Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINIUM POT BOTTOM PREHEATING METHOD
Technical area
The invention refers to the non-ferrous metallurgy, in particular to
electrolytic reduction of
aluminium, namely to pot bottom preheating methods for aluminium pots with
prebaked or inert anodes.
Technical level
Some of processes used in the aluminium industry require significant amount of
thermal energy
necessary for preheating of equipment before its start-up. In past, the lined
equipment preheating process
was often neglected, which for example resulted in cold starts of pots and
reduction of their life. Before
start-up of a pot, its cathode lining should be thoroughly and uniformly
preheated to minimise potential
damage from excessive temperature differences.
High-temperature differences and application of crude bottom ramming paste
upon bath pouring
to the pot may result in a heat shock, cracking of a cathode block, leaking,
and finally to reduction of the
pot life.
There are two basic pot bottom preheating methods:
¨ Electrical preheating;
¨ Preheating using gas or liquid fuel.
During preheating using gas or liquid fuel, it is difficult to control
generated amount of thermal
energy and heat distribution on the cathode surface / cathode lining
thickness. It is also difficult and
maybe even impossible to properly heat the side and end walls, when necessary.
There is probability of
non-uniform temperature distribution on the cathode surface with excessive
overheating of some areas, as
well as quite significant temperature differences throughout the cathode
lining.
The electrical preheating methods are based on current supply from anode bars
to the cathode
through a coke bed for the pot heating by means of electrical conductance and
heat radiation.
There is a well-known aluminium pot bottom preheating method including:
placement of
prebaked anodes on the pot bottom; attachment of the prebaked anode rod
assemblies to buses of the
anode busbar; raising of the prebaked anodes; pouring of liquid aluminium for
submersion of the
prebaked anodes into it; and connection of the pot to the electrical circuit
(G. Wolfson, V. Lankin.
Aluminium production in pots with prebaked anodes. Moscow: Metallurgy, 1974,
p.55, p.56).
A disadvantage of the well-known aluminium pot bottom preheating method is
that pouring of
liquid aluminium exposes the pot bottom to a heat shock, which may result in
formation of cracks in
cathode blocks and breakdown upon further operation of the pot. Another great
disadvantage is direct
contact of the pot bottom with liquid aluminium, which has low viscosity and
melting point. Aluminium
may penetrate deeply inside the pot bottom before solidification, react with
insulation, break it, or create a
thermal shunt.
There is also another well-known aluminium pot bottom preheating method
(patent #RU
2215825, 1PC C25C 3/06) including: covering of the pot bottom made of cathode
blocks and end
peripheral joints with a layer of carbon fill; placement of prebaked anodes on
it so that their soles come
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Attorney Ref.: 1671P 037CA01
into contact with the carbon fill layer throughout its area and rods of the
anode rod assemblies adjoin anode
buses of the pot anode busbar; attachment of the prebaked anode rod assemblies
to anode buses of the pot
anode busbar; passage of electrical current through the prebaked anodes,
carbon fill layer, and cathode
blocks; and control of current load on the prebaked anodes via their
controlled disconnection.
A disadvantage of the well-known aluminium pot bottom preheating method is
that up to 50% of
all prebaked anodes can be attached to anode buses of the pot anode busbar
using basic locks (rigidly).
Upon the pot bottom heating via natural burning of the carbon material, the
anodes attached using flexible
elements will go down, while the rigidly attached anodes will stay in place,
which will result in local
overheating spots of the pot bottom.
The pot bottom preheating method for aluminium pots with prebaked anodes
(patent #RU 2526351,
IPC C25C 3/06), which is the closest method by technical substance to this
application, includes: covering
of the pot bottom made of cathode blocks and steel bars with an electrically
conductive material; placement
of prebaked anodes with stubs on it; connection of the installed prebaked
anode rod assemblies to anode
buses of the pot anode busbar; passage of electrical current through the
electrically conductive material;
and control of current load on the prebaked anodes. At that, the electrically
conductive material is graphite
fill placed as truncated pyramid rows located in projections of stubs
throughout the prebaked anode length
with height of each row set in inverse proportion to passed amperage and
connection of all installed
prebaked anode rod assemblies to anode buses of the pot anode busbar using
flexible elements.
