CA1185435A - Fume recovery system in the carbothermic production of aluminium - Google Patents

Abstract

ABSTRACT

In the carbothermic reduction of alumina evolved carbon monoxide has a high content of Al fume and Al2O. A substantial part of this fume is recovered by allowing the temperature of the CO gas to fall by 50-150°C during passage through a moving bed of carbon particles at a temperature of 50-150°C.
The carbon particles move through the bed in either co-current or counter-current direction in relation to the CO gas and such bed may be retained in an enclosure which is rotatable about a vertical axis to a body of molten alumina, which lies beneath it.
Solid alumina may also be introduced into the carbon particle bed. This is conveniently done by introducing the alumina particles into the top end of the bed with a downwardly flowing CO gas stream.

Description

5~

IMPROV~D FU~E RECOVERY SY~EM l~ ~HE CARBOTHERMIC
PRODUC~ION OF ALU~NIUMt' ~ he prese~t i~vention relates to the carbothermic reduction of alumina to produce aluminium metal and in particular to an improved method and apparatus for recovery of fume generated in carbothermic reduction proce~es.
~ he reduction of alumina with carbon is highly endothermic and only proceeds to the production of alumi~ium met~l (in th~ absence of other reducible oxides) at temperatures in excess o~ 2050Co ~he pro-duction of alumi~ium metal at these ver~ high temper-atures is accomp~ied by evolutio~ o~ very large volumes of c~rbon monoxid~.
D 15 Many differen~ proposals for.carbother~ic reductio:cl of e~entially pure alumiIla have been put forward and some practic~ql succ~s~ has bee:rl obtained~
q!hus in U.Sf. :Pate~t ~o. 2,97~,032 a reactio~
~Ix~ure of carbon and ~lumi~a was heated ~rom above 20 with ~n ope~ arc from carbon ~lectrode~ at a temper-at~re ~ n excess of 2400C~
In U~S. Patent ~o. 3,783,167 it haa b~n pro-posed to produce ~lumi~ium by c~rbo*hermic reduction of alumina in the pla~ma of a pla~ma fur~ace.
I~ U~So Patent ~v. 1~9099,959 a molten alumi~a slag ~ containing dissolved alumi~ium carbide, i~
circulated ~uccessively through a zone of relatively low temperature, i~ which carbon f~ed material i8 added to the slag to react with the ~lumina to au~mo~t the aluminium carbide content of the slag ~ and a zone of relatively high temperature in which al~ium carbide react~ with alumina to relea~e alumi:~ium metal which i~
collect~d and ~eparated from the ~lag~ the alumi~ium carbide co~tent of the slag bei~K ~imultaneousl~y ~5 ~educed. ~he ~lag fr~m the h:igh temperature zo:ne may ~1~

543~;

be returned to the preceding low temperature zone in a

2-vessel 8~8tem or it m~y be forwarded to a succeeding ~ow temperat~re zone in a multi-~essel ~ystemO ~lumina is supplied at a suitable location, pr~fer~bly a high temperature zone, to replace the alumina consumed in the process.
In all the above-mentioned processes and~ indeed, in an~ process involving carbothermic reduction of alumina 9 the ac~ual production of aluminium metal involves an oporating temperature i~ the reaction zone (or final reaction ~o~e) of at least 2050C and usually higher.
At sueh high temperatures the partial pressures of Al Yapour and Al20, alumiDium suboxide, are substantial and th~e component~ back-react with the evolved carbon monoxide as the gas temperature is lowered. Such back-reaction is highly exothermic and reprefie~ts a very large pote~tial 1O-RS of ener~ urthermore, it gives ris~ to the fo~mation of deposit~ of al~mi~ium o~y-carbide, wh~ch are ~tick~ and tend to block up gas c~nduits.
. In U.S~ Patent ~o. 49099~59 the reactio~ in the low te~perature zone may be repreRented a~
2 ~l203 ~ 9C -- -~ Al4C3 (in solution) ~ 6C0 wh~reas the reaction in the high temperature zone may be repreaented as ~l4C~ (in ~olution~ ~ Al203 -- ~ 6A1 ~ 3C0 ~he~s reaction~ are both highly endothe~mic and proceed at temp~rature~ within the ranges of about 1g50 ~ 2050

