EP0084142B1

EP0084142B1 - Method and apparatus for electric current supply of pots for electrolytic production of metals, particularly aluminium

Info

Publication number: EP0084142B1
Application number: EP19820111772
Authority: EP
Inventors: Giancarlo C/O Alluminio Italia S.P.A. Solinas
Original assignee: Aluminia SpA
Current assignee: Aluminia SpA
Priority date: 1982-01-18
Filing date: 1982-12-18
Publication date: 1987-06-10
Also published as: EP0084142A3; DE3276543D1; EP0084142A2

Description

The present invention relates to a method for the electric current supply to pots for the electrolytic production of metals, in particular for the production of aluminium, said pots being connected in series and arranged transversally or longitudinally.

Background of the invention

As is known, aluminium is commercially produced in pots, electrically connected in series, by electrolytic reduction of alumina dissolved in molten cryolite, each of said pots essentially consisting of a rectangular crucible having the cathode (formed by joined carbon blocks which embody iron cathode bars) on the bottom, while the anodes (consisting of separate carbon blocks connected by iron yokes to aluminium anode rods) hang vertically over each pot and are immerged in the electrolytic molten bath of cryolite-alumina. The aluminium produced by electrolysis deposits onto the cathode, under said molten bath.
The electrical connection in series is accomplished by connecting, by means of aluminium conductor bars, the cathode outputs of an upstream pot with the anode bus bar of the adjoining downstream pot, said upstream and downstream positions being referred-as is usually conventional-to the prevailing current direction in the pot-line.
Generally, in the plants for the electrolytic production of aluminium, the pots of the pot-line are arranged, from a geometrical point of view, on one or more parallel rows, and are oriented in each of such rows in a so-called end-to-end relationship, or in a so-called side-by-side relationship, depending on whether the long side of said pots is arranged parallelly or perpendicularly to the predominant direction of the current in the series.
Each of the two arrangements (end-to-end and side-by-side) exhibits own typical known advantages and drawbacks of various kind, but for both said arrangements the electric current carried by whole conductor system (i.e. the anodes, the molten electrolyte and metal, the cathodes and the outer bus bar system) generates magnetic fields of comparatively high intensity, which cause in the electrolysis bath as well as in the molten metal, due to the interaction with the current flowing through said fluids, forces of electromagnetic nature which are generally dis- symetrically distributed.
Said magnetic fields and consequent forces (which are the more intense, the higher is the curent intensity involved, being also known that at present the tendency of the technique is to construct pots operating at 150-200 kA and over) cause serious disturbances to the behaviour of the pots, with related detrimental effects on the technical-economical results of the electrolysis process.
More particularly, said disturbances and detrimental effects may be summarized as follows:

- Localized erosions in the frozen ring of electrolytic bath and in the side lining materials of the pots, with consequent non-homogeneous operation of the pots and possible damages to the pot iron shell, such erosions and damages leading of course to a shortening of the life of said pots. The above-mentioned drawbacks are imputable chiefly to the motions induced in the bath and in the metal by the electromagnetic forces (the forces acting on the two individual fluids being, furthermore, different as to magnitude and distribution), which produce extended horizontal vortexes asymmetrically arranged with respect to the median axes of the pots and characterized by high peripheral speeds;
- Forming of waves of comparatively considerable amplitude on the bath-metal interface surface, such waves causing local reductions of the current efficiency as well as occasional short-circuits between cathodic metal and anode, with consequent high power specific consumption. Said forming of waves (which is strictly connected with the vorticose motions mentioned hereinabove) is prevailingly due to the dynamic friction interaction between the two above-said fluids in motion, said interaction bringing about phenomena of local accumulation of the mass of molten metal with respect of the bath and vice-versa.

