Cathode protection

A method of operating an aluminum smelting cell during the start-up phase of the cell is described. The method includes forming a layer of boron oxide on the exposed surface of the cathode of the cell, forming a layer of aluminum on the boron oxide layer, and starting the cell. This melts the boron oxide layer to form a barrier impervious to oxygen at a temperature from about 400.degree. C. to about 650.degree. C., and the aluminum layer is melted to form a barrier to oxygen at temperature above about 600.degree. C. to about 1000.degree. C. to reduce the development of oxidation products.

FIELD OF THE INVENTION 
This invention relates to the protection of refractory hard material 
cathodes used in aluminum smelting cells and to aluminium smelting systems 
incorporating such protected cathodes. 
BACKGROUND OF THE INVENTION 
In conventional designs for the Hall-Heroult cell, the molten aluminium 
pool or pad formed during electrolysis itself acts as part of the cathode 
system. The life span of the carbon lining or cathode material may average 
three to eight years, but may be shorter under adverse conditions. The 
deterioration of the carbon lining material is due to erosion and 
penetration of electrolyte and liquid aluminium as well as intercalation 
by metallic sodium, which causes swelling and deformation of the carbon 
blocks and ramming mix. Penetration of cryolite through the carbon body 
has caused heaving of the cathode blocks. Aluminium penetration to the 
iron cathode bars results in excessive iron content in the aluminium 
metal, or in more serious cases, a tap-out. 
Another serious drawback of the carbon cathode is its non-wetting by 
aluminum, necessitating the maintenance of a substantial height of pool or 
pad of metal in order to ensure an effective molten aluminum contact over 
the cathode surface. In conventional cell designs, a deep metal pad 
promotes the accumulation of undissolved material (sludge or muck) which 
forms insulating regions on the carbon cathode surface. Another problem of 
maintaining such an aluminium pool is that electromagnetic forces create 
movements and standing waves in the molten aluminium. To avoid shorting 
between the metal and the anode, the anode-to-cathode distance (ACD) must 
be kept at a safe 4 to 6 cm in most designs. For any given cell 
installation, where is a minimum ACD below which there is a serious loss 
of current efficiency, due to shorting of the metal (aluminium) pad to the 
anode, resulting from instability of the metal pad, combined with 
increased back reaction under highly stirred conditions. The electrical 
resistance of the inter-electrode distance traversed by the current 
through the electrolyte causes a voltage drop in the range of 1.4. to 2.7 
volts, which represents from 30 to 60 percent of the voltage drop in a 
cell, and is the largest single voltage drop in a given cell. 
To reduce the ACD and associated voltage drop, extensive research using 
Refractory Hard Materials (RHM). such as titanium diboride (TiB.sub.2), as 
cathode materials has been carried out since the 1950's. Because titanium 
diboride and similar Refractory Hard Materials which are wetted by 
aluminium, resist the corrosive environment of a reduction cell, and are 
excellent electrical conductors, numerous cell designs utilizing 
Refractory Hard Materials have been proposed in an attempt to save energy, 
in part by reducing anode-to-cathode distance. 
The use of titanium diboride current-conducting elements in electrolytic 
cells for the production or refining of aluminum is described in the 
following exemplary U.S. patents: U.S. Pat. Nos. 2,915,442, 3,028,324, 
3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061. 
Despite the rather extensive effort expended in the past, as indicated by 
those and other patents, and the potential advantages of the use of 
titanium diboride as a current-conducting element, such compositions have 
not been commercially adopted on any significant scale by the aluminium 
industry. 
Lack of acceptance of TiB.sub.2 or RHM current-conducting elements of the 
prior art is related to their lack of stability in service in electrolytic 
reduction cells. It has been reported that such current-conducting 
elements fail after relatively short periods in service. Such failure has 
been associated with the penetration of the self-bonded RHM structure by 
the electrolyte, and/or aluminium, thereby causing critical weakening with 
consequent cracking and failure. It is well known that liquid phases 
penetrating the grain boundaries of solids can have undesirable effects. 
For example, RHM tiles wherein oxygen impurities tend to segregate along 
grain boundaries are susceptible to rapid attack by aluminium metal and/or 
cryolite bath. Prior art techniques to combat TiB.sub.2 tile 
disintegration in aluminium cells have been to use highly refined 
TiB.sub.2 powder to make the tile, where commercially pure TiB.sub.2 
powder contains about 3000 ppm oxygen. 
