Note: Descriptions are shown in the official language in which they were submitted.
7~J6~
E~I~LD O~ T~l~ INVE:NTION
This invention relates to the electrolytic production
of aluminu~ from aluminous materials using an electrolyte
bath containing halides, more particularly, the present
invention relates to the electrodeposition of aluminum using
an anode as the sole source of aluminum in an electrolytic
cell maintaining dimensionally stable spacing between cathode
and anode at low bath temperatures to effect great energy
savings.
BAC~GROUND OF THE INVENTION
The commercial production of the alumiIIum in the world
has been by the Hall-~eroult process. In this well-known
process a purified source of alumina is dissolved in a
molten pximarily fluoride salt solvent, consisting essentially
of cryolite and ~hen reduced electrolytically with a carbon
anode according to the reactions
lJ2 A1203 ~ 3/4 C + 3e _________~_ Al ~ 3~4 CO~
and
1~ A1203 + 3/~ C ~ 3e ---------P- Al + 3/~ CO.
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Three characterlstics Df this system which are inherent in the
Hall-Heroult process include: first, carbon dioxide is produced
and the carbon anode is consumed at the rate of .33 to 1 pound
of carbon per pound of aluminum produced which results in a
required continual movement of the carbon anode downwardly
toward the cathode aluminum pool at the bottom of the cell
to maintain constant spacing for uniform aluminum production
and thermal balance in the cell;.second, the need to feed
. intermittently and evenly the solid alumina in a limited
concentration range to the "open type" cell to maintain peak
efficiency of operation in order to avoid "anode effects";
third, severe corrosion of cell materials due to the high
temperatures of 950-1000C and the fluoride salts resulting
in relatively low cell life and increased labor.
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A fourth characteristic not inherent in the system but
pxesent nonetheless is that the cell power efficiency is limited
to less than about 50% due to the practical requirement of main-
. taining a carbon anode to liquid aluminum distance greater than
~ one inch to reduce the magnetic fields' undulation of the
: 20 aluminum layer causing intermittent shorting with resultant
Faradaic losses due to the back reaction of aluminum droplets
~: with carbon dioxide,
:
~ Al ~ 3/2 C02 ~ 1/2 A12o3 ~ 3/2 CO~
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~72
The first three inherent limitations of the conventional
Hall-Heroult process can potent.ially be overcome either by use
of an aluminum chloride electrolysis process which in the
prior art would directly produce aluminum and chlorine gas
or through the use of all fluoride bath at temperatures of
670-750~C for the direct reduction of aluminum oxide.
.
The potential advantages of an aluminum chloride salt
electrolysis process include: (1) -the use of chloride salts
which are generally more economical than the fluorides of the
~all-Heroult salts, have a lower operating temperature of
670-800C are much less corrosive to cell construction
. .materials and have in general a lower specific gravity which can
: permit closer anode-cathode spacing; (2) the aluminum chloride
. electrolysis process requires a closed system reducing air
: pollution problems; (3) the chloride electrolytes, even at the
lower operating temperature of 670~-800C, have higher
cooductivities than that of the Hall-Heroult fluoride salts
at 950-1000C. This results in the production of aluminum
~ at lower energy consumption and at higher power and current
efficiencies; (4) tha use of the aluminum chloride electrolysis
I ~ process has a very broad operating range of aluminum concentra-
tion which results in no "anoàe effect"; (5) it is possible to
design the aluminum chloride electrolytic process cell with
bipolar electrodes which result in a much more compact cell
with increased production potential per unit volume.
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There are, however, potential advantages to the use of
an all fluoride bath if it is possible to use the Hall-Heroult
reaction mechanism system and yet continue to deposit metal.
The all fluoride bath potentially: (1) avoids substantial
structural changes in the cell if the aluminum oxide can be
directly reacted thereby making unnecessary the requirement
of the chloride system to close the top of the cell and (2) does
not evolve any ccrrosive, noxious anode gas, merely CO2. To
~ achieve these advantages the all fluoride bath must be used at
low temperatures of 670~-800~C but such is not possible in
accordance with prior art techniques because alumina, unlike
aluminum chloride, will not readily dissolve at such low
temperatures.
In the comparison of the commonly used Hall-Heroult
; alumina-fluoride process and the much less familiar aluminum
chloride process, there appear to be significant benefits in
the use of the aluminum chloride process, but a fair comparison
should not overlook the significant disadvantage of the aluminum
~ chloride electrolytic process in producing large quantities of
the corrosive gas chlorine liberated at the anode. The chlorine
entrains the chloride electrolyte to clog the exit ports and
deplete the bath. This-entrained electrolyte must be collected
and returned to the cell and the liberated chlorine must be
recycled to produce further aluminum chloride.
. ~
Although the potential advantages of utilizing an
aluminum chloride electrolysis process for the electrolytic
production of aluminum have been recognized for well over a
century, commercial realization of such a process has not oc-
curred. The lack of a sufficiently simple and economical process
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to produce large, commercial quantities of high purity anhydrous
aluminum chloride has been one of the reasons that an aluminum
chloride electrolysis process has never reached commercial
prominence.
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In general, the usual process known to the prior art
for producing aluminum chloride has been the conversion of an
alumina-containing material with chlorine in the presence
of carbon to yield aluminum chloride and a mixture of the gases
carbon dioxide and caxbon monoxide. This reaction,
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~ 10 A1203 + C ~ C12 ~ AlC13 ~ C02 and CO
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has been carried out under a wide range of conditions, each
variation having some alleged advantage. All of these
procedures for producing aluminum chloride have a common thread
however. Each involves the use ~f a source of carbon, a source
of chlorine, and an aluminum chloride reactor separate from
the electrolytic cell in which the metallic aluminum is
electrolytically produced.
The normal reaction temperature for the production of
~ aluminum chloride is generally in the range of 400C to 1000C
depending upon the form of the reacting agents. Unless a high
purity alumina source is used, other elements that are generally
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present such as iron, silicon, and titanium, are also
chlorinated and must undergo difficult separation from the
aluminum chloride. This contribu-tes to the size and cost
of the aluminum chloride producing plants.
The aluminum chloride electrolytic process would have
an unusual advantage beyond those advantages heretofore cited
if it were possible to avoid both the chlorine collection
and the independent production of aluminum chloride in a plant
separate from the electrolysis plant.
The electrodeposition of aluminum by the direct reduction
of alumina in an all fluoride bath is an attractive alternative
to the aluminum chloride system provided that the alumina
would dissolve at the low temperatures of 670-800C rather
- I than the 950-1000C consider,ed to be required for dissolution
in molten cryolite. Existing Hall-Heroult cells could be used
without substantial capital expenditures and yreat energy
savings would be possihle with such an all fluoride bath but
no such process for the electrodeposi-tion of aluminum is
available to those skilled in the art.
.
The fourth dlsadvantage of the Hall-Heroult cell, cell
power efficiency, has been considered by those skilled in
the art but it appears that the practical limit to energy saving
and efficiency in present Hall-Heroult cells has been reached
~, !!
¦¦ through careful design and operation of 150 to 225 Kamp cells
at anode current densities between 4.0 and 5.5 amps/in2. The
¦¦ lower energy limit appears to be about 5.6 to 6.0 Kwh/lb
.~ ¦ utilizing the most advanced currently known designs, computer
. I controls, bath modificatio~ and other improvements.
'r
It is known that larger cells capable of operating
¦~ at lower anode current densit.y consume considerably less
¦ energy. Lower anode current density, however, decreases
.. ¦ the production of aluminum per unit cell volume. The net
. 10 I result is that larger cells Froduce aluminum more economically
.~ I but at a lower production rat.e. If the anode current
.. ~ j density could be lowered but at the same time not reduce the
production rate of the cell, a substantial economy in the
¦ produc-tion of aluminum would rPsult.
. The prior art suggests that increasing the surface area
of the anode that is electrol.ytically active would lower the
current density. U.S. patent. 3,067,124 for instance discloses
. a type of electrolytic cell in which the electrodes are inclined
l towards the center in the shape of a pyramid or frustum of a
~20 ~ pyramid. It would normally be expected that the inherent
.~ advantages of utilizing the lower current density would be
' achieved with such structure. However, when such inclined
~ electrode system is embodied in the Hall cell system
requiring a high temperature bath at about 950C, the critical
spacing between cathode and anode is often lost. This
.. desired spacing cannot be maintained due to the
-- 8 --
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inherent dimensional instability of the cell structure
due both to such high temperatures of cell operation
and the inherent aggressive nature oE the salts required
for high temperature use.
,, .
Lower tempera-tures are not possible in the Hall
~` cell due to the lack of solubili-ty of aluminum oxide
in the cryolite at temperatures below about 940C and
`~ the fact that cryolite base salts have a freezing point
~ 10 in the range of 925~-950C. The lack of dissolved A12O3
;j~ present in the bath would r~esult in an anode effect
; which would at least increase the required voltage by
10-20 fold and cease aluminum deposition~ If a low
' temperature operation of such a cell would have been
possible, non-cryolite salts would permit both the use
of non-aggressive salt compositions and reduced tem-
~ perature gradients that would result in little or no
', dimensional ch-ange in the cell walls and bottoms and
,; consequently minimize the spacing changes between the
~ ~ 20 anode and cathode.
