Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a method for purifying aluminum alloys,
and more particularly to a method for electrolytically purifying aluminum
alloys such as aluminum-silicon alloys.
Alùminum-silicon alloys have been conventionally prepared by
adding to commercial grade aluminum a desired amount of silicon, normally
prepared independently, consequently resulting in a relatively high priced
alumin~m alloy product. In other processes, the aluminum-silicon alloys are
prepared directly from alumin~silica ore. For example, Seth et al U.S.
Patent 3,661,562 disclose that aluminum-silicon alloy can be prepared in
a blast furnace wherein coke or other suitable carbonaceous material is fed
into one reaction zone and a mixture of coke and alumina-silica ore is fed
into a second reaction zone. Hot carbon monoxide gases produced by combustion
of the coke are introduced into the second reaction for reducing the
alumina-silica ore. However, such or similar methods of producing
aluminum-silicon alloys often result in the alloy having very high silicon
and iron contents which normally have to be reduced or lowered for the alloy
to have commercial utility. One method of keeping the iron content low in
such alloys is to use alumina-silica containing ores with low iron content.
Another method involves the steps of lowering the iron content bv physieal
beneficiation prior to the reduction process. However, because of the
unfavorable economics and extra steps involved, it is preferred to start
with an alumina-silica containing ore having a high iron content, which, of
course, results in an alloy being high in silicon and iron as noted above
and the need for purification thereof.
Purification of aluminum alloys using electrolytic cells is
disclosed in the prior art. For example, Hoopes U.S~ Patent 673,364
discloses that if impure aluminum, in a melted state, is used as an anode in
an electrolytic cell, especially one in which the electrolyte contains
fused aluminum fluoride and a fluoride of a metal more electropositive than
aluminum, pure aluminum will be deposited at the cathode and fluorine is
set free at the anode when current is passed through the cell.
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In another method of purifying aluminum-silicon alloys, Sullivan
et al in U.S. Patent 3,798,140 disclose electrolytically producing aluminum
and silicon from aluminum-silicon alloys using a NaCl, KCl and AlC13 or
AlF3 electrolyte. The aluminum-silicon alloy is provided as an anode in a
perforated graphite anode crucible. A perforated graphite screen is
provided around a cathode and around an aluminacrucible to prevent any
fine silicon liberated during the electrolysis from floating into the
cathode department. However, production of purified aluminum in this
process is limited by its effective current density which is only 150 to
200 amps/ft in the chloride-fluoride electrolyte.
The present invention overcomes the problems in the prior art by
separating aluminum from alloying constituents such as silicon and iron
and the like in a highly economical manner.
An object of the present invention is to purify aluminum alloys.
Another object of the present invention is to purify aluminum
alloys containing high levels of alloying constituents such as silicon,
iron and the like.
Yet another object of the present invention is to provide an
electrolytic method of purifying aluminum.
Yet another object of the present invention is to produce high
purity aluminum.
These objects are accomplished by the process of the invention
which generally comprises the following steps: (a) providing the
aluminum alloy in a molten state in a container having a porous wall there-
in, said porous wall being capable of containing molten aluminum in the
container, the porous wall being permeable by a molten electrolyte; and
(b) Plectrolytically transferring aluminum through said porous wall to a
cathode in the presence of the electrolyte, thereby substantially purifying
said aluminum by separating it from its alloying constituents. This is
characterized in the facts that tl) said porous wall is a porous membrane,
(2) ~he electrolyte comprises at least one salt selected from the group
consisting of aluminum fluoride and aluminum ch~oride and at least one salt
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selected from the group consisting of sodium, potassium, lithium,
manganese and magnesium halide, and (3) the transferring is effected at a
current density of greater than 500 amps/ft2 and a voltage range of 1 to 5
volts and with the electrolyte being maintained within a temperature range
of 675 to 925C
In the drawings illustrating the invention:-
Figure 1 shows in cross section a form of apparatus suitable forpractising the process of the invention; and
Figure 2 is a schematic of an apparatus which can be operated on a
continuous basis to provide purified aluminum in accord with the process of
the invention.