With a view to obviating the drawbacks and disturbances cited hereinbefore, several methods and apparatus have been proposed (such as, e.g., the ones described in U.S. patents 3 969 213 and 4132621), which however are generally complicated or such as not to satisfactorily reduce the effects of the abovesaid magnetic forces.
Teachings in the known art are reported also in the German patents DE-B-1.161.695 and DE-B-1.049.108; namely: DE-B-1.161.695 teaches a method for reducing magnetic disturbances in series of electrolysis tanks said method comprising some particular arrangements of the external conductors connecting the adjacent tanks of the series, and conductors being ordered, when the effect of the adjacent row is negligible, in couples positioned in a symmetrical way in respect to the median vertical plane parallel to the current's general direction, and being on the contrary said conductors ordered, when the distortion effect of the adjacent row must be taken into account, in an unsymmetrical way in respect of the same said median vertical plane.
DE-B-1.049.108 teaches, in turn, a method relating to electrolysis pots disposed end to end, which method is characterized in that the main two conductors carrying the current from pot to pot through the whole series (said current having opposite direction for two adjacent rows) are as far as possible parallely positioned (side by side) on the median line of said two rows, as well as parallely are positioned the conductors connecting-perpendicularly to the general direction of the current in the series-said main two conductors to the single pots, further being symmetrically positioned the conductors which feed the current in and out of the single pots.

Summary of the invention

Thus, the object of this invention is to provide an improved method for the electric current supply to pots for the electrolytic production of metals, in particular aluminium, capable of minimizing the detrimental effects of the magnetic forces acting on the electrolysis pots, said pots being electrically connected in series, and arranged side-by-side or end-to-end in one row or more rows.
This and still other objects, which will be more clearly apparent to those skilled in the art from the detailed description given hereinbelow, are achieved by a method of the type which provides conductors in pairs symmetrically positioned in respect to the pots' rows and with the direction of current in one conductor flowing opposite to that in the other conductor, so that the magnetic fields tend to cancel each other, which method, according to the present invention is characterized in that all of the electric conductors and relevant currents in the circuit are symmetrically adjusted with respect to the two median vertical planes of each pot of the series. The double symmetrical current paths so provided permit to obtain a high compensation and the total symmetrization (with respect to both median vertical planes of each pot) of the magnetic field components and, by consequence, a total symmetrization of the magnetic forces induced by the interaction of said magnetic field with the pot current.
Said symmetrization of forces, in itself extremely advantageous for the purposes of a uniform distribution in the pot of the effects of said forces, is coupled, due to the particular arrangements of the conductors adopted in the invention, to a marked difference in the behaviour and action of said forces in respect of the prior art, what results, on the whole, in a substantial decrease of the abovesaid undesired drawbacks and detrimental effects in the abovementioned pots and relevant electrolytic process.
In fact, both the stress action on the side lining of the pot by the fluids in motion, and the "ondulation" of the bath-metal interface become practically homogeneous and are considerably reduced (in the four sectors into which each pot in question may be imagined to be divided by said two median vertical planes), the whole resulting in an excellent stability of the operation of said pots, in a sensible improvement of the current efficiency and in a substantial energy saving.

Brief description of the drawings

The invention, as schematically illustrated hereinbefore and the relevant advantages are described hereinafter more in detail, making reference to figures 1 to 11 for plants with side-by-side pots and to figures 12 to 18 for plants with end-to-end pots.
Said figures schematically show some arrangements of the conductors intended for carrying the current in plants for the electrolytic production of metals, in particular aluminium, figures 1 to 3 and 12 to 14 referring to conventional arrangements, while figures 4 to 11 and 15 to 18 refer to arrangements according to the present invention. More particularly, with reference to the side-by-side pots:

