COVER FOR AN ELECTROLYSIS CELL FOR PRODUCING ALUMINIUM

REVETEMENT D'ELECTROLYSEUR POUR LA PRODUCTION D'ALUMINIUM

English Abstract

?The invention relates to non-ferrous metallurgy, more particularly to producing aluminium by electrolysis, and even more particularly to a structural element for covering the space above a melt in an electrolysis cell for producing aluminium by the electrolysis of cryolite-alumina melts. In a cover for an electrolysis cell for producing aluminium, which is in contact with a vapour-gas phase when the electrolysis cell is in operation and which is in the form of central and peripheral sections which are moveably arranged relative to each other, the central and peripheral sections are made from a corrosion-resistant and erosion-resistant material which comprises 80.0-99.0 wt% of fluorophlogopite and 20.0-1.0 wt% of a refractory filler. The central sections of the cover may be permanently fixed on each anode rod, and the peripheral sections may be configured as convex panels rigidly and removably fixed on the top surface of a cathode and supported by the central section of the cover. Moreover, the refractory filler may be chosen from the following chemical substances: clay, calcium fluoride, rutile, sodium aluminosilicate, fluorapophyllite, nepheline, olivine, magnesium fluoride, and spinel. The end and side joints of the central and peripheral covers may be coated with a sealant layer in the form of a layer of alumina, and the central section of the cover may be provided with apertures. The use of this invention provides for the hermetic sealing of the cover, the reliability and safety of the structure, and a reduction in energy consumption.

French Abstract

L'invention se rapporte au domaine de la métallurgie des métaux non ferreux, notamment à la production électrolytique d'aluminium, et concerne un élément d'une structure de revêtement d'un espace au-dessus d'un bain de fusion d'électrolyseur afin de produire de l'aluminium selon un procédé d'électrolyse de bains de fusion de cryolite-alumine. Dans le revêtement d'électrolyseur de production d'aluminium sont formées des sections centrales et périphériques entrant en contact avec la phase de vapeur et de gaz lors du processus de fonctionnement de l'électrolyseur qui se présentent sous forme de sections centrales et périphériques disposées de manière à se déplacer les unes par rapport aux autres, et qui sont faites d'un matériau résistant à la corrosion et à l'érosion contenant 80,0-99,0 % en poids de fluorophlogopite et 20,0-1,0 % en poids d'une matière de charge réfractaire. Les sections centrales du revêtement peuvent être fixées rigidement sur chaque tige d'anode, et les sections périphériques peuvent se présenter sous forme de panneaux convexes fixés rigidement sur la surface supérieure de la cathode de manière amovible et reposant sur la section centrale du revêtement. En outre, la matière de charge réfractaire peut comprendre les composés chimiques suivants: de l'argile, du fluorure de calcium, du rutile, de l'aluminosilicate de sodium, de la fluoro-apophyllite, de la néphéline, de l'olivine, du fluorure de magnésium, de la spinelle. Sur les connexions d'extrémité et latérales des revêtements centraux et périphériques peut être appliquée une couche d'un agent d'étanchéification sous forme d'une couche d'alumine, et des ouvertures peuvent également être formées dans la section centrale du revêtement. L'utilisation de la présente invention assure l'étanchéité du revêtement, la fiabilité et la sécurité de la structure, et une diminution de la consommation énergétique.

Claims

7
CLAIMS
1. A cover for an electrolysis cell for producing aluminium, which is in
contact with a
vapour-gas phase when the electrolysis cell is in operation and which is in
the form of central
and peripheral sections which are movably arranged relative to each other,
characterized in that
the central and peripheral sections are made from a corrosion-resistant and
erosion-resistant
material which comprises 80.0-99.0 wt% of fluorophlogopite and 20.0-1.0 wt% of
a refractory
filler.
2. The cover according to claim 1, characterized in that central sections of
the cover are
permanently fixed on each anode rod.
3. The cover according to claim 1, characterized in that peripheral sections
are
configured as convex panels rigidly and removably fixed on the top surface of
a cathode and
supported by the central section of the cover.
4. The cover according to claim 1, characterized in that the refractory filler
may be
chosen from the following chemical substances: clay, calcium fluoride, rutile,
sodium
aluminosilicate, fluorapophyllite, nepheline, olivine, magnesium fluoride, and
spinel.
5. The cover according to claim 1, characterized in that the end and side
joints of the
central and peripheral covers are coated with a sealant layer in the form of a
layer of alumina.
6. The cover according to claim 1, characterized in that the central section
of the cover
is provided with apertures.