Moreover, fabrication further increases the cost of such tiles 
substantially. However, no cell utilizing TiB.sub.2 tiles is known to have 
operated successfully for extended periods without loss of adhesion of the 
tiles to the cathode, or disintegration of the tiles. Other reasons 
proposed for failure of RHM tiles and coatings have been the solubility of 
the composition in molten aluminium or molten flux, or the lack of 
mechanical strength and resistance to thermal shock. Additionally, 
different types of TiB.sub.2 coating materials, applied to carbon 
substrates, have failed due to differential thermal expansion between the 
titanium diboride materials and the carbon cathode block or chemical 
attack of the binder materials. To our knowledge no prior RHM containing 
materials have been successfully operated as a commercially employed 
cathode substrate because of thermal expansion mismatch, bonding problems, 
chemical crosion, etc. 
Titanium diboride tiles of high purity and density have been tested, but 
they generally exhibit poor thermal shock resistance and are difficult to 
bond to carbon substrates employed in conventional cells. Mechanisms of 
debonding are believed to involve high stresses generated by the thermal 
expansion mismatch between the titanium diboride and carbon, as well as 
aluminium penetrating along the interface between the tiles and the 
adhesive holding the tiles in place, due to wetting of the bottom surface 
of the tile by aluminium. In addition to debonding, disintegration of even 
high purity tiles may occur due to aluminium penetration of grain 
boundaries. These problems, coupled with the high cost of the titanium 
diboride tiles, have discouraged extensive commercial use of titanium 
diboride elements in conventional electrolytic aluminium smelting cells, 
and limited their use in new cell design. To overcome the deficiencies of 
past attempts to utilize Refractory Hard Materials as a surface element 
for carbon cathode blocks, coating materials comprising Refractory Hard 
Materials is a carbonaceous matrix have been suggested. 
In U.S. Pat. Nos. 4,526,911, 4,466,996 and 4,544,469 by Boxall ct al, 
formulations, application methods, and cells employing TiB.sub.2 /carbon 
cathode coating materials were disclosed. This technology relates to 
spreading a mixture of Refractory Hard Material and carbon solids with 
thermosetting carbonaceous resin on the surface of a cathode block, 
followed by cure and bake cycles. Improved cell operations and energy 
savings result from the use of this cathode coating process in 
conventionally designed commercial aluminium reduction cells. Plant test 
data indicate that the energy savings attained and the coating life are 
sufficient to make this technology a commercially advantageous process. 
Advantages of such composite coating formulations over hot pressed RHM 
tiles include much lower cost, less sensitivity to thermal shock, thermal 
expansion compatibility with the cathode block substrate, and less 
brittleness. In addition, oxide impurities are not a problem and a good 
bond to the carbon cathode block may be formed which is unaffected by 
temperature fluctuations and cell shutdown and restart. Pilot plant and 
operating cell short term data indicate that a coating life of from four 
to six years or more may be anticipated, depending upon coating thickness. 
The baking process should be carried out in an inert atmosphere, coke bed 
or similar protective environment to prevent "excessive air burn". In 
laboratory studies, it is possible to bake the test samples in a retort 
which maintains a high grade inert atmosphere and excludes air/oxygen 
ingress; however, this is not practical for commercial use. Baking under a 
coke bed is reported to give satisfactory protection for the TiB.sub.2 
/carbon composite material. 
Composite coatings have been tested in plants using full scale aluminium 
reduction cells (U.S. Pat. No. 4,624,766; Light Metals 1984. pp 573-588; 
A. V. Cooke et al., "Methods of Producing TiB.sub.2 /Carbon Composites for 
Aluminium Cell Cathodes", Proceedings 17th Biennial Conference on Carbon, 
Lexington, Kentucky (1985)). After curing, the coating is quite hard and 
the coated blocks may be stored indefinitely until baking. For baking, the 
coated blocks were placed in steel containers, covered with a protective 
coke bed, and baked using existing plant equipment such as homogenizing 
furnaces. Once baked, the blocks could be handled without further 
procautions during cell reline procedures. The integrity of the cured 
coating and substrate bond remained excellent after baking. No changes in 
cell start-up procedure were required for using the blocks coated with 
composite TiB.sub.2 material. No difficulties were encountered when the 
coated cathode cells were started-up using either a conventional coke 
resistor bake or hot metal start-up procedure. Core samples from the test 
cells demonstrated areas of good coating condition after 109 and 310 days 
of service in the operating cell, but performance was non-uniform. 