3 and 2050 ~ 2150C respectively~ ~he te~p~ratures in ~he low temperature zone(~) and the high temperature zone(~) are accordingly held within the above specified temperature r~n~e 5 .
~he large volumes of gas r~lea~ed i~ th~ low temperature zone(~) and in the high temperature Z0~(8) --3~
carry substantial quantitie~ of fume (both Al metal vapour and alumlnium suboxlde, A120). The amount of fu~e carried by the evolved C0 is~however considerably greater in the gas evolved in the high temperature zone than in 5 the gas from the low temperature zone because of the higher temperature and consequently higher vApour pressure of Al and Al~0.
In U.SO Patent No. 4,099,959 these fume components are removed from the evolved gas by passing 10 the gas throu~h the carbon feed material prior to introductlon of the feed material to the low temperature zone and contact with rarbon ~s clearly applicable for f~me removal from carbon monoxide evolved in any carbothermic proce3s for production of alumini.um from 15 alumina. It is an ob~ect of the present inventlon to ~llow back-rea~-tion of the fume component~ to proceed under controlled conditions which avoids the formation of m&~slve deposits which lead to blockag~s. It is a further ob~ect of the invent~on to allow back reactlon 20 to take place under such conditlons that at least a part of the heat released in exothermic bark reactions i~ recovered in a form directly relevan~ to the carbo~
thermic reduction process.
In accordance with the pre~ent invention the back 25 reaction takes place ~n a moving bed of carbon particles held at a temperature in the r~nge of 1950 - 2080C~ In one arrangement the gas is brought into contact with ~olld ~lumina ln such bed9 the solld a~umlna being converted to molten alumin~ which is then lntrvduced into 30 the carbothermic reduction reactor system. The solid alumina ls in one alternatlve suppl~ed to the top of the packed ~S9L3~

-4-bed of carbon particles. A molt~n alumina slag9 containlng dissolved aluminium carbide, flows out of the bottom of the packed bed.
As an alternative to supply of alumina to the top of the bed, ~ part of or all the solid alumina~
preheated if deslr~ed; can be fed into the molten slag in the region of the bottom of the reactor. In a further alternative, a part of or all the alumina may be injected directly in the form of solid par~icles into the fume-laden gas stream before entry to the bedO
The gas may be progressed counter-current or co-~.urrent to the flow o~ alumina slag in the packed bPd, which is preferably subjected to mild agitation to prevent the ormation of channels in the packed bed, and/or form~tion of solid bridges between the components of the ~ed and/or the bed and the wall. Since carbon i5 consumed in the scrubbing procedure ~additional~~o the carbon formed by back-reaction of aluminl~m valu2s (Al Yapour and Al~0) with carbon monoxide) addi~lonal carbon is ~upplied to the bed as well as the solid alumina feed. Some at least of the additional carbon is preferably in the form of coarse particles of a size typical of the particles of the packed bed to make good losses fro~ the bed in operation.
The scrubbing procedure is prefPrably carried out in a two-zone reactor, which comprises a somewhat tapering base section, to act as a collector for the molten slag and an upper ~ection, preferably generally 3~

cylindrical, mounted on the base section and free to move about its vertical axis in relation to the base section. The upper section then houses the packed bed, which at its bottom end is supported by the downwardly tapering waLl of the base section and the pool of molten slag collected therein. The upper section may be continuously or intermittently rotated or oscillated about its vertical axis to effect mild agitation of the packed bed. The base section is prefer-ably stationary but may in some circumstances be rotated or oscil-lated while still providing some relative movement with respect to the upper section.
The scrubbed gas issuing from the reactor and still contain-ing a minor proportion, for example 25%, of its fume content, is pre-ferably forwarded to a heat recovery stage in which a circulating stream of cooled alumina is heated by heat exchange with the gas and back-reaction of the remaining fume with CO, part of the -thus heated alumina being passed to and cooled in a heat-recovery, steam-raising boiler stage.
The solid alumina feed for the two-~one reactor is drawn from the circulating stream of alumina and the carbon requirement for the scrubbing procedure is, at least in part, introduced with this alumina. By correct control of the temperature reduction of the gas, as it passes through the scrubbing stage and the heat recovery stage, it would be possible for the ~ ~ ~ 5 4 ~ ~

carbon deposited in the alumina in the heat recovery stage to be rather exactly matched with ~he carbon feed requirement of the scrubbing stage. ~ke-up cold alumina feed is supplled to the circulating S al~mina stream in the heat recovex~ stage and small quantities of carbon may also be introduced into this feed or directly into the two-zone reactor of the scrubbing stage if required.
The fume-ree, but still hot, gas issuing rom the heat reoovery stage is then forwarded to a carbon feed prehea~ stage, in which the carbon feed for the carbother~ic re~ction is preheated to a temper~ture in the range of 800 - 1000C, for example~
In t~ scrubbing operPtion, in order to malnta1n the slag at a temperature above the "carbon line~9 in the phase diagram it is generally only permissible to reduce the g~s temperature by about 50C - 150C depending upon slag t~mperature and compo~ition during its pass~ge through the two-~one 20 reactor. However this permits about 75% or even --mor~ of the fume content of the gas to be recovered as A14 3 In feeding materials to the two-zone reactor alumina and carbon may be fed ~eparately. The carbon, where fed separately from the alumina feed, is prefer-~ ~35~3~