Description

CA 02972640 2017-06-28
COVER FOR AN ELECTROLYSIS CELL FOR PRODUCING ALUMINIUM
The invention relates to non-ferrous metallurgy, more particularly to
producing
aluminium by electrolysis, and even more particularly to a structural element
for covering the
space above a melt in an electrolysis cell for producing aluminium by the
electrolysis of
cryolite-alumina melts.
The most important factor defining electrolysis cell performance is the
hermetic sealing
of a cover above a melt. The hermetic sealing of the electrolysis cell cover
is directly associated
with aluminium production costs because of its impact on heat and energy
balance, raw material
consumption and environment.
The primary electrolysis cell cover has following main purposes:
1) It provides heat balance,
2) It minimizes gaseous emissions,
3) It stabilizes operating modes,
4) It minimizes raw material losses.
The surface area of a cryolite-alumina crust for covering the operating space
of
electrolysis cell having inert anodes is three times more than that of a
standard bath. The
cryolite-alumina crust cannot hermetically seal the electrolysis cell across
such big surface area
because of strength, porous structure, occurring chemical reactions and
instability of the surface
integrity.
Taking into account mentioned above factors, it is required to hermetically
seal an
electrolysis cell having inert anodes using a primary cover made of a material
which is resistant
to a corrosive atmosphere and gaseous fluorine-containing compounds, melt
droplets and
mechanical loads Such cover should ensure lower gas permeability, integrity,
heat insulation,
and strength. A cover material and a schematic diagram of cover fixturing
should be applicable
to all types of electrolysis cells.
US Patent 5582695, IPC CO4B 7/32, CO4B 14/04, CO4B 14/02, CO4B 14/30 published
on 10.12.1996 is known. The invention relates to structural elements of
aluminium electrolysis
cells contacting a gaseous phase, in particular, it relates to covers and
anode casings. The aim
of this patent is, as disclosed in the specification, to address the problem
of hermetic sealing of
the space between side boards and anodes of an electrolysis cell having a self-
baking anode, in
particular it is related to the replacement of a current cryolite-alumina
cover consisting of a
cast-iron (steel) gas-collecting bell and a cryolite-alumina crust with an
"artificial" cover. The
suggested artificial cover is made as one piece or as a combined piece made
of the refractory

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2
concrete having following composition: 15-30 wt% of hydraulic concrete, 5-10
wt% of
microsilica, 65-80 wt% of alumina.
This solution has the following main drawback:
According to this patent, during operation the cover material contacts with
the
electrolysis cell gas-vapour phase comprising not only fluorine and sulphur-
containing
compounds. The cover material of the electrolysis cell has to be corrosion
resistant to the
electrolysis cell gas-vapour phase containing fluorine and sulfur compounds in
the presence of
oxidants CO, CO2, 02, HF, which will enable the usage of the cover for
electrolysis cells having
a Soderberg anode and backed and inert anodes. The suggested material is not
totally inert, and
during operation, due to material porosity, this material is soaked with
electrolyte vapors. When
concrete is soaked, crystal and concrete bounds are destructed resulting in
product destruction.
In addition, a method for structural electrolysis cell cover from US Patent
.1µ12
2006124471 Al, IPC C25C3/14 published on 15.06.2006 is known. The invention
relates to
structural elements of a cover and a supply system of aluminium electrolysis
cells, in particular
to the creation of a sealing cover above electrolyte of the aluminium
electrolysis cell at low-
temperature operation. One of the conditions of the manufacturing technology
is the creation
of a gel-like layer on the electrolyte surface, which is provided by
electrolyte surface isolation
with an artificial cover structure of enhanced heat insulation properties.
The electrolysis cell
cover consists of a central section arranged in a central space between anodes
along the entire
length of a bath. The central section of the cover is secured to a gas pipe
beam and is fixed
against movement, can comprise several sections, the cover movement is not
envisaged. Lateral
sections of electrolysis cell cover are movable, can be provided in the form
of several lateral
sections, which are movable independently from each other and from the central
cover. The
electrolysis cell cover is made from a ceramic material (e.g., alumina).
This solution has the following main drawback:
The central section of the cover is not adapted for changing its own position,
and a
disassembling method can be performed only with changing operating position of
anodes.
Further, alumina suppliers are required to be disassembled prior to
disassembling the central
section.
Another significant drawback is a cover material:
¨ ceramics - can be damaged when subjected to temperature difference and
mechanical
damages.
US Patent X22005230265, IPC C25C3/08, B01D59/40, BO1D59/00, C25C3/00,
C25C5/04, C25C7/00 published on 20.10.2005 is known. This invention relates to
structural

