Note: Descriptions are shown in the official language in which they were submitted.
METHODS AND APPARATUSES FOR ALUMINUM AND ZINC RECOVERY FROM
DROSS AND METAL-RICH RESIDUES USING INDUCTION MELTING
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001]
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates to the field of treating
aluminum dross,
zinc dross and various metal-rich residues. More particularly, it relates to
methods
and apparatuses for recovering various metals from dross residues. For
example,
such methods can comprise recovering non-ferrous metals from dross such as
aluminum, zinc etc. For example, such methods can be carried out in an
induction
furnace. For example, such methods can be carried without the use of salts
i.e. salt-
free methods.
BACKGROUND OF THE DISCLOSURE
[0003] In a typical aluminum or zinc production plant a lot metal-
rich-plant
residues are generated such as, metal dross, autogenous mill residues,
crucible
cleaning residues, pit scrap, borings, pig bottom, mold machine skim and pot
bottom
[0004] In the case of aluminum dross, which generally comprises metal
oxides
and a considerable quantity of free (unreacted) metal, for economic reasons it
has
been found desirable to extract the free metal before discarding the residues.
[0005] For this purpose, several furnaces have been devised and some
are
presently being used; such furnaces are normally heated with an external heat
source, such as fuel- or gas-operated burners, plasma torches, or electric
arcs.
[0006] Fuel or gas can be used for heating the dross in a dross
treating
furnace, in order to recover the aluminum contained therein However, these
processes have the major drawback of requiring the addition of salt fluxes
such as
NaCI or KCI, used to increase the percentage of aluminum recovery. In addition
to the
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fact that such salt fluxes increase the cost of the operation, they also lead
to
increased pollution.
[0007] Some proposals and small scale tests were made regarding the
use of
furnaces for treating hot dross but these technologies were not efficient for
treating
cold dross without the addition of an auxilliary heat source such as fuel- or
gas-
operated burners, plasma torches, or electric arcs.
[0008] Processes that comprise extraction of the liquid metal from
dross by
mechanical compression of the hot dross removed directly from a furnace have
been
proposed. Such processes were found to be only used with hot dross right away
after
its skimming from the liquid metal holding furnace. Moreover, metal recovery
was very
poor.
[0009] Other techniques involving an electric arc were proposed but
comprised
several drawbacks such as the controlled feeding of the graphite electrodes
that
required to maintain the arc, Such a technology has thus been found difficult
to
implement at an industrial level due to its mechanical complexity in a high
electrical
voltage environnement.
SUMMARY OF THE DISCLOSURE
[0010] It would thus be highly desirable to be provided with a device,
system or
method that would at least partially address the disadvantages of the existing
technologies.
[0011] According to one aspect, there is provided a method for
treating
aluminum dross, zinc dross, or metal-rich residues the method comprising:
heating aluminum dross, zinc dross or metal-rich residues in an
induction furnace to induce electrical current in at least one metal contained
within the
aluminum dross, zinc dross or metal-rich residues and heat the at least one
metal at a
temperature above the melting point of the at least one metal for causing
melting and
agglomeration of the at least one metal at a bottom portion of the furnace;
and
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removing the molten at least one metal from the furnace so as to separate the
molten at least one metal from the aluminum dross, zinc dross or metal rich
residues.
[0012] According to another aspect, there is provided a method for
treating
aluminum dross, zinc dross or metal-rich residues, the method comprising:
heating aluminum dross, zinc dross or metal-rich residues in an induction
furnace to induce electrical current in at least one metal contained within
the
aluminum dross, zinc dross or metal-rich residues and heat the at least one
metal at
a temperature above the melting point of the at least one metal for causing
melting,
separation of a waste and the molten at least one metal, and agglomeration of
the at
least one metal at a bottom portion of the furnace;
removing from a furnace crucible the molten at least one metal;
transferring the recovered molten at least one metal to a holding furnace for
pouring in the melt;
removing the waste remaining in the crucible; and
scraping the wall of the crucible.
