Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINIUM REDUCTION CELL WITH A HEAT INSULATED
SIDE LINING
Field of the invention
The invention relates to the non-ferrous metallurgy field, namely, to the
design of reduction cells intended for the electrowinning of aluminium, and
can
.. be implemented in reduction cells of all types.
Background of the invention
In the existing designs of reduction cells intended for the electrowinning
of aluminium, the highest component of heat loss (about 35 %) falls on the
longitudinal side walls (Fig. 1) (see Zhaowen Wang et al., Heat Recovery of
the
Aluminium Reduction Cells. Production of Aluminium: Collection of Reports
of the 9th International Congress 'Non-Ferrous Metals and Minerals.'
Krasnoyarsk: North-Eastern University, 2017, 209-217). This amount of heat
loss negatively affects the thermal balance of the reduction cell making it
necessary to recover heat losses by increasing the voltage supplied to the
reduction cell, which entails increased electricity consumption and, as a
result,
an increase in the cost of aluminium produced.
An aluminium reduction cell with a side lining is known (application RU
94012661, C25C3/06, publication date: April 10, 1996), which allows forming
stable decking and ensuring a long service life of the reduction cell. The
main
distinguishing feature of this design is an inclined longitudinal side wall
made
of side graphitised carbon blocks. In this case, the side lining of the
reduction
cell is inclined to the horizon at an angle of 0.3-0.9 (I), where (I) is the
angle of
inclination of the side wall of the cathode shell to the same plane.
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There are known 'Aluminium reduction cell cathode device' (RU
1295786, C25C3/08, published in 1985), where the angle of inclination of the
side wall is 45-66 , 'Aluminium reduction cell' (SU 1788090, C25C3/08,
published in 1990), where the angle of inclination of the side wall is 20-45 ,
Tlectrolyser for production of aluminium' (RU 94009828, publication date:
April 10, 1996), where the angle of inclination of the side wall is 120 .
The disadvantage of these technical solutions is due to the fact that during
the electrolytic reduction process, the force from the thermal expansion of
the
bottom graphitised carbon blocks, acting on the inclined side wall, causes a
force directed perpendicular to the inclined walls of the shell. The
tangential
component, acting on the inclined side blocks, pushes them by a deckplate, the
loads on the deckplate increase significantly resulting in destruction of the
side
lining. Therefore, the use of inclined side walls requires an increase in the
rigidity of the cathode shell, in turn, an increase in the rigidity of the
shell is
associated with an increase in its weight and an increase in capital costs.
Also, the disadvantage is that with this version of the side lining, there is
oxidation and destruction of coal plates as a result of their exposure to
oxygen
of the air entering through the bath crust resulting in leakage of the bath
melt
into the lining and deterioration of thermal insulation properties. As a
result of
leaks, forces arise that push the coal plates away from the cathode shell and
subsequently destroy the side lining.
In addition, in case of inclined design of the side wall, stabilisation of the
thermal balance of the reduction cell by reducing heat loss from the sides
becomes possible only after the start-up period of the reduction cell, i.e.,
after
the formation of metal ledge and slag lining. Thus, the heat loss from the
sides
with an inclined longitudinal wall during the start-up period will remain
unchanged, which will require heat compensation by increasing the voltage
supplied to the reduction cell.
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A side lining of the aluminium reduction cell is known (patent RU
2072398, C25C3/06, publication date: January 27, 1997), in which a belt is
made of a non-conductive ceramic material based on aluminium nitride. The
belt is made of plates attached to each other by end faces with the help of
asymmetric protrusions and recesses using glue with additives of refractory
compounds. The disadvantage of this solution is the use of expensive
materials with low barrier resistance to the effects of the bath melt.