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According to the present invention, then, there
is provided in a process for the electrolytic production
of molten aluminum in a cell ha~ing a cathode with a
. cathode surface, a halide salt electrolyte and an anode
, body comprising an anodic mixture of an oxygen con-tain-
;~ ing compound of aluminum and an electrically conductive
reducing agent, the improvement comprising the steps
of positioning -the anode body in the electrolyte with
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at least one surface thereof in opposed relationship
to but spaced close to the surface of the cathode, con-
necting a source of electrical power -to the cathode,
connecting the source of electrical power to the anode
body by connecting the source directly by means of a
low resistance conductor to that portion of the anode
body at least approximately adjacent said one surface
of the anode body, energizing the source of electrical
power to apply a voltage across the anode body and
cathode with the principal anodic current substantially
bypassing the bulk of the anodic mixture and flowing
directly to that portion of the mixture at least ap-
proximately ad~acent said one surface for producing an
~: ~ electrolytic reaction at said one surface in which the
aluminum oxide in the mixture is converted to aluminum
ions recoverable as molten aluminum at the opposing
surface of the cathode, and maintaining the temperature
o~ the electrolyte above the melting temperature of
aluminum in the range of 670 to 810 during the elec-
:~ 20 trolytic process.
~: Conductor rods may be incorporated within the anode
;~ to enhance conduct.ivity of the anode.
: :
9a
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¦¦ Thus, electrolytic produc-tion of aluminum may be effected
¦ in a single cell from a molten halide salt bath containing
, ¦ aluminum and chloride ions which is depleted duriny elec-
trolysis and wherein aluminum ions are reproduced in situ
from the anode within the electrolytic cell. Aluminum ions
are produced at the anode by the reaction of an aluminous
` ¦¦ source and a reducing agent serving as the anode. The
~ ! aluminum ions are then deposited as aluminum metal at the
.~ I cathode. A unique porous membrane may be emplo~ed, which
' 10 passes electrolyte or other dissolved material while with-
holding undissolved impurities.
¦ Aluminum also may be deposited by the direct electrolytic
reduction of a dissociated and/or dissolved aluminum oxide to
produce molten metal at a temperature as 1QW as 670-810C with
the use of an all fluoride containing bath and an anode
containing aluminum oxide and reducing agent.
11
Dimensionally s-table cells for the electrodeposition of
aluminum may have sloped walls forming electrodes which reduce
anode current density and permit the maintenance of a reduced
anode-cathode spacing when using the low -temperature non-
~; 20 aggressive baths permissible with the par-ticular anode
compositi~
~1 10
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Embodiments of the invention will now be described with
¦ reference to the accompan~ing drawings in which:
Figure 1 is a schematic showing in cross section of the
electrolytic cell of the present invention containing a chloride
bath and illustrating the closed top of the cell along with
the relative positioning of the electrodes.
Figure 2 is a schematic showing partly broken away of an
electrode being used as an anode and having coated thereon the
mixture of aluminous material and reducing agent~
Figure 2A is a schematic vie~ in perspective of an
alternate embodiment of the electrode of Figure 2 showing a
¦ plurality of conductor cores within a matrix of the aluminous
material and reducing agent.
: ~ ~ .
Figure 2B is a schematic perspective view of a variation
of the electrode illustrated in Figure 2A.
.
Figure 3 is a schematic illustration partly broken away
of another alternative electrode.
. '
Figure 4 is a schematic illustration in cross section of
an open top electrolytic cell having an all fluoride bath and
an anode clamp providing a source of electric current to a
continuously introduced anode.
Figure 4~ is a schematic drawiny of allother alternative
electrode similar to the electrode of Figure 3
¦¦ - lOa -
117;~6~0
Figl~re S is a schematic view of an embodiment of the
present invention which illustrates -the use of a porous membrane
to contain the various anodic materials including an aluminum
containing material and a reducing agent.
¦ Figure 6 is a schematic view in cross section of another
alternate embodiment of an electrolytic cell illustrating the use
of bipolar electrodes.
Figure 7 (which is on the same page as Figures 12 and 13)
1s a schematic cross-sectional view of a combination of electro-
lytic cells with sloped sided electrodes and the composite anode
of complementary shape.
Figure 8 is a schema-tic cross sectional view of a
modification of the unique combination electrolytic cell and
composite anode.
'' ~ ' .
Figure 9 is a perspective illustration of the anode
o~ Figure 8.
.~ :
Figure 10 is a schematic cross sectional view of another
embodiment of the combination of Figure 8.
Figure 11 is a perspective illustration of the anode of
Figure 10.
Figure 12 is a schematic perspective view of a further
embodiment of the composite anode of the present invention
illustrating a lamincr construction.
,
Figure 13 is also a schematic perspective view of the anode
$ of Figure 12 and the anode clamp of Figure 4.
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The presen-t invention may employ a unique system for
electrolytically producing aluminum from a variety of raw
materials R containing aluminum in a low tempera-ture
electrolyte bath B. The system may employ an anode A
which is the sole source of the aluminurn being deposited
' on the cathode H. The anode A includes a combination of
an aluminous source usually alumina, A12O3, and a reducing
agent such as carbon. Conductors ~ may be incorporated
with the anode A to enhance conduct.ivity of the anode and
a membrane M may be used to con-tain the raw materials R.
An electrolytic cell C containing the anode A and
cathode H may take a variety of structural forms having
sloped wall electrodes or vertical wall electrodes. In
the preferred ~mbodiment sloped wall electrodes are more
economical and practical when used at low temperatures
with the unique anode.
The electrolytic bath B may be composed of chlorides
~: 20 or fluorides or mixtures thereof and does not require the
initial addition of an aluminum salt to the bath. In one
embodiment of the invention, the aluminum chloride cycle,
aluminum chloride is present in the bath and is maintained
at a constan-t concentration due to the reaction of the
composite anode A in the bath to form the aluminum ions
for reduction at the cathode. In another embodiment of
the invention, using an all fluoride ba-th, aluminum is
also ionized at the anode A for deposition at the cathode
H.
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The Electrolytic Cells
An electrolytic system embodying the present invention may
utilize an electrolytic cell C depicted in any one of the
Figures for the unique continuous production of aluminum.
In Figures 1-6 one form of the electrolytic cell structure
is shown generally at 10 as composed of an outer steel shell
having a refractory lining 14 that may serve solely as a thermal
insula-tor or as both insulator and electrode. The refractory
lining may be of any material resistant to the action of the
molten electrolytic bath 16. The refractory lining having
conventional vertical sides 15 and bottom 17 is designed to
maintain the desired thermal balance in the cell operation
and therefore ~ay be very thin in cross section in order to
achieve a small thermal gradient resulting in both a thin
layer of frozen salt on the surface of the refractory and a
hot outer wall on the surface of the steel shell 12. The
refractory lining may also be quite thick to achieve a
freeze-out layer of salt within the refractory lining resulting
in a cool surface on the steel shell although this is not
necessary in the vertical sided cell of Figures 1-6. In
contrast, in the slope sided electrode cell ~f Figure 7 cathode
19 is a conductive lining formed on both sides of the anode. A
tllerlllal and electrical insulation lining may be positioned
between the cathode 19 and the shell 12 if desired. The
freeze line should be within the boundaries of this conductive
lining or cathode 19 in order to prevent a solid layer of salt
collecting on the bath side of the electrodes. Such a salt
~layer would ac-t as an elec-trical insulator and prevent effective
- 30 Icurrent flow.
- 12 -
The lid 18 is provided on the top of the cell to produce
an air-tight closure and is only necessary in a chloride
containing bath. This lid thus prevents air and moisture
from seeping inside the cell or any vapors of the salt
composition 16 from leaking out to react with the environment.
The lid 18 may be lined with the refractory material 20
which may be the same as the refractory lininq 14 or any
other refractory material consistent with maintaining a
temperature balance in the cell as well as being chemically
inert to the salt composition 16. Seals 22 are supported on
the lid 18 and are secured against the electrodes 24, 25 and
26 to prevent atmospheric air and moisture from seeping into
the cell or the vapors from the cell exiting to the environment.
The sealing at the lid 18 and around the electrodes may be
by any means which prevents vapor leaks and may be standard
or conventional packing and gasket material capable of
withstanding the temperature o the operation while being
resistant to the electrolyte vapors. Acceptable materials
for such packing gasket use include asbestos, fibrous ceramics,
Teflo~, Vitron* silicones,liquid metal seals such as mercury,
liquid solder, tin, lead, etc.
~ : . l
Electrodes 24, 25 and 26 may be anodes, cathodes or
bipolar electrodes. They may include solid or coated conductors
to carry electric current for the cell operation. These
conductors may be any material that may withstand the
temperature within the cell which is the range of 150 to
1060C, stable to the halide composition 16 and is a good
clcctrical conductor. Materials tl~at are useful for this
purpose are carbon, graphite, and titanium carbides, nitrides
* Trademark
~ l ~7~6~3~
I
or bori~cs and alwninum me~al as al)uropriately sized for
heat transfer bqlance. The preferred materials for these
conductors have bcen found to be graphite and titanium
diboride when operating in the bipolar mode.
The aluminum chloride cycle cell also includes a stack
or exit tube 28 having a valve 30 to control the flow of any
gaseous elements from the stack and establish the pressure
builduu in the cell for continuous operation. Gaseous
vapors emanating from the cell are those of the oxidized
reducing agent and notably there is no chlorine gas detected ¦ -
at all with an alumlnum chloride containing salt. If any
chlorine is produced it would react at the anode 26
and be recycled a~ aluminum chloride. The molten aluminum
32 is tapped out by conventional tap 34 or otherwise drawn
out by vacuum through standard siphoning techniques well
known in the art.
Figure 4 illustrates a modification of the cell design
of Figure 1 again illustrating vertical sided electrodes 19.
