U.S. patent number 3,607,685 [Application Number 04/754,387] was granted by the patent office on 1971-09-21 for aluminum reduction cell and system for energy conservation therein.
Invention is credited to Arthur F. Johnson.
United States Patent |
3,607,685 |
Johnson |
September 21, 1971 |
ALUMINUM REDUCTION CELL AND SYSTEM FOR ENERGY CONSERVATION
THEREIN
Abstract
A compact electrolytic potcell for the reduction of aluminum in
fluoride fusions including a multiplicity of spaced and sized
electrical conductors extending from the reduced aluminum pad
underlying the fluoride fusion, through the refractory potlining,
and to an aluminum slab heat-sink on the potshell, the conductors
being molten where in contact with the pad and solid where in
contact with the aluminum slab and the temperature and voltage
difference of the slab relative to the pad being adjusted to as low
as feasible with cell operation to conserve energy for
reduction.
Inventors: |
Johnson; Arthur F. (Franklin
Lakes, NJ) |
Family
ID: |
25034574 |
Appl.
No.: |
04/754,387 |
Filed: |
August 21, 1968 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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528503 |
Feb 18, 1966 |
3434957 |
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Current U.S.
Class: |
205/374; 204/244;
204/247.4 |
Current CPC
Class: |
C25C
3/08 (20130101); C25C 3/085 (20130101) |
Current International
Class: |
C25C
3/00 (20060101); C25C 3/08 (20060101); C22d
003/12 (); C22d 003/02 () |
Field of
Search: |
;204/67,243-247,274 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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593,980 |
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Mar 1934 |
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DT |
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38,159 |
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Mar 1922 |
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NO |
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68,059 |
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Jul 1944 |
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NO |
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Primary Examiner: Mack; John H.
Assistant Examiner: Valentine; D. R.
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of my patent application
Ser. No. 528,503 filed Feb. 18, 1966, now U.S. Pat. No. 3,434,957.
Claims
I claim:
1. In a cell for the electrolytic reduction of aluminum from
fluoride fusions having an exterior metal potshell, an interior
refractory potlining for containing a reduced aluminum pad and the
overlying fusion, at least one anode for supplying current to the
fluoride fusion, a cathode heat-sink comprising an aluminum slab
and a multiplicity of spaced and sized conductors in the refractory
extending from the pad to the cathode heat-sink which is
electrically connected to an anode for another cell, said
conductors being molten where in contact with the pad, said
refractory being nonreactive with the fusion and aluminum, and said
slab being sufficiently thick to remove heat from the conductors to
maintain their attached portions solid.
2. A cell as defined in claim 1 which comprises aluminum conductors
encased in an electrically nonconductive refractory.
3. A cell as defined in claim 1 wherein said refractory potlining
consists essentially of hard, impervious, electrically
nonconductive, fused refractory.
4. A cell as defined in claim 3 wherein said refractory potlining
is formed of at least one of the materials alumina, calcium
fluoride, calcium oxide, aluminum nitride, aluminum fluoride,
sodium oxide and mixtures thereof.
5. A cell as defined in claim 4 in which the refractory is formed
essentially of a fusion of at least one of the materials dross,
bauxite, fluorspar, limestone and mixtures thereof.
6. A cell as defined in claim 1 wherein the cathode heat-sink
constituted at least part of the bottom of the potshell to which
the aluminum conductors are attached.
7. A cell as defined in claim 6 which comprises aluminum side and
end walls which are contiguous with the bottom slab and form a part
of the heat-sink the aluminum of the heat-sink being of such mass
as to freeze any of the fusion which may penetrate the potlining
refractory adjacent the potshell covered by the aluminum.
8. A cell as defined in claim 6 in which the heat-sink is insulated
to suppress the heat flow into it and has baffles restricting
atmospheric convection and heat insulation exterior to the
potshell.
9. A cell as defined in claim 7 in which the aluminum slab averages
at least 1 inch in thickness and covers at least most parts of
sides and ends and bottom so as to carry higher temperature heat in
the refractory enclosing the sides of the fluoride fusion to lower
temperature slab areas when they exist.
10. A cell as defined in claim 7 in which the entire potshell is
essentially aluminum and only such steel as needed for
reinforcement.
11. A cell as defined in claim 1 in which the current density
carried in the aluminum conductors averages between 200 and 1,000
amperes per square inch of cross section.
12. A cell as defined in claim 1 in which the cross section of an
aluminum conductor averages between 0.25 and 2 square inches.
13. A cell as defined in claim 1 in which the conductors are
essentially located in areas of the pad beneath the anode areas and
depend downward therefrom.
14. A cell as defined in claim 1 in which the number of conductors
averages at least 1 for every 2 square feet of anode area.
15. The cell of claim 1 in which the heat-sink is protected against
the attack of molten material in the cell by a thin, adherent
refractory coating.
