Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the treatment of
aluminous material comprising furnace skimmings, drosses
and metal residues with a view to recovery of metal values.
- In particular it relates to the recovery of aluminium from
furnace_skimmings arising--in the melting and processing of
aluminium and aluminium base alloys (hereinafter all
referred to as aluminium).
In the melting of aluminium, especially when dirty or
fine scrap is included in the metal charge, it is found
that a soft pasty layer of metal and oxide forms on the
metal surface. This is conveniently termed "skim". Skim
- is also formed when a body of liquid metal is stirred, e.g.
in the case of alloying operations, or on transfer from
one furnace hearth to another. In order to tap clean
- metal from the furnace it is necessary to remove the skim,
and normally much liquld metal lS entrained in it leading
to considerable losses of metal unless the skim is
Rubsequently treated. By sprinkling powdered salt flux
of "fluid" type on the skim it is possible to remove it
without dragging off so much metal, but it is more usual
to add a "drying" flux which will cause the skim to ignite
and in this way, although some aluminium is burnt, a
significant amount of metal is released from the skim ïnto
the metal bath and the remaining skim is more powdery in
nature and can be remo~ed with less metal entrainment.
Other methods of treating the skim involve skimming it off
the metal surface into a crucible, adding flux to cau~e
- some ignition of skim and/or agglomeratio~ of the oxide,
and mechanically stirring the skim to liberate some of the
entrainsd metal. These methods only give recoveries of
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from one-third to two thirds of the metal content of the
skim. Much oxide fume is reLeased when the crucibles
are emptied. Another method of treating skim is to
feed it into rotary salt furnaces, but although metal
recovery may be impr-oved thi~ in~olves an en~ironmental
problem owing to volatilisation of the ~alt from the
furnace.
More recently it has been proposed to recover metal
from sXim and drosse~ by mixing them cold with solid salt
flux and melting in an induction furnace. This has the
advantage that the salt is not overheated, the metal is
not oxidised by the products of gas combustion, and the
stirrin$ action of the induced current~ assists
agglomeration of the metal. However the capital
equipment involved is costly, the furnace lining requires
frequent replacement and the method will only treat ~-
metal-rich drosses. It is not therefore suitable for
treatment of skim which has undergone extensive slow
oxidation or has fired in the furnace before being
removed. All these method~ in which skim is allowed
to cool and is subsequently reheated for further
proce sing will inevitably involve some loss of metal
through oxidation as well as consumption of energy in
replacing the lost heat. -
We have now found that if fre~hly generated hot
skim is fed into a layer of liquid salt flux floating
upon a body of liquid aluminium, then substantially all
the metal content of the skim is quickly transferred
to the body of liquid aluminium when the skim i~ gently
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agitated with the flux. In this way the recovery of
metal from the skim is much greater than can be obtained
by even prolonged rabbling of thle skim upon the metal
~urface and ~prinkling o~ powdered flux upon it during
the operation. The layer of liquid flux used in the
process of the present invention should be deep enough
for the skim to be at least partially and preferably at
least Yubstantially immersed in the flux. In one aspect
the present invention provides a method of treating
aluminous material comprising furnace skim, drosse~ or
metal residue~ by means of salt fluxes in which the
aluminous material, preferably hot, is fed into a liquid
salt flux layer floating upon a body of liquid aluminium
and the aluminouR material is agitated with the flux
whereby substantially all the metal content of the
aluminous material passe~ into the body of aluminium.
The aluminouc material may be added batch wise in such
quantities a~ can be substantially immersed in the salt
layer and then gently ~tlrred to liberate the metal~
leaving the salt layer ready to accept a further
quantity of skim. Preferably skim is transferred
directly from the bath of a reverbatory furnace to the
salt layer. 1 ;
Thus the invention is capable of providing a simple
method of treating hot furnace skim to give very high
recovery of the metal co~tent without the need for
elaborate equipment~ without grave environmental problems
through evolution of oxide or salt fume, and without the
metal and energy losses involved in cooling and
reheating the skim.
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The flux layer is confined to a predetermined area
and the quantity of flux used in relation to the
predetermined area is such as to provide initially a
liquid layer of at least 1 cm and preferably at least
2 cm deep, though 5-10 cm depth iæ desirable ~ince more
skim can be added at one time.
The temperature of the met~l will not be less than
650 C and preferably not less than 700C. Con~iderably
higher temperatures may be used but above about 850C
the volatility of the flux may be con~idered objectionable.
