Note: Descriptions are shown in the official language in which they were submitted.
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Hot-dip galvanizing installation for steel strip
=
FIELD OF INVENTION
This invention concerns a hot-dip galvanizing installation for
steel strip.
BACKGROUND
Hot-dip galvanizing of continuously moving rolled steel strip
is a known technique that essentially has two variants, one
= where the strip leaving the galvanizing furnace drops obliquely
into a bath of molten metal comprising at least one metal
suitable for galvanizing such as zinc, aluminum, before being
diverted vertically upwards by a roll immersed in said bath of
molten metal. The other variant involves diverting the strip
vertically upwards as it leaves the furnace, before passing it
through a vertical channel containing molten zinc sustained
magnetically. The bath of molten metal is a zinc alloy with
variable proportions of aluminum or magnesium or manganese. For
the sake of clarity, this patent shall only describe the case
of an alloy of zinc and aluminum.
In both cases, the operation is intended to create on the
surface of the steel strip a continuous and adherent deposit of
a molten mixture of zinc and aluminum through which said strip
is passed. The kinetics of formation of this deposit is known
to the person skilled rin the art, it has been covered in
numerous publications including "Modelling of galvanizing
reactions", Giorgi et Al. in -La Revue de Metallurgie - CIT-,
October 2004. This documentation establishes that contact with
the molten mixture causes the dissolution of iron from the
steel strip that, firstly, participates in the formation, on
= the surface of the strip, of a compound layer of approximately
0.1 p of the compound Fe2A15Zn and, secondly, spreads to the
bath of molten mixture until the Fe2A15Znõ layer has formed.
continuously. The Fe2A15Znx layer serves to support the final
protective zinc layer while the dissolved iron contributes to
the formation of precipitates comprising iron Fe, aluminum Al
and zinc Zn known as "mattes" or "dross" in the molten mixture.
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These precipitates in the form of particles measuring between a
few microns to a few dozen microns may cause visible defects on
the coated (galvanized) strip that may be redhibitory, in
particular in the case of strips of sheet metal intended for
use in visible parts of car bodywork. Steel companies therefore
make significant efforts to limit or eliminate dross from
galvanizing baths.
The phenomenon of dross formation is known to the person
skilled in the art by, for example, publications such as
"Numerical simulation of the rate of dross formation in
continuous galvanizing baths", Ajersch et al. Depending on the
temperature of the molten zinc bath and its aluminum
concentration, the quantity of iron that can be dissolved
varies within reasonably wide limits. If iron concentration
exceeds the solubility threshold, nucleation and enlargement of
the Fe-Al-Zn compounds defined becomes possible. In normal
continuous galvanizing methods, a coating bath containing the
molten mixture to be deposited on the strip is always saturated
with iron, it follows that all of the iron dissolved from the
strip and spreading into the molten mixture is immediately
available to create dross in situ.
Of the different means available to attempt to control dross
or, at least, to reduce its quantity in the coating tank,
manual skimming of the surface of the molten mixture has long
been used. This method being rightly considered dangerous for
operators, means of mechanizing and robotizing this operation
have been considered, as described in JP 2001-064760.
Other techniques involving overflow, pumping or ejection have
been considered in order to remove the dross formed in the
coating tank. Thus, EP 1 070 765 describes a series of variants
of a galvanizing installation comprising, in addition to the
coating tank in which the dross forms, an auxiliary tank to
which the dross is evacuated.
EP 0 429 351 describes, in greater detail, a method and a
device intended to organize the circulation of molten mixture
between a coating zone of the metal strip and a cleansing zone
of the galvanizing bath containing the molten zinc, to ensure
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the separation of the dross in the cleansing zone, then to
return a:molten mixture "whose iron concentration is close to
or less than the solubility limit" to the coating zone.
However, if the physical principles present are properly
described, this document does not provide any details to enable
the person skilled in the art to implement them, in particular
how to simultaneously control cooling by a heat exchanger and
heating by induction of the same cleansing zone. There are also
no details on how to determine the circulation rate of the
= molten zinc.
= SUMMARY
One purpose of some embodiments of the invention is to provide
a hot-dip galvanizing installation for steel strip in a molten
mixture, for which a circulation circuit of the molten mixture
is thermally optimized.
