U.S. patent application number 10/684880 was filed with the patent office on 2004-05-13 for start-up of aluminium electrowinning cells.
Invention is credited to Duruz, Jean-Jacques, Liu, James Jenq, Nora, Vittorio de, Sekhar, Jainagesh Akkaraju.
Application Number | 20040089539 10/684880 |
Document ID | / |
Family ID | 32230765 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089539 |
Kind Code |
A1 |
Nora, Vittorio de ; et
al. |
May 13, 2004 |
Start-up of aluminium electrowinning cells
Abstract
A method of protecting during the start-up procedure a cathode
(1) of a cell for the electrowinning of aluminium where the cathode
(1) is optionally coated with an aluminium-wettable refractory
material (2) and on which cathode, in use, aluminium is produced.
The start-up procedure comprises applying before preheating the
cell one or more start-up layers (3) on the aluminium-wettable
refractory coating (2). The start-up layer(s) form(s) a temporary
protection (3) against damage of chemical and/or mechanical origin
to the aluminium-wettable coating (2), this temporary protection
(3) being in intimate contact with the aluminium-wettable coating
(2) and being eliminated before or during the initial normal
operation of the cell. The layers of the temporary protection (3)
may be obtained from at least one pliable foil of aluminium having
a thickness of less than 0.1 mm and/or an applied
aluminium-containing metallization, optionally in combination with
inter alia a boron-containing solution, a polymer, a phosphates of
aluminium-containing solution, or a colloid that gels while
preheating the cell, or combinations thereof.
Inventors: |
Nora, Vittorio de; (Nassau,
IT) ; Sekhar, Jainagesh Akkaraju; (Cincinnati,
OH) ; Duruz, Jean-Jacques; (Geneva, SZ) ; Liu,
James Jenq; (US) |
Correspondence
Address: |
Jayadeep R. Deshmukh
6 Meetinghouse Court
Princeton
NJ
08540
US
|
Family ID: |
32230765 |
Appl. No.: |
10/684880 |
Filed: |
October 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10684880 |
Oct 14, 2003 |
|
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09982552 |
Oct 17, 2001 |
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Current U.S.
Class: |
204/290.01 |
Current CPC
Class: |
C25C 3/06 20130101 |
Class at
Publication: |
204/290.01 |
International
Class: |
C25B 011/04 |
Claims
1. A method of starting-up a cell for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a
fluoride-based melt such as cryolite, the cell comprising a cathode
on which, in use, aluminium is produced and forms a layer or pool,
said method comprising applying one or more aluminium-containing
start-up layers on the cathode surface followed by preheating the
cell, the aluminium-containing start-up layer(s) temporarily
protecting the cathode surface during start-up, characterised in
that the method comprises applying at least one pliable foil of
aluminium which comes into and remains in intimate matching contact
with the cathode surface during preheating the cell and/or applying
at least one aluminium-containing metallization which is applied
and remains in intimate matching contact with the cathode surface
during preheating the cell to temporarily protect the cathode
against chemical attack by reaction with gases and/or fluids such
as melting electrolyte during cell start-up.
2. The method of claim 1, wherein the start-up layer(s) on the
cathode is/are eliminated by washing away the start-up layer(s)
and/or permanently integrating at least part of the start-up
layer(s) into the cathode surface by normal steady state operation
of the cell.
3. The method of claim 1, wherein the, cathode is made of
carbonaceous material, electrically conductive carbon-free
material, electrically non-conductive carbon-free material, or
combinations thereof.
4. The method of claim 1, comprising applying the start-up layer(s)
on a coating of aluminium-wettable refractory material forming the
cathode surface.
5. The method of claim 4, wherein the aluminium-wettable refractory
coating comprises refractory hard metal boride.
6. The method of claim 5, wherein the aluminium-wettable refractory
coating comprises particulate refractory hard metal boride in a
colloidal carrier.
7. The method of claim 1, comprising applying at least one pliable
foil of aluminium having a thickness of less than 0.1 mm onto the
cathode surface.
8. The method of claim 7, wherein said pliable aluminium foil is
from 0.03 to 0.05 mm thick.
9. The method of claim 8, wherein said pliable aluminium foil is at
least partly oxidised and at least partly incorporated into the
cathode surface as alumina.
10. The method of claim 1, comprising applying a metallization of
aluminium or an alloy or an intermetallic compound comprising
aluminium and at least one further metal selected from nickel,
iron, titanium, cobalt, chromium, vanadium, zirconium, hafnium,
niobium, tantalum, molybdenum, cerium and copper.