16. In a cell for the electrolytic reduction of aluminum from
fluoride fusions having an exterior metal potshell and a bottom, a
lining within the potshell for containing a reduced aluminum pad
and the fluoride fusion, the improvement which comprises a first
refractory lining in contact with the pad and fluoride fusion which
is hard, impervious and electrically nonconductive, a second layer
of heat insulating refractory underlying said first refractory
lining and overlying the cell bottom, a slab of aluminum in contact
with the cell bottom, and a multiplicity of spaced and sized
aluminum conductors in contact with the pad which pass through both
refractories and engage the aluminum slab.
17. In a cell as defined in claim 16 in which the slab is in
contact with aluminum side and end portions adjacent the areas of
the shell sides and ends, and means for restricting the escape of
heat from the aluminum slab.
18. In the reduction of aluminum by electrolysis of a fluoride
fusion overlying a reduced aluminum pad wherein cathode current is
withdrawn in a multiplicity of conductors in contact with the pad
and terminating in an aluminum slab heat-sink the improved process
which comprises restricting the escape of heat from the heat-sink
sufficiently that there is less than 20 kilowatts of heat removed
from the pad by the conductors.
19. The process of operating the cell of claim 1 in which the
voltage difference between the aluminum slab and the pad is less
than 0.2 volt.
20. The process of operating the cell of claim 1 in which the
temperature of the heat-sink in at least some parts exceeds
200.degree. C.
21. The process of operating the cell claim 1 in which at least
part of the heat from the heat-sink is utilized to evaporate water
into steam.
22. The combination of cells for the type defined in claim 1 which
comprises at least two cells arranged side-by-side the longitudinal
sides of which are close together, each cell having a plurality of
anodes transversally spanning the cell and being supported on means
providing upward and downward movement whereby each anode is
adjustable to unify current distribution, and a separate flexible
bus connecting the cathode heat-sink of one cell to the nearest
anodes transversally spanning the next cell.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrolytic cells and particularly to an
aluminum reduction cell having partially molten aluminum conductors
and a potlining of noncarbonaceous composition.
2. Description of the Prior Art
The electrolytic cell of the Hall-Heroult type in universal use
employs carbonaceous linings to provide a cathode vessel or pot
which is substantially, but not completely, nonreactive with the
components undergoing electrolysis or resulting therefrom. Such
linings are generally formed of baked carbon paste or blocks made
from calcined anthracite coal or foundry coke particles mixed with
coal tar pitch. It is common practice to embed iron or steel bars
in the carbonaceous lining to collect the cathode current and lead
it to copper or aluminum bus bars exterior to the cell. Although
such carbonaceous linings have been in use for about 80 years,
results have not been entirely satisfactory. Some of the drawbacks
known throughout the industry include wasteful losses of heat and
electric power (which increase with the age of the potlinings)
uneven current density resulting in local overheating, uneven and
excessive expansion and cracking, absorption of sodium causing
graphitization and injury to the refractory lining and penetration
of aluminum into cracks reaching the embedded iron or steel bars or
casing of the cell and damaging these, also impairing the purity of
the electrolyzed aluminum. The above cells have potlining lives
limited to a few years when the economics make relining
necessary.
The characteristic expansion of a carbonaceous cell lining requires
reinforcement of the steel potshell by steel buttresses or the like
having a weight and cost roughly equal to the potshell itself
besides occupying more valuable potroom floor space and requiring
longer pot to pot bus connections.
Many attempts have been made to obviate the need of carbonaceous
linings and thus overcome the above and other disadvantages.
Collector bars have been made at least in part of titanium diboride
and similar compounds resistant to molten aluminum and fluorides
and electrically conductive to a degree 100 or 1,000 times that of
carbonaceous lining so these could project through the potlining
and into the molten aluminum overlaying the potlining. With such
bars the potlining theoretically need not be electrically
nonconductive so that carbon may be eliminated and materials
refractory to attack but not conductive of electricity may be used.
However, as a practical matter refractory conductive compounds such
as titanium diboride have thus far been so expensive, subject to
such short life from breakage and erratic in amounts of current
carried from day to day that they have not been adopted on a
significant commercial scale.
Cells for reduction or refining such as described in U.S. Pat. Nos.
2,584,565, 2,685,566, and 2,866,743 suggest making connection with
the molten layer utilizing metal wells or troughs connected with
the cathode layer by an aperture and connected with the cathode bus
by metal connectors. So far as is known none of the above have led
to substantial commercial application due perhaps due to the added
cost of the wells and troughs and heat losses therefrom which might
exceed any electrical savings. In present commercial cells of
50,000 to 250,000 amperage capacity the electromagnetic circulation
and metal pileup caused by withdrawing current in a few large cross
section branching or curving cathode conductors would not be
conductive to a uniform anode-cathode spacing and hence uniform
current density over every square inch of the 10,000 to 50,000
square inches of fluoride fusion undergoing electrolysis. As a
practical matter the very essence of reduction cell efficiency is a
compact cell having uniform current density and temperature over
all anode and cathode areas and low heat losses and low electrical
losses which this invention provides.