Some embodiments of the invention therefore present a hot-dip
galvanizing installation for continuously moving rolled steel strip in
which the strip is immersed in a coating tank containing a
molten metal mixture, for example of zinc and aluminum, to be
deposited on the strip. The molten mixture is circulated
continuously between said coating tank and a preparation
device, in which the temperature of the molten mixture is
deliberately lowered in order to reduce the iron solubility
threshold and sufficiently raised to activate the fusion of at
least one ingot comprising a Zinc-Aluminum Zn-Al alloy in a
fusion zone of the preparation device thus assuring an
additional supply of molten mixture (Zn, Al), in sufficient
quantity to offset-the molten mixture deposited on the strip.
Furthermore:
- the preparation device includes a first and a second zone
linked by a transfer means for the molten mixture (or a
separation device in the form of a wall with a central
= aperture),
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- a flow path of the molten mixture is imposed sequentially
from the coating tank, via the first zone for ingot fusion and
dross settlement, via the transfer means (or the separation
device) to the second zone receiving a molten mixture cleansed
of dross, itself returned to circulation in the coating tank
via a return-flow path for the cleansed molten mixture, the
return-flow path being physically distinct from the flow path
such as by means of a loop,
- the thermal adjustment means are arranged along the flow path
of the molten mixture also providing a thermal loop between an
outlet of the flow from the second zone and an inlet of the
return-flow into the coating tank, the outlet and inlet being
distinct.
On account of the sequential physical and thermal loop of the installation
according to some embodiments of the invention, hot-dip galvanizing
for continuously moving rolled steel strip is advantageously
implemented wherein the strip is immersed in the coating tank
containing the molten zinc and aluminum mixture circulated
continuously between said coating tank and the preparation
device in which the temperature of the molten mixture is
deliberately lowered to reduce the iron solubility threshold.
For this purpose, the flow and return-flow paths are
established and managed such that:
- on the basis of the speed of the steel strip and its
thickness and its width on arrival at the coating tank, the
power supplied by said strip entering at a first temperature in
the molten mixture bath of the coating tank is determined. To
enable control of the heat balance of the operation, a second
temperature of the coating bath is fixed at a pre-set level
below the first temperature, -
- on the basis of the speed of the strip and its width and the
thickness of the intended coating, the power required to
maintain the quantity of molten mixture consumed in
consideration of a rate of consumption at the second pre-set
temperature is determined,
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- a comparator between the two aforementioned powers is then
activated, enabling the identification of two decision modes
(points a and b) on the thermal loop to be adopted:
a) if the power supplied by the strip is greater than that
required for fusion of the quantity of zinc consumed, a command
unit issues an instruction to reduce the temperature of the
strip, potentially associated with an instruction to reduce the
strip movement speed in order to maintain a balance or a
specific difference between the two aforementioned powers,
b) in the opposite case, and as a function of the first
measured rate of the molten mixture consumed (in the coating
tank or related to losses), the energy required to ensure
continuous fusion, in the preparation device, of Zn-Al alloy
ingots pre-heated or otherwise to a third temperature in
sufficient quantity to offset the mixture consumed is
determined.
Depending on these thermal conditions, the means of adjusting a
second circulation rate of the molten mixture between the
coating tank and the preparation device are then implemented in
order to provide in said preparation device the energy required
for the continuous fusion of the ingots while maintaining the
temperature of the molten mixture in the preparation device at
a fourth pre-set value, in all cases lower than the Second
temperature.
Finally, the temperature adjustment means make it possible to
set a fifth temperature for the molten mixture leaving the
preparation device in order to provide, as a function of the
first rate, the additional power required for the thermal
balance intended with a nearby return-flow inlet in the coating
tank.
Under these conditions, the means of controlling and
maintaining/adjusting the iron dissolution rate (rate of iron
concentration by unit of time) in the coating tank makes it
possible to check and maintain globally the iron concentration
of the molten mixture below its dissolution threshold.
Some embodiments of the invention include means for determining,
controlling or adjusting powers, temperature, rate (flow and
concentration) being thus sequentially and therefore suitably placed at
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several points of the physical flow and return-flow loop for
the molten mixture, to enable a suitable profile in terms of
zinc, aluminum and iron concentration resulting in a related
thermal profile and thermal balance in the loop as described
above and in the description below.
