Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM AND METHOD FOR CONTROL OF LAYER FORMATION IN AN
ALUMINIUM ELECTROLYSIS CELL
Background of the Invention
Technical Field
The invention relates to heat regulation in general and particularly improved
method and system for cooling over a large area, suitable for use for control
of layer
formation over an extended area in an aluminium electrolysis cell and
exploitation of
heat.
Background Art
During production of aluminium with electrolysis technology of today based on
so called Hall-Heroult cells, the operations of the cells depend on the
formation and
maintenance of a protective layer of frozen electrolyte in the side walls of
the cell.
This frozen bath is called side layer and protects the side lining of the
cells against
chemical and mechanical wear, and is an essential condition for achieving long
lifetime of the cells. The frozen bath operates simultaneously as a buffer for
the cell
with regards of changes in the heat balance. During operations the heat
generation
and the heat balance of the cell will vary due to unwanted disturbances of the
operation (changes in bath acidity, changes in alumina concentration, changes
in
interpolar distances, etc.) and desired activities on the cells (metal
tapping, change
of anode, fire, etc.). This causes the thickness of the layer of the periphery
of the cell
to change and in some cases the layer will disappear entirely in parts of the
periphery. Then the side lining will be exposed against the electrolyte and
metal,
which in combination with oxidizing gasses will lead to corrosion of the side
lining
materials causing these to erode. During operations over time run outs in the
side
can result from such repeated occurrences. It is therefore of importance to
control
formation of layer and layer stability in Hall-Heroult cells. For Hall-Heroult
cells with
high current densities model calculations show that it will be difficult to
maintain the
side layer of the cell due to large heat generation. For such cells and for
traditional
cells with heat balance problems it will therefore be a condition for a long
life cell that
one is able to maintain the layer protecting the side lining.
During production of aluminium in accordance with Hall-Heroult principle, this
takes place at present with relatively high use of energy as measured in kilo
watt
hours per kilo aluminium. The heat generation of the electrolysis cells takes
place as
a result of ohmic voltage drops in the cell, for instance in current feeds,
produced
metal and particularly in the electrolyte. Approximately 55 % of input energy
to the
electrolysis cell is used for heat generation in the cell. Data from
literature indicates
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that approximately 40 % of the total heat loss from the cells is lost through
the side
lining. Due to the high heat loss and the protecting frozen layer in the side
lining it is
a preferable place to place elements for heat regeneration in this area of the
cell.
There is a desire for optimizing control of layer formation and heat
regeneration. In order to optimize both of this purpose at the same time it is
important that heat regeneration takes place as close to the formed side layer
as
possible. This will lead to the control of and speed on layer formation is as
fast as
possible, and that temperature difference between input and output cooling
medium
is as large as possible. The latter is preferable for
exploitation/regeneration of
energy.
Furthermore, due to the large scale of electrolysis cells, it is also
desirable to
control said layer formation over an extended area since loss of layer
formation over
a small area can be damaging.
The traditional method of removing heat was to use air convection over the
entire surface area of the cell, resulting in limited potential for
exploitation of the
removed heat.
From the known art one should refer to granted patent NO 318012,
corresponding to WO/2004/083489. This describes a sidelining formed with
hollows
for flow-through of a cooling medium. The manufacturing process of this,
however, is
complex and requires the side linings to be moulded with hollows formed
preferably
before the material is sintered.
From the known art one should also refer to patent application NO 20101321,
brought into the PCT-phase as PCT/N02011/000263, of the present applicant.
This
describes a system for use for control of layer formation in an aluminium
electrolysis
cell and exploitation of heat comprising sidelining provided with at least one
hollow
for heat transfer and at least one heat tube, characterized in that the heat
tube is
provided by the hollow and that the hollow is at least one canal provided
along the
surface of the sidelining. The manufacturing process of this, however, is
complex
and requires providing the side linings with a large number of heat tubes,
typically
heat pipes, along the surface of the sidelining, each requiring separate
cooling.