SUMMARY OF THE INVENTION
This invention avoids the disadvantages of prior art by providing a
multiplicity of spaced and sized conductors housed in refractory
potlining with the part of the conductor adjoining the molten
aluminum cathode (hereinafter for brevity called the pad) being
molten and mixing therewith and the balance of the conductor being
in the solid state due to the outward conductance of heat therefrom
to a heat-sink such as a thick aluminum potshell or slab preferably
forming the ends and sides as well as the bottom of the cell and
cooled just enough by just enough circulating air on water to
maintain the lowest practical and feasible voltage and temperature
difference between the pad and slab to conserve the energy needed
for the aluminum reduction. The aluminum slab on the relatively
hotter sides and ends carries the heat (conventionally wasted to
the atmosphere there) down to the cell bottom, increasing the
temperature enough to oppose the flow of heat through the bottom
and the flow of heat through the aluminum conductors to the slab
heat-sink. Heat removal from the slab heat-sink may be regulated by
controlling the circulation of a heat absorbing fluid, such as air
into contact therewith. Slab temperature may be increased to as
much as 600.degree. C. but usually to about 150.degree. to
400.degree. C. Over a period of years the side and end potlining
exposed to the fluoride fusion usually wears thin enough so the
heat release from the fusion is just enough to keep a film of
fluoride fusion frozen against the sidelining sufficient to prevent
further attack. By this invention the unavoidable heat loss from
sides and ends is carried by the aluminum potshell or continuous
aluminum slab attached to a steel potshell to the potbottom where
refractory corrosion downward from the aluminum pad does not occur
and increased slab temperatures enormously decreases heat losses
from the cell bottom.
In addition, as a further aspect of the present invention, the
potlining is a noncarbonaceous material and preferably consists of
a layer of hard, impervious refractory compounds such as fused
alumina and calcium fluoride or oxide beneath the cathode layer of
molten aluminum. The hard, impervious potlining composition
adjacent the molten pad may be underlain by a heat insulating layer
having disconnected pores consisting, for example, of small, fused,
hollow spheres of alumina bound together by a refractory of lower
melting point such as alumina and calcium fluoride or oxide or a
castable refractory such as calcium aluminate cement.
The cell of the invention includes a multiplicity of aluminum
conductors depending from the aluminum pad to the aluminum slab at
the bottom, each conductor being encased, at least over part of its
length, in a hard refractory material. The conductors are carefully
spaced and sized for withdrawing current as uniformly as possible
over the entire aluminum pad cathode area, and consist preferably
of upright shafts of aluminum varying from 0.1 to 3 square inches
or more in cross-sectional area, the conductor or conductors
providing about one square inch of conductor cross section for each
square foot of pad directly beneath the anode. The lower size limit
of conductor is governed by the type of hard refractory channel or
tube surrounding the conductor as regards its surface tension, and
ability to wet but not react with molten aluminum containing
sodium. The upper limit of size is governed by the permissible
intensity of electromagnetically induced swirling action where the
conductor enters the pad. Too intense swirling prevents the desired
temperature gradient along the conductor length. In any size of
molten conductor having uniform cross section throughout its
length, the current density per unit area of aluminum cross section
is limited by that at which, on the high side, the electrical
resistance (voltage drop) and swirling is excessive, and on the low
current density side, by that at which the outward heat loss
through the aluminum conductors is excessive and freezes fluoride
fusion on the cell bottom. Electrical loss which is saved by making
the current density lower in the aluminum conductors can be
converted to the needed internal heat in the electrolyte itself by
merely increasing potline amperage, and hence production, or
alternately increasing the anode-cathode distance of an individual
cell (raising the anodes further from the pad) and this added
anode-cathode distance beneficially increases ampere efficiency
(aluminum production per cell). The range of current density per
square inch of aluminum conductor economically suitable to meet the
above conditions usually will be found to lie between 100 and 1000
amperes and most often between 400 and 1000 depending on the length
of conductor as Examples I, II, and III given herein illustrate.
The aluminum slab heat-sink which preferably constitutes a
continuous aluminum potshell on the order of 1 to 4 inches thick
forming the bottom, ends, sides and top deckplate distributes heat
so that any possibility of holes being eaten through the shell is
eliminated so higher average shell temperatures are safe. In fact,
the outside of the potshells may be insulated to restrict heat loss
to just that desired by the potroom operators to get best results
at the potline amperage carried. No sidelining has been found
entirely resistant to fluoride fusions. This invention uses
resistant, high-alumina refractories and in addition insures
sidewall temperatures uniform enough to prevent potshell damage but
high enough to allow much thinner sidelining than conventionally
used and without the loss of greater amounts of heat but actually
less heat. The thinner sidelinings and endlinings allow more anode
area (larger anodes) thus permitting greater amperage to be carried
in a cell of any given inside dimensions of width and breadth.
Furthermore, the compact cells of this invention may have less
depth for the reason bottom potshell temperatures may safely be
higher and the heat loss is slowed by such higher temperatures.
Ideally, all porous refractories (which deteriorate by absorption
of fluorides) are eliminated from the interior of the compact
aluminum potcell and only dense refractories used therein and
porous refractories of low heat conductivity used only external to
the potshell where insulating value is permanently retained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross section through a cell utilizing the
invention;
FIG. 2 is a plan view of the cell of FIG. 1 but with the fused
fluoride fusion and molten aluminum pad and anodes omitted for
better illustration;
FIG. 3 is a longitudinal vertical cross section at 3--3 of FIG.