ably fed into the top of the reactor in the form ofcoarse part~cles of approximately the same ~i~e a~ the particle~ of the packed bed~ ~he alumi~a may be fed in either at the top of the reactor or at the bottom of the reactor ~preferably entrained in the gas stream) 9 or b~th.
~ he gas is co~ve~iently led into t~e bottom of the reactor and is then preferably blown through the molten slag in the base section of the reactor~ ~his entrains molte~ slag from the pool in the base section and this slag separate~ from the gas as it pa~ses upwardly through the packed bed 80 that it trickles back throu~h the packed bed a~ a counter-current ~low in relatio~ to the upward gas streamn Heat tran~er from the gas to the alumina takes place to a large ex~ent at th~ extended surface presented b~ the slag trickli~g down~ardly cou~ter~curr~nt to the asce~ding ~ ga~ ~dditional alumina i~ formed at higher levels in the packed bed by rea~on o~ back-reaction betwee~ ~he fums compone~t~ ~nd carbon mo~oxide and trickles in a mo~ten sta*e downwardl~ in the bed~
In this arrangement the solid Alumi~a feed ~nd the c~rbon ~eed to the 2-zone reactor msy con~eniently be effected by peing pneumatically in~ected i~to the conduit conYeyi~g the g~ to the two-zone reactor~
Where the packed bed scrubber i~ operated in this w~y, there is a tendency ~or the p~cked bed to become flooded whe~ the gas throughput per unit area o~ the bed ri~es above a critical level at which the frictional drag on the dowDflowing ~lag counteracts the gravitational force on the slag~
I~ order to avoid the requirement for an exce~sively large diameter packed bed in the uppex ~ection of the reactor when very large gas flo~ are to be treated, it may be desirable to i~troduce the fume-laden gas iDto the top of the two-zone reactor.
In such case the alumina f eed and carbon feed material~
are also introduc0d into the top of the reactor so as to move co-current with the ga~ flow9 When thi~ co-current mode of operation i~
employed con~iderably higher ga~ flow rates are acceptable or a ~maller vessel for the ~ame flow rate.
The packed bed i~ the scrubber reactor i~
arranged in an entirel~ conventional manner for the 10 contact of a ~tream of liguid with a coun-ter current or co-current ~low Or gas,, ~ he particle ~ize OI the bed i8 selected to provide an extended ga~/liquid contact sllrf~ce and a rela~ively low resi~tance to ga~ flow (whether upward or downward). ~he equi~alent diameter ~f the carbon particle~ ~upplied to the packed bed is i~ the range of no~ more ~ha~ l~lO o~ th~ b2d diameter dow~ to 20 mm.
~ he "particle equivalent diameter" may be defi~ed aP the diameter of a sphere with the ~ame ~ur~ac~ ~rea as the average particle~ at the top of the bed.
- !~!he upper ~ection of the ~;cru~ber xeactor, in which t:he packed bed i~ contained~ is preferably con-~t~ucted with a very 81ightl~ larger diameter at the bottom end than at the top end so a~ to.permit the downward moYement of the carbon particle~ of the packed bed under the mild agitation ~pplied to the bed, ~uch mo~ement bei~g nece~ar~ because o~ the ~low co~sumption of the coarse particles of the bed in the operatio~ of the bed. The increase of diameter should prefera~ly be ~ufficiently ~m 1l to avoid the formation of inade-quate pa~king at the lower end of the upper section between the bed ~nd the surrounding wall. It i~
preferred that the ~lope of the wall of the upper sectio~
i8 of the order of 2 - 5 mm per metre (0~2 - 0O5%)~

~ 3 5 The upper section of the ~crubber reactor is preferably constructed ~ith a steel outer shell, lined with a layer of thermal insulation and having a carbon refractory inner llning. The thermal insulation is arranged to maintain the heat loss at a low val1le but is preferably arranged so that there is a $mall controlled heat loss sufficient to maintain the temperatur~ at the inner face of the lining at such value as to be below the "carbon line~' in the phase diagram so as to minimise erosion of the carbon lining by attack by molten alumina, As already explained the packed bed is formed of coarse carbon particles. These particles are prefer~bly formed of appropriately sized, coarsely broken calcined petroleum coke of ~ grade ~mployed for the production of carbon electrodes. This coke present~ less available area for reaction than coke feed particle~ suppli~d to th~ scru~ber reactor mixed with or separated from the alum1na feed.
However, as already stated~ part of the carbon supplled to the scrubber reactor is usually in the form of sized particles to replace losses from the packed bed~
In a further development of the invention, the packed bed for recovery o fume values is arranged above th~ lo~ temperature chambex(s) o~ a two-3~