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elements of aluminium electrolysis cells contacting a vapour-gas phase of an
electrolysis cell,
in particular to a cover structure of the upper part of electrolysis cells
having backed or inert
anodes. The aim of this patent is to eliminate heat losses of an electrolysis
cell, namely, to
reduce heat transfer from a cover to an environment, to reduce heat
consumption for dissolution
of the cover fallen into a melt, to reduce heat losses by means of heat
balance stabilization and
maintaining the operation space shape. In this patent, as well as in the
previously described
solution, an artificial cover for an electrolyte melt is made in the form of
sections movable
separately.
Because of its technical specs and the number of similar essential features,
the known
technical solution is selected as the closest analog (prototype).
According to the prototype solution, sections of an "artificial" cover above
an electrolyte
melt are made in the form suspended structures. Based on their location,
covers are peripheral
and central. Peripheral cover sections are made in the form of suspended
elements movable
separately from each other, from the cover central section and from anodes. A
cover section for
the electrolysis cell central space is made in the form of the one-piece
element or in the form of
several sections. The central cover is extended along the entire length of the
electrolysis cell;
cover sections are fixed by means of suspended fasteners and are movable with
respect to the
melt separately from anodes. The central cover structure is provided with
operational doors and
apertures, thus, allowing for production operations, bath feeding with raw
material, and work
with an anode assembly. The peripheral and central covers are made so that
cover lifting and
replacement can be done in compliance with the anode horizontal arrangement,
in other words,
actions with the cover wouldn't impact the stability of the current load.
Materials resistant to
oxygen and fluoride, e.g., ceramics or composite materials of a casing based
on the nickel-
titanium alloys and a heat insulator are suggested for use as the cover
material.
This solution has the following main drawback:
This artificial cover has the same structure and cannot be considered as
versatile for
usage in electrolysis cells having backed anodes and a Soderberg anode.
The central cover is secured in the way that makes the cover movable
independently
from anodes; it inhibits replacement of a failured cover without affecting
anodes and stability
of electrolysis cell operation. Taking into account the cover section
material, the need for
periodic replacement of cover section arises. Another significant drawback is
a cover material:
¨ ceramics - can be damaged when subjected to temperature difference and
mechanical
damages,
¨ composite material - is subjected to oxidation under the fluorine-oxygen
atmosphere.

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The object of the invention is to provide a multifunctional sectional
artificial cover
above electrolyte melt in electrolysis cells having an inert, backed anode and
a Soderberg anode,
which shows corrosion and erosion resistance to an aggressive vapour-gas
environment of
electrolysis cells.
The technical effect of the present invention is to provide a hermetically
sealed cover, a
reliable and safe structure, and a reduction in energy consumption.
Such technical result is achieved by that in a cover for an electrolysis cell
for producing
aluminium, which is in contact with a vapour-gas phase when the electrolysis
cell is in operation
and which is in the form of central and peripheral sections which are movably
arranged relative
to each other, the central and peripheral sections are made of a corrosion and
erosion-resistant
material which comprises 80.0-99.0 wt% of fluorophlogopite and 20.0-1.0 wt% of
a refractory
filler.
The central sections of the cover may be permanently fixed on each anode rod,
and the
peripheral sections may be configured as convex panels rigidly and removably
fixed on the top
surface of a cathode and supported by the central section of the cover.
Moreover, the refractory
filler may be chosen from the following chemical substances: clay, calcium
fluoride, rutile,
sodium aluminosilicate, fluorapophyllite, nepheline, olivine, magnesium
fluoride, and spinel.
The end and side joints of the central and peripheral covers may be coated
with a sealant
layer in the form of a layer of alumina, and the central section of the cover
may be provided
with apertures.
The subject matter of the present invention is as follows:
a cover is manufactured in the form of large-sized products in the form of
plates (Fig.1).
The cover is arranged above the top surface of the anode so that it doesn't
contact with the
electrolyte melt and consists of sections. One complete cover section consists
of a central
section 1 and a peripheral section 2. The central section 1 of the cover is
directly fixed on an
anode rod by means of protrusions, wherein the distance from the cover surface
to the
electrolyte melt is selected based on the electrolysis cell type and
specifications. This selected
distance makes allowance for the likelihood of brief exposure to electrolyte
run-ups onto the
cover working surface when changing anode-to-cathode distance in the
electrolysis cell. Cover
central sections are provided with apertures for alumina dispensers arranged
in accordance with
an electrolysis cell feeding scheme. The peripheral section 2 of the cover is
mounted so that
one of its sides is supported by the cover central section, and another side
is supported by the
top surface of the cathode (such as a flange, an edge, a lining). The width of
the peripheral