According to one aspect of the present invention, there is
provided a hot-dip galvanizing installation for continuously
moving rolled steel strip in which the strip is immersed in a
coating tank containing a molten metal mixture to be deposited
on the strip and circulated continuously between said coating
tank and a preparation device in which the temperature of the
molten mixture is deliberately lowered to reduce the iron
solubility threshold and sufficiently high to activate the
fusion of at least one metal ingot providing an additional
supply of molten mixture in one said fusion zone of the
preparation device, in sufficient quantity to offset the molten
mixture deposited on the strip, the preparation device includes
a first and a second zone linked by a molten-mixture transfer
means, a flow of the molten mixture is imposed sequentially
from the coating tank, via the first zone fusing the ingot and
causing the dross to settle, via the transfer means and to the
second zone receiving a molten mixture cleansed of dross,
itself returned to circulation in the coating tank via a
return-flow path of the cleansed molten mixture, the thermal
adjustment means are arranged along the flow of the molten
mixture providing a thermal loop between an outlet of the flow
from the second zone and an inlet of the return flow into the
coating tank, wherein the first zone of the preparation device
includes a local regulation means for lowering the temperature
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which may if necessary help to effect the lowering of the
temperature of the molten mixture required that is ideally
effected by selective dipping and removal of at least one ingot
in the first zone.
BRIEF DESCRIPTION OF THE DRAWINGS
Several advantageous exemplary embodiments of the installation
according to the invention are described to overcome the
drawbacks of the prior art.
Exemplary embodiments are provided using the figures described:
Figure 1 Schematic drawing of the installation,
Figure 2 Schematic drawing of a variant of the installation,
Figure 3 General drawing of the coating tank,
Figure 4 Arrangement of the installation according to a first
embodiment,
Figure 5 Arrangement of the installation according to a second
embodiment,
Figure 6 Arrangement of the installation according to a third
embodiment,
Figure 7 Arrangement of the installation according to a fourth
embodiment,
Figure 8 Arrangement of the installation according to a fifth
embodiment,
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Figure 9 Arrangement of the installation according to a
sixth embodiment,
DETAILED DESCRIPTION
Figure 1 is a schematic drawing of the installation according to an
embodiment of the invention. A steel strip (1) is introduced, ideally in
continuous movement, obliquely into a coating tank (2) via a
linking conduit to a galvanizing furnace (3) (not shown
upstream of the coating tank). The strip is diverted vertically
by a roll .(4) and passes through a molten coating mixture (5)
contained in said coating tank. The strip may be diverted by a
horizontal roll (4) supporting movement of the strip. A-channel
(6) enables the molten mixture to overflOw into a preparation
device (7) comprising two zones, a first zone (71) in which at
least one Zn-Al alloy ingot (8) is fused in.sufficient quantity
to offset the molten mixture deposited on the strip in the
coating tank and the inevitable (material) losses, and a second
zone (72) sequentially juxtaposed with the first zone in a flow .
path direction (FL) of the molten mixture (coating tank to
first zone then second zone).. These two zones (71, 72) may be
located in two. separate tanks placed side by side as indicated
in Figure 1 and linked by a transfer means (74) or they may be
combined in a single tank in which they are separated by a
separation device, =such as a wall with a central aperture.
A thermal adjustment means may include a cooling device (6, 62)
for the molten mixture leaving the coating tank or in the ingot
(8) fusion zone, said cooling resulting in a minimum
temperature threshold in the first zone (71) of the preparation
device that is sufficiently high to fuse the ingot. Under the
effect of the cooling (62) of the molten mixture on leaving the
channel (6), i,e. when entering and inside the first zone (71),
surface dross (81) and bottom dross (82) is formed and
retained, in the discharge flow direction (FL) by the end wall
of the first zone (71). Transfer means (74) withdrawing the
molten mixture between the two dross layers (81) and (82)
enable transfer to a second zone (72) of the preparation device
which therefore receives a cleansed molten mixture which may be
reheated by a heating means (75) preferably by induction. A
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pipe (9) recovers the molten mixture in the second zone (72)
and, in the case in Figure 1, under the action of a pumping
device (10) and a return-flow-path (11) pipe it resupplies the
coating tank (2) by means of a chute (12) at a cleansed molten
mixture rate. Devices enable the evacuation of dross from the
preparation device (first zone (71)). Advantageously, the first
zone (71) of the preparation device includes partitions (not
shown) isolating the portions of the molten mixture placed
between several ingots (8), arranged perpendicularly to the
flow path direction (FL). These may be implemented by means of
a wall with a central aperture, thus making it possible to
concentrate the bottom dross (82) and surface dross (81) ingot
by ingot as a function of their aluminum concentration.
