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Патент USA US3034982

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May 15, 1962
R. A. LEw?s
3,034,972
ELECTROLYTIC PRODUCTION OF ALUMINUM
Filed March 28, 1958'
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INVENTOR.
ROBERT A. LE W/S
BY
'
&2:7
ATTORNEV
3,@34372
tt
Patented May 15, 1962
l
2
highest e?icíency is favored by the lowest possible tem
&034372
perature of the molten electrolyte consistent with other
requirements, such as the solubílity of alumín-a in the elec
trolyte and general workability of the cell. In order to
maintain low electrolyte temperatures and prevent the
excessive generation of heart, it is necessary to operate
reduction cells employing conventional electrolytes at
ELECTRGLYTEC PROEUGTEON OF ALUMENUM
Robert A. Lev/is, Los Altos, Catif., assignor to Kaiser
Aluminum 8; Chemical Corporation, Oahland, Calif., a
corporation of Delaware
Filed Mar. 28, 1953, Ser. No., 724595
7 Claims. (CL 204-67)
low current densities.
This invention relates to` the production of aluminum
The minimum power required to produce alurninum
by electrolytic reduction of an aluminum-containing com 10 metal may be calculated by the following equation:
pound, e.g. alumina. More particularly, the invention
2.041 %C'E'
relates to an improved method for the electrolytic produc
P_
100
tion of aluminum.
(1)
_
1000
The production of aluminum by electrolysis of an alumi
num-containing compound, e.g. valumina, dissolved in a 15 where
molten salt electrolyte, e.g. cryolite, and deposition at the
P=the power invkw.
cathode is a very o?ld process. The alumina is broken
% C.E.=the percent current e?'íciency of the operation of
down into its components; the oxygen is liberated at the
the cell
anode and the aluminum -is deposíted at the cathode,
which forms the bottom of the electrolytic cell. In prac 20 2.*O4=the voltage required to sustain the cell reaction
I=cell current in amperes
tice, the pool of molten aluminum which is ?ormed in the
bottom portion of the cell in eftect constitutes the cathode
The voltage required to sustaín the reaction is calculated,
of the cell. Conventionally, use has been made of two
utilizing the beat of the overall cell reaction, by the
types of electrolytic cells, namely, that commonly referred
following equation:
to as a “pre-bake” cell and .that eornmonly referred to as 25
a Soderberg cell. With either cell, the reduction process
involves precisely the same chemical reactions. The
principal difference between the two cells is one of struc~
-AH
(2)
njF
Where
ture. In the pre-bake cell the carbon anodes are pre
E=the Voltage required to sustaín the cell reaction
baked before being installed in the cell, while in the Soder 30 AH-:the heat of reaction in calories
berg cell or continuous anode cell the anode is baked in
n=the valence change or number of Chemical equivalents
situ, that is, it is baked during operation of the electrolytíc
of substance reacting
cell, thereby utilizing part of the heat generated by the
j=constant for changing joules to calories=0.239
'reduction process.
coulombs of electricity passing through the
Conventionally, the total voltage drop across the cell 35 F=96,50O
cell for each Chemical equivalent of substance.
is about 4.5 to about 5 volts with a voltage drop between
the anode and cathode of about 3.5 to about 4 volts. The
Equation 2 may be expressed by
Voltage drop between the anode and cathode is composed
E
_AH
of (1) the voltage required to overcome the ohmic re
sistance of the electrolyte and (2) the sum of the reversible 4:0
(3)
decompositíon voltage required for the cell reaction and
the polarization voltages. The voltage required, eg., in
The overall cell reaction in the reduction of alumina is
“23,070n
as follows:
(l) above may be about 2 volts While the voltage of (2)
(4)
2A12Oa+3C _asc O2+4A1
may be about 1.7 volts. The remainder of the cell volt
(room (room
(cell
(cell
temp.) temp.)
temp.) temp.)
age is required to overcome the resistance of the lining, 4:5
the external conductors and connections, the anode, anode
The heat of reaction evolved in Equation 4 is expressed
effects, and various contact resistances. Power Consump
by:
tion per pound of aluminum produced generally falls with
in the range of `from about 7.5 to 8.5 kilowatt-hours
(lswh).
50
(5)
AH=4AHAI+3AHCOZ-2AHA1203-3AHC
Assuming a cell temperature of 975“ C. and room tem
Although the theory of the electrolysis of alumina in
perature of 2S° C.
molten cryolite has not been fully developed, a consider
able amount of knowledge has been gained over the past
seventy years in rega?'d to the electrochemical factors,
such as voltage relationships and current e?iciency, and 55
the thermal aspects, such as heat generation and heat dissi
pation, of the process. The knowledge of these factors
has been essential in the development of the electrolysis
AHCo2=AHCo2°+sensible heat (room temp. to cell
of alumina into a commercially feasible process. The
thermal aspects, in particular, are of utmost importance
for they determine the size, the design and the operational
conditions of the reduction cell.
