Note: Descriptions are shown in the official language in which they were submitted.
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A FURNACE AND A METHOD FOR COOLING A FURNACE
FIELD OF THE INVENTION
The present invention relates to a furnace and to a method for cooling a
furnace. More particularly, the furnace of the present invention is a furnace
in which
a high temperature process is conducted under oxidising conditions.
BACKGROUND TO THE INVENTION
Top submerged lance type furnaces are known. An example of a top
submerged lance type furnace is a furnace available from Xstrata Technology
Pty
Limited under the trademark ISASMELTTM . Figure 1 shows a schematic diagram of
such a furnace. The furnace 10 shown in figure 1 includes a barrel section 12
and an
offgas section 14. A bath of molten material 15 is held inside the furnace and
a lance
16 is lowered into the bath of material 15 such that the tip of the lance 16
is immersed
in the bath 15. Air or oxygen and a fuel, such as fuel oil or coal or coke, is
injected
through the lance. The fuel is combusted to heat the furnace. These furnaces
are used
in processes such as copper converting, lead smelting and the like. Such
processes are
operated under high temperature and under oxidising conditions due to the
injection
of air or oxygen through the lance into the furnace.
Top submerged lance type furnaces are typically constructed such that they
have an outer steel shell with an inside lining of refractory material. The
refractory
material protects the outer steel shell from the extremely high temperatures
experienced inside the furnace. The, inside lining of refractory material, is
sometimes
divided into an inner and an outer layer. The inner layer is sometimes
referred to as
the working lining and the outer layer is sometimes referred to as the backing
lining.
The backing lining comprises of a much more insulating refractory composition
compared to the working lining. Throughout this specification, the term
"working
lining" will be used to refer to the part of the lining that is adjacent the
hot contents of
the furnace and the term "backing lining" will be used to refer to the part of
the lining
that is adjacent the outer shell of the furnace.
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In a number of furnaces, efforts have been made to cool the outer steel shell
(and thereby increase the rate of heat removal from the furnace). Systems that
have
been used for external shell cooling comprise spray cooling or film cooling
systems.
In these systems, water is sprayed onto or runs down the external face of the
outer
steel shell. The water extracts heat from the outer steel shell, thereby
cooling the
outer steel shell. However, due to the system being exposed to the atmosphere,
combined with the relatively high shell and water temperatures, extensive
corrosion of
the outer steel shell can occur. Regular cleaning and maintenance of the
surface of
the outer steel shell is required to prevent the insulating corrosion layer
that would
otherwise form on the outer steel shell from inhibiting the heat transfer from
the shell
to the cooling water. Even with a clean outer shell surface, the heat transfer
coefficient between the shell and the cooling water is relatively low due to
the use of
low water velocities and pressures.
External shell-mounted forced cooling water systems have been used on
various types of furnaces. The external shell-mounted forced cooling water
systems
typically comprise steel channels welded to or formed on the external surface
of the
outer steel shell (or furnace steel shell), enabling the flow of water against
the furnace
steel shell under relatively high pressures and velocities, ensuring a high
heat transfer
coefficient between the water and the shell. This results in the effective
removal of
heat from the furnace shell whilst preventing contact between the water, the
cooled
surface, and the atmosphere. Furthermore, the quality of the water that has
passed
through the cooling channels can be controlled to prevent or minimise
corrosion of the
furnace steel shell. As a further safety advantage, as the cooling water
channels are
mounted or formed externally to the outer steel shell, any leaks that may
occur in the
cooling water channels result in water running down the outer face of the
outer shell.
In this regard, it will be understood that it is important that any water
leaks not cause
water to leak into the interior of the furnace as this could potentially cause
the furnace
to explode due to the rapid generation of steam from such water leaks.
Throughout the specification, the term "comprising" and its grammatical
equivalents shall be taken to have an inclusive meaning unless the context of
use
indicates otherwise.