With regard to ingot fusion, the first zone (71) of the
preparation device includes at least one supply of ingots (8 --
81, 82, ..., 8) whose concentrations are as required (Alt) by
the mixture in the preparation tank and it advantageously
includes supplies of ingots of which at least two have
different aluminum concentrations and of which at least one of
the ingots has a concentration higher than the concentration
required by the molten mixture in the preparation device.
Furthermore, the first zone (71) of the preparation device
. includes a means for regulating the rate of fusion of at least
two ingots, ideally by selective dipping and removal of at
least one ingot in the first zone (71). Finally, the first zone
(71) of the preparation device includes a local adjustment
means (6, 62) for lowering the temperature (T) which may if
necessary help to cause the lowering of the temperature of the
molten mixture required which is ideally effected by selective
dipping and removal of at least one ingot in the first zone
(71). By definition, a first temperature (T=T1) represents the
temperature of the strip (1) entering the coating tank (2), a
second temperature (T=T2) represents the temperature in the
coating tank and a third temperature (T=T3) represents the
temperature at the entrance and inside the first zone (71) of
the preparation device.
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Consequently, continuous fusion of the ingots (8) in the
preparation device (71) is assured at their full fusion rate.
It is then advantageous to dip a plurality of n ingots
simultaneously into the molten mixture bath, each potentially
having a different aluminum concentration and at least one of
them having an aluminum concentration higher than the
concentration required in the preparation device, in order to
make it possible to establish a concentration profile (or
fusion rate) that is variable over time. The required
concentration can itself be determined on the basis of the
aluminum consumption measured or estimated in the coating tank,
in the Fe2A15Znx compound layer formed on the surface of the
strip and in the dross formed in the preparation device.
Advantageously, the fusion rate of each of the n ingots can
also be controlled individually such as to adjust the aluminum
concentration in the preparation device to the required
concentration while maintaining the full fusion speed required.
Continuous fusion of the ingots in the preparation device
results locally in a cooling of the molten mixture from the
second temperature (T2) (outlet of the coating tank) to the
third pre-set temperature (T3) in the first zone (71) in order
to lower the iron solubility threshold and to enable the
localized formation of dross in said preparation device until
the solubility threshold at the pre-set temperature is reached.
Aluminum-rich "surface" dross then forms preferentially near to
the immersed aluminum-rich ingots then settles towards the
surface and the zinc-rich "bottom" dross forms preferentially
near to the immersed aluminum-poor ingots then settles towards
the bottom.
Following formation of the dross, the renewal rate of the
molten mixture entering the coating tank with an iron
concentration equal to the solubility threshold of iron at the
pre-set temperature makes it possible to keep the increase in
the dissolved-iron concentration below the solubility threshold
at the second temperature.
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The installation thus enables the implementation of a
galvanizing method characterized in that the following work
together:
- the coating tank (2) which comprises a first metal envelope
in contact with the molten mixture (5) and a second envelope of
refractory material separated from the first envelope by a
space in which the heating means are arranged. These heating
means are advantageously electrical resistors radiating onto
the metal envelope to guarantee a uniform heat distribution and
= to prevent hot spots inside the tank. Heating the coating tank
is primarily intended to offset the thermal losses caused by
the tank itself. It is not necessarily actively involved in the
general process for the thermal balance of the galvanizing
method related to the embodiments described.
- the preparation device that assures the fusion of the Zn-Al
alloy ingots in sufficient quantity to offset the molten
mixture deposited on the strip and the inevitable losses
comparable to a supplementary usage. In the first zone (71),
the controlled fusion of the ingots is accompanied by a
controlled reduction of the temperature of the molten mixture
that enables the localization of the formation of dross
exclusively in the preparation device. This dross is separated
in the preparation device in order to cleanse the molten
mixture before it is transferred to the coating tank.
- a circulation circuit that ensures the transfer of the molten
mixture, for example by pumping and by gravitational drainage,
between the coating tank and the preparation device and between
certain component parts of the preparation device.
The coating tank (2) is fitted with a sealing system ensuring
the link between the input of the strip moving towards said
=
tank and an output channel of the galvanizing furnace
downstream of said tank (not shown for the sake of clarity).
