GB2103657A

GB2103657A - Electrolytic cell for the production of aluminium

Info

Publication number: GB2103657A
Application number: GB08220410A
Authority: GB
Inventors: Daniel Baruch Miller
Original assignee: British Aluminum Co Ltd
Current assignee: British Aluminum Co Ltd
Priority date: 1981-07-18
Filing date: 1982-07-14
Publication date: 1983-02-23

Abstract

An electrolytic reduction cell for the production of aluminium having metallic side walls 12 with a refractory material lining in contact therewith, a cathode 13 disposed in the lower part of the cell and at least one anode 3 suspended above the cathode in spaced relationship thereto so that in operation with the cell containing a liquid flux 5 incorporating alumina and constituting an electrolyte in contact with the anode and the cathode, a pad 7 of liquid metal is maintained on the cathode with an interface between the electrolyte and the pad is characterised in that the lower part 15 of the lining has a different thermal conductivity from the upper part 20 of the lining, the change in thermal conductivity occurring at the region of the interface so that, in use, a frozen crust on the surface of the electrolyte extends downwardly and inwardly of the lining over the entire surface thereof. Erosion of the lining by molten aluminum is thereby prevented and the pad of molten metal more nearly conforms with the shadow of the anodes. Lining blocks 20 may have a thermal conductivity one third of blocks 15. Further rammed paste or block material 21 may be disposed between the lower part of each block 15 and the cathode block 13 and the lining may be further protected by rammed paste material 22. <IMAGE>

Description

SPECIFICATION Electrolytic reduction cell This invention relates to electrolytic reduction cells for the production of aluminium and to a method of operating such cells.

Aluminium is typically produced by the Hall-Heroult electrolytic reduction process wherein 1 to 10% aluminium oxide (alumina) dissolved in a bath containing a main component of cryolite (flux) is electrolysed at a temperature of from 900cm to 1 0000C. The process is usually conducted in a reduction cell which typically comprises an open topped relatively shallow steel shell the base of which contains a layer of refractory or other insulating material on top of which rests a lining of carbon material through which are embedded one or more steel conductor bars connected to a negative source of direct current. The said carbon lining forms the cathode of an electrolytic cell. The side walls of the steel shell are typically lined with a carbon material and the total carbon lining contains the liquid melt.

Many detailed proposals have been made for the precise construction of cell lining and many types of materials for example, carbon have been used for lining the walls of the shell. Such linings have included carbon blocks and rammed paste formed from particulate carbon material and a pitch or other carbon based binder capable of being baked to drive off volatile components. The choice of lining materials has been influenced by a number of factors including simplicity and cheapness of construction; longevity; ease of replacement and acceptable electrical and thermal efficiency of the cell.

One or more carbon anodes connected to the positive pole of a direct current supply are suspended in the melt. Upon application of current the alumina is continuously reduced to aluminium metal which collects on the cathodic carbon floor and is generally discharged periodically. A shallow pad or pool of molten aluminium is maintained on the carbon floor and functions as the active cathodic surface. A means of continuously or intermittantly replenishing the alumina consumed in the electrolytic process is always provided for in the design of the cell.

During operation the bath in contact with the air and with the cooier parts of the carbon lining freezes and forms a crust of solidified bath which seals the top surface between the anodes and between the anodes and the sidelining, and also usually extends downwardly from the top of the side walls of the cell interposing between the sidelining and the liquid flux and sometimes also upwardly from the bottom of these side walls interposing between the sidelining and the metal pad. The existence of and the thickness of this freeze on any part of the side walls is a function of the dynamic thermal equilibrium conditions set up at that particular point of the sidewalls.

The relationship between the many parameters involved in both the electrical and thermal efficiency of the cell are very complex but it is known to be desirable to reduce so far as possible, horizontal electrical currents both within the molten pad and between the pad and the electrolyte; to reduce the thickness of the side wall lining to enable anodes of larger cross section to be used but to protect the side wall lining in order to increase cathode life.

The most important cause of erosion of the side wall lining is contact by the edges of the molten metal pad during several thousand hours of continuous cell operation. An object of the present invention is to provide a lining which modifies the thermal balance of the cell so that the pad of molten metal more nearly conforms with the shadow of the anodes. A further object is to provide an improved lining.