AHA12o3= - 389.49 kcaL/mole
The current e?iciency (or Faraday ef?eiency) of the
aluminum reduction process is only about 80 to 88% of
the theoretical. This relatively low e?iciency is due to a 65 AH=4(9.1) +3(-82.75) - (2) (-389.49)
number of causes, particularly the formation of a "metal
AH: +567.1 kcaL/mole
tog" at the aluminum cathode and recombination of the
The voltage equivalent of the reactíon then is:
anodic and cathodic products, which are brought together
by stirring and di?iusion. The formation and reoxidation
of cathodic “metal tog„ are more active the higher the 70
'temperature of the metal and the electrolyte, and the
current ef?ciency thus becomes lower. Oonsequently, the
E: _
567,100
12( 23,070)
=
- 2.04 volts
The power e?iciency, that is, the percentage of the
3,o34,972
applied power used for the actual electrolysis, of a reduc
tion cell is expressed by the equation:
2.041( %C.E.
100 >><1s0
A.
cryolite constituent of the electrolyte or as an additive
thereto. It should also be noted that in an attempt to
reduce heat generation of the cell, in order to increase
power e?iciency, there has been a steady reduction in
the industry in current density used in operation of elec
trolytic cells.
The usual electrolyte composition consists essentially
of a mixture of the ?uorides of sodium and aluminum in
where
which alumina is dissolved. Generally speaking, it may
V=total voltage drop
10 be said that most electrolyte compositions consist essen
I V=total power into the cell
tially of cryolite and alumina and fall within the ranges,
In Equation 6, assuming a current et?ciency of about
80 to 85% and a voltage drop across the cell of about
4.5 to 5 volts, which ?gures are representative for opera
tion in the aluminum reduction industry, it is seen that
the power eí?ciency is on the order of 30 to 35%. Conse
in percent by weight of electrolyte, of sodium cryolite
(Na3AlF6)-80 to 90%, alum?'num ?uoride (AlF3)-O
to 10%, calcium ?uoríde (CaF2)-5 to 125%, and
alumina (Al2O3)_~2 to 6%. In operation, the electrolyte
quently, the major part of the total power supplied to the
cell is not utilized in producing aluminum.
In order to improve the power e?iciency, a reduction
composition is generally restricted to a sodium ?uoride
to aluminum ?uoride weight ratio in the range of between
1.20 .to 1.50 and the calcium ?uoride content restricted
to an amount within the range of 7 to 9%.
The freezing point of electrolyte compositions in the
of the heat losses caused by the various ohmic resistances 20
ranges set forth above is on the order of about 950” C.
in the cells is necessary. Consequently, many studies of
to 960° C. In operation, the temperature of the electro
the heat balance, that is, the heat generated and heat díssi
lyte generally is maintained above about 965° C. to avoid
pated, of alumínum reduction cells have been-made.
Operating difficulties due to excessive freezing and thick
Much experimenting has been done in an effort to de
crease the resistances (and her?ce improve thepower eiii 25 ening of the electrolyte crust which forms over the rnolten
electrolyte and the occurrence of ledges covering the in
ciency) of the various elements of the call, particularly
teríor sides and part of the bottom of the líning. Operat
the electrolyte which accounts for about tWo-thirds of the
ing temperatures on the order of 965° C. to 975“ C. are
total heat generated. Only a portion of the heat generated
generally used in the industry today. Higher temperatures
in the electrolyte is essential in heating and maintaining
the electrolyte at the Operating temperature. The rest of 30 present di?iculties due to reduced current eí?ciency result
ing from two factors. One factor is the effect of tem
the generated heat must be dissipated somehow in order
perature itself which reduces the current efliciency as
to prevent the cell from running at too high a temperature
temperature is increased. >The other factor is the interac
which would reduce the current ef?ciency.
tion of electrolyte and aluminum metal with the carbon
In an e?ort to reduce the heat generated in the elec
trolyte, prior work has been dírected to lower Operating 35 side walls of the cell lining and stray currents to the side
walls which result when the frozen electrolyte ledge cover
temperatures of the electrolyte and increased electrolyte
ing the side walls melts off as the temperature increases.
conductivity; the desired result being decreased cell volt
consequently, practical considerations of maximum cur
ages giving lower power consumption per pound of alumi
rent e?íciency dictate that reduction cells be operated with
num metal produced, as distinguished from increased pro
ductive Capacity of the cell. A lower limit to Operating 40 the electrolyte being at a temperature of not more than
about 10 to 15° C. above the freezing point. This results
temperature is ?xed -by the liquidus temperature of the
cryolite electrolyte. The liquidus of cryolite is lowered
by additions of sodium ?uoride, aluminu?n ?uoride, and
valurnina.
Sodium ?uoride additions are undesirable in
that they promote the formation of cathodic "metal fog,"
and it is therefore usual practice to operate with elec
trolytes containing an analytical excess of aluminum ?uo
in a rather thick crust forming over the top surface of the
electrolyte and ledges forming over the sides and partially
over the bottom of the cell lining, all of which tend to
limit the heat dissipation from the cell itself. This in turn,
limits the power input to the cell in order to prevent the
operating temperatures from rising above the desired level.
ride. However, aluminum ?uoride, due to its volatilíty,
This means that there is a de?nite limit to the current
ride in cryolite. Therefore, calcium ?uoride has been
cell in question.
