CA1184390A - Process and reactor for production of aluminium by carbothermic reduction of alumina - Google Patents

Abstract

A B S T R A C T
A process for the production of aluminium metal which comprises maintaining a body of molten slag in an essentially vertical column by passage of electric current through said slag between upper and lower electrodes in contact with such slag, maintaining layer of feed carbon and alumina in particulate form floating on said body of slag and passing evolved gases upwardly through the layer.
The slag at the upper end of the column may be in communication with a laterally displaced settlement chamber in which quiescent conditions are maintained to allow upward settlement of Al droplets to form a product layer.
The electrodes are graphite and are heavily cooled to protect them from attack by the slag. The upper electrode is conveniently arranged in the settlement chamber.

Description

3~3 ~1--"Process and Reactor or Production of aluminium by carbothermic reductîon of alumina'i The present invention relates to the productlon of alumini~m metal by the carbothermic redurtion of alumina~
S The production of aluminium metal by the performance of such reduction at temperatures in excess of 2150C in, for example, an electric arc furnace has already been describedO
A proce~s has also been described in Canadian Patent No. 1,084,974 in which the carbothermic reduction of alumina i~ performed in two stages by circulating a molten alumina slag be~ween one or more low temperature vessels, in which earbon is ~dded to the slag and re~ctq with the alumina to form Al~C3 in the slag, and one or more high temperature vessels in which Al4C3 in the slag reacts with alumina to release ~luminium met&l, whlch is removed from the system~
In this process the heat energy, to support the endothermic reactions involved, is primarily introduced into the sy~tem by electrical resi~tance heating of the slag during its pas~age in a conduit or conduits leading from a lo~ temperature vessel to a succeeding high temperature vessel~
The slag flow conduit or conduits is lined wlth A
layer of frozen slag to protect an underlying lining of refractory.
The layer of frozen slag results from the applicatlon of coolant to the outer steel shell or ~acket, in whi~h the refractory is supported. Since the conduits are inclined to the longitudinal and larg~ volumes o gas are released th~rein lt is found very difficul~ to maintain a uniform layer of fro~en slag in the conduits. The object of the present invention is to provide a new form of ~pparatus for the i~

carbothermic reducti~n of alumina, in which this d~fficulty i8 obvlated.
According to the present invention the carbo-thermic reduction of alumina is performed in a~ appara-tu~
in which a bbdy of molten alumina/aluminium carbide slag is contained within a single vertical column and the heat energy input is achieved by passing electrlc current between a lower electrode at the bo-ttom of the vertical colum~ and an upper electrode arranged in electrical contact with the slag at the top of said column. ~he upper electrode is preferably located in a laterally displacea chamber communicating with -the upper end of the column. This preferred arrangeme~t allows an upw~rd flow of evolved ga~ in an e~entially axial direction ln the oolumn and an essentially equallsed radial hea-t flow from the slag ln the column to the walls of the colum~
i and consequently it is possible to maintai~ a substantially j equallsed layer of frozen slag around the column of molten slag.
By placing the upper electrode to or~e side of the column of slag it ~ possible to maintain a complete superDatant layer of feed materials, coke and alumina 9 o~ the column of slag. lhi~ allows a substantial proportion of the fume conte~t (Al vapour and A120) of the gas to back react exothermically with carbo~ to form A14C3 i~
situ on the carbon feed a~ it passes upwardly through the feea material layer~ The heat energy thus recovered from the fume conte~t serves to preheat the feed materials, both carbon and alumi~a, before dissolution in the slag colum~ and may result in a proportion of the alumina melti~g before entry into the mol-te~ ~lag column or in partial reaction of the alumina a~d carbo~.
O~e form of reactor for putting the lnvention lnto effect is illustratea in the accompshying drawing.
~he reactor i~ housed within an outer steel shell 1, which allows heat to be withdraw~ as necessary from selected areas by external application of coolant.
The steel shell 1 defines a main reaction chamber 2 and a metal settlement chamber ~, the headspaces of which are divided from each other by a curtain wall 4, which is inter~ally water co~led.
The steel shell 1 is integral with a column 5, in which the major part of the heat input takes place.
~he heat input to the system relies on passage of current between graphite electrodes 6 and 7, which are in direct contact with the molten slag. The electrodes 6 and 7 are ~ubjected to heavy, but controlled9 cooling, arranged so that the molten slag in contact with such electrodes is maintained in a molten condition but at sufficiently low temperature as essentially to avoid reaction with the slag (a~ opposed to normal erosion). The vertical column 5 i9 arranged to be sufficiently electrically resistant (in relation to the voltage applied between electrodes 6 and 7) in both the radial and axial direction to avoid substantial leakage currents. ~his is achieved by main-taining a solidified slag layer in the colwmn 5. ~or this purpose it may be de~irable to employ a refractory lining in -the column, which is adequately heat cond~cti~e in the radial direction, as described in co-pending~ Patent Application Serial No.~9~ he solidified slag layer 5a p:rotect~ the column lining from direct contact by molten ælag and is maintained by the application of coolant to the external surface of the steel ~hell of column 5~ lhe cross sectional dimension of column 5 i9 ~mall in xelation to that of the remainder of the current path between electrodes 6 and 7,so that most o~ the heat developed by passing current between electrodes 6 and 7 is generated wit~i~ the column 5. In consequence the temperature of the slag within the column 5 i~ somewhat higher than in the reaction chamber 2~ Co~sequently there is continuous convective circulation o~ slag between the column 5 and the reaction chamber 2 where the alumina slag reacts wlth A14C3 to release aluminium metal with accompanying evolution of large volumes of C0 gas, which .