~lOo zone oarbothermlc reductlon system of the type describedin U.S. Patent No. 4,099,959 with the result that alumina in a molten state enters the molten slag in the chamber and the carbon, required to replace carbo~
consumed-l~ the carbothermic reduction reactions, enters the slag already pr~heated to a temperature almo~t equal to th~ temperature of the lag in the low temperature ~h~mberO
In a further alternative method of putting lV the invention into effect the baok reaction i~ allowed to t~ke place in a packed carbon bed which oonsl~ts wholly o or in p~rt of ~ctive" carbony so that a ~ub3tanti~1 part of the Al and Al~0 fume content is convert~d ~irectl~ to Al4C3. "Active" car~on for the pre~ent purpose c~n be consldered to ~e any form of carbon poS8eq~ing A large ~peci~it surface area and - conseguently a relatlvely low ~trength, ~o that the 'reQulting Al4C3 reaction product doe~ not adhere 8trongly to the carbon particles and/or is very pornus and open, 80 t~a~ the deposition o the reaction product doe~ not result in cementing of the carbon ~art~les to one a~otherO
In thi~ alternative a small amount of alumina slag is ~180 depo~ited in the c~rbon bed, but i~ is not the pu~pose o~ ~his alternatlve to generate molten al~mi~a for introduction into ~he Garbothermi~ reduction reactor. Therefore prefera~ly there would not ~l~o be an addltion of ~olid alumina to the be~9 ~lthough in 80me ln~tances it ~ight be convenien~ to introduce 80me ~olid by means of a scn~b~er system a~ already de~cribed above.

~5~ 5 --11~
The car~on dr~n off from the bottom of the packed bed ~crubber may be fed directly to the carbo~
thermic re~ctor as feed or it may in some instance~ be preferred to pass it to a cooler~ The cooled carbon may then be ground ~nd employed as feed or the carbo thermic reduction reactor without further treatment.
I~ many instances, howev~r, it is preferred for the carbon feed rate to the packed bed scrubber to be ln excess of the eed rate of carbon required by the reactor. In such c~se the collected carbon i~
cla3sified ater grinding and the fine fr~ction is employed as feedO ~ major part of the depo~ited Al~G3 i5 normally found in the relRtively fine feed materlal.
According-to ~ further aspect of the inYention 1~ deposition of ~olids in the conduit or condult~ leading to the scrubber reactQrs is reduced or avoided by generatio~ o heat in the gases passing throu~h the condult or conduits and/or by dilution of the gases to redu~e the par i~l pre~sure of Al fume ~nd A120 in thP
8a~. In mo~t instance.~ carbon dioxide or water ~either as liquid or steam~ i8 lnjected into the gas st~eam as it enters the ~ondu~t le~di~g from the reactor to the packed bed ~crubber. This ~erves both a~ a diluent ~nd a~ a mild ox~dant~ which conYerts the Al vapour tQ A120 Alternatively hea~ may be generated in the conduit by in~ection of carefully controlled small qu~ntltles of oxygen which are intended to produce ~ufflcient heat to m~ke good the heat 108~ from the conduit ~o that the ga~
stre~m enter~ the packed bed ~crubber at sub~tAntially the ~ame temperature a~ it left the reactor~

S~3~;

The preventlon of formation of deposits in an outlet conduit from a carbothermlc reactor by dilutlon ~nd/or oxidation of the fume content of the gas is of general applic~tion and is not conflned to the method of thP pre~en~ lnvention in which the fume-laden ga~
is cooled in contact with a moving packed bed of carbon parti~les.
Referring now o the accompanying drawings~
Figure 1 is a diagr~m o a romplete fume and heat recovery system for a carbothermic urnace ~or aluminium production.
Flgure 2 ~s a dlagrammatic represen~at~on of a two-zone scrubber reactor for counter-current flow~
Figur~ 3 i~ a diagrammatic representation o a ~wo-zone reactor arranged for co-current flow.
Figure 4 shows the partial phase dl~gr~ for the ~y3tem Al-O~C.
Flgure S i~ ~ diagrammat~c represen~ation of the ~crubber Qf the ~re~ent in~ention poai~ion~d above a low temperatur ch~nber of a carbothermic reactor - ~y~tem.
Flgure 6 i~ a diagram of a ~ur~her orm of ume recovery ~ystem ~n ~crordance with ~he in~entionr In the accompanying Figure 1 there is ill~strated ~ 5~ 3 ~