The voltage drop in the electrolyte, caused by the ohmic
resistance of the electrolyte and by which the correspond
density which can be used in practice for a given cell
can be used only in restricted amounts, e.g. up to 10%
in excess of the stoichiometric amount of aluminurn ?uo 50 design which in turn limits the productive capacíty of the
used in amounts of up to l2.5% to further lower the
liquidus which has been partly lowered by the addition of
ing resistance heat generation can bë measured, is ex
aluminum ?uoride. The practical limit of solubílity of
alumina is about 8% and, also, the alumina content di 55 pressed by the equations:
minishes as the electrolysis proceeds. However, the addi
(7)
Ve:]Re
tion of calciurn ?uoride as well as the additions of alumi
where
Re=the
resistance
of the electrolyte and
num ?uoride and alumina, undesirably reduce the con
ductivity of the electrolyte. Furthermore, ealeium ?uo
V„=T-lI=ø-ld
ríde undesirably reduces the density differential between 60 (s)
a
the electrolyte and the molten aluminum.
where
It was recognized in the prior art that some increase
r=the speci?c resistivity of the molten electrolyte
in current el?ciency and decrease in carbon Consumption
l=the anode-cathode distance
should result if the Operating temperatures could be
zz=average cross-sectional area of electrolytic path over
lowered. consequently, the prior art is replete with sug 65
distance l
gestions or proposals for modi?cation of the basic elec
dzaverage current density in the electrolyte
trolyte composition in order to accomplish this objective.
In Equation 8 all of the factors are theoretically varíable,
In general, these proposals have been to add other alkali
but from a practical standpoint only d, current density,
?uorides, cryolites, or chlorides as an equivalent or substi
tute for the sodium ?uoride or sodium cryolite. It has 70 may be varied a considerable amount. The temperature
cannot be increased for the purpose of reducing the speci?c
been proposed to use sodium chloride as an additive or as
resistivity because, as stated previously, an increase in tem
a substitute for part of the sodium ?uoride of the con
perature will result in lower current ef?ciency. The re
ventional electrolyte. Also, it has been proposed to use
potassium ?uoride or cryolite and lithium ?uoride or
duction of anode-cathode distance, l, would result in in
cryolite as a substitute for the sodium ?uoride or sodium 75 creased reoxidation of the cathodic metal. consequently,
3,o34,972
6
the current density becomes the basic characteristic of the
cell, and a reduction of current density has been ,the
means used for reducing the heat generated in the cell;
electrolyte under cell Operating conditions, is wettable by
In View of the numerous and complex interrelated fac
tors affecting the operation of an altuninu?n reduction cell,
it is only natural that the philosophy of those engaged in
the reductíon ?eld has been to limit the current densityin
order to reduce the heat generated in the cell, thereby im
proving the current e?iciency and the power elž?ciency.
This course of action, although it directly limits the pro
ductive capacity of the cell, has been deemed expedient
and necessary in order to have economic harmony among
the many interrelated factors of the cell. In order to in
crease the production of aluminum, cells of larger and
larger dimensions are being built. With cells of these 15
larger dirnensions, the problem of power ef?ciency be
comes greater.
referred to as refractory hard metals. If desired, the
who-le of the current-conducting element may consist
essentially of one or more of such materials.
The expression “consists essentially,” las used herein
after in the speci?cation and claims, means that the por
tion of the element made of one or more of the carbides
and :borides referred to above does not contain other sub
stances in amounts suf?cient materially to ?a?Ëect the de
sirable characteristics of the current-conducting element,
although other substances may .be present in minor
The rate of heat generation of a cell
increases faster with increase in size than its ability to dis
sipate heat for the same electrolyte temperature. Conse
quently, under the prior mode of operation, further reduc
molten aluminum under cell Operating conditions, and has
good stability under the conditions existing at the cathode
of ,the cell. It has been found that the carbides and
borides of titanium, zirconium, tautalum and niobium and
mixtures thereof, eXhibit all or substantially all of the
properties hereinabove set forth. These compounds are
amounts which do not materially affect such desirable
char?acteristics, for example, small proportions of oxygen,
It is preferred that
the refractory materials in the current-conducting elements
20 iron, or nitrogen in titanium boride.
tions in current density are`.mandatory in order not to
generate excess heat in the electrolyte, even though this
measure is at the expense of cell productivity.
Accordingly, it is the prímary purpose and object of this
be essentially free from elements or compounds which
Would lead to undesirable contamination of the aluminum
produced. Nevertheless, the current-conducting elements
invention to provide an improved method for Operating an 25 may contain initially, among others, certain compounds
electrolytic cell for the production of aluminum which
which dissolve out when the element is ?rst put into use
results in a substantial increase in the productive capacity
and which do not materially ?afiect the element.
or output of the cell.
esírably, that portion of the element consisting essen
A further object of this invention is to provide an im
tially of one or more of the aboveementioned refractory
proved method for the electrolytic production of alumi 30 materials should be composed of at least 90% by Weight
num metal Wherein the electrolytic cell productive capacity
of such materials. The elements have been found to pos~
sess a relatively high electrical conductivity (substan
is increased with little or no increase in power Consump
tion per pound of aluminum metal produced and With a
tially better than that of canbon), a good resistance to
attack by molten electrolyte, a very low solubility in
decrease in capital investment, maintenance, administra
tion, and labor costs per pound of aluminum metal 35 molten aluminum at cell Operating temperatures, are Wet
produced.
by molten aluminum under cell Operating conditions, and
have a resistance to oxidation considerably better than
A further object of this invention is to provide an im
that of carbon. Such elements can he produced in suita
proved method ?or the electrolytic production of alumi
ble forms With good mechanical properties.
num which results in better heat dissipation, greater cur
The modi?cation of the electrolyte or bath by the pres
rent ef?cíency and rlower carbon Consumption per pound 40
of aluminum produced.
