Note: Descriptions are shown in the official language in which they were submitted.
CA 02252203 1998-10-29
PATENT
DOCKET N0. 64806
HIGH PURITY SALT ALUMINUM FLUB
FIELD OF THE INVENTION
This invention provides a composition and method of
using that composition in the recycle of aluminum,
especially scrap aluminum such as found in used beverage
containers. More particularly, this invention relates to
a high purity salt flux composition and additive
composition for use with a high purity salt for making
the high purity salt flux composition, which salt flux
composition can be used during the remelting of scrap
aluminum. The method of the invention is directed to
using the high purity salt flux composition during the
aluminum recovery process to improve coalescence of the
molten aluminum.
BACKGROUND OF THE INVENTION
Use of molten salt fluxes in the secondary aluminum
industry is known to improve direct recovery of aluminum
in remelting processes. Aluminum and scrap aluminum,
such as used beverage containers (UBCs), are treated
using such processes. Remelting of the aluminum in a
furnace is carried out under cover of a layer of molten
salt to prevent oxidation of the aluminum in the furnace
atmosphere and to promote coalescence of the molten
aluminum so as to maximize recovery of aluminum. During
processing, an oxide film tends to form on the surface of
the molten aluminum droplets. The oxide film inhibits
coalescence of the molten aluminum, causing smaller
particles to be lost in the process thereby reducing the
amount of aluminum recovered. The unrecoverable aluminum
droplets having the oxide film are sometimes referred to
as dross.
Use of a salt flux in the furnace helps to strip
away and suspend the oxide film so that coalescence of
the droplets increases and dross formation decreases.
The salt flux wets the oxide film and initiates
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disintegration of the film, stripping it from the surface
of the molten aluminum droplets. Fragments of the oxide
film stripped from the aluminum remain suspended in the
flux. The aluminum droplets, which have a density
greater than the flux, then form a continuous molten pad
beneath the flux layer. The flux also prevents further
oxide formation by keeping the metal protected from the
atmosphere of the furnace.
One type of salt flux is primarily composed of a
mixture of high purity sodium chloride and potassium
chloride. The high purity salts used in such processes
are solution mined and purified by complex, highly
developed methods.
An object of this invention is to provide for
improved aluminum recovery in a recycle process which
includes scrap aluminum and salt flux.
Another object of the invention is to provide for
improved coalescence of the molten aluminum in a process
which includes melting scrap aluminum with a high purity
salt flux composition, especially aluminum from UBCs.
Yet another object of the invention is to use an
additive with a high purity salt flux composition, which
additive includes an alkaline agent and a fluoride
source to increase the coalescence of the remelted molten
aluminum, thereby improving recovery of the metal.
Further objects and advantages of the invention will
be found by reference to the following specification.
SUMMARY OF THE INVENTION
The present invention is directed to a method and
composition for increasing aluminum recovery in the
recycle of aluminum, and in an important aspect, scrap
aluminum such as aluminum from used beverage containers
(UBCs). The salt flux composition of the invention
protects the molten aluminum from oxidation, strips a
protective oxide film from the molten aluminum so that
molten aluminum droplets can coalescence and holds the
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oxide film in suspension so that the molten aluminum can
be recovered.
Broadly, the salt flux composition of the invention
comprises high purity NaCl and/or KCl, an alkaline agent
and a fluoride source. The amounts of alkaline agent and
fluoride source, along with the high purity NaCl and/or
KC1, are effective for improving coalescence and reducing
aluminum loss in the recovery of aluminum from molten
scrap aluminum, where the improvement is relative to a
salt flux composition which comprises a high purity salt
without an alkaline agent and a fluoride source.
In an important aspect, the invention includes a
salt flux composition which comprises high purity NaCl
and/or KC1, an alkaline agent and a fluoride source,
where the amounts of the alkaline agent and fluoride
source, along with a high purity salt in the salt flux
composition, are effective for improving coalescence and
reducing aluminum loss in the recovery of aluminum from
molten scrap aluminum, especially UBCs which comprise
specific alloys of aluminum. Such an improvement is
relative to a process using like conditions and a high
purity salt flux composition consisting essentially of a
high purity salt without the alkaline agent and fluoride
source. Generally, the salt flux composition comprises
at least about 0.2 weight percent and preferably from
about 0.2 to about 3 weight percent alkaline agent and at
least about 3 and preferably from about 3 to about 7
weight percent fluoride source, all based upon the weight
of the salt flux composition. Generally, the salt flux
composition comprises from about 90 to about 96.8 weight
percent NaCl and/or KC1 in this aspect of the invention.
