Field of the Invention
[0001] The present invention relates to the operation of electrolytic aluminum production
cells. More particularly, the invention relates to the reduction of sulfur impurities
in inert anode aluminum production cells in order to increase current efficiencies
of the cells.
Background of the Invention
[0002] Aluminum is conventionally produced in electrolytic reduction cells or smelting pots
which include an electrolytic bath comprising molten aluminum fluoride, sodium fluoride
and alumina, a cathode, and consumable carbon anodes. The energy and cost efficiency
of aluminum smelting can be significantly reduced with the use of inert, non-consumable
and dimensionally stable anodes. Replacement of traditional consumable carbon anodes
with inert anodes allows a highly productive cell design to be utilized, and may provide
environmental benefits because inert anodes produce essentially no CO
2 or CF
4. Some examples of inert anode compositions comprising nickel ferrite-based ceramic
materials and/or metal alloys are provided in
U.S. Patent Nos. 5,794,112,
5,865,980,
6,126,799,
6,217,739,
6,332,969,
6,372,119,
6,416,649,
6,423,195 and
6,423,204.
[0003] During aluminum smelting operations, deleterious impurities such as sulfur, iron,
nickel, vanadium, titanium and phosphorous may build up in the electrolytic bath.
For example, in inert anode cells, sulfur species can build to higher concentrations
in the bath because it is no longer removed as COS or other sulfur-containing species
as in consumable carbon anode cells. The presence of sulfur or other multi-valence
elemental impurities in the bath causes unwanted redox reactions which consume electrical
current without producing aluminum. Such impurities can significantly reduce the current
efficiency of the cells. Sulfur species have a high solubility in the bath and act
as oxidizing agents to react Al to form Al
2O
3. This can cause unwanted back reaction of the aluminum which also reduces the current
efficiency of the cell. Furthermore, sulfur, iron, nickel and other impurities in
the bath can lower the interfacial energy between the bath and the molten pad of aluminum
formed in the cell, thereby reducing coalescence or promoting emulsification of the
surface of the aluminum pad.
[0004] The present invention has been developed in view of the foregoing, and to address
other deficiencies of the prior art.
Summary of the Invention
[0005] The present invention recognizes the build up of sulfur impurities in inert anode
aluminum production cells, and reduces such impurities in order to increase current
efficiencies of such cells. Sulfur impurities may be reduced and removed in regions
of the bath in order to achieve high current efficiencies. Gaseous emissions may be
scrubbed prior to dry scrubbing with alumina in order to minimize the recirculation
of impurities into the bath while maintaining acceptably low sulfur concentrations.
Sulfur content of materials introduced into the bath may be controlled.
[0006] An embodiment of the present invention provides impurity reduction zones in the bath
of inert anode aluminum production cells which reduce or eliminate unwanted impurities.
In one embodiment, the impurity reduction zone is provided by a purifying electrode
having an electrochemical potential that is controlled within a selected potential
range which reduces or oxidizes sulfur impurities, thereby facilitating removal of
the impurities from the bath. For example, reduced sulfur species have much lower
bath solubility than oxidized sulfate impurity species, and the reduced sulfur species
can escape relatively easily from the bath while avoiding a redox cycle caused by
the oxidized sulfate species. In another embodiment, the impurity reduction zone comprises
a volume of the bath in which oxygen is reduced or eliminated, e.g., oxygen generated
during operation of an inert anode cell is prevented from entering a region of the
bath. In a further embodiment, the impurity reduction zone is created through all
or portion of the bath by adding a reductant such as Al, carbonates (e.g., Na, Ca,
Li, Al and Mg carbonates), CO and/or CO
2. In another embodiment, electric current flow is interrupted through some or all
of the electrodes of a cell, or electrodes are not positioned in certain areas of
the cell, in order to allow sulfur-containing gas to escape from the bath. These embodiments
in which impurity reduction zones are provided in the bath may be used alone or in
various combinations.
[0007] Another embodiment of the present invention removes sulfur impurities from gaseous
cell emissions by techniques such as scrubbing with activated carbon to remove SO
2 before it is absorbed by the alumina that is returned to the inert anode cell.
[0008] A further embodiment of the present invention reduces sulfur impurities to acceptable
levels by controlling the sulfur content of materials added to the bath, such as the
sulfur content of alumina and aluminum fluoride fed to the bath. Mass balance calculations
may be used in order to select acceptable sulfur content of alumina and other materials
added to the bath.