[0013] According to another aspect, there is provided a method for
treating
aluminum dross, zinc dross or metal-rich residues, the method comprising:
heating in an induction furnace a molten metal heel to induce electrical
current in the molten heel metal to keep the metal molten and contacting the
molten
metal heel with aluminum dross, zinc dross or metal-rich residues in order to
heat
the aluminum dross, zinc dross or metal-rich residues charged into a furnace
crucible by conduction of heat from the molten metal heel surface, for causing
melting, separation of a waste and the molten at least one metal, and
agglomeration
of the at least one metal at a bottom portion of the furnace;
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removing the molten at least one metal from the furnace so as to separate the
molten at least one metal from the waste, while maintaining the molten metal
heel in
the crucible at a predetermined level.
[0014] According to another aspect, there is provided an apparatus for
treating
aluminum dross, zinc dross or metal-rich residues, the apparatus comprising:
an induction furnace suitable for treating aluminum dross, zinc dross or metal-
rich residues by heating the aluminum dross, zinc dross or metal-rich residues
via an
eddy electrical current induced, at a temperature above a melting point of at
least one
metal to be recovered therefrom, the furnace having an opening for passing
material
therethrough for charging and discharging a furnace crucible and a cover for
closing
the opening;
an injector for injecting an inert gas into the furnace; and
a temperature controlling device for monitoring and controlling temperature in
the furnace.
BRIEF DESCRIPTION OF DRAWINGS
[0015] The following drawings represent non-limitative examples in
which:
[0016] The following examples are presented in a non-limiting manner.
[0017] FIG. 1 is a side elevation view of a single coil induction
furnace in
accordance with the present disclosure;
[0018] FIG. 2 is a side elevation view of a double coil induction
furnace in
accordance with the present disclosure;
[0019] FIG. 3 is a side elevation view of a single-coil-continuous-
tapping
induction furnace in accordance with the present disclosure; and
[0020] FIG. 4 is a side elevation view of the furnace without the
plunger used to
illustrate the formation of a bridge well above the surface of the melt.
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DESCRIPTION OF VARIOUS EMBODIMENTS
[0021] The following examples are presented in a non-limitative manner.
[0022] The word "a" or "an" when used in conjunction with the term
"comprising" in the claims and/or the specification may mean "one", but it is
also
consistent with the meaning of "one or more", "at least one", and "one or more
than
one" unless the content clearly dictates otherwise. Similarly, the word
"another" may
mean at least a second or more unless the content clearly dictates otherwise.
[0023] In understanding the scope of the present disclosure, the term
"comprising" and its derivatives, as used herein, are intended to be open
ended terms
that specify the presence of the stated features, elements, components,
groups,
integers, and/or steps, but do not exclude the presence of other unstated
features,
elements, components, groups, integers and/or steps. The foregoing also
applies to
words having similar meanings such as the terms, "including", "having" and
their
derivatives. The term "consisting" and its derivatives, as used herein, are
intended to
be closed terms that specify the presence of the stated features, elements,
components, groups, integers, and/or steps, but exclude the presence of other
unstated features, elements, components, groups, integers and/or steps. The
term
"consisting essentially of", as used herein, is intended to specify the
presence of the
stated features, elements, components, groups, integers, and/or steps as well
as
those that do not materially affect the basic and novel characteristic(s) of
features,
elements, components, groups, integers, and/or steps.
[0024] Terms of degree such as "about" and "approximately' as used
herein
mean a reasonable amount of deviation of the modified term such that the end
result
is not significantly changed. These terms of degree should be construed as
including
a deviation of at least 5% or at least 10% of the modified term if this
deviation would
not negate the meaning of the word it modifies.
[0025] For example, the heating can be carried out under an inert gas
atmosphere.
[0026] For example, the inert gas is Ar or N2.
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[0027] For example, the inert gas is injected at a controlled rate.
[0028] For example, the molten at least one metal is removed from the
furnace
by means of a tap hole at a bottom portion of a furnace crucible.
[0029] For example, the molten at least one metal is removed from the
furnace
by tilting the furnace and emptying the molten at least one metal by a furnace
crucible
spout.
[0030] For example, the molten at least one metal is removed from the
furnace
by tilting the furnace and emptying the molten at least one metal by a furnace
crucible
spout while preventing, the flow of a solid waste floating at the surface of
the melt.
[0031] For example, the molten at least one metal is removed from the
furnace
by tilting the furnace and emptying the molten at least one metal by a furnace
crucible
spout while preventing, with a grid, the flow, with the molten at least one
metal, of a
solid waste floating at the surface of the melt.
[0032] For example, the at least one metal is aluminum or zinc.
