TREATMENT OF GASES FROM A CARBOTHERMIC REACTOR FOR THE PRODUCTION OF ALUMINUM

TRAITEMENT DES GAZ D'UN REACTEUR CARBOTHERMIQUE, POUR LA FABRICATION DE L'ALUMINIUM

English Abstract

A B S T R A C T
In the production of aluminium by the
carbothermic reduction of alumina the evolved CO gas,
laden with Al and Al2O fume, is contacted with
gaseous chlorine at a location within the carbothermic
reduction furnace or at or near the gas outlet from
the furnace and before the evolved gas has undergone
significant cooling. The amount of chlorine employed
is preferably in excess of the requirement for
conversion of all the fume content to AlCl3. Further
quantities of AlCl3 may be generated by injecting
alumina and chlorine into the CO gas stream at a
carbochlorination reactor downstream from the
reduction furnace.

Claims

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The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. In a process for the production of aluminium by
the carbothermic reduction of alumina, the improvement which
comprises contacting the evolved CO gas, laden with Al and
Al2O fume, with gaseous chlorine at a location within the
carbothermic reduction furnace or at or near the gas outlet
and before the evolved gas has undergone significant cooling.
2. A process according to claim 1 in which fume-laden
CO gas is contacted with chlorine in amount in excess of the
stoichiometric quantity for conversion of the Al + Al2O fume
content to AlCl3.
3. A process according to claim 2 in which alumina is
injected into said CO gas for reaction with said CO and
chlorine for production of extra quantities of aluminium
chloride.
4. A process according to claim 3 in which a first
portion of chlorine gas is introduced into the fume-laden CO
gas stream before significant cooling of said gas stream and
a second portion of chlorine gas is introduced into said CO
gas stream at a carbochlorination reactor, at which alumina
is introduced into said gas stream.
5. A process according to claim 2, 3 or 4 in which
aluminium chloride is separated from said gas stream and is
supplied to an electrolysis cell for production of aluminium,
the chlorine in said electrolysis cell being recycled to the
treatment of said CO gas.

Description

~ S'~ 3
The presen~ inventi.on relat~s to the production of
aluminium by the carbothermic rcduction o~ alumina.
Variou~ method~ for the production of all~nina by
carbothermic r~ductlon have been de~cribed. All such
methods suffer rom the dr~wb~ck that the carbon monoxide gaS3
evolved at the high tempe~ature~ a~ociated with khe
productlon of aluminium me~al ~y thîs route, has ~ high content
o~ gaseous Al fume and aluminium suboxideO These sub~ta~ces
back react rapidly wlth carbon monoxide to yield alumina and
carbon as the ga9 iS cooled. Such back reaction involve~ a
sub~tantial process lo~ in the energy employed. The back
reaction also results in very hard dense deposits of ~lumina
~nd aluminium oxycarbide at the ga~ outlets ~rom the furn~ce
where the evolved g~5 i3 inevitably ~ubjected to cooli.ng and
1~ con~eguent back reactlon of carbon monoxide wi.th the ft~e
rarried by the ga~.
It is an object of the present inventlo~ to overcGme
the problem resu].ting from the ~ormati.on o deposits at
locations where the local temperature conditions are only
911ghtly below the temperature of the gases in the furnace~
A~cording to the present :Lnventlon a process for
the production of aluminillm by carbothermic recluction o
alumi.na is characterlsed in that chlorine is injected into the
evolved ga~es either withln the furnace or at or near the gas
outlet rom the ~urnace, i.e~ at a location where the local ~as
temperature is ~bove 2000C. The injected chlorine gas reacks
preferen~ially to C0 with the Al fume and A120 ~o form reaction
products which rema:ln in the ga~eous phase down to relati.vely
low temperature~
In mc~st instance~ chlorine i~ in~ected in an ~moun~

