U.S. patent application number 10/932846 was filed with the patent office on 2006-03-02 for method using single furnace carbothermic reduction with temperature control within the furnace.
Invention is credited to Richard J. Fruehan.
Application Number | 20060042413 10/932846 |
Document ID | / |
Family ID | 35941150 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060042413 |
Kind Code |
A1 |
Fruehan; Richard J. |
March 2, 2006 |
Method using single furnace carbothermic reduction with temperature
control within the furnace
Abstract
A start-up process of making aluminum using a single
carbothermic reactor/furnace (11) having a single hollow reactor
compartment with bottom resistance heating electrodes (16) (13) in
the side of the reactor, involves adding Al.sub.2O.sub.3 and C
(carbon) for start-up and melting the mixture to provide a
(Al.sub.2O.sub.3--Al.sub.4C.sub.3) slag having a temperature
between about 1875.degree. C. and 2000.degree. C.; and then adding
Al.sub.4C.sub.3 to the slag and raising the temperature of the
furnace (11) to form a top Al phase with 6 to 8 wt % C (21) and a
bottom slag phase (22); and then adding Al.sub.2O.sub.3 is added to
the Al--C/slag (21, 22) to produce an Al.sub.2O.sub.3 rich slag
lower the reactant temperature and produce a decarbonization
reaction (step 30) providing a top Al phase with less than 5 wt % C
(23) which is then tapped after step (40). The remaining slag is
the starting material.
Inventors: |
Fruehan; Richard J.;
(Murrysville, PA) |
Correspondence
Address: |
ECKERT SEAMANS CHERIN & MELLOTT, LLC;ALCOA TECHNICAL CENTER
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
35941150 |
Appl. No.: |
10/932846 |
Filed: |
September 1, 2004 |
Current U.S.
Class: |
75/10.27 |
Current CPC
Class: |
C22B 5/10 20130101; C22B
4/02 20130101; C22B 21/02 20130101 |
Class at
Publication: |
075/010.27 |
International
Class: |
C22B 4/02 20060101
C22B004/02 |
Claims
1. A method of using a single carbothermic reactor to produce
aluminum with low carbon content, comprising: (a) providing a
single furnace having a single hollow, interior reactor compartment
with a plurality of bottom resistance heating electrodes in the
furnace walls and one or more optional vertical electrodes; (b)
adding Al.sub.2O.sub.3 and C for start-up of the process to the
inside of the furnace and melting their mixture, to provide a
(Al.sub.2O.sub.3--Al.sub.4C.sub.3) and excess Al.sub.4C.sub.3 slag
having a temperature between about 1875.degree. C. and 2000.degree.
C.; (c) adding Al.sub.4C.sub.3 to the slag, and raising the
temperature of the furnace to form a top Al phase with about 6 wt %
to 8 wt % C and a bottom slag phase having a temperature between
about 2050.degree. C. and 2100.degree. C.; (d) adding
Al.sub.2O.sub.3 to the Al--C/slag which Al.sub.2O.sub.3 addition
results in producing an Al.sub.2O.sub.3 rich slag and in lowering
the temperature to between about 1800.degree. C. and 1900.degree.
C., to produce a decarbonization reaction within the single reactor
compartment, providing a top Al phase with less than <5 wt % C
and a bottom (Al.sub.2O.sub.3 rich-Al.sub.4C.sub.3) slag having a
temperature between about 1800.degree. C. and 1900.degree. C.; and
(e) tapping the top Al<5 wt % C phase; and (f) repeating steps
(b) to (e).
2. The method of claim 1, wherein at least one vertical top
electrode is used to provide arc heating in step (b).
3. The method of claim 1, wherein, in step (d) addition of
Al.sub.2O.sub.3 changes the slag composition, transferring C from
Al to the slag, and where in step (d) the top Al phase has <3 wt
% C, which is tapped in step (e).