11. The method of claim 10, wherein said metallization is obtained
from metallic powder(s) applied in an aqueous or non-aqueous
liquid, or in an aqueous liquid containing organics.
12. The method of claim 11, wherein the liquid is a polymer, such
as polyurethane, ethylene glycol, polyethylene glycol, resins,
esters or waxes.
13. The method of claim 10, wherein the metallization is an
intermetallic compound comprising aluminium and at least one
further metal selected from nickel, iron, titanium, cobalt,
chromium and zirconium.
14. The method of claim 13, wherein said intermetallic compound is
NiAl or Ni.sub.3Al.
15. The method of claim 13, wherein said intermetallic compound is
obtained by applying aluminium in the form of a powder, sheet,
porous body or mesh onto a sheet, porous body or a mesh of said
further metal, or vice-versa.
16. The method of claim 15, wherein said intermetallic compound is
obtained by heating the aluminium and said further metal on top of
the cathode surface in the aluminium electrowinning cell to a
sufficient temperature to initiate a reaction for the formation of
said intermetallic compound, before or during preheating of the
cell.
17. The method of claim 1, comprising applying at least one
additional start-up layer on the cathode surface, said additional
layer being obtained at least partly from a boron-containing
solution forming a glassy layer.
18. The method of claim 17, wherein the boron-containing solution
contains boron oxide, boric acid or tetraboric acid.
19. The method of claim 17, wherein the boron-containing solution
comprises a boron compound dissolved in a solvent selected from
methanol, ethylene glycol, glycerine, water and mixtures
thereof.
20. The method of claim 1, comprising applying at least one
additional start-up layer on the cathode surface, said additional
layer being obtained at least partly from a polymer or a polymer
precursor.
21. The method of claim 20, wherein the polymer is selected from
polyurethane, ethylene glycol, polyethylene glycol, resins, esters
or waxes.
22. The method of claim 1, comprising applying at least one
additional start-up layer on the cathode surface, said additional
layer being obtained at least partly from a solution comprising
phosphates of aluminium.
23. The method of claim 22, wherein the phosphates of aluminium are
selected from monoaluminium phosphate, aluminium phosphate,
aluminium polyphosphate, aluminium metaphosphate and mixtures
thereof.
24. The method of claim 1, comprising applying at least one
additional start-up layer on the cathode surface, said additional
layer being obtained at least partly from a colloid solution that
gels during preheating.
25. The method of claim 24, wherein the colloid is selected from
colloidal alumina, silica, yttria, ceria, thoria, zirconia,
magnesia, lithia, monoaluminium phosphate, cerium acetate or
mixtures thereof.
26. The method of claim 24, wherein the colloid is at least partly
integrated into the cathode surface or an aluminium-wettable
refractory coating on said surface.
27. The method of claim 24, wherein the colloid solution comprises
a particulate conductor.
28. The method of claim 27, wherein the particulate conductor is
selected from aluminium, nickel, iron, titanium, cobalt, chromium,
zirconium, copper and combinations thereof.
29. The method of claim 1, comprising applying at least one
start-up layer containing carbides and/or borides of metals, in
particular of metals selected from the group comprising aluminium,
titanium, chromium, vanadium, zirconium, hafnium, niobium,
tantalum, molybdenum and cerium.
30. The method of claim 1, comprising applying at least one
start-up layer containing particulate aluminium.
31. The method of claim 1, comprising applying at least one thick
sheet of aluminium having a thickness of 1 to 5 mm on top of the
start-up layer(s).
32. The method of claim 1, wherein each applied start-up layer is
electrically conductive, the start-up layer(s) being covered with a
layer of electrically conductive material, heat being generated by
passing electric current via anodes, through the conductive
material and the conductive start-up layer(s) into the cell bottom
to heat the cell by the Joule effect.
33. The method of claim 32, wherein a relatively thick sheet or
sheets of aluminium, usually from 1 to 5 mm thick, is placed
between each anode and the temporary protection.
34. The method of claim 33, wherein a relatively thick sheet or
sheets of aluminium, usually from 1 to 5 mm thick, is placed on the
temporary protection.
35. The method of claim 32, wherein said conductive material
contains coke.
36. The method of claim 1, wherein the cell bottom is flame
preheated by burners.
37. The method of claim 1, wherein the cell bottom is preheated
using infrared radiation.
38. The method of claim 4, wherein the aluminium-wettable
refractory coating protected by at least one start-up layer is
covered with a halide-based electrolyte having a point of fusion in
the region 660.degree.-760.degree. C., said fluoride-based melt
being added to the cell when the temperature of the cell exceeds
the point of fusion of said halide-based electrolyte added to the
conductive material.