[0009] An aspect of the present invention is to provide a method of operating an inert anode
electrolytic aluminum production cell. The method comprises providing a cell comprising
an electrolytic bath, a cathode and at least one inert anode positioned at or above
a level of the cathode, passing current between the inert anode and the cathode through
the electrolytic bath, and maintaining a sulfur impurity concentration in the electrolytic
bath of less than about 500 ppm. In a preferred embodiment, the sulfur impurity concentration
is maintained below about 100 ppm.
[0010] Another aspect of the present invention is to provide a method of reducing sulfur
impurities in an electrolytic aluminum production cell. The method comprises providing
an impurity reduction zone within an electrolytic bath of the cell. In a preferred
embodiment, the cell comprises inert anodes.
[0011] A further aspect of the present invention is to provide a method of producing aluminum.
The method includes the steps of providing a cell comprising an electrolytic bath,
a cathode and at least one inert anode located at or above a level of the cathode,
passing current between the at least one inert anode and the cathode through the electrolytic
bath, maintaining a sulfur impurity concentration in the electrolytic bath of less
than about 500 ppm, and recovering aluminum from the cell.
[0012] Another aspect of the present invention is to provide an inert anode electrolytic
aluminum production cell comprising means for reducing sulfur impurities contained
in an electrolytic bath of the cell during operation of the cell.
[0013] A further aspect of the present invention is to provide an inert anode electrolytic
aluminum production cell comprising a cathode, at least one inert anode located at
or above a level of the cathode, an electrolytic bath communicating with the cathode
and the at least one anode, and a sulfur impurity reduction zone within the electrolytic
bath.
[0014] Another aspect of the present invention is to provide an inert anode electrolytic
aluminum production cell comprising a cathode, at least one inert anode, an electrolytic
bath communicating with the cathode and the at least one anode, and a purifying electrode
at least partially submerged in the electrolytic bath for providing a sulfur impurity
reduction zone within the electrolytic bath.
[0015] A further aspect of the present invention is to provide an inert anode electrolytic
aluminum production cell comprising a cathode, at least one inert anode, an electrolytic
bath communicating with the cathode and anode, and a purifying electrode at least
partially submerged in the electrolytic bath for providing an impurity reduction zone
within the electrolytic bath.
[0016] These and other aspects of the present invention will be more apparent from the following
description.
Brief Description of the Drawings
[0017]
Fig. 1 is a graph illustrating the build up of sulfur impurity levels during operation
of an inert anode aluminum production cell.
Fig. 2 is a partially schematic side sectional view of an aluminum smelting cell including
an anodic purifying electrode which utilizes the power supply of the cell in accordance
with an embodiment of the present invention.
Fig. 3 is a partially schematic side sectional view of an aluminum smelting cell including
an anodic purifying electrode which utilizes a separate power supply in accordance
with an embodiment of the present invention.
Fig. 4 is a partially schematic side sectional view of an aluminum smelting cell including
a cathodic purifying electrode with an interior cathode connection in accordance with
an embodiment of the present invention.
Fig. 5 is a partially schematic side sectional view of an aluminum smelting cell including
a cathodic purifying electrode with an exterior cathode connection in accordance with
an embodiment of the present invention.
Fig. 6 is a partially schematic side sectional view of an aluminum smelting cell including
an oxygen barrier tube submerged in the electrolytic bath in accordance with a further
embodiment of the present invention.
Fig. 7 is a graph of sulfur impurity concentration versus operation time of an inert
anode aluminum production cell incorporating a purifying electrode in accordance with
an embodiment of the present invention.
Fig. 8 is a graph of current efficiency versus sulfur impurity concentration within
an electrolytic bath, showing substantially reduced current efficiencies at higher
sulfur impurity levels.
Fig. 9 is a graph of current efficiency versus sulfur impurity concentration within
an electrolytic bath and total impurity levels in the produced aluminum, demonstrating
substantially reduced current efficiencies at higher sulfur impurity levels and higher
aluminum impurity levels.
Figs. 10a-10d are photographs of solidified baths. Fig. 10a shows a solidified bath
with minimal sulfur impurities in which a coalesced aluminum pad has been formed.
Figs. 10b-10d show solidified baths containing high levels of sulfur impurities, illustrating
the formation of several uncoalesced aluminum spheres throughout the frozen bath.
Fig. 11 is a partially schematic diagram of a bath emission scrubber system in accordance
with an embodiment of the present invention.