One should also refer to flat heat pipes, also known as two-dimensional heat
pipes, based on plates forming thin planar capillaries. This design is useful
for heat
spreaders in height sensitive applications, however as the capillaries are
small and
thin the total heat transfer is small. Also this design features large metal
areas that
are not actual parts of the capillaries, further reducing the total heat
transfer.
Furthermore this design is typically flat whereas some surface roughness of a
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sidelining should be expected, leading to poor thermal contact. This means
that flat
heat pipes are not suited for cooling a sidelining.
In general it is a problem that efficient cooling over a large area requires a
large number of parts which in turn adds complexity and cost while also
reduces
overall reliability.
There is therefore a need for a method and a system overcoming the above
mentioned problems.
Summary of the Invention
Problems to be Solved by the Invention
Therefore, a main objective of the present invention is to provide a method
and
system for use for control of layer formation over an extended area in an
aluminium
electrolysis cell and exploitation of heat.
A second object of the invention is to provide a method and system for use for
control of layer formation suited for retrofitting to an aluminium
electrolysis cell.
Means for Solving the Problems
The objective is achieved according to the invention by a system for use for
control of layer formation in an aluminium electrolysis cell as defined in the
preamble
of claim 1, having the features of the characterising portion of claim 1, and
a method
for control of layer formation in an aluminium electrolysis cell as defined in
the
preamble of claim 8, having the features of the characterising portion of
claim 10.
The present invention attains the above-described object by a flat heat tube
for
attachment to the steel casing of an aluminium electrolysis cell. This heat
tube can
be a heat pipe or a thermosyphon, or even a cooling pipe using a fluid not
undergoing substantial phase transition. Preferably the heat tube is provided
with a
substantially flat surface. Preferably the heat tube has a meandering shape.
In a first embodiment a system for use for control of layer formation in an
aluminium electrolysis cell and exploitation of heat, said electrolysis cell
comprising a
sidelining and a shell is provided, further comprising a surface attached heat
tube
provided with means for attachment to said shell.
In a preferred embodiment the heat tube is a heat pipe.
In a another embodiment the heat tube is a thermosyphon.
In a further embodiment the thermosyphon is provided with a substantially
downward
inclination.
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In a preferred embodiment the heat tube has a meandering shape.
In a preferred embodiment the heat tube is provided with at least one flat
face suited
for attachment to the surface of the steel shell 8 of the electrolysis cell.
In a embodiment the flat face is provided with a longitudinal track.
In a preferred embodiment the longitudinal track runs parallel to the heat
tube.
In a another preferred embodiment the longitudinal track runs in a meandering
path
with respect to the heat tube.
In a preferred embodiment a method for control of layer formation in an
aluminium
electrolysis cell said electrolysis cell comprising a sidelining and a shell
is provided,
further comprising a heat tube provided with means for attachment to said
shell is
attached to said shell, conducting the heat away using said surface attached
heat
tube.
Effects of the Invention
The technical differences over prior art is that the present invention is for
attachment to the steel shell covering a ceramic block, and freezes out a
sidelayer in
the electrolyte bath by extracting heat through the ceramic which is in
thermal
contact with the steel shell. Prior art in contrast describes cooling by
embedding
fooling features in or embedded in the ceramic block.
The technical effect of the flat heat tube is that it extracts heat from a
large
area. Also by using a meandering shape a large surface area can be covered by
a
single heat tube.
These effects provide in turn several further advantageous effects:
= it makes it possible to provide a convenient solution having few parts
for
cooling a large area,
= it provides a robust and greatly simplified production and installation
without
the need of purpose formed ceramic blocks,
= it can be retrofitted to existing cells, even during uninterrupted
operation
= reduction of risks in emergency situations
= maintenance during operation, and
= the heat pipe can be changed during operation
For heat tube employing substantially phase transition some further
advantages are:
= the heat pipe converts between low heat flux in the evaporation side to high
heat flux on the condensation side, enabling a small size heat exchanger,
= only a limited amount of working fluid is required, and
= the surface of the heat pipe is isothermal.
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It should be noted that the present invention differs from other heat pipe
solutions such as circulating heat pipes where fluid in vapour phase flows in
separate tubes from tubes conducting fluid in liquid phase, thus adding extra
tubes
5 and further complexity.