A disadvantage of this aluminium pot bottom preheating method is that the
graphite material is
filled as rows in projections of stubs throughout the length on all prebaked
anode blocks. This filling
method of the graphite material does not allow uniform heating of the pot
bottom in the first half of the
preheating process, since current will flow to the pot middle as heated if the
graphite material under the
anode has the same section. As a consequence, the pot ends will be heated more
slowly, which will result
in a significant temperature gradient.
Invention disclosure
The task of the proposed invention is to ensure uniform heating of the
aluminium pot bottom
throughout the preheating process.
The technical result is solution of this task, safe start-up, and extended
life of the aluminium pot.
The technical result achieved upon implementation of this application also
consists in non-uniform
current distribution in the pot bottom to get its uniform heating up to 900 C
for less than 60 hours as during
gas-flame preheating.
In a further aspect, this document discloses a method of pre-heating a pot
bottom of aluminium
pots with pre-baked anodes, said method comprising: covering the pot bottom
with an electrically
conductive material, placing pre-baked anodes onto the electrically conductive
material, said anodes having
anode rod assemblies being connected to anode buses of an anode busbar of the
pot by means of flexible
electrical connection elements, passing an electric current through the
electrically conductive material, and
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controlling a current load on the anodes for pre-heating, wherein the
electrically conductive material under
the anodes is formed so that the quantity of the electrically conductive
material is less under the anodes
located in the middle of the pot than under the anodes located near the
extreme end anodes, and the quantity
of the electrically conductive material is less under the anodes located near
the extreme end anodes than
under the extreme end anodes.
The invention substance is explained by drawings, where:
- Fig. 1 shows geometry of the electrically conductive material (graphite
'bed') - top view using the
example of a pot with 24 paired anodes;
- Fig. 2 shows a stencil for knurling of the graphite 'bed' up to 200 kA;
- Fig. 3 shows a stencil for knurling of the graphite 'bed' over 200 kA;
- Fig. 4 shows knurling of the graphite 'bed' at the end anodes;
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- Fig. 5 shows knurling of the graphite 'bed' at the anodes located near the
end anodes;
- Fig. 6 shows knurling of the graphite 'bed' at other anodes;
- Fig. 7 shows a temperature pattern of the pot bottom before the pot start-up
with non-uniform
heating of the pot bottom due to non-optimal filling of the graphite material;
- Fig. 8 shows surface temperature of the pot bottom in 1 hour before the pot
start-up;
- Fig. 9 shows amperage measured using 'pliers' on the end anodes (1, 12, 13,
24) throughout the
pot preheating process with modification of the graphite material
configuration (refer to Fig. 1);
- Fig. 10 shows a heating trend of the pot bottom at check points;
- Fig. 11 shows the proposed flexible elements for connection of anode rods
with the anode bus to
carry out independent preheating;
- Figs. 12, 13 show alternative flexible elements.
Invention implementation
The graphite 'bed' is installed (Fig. 1) using one of the proposed stencils
(Figs. 2, 3) depending on
the pot amperage.
Knurling of the graphite 'bed' at the extreme end anodes is carried out as
follows.
The stencil (Fig. 4) is placed onto the pot bottom in the knurling area of the
graphite 'bed' on the
anode projection (arrangement of bars: #1 - side-anode; #10 - row spacing).
The graphite material is filled
up to the upper face (flush) to the space between rails. The graphite material
is levelled without ramming
using the bar edges as supports. The excessive graphite material is removed,
for example, using a
levelling scraper. The stencil is demounted from the pot bottom and the
excessive graphite material is
removed.
Knurling of the graphite 'bed' at the anodes located near the end anodes is
carried out as follows.
The graphite 'bed' is installed using one of the proposed stencils (Figs. 2,
3) depending on the pot
amperage. The stencil is placed onto the pot bottom in the knurling area of
the graphite 'bed' on the anode
projection (arrangement of bars: #1 - side-anode; #10 - row spacing). The
graphite material is filled up to
the upper face ('flush') to the space between rails. The raw material is not
filled to the space between the
7th and 8th bars of the stencil (Fig. 5). The graphite material is levelled
without ramming using the bar
edges as supports. The excessive graphite material is removed, for example,
using a levelling scraper. The
stencil is demounted from the pot bottom and the excessive graphite material
is removed.