4. The method of claim 1, wherein off-gas comprising Al.sub.2O, CO
from step (b) and step (c) Al result are fed to a reactor to
produce Al.sub.4C.sub.3 and Al.sub.2O.sub.3 or
Al.sub.4C.sub.3--Al.sub.2O.sub.3 slag which is added to the slag in
step (c).
5. The method of claim 1, wherein, in step (d) the top Al phase has
<3 wt % C, which is tapped in step (e).
6. The method of claim 1, wherein after step (e) the temperature is
increased to from about 1875.degree. C. to 2000.degree. C. and
Al.sub.2O.sub.3 and C are added to begin step (a).
7. The method of claim 1, wherein, in step (d) during
decarbonization C is transferred form the Al phase to the slag.
8. The method of claim 1, wherein, Al.sub.2O, CO, and gaseous Al
are passed to an off gas reactor, where C is added to produce solid
Al.sub.4C.sub.3 and Al.sub.2O.sub.3 and
Al.sub.4C.sub.3--Al.sub.2O.sub.3 slag which is returned to the
furnace.
9. The method of claim 8, wherein the Al.sub.4C.sub.3 is returned
during step (c).
10. The method of claim 1, wherein the bottom resistance heating
electrodes are disposed in the side of the furnace adjacent the
bottom slag phase and are of a material selected form the group
consisting of carbon, graphite or non-consumable inert anode
material comprising ceramic.
11. The method of claim 1, wherein the slag from step (d) is used
as a starting material for the next cycle, along with additional
Al.sub.2O.sub.3 and C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of producing low
carbon aluminum in a single reactor compartment carbothermic
furnace with control to lower or raise the temperature of reactants
within the interior of the reactor compartment.
BACKGROUND OF THE INVENTION
[0002] The direct carbothermic reduction of alumina has been
described in U.S. Pat. No. 2,974,032 (Grunert et al.) and U.S. Pat.
No. 6,440,193 B 1 (Johansen et al.) as well as in Proceedings
6.sup.th Conference on Molten Slags Fluxes and Salts, Edited by S.
Seetharaman and D. Sichen "Carbothermic Aluminum", K. Johansen, J.
Aune, M. Bruno and A. Schei, Stockholm, Sweden-Helsinki Finland,
Jun. 12-17, 2002. It has long been recognized that the overall
reaction: Al.sub.2O.sub.3+3C=2Al+3CO (1) takes place, or can be
made to take place, generally in steps such as:
2Al.sub.2O.sub.3+9C=Al.sub.4C.sub.3+6CO (vapor) (2)
Al.sub.4C.sub.3+Al.sub.2O.sub.3=6Al+3CO (vapor) (3)
Al.sub.2O.sub.3+2C=Al.sub.2O (vapor)+2CO (vapor) (4)
Al.sub.2O.sub.3+4Al=3Al.sub.2O (vapor) (5), and Al=Al (vapor)
(6).
[0003] Reaction (2) takes place at temperatures below 2000.degree.
C. and generally between 1900.degree. C. and 2000.degree. C.
Importantly, reaction (3), which is the aluminum producing
reaction, takes place at higher temperatures of about 2050.degree.
C., and requires substantial heat input. Very importantly, in
addition to the species stated in reactions (2) and (3), volatile
species including gaseous Al, reaction (6), and gaseous aluminum
suboxide that is Al.sub.2O, are formed in reaction (4) or (5). In
the overall carbothermic reduction process, the Al.sub.2O and Al
gases are recovered by reacting them with carbon in a separate
reactor usually called the vapor recovery unit or vapor recovery
reactor.