Figs. 12-17 are graphs of sulfur impurity concentrations in electrolytic baths versus
cell operation times, illustrating mass balance calculations for cells operated with
varying sulfur impurity levels in the alumina feed, cells operated with and without
a purifying electrode, and cells operated with and without activated carbon SO2 scrubbers.
Detailed Description of Preferred Embodiments
[0018] The present invention reduces sulfur impurities during aluminum smelting processes
which have been found to adversely affect current efficiency of the electrolytic cells.
Additional types of impurities to be reduced or eliminated include iron, copper, nickel,
silicon, zinc, cobalt, vanadium, titanium and phosphorous impurities. The "current
efficiency" of a cell can be determined by the amount of aluminum produced by a cell
during a given time compared with the theoretical amount of aluminum that could be
produced by the cell based upon Faraday's Law.
[0019] Sulfur is a particularly harmful impurity which has been found to significantly adversely
effect current efficiency of inert anode cells. For example, in inert anode cells,
sulfur in ionized forms such as sulfates, e.g., Na
2SO
4 and Na
2SO
3, may be present in various valence states, e.g., S
-2, S
0, S
+2, S
+4 and S
+6. The S
+6 species is particularly disadvantageous in inert anode cells because it can be easily
reduced and subsequently reoxidized. The sulfur impurities form redox couples between
the anodes and cathodes of the cells which consume electricity without producing aluminum.
Furthermore, sulfur impurities adversely affect the bath/aluminum interfacial energy
such that uncoalesced aluminum is dispersed in the bath where it can be more easily
oxidized. Current efficiency is significantly reduced as a result of sulfur impurities.
It is therefore desirable to eliminate some or all sulfur species from the bath. It
is typically desirable to maintain sulfur impurity levels below about 500 ppm in the
bath, preferably below about 250 ppm. In a particularly preferred embodiment, sulfur
impurity levels are maintained below about 100 ppm.
[0020] Iron impurities are disadvantageous because iron can also form redox couples which
adversely affect current efficiency of the cell. Furthermore, it is desirable to minimize
the amount of iron impurities contained in the aluminum produced by the cell. Iron
impurity levels in the produced aluminum are preferably maintained below about 0.5
weight percent, typically below about 0.25 or 0.2 weight percent. In a particularly
preferred embodiment, the iron impurity level is below about 0.18 or 0.15 weight percent.
Copper impurity levels in the produced aluminum are preferably maintained below about
0.2 or 0.1 weight percent, more preferably below about 0.04 or 0.03 weight percent.
Nickel impurity levels in the produced aluminum are preferably maintained below about
0.2 or 0.1 weight percent, more preferably below about 0.03 weight percent. The produced
aluminum also preferably meets the following weight percentage standards for other
types of impurities: 0.2 maximum Si; 0.03 maximum Zn; and 0.03 maximum Co.
[0021] Individually, sulfur and iron impurities have been found to significantly reduce
the current efficiency of inert anode aluminum production cells. For example, sulfur
levels above about 500 ppm in some inert anode cells have been found to reduce the
current efficiency of the cells below about 80 percent. The combination of sulfur
and iron impurities has been found to be particularly disadvantageous in inert anode
cells. The build-up of combined sulfur and iron impurity levels can actually cause
aluminum produced during operation of the cell to be removed.
[0022] It has been found that during the operation of inert anode cells, the amounts of
sulfur and other impurities may initially be within acceptable levels, but may increase
to unacceptable levels during continued operation of the cell. In comparison with
consumable carbon anode cells which produce COS, inert anode cells have been found
to build up sulfur impurities in the bath to levels above 500 ppm, often above 1,000
ppm. Fig. 1 is a graph illustrating the build up of sulfur impurity levels during
operation of an aluminum production cell after the consumable carbon anodes of the
cell have been replaced with inert anodes. After several days of operation with the
inert anodes, the sulfur impurity level increases above 500 ppm.
[0023] In accordance with an embodiment of the present invention, impurity reduction zones
are provided in aluminum production cells. Figs. 2-5 illustrate embodiments in which
reduction zones are created through the use of at least one purifying electrode positioned
in the bath.
