Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02570361 2006-12-14
Casting method and casting machine for aluminium and/or aluminium alloys
The invention concerns a casting method for aluminium and/or aluminium alloys
in
accordance with the generic term of claim 1 and also a casting machine for
implementation of the method.
Casting methods and casting machines for aluminium and/or aluminium alloys are
known
in the art, in which solid or liquid aluminium is melted in a smelting furnace
and then
retained in a refining furnace, from which the molten aluminium flows via a
long channel
to a casting station and is thereby exposed to different treatments. Thus,
alloy additions
are added to the molten aluminium flowing down the channel and an inert
purging gas
(argon) is injected, before the melt, via a filter, reaches the casting
station, in which it is
cast into semi-finished products (cf. Fig. 1 in which an ordinary casting
machine for
aluminium is shown diagrammatically). Only limited homogenisation of the
molten
aluminium is possible. The treatment times are tied to the casting process and
thus
predetermined and limited in terms of time.
The individual treatment stations have to be co-ordinated with each other
perfectly. If one
of the stages of this in-line machine does not function, then the entire
casting machine has
to be shut down. The long channel through which the molten aluminium flows
means a
loss of temperature so that the material has to be overheated in the refining
furnace so that
a sufficient temperature can be reached on arrival at the casting station.
Extensive casting
times mean that the smelting and refining furnace has to be available for the
entire casting
time, before the next batch of melt is used. The energy consumption of the
furnaces is
correspondingly high. Normally, reverbatory furnaces with hydrocarbon fuel are
used,
giving rise to the disadvantage of a rapid absorption of hydrogen from the
burner flame.
Furthermore, greenhouse gases and other pollutants which load the atmosphere
are also
created. The long, open channel through which the molten aluminium runs,
however,
also means that the metal absorbs hydrogen from the atmosphere and causes the
formation of dross.
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The present invention is based on the problem of proposing a more economical
and more
flexible casting method for aluminium and/or aluminium alloys and also of
creating a
casting machine to implement the method, enabling an optimal period for the
treatment
and casting of the molten aluminium and thereby achieving an improved quality
of the
semi-finished products to be produced.
This problem is solved according to the invention by a casting method with the
characteristics of claim 1 and by a casting machine with the characteristics
of claim 13.
Preferred refinements of the casting method according to the invention and of
the casting
machine according to the invention form the subject matter of the dependent
claims.
By making use of ladles in accordance with the invention for the treatment and
feeding of
molten aluminium in controllable sequences, preferably to several casting
stations, this
phase of the method is decoupled in terms of time from the actual casting
process. The
individual treatments are no longer fixed and temporally restricted, but can
if necessary be
adapted until the desired quality of the molten aluminium to be cast is
achieved in the
relevant ladle.
The method according to the invention is considerably more efficient than the
in-line
method since there is no longer any need for large refining furnaces. If used
at all, the
furnaces are used for smelting and heating, but not for keeping warm over
longer periods.
These can be in the form of energy-efficient and ecologically advantageous
induction
furnaces.
The invention is next explained in more detail with the aid of the drawings,
which show:
Fig. 1 diagram of an embodiment of a state of the art casting machine for
aluminium; and
Fig. 2 diagram of an embodiment of an aluminium casting machine according to
the invention.
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Fig. 1 shows a state of the art casting machine 1 for aluminium and/or
aluminium alloys.
Liquid or solid aluminium is introduced as basic material in a first stage or
station 2,
which comprises a smelting furnace 3 and a refining furnace 4 attached
thereto. For
example, the aluminium can be supplied from a potroom by means of a transfer
ladle 5 or
as a scrap batch. The furnaces 3, 4 are usually large reverbatory furnaces
using
hydrocarbon fuel. The melt produced in the smelting furnace 3 is heated in the
refining
furnace 4 to the necessary temperature and partly homogenised by agitation.