[0004] Other patents relating to carbothermic reduction to produce
aluminum include U.S. Pat. No. 4,099,959 (Dewing et al.), where
dual reaction zones are described and where off gases are passed
through granular carbon material and countercurrent to fresh coal
or "green" coke in a gas scrubber. U.S. Pat. Nos. 4,033,757 and
4,388,107 (both Kibby) teach reduction of carbon content by heating
the surface of the melt to about 2100.degree. C. while maintaining
a lower temperature of about 1850.degree. C. in the slag thereby
lowering C (carbon) in the metal. This however would seem to be
difficult in operation and would appear to cause excessive
vaporization. The former Kibby '757 patent uses arc heating and a
plasma jet in a process that starts at 1850.degree. C.-1950.degree.
C., then arc heats to 2100.degree. C., producing Al with <10 wt.
% C. The latter Kibby '107 utilizes a secondary furnace or separate
decarbonization zone requiring transfer of very hot metal and slag
to and from the furnace.
[0005] Other art in this area, includes, for example, U.S. Pat.
Nos. 4,334,917 and 4,533,386 (both Kibby) which appear to teach
either multiple reactors or additional decarbonization zones. U.S.
Ser. No. 10/646,507, filed Aug. 23, 2003 (J. Aune et al.-Docket
03-0668) teaches an electrode arrangement for a single reactor
compartment carbothermic furnace, where side wall electrodes, each
connected to the other, substitute for a bottom lining as an
electrical contact and vertical electrodes are submerged in the
liquid slag both.
[0006] In the carbothermic process, the use of dual reaction zones
or a plurality of furnaces, adds expense to the process, and
unnecessary complication. What is needed is an efficient and simple
method for recovering lower carbon containing aluminum. Therefore,
it is one of the main objects of this invention to provide a more
cost and energy effective, improved aluminum production process, by
use of a single reactor compartment, carbothermic furnace with
temperature control of the reactor compartment.
SUMMARY OF THE INVENTION
[0007] The above needs are met and the above problems solved by
providing a method of using a single carbothermic reactor to
produce aluminum with low carbon content, comprising: (a) providing
a single furnace having a single hollow, interior reactor
compartment with a plurality of bottom resistance heating
electrodes and one or more optional vertical top electrodes; and
then; (b) adding Al.sub.2O.sub.3 and C for start-up of the process
to the inside of the furnace and melting their mixture, to provide
a (Al.sub.2O.sub.3--Al.sub.4C.sub.3) slag and excess
Al.sub.4C.sub.3 having a temperature between about 1875.degree. C.
and 2000.degree. C.; and then (c) adding Al.sub.4C.sub.3 to the
slag, and raising the temperature of the furnace to form a top Al
phase with about 6 wt % to 7 wt % C and a bottom slag phase having
a temperature between about 2050.degree. C. and 2100.degree. C.;
and then (d) adding Al.sub.2O.sub.3 to the Al--C/slag, which
Al.sub.2O.sub.3 addition results in producing Al.sub.2O.sub.3 rich
slag and in lowering the temperature to between about 1800.degree.
C. and 1900.degree. C., to produce a decarbonization reaction
within the single reactor compartment, providing a top Al phase
with less than (<) 5 wt % C and a bottom (Al.sub.2O.sub.3
rich-Al.sub.4C.sub.3) slag having a temperature between about
1800.degree. C. and 1900.degree. C.; and then (e) tapping the top
Al<5 wt % C phase; and (f) repeating steps (b) to (e). This slag
is then used to begin the next cycle. The next cycle is begun by
adding some C and Al.sub.2O.sub.3 to the bottom slag and repeating
steps (c) to (e). Preferably the tapped aluminum phase is Al<3
wt % C and the Al.sub.4C.sub.3 added in step (c) is from a vapor
recovery unit associated with the reactor.