The cell structures,including the shell 12 and refractory
14, are the same as that previously described, the electrode
44 serving as the anode may be ei-ther one of the anodes
shown in Figures 2, 2A or 2B but preferably Figure 3. The
anode 44 is immersed in the electrolyte containing fluoride
or chloride salts or mixtures thereof and heated to a temperature
gcnerally between G70 and 810C. At the bottom of the
cell, and resting uuon cathode bar 45 positioned over the
refractory insulation 14 is a block 46 which preferably is
slightly wider than the anode 44 and serves as the cathode
through suitable electrical connection to cathode bar 45.
1'1 ~
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~7;~6~ 1
c ~lock 4G may bc madc fron~ any o~ ~he proviously dcscribed
electrode materlals. Thc block 4G should extend close to
the base 50 of the anode 4q which is the only surface for
erosion of the anode. Closer anode-cathode spacing for such
electrode confiyuration is ~ossible when the block 46 also
rises above the level of the molten aluminum 32. As the
aluminum is deposited on the cathode block 46, its surface
is wetted and the aluminum runs off the block into the pool
32 at the bottom of the cell to be tapped off as desired at
34.
Figure 4 also illustrates a po~ler attachment clamp 47,
shown schematically, in contact with the anode 44 either
above but preferably below bath level and adjacent to the
; bottom of the anode to minimize the power loss due to the
resistance of the anode. Anode 44 may be structured for
instance as shown in Figures 3, 12 and 13. The clamp does
~; not act as an anode. Rather, the composite anode 44 dissolves
in the bath in the anode reaction. The clamp 47 may partially
or completely surround the anode 44 so that the anode 44 may
~20~ be ~ed continuously into the bath while maintaining electrical
contact with clamp 47. The clamp is composed of any suitable
nert material that is electrically conductive. Among these
materials are graphite, carbon, TiB2 or mixtures of these.
The electrical contact between the clamp and the anode may be
through protruding contact point or nub 48. The po-~er attach~
ment to the clamp 47 is through suitable split cyli~drical
conductors 49 that extend above the cell top.
~` ~ ~l
In lieu of charlgillg thc anodcs poriodically to supyly
fresh aluminous material, the present invention is adaptable
to a feed mechanism for continuous operation as shown in
Figure 5 or the continuous feed of an electrode as shown in
Figure 4 of the prebaked or Soderberg type.
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¦ Protruding up through the cell C oE Figure 5 is an
anode electrode 52 which penetrates deeply into the melt 15
but remains above the molten aluminum pool of aluminum 32 or
the cathode block 46. Surrounding the anode 52 are the
anode raw materials, shown generally at R, comprising the
aluminous material and the reducing agent. This anodic
mixture may be formed into small particle size from a .001
inch approximately to 1.0 inch or more and may have been
formed by extrusion, molding or the like and fed into the
cell by the hopper 54. The raw material particles of aluminous
material and reducing agent are identified specifically at
58 and are in close contact with the anode 52 to provide the
necessary source of aluminum and the reducing agent.
. .
. ~ These anodic raw materials are held in close contact
with each other and with the anode 52 by being contained in
a porous membrane container 60 which surrounds the anode 52.
As the anode materials 58 are used up and their level drops
substantially below the level of the molten bath 16, feed 54
is operated to add additional anodic materials 58 into the
porous membrane container 60.
''
In the embodiment of Figure 6 there is illustrated a
bipolar cell. Again, like structure has been designated
with the same identifying numerals.
,s, .
16
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~7~ ~
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l ll
¦ The same basic principle in operation of the bipolar
cell exists except that there is a pair of electrodes at
either end of the cell which are connected to a suitable
electric source. One of the electrodes 64 is a cathode and
at the opposite end'is an anode 66. Between the electrodes 64
and 66 is a group of spaced electrodes 68 which are unconneeted
to each other or to any electrical source. Secured to eaeh
of the electrodes 68 and the anode 66 is a porous membrane
container 60 of the same type as that described at 60 in
Figure 5. The porous membrane 60, however, in the bipolar
cell has as one side, one of the electrodes 66 or 68 that
form the enclosure for the anodic raw materials 58.
: : ~ .
In the bipolar cell the side of the electrode 68 nearest
the anode 66 becomes negatively charged and the side of the
electrode 68 facing the eathode 64 becomes positively charged.
This side 72 of the electrode 68 will act as the anode and
is the side that is in contact with the anodic raw materials
58. The electrolysis then produces aluminum on the negative
side of the electrode 68 and CO2 on the positive or anodie
side of the same eleetrodes~ The aluminum falls to the pool
32 at the bottom to be eollected in the usual manner.
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In Figures 7 through 11 there is illustrated the sloping
sided electrode-electrolytic cell which in combination with
the anode composition of the present invention results in
' ¦ substantial economies in the electrodeposition of aluminum.
In typical Hall cell procedures aluminum reduction
cells have an anode-cathode spacing which must take into
consideration the magnetic field effect and the "back reaction"
due to the undulations of the aluminum pool. Such considerations
prevent any closer spacing than about 1.5 and 2.0 inches
between the bottom surface of the anode where all erosion
occurs and the top of the aluminum pool or the cathode
electrode. A further and equally significant reason for the
requirement of greater spacing between the cathode and anode
whether in the llall cell construction using vertical sides
or any attempt to use a sloping side electrode is the serious
diffi~ulty of maintaining dimensional stability due to the
high temperatures required and the aggressive salts that
necessarily were included to retain a high temperature for
the dissolution of th~ alumina. In the combination of the
slope sided electrode cells and the anode utilizing aluminum
. oxide and a reducing agent to provide the sole source of
aluminum the use of temperatures as low as just above the
melting temperature of aluminum minimizes any of the problems
regarding dimensional instability and therefore enable the
cells of the present invention to be structured with a
'closer anode-cathode spacing unattainable in -the past. Thus
it is the particular combination of -the anode and the sloped
walls for the construction of the cell that achieves a lower
IR power drop in the salt due to the close spacing permissible
'30 , between the sloped walls and the reduction in the anode
current densi-ty.
18
1~l7;~ )0
~ ssentially the cells oE Figures 7 tllrougll 11 are
similar to those previously described except for the sloping
surfaces forming the electrodes. I~ith this cell structure
the anode 74 is provided with sloping sides 76 which as
shown are external and directed downwardly and inwardly
although the direction o~ the angle is not at all critical.
The slope of the sides may be in any direction or any angle
from the level of the bath B. The angle may even vary from
10 to 80 or more from the bath level. Through the use of
the sloping sided electrode's anode bottom and that portion
of the sloping anode side that is immersed in the bath 16, the
anode will erode over a greater surface area and supply the
aluminum for ultimate deposit on the cathode.
¦ The cathode 78 has surfaces 79 of complementary shape
to the sloping sides 76 of the anode to provide for an
¦electrode spacing on the sides as shown by the spacing Y.
~ I~This spacing may be between 0.25 and 2.5 inches. Greater
; spacing produces greater energy consumption. The spacing
between the bottom 80 of the anode 74 in Figure 7 and the
~20 surface of the aluminum layer 82 forming a part of the
aluminum pool 84 is shown at X and may be 0.25 to 2.5 inches.
Preferably the spacings X and Y should be between about 0.25
to 1.0 inches.
The spacing between the anode and the cathode above the
solidificd bath laycr 86 is not significant to the utility
of tllc invelltion. Ilowever the spacings X and Y between the
anode and the cathode may be equal or different depending
upon the desired current density and anode erosion but when
set as close as specified above will result in substantial energy
consumption savings.
~ - 19-
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The linincJ 78 formin~ the cathode of the cell may be of
typical material used for electrolytic cells such as carbon,
titanium diboride, or the like and is shaped as previously
stated to conform to the external shaping of the anode 74.
~dditionally, the base of the lining has an inclined floor
90 for the aluminum pool leading into a catch well 92 for
the aluminum. ~s can be seen the sloping floor 90 is such
as to retain only a limited depth of aluminum layer which
can be reyulated through draw-off means (not shown) of the
aiuminum from the catch well. The purpose of the thin
aluminum layer below the base 80 of the anode is substantially
to eliminate the ripple or wave like undulations of the
molten aluminum layer due to the magnetic effects within the
cell.
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In other respects the cell of Figure 7 is like that of
:: ~ Figure 1 in that a lid 18 is provided with an exhaust port
28 being part of shell 12. Refractory insulation of any
~ suitable form as shown.at 14 may also be included.
: : The combination of the use of the anode with sloped
sides to conform to the sloped cathodes enables the configura-
tions of the cell and anode to vary substantially as shown in
Figures 8 through 11. .
In Figures 8 and 9 the shape of the anode 74 is varied
and has centrally located divergently sloped sides 94 which
form an ape~ 9G in the anode. The carbon or other lininq
material such as Til12, etc. serVincJ as tl-c cathode projects
upwardly to complement the internal shaping of the anode as
best shown in Figure 8. The operation of such a cell as
shown in Figures 8 and 9 is essentially the same as that
described in Figure 7 particularly with regard to the increased
erosion surfaces 94.
- 20 -
7~
In Figures 10 and 11 dual anodes 100 and 102 with
oppositely shaped sloped sides 104 and 106 respectively are
positioned in a cell with cathode 98 shaped essentially
identically to that described in Figure 8.
Tl,e use of the sloped cathode concept of electrolytic
cells shown in Figures 7 through 11 has been found to require
that no frozen salt layer be permitted on the su~faces of
the sloped cathode wall immersed in the bath and confronted
with a portion of the anode surface. Otherwise the desired
spacing between cathode and anode cannot be maintained.