diagrammaticall~ a complet~ system for the recover~ of thermal and chemical energy from fume-laden C0 off-g~
from a carbothermic furnace for the production of alumi~ium, In ~igure 1 gas from the carbothermic furnace is led through a conduit 1 at a -temperature of the ordex of 2030-2050C (in this example~. At that temperature the fume content of the gas in one typical example is 35 - 40Yo by weight and is composed of Al vapour and ~0 ~l20 in the ratio of appro~imatel~ 1 6O ~he efficie~t recoYe.~ of the chemical and thermal energ~ repre~ented b~ this fume content is therefore ex~remely important in relation to ths economical performance of the carbo-thermic reduction o~ alumina to all~minium metal, a~ is al~o the ef~icient recovery of the sensible heat and chemical ~nergy vf the emitted CO gas.
The initial treatment of fume-laden gaB i8 - performed in scrubber reactor 210 In the ro~nter~
current mode of treatment ga~ e:~ters the bottom of the ?0 reactor ~nd exits ~rom the top a~ indicated diagram-matic~ in ~ull lines in Figure 10 In the co-curre~t mode t~e gas e~ters ~he top of scrub~er reactor 21 a~d exits ~rom the bottom a~ indicated in dotted 11I1~; o In the counter-current mode the ~a~ fro~ condui~
1 enter~ scrubber reactor 21 (as ~howrl in ~igure 2) and i8 bubbled through a pool 22 of molten ~lag co~tai~ed in a statio:nary base ~ectiorL 23 of said r~acto~ he ga~, in emergi~g from the slag pool 229 throw~ u~
molte~ ~lag into the lower part o~ a packed bed 2~ of coarse carbon particlea housed in an upper section 25 of the rsactor 21 a~ already described. The gas pas~es up through the packed bed 24 and emerge~ through an axial exit passage 26~ ~he alumina and carbon ~esd requireme~s of the 8y8tem may be supplied via a p ~S~3~;

4~

26a above the packed bed or m~ be injected pneu-matically through a passage 27 into the incoming ga~
~tream in conduit 1 or may be divided between these two routes.
The ga~, exiting from the two-~one scrubber reactor, is at a temperature of 1950 - 1980C and flow~
through conduit 2 to a venturi feeder reactor 28~
The ~as, which ~till has a ~ubstantial fume co~te~t 9 is brought into co~tact with a much larger ma~ of r~lativel~ cool ~olid alu~ina in reactor 28. The alumi~a, which already contain~ a proportion o~ carbon~
i~ heated by heat exchange with the ga~ b~ for example, 1000C in reactor 28. ~lumina and carbon are formed by back-rea~tion of the remaining fume content with carbon mo~oxideD
~ he ga~ with entrained alumina i~ forwarded from v~turi reactor 28 through conduit 29 to c~clone 30.
~he separated al~mina, contai ~ g about 5-1~/o carbon, i8 led out of c~clo~e 30 through conduit 64 '~he O .re~uired amount o~ alumina for fseding the scrubber r~actor 21 is su~plied to that reactor throu~h ~o~duit 3 aQd co~titutes ~bout 25-3~ of the lumina led out from the c~clone 30. ~he remainder of the hot alumi~a is led through conduit 8 to a heat exch~ng~ boiler 31, 2~ in which it give~ up heat to raise steam. ~he cooled al~mi~a from boiler 31 i~ forwarded through conduit 10 to a mi~er 32, in which it is mixed with cold make-up feed alumina, together with a small amount of make up carbon feed, entering throu~h conduit 9 And i 9 then ~0 recirculated through conduit 5 to the venturi reactor l~ the ~oregoi~g des~ription o~ the recovery of heat from the off-gas from the ~c~ubber reactor 21 it i~ a~sumed that the aluminaJc~rbon ratio of the ~lumi~a iss~ g from cyclo~e 30 through conduit 6 has been S~3~;

