Note: Descriptions are shown in the official language in which they were submitted.
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improved apparatus and process for sepcwating aluminium from a mixture
of al umini um an d al cunini um dross
Field of the Invention
This invention relates to an apparatus and a process for separating
aluminium from a mixture of aluminium and aluminium dross. In particular,
the invention relates to a process for recycling aluminium from dross
produced during an aluminium melting processes, and to an apparatus fox
carrying out that recycling process.
Background of the Invention
When aluminium is melted e.g. for manufacture of extrusions ingots
and billets, because of the influence of oxygen frcm environmental air on the
aluminium and the existence of impurities, particularly oxides, nitrides and
carbides, in the molten aluminium,. a layer of sludge, also known as dross
rises to the surface of the molten alunliniul:n. This layer of dross has to be
removed from the molten aluminium beforE: the molten aluminium can be
cast. This is done by the use of a suitable ladle. In a rather crude process,
known as skimming, the ladle is dragged across the top of the molten
2o aluminium and the dross is scraped into a suitable receptacle: During the
Ski:IIlmIIlg prOCBSS, as well aS I'eInOVlIlg sludge including oxides and other
impurities, pure aluminium is also removed'.. The quantity of pure
aluminium removed depends on the depth to which the ladle is inserted in
the aluminium to ensure removal of alI the dross and to a large extent
depends an the skill of the furnace worker handling the ladle. However,
typically 30 to 60% of the mixture/dross, by weight is aluminium.
The term dross. as used herein, refers to the impurities such as oxides
which float to the surface of the molten alurniniunl, but the term is also
used
in the art to refer to the mixture of aluminium and the impurities.
3o Because of the amount of aluminium iin the mixture, it is obviously
desirable to remove as much aluminium from the dross/aluminium mixture
as possible. Almost all recycling is done by a process known as rotazy salt
furnace processing. In that process, the dross containing pure aluminium is
first ailo~ved to cool. The longer the aluminium spends hot, the more
oxidation occurs arid less aluminium is recovered in the recycling process, so
often cooling is encouraged and accelerated. In some cases some initial
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separation of aluminium from the mixture i.s first carried out by one of two
rather inefficient devices know as drain pans and dross presses, respectively.
In the former the mixture is allowed to sit v~~hile molten and some of the
aluminium will sink to, and agglomerate in, the bottom of the pan. In the
lattez~, the mixture is compressed and the aluminium droplets tend to stick
together. These processes are inefficient anal have to be followed by rotary
salt processing or other methods of external dross processing. Because the
mixture is kept hat longer for the drain pan or dross press process, the
recovery rate in the subsequent rotary salt process is reduced, so diain pans
1o and dross presses are generally not commercially viable, and not often
used.
Recycling is not generally done at the furnace, but is usually is done by
specialist metal-recycling companies. In the rotary salt recycling process,
the
dross is heated and remelted and various salts and fluxes are added in order
to separate the aluminium from the oxides and other impurities. While the
process is highly efficient in terms of the quantity of aluminium removed
from the dross, removing approximately 85% of the available aluminium, the
waste prOdLlCt from the recycling process, ie~ the mixture of salts and the
oxides, is unpleasant, very environmentally unfriendly. and difficult to
dispose of safely. Further, the process requires the transporting of the dross
to the recycles in lorries or the like which is also undesirable from an
environmental point of view, and inefficient: in terms of fuel. Also, the
dross
has to be remeited in order to extract the aluminium in the recycling process
which requires a substantial amount of energy. There are some smelting
plants which have their own rotary salt recycling furnace, however, the
process of cooling and transporting the coolE:d mixture to the furnace remains
the same, although savings are made in total transport costs.
Proposals have been made for separating aluminium from dross in the
past. GB 1533696 and US 3689049 disclose -two different device for
separating aluminium from dross. Neither device has had any commercial
success, perhaps because they are over-complicated and too unreliable far
the extreme environment in which they havE; to operate.
The present inventor has also invented an apparatus and process,
disclosed in AU 56260/98, that provides a simpler and more cast effective
method of recycling aluminium from dross. The present invention is
directed to improvements in the apparatus a<r.rlier developed by the inventor.