Brief Description of the Drawings
The invention will be further described below in connection with exemplary
embodiments which are schematically shown in the drawings, wherein:
Figure 1 shows state of the art of a Hall-Heroult cell in the form of a
sidelining block,
and a steel shell or casing,
Figure 2 shows a detail section of the embodiment of figure 1 together with
section
as seen from the side,
Figure 3 shows state of the art of a Hall-Heroult cell in the form of a
sidelining block
with hollows provided with heat tube, and a steel shell or casing,
Figure 4 shows the present invention installed on a cell
Figure 5a shows an end view of a first embodiment,
Figure 5b shows a side view of the first embodiment,
Figure Sc shows a front view of the first embodiment,
Figure 5d shows a cross section A-A of the first embodiment,
Figure 5e shows a cross section A-A of the first embodiment having a
longitudinal
track,
Figure 5f shows a cross section B-B of the first embodiment,
Figure 6a shows an end view of a second embodiment,
Figure 6b shows a side view of the second embodiment,
Figure 6c shows a front view of the second embodiment,
Figure 6d shows a cross section A-A of the second embodiment, and
Figure 6e shows a cross section A-A of the second embodiment having a
longitudinal track.
Description of the Reference Signs
The following reference numbers and signs refer to the drawings:
1 Anode hanger
2 Anode carbon block
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3 Liquid electrolyte
4 Liquid aluminium
Cathode carbon
6 Frozen electrolyte
7 Insulating brickwork
8 Steel shell
9 Ramming paste
Heat insulation
11 Sidelining block
12 Heat tube
13 Condensation unit for heat tube
14 Condensation fins
Thermal paste between sidelining block and steel shell
100 Surface mounted heat tube
110 Heat tube cold end
120 Condensation unit for heat tube
122 Condensation fins
124 Fluid connector
130 Heat tube hot end
132 Lower end of heat tube
140 Flat face
142 Flat face attachment holes
144 Transverse member
146 Transverse member attachment holes
150 Longitudinal track
152 Entry hole to longitudinal track
154 Exit hole to longitudinal track
Detailed Description
The invention will in the following be described in more details with
references
5 to the drawings showing embodiments and where figure 1 shows state of the
art of a
Hall-Heroult cell in the form of a sidelining block 11 and a steel shell 8 or
casing.
Details are shown in figure 2. A state of the art cell using active cooling as
known
from previously mentioned prior art is shown in fig. 3.
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With sidelining one should here understand this to mean sidelining block 11,
optionally in the case of state of the art together with the heat insulation
10, wherein
the sidelining block is optionally provided with heat tube 12. The sidelining
block 11
is typically a ceramic block, typically in the form of silicon carbide (SiC).
Principles forming the basis of the invention
By heat tube 12, 100 there are two embodiments intended: "heat pipe" where
a wick or other capillary effect pulls the liquid back to the hot end, and
"thermosyphon" where the gravity pulls the liquid back to the hot end. The hot
end is
also known as the evaporation section. Both principles can be applied for this
invention, though a thermosyphon it is preferred that the tube body is
provided with a
substantially downward inclination so that fluid in the liquid phase can run
down the
length of the tube. Since heat tubes of either type operate by removing heat
by
phase transition liquid to gas, it is preferred that the heat tube allows
liquid to reach
the lowest point in the heat tube.
A typical Hall-Heroult cell comprises a steel casing or shell 8, surrounding a
sidelining block 11. The steel casing is in good thermal contact with
sidelining block
due to a thermal paste. The sidelining block, on the opposite side from the
steel
casing, is in contact with the electrolyte containing aluminium (Al). By use
of thermal
control the heat extracted from the electrolyte builds up a layer of frozen
electrolyte
on the sidelining, leaving the remaining part of the electrolyte 3 in the
liquid phase.
Central in the invention is the realisation that it is possible to remove a
sufficient amount of heat by attaching a heat tube 100 to the steel casing.
When a
heat tube operates to remove heat, the steel shell does not become overheated,
and
with the high thermal conductivity present through the metal and the thermal
paste,
the layer of frozen electrolyte 6 can be maintained.