5~
arranged to be that required for supply to the scrubber reactor 21~ If the alumina/carbon ratio ~
not that required for reactor 21, additional alumina or c~rbon is supplied to that reactor to correct the ratio and where additional alumina is required, then there will be an increase of the proportion of the circulating alumina stream diverted to the reactor 21.
In many instances it will be preferred to introduce coarse make-up carbon feed particles direct to reactor 21 to make good losses from the packed bed.
~ he cooled C0 gas issui~g from c~clone 30 i8 es~entially free from ~ume components, but still has a high ~ensible heat content9 which i~ employed to pre-heat the carbon feed to the carbothermic f~rnace.
~he gas from cyclone 30 i8 led through co~duit 7 to a venturi reactor ~ in which it i~ contacted by cold feed carbon entering through conduit 110 ~he carbon iB heated to 800 - 900C by co~tact with the gas and is forwarded via conduit 12 and i8 separated from the-gas i~ a ~econd cyclone 34~ from which it i~
led to th~ carbothermic furnace through conduit 14.
~ he carbon heated in this sy~tem usually con~
Qtitutes i~ exceas of 95% of the total car~o~ i~put to the carbothermic ~rnace, the remaining carbo~
feed i~troduced through conduit 9 or directly into the scrubber-reactor ~1 bei~g rorw~rded to the fur~ace in the form of ~l~C~ as part of the ~l4C3 cont~nt of the slag which collects in the base ~ction 23 of the reactor ~1. This collected slag is returned to the carbothermic furnace throu~h co~dui~ ~.
~ he C0 gas stream from the cyclone 34 is passed through a conduit 13 to a gas clsaning unit 35, from whence it is passed to a gas holder via a c~nduit 15u A~ already explained~ it i~ an important .35 feature of the method of the in~ention that the packed bed in the re~ctor 21 is preferably subjected to mild agitation to break up bridges that may be formed betw~en the carbon particles forming the bed and to maintain the bed in a substantiall~ homogeneous condition tv aYoid ch~nnelling, which could lead to local overheating of the bed along the channels. Such agitation also aid~ heat transfer between the gases ~nd the slag. ~or this rea~on in the construction o~ the reactor 21 9 8hown i~ ~'igure 2, the statio~ar~ base 10 section 23 i~ somewhat tapered to contain a pool 22 of molten slag, which~ together with the inclined wall of tapered base ~9 su?ports a packed bed 24 of carbon particles. The base section is provided with a gas inlet 40 to admit ga~ from the conduit 1 and a slag 1 5 outlet port 419 leading to conduit 4 , in~o which a gas inlet 42 is preferably pro~ided to permit i~trod~ctio~
of an inert gas, ~uch a3 argon, tc drive slag through co~duit 4 to the carbothe~mic furnace~
I~ ~iguxe 2 the upper ~ec-kion 25, which is e~entially cylindrical in characterg is ~upported by a - radially pro~ecti~g flange 44 of the lower sectio~O
~he upper sectio~ 25 ïs rotated, ~ither co~tinuously or i~termittently9 or oscillated by two or more driYe roll~
45 ~ s~mmetric~l ly arranged around the upper sectio~ 25~
I~ order to pro~ide a gas-proof seal between the rotatable upper ~ection and the stationary lower ~ec-tion, the shell oî the upper section is provided with a flange 46, parallel to the flange 4~0 A mea~ured stre~m of inert gas i8 I~troduced under pre~uxe through orifice~ 47 ~o as to purge Iurnace gases ~rom this regio~, A layer 4~ of powdered graphite i~ main-tained above the flange 46 to act a~ a ga~ ~eal to preve~t outflow oî gas.
re 3 ~;hows a modified co~truction of sc~u~ber 35 reactor intended ~or co-current operation~ l~ Figure ~

~ ~ ~ S~ 3 llke parts are indicated by the same reference ~umerals as in Figure 2~
In the oonstructio~ of Figure 3 both the fume-laden gas and -the feed material are introduced through a~ axial top-openi~g 26i, qo that the feed and the ga~
traYel oo-current dow~wardly through the packed bsd 240 The ga5 iS ~oharged through a series of ports 50 in the wall o~ the upper sectio~ 21 into a gas manifold 51, ~ecured to the casing o~ the stationary lower section 23. Gas i~ dl~charged from manifold 51 via one or more outlets ~not ~hown) to venturi reactor 28.
At the top of the ~anifold 51 a gas-proof 3eal is provided betwee~ a flange 52 ~arried b~ the oa~ing of upper ~ection 21 a~d a corresponding flange 54 at the top o~ the manifold.
A~ already explained the co-current arra~gement allows faster ga~ flow rate~ through the pao~ed bed without flooding the bed a~d therefore a smaller ~¢rubber apparatus may be ~mplsyed to handle 8 giYen ga~ flow. ~he apparatus is both cheaper to con~truct and may be operated with a lower overall heat loss because a ~maller area of carbo~ lining is required to be maintained at a temperature below the "carbo~ line'l.
On the other hand, cou~ter-curre~t flow provides better gas-~lag co~tao~ a~d therefore more efficient ~crubbing.
Since the counter-current reactor is ~aces~arily larger, the pressure drop across the reactor vessel will be lessO
In operati~g the device in either co-curre~t or eounter-current mode it i~ o~ly nece~sary to rotate the upper section a few times in an hour in order to obtain the required movement of particles i~ the bed to avoid channelli~g and bridging.