Other objects and advantages of the present invention
will be apparent from the following detailed description.
According to› the present invention, it has been found
that an electrolytic cell Operating with a given Voltage
drop across the cell can be operated with signi?cantly
greater productive `Capacity or output and with better heat
ence of lithiurn can be ?accomplished by an addition or
incorpor-ation of ?a suitable lithiu-m-containing material to
the conventional „molten electrolyte in the cell or as a
substitnte for calcium ?uon'de or soclium ñuoride or both.
Alternatively, the lithium-containing material may be in
corporated by making up the electrolyte completely out
side of the cell. By practice of the instant invention the
oalcium ?uoride can be maintained at a minium and in
dissipation, greater current e?iciency, and lower carbon
the preferred operation of the invention the electrolyte
consumption by carrying out the reductíon of the alumi
num-containing compound, eg. alumina, by a method 50 should not include intentionally ;added calciurn ?uon'de.
According to the invention, lithium should he present
Wherein a molten salt electrolyte consisting essentially of
in the electrolyte in an amount equal to that resulting
cryolite and alumina is modi?ed by the presence of or
from the addition of lithiurn ?uoride in van amount from
containing a predetermined amo-unt of a lithium-contain
about 2% to 20% by Weight of the molten electrolyte,
ing compound and whereín use is made of a substantíally
increased current or, in other Words, Where there is estab 55 preferably from about 3 to 8%. The lithium material
may take the form of various compounds which are com
lished a current ?ow so that the resultant voltage drop
patible With the other electrolyte con-stituents and do not
across the cell is maintained at least substantially as great
introduce excessive lamounts of impurities into the cell.
as that existing in the cell devoid of the lithium addition
For example, use can be made of lithium fluoride, lithium
to the electrolyte. The power 'Consumption per pound of
aluminum produced 'generally is substantially the same as 60 aluminum ?uoride (lithiurn cryolite), lithium canbonate,
lithium hydroxide, or lithium aluminate. In providing
that for conventional cells. However, the present inven
the necessary lithium content in the electrolyte, such com
tion also contemplates in some instances using a cell volt
pounds can be used singly or in combirration. Regard
age and power Consumption greater than that of conven
less of the lithium compound used from the above list,
tional cells but wherein the extent of pro-?t from increased
production far eXceeds the added cost of power.
65 the end result in the molten electrolyte, as far as com
position is concerned, ?will be the same. Although various
An even greater increase in produotive capacity of elec
lithium-containing compour?ds :are suitable. for purposes
trolytic reduction cells can be achieved by, in addition to
the above features, passing electric current through such
electrolyte from the anode to at least one cathodic current
conducting element exposed to the pool of molten alumi 70
num at the base of the cell, which pool in effect constitutes
the cathode, and wherein at least that portion of such
element in 'contact with the molten aluminum consists
essentially of a material possessing a low electrical re
sistivity, a low solubilíty in molten aluminum and molten 75
of the present invention, from the standpoínt of handling,
moisture pick-up, gas evolution, etc., the presently pre
ferred source materials ?are lithium ?uoride, lithium cryo
lite, and lithium aluminate.
As discussed hereinbefore, the present invention in
voives the presence of lithium in the electrolyte in a pre
determiued amount coupled with maintenance of a volt
age drop across the cell at least substantially equal to
3334372
3
of that required for melting) in the electrolyte of the
that existing in the cell devoid of the lithium content in
the electrolyte. Or, stated another way, the present in
invention, While still maintaining a lower Operating tem
perature, may be substantially greater than that existing
in a conventional _cell_ This may account at least in part
the resultant voltage drop is at least substantially as great
as the ?rst named voltage drop of the cell devoid of the Cn for less bottom ledging, lower cathode resistance and
thinner top crust permitting better dissípation of heat.
lithium content. The current is maintained at greater
In addition, the lower Operating temperature of reduc
than a 5% increase over normal operation, that is, op
tion cells according to the present -invention results in a
eration of the cell devoid of the lithium content in the
lower net carbon consumption from anodes per pound
electrolyte. The .anode current density used in the op
eration of the conventional “pre-bake” cells is in the range 10 of aluminum produced. It will be understood that the
above discussion is in no way to be a limitation on the
of from about 6.0 to 8.0 rarnperes per square inch and in
invention :but is given merely as a possible explanation
the case of the conventional “Soderberg” cells, in the
for the signi?cant results achieved -by the present inven
range of from- ›about 4.5 to 6.5 amperes per square inch.
tion.
The anode current density determinations cited herein are
vention utilizes a substantially increased current so that
calculated by dividing the current supplied to the cell by 15
It has been found. also that because of the marked
the nominal bottom\ area of the anodes.
etfect of lithium on the freezing point and electrical con~
The nanges given above for current densities used in
conventional cells `are due to the size and heat balances
ductivity of the electrolyte, it is possible, by making
relatively small changes in `lithium content, to fully com
pensate for di?erences in -heat díssip'ation with age of a
of various cells. Smaller cells in general operate at
higher current densities than do larger cells because of 20 given reduction cell in a cell line. This can be of sub
stantial signi?cance in reduction cell line operation in
better heat dissipation. With the use of the lithium ad
compensating for irregularities and poor current el?
dition of the instant invention, the current density (and
ciency oftentimes encountered in individual cells in a given
the current) for any given cell can be maintained at
greater than a 5% increase in order to increase the pro_
line of cells, particularly with regard to new cells placed
25 in operation.