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Summary of the Invention
According to a first aspect, the prese.rzt invention is an apparatus for
separating molten aluminium from a nxixtu.re of nxolten aluminium and
aluminium dross including:
a table for supporting an insulated crucible containing the mixture of
molten aluminium and aluminium dross, tlxe table being adapted to vibrate
so as to vibrate the crucible;
a frame adapted to support a paddle :means or stirring means, means
for rotatizxg the paddle means and means for relatively lowering the same into
1o the mixture in the crucible for stirring the naixtu re with the paddle
means
enabling simultaneous vibration of the crucible and stirring of the mixture;
and
a screenizxg means able to substantially surround the crucible when it
is supported by the table, the screening means having at least a portion
25 tlxereof movable between at least a first retracted position to allow
access to
the crucible and at least a second position where it prevents access to the
crucible.
The screening means can be removably mounted to the frame. In this
embodiment, the~movable portion may comprise a door that is retractable to
20. allow access to the table of the apparatus. T'he door can be adapted to
move
upwardly, sidewardly or. downwardly to provide a portal for access to the
table. The door can be slidably nxovable relative to the remainder of the
screening means. In another embodiment, t:he door can be pivotally mounted
to the wall. Vhhen the door is fully retracted, tlxe portal must be of
sufficient
25 size to allow the crucible to pass through thE: portal.
In another enxbodiment, the entire screening means can be movable
between the first retracted position and the second position. In this
embodiment, the screening means can extend downwardly from the lowering
means for the paddle means and is so movable reidtive to the table and
3o crucible in concert with the operation of the lowering nxeans.
The screening means is preferably adapted to prevent or at least
ameliorate the escape of heat from the viciniity of the crucible. By
preventing
the escape of heat, the screening nxeans servE;s to protect workers working
near the apparatus. It also serves to slow the rate of cooling of the crucible
35 and its contents which is desirable. The screening means can also
preferably
be adapted to prevent or at least substantially prevent the escape of dust and
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flame from the vicinity of the cI°LICZble. The screening means can also
be
adapted to safeguard workers working in tlae vicinity of the apparatus from
explosions of aluminium and dross from th.e crucible.
The screening means is preferably formed from a mufti-layer structure,
comprising at least an inner layer and an outer layer. The inner and outer
layer can be fabricated from a suitable metal or other material, including
refractory or cetnentitious materials.
The paddle ar stirring means can comprise a plurality of blades
mounted on a shaft supported by a frame, t:he shaft being adapted to be
1o raised or lowered so that the blades may be raised when a new crucible is
placed on the table located underneath the blades. In one embodiment, in
use, the shaft is positioned at or near the centre of the crucible and is
rotatabie by a rotating means about its longitudinal axis. In a further
embodiment, the shaft, in use, is radially offset froze the centre of the
crucible and is in addition to rotating about its longitudinal axis is also
rotated by the rotating means about the cen9_re of the crucible
thez°eby
providing more thorough stirring of the crucible's contents. In a still
further
embodiment, a gear is provided such that the shaft undergoes a planetary
motion as it is rotated by the rotating means about the centre of the
crucible.
2o This fur then rotation of the blades leads to a even more thorough stirring
of
the crucible's contents.
The crucible may be made of a refractory material having a metallic
outer skin spaced froze the refractory material by a layer of insulation. The
crucible can be provided with a covering means that at least partially covers
the upper opening of the crucible. The cover can be retractable or otherwise
removable froze the crucible when the paddle or stirring means is relatively
lowered into the crucible. If desired, the cover can be adapted to surround
the shaft of the paddle or stirring means while the paddle or stirring means
is
stirring the crucible's contents.
The table may be mounted on springs ito assist in vibrating the
crucible. Vibration of the table and crucible may be achieved by the
mounting of at least ozae ultrasonic transducer, electric vibrator, mechanical
vibrator or a combination of one or mare of these transducers to the crucible
and/or table. The table can be vibrated such 'that it oscillates in a
horizontal
axis, a vertical axis on in both axes.
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The frame may consist of two or more upright pillars. A cross-beam
may be supported by the frame and that cross-beam may carry the rotating
means. The shaft for the blades may depend down from the cross-beam with
the blades mounted on the lower end of that shaft.
5 The apparatus can include one or more exhaust pipes that allow
gaseous product from the process performed by the apparatus to be vented to
atmosphere outside the screening means. 'rhe exhaust pipes can incorporate
bagging systems that collect the exhaust gaseous product for later disposal.