2;
FIG. 4 is a vertical cross section of another type of cell
utilizing the invention;
FIG. 4a is a cross-sectional view taken along lines 4a--4a of FIG.
4;
FIG. 5 is an electrically downstream side view of the cell of FIG.
4;
FIG. 6 is a partial vertical cross section of a variation of the
cell of FIG. 1;
FIG. 7 is an enlarged vertical cross section of one of the current
conductors of the cell of FIG. 1;
FIG. 8 is an isometric view of a refractory block of the cell of
FIG. 1;
FIG. 9 is a cross-sectional view of a modified embodiment of the
electrical conductor of the invention;
FIG. 10 is a cross-sectional view taken along the lines 10--10 of
FIG. 9; and
FIG. 11 is a cross-sectional view of another type of cell utilizing
the invention with a very compact aluminum potshell externally heat
insulated.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The compact cell embodiment illustrated in FIGS. 1, 2 and 3
comprises a steel potshell bottom 1 tightly attached to an aluminum
slab 2 on the under bottom of the cell which may have fins 3 to
extend its surface for cooling by atmospheric convection or air
blowers. An aluminum slab 2a is attached to the sides 4 and 5. The
electrically upstream side 4 and downstream side 5 and ends 6 and 7
may be made of steel or aluminum or preferably steel attached to
aluminum slabs inside or outside the steel shell. If the potlinings
do not contain carbonaceous material, the entire potshells may be
made of aluminum several inches thick (as shown in the compact cell
of FIG. 11) without the need of conventional steel buttresses to
keep a cell from deforming. Of course, if very high shell
temperatures of 300.degree. to 500.degree. C. or more are carried a
steel shell outside the aluminum slab is advisable for strength.
The aluminum slab equalizes temperatures and electrical potentials
over the entire shell and permits heat to be dissipated as desired
from the most convenient location. It also permits the heat to be
converted to steam power. The steam pipes of this invention are
electrically insulated from the potcells by a thin, heat conductive
refractory, so that the steam can be joined from many potcells at
various electrical potentials without the pipes constituting a
"short circuit" hazard. Since it usually takes less than 1 gallon
of water per minute evaporated to steam to recover all the waste
heat possible to recover from a 100,000 ampere cell, it is very
convenient to use this form of cooling on cells; although not
previously feasible without proper electrical insulation. On very
large cells this invention provides that the steam pipes be in
direct metallic contact with the potshells to absorb highest
temperature heat from them and each cell have its own high pressure
steam turboelectric generator which feeds electric power back into
the potline. It is theoretically possible to reduce electric power
requirements in a reduction plant by about 10 percent in this
manner assuming that about 25 percent of the heat now being wasted
can be converted to electric power. In any case, whatever saving
can be made in heat now wasted from the potshells constitutes an
addition to a direct saving in voltage of about 10 percent made by
this invention since cells often operate at about 4.5 volts and
have potlining drops conventionally of about 0.5 volts which this
invention reduces to about 0.05 volts. Of course, elimination of
the waste heat from the potroom atmosphere greatly improves working
conditions therein.
The potshell may be insulated against loss of heat from the molten
contents by a relatively lightweight heat insulation 9 having
disconnected pores which may advantageously be made of hollow
spheres of fused alumina suspended in an impervious refractory of
70 percent to 80 percent calcium fluoride and the balance alumina
which melts at a temperature about 700.degree. C. lower than the
fused alumina spheres but about 300.degree. C. above the fluoride
fusion used in reduction cells. Less positive disconnected porosity
may be obtained in the fused refractories of this invention by gas
bubbles released and purposely retained by rapid freezing during
manufacture. The impervious fused refractories prevent absorption
into the pores or spheres and thus slow loss of heat insulating
qualities desired during the 10 or 20 years of potlining life
sought by this invention. In FIG. 11 no porous refractory is used
inside the potshell but only on the exterior where it should last
indefinitely.
The bottom layer of hard, impervious refractory 10 on which the
molten aluminum cathode pad 12 rests and is contained may be made
of fused alumina or such fused mixture containing 70 percent to 80
percent calcium fluoride or oxide and sodium oxide (as soda ash)
and the balance alumina as will melt below 1300.degree. C. compared
to 2000.degree. C. for alumina and is resistant to attack by molten
alumina containing sodium.
The pot sidelining of hard, impervious refractory 10 which
surrounds and contains the molten aluminum fluoride fusion may be
made of fused alumina or a fused mixture containing 70 percent to
80 percent of calcium fluoride or oxide and aluminum fluoride and
the balance alumina which is resistant to attack by the fluoride
fusions undergoing electrolysis which often contain cryolite plus
an excess of aluminum fluoride necessary to the cryolite formula
Na.sub.3 A1F.sub.6.
