Note: Descriptions are shown in the official language in which they were submitted.
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A method and equipment for positioning when replacing anodes in an
electrolysis cell
The present invention concerns a method for positioning when replacing anodes
in
an electrolysis cell, and equipment for carrying out this method.
Electrolysis cells of the Hall-Heroult type with prebaked anodes for the
production of
aluminium require regular replacement of used anodes with new ones during
operation. Such prebaked anodes comprise a prebaked or calcined carbon block
to
which an anode suspender is attached via nipples that are fastened to the
carbon
block. The anode suspender and its nipples are of a metallic material. The
combination of carbon block and anode suspender are usually called the anode,
and
this is fastened via the anode suspender to the superstructure of the
electrolysis cell,
more precisely to an anode beam that may run in the longitudinal direction of
the cell.
A standard cell design comprises two anode beams, and a certain number of
anodes
may be arranged side by side along each of the beams. Often each anode beam
may have 8-10 anodes. The carbon material in the anodes is consumed during the
electrolysis process and must be replaced before the metallic material in the
nipples
is revealed. This process takes approximately 28 days and, in an electrolysis
hall
with several tens of cells, there will be an extensive need for the removal of
used
anodes and the insertion of new ones. In this operation, it is important for
the lower
side of the new anode that is inserted to be positioned as correctly as
possible in the
position of the used anode in terms of height. This is because the interpolar
distance
(the distance between the anode and the cathode) is an important parameter in
the
cell.
Today this operation is increasingly performed using a traversing crane
mounted on
rails that runs along the rows of electrolysis cells and is usually positioned
above
them. The operation stated is one of the more labour-intensive and frequent
operations during the operation of electrolysis cells, and the various players
within
the industry have developed improvements to simplify and rationalise this work
while
focusing on the safety and working environment of the operators. An
established
method of determining the insertion height of the new anode is to place it on
a table
beside the anode removed and to mark a common reference level on the anode
suspender with chalk. A measuring stand is used as an auxiliary tool for this
operation.
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US patent no. 4,221,641 concerns a method and an arrangement for replacing
electrodes in a reduction cell for the production of aluminium. The method
involves
the transport path of a used anode from the cell to a first level above the
cell being
registered so that the length of the electrode is measured, a new electrode
being
lowered, whereby the distance from a second level parallel to the first level
is
detected to measure the length of the new electrode, measurement of the
distance
between the levels stated and the new electrode being lowered further from the
second level for a distance that is equal to the transport path of the used
anode
minus the distance between the two levels stated. To determine the level, this
solution comprises an arm which is activated by the electrodes and which also
has a
detector to detect the arm's motion. The detector consists of a beam of light
and a
receiver, and the beam of light is broken by a screen mounted on the arm. The
detector solution is of the optical/mechanical type with moving parts that
easily can
move out of position over time and/or require extensive maintenance/inspection
to
ensure the mobility and correct function of the components. This principle of
measurement is based upon the use of an incremental counter in addition to
limit
switches, where the relative movement of the crane is counted.
The present invention represents a method and equipment for the replacement of
anodes in an electrolysis plant that provide very precise positioning of the
anodes
and are more robust in the face of dust, wear and mechanical stress than prior
art
solutions. Moreover, the solution requires little inspection and has a good
user
interface for the operator who operates the crane.
In accordance with the present invention, there is provided a method for
positioning
when replacing anodes in a electrolysis cell [Hall-Heroult] with prebaked
anodes, in
which a crane with an anode gripper is used to lift out used anodes and to
insert new
anodes, the gripper act in a predetermined, fixed point in the hanger of the
anodes,
and in which a new anode is inserted at a height in accordance with a
calculated
height based on the height of the anode removed, the height of the anode
removed
and the height of the new anode being measured against a common reference
level,
characterised in that laser-based measuring equipment for length measurements
is
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arranged between a point on the crane, which is stationary in terms of height
during
the operation, and a point on the anode gripper, which moves together with the
anode, that the equipment measures the heights stated and that the measured
values are processed by a PLC-based system, which determines the insertion
height
of the new anode in accordance with a specific algorithm.