[0008] In step (b), arc heating using retractable, at least one
vertical top electrodes are preferably used to provide slag. In
step (d), addition of Al.sub.2O.sub.3 at this stage, very
importantly, lowers the temperature within the furnace and changes
the slag composition transferring a substantial amount of C from
aluminum to the slag. This provides a very simple method to produce
lower carbon containing aluminum, where only one furnace or reactor
is used in the process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention is further described with reference to the
accompanying non-limiting drawings in which:
[0010] FIG. 1 is a flow sheet showing one example of a previously
conceptualized system of a carbothermic reduction process for the
production of aluminum, including an off-gas vapor recovery reactor
to recover the Al.sub.2O and Al vapors as Al.sub.4C.sub.3 and/or
Al.sub.2O.sub.3 solids (and Al.sub.4C.sub.3--Al.sub.2O.sub.3 slag);
and
[0011] FIG. 2 is flow sheet showing the steps involved in this
invention to produce low carbon aluminum utilizing a single
reactor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0012] FIG. 1 is a simplified illustration of one embodiment of a
carbotherimc reaction process to produce Al and, recover A1,
Al.sub.2O and CO in the off-gases as Al.sub.4C.sub.3,
Al.sub.2O.sub.3 and slag and passes this material to the smelting
furnace. In FIG. 1, gas flows are shown as dashed lines and flows
of solids and molten substances are shown as solid lines.
[0013] In FIG. 1, the off-gas from a carbothermic smelting furnace
here, for simplicity, comprising a first stage 1 and possibly a
second stage 2 is forwarded via conduits 3 and 4 to an enclosed
off-gas reactor 5 operating at a temperature of about 1600.degree.
C. to 2050.degree. C. depending on the type reactor. There could be
more than one such reactor, for example, one for stage one smelter
1 and one for stage two smelter 2. The reactor 5 could be a
counter-current moving bed reactor or a fluid bed or a series of
fluid beds. The Al-components of the off-gas entering the reactor 5
react with the carbon to form Al.sub.4C.sub.3, Al.sub.2O.sub.3 and
Al.sub.4C.sub.3--Al.sub.2O.sub.3 slag material. Conduit 6 can be
used to pass this material to stage 2.
[0014] The gas from reactor 5 contains primarily CO, and possibly
some H.sub.2 from the volatile part of the charcoal reactor charge
and little or no Al or Al.sub.2O. The off gas from reactor 5 has a
high energy value as hot CO and could be used to produce electrical
energy in a gas turbine or conventional boiler. The aluminum vapor
species will have reacted to carbide, condensed to Al.sub.2O.sub.3
and C or formed an Al.sub.2O.sub.3--Al.sub.4C.sub.3 slag. The
Al.sub.4C.sub.3--Al.sub.2O.sub.3 slag and unreacted carbon is fed
into the second stage of the carbothermic smelter via conduit 6. An
Al--C liquid alloy exits smelter stage 2 as shown in FIG. 1, where
(s) means solid, (v) means vapor and (liq) means liquid in FIG.
1.
[0015] FIG. 2 illustrates the basic steps, reactions and reactants
in the method of this invention. This new process uses a single
furnace, so no slag recycle is required, and slag resistance
heating to avoid excess vaporization. In the first step of the
process, Al.sub.2O.sub.3 and carbon are added and
Al.sub.2O.sub.3--Al.sub.4C.sub.3 slag is produced which can contain
excess Al.sub.4C.sub.3 above saturation. The furnace operates at
about 1875.degree. C. to 2000.degree. C. to produce slag. The
second step produces an Al-6-8 wt % C alloy at about 2050.degree.
C. to 2100.degree. C. and requires additional energy and additional
Al.sub.4C.sub.3, part of which is the excess from the first step
and the remainder is from the vapor recovery unit. Next, room
temperature Al.sub.2O.sub.3 is added to the furnace, an
Al.sub.2O.sub.3 rich slag is produced and the temperature decreases
to about 1850.degree. C. This removes the carbon into the alumina
rich Al.sub.2O.sub.3--Al.sub.4C.sub.3 slag. This will remove about
65 wt % of the carbon, that is, the carbon in the aluminum will
decrease from 6 wt % to 2 wt %. The overall process is much simpler
since it is not a multi-stage reactor and does not require transfer
of hot liquid slag.