[0024] Fig. 2 is a partially schematic side sectional view of an aluminum smelting cell
10 in accordance with an embodiment of the present invention. The cell 10 includes
a refractory wall 11 and a cathode 12. During operation, the cell 10 is partially
filled with a molten electrolytic bath 13 which is contained by the refractory wall
11. During the aluminum production process, a molten pad of aluminum 14 forms at the
bottom of the cell 10. An anode assembly 15 includes anodes 16a and 16b which are
partially submerged in the bath 13. The anodes 16a and 16b are positioned above the
level of the cathode 12 in the embodiment shown in Fig. 2. However, other anode/cathode
configurations known in the art may be used in accordance with the present invention
in which at least a portion of the anode(s) are positioned at the same level as the
cathode(s). With these configurations, sulfur impurities tend to build up in the bath
13 without contacting the aluminum pad 14 that is formed at the bottom of the cell
10. The anodes 16a and 16b preferably comprise inert anodes, for example, as disclosed
in
U.S. Patent Nos. 6,162,334,
6,217,739,
6,332,969,
6,372,119,
6,416,649,
6,423,195 and
6,423,204 comprising ceramic and/or metallic compositions. A purifying electrode 17 is partially
submerged in the bath 13. The purifying electrode 17 may be made of any suitable material
such as carbon, graphite, TiB
2, W, Mo, carbon steel or stainless steel.
[0025] In the embodiment shown in Fig. 2, the purifying electrode 17 is connected to the
power supply of the cell 10. An oxygen barrier 18 is provided in the bath 13 between
the anode 16b and the purifying electrode 17. The oxygen barrier 18 may be made of
any suitable material such as TiB
2, BN or ferrites. During anodic operation of the cell 10, current supplied to the
purifying electrode 17 creates a positive potential of sulfur, such that sulfur species
are oxidized, e.g., to gaseous phases such as COS and SO
2. The cell 10 is typically a commercial scale cell operated above 50,000 Amps for
the commercial production of aluminum.
[0026] Fig. 3 is a partially schematic side sectional view of an aluminum smelting cell
20 in accordance with another embodiment of the present invention. The cell 20 is
similar to the cell 10 shown in Fig. 2, with the exception that the purifying electrode
17 is connected to a separate power supply 19.
[0027] Fig. 4 is a partially schematic side sectional view of an aluminum smelting cell
30 in accordance with a further embodiment of the present invention. The cell 30 is
similar to the cell 10 shown in Fig. 2, except the cell 30 includes a purifying electrode
37 which operates in a cathodic mode through its contact with the molten aluminum
pad 14 which, in turn, is electrically connected to the cathode 12. The purifying
electrode 37 operates at a negative potential of sulfur, such that sulfur species
are reduced, e.g., to elemental S or gaseous S
2.
[0028] Fig. 5 is a partially schematic side sectional view of an aluminum smelting cell
40 in accordance with another embodiment of the present invention. The cell 40 is
similar to the cell 30 shown in Fig. 4, except it includes a purifying electrode 47
that is externally connected to the cathode 12.
[0029] Fig. 6 is a partially schematic side sectional view of an aluminum smelting cell
50 in accordance with a further embodiment of the present invention. The cell 50 is
similar to the cell 10 shown in Fig. 2, except the cell 50 does not include a purifying
electrode and is provided with an oxygen barrier tube 52 partially submerged in the
bath 13. The oxygen barrier tube 52 may be made of any suitable material such as alumina,
TiB
2, BN or ferrites. The interior 53 of the oxygen barrier tube 52 contains a portion
of the bath 13 which is isolated from gaseous species generated at the interface between
the anodes 16a and 16b and the bath 13. For example, when the anodes 16a and 16b comprise
inert anodes, oxygen generated at the anode/bath interface is prevented from entering
the interior 53 of the barrier tube 52. This substantially oxygen-free zone allows
sulfur-containing species such as SO
2 to vent from the bath 13 through the barrier tube 52 rather than creating unwanted
oxygen-containing reaction products in the bath 13.
[0030] Fig. 7 is a graph of sulfur concentration versus operation time of bench scale aluminum
production cells operated with a single inert anode. In Fig. 7, the dashed lines represent
tests performed with no purifying electrodes, while the solid lines represent tests
performed with TiB
2 purifying electrodes. The dashed lines in Fig. 7 show sulfur levels in the test cell
operated without a purifying electrode, after doping with 200 ppm sulfur (lower dashed
line) then doping with 300 ppm sulfur (upper dashed line). Doping was done using Na
2SO
3. The same results were achieved using Na
2SO as the dopant. The sulfur concentration remained substantially constant or slightly
increased in these cells operated without a purifying electrode The round points in
Fig. 7 are from a test cell similar to those illustrated in Figs. 2 and 3 incororating
a TiB
2 purifying electrode which was maintained at an electrode potential of E=0 V relative
to the aluminum potential. In this cell, the sulfur concentration decreased from an
initial level of about 560 ppm to about 110 ppm within 2 hours. The square points
in Fig. 7 are from a test cell similar to that shown in Fig. 4 with a TiB
2 purifying electrode immersed into the metal pad. In this cell, the sulfur concentration
decreased from about 250 ppm to about 110 ppm within 2 hours. The triangular points
in Fig. 7 are from a test cell similar to that shown in Fig. 5 in which a TiB
2 purifying electrode was externally connected to the cathode. In this cell, the sulfur
impurity level decreased from about 160 ppm to about 120 ppm in 2 hours.