The bagging systems can be adapted to also or instead extract dust from the
20 gaseous product. The exhaust systems can also include filtering or
scrubbing
means that clean or substantially clean the gaseous product from the process
of harmful or potentially harmful constitue:rzts.
The apparatus can include a weighing means, such as an electronic or
mechanical scale, that allows measurement of the weight of the crucible and
its contents while the cz°ucible is sitting on the table. The apparatus
can also
include a control means. The control means is preferably adapted to allow
an operator to operate all features of the apparatus from a central location.
In
a further preferred embodiment, the control means can be automated such
that the apparatus i°uns normally without operator input.
Zo Thus in a further aspect of the present iIlVeIltion there is provided an
apparatus for separating molten aluminium from a mixture of molten
aluminium and aluminium dross including:
a table for supporting an insulated Cl°Ltclble COIItalIllllg the
mixture of
molten aluminium and aluminium dross;
a weighing means, such as an electror:~ic or mechanical scale for
measuring the weight of the cr ucible and its contents while the crucible is
sitting on the table;
a frame adapted to support a padc~e means or stirring means, means
foz° rOtatlz2g the paddle means and means for relatively lowering the
same into
3o the mixture in the crucible for stirring the mixture with the paddle means
enabling S1II1LlltaIleoLlS Vlbl'atI0I1 Of the crucible and stirring of the
mixture;
a sensor means for sensing the pressure on the paddles during stirring;
and
a control means arranged to determine the optimum stirring time tiom
the measurements of pressure on the paddles. and/or the weight of the
crucible and contents.
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The control means may also control the sequence and rate of the
vibrations.
While the size of the crucible containing the dross is not important,
typically the crucible should be large enough to carry between 800 kilograms
to 1.5 tonne of dross. The process takes approximately 10 minutes and
removes up to 95% of the aluminium in thE; dross, hence the present
invention can separate approximately 1000 kilograms of aluminium from
dross every 4-10 minutes.
Typically, the crucible will include plugholes which are plugged prior
1o to carrying out the separation process with for example a cone or a sand
and
clay solution, and are unplugged to drain tl:ue crucible into a draining pan.
The draining pan preferably has a cover, having an inlet port, that encloses
the draining pan. Iii use, aluminium being drained from the crucible at the
completion of the process cycle passes through the inlet port and into the
further draining pan. As there is risk of an f:xplosion if the molten
aluminium comes into contact with moisture in the further draining pan, the
covez° preferably contains any such explosion should it occur thereby
ensuring the safety of workers close thereby.
In an alternative embodiment, the apparatus can include an additional
2o drain pan adapted to be positioned directly'~beneath the crucible which is
attachable to the crucible and so movable, if' desired, with the crucible when
the crucible is moved IIl al2d out of the apparatus. Where the crucible has a
drain pan, the crucible ~h~ould preferably have one or more drain holes in or
adjacent its bottom surface and in fluid communication with the drain pan so
that aluminium can drain into the drain pan.. The provision of a drain pan is
advantageous as it allows for collection of aluminium which moves to the
bottom of the crucible to be readily retrieved front the crucible. It has
particular application in those instances where the aluminium content of the
aluzniniu zn/dross mixture is relatively high, ie about 65-85% aluminium. If
the aluminium cannot drain in these instancfss, it is possible that previously
molten aluminium in the mixture closer to the top of the crucible will begin
to solidify before completion of the stirring and vibrating process according
to the present invention. By lowering the level of the mixture during the
process, using the drain pan, it lessens the li~;elihood of solidification in
the
cz~.icible before completion of the process.
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The one or more drain holes that axe in fluid communication with vthe
drain pan can be sealable by a sealing mea:as. Each sealing means can be
mounted to the crucible at all times and be moved in or out of sealing
engagement with its respective drain hole as desired.
It will be appreciated that the apparaitus could be adapted to receive
more than one crucible at a time. For example, the apparatus could comprise
two or more tables and stirring means to all!.ow the contents of two or more
crucibles to be stirred simultaneously. In another example, the apparatus
could comprise two or more tables but only one stirring means. In this
embodiment, the contents of each crucible would be stirred sequentially. As
Sllcll, the option would be open for the coni'.ents of one crucible to be
stirred
and vibrated while another crucible is being filled with dross mixture and
positioned in the apparatus. When the process cycle was complete on the
first crucible, the stirring means would be relatively raised, moved and then
25 relatively lov~Tered into the next crucible. VL'hile this is occurring, the
recovered aluminium can be drained from t:he first crucible and it can be
removed from the apparatus, filled with the next quantity of dross mixture to
be processed and then returned to the apparatus ready for the stirring means
to again be moved back and lowered again into this crucible. This sequential
mode of operation is particularly advantageous as it minimises the down
time where the stirring means is not in operation.