which i3 essentially the stoi.chiometric quarltity required to
converlt the whole o: the Al ftune and Al 2 content of the ~as
(which may ~mount to 40~0 by we-Lght of the :~me) to aluminium
tr:lch10ride~ which condenses to the solid ~tate at about 200C
S or can be absorbed in mo1ten mixes of NaCl/AlCl3 ~t ~omewhat
higher temperatures.
Alumin-Lum trichloride is ~ valuab1e chemical ~ubstance
and can also be u~ed as feed materi~1 for prodllction of
aluminlum by electrolysis in a mu1tipo1ar ce111
It is already known that aluminium trichloride can
be produced by reaction of alumina with gaseous chlorine and
carbon monoxide. Thus the ume laden exhaust gas from a
carbotherrE ic reduction furnace can be uti 1i~ed as a source of
a1uminium trichlor1de by direct reaction of chlorine with the
Aluminium content in the ~ume and it may be additiona11y
employed with the chlorlde or the productiQn of alumin:3um
chloride by reacti on of aluminitlm oxideO The required amount
of ch1Orine or utilisation of the C0 rnay be in~cted into the
cArbothermic reduction ~u~ace or at the outlet~ Addit:Lon of
chlorine may ~18O be made in part ~t the carbo-ch1Orination
Ytage where addit:i.onal alumina may optionally be brought lnto
contact with th~ ~aseous cn for convers.ion to AlC13.
One sy~tem for car~ying Otlt the invention i.s fu~ther
describ~d with reference to the accompanying drawing,
The carbothenmic reactor, shown diagrammatically at 19
1~ constructed and designed to operate as described with
reference to Figure 8 of Ca~adian Patent No. 17084,974~
The reactor 1 has a low temperature zone ve~el 2
and R high temperature zone ves~el 3, re~pectively having gas
outlets 4 and 5.

--3~
Feed materials, C0 an~ Al~037 ~re supplied to
vessel 2 and aluminium product met~l is taken off from
vessel 3.
Except for h ~mall propor~ion taken off a~ Al~G3
dissolved in the A1 product mekal, th~ carbon ~eed leav2~
the reactor 1 in the form of C0 gas~ The ma ~ of G0 gas
liberated in ve~sel 2 i5 approxim~tely double the mass of ~as
liberated in veæsel 3. However, bec~use of ~he substantially
lower temperature condi~ions in vessel 2 than in vessel 37 the
ftIme content o the ga~ exiting from v@ssel 2 is low and in
many C~3~8 it i~ unneces~ary to subject it to treatment wlth
chlorine.
In performing the proces~ o the lnvention chlorine
ga~ is in~ected at the gas outle~ S ~t the point where lt
leave~ the vessel 3.
A~ shown in th~ accompanying drawing chlorlne is
in~ected into the C0 gas ~tream ~t 6. T~e ~mount of chlor~ne
~ .cted at 6 is preferably suficient to convert the whole
Al ~ A120 ~ume content to AlC13. The thus-treated C0 gas
9tream i9 led via conduit 7 to a carbo-chlorination reactor 8.
The gas ~tre~m tmdergoe3 substantia~ cooling during its
p~s~e through condui.t 7 but no signi~icant ~olid clepositæ
occur ~s ~ result of back re~ction. Feed al~mina may
optionally be supplied ts reactor 8 via inlet 9 and a gas
~5 stream compo~ed ~ C02 + AlC13 vapour i~ led out at 103
Additional chlorln~ may be led inkc the reactor 8 at 11.
Additlonally the C0 gas stream from the vessel 3 may optvnally
be led to the reactor 8 via conduit 12 and in such case ik .i~
pre~er~ble that chlorine in sufficient amount to avold
fo~natiGn of deposits in conduit 12 is in~ected at 14.

5~3
~4--
The gas led out rom the carbochlori.nation
reactor 8 via 10 is passed into a desublimer 15, in whi.ch ~he
gas is chilled to a temperature below the sublim~tion
temperatur~ of AlC13. The non~condellsible gas, es.~en~ially
C2 is exhausted through the outlet 16, The cond~nsed AlC13
i~ removed via 17.
In one arrangement the AlG13 may be supplied directly
to an electrolysi.s cell and the resul~ing chlorin~ may be
recycled back to the carbothermlc reductlon urnace5 either
directly or via the medium o~ transport cylinders.
Although ln this example the inventi~n is applled to
the CO of~g~s from a reactor o the type de~cribed, it is
equally applicable in any sy~tem where aluminlum metal ls
produced by the carbothermic reduction of ~luminium~