Once the molten aluminium has reached the necessary temperature, it is guided
out of the
refining furnace 4 via a long channel 6 to a casting station 7, whereby it
flows through
various treatment stations 11, 12, which together with a filter 13 connected
upstream of
the casting station 7 form a second stage 10 of the casting machine 1. In the
treatment
station 11, various alloy additions are added to the molten aluminium. Gas
purification
occurs in the treatment station 12.
The casting station 7, in which the molten aluminium is cast into semi-
finished products,
can be operated continuously or semi-continuously in a way known in the art
and
therefore not described in more detail.
The treatment periods in the second stage 10 are tied to the casting process
to be carried
out in the casting station 7 and thus predetermined and restricted. The
individual
treatment stations 11, 12 must be perfectly synchronised with each other in
their function.
If one of the stages of this in-line machine does not function, then the
entire casting
machine 1 has to be shut down. The long channel through which the molten
aluminium
flows means a loss of temperature so that the material has to be overheated
(e.g. to 730 C)
in the refining furnace 4 so that a sufficient temperature (e.g. 700 C) can be
reached on
arrival at the casting station 7. Extensive casting times mean that the
smelting and
refining furnace 4 has to be available for the entire casting time, before the
next batch of
melt is used. The energy consumption of the furnaces 3,4 is correspondingly
high.
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In reverbatory furnaces with hydrocarbon fuel, the disadvantage arises of a
rapid absorption of hydrogen from the burner flame. Furthermore, greenhouse
gases and
other pollutants which load the atmosphere are also created. The long, open
channel 6
through which the molten aluminium runs, however, also means that the metal
absorbs
hydrogen from the atmosphere and causes the formation of dross.
Fig. 2 shows a diagram of a casting machine 1 according to the invention for
aluminium
and/or aluminium alloys. The first stage of the casting method according to
the invention
occurs in a filling station 21, in which hot molten aluminium is poured into a
number of
ladles 25. The ladles can for example have a capacity of 15 t. Either liquid,
hot
(temperature approx. 900 C) aluminium from a potroom can be poured directly
into the
ladles 25 using transfer ladles or at least one, preferably several, furnaces
22, 23, 24 are
assigned to the filling station 21 and are responsible for delivering the
molten aluminium,
whereby in addition to liquid aluminium, scrap aluminium or ingots provided
for
recasting can serve as basic material. The molten aluminium can for example be
poured
into one of the ladles 25 at half-hourly intervals. Advantageously molten
aluminium of
varying quality (with different degrees of aluminium purity) can be poured
from the
individual furnaces into the ladles 25, whereby the filling of the ladles 25
with molten
aluminium, possibly also with mixed material from various furnaces 22, 23, 24,
can be
computer-controlled.
Preferably, electrical induction furnaces can be used as furnaces 22, 23, 24,
which are
considerably more efficient in energy terms than reverbatory furnaces. These
can for
example be induction furnaces with a capacity of 20 t, from each of which 15 t
of molten
aluminium can be poured into one of the ladles 25 and the remaining 5 t can
serve when
smelting a further charge.
The casting machine 1 according to the invention has a purification and
preparation
station 30, from which purified and pre-heated ladles 25a are transported to
the filling
station 21 for filling (ladles 25 on a transport section are generally
designated in Fig. 2 by
the letter T). By pre-heating the ladles 25 to, for example, 900 C, the molten
aluminium
CA 02570361 2006-12-14
poured from the furnaces 22, 23, 24 operated at a temperature of approx. 800 C
can
remain for longer in the ladles 25 until it falls to the typical casting
temperature of 700 C,
as would be the case without pre-heating.
After the respective ladles 25 have been filled, the dross is skimmed off the
surface of the
melting bath (dedrossing) by tilting the ladle.