The cooled cathode bus 14 is attached to the aluminum outer
potshell 2. The cathode bus is advantageously formed of flexible
sheets of copper or aluminum and is connected to an anode bus 15,
of the next electrically downstream cell in the potline (not
shown). The anode bus 15 is supported above the potshell on
adjustable jacks 16 and 17 resting on the steel deckplate which
covers the sidelining. Steel stubs 18 carry the current from bus 15
to the anodes 20 and thence through the fluoride fusion 21 on which
the crust 22 forms, then down into the molten aluminum cathode
layer or pad 12. As shown in FIG. 6 the hard potlining refractory
10 may have bowl-shaped depressions 23 adjoining the conductor
assemblies 24. As shown in FIG. 7, these assemblies each consist of
a hard, fused refractory tube or coating 25 and an aluminum core
conductor 26.
FIG. 4, 4a, and 5 illustrate how aluminum conductor assemblies 40
may be used in the electrically downstream side of a compact cell
to carry all the current a relatively short aluminum conductor path
through the pot sidelining either horizontally or inclined somewhat
upward or downward from the horizontal. As shown in FIGS. 4 and 4a
the molten aluminum pad should be thicker on the electrically
downstream side of the cell where a metal well or trough 43 may
serve to feed the electric current accumulated in the pad to the
horizontal aluminum conductors 40 which lead to the cathode bus
exterior to the potcell.
If desired, and particularly where the aluminum conductor is
molten, the aluminum conduit may be partly filled with fused
refractory rods or refractory fibers 41 of FIGS. 9 and 10 which may
be formed by casting or drawing fused refractory material such as
that mentioned above. The rods or fibers are laid parallel to the
conduit to restrain metal circulation and to distribute the current
carrying paths over a greater area of the side of the cell without
greatly increasing the amount of heat lost from the cell as shown
in FIGS. 4 and 4A. Such a mixture of refractory rods or fibers 41
and aluminum 42 may have as little as one-sixth the heat and
electrical conductivity as a solid mass of aluminum, and more
importantly, they screen out unwanted suspended solids from
entering the conduit and likewise restrain metal circulation which
would otherwise wear the conduit and prevent the desired high
temperature-gradient therein. Where the conductor is molten, any
refractory granules or pebbles mixed with the metal have the
undesired feature of causing eddy current circulation of metal and
local overheating. Likewise particles may bring on the "pinch"
phenomenon as originally explained by Carl Hering about 1907. If
the aluminum cathode connection is not attached to the cell
sidewall, which acts as a heat sink since it is cooled by the
atmosphere, it must be attached to an aluminum bus 27 connecting
all the cathode connections of the cell together to eliminate
slight voltage differences between sad individual cathode
connections. This bus 27 may be an aluminum tube or attached to an
aluminum tube 44 and cooled by forced air or water circulation. The
heat from the anode bus 15 may be carried back by the flexible
leaves 14 to the cathode bus 27 having a hollow interior duct 44
cooled by air but preferably by water which boils to produce
byproduct steam with only a fraction of a gallon per minute of
water being required per potcell to recover the heat as discussed
previously. To cause less heat to be lost from the molten fusion
the potshell temperatures are raised by restricting atmospheric
airflow over the bottom, sides and ends by, shields, baffles, or
the heat insulation 46 illustrated in FIG. 11. The higher the
potshell temperatures used the larger are the aluminum conductors
used to keep down the voltage losses within the conductors as
illustrated in Example II. In FIG. 11 water is evaporated to make
steam in the steel pipes 44 to which are welded steel plates 45 to
pick up heat from the potshell through a thin electrical insulator
but heat conductive refractory 8 which electrically insulates the
potcell from the steam pipes. The thin refractory 8 can be made of
a thickness necessary to carry just the steam pressure desired in
the system. At temperatures up to about 200.degree. C. gasket
materials are available to electrically insulate steam lines
gathering steam from potcells operating at different electrical
potentials.
The aluminum conductors may be housed in grooves in the fused
refractory blocks 10 instead of making special conduits. Such
grooves may be horizontal as grooves in the bottom lining of the
potcell of FIG. 4 or be vertical in the bottom lining of the
potcells of FIGS. 1, 3, 6 or 11. Rather than make grooves in the
potlining blocks, the edges of adjoining blocks may be beveled or
rounded to provide T-shaped or star-shaped apertures bounded by
inward curves which slow circulation of the molten portion of
aluminum conductor housed therein. FIG. 8 shows a block 10 with
vertical groove 24a.