- figure 1 shows the arrangement, in a plan view, of a pot line and relevant bus bar conductors in a known conventional plant, in which said line extends on two parallel rows;
- figure 2 shows more in detail a conventional supply scheme to a downstream pot C_v by the corresponding upstream of pot C_M of the same row;
- figure 3 shows a simplified scheme relating to a conventional plant of the "mono-row" type in which, as the pots are arranged on a single row, the closing of the series circuit is secured by two independent reverse bus bars;
-figure 4 shows a simplified scheme concerning a plant with side-by-side arrangement according to the present invention, in which, by way of example, the pot-line extends on two parallel rows;
-figure 5 shows a scheme of a type of arrangement of the conductor bus bars according to this invention and referred to as "cross symmetrical arrangement". (In this figure, like in the following figures 6 to 10 and 16-17, the bus bars lying underneath the bottom plane of the pots are indicated with dashed lines);
- figure 6 shows a variant of the arrangement of fig. 5, referred to as "rhombus symmetrical arrangement";
- figure 7 shows a scheme of another type of arrangement of the conductor bus bars according to the present invention, referred to as "corridor symmetrical arrangement";
- figure 8 shows a variant of the arrangement of fig. 7, referred to as "hexagon symmetrical arrangement";
- figure 9 shows a scheme of another type of arrangement of the conductor bus bars according to the present invention, referred to as "rectangle symmetrical arrangement";
-figure 10 shows a scheme of still another arrangement of the conductor bus bars according to the present invention, referred to as "sheaf symmetrical arrangement;
-figure 11 shows, finally,-in axonometric projection-the whole electric circuit of the cross arrangement as per figure 5 (for the sake of illustrative completeness, the drawing shows also the series reverse bus bars).

Making now reference to the pots arranged in end-to-end relationship:

- figure 12 shows the arrangement, in a plan view, of a series of pots and relevant bus bars in a known conventional plant, in which said series extends on two parallel rows;
-figure 13 shows in greater detail a known conventional scheme of the supply to a downstream pot C_v by the corresponding upstream pot C_M, the two pots being positioned in the same row;
- figure 14 shows a simplified scheme relating to a known plant of the "mono-row" type in which, since the pots are arranged on a single row, the closing of the series circuit is secured by two independent reverse bus bars;
-figure 15 shows a simplified scheme concerning a plant with the end-to-end arrangement according to the invention in which, by way of example, the pot-line extends on two parallel rows;
-figure 16 shows a scheme of a type of arrangement of the conductor bus bars according to the present invention, referred to as "sheaf symmetrical arrangement";
- figure 17 shows a scheme of another type of arrangement of the conductor bus bars according to the present invention, referred to as "corridor symmetrical arrangement";
-figure 18 finally shows-in axonometric projection-the whole electric circuit of the corridor arrangement of figure 17 (for the sake of illustrative completeness, the drawing shows also the series reverse bus bars).