Using a lid covering the coating tank, the entire surface of
the molten mixture is therefore also protected against
oxidization, by the neutral atmosphere of the galvanizing
furnace on the strip-input side of the coating tank and, on the
strip-output side of the same tank, by a slight overpressure of
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neutral gas introduced by a pipe (61) which also protects the
surface of the molten mixture in the preparation device.
The preparation device (7) may comprise two tanks, one for
fusing the ingots and localizing dross formations, the other
localizing the reheating means of the molten mixture, the
molten mixture being transferred from one tank to the other by
pumping or by gravity by means of filter chutes which may be
supplied alternately or together by valves. This aspect will be
further detailed below.
The preparation device (7) may also comprise a single tank
comprising the first and the second zone (71, 72) separated,
for example, by a filter wall, the first zone fusing the ingots
and localizing the dross formations, the second zone (72)
receiving the cleansed molten mixture. In this case, the second
zone is fitted with a heating means (75), advantageously
induction heating, reheating the cleansed molten mixture before
it returns to the coating tank, such as to provide a return-
flow path (RFL) thermal loop at the end of the flow path to the
start of a new flow (FL).
The circulation circuit may include at least one lift pump (10)
drawing via a duct (9) in the cleansed zone of the preparation
device and, having passed through a return-flow-path (RFL) duct
(9), supplying either the return chute (12) in the coating tank
(2) directly, or interchangeable filter chutes supplying an
additional tank fitted With an induction-heating means
reheating the molten mixture before it is returned by gravity
to the coating tank via the return chute. In order to reduce
the lift height of the pumps, at least one pump may
advantageously be used between the cleansed zone (72) of the
preparation device and the additional tank and at least one
other pump between the additional tank and the chute of the
coating tank. This shall also be further described below.
In summary, Figure 1 is a first drawing of the hot-dip
galvanizing installation for continuously moving rolled steel
strip (1) in which the strip is immersed in the coating tank
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(2) containing a molten metal mixture (5), such as of zinc and
aluminum, to be deposited on the strip moving continuously
between said coating tank and a preparation device (7) in which
the temperature of the molten mixture is deliberately lowered
to reduce the iron solubility threshold and sufficiently high
to activate the fusion of at least one Zn-Al ingot (8) in a
fusion zone of the preparation device, in sufficient quantity
to offset the molten mixture deposited on the strip.
The installation is defined by the following characteristics:
- the preparation device (7) includes a first and a second zone
(71, 72) in two separate tanks or in a single tank where they
-are separated by a transfer means (74) or a separation device,
- a flow of the molten mixture is imposed sequentially from the
coating tank, via the first zone (71) fusing the ingot,
potentially via the transfer means (74) or the separation
device (73) designed to filter the dross from the molten
mixture in the first zone and to transfer the molten mixture
filtered of dross to the second zone (72) receiving a molten
mixture cleansed of dross, itself returned to circulation in
the coating tank via a return-flow path (11) of the cleansed
molten mixture,
- the thermal adjustment means are arranged along the flow of
the molten mixture providing a thermal loop between an outlet
(9) of the flow from the second zone (72) and an inlet (12) of
the return flow in the coating tank.
One of the thermal adjustment means includes a first heating
means (75) for the molten mixture cleansed in the second zone
(72). Advantageously this enables looped thermal continuity
between respective inlets and outlets of the flow path and the
return-flow path.
One of the thermal adjustment means includes a second heating
means (1) for the molten mixture in the coating tank. This
heating means and, at least its maintenance and adjustment
around a temperature threshold, is also ensured or complemented
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by the strip itself leaving the galvanizing furnace and
dropping into the coating tank at a temperature higher than
that of the molten mixture in the coating tank. This beneficial
aspect constituting a second heating means is therefore
effected by thermal transfer by providing the motive force of
the strip to be immersed in the molten mixture (5) required to
bring a quantity of molten mixture to a required temperature.
It should also be noted that the temperature of the molten
mixture in the coating tank undergoes, after heating or
maintenance of the temperature using the moving strip, the
temperature drop described above at the entrance to the first
ingot-fusion zone (71). A basic thermal looping stage on the
flow path is therefore advantageously provided.