Aluminum alloy as referred to herein is an alloy containing
typically not more than 99.9 wt.% aluminum. However, alloys which can be
purified in accordance with the present invention can contain large amounts
of impurities. For example, the aluminum alloys can contain as much as
50 wt.% Si. Also, the alloys can contain large amounts of Fe, for example,
20 wt.%. In addition, other alloying constituents normally associated with
aluminum, e.g. Ti, can usually be re ved in accordance with the present
invention. Also, the alloying constituents can be reduced to a very low
level. That is, the present invention can be useful in providing high
purity aluminum, even when the starting material is relatively pure.
By reference to Figure 1, there is shown an electrolytic cell
configuration 10 in which an aluminum alloy can be purified substantially
in accordance with the present invention. The cell comprises an outer
container 20 which, at least a portion thereof, is constructed of graphite
or a like material which can act as a cathode in the cell. For example, the
cell
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may be constructed such that only bottom 21 or a portion thereof
may serve as a cathode. Electrolytic cell 10 further comprises a
second container 30 in communication with the cathode referred to
by means of electrolyte 24. Container 30 serves as a vessel, as
shown in Figure 1, in which aluminum alloy 32 is provided in
molten form. Container 30 should be constructed of a material
resistant to attack by molten aluminum alloy 32 and electrolyte
24 and must have a wall or a portion of a wall thereof permeable
or penetrable by an ion containing one or more aluminum atoms
which can be electrolytically transferred or transported through
the wall to the cathode.
Container 30 can be constructed from a conductive or
non-conductive porous material. If container 30 is constructed
from non-conductive porous material, an anode has to be projected
into aluminum alloy 32 in order that the aluminum can be electro-
lytically transported to the cathode. If container 30 is made
from a conductive, porous material, then the container can act as
the anode as shown in Figure 1.
With respect to the permeable wall, it is preferred
that such material be a carbonaceous material when separation of
constituents such as silicon, iron and the like from aluminum is
desired. However, it is within the purview of the present
invention to select other materials permeable by an ion contain-
ing one or more aluminum atoms but which restricts the passage of
constituents such as those just mentioned. The preferred carbo-
naceous material suitable for use in the present invention is
porous carbon or porous graphite having a maximum average pore
diameter of 635 microns. An average pore diameter in the range
of 5 to 425 microns can be used, with a preferred diameter being
in the range of 20 to 220 microns. Porous carbon, obtainable
from Union Carbide Corporation, Carbon Products Division, Niagara
Falls, ~ew York, and referred to as PC-25 having an effective
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porosity of about 48% and an average pore diameter of about 120
microns has been found to be quite suitable. Porous carbon or
other porous material used in this application is further char-
acterized by being impenetrable or impermeable to molten aluminum
and alloying constituents thereof in the absence of electric
current being passed through the cell but permeable by molten
salt used as the electrolyte.
With respect to the pore size, it should be noted that
its size can vary depending on the amount of head, the tempera-
ture of the molten aluminum, and the wettability of the porous
member. Also, the electrolyte employed as well as the alloying
constituents can affect the size of the pore which will be
impenetrable or impervious to molten aluminum and alloying
constituents thereof in the absence of electric current being
passed through the cell. Thus it will be seen that in certain
instances porous members having pores therein having a larger
maximum pore diameter or having an average pore diameter larger
than that indicated in the range above can be used in the instant
invention and will be impermeable to the molten aluminum.