Knurling of the graphite 'bed' at other anodes is carried out as follows.
The graphite 'bed' is installed using one of the proposed stencils (Figs. 2,
3) depending on the pot
amperage. The stencil is placed onto the pot bottom in the knurling area of
the graphite 'bed' on the anode
projection (arrangement of bars: #1 - side-anode; #10 - row spacing). The
graphite material is filled up to
the upper face ('flush') to the space between rails. The raw material is not
filled to the space between the
3rd, 4th, 7th, and 8th bars of the stencil (Fig. 6).The graphite material is
levelled without ramming using
the bar edges as supports. The excessive graphite material is removed, for
example, using a levelling
scraper. The stencil is demounted from the pot bottom and the excessive
graphite material is removed.
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After installation of all anodes, the start-up charge (cryolite, crushed hard
bath, soda) is loaded to
the side-anode space and the anode body is covered with cryolite on top.
All installed prebaked anode rod assemblies are connected to anode buses of
the pot anode
busbar, for example, using a set of flexible aluminium tapes and full
electrical current is passed through
the graphite material layer. Current load of the prebaked anodes is controlled
via disconnection of anodes
with high load or local bottom overheating.
Fig. 7 shows a temperature pattern of the pot bottom before the pot start-up
with non-uniform
heating of the pot bottom due to non-optimal filling of the graphite material.
It is clear that the pot middle
is heated up to 800-750 C, while the pot ends have temperature below 400 C.
The ends are heated in the
second half of the preheating process via heat transfer from the middle and,
as a consequence, uniform
temperature of the pot bottom is achieved at the end of the preheating
process.
Fig. 8 shows surface temperature of the pot bottom in 1 hour before the pot
start-up. Fig. 9 shows
amperage measured using 'pliers' on the end anodes (1, 12, 13, 24) throughout
the pot preheating process
with modification of the graphite material configuration (refer to Fig. 1),
i.e. it shows a trend of amperage
on the end anodes. It is clear from the diagram (Fig. 9) that current on these
anodes is higher by 20-25%
of the nominal value due to more strips of the graphite material (Fig. 1).
It is clear from Figs. 8, 9 that the new graphite fill geometry allows us to:
1) Uniformly heat the pot bottom surface up to the target values for 48
hours;
2) Redistribute current to the end anodes.
Fig. 10 shows a heating trend of the pot bottom at check points. It is clear
that the average pot
bottom surface temperature is achieved at the check thermocouples located as
follows:
1. In the row spacing - 949 C (target - over 900 C);
2. At the 1st stub on the 'inlet' and 'outlet' sides - 808 C (target - over
800 C);
3. At the pot ends - 736 C (target - over 550 C).
So, the proposed pot bottom preheating method for aluminium pots with prebaked
anodes
includes: covering of the pot bottom with an electrically conductive material;
placement of prebaked
anodes on it; their connection to anode buses of the pot anode busbar; passage
of electrical current
through the electrically conductive material; and control of current load on
the anodes for preheating as
inherent for the pilot model. At that, uniform preheating is ensured via
proper selection of the electrically
conductive material quantity under the anodes. Namely, quantity of the
electrically conductive material
under the anodes is selected so that the material quantity is less under the
anodes located at the pot middle
than under the anodes located near the extreme end anodes, while the material
quantity is less under the
anodes located near the extreme end anodes than under the extreme end anodes.
The electrically
conductive material is preferably graphite with fraction from 0.1 mm to 10 mm.
It is reasonable to set
height and length of each row of the electrically conductive material under
the anodes in inverse
proportion to passed amperage. The installed prebaked anode rod assemblies are
usually connected to
anode buses of the pot anode busbar using flexible elements (Fig. 11).
