Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for industrially
advantageously producing aluminum by electrolyzing
aluminum chloride in molten salt.
2. Description of the Prior Art
Aluminum has been heretofore produced by a so-called
Hall-Heroult process wherein aluminum oxide (alumina) is
dissolved in an electrolytic bath of a metal fluoride
molten salt consisting predominantly of molten cryolite
and is electrolyzed in the electrolytic cell. However, in
the Hall-Heroult process, in principle, such large amounts
of electric energy are required to electrolytically reduce
alumina that, in fact, the electric power required to
produce one ton of aluminum is 15,000kwh or more; there-
fore, the development of a process for producing aluminum
with reduced electric energy consumption is highly
desirable.
As a prospective energy saving aluminum producing
process to take the place of the Hall-Heroult process,
there is known an aluminum chloride electrolyzing process
wherein aluminum chloride is dissolved in a molten salt
bath of alkali metal chloride such as NaCl or KCl (some-
times accompanied by small additions of alkaline earth
metal chloride) and is electrolyzed. Although this
aluminum chloride melting electrolyzing process has
definite advantages in that it can be operated at an
electrolyzing temperature of about 700C., which is about
300C. lower than in the Hall-Heroult process, and in
that, as the anodic reaction releases chlorine, the graph-
ite electrode used for the anode will not be consumed, it
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has been long ignored in the industry for the reasons that
it is troublesome to handle aluminum chloride and chorine
gas at high temperatures and that no adequate bath-
resistant material was industrially available. However,
an aluminum chloride electrolyzing process (the so-called
ALCOA* process) employing a new electrolyzing apparatus
and an electrolytic bath having new composition has been
recently suggested by ALCOA (Aluminum Company of America),
U.S.A. (U.S. Pat. No. 3,822,195) and has quickly received
industrial attention.
In this ALCOA* process, the electrolysis is carried
out at a bath temperature of about 700C., an electrode
distance of about 15mm and a current density of about 1
ampere/cm2 in an electrolytic bath composed of a molten
salt of an AlC13-LiCl-NaCl system in which the LiCl is
mixed at a high concentration by means of an electrolytic
cell made by horizontally arranging bi-polar electrodes
formed of carbon (graphite) electrode plates with a proper
clearance between them in a cell lined with a refractory
material having a nitride base so that chlorine gas may be
produced on the anode surface and molten metallic aluminum
may be produced on the cathode surface. The special fea-
tures of this process include the use of a special material
high in the fireproofness and bath corrosion-resistance
corrosion is used for the electrolytic cell container, the
introduction of LiCl high in electric conductivity in the
molten salt electrolytic bath composition to reduce the
voltage drop of the bath, and a reduction in the distance
between the electrodes to lower electric power consumption.
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SUMMARY OF THE INVENTION
However, lithium chloride to be used in the electro-
lytic bath is an expensive industrial commodity and, the
electric current efficiency for an electrolytic bath of
the AlC13-LiCl-NaCl system is about 85% at most; thus,
the extent of possible improvement (i.e. reduction), in
electric power consumption is limited consequently it is
desirable to develop a more industrially advantageous
process for producing aluminum. As a result of making
various researches particularly on the composition of an
electrolytic bath, the present inventors have discovered
that, when aluminum chloride is electrolyzed by using an
electrolytic bath which contains, a relatively large amount
of CaC12 or MgC12 in the bath of AlC13-NaCl system,
aluminum can be produced at a very high current efficiency.
According to the invention there is provided in a
process for electrolytically producing metallic aluminum
by melting and electrolyzing aluminum chloride together
with an alkali metal halide in a cell lined with a refrac-
tory material and containing graphite electrodes to produce
chlorine gas at the anode surface and molten aluminum at
the cathode surface, the improvement wherein the electro-
lytic bath of a mixed molten salt consists essentially ~ -
of 2 to 15% by weight of AlC13, 15 to 70~ by weight of
at least one chloride selected from the group consisting
of CaC12 and MgC12, provided that the amount of CaC12 does
not exceed 40~ by weight of the totaI weight of the bath, 83
to 15% by weight NaCl and 0 to 10% by weight of BaC12.
By means of the present invention, at least in prefer-
30 red forms, metallic aluminum can be produced by electrolyz-
ing aluminum chloride at a high current efficiency reaching ~;
about 90 to nearly 100% and therefore the electric power
needed for producing the aluminum can be greatly reduced.