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BRIEF DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a furnace and a method for
cooling a furnace that is appropriate for use in furnaces in which oxidising
conditions
are encountered within the furnace.
In a first aspect, the present invention provides a furnace in which a high
temperature process is conducted under oxidising conditions in the furnace,
the
furnace comprising an outer shell made from a metal, one or more cooling
channels
formed on or joined to the outer shell and a furnace lining, the furnace
lining
comprising a backing lining comprising a relatively high thermal conductivity
layer
positioned adjacent to an inner wall of the outer shell and a working lining
positioned
inwardly of the layer of relatively high thermal conductivity.
Throughout this specification, a furnace is to be taken to be operating under
oxidising conditions if the partial pressure of oxygen in the furnace
atmosphere is
greater than 10-9 atm.
The working lining may be positioned against the backing lining.
In embodiments of the present invention, the backing lining has a thermal
conductivity that is significantly higher than the thermal conductivity of the
working
lining. In some embodiments, the backing lining has a thermal conductivity
that is
similar to or even higher than the thermal conductivity of the outer shell.
In some embodiments, the backing lining comprises a graphite layer or a layer
made from a material including graphite or a layer of a carbon-based material.
In
some embodiments, the backing lining comprises a graphite layer.
In one embodiment, the outer shell of the furnace comprises a steel shell.
In some embodiments of the present invention, the one or more cooling
channels may be welded to an external surface of the outer shell.
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The one or more cooling channels may comprise cooling water channels for
receiving cooling water. The cooling water channels may receive cooling water
that
has a high pressure and a high velocity of travel through the cooling water
channels.
The one or more channels may be arranged in a serpentine pattern. The one or
more channels may comprise a plurality of channels that are spaced apart from
each
other.
The furnace may comprise a top submerged lance furnace.
The relatively high thermal conductivity backing lining, such as a graphite
layer or a layer made from a material including graphite or a layer of a
material of
graphite or a layer of a carbon-based material may be positioned throughout
all of the
lining of the furnace. Alternatively, the layer may be positioned in only a
portion or
portions of the furnace.
The graphite layer or layer made from a material including graphite or layer
of
a material of graphite or a layer of a carbon-based material may comprise a
plurality
of graphite tiles or graphite bricks or tiles or bricks made from a material
including
graphite or a material of graphite or a layer of a carbon-based material that
are glued
or cemented or otherwise affixed to the inside surface of the outer shell of
the furnace.
Where a cement or glue is used for this purpose, the cement or glue may be
graphite
or carbon-based with a high thermal conductivity.
The graphite layer or layer made from a material including graphite or layer
of
a material of graphite or a layer of a carbon-based material may have the
thickness of
between 30 and 250mm, more suitably between 50 and 100 mm. A thickness of
approximately 70 mm may be appropriate.
The working lining may comprise any suitable refractory material known to
the person skilled in the art. The working lining may have a thickness that is
greater
than the thickness of the backing lining.
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In a second aspect, the present invention provides a method for cooling a
furnace in which a high temperature process is conducted under oxidising
conditions,
the method comprising providing a furnace comprising an outer shell made from
a
metal, one or more cooling channels formed on or joined to the outer shell and
a
5 furnace lining, the furnace lining comprising a relatively high thermal
conductivity
backing lining positioned adjacent to an inner wall of the outer shell and a
working
lining positioned inwardly of the backing lining, operating the process in the
furnace
and passing cooling water through the cooling channels to cool the furnace.
The working lining may be positioned against the backing lining. The
working lining may be a refractory based lining.
In embodiments of the present invention, the backing lining has a thermal
conductivity that is significantly higher than the thermal conductivity of
refractory
based working lining. In some embodiments, the backing lining has a thermal
conductivity that is similar to or even higher than the thermal conductivity
of the outer
shell.
In some embodiments, the backing lining comprises a graphite layer or a layer
made from a material including graphite or a layer of a material of or
including
graphite or a layer of a carbon-based material. In some embodiments, the
backing
lining comprises a graphite layer.
