Note: Descriptions are shown in the official language in which they were submitted.
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CARBON BAKING FURNACE
The present invention relates to the baking of carbon, in particular to the
baking of carbon
anodes such as for use in aluminium smelters. The present invention further
relates to a
carbon baking furnace, to a process for the baking of carbon articles and
articles so baked.
The conversion of alumina to aluminium metal by electrolysis results in a
substantial
consumption of carbon anodes. Molten aluminium is deposited onto a carbon
cathode and
simultaneously oxygen is deposited on and consumes the carbon anode of the
electrolytic cell.
Typically, up to 0.4 tonnes of carbon are consumed for every tonne of
aluminium produced.
As a result, aluminium smelters have a requirement for a substantial and
continuous supply
of carbon electrodes. It is common for smelters to manufacture carbon anodes
on site as an
integral part of the aluminium production process.
The manufacture of carbon anodes comprises producing "green" anode blocks and
baking the
"green" blocks to produce anodes suitable for use. The production of "green"
blocks involves
the mixing of crushed coke or anthracite with a binding agent which, for
example, contains
coal tar pitch. The viscous mixture is then pressed to form "green" anode
blocks. Depending
on the smelters' requirements, "green" anodes may typically weigh from a few
hundred
kilograms to more than a tonne. The mixture of coke and pitch binder is
generally solid at
room temperature and softens at temperatures over about 50 C. Volatile
components are
released at temperatures between 50 C and 400 C. When subjected to further
heating over
a period of time, to about 1200 C, the anode hardens, resulting in improved
physical
properties, such as electrical conductivity and resistance to oxidation.
Carbon anodes are typically manufactured in carbon baking furnaces which are
often referred
to as ring-type furnaces where "green" anode blocks are loaded into large
pits, covered with
sacrificial coke and baked. The furnace is generally a concrete tub lined with
refractory
materials to provide a number of pits into which a column of green anodes may
be loaded for
baking, usually about six deep. Each pit is typically surrounded by two flues
for heating the
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anodes, the flue-walls also being lined with refractory materials. As each pit
is loaded,
"green"anodes are packed in with sacrificial coke. These furnaces typically
require baking
cycles of about 2 to 3 weeks which includes preheating and the time that the
baked anode
is left in the pit to cool prior to removal.
The thermal cycling of these types of carbon baking furnaces causes adverse
effects on
the refractory materials, concrete and other ceramic components. The
refractory materials
deform with heat and time, resulting in altered brick dimensions. Further,
packing coke
material may lodge into expansion gaps which, together with temperature
cycling, leads
to large structural deformation of walls and ultimately failure.
Deformed flue walls must be regularly replaced as excessive tub-wall
deformation results
in an inefficient operation and necessitates furnace rebuilding. Costs
associated with
regular flue-wall replacement and maintenance of joints and tub-walls can be
as high as
50% of the overall anode baking costs.
We have now found a furnace configuration which overcomes or at least
substantially
alleviates the problems associated with the conventional ring-type furnaces.
According to the present invention there is now provided a carbon baking
furnace
comprising a refractory lined kiln defining a substantially vertical baking
path comprising
at least a first heating zone capable of heating the green carbon article to
remove volatile
organic compounds and at least a second heating zones for baking the carbon
articles, the
kiln having upper and lower ends, the kiln further comprising: a receiving
zone adjacent
said upper end of said path comprising means for substantially continuously
receiving
green carbon articles; means for packing said green carbon articles in a
sacrificial
medium; heating means for heating the second heating zone, the heating means
including
heat ducts disposed perpendicular to the vertical baking path of the kiln;
means for
substantially continuously displacing the packed carbon articles through said
baking path;
and means for substantially continuously removing baked carbon articles from
said lower
end of the baking path.
Also in accordance with the present invention there is provided a carbon
baking furnace
comprising a refractory lined kiln defining a substantially vertical baking
path comprising
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at least one heating zone and upper and lower ends, the kiln further
comprising: a
receiving zone adjacent said upper end of said path comprising means for
substantially
continuously receiving green carbon articles; means for packing said green
carbon articles
in a sacrificial medium; means for substantially continuously displacing the
packed
carbon articles through said baking path; and means for substantially
continuously
removing baked carbon articles from said lower end of the baking path.
In a particular embodiment, the means for substantially continuously receiving
green
carbon articles includes a conveyor and a hydraulic ram whereby the conveyor
positions
the green carbon article adjacent to said upper end of the vertical baking
path and the
hydraulic ram positions the green carbon article into said upper end of the
baking path.