[0033] For example, heating is stopped upon reaching the required
temperature
of the aluminum dross, zinc dross or metal-rich residues by induction of eddy
current
into the aluminum dross, zinc dross or metal-rich residues charged into the
furnace
crucible.
[0034] For example, the at least one metal is aluminum or zinc.
[0035] For example, the predetermined level of molten heel is a same
level
than before introducing the charge into the furnace.
[0036] For example, the heating is stopped upon achieving the required
temperature of the material by transfer of heat from the molten metal heel.
[0037] For example, the method further comprises monitoring and
controlling
the temperature at different levels in the crucible to avoid overheating of
the liquid at
least one metal.
[0038] For example, the eddy current is generated by electromagnetic
induction
produced by at least one coil surrounding the crucible.
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[0039] For example, the method comprises heating aluminum dross.
[0040] For example, the method comprises heating metal rich residues
from an
aluminum plant
[0041] For example, the waste comprises aluminum oxide.
[0042] For example, the waste comprises at least 95, 96, 97, 98, 99 or
99.5 %
of aluminum oxide.
[0043] For example, the waste is recycled for use in an electrolytic
cell.
[0044] For example, the waste is recycled for use as a cover in an
electrolytic
cell.
[0045] For example, the method comprises heating zinc dross.
[0046] For example, the method comprises heating metal residues from a
zinc
plant.
[0047] For example, the waste comprises zinc oxide.
[0048] For example, the waste comprises at least 95, 96, 97, 98, 99 or
99.5 %
of zinc oxide.
[0049] For example, the waste is recycled for use in a zinc leaching
step.
[0050] For example, the method comprises heating metal-rich residues.
[0051] For example, the metal-rich-plant residues are chosen from
residues
generated in operating aluminum and zinc plants, metal dross, autogenous mill
residues, crucible cleaning residues, pit scrap, borings, pig bottom, mold
machine
skim, pot bottom and mixtures thereof.
[0052] For example, the crucible is dimensioned to operate with a molten
metal
heel of about 65 % to about 85 % of a capacity of the crucible.
[0053] For example, the crucible is a non-conducting crucible.
[0054] For example, the method further comprises monitoring and/or
controlling
the temperature at different levels in the crucible.
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[0055] For example, the temperature monitoring and/or controlling is
carried out
by means of a temperature controlling device that is effective for monitoring
and/or
controlling temperature of a charge inside the crucible.
[0056] For example, the temperature controlling device comprises a
plunger for
inserting into the crucible, the plunger defining a hollow chamber for
receiving at least
one thermocouple for monitoring and/or controlling the temperature.
[0057] For example, the temperature controlling device comprises a
plunger for
inserting into the crucible and contacting the charge, the plunger defining a
hollow
chamber for receiving at least two thermocouples for monitoring and/or
controlling the
temperature at different levels inside the crucible.
[0058] For example, the plunger is a steel plunger.
[0059] For example, the surface of the plunger is covered with a
protective
coating for protecting the plunger against molten aluminum.
[0060] For example, the protective coating comprises WC-Co.
[0061] For example, the method further comprises moving the plunger is
movable upwardly, downwardly and/or laterally for stirring the charge.
[0062] For example, the method further comprises moving the plunger is
movable upwardly, downwardly and/or laterally for stirring the charge, thereby
breaking accumulation of solids on top on molten metal.
[0063] For example, the method further comprises further moving the
plunger is
movable upwardly, downwardly and/or laterally for stirring the charge, thereby
breaking accumulation of solids bridging portion(s) of an internal wall of the
crucible.
[0064] For example, the eddy current is generated by electromagnetic
induction
produced by at least one coil surrounding the crucible.
[0065] For example, the eddy current is generated by electromagnetic
induction
produced by at least two coils surrounding the crucible.
[0066] For example, the eddy current is generated by electromagnetic
induction
produced by a double coil surrounding the crucible.
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[0067] For example, the furnace crucible comprises a tap hole for
tapping at
least one recovered molten metal.
[0068] For example, the furnace comprises a device for tilting the
furnace.
[0069] For example, the apparatus further comprises a controller for
controlling
inert gas injection.
[0070] For example, the apparatus further comprises means for conveying
the
molten at least one metal into a holding furnace.
[0071] For example, the apparatus further comprises means for pouring
recovered molten metal into the holding furnace.
