US2685566A

US2685566A - Molten metal electrolysis cells

Info

Publication number: US2685566A
Application number: US112347A
Authority: US
Inventors: Schmitt Hans
Original assignee: Pechiney SA
Current assignee: Pechiney SA
Priority date: 1949-08-25
Filing date: 1949-08-25
Publication date: 1954-08-03
Anticipated expiration: 1971-08-03

Description

g- 1954 H. SCHMITT MOLTEN METAL ELECTROLYSIS CELLS 2 Sheets-Sheet 1 Filed Aug. 25, 1949 INVENTOR. Hans Schmi'tt g- 1954 H. SCHMITT MOLTEN METAL ELECTROLYSIS CELLS Filed Aug. 25, 1949 2 Sheets-Sheet 2 INVENTOR.

Hams Schmitt H or ey Patented Aug. 3, 1954 MOLTEN METAL ELECTROLYSIS CELLS Hans Schmitt, Saint Jea assignor to Pechine in de Maurienne, France, y Compagnie de Produits (lhimiques et Electrometallurgiques, a corporation of France Application August '25, 1949, Serial No. 112,347

8 Claims. 1 This invention relates generally to molten metal electrolysis cells or furnaces of the type used in refining metals such as aluminum, and it more specifically deals with electric connections for electrolysis cells, of the kind comprising in operation three electrically conductive layers in superposed relationship, the topmost and lowermost of said layers being metallic. In refining furnaces or cells of this type it has been proposed to establish the requisite electrical connections with the cathodic metal layer by providing a body of metal similar in nature to the said cathodic metal, but in the solidified state, within a container arranged outside and to one side of the cell, said body of metal communicating with the main body of molten cathodic metal within the cell by means of a conduit extending through a side wall of said cell and filled with the same metal in the molten state, and using metal conductor bars making electric contact with said solidified metal in the container.

Connections of the type just described present very definite advantages. For one thing, the electrical resistance thereof or voltage drop therethrough in operation is substantially lower than that found in any other known type of connecting means suitable for the purpose contemplated, such as the usual means comprising electrodes of carbon or graphite dipping into the molten metal.

This advantage is especially desirable in the case of electrolysis cells used for refining aluminum. Then, all the drawbacks usually attending employment of the conventional electrodes are of course minimized.

On the other hand, however, connecting means of the molten metal type described above are subject to certain shortcomings which in the past have considerably restricted extensive commercial use thereof. Basically those shortcomings may be ascribed to the high thermal conductivity of the metal connection, which results in considerable heat losses through the molten metal connection, which losses may be so high as to offset the saving accruing from the reduced voltage drop through said connection. If it is desired to reduce said heat losses, it becomes necessary to provide the metallic connecting line or vein with as is restricted a cross-section as consistent with suitable electrical conduction. This restriction in the sec-tion area of the metal connecting vein in turn will practically always be found to lead to the occurrence of the peculiar phenomena caused by so-called pinching effect, viz., constrictions and formation of whirling currents in the said molten metal vein.

Such pinching eiTects then generate circulatory movements in the molten metal between the molten electrode metal within the cell and the molten metal in the connecting channel. These movements result in the introduction of slag and oxide into said channel, ultimately clogging it. Moreover said movements entail further heat losses, because hot metal is led into the connecting channel, in which it cools down, and is then led back into the cell. Moreover, each time the connecting channel is cleaned, the thermal balance within the cell becomes disturbed, so that it becomes impossible to maintain a uniform temperature therein.

It has been stated above that in order to minimize the heat losses resulting from the high thermal conductivity of the molten metal connection, the cross sectional area of the connecting channel should be restricted to a minimum, in order to minimize. the heat losses through convection, radiation and conduction. It has also been stated that a limit is imposed in this direction by the occurrence of pinching efiect. To determine this minimum permissible cross sectional area S of the connecting channel below which undesirable pinching effects occur, one must start with the assumption that the electromagnetic force set up in the molten metal is balanced by the hydrostatic pressure head therein. The electro-magnetic force F is expressed by the formula And the pressure head acting on the molten metal in the channel to balanc the constriction produced by said pinching force is (2) P=cd-l0- kilograms per square centimeter, wherein c is the vertical height of constriction due to the pinching efiect, and d is the specific gravity of the metal.

It is a general object of the present invention to provide, in a molten metal electrolysis cell or furnace of the type specified'a connection of the molten metal type which is generally free of the above-described drawbacks.