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The 'anode bus - anode rod' flexible element features the following design
solutions
distinguishing it from alternative options:
¨ Section of contacts, area, and holddown pressure ensure current density:
for contact of parts -
not more than 0.6 A/mm2; for flexible conductors - not more than 1.2 A/mm2;
¨ Overall and
connection dimensions allow unhampered installation and disconnection of the
flexible element;
¨ Size and thread pitch of nuts are unified; design of screws allows using
a traversing
mechanism (wrench) for tightening of anode locks.
After installation of all anodes, the start-up charge (for example, cryolite,
crushed hard bath,
soda) is loaded to the side-anode space and the anode body is covered with
cryolite on top. At that, all
installed prebaked anode rod assemblies are connected to anode buses of the
pot anode busbar using a set
of flexible aluminium tapes and electrical current is passed through the
graphite material layer. Current
load of the prebaked anodes is controlled as well via disconnection of anodes
with high load or local
bottom overheating.
It should be noted that in connection with the current economic situation at
present, smelters
should take measures to detect and eliminate operational / general production
costs affecting production
cost of commodity products at all production stages without reduction in
quality of products. One of
aspects directly affecting production cost of primary aluminium is relining
and process maintenance of
metallurgical equipment via preheating.
The pot preheating stage before connection and start-up is one of the most
important operations
during their operation. The pot life, quality of produced aluminium, and key
performance indicators to a
large extent depend on quality of the preheating operation. During preheating,
it is important to ensure
uniform and smooth heating of the pot cavity and cathode.
Requirements to the pot preheating before its start-up consist in the
following:
¨ Ensure smooth transition from the cold state to the reduction temperature
conditions;
¨ Exclude thermal 'shocks' including those during bath pouring;
¨ Achieve minimum thermal pressure on the cathode in both vertical and
planar directions;
¨ Ensure proper baking of bottom ramming paste;
¨ Ensure full drying of the pot bottom pedestal after its lining using
liquids.
In the world practice, the following three basic pot preheating methods are
used depending on the
heating principle:
1. Preheating with electrical current, where heat radiation is defined by
Joule-Lenz's law:
1.1. On both fine-dispersed and coarse carbon materials;
1.2. On liquid metal or aluminium chips;
1.3. With moulding of a new anode (Soederberg);
2. Thermal preheating, where heat-transfer medium is natural gas or oil
product;
3. Start-up without preheating with bath and metal pouring immediately to a
cold pot.
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Before 1995, the pot preheating operation was carried out at the Sayanogorsk
Aluminium Smelter
with two methods:
¨ On S-175M2 pots - via flame preheating (by a preheating unit designed by
VAMI);
¨ On S-255 pots - with electrical current on carbon grits ('seeds), where
anodes after their
placement on the 'seed' layer were rigidly pressed to the anode busbar with
standard clamps.
Since 1995, the following measures were implemented at the Sayanogorsk
Aluminium Smelter
under the pot life extension programme for optimisation of the preheating
process:
¨ Flexible connections of anode rods to anode buses for independent
preheating with electrical
current on all pot types;
¨ Improved control of supplied pot power via disconnection of rheostat shunts
with increased
quantity of disconnection steps from 2-3 to 6- 8, which significantly improved
heating quality;
¨ Arrangement of a specialised team for pot preheating and start-up
operations.
During electrical preheating on carbon grits, anodes were rigidly pressed to
the anode busbar with
standard clamps. After 1995, flexible connections of anode rods to anode buses
were applied. The main
disadvantages of this electrical preheating on coke are as follows:
¨ Problems with control of heating rate (disconnection of rheostat shunts);
¨ Non-uniform pot bottom heating due to applied raw material (coke) and non-
uniform
adjoining of anode soles (knurling, design of connection tapes);
¨ High labour intensity during pot start-up (coke removal).
Since 2004 and at present after adjustments of RA-300 technology and start-up
of the Khakas
Aluminium Smelter, all pots at the Sayanogorsk Aluminium Smelter are preheated
with the gas-flame
method. The existing preheating and start-up procedure for RA-300 and RA-400
pots is schematically
represented as follows:
Gas-flame preheating => Bath pouring => Pot connection to circuit without
potline disconnection
=> Adjustment of parameters to target values
The disadvantages of the gas-flame preheating method are as follows:
1) For lining preheating in volume and achievement of target temperatures,
preheating duration
should be increased from 72 hours to 96 hours (topical in the cold season).
