Note: Descriptions are shown in the official language in which they were submitted.
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METHODS AND APPARATUS FOR REDUCING SULFUR IMPURITIES AND IMPROVING CURRENT
EFFICIENCIES OF INERT ANODE ALUMINIUM PRODUCTION CELLS
Field of the Invention
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
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 CO2 or CF4. 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.
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
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form A1203. 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.
The present invention has been developed in view of the foregoing, and to
address other deficiencies of the prior art.
Summary of the Invention
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.
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
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bath by adding a reductant such as Al, carbonates (e.g., Na, Ca, Li, Al and Mg
carbonates), CO and/or COa. 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.
Another embodiment of the present invention removes sulfur impurities
from gaseous cell emissions by techniques such as scrubbing with activated
carbon to remove SO2 before it is absorbed by the alumina that is returned to
the
inert anode cell.
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.
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.
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.
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
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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.
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 batli of the cell during operation of
the cell.
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.
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 electrol ~tic 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.
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 impuriity reduction zone within the electrolytic bath.
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In one particular aspect, the invention provides a
method of operating an inert anode electrolytic aluminum
production cell to maintain a low sulfur impurity
concentration, the method comprising: providing a cell
comprising a molten electrolytic bath comprising fluoride
and alumina,a cathode and at least one inert anode; passing
current between the at least one inert anode and the cathode
through the electrolytic bath to produce aluminum;
maintaining a sulfur impurity concentration in the
electrolytic bath of less than about 500 ppm, wherein the
sulfur impurity concentration is maintained by providing an
impurity reduction zone in the electrolytic bath during the
passing step; and recovering aluminum from the cell.
In a further particular aspect, the invention
provides a method of reducing sulfur impurities in an inert
anode electrolytic aluminum production cell, the method
comprising providing an impurity reduction zone within an
electrolytic bath of the cell, while producing aluminum in
the cell, wherein aluminum produced by the cell has an iron
impurity level of less than about 0.5 weight percent.
These and other aspects of the present invention
will be more apparent from the following description.
Brief Description of the Drawings
Fig. 1 is a graph illustrating the build=up of
sulfur impurity levels during operation of an inert anode
aluminum production cell.
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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. l0a-lOd 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. lOb-lOd show solidified baths containing high levels of
sulfur impurities, illustrating the formation of several uncoalesced aluminum
spheres throughout the frozen bath.
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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
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.
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., Na2SO4
and
Na2S03, may be present in various valence states, e.g., S-2, S , 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
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ppm. In a particularly preferred embodiment, sulfur impurity levels are
maintained below about 100 ppm.
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.
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.
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
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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.
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.
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, TiB2, W, Mo, carbon
steel or stainless steel.
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
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be made of any suitable material such as TiB2, 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 SO2. The cell 10 is typically a commercial scale cell
operated above 50,000 Amps for the commercial production of aluminum.
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.
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 ce1130 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 S2.
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.
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 ce1150 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, TiB2, 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
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interior 53 of the barrier tube 52. This substantially oxygen-free zone allows
sulfur-containing species such as SO2 to vent from the bath 13 through the
barrier
tube 52 rather than creating unwanted oxygen-containing reaction products in
the
bath 13.
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 TiB2 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 Na2SO3. The same
results were achieved using Na2SO 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 TiB2 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 TiB2 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 TiB2 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.
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
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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.
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.
After running a test in an inert anode cell at 4 amp/cm2 for 30 mins, 500
ppm of S as Na2SO3 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.
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, Na2CO3a CaCO3, Li2CO3,
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MgCO3, CO and CO2. 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 CO2 may be
added to the bath by means such as standard sparging techniques.
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.
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.
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
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solution into the exhaust gases. Dry scrubbing uses materials having high
surface
areas, such as active carbon or lime, which react with the gases.
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
SO2
onto activated carbon occurs in two steps. In the first step SO2 is
catalytically
oxidized on the carbon to SO3. Then the SO3 hydrolyzes in the presence of
water
vapor to form sulfuric acid, which condenses in the pores of the carbon:
SO2+1hOZCarS03
SO3 + H20 - H2SO4 (condensed).
Fig. 11 is a schematic diagram of a sulfur scrubbing system 60 including a
ce1162 equipped with a hood 64. Pot gases 66 comprising oxygen, sulfur-
containing species such as SO2 and fluorides flow from the cel162 to an
activated
carbon bed 68 where the SO2 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 SO2
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 ce1162 via contact with the alumina 82 in the dry scrubber 78.
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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.
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 SO2 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 SO2 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.
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-
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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.
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.