.the said molten metal in the It is an object, more specifically, to provide, in a cell or furnace of the type specified, electric connecting means to and from the metal in said cell, said means comprising a solidified body of metal similar in nature to said metal in the cell, situated in a container arranged, outside and to one side of said cell and communicating with said molten metal in the cell through a channel extending through a side wall of the cell and filled with said metal in molten condition, said connecting channel being so con-figurated as to minimize the harmful effects of electromagnetic pinching forces acting on the metal within said channel.

It is another object to provide such a molten metal connecting means wherein inflow of slag and other foreign material into the channel, tending to obstruct said channel, is reduced or prevented.

A further object is to provide a molten metal connection of the type described wherein heat losses may be reduced to a minimum without at the same time creating prohibitively high pinching forces within the metal in the molten connection.

It is a further object to provide a molten metal .connectionof the type described in which whirling currents tend to form, and all foreign nonmetallic impurities tend to collect, in a localized inner zone or section of the connection, leaving a major extent or section of said connection in a quiescent state free of said impurities.

Another object is to provide such a molten metal connection in which non-metallic impurities, such as oxide and lag, tend to collect in a localized inner section of the connection, from which they may be readily removed without unduly disturbing the thermal balance and uniform temperature prevailing in said cell.

Another object is to provide such a molten metal connection in which the molten metal in the connection will solidify quiescently toward the outer end of the connection so as to merge gradually with the solidified metal in the outer container.

Other objects and advantages will appear as the description proceeds.

The invention essentially provides, in an electrolysis cell or furnace of the above specified type, a molten metal connection which comprises a solidified body of metal similar in nature to the metal within the cell, and situated in a container outside and to one side of said cell, one or more metallic bar conductors in electrical contact with said body of solidified metal, and a connecting channel extending from said container through a side wall of the cell into communication with cell, said channel being such in longitudinal configuration as to comprise a first or inner section at that end thereof communicating with said molten metal in the cell, which is comparatively short and wide, and a second or outer section, at that end of the channel communicating with the solid metal in the container, which is relatively long and narrow. Further according to the invention, said short inner section, at the molten metal containing end of the channel, is provided with a cross sectional area so calculated that the drop in the liquid level therein below the level of the molten metal in the cell, and as a result of the afore- ,mentioned balance struck between electro-magnetic pinching forces and hydrostatic pressure head acting on the metal in said inner section,

may reach a value as high as about 0.8 centimetre; while said longer outer channel section, at the solidified metal containing end of the channel, is provided with a larger cross-sectional area, so calculated that the drop in liquid level therein below the level of the molten metal in the cell, and as a result of the afore-mentioned balancing forces, remains less than about 0.5 centimetre. In said outer channel section, there is thus provided a quiescent zone free of foreign infiltrations due to pinching effect, and in which the molten metal may solidify in satisfactorily quiescent conditions.

The cross-sections of the two portions of the channel are calculated from the equations above by letting P=F;

2 I B= 3 3 S 10 cd 10 from which #2 it. -5 S 3 cd 10 in the first portion of the channel in accordance with the invention, 0 can reach the value of 0.8 cm, then the cross-section S1 of this portion of the channel can reach the minimum value:

2 ii. 5 elf 5,2 03d 10 -O.833 10 d The cross-section of the second portion of the channel should be at least equal to:

Thus the valves of c, given by the present invention, allow calculating suitable cross-sections S1 and S2 for each particular case of the current I and the density d of the liquid metal.

The accompanying drawings illustrate some exemplary forms of embodiment .of the connecting means of the invention.

Fig. l is a horizontal cross-sectional view; and

Fig. 2 is a vertical cross-sectional view of a connecting channel and adjacent parts, according to a first embodiment; while Y Fig. 3 is a horizontal, and

Fig. 4 a vertical, cross-sectional View relating to a modification.

Similar or corresponding parts have been designated by identical references in all the figures.