39. The method of claim 37, wherein said halide-based electrolyte
is mixed with or covers a layer of conductive material such as
coke.
40. The method of claim 1, wherein the or at least one start-up
layer extends up the side walls of the cell.
41. The method of claim 40, wherein the or at least one start-up
layer extends above the level of the fluoride-based melt during
normal use of the cell.
42. The method of claim 1, wherein the or at least one start-up
layer is applied on the cathode surface using painting methods,
such as brushes, rollers or spraying.
43. The method of claim 1, comprising applying at least one
start-up layer by hot-spraying molten metal.
44. The method of claim 1, comprising applying at least one
start-up layer by CVD, PVD, plasma spraying, electro-deposition,
chemical deposition, adhesive application or hot-pressing.
45. The method of claim 1, comprising applying the or at least one
start-up layer with an automated or a partly automated system.
46. A method of electrowinning aluminium comprising a cell start-up
procedure as described in claim 1 followed by producing aluminium
by the electrolysis of alumina dissolved in a fluoride-based
melt.
47. The method of claim 46, comprising operating the fluoride-based
melt at a temperature comprised between 700.degree. and 970.degree.
C.
48. The method of claim 46, comprising evolving oxygen on a
non-carbon anode.
49. The method of claim 48, comprising operating the fluoride-based
melt at a temperature comprised between 750.degree.and 850.degree.
C.
50. The method of claim 46, comprising producing aluminum on an
aluminium-wettable drained cathode.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the starting up of cells for the
electrowinning of aluminium by the electrolysis of alumina in a
cryolite-based melt, which cell comprises a conductive cell bottom
on which, in use, aluminium is produced and forms a layer or pool
atop which is the molten cryolite electrolyte. The invention is
particularly but not exclusively concerned with the start up of
such cells where the cathode surface is protected by an
aluminium-wettable refractory coating.
BACKGROUND OF THE INVENTION
[0002] Aluminium is produced conventionally by the Hall-Hroult
process, by the electrolysis of alumina dissolved in cryolite-based
molten electrolytes at temperatures up to around 950.degree. C. A
Hall-Hroult reduction cell typically has a steel shell provided
with an insulating lining of refractory material, which in turn has
a lining of carbon which contacts the molten constituents.
Conductor bars connected to the negative pole of a direct current
source are embedded in the carbon cathode substrate forming the
cell bottom floor. The cathode substrate is usually an anthracite
based carbon lining made of prebaked cathode blocks, joined with a
ramming mixture of anthracite, coke, and coal tar or resins.
[0003] In Hall-Hroult cells, a molten aluminium pool acts as the
cathode. The carbon lining or cathode material has a useful life of
three to eight years, or even less under adverse conditions. The
deterioration of the cathode bottom is due to erosion and
penetration of electrolyte and liquid aluminium as well as
intercalation of sodium, which causes swelling and deformation of
the cathode carbon blocks and ramming mix. In addition, the
penetration of sodium species and other ingredients of cryolite or
air leads to the formation of toxic compounds including
cyanides.
[0004] When they are put into service, aluminium electrowinning
cells must be preheated. When the cell has reached a sufficient
temperature, molten cryolite electrolyte is added and the start-up
is continued until the cell reaches an equilibrium operating
condition.
[0005] One known cell start-up procedure comprises applying a layer
of coke or similar conductive material to the cell bottom and
passing an electric current via anodes and through the coke into
the cell bottom to heat the cell by the Joule effect. Another known
cell start-up procedure uses flame burners. In U.S. Pat. No.
4,405,433 (Payne), it has been suggested that refractory fibrous
materials of aluminium silicate be placed over refractory hard
metal cathode assemblies prior to preheating of the cathode
assemblies.
[0006] U.S. Pat. No. 5,651,874 (Sekhar/de Nora) has proposed
coating the carbon cell bottom with particulate refractory hard
material in a colloidal carrier to produce a hard adherent
aluminium-wettable surface coating. These aluminium-wettable
refractory coatings have by far outperformed all previous attempts
to use such materials to protect carbon cell bottoms.
[0007] To facilitate cell start up, in particular when using these
improved coatings, it has already been proposed to place an
aluminium sheet on top of the coating before preheating (see
Cathodes in Aluminium Electrolysis, 2nd Edition, 1994, M. Sorlie
and H. Oye, published by Aluminium Verlag, page 70). The purpose of
this aluminium sheet was to avoid possible hot-spots due to uneven
current distribution. Because of the high current densities
employed and the need to ensure an even heat distribution,
aluminium sheets with a thickness of 1 to 5 mm were used. This
aluminium sheet melts during the start-up procedure and is
integrated into the pool of product aluminium.