Where conduits are used to house the aluminum conductor, FIG. 7
illustrates in vertical cross section an aluminum conduit assembly
24 of the invention comprising a fused refractory tube or coating
25 on the aluminum conductor core 26. The fused refractory tube may
be fused alumina or lower melting point fusible mixtures of the
oxides, nitrides or fluorides of calcium, sodium, aluminum or
magnesium which best resist attack of molten aluminum and dissolved
sodium therein present in the reduction cells. Small additions of
zirconium, titanium or boron may be used in the refractory mixtures
to the extent refractory or wetting properties are improved. For
example, titanium boride added to or coated by rubbing or
application of slurries on a tube or fiber of alumina makes
aluminum wet the tube or fiber better so smaller tubes or channels
may be used. Other coatings which may be used are the carbides of
titanium, zircon, hafnium, vanadium, columbium or tantalum. The
refractory tube may be formed first and the aluminum forced therein
by pressure or aluminum wire or rod may be dipped into fused
refractory one or more times to acquire a coating. The assembly 24
may then be inserted into a groove 24a in the vertical side of a
fused refractory block as illustrated in FIG. 8 or such grooves may
simply be filled with aluminum without another refractory 25. In
any case the aluminum conductor 26 extends through a hole in the
steel bottom and is welded to the aluminum outer potshell 2 which
is shown in FIGS. 1, 3 and 6 to make mechanical and electrical
connection therewith. The lower end of the refractory tube may be
protected against bending or breakage and its contained aluminum
further protected against possible oxidation as well as bending and
breakage by a steel pipe casing (not shown) which can be welded to
the steel potshell. Any such steel pipe should be protected inside
and outside by a coating such as aluminum silicate commercially
sold as fiberfrax cement coating.
The aluminum slab 2 may be poured as a molten mass on the bottom
(or sides if desired) of the steel potshell by turning it upside
down (or on the proper side or end) for this casting operation, or
the slab may be poured inside the shell as disclosed in my
copending Pat. Application Ser. No. 607,330, filed Jan. 4, 1967,
now U.S. Pat. No. 3,434,958. The projecting aluminum conductor 26
of each assembly 24 is thereby fused or welded into an aluminum
outer or inner potshell which constitutes a heat sink. The upper
end of conductor 26 extends into the molten aluminum pad and melts
and mixes with it as soon as the cell is started.
Instead of the aluminum conductors being made of commercial primary
grade aluminum of 99.50 percent to 99.90 percent grade and the
balance mostly iron and silicon impurities, the conductors may be
made of heavier aluminum alloys and preferably those which have
higher melting points and higher viscosities than commercial
primary aluminum. Moreover, conductors as installed in the
potlining may be made of steel similar to those described and
illustrated in my copending Pat. application Ser. No. 528,503 filed
Feb. 18, 1966, now U.S. Pat. No. 3,434,957, but with the conductors
extending upward through the potlining into the molten aluminum pad
with the result that the aluminum first reduced in starting the
potcell operation alloys with each steel conductor in at least the
upper inch or so of its length until a stable liquid-solid
interface is formed. This results in at least some part of the
liquid or semiliquid portion of the conductor being an alloy of
iron and considerably different in composition from the solid
conductor which is principally steel. In the preferred
construction, however, the conductors are the same approximate
chemical composition as the primary aluminum or aluminum alloy
produced in the potcell where used since thereby the aluminum
produced is not contaminated, high electrical conductivity is
achieved and the needed simplicity in construction and operation is
secured. "Aluminum" as used herein is intended to include all
aluminum base alloys.
FIGS. 9 and 10 show a variation in construction. The conductor 40
of FIGS. 9 and 10 consists of a plurality of refractory rods or
fibers 41 surrounded by molten aluminum 42 and encased in a
refractory tube conductor 44. This type of conductor 40 gives the
added advantages that circulation of the molten metal is
constrained, and granular material such as aluminum oxide and
aluminum carbide may be kept from settling into the tube
conductors. Such granular foreign matter tends to cause eddy
currents in molten metal and thus prevents the desired temperature
gradient developing in the conductor as the distance from the
molten aluminum cathode increases and the distance to the heat sink
of the potshell or exterior to the potshell decreases. On the other
hand, refractory fibers or rods running parallel to the conductors
keep the electrical current running parallel to the conductor and
restrain the circulation of molten metal. The rods or fibers may be
made of graphite or graphite impregnated with aluminum or made from
the oxides, nitrides, borides and fluorides of aluminum, sodium,
titanium and zirconium. The rods may be made principally of fused
aluminum oxide which is not wet by aluminum but this can be
overcome as previously explained by coatings on the aluminum oxide
which will make aluminum wet it. Consequently, the wettable fibers
result in the aluminum being drawn between the fibers by the
capillary action of a wick. The finer fibers wet by the aluminum
then practically filter out foreign particles which would otherwise
tend to work down into the conductors. Aluminum nitride or boron
nitride resist attack by molten aluminum and are examples of a
useful refractory body for the above rods or fibers or coating
thereon. The rods or fibers may be 0.01 to 0.3 inches in diameter
and need be no longer than the length of the conductor which
remains molten.