Additionally, the frozen salt that would adhere to the wall
of the cathode is a good electric insulator and thus would
inhibit current flow from the anode to the sloped cathode
side wall. In prior use of such sloped walled electrodes
the problem of salts freezing on the sides as well as dimensional
instability of the lining prevented any extensive use of
such cells. However with the anode composition of the
present invention and the lower bath temperatures a variety
of low melting salt compositions which will not freeze out
on the side wall can readily be utilized. Ideally the
melting point of the salt and the ceil thermal balance is
adjusted such that the freeze line of the salt is within the
lining or at the steel shell rather than at the lining or
cathode-bath interface. It is not importan-t where the
freeze line is located so long as the freeze line is within
the lining and that the salt is maintained in a liquid state
on the surface of the cathode lining immersed in the bath.
In such instance the proper cathode-anode spacing is maintained
without difficulty.
~7~
The Proc s
_ e 5
a. Chloride Containing Bath
The electrolytic process of the present inven-tion
for the continuous production of aluminum ions at the anode
may utilize the closed top electrolytic cell depicted in
Figure 1 or any of the other cells disclosed herein, pre-
ferably with the top closed or adequate provision being
made to pre~ent. (a) moisture from contac-ting the chloride
electrolyte, or (b~ oxidation of the aluminum chloride,
while containing the vaporized bath salts. The benefits
of using the chloride containing bath are derived not only
from the continuous in situ production of aluminum ions
at the anode but also from the use of a substantially lower
energy requirement to produce a high quality aluminum with
the total absence of chlorine gas exiting from the cell.
; The continuous production of aluminum ion at the anode
20~ is brought about through the formation of the anode from
~ ~ :
an aluminous material containing aluminum oxide and a re-
ducing agent. This anode is immersed in a molten bath
containlng alkali metal and~or alkaline earth metal halide
salts of any composition provided that aluminum chloride
is present in the bath. Upon electrolysis, ionized aluminum
in the bath is deposited as aluminum metal on the cathode
while the reaction at the anode also forms CO2 in addition
to the aluminum ion. The aluminum is collected as molten
aluminum and drawn o~f but it is the reac-tion at the anode
to reform aluminum ions that should be particularly noted.
! ~7~
It is possible the halogen chlorine, whether it is the
chloride ion~ atomic chlorine or chlorine gas may take part in
the chloride reaction with the aluminum oxide of the aluminous
material and the reducing agent of the anode to produce
aluminum ions plus the reducing agen~ oxide. Aluminum from
the anode is ioni~ed-in the molten bath for continuation of
the cycle and the anions which may be chloride, oxide or other,
maintain the charge balance with the aluminum ions.
The aluminum produced at the cathode generally is
as pure as the aluminous material forming the anode. It
is possible to produce ultrapure aluminum by u-tilizing
a very pure alumina source or to produce a slightly im-
pure aluminum by the direct use of aluminous ore materials
such as bauxite or aluminum bearing clays such as kaolin
or mixtures of these ores. In general it is possible to
obtain purity of aluminum of at least 99.5~.
::
It is known in the Hall-Heroult cell reaction that the
carbon of the anode contributes to the overall reaction of
: winning aluminum by decreasing the decomposition voltage of
A12O3, For example the decomposition of A12O3 in cryolite on
a platinum anode is about 2.2 volts but on a carbon electrode
considering about 50Vol% CO produced and 50% CO2, the
decomposition vol-tage is abou-t 1.2. Appro~imately~ the same
decomposition voltage is obtained from ~12O3 if methane is
injected under the pla~inum anode to produce mainly CO2.
23
,
~7~6~
The use of the composite anode resul-ts in a lower
decomposition voltage than would be obtained if AlC13 were
decomposed with -the discharge o C12 gas on the anode.
In any electrochemical reaction if the current voltage
curve is extrapolated to 0 current, a number approxima-
ting the decomposition voltage is obtained. In an aluminum
chloride electrolysis process when a graphite anode is
used, a decomposition of 1.8 to 2.0V can be obtained which
is consistent with values reported in the literature and
the theoretical value calculated from thermodynamics.
It was found that the decomposition voltage varies
slightly with electrolyte composition. With pure NaAlC14
the decomposition voltage is the lowest but as the AlC13
component of the electrolyte decreased, the decomposition
voltage tended to increase slightly. The lowest decomposi-
tion voltage obtained was 0.5 volts and the highest 1.5
volts. The average value was 1.2 volts~ Utilizing the
most prevalent average value of 1.2 decomposition voltage,
;; 20 it ~an be ohserved that in the present invention the de-
composition voltage is less by 0.6 volts than that for
AlC13 when chlorine is discharged and the presently obtained
value approximates that of A12O3 and carbon which suggests
that the same overall reaction mechanism occurs both in
the Hall-Heroult cell and in the present inven-tion. This
lower decomposition voltage results in a considerable energy
saving for the elec-trolytic production of aluminum not
only compared to classical aluminum chloride systems where
chlorine is discharged at the anode but also when considering
30 the additional energy necessary to produce AlC13 from A12O3,
carbon and chlorine.
24
,
The process conditions for the electrolytic production
of aluminum have not been found to be criti~al with respect
to the voltage applied or the current density. The temperature
of the bath may vary considerably and is simply that necessary
to maintain the bath molten which, depending upon the
composition of the halide salts present may be achieved within
the temperature range of 150 to 1000C but generally may be
in the range of between the melting point of aluminum and the
boiling point of the cell components, preferably 10 to 400DC
and most preferably 10 to 150C up to less than 250C above
the melting point of the aluminum. The pressure conditions
within the enclosed cell are not critical particularly
inasmuch as there is no chlorine gas escaping as in prior art
aluminum chloride salt processes. While CO or CO2 or both may
be generated from the present process, these gases are not as
corrosive as chlorine. The pressure conditions, not being
; ~ important, may range from atmospheric to 10 or more psig.
b. All Fluoride Containing P,ath
The ~all cell operates chemically based upon the fact
that alumina will dissolve in the cryolite-fluoride salt bath
at a temperature of 950-1000C. Bayer alumina is soluble
in the cryolite containing bath at a minimum temperature of at
least 900C or above. Any fluoride containing bath at a
temperature belo~ about 900C will not readily solubili~e
ordinary processed Bayer alumina and, therefore, alumina, as
the source of aluminum, cannot enter the reduction reaction
,.. , ., , ~ .. ,~ . ..... . . ..
I ~7~
nor is it possi~le for aluminum to be deposited at the cathode.
Without this general solubility of alumina in the ~luoride
salt bath, it is no-t feasiblc to electrowin aluminum.
It has been discovered that in all fluoride containing
baths the temperatures may be in the range of between the melt-
ing point of aluminum and the boiling point of the cell com-
ponents, preferably 10-400C and most preferably 10 to 150C
up to less than 250C above the melting point of the aluminum.
10To electrowin aluminum from its corresponding oxide or other
oxygen containing compound the range of bath temperatures
generally would be about 670-800C and preferably 700-750C.
. I
The important aspect of this discovery which differentiates
it from the conventional procedures of the Hall-Heroult cell
lS that the composite anode containing the mix-ture of
aluminum oxide and reducing agent effects a transformation of
the aluminum oxide and produces ionic aluminum in the low
temperature fluoride bath. The overall reaction, however,
is believed to be essentially the same as the Hall cell reaction
20 ~ as prevlously stated. The aluminum is produced in liquid form
on the liquid metal pool serving as the cathode. It is
presumed that a reaction occurs at the anode surface in a
unique manner that results in the reaction of aluminum oxide to
produce aluminum ions similar to the mechanism that occurs in
the llall cell even though -the temperature is only slightly
above the melting point of aluminum.
~7~
The importance oE utilizing the composite anode in
the present invention should be quite clear because under
the same conditions as that of the present invention but
using a carbon or other non-consumable anode, the addi-tion
of aluminum oxide to the bath will not result in either
the dissolution of the aluminum oxide or the electro-
deposition of the aluminum. Utilizing the present com-
posite anode in a low temperature from 670-~00 with an
all fluoride electrolytic bath, the Hall cell can be operated
in a manner such as Figure 4 without the closed top re-
quired in the operation of the chloride bath as shown in
Figure 1. The bath composition, current densities and
other process parameters are not critical to the operation
of the chloride bath or fluoride bath containin~ cell.
~'7
The Anode
The anode provides the sole source of aluminum ions
for electrolytic reduction to aluminum at the cathode as well
as~ with a carbon reducing agent, the means to conduct
electrical current thxough the dielectric aluminum oxide
to the reaction site for the aluminum oxide in contact with
and immersed in the electrolyte. The anode also preferably
provides at least in part a necessary source of a reducing
agent that enables the aluminum oxide to react in the anodic
environment to produce the al~ninum for deposition at the
cathode as aluminum metal.
~; :
The reducing agent is preferably, at least in part,
I intermixed with the aluminum oxide to provide intimate contact
between the reduciny agent and the aluminum oxide. The reducing
agent, if properly selected, to be conductive may when inter- i
mixed with the aluminum oxide al50 fulfill the function of a
conductor of electrical current to the reaction site for the
aluminum oxide. Following the reaction of a particle of
aluminum oxide a-t a particular si-te in contact with the
electrolyte and having present an electrical current, another
particle at the same site now is uncovered and can react. This
¦pattern occurs throughout the surface oE the anode and continues
¦until there is no more aluminum oxide to react. If the reducing
agent is not conductive and is not intermixed with the aluminum
loxide, the electrical conductor function must be other~Jise
¦achieved by conductor rods to maintain the aluminum oxide anodic
¦at the reaction site.
- 28 -
~7~
In an aluminum chloride salt bath, the anode functions to
provide a reducing agent that aida in the theorized reaction of
the aluminous source with the chloride or oxygen or both to
maintain a constant concentration of aluminum chloride. That is,
the reaction which results from the dissolving of the A12O3-C
mixture releases Co or Co2 gas rather -than chlorine gas, so
that the chlorine of the aluminum chloride is not dissipated, as
occurs in former processes using aluminum chloride electrolyte.