In a further aspect, the present inventiion is an apparatus for separating
molten aluminium froze a mixture of molten aluminium and aluminium
dross including:
a table for supporting an insulated crucible containing the mixture of
molten aluminium and aluminium dross;
at least one ultrasonic transducer, electrical vibrator or mechanical
vibrator, adapted to vibrate the table either directly or indirectly:
a frame adapted to support a paddle means or stirring means, means
3o fOT rOtatlIlg the paddle means and means for relatively lowering the same
into
the mixture in the crucible for stirring the mixture with the paddle means
enabling simultaneous vibration of the crucible and stirring of the mixture.
According to a further aspect, the presE:nt invention comprises an
insulated crucible able to contain a mixture of molten aluminium and
aluminium dross, and a drain pan that is positioned immediately below or
adjacent and is reznovably attachable to the c~°ucible, the drain pan
being
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adapted to receive molten aluminium that: flows through at Ieast one sealable
drain hole in the crucible.
The one or snore drain holes in the crucible are preferably sealable by a
sealing means that is mounted on the crucible and is movable in or out of
sealing engagement with its respective drain hole as desired.
In yet a further aspect, the inventior.~ involves a method for separating
molten aluminium from a mixture of aluminium and aluminium dross
comprising the steps of:
removing the mixture from a furnace containing molten aluminium;
20 transferring the hot dross to an insulated crucible;
trallSferrlIlg the crucible to a table means;
inserting a paddle means into the mixtu he in the crucible:
relatively lowering a screening means such that it substantially
surrounds the crucible;
stirring the mixture with the paddle :means and simultaneously
vibrating the table and crucible to cause the oxide skin on aluminium
droplets in the dross to break, and to cause the aluminium droplets to
coalesce, such that the droplets enlarge in volume and in weight and tend to
sink to the lower part of the crucible; and
removing aluminium from the lower :part of the crucible.
h still yet a further aspect, the invention involves a method for
separating molten aluminium from a mixture of aluminium and aluminium
dross comprising the steps of:
removing the mixture from a furnace containing molten aluminium;
transferring the hot dross to an insulated crucible;
transferring the crucible to a table means;
inserting a paddle means into the mixture in the crucible;
stirring the mixture with the paddle rrleans and simultaneously
vibrating the table and crucible using at least one ultrasonic transducer,
electric vibrator, mechanical vibrator or a combination of one, two or alI to
cause the oxide skin on aluminium droplets in the dross to break, and to
cause the aluminium droplets to coalesce, such that the droplets enlarge in
volume and in weight and tend to sink to thE; lower part of the crucible; and
removing aluminium from the lower part of the crucible.
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The pure aluminium can then be either cast as an ingot or recycled
straight back into the furnace containing molten aluminium or drained into a
crucible.
The process is carried out without substantially cooling the dross,
preferably at a temperature of about 750°C.
In a yet further aspect the invention provides an apparatus for
separating molten aluminium from a mixtulre of molten aluminium and
aluminium dross including:
a table for supporting an insulated crucible containing the mixture of
1o molten aluminium and aluminium dross;
at Ieast one heating means such as a ;gas burner of the like adapted to
either preheaf the paddles or heat the contents of the crucible or heat both.
a frame adapted to support a paddle means or stirring means, means
for rotating the paddle means and means for relatively lowering the same into
I5 the mixture in the crucible for stirring the n:Iixture with the paddle
means
enabling simultaneous vibration of the crucible and stirring of the mixture.
The gas burners of the like can be used to maintain the heat in the
contents of the crucible during the stirring process. Tn contrast with prior
art
devices SLlCh aS dross presses maintaining heat during the separation process
2o has been found to be desirable.