In one embodiment, the method of the present invention is operated such that
the maximum temperature reached in the graphite layer does not exceed 500 C,
preferably not exceed 400 C, more preferably not exceed 250 C.
In another embodiment, the method of the present invention is operated such
that heat is removed from the furnace at a rate of 5 kW/ma under normal
operating
conditions and a new working lining, up to 25 kW/ma for a worn working lining,
and
not exceeding a localised heat flux of 120 kW/m2 under extreme operating
conditions
and localised failure of the working lining.
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In some embodiments of the present invention, cooling water flows through
the cooling channels at an average rate of 1 to 2 m3/h per m2 of furnace shell
area, and
at a minimum velocity in the cooling channels of 1 m/s, preferably above 2
m/s.
Graphite layers are suitably used as the backing lining in some embodiments
of the present invention and, for convenience and brevity of description, the
present
invention will be described hereinafter with reference to a graphite layer.
However, it
will be understood that the present invention also encompasses layers made
from
other materials such as a layer made from a material including graphite or a
layer of a
material of graphite.
In the furnace and method of the present invention, the thermal conductivity
of
the graphite lining is three to four times higher than the thermal
conductivity of the
outer steel shell. As a result, the graphite lining layer will conduct and
spread heat
sideways along the shell before the heat exits the shell into the forced
cooling water
system. Therefore, the graphite layer will assist in removing heat from the
working
lining adequately to reduce the wear rate of the working lining due to lower
operating
temperatures, especially for a worn working lining. Furthermore, this design
prevents
or minimises the formation of localised hot spots on the shell between the
external
shell mounted forced cooling channels.
This is in contrast to the prior art linings used in furnaces, for example,
top
submerged lance furnaces, in which oxidising processes take place. In such
furnaces,
the working lining is positioned against a more insulating backing lining,
which in
turn is positioned against the inner wall of the outer steel shell. The
thermal
conductivity of the insulating backing lining is approximately 150 times less
than that
of the steel shell. Combining the insulating backing lining with an external
shell
cooling system will not be advantageous for the side wall lining campaign life
because the insulating backing lining will insulate the working lining from
the shell
cooling system, resulting in a higher wear rate for the working lining due to
higher
operating temperatures, even for a worn working lining. Furthermore, a
localised
high heat load on the sidewall could result in a hot spot on the shell between
the
external shell mounted forced cooling channels. Operating experience has also
shown
that the temperature of the outer steel shell in such furnaces can approach or
even
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exceed 200 C. This high temperature on the outer wall of the furnace
represents an
occupational health and safety problem in the working environment for
operators of
the furnace.
In contrast, using a furnace in accordance with the present invention can
result
in the temperature of the outer surface of the outer steel shell being in the
range of
from 40 to 80 C. It will be appreciated that this provides a safer and more
comfortable working environment for the operators of the furnace.
In all aspects of the present invention, the furnace may be continuously
operated under oxidising conditions. In other embodiments, the furnace may
operate
under oxidising conditions for a period of time and then operate under
reducing
conditions. Operation of the furnace may sequence between operation under
oxidising
conditions and operation under reducing conditions.
Other benefits and advantages arising from the present invention will be
described in the following description of a preferred embodiments of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a schematic diagram of a top submerged lance furnace;
Figure 2 shows a schematic cross sectional view of a side wall lining/cooling
system arrangement used in a top submerged lance type furnace in accordance
with an
embodiment of the present invention; and
Figure 3 shows a temperature profile through the side wall of the furnace
shown in figure 2 in the event that the working lining becomes completely worn
away.
DETAILED DESCRIPTION OF THE DRAWINGS
It will be appreciated that the drawings have been provided to illustrate
features of preferred embodiments, of the present invention. Therefore, it
will be
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understood that the present invention should not be considered to be limited
solely to
those features as shown in the drawings.
Figure 1 is a schematic diagram of a prior art top submerged lance furnace.