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The reason why a high current efficiency can be
obtained when aluminum chloride is electrolyzed in molten
salt electrolytic bath constituted as described above is
not clear. However, from the fact that the greatest cause
of loss in current efficiency in the electrolysis of
aluminum chloride is said to be based on a reverse reaction
in which a part of the aluminum deposited at the cathode
surface dissolves in the electrolytic bath and reacts with
the chlorine gas generated on the anode surface, it is pre-
sumed that the mixed molten salt of the AlC13-MgC12-NaCl,
AlC13-CaC12-MgCl-NaCl or AlC13-CaC12-NaCl system having
the above mentioned composition range has an action of
effectively inhibiting the above mentioned reverse reaction
on account of the solubility of aluminum in the bath, the
viscosity of the bath and the wettability of aluminum with
the bath.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In molten salt electrolytic bath systems as described
above in~the present invention, when the concentration of
AlC13 in the bath exceeds 15% by weight, the electric
conductivity will be so remarkably reduced and the vapor
pressure of the bath will become so excessive as to cause
the cell voltage to rise and cell operation to become
unstable. Therefore, it is desirable to retain the AlC13
concentration below 15% by weight. If it is less than 2%
by weight, the concentration will be so low that the elec-
tric power will likely be locally consumed for other
purposes than the production of aluminum and it will be
troublesome to control the supply of AlC13.
Further, in the bath of this system, the content of a
total amount of 15 to 70% by weight MgC12 or CaC12 has
an effect of elevating current efficiency during the
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electrolysis. When the total amount of MgC12 and/or
CaC12 is less than the lower limit value, the current
efficiency improving ~ffect will be poor and, when the
upper limit value is exceeded, the electric conductivity
of the bath will be remarkably reduced. When the CaC12
concentration in the bath of the AlC13-CaC12-NaCl system
or AlC13-MgC12-CaC12-NaCl system exceeds 40% by weight,
the bath will be separated into two layers and no normal
electrolytic operation will be possible. Therefore, it is
desirable to keep the CaC12 concentration below it.
In the AlC13-MgC12-NaCl bath system, NaCl and MgC12
form together with AlC13 a uniform mixed bath over a com-
paratively wide composition range and therefore can be
used very stably. The higher the weight ratio of MgC12
to NaCl, the higher the current efficiency in the electro-
lysis tends to be. Therefore, it is desirable that the
NaCl and MgC12 be present in amounts sufficient to give
a weight ratio of MgC12/NaCl of 1 or more to obtain 97%
or higher current efficiency. However, when the MgC12
content exceeds 70~ by weight, the electric conductivity -~
of the bath will drop remarkably. Therefore, it should be
less than 70~ by weight or preferably less than 65% by
weight. If it is less than 15% by weight, the effect of
increasing the current efficiency will be poor. Therefore,
more than 15% by weight should be present.
In the AlC13-CaC12-MgC12-NaCl bath system, it is desir-
able that the contents of CaC12, MgC12 and NaCl keep a weight
ratio of [~aC12~MgC12]/NaCl of 1 or more to obtain 97%
or more of current efficiency.
30The electrolyzing conditions under which stable
operation of the electrolysis of aluminum chloride is possible
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with the electrolytic bath of the present invention are
different depending on the type and capacity of the cell
but are generally of a bath temperature of 680 to 780C.,
an electric current density of 0.5 to 2.0A/cm2 and an
electrode spacing distance of 10 to 25mm. When electroly-
sis conditions are selected within this range, the electro-
lytic production of aluminum will proceed continuously with
a current efficiency of more than about 90%. Further, when
the electrolytic bath of the present invention is used, the
cathode collapsing phenomenon occasionally seen in this
kind of electrolyzing process will not be caused.
Incidentally, in electrolyzing aluminum chloride
according to the present invention, such horizontal multi-
electrodes as are known in the ALCOA process can be used
in the electrolytic cell. However, generally, in this
kind of electrolyzing process, the distance between the
electrodes is so comparatively small (10~25mm~ that it
is desirable from the standpoint of increasing current
efficiency to quickly remove the aluminum produced at the
anode surface and prevent its reaction with the chlorine
gas produced at the anode surface. As a result of inves-
tigating the electrodes from this viewpoint, it has been
found that, when the electrodes are inclined, the produced
aluminum will flow easily and remove itself from between
the electrodes. The angle of inclination of the electrode
plates from the horizontal plane should be less than 50
degrees or preferably 10 to 45 degrees. If the angle of
inclination exceeds 50 degrees, the current efficiency
will be rather less.
