US3728243A - Electrolytic cell for the production of aluminum - Google Patents

Abstract

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM HAVING PRESCRIBED MAXIMUM HORIZONTAL DISTANCE BETWEEN OUTER LOWER EDGES OF THE ANODES AND ADJACENT WALL SURFACES OF THE POT LINING, POT LINING WITH SPECIFIED THERMAL ISOLATING POWER, CATHODE BARS INSULATED BENEATH THE SIDE WALL OF THE LINING AND SPECIFIC RATIO OF IRON CROSS SECTION TO CARBON CROSS SECTION THROUGH THE BOTTOM OF THE LINING, IN ORDER TO REDUCE THE HORIZNTAL COMPONENTS OF THE ELECTRIC CURRENT IN THE CELL.

Description

April 1973 w. SCHMIDT HATTING 3,

ELEC'I'ROLY'IIC CELL FUR THE PRODUCTION OF A'LUM INUM {"1 led Aug. 25, 1971 4 Sheets-Sheet 1 21 '1 P P rj 0 2s /19 2o Apri 3 w, SCHMIDT HATTING 3, 3

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ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed Aug. 25, 1971 4 Sheets-Sheet 3 Fig.5

April 17, 1973' w, SCHMIDT HA TING 3,728,243

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed Aug. 25, 1971 v 4 Sheets-Sheet 4.

Fig.

United States Patent 3,728,243 ELECTROLYTIC CELL FOR THE PRODUCTION 0F ALUMINUM Wolfgang Schmidt-Hatting, Chippis, Switzerland, assignor to Swiss Aluminium Ltd., Chippis, Switzerland Filed Aug. 25, 1971, Ser. No. 174,892

Claims priority, application Switzerland, Sept. 1, 1970,

13,101/70 Int. Cl. C22d 3/02, 3/12 US. Cl. 204-243 R 2 Claims ABSTRACT OF THE DISCLOSURE To obtain aluminium by electrolysis, aluminium oxide (A1 0 alumina) is dissolved in a fluoride melt. Electrolysis is carried out in a temperature range of about 940 to 975 C. The cathodically deposited aluminium collects under the fluoride melt on the bottom of the cell. Anodes of amorphous carbon are dipped from above into the melt. The electrolytic decomposition of the alumina causes oxygen to form on the anodes, and this combines with the carbon of the anodes to form CO and CO A typical aluminium electrolysis cell is shown diagrammatically in FIGS. 1 and 2 of the accompanying drawings, which are a longitudinal section and a transverse section.

A fluoride melt (the electrolyte) is contained in a steel pot in which is a layer of insulation 13 and a carbon lining 11. The insulation 13 is of refractory thermally insulating material. Cathodically precipitated aluminium 14 collects on the bottom 15 of the cell. The surface 16 of the liquid aluminium acts as the cathode. Iron cathode bars 17 are embedded in the bottom of the carbon lining 11 and serve to conduct current from the bottom of the cell to the exterior.

Anodes 18 of amorphous carbon dip into the fluoride melt from above so as to conduct the direct current to the electrolyte. They are fixed by rods 19 and clamps 20 to two anode bus bars 21. These together constitute an anode beam. The electrolyte 10 is covered with a crust 22 of solidified melt and on top of this is a layer 23 of alumina.

The distance d from the underside 24 of the anode to the upper surface 16 of the aluminium (also called interpolar distance) can be varied by raising or lowering of the anode beam 21, 21 with the aid of the lifting mechanisms 25 which are mounted on columns 26. As a result of attack by the oxygen liberated in the electrolysis, the anodes are consumed on their underside to an extent of about 1.5 to 2 cm. each day, according to the particular construction of the cell.

The cathode bars 17 have two tasks. They collect the current from the active part of the carbon bottom beneath the anodes 18 and they conduct it out of the cell. Where they collect the current and are designated by 29, the current intensity in the cathode bar increases on both sides towards the exterior. At 28, outside the active part 27 of the carbon bottom, the cathode bars serve as pure current conductors. From each cell, cathode bus bars 30 conduct the current from terminals at the outer ends of the cathode bars 17, to the anode beam 21, 21 of the following cell.