In accordance with another aspect of the present invention, there is provided
equipment for positioning when replacing anodes in a electrolysis cell [Hall-
Heroult]
with prebaked anodes, comprising a crane with an anode gripper to lift out
used
anodes and to insert new anodes, the gripper acts in a predefined point of the
anode's hanger, with which a new anode is inserted at a height in accordance
with a
calculated height based on the height of the anode removed, the height of the
anode
removed and the height of the new anode being measured against a common
reference level, characterised in that laser-based measuring equipment for
length
measurements is arranged between a point on the crane, which is stationary in
terms
of height during the operation, and a point on the anode gripper, which moves
together with the anode, and that the equipment measures the heights stated
and
transfers the data signals to a PLC, which processes measured, saved values
and
determines the insertion height of the new anode in accordance with a specific
algorithm.
The present invention will be described in further detail in the following by
means of
figures and examples, where:
Figs. 1 a-b show elementary diagrams with vital parts of the measuring
equipment,
with and without an anode;
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Fig. 2 shows an elementary diagram of how the measurements are
performed.
As stated above, one intention of the present method is to achieve more
precise
insertion of anodes by means of more precise distance measurement using
improved
equipment involving distance measurement using a laser. This avoids random
anode
hoist measuring errors, and there is less possibility of incorrect operation
thanks to
the interlocking technology chosen. The measuring principle and the sequence
used
eliminate measuring errors in connection with the vertical deflection of the
crane
bridge and random play in the coarse structure.
Compared with manual measurement with a measuring stand and reference level,
replacing anodes is less labour-intensive with this method - no operator is
required
on the floor for insertion and for positioning of the anode height, and the
anode
insertion height is more precise, which is important for the operation of the
cell.
With reference to Figure 1 a, the measuring equipment comprises a laser cell 3
installed inside a tight cabinet 4 with a vertical protection tube 5. The
laser beam 7
shines through the tube towards a reflective tag 6 placed on the anode gripper
2.
Under these circumstances, the laser cell will measure the precise distance.
The tight
cabinet, together with the protection tube and air overpressure that is
supplied to the
cabinet 4 via an air supply pipe 8, prevent fluoride dust and gas from
reaching the
cell's lens. This combination will produce precise measurement without random
measuring errors. Cables for communication with the PLC run from the cabinet.
A lifting device, the anode hoist, is mounted on a rotation crab on the
combination
crane that is able to lift a burned-out anode out of an electrolysis cell and
replace it
with a new anode. The anode hoist is hydraulically controlled, i.e. the anode
and
anode suspender are lifted out of and lowered into the cell by means of
hydraulic
force.
The anode hoist is equipped with an anode gripper. This is a gripping device
that is
fastened to the anode hoist. It can grip the anode suspender 10 that is
fastened to
the anode 11 (see Fig. 1 b). Conventionally, the anode suspender has a hole in
its
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upper part that can be gripped by a gripping device. Such gripping devices may
comprise one ore more studs that can be entered into the hole in the
suspender.
PLC means Programmable Logic Control. The PLC can control output signals (0)
by
means of input signals (I) and a logically constructed program. The PLC
consists of
several microprocessors. The PLC processor is located on the crane structure
while
several decentralised I/O racks are located on the mobile crabs and on the
driver's
cab. The decentralised I/O racks are linked to the PLC processor by means of
high-
speed data communication, which is very noise immune. This avoids having many
signal cables, which may be vulnerable to noise and error signals.
In this connection, laser measuring equipment can measure the distance between
the laser cell and a reflective tag. The precision of this combination is
approximately
1 mm. The laser cell used in the example has RS-232 communication with the
decentralised racks. The laser cell is expediently located inside a tight
cabinet with
overpressure attached to a flanged protection tube, for example of diameter 50
mm
and length approximately 2.5 m. The laser beam is designed so that it shines
from
the cabinet through the tube and down towards the reflective tag on the anode
gripper.