Electrolyte 24 is an important aspect of the present
invention. The electrolyte should comprise an aluminum fluoride
or chloride and at least one salt selected from the group con-
sisting of lithium, potassium, sodium, manganese and magnesium
halide with a preferred electrolyte comprising aluminum fluoride,
lithium chloride and potassium chloride. The use of lithium
chloride permits the use of high current densities without
adversely affecting the operation of the cell as by heat genera-
tion due to high resistance encountered in the electrolyte. The
potassium chloride aids in the coalescence of purified aluminum
26 deposited at the cathode. That is, when lithium chloride is
used without potassium chloride, aluminum deposited at the
cathode can remain in divided particle form making its recovery
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from the cell difficult.
The electrolyte can comprise, by weight percent, 5 to
95~ LiCl, 4 to 70~ KCl and 1 to 25% AlF3. Preferably, the
composition is 38 to 90% LiCl, 8 to 50% KCl and 2 to 12% AlF3.
AlC13 or MgC12 can be used instead of AlF3; NaCl can be used
instead of KCl; and LiF can be used instead of LiCl but on a less
preferred basis. It will be appreciated that combinations of the
above salts can also be used but again on a less preferred basis.
The temperature of the electrolyte can affect the
overall economics of the process. If the electrolyte temperature
is too low, the purified aluminum can be difficult to collect.
Also, low temperatures can result in low electrolyte conductivity
and consequently low cell productivity. Too high operating
temperatures can diminish the useful life of the anode and
cathode as well as cause vaporization of the salt. Thus, while
the ternperature can range from 675 to 925C, a preferred tempera-
ture is in the range of 700 to 850C.
In the process of the present invention, the cell can
be operated at high current densities resulting in high yields of
purified aluminum. Also, the cell can be operated at high
current densities without encountering high resistances in the
electrolyte and the resulting generation of undesirable heat and
its attendant problems. The cell can be operated at a voltage of
1 to 5 volts and a current density in the range of 200 to 3000
amps/ft2, or in certain cases higher, with a preferred voltage
being in the range of 1.5 to 4.5 volts and a minimum current
density which should not be less than 200 amps/lt2 and preferably
at least 300 amps/ft2.
In operation of the electrolytic cell, molten electro-
lyte 24 is provided in container 20 and preferably kept at atemperature in the range of 700 to 850C. Aluminum alloy in
molten form is placed in container 30. An electrical current is
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passed from the anode to the cathode and aluminum is transported
by virtue of the electrolyte through the porous carbon to the
cathode where it is deposited and collected. The porous wall
restricts the passage of alloying constituents such as silicon
and iron and other residues and hence prevents the contamination
of the purified aluminum under these operating conditions. If
container 30 is constructed from a conductive, porous material,
purified aluminum 26 should not be permitted to accumulate in
container 20 until it touches container 30 since this would
short-circuit the cell.
It will be appreciated by those skilled in the art that
a number of anode containers, such as shown in Figure 1, may be
positioned within the cathode or outer container 20 to increase
the production of the cell. Also, it will be appreciated that
other configurations employing the permeable membrane may be
used. For example, container 20 may be constructed from a non-
conductive material and the porous membrane may be used to divide
the container, providin~ an area to contain the impure molten
aluminum 32 and another area or space in which to provide the
electrolyte. The aluminum may be purified by providing an anode
in the impure aluminum and a cathode in the electrolyte and
passing electric current therebetween.
By reference to Figure 2, there is shown an alternate
embodiment of the electrolytic cell which can be operated on a
continuous basis. The cell 10' comprises outer container 20'
constructed of a material resistant to attack by purified alumi-
num 26 or molten electrolyte 24 and a second container 30' which
serves as a vessel in which aluminum alloy 32 is provided in
molten form. The cell has a cathode 22 which projects into
3Q electrolyte 24. Underneath cathode 22, a receptacle 23 is
positioned to receive purified aluminum 26 precipitated or
deposited at the cathode. Receptaclè 23 has an outlet 27 through
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whi~h purified aluminum 26 can be removed continuously at a rate
substantially commensurate with the rate of deposition thereof at
cathode 22. Container 30', in the embodiment illustrated in
Figure 2, has a porous wall 29 permeable or penetrable by an ion
containing one or more aluminum atoms which can be electrolyti-
cally transported through wall 29 to the cathode. An outlet 34
is provided so that residues or alloying constituents 36 remaining
after aluminum has been separated therefrom can be removed. In
the particular embodiment illustrated in Figure 2, side 29 of
container 30' serves as the anode of the cell.