[0072] For example, the apparatus further comprises a conveyor to charge
the
material in the furnace.
[0073] For example, the apparatus further comprises a vibratory conveyor
suitable for charging in a heel melting operation.
[0074] For example, the apparatus further comprises a suitable container
that is
an insulating refractory lined ladle for transporting the recovered molten
metal and for
pouring the molten metal into a plant molten metal holding furnace.
[0075] For example, the temperature controlling device is effective for
monitoring and controlling temperature of a charge inside the crucible.
[0076] For example, the temperature controlling device comprises a
plunger for
inserting into the crucible, the plunger defining a hollow chamber for
receiving at least
one thermocouple for monitoring the temperature.
[0077] For example, the temperature controlling device comprises a
plunger for
inserting into the crucible and contacting the charge, the plunger defining a
hollow
chamber for receiving at least two thermocouples for monitoring the
temperature at
different levels inside the crucible.
[0078] For example, the plunger is a steel plunger.
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[0079] For example, the surface of the plunger is covered with a
protective
coating for protecting the plunger against molten aluminum.
[0080] For example, the protective coating comprises WC-Co.
[0081] For example, the plunger is movable upwardly. downwardly and
laterally
for stirring the charge.
[0082] For example, the metal-rich-plant residues are chosen from
residues
generated in operating aluminum and zinc plants, metal dross, autogenous mill
residues, crucible cleaning residues, pit scrap, borings, pig bottom, mold
machine
skim, pot bottom and mixtures thereof.
[0083] For example, the dross can be obtained from skimming of a metal
holding furnace.
[0084] For example, the crucible can be a non-conductive crucible.
[0085] For example, a nonconductive crucible holding the charge of
material to
be heated, can be surrounded by a coil (for example of copper wire). An
alternating
current can flows through the coil. For example, the coil can create a rapidly
reversing magnetic field that penetrates the metal present in the material.
The
magnetic field can induce eddy currents such as circular electric currents,
inside the
metal, by electromagnetic induction. The eddy currents, flowing through the
electrical
resistance of the bulk metal, heat it by Joule heating. For example, once
melted, the
eddy currents can cause vigorous stirring of the melt, thereby assuring
appropriate
mixing.
[0086] For example, once melted, the eddy currents can cause vigorous
stirring
of the melt, assuring good mixing.
[0087] For example, the magnetic field can induce eddy currents,
circular
electric currents, inside the metal, by electromagnetic induction. The eddy
currents,
can be flowing through the electrical resistance of the bulk metal, heat it by
Joule
heating. The current in the at least one coil can be fed and controlled
independently in
order to heat independently some section of the crucible in order, for
example, not to
overheat the bottom of the crucible when full with molten metal.
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[0088] For example, it is possible to select a low frequency power
supply to get
more important stirring at the metal line, and at the same time the lower
frequency,
giving larger wave length, provided more penetration of the electromagnetic
field into
the charge, which will pass straight through the entrapped metal at the dross
line and
not superheat the entrapped metal.
[0089] For example, the methods can comprise
charging a batch of dross , resulting from skimming of a metal holding
furnace in an aluminum plant, or of aluminum-rich-plant-residue into the
nonconductive crucible of an induction furnace (for example to insure that the
charge is thereby heated above the melting point of the metal to be recovered
by
electromagnetic induction of electrical eddy currents in the partially
metallic charge).
For example, the eddy currents, flowing through the electrical resistance of
the bulk
metal, heat it by Joule heating.
providing an inert atmosphere in the furnace by filling the furnace with inert
gas, such as Ar or N2, for example to prevent oxidation of the metal during
the
process.
energising an electrical current in the induction coils surrounding the
nonconductive crucible of the furnace (for example in order to induce a
heating
electrical eddy current in the charge to bring it to a temperature above the
melting
point of the recoverable metal), its separation from the waste also contained
in the
dross or residue and its agglomeration at the bottom of the furnace. For
example,
once melted, the eddy currents cause vigorous stirring of the melt, assuring
good
mixing.
removing from the furnace crucible the recoverable free metal (for example by
tilting the crucible or by by means of a tap hole at a bottom portion of a
furnace
crucible. The remaining waste can also be removed by tilting the crucible and
by
scraping the crucible walls if required.
charging into the furnace a new batch of material for recycling and repeating
the process.