Also, the lithium additions to the electrolyte elfect a
of the instant invention, that the increase in current density
greater density differential between the electrolyte and
(and current) may be 25% or higher.
Practice of the invention has produced important and
the aluminum metal. A greater density differential be
unexpected results in operation of aluminum electrolytic
tween the electrolyle and the aluminum metal reduces the
reduction cells, the most important being that of pro 30 tendency for metal to rise into the electrolyte under forces
ducing signi?cant increases in production capacity of a
caused 'by electromagnetic ?elds, discharge of anode
gases, and thermal convection. This not only allows
conventional reduction cell, eig. 12% increase in pro
for better se'paration of the metal from the electrolyte
duction per cell. Still greater increases in productive
ductive capacity. It appears possible, through practice
capacity have been found, according to the invention,
when use is also made of cathodic current-conducting ele
ments made of refractory hard metals, as discussed here
inbefore, e.g. 25% increase or greater, in productive ca
but also has a clamping effect on the tunbulence induced
35 in the metal and the electrolye by the magnetic ?elds re
sulting from the heavy electrical Currents. Turbulence
from electromagnetic forces, which becomes particularly
pacity.
acute in large size cells, results in reoxidation of the metal
which will seriously impair the current e?iciency. With
Moreover, the presence of lithium ion in the electrolyte
results in little effect on the deposition ef?ciency for 40 the clamping of the turbulence by the lithium-containing
aluminum. Any adverse e?ect is more than oil-set by the
electrolyte, the working distance, that is, the anode-Cath
corresponding decrease in Operating temperature made
possible by the lower freezing point of the electrolyte
ode distance, can be decreased.
For purposes of further understanding the invention,
Table I below sets fcrth comparative, average Operating
resulting from the .addition of lithium. Moreover, lithium
shows no signs of increasing attack on the lining.
45 data between two groups of nine conventional horizontal
The signi?cant increase in productive Capacity of the
stud Soderberg cell-s in a commercial operation, one group
reduction cell is believed primarily due to the lower elec
Operating with and one group without the presence of
trolyte resistance and to an increase in heat dissipation
lithium in the electrolyte or bath.
of the cell. Moreover, it is believed that the increased
Table I
heat dissipitation of the cell is due to less bottom and side 50
ledging :and a thinner top crust.
Horizontal Horizontal
It is to be noted that the initial freezing points of the
Stud Cclls Stud Cclls
lithium-'containing electrolytes are substantially lower
(N 0 Li)
(XVith Li)
than those of the conventional electrolytes. The freez
No. of Calls _________________________________ ._
9
9
ing point of an electrolyte containing 3% lithium ?uoride
is about 940” C., while an electrolyte containing 7%
ays ____________________________________ ..
77
77
Pot Days__ ________ _.
638
632
lithium ?uoride has a freezing point of about 890° C.
Total Production, Lbs _____ __
660, 568
708, 639
Therefore, with the lithium addition it is practical to use
Production, Lbs. per cell daym
1, 035
1 2
Increase in Produstion, Perccnt
__________ __
?S. 2
a lower Operating temperature. It has been found that
Material...
Carbon
carbon
with an electrolyte consisting of 34% lithium cryolite 60 L?nmg
Current, Amperes
, 114
72 236
Perigd:
(corresponding to about 165% lithium ?uoride), 65%
sodium cryolite and 1% alumina, operation of the cell
Voltage Drop Across Cell ___________________ ..
5. 06
5. 03
Voltage Drop Between Anodc and Cathodc_ „_
3. 70
3. 64
could be car?'ied out at 845° C. With an electrolyte con
Anode Current Density, Amps/sq, in _______ ..
5. 94
taining 15% lithium cryolite (corresponding to about
Bath Temperatura, ° C ___________ -.
7 .5% of lithium ?uoride), it was found that the freezing
point was such that operation of the cell could be carried
out at 910° C. At the lower temperatures possible with
Current Increase, Percent..
Bath Ratio (Nail/AIF@---
Y
G
0. 3
970
9.51
1.33
1.33
" Bath Frcezing Point, ° C..
957
930
Bath, Pcrccnt Cam."
6. 5
1 3. 9
Bath, Percent AlaOa __________ __
5. 5
the higher lithium additions, the alumina solubility and
solution rate is decreased; however, with the improve
ment of feeding techniques these higher additions will 70
be of considerable value to the reduction operation.
Utilizing a lithium addition corresponding to 3 to 8%
. __________ ._
Bath, Perccnt LiF (Source oi L
.......... ._
Anode-Cathode Distanca, Inchcs____
1.
Lbs., C/Lb. All _________________ __
5. 5
4, 35
Li CO
< 2 1.
0. 521
0. 508
Current Eíüeiency, Perccnt..
85. 6
K.w.h. per Lb. Al ____________ __
7. 90
7. 78
0. 574
0.610
Cathode Voltage Drop, Volts
Bath Depth, Inches ________ __
87. 5
.___
5
5
Metal Depth, Inches ________________________ _.
10
10
lithium ?uoride, the cell Operating temperature ?nay be
from about 910° C. to 955° C.
The amount of super-heat (that is, the heat in excess
'I CaFz not intcntionally added-introduccd into electrolyte as impuri~
ties from raw materials.