According to Figure 1, the preparation device includes the
transfer means (74) linking the two separate zones or tanks
(71, 72) placed side by side between which the molten mixture
is transferred. The transfer means (74) includes a pump (742)
or a link channel. The transfer means (74) in fact includes a
lifting pump (742) with a pump inlet (741) located at a central
height of the first zone (71) and a pump outlet (743) in the
second zone (72), said first and second zones (71, 72) being
separated physically in the form of two different tanks. The
level of the pump inlet (741) in the first zone (71) or the
level of the link channel are advantageously located between
the upper settling zone for surface dross (81) and the lower
sedimentation zone for bottom dross (82) or in the middle third
of the height of the first zone (71). It is essential that the
pump inlet (741) is located in an interstice free of dross so
that it is not pumped. The settling and sedimentation zones
form a gradually increasing accumulation that for a given
molten mixture rate in the flow path (FL) effectively ensures
that there is a dross-free pumping window in the first zone
(71).
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Figure 2 is a variant of the schematic drawing of the
installation according to Figure 1 in which the initial coating
tank is subdivided into a first strip-diversion compartment
(15) (with no molten mixture) and a coating tank (13)
comprising a molten mixture bath (5) supported by magnetic
suspension. Principally, the installation implements a variant
of the method in which the molten mixture bath (5) is supported
by magnetic suspension in a coating tank (13) connected to the
preparation device as in Figure 1. The suspension effect is
= provided, continuously, by electromagnetic devices (14). A
compartment (15) links the furnace and the diversion of the
strip (1) by the roll (4).
Figure 3 is a general drawing of the coating tank according to
the variant described by Figure 1. This type of tank (if
= emptied) may also be adapted for the coating tank according to
Figure 2 as a means of introducing the strip into the magnetic-
suspension coating tank. The steel strip (1) coming from the
galvanizing furnace (not shown) is diverted vertically upwards
by the roll (4) (on a horizontal axis of rotation) immersed in
the molten mixture (5). Following diversion by the roll (4),
the vertically moving strip then comes into contact with an
anti-crossbow roll (41) and a roll determining a pass line (42)
through an upper opening of the coating tank. The coating tank
is made up of a first metal envelope =(2) whose shape with
dimensions similar to the route along which the strip moves is
designed to reduce the volume of the molten mixture and thus to
enable its rapid renewal using pumps with a capacity of close
to, for example 100 metric tons per hour. A second envelope
= made of refractory material (not shown) protects the tank
environment from radiated heat and enables heat loss to be
limited. Advantageously, there are heating resistors (not
shown) between these two envelopes in order to offset the low
heat loss from the tank. Discharge chutes (6) and return chutes
(12) enable the tank to be placed easily into the circulation
circuit (flow path / return-flow path) of the molten mixture. A
mobile sealing system (31) enables the inlet of the tank to be
linked to the outlet channel of a galvanizing furnace
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downstream of the movement. The free surface of the molten
mixture is protected, in this zone, against oxidization by the
inert atmosphere of the furnace.
Figure 4 shows an arrangement of the installation according to
a first embodiment. A coating tank (2) with immersed roll as
described in Figure 1 or 3 or a coating tank with magnetic
suspension (13) as described in Figure 2 overflows its molten
mixture into the preparation device (7), specifically into its
first zone (71). This preparation device is in fact here split
into two zones (71) and (72) as in Figure 1. In the first zone
(71) of the preparation tank the fusion of the ingots (8) and
the localized precipitation of dross take place. The molten
mixture cleansed by natural separation of bottom dross (by
sedimentation) and surface dross (by settling) is collected in
the second zone (72) where it is heated by the induction device
(75). Transfer from the first to the second zone may be
effected using the transfer means (74) (by lifting pump (742)
as shown in Figure 1) or by simple linking channel. In this
case, at least one lifting pump (10) circulates the molten
mixture between the cleansed zone (72) of the preparation
device and the chute (12) of the coating tank via a return duct
(return-flow path). Advantageously, two lifting pumps (10) are
placed in parallel, one being in use and the other on stand-by
in case the first lifting pump requires maintenance, or
develops an operating fault or a malfunction due to wear.
For all installation variants, the surface and bottom dross
(81, 82) is collected and discharged from the preparation
device by classical means such as mechanical skimming, pumping,
centrifuging or magnetic separation.