[0031] An electrochemical test was conducted to determine the affect of sulfur impurity
concentrations on the current efficiency of a test cell comprising an inert anode.
The test was conducted by setting up an electrolytic cell using commercial Hall-bath
and a cermet inert anode, adding different concentrations of S as sulfide/sulfate
into the bath, and using standard cyclic voltammetry and chronopotentiometry methods
to determine the effect of S concentration in the bath on current efficiency. Fig.
8 is a graph of current efficiency versus sulfur concentration in the bath, demonstrating
significant decreases in current efficiencies as the sulfur impurity levels increase.
At sulfur concentrations above 500 ppm, the current efficiency of the cell decreases
below 70 percent.
[0032] Fig. 9 is a graph showing current efficiency versus sulfur impurity levels in a bath
and total impurity levels in the produced aluminum. A test was performed to determine
the influence of sulfur on current efficiency at a relatively large scale. An electrochemical
cell including one inert anode and was operated at 950 Amperes. Initially the electrolyte
was low in sulfur and the contaminates in the aluminum produced by the cell were at
low levels. Since the alumina is decomposed to oxygen and aluminum, oxygen evolution
from the cell was used to determine the current efficiency of the cell. Aluminum contaminants
such as iron, nickel and copper were added to the cell to determine their effect on
current efficiency. Fig. 9 is a summary of the results of this test. At low sulfur
levels in the electrolytic bath and low aluminum impurities, the current efficiency
was above 90 percent. As sulfur and contaminants were added the current efficient
initially fell below 80 percent, then 70 percent, and eventually dropped to less than
50 percent. As shown in Fig. 9, current efficiency is substantially decreased by sulfur
impurities in the bath and impurities contained in the aluminum produced by the cell.
[0033] After running a test in an inert anode cell at 4 amp/cm
2 for 30 mins, 500 ppm of S as Na
2SO
3 was added to the bath. The metal at the end of the test was not coalesced. Several
aluminum spheres were present in the solidified bath, and a few aluminum spheres were
seen in the solidified bath. Photographs of uncoalesced aluminum spheres are provided
in Figs. 10b-10d. For comparison purposes, a photograph of solidified bath having
a coalesced aluminum pad from a cell having a minimal sulfur impurity level is shown
in Fig. 10a.
[0034] In accordance with another embodiment of the present invention, the impurity reduction
zone is created through all or a portion of the bath by adding or controlling the
distribution of reductants such as Al, Na
2CO
3, CaCO
3, Li
2CO
3, MgCO
3, CO and CO
2, When Al is used to reduce impurities, it may be added in the form of recirculated
aluminum produced by the cell, or the aluminum may be added as pellets, rods or slabs.
The aluminum reductant may be continuously or intermittently added to the bath. Gaseous
reductants such as CO and CO
2 may be added to the bath by means such as standard sparging techniques.
[0035] In accordance with a further embodiment of the present invention, electric current
flow may be interrupted through some or all of the electrodes of a cell in order to
allow impurities to escape from the cell in gaseous forms. For example, electrode
current may be interrupted to some or all of the inert anodes of a cell in order to
allow sulfur-containing gas such as sulfur dioxide to escape from the bath. Alternatively,
selected regions of the cell may not include anodes in order to provide a region or
regions within the cell where oxygen generation is reduced or eliminated.
[0036] The various embodiments for producing impurity reduction zones as described herein
may be combined. For example, when an oxygen barrier tube as show in Fig. 6 is used,
a purifying electrode such as shown in Figs. 2-5 may be positioned within the tube.
Alternatively, purifying reductants such as aluminum may be introduced into the bath
through such an oxygen barrier tube, with or without the additional use of a purifying
electrode.