The salt flux u~ed may consist of mixture~ of the
alkali ænd alkaline earth metal chlorides, incl~ding
MScl2~ and may contain one or more fluorides of the
alkali and alkaline earth metal~, including MgF2, and
will--be thinly--fluid at tempera*ure~ above abou* 675C.-
Suitable compositions are based on KCl and NaCl in
eutectic proportions or in equal portion~ by weight-with
additions of 0-5% CaF2~ 0-25% NaF, or 0-20% MgC12. With
CaF2 additions, the flux i~ fumele~s in operation and
haa adequate coagulating power for dispersed metal.
With the NaF additions the coagulating power of the
flux is increased but slight fuming may occur, whereas
with MgC12 additions the wetting power of the flux i8
increased but some evolution of HCl wi~l be encountered,
particularly in damp atmosphere~.
In one embodiment of the invention aluminium is
heated in a reverberatory furnace provided with a side
well~ a layer of liquid salt flux is provided upon the
metal in the side well~ and the skim formed in the main ~-
hearth i~ transferred to the sid,e well, said ~kim being
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agitated with the flux whereby su~stantially all the
metal content of the skim pa~seq into the body of liquid
metal in the side well. The metal in the furnace and
the metal in the side well are preferably in
communication below the layer of flux.
When the invention is operated in conjunction with
a side well or forewell~ the metal in the main hearth can
be conveniently skimmed by pushing or drawing the skim
directly into such sidewell or forewell. This has many
advantages. The skimming can be done as soon as the
metal charge is melted and before the floating skim has
undergone significant oxidation. The skim is already
above the melting point of the metal and will disintegrate
rapidly on entering the flux layer, very little stirring
being needed. The skimmed-metal in the main hearth will
then be directly exposed to the heat of the furnace and
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will not be partially insulated therefrom by a layer of
skim, 80 making for thermal economy in the furnace
operation. Moreover the heat contained in the skim is
preserved and is not lost through allowing the skim to
cool down with subsequent need to remelt it for later
processing. There is no oxide fumesuch as is frequently
evolved when skim is taken from the furnace and allowed
to cool; nor i8 there any ,108s of aluminium incurred
through deliverate burning part of the skim in order that
some metal will be liberated through the increase in
temperature. In this preferred process, should any qkim
be allowed inadvertently to catch fire in the main hearth,
then burning is immediately extinguished when the ~kim is
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fed into the flux covered side well. There i9 no
objectionable salt fume generated. Finally all need for
equipment for crushing, grinding~ screening and remelting
of skim (e.g. in induction furnaces) i4 avoided.
The sidewell may conveniently be fitted with
insulating lids, and provided with a gas burner or other
heating device whereby the temperature of the metal can,
if desired~ be raised above that in the main hearth. The
side well may extend alongside a short length only of one
wall of the furnace, for example alongside one or more
drossing doors. Alternatively the flux layer may be
confined to part only of a side well, e.g. by means of a
baffle wall or floating barrier.
It is also possible to portion off part of the furnace
hearth e.g. by means of a baffle wall, and to carry out
the process of the present invention within such portion
of the furnace.
The process of the present invention is not confined
to skim but may be used to treat metal-containing residue~,
dro~ses or finely divided scrap metaI. Such materials
are preferably preheated prior to feeding into the side
well, otherwise the rate of melting will be comparatively
slow. Skim formed on earlier occasions may also be fed
into the side well 9 and i~ again desirably preheated
preferably to a temperature of at ~east 500C. Pre-
warming to remove moisture should be of course carried
out on grounds of safety. If desired, a metal pump may
be used to increase the circulation of hot metal between
the main hearth and the side well.
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Another say of carrying out the process is to use
a small reverberatory furnace with side well, in wh~ch
the main hearth iq not employed as a melting furnace but
merely aq a means of providing hot metal to the side well.
Alternatively a brick lined box may be used in which
there is provided a baffle wall which divides the box
into two compartments that are in communication below
the base of the baffle wall or by means of channels
passing through the baffle wall. Means is provided for
heating liquid metal in one of the chambers, for example
conventional gas or oil burners, electric radiant tubes,
a channel inductor, or gas fired immersion heaters. The
box is partly filled with liquid metal, and the
temperature maintained at the desired level by application
of heat to one of the chambers. The ~lux required for -
carrying out the process is applied to the other chamber.
tap hole may be provided in order to lower the level of
liquid metal when required, or alternatively an overflow
pipe may be provided. Either the small reverberatory
furnace or the divided brickbox may be made portable
and lifted or wheeled from one large melting or holding
furnace to another in order to collect and treat the
skim generated in ~uch large furnace.