In another embodiment, the means for substantially continuously removing baked
carbon
articles from the baking path includes a plurality of hydraulic rams and
conveyor belt
wherein the bottom-most baked carbon article is supported by a hydraulic ram
while the
adjacent baked carbon article is engaged and supported by a second pair of
opposed rams
and wherein the bottom-most baked carbon article is subsequently positioned by
the first
mentioned hydraulic ram onto a conveyor belt.
Also in accordance with the present invention there is provided a process for
baking
carbon articles, said process comprising the steps of. substantially
continuously loading
green carbon articles into a refractory lined kiln, said kiln defining a
substantially vertical
baking path comprising at least one heating zone and upper and lower ends,
wherein said
articles are loaded into a receiving zone adjacent said upper end of the
baking path;
packing said green carbon articles in a sacrificial medium; substantially
continuously
displacing the packed carbon articles through said baking path; and
substantially
continuously removing baked carbon articles from said lower end of the baking
path.
In a particular embodiment, the green carbon articles are received using a
conveyor and a
hydraulic ram whereby the conveyor positions the green carbon article adjacent
to the
upper end of the vertical baking path and the hydraulic ram positions the
green carbon
article into the upper end of the baking path.
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In another embodiment, the baked carbon articles are removed from the baking
path using
a plurality of hydraulic rams and conveyor belt wherein the bottom-most baked
carbon
article is supported by a hydraulic ram while the adjacent baked carbon
article is engaged
and supported by a second pair of opposed rams and wherein the bottom-most
baked
carbon article is subsequently positioned by the first mentioned hydraulic ram
onto a
conveyor belt.
Also in accordance with the present invention, there is provided a carbon
baking furnace
for carbon anode blocks comprising a refractory lined kiln defining a
substantially vertical
baking path through which plural carbon anode blocks are displaced as they are
baked, a
receiving zone adjacent a first end of the baking path in which green carbon
anode blocks
are substantially continuously placed into said first end of the baking path,
a supply of
sacrificial medium for packing said green carbon anode blocks in said
sacrificial medium
prior to the packed carbon anode blocks being substantially continuously
displaced
through said baking path to form baked carbon anodes, and a discharge zone
adjacent a
second end of the baking path in which said baked carbon anodes are
substantially
continuously discharged from the kiln.
Further in accordance with the present invention, there is provided a process
for baking
carbon anode blocks, said process comprising the steps of substantially
continuously
loading green carbon anode blocks into a refractory lined kiln, said kiln
defining a
substantially vertical baking path through which plural carbon anode blocks
are displaced
from a first end to a second end as they are baked, packing said green carbon
anode
blocks in a sacrificial medium prior to displacing the packed carbon anode
blocks through
said baking path, substantially continuously displacing the packed carbon
anode blocks
through said baking path to form baked carbon anodes, and substantially
continuously
discharging the baked carbon anodes from the kiln at the second end of the
baking path.
There is also provided a process for baking carbon articles comprising the
steps of:
loading a plurality of green carbon articles into a baking path of a
refractory lined kiln,
said baking path of the refractory lined kiln being substantially vertical and
having an
upper end and a lower end; displacing the green carbon articles through said
substantially
vertical baking path from the upper end toward to lower end where the green
carbon
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articles are baked to form baked carbon articles; and removing the baked
carbon articles
from the kiln at the lower end of the baking path.
Advantageously, the present invention provides carbon articles which have been
subjected to more consistent temperature treatments. This results in articles
of improved
baked quality. In the manufacture of baked carbon anodes for use in aluminium
smelting
operations, improved baked quality contributes significantly to cell
efficiency.
Temperature gradients in excess of 150 C in a single pit of ring type furnaces
are not
uncommon. Such high temperature gradients may result in thermo-mechanical
degradation of the anode matrix. The present invention may reduce or eliminate
cracking
of the anodes which may otherwise result from excessive temperature gradients
in the
baking process.
It will be understood that the term "substantially continuously" refers to a
continuous
mode of operation whereby carbon articles are passed continuously through the
kiln. The
carbon articles are passed through the kiln at either a uniform rate or may
involve a
periodic or step-wise passage through the kiln. This will be determined
primarily by the
means for receiving and removing the articles from the kiln.
The refractory lined kiln may be any convenient refractory lined kiln which
incorporates a
baking path through which the carbon articles may pass and which is capable of
heating
the carbon articles up to the desired baking temperatures, typically bout 1200
C to
1300 C.
Preferably the refractory lined kiln may comprise a number of heating zones
whereby
each heating zone is capable of maintaining the carbon articles at a desired
temperature.