[0037] In accordance with another embodiment of the present invention, sulfur contained
in gaseous emissions from inert anode cells is removed by scrubbing techniques. During
inert anode cell operations, the hot gases emitted from the cell may be recovered
and used to heat the incoming alumina feed by passing the hot gases over the alumina.
When sulfur and other impurities contained in the gaseous emissions contact the alumina,
they are absorbed and carried back to the cell by the incoming alumina. Scrubbing
removes sulfur in the off-gas flow, e.g., by electrostatic or chemical (wet or dry
scrubbing) means. Electrostatic techniques use electrically charged plates or electrostatic
precipitators, which attract the charged sulfur species. The surface is periodically
cleaned to remove deposited sulfur species. Wet scrubbing means injecting water or
a chemical solution into the exhaust gases. Dry scrubbing uses materials having high
surface areas, such as active carbon or lime, which react with the gases.
[0038] Sulfur removal may be achieved by passing the gaseous emissions through a bed of
reactive material such as activated carbon or the like. Adsorption of SO
2 onto activated carbon occurs in two steps. In the first step SO
2 is catalytically oxidized on the carbon to SO
3. Then the SO
3 hydrolyzes in the presence of water vapor to form sulfuric acid, which condenses
in the pores of the carbon:
[0039] Fig. 11 is a schematic diagram of a sulfur scrubbing system 60 including a cell 62
equipped with a hood 64. Pot gases 66 comprising oxygen, sulfur-containing species
such as SO
2 and fluorides flow from the cell 62 to an activated carbon bed 68 where the SO
2 and other sulfur-containing species are removed. Carbon and sulfuric acid 70 from
the activated carbon bed 68 are treated in a regeneration chamber 72, and regenerated
carbon 74 is reintroduced into the activated carbon bed 68. The activated carbon can
be regenerated by treatment with water in the regeneration chamber 72 to form an effluent
73 such as dilute acid or chemicals such as gypsum. Oxygen and fluoride gases 76 exit
the activated carbon bed 68 and pass through a dry alumina scrubber 78 to remove fluoride
values so they can be returned to the cell 62, thereby recycling the fluoride values
and minimizing fluoride emissions to the atmosphere. Gases from the scrubber 78 are
vented 80 to atmosphere. Alumina 82 is fed to the dry scrubber 78. As described in
more detail below, the alumina 82 may comprise various sulfur impurity contents. After
the alumina 82 is contacted with the oxygen and fluoride gases 76 in the dry scrubber
78, the alumina and absorbed fluorides 84 are recycled 86 to the cell 62. It is important
that the SO
2 scrubbing in the activated carbon bed 68 does not remove a significant amount of
the fluoride from the pot gases 66 so the maximum amount of fluorides can be recycled
to the cell 62 via contact with the alumina 82 in the dry scrubber 78.
[0040] In addition ot the system 60 shown in Fig. 11, alternative scrubbing or stripping
systems that may be used in accordance with the present invention include other types
of reactive beds such as lime beds, aqueous leaching systems, electrostatic precipitators,
and the like.
[0041] In accordance with a further embodiment of the present invention, the sulfur content
of various materials introduced into the bath is controlled. Figs. 12-17 illustrate,
through mass balance calculations, the influence on the steady state concentration
of sulfur in the cell of the following parameters: the use of cleaner raw materials;
scrubbing SO
2 from the pot gas to reduce recycle back to the cell; and providing an impurity reduction
zone in the cell. Fig. 12 shows that with a sulfur content in the alumina fed to the
cell of 60 ppm, and considering 40 percent efficient dry scrubbing, the steady state
sulfur in the bath would be under 100 ppm. As shown in Fig. 13, with 110 ppm sulfur
in the alumina, the use of an activate carbon bed also can achieve 102 ppm sulfur
in the bath. As shown in Fig. 14, with 110 ppm sulfur in the alumina and without the
activated carbon bed, the sulfur increases to 170 ppm. Increasing the sulfur in the
alumina to 250 increases the sulfur in the bath to 374 ppm, as shown in Fig. 15. The
use of an impurity reducing zone in the cell would increase the SO
2 removal four-fold, allowing the use of 250 ppm sulfur alumina while achieving a sulfur
level in the bath of less than 100 ppm, as shown in Fig. 16. The combination of an
impurity reducing zone in the cell with activated carbon scrubbing can permit the
use of alumina containing as much as 450 ppm while still achieving a sulfur level
in the bath of 100 ppm, as shown in Fig. 17.