It will be seen that in these embodiments of the
invention a method is provided of treating aluminou~
material by means of salt fluYes in which the aluminous
material i~ transferred to a layer of liquid salt flux
floating upon a body of liquid aluminium in such
quantities at a time as can be at least substantially
immersed in the flux layer, said body of liquid
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aluminium being in sub-surface communiCation with another
body of liquid aluminium for which means of heating iS
provided~ and said skim being agitated with the fluX whereby
substantially all the metal Content of the skim pasSes into
the body of the metal.
Another form of portable treatment unit whiCh has
proved very suitable for aluminous material~ particularly
when the quantities to be treated at one time are not very
large~ is comprised of an externally heated non-metallic
crucible divided by a baffle wall into chambers ~hich are
in communication below the baffle wall.
SuCh eqUipment is advantageous where the aluminous
material to be treated i8 below the desired temperature
and may give rise to partial freezing of the liquid salt
flux layer. In such caseS the ability to apply external
heating to the treatment unit enables the fluX to be
remelted quic~ly.
Where metal at temperatureS of 800C or more is
readily available~ e.g. at aluminium smelters~ it is
possible to carry out the treatment of skim in accordance
with the present invention using very simple equipment.
For example a trough lined with suitable refractory
material and without either a baffle wall or separate
heatins means may be employed.
The prewarmed trough may be placed below the drossing
door Of a reverberatory furnace~ metal at 800-850 C
poured into the trough~ fluX added~ and the furnace skim
drawn from the metal surface directly into the trough.
After a short period of agitation~ the liberated metal
may be collected through ; tap hole near the base of the
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trough.
In yet another embodiment of the invention the skim
or the like i8 sub~tantially immersed in a pool of liquid
flux floating on a liquid metal bath, the flux pool ~ -
being confined by means of a container partially
immersed in the liquid bath and in communication therewith
through one or more holes in the lower part of the
container, and a stream of liquid metal is fed into the
container so as to agitate the skim with the liquid flux.
Preferably the container is of round section.
Preferably the metal stream is fed into the container
in a broadly tangential direction, so that the metal flows l
round the ~ide of the container, thereby creating a ,
vortex in the container. It is undesirable however to
create sufficient circular motion to cause the liquid
flux to be carried down into the bath of metal. One -
or more baffles may be applied to the central part of the
metal surface within the container in order to prevent a
central vortex from forming whil~t still retainiD~ the
circular motion of the metal round the periphery of the
container. Two baffles intersecting at right angles to
form a cross may be used, the length of the baffles being
preferably fro~ one tenth to one half of the diameter of
the container and the baffles extending at least 1 in. .
below the level of the metal during operation of the
- equipment.
The container may be a crucible of plumbago, silicon
carbide bonded with alumina, a mixture of zircon and
alumina~ or any other refractory material which is
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Rufficiently re~i~tant to the acti~n of liquid aluminium
and to therm~l shock.
S~im to be treated by the process i8 preferably fresh
and hot but cold skim can also be treated by this
embodiment. ~ith cold skim it is particularly desirable
that the liquid metal stream should not be too cold and a
temperature of 725-800C is preferred.
The container may be conveniently supported from above
by meanR of a ring in known manner. Alternatively, when
the "hole" in the container is 90 large that there is no
bottom wall at all and the container is in effect a ring,
the ring may be allowed to float upon the metal ~urface
in which case the metal stream may be applied in a
direction other than tangential~ for example vertically
downwards~ A handle may be attached to the ring and the
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ring moved about underneath the metal stream so that the
metal stream is brought successively into contact with all
the skim contained within the ring.
If desired the metal stream may be dispensed with and
the skim agitated with the flux by moving the ring
backward and forwards over the metal surface.
Embodiment~ of the present in~ention will now be
described by way of example only with reference to the
accompanying drawings which show diagrammatically
alternative forms of apparatus for carrying out the
process of the present invention.
In the drawings:
-Figure~ 1, 2 and 5 are vertical sections through
different apparatus for carrying out the invention,
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Figure 3 i~ a plan view of the apparatus of
Figure 2, and
Figure ~ i8 a plan view of another apparatus for
carrying out the invention, and
Figures 6 and 7, Figures 8 and 9 and Figures 10
and 11 are respectively vertical sections and plan
view~ of apparatus for carrying out other embodiment~
of the invention.