Extensive testing of TiB.sub.2 /carbon composite materials have been 
performed in both laboratory and plant tests. The improved laboratory 
tests and more detailed cell autopsies have shown a variability in 
material performance not observed in previously reported tests. The x-Ray 
Diffraction (XRD) analysis was used to measure the trace impurities in the 
test samples. It was discovered that the poor performance of a test 
material had a direct correlation with the presence of oxidation products 
of Ti and B such as TiO and/or TiBO.sub.3, within the structure of the 
material. A similar variation was detected in the RHM coating applied to a 
carbon cathode. 
Laboratory tests demonstrated that none of the conventional methods (e.g. 
coke bed, inert gas, liquid metal, boron oxide coating on anodes) for 
preventing/controlling carbon oxidation was adequate to prevent the 
formation of TiBO.sub.3 or similar oxidation products during the bake 
operation and/or the cell start-up. 
In addition to the above described problems associated with RHM cathodes, 
the start-up phase of operation of conventional cells can also result in 
oxidation damage leading to reduced operational life, and the present 
invention is not therefore limited to cells have RHM cathodes. 
BRIEF DESCRIPTION OF INVENTION AND OBJECTS 
It is a primary object of a present invention to provide a method of 
protecting aluminium smelter cathodes against deterioration in use, and 
more specifically to provide an improved start-up procedure by means of 
which the life of aluminium smelter cell cathodes may be extended. 
In its broadest form, the invention provides an improved start-up procedure 
for aluminium smelting cells characterized by the creation or 
establishment of conditions which reduce the formation of oxides from 
external oxidant sources in cathode materials during the start-up period 
of the cell. This reduction in the formation of oxides will result in 
cathode materials having superior longevity when compared with Refractory 
Hard Materials and other cathode materials which have not been similarly 
protected against the development of oxide products. 
In one currently preferred form of the invention, the desired conditions 
are established in the smelting cell by the formation of a barrier which 
is liquid or molten during the start-up temperatures above about 
400.degree. C., which is in intimate contact with the exposed surfaces of 
the cathode, which is stable and effective at temperatures up to about 
1000.degree. C. and which is substantially impervious to oxygen throughout 
the start-up period of the cell. 
One of the major advantages of the use of a barrier which is liquid or 
molten is that it allows outgassing from the refractory material during 
the start up procedure while preventing the return of such gases or other 
oxidants to the cathode material. This would not be the case where say a 
gaseous barrier is present since the outgasses and other oxidants may 
readily mix with the barrier gas and will, therefore, be free to react 
with the cathode material. 
The barrier may be formed of two materials, one which is effective up to 
one temperature and the other effective from said one temperature to 
temperatures up to about 1000.degree. C. 
In one form of the invention, this is achieved by the use of boron oxide 
(B.sub.2 O.sub.3), which melts at about 450.degree.-470.degree. C. or 
lower due to impurities, or some other suitable material which is liquid 
or molten at temperatures above about 400.degree. C., which is 
substantially impervious to oxygen transport and which wets carbon. This 
material provides a barrier which substantially prevents the Refractory 
Hard Materials (or other cathode materials) of the cathode from being 
oxide contaminated. At temperatures above about 650.degree.-700.degree. C. 
at which the boron oxide material is likely to be less effective, 
aluminium pellets or the like which are added to the cell with the boron 
oxide and form a molten aluminium barrier which functions during start up 
until the cell starts producing aluminium which functions as a barrier for 
the remainder of the operating life of the cell. Thus, by establishing a 
substantially oxygen impermeable barrier which essentially prevents 
formation of oxides during the start-up period, the cathode of the cell is 
protected against subsequent damage of the type outlined above. 
The boron oxide can be used directly or alternatively can be formed in situ 
by controlled oxidation of a TiB.sub.2 containing material such as the 
refractory hard material coating or a commercially available product such 
as Graphi-Coat. 
In another aspect, the invention provides a method of reducing the 
development of oxidation products in Refractory Hard Material or other 
cathodes during the cell start-up procedure, comprising the step of adding 
to the cell at least one material which is liquid or molten at 
temperatures above about 400.degree. C. and which is stable at 
temperatures up to about 1000.degree. C., which covers the cathode of the 
cell and thereby forms a barrier to oxygen, and which does not materially 
affect the operation of the cell. 