The refractory 10 may be made of hard, fused alumina or lower
melting mixtures previously mentioned and pressure cast in graphite
molds to form blocks and then grooves for conductors or made more
true to size with a diamond saw. As stated above the use of grooves
may eliminate the need for the refractory casing 25; and if many
small grooves are used the need for fibers or rods therein is
eliminated. The face of a CaF.sub.2 -A1.sub.2 0.sub.3 block cooled
by casting against graphite will be higher in alumina and should be
used adjacent to molten aluminum or fluoride fusion as the cooled
face is more resistant to corrosion. The heat insulating refractory
9 may be alumina powder or other type of heat insulating powder or
brick suitable for the temperatures and possible chemical attack
involved by breakthroughs in 10 or 11 permitting molten aluminum
and cryolite fusion to percolate downward. The preferred refractory
9 of this invention provides maximum permanent insulation by
providing as an aggregate, fused alumina spheres and as a binder
for the aggregate a castable refractory such as calcium aluminate
cement of a fused refractory composed chiefly of calcium fluoride
or oxide and alumina. Such fused refractory is relatively cheap to
make since calcium fluoride is mined as fluorspar or calcium oxide
as limestone and alumina may be provided as low-silica bauxite with
iron impurities being removed in the fusion process or provided in
dross from skimmings from aluminum reduction plant crucibles,
holding furnaces or remelt operations after contained aluminum
particles are screened out.
In the operation of cells of the invention it will generally be
found desirable to carry about 3.5 to 7 amperes per square inch of
current density in the anode area corresponding to about 500 to
1,000 amperes per square foot of cathode area pad laying under the
anodes. An average of about one or two or more conductors per
square foot will ordinarily be required to carry 100 to 1,000
amperes of current per conductor. To prevent metal circulation and
pile up in the pad, conductors are confined to areas directly under
the anodes and are spaced uniformly there. The conductor of
aluminum carrying current from the molten aluminum at 950.degree.
C. to a heat sink at 150.degree. C. is appreciably more efficient
than conventional carbon as regards power losses of heat and
electricity providing the conductors are properly proportioned in
size and uniformly spaced as exemplified in FIG. 1. Table I shown
below shows the approximate advantage of conducting electricity out
of the cathode pad with aluminum instead of carbon at the different
temperature zones through which the conductor must pass.
##SPC1##
In making the comparison of Table I it must not be overlooked that
the conventional carbon potlining serves the double purpose of
conducting cathode current from the pad as well as containing the
fused contents at 950.degree. C. and that substituting aluminum for
carbon as the electrical conductor outward from the molten cathode
pad makes desirable substituting a suitable refractory for the
carbon potlining that will have a much lower specific heat
conductivity. In this respect fused alumina refractories can have a
heat conductivity about equal or less than conventional fluoride
saturated carbon potlining. Fused calcium fluoride-alumina mixtures
containing discrete and unconnected pore spaces afforded by fused
alumina bubbles can have as little as one-sixth of the heat
conductivity of conventional potlining. Protected by a layer of
fused alumina the porous refractories 9 should preserve heat
insulating qualities for perhaps 10 or 15 years whereas porous heat
insulation used beneath the carbon potlining suffers great
deterioration in insulating power in 5 years due to the ready
absorption of sodium and fluorides by the carbon and any
conventional porous insulation backing it. The cell of FIG. 11 not
having porous refractories inside the aluminum potshell eliminates
absorption.
Although carbonaceous potlining is made unnecessary by this
invention and is not to be desired, it will be understood that
carbonaceous potlining can be used with the aluminum conductors of
this invention with the resulting improvement over conventional
potlinings that the electrical loss is greatly reduced between the
molten cathode pad and the cathode bus. To appreciate the advantage
of the aluminum conductors of this invention it must be realized
that conventional potlinings suffer not only from the poorer
electrical conductivity of carbon compared to aluminum shown in
Table I but also in the contact drops inherent between the molten
aluminum pad and carbon and between carbon and the steel collector
bars as well as a further contact drop between the steel collector
bar and the aluminum cathode bus. These contact drops may
constitute one-third or one-half of the 0.4 to 0.6 volt drops
between the molten aluminum pad and cathode bus of conventional
cells. With the present invention the above total voltage drop may
be only 5 percent to 10 percent of conventional values. Also this
invention eliminates the highly undesirable characteristic of
carbon potlinings to "hog" or concentrate current in any portion of
the potlining that becomes overheated and persists since the
electrical resistance of carbon decreases greatly with increasing
temperature which is opposite to the behavior of most metals
including aluminum whose electrical resistance increases about
eight times in going from solid aluminum at room temperature to
cathode pad temperature near 1000.degree. C. The value of the
aluminum conductors is further illustrated in the following
discussion on conductor design.
The heat conductivity of some molten aluminum between 960.degree.
C. and its freezing point just below 660.degree. C. can average
about 2.4 watts/inch-1/.degree. C. in comparison to 4.5 identical
units in solid aluminum from about 660.degree. C. down to
160.degree. C. potshell (heat sink) temperature. As regards
electrical conductivity or inversely electrical resistance
(RI.sup.2 loss) this aluminum would average about 11 watts per inch
of length when carrying 1,000 amperes per square inch of conductor
cross section while the solid portion average about 3 watts per
inch length RI.sup.2 loss. With such information it is easy for
those versed in the art to calculate the length of conductor
illustrated in FIG. 7 that will be molten in the end joining the
pad at 960.degree. C. and the remaining length that will be frozen
solid towards the outer aluminum potshell at 160.degree. C. If it
is assumed that the liquid-solid interface is stable, then the heat
carried down from the molten metal pad taking into account that
generated by RI.sup.2 loss will be equal to that carried away from
the stable interface taking into account the RI.sup.2 generated in
the solid portion. If high enough current density is carried in the
aluminum conductor, the RI.sup.2 heat generated in the molten
portion may be made enough so temperature gradient near the upper
end of the conductor is small and there is practically no heat
conducted out of the pad by the conductors. However, this is rarely
desirable since it does not result in the lower range of electrical
losses which are more desired since heat losses can be saved by
better heat insulation in the potlining or outside the potshell as
described above.