According to one aspect of the invention there is provided an electrolytic reduction cell for the production of aluminium having metallic side walls with a refractory material lining in contact therewith, a cathode disposed in the lower part of the cell and at least one anode suspended above the cathode in spaced relationship thereto so that in operation with the cell containing a liquid flux incorporating alumina and constituting an electrolyte in contact with the anode and the cathode, a pad of liquid metal is maintained on the cathode with an interface between the electrolyte and the pad characterised in that the lower part of the lining has a different thermal conductivity from the upper part of the lining, the change in thermal conductivity occurring at the region of the interface so that, in use, a frozen crust on the surface of the electrolyte extends downwardly and inwardly of the lining over the entire surface thereof. The lower part of the lining may have greater thermal conductivity than the upper part.

Preferably the lower part of the lining is separated from a cathode within the cell by further refractory material having a different thermal conductivity from said lower part so that in use said crust extends further inwardly of the lining from the lower part of the side wall towards the edge of the anode shadow.

The further refractory material may have a thermal conductivity lower than that of said lower part.

The junction between the upper and lower parts is preferably stepped so that the surface of the lower part remote from the side wall is higher than the surface adjacent the side wall.

The present invention also provides a method of operating an electrolytic reduction cell for the production of aluminium comprising varying the thermal conductivity of the wall lining of the cell between the upper and lower parts thereof to create an unbroken frozen crust from the upper surface of the electrolyte downwardly over the lining.

The above and other aspects of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows diagrammatically in vertical section part of a cell with unacceptable molten metal pad configuration.

Fig. 2 is a similar view showing a desirable configuration, and Fig. 3 shows, to a larger scale, a cell wall lining according to the present invention.

Referring to Fig. 1 this shows diagrammatically part of an aluminium reduction cell 1 having a cathode 2 and an anode 3. In operation a frozen crust 4 of flux and alumina forms over the electrolyte 5.

The freeze line 6 of this crust is frequently of such shape and disposition that electrolyte and the molten aluminium pad 7 are in direct contact with the cell wall 8. The chain line 9 indicates the kind of erosion which is commonplace. The freeze line 6 has been shown to indicate a further quantity of frozen material 9 in the bottom corner of the cell. This does not always occur in which case cell wall erosion is increased.

Fig. 2 shows a desired arrangement in which the freeze line 6 occurs wholly inwardly of the cell wall 8 and the pad 7 lies more nearly only in the shadow of the anode 3.

Fig. 3 shows a cell construction which enables the freeze line configuration of Fig. 2 to be achieved without altering any of the operating parameters of the cell that would otherwise produce the undesirable result of Fig. 1.

In Fig. 3 the cell comprises a steel shell 10 having a base 11 and the side wall 12. A cathode block 13 is disposed in the cell on a refractory layer 14 such as alumina or insulating brick all in conventional fashion.

The side wall lining comprises blocks 1 5 resting on the layer 14 and having their outer surfaces 1 6 bonded to the wall 12 with a mortar (not shown). The upper part of each block 1 5 has a horizontal face 17, an upwardly and inwardly inclined face 18 and a horizontal face 19. The stepped upper part of each block 1 5 is desirable but not essential. The lining 20 for the upper part of the wall 12 is of blocks or cold rammed paste. If blocks are used to form the lining 20 then these are also bonded to the wall 12 by a suitable mortar (not shown).The mortars used for the blocks 15 and lining 20 may have thermal conductivities approximately to that of the respective blocks but this is not important since the mortar layer is sufficiently thin that for purposes of thermal conductivity considerations the blocks 1 5 and 20 are in contact with the wall 12. The lining 20 has a thermal conductivity of about one third that of the blocks 1 5. The particular ratio of thermal conductivities may be chosen, for example, to be approximately the same as the ratio of heat transfer coefficients between liquid aluminium and the crust and that between the liquid bath and the crust.Further rammed paste or block material 21 is disposed between the lower part of each block 1 5 and the anode block 13 and the wall lining above described may be further protected by rammed paste material 22 which extends downwardly of the wall lining and across the top of the material 21.