In one preferred form, the method includes adding a first material which is 
liquid or molten at temperatures above about 400.degree. C. and which is 
substantially impervious to oxygen transport, as well as a second material 
which is liquid or molten at temperatures above about 600.degree. C. and 
which forms a substantially impervious barrier to oxygen transport. 
While a currently preferred first material is boron oxide (B.sub.2 
O.sub.3), other materials which are liquid or molten at about 400.degree. 
C. and which form a carbon wetting film substantially impervious to oxygen 
at temperatures above 400.degree. C. may be used. For example, materials 
such as mixtures of chloride or fluoride salts or liquid melts such as 
lead tin alloys may be used, although they are currently considered to be 
less practical than boron oxide. The boron oxide can be used directly or 
alternatively can be formed in situ by controlled oxidation of a TiB.sub.2 
containing material such as the refractory hard material coating or a 
commercially available product such as Graphi-Cost (trade mark). While use 
of this alternative method may result in an outer skin of oxide 
contaminated RHM, this skin may be regarded as a sacrificial layer which 
an operator is willing to lose in return for a protection system which is 
less complex and costly to operate. The effectiveness of this alternative 
protection method will be dependent on the porosity of the refractory hard 
material with lower porosities giving better results. 
Clearly, the most preferable second material, for practical reasons, is 
aluminium metal since this is present in the cell in any event. However, 
other metals or compounds, which are fluid at about 600.degree. C. and 
above, which completely cover the carbon to create a substantially 
impervious barrier to oxygen transport may be used. 
In the post-start-up phase of operation of the cell, it may be necessary or 
desirable to remove the viscous boron oxide layer, or other viscous layer 
derived from the boron oxide coating, which adhere to the surface of the 
cathode. While this removal may be achieved in a number of ways, such as 
flushing the cell with fresh metal to physically remove the layer, it is 
presently preferred to remove the layer chemically by converting the boron 
oxide into a more innocuous boron-containing phase such as by contacting 
the boron oxide phase with Ti-containing species, leading to the 
precipitation of TlB.sub.2. For example, Ti-bearing additions such as 
TiO.sub.2 may be added to the electrolyte or Ti-Al alloys may be added to 
the metal. Other transition metal species in the fourth to sixth groups of 
the periodic system which are able to form borides from the boron oxide 
layer may also be used with acceptable results, such as Zr, Hf, V, Nb, Ta, 
Cr, Mo and W.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In the following description, the conditions under which RHM material can 
be heated above 400.degree. C. without degrading its consistency and 
service life in an aluminium cell will be outlined in greater detail. Two 
types of TiB.sub.2 /carbon composite materials were evaluated in 
laboratory and plant exposure tests to determine their uniformity and 
service life when used to form an aluminium wetted cathode surface for the 
electrolytic winning of aluminium from a molten cryolite based bath. The 
cathode coating material was formulated, mixed, applied to the cathode 
block top surface and cured as taught in U.S. Pat. No. 4,526,911 to Boxall 
et al. The cured coating blocks were then baked under a fluid coke bed as 
described by Boxall et al. A nitrogen purge was maintained through the 
metal box containing the coated blocks and fluid coke to prevent any 
ingress of air during the bake procedure. After cooling to less than 
200.degree. C., the baked coated blocks were removed from the coke bed. 
Normal cell construction procedures were used to construct a conventional 
pre-bake cathode using the coated blocks. 
The cathode tiles were molded, cured and baked as taught in U.S. Pat. No. 
4,582,553 by Buchta. A fluid coke bed with a nitrogen purge was used to 
protect the tiles from "excessive air burn". The tiles were attached to 
the top of the cathode blocks in a conventionally rammed cathode using 
UCAR C-34 cement as described by Buchta. 
A conventional resistor coke bed start-up procedure was used to heat the 
coated lined cathode cell up to about 900.degree.-950.degree. C. before 
fluxing with molten bath transferred from other cells in the potline. The 
test cells were operated as regular cells for approximately 6 weeks before 
the shut down for autopsy. Most of the bath and metal were tapped from the 
cell during the shutdown procedure. After cooling, the remaining bath and 
metal were removed from the cathode surface to expose the coated tiled 
surface. Visual inspection and photographs of the cathode surface were 
used to evaluate the condition of the exposed cathode coating tiles. Core 
samples were taken for metallurgical and chemical analysis. 
