 to which are attached fins 95 and a' tube system (not shown) similar to that illustrated in Figs. 1 to 5. This tube system may be supported by the cover 81. I may remove excess heat by such a heat transfer system to control the quantity of heat removed, thereby decreasing the temperature and increasing the density of the descending cathode stream, and I also control the direction of the cathode stream, preventing indiscriminate mixing'of the stream with the bath. In further controlling the circulation of the cathode stream, I provide means separating the cooled descending stream from the ascending hot stream to repress the flow of heat from. the ascending stream into the descending cooled stream.

The anode stream flows in the` vdirection of the arrows 91 and the thermal insulation 11 represses the flow of heat from the anode stream through the walls of the cell, thus forcing the heat to flow into the cathode stream, raising the temperature thereof and thus facilitating its circulation. By varying the covering 11 or by varying the circulation of air over the outside oi the side walls of the cell, I may eect a control over the density of the electrode stream and also its circulation.

In some cases, it will be advantageous to repress the flow of heat through the cathode body by means and methods described in my abovementioned Patent No. 2,315,443. In these ways', I may maintain a maximum difference in temperature and density between the electrolyte flowcathode stream in the vicinity of the crest of the loop and preventing the ascending and descending portions of the stream from intermingling thereby increasing the density of the electrolyte in the descending part of the stream and facilitating its circulation.

2. In a method for the electrolysis of fused baths in a cell having connected upright spaces adjacent the cathode one of which is between the cathode and a diaphragm, the improvement which comprises circulating in the connected upright spaces an ascending cathode stream between the cathode and the diaphragm and a descending cathode stream in another space adjacent the cathode, preventing the ascending and descending streams from intermingling near the top of the cathode, controlling the dissipation of heat from the electrolyte adjacent the anode by repressing the flow of heat out of the upper portions of the cell by means of a thermal barrier placed in engagement with the upper portions of the cell, whereby more heat is forced to flow into the ascending cathode stream increasing the temperature thereof, interposing a thermal barrier between the ascending and descending streams to repress the flow of heat into the descending stream, and withdrawing a large part of the excess heat from the upper portion of the descending stream to increase its density.

3. In a cell for the electrolytic production of light metals having an upright cathode, an u'pright anode, a diaphragm between the active surfaces oi the cathode and anode forming a space wherein electrolyte ascends between the cathode and diaphragm, means defining an upright space along the inactive side of the cathode, said upright space connecting above and below to the space between the diaphragm and the active sur'- face of the cathode and a metal collecting hood above the cathode, the improvement which comprises heat exchange means mounted near the top of the cathode and under the hood in the electrolyte in the upright space for withdrawing heat from the electrolyte to increase its density causing the electrolyte to descend in the upright space in a strong stream, whereby the ascending and descending streams flow in a loop around the cathode.

4. In a cell for the electrolytic production of light metals having an upright cathode, an upright anode, a diaphragm between the active surfaces of the cathode and anode forming a space wherein electrolyte ascends between the cathode and diaphragm, wall means defining an upright space along the inactive side of the cathode, said upright space connecting above and below to the space between the diaphragm and the active surface of the cathode, and a metal collectinc.r hood above the cathode, the improvement which comprises heat exchange means mounted near the top of the cathode and under the hood in the electrolyte in the upright space for withdrawing heat from the electrolyte to increase its density causing the electrolyte to descend in the upright space, and heat insulating means mounted along the cathode between the active side of the cathode and the wall means for repressing the flow oi' heat from the ascending electrolyte into the descending electrolyte through the bodv of the cathode.

5. In a cell for the electrolyticl production of light metals having an upright cathode, an upright anode, a diaphragm between the active surfaces of the cathode and anode forming a space wherein electrolyte ascends between the cathode and diaphragm, wall means defining an upright space along theinactive side of the cathode. said upright space connecting below to the space between the diaphragm and the active surface of the cathode, and a metal collecting hood above the cathode, the improvement which comprises a baille the lower edge of which is laterallv disposed outside the cathode, said baiiie extending above the cathode and the metal collecting hood to the` top portion oi the bath in the cell. and being so located that the ascending electrolyte continues upward along one side of the baille, over the top of the baiiie and then descends into the said upright space through which it continues to descendv and flowing under the cathode enters the space between the active surface of the cathode and the diaphragm.

6. In a cell for the electrolytic production of light metals having an upright cathode, an upright anode, a diaphragm between the active surfaces oi the cathode and anode forming a space,

wherein electrolyte ascends between the cathode and diaphragm, means defining an upright space along the inactive side of the cathode, said up-4 the other side of the bale into. the said upright space wherein the electrolyte descends, said baille repressing the flow o1' heat from the ascending lyte from intermingling, and heat exchange means 00 under the auxiliary hood in the descending electrolyte for withdrawing heat from the descending A eletrolyte to increase its density and its descending flow.

7. In a cell for the electrolysis of light metals, the improvement which comprises a hollow up# right cathode, an .upright anode around the cathode, a diaphragm between the anode and cathode forming an upright space, a metal collecting hood above the cathode, means causing electrolyte to ascend in the upright space, flow over the cathode and descend in the hollow cathode, and heat exchange means under the hood and in such -proximity to the cathode that the descending above the cathode, the improvement which comprises chilled surfaces near the top of the-cathode and under the metal collecting hood which are submerged in electrolyte in the upright space, said chilled surfaces forming part of a heat transfer system comprising a heat exchange means outside of the cell and tubes connecting the chilled surfaces with the heat exchange` means for circulating therethrough a fluid mediumfwhereby heat absorbed by the chilled surfaces is transferred to the iiuid medium and removed from the descending electrolyte.

9. In a method for the electrolysis of fused baths in a cell having connected upright spaces adjacent the cathode one of which is between the cathode and a diaphragm located intermediate the cathode and the anode and the other is on the opposite side oi' the cathode, the improvement which comprises circulating in the connected upright spaces under the action of gravity an ascending cathode stream between the cathode and the diaphragm and a descending cathode stream in the space 'on the opposite side oi' the cathode, and directing the ascending stream above the cathode into the proximity of the upper surface of the bath and then downward as the descending stream, and preventing the ascending and descending streams from intermingling above the cathode, thereby maintaining distinct and positive streams o1' electrolyte.

10. 'Ihe method according to claim l in which the flow of heat from the ascending stream into the descending stream is repressed by means oi a thermal barrier.

4ROBER'J'.' J. McNI'I'I.,

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