[0016] In the first step 10 of FIG. 2, slag is produced. In the
second step 20 metal 21 is produced with about 5 to 7 wt % C on top
of a slag phase 22 and gases are released (not shown for the sake
of simplicity). In the third step 30, an extraction or
decarbonization reaction is provided, at lowered temperatures to
reduce vapor loss, where added Al.sub.2O.sub.3, is at ambient
temperature (about 20.degree. C. to about 35.degree. C.), and
importantly, helps lower both temperature substantially and
provides an alumina rich slag in step 40. Here, C is transferred
from the Al phase to provide an aluminum phase containing less than
(<) 5 wt % C phase, preferably a <3 wt % C phase 23, which is
then tapped. Steps 30 and 40 merge somewhat.
[0017] In summary, in the process, we have:
[0018] Slag Making: To start up the process, Al.sub.2O.sub.3 and
carbon are added to make a liquid slag, 77% Al.sub.2O.sub.3-23%
Al.sub.4C.sub.3 (mole percent) at about 1900.degree.
C.-2000.degree. C. and some excess Al.sub.4C.sub.3. Some Al.sub.2O
and Al vapors are formed and go to the vapor recovery reactor 5.
Once the process is at a steady state, the starting point for slag
making is the slag remaining after decarburization in the previous
cycle.
[0019] Metal Making: Metal is produced by the following reaction at
about 2050.degree. C.-2100.degree. C.:
Al.sub.2O.sub.3+Al.sub.4C.sub.3=6Al+3CO Aluminum carbide is added
from the vapor recovery reactor 5. About 17% of the Al will
vaporize as Al.sub.2O and Al. It is not possible to react all of
the slag since the energy is supplied by slag resistance heating
through the slag and some slag must remain in the furnace. About
20% of the slag does not react and remains for resistance heating.
Some decarburization can occur by raising the temperature after all
the carbide is added and reducing the carbide content of the slag
and carbon in the metal but this will result in large amounts of
Al.sub.2O and Al vaporization.
[0020] Decarburization: Al.sub.2O.sub.3 is added to the furnace to
remove carbon from the metal. Some electric power is necessary to
heat and melt the Al.sub.2O.sub.3 while some of the energy comes
from the sensible heat of the slag since its temperature is higher
than required for decarburization The slag-metal system is allowed
to cool to about 1850.degree. C. The slag becomes rich in
Al.sub.2O.sub.3 and carbon is transferred from the metal to the
slag (Al.sub.2C.sub.3). The metal is tapped and the resulting
Al.sub.2O.sub.3 rich liquid slag is the starting point for return
to slag making.
[0021] After the metal is tapped the temperature is increased to
about 1900.degree. C.-2000.degree. C. and Al.sub.2O.sub.3 and
carbon are added once more, to produce the desired liquid slag
compositions and excess Al.sub.4C.sub.3 for metal making. In the
process, substantial amounts of CO are produced which carry Al as
Al and Al.sub.2O gaseous species. These are converted to
Al.sub.4C.sub.3 in the vapor recovery reactor 5 and returned to the
furnace during metal making, all as shown in FIG. 2.
[0022] Generally, in step 10, a single furnace 11, having side
walls and a bottom, and a single, hollow reactor compartment 13, as
shown in FIG. 2, is used solely in this invention; without interior
underflow partition walls/baffles or the like. The furnace can have
a substantially rectangular, square, circular or oval shape. Within
the side walls of the furnace are bottom resistance heating
electrodes 16, preferably located in the side(s) of the reactor as
shown. In step 10, preferably, at least one top vertical
retractable exterior electrode 12 is used. It can provide an arc to
melt the solid Al.sub.2O.sub.3 and C at start-up or at steady
state, added to producing molten slag phase
Al.sub.2O.sub.3--Al.sub.4C.sub.3 slag plus additional
Al.sub.4C.sub.3.