The high purity salt flux will have less than about
0.2 weight percent sulfate. The composition and process
of the invention also will at least maintain or improve
aluminum coalescence as compared to a process using a
salt flux consisting essentially of a high purity alkali
metal salt such as NaCl and/or KC1.
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In another aspect, the invention is directed to an
additive composition which consists essentially of an
alkaline agent and fluoride source for addition to high
purity NaCl and/or KC1 to provide a high purity salt flux
composition which improves aluminum coalescence in a
process which involves mixing scrap aluminum metal, the
additive and high purity salt to recover aluminum, as
opposed to a process in which aluminum is recovered from
dross resulting from previous aluminum processes without
the further addition of scrap aluminum.
In an important aspect, the alkaline agent is
selected from the group consisting of Na2C03, NaOH, KOH,
KZC03 and mixtures thereof and the fluoride source is
selected from the group consisting of MF, CaFz, MA1F4
M3A1F6 and mixtures thereof, where M is K or Na.
In another important aspect, the additive
composition comprises alkaline agent in an amount of from
about 3 to about 50 weight percent and fluoride source in
an amount of from about 50 to about 97 weight percent,
each based upon the weight of the additive composition.
In another important aspect, the additive composition
comprises between about 4 to about 10 weight percent of
the salt flux composition.
The salt flux composition of the invention generally
is used in the process of the invention at a level of at
least about 1 weight percent, based upon the weight of
aluminum being processed.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
As used herein, "scrap aluminum" means aluminum
stock left over from equipment or structural manufacture
or used beverage cans.
As used herein, scrap aluminum from UBCs includes
3003 aluminum alloy, 3004 alloy and 5182 alloy.
As used herein, a "standard purity salt" means
sodium chloride having at least about 0.3 weight percent
sulfate or potassium chloride having at least about 0.02
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weight percent sulfate, and a "high purity salt" means
sodium chloride having less than about 0.2 weight percent
sulfate or potassium chloride having less than about 0.01
weight percent sulfate.
As used herein, "high purity salt flux composition"
or "high purity salt flux" means a flux composition which
comprises a high purity sodium or potassium chloride or a
mixture thereof. A high purity flux composition has at
least about the same or a lesser amount of sulfate than
which is in high purity NaCl, KC1 or blends thereof,
which is used in the high purity flux composition.
As used herein, "additive composition" means a
composition which comprises an alkaline agent and a
fluoride source for use with an alkali metal salt to
provide a salt flux composition which may be used in the
recovery of aluminum from scrap aluminum.
As used herein, "dross" means the formation of
unrecoverable aluminum droplets having an oxide film
covering the outer surface which are entrapped within the
salt flux layer in the furnace.
As used herein, "recovery yield" means the yield of
recovered aluminum metal from a recycling process where
aluminum metal scrap is mixed with a high purity salt
flux composition and where the yield of recovered
aluminum is based upon the weight of the aluminum which
is put into the process as a starting material.
Salt Flux Composition
The invention provides a method and compositions
for enhanced aluminum recovery when scrap aluminum is
mixed with the high purity salt and additive and the
blend is processed for recycle. The invention provides a
high purity salt flux composition, which comprises high
purity NaCl and/or KC1, an alkaline agent and a fluoride
source, to be used in a furnace during the recycle of
aluminum, especially scrap aluminum such as found in used
beverage containers (UBCs). The salt flux composition of
the invention is melted along with scrap aluminum, which
is not aluminum which is part of a dross blend, to
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provide a molten mixture in the furnace, wherein the salt
flux composition promotes the coalescence of the molten
aluminum droplets and prevents oxidation of the aluminum
so as to increase the recovery yield of aluminum from the
recycle process.