The following examples are given to show the advantages and
superior operating results obtained with the invention.
EXAMPLE I
A 100,000 ampere cell of this invention has 100 vertical conductors
each about 1 square inch in cross section and each about 24 inches
long reaching from the cathodic pad to the 2 inch thick, finned,
aluminum potshell bottom maintained at about 160.degree. C. by
controlled atmospheric convection. By the methods disclosed above
it is computed that about 8 inches of the upper end of each
conductor remains molten and the remaining lower 16 inches solidify
so that practically no heat leaves the metal pad and only about 9
kilowatts of RI.sup.2 heat are carried into the finned potshell
bottom. There is a total electrical RI.sup.2 loss in the molten and
solid portions of the aluminum conductor corresponding to a voltage
loss of 0.14 volts which is about one-third or one-fourth that
common in conventional cells.
EXAMPLE II
A 100,000 ampere capacity cell of this invention is built with 200
vertical conductors each 1 inch square in cross section and 24
inches long terminating in aluminum bottom slab maintained as a
heat sink initially at 160.degree. C. By calculation, about 6
inches of each conductor remains molten and the remaining 18 inches
solid. The conductors as a whole carry about 22 kilowatts of heat
out of the molten pad and about 23.5 into the slab heat sink. There
is an RI.sup.2 heat loss in the conductors of only about 3
kilowatts corresponding to a voltage loss of about 0.03. With such
a low voltage difference between molten metal and potshell there is
practically no danger remaining from a crack in the potlining
allowing molten metal from the pad to eat a hole through the
potshell. Consequently the steel shell, which is covered on sides
and ends as well as bottom with a continuous aluminum slab 3 inches
thick, is brought up to a temperature of 360.degree. C. by enough
baffles to restrict atmospheric convection cooling as well as by
partial covering with heat insulation. By this change in heat sink
temperature the length of conductor molten changes to 9 inches and
only about 13.5 kilowatts of heat are carried out of the molten
metal pad and about 15 delivered to the heat sink. The RI.sup.2
heat loss is increased to about 4 kilowatts corresponding to a
voltage loss of 0.04 volts compared to the 0.03 initially when
shell temperature was 160.degree. C. instead of 360.degree. C.
Likewise there is a heat saving of about 25 percent of about 60
kilowatts initially lost by conduction through the potlining to the
bottom, sides and ends. Consequently by raising the shell
(heat-sink) temperature about 200.degree. C. the heat loss from the
pad via the aluminum conductors is fully compensated by lower
conduction losses through the potlining while the electrical saving
amounts to about 10 percent of normal cell voltage of about 4.5
volts so that 10 percent more potcells are then added to a potline
of this invention to add 10 percent more production from the same
amount of potline power as employed by a conventional cell
potline.
EXAMPLE III
A 100,000 ampere capacity cell of this invention and the type
illustrated in FIG. 4 is equipped with horizontal side conductors
having a length of about 10 inches and a total cross-sectional area
of about 100 square inches of aluminum conductor. In operation
about 3.5 inches length of a conductor is molten and 6.5 inches
solid. Under these circumstances the heat lost from the pad amounts
to about 15 kilowatts. There is an electrical loss of about 6
kilowatts RI.sup.2 loss corresponding to a voltage loss of about
0.06 volts from the molten pad to the heat sink. The steel shell is
heat insulated sufficiently so that the 15 kilowatts of heat lost
through the conductors is more than compensated, compared to
conventional cells, by lower heat loss conducted through the
potlining.
EXAMPLE IV
A conventional potline of 100,000 amperes having 100 cells
operating in series at 4.4 volts each and thus utilizing a total of
44,000 kilowatts is converted to the process of this invention by
the use of thinner walls of potlining on the sidelinings and
endlinings, aluminum slabs 3 inches thick on top of the deckplates
as well as on all outsides of ends, sides and the bottom of the
shell, 200 aluminum conductors each having 1 square inch cross
section and heat insulation outside the potshell to raise shell
temperature from a previous average of 200.degree. C. to a new
average of 400.degree. C. Enough greater fusion area is created so
anodes of 10 percent greater cross section may be used than
previously in conventional practice and the cells only average 4.0
volts at 110,000 amperes and aluminum production is increased about
10 percent with the same total amount of power as previously. No
shell burnouts are experienced with the thinner sidelinings and
higher amperage and shell temperatures because the thick aluminum
slabs freeze all fusion and molten aluminum penetration through
cracks, and because pad-slab voltage difference is only 0.04
volts.
* * * * *