In the cell of the present invention, the distance "x"
(shown in Figure 2) between the anode and cathode should be
closely controlled in order to aid in minimizing the voltage drop
across the cell. Thus, the distance "x" between the cathode and
anode should not be more than 1.0 inch and preferably not more
than 0.5 inch.
The present invention is advantageous in removing
silicon and iron and the like in aluminum alloys to a very low
level. In addition, the present invention is capable of
separating magnesium and the like from aluminum. That is, if the
aluminum alloy to be purified contains magnesium or the like,
i.e. less noble than aluminum, such included metal can pass
through the porous membrane but is not normally deposited at the
cathode. Magnesium and the like are normally dissolved in the
bath and thus, are not prone to contaminate the purified aluminum
deposited at the cathode.
The present invention, as well as providing purified
aluminum, is advantageous in that it can provide high purity
silicon. In addition, ferro-silicon compounds can be recovered
since these materials do not pass through the porous membrane.
Furthermore, while it has been noted hereinabove that the inven-
tion was particularly useful with respect to purifying aluminum
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alloys obtained from the high silicon ores, it is also useful in
purifying aluminum scrap containing iron and silicon materials.
Also, the invention can be used to purify aluminum used in clad
products, e.g. brazing alloy.
The following examples are still further illustrative
of the invention.
Example I
An aluminum alloy containing 11.4 wt.% silicon and 0.21
wt.% iron was provided in molten form in an anode section of a
cell~ A molten electrolyte consisting of 5 wt.% aluminum fluoride
and 95% lithium chloride was used. The electrolyte temperature
was 750C. The anode section was fabricated from porous carbon
having an average pore diameter of 120 microns and a porosity of
48~. The distance between the anode and cathode was 0.4 inch.
An electric current, amperage 125 and voltage 4.2 at a current
density of 650 amps/ft2 was passed across the cell. Purified
aluminum collected at the cathode contained only 0.011 wt.%
silicon and 0.05 wt.~ iron.
Example II
The aluminum alloy of Example I was purified as in
Example I except the electrolyte contained 5 wt.% AlF3, 10 wt.~
KCl and 85 wt.% LiCl. The cell was operated at 4.2 volts and a
current density of about 700 amps/ft2. The purified aluminum
collected at the cathode contained 0.009 wt.% Si and 0.015 wt~
Fe.
Example III
A clad product having a core of aluminum alloy 3105
(0.5% Mn, 0.5% Mg, remainder essentially Al) and a cladding on
both sides thereof (composition being 9.75% Si, 1.5% Mg, remain-
der essentially Al) was melted to provide an alu~inum alloy
composition having 3.10% Si, 0.45% Fe, 0.11% Cu, 0.16% Mn and
0.56% Mg. For purposes of purification, the melt was provided in
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an anode section and treated as in Example I except the electro-
lyte composition was 10% AlF and 90% LiCl and the current density
was 500 amps/ft2. Analysis of the purified aluminum showed only
0.002% Si, 0.004% Fe, 0.001% Cu, 0.004% Mn and 0.0003~ Mg, thus
providing substantially 99.99% aluminum.
From the above examples, it can be seen that silicon
and iron content of the aluminum were reduced rather signifi-
cantly. Further, it can be seen that the invention is capable of
producing high purity aluminum metal.
Various modifications may be made in the invention
without departing from the spirit thereof, or the scope of the
claims, and therefore, the exact form shown is to be taken as
illustrative only and not in a limiting sense, and it is desired
that only such limitations shall be placed thereon as are imposed
by the prior art, or are specifically set forth in the appended
claims.
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