Brief Description of the Drawings
By way of example only, preferred embodiments of the invention will
now be described with reference to the accompanying drawings, in which:
Figure 2 is a schematic and simplified illustration of the present
25 invention:
Figure 2 is a detailed drawing of apparatus embodying the present
invention;
Figure 3 is a detailed drawing of the apparatus depicted in Fig. 2 but
with additional features:
Figure 4 is a cross-sectional view of another embodiment of a crucible
for use in the present apparatus;
Figure 5 is another view of the cruciblE~ of Figure ~:
Figure 6 is a simplified plan view of the crucible of Figure ~;
Figure 7 is a side view of a variation on. the separation apparatus
3~ S110WI1 in FIgLITe 2; and
Figure 8 is a drawing of a bucket with lid.
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Detailed Description of the Preferred Bmb~odiment
Referring the to the drawings, Figure 2 shows a forklift truck 30
ta~ansporting a crucible containing dross at .a temperature about 750°C
to an
apparatus, generally indicated at 14, for removing aluminium from the dross.
5 The crucible is placed on a table 16 of the apparatus. Blades 18 are lowered
into the molten dross, the table 16 is vibratc;d in a vertical direction, as
illustrated by the arrows AA, the blades 18 rotate in the molten dross, and by
viz~tue of that stirring and vibration, the alu zninium droplets and particles
in
the dross coalesce to form larger droplets azrd gradually sink to the base of
1o the crucible. The aluminium can thEll be drained out into a bucket 20 and
either transferred directly back to the furnace or used to produce an
aluminium ingot 22.
Figure 2 shows a more detailed drawing of the apparatus of the present
lIlVeI1t10I1. The device includes a frame comprised of a number of upright
Zs pillars or rails 50, a base 52 arid cross beam .54. In plan view the frame
includes four pillars located on the corners of a square. A table 5~ is
mounted on squat pillars 56 which rise up from the base 52. A number of
springs 58 also extend between the base 52 a~.nd the table 5~. On top of the
table a crucible 59 is located. The crucible can be generally square or
2o ClrClllar in plan view and has sides which taper outwardly. The inside of
the
base of the crucible is shaped like a pyramid so that no sharp corners exist,
so that the dross can be stirred properly and .so that the crucible can be
cleaned easily. A pipe or channel 60 is formed in the base of the crucible
which, when unblocked, allows molten metal to flow out from the crucible.
25 The crucible is made of a refractory marterial, having an outer skin of
metal and an insulating layer disposed between the skin and the refractozy
n zaterial. Although refractory material has insulating properties, the
additional layer further helps to prevent the alross cooling as it is
transported
to the table. A shaft 62 depends from the cross-beam 54 and an the lower
3o end of the shaft there are a series of blades or paddles 64 for stirring
the
contents of the crucible. The shaft is movablE; relative to the cross-beam in
the vertical direction to raise and lower the blades 6~. A motor is also
provided on the shaft to rotate the shaft in the direction B to thus stir the
contents of the crucible by means of the blades 64. While not depicted, it
5 can be envisaged that each of the blades 64 could be smaller than depicted,
with the shaft 62 adapted to rotate both about its own axis and that of the
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cz-czcible 59. Other stirring arrangements for the blades 64 such as described
above could also be readily envisaged.
In use, dross is taken from the furnace and placed straight into a .
crucible 59 and the crucible moved by a forklift or the like and placed
directly on the table 54. Once the crucible is in place on the table, the
blades
are lowered into the dross. The table on wl;~ich the crucible sits is then
vibrated in the vertical direction, at a rate ctf between 500 to 5,000 or more
vibrations per minute. In the embodiment depicted in Fig. 2, the vibration of
the table is achieved through LISe Of electrical vibrators 70 mounted under
the
table 5~. In other embodiments, the vibration may be achieved through use
of one or more ultrasonic transducers, mechanical vibrators, or a
combination of such vibrators mounted to t:he table 54. It is also possible to
mount the electrical vibrators on the sides of the crucible or frame to
provide
horizontal vibration in addition to, or instead of, the vertical vibrations.
In normal operation, the blades 6~ turn at a rate of between 3 to 40
rpm. The vibrations break the oxide layer surrounding the droplets of
aluminium in the dross and allow the metal droplets to coalesce with the
result that the droplets then become larger i:n volume and tend to sink to the
bottom of the crucible where they can flow through the hole 60 into another
2o bucket or drain pan. As shown in Figure 9 t:he other bucket or drain pan
ZOa
can have a cover 80 having an inlet orifice 8:2 to allow aluminium to drain
into the bucket in a safe manner. The rate o:F vibration can change during the
process and tends to start more slowly and then increase later for best
results.