The seven day laboratory exposure test was performed in a Hollingshead cell 
comprising an inconel pot, a graphite crucible, a variable height graphite 
stirrer driven by a 60 r.p.m. geared motor and insulating lid of 
pyrocrete. 
Test samples of TiB.sub.2 /C composite were glued to the bottom of the 
crucible with UCAR C-34 cement and were coated with boron oxide paste. 
Samples were then buried in synthetic cryolite (2 kg) and about 2 kg of 
aluminium metal granules were placed on top. The temperature was raised at 
40.degree./hr to 980.degree. C. and the stirrer was immersed so that it 
mixed both metal and bath. After seven days of operation at 980.degree. 
C., the graphite crucible and contents were allowed to cool and then cross 
sectioned to enable visual and chemical analysis of the test samples. Test 
results confirmed that this long term dynamic exposure test can be used to 
screen RHM cathode materials, glues, formulations and baking rates in the 
laboratory prior to their use in industrial scale cells. 
The following TiB.sub.2 composite failure mechanisms observed in the 
industrial cells were reproduced in the test cell: 
(a) delamination cracking of tiles and coatings; 
(b) complete debonding of tiles due to stresses set up by sodium swelling; 
(c) partial debonding of tiles due to chemical attack of the glue, and 
(d) deformation of tiles. 
Furthermore, the dynamic exposure testing of TiB.sub.2 composite materials 
also confirmed the following observations made during cell autopsies and 
laboratory investigations: 
glued joints between tiles and cathode block are subject to chemical 
attack; 
coating produced and baked under laboratory conditions performs much better 
than that produced and baked in the plant; 
order of rank of laboratory performance is coated anthracite block&gt;coated 
MLI block&gt;tiled anthracite block&gt;tiled graphite block; 
structural integrity of the laboratory baked coatings is better than the 
laboratory baked tiles and much better than the plant baked coatings; 
the bonding interface between coating and anthracite block is at least as 
resistant to bath and sodium as the coating itself. 
A large variation in coating/tile quality was found on the cathode surface 
of the autopsied test cells. There appeared to be a random distribution of 
good, poor and missing coating/tile areas over the cathode surface. The 
presence of well bonded undeformed areas of coating/tile demonstrated that 
the material could survive the aluminium cell environment provided a more 
consistent material could be produced. 
No correlation between the material test results and the mixing, spreading, 
molding and curing process parameters could be established to explain the 
variability observed in the plant tests. 
It was discovered that the condition of the exposed coating/tile material 
was related to the presence of oxides of titanium, including mixed oxides, 
in the material, the oxide content being determined using known X-ray 
Diffraction (XRD) analysis. 
TABLE 1 
______________________________________ 
TiB.sub.2 /Carbon Composite Baking Tests 
Oxides of 
Titanium Rel- 
Test Protection Where ative XRD 
Sample Systems Baked Peak Height 
______________________________________ 
Coatings 
BN1 Coke bed Lab 10 
BN1 B.sub.2 O.sub.3 only 
Lab 6 
BN1 B.sub.2 O.sub.3 only 
Lab 5 
BN1 Al powder Lab 10 
BN1 B.sub.2 O.sub.3 + Al 
Lab 1 
BN1 Graphicoat Lab 6 
BN1 TiB.sub.2 /C icing 
Lab 5 
BN1 B.sub.2 O.sub.3 
Lab 7 
BN1 Graphicoat Lab 5 
BN1 TiB.sub.2 /C icing 
Lab 7.5 
BN1-2C Coke bed Plant-28/5/87 
4 
BN1-4C " " 10 
BN1-6C " " 4 
BN1-7C " " 10 
BN1-8C " " 24 
BN1-1C B.sub.2 O.sub.3 + Al 
Plant-4/8/87 
1 
BN1-3C " " 2 
BN1-6C " " 2 
Pitch Bonded 
Coke bed + Ar Lab 34 
Pitch Bonded 
Coke bed + Ar 
Lab 34 
BM1 Graphi-Coat + Al 
Plant Test 2 
BM1 TiB.sub.2 /C icing + Al 
Plant Test 2 
Cast Tiles 
BR7 Coke bed + Ar Lab 6 
BR7 Coke bed " 8 
BR7 B.sub.2 O.sub.3 only 
" 5 
BR7 B.sub.2 O.sub.3 + Al 
" 2 
______________________________________ 
The preferred H.sub.2 O.sub.3 /Al protection system was found to provide 
the best results, although the use of a sacrificial layer or coating, such 
as Graphi-Coat or TiB.sub.2 /C icing, in licu of the B.sub.2 O.sub.3 
component also produced acceptable results. 
