GB2065174A

GB2065174A - Cathodes for electrolytic furnaces

Info

Publication number: GB2065174A
Application number: GB8038942A
Authority: GB
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1979-12-05
Filing date: 1980-12-04
Publication date: 1981-06-24
Also published as: FR2471425A1; JPS5693888A; DE3041680A1; FR2471425B1; AU6465080A; AU538292B2; DE3041680C2; GB2065174B; CH643600A5

Abstract

The invention relates to cathodes of electrolytic furnaces, for use in the production of metals, in particular aluminium, where the cathode is formed by a liquid metal (14). The liquid metal (14) contains, apart from an uppermost freely movable layer ("a") of at least 2 mm, a bed (16) of particulate material, which rests on a floor (10) of the furnace and is inert and solid at operating temperatures of the furnace. As a result, the movement of the liquid metal (14) is suppressed or retarded, and the interpolar distance ("d") to the anode (22) can be lowered to 10 to 25mm. The bed in the liquid metal can be subdivided into smaller regions by dividing walls (20), and formed of TiB2, TiC, TiN, ZrB2, ZrC and/or ZrN, or SiN bonded SiC or SiON. <IMAGE>

Description

SPECIFICATION Cathodes for electrolytic furnaces The present invention relates to electrolytic furnaces, for use in the production of metals, in particular aluminium, and relates more specifically to cathodes for use in such electrolytic furnaces.

The electrolytic production of aluminium from aluminium oxide involves dissolving the latter in a fluoride melt made up, for the greater part, of cryolite. The cathodically deposited aluminium collects under the melt, on a carbon floor of the furnace, and the upper surface of the liquid aluminium forms the cathode. Dipping into the melt from above, through a crust of the melt, are anodes which, in conventional processes, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminium oxide, oxygen is formed at the carbon anodes; this combines with the carbon of the anodes and forms CO2 and CO. The electrolytic process takes place at about 940-9700C. During the process, the electrolyte becomes depleted in aluminium oxide.At a lower concentration of 1 to 2 wt % aluminium oxide in the electrolyte, the so-called anode effect occurs whereby a sudden increase in voltage from e.g. 4 to 4.5V takes place up to 30V and more. Then, at the latest, the crust has to be broken open and the aluminium oxide concentration raised by adding new aluminium oxide (alumina) to the furnace.

Under normal conditions the furnace is usually fed with alumina, at regular intervals, even if no anode effect occurs, and this is done by breaking open the crust and adding alumina to the furnace.

It is known that at high current levels e.g. above 50 kA (kilo ampere), the combined effect of vertical components of magnetic field and horizontal components of current can lead to an undesirable deformation of the upper surface of the liquid aluminium and can cause pronounced streaming of the aluminium. When the interpolar distance is small, these undesirable deformations can become so great that the aluminium touches the anodes, producing short circuiting.

Furthermore, the turbulence at the upper surface of the aluminium due to this upward doming leads to increased chemical dissolution of the aluminium in the molten electrolyte, causing a cloud of finely dispersed aluminium to form which, as is known, results in reduced current efficiency.

It is therefore impossible to operate with an interpolar distance below a certain critical limit.

On the other hand, the loss of electrical energy is larger the greater the interpolar distance for the same current density. In principle reducing the current density would be advantageous, however this would require larger capital costs for the furnace and the smelter building i.e. costs which are out of proportion to the benefit and are therefore unacceptable.

Various measures and constructions aimed at reducing the vertical components of the magnetic field and the horizontal components of the current are known. Also known are cathode constructions which are wet by aluminium and feature only a thin aluminium layer which can be moved only slightly in the direction vertical to the cathode, and therefore to a large extent eliminate the classical surface deformations i.e. both stationary doming and waves. These wettable materials are, however, very expensive and have yet to prove they have long service lives. The greatest disadvantage of these arrangements is, however, that circulation of the electrolyte between anode and cathode is made more difficult, as a result of which the cryolite melt is depleted in alumina by the precipitation of aluminium, and the furnace becomes susceptible to the anode effect.