Best Modes of Carrying Out the Invention
The embodiment of the apparatus according to the best mode of invention
shown in Fig. 5 and 6 comprises a meandering thermosyphon 100 hot end having a
substantially continuously downward component with reference to gravity, and
an
alternating right and left component horizontally. At the upper end a cold end
is
attached. The cold end is also known as the condensation end. At the cold end
the
condensation unit is placed having condensation fins attached to the cold end
for
efficient heat transfer. Fluid enters a first fluid connector, into the
condensation unit
where heat is removed from the cold end, and out through a second fluid
connector.
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As working fluid in the liquid phase enters the hot end 130 the fluid runs
downwards, evaporating as it flows and thus removes heat from the evaporator
end.
It is preferred that a sufficient amount of heat reaches the lower end 132 of
the heat
tube 100 in order to extract heat along the entire length of the heat tube
from the cell
through the sidelining block 11.
Some horizontal extents of the heat tube can be accepted, also small
amounts of depressions. Any depressions will catch fluid in the liquid state,
limiting
the amount that continues downward.
As heat is absorbed by the fluid in the liquid state it will make a phase
transition to the vapour phase, boiling, and the fluid in the vapour phase
will move
upwards to the top where the cold end 110 or condensation end is positioned.
Condensation to the liquid phase takes place using a heat exchanger 130,
transferring the heat typically to an oil or molten salt circuit.
The heat tube 100 is typically in the form of a thermosyphon, since gravity is
sufficient for ensuring fluid in the liquid phase is transported down the heat
tube.
In a preferred embodiment the heat tube is provided with at least one flat
surface suited for attachment to the surface of the steel shell 8 of the
electrolysis
cell. This can be made through moulding as well as by welding a traditional
heat tube
to a suitably formed part having a flat surface or flat face 140.
For the attachment it is preferred that the surface attached heat tube is
provided with holes 142 for attachment to the steel shell. Holes can be
provided
along the sides of the flat face of the heat tube. The flat surface is then
provided with
thermal conductive paste, of which many are well known in the art, and then
attached to the steel shell using the holes by bolts, screws or other methods
well
known in the art.
For improving efficiency of extraction of useful heat the steel shell assembly
with the heat tube is provided with thermal insulation, preventing heat from
leaking
into the surroundings. This is shown in fig. 4.
Alternative Embodiments
A number of variations on the above can be envisaged. For instance one can
envisage a hybrid heat pipe ¨ thermosyphon solution, wherein a wick structure
raises the liquid phase through capillary action up along the part of the tube
close to
the external flat face of the heat tube, thus increasing the effective area
for phase
transition.
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Several variations can be envisaged for attachment holes. In addition to the
holes described above, one can also provide the surface attached heat tube
with
transverse members 144, extending substantially transverse to the heat tubes.
Said
members provide a better working distance for providing holes 146 and
attachment,
with respect to the more fragile heat tubes 12.
Providing sufficient and even distribution of heat paste is important for
optimal
heat transfer. While manual application of thermal paste prior to attachment
is a
simple solution it is not certain to be sufficient. First of all the outer
surface of the
steel shell 8 might not be as smooth and level as the flat face of the heat
tube.
Furthermore it might become necessary later to add more or replenish lost heat
paste. One solution is to provide a longitudinal track 150 in the flat face
and
preferably also an access hole to 152 this track for applying heat paste under
pressure through said hole and into said track, preferably until heat paste
starts
exiting through an exit hole 154. The longitudinal track can be straight or
meandering. The effect of this is to simplify application of heat paste and
enable
replenishing of heat paste without having to interrupt operations of the cell.
Alternatively adhesive or thermal glue can be used.
Also the steel shell and the heat tubes might not be as separate parts, rather
the heat tubes could be moulded into the steel shell during manufacture, as a
monolithic unit.
While the shell of the cell is described as being made of steel, it should be
clear that any other material will also work as long as it can conduct heat
and
withstand the temperatures involved. Several alternatives can be envisaged,
such as
ceramic materials.
Industrial Applicability
The invention according to the application finds use in control of layer
formation in an
aluminium electrolysis cell and exploitation of the heat.
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