FR2508496A2 - Aluminium production in a Hall-Heroult cell - with removable refractory cathode elements - Google Patents

Abstract

In a Hall-Heroult cell for the production of aluminium the carbon cathode base carries removable cathode elements. These comprise active components, e.g. titanium diboride balls, on an inactive intermediate support, which may be electrically conductive, functioning as described in the main patent. The elements have an effective density greater than that of liquid aluminium, at the operating temperature of the cell. The balls are held in place on the intermediate support by a surrounding rim, allowing damaged or worn elements to be removed and replaced as units. A variety of alternative configurations is described. Easy removal and replacement of the elements, while the cell is operating, is most useful. As no fixings are required, liquid aluminium is allowed free passage around the active elements, and through the inactive support.

Description

REMOVABLE CATHODIC ELEMENTS IN ELECTRICALLY CONDUCTIVE REFRACTORY
FOR THE PRODUCTION OF ALUMINUM BY THE HALL-HEROULT PROCESS
The present invention relates to refractory electroconductive cathode elements and, in particular, titanium diboride elements for the production of aluminum by the Hall-Héroult process. It constitutes an improvement to the cathode elements which are the subject of the main patent application.

In this request, we have described and rev-endiqtté in particular - a process for the production of aluminum using the Hall-Héroult technique
consisting in electrolyzing alumina dissolved in cryolite
fondue, at a temperature of the order of ~ 930 to 9600C, between a system
cathodic comprising a carbonaceous substrate permanently covered by
a layer of liquid aluminum and an anode system comprising at least
minus a carbon anode, characterized by what we say on the sub
carbon cathodic stratum a plurality of titanium boride elements,
not linked to said substrate and not linked to each other, drilling on said sub-
strat a bed of regular thickness, in that we adjust the thickness
of the layer of liquid aluminum to a value at most equal to.
of the bed of elements made of titanium boride and in that the
distance between the plane of the anode system and the upper plane of the bed
of titanium boride elements at a value between 30 and 10
millimeters - a cathode for implementing this process, characterized in
that it includes a carbonaceous substrate using a plurality of elements
titanium diboride, not linked to the substrate and not linked between
them, forming on said substrate a bed of regular thickness.

According to the main patent, this cathode may include an intermediate carbon support placed on the basic carbon substrate, and supporting the bed of particles of titanium diboride.

We have discovered that we can very significantly facilitate the placement and removal of the active elements of titanium diboride, the electrolytic cell being in operation, if these active elements were made integral with the intermediate support, which rests ui - n .eiip gravity without mechanical connection on the basic carbon substrate, this connection between active elements and intermediate support preferably comprising neither collage, neither sealing, nor screwing, mc ?is a simple interlocking suf fisamnent loose to allow a circulation of the liquid aluminum produced between the inert intermediate support and the active electroconductive refractory elements.

The object of the present application is therefore removable cathode elements comprising an inert intermediate support and active elements in electroconductive refractory, such as TiB2, integral but separable from said support, the assembly formed by the inert intermediate support and the active elements having a density greater than the density of liquid aluminum at the temperature of electrolysis.

In all that follows, we will designate by a) Basic carbon substrate: the electrically conductive carbon coating separating the metal or the bath, thermal insulating refractories lining the tank, this coating being able to be formed of precooked blocks or of a mass- carbonEe cooked in place and having horizontal or vertical metal bars sealed in this carbon coating, to ensure the collection and evacuation from the tank of cathode electric current, according to the usual structure of tanks for the production of aluminum by the process Hall-Héroult.

b) Inert intermediate support: an intermediate body resting by gravity on the basic carbon substrate and supporting the active cathode elements. The term inert means that this intermediate support does not have the main function of serving as a cathode for the electrochemical deposition of metallic aluminum.

c) Active cathode elements: elements made of an electroconductive refractory body, such as titanium diboride, the main purpose of which is to serve as an active cathode for the electrochemical deposition of metallic aluminum from the electrolysis bath. Their unit weight is generally less than about 1,000 grams.

d) Titanium boride (or titanium boride): this is a general term covering all electroconductive mixtures containing at least 90% of pure titanium diboride TiB2
Figures 1 to 11 show different embodiments of the invention.

Figure 1 recalls, on the right part of the line AA ', the structure of a conventional electrolysis tank of which only the essential elements are referenced: the metal casing (1), the refractory lining (2), the lining carbon (3), the carbon cathode (4), the cathode bars (5), the sheet of liquid aluminum (6), the electrolyte (7), the frozen electrolyte crust (8), the anodes (9 ), the alumina feeder (10) and, on the left side of the line
AA ', the structure of a modified tank, according to the main patent, by introduction of cathode elements made of TiB2 (11), with the central collector (12) in which the liquid aluminum collects. The level of the liquid aluminum sheet is adjusted by the position of the flow channel (13).