According to US patent 4071 420, circulation of the cryolite melt is improved by having cathode elements in the form of wettable pipes, which are closed at the bottom and which project, in the region of the anodes, out of the liquid aluminium which lies on the rest of the furnace floor. The pipes are completely full of aluminium, and the interpolar distance can be kept small. The new metal formed by the electrolytic process then flows into a sump of aluminium at a lower level.

Between the aluminium in the closed pipes and the conductive furnace floor there must be an electrical connection e.g. the pipes may be made of an electrically conductive material.

Apart from the difficult and expensive production of the wettable pipes, this arrangement is effective only when the aluminium surfaces facing the anodes are small. This means that the ratio of the wettable material to the cathodic functioning surface is high and there is therefore no cost saving compared with other cathodes made of wettable materials.

It is therefore an object of the present invention to develop a cathode for an electrolytic furnace, for use in the production of metals, in particular aluminium, which immobilises the cathodic acting surface of the liquid metal at a much more favourable ratio of capitai costs for the cathode construction to the cathodic acting surface.

According to the present invention an electrolytic furnace, for use in the production of metals, in particular aluminium, comprises a cathode formed by a liquid metal which is separated from an anode of the furnace by an interpolar distance of 10 to 25 mm, and which, apart from an uppermost freely movable layer of at least 2 mm, contains a bed of particulate material resting on a floor of the furnace, the particulate material of the bed being inert and solid at operating temperatures of the furnace.

It is essential that the bed of particulate material never projects out of the liquid metal into the molten electrolyte. The uppermost layer of the liquid metal above the bed of particulate material is, however, preferably only 2 to 3 mm thick.

In general at least one tapping hole not covered by the bed is provided to allow the liquid metal to be taken from the furnace. The particulate material of the bed can be added by means of a manipulator or other servicing or replenishing vehicle.

The particle size of the particulate bed material can be between 0.1 and 100 mm. This particle size must however in any case be less than half of the thickness of the bed, which in general lies between 10 and 100 mm. The bed is however preferably 10 to 50 mm thick. The liquid metal penetrates the spaces or pores in the bed and fills them. The movement of the metal is thus, apart from the uppermost free layer, retarded mechanically. No waves which would be harmful to the operation of the furnace can then form in the metal; instead, this is prevented or retarded by an inexpensive material situated below the metal surface.Such materials can, for example, be: metallic, conductive materials which are wet well by aluminium, such as TiB2, TiC, TiN, ZrB2, ZrC and/or ZrN; or aluminium resistant materials which do not conduct electricity well and have a higher specific weight than aluminium, such as silicon nitride bonded silicon carbide or siliconoxynitride.

The particulate bed material in the liquid metal can spread uniformly over the whole of the furnace floor. However, fence-like or weir-like dividing walls can also be provided, projecting up close to the upper surface of the bed i.e. the surface facing the anodes. This causes the bed to be divided into regions so that movement in the horizontal direction is limited by the dividing walls.

To facilitate removal of the liquid metal from the furnace the dividing walls may be intermittently open.

The walls dividing up the bed can be made of materials which are wet to a greater or lesser degree by aluminium (e.g. silicon nitride bonded silicon carbide or carbon), and can be electrically conductive or non-conductive. It is important, however, that the dividing walls are resistant to dissolution and erosion by the liquid metal at the operating temperatures of the furnace.

In order to reduce the horizontal component of the electric current, those parts of the furnace floor in contact with the liquid metal, and lying outside a vertical projection of each of the anodes, can be covered with a coating formed of material which is compatible with aluminium and which is a poor electrical conductor. The coating functions in that the electrical current is led only in the regions of the liquid metal lying directly vertically beneath the anodes. Suitable materials for the coating are silicon nitride, silicon carbide, silicon nitride bonded silicon carbide or silicon-oxynitride.