3,728,243 Patented Apr. 17, 1973 To reduce heat losses here, the cross-section of the iron cathode bars is reduced outside the active part 27 of the carbon bottom. Thus the flow of heat out of the melt through the bars to the exterior is reduced. This reduction is the subject of our patent application No. 139.154 Apr. 30, 1971.

The electrolyte possesses a substantially poorer electric conductivity than the liquid aluminium which is situated on the bottom of the cell. The ratio of the two conductivities is between l0 :1 and 10- 1. If the current withdrawal through the carbon bottom does not locally exactly correspond to the current feed through the anodes into the electrolyte, horizontal current density components must occur in the melt, which are caused by the local difference between feed and withdrawal of the current. The great difference of the electrical conductivties of the two Stratified liquids has the effect, according to the tangent law of electric current dynamics, that a kink occurs in the current lines at the boundary surface between electrolyte and liquid aluminium. The result is that the current lines in the electrolyte, to a first approximation, extend vertically. On the other hand in the metal major hori zontal current density components can occur which can be locally greater than the vertical. The different current density components in the electrolyte and in the liquid aluminium, in cooperation with the magnetic induction between the two media, result in differences in the pressure which can be compensated only by a doming up of metal. This can amount to many centimetres in height since the domed-up metal is covered by the electrolyte and thus has only an effective specific gravity corresponding to the difference of density between electrolyte and metal.

Furthermore, the horizontal current density components in cooperation with the magnetic induction can cause a force field distribution in the liquid metal which is not free from rotation. The consequence of this is a flow of metal combined with a major doming up of metal, which in turn is caused by current density components induced by this movement of a current conductor in a magnet field. Doming up and movement of metal are detrimental to the electrolytic efficiency (ratio of the quantity of aluminium actually obtained to the quantity theoretically precipitated according to Faraday). If the electrolyte efiiciency falls, the electric energy consumption rises (kWh/kg. Al).

If therefore only vertical current density components are present in the metal and in the melt, a doming up of metal without metal movement is impossible. Nevertheless, there may be rotation drive in the metal, as shown by the following equation of the volume forces k:

ii 5 6B, 6B, 6B, 6y By+ 6a 6x 6y 62 Here j j and i signify the current density components in the metal in the three axial directions and B B and B the corresponding components of the magnetic induction.

If it is ensured that the current withdrawal through the carbon bottom at the underside of the liquid metal corresponds to the current feed at the upper side of the.

metal, the following components are zero: i and j and thus also the three partial derivatives of j Only the last member of the rotary drive must be caused to disappear by making eB /fi become little or zero, since i is always present (normal electrolytic current).

Normally, horizontal current density components occur in both axial directions. In the case of incorrect dimensioning of the thermal insulation on the walls of the cell, a direct flow of current from the anodes to the cell rim is possible, which generates horizontal current density components.

The component transverse to the cell longitudinal axis may be augmented due to the fact that the cathode area, often on account of an excessively great distance between the outside of the anode and the carbon lining of the side walls, is larger than the anode area. Moreover the iron cathode bars outside the active part of the carbon bottom can also take up current, if there they are not sufiiciently electrically insulated from the carbon lining of the walls of the cell. Moreover, if the cross section of the iron cathode bars in the active part of the carbon bottom is too small, a great outward displacement of the electrolytic current takes place in the carbon bottom, likewise generating powerful horizontal current density components.

There is also the fact that due to incorrect dimensioning of the cross-sections of the cathode bus bars which conduct the current from one cell to the following cell of the series, powerful horizontal current density components can occur in the liquid aluminium which, in some cases, are locally greater than the vertical.

We have faced the problem of largely suppressing the horizontal current density components in an aluminium electrolytic cell for a current intensity of 70 ka. and above.

On the basis of extensive experiments we have developed cells which have a series of inventive features which are described below and then summarised and which in conjunction ensure success.

One feature is that the horizontal distance between the outer lower edges of the anodes and the inner faces of the walls of the steel pot does not exceed '55 to 60 cm. If 20 cm. are deducted for thermal insulation and carbon lining, a horizontal interval of at most 40 cm. remains between the outer lower edge of the anodes and the furnace rim, that is the inner surface of the walls of the carbon lining. The minimum horizontal distance between the outer lower edge of the anodes and the furnace rim is 25 to 30 cm.