[0042] In accordance with an embodiment of the present invention, the sulfur content of
alumina may be selected within various ranges while maintaining acceptable sulfur
impurity levels in the bath. For example, low-sulfur alumina having a sulfur content
within a range of from about 40 to about 100 ppm may be used with no additional sulfur-reducing
steps, or with minimal additional sulfur-reducing techniques. Medium-sulfur alumina
having a sulfur content within a range of from about 100 to about 250 ppm may be used
with selected sulfur-reducing techniques of the present invention necessary to achieve
the desired sulfur concentration in the bath. High-sulfur alumina having a sulfur
content of from about 250 to about 600 ppm or higher may be used in combination with
the present sulfur-reducing techniques in order to maintain the desired sulfur concentration
in the bath.
[0043] Having described the presently preferred embodiments, it is to be understood that
the invention may be otherwise embodied within the scope of the appended claims.
1. A method of operating an inert anode electrolytic aluminium production cell (10, 20,
30, 40, 50), the method comprising:
providing a cell (10, 20, 30, 40, 50) comprising an electrolytic bath (13), a cathode
(12) and at least one inert anode (16a, 16b) located at or above a level of the cathode
(12);
passing current between the at least one inert anode (16a, 16b) and the cathode (12)
through the electrolytic bath (13); and
maintaining a sulfur impurity concentration in the electrolytic bath (13) of less
than 500 ppm; wherein the sulfur impurity concentration is maintained by providing
an impurity reduction zone in the electrolytic bath (13), the impurity reduction zone
being provided by either
(a) a purifying electrode (17, 37, 47) at least partially submerged in the electrolytic
bath (13),
(b) an oxygen barrier member (18, 52) at least partially submerged in the electrolytic
bath (13),
(c) adding a purifying reductant to the electrolytic bath (13),
(d) removing at least one inert anode from a region of the cell, or
(e) interrupting electrical current through at least one electrode of the cell.
2. The method of Claim 1, wherein the sulfur impurity concentration is maintained below
100 ppm.
3. The method of Claim 1 or claim 2, wherein the cell operates at a current efficiency
of at least 80 percent.
4. The method of Claim 1 or claim 2, wherein the cell operates at a current efficiency
of at least 90 percent.
5. The method of Claim 1, wherein aluminum produced by the cell has maximum impurity
levels of 0.5 weight percent iron, 0.2 weight percent copper and 0.2 weight percent
nickel.
6. An inert anode electrolytic aluminum production cell (10, 20, 30, 40, 50) comprising:
a cathode (12);
at least one inert anode (16a, 16b) located at or above a level of the cathode (12);
an electrolytic bath (13) communicating with the cathode (12) and the at least one
anode (16a, 16b); and
a sulfur impurity reduction zone within the electrolytic bath (13), provided by
(a) a purifying electrode (17, 37, 47) at least partially submerged in the electrolytic
bath (13), or
(b) an oxygen barrier member (18, 52) at least partially submerged in the electrolytic
bath (13).
1. Verfahren zum Betreiben einer elektrolytischen Inertanodenaluminiumproduktionszelle
(10, 20, 30, 40, 50), wobei das Verfahren Folgendes umfasst:
Bereitstellen einer Zelle (10, 20, 30, 40, 50), die ein elektrolytisches Bad (13),
eine Kathode (12) und mindestens eine Inertanode (16a, 16b), die sich an oder über
einer Höhe der Kathode (12) befindet, umfasst;
Leiten von Strom zwischen der mindestens einen Inertanode (16a, 16b) und der Kathode
(12) durch das elektrolytische Bad (13) und
Aufrechterhalten einer Schwefelverunreinigungskonzentration in dem elektrolytischen
Bad (13) von weniger als 500 ppm; wobei die Schwefelverunreinigungskonzentration durch
Bereitstellen einer Verunreinigungsverringerungszone in dem elektrolytischen Bad (13)
aufrechterhalten wird, wobei die Verunreinigungsverringerungszone durch eine bzw.
eines der folgenden bereitgestellt wird:
(a) eine Reinigungselektrode (17, 37, 47), die zumindest zum Teil in das elektrolytische
Bad (13) eingetaucht ist,
(b) ein Sauerstoffbarriereelement (18, 52), das zumindest zum Teil in das elektrolytische
Bad (13) eingetaucht ist,
(c) Zugeben eines Reinigungsreduktans zu dem elektrolytischen Bad (13),
(d) Entfernen mindestens einer Inertanode aus einer Region der Zelle oder
(e) Unterbrechen eines elektrischen Stroms durch mindestens eine Elektrode der Zelle.