Detailed description of the invention

Referring to the above figures, there is completed hereinbelow the illustration of the known technique, and is provided, in comparison with said technique, a detailed description of the present invention.
As is known, the magnetic forces F which influence the behaviour of an electrolytic pot are generated by the interaction of the magnetic induction fields created by the currents flowing through the outer and inner conductors (in respect of the pot), with the current density field7 existing inside the molten areas of the pot (more specifically: bath and metal) according to the vectorial relationship
wherein both F and J and B are intended in general to be functions of the point of observation, being such point referred-in the present case-to an orthogonal reference system having the center coinciding with the center of the cathode plane of the pot (metal bottom), the x axis directed as the general flow of the current in the potline, the y axis arranged at 90° counterclockwise and the z axis directed upwards.
At the design and assembling stages there is usually devised and carried out a perfect geometrical-structural symmetry of the pot with respect to its two median vertical planes, such geometry and structure theoretically securing a symmetrical distribution of the current, and therefore ofJ_; inside the cavity containing the fluids (bath and metal).
A possible dissymmetry in the magnetic force field F in said cavity is therefore substantially ascribable to the field B Such dissymmetry is a consequence, in turn, of the dissymmetry of the outer conductor bus bar system of. the pot, since the inner conductors (from the anode rod system to the cathode block) are usually symmetrically arranged.
In the conventional known plants for electrolytic production of aluminium there are two prevailing types of dissymmetry of the field S: one is typical of the bi-row (or multi-row) plants and is due to the presence and consequent influence of a pot row in respect of the other row (adjoining row), since in said two rows the electric current carried by the bus bars flows in opposing directions, as is shown in figures 1 and 12.
The electric current which flows-with direction parallel to x-through the adjoining row causes in the pots of the other row a systematic unbalance of the fieldB wifih respect to the median vertical plane xz ("transversal plane" in the transversally arranged pots and "longitudinal plane" in the longitudinally arranged pots).
The second type of dissymmetry, as is shown in figures 2 and 13, is conversely due to the non- symmetrical arrangement of the conductor bus bars and of the currents with respect to the plane yz, i.e. to the median vertical plane of the single pot perpendicular to the direction of the current in the row ("longitudinal plane" for transversally arranged pots and "transversal plane" for longitudinally arranged pots), and causes an unbalance of field B between the upstream and the downstream halves of the pot.
In the known mono-row pot-lines (as is shown in figures 3 and 14), the former of said two types of dissymmetry is not present.
The same may apply to the pot-lines (either mono-row or multi-row) according to the present invention, since (as figures 4 and 15 show) the effect of the adjoining row-which is actually only the effect caused by the conductor bus bars of such row which are parallel to x and carry the current from each pot to the succeeding one-is completely annulled by the presence of the reverse bus bars positioned side-by-side thereto. Therefore, for the purpose of eliminating the drawbacks and detrimental effects cited hereinabove, reference is to be made in the present case only to the second type of dissymmetry mentioned hereinabove.
In the connection between pots of the known conventional type (particular reference is to be made to figure 2 for the case of pots arranged side-by-side, and to figure 13 for the case of pots arranged end-to-end) it is apparent that the arrangement of the bus bars is remarkably dis- symmetrical with respect to median vertical plane yz (namely between upstream and downstream).
Such dissymmetry concerns above all, and with serious effects in terms of the generated magnetic field, the direction of the current flowing in the bus bars parallel to x, said direction being oriented exclusively from upstream to downstream (and therefore being completely absent, for restoring the symmetry, any corresponding bus bars directed from downstream to upstream). Furthermore, although with different alternatives connected with different schemes of collecting the currents carried by the output cathode bars, the total current carried on the two sides of the row from the two sets of cathode bus bars parallely to the x direction is never constant, as is clearly apparent from the examples illustrated in the above-mentioned figures 2 and 13.
Turning now to the detailed description of the present invention-in the form of preferred but not exclusive embodiments thereof-particular reference is made to figures 5 through 11 (with greater detail in figure 11) for pots arranged side-by-side and to figures 16 through 18 (with greater detail in figure 18) for pots arranged end-to-end.

Pots arranged side-by-side:

a) The total current carried by the output cathode bars is always symmetrically collected in four equal portions along the two long sides of the pot and conveyed through the cathode bus bars to the four points A₁, A₂, B₁, B₂ symmetrical with respect to the two median vertical planes of the pot, said points being positioned

- for the arrangements as per figures 5 and 6: at the four cathode outputs closest to the transversal median plane;
- for the arrangements as per figures 7 and 8: at four intermediate cathode outputs on the four half long sides;
- forthe arrangements as per figures 9 and 10: at the four cathode outputs farthest from the transversal median plane.