This figure has been described in the background section of this specification
and
need not be described further.
Figure 2 shows a side wall lining/cooling system arrangement for use in an
embodiment of a furnace in accordance with the present invention. The furnace
may
be a top submerged lance type furnace. The side wall lining/cooling system
comprises
an outer steel shell 30. Cooling water channels 32, 34 are welded to the
outside of the
outer steel shell 30. The cooling water channels are placed into fluid
communication
with a source of high pressure cooling water in a manner that will be known to
a
person skilled in the art.
The furnace lining includes a backing lining in the form of a graphite layer
36.
The graphite layer may be formed from a plurality of graphite tiles having a
thickness
of approximately 70 mm that are glued or cemented to the inside surface of the
steel
shell 30. The graphite layer may alternatively be made from graphite bricks
having a
thickness of up to 250mm or even greater. The backing lining may alternatively
be
made from a material including graphite or a material of graphite or a layer
of a
carbon-based material. The cement used for this purpose is suitably graphite
or
carbon-based and it has a very high thermal conductivity. As will be
appreciated by
persons skilled in the art, the graphite layer 36 provides a layer having a
high thermal
conductivity. Indeed, the thermal conductivity of the graphite layer 36 may be
three
to four times higher than the thermal conductivity of the outer steel shell
30.
The furnace lining also includes a working lining, in this case in the form of
a
refractory lining 38. The layer 38 constitutes the working lining of the
furnace. The
hot environment of the furnace is denoted by reference numeral 40. As can be
seen
from figure 2, the working lining 3 8 is positioned between the hot
environment 40 and
the graphite layer 36.
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As mentioned above, the thermal conductivity of the graphite layer 36 is three
to four times higher than the thermal conductivity of the furnace steel shell
30. As a
result, the graphite backing lining layer 36 will conduct and spread heat
sideways
along the furnace steel shell 30 before the heat exit the shell into the
forced cooling
water channels 32, 34. Therefore, the graphite backing lining layer 36 assists
in
removing heat from the working lining 38 adequately to reduce the wear rate of
the
working lining due to the lower operating temperatures in the working lining.
This is
especially so for a worn working lining. Furthermore, the graphite backing
layer 36
prevents or minimises the formation of hot spots on the outer steel shell 30
between
the external shell mounted forced cooling water channels 32, 34.
The operating temperature of the furnace can vary between 900 C to 1600 C
under extreme conditions. Heat transfer to the furnace sidewall is through
convection
adjacent to the liquid furnace bath, and through conjugate convection and
radiation
above the liquid furnace bath. The resulting heat flux through the furnace
sidewall
could vary between 5 and 25 kW/m2 depending on the working lining condition
and
operating conditions. Under extreme operating conditions and in areas where
the
working lining is damaged or completely worn away, localised heat fluxes of up
to
120 kW/m2 can be experienced. The operating temperature of the graphite layer
will
vary between 55 and 110 C depending on the working lining and operating
conditions. Under extreme operating conditions and with the working lining
worn
back completely, the graphite temperature may rise to a maximum of 400 C. The
average temperature of the external surface of the steel shell and cooling
water
channels will vary between 40 to 80 C depending on the working lining and
operating
conditions. The increase in cooling water temperature through the cooling
water
circuits may vary between 5 and 15 C. The cooling water outlet temperature may
reach a maximum of 65 C, depending on inlet water temperatures and heat load.