In Figure 1 a crucible 1, which may for example be
made of cast iron, plumbago or ~ilicon carbide, is
heated externally by means of a gas burner 2. The
crucible is filled partly with liquid aluminium 3, upon
which float~ a layer of liquid flux 4. Skim 5 is
charged to the crucible to be substantially immersed
in the layer and is gently agitated with the stirring
tool 6.
Figure 2 showc a section of a reverberatory furnace
21, fitted with a side well, 22. Metal 23, in the
side well is in communication with that in the main
hearth 24~ by channels 25, e~tending through or belou a
baffle wall 26~ A salt flux layer 27, is provided on
the metal in the side well and skim 28, forming on the
metal surface in the main hearth i~ drawn into the ~ide
well using the ~craping tool 29.
Figure 3 shows a plan view of the same furnace in
which the main hearth 31, i4 separated from the side
well, 32~ by means of the baffle wall 33. A stirring
tool 34, is used to agitate the skim with the flux in
the side well 32.
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Figure 4 shows a plan view of a furnace for ¦ .
treatment of ~kim. The furnace consist~ of a refractory l .
lined box 41, divided by a baffle wall ~2, into two :
chambers in communication through channels in the baffle .
wall. One chamber 43, is provided with a ga~ burner 44,
whilst the other cham~er 45, receives a layer of flux and
is used to treat the skim. :
Figure 5 show~ a sectional view of a mobile furnace .
for skim treatment which can be taken to large
reverberatory furnaces for charging with skim. The
mobile furnace consi~t~ of a refractory lined box 51, ~ :
as shown in Figure 4, comprising a heating chamber 52, ;
and a qkim treatment chamber 53, having a layer of flux
floating on the metal therein. The mobile furnace i~
placed adjacent to a large reverberatory furnace 54,
and the skim 55, formed on the metal 56, in the large
furnace, is Rcraped into the treatment chamber 53, of
the mobile furnace u~ing the tool 57.
In the embodiment of Figures 6 and 7, a crucible
61 contains a central hole at the bottom 62~ and
optionally one or more holes 63, in the side wall~ and
is immersed in molten aluminium with the holes below .
the metal level 64. A jet of liquid aluminium 65, from :~ .
a conventional pouring device impinges on the wall of ~
the crucible, producing a circular motion therewith. ~: .
The crucible contains a layer of molten salt flux .
and skim 66 is added ~o that it is substantially
immersed in the flux. The motion of the metal in the .
crucible causes agitation of the ~kim in contact with ~.
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the liquid ~alt.
Figures 8 and 9 show another method of carrying
out the invention. In thi~ case the crucible ha~ the
whole of the bottom removed and a cruciform baffle 80
is placed in such a position aq to prevent formation of
a vortex even with a faxt moving metal stream.
Figure~ 10 and 11 sho~ a further method of
carrying out the invention. The refractory ring 111,
floats on the liquid metal 64, and i8 moved about by
means of the handle 112, so that all parts of skim 113,
contained within the ring can be brought under the metal
stream 114, and thereby a~itated with the salt flux 115.
The invention will be illustrated by the following
Example.
Example
In one experiment to illu~trate the efficacy of the
process, 17.8 kg of aluminium was melted in a crucible
(Crucible A) and converted into skim by prolonged blowing
of compressed air into the metal. The skim produced
was periodically removed and fed into a salt pool
approx. 10 cm deep floating on 17.2 kg of liquid metal
in a second crucible (Crucible B). The skim was
agitated in the liquid flux pool for about 15 secg.
after each addition. When all the q~im had been added,
the contents of Crucible B were stirred for about 30 ;
secs. and then carefully caqt~ 33.3 kg of clean metal
being obtained. A small amount of metal remaining in
Crucible A was also weighed (1.0 kg), so that the
quantity of clean metal used to produce the skim
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transferred to Crucible B could be ascertained by
difference. Allowing for the weight of metal
originally present in Crucible B, it will be seen that
16.8 kg of metal was converted into skim and 16.1 kg :
was recovered from this skim in the form of clean metal,
making a recovery of 95.8%. The metal temperature in :
each crucible was 700-720 C.
In the course of many tests carried out in
accordance with the present invention on ~luminous
10 materials with the metal content ranging from about 40 .
wt.% to over 95 wt.%~ we have found that the metal
recovery obtained con~istently exceeds 90 wt.% and
generally exceeds 95 wt.% of the metallic content of the -
material. -
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