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section is equal to that of the central section and is a parabola in shape,
thus enabling quick
cover removal from the bath for technological operations to be performed. In
this way, central
and peripheral sections of the cover define a cover section for one anode on
the blank or front
side, and the number of cover sections corresponds to the number of anodes.
Each cover section
is movable together with the anode and can change own position separately from
neighbor cover
sections and neighbor anodes. The electrolysis cell cover permanently contacts
the gas-air
environment of the electrolysis cell and periodically contacts the electrolyte
melt, that is why
the cover is manufactured of the corrosion and erosion resistant material,
proof to the aggressive
vapour-gas environment of electrolysis cells having backed, self-baking, or
inert anodes. The
used cover material is not soaked and wetted with the cryolite-alumina melt.
For cover
manufacturing, fluorophlogopite or alumina slurries can be used. Independent
movement of the
cover allows for hermetic sealing of the operation space of the electrolysis
cell having inert
anodes and makes technological operations simple and mobile. Any cover section
can be
disassembled independently from the others and replaced with a new part.
One of the factors of variance for a fluorophlogopite cover is a chemical
composition
of an agglomerate in terms of the main component KMg3(Si3A1)010F2*, because
the change of
material chemical purity results in the change of physical and chemical
properties of the
material, and consequently, its mechanical strength, heat conductivity, and
corrosion and
erosion resistance. When fluorophlogopite is used as the cover material, it is
required to use the
material containing the main component, fluorophlogopite, in the range of 80-
99%, which will
allow flexible usage of materials for different types of electrolysis cells.
In addition to hermetic
sealing, heat insulation of the operation space above a melt in an
electrolysis cell,
fluorophlogopite ensures melt and aluminium purity. Operational apertures in
the central
section of the cover allow using the cover for electrolysis cell feeding,
cover installation and
disassembling, and maintenance operations. To seal the joints of cover
sections alumina is used,
which is chemically similar to the fluorophlogopite material. Operational
through-apertures in
the ends of the cover central section in the electrolysis cell center provide
removal of the gas-
air mixture from the electrolysis cell to a gas removal system.
The procedure for preparation of the electrolysis cell having the claimed
cover includes
following steps:
Central and peripheral cover sections are produced from the machined blanks.
Then,
appropriate apertures are made in the produced sections according to location
and position
markings in the electrolysis cell. Next, these cover sections are placed on
anodes mounted or
being mounted in the following sequence: at first, the central section and
then the peripheral

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section. The cover made of fluorophlogopite can be placed in an active
electrolysis cell because
this material is resistant to thermal shocks.
The electrolysis cell cover according to the suggested solution features
improved
reliability, provided by not only the corrosion and erosion resistance of a
cover material but
also by an independent structural feature of the sectional cover allowing
access to any part of
the electrolysis cell in a short time without significant intervention in the
process. In addition,
thermal expansion of the cover material is prevented due to the material
properties, allowing
free movement of the cover along adjacent covers.
The ease of the suggested cover manufacture process is ensured by the
processability of
the inventive cover material (fluorophlogopite) and the simplicity of a
fastening system on an
anode rod allowing for section functionality ensured by means of the product
strength and the
shape of fastening surface and by supporting the central cover.
Recently conducted long-term laboratory and pilot-scale tests of the claimed
fluorophlogopite cover according to the suggested technical solution have
shown its functional
ability and efficiency.

Representative Drawing