By preventing this low level oxidation of the TiB.sub.2, the composite 
structure remains intact and a long service life is maintained. 
The appreciable oxidation of TiB.sub.2 evident during unprotected start-up 
was not anticipated since data sheets for TiB.sub.2 indicate a high 
resistance to air oxidation at temperatures up to 1100.degree. C. (ICD 
Group Inc., New York, N.Y., technical bulletin dated October 1979). Based 
on this data, the use of a coke bed to prevent air burn of the carbon 
matrix and the carbon matrix itself was relied upon to provide adequate 
oxidations protection for the TiB.sub.2. 
The data in Table 1 show that the conventional methods for protecting 
carbon from air burn are inadequate and that an unexpected synergism was 
found when a combination of B.sub.2 O.sub.3 (or a suitable `sacrificial` 
layer) plus Al was used to protect the TiB.sub.2 material. 
According to one practical embodiment, the B.sub.2 O.sub.3 /Al protection 
system and cell start up procedure according to one embodiment is as 
follows: 
1. B.sub.2 O.sub.3 powder is evenly distributed over the cured composite 
surface of the cathode. An amount of about 80 kgs was used in the 100K 
ampere test cell. For difficult or vertical surfaces, a H.sub.3 BO.sub.3 
powder added to water to form a viscous paste is used. 
2. Cover the B.sub.2 O.sub.3 with aluminium foil to protect the powder 
against disturbance during subsequent operation. Overlapping strips of 
1200 mm wide heavy duty foil has been found to be sufficient. 
3. Cover the foil with aluminium "pellets". The amount should be calculated 
to provide at least 20 mm of molten metal over the highest part of the 
cathode. About 4 tons of pellets was found sufficient for the 100K ampere 
test cell. 
4. Baking is carried out by directing oil fired burners between the anodes 
and the pellets, and heating at a rate of about 50.degree. C./hr. After 
the aluminium has melted, the anodes can be lowered, current applied and 
the baking process continued. 
It will be evident from the above discussion that the improved start-up 
procedure embodying the invention provides the following advantages over 
the prior art practices: 
1. Provides improved protection for materials from oxidation damage at 
temperatures in excess of 400.degree. C. 
2. Provides low oxygen activity environment required to prevent oxidation 
of RHM and RHM containing composites when heated above 400.degree. C. 
3. Provides a quality control test for vendor supplied RHM composite 
articles (XRD analysis procedure for critical oxide impurities). 
4. Improves reliability, uniformity and service life for RHM type cathodes. 
5. Enables the use of RHM cathode materials which were previously 
unacceptable due to poor service life. 
The above described start up procedure leaves a viscous boron oxide layer, 
or other layer derived from the boron oxide coating, on the surface of the 
cathode. The continued presence of the viscous boron oxide layer prevents 
a sloping cathode cell from operating in its desired manner. That is, the 
aluminium metal is restricted from draining to the metal sump. Other 
operational difficulties may also occur, as described elsewhere (E. N. 
KARNAUKIIOV et al, Soviet Journal of Non-Ferrous Metals Research, English 
version Vol. 6 No. 1 1978, p. 16). Our own experience has shown that metal 
pooling may occur on the cathode surface, leading to uneven anode burning 
and/or short-circuiting, low current efficiency and general cell 
instability. The transition from start-up conditions to normal stable cell 
operation may therefore become problematic unless the boron oxide layer 
can be effectively removed at the end of the start-up phase. We have found 
that the establishment of stable operating conditions can be accomplished 
more efficiently by accelerating the rate of removal of the boron oxide. A 
number of methods have been found successful for achieving this removal. 
For instance, by flushing the cell with fresh metal the removal of the 
boron oxide has been promoted. However, the transferring of large volumes 
of molten metal into and out of the cell, whilst effective, is 
inconvenient, hazardous and undesirable. 