Two electrolytic furnaces in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical section through part of a furnace with current supply from below; and Figure 2 is a vertical section through another furnace with current supply at the sides.

Figure 1 shows a layer of electrolytically deposited liquid aluminium 14, which forms the cathode of the furnace and lies on a carbon floor 10 in which a cathode bar 12 is embedded.

Likewise on the carbon floor 10 is a bed 16 of solid particulate material, the upper surface of which is a distance "a" of 2 to 3 mm from the upper surface of the liquid aluminium. This distance "a" is very important as the bed 16 must not project up into electrolyte 18. The bed 16 is permeated by the liquid aluminium 14, with the particulate material of the bed 16 being inert and solid at the operating temperatures of the furnace.

The bed 16 is divided into regions by dividing walls 20 which reach almost up to the upper surface of the bed 16.

Immobilising the cathodic acting surface of the aluminium by the bed of particulate material makes it possible to lower interpolar distance "d" between the liquid aluminium 14 and the anode 22 to lotto 25 mm.

The electrolytic furnace shown in Figure 2 has electric current fed to the sides of a pot 24 which may be made of carbon or, advantageously, be made of concrete. The pot 24 is penetrated at the sides by cathodic current supplying elements 26, e.g. of aluminium or copper, carrying electrically conductive elements 28 made of material e.g. TiB2 which is resistant to the liquid aluminium 14. As a result of the current supply from the sides, the voltage drop can be reduced and the service life of the pot increased, especially when this is made of concrete. Furthermore, this feeding from the side permits higher current densities which in the upper, free layer of the liquid aluminium 14 results in higher streaming rates.

Due to the inclination of furnace floor 30, the metal precipitated out in the process flows to a tapping hole (not shown) which is not covered with particulate bed material.

Claims

1. An electrolytic furnace, for use in the production of metals, in particular aluminium, comprising a cathode formed by a liquid metal which is separated from an anode of the furnace by an interpolar distance of 10 to 25 mm, and which, apart from an uppermost freely movable layer of at least 2 mm, contains a bed of particulate material resting on a floor of the furnace, the particulate material of the bed being; inert and solid at operating temperatures of the furnace.

2. An electrolytic furnace according to claim t in which the thickness of the bed is between 10 and 100 mm.

3. An electrolytic furnace according to claim 2, in which the thickness of the bed is between 10 and 50 mm.

4. An electrolytic furnace according to any preceding claim, in which the particle size is between 0.1 and 100 mm, for the particulate material of the bed, and is at most less than half of the thickness of the bed.

5. An electrolytic furnace according to any one of claims 1 to 4, in which the bed is formed of material which is metallic, conductive and wet well by aluminium.

6. An electrolytic furnace according to claim 5, in which the bed is formed of TiB2, TiC, TiN, ZrB2, ZrC and/or ZrN.

7. An electrolytic furnace according to any one of claims 1 to 4, in which the bed is formed of material which is resistant to aluminium, has a low electrical conductivity and has a higher specific weight than aluminium.

8. An electrolytic furnace according to claim 7, in which the bed is formed of silicon nitride bonded silicon carbide or silicon-oxynitride.

9. An electrolytic furnace according to any preceding claim, in which the bed is divided into regions by fence-like or weir-like walls which extend towards the upper surface of the bed.

10. An electrolytic furnace according to any preceding claim, in which the floor of the furnace in contact with the liquid metal, and outside a vertical projection of the anode, has a coating formed of material which has a low electrical conductivity and is compatible with aluminium.

1 An electrolytic furnace according to claim 10, in which the coating is formed of silicon nitride, silicon carbide, silicon nitride bonded silicon carbide or silicon-oxynitride.

12. An electrolytic furnace according to any preceding claim, in which the uppermost freely movable layer of the liquid metal is 2 to 3 mm thick.

13. An electrolytic furnace according to any preceding claim, in which there are a plurality of the anodes and the floor slopes.

14. An electrolytic furnace according to claim and substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.