b) The current so conveyed is carried downward by means of two pairs of conductors a₁, a₂; b₁, b₂ (fig. 11) of equal length and cross-section, preferably vertically arranged and symmetrically positioned with respect to the two median vertical planes, up to the four symmetrical points A₁', A₂', B₁', B₂' (fig. 11) arranged on a plane (hereinafter referred to as "lower plane") lying beneath the bottom of the pot and parallel to the reference plane xy.
c) The current so carried to the four abovesaid points A₁', A₂', B₁', B₂' is conveyed by two pairs of conductors of equal length and cross section c₁, d₁; c₂, d₂, preferably horizontal, which follow symmetrical paths with respect to median vertical planes xz and yz-according to the abovesaid alternative "cross arrangement" (fig. 5), "rhombus arrangement" (fig. 6), "corridor arrangement" (fig. 7), "hexagon arrangement" (fig. 8), "rectangle arrangement" (fig. 9)-to two points M₁, M₂ of the longitudinal median plane, symmetrical with respect to the transversal median plane, being it also possible to assemble said points M₁, M₂ to a single point M-in the "sheaf arrangement" alternative (fig. 10)-corresponding to the vertical projection of the pot centre on the lower plane.
d) From said points M₁, M₂ (or M), by means of two pairs of side-by-side conductors e₁, f₁; e₂, f₂ of same length and cross-section, the current is carried, parallelly to the general flow of the current in the series, from the longitudinal median plane of the upstream pot to the longitudinal median plane of the downstream pot, up to the points N₁, N₂ (or N in figure 10), substantially homologous-when taking into consideration the connection in series of all the pots-to the aforesaid points M_i, M₂ (or M).
e) From said points N₁, N₂ (or N), by means of two pairs of side-by-side conductors g₁, h₁; g₂, h₂ (figure 11) of same length and cross-section, the current is carried upward-according to paths symmetrical with respect to the two vertical median planes-up to the anode bus bar t in the two symmetrical points P₁, P2 (fig. 11), said anode bus bar being so symmetrically supplied at the heads of the pot.
f) The total feeding current I which, coming from the rectifier station, supplies the first pot in the series is distributed (at least from a certain point onward) in four equal portions in two parallel pairs of side-by-side conductors of equal length and cross-section, said pairs being lined, up to the intersection points with the longitudinal median plane of the abovesaid first pot, with the aforecited pairs of conductors e₁, f₁; e₂, f₂, so that a "continuous" is practically obtained from the rectifier station to the longitudinal median plane of the last pot in the series, such "continuous" being represented by two pairs of side-by-side conductors, each pair carrying, in the x direction, a current equal to 1/2.
g) The four reverse bus bars are divided into two pairs m₁, n_l; m₂, n₂ (fig. 11) of equal length and cross-section, arranged all through the pot- line and up to the rectifier station in proximity of and parallely to the two cited pairs representing the "continuous" mentioned in point f) above, each of said pairs of reverse bus bars carrying a current equal to I/2 having a direction opposite to the general flow direction of the current in the potline, wherefore a complete compensation between the current feeding the series and the reverse current is provided in the four pairs of said conductor bus bars; as a consequence thereof, the magnetic field generated by them is fully annulled and, therefore, any cause of dissymmetry due to the currents with respect to the longitudinal vertical planes of the pots is removed.