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[0090] For example, the apparatus for recovering metal, such as aluminum
or
zinc, contained in a dross or a plant aluminum or zinc residues, comprising:
an induction furnace adapted for high temperature treatment of drosses and
metal-rich-plant-residues, for example, the furnace comprises a nonconductive
crucible holding the charge of material to be heated, surrounded by a coil
(for
example of copper wire). An alternating current flows through the wire. The
coil
creates a rapidly reversing magnetic field that penetrates the metal present
in the
material. The magnetic field induces eddy currents, circular electric
currents, inside
the metal, by electromagnetic induction. The eddy currents, flowing through
the
electrical resistance of the bulk metal, heat it by Joule heating. The furnace
can have
an opening through which the material to be treated may be charged into the
crucible and the recovered molten metal and the waste can be discharged from
the
crucible, as well as a cover for closing the opening during treatment of the
charge.
optionally means for tilting the the furnace and lifting the furnace cover;
optionally means for injecting an inert gas into the furnace;
optionally means for monitoring the temperature of the charge inside the
crucible. For this purpose, a plunger (for example made of steel) can be
provided
which is inserted through the furnace cover, for example axially and all the
way
down into the crucible; the plunger can be hollow in order to contain several
thermocouples devices to be used for monitoring the temperature at different
levels,
inside the crucible; the external surface of the plunger can be covered with a
protective coating (for example WC-Co), against the molten aluminum attacks.
The
steel plunger can also be equipped with means to move it slightly up and down
and
sideways in order to slightly stir the charge and break the possible formation
of a
solid cap or "bridge', above the melt;
optionally means for returning the recovered metal in the molten state to the
holding furnace;
optionally means for pouring the recovered molten metal into the holding
furnace;
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optionally means for supplying the electrical current to the coil surrounding
the crucible which, by electromagnetic induction, will generate the electrical
eddy
currents required for the Joule heating of the metallic charge contained in
the
crucible;
optionally means to water cool the coil surrounding the crucible; and
optionally means for the control of the operation and the recording and time
display of the measured data such as the electrical power, furnace current,
furnace
frequency, capacitor voltage, ground leakage and charge temperature.
[0091] For example, the apparatus for recovering metal, such as aluminum
or
zinc, contained in a dross or a plant aluminum or zinc residues or metal-rich
residues,
can comprise:
an induction furnace adapted for high temperature treatment of drosses and
metal-rich-plant-residues, the furnace comprises of a nonconductive crucible
holding
the charge of material to be heated, surrounded by at least one or at least wt
two
independent coils (for example of copper wire). An alternating current flows
through
the coil wires. For example, the coils can create a rapidly reversing magnetic
field
that penetrates the metal present in the material. The magnetic field can
induce
eddy currents, circular electric currents, inside the metal, by
electromagnetic
induction. The eddy currents, can be flowing through the electrical resistance
of the
bulk metal, heat it by Joule heating; the current in each coil can be fed and
controlled independently in order to heat independently some section of the
crucible
in order, for example, not to overheat the bottom of the crucible when full
with molten
metal. In addition, the furnace can have an opening through which the material
to be
treated may be charged into the crucible and the recovered molten metal and
the
waste can be discharged from the crucible, as well as a cover for closing the
opening during treatment of the charge;
optionally means for tilting the the furnace and lifting the furnace cover;
optionally means for injecting an inert gas into the furnace;
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optionally means for monitoring the temperature of the charge inside
the crucible. For example, a plunger (for example made of steel) can be
provided
which can be for example inserted through the furnace cover, for example,
axially
and all the way down into the crucible; the plunger can be hollow in order to
contain
several thermocouples devices to be used for monitoring the temperature at
different
levels, inside the crucible; the external surface of the steel plunger can be
covered
with a protective coating (such as WC-Co) against the molten aluminum attacks.
The
steel plunger can also be equipped with means to move it slightly up and down
and
sideways in order to slightly stir the charge and break the possible formation
of a
solid cap or "bridge', above the melt.
optionally means for returning the recovered metal in the molten state to the
holding furnace;
optionally means for pouring the recovered molten metal into the holding
furnace;
optionally means for supplying the electrical current to each coil surrounding
the crucible which, by electromagnetic induction; will generate the electrical
eddy
currents required for the Joule heating of the metallic charge contained in
the
required section of the crucible;
optionally means for supplying the amount of electrical current in each of the
coils as required to obtain the temperature profile in the crucible required
for the
metal recovery treatment of the material charged into the crucible;
optionally means to stop the induction heating of the material to prevent
overheating of the charge;
optionally means to water cool the coils surrounding the crucible; and
optionally means for the control of the operation and the recording and time
display of the measured data such as the electrical power, furnace current,
furnace
frequency, capacitor voltage, ground leakage and charge temperature.