2. Verfahren nach Anspruch 1, wobei die Schwefelverunreinigungskonzentration unter 100
ppm gehalten wird.
3. Verfahren nach Anspruch 1 oder 2, wobei die Zelle mit einer Stromausbeute von mindestens
80 Prozent arbeitet.
4. Verfahren nach Anspruch 1 oder 2, wobei die Zelle mit einer Stromausbeute von mindestens
90 Prozent arbeitet.
5. Verfahren nach Anspruch 1, wobei von der Zelle produziertes Aluminium maximale Verunreinigungsgrade
von 0,5 Gewichtsprozent Eisen, 0,2 Gewichtsprozent Kupfer und 0,2 Gewichtsprozent
Nickel aufweist.
6. Elektrolytische Inertanodenaluminiumproduktionszelle (10, 20, 30, 40, 50), die Folgendes
umfasst:
eine Kathode (12);
mindestens eine Inertanode (16a, 16b), die sich an oder über einer Höhe der Kathode
(12) befindet;
ein elektrolytisches Bad (13), das mit der Kathode (12) und der mindestens einen Anode
(16a, 16b) in Verbindung steht; und
eine Schwefelverunreinigungsverringerungszone in dem elektrolytischen Bad (13), die
durch Folgendes bereitgestellt wird:
(a) eine Reinigungselektrode (17, 37, 47), die zumindest zum Teil in das elektrolytische
Bad (13) eingetaucht ist, oder
(b) ein Sauerstoffbarriereelement (18, 52), das zumindest zum Teil in das elektrolytische
Bad (13) eingetaucht ist.
1. Procédé pour faire fonctionner une cellule de production d'aluminium électrolytique
à anode permanente (10, 20, 30, 40, 50), le procédé comprenant :
se procurer une cellule (10, 20, 30, 40, 50) comprenant un bain électrolytique (13),
une cathode (12) et au moins une anode permanente (16a, 16b) située à ou au-dessus
d'un niveau de la cathode (12) ;
faire passer un courant entre la au moins une anode permanente (16a, 16b) et la cathode
(12) à travers le bain électrolytique (13) ; et
maintenir une concentration d'impuretés à base de soufre dans le bain électrolytique
(13) de moins de 500 ppm, la concentration en impuretés à base de soufre étant maintenue
par disposition d'une zone de réduction d'impuretés dans le bain électrolytique (13),
la zone de réduction d'impuretés étant fournie par l'un ou l'autre de
(a) une électrode de purification (17, 37, 47) au moins partiellement immergée dans
le bain électrolytique (13),
(b) un élément barrière à l'oxygène (18, 52) au moins partiellement immergé dans le
bain électrolytique (13),
(c) addition d'un réducteur de purification au bain électrolytique (13),
(d) retrait d'au moins une anode permanente à partir d'une région de la cellule, ou
(e) interruption du courant électrique à travers au moins une électrode de la cellule.
2. Procédé selon la revendication 1, dans lequel la concentration en impuretés à base
de soufre est maintenue au-dessous de 100 ppm.
3. Procédé selon la revendication 1 ou la revendication 2, dans lequel la cellule fonctionne
à un rendement en courant d'au moins 80 pour cent.
4. Procédé selon la revendication 1 ou la revendication 2, dans lequel la cellule fonctionne
à un rendement en courant d'au moins 90 pour cent.
5. Procédé selon la revendication 1, dans lequel l'aluminium produit par la cellule a
des taux d'impuretés maximaux de 0,5 pour cent en poids de fer, 0,2 pour cent en poids
de cuivre et 0,2 pour cent en poids de nickel.
6. Cellule de production d'aluminium électrolytique à anode permanente (10, 20, 30, 40,
50) comprenant :
une cathode (12) ;
au moins une anode permanente (16a, 16b) située à ou au-dessus d'un niveau de la cathode
(12) ;
un bain électrolytique (13) communiquant avec la cathode (12) et la au moins une anode
(16a, 16b) ; et
une zone de réduction d'impuretés à base de soufre à l'intérieur du bain électrolytique
(13), fournie par :
(a) une électrode de purification (17, 37, 47) au moins partiellement immergée dans
le bain électrolytique (13), ou
(b) un élément barrière à l'oxygène (18, 52) au moins partiellement immergé dans le
bain électrolytique (13).