A second feature is that the thermal resistance of the walls of the insulation 13, between the walls of the carbon lining 11 and the walls of the steel pot 12, lies between 0.5 and and As a consequence, a solid lateral cryolite crust is formed by removal of heat which reduces the cathodic, currentcollecting aluminium area and effectively limits the lateral current flow into the furnace rim.

A third feature is that the parts of the cathode bars beneath the side wall of the carbon lining are surrounded by insulation. This is shown in FIGS. 3 and 4 of the accompanying drawings which are a diagrammatic longitudinal section and transverse section of a cell embodying the features of the present invention.

The cathode bars have their lower faces flush with the interface of the bottom of the carbon lining and the bottom of the insulation. Thus there is no carbon beneath the bars. Furthermore, outside the active part 27 of the carbon bottom the bars are surrounded with insulation 31. Thus there can be no current flow into the bars outside the active part of the carbon bottom. We recognise that this feature per se has been proposed previously.

The outward current displacement cannot be entirely avoided since the cell bottom (carbon and cathode bars) has a substantially poorer electrical conductivity than the liquid aluminium situated above it. A fourth feature is to place into the active part of the carbon bottom the largest cathode bar cross-section 29 which is compatible with mechanical strength of the carbon bottom. The ratio of iron to carbon should amount to at least 17:100and at most 20:100. If a smaller iron cross-section is provided, unacceptably high horizontal current density components occur in the liquid aluminium. If, on the other hand, a greater iron cross section is provided, a mechanical weakening of the carbon lining occurs, this weakening being caused by the larger thermal coeflicient of expansion of iron in comparison with that of carbon.

To summarise: according to this invention an electrolytic cell for the production of aluminium by electrolysis of alumina in a melt, comprises a pot body of steel; a layer of thermal and electrical insulation against the inside of the body, a lining of carbon against the inside of the insulation layer; the body, the layer and the lining each consisting of a bottom, two side walls and two end walls; iron cathode bars each having at least a part within the lining and a part passing through a side wall of the insulation layer and through a side wall of wall of the insulation layer and through a side Wall of the body; and anodes arranged to dip into electrolyte in the pot; wherein the horizontal distance between outer lower edges of the anodes and adjacent wall surfaces of the lining does not exceed 40 cm.; and thermal resistance of the walls of the insulation layer is between C h. C.

' and 1X10 the parts of the cathode bars beneath the side wall of the lining are surrounded by insulation; and the ratio of iron cross section to carbon cross section in any vertical plane from end to end of the cell through the bottom of the lining is between 10:100 and 201100.

By means of the invention transverse horizontal current density components are largely suppressed, and longitudinal components are reduced.

No continuous iron current conductors are present in the longitudinal direction of the cell. Neverthless, substantial horizontal current density components can persist in the cell longitudinal direction in the liquid aluminum unless by suitable dimensioning of the cathode bars which conduct the current from the one cell to the anodes of the following cell of the series it is ensured that each cathode bar of the cell bottom as far as possible carries the same current.

This can be achieved by a circuit arrangement which is illustrated in FIG. 6 of the accompanying drawings and which is per se the subject of my co-pending application Ser. No. 174,890 filed Aug. 25, 1971 (corresponding to Swiss appl. No. 13,100/70). According to this arrangement a plant comprises a plurality of electrolytic cells; each cell including at least one terminal outside the pot on each cathode bar, and an anode beam carrying the anodes, and electrical connecting means comprising a plurality of cathode bus bars each of which connects a respective group of at least one of the cathode bar terminals of one cell to the anode beam of the next cell, the cross sections of the individual bus bars being such that, when an equal current flows through each cathode bar,

- then the voltage drop is the same along each bus bar from the respective bar terminal nearest to the anode beam to a point midway along the anode beam.

FIG. 6 shows a resistance substitute circuit diagram calculated from the liquid aluminium of one cell to the middle M of the anode beam of the following cell.

R is the proportional bottom resistance for an iron cathode bar, calculated from the liquid aluminium to the outer end of the cathode bar.