[0008] However, it has been found that whereas use of such
aluminium sheets has been effective in reducing hot-spots, they do
not protect against oxidation of the cathode. The use of thick
aluminium sheets has not addressed this problem.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of the invention to provide a
start-up procedure which is entirely reliable as regards the
avoidance of any damage to the cathode surface by using a material
which when applied to the cathode forms a thin and uniform
protective layer.
[0010] Another object of the invention is to protect the cathode by
covering it with a temporary protective material before preheating
the cell. A further object of the invention is to provide a
temporary protective material which is at least partially
eliminated by the beginning of normal use of the cell, such that it
does not contaminate the product aluminium with the temporary
protective material.
[0011] The invention in particular relates to a method of
starting-up a cell for the electrowinning of aluminium by the
electrolysis of alumina dissolved in a fluoride-based melt such as
cryolite, the cell comprising a cathode on which, in use aluminium
is produced and forms a layer or pool. The start-up method
comprises applying one or more aluminium-containing start-up layers
on the cathode surface followed by preheating the cell, the
start-up layers temporarily protecting the cathode surface during
start-up.
[0012] The method of the invention comprises applying at least one
pliable foil of aluminium which comes into and remains in intimate
matching contact with the cathode surface during preheating the
cell and/or applying at least one aluminium-containing
metallization which is applied and remains in intimate matching
contact with the cathode surface during preheating the cell. The
start-up layer(s) temporarily protect(s) the cathode against
chemical attack by reaction with gases and/or fluids such as
melting electrolyte during cell start-up.
[0013] The start-up layer(s) form(s) a temporary protection against
damage of chemical or chemical/mechanical origin to the cathode,
this temporary protection being in intimate contact with the
cathode surface and being usually at least partly eliminated before
or during the initial normal operation of the cell. The temporary
protection may be "washed away" by normal operation of the cell or
permanently integrated into the cathode surface.
[0014] In contrast to the prior art, the temporary protection
remains in intimate contact with the cathode surface below a layer
of molten aluminium during the cell start-up. When only a thick
sheet of aluminium is applied on the cell bottom, the applied layer
melts during start-up and is merely integrated to the pool of
product aluminium without preventing melting electrolyte from
attacking the aluminium-wettable coating.
[0015] For the purpose of this invention, start-up layers may for
example be obtained from the following materials: at least one
pliable foil of aluminium having a thickness of less than 0.1 mm;
and/or an applied metallization of aluminium or an alloy or an
intermetallic compound comprising aluminium and at least one
further metal selected from nickel, iron, titanium, cobalt,
chromium, vanadium, zirconium, hafnium, niobium, tantalum,
molybdenum, cerium and copper.
[0016] In combination with the aluminium-containing start-up
layers, additional start-up layers may be used such as:
[0017] a) a boron-containing solution forming a glassy layer;
[0018] b) a polymer or a polymer precursor;
[0019] c) a solution containing phosphates of aluminium;
[0020] f) a colloid; and combinations of the aforesaid.
[0021] Normally the cell bottom is made of carbonaceous material
such as carbon blocks. The cathode mass can be made mainly of
carbonaceous material, such as compacted powdered carbon, a
carbon-based paste for example as described in U.S. Pat. No.
5,362,366 (Sekhar et al), prebaked carbon blocks assembled together
on the shell, or graphite blocks, plates or tiles.
[0022] It is also possible for the cathode to be made mainly of an
electrically-conductive carbon-free material, of a composite
material made of an electrically-conductive material and an
electrically non-conductive material, or of an electrically
non-conductive material.
[0023] Such non-conductive carbon-free materials can be alumina,
cryolite, or other refractory oxides, nitrides, carbides or
combinations thereof and the conductive materials can be at least
one metal from Groups IIA, IIB, IIIA, IIIB, IVB, VB and the
Lanthanide series of the Periodic Table, in particular aluminium,
titanium, zinc, magnesium, niobium, yttrium or cerium, and alloys
and intermetallic compounds thereof.
[0024] The composite material's metal preferably has a melting
point from 650.degree. C. to 970.degree. C.
[0025] The composite material is advantageously a mass made of
alumina and aluminium or an aluminium alloy, see U.S. Pat. No.
4,650,552 (de Nora et al), or a mass made of alumina, titanium
diboride and aluminium or an aluminium alloy.