There is a known reduction cell for producing aluminium and a method
for maintaining a crust on the side wall and regenerating electricity
(application
RU 2002135593, C25C3/06, publication date: May 29, 2001), wherein a high-
temperature, heat-resistant and heat-insulating material is located inside, on
the
inner part of the side walls of the steel shell, characterised in that all the
side
walls of the reduction cell are equipped with cooling evaporative panels. The
disadvantage of this solution is that the thermal balance of the reduction
cell of
the claimed design depends on the metal ledge and slag lining intensively
formed by cooling the sides with evaporative panels, in each case of deviation
from the preset process parameters of the reduction cell operation and change
of
the shape of the working space, the thermal balance of the reduction cell will
become unstable. The location of the cooling evaporative panels inside the
cathode shell is critical, since the contact of the panels with the aggressive
bath
environment will significantly reduce their service life, and the protection
of the
panels by coating them with a heat-resistant, heat-insulating material is
associated with a significant increase in capital costs.
The closest in terms of technical substance to the claimed invention is the
side lining of a reduction cell (copyright certificate SU 377419, C25C3/08,
publication date: April 17, 1973), wherein the side lining is made of
materials
with different resistance to melt, while its upper part is made of a material
with
increased resistance ¨ graphite plates, and its lower part is made of a
material
with reduced resistance ¨ baked carbon blocks. The disadvantage of this
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prototype is that after sintering with an adhesive layer at a high speed of
melt
circulation, the insert is destroyed due to deformation shifts occurring in
the
upper parts of the cathode shell.
Disclosure of the invention
The objective of the proposed invention is to reduce heat losses from the
side walls of the reduction cell and to stabilise the thermal balance of the
reduction cell, as well as to ensure formation of a stable metal ledge and
slag
lining, which ensures a long service life of the reduction cell, a stable
behaviour
of the electrolytic reduction process and allows to significantly reduce
electric
power consumption thanks to the operation of the reduction cell at a lower
operating voltage.
The technical result consists in solving the task at hand, reducing the
operating voltage of the reduction cell by reducing heat losses from the side
walls of the reduction cell, stabilising the thermal balance and increasing
the
MHD (magnetohydrodynamic) stability of the reduction cell.
As a result of the stabilisation of the thermal balance of the reduction cell,
the above technical result also includes a reduction in the electric power
consumed, while an increase in MHD stability allows to increase the current
efficiency, which in turn allows to reduce the cost of aluminium produced.
The task is addressed, and the technical result is achieved due to the fact
that in a reduction cell intended for the electrowinning of aluminium,
including
a metal cathode shell, heat-insulating and refractory lining, a bottom made of
bottom blocks with cathode current-carrying rods, a side lining, the novelty
consists in the fact that the side lining is made of silicon carbide and/or
carbon
plates with an additional moulded refractory layer with lower thermal
conductivity installed between the walls of the metal cathode shell and
silicon
carbide and/or carbon plates.
To implement the proposed invention, it is advisable to use carbon plates,
silicon carbide materials, chamotte. Silicon carbide is a material obtained by
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combining powdered silicon carbide with soot, characterised by high density,
high degree of heat resistance, electrical conductivity and resistance to the
process of scale formation.
Multi-layer structures can be used, where the first layer is formed by a
silicon carbide plate or a cheaper carbon one. The material of the first layer
should be erosion-resistant, with low electrical and thermal conductivity,
with
low porosity and high density. The refractory layer can be made of chamotte
material or represent a multi-layer structure. The dimensions of the
refractory
layer are determined in accordance with the design of the cathode shell, and
depend on the target technological parameters of the reduction process. It is
important that the first silicon carbide layer is erosion-resistant to an
aggressive
electrolysis medium (electrolyte), and the second, for example, from chamotte,
is heat-resistant to reduce heat losses.
The moulded refractory layer can be made of monolithic refractory plates
and/or refractory bricks, can contain inserts of additional heat-insulating
material located inside the masonry of refractory plates and/or refractory
bricks,
while the inserts can be made of moulded and/or unmoulded thermal insulation
material.
The additional refractory layer can also be made of unformed material. In
this case, the installation of the side lining provides for the manufacture of
a
special tooling (frame), which is filled up with the unformed material for its
further ramming, drying and solidification into a refractory layer in the
shape
given by the tooling.