The refractory lined kiln of the present invention may be operated
continuously thereby
permitting each heating zone to be operated at an equilibrium temperature with
little or no
thermal cycling. The operation at equilibrium temperatures permits greater
fuel efficiency
as fuel which would otherwise be spent in the reheating of furnace components
in ring-
type furnaces may be eliminated. In the present invention fuel need only be
used to
provide the necessary calorific values required to bake the anodes (not
heating and
reheating refractory substructures).
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Furthermore, the operation of the refractory lined kiln under equilibrium
conditions allows
structural materials such as refractories, concrete supports and other ceramic
products to be
maintained at relatively constant temperatures over an extended period,
substantially reducing
thermal cycling of these materials. The furnace may then be able to operate
for longer
periods without extensive maintenance or rebuilding. Furnace rebuilds costing
tens of
millions of dollars are standard in ring-type furnaces. Such rebuilds may take
place at
approximately five- yearly intervals. Thermal cycling of refractories will be
substantially
reduced according to the present invention and concomitantly the useful life
of the furnace
will be greatly extended.
Refractories may also advantageously be selected according to the heating zone
in the kiln in
which they are intended for use thereby permitting the efficiency and
operating life of the
furnace to be optimised. For example, in a preheat/volatiles extraction zone,
low
permeability refractories with high resistance to volatiles attack, such as
those refractories
used in incinerators are preferred. In the high temperature zone, refractories
with high
resistance to thermomechnical degradation are preferred. Alumina silicates,
such as those
having greater than 45 % alumina may be used in these and other zones of the
kiln.
The refractories may include guides, such as protrusions, to position the
carbon articles within
the baking path defined by the refractory lined kiln. It is desirable that the
baking path be
substantially linear so as to permit the carbon articles to be readily
conveyed along the baking
path with a minimum requirement for actuators or conveyors to operate or be
located within
the kiln. Preferably, the sacrificial medium is sufficient to support and
guide the carbon
articles without the need for actuators to be located in the high temperature
regions of the
kiln.
Preferably the kiln may comprise a number of heating zones in order to provide
optimum
control of temperature treatment of the carbon articles. Defined thermal
gradients along the
carbon baking furnace make it possible to separate heating and volatile
extraction zones. It
is preferable that the kiln comprise a first heating zone whereby the carbon
article is heated
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to a temperature whereby any volatile materials are removed from the furnace
to reduce
chemical degradation of the refractories and/or other kiln components and to
reduce the
possibility of explosion within the kiln. The extraction of volatile organic
compounds from
the kiln permits these potentially toxic compounds to be contained. Optionally
these volatile
organic compounds may be extracted and pumped into the heating zone with the
fuel and
combusted as part of the heating of the kiln. This minimises the likelihood of
any emissions
of volatile materials such as pitch fumes from the furnace into the
environment.
Furthermore, the containment of pitch fumes is particularly desirable as pitch
condensates are
extremely difficult to handle and costly to eliminate. Current technologies,
due to design
limitations, are unable to fully combust or efficiently contain these toxins.
Pitch fumes
escape from the furnace environment or condense within the ringmains. Pitch
condensates
are extremely difficult to handle and are costly to eliminate. The present
invention makes it
possible to reduce the risk of exposing workers to these highly toxic
substances.
Second and subsequent heating zones may be used to control the rate of
increase of the
temperature of the carbon articles as they substantially continuously pass
through the baking
path. The rate at which the carbon articles pass through the baking path and
the configuration
of the heating zones in the kiln will determine the temperature gradient
across the carbon
articles and the heat treatment profile to which the carbon articles are
subjected.
Preferably, the kiln defines a vertical baking path whereby the carbon
articles are displaced
downwardly through the baking path as a result of the weight of the carbon
article and the
weight of the sacrificial medium and subsequent carbon articles stacked above.
The rate at
which the carbon articles pass through the baking path may conveniently be
regulated by
substantially continuously removing the bottom most baked carbon article from
the kiln in a
manner whereby the weight of the carbon articles and the sacrificial medium is
supported
within the kiln and the remaining carbon articles are substantially
continuously displaced
through the baking path.
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Alternatively the kiln may define a substantially horizontal baking path or a
baking path on
an inclined plane whereby the carbon articles are substantially continuously
displaced through
the baking path by a push rod or other convenient means.
The means for substantially continuously receiving green carbon articles may
be any
convenient means, dependent on the configuration and orientation of the
refractory lined kiln
and the baking path. We have found that the use of a conveyor and a hydraulic
ram is
particularly suited to a kiln having a vertical baking path wherein the green
carbon articles
are received at the top of the kiln. The conveyor may position the green
carbon article
adjacent to the top of the kiln and the hydraulic ram place the green carbon
article into the
top of the baking path.