Figure 5 shows an arrangement of the installation according to
a second embodiment. The general principle being the same as
the first embodiment according to Figure 4, at least one
lifting pump (10) (such as the pump (742) of the transfer means
(74), hence saving on one of the pumps (10, 742)) circulates
the molten mixture from an outlet of the first zone (71) of the.
preparation device to the second zone (72) provided with
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induction heating means (75) and placed just upstream of the
feed chute (12) of the coating tank (2) that it feeds by
gravity. In this way, a control of the temperature of the
molten mixture destined for the return flow (11) towards the
coating tank is more efficient, since the heat losses on the
return-flow path from the outlet of the cleansing tank may be
more closely offset (maintaining the temperature in the coating
tank is in fact essential to ensure the correct operation of
the installation). The molten mixture may be transferred from a
= lifting-pump outlet channel in the second zone (72) via at
least one filter chute (76), in this case two interchangeable
chutes designed to be used alternately. Also in this case, one
chute is in use, while the other is on stand-by. An additional
chute may also be used and supported, while the other two are
attached to the installation. The molten mixture filtered and
= reheated in the second zone (72) is reintroduced via a gravity
outlet in the chute (12) of the coating tank to ensure the
final stage of the return-flow path.
Figure 6 shows an arrangement of the installation according to
a third embodiment. The general principle being the same as the
second embodiment according to Figure 5, the molten mixture is
transferred in two stages: firstly by pumping the cleansed
molten mixture from the first zone (71) of the preparation
device to the second zone (72) then by pumping from said second
zone (72) to the feed chute (12) of the coating tank. For this
purpose, the second zone (72) may be arranged close to the
outlet of the first zone (71) of the preparation device.
Comparably with the second embodiment of Figure 5, this
arrangement makes it possible to reduce the lifting height of
each of the two lifting pumps (742, 10) arranged in series on
the return-flow path. An outlet of the second zone (72) is
linked to an inlet of the second lifting pump (10) one outlet
of which leads to the feed chute (12) of the coating tank.
Potentially, several filter chutes (76) are interchangeable
between the outlet of the first lifting pump (10) and the inlet
of the second zone (72).
=
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Figure 7 shows the arrangement of the installation according to
a fourth embodiment similar to Figure 4 from which it differs
in that the transfer means (74) of the molten mixture between
the first zone (71) and the second zone (72) of the preparation
device is realized by gravity through filter chutes (76) fed
alternately, for example by putting one in use and the other on
stand-by. An additional filter chute may then be supported by
distributors (77) holding the filter chutes above the second
tank (7b). The inlet of an arm serving the filter chutes (77)
is placed as described above at a wall height free of any
accumulation of dross. In this way, the use of a lifting pump
(742) for the transfer means (74) is advantageously saved.
Figure 8 shows an arrangement different to the principle
described in Figure 1 in which the preparation device (7)
comprises two zones, a first zone (71) in which at least one
ingot (8) is fused in sufficient quantity to offset the molten
mixture deposited on the strip in the coating tank and the
inevitable (material) losses, and a second zone (72)
sequentially juxtaposed with the first zone (71) in the flow-
path direction (FL) of the molten mixture (coating tank to
first zone then second zone). These two zones (71, 72) are
localized in the same tank as indicated and separated by a
separation device (74, 73), such as an wall with an aperture or
at least a dross filter in its central part (731). The first
zone (71) fuses the ingots and localizes the formation of dross
outside the central part (731), the second zone (72) receives
the cleansed molten mixture through the central part (731). In
this case, the second zone is fitted with an induction-heating
means (75) reheating the cleansed molten mixture before it
returns to the coating tank via the lifting pump (10), such as
to provide a return-flow path thermal loop at the end of the
flow path to the start of a new flow path.
The aperture of the separation device (73) may be fitted with a
filter cap intended to retain the dross that does not settle on
the surface or the bottom on the tank. It may also be replaced
by an interchangeable filter wall.
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This embodiment is also applicable jointly with an auxiliary
reheating tank. In this case, the preparation device has no
induction-heating means and the relative arrangement of the
preparation device and the reheating tank may be one of those
described between the first and second zone of the preparation
device in Figures 4, 5, 6 and 7.
So as not to overload the description and the number of
figures, it is specified that the transfer means (74) or at
least a vertically central part of the preparation device (see
Figures 1, 2, 4, 6, 7) may additionally be fitted with a filter
wall (73) as in Figure 8, for example located such as to
isolate the pump inlet (741) of the transfer means (74) of a
=
first (ingot fusion) part of the first zone (71). This ensures
that the pump inlet is never blocked by dross.
Equally, the transfer means (74) may include, instead of a
pumping device, a separation device in the form of a single
vertical wall (73) with a central aperture (731), as in Figure
8.