[0026] The composite material can also be obtained by micropyretic
reaction such as that utilizing, as reactants, TiO.sub.2,
B.sub.2O.sub.3 and Al.
[0027] The cathode can also be made of a combination of at least
two materials from: at least one carbonaceous material as mentioned
above; at least one electrically conductive non-carbon material;
and at least one composite material of an electrically conductive
material and an electrically non-conductive material, as mentioned
above.
[0028] Advantageously the cathode surface is coated with an
aluminium-wettable refractory material, such as a refractory hard
metal boride. Particulate refractory hard metal boride may for
instance be included in a colloidal carrier and then applied to the
cathode surface, i.e. according to the teaching of the aforesaid
U.S. Pat. No. 5,651,874 (Sekhar/de Nora).
[0029] When an aluminium foil is used as a start-up layer, the foil
is preferably from 0.03 to 0.05 mm thick. Eventually, the foil may
be oxidised during heating and incorporated (as alumina) into the
cathode surface or into a coating of aluminium-wettable refractory
material.
[0030] The protection involving aluminium foils is contrasted with
the use of a thick aluminium sheet, known from the prior art, which
helps to avoid possible hot-spots due to uneven current
distribution. Thick sheets of aluminium cannot be in intimate
contact with the cathode surface because of their poor malleability
and therefore cannot sufficiently protect the cathode from fluid
and/or gaseous attack during start-up. Such thick sheets of
aluminium merely ensure a good current distribution to avoid
hot-spots. In contrast, thin foils of aluminium intimately match
the surface of the cathode which may be porous and protect the
cathode from undesired attacks during preheating of the cell.
[0031] However, as explained below, it is possible to use a thick
aluminium sheet in combination with a protective layer according to
the invention, e.g. including aluminium foils.
[0032] As stated above, an aluminium-containing metallization may
be used in order to protect the cathode surface. The metallization,
which can be intimately bonded to the cathode surface, combines
chemical, mechanical and electrical properties useful for the
start-up procedure. This type of start-up layer prevents damage to
the cathode of chemical and/or mechanical origin and additionally,
the good conductivity of the material may be advantageously used
during the cell heating procedure when achieved by the Joule effect
as described hereafter. Typical metals which may be used for a
metallization are aluminium, or an alloy or intermetallic compound
comprising aluminium and at least one further metal selected from
nickel, iron, titanium, cobalt, chromium, vanadium, zirconium,
hafnium, niobium, tantalum, molybdenum, cerium and copper.
[0033] The use of metallic paints obtained from metallic powder
applied in an aqueous, non-aqueous liquid or in an aqueous liquid
containing organics, in particular in a polymer, such as
polyurethane, ethylene glycol, polyethylene glycol, resins, esters
or waxes may be very convenient due to their good protective
properties and the ease with which such a material may be applied
on the surface of the cell bottom.
[0034] Generally, the constituents and the amount of the protective
metallization will be chosen so that there is an adequate
protection during cell start-up but no undesirable contamination of
the aluminium produced. Such metallization may also assist wetting
of a porous refractory surface with molten aluminium. The
protective metallization will usually not remain permanently on the
surface of the cathode but will be either "washed away" or
integrated into the surface of the cathode by the time the cell
reaches its normal steady state operation.
[0035] For instance an aluminium paint can be applied on the cell
bottom and then covered with a plurality of foils of aluminium as
described above.
[0036] Advantageously intermetallic compounds may be used to
protect the cell bottom. These compounds may comprise aluminium
with a further material selected from nickel, iron, titanium,
cobalt, chromium, zirconium or combinations thereof. Furthermore
such a layer of intermetallic compound is advantageously formed by
applying on top of the cell bottom a combination of either
aluminium powder, sheet, mesh or porous body such as foam on top of
a sheet, mesh or porous body of said further material, or
vice-versa. Heating the two metals before or during preheating of
the cell initiates a spontaneous reaction for the production of an
intermetallic component. Normally, such a layer is then evacuated
with the produced molten aluminium before or during the initial
normal operation of the cell.
[0037] The use of NiAl or Ni.sub.3Al as an intermetallic layer is
particularly beneficial. For instance, NiAl is remarkably stable
when exposed to heat, the melting point being at about 1600.degree.