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[0092] For example, the apparatus for recovering metal, such as aluminum
or
zinc, contained in a dross or a plant aluminum or zinc residues or metal-rich
residues,
can comprise:
an induction furnace adapted for high temperature treatment of drosses and
plant aluminum residues, the furnace comprises of a nonconductive crucible
holding
the charge of material to be heated, surrounded by a coil (for example a
copper
wire). An alternating current can flow through the wire. The coil can create a
rapidly
reversing magnetic field that penetrates the metal present in the material .
The
magnetic field can induces eddy currents, circular electric currents, inside
the metal,
by electromagnetic induction. The eddy currents, can be flowing through the
electrical resistance of the bulk metal, heat it by Joule heating. For
example, the
furnace can have an opening through which the material to be treated may be
charged into the crucible and the waste can be discharged from the crucible,
as well
as a cover for closing the opening during treatment of the charge. A tap hole
can be
provided at the bottom of the furnace crucible for tapping of the molten
metal;
optionally means for tilting the furnace and lifting the furnace cover;
optionally means for injecting an inert gas into the furnace;
optionally means for monitoring the temperature of the charge inside the
crucible. For example, a plunger (for example comprising steel) can be
provided
which is inserted through the furnace cover, for example axially and all the
way
down into the crucible; the steel plunger can be hollow in order to contain
several
thermocouples devices to be used for monitoring the temperature at different
levels,
inside the crucible; the surface of the steel plunger can be covered with a
protective
coating such as WC-Co, against the molten aluminum attacks. The steel plunger
can
also be equipped with means to move it slightly up and down and sideways in
order
to slightly stir the charge and break the possible formation of a solid cap or
"bridge',
above the melt;
optionally means for returning the recovered metal in the molten state to the
holding furnace;
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optionally means for pouring the recovered molten metal into the holding
furnace;
optionally means for supplying the electrical current to the coil surrounding
the
crucible which, by electromagnetic induction, can generate the electrical eddy
currents required for the Joule heating of the metallic charge contained in
the
crucible;
optionally means to water cool the coil surrounding the crucible; and
optionally means for the control of the operation and the recording and
display
of the measured data such as the electrical power, furnace current, furnace
frequency, capacitor voltage, ground leakage and charge temperature.
[0093] For example, the apparatus for recovering metal, such as aluminum
or
zinc, contained in a dross or a plant aluminum or zinc residues or metal-rich
residues,
can comprise:
an induction furnace adapted for high temperature treatment of drosses and
plant aluminum residues, the furnace consists of a nonconductive crucible
holding
the charge of metal to be melted, surrounded by a coil (for example comprising
copper wire). An alternating current can flow through the wire. The coil can
creates a
rapidly reversing magnetic field that penetrates the metal contained in the
crucible.
The magnetic field can induce eddy currents, circular electric currents,
inside the
metal, by electromagnetic induction. The eddy currents, can be flowing through
the
electrical resistance of the metal, heat it by Joule heating. The induction
heating, in
this case, can be provided to heat the pure metal, (for example which can fill
the
crucible at about 60 to about 85% or at about 70 to about 80% of its
capacity), thus
forming what is called a Molten Metal Heel. The material to be treated, can be
placed on the surface of the molten metal heel, and can be heated by heat
transfer
from the molten metal heel and also by induction. The amount of heat provided
by
the molten metal heel can be greater than the heat provided by the induction.