Figure 2 recalls one of the modes of implementation of the main patent.

The cathode elements (14) made of TiB2 - here, balls - are here simply placed in several layers on a carbon intermediate support (15) placed itself on the carbon cathode (4). The support (15) has orifices (16) opening into channels (17) to collect the liquid aluminum which collects in the central collector (12) of FIG. 1.

FIG. 3, which relates to the invention which is the subject of this Addition Certificate, represents a first mode of implementation. The cathode elements (14) - here balls - are integral with an intermediate support (15), the upper part of which has the shape of a basket comprising peripheral flanges (18). The term "united" should be interpreted, throughout this presentation, as meaning: not being able to separate spontaneously, and forming a whole, without there being a permanent connection, so as to allow disassembly without damaging either the substrate nor the TiB2 elements.

This arrangement makes it possible, in fact, to prepare the intermediate support (15), to place therein the elements (14) made of TiB2, possibly to preheat them then to introduce them into the electrolysis tank in operation and, even, d '' remove the assembly, for example, to renew the worn or damaged elements (14), or the inert intermediate support itself if it turns out that it has been damaged or has worn out faster than the active elements. The substrate may, for this purpose, be provided with any handling means known from the fall of art.

FIG. 4 represents a variant, in which the intermediate support is a plate (19) pierced with holes (20), the cathode elements (21) made of TiB2 are studs with a substantially flat head (22) and a vertical tail (23 ). When the base of the vertical tail (23) does not come into contact with the basic carbon substrate (4), the underside of the head preferably carries peripheral bosses (24), three for example, or fins or radial ribs (25), and sufficient clearance has been provided between the tail (23) and the hole (20) to ensure the flow of liquid aluminum.

It can also be provided that the tail of such elements comes into contact, by gravity, with the basic carbon substrate: in this case, the grid or the plate pierced with holes serving as an intermediate support will have a simple role of lateral support and it is either the tail of the active elements, or the holes of the support grid which will advantageously be provided with fins (25) or bosses (26) ensuring this lateral guidance. The height of this intermediate support will be less than the length of the tail of the active TiB2 elements.

The shape of the studs is susceptible of several variants, for example stud with circular head and cylindrical tail, and three support bosses (FIG. 5A), square head, parallelepipedic tail and four support bosses (FIG. 5B), or circular head and cylindrical tail with support ribs (Figure 5C), these examples being given without limitation.

it is obvious that the assembly of the intermediate support and of the TiB2 elements that it supports must have a density greater than that of liquid aluminum at working temperature (930-9600C). These densities are of the order of 1.8 to 2 for a graphite support, 4 to 4, s ç-'our the TiB2 elements and 2.3 for the aluminitm at 9500C.

Those skilled in the art will be able to determine by a simple calculation the necessary relationships between the volume of TiB2 and the volume of, -> phite.

In the case of FIG. 4, if the vertical tail (23) of the studs (21) is long enough, and if the cn ensures that the free space (27) under the perforated slab (19) remains permanently occupied by
liquid produced, the passage of the electrolysis current takes place directly through the studs towards the liquid aluminum produced, and the intermediate support can be made of a material which is little or not conductive provided that it resists the action of electrolyte and liquid aluminum (certain aluminous refractories or carbonitride, or silicon oxynitride for example).

Figures 6, 7 and 8 show three other variants, relating to the same principle: the TiB2 elements (28, 29, 30) are threaded on kinds of rails (31, 32, 33), which are integrated into the intermediate support ( 34).

 As in the case of the previous figures, the intermediate support can be made of a material that is not very conductive or not if the level of the sheet of liquid aluminum is such that it remains in permanent contact with the TiR1 elements. it must be made of conductive material (most often carbon).

FIG. 9 represents a last variant, always according to the same principle: the elements of TiR2 (35), of annular shape, are threaded with a large clearance on rods (36) of substantially horizontal axis, the ends of which are engaged in recesses (37) provided in the main peripheral frame (38) forming an intermediate support.

it is not necessary that these rods and the main peripheral frame are electrically conductive, since provision has been made for the titanium diboride rings to cross the interface between the sheet of metal and the bath.

The recesses of the main frame can be blind so as to avoid longitudinal displacement of the rods supporting the active TiB elements.

In all cases, the inertial intermediate substrate comes to rest on the flat surface of the carbonaceous cathode blocks (4) with which it ensures, if necessary, the electrical contact for the passage of the electrolysis current.