Pots arranged end-to-end

h) The total current carried by the output cathode bars, always symmetrically collected-by means of the cathode bus bars-in two halves at middle points A, B of the sides of the pot, is carried downward-by a pair of conductors a, b of equal length and cross-section belonging to the transversal median plane, symmetrically positioned with respect to the longitudinal median plane-up to a plane lying; optionally, underneath the pot bottom (hereinafter referred to as "lower plane") in two points C, D symmetrically arranged with respect to the longitudinal median plane. In the "sheaf arrangement" of fig. 16, said two points C, D are assembled in a position corresponding to the vertical projection of the pot centre; in the "corridor arrangement" of figures 17 and 18, the two points are separated, being respectively outside (figure 17) or at (figure 18) the vertical projections of abovesaid points A, B.
i) From said points C, D-by means of a pair of conductors c, d of equal length and cross-section the current is carried, according to a path parallel to the general direction of the current in the series, from the transversal median plane of the upstream pot to the transversal median plane of the downstream pot, up to points C', D', which are substantially homologous-when taking into consideration the connection in series of all the pots-to the aforesaid points C, D. In the "sheaf arrangement" alternative of figure 16, said conductors c, d are positioned side-by-side.
I)-From the abovesaid pair of points C', D'-by means of a pair of conductors a', b' of equal cross-section, which follow paths parallel to the preceding pair of conductors a, b (and therefore having the same length of the latter and being likewise symmetrically arranged with respect to the longitudinal median plane)-the current is carried again from said lower plane to the level of the cathode outputs of the pots, till reaching, at the long sides of the downstream pot, the two points A', B', which are substantially homologous to the two points A, B of the upstream pot.
Taking into account the connection in series of the pot-line there are by consequence-in a same pot-two pairs of side-by-side conductors (namely a, a'; b, b') symmetrically arranged, on the transversal median plane, with respect to the longitudinal median plane; furthermore-within each couple-the two conductors carry the same currents (equal exactly to 1/2), but directed in opposing directions.
m) From preceding points A', B'-by means of two conductors e, f (anode risers) of equal length and cross-section, and arranged on the transversal plane, which follow symmetrical paths with respect to the longitudinal median plane-the current is finally carried upward, up to anode bus bar t, at points E, F (figure 18), so that such anode bus bar is symmetrically supplied in correspondence of the central points of the long sides of the pot.
n) The total feeding current I which, coming from the rectifier station, flows to supply the first pot in the series, is divided (at least from a certain point onward) into two equal portions on a pair of parallel conductors of equal length and cross-section, said pair being lined, up to the intersection points with the transversal median plane of the abovesaid first pot, with the cited pair of conductors c, d so that a "continuous" is practically provided from the rectifier station to the transversal median plane of the last pot in the series, such "continuous" consisting of a pair of conductors parallel to direction x, each carrying a current equal to 1/2.
o) The reverse current is carried by two conductors m, n (fig. 18) of equal length and cross-section, arranged along the all pot-line and up to the rectifier station close and parallely to the two conductors constituting the "continuous" mentioned in point n) above, each of said reverse conductors carrying a current equal to I/2 having a direction opposite to the general direction of the current in the series, wherefore a complete compensation between feeding current and reverse current is obtained in the two pairs of abovesaid conductors; as a consequence thereof, the magnetic field generated by them is completely annulled, and any cause of dissymmetry of the currents with respect to the transversal vertical planes of the pots is therefore eliminated.
From what has been explained and exemplified hereinbefore there are apparent the advantages of the present invention, which are listed and illustrated in the following:
I) The conductors utilized for carrying the cathode current can have uniform cross-sections in the whole plant. This depends on the fact (clearly evidenced by figures 11 and 18) that from the current collecting points at the long sides of each single upstream pot up to the joins to the anode bus bar of the adjoining downstream pot, the conductors-having the same length and cross-section in the single sections-follow total paths of equal length and a constant current equal to 1/2 flows therein on each side of the row.
In the prior art, conversely (for example according to the arrangement of figures 2 and 13), the connection between upstream pot and downstream pot is provided by conductors which follow paths of different length and carry different currents, what forces-in order to secure the electrical balance of the pot-to utilize conductors with different cross sections.
Besides, the calculation of these sections, due to the presence of parasite contact resistances of various types in the circuits, is often affected by errors which involve non-homogeneity in the output of current from the pot, especially between upstream and downstream.
The utilization of the abovesaid uniform cross-section, extensible, on the basis of what illustrated at points f), g), n) and o) hereinabove, also to the separate bus bars from and to the rectifier station, involves undeniable technical advantages and economical saving as regards design, assembly and maintenance.
II) The working floor of the pots is left fully free-in all of the alternatives-with the exception of the space required for the anode risers. This results in advantages and simplifications with regard to the operation of the mechanical equipments around the pots.
III) The ordered and simple geometrical arrangement of the conductors results in a simplification of the bar system assembly and disassembly operations, as well as in a uniformity and homogeneity of access to the outer walls of the iron pot-shell under operative conditions (for example, for accomplishing measurements and checks).
IV) The presence of only one direct connection between the median planes yz of two succeeding pots facilitates the by-pass operation of one or more pots which need to be separated from the remaining pots in the series (a comparison should be made with the more complex situation existing in the case of the known arrangements of figures 2 and 13).
V) The conductor arrangement adopted reduces to the essential the portions of outer conductors which are unshielded, from a magnetic point of view, by the steel shell of the pot.
Furthermore it should be noted that a beneficial and symmetrical shielding action is exerted by the iron structures employed for the containment and support of the pot-shell on the conductors, if any, placed underneath the bottom of the pot.
VI) The double geometrical and structural symmetry of the steel shell and of the totality of conductors secures, finally, a double symmetry of the steel shell shielding action.
VII) The symmetrical distribution of the "residual" conductors (i.e. the ones which determine a non identically null field) with respect to the two median vertical planes (therefore, a "doubly symmetrical" distribution) causes in the bath and in the metal "doubly" and totally symmetrical distributions of the magnetic induction field B and of the magnetic forces field r, what generally involves a uniform operation of the pot in the four portions whereinto it may be imaginarily supposed to be divided by the above- said median vertical planes, the unbalances between upstream and downstream and between right and left side (with respect to the general direction of the current flow in the series) having been eliminated.
VIII) The "doubly" symmetrical distribution of the magnetic forces in the pot leads to a "doubly" symmetrical distribution of the horizontal velocity field in the bath and in the metal, with consequent:

-elimination of the extended asymmetrical horizontal vortexes typical of the pots with conventionally arranged conductors;
- generalized reduction of the velocities in the metal;
- reduction of the waves amplitude at the bath-metal interface.

IX) There is obtained a substantial absence of abnormal behaviours in the pots placed at the beginning and at the end of the series-as is found in the known conventional plants-because the magnetic effects induced by each pot in the adjoining ones are extremely reduced.
This fact permits also to avoid known extensions (for magnetic compensation) of the bus bars connected to the terminal pots of the rows.
X) In the side-by-side pots, the conductor arrangements, as previously described and illustrated in figures 4 through 11, cause a considerable reduction, in all the area of the molten metal, of the detrimental portion of the longitudinal component of the magnetic force, such detrimental portion being expressed by -J_xB_z, where J_x is the transversal component of current density J and B_z is the vertical component of magnetic induction 8. Such reduction has a considerable practical advantage as it permits to the pot to operate-without any harmful effects-with a transversal horizontal component J_x of the current by far higher than the one existing in the conventional pots. This permits to sensible reduce the height of liquid metal deposit in the pot, with consequent saving in the deferred costs of the metal.
XI) In the end-to-end pots, the side-by-side arrangement of conductors in countercurrent all through the bus bar system-from the points collecting the cathode outputs in correspondence of the centers of the long sides of the upstream pot up to the homologous points on the long sides of the downstream pot-secures the total elimination of the magnetic effects caused by said conductors. In other words, the "residual" magnetic field (i.e. non identically null field) is only the one generated by outer portions of cathode by the bars running along the pot long sides, by the risers, by the anode bus bar and by the conductors inside the pot; all the abovesaid conductors being symmetrically arranged with respect to both median vertical planes.
It should be pointed out, in particular, that there. is eliminated the magnetic field generated (in the conventional plants) by the currents parallel to y, carried by the conductors arranged in the corridors between one pot and the succeeding one, such currents-as is known-causing very high values of the vertical magnetic field at the heads of the pots.
It should be also noted in particular that, as the anode risers are placed in correspondence of the middle points of the pot long sides, there is a lowering-with respect to the known arrangements-of the transversal component By of the magnetic field, which is responsible for the detrimental longitudinal forces, against a rising of the longitudinal component Bx, responsible for the less harmful transversal forces.
XII) In the end-to-end pots, the conductors arrangements (as previously described and illustrated in figures 15 through 18) involve the practical elimination-in the fluid areas of metal and of bath-of the harmful total longitudinal component of the magnetic force, expressed now by F_x=J_yB_z-J_zB_y, such practical elimination being due to the simultaneous sensible reduction of the values of By and B_z.
In particular, in the whole area of molten metal, an almost total elimination of term JyB_z is obtained, such term representing the product of the transversal component of the current by the vertical component of the magnetic field. Said elimination of JyB_z permits the pot to operate-without detrimental effects-with a transversal component of the current Jy much higher than the one existing in the known conventional pots. This entails the possibility of remarkably reducing the liquid metal deposit inside the pot, with a consequent appreciable saving in the deferred costs for the metal.

Claims

1. A method for electric current supply to pots for the electrolytic production of metals, particularly aluminium, arranged in side-by-side or end-to-end relationship and connected in series in one or more rows, said method being of the type which provides conductors in pairs symmetrically positioned in respect to the pots' rows and with the direction of current in one conductor flowing opposite to that in the other conductor, so that the magnetic fields tend to cancel each other, characterized in that all of the electric conductors and relevant currents in the circuit are symmetrically adjusted with respect to the two median vertical planes of each pot of the series.