The maintenance of a constant concentration of aluminum chloride
eliminates the necessity for any external replenishment of the
aluminum chloride being electrolyzed or the discharge of chlorine
on the anode.
In the all fluoride bath process, the anode of this inven-
tion as in the case of the chloride cycle provides the aluminum
oxide that reacts in the fluoride bath to form aluminum ions at
a uniquely low temperature in the 670-800C range. The cell
may also be open as in Figures 4, 5 or 7.
The source of the aluminum is alumina, A12O3, but also it
could be any aluminum oxide bearing material such as bauxite or
a e].ay such as kaolin or other material which would react at the
anode to produce aluminum ions to be reduced to the molten metal
at the eathode as in the fluoride or chloride cycle processes.
When the intermixture forms the anode, the ratio is in an
amount that ranges from at least 1.5 up, with acceptable with ac-
ceptable upper limits of 7.5, 20.0 or even 50~0 or more parts by
weight of aluminum oxide in the aluminous material per part of the
weight of the redueing agen-t. Preferably, for the purposes of the
present invention, the amount of aluminum oxide in the aluminous
material intermixture will be 2.0 to 6.5 and mos-t preferably
2.5-6.0 parts by weight aluminum oxide per part redueing agent.
29
" il ~
~7?~
The reducing ayent that may be used in accordance with
¦the present invention is not limited to any particular material,
but could be any of those materials known to be effective to
l react with the aluminum oxide. Th~ reaction in the fluoride and
! chloride baths is not clearly defined but it may be that the
reducing agent reacts with the A12O3 to produce aluminum ions
¦that eventually deposit on the cathode and CO2 at the anode.
The reaction mechanism may be the same in all chloride, all
fluoride or mixed chloride/fluoride salt electrolytes.
¦ Among the reducing agents that are particularly useful
for alumina and other oxides are carbon or a reducing
carbon compound used in the intermixture. Sulfur, phosphorUS
or arsenic may also be used independently or in combination
with carbon. Carbon is particularly preferred because it
characteristically has the dual capability of carrying current ;
to the reaction site of the aluminum oxide as well as main-
;~ taining a reducing function and giving off a gaseous product
at the anode.
~ ,
The source of carbon in the intermixture can be any
~20 organic material particularly those having a fossil origin such ;
as tar, pitch, coal and coal products, reducing gases, ~or
example carbon monoxide, and may also include natural and
synthetic resinous materials such as the waxes, gums, phenolics,
epoxies, vinyls, etcO and the like which may if desired be
cokcd even while in the presence o~ the aluminous material.
Coking of the carbon source intermixed with the aluminum oxide
compound can be accomplished by known art techniques such as
those used in prebaked anodes that are utilized in the Hall-
Heroult cell. This is accomplished by castiny, molding,
extruding, etc., a composite anode such as A12O3-pitch in the
~ 30
&~
desired ratio of, for example 6.5 parts aluminum oxide to
¦one part carbon in the coked condition, and slowly heating the
formed anode in a nonoxidizing atmosphere to a coking temperature
of 700 to 1200C. After coking, the composite anode is then
ready for use.
It is also, for instance, contemplated within the scope
of the present invention to produce carbon as a reducing agent
in the intermixture with aluminum oxide by coking the carbon
source in the molten electrolytic bath both prior to and during
electrolysis. Bath temperatures typically in the range of
670 to 850C are adequate to coke the carbon source to produce
the carbon necessary. The time to achieve such coking is not
critical but it may require several minutes to several hours
depending upon the temperature of the molten bath and the mass
of the mixture of aluminous source and the reducing carbon
source.
~ ~ : .
I Continuous coking is possible using the attachment clamp
of Figure 4 by introducing one anode on top of the last and as
consumption occurs the anode is continuously lowered until
one is completely consumed and the next takes its place,
and so on. The anode may be fed continuously to the cell
in the green state as in the case of a traditional Soderberg
electrode. In this case, steel pins are traditionally used to
make contact but the contacts could also be graphite, carbon,
TiB2, alun~inum or composites of these. The green composite
anode material is gradually coked from the heat of the cell
~ i
~l7~
such that the end of the anocle in -the salt is always fully
coked to the operating temperature of -the cell. Coking in the
Soderberg fashion in the cel:L at 670 - ~50 produces a lower
conductivity anode compared 1:o composite anodes prebaked at
much higher temperatures.
The source of the entire reducing agent, as previously
stated, need not be intermixed with the aluminum oxide source
to form the anode. It has been found, for instance, that the
only requirements for the reclucing agent are that it be in
contact with the anodic alum:inum oxide and present in sufficient
amounts to produce aluminum metal at the cathode. It is
manifest however that electr:ic current must be transmitted
to the reaction site to enab:Le -the reaction to proceed.
If the reducing agent such as carbon is not intermixed
with aluminum oxide to carry the current, it is conceivable
that another conductor, compatible with the cell and its
contents, could be used. For instance, aluminum or noble
metals or high melting condustive oxides such as silver-tin
oxide or Tis2, either alone or as composites with carbon
or graphite, may be intermixed with the aluminum oxide in
amounts merely sufficient to carry electric current to the
reaction site. Such amount :Ls not critical provided the
aluminum-o~ide is made anodic at that reaction site. Amounts
as low as about O.OOl up to at least about 0.75 parts
conductive material per part aluminum oxide may be used.
Greater amounts increase the conductivity at the expense of
the availability of the reac1:ive material but are possible
without any actual upper lim:Lt. Of course, there still
must be present a reducing agent to achieve the necessary
reaction.
~7;?~6q~ 1
~ he C~(! Or cllUlllirla .1.; tZ~C alunlinou~; material, thc
use of hydratecl or calcined alumilla may be used. Anodes
Lormed ~rom hydratcd alumina can show improvecl conductivity
compared to calcined alumina but hydrated alumina, A12O3 x
3~20 or Al (Oll)3 has the tenclency to crack during prebaked
type coking and whell placed in the hot bath, due to the
water driven off during the coking operation.With an aluminum
chloride containing salt utilizing an in bath coking of the
hydrated alumina the water driven off could undesirably
hydrolyze the AlC13.
Any cracking or breaking of the anode due to the
expelled moisture causes no difficulty provided the membrane
as shown in Figure 5 surrounds the anode. Any particles of
the anode that drop off will be cont~ained in the membrane
for continual reaction. The anode may also contain any proportion
of hydrated and calcined oxide to minimize the cracking.
The maximum amount of hydra-ted oxide that can be used affects
an energy saving in calcining.
The size and surface area of the particles making up
~20~ the anode containing the aluminum oxide have no-t shown
any sensitivity regaxding anode reaction rate. This char-
1~ ~acteristic is in contrast to prior art experience in the
reaction of A12O3 and carbon with chlorine as a gas-solid
reaction in a furnace. In the past it has been found that
the reaction temperature and rate are highly sensitive to
the particle size and particle surface areas.
. I
It is generally desired in the prior art to utili~e
alumina with a surface area in the range of 10 to 125 m2/g
in the AlC13 reaction. I-~owever, in the present invention,
33
:~7~
~no sensitivity was detect2d with regard to reaction rate of
the-anode based upon particle size or surface area. That
~is, A12o3 with a surface area of .5m2/g or less apparently
reacted as readily as A12O3 with a surface area of 100
m /g. These results are based upon experiments run with
anodes containing alumina having particles with differing
surface area and sizes. Anode current densities ranging from
2 to 40 amps/in2 were run in cells with the exhaust line
jconnected to a starch-iodine indicator for chlorine detection.
¦NO chlorine gas was detected regardless of the curren-t
¦density or the surface area of the alumina. This suggests
¦that if any chlorine is produced at the anode it all reacts
to reform aluminum chloride or that only aluminum ions form
at the anode from the A12O3 while the oxygen from the A12O3
combines with the carbon producing CO2. It is believed that
to produce chlorine at the anode it would be necessary to
raise the potential so high as to overcome the decomposition
potential of the AlC13 but even then the produced chlorine would
probably react with the A12O3 and carbon to produce more AlC13
rather than evolve chlorine at the anode.
...
~ I
1~'7261!10
Anodes for use in electrolysis cells may be produced
in a variety of forms and by a variety of fabrication processes.
mixture of aluminum oxide material and the reducing agent may
form the anode in any convenient manner. For instance, a
mixture may be bonded to a typical electrode to form a
coating surrounding all or one side of the electrode as shown
in Figure 2 of the drawings~ It is also contemplated that the
~node material may form the anode on being moulded or otherwise
formed into a suitable shape to which is attached one end of the
lQ electrode rod or pin in the manner shown in Figure 3 of the
drawings. It is also possible to meet the requirements of the
present invention to form the anode in the manner other than
having any physical bonding directly to the electrode. It is I~
desirable, however, that the aluminous material be in intima~e i
physical contact with the carbonaceous material or other
reducer. The latter concept may be brought into being if the
~` ~ mixtures of the aluminous material and reducer are in the form
of a homogeneous mixture of powders, small pellets of the mi~ed
~ powders, or larger composite briquettes of such mixed materials
20~ that may have been formed by molding or extrusion into various
sizes from .001 inch to 1 inch or more. Uniformity of the
distribution of the carbon and aluminum oxide has been found
to be desirable to attain maximum anode efficiency during its
dissolution or reaction under electrolysis.
::
To hold the aluminous material and the reducing agent
forming the anodic materials in the region of the electrode
and thus in combination forming the anode, a container in the
form of a porous membrane may be utilized.