We have discovered that the removal of boron oxide can be most conveniently 
facilitated by the chemical conversion in situ to a separate and more 
innocuous boron-containing phase that does not interfere with the draining 
of the cathode metal to the sump. By contacting the B.sub.2 O.sub.3 phase 
with a Ti-containing species, chemical interaction between Ti and B is 
achieved leading to the conversion of B.sub.2 O.sub.3 to TiB.sub.2 and the 
precipitation thereof. Importantly, this chemical conversion process 
provides for the removal of the potentially problematic boron oxide 
viscous phase, which in turn allows for a rapid transition to stable and 
efficient drained cathode cell operation, as evidenced by normal bath 
temperatures and the uninterrupted filling of the metal sump at a rate 
consistent with the expected metal production rate. 
Alternatively, it may be possible to use Ti in the form of an alloy of 
aluminium (e.g. Ti-Al) to provide close contact between the B and Ti 
species, respectively. The Ti-Al alloys are a preferred form of Ti 
addition since they are readily available as master alloys in the 
aluminium foundry industry. Furthermore, it is well known in aluminium 
foundry practice (e.g. AU 21393/83 "Removal of Impurities from Molten 
Aluminium") that the removal of metal impurities from molten aluminium can 
be achieved in a straightforward manner by contacting molten aluminium 
with a boron-containing material, thus leading to the generation of 
insoluble metal borides (e.g. (Ti, V) B.sub.2). The formation and 
deposition of TiB.sub.2 is, therefore, readily accomplished. However, the 
use of Ti-Al alloys for the removal of viscous boron-containing layers on 
the cathode surface, by the chemical conversion to another phase, has not 
been previously demonstrated. 
While the use of Ti species is preferred for the above reasons, any RHM 
species, such as the metals in the fourth to sixth groups of the periodic 
system (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W), which can form borides from 
the boron oxide layer may be used with acceptable results. 
In one preferred form of the process, Ti-bearing additions, or other RHM 
boride forming species, such as those mentioned above, may be made 
directly to the electrolyte. Cryolite electrolytes are good solvents for 
oxide ores, so a convenient form of the Ti-containing species is as 
TiO.sub.2, although other additives may also be employed. The 
Ti-containing species reacts with the B.sub.2 O.sub.3 to form at least a 
TiB.sub.2 precipitate, although other equally acceptable precipitates may 
form. 
In each of the above cases, an aluminium-RHM diboride alloy phase is formed 
on the cathode surface, and this may offer additional restorative and 
other benefits to the cathode surface. 
In laboratory tests, it was observed that a 1.875 g addition to the bath of 
TiO.sub.2 effectively removed a 0.975 g layer of B.sub.2 O.sub.3 
originally located at the interface between the composite and the metal 
(i.e. no B.sub.2 O.sub.3 could be detected at the interface by either 
visual or chemical microprobe methods). The mass of TiO.sub.2 was chosen 
to be in excess of that needed for stoichiometric conversion to TiB.sub.2 
to ensure that all the B.sub.2 O.sub.3 was removed. The mass ratio of Ti/B 
in TiB.sub.2 is 2.218:1, and the mass ratio of Ti/B actually used was 
3.71:1, which equates to a Ti mass excess of 67%. Thus, a TiO.sub.2 
/B.sub.2 O.sub.3 mass ratio of 1.875/0.975=1.92 (i.e. .apprxeq.2) is 
effective for removing the B.sub.2 O.sub.3 layer at the cathode surface. 
The TiB.sub.2 precipitate is formed as randomly distributed and irregularly 
shaped fine particles ranging in size from less than 1 um to about 10 um. 
These particles sometimes aggregate as clusters consisting of from 3 or 4 
to 30 or 40 particles. Because of the much higher density of TiB.sub.2 
compared to Al (i.e. 4.5 g/cm.sup.3 vs 2.3 g/cm.sup.3), the TiB.sub.2 has 
been observed to form a sediment on the cathode surface and may, 
therefore, provide restorative and other benefits for cathodes containing 
RHM, such as TiB.sub.2 (e.g. reduces solubility of the RHM). Similar 
comments apply equally to the other RHM boride forming species referred to 
above. 
The above described post-start-up operations provide the means for 
enhancing the removal of a major portion of the boron oxide phase that is 
potentially disruptive to normal cell operation. The enhanced rate of 
removal facilitates the smooth transition from the start-up phase in which 
the boron oxide layer performs a useful protective function-to cell 
operation.