I-iigh vibrational rates are used to bOlld very small drops of liquid metal
together and by utilising higher vibrational speeds a larger percentage of
pure
metal can be recovered. As discussed above, generally, the content of pure
metal in dross from aluminium furnaces is usually between 30 to 60oro, and
once the process described above has been carried out on the dross the
content of the remaining aluminium in the dross tends to be in the range of 3
3Q to 5~%. Both stirring and vibrating is needed.
The specific embodiment described above refers to vibrating the
crucible in a generally vertical axis, and generally horizontal stirring,
stirring
and vibration could take place in other axes and by other methods. The
important feature of the invention is that sufficient agitation, vibration, or
stirring occurs at rates which cause the aluminium droplets to coalesce.
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In_a further embodiment of the invention as depicted in Figure 3, walls
71 are shown mounted to the frame 50 of the apparatus 14. The walls are a
mzzlti-layer structure comprising a metallic inner layer and a metallic outer
layer. The walls 71 serve to prevent heat, dust, flame and explosion from
escaping the vicinity of the crucible and endangering any workers working
nearby. The walls 71 also serve to lessen t:he rate of temperature drop of the
crucible and ifs contents during operation of the apparatus. Disposed on one
face of the apparatus is an opening 72 provided by a door that can be raised
or lowered as desired. In the depicted embadi.ment, the door is not visible as
1o it has been retracted behind the upper wall 71. The opening is of
sufficient
size to allow the forklift to insert the crucible into the apparatus. In Fig.
3,
the device is depicted just after the crucible has been placed on the table 54
and with the blades 6~ lowered into the cmicible ready to stir the contents of
the crucible. In normal operation, it would. be anticipated that the door
would be left open for a short as time as possible. Accordingly, in normal
operation, it would be expected that the opening 72 would be closed by the
door immediately after the crucible is placE;d on the table or as the blades
64
are lowered into the crucible.
An alternative crucible to that depicted in Figures 1 to 3 but which
2o could be used in the apparatus and process described herein is depicted
generally as 75 in Figures ~ to 6. In this embodiment, the crucible 75 has a
drain pan 76 positioned in rmediateIy under and removably attached to the
lower surface of the crucible 75. Located in the lower surface of the crucible
are a plurality of drain holes 77 through which the aluminium can drain into
the da~ain pan 76 in use. 'The drain pan 76 is particularly advantageous for
those smelters having a high proportion of metal to dross in the dross
mixture. In such cases, the higher proportion of aluminium can lead to the
aiuminiu zzz solidifying near the top of the mixture before the cycle of
stirring
and vibration is complete. By providing the drain holes 77, the aluminium
falling to the bottom of the crucible 75 can drain from the crucible 75
thereby
lowering the level of dross mixture in the crucible and so reduce the
likelihood of the aluminium in the remaining; mixture solidifying before
completion of the cycle.
Figure 7 shows a variant of the apparatus shown in Figures 2 and 3
having a number of additional features. First: a hood 83 is provided and
exhaust pipe 8~ are also provided to allow safe extraction of dust and/or
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13
gaseous product from the process. A bagging system 86 for the capture of
dust passing through the exhaust pipe is aI:>o provided so that gases leaving
the exit 88 of the pipe are clean.
Figure 7 also illustrates the provision of two gas burners 88 and 90.
The gas burner 88 is used to preheat the paddles and burner 90 can be used
to heat the surface of the contents of the crucible to assist in preventing
solidification of the aluminium.
A further additional feature is the provision of a weighing means such
as electronic scales adapted to weigh the crucible and contents and transmit
that information to a computer control unit 94. The control unit also
receives inputs from sensors 96 associated with the paddle drive means
which measure the force required to turn the paddles, Fl'OIIl this information
the control means can determine the optimal stirring time and feed control
signals to a control box 94 which controls the rate and duration of stirring.
The control means can also be used to control the sequence and rate of
vibe ations.
Although the above process has been described as being used with
aluminium, it could be possible to employ the process in other processes
having metals of similar properties to aluminium.
z6 It will be appreciated by persons skilled in the art that numerous
variations and/or modifications may be made to the invention as shown in
the specific embodiments without departing from the spirit or scope of the
invention as broadly described. The present embodiments are, therefore, to
be considered in all z~espects as illustrative and not restrictive.