As shown in Figs. 1 and 2, the reference numeral l6 designates an electrolysis cell of the type described, and I5 is the connecting channel extending through a side wall H of the cell In and filled with molten metal similar in character to the metal 12 contained in cell [0 and with which an electrical connection is to be made. The channel l5 leads to a container l8 outside the cell, which contains a body of the solidified metal. Metallic bar conductors 19 have their lower ends immersed in the body of solidified metal in container 13. As shown, the channel it longitudinally comprises two sections differing in crosssectional area and configuration, so that the vein of molten metal contained in each of said sections is subjected to vertical constrictions differing in extent, due to the different electromagnetic forces acting in each of them. The section It of the channel l5, which connects with the molten metal 12 within the cell 19, is smaller in cross sectional area than the channel section l'i, containing the molten metal merging with the solidified metal in the container IS. The crosssectional area of the inner section 16 of the channel I5 is, according to the invention, so calcu- 5. lated, in'accordance with Formulae '1 and 2 given hereinabove, that the drop in liquid level (or vertical constriction) therein below the level of the molten metal I2 in the cell, as resulting from the afore-described balance struck between electro-magnetic pinching forces acting on said metal in section I6 and the hydrostatic pressure head thereof, respectively, may reach a value a high as about 0.8 cm. And the cross-sectional area of the outer section ll of the channel I is 50 cal .culated in a similar manner that the drop in liquid level (or vertical constriction) therein below the level of molten metal I2 in the cell, remains less than about 0.5 cm. The vertical constriction of the molten metal, in the inner section I6, of the channel, i greater than that in the outer section I1; in the outer section H, the level of the molten metal more closely approaches the liquid level of the metal I2 within the cell, and is at all times higher than the level in the section I6. The cross-sectional configuration of the inner section I8 may suitably be formed as a flat rectangle; for practical reasons, the height of metal available may be limited, so that in order to obtain a sufficiently large cross-sectional area in view of the requirements of the invention, said inner channel section is provided with a width dimension larger than the vertical dimension of the metal therein, as shown by a comparison of Figs. 1 and 2. As concern the length of the inner channel section I6, this should be as short as possible. Practically said length will generally be determined by the thickness of the side wall II of the cell.

As regards the outer section 1 l of the channel, its cross-sectional area, as stated above, should be so predetermined that the electromagnetic pinching forces created therein will be lower than in the inner section I6. Accordingly, the section I1 is provided with a greater vertical dimension and a smaller width dimension than the corresponding dimensions of the section It, those dimensions being such that the resulting crosssectional area of section I! is larger than that of section IS.

The advantageous results produced by the arrangement according to the invention may be described as follows: Major pinching efiects tend to occur predominantly in the intermediate zone ,or inner section It of the channel rather than in the outer section IT. The section It may be said to form an electromagnetic barrier between the molten metal in the cell and the metal I! acting as an electrical connection. Rather than occurring throughout the entire length of the molten metal connection, the harmful pinching effects are thereby restricted to the inner section thereof, wherein the drop in liquid level due to vertical construction is greatest. Thus the previously described circulatory movements, created I in the molten metal by said pinching effects, re-

main restricted to said inner section I6 and are substantially entirely arrested on reaching the zone of transition between the channel sections I6 and I1. Thus said circulatory movements and the accompanying heat losses occur almost exclusively Within the very short section I6 so that said heat losses are minimized, and the heat'remains more nearly localized within the cell. Also any oxide, slag and similar impurities accompanying said circulatory movements are prevented from entering the section I1, and, due to the reduced liquid level in the section. I6 such impurities tend to settle therein. Because of the increased sectional area in-the channel section I! over that in I6, the metal in 'thesection II remains in a substantially quiescent "condition, and the circulatory movements produced in the section It are positively arrested, asstated previously, on reaching the transition between I6 and I l. The molten metal in the outer end of section I6 toward chamber I8 is in ideal condition for proper solidification. It is even found in practice that any impurities, suchas oxides, slag, etc., that may be present in the metal of the outer section II, become effectively transported into the inner section I 6 where, as stated, they will settle.

such impurities may be removed from the inner channel section It in which they collect and the thermal balance of the cell is disturbed to a sub stantially reduced degree as a result of such a cleaning operation.

In the modification shown in Figs. 3 and 4, a channel section 26 forms a -bend or knee at an outlet from an inner section 25. This tends to enhance the above-mentioned desirable effects and produce still greater quiescence in an outer section 25. In this embodiment, the bottomof the section 26 is further shown (see Fig. 4) as sloping downwardly in the outward direction.

Still further to increase the barrier effectof the inner channel section 25, physical separating means, such as grating structures made of graphite or other suitable material, may be interposed between inner and

outer channel section

25 and 26.

The cooling of the molten metal in the channel should be so controlled that the molten metal will tend to solidify substantially at the outer end of the channel section 26, adjacent to the container 21. Moreover, it is preferable that the contact or transition from the molten to the solid metal in section 25 should occur along a vertical plane or substantially so. For this purpose, and as shown in Figs. 3 and 4 by way of example, ducts or vents 29, allowing a circulation of cooling air, may be formed around the walls of the channel near container 27 in order to promote cooling of the molten metal in that area.