The fitting or removal of the lined supports can be carried out using pliers handled by the same service machine as the anodes. This clamp has a vertical rod urie section similar to that of an anode rod. Slow vertical removal or removal of the trimmed supports is accomplished by applying the pliers rod against the spider and guiding it using the anode rod connector. It is thus ensured that the trimmed supports will be positioned very exactly at the right of the anodes and can be taken up, if necessary, without particular difficulties, their position being perfectly defined.

Finally, it is possible to protect all of the cathode elements inert support + active elements made of TiB2 - against the accidental fall of an anode, by providing bosses (39) or prominent rails on the upper face of the intermediate support (19). the upper level slightly exceeds (by a few millimeters to about ten millimeters), the upper level of the active elements of TiB2, these protruding bosses or rails being situated vertically with feet or intermediate walls (40) of the support resting on the carbon-containing substrate. base (4).

Another solution consists in having vertical studs (41) of a refractory body denser than liquid aluminum, resistant well to the electrolysis bath and to liquid aluminum, resistant well to mechanical shocks and to compression. -1000 C, the base of these studs resting by gravity on the basic carbon substrate and their upper face being closer to the anode than the upper face of the active TiB2 elements, these studs passing through the intermediate support and being arranged so te1 le that at least three of them possibly belonging to several different intermediate supports, are located under each anode, so that at least one polygon formed by the studs located under the same anode, contains the vertical projection of the center of gravity of the base of the anode.

Claims (8)

CLAIMS '10 / - Removable cathode elements, the active element of which is an electroconductive refractory, for implementing the process for the production of aluminum by electrolysis, according to claim 1 of the main patent, and intended to be placed on the carbon cathode , characterized in that they comprise an inert intermediate support and active elements in electrically conductive refractory and, in particular, in titanium diboride, integral but separable from said support, the assembly formed by the inert intermediate support and the active elements having a density greater than the density of liquid aluminum at the temperature of electrolysis. 20 / - Removable cathode elements, according to claim 1, characterized in that the inert intermediate support is electrically conductive. 30 / - Removable cathode elements, according to claim 1, characterized in that the inert intermediate support has an electrical conductivity substantially zero or negligible compared to that of liquid aluminum and titanium diboride. 40 / - Removable cathode elements, according to claim 1, characterized in that the inert intermediate support is provided with a peripheral rim which retains the active elements. 50 / - Removable cathode elements, according to claim 1, characterized in that the active elements are threaded on a profile or a rod with a substantially horizontal axis.  6 / - Removable cathode elements, according to claim 5, characterized in that the shape of the active elements and the shape of the profile cooperate so that the active elements on the profile can only be engaged or removed by a translation along the 'axis of the profile. 70 / - Removable cathode elements, according to claim 1, characterized in that the active elements are formed of a head whose upper face is substantially planar and of a tail perpendicular to the head, and in that the inert intermediate support comprises a plurality of orifices with a vertical axis into which the tail of the active elements can be introduced with sufficient clearance to allow the passage of liquid aluminum = 80 / - Removable cathode elements according to claim 7, characterized in that the underside the head of the active elements is provided with a plurality of bosses which come to bear on the inert substrate on the periphery of the orifices of vertical axis. 90 / - Removable cathode elements, according to claim 7, characterized in that the tail of the active elements is provided with a plurality of radial fins over at least part of their length at the level of the upper plate of the intermediate support, so to ensure their centering in the vertical axis hole. 100 / - Removable catllodic elements according to claim 7, characterized in that the vertical axis orifice of the inert intermediate support is provided with a plurality of radial fins to ensure the centering of the tail of the active elements. 110 / - Removable cathode elements, according to any one of claims i to 10, characterized in that the inert intermediate support comprises, on its upper face, protruding bosses or rails, the upper level of which slightly exceeds the active elements in TiB2, these protruding bosses or rails being situated vertically with feet or intermediate walls of the support resting on the basic carbon substrate.  120 / - Removable cathode elements, according to any one of claims 1 to 10, characterized in that they comprise vertical studs of a refractory body denser than liquid aluminum, resistant well to the electrolysis bath and to liquid aluminum, resistant well to mechanical shock and compression up to at least 1000 C, the base of these studs resting by gravity on the basic carbonaceous substrate, and their upper face being closer to the anode than the upper face of the active elements of TiB2, these studs passing through the inert intermediate support and being arranged in such a way that at least three of them, possibly belonging to several different inert intermediate supports, are located under each anode, so that 'at least one polygon, formed by the studs located under the same anode, contains the vertical projection of the base center of gravity of the anode.