The ladles 25 filled in the filling station 21 are transported to a treatment
station 32, in
which the second stage of the casting process occurs. Firstly alloy additions
are added to
the molten aluminium (cf. the ladles referred to as 25b in Fig. 2). (However,
it is also
possible first to place at least part of the alloy additions in the cleaned
ladles 25a before
pouring in the melt.) After that the molten aluminium is homogenised and
purified (cf
ladles 25c). To this end, the ladles are placed beneath a rotary impeller
immersible in the
respective ladles 25c to inject inert gas, e.g. argon or nitrogen, whereby
combined
hydrogen removal, homogenisation and/or thermal regulation of the molten
aluminium
can occur. The injection of argon eliminates the absorption of hydrogen from
the
humidity present in the atmosphere and the formation of dross is reduced. To
remove
alkali trace contaminants, small amounts of chlorine can be added into the
purge gas.
Following treatment of the molten aluminium the ladles 25 can be kept in
storage stations
provided for this purpose (in Fig. 2 such storage stations are generally
designated by the
letter S), until a casting station 33 or 34 is available. The casting machine
20 preferably
has several such casting stations (two shown in Fig. 2), to which the ladles
25 can be
transported from the treatment or storage station, and in which the molten
aluminium is
cast into semi-finished products.
The temperature of the molten aluminium is maintained advantageously by
covering the
ladles 25 with a cover.
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During the time spent in the storage station S, the temperature in the ladles
25 can
be reduced by the injection of argon through a porous plug in the base of the
ladle or
maintained or increased by means of a small burner built into the ladle cover.
The emptying of the ladles 25d at the respective casting station 33, 34 occurs
through the
base of the ladle by means of controllable opening of a sliding closure,
whereby the
outflowing molten aluminium is guided into a collector spout, preferably
encased by an
inert gas. During this phase, too, argon can be blown in through the porous
plug in the
ladle base, agitating and purifying the melt. By covering the ladles 25d,
inert
atmospheres can be created in their upper area, which reduces oxidation and
absorption of
hydrogen.
The casting stations 33, 34 are each equipped with a filter system in a way
known in the
art and are operated continuously or semi-continuously.
After emptying the ladles 25d, these are transported to the aforementioned
cleaning and
preparation station 30 where they are cleaned (cf. ladle 25e) and prepared for
re-use, in
particular pre-heated (cf. ladle 25a). The emptied ladles can also be stored
until further
use in the storage stations S provided for this purpose.
There are multiple routes provided to transport the ladles 25 from one station
to the next
or to the storage stations (S), whereby the ladles 25 can be transported on
rails or by
means of overhead hoists.
The casting machine according to the invention is equipped with a control
system to
control the charges to be poured out of the individual furnaces 22, 23, 24
into the
individual ladles 25, the alloy additions, heating, cooling, gas supply and
treatment times,
so that the molten aluminium reaches the casting stations 33, 34 in the
desired quality, at
the desired temperature and fully homogenised.
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By the use of ladles 25 according to the invention for the treatment and
feeding of
molten aluminium in controllable sequences to preferably several casting
stations 33, 34,
this process phase is decoupled in terms of time from the actual casting
process. The
individual treatments are no longer predetermined and limited in terms of
time, but can be
adjusted as required, until the desired quality of the molten aluminium to be
cast is
achieved in the respective ladle. If, for example, a lower hydrogen content is
required, the
gas purification period can be extended. This option did not exist in the
traditional in-line
process according to Fig. 1. The production output of the casting machine
depends on the
actual casting process, solely at the casting stations, which can be continued
until the
supply of the treated molten aluminium to the casting stations is interrupted
as required.
The method according to the invention is considerably more efficient than the
in-line
process, since there is no longer any need for large refining furnaces. If
used at all, the
furnaces are used for smelting and heating, but not for keeping the melt warm
over longer
periods. These can be in the form of energy-efficient and ecologically
advantageous
induction furnaces. By pre-heating the ladles, the smelting temperature
achievable in the
furnaces can be lower.