Preferably an evacuating duct or outlet may be formed in a side of the channel section 26 as shown at 38 in Figs. 3 and 4, said outlet being normally sealed by a plug as of bricks 3 I.

The inner channel section 25 is provided with a top aperture for cleaning purposes, normally sealed with a heat-insulating cover 22, so arranged as to provide a free gap 23 over the surface of the molten metal in the channel section 25. The side-walls of the outer channel section 26 may be suitably heat-insulated to minimize heat losses therein.

The electric connection from the bar conductors 28 to the solid metal within container 21 may be made in any suitable conventional manner. Thus the connection may include a threaded or a welded joint or the like. Alternatively, the bar conductors 28 may be simply made to dip into the molten metal, which is then allowed to solidify around them. In the embodiment of the invention shown in Figs. 1 and 2, the bar conductors I9 are shown as being vertical, while in the embodiment shown in Figs. 3 and 4 they are illustrated as horizontal.

It will be understood that the specific embodiments of the invention described hereinabove and illustrated in the drawings should be regarded as exemplifications of the invention and not as restricting the scope thereof, the said scope being defined exclusively by the ensuing claims.

Whati claim is:

1. An electrolytic cell -for an electrolytic process intwhich a layer of molten metal floats on the electrolyte as an electrode, comprising: a main container having substantially vertical side walls, a refractory body attached to a side wall and having a substantially horizontal upper surface, achannel extending through said walland along said body such that a portion of said layer may reside therein, said channel having an inner portion openinginto the main ontainerand an outer portion residing away from said container, the channel being open-topped, a conductor in the outer portion at a part thereof away from said inner portion, the cross-sectional area of said inner portion being substantially smaller than the cross-sectional area of said outer portion, whereby pinching effects are avoided, and conductive means extending into the outer portion of said channel for connection to a source of electric current.

2. A molten metal electrolysis cell providing an electrical connection from the molten metal to an external source of electric current and particularly adapted to preventing current breakage due to pinching effect, which cell comprises a main container, a body of refractory material fitted to a side wall of the main container of said cell, bar conductors fixed in an end part of the body "for connection to said source, a substantially horizontal channel in said body having one end thereof extending through said side wall of the cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating with the atmosphere, said channel including a first portion beginning at the point at which said channel opens into the cell interior and including a remaining portion which is substantially larger in cross-sectional area than the first portion and which extends from the end of said first portion asfar as the'bar conductors.

3. A molten metal electrolysis cell providing an electrical connection from the molten metal to an external source of electric current and particularly adapted to preventing current breakage due to pinching effect, which cell comprises a main container, a body of refractory material fitted to a side wall of and outside of the main container of said cell, metal bar conductors fixed in an end part of the body for connecting to said source, a substantially horizontal channel in said body having one end thereof extending through said side wall of the cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating at every point of its extent with the atmosphere; said channel including av first and shorter portion beginning at the point at which saidchannel opens into the cell interior and including a remaining longer portion which is substantially larger in cross-sectional area than the first portion and which extends from the end of said first portion as far as the bar conductors.

41A molten metal electrolysis cell providing an electrical connection from the molten metal to an external source of electric current and particularly adapted to prevent current breakage through pinching effect, which cell comprises a main container, a body of refractory material fitted to aside wall of andoutside of the main containermof said cell, metal bar conductors fixed in and and part of. said bodyfor connectingto said source, a substantially horizontal channel in said body having one end thereof extending through said side wall of said cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating at every point of its extent with the atmosphere, and said channel including a first and shorter portion beginning at the point at which said channel opens into said cell interior, and a remaining and longer portion which is substantially larger in cross-sectionalarea than the said first portion and which extends from the end of said first portion as far as said bar conductors, the bottom of the channel in said first portion lying on a level higher than the level of the bottom of the channel in said remaining portion.

5. A molten metal electrolysis cell providingan electrical connection from the molten metal to an external source of electric current and par ticularly adapted to preventing current breakage due to pinching efiect, which cell comprises a main container, a body of refractory material fitted to a side wall 01" and outside of the main container of said cell, metal bar conductors fixed in an end part of said body for connecting to said source, a substantially horizontal channel in'said body having one end thereof extending through said side wall of the cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communieating at every point of its extent with the atmosphere, said channel including a first and shorter portion beginning at the point at which said channel opens into the cell interior and including a remaining longer portion which is substantially larger in cross-sectional area than the first portion and which extends from the end of said first portion as far as the bar'conductors, and the breadth of the first portion of the channel being greater than the breadth of the said remaining portion.