The means for packing the green carbon articles in a sacrificial medium may be
any
convenient means dependent on the morphology of the sacrificial medium.
Preferably the
sacrificial medium is a friable packing coke. The means for packing the green
carbon articles
in a friable sacrificial medium may include a hopper fitted with a suitable
nozzle whereby the
sacrificial medium is spread over and around the green carbon article.
Preferably the
sacrificial medium occupies the remaining space in the kiln once the carbon
articles have been
located therein. It is desirable to reduce the amount of free space in the
kiln.
Refractory spacers are preferably used to separate the carbon articles in the
kiln so as to
prevent the carbon, articles being baked together and to alleviate
difficulties in separating the
baked carbon articles when they are removed from the baking path. Suitable
spacers may be
made from consumable materials such as aluminium sheets, cardboard, or paper.
Alternatively the spacers may be made from refractory or carbonaceous
materials, such
spacers may be reused or recycled.
The means for the substantially continuous displacement of the carbon articles
through the
baking path may include means whereby the carbon articles are forced or pushed
through the
baking path. Preferably gravity, or the weight of the column of carbon
articles in a vertical
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baking path is used to urge the carbon articles through the baking path and
the substantially
continued displacement of the carbon articles is achieved by retarding or
braking the
movement of the lower or lowest carbon articles thereby exercising control
over the rate at
which the carbon articles pass down the kiln.
The means for substantially continuously removing the baked carbon article
from the kiln may
be any convenient means depending on the configuration and orientation of the
kiln. In the
preferred configuration and orientation of the kiln where the baking path is
substantially
vertical and the carbon articles pass down the kiln under their own weight and
the weight of
the sacrificial medium, a preferred means for substantially continuously
removing the baked
carbon articles from the kiln includes the use of hydraulic rams and conveyor
belts. The
bottom-most baked carbon article is preferably supported such as by a
hydraulic ram while
the adjacent baked carbon article is engaged by a second pair of opposed rams
so as to
restrain or support all but the bottom-most baked carbon article. The bottom-
most baked
carbon article is then lowered or positioned by the first mentioned hydraulic
ram onto a
conveyor belt for storage and/or use.
The sacrificial medium may be any medium which will protect the carbon
articles during the
baking process. The sacrificial medium may preferentially react with or absorb
the oxygen
in the kiln prior to its reaction with the carbon in the articles being baked.
It is preferred that
the sacrificial medium be friable to permit it easy incorporation around the
carbon articles and
to permit the easy movement of the carbon articles within the kiln. The
sacrificial medium
may be selected so as to provide an optimum balance of rendering the carbon
articles easily
moveable through the furnace and providing sufficient; protection of the
carbon articles from
oxygen contained within the kiln. Preferably the sacrificial medium is packing
coke having
a maximum particle size of less than 15mm.
It is also desirable for the sacrificial medium to conduct heat efficiently to
the carbon articles.
For this reason, it is desirable that the amount of sacrificial medium used is
sufficient to
provide adequate protection for the carbon articles from oxygen, permit the
easy movement
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of the carbon articles in the kiln and to provide efficient heat transfer.
Desirably the sacrificial medium may be collected from the furnace and, so far
as is capable,
be recycled for packing further green carbon articles. For example, the flow
of this medium
may be controlled by the use of a pendulous carbon trough, the flow rate being
proportional
to the period of oscillation.
The furnaces of the present invention may be arranged such that a plurality of
such furnaces
are positioned adjacent to one another so as to permit the efficient use of
equipment and
heating values in the fuel.
We have found that improved control of temperature permits more even heat
treatment of the
carbon articles and the manufacture of carbon articles having higher baked
quality.
Additionally, anode deformation, otherwise known as "slumping", during baking
which a
problem in ring furnaces is insignificant. Green anodes subjected to loads at
varying
temperatures reveal dimensional instability in the 25 C to 150 C temperature
range. This is
directly related to the temperature zone at which anode volatiles are emitted.
We have found
that anodes (regardless of load) maintain their structural integrity at
temperatures beyond
150 C and the degree of permanent deformation is less than 0.5 %
Current carbon baking furnace designs operate within narrow parameters, the
distance
between the anodes and flue walls is critical to achieving optimum baking. In
the past this
inflexibility in design has presented operators with costly refurbishment when
anode design
changes are required. The carbon baking furnace of the present invention may
more easily
be modified to accommodate anode design changes and. to control carbon anode
temperature,
such as by varying the rate at which they are passed through the various
temperature zones
and the temperature of said zones. Furthermore, baking temperatures may be
optimised to
improve in anode baking.