9 ,
It will -be apparent- from the above table that the pres
ent invention, as applied to conventional electrolytic re
usual manner an insulating layer 2 which can be any
desired material, e.g. alumina, bauxite, clay, aluminum
duction cells for the production of aluminum, results in
silicate brick, etc. Within the insulating layer 2 is dis
a Very signi?cant increase in the productive Capacity' of
posed cell lining 3 which can be of any desired material,
the cell and, also, a signi?cant decrease in the consump
eg., carbon, alum?'na, ,fused alumina, silicon carbide, sili
tion of canbon per pound of aluminum produced as well
con nitride bonded silicon carbide or other desired ma
as other advantages.
terials. Most commonly the lining is made up of a plu
Calculating the power ef?ciencies by use of Equation
rality of carbon blocks or is a rammed carbon mixture
6 above, the percent power e?iciency of the Sodenberg
or a combination of a ramrned carbon rniXture for the
cells in Table I Operating without the lithium electrolyte 10 bottom of the -lining with side and end walls constructed
34.6% while those Operating with the lithium bath is
of blocks of carbon. Alternatively, the side and end
362% , a substantial improvement.
walls can be constructed of blocks of silicon carbide or
Table I'I records data which shows a comparison be
tween pre-bake cells,
and without the presence of
lithium in the electrolyte.
other suitable refractory. The -lining 3 de?nes a chamber
which contains a pool of molten aluminum 4 and a body
15 of molten electrolyte or bath 5, as described.
When carrying out the method of this invention and
at the time when aluminum is being produced, electro
lyte 5 and aluminum pool 4 are both in the molten state.
Table Il
Suspended from above the electrolyte, and partially im
No. of Cells _____________________________ __
Pre-Bake
Cell
Pre-Bake
Cell
(No Li)
(With Li)
20 mersed therein, is anode 6 of the conventional carbon
type and which can be either of the "pre-bake" or “Soder
berg” (self-baking) type known to the art. Molten elec
trolyte 5 is covered by a crust 7 which consists essentially
1
1
Period, Days __________ __
Total Production, Lbs ________ _-
._
252. 6
37, 450. 8
251.4
41, 742. 8
Production, Lbs. per Cell Day..
__
148. 2
166. 0
Increase in Production, Pereent_--
_ ____________ _-
Lining Material __________ __
Current, Amperes _____ _.
Current Increase, Percen
Carbon
Carbon
9, 716
10, 783
____________ _.
Voltage Drop Across Cell--
12
..
11
-
5. 18
5. 11
Cathode ______________________________ __
4.01
4. 13
Anoda Current Density, Amps/sq. in__-_
7. 64
8. 48
Voltage
Drop
Between Anode and
Bath Temperature, °
____
.__
974
961
Bath Ratio, NaF/A1F3___
Bath Freezing Point, ° C
Bath, Pereent CaF2.__-_
_
.
1. 39
965
7. 7
1. 42
942
I 2. 8
Bath, Pereent Alaoa (Average) ____ ._
_
3, 0
2.1
Bath, Pereent LiF (Source of LiF) ____________________ ..
4. 93
of frozen electrolyte constituents and additional alumina.
25 As ?alumina is consumed in electrolyte 5, the frozen crust
is broken and more alumina fed into the electrolyte.
The anode is connected by suitable means (not shown)
to the positive pole of a source of supply of electrolyzing
current. For purposes of completing the electric circuit
30 use is made of cathodic current-conducting elements 8.
The elements 8 extend through suitable openings provided
_
in the metal shell insulation layer and lining with the
inner end thereof projecting into the pool of molten
aluminum. The outer ends of such elements are con
35 nected by suitable means to negative bus-bars 9.
(LiF, L?OH
and LiCO3)
Anoda-cathode Distanee, Inches ________ __
Lbs., C/Lb. Al ___________ __
2. 11
0. 473
2.15
0. 450
Current Ei?eiency, Perce??t
86. 0
K w.h. Per Lb.
8. 15
7. 96
0. 68
0. 457
6. 7
5 5
7.0
5. 1
_______ __
Cathode Voltage Drop, VoltsBath Depth, Inches ___________ __
Metal Depth, Inehes ____________________ __
86.8
1 CaFa not intentio??ally added-introduced into electrolyte as impnri
ies rrom raw materials.
4-0
It will be understood that the drawing is but one of
many apparatus embodiments using such cathodic current
conducting elements that can be used for carrying out
the method of this invention. For example, other modi
?cations involve the current-conducting elements project
ing upwardly from the base of the cell lining with the
upper end of such elements projecting into the molten
aluminum pool or wherein the elements extend down
wardly from above the cell through the electrolyte and
45 with the lower end of the elements immersed in the pool
of molten aluminum.
In addition, it is not necessary
that the elements project completely through the lining,
insulating ilayer and metal shell of the cell structure. In
From Table II it is seen that with the practice of the
invention a substantial increase in productive Capacity is 50 stead, such elements may terminate short of the metal
shell or insulation lining and suitable electrical connec
realized. Also, a sígni?cant decrease in the Consump
tion means made between the outer end of the element
tion of carbon is realized. The power ef?ciencies (based
and the negative bus bar. Moreover, as mentionecl here
on Equation 6 above) are 338% for the cell Operating
without lithium and 34.6% for the cell Operating with
ír?before, such elements may be made up entirely of the
lithium.