RS-232 (ASCII characters - port to port) communication is a data communication
method that precisely transfers signals from the laser cell to the PLC.
A PanelView (not shown) is a screen-based visualisation system that
communicates
with the PLC via the same communication method as for decentralised I/O racks.
This visualisation system can read off all values saved as described below so
that
the crane driver in the cabine can read the values.
A light panel (not shown) comprises a column of light with 5 lights of
different colours.
This light panel has 2 function modes. Mode 1 is used when removing the anode
and
indicates the measuring sequences step by step. Mode 2 is positioning
indication
when inserting the anode. This indicates when the anode is too high, too low
or in the
correct position. This is based on an algorithm that is programmed in the PLC.
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1) PLC Interlocking (PLC interlocking in the PLC-program)
The sequence is interlocked so that mechanical play is always eliminated. This
is done by the operator having to use the joystick for the anode hoist UP
STAGE 1 for a minimum of 2 seconds and to use the REMEMBER
5 MEASURED VALUE switch at the same time before the anode gripper
position is saved in the PLC.
When the joystick for the anode hoist is in the position UP STAGE 1, the
hydraulic aggregate ensures, with the appropriate valves, that the anode hoist
has approximately 60-70% lifting force in relation to the weight of a burned-
out
-10 anode.
2) Integrated PLC Solution
The PLC installation results in the signal transfer between the laser cell and
the PLC being very noise immune, the signal transfer being reliable and
random error measurements not being produced.
3) Visualisation
Simple visualisation shows the sequence and produces reliable positioning of
the new anode.
The crane in the example is controlled by the PLC with intelligent
decentralised I/O
racks. The distance laser measuring equipment communicates with the
decentralised
I/O racks. The laser cell and one decentralised rack must be positioned on the
anode
hoist as the laser cell transfers measured values to the PLC processor via an
intelligent decentralised I/O rack. It is important for the transfer of values
between the
laser cell and the PLC to be as noise immune as possible in relation to
electromagnetic and electrostatic beams and not to be affected by temperature
fluctuations in order to avoid error signals. This is achieved with the PLC
structure
chosen.
The laser measuring equipment with the laser cell is, as stated above, located
inside
a dust-tight cabinet 4 (the electrotechnical designation is IP 56) on the
rotating anode
hoist crab. The cabinet has a flanged tube 5 of diameter 50 mm and length
approximately 2.5 m mounted on it, which extends down towards the reflective
tag 6.
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The reflective tag is fastened to the anode gripper 2. The laser measuring
equipment
measures the distance between a fixed point in this tight cabinet (the
installation
location) on the anode hoist through the tube and down towards the reflective
tag on
the anode gripper. The tight cabinet is provided with overpressure and a long
tube as
stated so that the powerful upward air flows (hot air with a lot of gas
containing
fluorine and dust in connection with anode replacement) do not reach the lens
of the
laser cell. This is an important detail in the prevention of error
measurements. The
laser measuring principle described is a central part of the present invention
as it is
not subject to mechanical wear and does not require maintenance.
A PanelView 550 visualisation panel is installed inside the crane cab so that
all
measurements and stages in the sequence can be read off. This is controlled by
the
PLC.
A function light panel is installed in front of the operator with 5 indicator
lamps. This
function display has 2 modes. This is made for the operator to let him monitor
the
functions in connection with the insertion of anodes, in particular the
mounting height
for the new anode (for more information, see the functional description).
The operator's controls comprise a joystick on the left side, which has 3
stages for
upward movement of the anode gripper.
UP STAGE 1 exerts a lifting force on the anode gripper of approximately 60-70%
of
the weight of the burned-out anode and has a relatively low lifting speed.
This force
setting is used when the anode gripper height needs to be measured using the
laser
equipment.
UP STAGE 2 exerts a lifting force on the anode gripper of approximately 300%
of the
weight of the burned-out anode (breakaway force when removing an anode) and
has
the same speed as UP STAGE 1.