The carbon baking furnace of the present invention is able to accommodate
anodes of varying
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size with minimal structural change. Temperature profiles can be tailored by
altering
configuration of the columns heat exchange systems.
Advantageously, the carbon baking furnace of the present invention may result
in a significant
reduction in fuel consumption, more uniform baking of anodes and efficient
toxic volatiles
elimination. The substantially continuous flow of anodes and coke through the
carbon baking
furnace eliminates the need for expensive and labour intensive loading and
unloading
procedures as in existing furnace designs. Large multipurpose cranes costing
more than 10%
of the overall capital budget of ring type furnaces requiring high ongoing
operating costs may
be eliminated. The present invention permits a fully automated anode and
packing coke
loading and unloading system, along with the resultant increased productivity.
In general, the continuous furnace of the present invention will require
approximately one
fifth of the surface area needed to accommodate an equivalent ring-type
furnace. The
continuous furnace of the present invention may typically be between 20m and
30m high,
with anode velocities between 3 and 4.5m/day.
Throughout this specification and the claims which follow, unless the context
requires
otherwise, the word "comprise", or variations such as "comprises" or
"comprising", will be
understood to imply the inclusion of a stated integer or group of integers but
not the exclusion
of any other integer or group of integers.
The present invention will now be further described with reference to the
accompanying
drawings. In the drawings the carbon articles are represented by anodes for
use in the
aluminium smelting industry. It will be understood that the present invention
applies equally
to the baking of other carbon articles.
Figure 1 is a cross sectional view of the carbon baking furnace of the present
invention.
Figure 2 is a cutaway perspective diagram of a preferred embodiment of the
carbon baking
furnace of the present invention.
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The carbon baking furnace(1) shown in Figure 1 incorporates a refractory lined
kiln(2) and
a heating means(4). Green carbon anodes(3) are encased in packing coke(6) and
are
substantially continuously fed through the heating zone generated by the
heating means(4).
Baked carbon anodes(5) are produced after passing through the heating zone.
Means for
receiving green carbon anodes (not shown) and means for removing baked carbon
anodes (not
shown) assist to control the rate at which the anodes pass through the heating
zone and the
temperature profile to which they are subjected.
Figure 2 shows a preferred configuration of the carbon baking furnace of the
present
invention. Figure 2 is a cutaway perspective drawing. Green carbon anodes(3)
are positioned
adjacent the refractory lined kiln(2) by conveyor beit(7). Hydraulic rams(8)
position the
green carbon anodes (3) over the baking path of refractory lined kiln(2). The
bottom-most
baked carbon anode(5) is lowered by hydraulic ram(9) after the adjacent carbon
anode(10)
is engaged by hydraulic rams(11). Baked carbon anode (5) is then positioned by
hydraulic
ram(9) on conveyor belt(12) for storage and/or use. Hydraulic ram(9) engages
baked carbon
anode(10), hydraulic rams (11) disengage baked carbon anode(10) and hydraulic
ram(9)
lowers baked carbon anode(10) into the position previously occupied by baked
carbon
anode(5) as a result the column of anodes moves downwards in a substantially
continuous
manner.
Green carbon anode(3) is positioned on a spacing elernent(13). Packing coke(6)
is then fed
by hopper(14) through nozzle(15) to fill the space surrounding the green
carbon anode(3) in
the kiln(2). As the carbon anodes pass through the kiln(2) the carbon anodes
enter a volatile
extraction zone(16). Volatiles such as pitch fumes are extracted through holes
in the
refractory materials(17) through extraction unit(18). The extracted fumes are
fed into the
heating unit(19) in addition to fuel in order to provide sufficient calorific
values to raise the
temperature of the carbon anodes to the desired baking temperature. The fuel
is fed into the
heating unit(19) through nozzle(20).
After the anodes have passed through the high temperature zone adjacent to the
heating
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unit(19) the packing coke(6) is removed from the baked anodes by scrapers(21).
The packing
coke(6) which has been removed from the baked carbon anodes is then
transported on
conveyor belts(23) and (24) and returned to the hoppers(14) via continuous
buckets(25) and
conveyor belt(26).
Those skilled in the art will appreciate that the invention described herein
is susceptible to
variations and modifications other than those specifically described. It is to
be understood
that the invention includes all such variations and modifications which fall
within its spirit and
scope. The invention also includes all of the steps, features, compositions
and compounds
referred to or indicated in this specification, individually or collectively,
and any and all
combinations of any two or more of said steps or features.