55 refractory hard metal, as described, or only in part, it
It is not the principal aim of the present invention,
being essential that that portion of the surface of the
however, to improve the power ef?ciency of the reduc
end of the element which is in contact with the pool
tion operation. There are instances in the practice of
of molten aluminum and electrolyte consists essentially
the invention wherein the power efñcíency may be less
of such materials. Furthermore, the negative current
than that of the conventional cell operation, but the sacri 60 conducting elements can take the form of a sheet, plate,
?ce of power eñiciency in these cases with the use of the
or other suitable shape and, also, Where in e?ect it
present invention will be more than off-set by the in
would function as the cathode of the electrolytic cell in
crease in productive Capacity.
place of the pool of molten aluminum.
As set forth hereinabove, it has also been found, ac
Examples of practice of the present invention, and
cording to the invention, that the use of the lithium-con 65 involving use of a reduction cell structure similar to
taining electrolyte combined With use of a cathodic cur
that schernatically shown in the drawings, are set forth
rent-conductíng element or elements of the refractory
in Table III below. In Table III data are given for two
hard metals, described above, results in still further sig
pre-bake cells (A and B) employing refractory hard
ni?cant increases in productive capacity of the aluminum 70 metals as cathodic current-conducting elements and hav
reduction cell.
ing lithium present in the electrolyte. For comparison,
One embodiment of a reduction cell suitable for carry
ingout the method of this invention is shown schemat
ically in the drawing. In this embodiment, 1 is a metal
Operating data are given for a prebake cell not employ
ing refractory hard metal cathodic current-conducting
elements and not involving a lithium-containing electro
shell, generally steel, within which is disposed in the 75 lyte.
Table lll
Prebake
Cell
(No Ll)
No. oi Cells.,.
Period, Days_
Total Production, Lbs
A
B
Prebake Cell
Prcbake Cell
Elements 1
Elements 1
(With Li)
(With Li)
Reiractory
Hard Metal
Cathodic
Refractory
Hard Metal
Cathodic
1
1
1
140. G
19, 587
144. 5
27, 725. 4
143
27, 105. 2
Production, LbsJCell Day_ , _ ___
__ __
Increase in Production, Percent..
..._ __________ ._
139. 3
Llning Material __________________________ _,
Carbon
Current, Amperes ........................ __
9, 366
Current Increase, Pereent ______ __
191. 9
189. 5
37.8
36
Bottom-Car-
Bottom-Sic
bon Wallš-C
?
Walls-SIC
ll, 895
11,806
27
26
5. 46
5. 02
34 93
Voltage Drop Across Cell, Volts __________ ..
Voltage Drop Between Anode and Cathode,
4. 73
4. 27
74 37
9. 35
9. 30
Batl? Tenipemture, ° C_.
Volts ________________________ ..
__
974
941
946
Bath Ratio, NaF/AIFL›
1.49
1.32
1.32
963
937
937
Both, Percent CaF? __________ __
7. 48
2 2. 42
2 2. 5
Bach, Percent AlzOa (Average).__
3. 76
2. 48
2.49
Anode Current Density, Amps /s
Both Freezing Point, ° 0._
__..
Both, Perccnt LiF (Source of LlF) ____________________ ..
Anode-Cathode Distanca, Inches .......... __
Lbs., C/Lb. Al ................. ..
_.
2. 12
0. 482
84, 7
4. 97
4. 97
(LlF)
(LiF)
2.05 _
0.407
90. 9
2. 08
0. 423
Current E?iciency, Percent..
._
K.w.h. per Lb. Al ___________ ._
_.
8. 19
8.13
7. 51
Cathode Voltage Drop, Volts. _
,_
0. 679
0. 269
0. 208
90. 5
Batl? Depth, lnches _________ _.
Metal Depth, Inches ____________ _.
._
.
6. 8
5. 4
6, 5
5. 3
6. 5
6.2
1 A typical analysis of the reiractory hard metal elements is as follows (in percent by
weight): 'l`iB2~94.5%; TlC-2.5%; B4C_1.5%; 'EN-&8%; Fe-O.25%; Rnor-025%; Free
Carbon-0.20%.
2 OaFz not intentionally added-introduced into electrolyte as impuritics from raw ma
terlals.
lt is thus seen from a comparison of the examples in 35 tiron without departing from the spirit and scope thereof
and, `as such, the invention is not to be limited except by
Table IlI that the productive Capacity of the cell is vastly
the appended claims, wherein
increased, the carbon Consumption per pound of alumi
What is claimed is:
num produced is reduced, the current ei?ciency increased,
1. The method of increasing the output of an electro
and wherein the power per pound of aluminum produced
is substantially the same as that resulting from operation 40 lytic aluminum reduction cell, which cell comprises a
carbon anode, a molten aluminum cathode and a molten
of a conventional electrolytic reduction cell.
salt electrolyte consísting essentially of cryolite and
The power e?iciencíes of the examples in Table III
alumina, and in the operation of which current is passed
are 34%, 34% and 368% for the prebake cell without
from said anode through said electrolyte to said cathode
refractory hard metal cathodíc current-conducting ele
with a given voltage drop across the cell, which method
ments, Cell A, and Cell B, respectively.
