CA1188523A - Treatment of gases from a carbothermic reactor for the production of aluminum - Google Patents

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.

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~

Claims (5)

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.