I
l 3S
~1
o " li I .
7~6~0
I
I .
For successful commercial use, the anode should be as
conductive as possible. Since the anode of the present
invention is not solid or pure carbon as is traditionally used
in the Hall cell, it will be less conductive because of the
presence of the aluminous compound~ If the anode were permitted
to become as resistive as the salt electrolyte then the heat
balance can be affected due to overheating that can occur as a
result of passing the same current through the more resistive
anode. For instance, when using a solid composite anode such
as shown in Figure 3 in the cell of Figure 1, it is necessary
for the electric current to travel through the anode from top
to bottom, with power losses translated to heating of the bath.
It is therefore desirable to construct an anode to have as high
a conductivity as possible. Obviously, the more conductive
the anode material, the lower the power consumption for winning
metal but in any event the conductivity of the anode should be
greater than the conductivity of the salt for optimum operation.
Particularly when it is desired to achieve the goal of maximum
production of aluminum with minimum power usage, the resistance
of the anode becomes significant.
I
It has been found that the conductivity of the anode
varies considerably depending on the manufacturing process.
The parameters which have been found to affect conductivity are
the ratio of binder carbon material such as pitch, carbon or
coke particles included in tlle comuosite anode as the source
of the reducing agent and the type of aluminum oxide. The
greater the carbon content of the anode, within the previously
specified ratio of aluminum oxide to reducing agent, the greater
~1 .
36
I
7~6
~1 '
the conductivity. It is possible, for example, when using
j¦ a ratio in the range of 4/l to 6/l aluminum oxide to carbon
¦ to construct a solid composite anode that has at least a tenth
¦ the conductivity of a standard Hall-lleroult anode.
In order to reduce the power loss through the composite
anode, several al-ternatives are also shown in Figures 2, 2A,
2B and 4A.
To achieve higher conductivity and reduce power loss
¦ through the composite prebaked anode another embodiment utilizes
.0 1 one or more conductive cores 36 or 37 positioned in the anode
11 as shown in Figures 2, 2A and 2~ or a plurality of vertically-
I ¦ arranged conductor pins 39 as shown in Figure 4A.
The composite anode 26A shown in Figure 2 has a conductive
I central core 36 that can be carbon or graphite molded into the
: composite anode with the composite anode material
38 molded or coated into an annulus thereabout.
he central core 36 may also be a metal such as the same
: I metal being deposited, for example, aluminum. The exterior
of the conductor 36 is coated on one side for bipolar use or
~O surrounded on both sides for monopolar use by a matrix 38 of
composite anode material comprising the mixture of aluminum
oxides and reducing agent as previously described. ~1hen
coated 011 a single side a bipolar operation is anticipated.
The term "oxides" should be i.nterpreted to include the silicates
which often are a combination of the metal oxide and silicon
oxide or any other oxygen corltaining compound of the aluminum
¦ to be ~pos.ited.
' 37
6~ 1
For larcJc siz~ anodes anl3tller altcrnate embodiment isshown in Figure 2~ and 2B. To improve conductivity, primary
grade purity alu~inum rods 36 and 37 are preferred to be
used as electrical conduction buses in a matrix of the composite
anode composition 38 that may be of the prebaked or Soderberg
type. Since primary grade aluminum is used to form the
conductor rods, it will melt as the anode is consumed and
join the cathode metal for a continuous cycle. The rods
are spaced such that the voltage drop is minimized relative
to the conductivity of the composite anode. In Figure 2B
the conductor rods 36 are shown to be connected to a plate 40
supported by a central conductor 41.
The number and size of the conductors 36 and 37 are
selected based on anode size, current density of the anode,
cell size, operating temperature and heat transfer such that
the aluminum conductors 36 and 37 melt at the same rate that
the matrix 38 of the anode is consumed. T~e~ unique advantage
of the anode embodiments shown in Figures 2A and 2B is the
avoidance of large voltage drops in the relatively highly
resistive anode so as to permit the process to be operated
at substantially reduced power consumption. The size of the
aluminum rods may fall within the diameter range of 0.0625 to 3.0 !
inches preferably 0.125 to 2.0 inches most preferably 0.25
to l.0 inch.
For achieving desirable conductivity in the anode,
the spacing between the outer surface of the composite anode
material 38 and the surface of any aluminum rod as in Figure
2, 2A or 2B and spacing between the outer surfaces of these
i~7~600
al~ninum rods in Figure 2A and 2B is not critical and may
ranye from 0.125 to 2~ inches, preferably 1.0 to 6.0 inches
and most preferably 1.5 to 4.0 inches. As an example, if
the conductivity of the composite anode is approximately 0.1
of a standard prebaked Hall cell anode then aluminum rod
spacing of approximately 3.0 inches will result in an acceptable
voltage dropO
Since the operating tempera-ture of the cell is usually
in the 700 - 750C range the aluminum rods can be sized
such that they will melt approximately at the same rate as
the anode is consumed and will thus conduct power to the
bottom of the anode. If the diameter of the al~ninum rod is
too large, it will not melt and salt will freeze over its
surface which results in the anode being consumed leaving an
aluminum stub that will short to the cathode as the anode is
advanced. If the rod diameter is too small it will melt
back too far into the anode which results in too large a
voltage drop due to the longer conductivity path. It is
desirable that the aluminum rods melt back into the anode to
a slight degree rather than remaining flush with the bottom ¦`
surface of the anode. This is so that anodic oxidation of
the aluminum rods will be minimized. Desirable melt back
distance is based upon that which provldes the minimum
voltage drop coupled with the minimum anodic oxidation of
the aluminum rods. Should the rods remain flush with the
bottom surface of tl-e anode, there would be a tendency for
aluminum ions to ~ass into the bath from the rods (as in a
refining operation) as well as from the composite anode
material, thus lowering the cell's Faradaic efficiency.
39
llcat call hc b~ cecl such t]Ult ~he concluctance from the bath
up throu~h the anode and power generated through the conduetors
¦is balanced to achieve the desired amoun-t of melting of the
¦conductor aluminum rods.
¦ Figure 3 discloses anothcr alternative embodiment of 1.
the composition of an anode el.ectrode as shown at 26B. In
the embodiment electrode 26B i.s composed of a composite 38
which is the same material as the coating 38 in Figure 2 and is
l formed into a suitable shape f'or'use as an electrode. This
1 form of the electrode may be molded about a stub or pin
electrode 42 which extends out. from the upper end of the
body of the electrode 26B for connection of the usual electrieal
circuit. Alternatively electrode 26B i5 molded and then stub
; : I 42 is inserted by known art techniques such as utili~ed with
l pre.~aked Hall cell anodes.
:: i
Another alternative emhocliment is that shown in Figure
4A wherein an anode 26B is ill.ustrated having the same
: eomposition as previously described but being provided with
a plurality of pairs of vertically arranged co~ductor pins
39 to which lower pair of pins~ a bus 39A is attached direetly
as in the Soderberg type connections. The positioning of
the pins 39 is not critieal and they may be perpendicular to
the anode axis or angled as shown. As the composite anode
material is consumed the bus c:onnections 39A are moved
u~wardly to the next higher pair of pins. The pins being
composed o~ primary ~rade aluminum are also consumed and as
previously mentioned are addecl to the pool of deposited
.
~7~6~ 1
alumillum. ~ltern~tivcly thc L)ills may be of iron as typically
used in the Soderberg connection and the pins are removed as the
anode is consumcd.
The embodiments of Figures 12 and 13 illustrate a variation
of conducting electrical energy to the working surface of the
anodeO As shown blocks 94Of the composite anode A are laminated
with sheets of aluminum metal 108. These sheets act precisely
as the aluminum rods in Figures 2~ and 2B. The shape and
number of laminae are not critical~ The blocks may lie in
continuous form and fed into the cell through clamp 47 as in i
Figure 4 to which along with the aluminum sheets 108, the
electrical connection is made.
. 1l
The number, spacing and thickness of the aluminum sheets 108 ¦
are determined by the same factors as described with respect
to the condNctors 36 and 37. Generally the aluminum sheet
thickness wlll range from .001 to .5 inches thick and
preferably .010 to .375 inches and most preferably .010 to .25
inches thick. The aluminum sheets must be of sufficient
I thickness to conduct the necessary current to avoid ~ajor
voltage drop and also melt into the cathode pool as the anode
is consumed. The spacing between the aluminum sheets 108 is
such as to avoid excessive voltage drop through the composite
block as set forth with respect to conductors 36 and 37.
Generally the spacing will range from .125 to 24.0 inches
prefera~ly from 1.0 to 6.0 inches and most pref2rably 1.5 - 4.0
inche~.
:~7~6~D~
The Membrane
The membrane as shown in Figure 5 of the drawings is
designed to have a tripartite function or capabili-ty.
First, the membrane acts as a separator or quiescent
barrier between the molten cathodic me-tal phase and the
source of anode material to be electrolyzed. With the use
of the membrance, the spacing can be reduced substantially
to achieve significant increases in conductibility and
efficiency without any turbulent effects that could other-
wise produce a reduction in the efficiency or quality of
the aluminum product.
~ ~ .
Second, the membrane physically restrains materials
of the composite anode that, fo~ instance, may include the
aluminous raw material and the reducing agent. This
restraint maintains these materials close to the electrode
.
to form an anode for production of aluminum ions in the most
efficient manner. The membrane also prevents mixing of the
raw materials with the molten aluminum at the cell bottom.
Should a hydrated metal oxide, such as the hydrated alumina,
be used as one of the anodic materials, the membrane holds
any of the pieces of the anode that may crack off due to
the evolution ~f moisture from the alumina during bath
coking. These pieces continue to be a source of aluminum
through the reduction reaction as long as they are within
the anode circuit within the membrane.