UP STAGE 3 exerts a lifting force on the anode gripper of approximately 200%
of the
weight of the burned-out anode and has a high speed for rapid anode handling.
On the same joystick as stated above, there is a press-button switch at the
front. This
is used to operate REMEMBER ANODE GRIPPER POSITION.
Functional Description, see Figure 2
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All the functions described are carried out by the operator in the crane cab.
The
operator first sets the offset value on the PanelView, i.e. how much higher
the new
anode is to be inserted in relation to the burned-out anode (the typical
setting is 20
mm). The operator guides the crane so that he grips the anode suspender (the
anode) with the anode gripper. He then lifts the burned-out anode by operating
the
anode hoist UP STAGE 1 with the joystick (see Figure 2a). This involves the
anode
hoist lifting with a preset force that is approximately 60-70% of the anode
weight
(constant force). This results in the elimination of play. While the lifting
is taking
place, the operator presses the REMEMBER POSITION switch on the joystick. The
anode gripper position is saved in the PLC if UP STAGE 1 lifting has been
active
continuously for 2 seconds. If the operator uses UP STAGE 2, the anode gripper
position will not be saved in the PLC. This is an important interlocking
function in the
PLC to eliminate play and to ensure that the crane is deflected in
approximately the
same way for all measurements. When this has been done in accordance with the
method, the PLC will acknowledge this with a yellow light on the light panel.
This
means that measurement A has been completed. The anode clip is loosened and
the
burned-out anode is removed from the cell.
The burned-out anode is placed down on the reference level (checker plate),
see
Figure 2b. The anode is lifted up by using anode hoist UP STAGE 1, i.e. the
anode
hoist will again lift with approximately 60-70% of the anode weight. The
burned-out
anode will not be lifted from the reference level, but the crane structure is
extended in
the same way as when lifting the anode out of the cell. This results in the
play in the
crane being eliminated. While the lifting is taking place, the operator
presses the
REMEMBER POSITION switch on the joystick. The position is saved in the PLC if
the UP STAGE 1 lifting was active for 2 seconds. When this has been done
correctly,
the PLC will acknowledge this with a green light on the light panel. This
means that
measurement B has been completed.
The old anode is then placed in a waste bin. The new anode is fastened to the
anode
gripper. This is then placed on the same reference level as for the burned-out
anode,
see Figure 2c. The anode is lifted up by using UP STAGE 1, i.e. the anode
hoist will
again lift with approximately 60-70% of the anode weight. While the lifting is
taking
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place, the operator presses the REMEMBER POSITION switch on the joystick. The
position is saved in the PLC if the UP STAGE 1 lifting was active for 2
seconds.
When this has been done correctly, the PLC will acknowledge this with a red
light on
the light panel. This means that measurement C has been completed. All the
measurements have now been completed.
The PLC calculates the insertion height of the new anode using this formula -
see
Figure 2d:
D=A-B+C-X
D is the desired position of the new anode.
A is the position of the burned-out anode in the cell.
B is the position of the burned-out anode on the reference level.
C is the position of the new anode on the same reference level.
X is the additional height of the new anode in the cell in relation to the
burned-out
anode.
The new anode is inserted in the cell at anode position = D (+/- tolerance,
typically +/-
3 mm - the tolerance can be adjusted from the operator panel).
The indicator panel then switches to Mode 2 for indication. The operator
inserts the
anode in the cell in accordance with the light indication in Mode 2, i.e. if
the yellow
lights light up (1 or 2 yellow lights), the anode is too high, and if the red
lights light up
(1 or 2 red lights), the anode is too low. The anode has the correct height
position if
the green light lights up, i.e. the anode is in position D +/- 3 mm. When the
anode is
positioned in the cell and the green light lights up, the anode is fixed with
the anode
clamp. The sequence has been completed, and the equipment can be used to
replace an anode in another cell.
Additional insertion height for a new anode is important to avoid anode
deformation.
The additional height means that the anode does not draw "full power", and it
is
allowed to heat up gradually before full current flows through it.