C. Additionally it presents good mechanical characteristics.
[0038] An additionally applied protective start-up layer on the
cathode surface may be obtained at least partly from a
boron-containing solution that forms a glassy layer. The boron
solution can be made from boron oxide, boric acid or tetraboric
acid in an aqueous, non-aqueous or aqueous containing organics
solvent such as methanol, ethylene glycol, glycerine, water or
mixtures thereof. Optionally, the solution may comprise particulate
metal that enhances the conductivity of the layer. Good results
were obtained where a mix of aluminium and optionally borides
and/or carbides of metals from the group comprising aluminium,
titanium, chromium, vanadium, zirconium, hafnium, niobium,
tantalum, molybdenum and cerium were added to the solution.
Application of a boron-containing layer covered with a plurality of
aluminium foils was successfully tested as well.
[0039] Alternatively a further applied protective start-up layer on
the cathode surface may be obtained at least partly from a polymer
or polymer precursor, such as polyurethane, ethylene glycol,
polyethylene glycol, resins, esters or waxes. As described above,
the electrical conductivity of such a layer may be enhanced by
adding particulate conductive material, optionally mixed with
borides and/or carbides.
[0040] A further alternative is to form an additional protective
start-up layer on the cathode surface from an aqueous and/or
non-aqueous solution containing phosphates of aluminium, such as
monoaluminium phosphate, aluminium phosphate, aluminium
polyphosphate, aluminium metaphosphate and mixtures thereof, for
instance dissolved in water. Such a protective start-up layer is
advantageously covered with a plurality of foils of aluminium as
described above.
[0041] When an additional protective start-up layer is obtained at
least partially from a colloid solution, the colloid is preferably
selected from colloidal alumina, silica, yttria, ceria, thoria,
zirconia, magnesia, lithia, monoaluminium phosphate, cerium acetate
or a mixture thereof. The colloid solution usually gels during
preheating of the cell and protects the cathode and if present the
aluminium-wettable refractory coating during this critical start-up
phase. Optionally, the colloid from the start-up layer may be at
least partially incorporated into the cathode surface or the
aluminium-wettable coating.
[0042] Such an additional protective start-up layer advantageously
contains a particulate conductor, such as particulate aluminium,
nickel, iron, titanium, cobalt, chromium, zirconium, copper and
combinations thereof, in order to improve the conductivity of the
start-up layer and additionally to avoid uneven current
distribution. Likewise, a protective start-up layer comprising
colloidal alumina can be used in combination with at least one
pliable foil of aluminium having a thickness of less than 0.1 mm.
For instance one or more aluminium foils can be sandwiched with the
colloidal alumina.
[0043] As previously described, protective material is either
intimately merged or is in intimate contact with the porous surface
of the aluminium-wettable refractory coating and may be partly (or
wholly) permanently incorporated in this coating, or partly or
wholly removed as the cell reaches its steady state operation.
[0044] Additives such as particulate aluminium, or borides and/or
carbides may be added to any protective start-up layer described
hereabove before preheating the cell. Such addition enhances the
protective effect of the temporary protection. The borides and/or
carbides may be selected from the following metals: aluminium,
titanium, chromium, vanadium, zirconium, hafnium, niobium,
tantalum, molybdenum and cerium.
[0045] Similarly, pliable foils or thick sheets of aluminium may be
added on top of any protective start-up layer described
hereabove.
[0046] In the method of the invention, heat can be generated in the
conductive material to preheat the cell bottom by passing electric
current via anodes and through conductive material such as resistor
coke into the cell bottom to heat the cell by the Joule effect.
[0047] For this configuration the temporary protection preferably
has good electrical conductivity. The conductivity of a layer may
however be enhanced by adding conductive material as described
above. For instance aluminium powder may be incorporated to a
non-inherently conductive or poorly conductive material and
therefore even thick layers of poorly conductive based material may
be used.
[0048] When this electrical resistance preheating is used, a
relatively thick sheet or sheets of aluminium, usually from 1 to 5
mm thick, preferably 3 to 5 mm, can placed between each anode and
the temporary protection, in particular on top of the temporary
protection. This thick sheet of aluminium, as is known, serves to
assist the distribution of the electrical current evenly, and hence
avoid hot-spots.
[0049] Alternatively, the cell bottom can be flame preheated by
burners providing precautions are taken to avoid direct contact of
the flame with a thin protective start-up layer, e.g. by covering
it with a layer of coke or other heat-conductive material. This
method offers the advantage of not being dependent upon the
electrical conductivity of the materials involved.
[0050] Another method to heat the cell during the start-up
procedure, which is not dependent upon the material involved,
involves radiation techniques. A light source may be used in order
to transfer heat to the cell in form of light waves. The preferred
emitted wavelengths correspond to the infrared spectrum. This
technique offers the advantage of avoiding pollution of the cell
with undesired elements as when using the flame or the carbon
resistor technique.