2. A method according to claim 1, in which the pots are arranged side-by-side, characterized in that:

- the total cathode output current of each single pot in the series is symmetrically conveyed in four equal portions

- by means of cathode bus bars-to four points (A₁-A₂, B₁-B₂) symmetrically positioned with respect to both median vertical planes-longitudinal and transversal, respectively-of said pot; whereafter

-from said four points, the current is carried downward

- by four conductors of equal length and cross-section (a₁―a₂, b₁-b₂), symmetrically arranged with respect to both said median vertical planes-up to four points (A₁'-A₂', B₁'-B₂') lying on a lower plane underlying and parallel to the bottom of said pot, said four points resulting to be symmetrically positioned with respect to the abovesaid two median vertical planes; whereafter

-the current is conveyed-by two pairs of conductors of equal length and cross-section (c₁-d₁, c₂-d₂) which follow horizontal paths symmetrically arranged with respect to the aforesaid median vertical planes-to two points (M_l-M₂) in the median longitudinal plane, such points resulting to be symmetrically arranged with respect to the median transversal plane; whereafter

- from the abovesaid two points (M₁-M₂)-by means of two pairs of side-by-side arranged conductors of equal length and cross-section (e₁-f₁, e₂-f₂)-the current is carried, parallely to the general direction of the current in the series, up to the longitudinal median plane of the adjoining downstream pot, being said plane intersected by said conductors in two points (N₁-N₂) homologous to the aforecited preceding points; and finally

- the current is carried upward-by two pairs of side-by-side arranged conductors of equal length and cross-section (g₁-h₁, g₂-h₂) which extend symmetrically with respect to the median transversal plane of the abovesaid downstream pot-up to the level of the anode bus bar (t) in order to allow the symmetrical supplyto said bus bar in correspondence of the median area of the heads of said pot.

3. A method according to claim 1 in which the pots are arranged end-to-end, characterized in that:

- the current carried by the cathode output bars of each single pot in the series is symmetrically conveyed by four identical cathode bus bars, to two collecting points (A-B) belonging to the transversal median plane and symmetrically positioned with respect to the median longitudinal plane; whereafter

- the current, from the two aforesaid collecting points, is carried-by a pair of conductors of equal length and cross-section (a, b), which, on the median transversal plane, are symmetrically arranged with respect to the longitudinal median plane-up to two symmetrical points (C, D) of the median transversal plane, lying on a lower plane underlying the bottom of the pot; whereafter

- from the aforesaid points (C, D)-by means of a pair of conductors of equal length and cross-section (c, d), arranged side-by-side-the current is carried, parallely to the general direction of the current in the series, up to the median transversal plane of the adjoining pot in downstream position, being this plane intersected by said conductors in two points (C', D') homologous to the preceding points; whereafter

- the current is carried-by a pair of conductors of equal length and cross-section (a', b') which, on the transversal plane of the downstream pot, are parallely arranged to the abovesaid conductors (a, b) on the transversal plane of the preceding upstream pot (and therefore have the same length of the latter and are equally symmetrically arranged with respect to the median longitudinal plane)-up to two points (A', B') of the long sides of said downstream pot, which are homologous to the two above-mentioned collecting points (A, B) of the upstream pot; finally

-the current is carried upward-by two conductors of equal length and cross-section (e, f) which, on the median transversal plane, are symmetrically arranged with respect to the longitudinal median plane-up to the anode bus bar (t) of said downstream pot in correspondence of the median areas of the long sides.

4. A method according to the preceding claims, characterized in that the reverse conductors to the rectifier station, divided into symmetrical pairs, are positioned side-by-side to those parts of the feeding conductors, parallel to the general direction of the current in the series, which directly connect each pair of succeeding pots along the total length of the series.

5. A method according to the preceding claims, characterized in that the total set of conductors of the circuit external to the pots, i.e. transport cathode bars, anode risers and reverse conductors, have the same cross-section.