The
furnace can have an opening through which the material to be treated may be
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charged into the crucible and the waste can be discharged from the crucible,
as well
as a cover for closing the opening during treatment of the charge.
optionally means for tilting the the furnace and lifting the furnace cover;
optionally means for injecting an inert gas into the furnace;
optionally means for monitoring the temperature of the charge inside the
crucible. For this purpose, a plunger (for example comprising steel) can be
provided
which is inserted through the furnace cover, for example axially and all the
way
down into the crucible; the steel plunger is hollow in order to contain
several
thermocouples devices to be used for monitoring the temperature at different
levels,
inside the crucible; the surface of the steel plunger can be covered with a
protective
coating (for example such as WC-Co), against the molten aluminum attacks. The
plunger can also equipped with means to move it slightly up and down and
sideways in order to slightly stir the charge and break the possible formation
of a
solid cap or "bridge', above the melt.
optionally means for returning the recovered metal in the molten state to the
holding furnace; and
optionally means for pouring the recovered molten metal into the holding
furnace.
optionally means for supplying the electrical current to the coil surrounding
the
crucible which, by electromagnetic induction, will generate the electrical
eddy
currents required for the Joule heating of the metallic charge contained in
the
crucible.
optionally means to water cool the coil surrounding the crucible.
optionally means for the control of the operation and the recording and
display
of the measured data such as the electrical power, furnace current, furnace
frequency, capacitor voltage, ground leakage and charge temperature.
[0094] In Figs. 1-3, the numbers identify the following components:
(1) Thermocouples
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(2) Steel Plunger
(3) Argon inert gas injection pipe
(4) Furnace cover
(5) Non-conductive crucible
(6) Coil cooling water in pipe
(7) Copper induction coil
(8) Cooling water out pipe
(9) Coil cooling water in pipe for second coil
(10) Copper induction second coil
(11) Coil cooling water out pipe for second coil
(12) Crucible taphole and plug
[0095] FIG. 4 is a side elevation view of the furnace without the
plunger used to
illustrate the formation of a bridge well above the surface of the melt. As
shown, a
solid cap or bridge (a) is formed at the top of the furnace which prevents the
fall of the
charge on the molten metal and as the void (b), between the two, acts as an
insulator the molten metal (c) will superheat and its temperature will rise
rapidly
therefore there is a need to monitor the temperature to stop the process if
needed to
avoid equipment damage.
[0096] For example, the processing of the dross can be carried out under
inert
atmosphere, such as Argon gas, in order to prevent oxidation of the
recoverable
metal;
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[0097] For example, in order to avoid dross build up at the molten metal
line
and superheating of entrapped metal at the same level, it is possible to
select a low
frequency power supply to get more important stirring at the metal line, and
at the
same time the lower frequency, giving larger wave length, provided more
penetration
of the electromagnetic field into the charge, which will pass straight through
the
entrapped metal at the dross line and not superheat the entrapped metal.
[0098] As indicated, for example, by Tabatabaei and Turner,
(http://www.foundnimad.com/feature/molten-metal-splash-and-furnace-refractory-
safety) and as illustrated in FIG. 4:
"When charge material in the top portion of the furnace is not in contact
with the molten metal below it, the dangerous condition known as bridging
exists. When bridging occurs, charge material is no longer serving to
moderate the temperature of the bath during the melting cycle. Also, the air
gap between the molten metal and the bridge can act as an insulator. The
molten metal below the bridge, under the impact of full melting power, will
superheat."
[0099] It is precisely to prevent bridging to occur that, in accordance
with the
present disclosure and as illustrated in FIG. 1, 2 and 3, it is proposed to
use a plunger
equipped with several thermocouples for a continuous monitoring of the
temperature
at different levels in the crucible, thus avoiding the superheating of the
melt.
[00100] Melting in a large heel of molten metal has the advantage that
the power
supply see always approximately the same load and therefore the efficiency is
maximized. The furnace as shown FIG. 1 could be operated in this manner.
[00101] In addition, by keeping a high metal line in the furnace after
tapping, can
allow the operator easier access to scrape the walls at the metal line,
keeping dross
from building up.
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[00102] It is also possible to avoid superheating of the melt, by using
a furnace
with two induction coils, as shown FIG.2, or more induction coils in order to
be able to
adjust the induction heat delivered in different levels of the furnace as
required to
obtain uniform temperature.
[00103] The furnace illustrated in FIG. 3 can be operated for stack
melting with
continuous feeding and tapping. The temperature monitoring, made by the
plunger,
can be used to adjust the tapping flow, ensuring that the level of the melt
remain the
same in the furnace.
OPERATION PROCEDURE REVIEW
[00104] In industrial operation safety issues can require possible
charging of hot
dross and discharging of both the molten metal and the hot waste at high
temperatures have to be addressed as well as issues of protection of the
environment, non-discharge of greenhouse gas and great attention to energy
savings.