In an important aspect, the salt flux composition
consists essentially of high purity salt, alkaline agent
and fluoride source which flux composition is mixed with
scrap aluminum from UBCs comprising alloys of aluminum,
such as 3003, 3004 or 5182 aluminum alloys which aluminum
is not a part of any dross blend from previous aluminum
processing. The UBCs also may contain up to about 2%
magnesium.
In its broadest aspect, the salt flux composition of
the invention comprises high purity NaCl and/or KC1, an
alkaline agent and a fluoride source. The amounts of
alkaline agent and fluoride source, along with the high
purity NaCl and/or KC1, are effective for improving
coalescence and reducing aluminum loss in the recovery of
aluminum from molten scrap aluminum, where the
improvement is relative to a salt flux composition which
comprises a high purity salt without an alkaline agent
and a fluoride source. Generally, in this aspect, the
salt flux composition comprises at least about 0.2 weight
percent and preferably from about 0.2 to about 3 weight
percent alkaline agent and at least about 3 and
preferably from about 3 to about 7 weight percent
fluoride source, all based upon the weight of the salt
flux composition. Generally, the salt flux composition
comprises from about 90 to about 96.8 weight percent NaCl
and/or KC1 in this aspect of the invention.
When a mixture of high purity salts is used, the
ratio of NaCl to KCl is from about 30:70 to 70:30.
Preferably an essentially equimolar mixture of high
purity NaCl and KC1 is used in the high purity salt flux
to provide a lower melting temperature for the high
purity salt flux composition, as well as to lower the
cost of the salt flux. More particularly, it is
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desirable to provide a mixture of high purity salts
having a composition at or near the eutectic point of the
NaCl and KC1 blend so as to minimize melting temperature.
It is possible, however, to use only NaCl or KC1 with
similar recovery results. Of course, the presence of the
alkaline agent and fluoride source in the salt flux
composition also will affect the melting temperature of
the salt flux composition. The melting point of the
eutectic mixture of high purity salts is about 750° C.
The additive composition of the invention is added
to a high purity salt to provide the salt flux
composition of the invention. The additive composition
comprises an alkaline agent and fluoride source in
amounts effective for increasing the recovery yield of
aluminum during recycle when the additive composition is
added to a high purity alkali metal salt. In an
important aspect the additive consists essentially of the
alkaline agent and fluoride source and the salt flux
composition consists essentially of the high purity salt
(NaCl and/or KC1), the alkaline agent and fluoride
source. The additive composition comprises at least
about 4 weight percent of the high purity salt flux
composition and preferably between about 4 to about 10
weight percent.
The alkaline agent may be either high or low bulk
density soda ash, K2C03, NaOH or KOH. Preferably, the
alkaline agent is soda ash. Preferably, the alkaline
agent is added in an amount between about 0.2 to about 30
based on the weight of the salt flux composition,
although use of an amount greater than about 2% provides
only a limited increase in benefit.
While not intending to be bound by any theory, the
alkaline agent increases coalescence of the molten
aluminum, even as compared to certain salt fluxes
comprising high purity salts. Another benefit associated
with the alkaline agent is that it promotes the formation
of the non-reactive forms, Ca0 and MgO.
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The fluoride source may be KF, NaF, CaF2, Na3A1F6
(cryolite) , K3A1F6, NaAlF9 (SATF) , KA1F9 or mixtures
thereof. Preferably, it is cryolite or SATE. While not
intending to be bound by any theory, a fluoride source in
the additive composition improves coalescence of the
molten aluminum by increasing the dissolution of the
oxide film on the molten aluminum droplets. The fluoride
source is in an amount effective for improving
coalescence of the molten aluminum. Preferably, the
fluoride source is present in an amount between about 30
to about 7a based on the weight of the salt flux
composition. Addition of a fluoride source and/or the
alkaline agent to the high purity salt flux also may
reduce the melting temperature of the high purity flux
composition.
Method of the Invention
The method of the invention is directed to enhanced
recovery of aluminum in a remelting process. If UBCs are
to be recycled, pre-processing includes the mechanical
shredding of the UBCs into strips of about 12 inches long
and about '~ to about 1 inch wide. If other forms of
aluminum are to be processed according to the method of
the invention, they should be pre-processed, if
necessary, to provide similarly sized particles.