6. A molten metal electrolysis cell providing an electrical connection from the molten metal to an external source of electric current and particularly adapted to preventing current breakage due to pinching efiect, which cell comprises a main container, a body of refractory material tted to a side wall of the main container of said cell on the outside thereof, metal bar conductors fixed in an end part of said body for connecting to said source, a-substantially horizontal channel in said body having one end thereof extending through said side wall of the cell to open into the inter-ior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating at every point of its extent with the atmosphere, said channelincluding a first and shorter portion beginning at the point at which said channel opens into the cell interior and including a remaining longer portionwhich is substantially larger in cross-sectional areathan the first portion and which extends from the end of said first portion as far as the bar conductors, the bottom of the channel in the first portion 1ying on a level higher than the level of the bottom of the channel in the said remaining portion, and the breadth of the first portion being greater than the breadth of the saidremaining portion of the channel.

'7. A molten metal electrolysis cell providing anelectrical connection from the molten metal to an external source of electric current and particularly adapted to preventing current breakage due to pinching efiect, which cell comprises a main container, a body of refractory material fitted to a side wall of the main container of said cell on the outside thereof, metal bar conductors fixed in an end part of said body for connecting to said source, a substantially horizontal channel in said body having one end thereof extending through said side wall of the cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating at every point of its extent with the atmosphere, said channel including a first and shorter portion beginning at the point at which said channel opens into the cell interior and including a remaining longer portion which is substantially larger in cross-sectional area than the first portion and which extends from the end of said first portion as far as the bar conductors, and the said channel being horizontally bent in a zone of said body lying between the outer face of the side wall of the cell and the metallic bar conductors.

8. A molten metal electrolysis cell providing an electrical connection from the molten metal to an external source of electric current and particularly adapted to preventing current breakage due to pinching effect, which cell comprises a main container, a body of refractory material fitted to a side wall of the main container of said cell on the outside thereof, metal bar conductors fixed in an end part of said body for connecting 10 to said source, a substantially horizontal channel in said body having one end thereof extending through said side wall of the cell to open into the interior of said cell and having the opposite end extending to said bar conductors, said channel having a bottom, side walls and an open top communicating at every point of its extent with the atmosphere, said channel including a first and shorter portion beginning at the point at which said channel opens into the cell interior and including a remaining longer portion which is substantially larger in cross-sectional area than the first portion and which extends from the end of said first portion as far as the bar conductors, and the bottom of the channel in said first portion sloping downwardly towards the bottom of the channel in said remaining portion.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 795,886 Betts Aug. 1, 1905 2,584,565 Ferrand Feb. 5, 1952 FOREIGN PATENTS Number Country Date 220,856 Germany Apr. 8, 1910 593,980 Germany Mar. 7, 1934 670,658 Germany Jan. 21, 1939 38,159 Norway Oct. 29, 1923 OTHER REFERENCES Ser. No. 369,610, Helling (A. P. C.), published May 18, 1943.

Claims

1. AN ELECTROLYTIC CELL FOR AN ELECTROLYTIC PROCESS IN WHICH A LAYER OF MOLTEN METAL FLOATS ON THE ELECTROLYTE AS AN ELECTRODE, COMPRISING: A MAIN CONTAINER HAVING SUBSTANTIALLY VERTICAL SIDE WALLS, A REFRACTORY BODY ATTACHED TO A SIDE WALL AND HAVING A SUBSTANTIALLY HORIZONTAL UPPER SURFACE, A CHANNEL EXTENDING THROUGH SAID WALL AND ALONG SAID BODY SUCH THAT A PORTION OF SAID LAYER MAY RESIDE THEREIN, SAID CHANNEL HAVING AN INNER PORTION OPENING INTO THE MAIN CONTAINER AND AN OUTER PORTION RESIDING AWAY FROM SAID CONTAINER, THE CHANNEL BIENG OPEN-TOPPED, A CONDUCTOR IN THE OUTER PORTION AT A PART THEREOF AWAY FROM SAID INNER PORTION, THE CROSS-SECTIONAL AREA OF SAID INNER PORTION BEING SUBSTANTIALLY SMALLER THAN THE CROSS-SECTIONAL AREA OF SAID OUTER PORTION, WHEREBY PINCHING EFFECTS ARE AVOIDED, AND CONDUCTIVE MEANS EXTENDING INTO THE OUTER PORTION OF SAID CHANNEL FOR CONNECTION TO A SOURCE OF ELECTRIC CURRENT.