Although the invention is not so limited, it will be 45 comprises incorporating lithium-containing material in
said electrolyte in an amount to provide in said electro
noted by way of the examples heretofore discussed that
lyte a lithium content equal to that resulting from an
the cell voltage drops are within the range of about 4.5
to 5.5 volts.
addition of lithium ?uoride in an amount from about 2 to
20 percent by weight of the electrolyte, and establishing
lt Will be apparent from the above description that by
practice of the present invention sígni?cant increases in 50 a current ?ow so that the resultant voltage drop across the
cell is at least substantially as great as said ?rst named
productive Capacity or output can be accomplished with
voltage drop, the temperature of the lithium-modi?ed
existing conventional electrolytic reduction cells, eg.,
electrolyte being above about 845 ° C.
“prebakd” cells and Soderberg cells of either the hori
2. Method according to claim 1 wherein the lithium
zontal stud or Vertical stud type, wherein power con
content of the lithium-containing material incorporated in
sumption per pound of aluminum produced is substantial
the electrolyte is equal to that resulting from the addition
ly the same coupled with lower net carbon Consumption
of lithium ?uoríde in an amount from about 3 to 8% by
per pound of aluminum and higher cell current and power
weight of the electrolyte.
e?lciencies. Moreover, by modi?cation of the cell struc
3. Method according to claim 1 wherein said lithium
ture to include the provision of cathodic current-conduct
ing elements, as described, still further increases in produc 60 containing material is at least one substance selected
from the group consisting of lithium ?uoride, lithium
tive capacity can be accomplished. In addition to conven
cryolite, lithium aluminate, lithium carbonate and lithium
tional electrolytic cells, the invention ís also applicable to
hydroxide.
other electrolytic cells, for example a multiple-cell alumi
4. The method of increasing the output of an electro
num reduction furnace having inclined bipolar electrodes
as disclosed in French Patent l,1l9,832 and a cell em
ploying sloping cathode walls as disclosed in French
Patent l,ll9,82i. Manifestly, from these signi?cant in
creases in productive Capacity or output stern important
05 lytíc aluminum reduction cell which cell comprises a car
bon anode, a solid cathodic current-conducting element
wherein at least the operative surface thereof consists es
sentially of at least one of the materials selected from
the group consisting of the borides and carbides of ti
economic .advantages. Capital investment in electrolytic
furnaces per pound of aluminum produced will be de 70 tanium, zirconium, tantalum and niobium, a pool of
molten aluminum and a molten salt electrolyte consisting
creased. Also, labor, maintenance, and administration
essentially of crylolite and alumina, and in the operation
costs per pound of aluminum produced will be substan
of which current is passed from said anode through said
tially reduced.
electrolyte to said element with a given voltage drop
lt will be understood that various changes, modi?ca
tions and altcrations may be made in the instant inven 75 across the cell, which method comprises incorporating
&034,972
13
lithium-containing material in said elect?'olyte in an
amount to provide in said electrolyte a ljthium content
equal to that resulting from an addition of lithium ?uo
ride in an amount from about 2 to 20% by Weight of the
electrolyte and establishing a current ?ow so that the
voltage drop is at least substantially as great as said ?rst
named voltage drop, the temperature of the lithíum
modi?ed electrolyte being above about 845° C.
5. Method according to claim 4 wherein the lithium
content of the lithium-containing material incorporated 10
in the electrolyte is equal to` that resulting from the addi
tion of lithium ?uo?ide in an amount from about 3 to 8%
by weight of the electrolyte.
6. A method of increasing the output of an electrolytic
alumir?um reduction cell comprising the steps of passing a
current from a carbon anode through a m'olten salt elec
trolyte consistíng essentially of cryolite and alumina, and
14
?ow so that the voltage drop across the cell is in the
range of about 451150 5.5 volts, the temperature of the
electrolyte being above about 845° C.
7. Method according to claim 6 wherein said electro
lyte contains a lithium content equal to that resulting
from the addition of lithiurn ?uoride in amount from
about 3 to 8% by Weight of the electrolyte.
u References Cited in the ?le of this patent
UNITED STATES PATENTS
400,664
886,757
1,566,694
1,709,759
2,062,340
2,919,234
Hall _________________ __ Apr. 2,
Blackmore ____________ __ May 5,
Railsback ____________ __ Dec. 22,
Weber et al ___________ __ Apr. 16,
Weaver ______________ __ Dec. 1,
Slatin ________________ __ Dec. 29,
687,758
1,149,468
Great Britain _________ __ Feb. 18, 1953
France ______________ __ Dec. 26, 1957
to a molten aluminurn cathode, `said electrolytevcontain
ing a lithiurn content equal to` that resulting from the addi
tion of lithíum ?uoride in an amount from about 2 to 20% 20
by weight of the electrolyte; and increasing the current
1889
1908
1925
1929
1936
1959
FOREIGN PATENTS
UNITED STATES PATENT OFFICE
CERTIFICATE OF CORRECTION
Patent No. &034.972
May 15, 1962
' _Robert A. Lewis
ppears in the above numbered pat
It is het-aby certified that error a id Letters Patent should read as
ent requiring correction and that the sa
oorrected below.
Column 3, line 26, for "call" read -- cell --; column 8il
line 29, for "electrolyle" read -- electrolyte --; line 35v for
"electrolye" read -- electrolyte --; same column 8, Table I?
"Produstion" read -- Production
first,
column, Seventh item, for
-, column 9. Table II, column 3, line 19 ther-cof, for
"LiCO3" read -- Li2CO3 --.
Signed and sealed this lst day of January 1963.
(SEAL)
Attes?:
DAVID L. LADD
ERNEST W. SWIDER
Attestiug Officer
Commissioner of Patents
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