Preferably, the aluminum also is delacquered, if
necessary. Any method known to one skilled in the art
can be used to shred and to delacquer the aluminum in
preparation for the remelting process.
Preferably, the components of the salt flux
composition of the invention or a high purity salt and
the additive are combined to form a dry mixture prior to
being charged to a furnace. The salt flux composition
then is charged to the furnace, such as a vertical muffle
or rotary furnace or other suitable, commercially
available furnace, for melting either prior to or
concurrently with addition of the aluminum. The furnace
should have as its melting zone a container that is
relatively inert to the molten salt flux so that
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impurities are not introduced into the flux composition
from the container. The temperature of the furnace is
held between about 750°C and about 800°C. The salt flux
composition is melted at from about 740°C to about 750°C
and may be held in the molten state for about 300
minutes. Preferably, a reducing atmosphere is maintained
in the furnace to reduce oxidation of aluminum to
aluminum oxide.
The shredded scrap metal then is added to the molten
salt flux composition in the furnace as a batch process.
The purified aluminum which has coalesced beneath the
salt flux layer is decanted from the furnace after
approximately 30 minutes. Generally, the recovery yield
of aluminum decreases the longer the aluminum remains in
the furnace. The process also may be modified for
continuous processing of the scrap aluminum.
The amount of flux composition used in the furnace
is at least about 1 weight percent and, in an important
aspect, is from about to to about 500, based upon the
weight of aluminum. Preferably, the amount of flux used
in the process is from about 2o to about 5o based upon
the weight of the aluminum. With each batch of aluminum
processed, fragments of oxide film, particles of aluminum
coated with the oxide film and other impurities become
entrapped in the flux composition layer, causing it to
become more cloudy and viscous. The salt flux
composition may be re-used in the furnace until the flux
composition becomes too viscous, which makes it difficult
to remove purified aluminum. Generally, the salt flux
composition may be re-used approximately 6 times. After
the initial charge of salt flux composition, an amount of
flux composition is added to the furnace along with each
batch of aluminum for each re-use of the flux
composition. Approximately 5o to 15~ of the total weight
of flux composition initially charged to the furnace is
added with each subsequent batch of aluminum.
The following example illustrates a method for
carrying out the invention and should be understood to be
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illustrative of, but not limiting upon, the scope of the
invention which is defined in the appended claims.
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ERAMPLE 1
Various flux compositions, as described below in
Table l, were tested in duplicate in this experiment.
Compositions 1 and 2 were included in the experiment as
control compositions. Compositions 3-6 relate to the
invention. In all compositions, the fluoride source was
SATE, the carbon source was coal and the alkaline agent
was soda ash. SATF was added to the salt flux
composition in an amount of about 5 weight percent, based
on the weight of the salt flux composition. Soda ash,
when used, was added to the salt flux composition in an
amount of about 1 weight percent, based on the weight of
the salt flux composition.
Table 1
Composition Components
1 High Purity Salt + Fluoride Source
2 High Purity Salt + Fluoride Source
High Purity Salt + Fluoride Source +
3 Carbon Source + Alkaline Agent
High Purity Salt + Fluoride Source +
4 Carbon Source + Alkaline Agent
High Purity Salt + Fluoride Source +
5 Alkaline Agent
High Purity Salt + Fluoride Source +
6 Alkaline Agent
Forty-five grams of flue; composition was heated in a
crucible and allowed to melt for 20 minutes. After 20
minutes, 2.25 grams of delacquered, shredded aluminum
beverage cans was added to the molten flux. The flux
with aluminum was maintained in a molten state at 800°C
for one hour without agitatian. The crucible was
inclined at a 20° slope from the horizontal to bring the
molten aluminum drops into contact with one another. The
crucible then was cooled to room temperature. Water
soluble material was removed by dissolving in water. The
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remaining water insoluble materials, including aluminum,
were dried. The aluminum particles were removed.
A visual inspection of the recovered aluminum
revealed that coalescence is improved with a salt flux
composition comprising high purity salt, an alkaline
agent and fluoride source as compared to a high purity
salt flux composition without the additive. The use of a
carbon source in the salt flux composition provides no
visible improvement to coalescence when a high purity
salt flux composition with alkaline agent and fluoride
source is used.
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