A first cathode bus bar collects the current from n cathode bar terminals. To the commencement of the anode beam of the following cell, it has the resistance R Analogously a second cathode bus bar with its own resistance R collects the current from n terminals, a third cathode bus bar with its own resistance R the current from n;, terminals and so on. R is the resistance of the anode beam of the following cell, calculated to the middle M. of the anode beam.

I is the total cell current.

Each cathode bar should conduct the same current 1 No horizontal current density components occur in the longitudinal direction of the cell in the liquid aluminium if the cross sections of the individual bus bars are so chosen that the voltage drop in each cathode bus bar, from the point of feed to the last iron cathode bar (points A, B, C, etc.) to the middle M of the anode beam of the following cell is the same. In this case in the first bus bar a current 11 1 flows, in the second bus bar a current 11 1 in the third bus bar a current 11 1 and so on. The calculation must take place as if the current I from the anode beam of the following cell were not tapped continuously but at a point exactly in the middle of the cell (point M).

FIG. 5 of the accompanying drawings is a diagrammatic plan of an actual layout. It shows a series of three cells A, B, C. In this example each cell includes three groups D, E, F of iron cathode bars on each side. Each group comprises three iron cathode bars G, H, I and a respective bus bar K. In this example two bus bars K are connected to the left end of the anode beam and one bus bar K to the right end. L denotes the direction of the pot line current.

A complete series comprises from a few cells up to 100 or more. At the first cell of the series the invention is only to be applied to the bus bar connection to the second cell. At the end of the series all bus bars are connected together.

The number of the iron cathode bars depends on the size of the cell, on the current intensity and on several other factors; for example a 100,000 ampere cell can include between and 20 cathode bars (meaning between 10 and 20 protruding ends on each side; often the cathode bars are divided in the middle of the carbon bottom, that is to say that the two halves are disposed in such a way that they have a common axis but do not touch eah other). As to the number of bus bars, there are many possibilities from one bus bar for each cathode bar to only one bus bar for all cathode bars together on each side.

In FIG. 5 the cells are end to end. They may alternatively be side by side.

The anode beam can consist of one or more single anodic bus bars. In the FIGS. 2 and 4 the anode beam 21 consists of two anodic bus bars.

I claim:

1. An electrolytic cell for the production of aluminium by electrolysis of alumina in a melt, comprising:

a pot body of steel,

a layer of thermal and electrical insulation against the inside of the body, a lining of carbon against the inside of the insualtion layer,

the body, the layer and the lining each consisting of a bottom, two side walls and two end walls,

iron cathode bars each having at least a part within the bottom of the lining, a part beneath a side wall of the lining and a part passing through a side wall of the insulation layer and through a side wall of the body,

and anodes arranged to dip into electrolyte in the pot,

wherein the horizontal distance between outer lower edges of the anodes and adjacent Wall surfaces of the lining does not exceed 40 cm.,

the thermal resistance of the walls of the insulation layer is between h. C. h. C.

kcal. kcal.

the parts of the cathode bars beneath the side wall of the lining are surrounded by insulation,

and the ratio of iron cross section to carbon cross section in any vertical plane from end to end of the cell through the bottom of the lining is between 17:100 and 20:100.

2. A plant comprising a plurality of cells according to claim 1 in series, each cell including at least one terminal outside the pot on each cathode bar, and an anode beam carrying the anodes, and electrical connecting means between each cell and the next in the series, each connecting means comprising a plurality of cathode bus bars each of which connects a respective group of at least one of the cathode bar terminals of one cell to the anode beam of the next cell, the cross sections of the individual bus bars being such that, when an equal current flows through each cathode bar, then the voltage drop is the same along each bus bar from the respective bar terminal nearest to the anode beam to a point midway along the anode beam.

0.5)(10 and 1 X10 References Cited UNITED STATES PATENTS 2,786,024 3/1957 Wleugel 204243 R 3,562,136 2/1971 DeVarda et a1. 204243 R 3,607,685 9/1971 Johnson 20467 3,649,480 3/1972 Johnson 204243 R HOWARD S. WILLIAMS, Primary Examiner D. R. VALENTINE, Assistant Examiner US. 01. X.R. 204-244

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