Furthermore, recovery of the metal can be achieved without any use of salt
fluxes and
with a significantly reduced off-gas generation requiring much smaller gas
cleaning
equipment. For example, the process for recovery of the valuable metal,
contained in
the dross and the plant residues described earlier, can comprise :
injecting an inert gas, such as Ar or N2, to fill the crucible in order to
avoid
any oxydation of the metal contained in the charge;
charging the material to be treated using an appropriate conveyor (for
example a vibratory conveyor especially in the case of heel melting operation
where
the amount of feed has to be monitored to avoid splashing). Heel melting can
also
require drying of the feed prior to charging;
inserting the plunger, if required, to continuously monitor the temperature
at different level in the crucible;
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energysing the induction coil in order to heat the charge using a Melting
Control System which calculates the energy required to melt the charge as a
function
of the charge weight and of the estimated metal content in the charge and the
heat
content required to heat and melt it. . A slight flow, in the crucible, of
inert gas, such
as Argon, can be maintained during that operation in order to prevent
oxidation of
molten metal; and
for added control of the operation the plunger thermocouples can be
connected to an alarm and to the Melting Control System to interrupt the
heating
operation in case of charge overheating;
[00105] Once all the energy required for the operation or once the
thermocouples of the plunger indicate that the required temperatures have been
reached, the tapping of the molten metal can be done by tilting the furnace.
Use can
be made of a special metal grid placed in the crucible, just in front of the
furnace
spout, to prevent the flow, with the liquid metal, of the waste floating at
the surface
of the melt.
[00106] Once all the liquid metal has been tapped, the crucible is
scrapped to
remove all the waste and specially the dross stuck on the crucible wall.
[00107] A slight overpressure of inert gas, such as argon or dinitrogen,
can be
used during the processes described hereinabove, to prevent any air inflow
into the
furnace crucible which otherwise would oxidize some of the metal during the
steps of
charging, in the case of hot dross, and processing or discharging from the
furnace.
RECOVERY OF METAL FROM DROSS AND WASTE FEASIBILITY STUDY
[00108] Three series of induction melting tests were conducted at the
CANMET
facility, Hamilton Canada, to determine the feasibility of recovering aluminum
metal
from dross and from metal-rich-plant-residues including autogenous mill
residues,
crucible cleaning residues, pit scrap, borings, pig bottom, mold machine skim
and pot
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bottom, in a conventional induction furnaces and without the use of any salt
addition
to the charge material.
[00109] Two induction furnaces of different capacty were used:
- a small 100 kW, 2500 Hz induction furnace, 10' ID, 15" deep
- a larger 2 500 kW, 1000Hz tilting induction furnace, 15" ID, 23 "deep
The total amount of material tested was 385 kg,
[00110] The following Table 1, gives the total amount tested for each
kind of
residue, in kg, the corresponding amount of metal recovered and the
corresponding
percentage of recovery.
TABLE
Kil ND OF WASTE AMOUNTCharged METAL recovered % RECOVERY
Pit Scrap 44.5 kg 38.8 kg 87 %
Borings 66.8 kg 65.5 kg 98 %
Mill Residues 30 kg 11.15 37.2%
Dross 244 kg 168.4 69%
[00111] In the case of the tests performed with a dross charge two
melting
methods were investigated : a start from a 100% solid charge and a start with
50%
molten metal heel with solid dross samples added.
[00112] The test results indicate that aluminum dross and aluminum-rich-
plant-
residues are suitable charge materials for treatment in an induction furnace.
The
results indicate also that metal recovery is comparable to that obtained with
other
treatment technologies.
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[00113] It will be appreciated that, for simplicity and clarity of
illustration, where
considered appropriate, reference numerals may be repeated among the figures
to
indicate corresponding or analogous elements or steps. In addition, numerous
specific
details are set forth in order to provide a thorough understanding of the
exemplary
embodiments described herein. However, it will be understood by those of
ordinary
skill in the art that the embodiments described herein may be practiced
without these
specific details. In other instances, well-known methods, procedures and
components
have not been described in detail so as not to obscure the embodiments
described
herein. Furthermore, this description is not to be considered as limiting the
scope of
the embodiments described herein in any way but rather as merely describing
the
implementation of the various embodiments described herein.
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