[0023] The electrodes 12 and 16 can be made from carbon, graphite,
or non-consumable inert ceramic materials, where each is
individually supplied with electricity by electric current means
19. The bottom resistance heating electrodes are preferably
horizontal and used in metal making to reduce super heating the
metal and causing excessive vaporization. The bottom electrodes 16
are also preferably disposed at/adjacent to the bottom phase molten
slag phase/level 22, as shown in steps 20, 30 and 40. In step 10
and 20, Al.sub.2O, vapor, CO and Al exit as streams 3 and 3'. The
Al.sub.2O.sub.3, C, Al.sub.4C.sub.3 supply means in steps 10 to 30
are preferably gas tight. The purified aluminum stream 26 may then
be passed to any number of apparatus, for example, degassing
apparatus to remove, for example, H.sub.2 fluxing apparatus to
scavage oxides from the melt and eventually to casting apparatus to
provide unalloyed primary shapes such as ingots or the like of
about 50 lb. (22.7 Kg) to 750 lb. (341 Kg). These ingots may then
be remelted for final alloying in a holding or blending furnace or
the melt from fluxing apparatus may be directly passed to a furnace
for final alloying and casting as alloyed aluminum shapes.
[0024] Whereas particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to
those skilled in the art that numerous variations of the details of
the present invention may be made without departing from the
invention as defined in the appended claims.
[0025] An example of how this process would work beginning with the
metal making stage, 100 moles of a 77% Al.sub.2O.sub.3-23%
Al.sub.4C.sub.3 slag. As the temperature increases, reaction (3)
occurs. Al.sub.4C.sub.3 would be added to the slag to maintain the
slag composition. The reaction proceeds until there are 15 moles of
Al.sub.2O.sub.3 and 5 moles of carbide remaining in the furnace.
The process will produce 372 moles of Al but 62 moles will vaporize
leaving 310 moles of liquid Al containing about 7.5 wt % C.
[0026] The Al vaporized will produce about 15 moles of carbide.
During slag making enough Al is vaporized to produce 10 moles of
carbide. A total of 62 moles of carbide are required in the metal
making step. With 28 moles of carbide reacting from the slag and
about 25 moles from the vapor recovery reactor ("VRR") there is a
deficit of about 9 moles of Al.sub.4C.sub.3. This additional
carbide can be produced in slag making so the actual starting point
is: [0027] 100 k moles of slag containing 77% Al.sub.2O.sub.3-23%
Al.sub.4C.sub.3 and 9 k moles of Al.sub.4C.sub.3 About 25 k moles
of Al.sub.4C.sub.3 are added from the VRR.
[0028] For metal making, the slag +Al.sub.4C.sub.3 is heated to a
higher temperature (2050.degree. C.-2100.degree. C.) producing 310
k moles aluminum metal containing about 7.5 wt. % C. About 20 k
moles of slag remain for resistance heating.
[0029] For decarburization 75 k moles of Al.sub.2O.sub.3 is added
making the resulting slag 90 k moles Al.sub.2O.sub.3-and 12 moles
Al.sub.4C.sub.3. The temperature is decreased to about 1850.degree.
C. At the lower temperature the carbon distribution ratio between
slag and metal increases. The carbon content of the metal is
reduced from about 6.0% to 2.5%. This is based on a carbon
distribution ratio between slag and metal of 2 and 8904 kg of metal
and 9900 kg of slag. The metal is tapped and the remaining slag, 90
k moles Al.sub.2O.sub.3-12 k moles Al.sub.4C.sub.3, is the starting
point for slag making.
[0030] After the metal is tapped the temperature is increased to
about 2000.degree. C. and Al.sub.2O.sub.3 and carbon are added to
produce the desired liquid slag composition and excess
Al.sub.4C.sub.3 for metal making. This will require about 225 k
moles of C and 37 k moles of Al.sub.2O.sub.3. After the slag is
made the metal making step is repeated.
[0031] 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.
* * * * *