The present inventor is aware that a similar furnace lining/cooling system in
which an external shell mounted forced cooling water system is combined with a
high
thermal conductivity graphite backing lining has been used in other types of
furnaces
(such as electric furnaces) in which high temperature processes are conducted
under
reducing conditions. However, such cooling systems/furnace linings have not
been
used in furnaces in which oxidising processes take place. The reason that
persons
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skilled in this art have heretofore not considered such furnace linings to be
suitable for
use in furnaces in which high temperature processes take place under oxidising
conditions is that the graphite layer is itself readily oxidisable if it ever
becomes
exposed to the hot environment of the furnace. Therefore, if wear of the
working
5 lining of the furnace takes place to a degree such that the working lining
is essentially
worn away in a part of the furnace such that the graphite layer is exposed to
the hot
environment of the furnace, conventional thought was that the graphite layer
would
very quickly become oxidised by the oxidising conditions to which it was being
exposed. Effectively, it was thought that if the graphite layer was exposed to
the hot,
10 oxidising conditions inside the furnace, the graphite layer would
essentially very
quickly burn away. This dilemma is, of course, not of concern in furnaces
operated
under reducing conditions.
Surprisingly, the present inventor has found that in the event that the
working
lining 38 becomes completely worn away in a part of the furnace, the rate of
cooling
through the graphite layer 36 is sufficiently high such that instead of the
graphite layer
36 becoming quickly oxidised, a protective and stable freeze layer will form
on the
hot face of the graphite lining 36, thereby limiting the heat loss through the
side wall
and protecting the graphite lining 36 from other wear mechanisms, such as
erosion
and oxidation. The hot face temperature of the graphite lining is maintained
well
below 500 C, thereby preventing significant oxidation of the graphite taking
place in
the medium to long term. As mentioned above, this finding is contrary to
conventional thinking.
Figure 3 demonstrates the formation of a stable protective freeze layer on the
hot face of the graphite layer in the event that the graphite layer becomes
exposed by
virtue of the working lining 38 becoming worn away. In figure 3, the steel
shell 30
and the graphite layer 36 are shown. A stable freeze layer 42, which forms on
the
graphite layer 36, is also shown. The stable freeze layer may, for example,
have a
thickness of approximately 15 mm. As can be seen from figure 3, the furnace is
operated at a temperature of approximately 1100 C. However, due to the quite
high
rate of heat transfer through the graphite layer, the freeze layer 42 is
formed over the
exposed graphite layer 36. Typically, the freeze layer is formed within around
30
minutes of the graphite layer becoming exposed. This minimises the amount of
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oxidation of the exposed graphite layer that takes place. Further, the maximum
temperature in the graphite layer is well below 500 C, and typically
maintained below
250 C, thereby avoiding further oxidation of the graphite layer. Figure 3 also
shows
that there is a steep temperature gradient through the protective freeze layer
42.
The furnace and the method for cooling the furnace in accordance with the
present invention has a number of further advantages:
= When the working lining is new, the furnace and method for
cooling the furnace of the present invention does not result in a lot of
difference to the lining temperature (when compared with prior art linings
used in top submerged lance furnaces). However, it does make a large
difference to the external temperature of the steel shell of the furnace.
Normally, a top submerged lance furnace, without water channel cooling has
an outside steel shell temperature of approximately 200 to 300 C. However,
furnaces operated in accordance with the present invention have an outside
steel shell temperature of around 40 to 80 C.
= As the working lining wears away, a cooler temperature is
established in the working lining, which reduces the wear rate of the working
lining.
= If the working lining becomes completely worn away, the
graphite layer extracts heat away from any developing hotspots within the
furnace sidewall and prevents the formation of hotspots on the furnace steel
shell. Further, a frozen slag layer forms on the graphite surface, which
protects the graphite layer and reduces heat loss through the graphite layer.
= The externally mounted cooling channels can be spaced from
each other such that a large portion of the external surface of the outer
shell of
the furnace is exposed. This allows for visual inspection of the outer shell
to
take place. It is also possible to mount thermocouples to the outer surface of
the outer shell in order to monitor the temperature of the outer shell. This
is
not possible if panel cooling (in which water covers the whole outer shell of
the furnace) is used.
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Those skilled in the art will appreciate that the present invention may be
susceptible to variations and modifications other than those specifically
described. It
will be appreciated that the present invention encompasses all such variations
and
modifications that fall within its spirit and scope.