42
I
7~ 0
Third, the membrane permits the free passage of ionic
substances and dissolved solids in the electrolyte but will not
pass and will substantially reject molten aluminum and
undissolved solid materials that constitute the usual
impurities present in the aluminous source and prevent the
contamination of the cathodic deposition.
The external shape of the membrane is not important
and may be in the form of a cylinder, prism, etc., or portion
thereof. For instance, the membrane may have a three or four- ¦
sided shape with a bottom and thus form an enclosed container.
This container is so designed to hold the anodic raw materials
for reaction in the salt bath.
Due to the corrosive nature of the molten salt bath,
the selection of the materials to form the membrane is important
to the life of the cell and the success of the process. If
the electrolyte to be used is an all chloride bath, the
choices for the membrane are sornewhat greater due to the
reduced corrosive character of such a bath as compared to a
bath containing fluorides. Baths containing some fluorides
"20 are preferred, however, because of their lower volatility.
42a
.7,~6~
11 1
`
The all fluoride bath possesses other advantages as set forth
above. Materials suitable for use in a fluoride bath would
of course be useful in the less corrosive chloride bath.
.
Among the materials that have been found to be useful
I include vitreous carbon foam, carbon or graphite as a porous
solid or porous solids of refractory hard metals such as:
the nitrides of boron, aluminum, silicon (including the
oxynitride), titanium, hafnium, zirconium and tantalum; the
~ silicides of molybdenum, tantalum and tungsten; the carbides
of hafnium, tantalum, columbium, zirconium, titanium, silicon,
boron and tungsten; and the borides of hafnium, tantalum,
zirconium, columbium, titanium and silicon. Other refractory
hard metals as known in the art may be found useful to form
the membrane provided that they are resistant to the molten
salt bath.
'~ : il
; The refractory hard metals forming the membrane of the
present invention may be made into the form of a cloth, mat,
felt, foam, porous sintered solid base or simply a coating on
~; such a ~ase, all of which are known in the art for other
purposes. The membrane must also meet particular standards
of through passage porosity and connected pore si2e.
:~
These two characteristics may be defined as follows:
: :~
through passage porosity - the percentage of the total
volume o~ the men~rane that is made up of passages
that pass through from one side of the membrane to the
other;
43
jl
7~6~0
connected p~re s-ze - the smallesL dlameter of a
passage through the membrane.
The through passage porosity varies with the nature of
the membrane material, the temperature of the molten bath and
the salt composition but the common characteristic of useful
membranes is that the porosity must be sufficient to pass
all the metal ions such as aluminum and all the electrolyte
salts without passing the undissolved impurities. It has been
found that the greater the porosity, the greater is the current ;
flow and, therefore, the greater the electrical efficiency of the
cell. The porosity may vary from 1% to 97% or more, but
generally is in the range of 30~ to 70~. The preferred
porosity to achieve the greatest efficiency is in the 90% to
97% range. A vitreous carbon foam, for instance, is capable
of yielding such a high porosity and retain sufficient mechanical
streng~h.
~ . '
The connected pore size must be small enough to reject
the solid impurities that have not been dissolved but large
~ enough to pass the ionic and dissolved particles. Generally,
the acceptable pore size is between one micron ~nd one cm.
The thickness oE the membrane material is a function of
its porosity, pore size and ability to retain undissolved
impure solids and molten metal~ Obviously the thicker the
membra , tbe grea-er the eloctrical reslstance. It is
44
~ l l
` I ~L~7~
therefore eslrable to use as thin a membra~e as is practical
consistent with the porosity and pore size standards as well
¦as the mechanical strength of the membrane in position in the
¦cell. The preferable thickn~ss is .125 to .5 inch but may be
as thick as 2.0 inches or more.
Typical membrane materials that have been found useful
include but are not limited to vitreous carbon foam, carbon or
graphite in the form of a porous solid, felt or cloth, aluminum
nitride, silicon nitride, silicon carbide, silicon oxynitride,
boron nitride and titaniwn nitride as a porous solid, as a
cloth or as a coating on the surface of a vitreous carbon foam
or porous graphi-te. Aluminum nitride appears to be the most
desirable material. It has been found that aluminum nitride
; can conveniently be formed in a porous structure by first
making a porous alumina structure then impregnating with carbon
followed by heating to 1750C in a nitrogen atmosphere to
convert the alumina to aluminum nltride. Such a procedure
results in a strong porous structure that is chemically
compatible with the corrosive salt environment and the molten
~20 aluminum.
~ ~:
.~ The Molten Bath Composition
.
The electrolytic bath of the present invention can vary
considerably in comparison to the -typical Hall cell salt
comyosition. In the ~resent invention the bath composition
may include any halide salt, particularly, chloride and fluoride
are favored. Any alkaki or alkaline earth metal such as
particularly sodium, potassium, lithium, calcium, magnesium,
barium and the like may be used to form the halide salts.
.
~ 45
~7~
There is no critical composition or range of proportions desired
or necessary. It has been observed that no aluminum salt
need initially be present in the electrolyte to produce
aluminum under electrolysis utilizing the composi-te anode.
For example, a salt electrolyte containing only alkali and/or
alkaline earth halides will produce aluminum metal at the
cathode utiliziny the composite anode and with no l'anode effect~"
It is generally preferred for the salt bath to initially
contain an aluminum halide, although this is not necessary to
practice the invention. In the case of the AlC13 containing
baths which may contain only chloride anions or both chloride
and fluoride anions, the aluminum chloride concentration may be
2 to 60% but may also be in the range of 1~ to 95% by weight
AlC13. The all fluoride bath may include the same fluoride
; ~ salts as set forth above and may as well contain aluminum
fluoride in any proportion desired.
~ .
Among the advantages and disadvantages of the various
electrolyte -types are that the all chloride bath has very low
tolerance to oxide contamination, but has very high conductivity
and is t least,corrosive to refractories and cell components.
46
l .'
In fluoride containing electrolytes the
aluminum deposits as droplets which agglomerate and pool
readily, but the corrosivity of the electroly-te to refractories
and cell components is greatly increased. ;
A lithium component of any electrolyte will increase
the conductivity but is expensive and increases the cost of the
electrolyte. This has to be balanced in any operation as to
the electrolyte cost, conductivity of the electrolyte and the
resultant power consumption of producing the aluminum.
I;
The preferred electrolyte is a balance of economics of
the salt components, conductivity, corrosiveness to refractories
¦ and cell components, tolerance to oxide contamination and
; I agglomeration of the deposited aluminum into a pool for easy
harvesting.
'~ ,
~ ~ 47
-``` o
EXA~IPLES
Example l
.
In Figure 1, the anodes are graphite plates and the
cathode electrode is a titanium diboride plate. The anodes 1
were prepared with a coating 38 as shown in Figure 2 which ,,
consisted of Bayer Process purified A12O3 calcined to 1000C ,
and mixed in a wei~ht proportion of fiv~ parts A12O3 to one
part carbon in the coked stage. The carbon was obtained by
mixing the A12o3 ~ith a phenolic resin and gradually heating
to 1000C in an inert atmosphere for coking the phenolic
resin to carbon. The electrode coating was prepared by mixing
the A12O3 and phenolic resin, truwelling or otherwise applying
the mixture on the electrode, and heating to coking temperature.
:
The ele~trolyte consisted of an e~uimolar mixture of
sodium chloride and aluminum chloride forming the double
salt NaAlC14 at about 150C. The temperature of -the cell
was raised to 700C and electrolysis of the A12O3 conducted
for several hours which produced a layer of molten aluminum
~ on the bottom of the cell. Examinakion of the anode revealed
20~ that the coating had dissolved and aluminum was deposited at
the cathode. This deposition of aluminum was equivalent to
the aluminum content of the A12O3 dissolved at the anode.
The overall controlling reaction is believed to be the ionization
of the ~12O in the anode with the carbon reacting to form
primarily CO2. During the electrolysis there was no evidence
of any chlorine gas being liberated at the anodes and in the
exit tube. The exit gas was analyzed and determined to be
primarily CO2.
".:
u`- 48
~L~7~
l~xam~le 2
.
The electrolyte salt composition consisted of 63% NaCl,
17% LiCl, 10% Li~, 10% AlC13 and the electrode coating of
Figure 2 was prepared from standard bauxite ~1203 and a
petroleum tar pitch which was coked to produce an A1203 to
carbon (as coked) ratio of 5.7 to 1. The electrolysis was
conducted in the Figure 1 cell at a temperature of 750C.
The spacing between anode and cathode was 1/2 inch whi~h
produced an electrode current density of 15 amps/in2 at an
imposed voltage of 2.5 volts. There was no chlorine gas
detected as being released from the anode which is indicative
of the A1203 in the bauxite reacting so as to prevent any free
chlorine from being formed in the anodic cycle. Aluminum
was deposited which settled to the bottom oE the cell. The
harvested aluminum was produced at a Faradaic efficiency of
92~ with an energy consumption o~ 3.67 Kwh/lb.
:: .