~5~35 ~ he described sy~tem enRures that a sub~tantial portion of the chemical energy due to the fume content and the sensible heat of the off-gas from a carbo-thermic reduction furnace i8 reco~ered in the form of Al4C~ and molten alumina to suppl~ to the materials addition~ ch~mber of a carbothermic reduction system as for example described in U~S. Patent ~oO ~,099,959.
It also enables the carbon feed for such a s~stem to be preheated to a substantial extent towards reaction temperature, B~ ~irtue of the supply of a large pa~t of the re~uired ~14C3 and by virtue of ~upply of molten alumi~a ~nd preheated carbon feed to the materials additions chamber of a ~ystem of the type described in U.~0 Patent ~o. 49099,959 the heat input requirements of such cham~er are very much reduced.
I~ the operation of this system of the present i~Yention increase in the feed of alumina to the sc~u~er reactor results i~ i~creased cooling of the gas a~d increased reaction in the scrubber reactor.
However if the feed rate is excessi~e the temperature goe~ below the carbon line so that the reactio~ product i5 not ~14C3~ but o~ly al ~ ~a ~nd carbon. 0~ the other hand 9 if the temperatuxe of the bed goes too high, the proportion of the ~ume which i~ converted to Al4C3 in the reactor decreases. Consequentl~ it i8 preferred to co~trol th~ ~lumlna ~eed rather carefully to ensur~
that the packed bed conditions are o~ly ~lightl~ above the c~rbon lineO
In one exa~ple~of an efficiently operated scrubber treating the fume content of the off-ga~es from a carbother~ic furnace having an output of 1 tonne Al~hr the dimensions of the packed bed are 106 metres dia-meter and 4.5 metres high when operated in t~e counter~
35 current mode~ 1~ the co-current mode the required di-men~ions are 103 metres diameter and 4 metre~ hi~ho The incorporation of the fume recovery system of the present inv~ntion into a carbothermic reaction system of the type described in U.S. Patent No, 4,099,959 is illustrated in Figure 5.
In the apparatus of Figure 5 the system incorporates a low temperature reaction chamber 61 and a high temperature reaction chamber 62, connected by a forward slag duct 63 and a return slag duct 64, which preferably includes means for controlling the electrical resistance of the slag stream therein as described in our co-pendlng Patent Application No. 2011475.
The slag, composed of molten alumina and dissolved aluminium rarbide, is circulated through the 15 5y9tem formed of cham~ers 61 and 62 by generation of gas in the forward duct 63 as a result of heating the slag in duct 63 by electric resistsnce heating. For this purpose electrodes 65 and 66 are respectively provided in chambers 61 and 62.
- 20 The gas generated by the reaction of A14C3 and A1~03 to release Al in duct 63 and chamber 62 is returned from chamber 62 to ch~mber 61 by gas return duct 67~ which enters chamber 61 above the level of molten slag, but as close to that level as is practicable~ The gas from chamber 62 becomes mixed with the gas generated in cham~er 61 in the lower part of the column 68, Eormed by feed materials in a scrubber upper section 69 of the chamber 61.

5 ~ 3 The scrubber section 69 is constructed in a manner similar to the scrubber illu~trated in Figure 3, except that the gas duct 67 from chamber 62 enters the manifold 51 so that the gas flows upwardly through the column 68 countercurrent to the descending feed material) of which the column 68 is composed.
In this arrangement the feed to the column 68 is composed of A1203 and C in proportions req~ired by the process, taking into account the relevant proportions o~ the alumina, which will be carried out as Al and A120 in the exhaust gas exiting from the upper section 69.
The feed to column 68 ran consist of a mixture of carbon and alumina intimately mixed as pellets, Alternatively the feed may consist of individual pellets of A1203 and carbon lumps.
Successful execution of the proposed scheme requires that the feed be properly sized to avoid chan~æ~ng and hot spots in the column, and excessive pressure bulld up in the carbothermic reduction furnace fonmed ~y chambers 61 and`62~ -During the upward passage of the gasthrough the packed bed column 68~liquid slag is liable to be ~arried into the upper regions of the column.
Liquid slag 1s also produced due to scrubbing reactions.
Th~ liquid slag perrolates downwards unless the bed becomes flooded, as explained above~ The dimensions 5 ~ 3 5 -21~

of the chamber 1, p~rticularly of its invert~d conical upper part, are selected to avoid flooding in the column.
Control of composition of the slag is achi~ed by lining chamber 61 with a graphite or other suitable carbon lin~ng 70 to act as a oounter electrode to ce~tral electrode 65 and pa8~ing current from the oentral electrode 65 ~Q
thi~ lining. Heat ca~ thu~ be lmparted to the 10 co~tent3 of zo~ 61 where carbon is ava~lable~ -i~creasing the formation o~ ~14C~.