Previously, a two-layer lining was not used, i.e., there was no lining made
of two different materials, either carbon lining (cheap, with low resistance
to
overheating, and low thermal conductivity) or silicon carbide lining (more
expensive, more corrosion resistant, but also heat conducting with high heat
losses) were used.
The invention essence is explained by the drawings.
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Fig. 2 a) presents the results of modelling the temperature field and the
shape of the working space of a reduction cell with a two-layer insulated side
lining in comparison with a reduction cell of the existing serial design b);
Fig. 3 shows a reduction cell intended for the electrowinning of
aluminium;
Fig. 4 shows the additional refractory layer.
Explanations of the structural elements shown in the drawings:
1 ¨ metal cathode casing;
2 ¨ heat-insulating lining;
3 ¨ refractory lining;
4 ¨ bottom made of bottom blocks;
5 ¨ cathode current-carrying rods;
6 ¨ carbide-silicon plates;
7 ¨ additional moulded refractory layer;
8 ¨ inserts of additional heat-insulating material;
9 ¨ refractory bricks/plates.
Until now, a side lining made of carbon blocks, later a side lining made of
silicon carbide plates was used in the reduction cells. Carbon blocks are
mostly
cheap and have low thermal conductivity, which minimises heat losses from the
sides of the reduction cell. At the same time, carbon blocks have relatively
low
erosion resistance (low resistance to high overheating, as well as to the
aggressive reduction process environment ¨ the bath), which reduces the
service life of the reduction cell. Expensive silicon carbide plates, on the
contrary, have a high resistance to overheating and high resistance in the
bath
environment. A significant disadvantage of silicon carbide plates is their
high
thermal conductivity, which entails large heat losses from the sides of the
reduction cell, which require compensation in the form of voltage to stabilise
the thermal balance, which leads to increased electricity consumption ¨ an
increase in the cost of aluminium.
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The proposed heat-insulated structure of the side lining, consisting of
silicon carbide plates bordering the bath and an additional refractory layer
between the silicon carbide plates and the reduction cell casing, combines
both
high overheating resistance and high resistance to the bath, and low thermal
conductivity, which ensures a long service life with a stable thermal balance
of
the reduction cell, which will allow implementing reduction technologies at
lower voltage, with lower electric power consumption, with lower cost of
aluminium produced.
Below are the results of mathematical modelling of a reduction cell with a
side lining of the claimed design.
The calculation results showed that to ensure the thermal balance of a
reduction cell with a two-layer (or more) heat insulated side lining, with the
bath temperature of 956 C, an interpolar distance of 48.3 mm is necessary.
Under these conditions, the total heat loss from the anode device will be
106.0
kW (42.1 %), from the longitudinal walls of the cathode device ¨ 86.5 kW
(21.5 %), from the end walls of the cathode device ¨ 37.3 kW (14.9 %), from
the bottom of the cathode device ¨ 22.8 kW (8.6 %). The maximum
temperature of the longitudinal wall of the cathode device will be 235 C, the
maximum temperature of the end wall of the cathode device will be 155 C, the
maximum temperature of the bottom of the cathode device will be 70 C. The
length of the metal ledge along the longitudinal wall will be 150 mm, along
the
end wall ¨ 162 mm under the anode. The minimum thickness of the slag lining
will be 30 mm along the longitudinal wall and 64 mm along the end wall at the
level of the carbon block insert. The average voltage will be 4,050 V, and the
electric power will be 12,823 kWhit.
Within the framework of pilot scale tests of insulated double-layer (and
more) side lining, the additional refractory layer was glued to the cathode
shell
of the reduction cell, a layer of silicon carbide plates was also glued to the
additional refractory layer. During the installation of the two-layer side
lining,
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no special requirements were imposed on the gluing materials, as well as on
the
laying operations. During industrial tests on an experimental reduction cell,
thanks to the insulated design of the side lining, a reduction in electric
power
consumption by 300 kWhit was achieved compared to the existing reduction
cells provided with silicon carbide side lining.
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