2) Insufficient quantity of burners in the Hotwork unit for long pots. Small
quantity of
temperature check points. Problems with operation of the unit in magnetic
fields and during heavy frost.
3) No data on temperature of the pot bottom during preheating - temperature of
gas-air
environment is measured.
4) Troubled operations of connection/start-up at full amperage:
¨ Personnel safety;
¨ High probability of unscheduled current drops;
¨ Long start-up duration (pouring of more bath), high-voltage anode effects
during start-up.
The obtained experience from start-up of pots in the RA-400 pilot area shows
that gas-flame
preheating does not meet the process requirements in the cold season (longer
preheating is necessary to
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achieve the minimum required pot bottom temperature). This fact is not
acceptable for quick
commissioning of the Taishet Aluminium Smelter, since Taishet (Irkutsk Region,
Russia) has a negative
average monthly ambient temperature in seven months per year according to the
climatic parameters.
The main condition for connection of pots at the Taishet Aluminium Smelter
taking into account its
production capacity is also connection of RA-400 pots to the circuit without
disconnection of the potline
process load to exclude high loads on the Siberian energy system.
The main technical solution allowing excluding the above-listed disadvantages
is to substitute
gas-flame preheating of pots with electrical current preheating. Application
of the electrical preheating
process will allow us to:
¨ Reliably connect a pot to the circuit without the potline disconnection or
current drop;
¨ Exclude costs for expensive preheating equipment and fuel (exclude the
limiting factor for
quick commissioning of the smelter and environmental impact);
¨ Reduce duration of the pot preheating operation.
The achieved key performance indicators are as follows:
1. Reliable and safe connection of a pot at full current of the potline.
2. Reduction of the pot preheating duration from 72 hours to 54 hours.
3. Exclusion of costs for expensive preheating equipment and fuel (reduction
of
environmental impact).
The fundamental distinctions of the proposed technical solution are as
follows:
1) Preheating at full amperage without rheostat shunts;
2) Application of graphite materials;
3) Differentiated knurling of the graphite 'bed';
4) Optimal design of flexible contact parts:
¨ Anode freedom in three directions (X, Y, Z);
¨ Immediate control via current distribution on anodes;
5) Automated temperature monitoring.
Taking into consideration the above description of the method, examples, and
distinctions, this
document discloses a pot bottom preheating method for aluminium pots with
prebaked anodes
including: covering of the pot bottom with an electrically conductive
material; placement of prebaked
anodes on it; their connection to anode buses of the pot anode busbar; passage
of electrical current
through the electrically conductive material; and control of current load on
the anodes for preheating.
This method is distinguished by the fact that uniform preheating is ensured
via proper selection of the
electrically conductive material quantity under the anodes. Namely, quantity
of the electrically
conductive material under the anodes is selected so that the material quantity
is less under the anodes
located at the pot middle than under the anodes located near the extreme end
anodes, while the material
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quantity is less under the anodes located near the extreme end anodes than
under the extreme end
anodes.
Further, this document discloses the above method distinguished by the fact
that the
electrically conductive material is graphite with fraction from 0.1 mm to 10
mm.
Further, this document discloses the above method distinguished by the fact
that height and
length of each row of the electrically conductive material under the anodes
are set in inverse proportion
to passed amperage.
Further, this document discloses the above method distinguished by the fact
that the installed
prebaked anode rod assemblies are connected to anode buses of the pot anode
busbar using flexible
elements ensuring the anode freedom in three directions (X, Y, Z).
Further, this document discloses the above method distinguished by the fact
that after
installation of all anodes, the start-up charge (for example, cryolite,
crushed hard bath, soda) is loaded to
the side-anode space and the anode body is covered with cryolite on top.
Further, this document discloses the above method distinguished by the fact
that all installed
prebaked anode rod assemblies are connected to anode buses of the pot anode
busbar using a set of
flexible aluminium tapes and electrical current is passed through the graphite
material layer.
Further, this document discloses the above method distinguished by the fact
that current load
of the prebaked anodes is controlled via disconnection of anodes with high
load or local bottom
overheating.
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