The block 1 5 may be of electrically calcined carbon or graphite with a pitch binder, silicon carbide blocks or any similar refractory with the requisite thermal properties. The material 20 may be water or pitch based carbonaceous ramming material. It can also be carbon or other refractory block. The mortar may be a water based or pitch based mortar or a silicon carbide cement paste of graphite or other particulate carbon. The material 21 may be selected from the same materials as the material 20 and the material 22 may be a hot or cold worked carbonaceous ramming material of silicon carbide mortar.

It has been found desirable that the inner face 23 of the block 1 5 extends upwardly from the upper surface of the material 21 approximately two thirds the height of the side-lining. While the face 1 8 has been shown as sloping it could be stepped.

The following examples relate to a cell having an operating depth of approximately 24 inches and a cell wall lining thickness of about 8 inches.

EXAMPLE 1 Thermal Conductivity Reference No. Material (Wm-'K-1)

21 Rammed carbon 3-4 22 Rammed carbon 20 Rammed carbon 8-10 1 5 Electrically calcined Anthracite 25-30 In this example the rammed carbon material 22 did not extend over the upper surface of the material 21 and in normal cell operating conditions an unbroken freeze extended over the entire vertical surface of the side wall lining having a minimum thickness of 5 inches. At the bottom of the cell the freeze extended from the side wall lining to a position approximately to the edge of the anode shadow.

EXAMPLE 2 Thermal Conductivity Reference No. Material (Wm-'K-') 21 Rammed carbon 8-10

22 Rammed carbon 8-10 20 Rammed carbon J 1 5 Silicone carbide block in two courses 22 In this example the material 22 extended over the upper surface of the material 21 and in normal operating conditions a generally similar freeze developed. However a thicker layer of freeze developed on the bottom of the cell with its inner edge slightly inwardly of the edge of the anode shadow and the minimum thickness of the freeze on the side wall occurred at a high position than in example 1.

EXAMPLE 3 as example 1 but 21 replaced by carbon block, thermal conductivity 12 Wm-'K-1 In general:- Ref. (15) has range 15--100 Wm-'K-' Ref. (21) has range 3-20 Ref. (22) has range 3-20 Ref. (20) has range 3-20 In general it is desirable that the thickness of the freeze along the side wall lining should be a minimum of 1 inch and it is apparent from the above examples give a considerable amount of flexibility in design to accommodate widely varying cell dimensions and operating conditions.

As described above it has been assumed that the lower part of the sidewall lining should have a thermal conductivity greater than that of the upper part. This is in order to correct a situation in which the freeze line 6 (Fig. 1) would otherwise extend into the sidewall.

It will be understood however that the opposite problem may occur, that is to say, that there is too much freeze in the area of the molten metal pad so that the latter has an area significantaly less than the shadow of the anode. In such event it would be desirable to arrange that the lower part of the sidewall lining has a thermal conductivity less than that of the upper part.

Claims

1. An electrolytic reduction cell for the production of aluminium having metallic side walls with a refractory material lining in contact therewith, a cathode disposed in the lower part of the cell and at least one anode suspended above the cathode in spaced relationship thereto so that in operation with the cell containing a liquid flux incorporating alumina and constituting an electrolyte in contact with the anode and the cathode, a pad of liquid metal is maintained on the cathode with an interface between the electrolyte and the pad characterised in that the lower part of the lining has a different thermal conductivity from the upper part of the lining, the change in thermal conductivity occurring at the region of the interface so that, in use, a frozen crust on the surface of the electrolyte extends downwardly and inwardly of the lining over the entire surface thereof.

2. A cell according to claim 1 in which the lower part of the lining has greater thermal conductivity than the upper part.

3. A cell according to claim 1 or claim 2 in which the lower part of the lining is separated from the cathode within the cell by further refractory material having a different thermal conductivity from said lower part so that in use said crust extends further inwardly of the lining from the lower part of the side wall towards the edge of the anode shadow.

4. A cell according to claim 3 in which the further refractory material has a thermal conductivity lower than that of said lower part.

5. A cell according to any one of the preceding claims in which the junction between the upper and lower parts is stepped so that the surface of the lower part remote from the side wall is higher than the surface adjacent the side wall.

6. A method of operating an electrolytic reduction cell for the production of aluminium comprising varying the thermal conductivity of the wall lining of the cell between the upper and lower parts thereof to create an unbroken frozen crust from the upper surface of the electrolyte downwardly over the lining.