Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for the
treatment of liquid dross and waste metal obtained in
smelting and remelting plants and which is poured from the
melting furnace into a skimming trough whereupon the dross
is conveyed from the trough into a rotating, substantially
horizontal cooling pipe in which it passes from one end to
the other thereby becoming cooled below the melting point to
obtain a granulated form. The invention further relates to
apparatus for carrying out the method.
It is well known that waste metal and liquid dross
obtained in the smelting and remelting of metal generally
contain a substantial percentage of pure metal. For this
reason, it is common practice for such waste metal and liquid
dross to be collected and conveyed to separate recovery
plants. According to the arrangement illustrated in German
Offenlegungsschrift 27 13 281, which concerns the recovery
of aluminum from aluminum dross, the hot aluminum dross is
charged into a rotating open cooling drum in which it is
cooled. The cooling and circulation of the dross in an open
drum will, of course, result in oxidation of at least a por-
tion of the aluminum due to contact with air. This results,
on the one hand, in an increase in the temperature of the
dross which must, of course, be compensated for through
increased cooling action and, on the other hand, a decreased
yield in the amount of pure aluminum recovered. Since
aluminum oxide has a lower market value than pure aluminum,
the economy of the procedure is not as high as is desired.
Accordingly, the main object of the present inven-
tion is to provide new and improved methods and apparatus for
treating liquid dross so as to increase the amount of pure
metal recovered therefrom.
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It is understood that the method and apparatus of
the present invention is applicable to the recovery of most
metallic substances, such as, for example, aluminum, zinc,
lead, etc.
Briefly, in accordance with the present invention,
this and other objects are obtained by providing a method
wherein the hot liquid dross is cooled in the absence of air
or oxygen and preferably while in an inert gas atmosphere.
The apparatus is constructed such that the space occupied
thereby is relatively small so that the recovery apparatus
can be erected proximate to the location from which the dross
is obtained.
According to the method of the invention, the cooled
dross in granulated form is subjected to a subsequent grinding
operation since the pure metallic material is found in greatest
quantities in larger particles and the majority of the oxides
and other impurities are present in the dust or fine-grained
phase. Furthermore, yields are improved by the removal of
dust in a screening station to which the dross is directed
subsequent to the grinding operation since the oxide-rich fine
dust can then be separated from the other fractions.
Thus, according to the present invention, the hot
liquid dross is cooled in the absence of air or oxygen, such
as under an inert gas atmosphere, after which the granulated
dross is subjected to an autogenous grinding process. The
ground dross is then separated, preferably by screening, into
a granulated metal fraction and into a fine-grained fraction
containing substantially powdered metal oxide.
According to the apparatus of the invention, a hood
is located over the inlet side of the cooling pipe into which
the liquid dross is charged so that the entrance of air into
the cooling pipe through the inlet opening is prevented. The
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skimming trough from which the dross is charged into the
cooling pipe is sealingly engaged to the hood so that the
dross is charged into the cooling pipe without any substan-
tial contact with air.
Other features of the method and apparatus of the
present invention will be described in greater detail herein-
below.
A more complete appreciation of the present inven-
tion and many of the attendant advantages thereof will be
readily appreciated as the same becomes better understood by
reference to the following detailed description when
considered in connection with the accompanying drawin~s, in
which:
FIG. 1 is a side elevation view in schematic form
illustrating one embodiment of treatment
apparatus according to the present inven-
tion' and
FIG. 2 is a ~ection view taken along line A-A of
Fig. lo
Referring now to the drawings wherein like reference
characters designate identical or corresponding parts through-
out the several views, the treatment apparatus illustrated in
the Figures are based on an embodiment of the invention for
treating aluminum dross. However, as noted above, the present
invention is equally applicable to the recovery of metals
other than aluminum.
The treatment apparatus according to the present
invention comprises a rotatably mounted substantially hori-
zontal cooling pipe 1 into the inlet side of which the liquid
dross is charged from a skimming trough 8 by means of a `~
bucket elevator 5. A hood 6 is 1ocated over the inlet side
of the cooling pipe 1. The skimming trough 8 sealingly
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engages hood 6~ Thus, hood 6 includes sealing means 6a and
the skimming trough 8 is urged to sealing engagement with
seal means 6a such as by means of a table schematically shown
at 9 on which the skimming trough 8 is placed by a fork lift
or the like. The hood communicates with a source of inert
gas.
In the above manner, entry of air into the cooling
pipe is substantially prevented and the dross is charged from
the skimming trough into the cooling pipe without substantial
contact with air. This has the beneficial result that the
aluminum (or other metal) no longer oxidizes as the dross moves
through the cooling pipe 50 that increased yields of pure
metal can be recovered and the temperature of the dross will
not increase due to oxidation thereby decreasing the energy
required for cooling.
The dross is charged from the skimming trough into
the cooling pipe 1 by a bucket elevator 5 which substantially
comprises two shovel-type arms which alternately lift the
dross from the skimming trough 8 and conduct the dross in the
direction of the axis of rotation through the inlet side of
cooling pipe 1 as shown by arrow 20n In this manner, the
dross is charged from the skimming trough into the cooling
pipe in an intermittent manner which facilitates the cooling
thereof.