Also, in the process of the present invention! it is
advantageous to add a small amount of BaC12 to the
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electrolytic bath, because the addition of BaC12 in the
amoun~ of up to 10% of the total amount of the electro-
lytic bath functions to reduce interfacial tension of the
aluminum metal bath so as to effectively promote the flow
of the metal from the cathode surface, though it is not
effective for improving current efficiency.
The present invention shall be explained more particu-
larly by means of the following concrete examples.
EXAMPLE 1
29.6g of aluminum were obtained by continuing an
electrolysis for 4.5 hours at an electrode spacing
distance of 14mm, a bath temperature of 750C., an electric
current of 20A, a current density of lA/cm2 and a cell
voltage of 3.30V using a mixed molten salt of the AlC13-
MgC12-NaCl system composed of 8.3% by weight AlC13,
56.7% by weight MgC12 and 35.0% by weight NaCl as the
electrolytic bath and using inclined graphite electrode
plates (having an effec~ive reaction surface of 80mm x 25
mm) extending at an angle of 30 degrees to the horizontal
within an electrolytic cell lined with a refractory
material of alumina. The current efficiency at this time
was 98%.
EXAMPLE 2
29.09 of aluminum were obtained by continuing an
electrolysis for 4.5 hours at an electrode spacing distance
of 14mm, a bath temperature of 750C., an electric current
of 20A, a current density of lA/cm and a cell voltage
of 3.20V using a mixed molten salt of the AlC13-CaC12-NaCl
system bath composed of 10.0% by weight AlC13, 38.4% by
weight CaC12 and 51.6% by weight of NaCl as the electrolytic
bath and using inclined graphite electrode plates (of an
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effective reaction surface of 80mm x 25mm) of an angle
of 30 degrees with the horizontal within an electrolytic -
cell lined with a refractory material of alumina. The
current efficiency at this time was 96%.
EXAMPLE 3
29.3g of aluminum were obtained by continuing an ,~
electrolysis for 4.5 hours at an electrode spacing distance
of 14mm, a bath temperature of 750C., an electric current
of 20A, a current density of lA/cm2 and a cell voltage of
3.45V using a mixed molten salt of the AlC13-CaC12-
MgC12-NaCl system composed of 6.7% by weight of AlC13,
23.3% by weight CaC12, 42.0% by weight MgC12 and 28.0% by
weight NaCl as an electrolytic bath and using inclined
graphite electrode plates (of an effective reaction surface
of 80mm x 25mm) of an angle of 30 degrees with the horizon-
tal within an electrolytic cell lined with a refractory
material of alumina. The current efficiency at this time
ws 97%.
EXAMPLE 4
28.7g of aluminum were obtained by continuing an elec-
trolysis for 4.5 hours under the same conditions as in
; Example 3 (except for a cell voltage of 3.23V) by using a
mixed molten salt of the AlC13-CaC12-MgC12-NaCl system
having a composition of 11.0~ by weight AlC13, 14.0% by
weight CaC12, 24.0% by weight MgC12 and 51.0% by weight NaCl
as the electrolytic bath. The current efficiency at this time
was 95.0%.
EXAMPLE 5
27.6g of aluminum were obtained by continuing an
electrolysis for 4 hours under the same conditions as in
Example 4 using,a mixed molten salt of the AlC13-MgC12-
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BaC12-NaCl system having a composition of 14.0~ by weight
AlC13, 27.0% by weight MgC12, 8.0% by weight BaC12 and
51.0% by weight NaCl as an electrolytic bath. The current
efficiency was 91.2~.
EXAMPLE 6
59.2g of aluminum were obtained by continuing an
electrolysis for 4 hours at a bipolar electrode (of
the type disclosed in our co-pending Canadian Patent Ap-
plication Serial No. 290,930 filed on November 15, 1977)
located between cathode and anode electrodes at a spac-
ing of 14 mm from each, a bath temperature of 750C.,
an electric current of 20A and a cell voltage of 6.5V
using a mixed molten salt of the AlC13-MgC12-NaCl
system composed of 9.5% by weight of AlC13, 54.5% by
weight of MgC12 and 36.0% by weight of NaCl as the
electrolytic bath and using inclined graphite electrode
plates (of an effective reaction surface of 80mm x 25mm)
making an angle of 30 degrees with the horizontal within
an electrolytic cell lined with a refractory material of
alumina. The current efficiency at this time was 96.4%.
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