Example 3
: ~:
~::
I The electrolyte salt composition consisted of 10% NaC1,
50~ CaC12, 20% CaF2, 20% AlCl30 The electrode coating of
Figure 2 was prepared as in Example 2 but only on one side
of the electrode. The electrical connections were made such
that the anode adjacent to the exit -tube was connected to
the positive terminal and the negative terminal to the
electrode most remote to the exit tube. The coated sides of
the clectrodes 25 and 26 each faccd away from the exit tube
and toward the cathode. Electrode 24, the cathode, was not
coated. This results in electrode 25 not being physically
connected to the direct current power supply. That electrode
¦then becomes bipolar. The side coa-ted with the A1~03-C
- 49 -
7~6~
mixture is thus positively charged. The side of bipolar
electrode 25 nearest the cxit tu~e becomes negatively charged
upon which aluminum is deposited and sinks into the molten
pool~ Aluminum also deposits on the negatively charged
electrode 24 and sinks into the molten pool. The temperature
of the cell operation was 800C and the imposed voltage was
3 volts with respect to each electrode or a total of 6 volts
across the terminals. This imposed voltage with an electrode
spacing of 3/4 inch resulted in an electrode current density
of 12 amps/in2.
xam~le 4
:~ :
:
The anode electrodes were composed of titanium diboride
rods and the cathode electrode was also titanium diboride.
The anodes were coated with bauxite as in Figure 2 which has
been calcined at 600C, mixed with phenolic resin, and coked ,
; at 800C. The ratlo of aluminum oxide in the bauxite to
~; carbon after coking was 6 to 1. The electrolyte salt composition
; was 20% NaCl, 30% CaC12, 10~ CaF2, 4% NaF, 36~ AlC13 and
was operated at 750C at an electrode density of 15 amps/in2.
This resulted in 4 volts at an electrode spacing approximately
; ~
3/4 inch~ No chlorine gas was observed in the discharge
exit port which shows that if any chlorine was generated at
the anode it reacted with the bauxite to reform metal chlorides
which were then deposited as metal at the cathode. The
composition of the aluminum deposited in the molten pool was
97% pure COntaininCJ .5% Si, 1.5% Fe and .9% Ti with minor
other constituents.
-$ I
~ 7~
The electrolyte salt composition consisted of 65%
CaC12, 20% CaF2, 5~ NaF, and 10~ AlC13. The anode electrodes
were as shown in Figure 3 made an aluminum oxide to carbon
ratio of 5.5 t,o 1 using a copper bus pin. The aluminum
oxide was commercial grade Alco~ ~-1 and the carbon was
obtained from a mixture of phenolic and pitch which was
coked to 1100C. Electrolysis in a cell as shown in Figure
1 produced aluminum metal that settled into the pool at the
~10 bottom of the cell. No chlorine gas was detected in the
exit tube. The aluminum produced had a purity of 99.9%.
:
~xample 6
~ , ~ .
The electrolyte salt composition consisted of 30% NaCl,
8% LiCl, 27% CaC12, 20~ CaF2, 10~ I,iF and S~ AlC13. The
anode electrodes were graphite coated with a clay mineral
kaolin and carbon as in Figure 2 to yield a ratio of 5.6
~ A12o3 in the clay to 1 carbon after coking. Electrolysis
; yielded aluminum without any chlorine gas being detected in
the exit tube while the anode coating dissolved as a result
of electrolysis.
* Trademark
:
~ I
~72~
I
Example 7
The electrolyte of ~xample 4 was used and the anode
electrode of Figure 2 was prepared by mixing bauxite and a
phenolic resin in a consis-tency to approximate that of a
viscous gel and which would yield a ratio of contained
aluminum oxide to carbon of 5.5 to 1 upon coking. The
¦bauxite-phenolic was troweled onto the graphite for use as
an anode and dried to 150C which produced a hard coating
but not one fully cured. The electrode was then gradually
lowered into the salt electrolyte which was at a temperature
of 780C. After a five minute period to allow volatiles
¦from the phenolic to escape and coking to occur, electrolysis
¦was conducted which produced aluminum and anode dissolution
without the evolution of any chlorine gas in the exi-t tube.
:
~:
.
The cell in Figure 5 utilized a porous membrane of
aluminum nitride material 3/16 inch thick haviny 50% porosity
with a pore size in the range of 12 to 24 microns. The
aluminum nitride was obtained by impregnating an alumina
~20 porous body with carbon and then heating to 1750C in a
nitrogen atmosphere. The anode conductor was a graphite rod
and the anode aluminous material was a Bayer A12O3 and
carbon mixed powder in a ratio of 6 to 1. The electrolyte
salt COMpOSitiOn was 20~ NaCl, 25% LiCl, 30% LiF, 25% AlC13
and electrolysis was conducted at 720C. The spacing between
the membrane and the aluminum pool was approximately 1/2
inch and electrolysis was run at an anode current density of
1 .
~ 1 52
l . I
- l~ :
~ ll
10 amps/in2. T~lis resulted in a voltage of 2.8 d The aluminum
was produced at an efficiency of 92~ and had a purity of
99>5%.
Example 9
A salt composition consisting of 12~ NaF, 25% LiF, 28%
NaCl, 15~ LiCl, 10% AlF3 and 10~ AlC13 was rnelted into a
cell with straight side walls. A 2" thick aluminum pad was
melted on the bottom of the cell and the operation temperature
was adjusted to 700~C. Utilizing an anode as shown in
~10 Figure 2A a spacing between the bottom of the anode and the
aluminum pad of 1-3/4 inches was set. At an anode current
density of 6 amps/in2 the cell potential was 3.5 volts.
After 8 hours electrolysis, the anode was removed from
~ ~ the cell and placed in a cell with 45 side walls surh as
;~ ~ shown in Figure 7. After a few hours electrolysis the anode
~; ¦had eroded such that its sides were parallel to the cathode
side walls. The anode immersion depth in the salt was three
~; inches. Utilizing the same total curren~ as in the straight
sided cell, the potential was 2.45 volts. This lowered
~0 potential due to side anode erosion at 90% current efficiency
and at a constant production rate reduces the power consumption
from 5.25 kwh/lb to 3.68 kwh/lb which is a reduction o~ 1.57
- kwh/lb.
53
-1
~
~7~6~i
l'xaml)le 10
A salt composition consisting of 20~ NaCl, 25~ LiC1,
25% LiF, 10~ NaF, 10% A1~3, 10% AlC13 was melted in a cell
with straight side walls. ~ 2" thick aluminum pad was
melted on the bottom o~ the cell and the operatio~ temperature
adjusted to 700~C. Utilizing a composite anode 12 inches
long with a copper bus bar fitted into one end in the traditional :
manner, the anode-cathode spacing was adjusted to 1-3/4
inches. At an anode current density of 6 amps/in2 the cell
potential was 5.75 v. after equilibrium had been reached.
An identical anode but with a 45 slope on the end :
opposite the bus bar was inserted into a cell with 45 side
walls such as shown in Figure 7. Immersion depth of the
anode in the salt was three inches. After a few hours
electrolysis to assure the angle on the anode was the same
as the side walls of the cell the potential required to
achieve the same total current as had baen used in the
straight sided cell was 4.4 v. This lowered potential due
to side anode erosion at 90~ curren-t efficiency and at
constant production rate between the two cell types, reduces
the power consumption from 8.63 kwh~lb to 6.6 kwh/lb which
is a reduction of 2.03 kwh/lb.
The reduction in power consump-tion at a cons-tant current
between that used in the traditional cell where the
bottom o~ the anode only is erodcd and that in a sloped .
catllode cell is obvious from this example.
5~
,~
~ ~7~
~xam~le 11
An anode was made utili~ing ~lco~ ~ 1 A12O3 mixed with
cold tar pi-tch and phenolic and molded in a closed die with
heat applied to harden the phenolic component. The ratio
of components was such that after coking the composite anode
contained 17% carbon and 83~ Al o . The electrolyte consisted
of 20~ NaCl, 25% LiCl, 30% LiF and 25% NaF and was
operated at 700C. A cell as shown in Figure 7 was used but no
aluminum pad was'added. After several hours electrolysis at
about 700~C aluminum collected in the well showing the
composite anode will produce aluminum under electrolysis
without the initial use of an aluminum salt in the electrolyte.
These results suggest the reaction mechanism of the composite
anode is to release aluminum ions into the salt electrolyte
which are reduced at the cathode and the carbon in the anode
reacts to produce CO2O
xample 12
~'~
A straight sided cell such as shown in Figure S was
utilized, but without the membrane 68 and anode rod 52. Instead
~20 a carbon cylinder was mounted just above the salt electrolyte
layer into which was inserted a short anode section such as
shown in Figure 2A. On top of the prebaked short anode and
around the conductor rods 37 a mixture of A12O3, petrolPum coke
powder and a mixture of tars and pitches were added. The
mixtures of ~12O3~ coke powder and tars/pitches were such as to
yield 18% carbon and ~2~ Al~O3 when coked to the salt
electrolyte temperature of 740C. ~ salt electrolyte consisting
* Trademark
_~3
7~
1l .
of 10~ NaF, 25~ LiF, 20~ NaC1, 15% LiCl, 20% AlF3 and 10
AlC13 was utilized. Electrical connection was made to
conductor rods 37 with clips in the cool area above the level
of Soderberg type anode composition in the carbon cylinder.
Electrolysis was conducted at 10 amps/in2 anode current
density with a spacing of 1-1/4 inches between the aluminum
pool and anodeO Electrolysis was continued with additions of
A123-carbon-tar/pitch in the carbon cylinder and continuous
feed of the anode which hardened and coked as it ente~ed the
salt electrolyte. The aluminum conductor rods melted as the
anode was consumed and joined the cathode pool 32 of aluminum.
. .
A similar run was made utilizing a carbon rectangle
rectangular prebaked blocks and aluminum sheet as shown in
Figure 13. The aluminum sheets were .060 thick and the
prebaked blocks were 2.0 inches thick. Electrical connection
was made utilizing rollers on each aluminum sheet. As the anode
was advanced additional prebaked blocks were inserted between
the alur,~inum sheets.
.~. ~ .
~ I