Finally, Figure 9 shows an embodiment of the installation (top
view as opposed to the side views in the preceding figures)
concerning all of the embodiments requiring at least one
lifting pump placed on the return-flow path of the molten
mixture. In fact, the preparation device includes at least a
flow-path portion (FL) of the molten mixture coming from an
outlet (Cl) of the coating tank (2, 13) being juxtaposed side-
by-side with a return-flow-path (RFL) portion of the molten
mixture via an inlet (02) in the coating tank. In other words,
the flow and return-flow paths are parallel in this top view,
or at least they form a channel with a half-turn leaving and
rejoining the coating tank. Ideally, the flow-path portion is
in the first zone (71) and the portion of the return-flow path
is in the second zone (72) according to the definitions of the
zones described in the preceding figures. This configuration
therefore makes it possible to implement the return-flow path
using the second zone (72) as a cleansing tank. Return-flow
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piping (11) is therefore no longer necessary. This embodiment
also advantageously makes it possible to do without a lifting
pump. The thermal loop is also simplified, given that the
return-flow heat losses through the pipes leaving the pump are
avoided.
In this example, the flow-path portion and the return-flow-path
portion include extremities opposite the coating tank being
linked by at least one link (CR) (in this case a channel) to
ensure a change of flow direction of the molten mixture. The
link channel may however have another form, for example a half-
ring extending the outlet of the flow path and the inlet of the
return-flow path or be a central aperture between the two
common sides of the flow path and the return-flow path. Thus, a
separation device (73) such as the one described in Figure 8 is
arranged upstream of the link channel in the flow direction of
the molten mixture. If the two juxtaposed tanks (71, 72) are
placed side by side, a side aperture between the two tanks
fitted with a filter wall is alone sufficient to fulfill the
role of link channel.
In order to facilitate the loop circulation from and to the
coating tank, in particular with horizontal-circulation flow
and return-flow paths, the return-flow-path portion may include
at least one delivery pump (PUMP) near to its outlet in the
coating tank, in particular located in the second cleansing
zone (72). Other delivery pumps (not lifting) may also be
arranged as required on the full circulation loop for the
molten mixture (5). It is also possible for the flow-path
'portion, the link channel and/or the return-flow-path portion
to have at least one negative-slope drainage section to
facilitate one-way drainage through the action of gravity after
the outlet (Cl) of the coating tank.
The lifting-pump and gravity-drainage devices prevent the risk
of the mixture blocking the pipes. For drainage at the same
level as shown in Figure 9, it is advisable to provide for the
option of heating the pipes.
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Finally, in accordance with all of the embodiments described
according to the invention:
- means, ideally activatable continuously, for measuring the
temperature and concentration of one or more elements of the
molten mixture, for example, aluminum, shall be provided on at
least the flow path from its inlet in the coating tank to the
outlet of the preparation device;
- means, ideally activatable continuously, for measuring the
molten mixture level shall be arranged in the preparation
device;
- means, ideally activatable continuously, for maintaining and
adjusting the rate and temperature of the molten mixture shall
be placed on at least one point of the flow path;
- means, ideally activatable continuously, for maintaining and
adjusting the temperature of the strip leaving the galvanizing
furnace linked to the coating tank shall be placed downstream
of the coating tank and/or at its input;
- means, ideally activatable continuously, for maintaining and
adjusting the movement speed of the strip shall be taken into
account in the thermal loop;
- means, ideally activatable continuously, for measuring strip
width and thickness downstream of the coating tank shall also
be taken into account in the thermal loop;
- means, ideally activatable continuously, for maintaining and
adjusting an insertion dynamic for the ingots in a fusion zone
of the preparation device shall be placed preferably above the
first zone (71) of the preparation device;
- a dynamic-parameter measurement control unit and an
adjustment unit for parameters related to the strip, at the
coating tank and at the preparation device shall be linked to
the measurement and adjustment (or maintenance) means. In
particular, the adjustment unit may include predictive
parameter commands, a real-time control system and/or a self-
learning process. Furthermore, the adjustment unit may include
external-command inputs parallel to the adjustment unit to
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enable manual or override adjustments, for example in the event
of re-adjustment of parameters as a result of new measurement
values for the concentration of an alloy element, for example
aluminum, a temperature variation, a variation in a property of
the moving strip, etc.