[0051] Furthermore, the layer of coke or other conductive material
can be mixed with and/or covered with a halide-based electrolyte
having a point of fusion in the region 660.degree.-760.degree. C.
Molten cryolite is added to the cell when the temperature of the
cell exceeds the point of fusion of said halide-based electrolyte
added to the conductive material. This halide-based electrolyte can
be mixed with or covered with a layer of conductive material such
as coke.
[0052] Optionally protective start-up layers may extend to cover
other components, such as cell side walls. The protection may even
extend above the level of the fluoride-based melt reached during
normal use of the cell.
[0053] Several methods are available in order to apply protective
start-up layers on the cell bottom. When the precursor of a layer
is of liquid form various painting methods may be applicable such
as using brushes, rollers or spraying techniques. In case of a
metallization hot-spraying may be used to apply molten metal.
[0054] Vapour deposition techniques such as chemical vapour
deposition (CVD) or physical vapour deposition (PFD) may be used,
or plasma spraying. Chemical or electro-deposition may be
advantageously used considering the electrolytic cell
environment.
[0055] When a foil is included in the protective layer, such as an
aluminium foil, it can be secured on the cell bottom using adhesive
application or hot-pressing which gives good results in case of
powdery precursors as well.
[0056] All these techniques for applying the temporary protection
on the cell bottom may be advantageously done using an automatic
device like for instance an apparatus as described in international
application WO98/20188 (Sekhar/Berclaz). However, other types of
system may be envisaged, like an angular (cylindrical or SCARA) or
a parallel type robot. Partly automated system may also be
used.
[0057] The invention also relates to a method of electrowinning
aluminium. The method comprises two steps namely a start-up
procedure with a temporary protection as described hereabove
followed by the production of aluminium.
[0058] The fluoride-based melt is usually operated at a temperature
comprised between 700 and 970.degree. C.
[0059] Aluminium may be produced by using oxygen-evolving
non-carbon anodes, in particular metal-based anodes, as for
instance disclosed in co-pending applications PCT/IB99/00017 and
PCT/IB99/00018 having an oxide based electrochemically active
surface, such as a hematite coating which may be maintained
dimensionally stable as disclosed in co-pending applications U.S.
Ser. No. 09/126,839, PCT/IB99/00015 and PCT/IB99/00016. As
described in these references, the fluoride-based melt is
preferably operated at reduced temperature, i.e. between 750 and
850.degree. C., to limit contamination of the melt and of produced
aluminium by anode constituents.
[0060] The above described method is suitable for conventional
Hall-Hrqult aluminium production cells. However, it can also be
utilised for advanced cell designs, in particular designs
incorporating an aluminium-wettable drained cathode, as disclosed
U.S. Pat. No. 5,683,559 and in co-pending application
PCT/IB99/00222 and/or in cells operating without electrolyte crust,
for instance fitted with an insulating cover as disclosed in
co-pending application WO99/02763.
BRIEF DESCRIPTION OF THE DRAWING
[0061] FIG. 1 is a schematic view of part of a cell for the
electrowinning of aluminium arranged for carrying out the start-up
procedure of the present invention.
DETAILED DESCRIPTION
[0062] FIG. 1 shows part of an aluminium electrowinning cell
comprising a cathode cell bottom 1, for example carbon, coated with
an aluminium-wettable refractory material 2, in particular a
slurry-applied titanium diboride coating as described in U.S. Pat.
No. 5,651,874 (Sekhar/de Nora). The coating 2 is covered with a
temporary protection 3, for example a couple of thin aluminium
foils each having a thickness of 0.04 mm, against damage of
chemical and/or mechanical origin to the aluminium-wettable coating
2 during the start-up procedure. Alternatively, other protective
layers can be applied, for example those described in Examples II
to V below. On the temporary protection 3 a thick sheet of
aluminium 4 (i.e. having a thickness of 4 mm) is applied. The
aluminium sheet 4 is covered with resistor coke 5 up to the bottom
of the anode 6 which is facing the cathode. The resistor coke 5 may
extend along the temporary protection or be confined under the
anode 6 as indicated by the dashed line 5a. The temporary
protection 3 extends up a wedge 7 connecting the cathode cell
bottom 1 to the cell side wall 8 on which frozen alumina-containing
cryolite 9 is located. The temporary protection 3 and the aluminium
sheet 4 are shown out of proportion in FIG. 1.