Cooling pipe 1 comprises a double-walled jacket
through which a cooling liquid such as, for example, water,
flows. In order to promote the forward movement of the dross
in the cooling pipe 1, the inner wall of the pipe has an
inwardly tapered or conical configuration 12 at the inlet end
thereof. Thus, a downward sloping surface is presented to the
dross at the initial end of the cooling pipe 1 so that the
dross is prevented from backing up in the cooling pipe and
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possibly falling back into the skimming trough 8. The cooling
pipe l is continually rotated in a manner described below so
that the dross will wet only a portion of the circular cross-
section of the inner surface thereof in a manner similar to
an autogenous mill and is urged forwardly through the cooling
pipe under its own weight. During this movement through the
cooling pipe, the dross is cooled through contact with the
inner wall and discharged in granulated form through a
discharge opening 13 formed in pipe l into the inlet 14 of
autogenous mill 2.
The autogenous mill 2 comprises a grinding drum
having a substantially circular cross-section and defined by
end faces 3 and 4. The cooling pipe 1 is mounted substantially
coaxially with respect to the mill 2 and in the illustrated
embodiment is fixed thereto at end wal:Ls 3 and 4 for rotation
therewith. In this connection, drive wheels 16 rotate both
the cooling pipe l and grinding mill 2 as a fixed unit so that
both rotate at the same speed.
The section of mill 2 comprising inlet 14 has a
reduced diameter cross-section in which substantially helical
webs 15 are provided onto which the granulated dross falls
from the discharge opening 13 of pipe l. The helical web l~
acts on the dross not only to direct the same into the main
section of the mill 2 but, additionally, to provide that dross
will be maintained at the inlet section in amounts such that
the same will provide a simple seal. Thus, the dross is
directed as shown by arrow 21 from the discharge 13 of the
cooling pipe into the inlet 14 of the mill wherein the helical
webs 15 direct the same into the larger diameter portion
thereof and at the same time accumulates the dross in a manner
so as to seal the opening.
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The autogenous mill is in constant rotation and
crushes the particulate solid dross so as to produce a dust
fraction and a granular fraction, the latter constituting both
a coarse grained fraction 17 and a fine grained fraction 18.
Since larger particles are deposited on the larger circum-
ference of the autogenous mill, the discharge from the mill
can be conveyed into a screening station 7 by discharge
apparatus, designated 10, similar to bucket elevator 5 and
which is rigidly connected to the drive of mill. Screening
station 7 separates the discharged material into the coarse
grained fraction 17 and the fine grained fraction 18, the
fractions being themselves discharged into appropriate con-
tainersO The coarse grained fraction 17 will contain substan-
tially pure aluminum while the fine grained fraction 18 will
contain the bulk of aluminum oxide and other powdered
impurities.
According to another feature of the present invention,
the screening station 7 is preferably ventilated during opera-
tion by means of a dust exhaust 11 whereby the dust content of
the fine grained fraction 18 is reduced and the relative con-
tent of aluminum metal increased.
In a modification of the embodiment illustrated in
the Figures, the cooling pipe 1 can be provided with a separate
drive so that its rotation will be independent with respect -to
that of the autogenous mill. In this connection, the cooling
pipe 1 can be mounted in its own bearings, designated in
phantom at 22 on end faces 3 and 4 of the autogenous mill.
Such construction is advantageous in that both the cooling pipe
1 and the mill 2 can be rotated at respective optimum speeds
so that these components can be designed independently of each
other.
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The treatment apparatus of the present invention may
also be provided with a feeding device 19 on the side opposite
to that of bucket elevator 5 through which cooled dross can
be fed into the mill 2 directly through discharge opening 13
and inlet 14 whereupon the cooled dross is crushed to the
desired grain size. The feeding device 19 is hermetically
sealed during the cooling operation so no air can enter since
it is important according to the present invention that the
liquid dross be cooled rapidly in the absence of air.
In one illustrative embodiment of the present inven-
tion, if it is assumed that the volume of the cooling pipe and
associated apparatus is about 2 cubic meters, such volume will
contain about 80 decagrams of oxygen with which about 1 ~ilo-
gram of metallic aluminum can be oxidized. This will corres-
pond to a loss through oxidation of less than 1% in a charge
of 300 kilograms of dross having an a].uminum content of about
50%. Of course, such loss is insignii-icant in practice.
It is important for the inner wall of the cooling
pipe in the inlet region of the liquid dross to be smooth and
have no fittings so that the dross cannot settle on such
fittings. A self-smoothing effect has been observed in the
operation of such cooling pipes, i.e., after a short time of
operation, a thin aluminum film is formed on the inner pipe
wall which prevents further caking of aluminum.
As seen in Figure 2, the discharge opening 13 of
cooling pipe 1 has a relatively large passage which will limit
the maximum grain size of the dross. Thus, if larger pieces
of solid dross are charged, the same must be crushed in
another manner. The helical web 15 of inlet 14 of mill 2
preferably comprises a single unitary member and, as noted
above, promotes the advancement of the granular dross. In
the illustrated embodiment, a pair of rollers 16 drive the
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mill along with screening station and cooling pipe. ~oth
rollers 16 are driven by a common motor which is not shown.
Obviously, numerous modifications and variations of
the present invention are possible in the light of the above
teachings. It is, therefore, to be understood that within
the scope of the claims appended hereto, the invention may be
practiced otherwise than as specifically disclosed herein~