The major beneficial aspects of the system of Figure 5 are Sa) Recovery of Al values from the carbathermic furnace fume on to the feed material that is being continuously fed to the furn~ceO
(b~ Efficient preheating of the feed in a counter current manner by the fumes from the reduction - furnace. Such counter current preheating is highly energy effici~nt.
(c) Efficient scrubbing of any fine 2S particulate matte.r formed by back reactions in the fumes.
Packed bed scrubbing is known to be an efficient process for removal of fine sollds.

3~;

(d) Self regulating feeding mechanism as the charge moves dowmwards at a rate equal to its consumption.
In the system shown in Figure ~ a carbo-thermlc reduction reactor is indlcated diagrammatlcally at 71. The stream of gas generated in the reactor 71 ~s led through a conduit 72 to a scrubber 73.
One or more gas iets ~4 lead into conduit 72 ~O Mildly oxldiæing steam or C02 may be supplied through the ~et~ 74. Alternatively the jets may be hydrocarbon~
mantled o~yge~ ~ets.
~ he scrubber 73 may consist of a stationary baAqe p~rt 75 and ~ rotatable upper 2art 76, operat~ng on ~he general principles of the apparatu~ of Figures 2 3 to keep the arbon particles in motion~
The conduit 72 convey~ ga~ to the st~tionary manifold 77, from which it enters the upper part 76 through port~ (not 3hown~. The g~s fiowQ up the reactor and is led out axially through a conduit 78 to a further tre~tment ~tage, illustr~ted diagrammat~ally at 79~
The gas stream ~ypically unde~goe~ a temperature recluction o 50-1~0C during it~ upward progres~ through the carbon bed in the scrubber 73 countercurren~ to th~ descending ~5 carb~n p~rticles in the upper part 7$~ Durin~ de~cen~
aluminium carbids i~ formed by rea~tio~ of the ~l vapour and A120 with the a~tl~ ~arbon parti~le~ i~
ths pa~kea bed.

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The carbon p~rticles are withdr~wn at controlled rate from the scrubber 73 by means of a rotatable extractor 80 and discharged to a cooled storage unit 81~ The cooled carbon from the unit 81 is preferably passed through a grinding stage (not ~hown) to a classification stage 82, from which a coarse fraction, composed of carbon and some Al4C3, is returned via conduit 83 to the upper end o ~he scrubber 73. "Active"
carbon, preferably preheated~ is introduced via conduit 84 ~0 in an amount sufficient to balance the rate of feed carbon (containing Al4C3) via co~duit 85 from the cl~ssification stage to the reactor 71.

Claims (10)

1. A method for the treatment of fume laden carbon monoxide from a carbothermic reduction of alumina which comprises introducing the carbon monoxide gas into a moving bed of carbon particles held at a temperature of 1950-2080°C and allowing the temperature of such gas to fall by 50-150°C during passage through said bed.

2. A method according to claim 1 in which such gas is passed through said bed of carbon particles in a counter-current direction to the moving carbon particles.

3. A method according to claim 2 in which said carbon particles consist wholly or in part of active carbon particles.

4. A method according to claim 1 in which said moving bed of carbon particles is maintained above a body of molten aluminous slag.

5. A method according to claim 4 in which said bed of moving carbon particles is maintained in an enclosure which is rotatable about a vertical axis in relation to a container for said body of molten aluminous slag.

6. A method according to claim 5 in which solid alumina particles are supplied to said moving bed of carbon particles.

7. A method according to claim 6 in which said solid alumina particles and said fume-laden carbon monoxide gas is introduced into the top end of said moving packed bed and progressed co-current through said packed bed,

8. A method according to claim 7 in which said carbon monoxide is withdrawn radially outwardly near the bottom end of said packed bed and molten alumina is withdrawn downwardly from said packed bed into said body of molten aluminous slag.

9. A method according to claim 6 in which said fume-laden carbon monoxide is introduced through said body of molten aluminous slag for upward flow through said moving bed of carbon particles.

10. A method according to claim 9 in which solid alumina particles are injected into the fume-laden carbon monoxide stream before entry into said body of molten aluminous slag.