[0063] When current is passed from the anode 6 to the cathode 1 via
the resistor coke 5 and the thick aluminium sheet 4 and aluminium
foils 3 heat is generated mainly in the resistor coke 5. The thick
aluminium sheet 4 helps to avoid hot-spots as described in the
prior art. The heat generated in the resistor coke 5 enables first
the aluminium sheet 4 and foils 3 and then the frozen cryolite 9 to
melt and fill the cell. The presence of the thin foils of aluminium
3 which come into intimate contact with the aluminium-wettable
coating 2 while preheating the cell prevents melting cryolite from
coming into contact with the coating 2.
[0064] When molten cryolite fully covers the cell bottom up to the
level of the anode bottom 6 electrolysis of alumina dissolved in
the molten cryolite may begin. The evacuation of free elements
originating from the thin foils of aluminium 3, the aluminium sheet
4 and the resistor coke 5 takes place before or during initial
normal operation of the cell.
[0065] The feasibility of the invention has been demonstrated in
the following laboratory tests:
EXAMPLE I (COMPARATIVE)
[0066] In order to show the oxidation of TiB.sub.2 into TiO.sub.2
and B.sub.2O.sub.3, when applied in a colloid onto the cell bottom,
the following laboratory test was carried out.
[0067] Platelets (about 20.times.40.times.3 mm) of an
aluminium-wettable refractory coating material were prepared by
slip casting of a slurry made of 14 ml of colloidal alumina, 12 ml
of colloidal silica and 50 g of TiB.sub.2 powder on a porous
plaster substrate.
[0068] One platelet was weighed, subjected to a heat treatment in
air at 800.degree. C. for 15 hours in a box furnace and weighed
again after cooling. Under these conditions, which simulate the
oxidising conditions during cell-start-up the weight uptake
resulting from oxidation of the TiB.sub.2 components to form
TiO.sub.2 and B.sub.2O.sub.3was 0.69 g or about 12% of total
TiB.sub.2 content.
EXAMPLE II
[0069] A similar procedure was carried out as in the previous
example but with the TiB.sub.2 coating protected by foils of
aluminium.
[0070] A platelet as in Example I was wrapped in three layers of
aluminium foil (0.02 mm thickness) and submitted to the same
treatment as for the previous example. The weight uptake, taking
into consideration the oxidation of the aluminium, was found to be
5%, therefore demonstrating the efficiency of the protective layer
against oxidation.
EXAMPLE III
[0071] In order to demonstrate the effectiveness of a protective
layer originating from an aluminium paint the following test was
carried out.
[0072] A platelet as in Example I was metallized on all its surface
by spraying an aluminium powder (<1 .mu.m) in suspension in an
organic carrier and synthetic resin three times, allowing each
layer to dry at room temperature until a layer of about 80-100
.mu.m was obtained. The coated sample was submitted to the same
treatment as in the first example. The weight uptake, taking into
consideration the oxidation of aluminium, was found to be 2%,
therefore demonstrating the efficiency of the protective layer
against oxidation.
EXAMPLE IV
[0073] Furthermore, the efficiency of a combination of an aluminium
foil and a polymer was tested in a similar way as in the preceding
examples.
[0074] A platelet as in Example I was coated by brushing on all its
surface with one layer of polyurethane in an organic solvent and an
aluminium foil (0.06 mm thickness) was applied on top immediately
after, so that the aluminium foil was firmly fixed after the
polyurethane solution has dried out. The coated sample was
submitted to the same heat treatment as for Example I. The weight
uptake was found to be 0.5%, therefore demonstrating the efficiency
of this protective layer against oxidation.
[0075] It will be understood that modifications may be made in the
present invention without departing from the spirit of it. Thus,
the scope of the present invention should be considered in terms of
the following claims, and is understood not to be limited to the
details of operation described in the specification.
EXAMPLE V
[0076] Finally, the efficiency of a combination of an aluminium
foil with an aluminium paint was tested.
[0077] The patelet of Example I was metallized as in Example 3 on
all its surface by spraying a paint of aluminium powder (<1
.mu.m) in suspension in an organic carrier and synthetic resin
three times, allowing each layer to dry at room temperature until a
layer of about 80-100 .mu.m was obtained. An aluminium foil (0.06
mm thickness) was applied thereon immediately after the last layer
of paint was sprayed, so that the aluminium foil was firmly fixed
after the last paint layer had dried out.
[0078] The coated sample was submitted to the same treatment as in
the first example. The weight uptake, taking into consideration the
oxidation of aluminium, was found to be less than 2%, therefore
demonstrating the efficiency of the protective layer against
oxidation.
* * * * *