is heavily fume-laden with Al vapour and A120.
~eed material is supplied via an entry conduit 8 to a floati~g bed 9 of carbon and alumina feed particles maintained on the molten slag in chamber 2 so that the evolved gas undergoes some degree of scrubbing of the fume content as a result of co~tact with the bed 9 before exiting via a gas outlet conduit 10 to further fume scrubbing stages (not shown).
;lhe conditio~s in the reaction chamber 2 are very turbulent as a result of the very large volume~
; of gas evolved, 0~ the other ha~d the conditions i~ the slag in column 5 are relatively quiescent 9 since the release of gas in that region is impeded by the sub-stantial head of slag above it. ~he pressure head of slag has the effect of raising the temperature at which the alumina content of -the slag can react with its A14C3 conte~t.
In the system of the inve~tio~ the aluminium metal is released in very fine droplet form, which renders it~ settlement (upwards~ because it is less dense than the slag) very difficult in the turbulent conditions of the reaction chamber. However becau~e of the turbulence in the reaction chamber there iæ steady slag circulation between the reaction chamber 2 and the settlement chamber 3 9 in which the aluminium metal forms a supernatant layer 11, from which it may be draw~ off at appropriate ; intervals. lhe headspace in the chamber 3 is vented at 12, essentially a~ a precautionary measure against gas build-up in the eve~t of gas bubbles being drawn through into the chamber 3 in the slag.
The arrangement of the upper electrode 69 SO
that it projects upwardly into the slag in chamber 3, ensures that the exposed portion of the electrode is wholly submerged in slag and the remai~der o~ the electrode projects outwardly of the reactor shell for applicatlon of protective cooling.

Claims (4)

1. A process for the production of aluminium metal which comprises maintaining a body of molten slag in an essentially vertical column by passage of electric current through said slag between upper and lower electrodes in contact with such slag, maintaining a layer of feed carbon and alumina in particulate form floating on said body of slag and passing evolved gases upwardly through the layer.

2. A process according to claim 1 further comprising arranging a reaction chamber above said column, arranging said upper electrode in a laterally displaced chamber communicating with said reaction chamber and arranging flow of slag between said reaction chamber and said laterally displaced chamber.

3. A process according to claim 2 further comprising isolating the headspace over the slag in said reaction chamber from the headspace over the slag in said laterally displaced chamber, introducing the feed materials into the headspace in the reaction chamber and withdrawing evolved gas from such headspace, permitting aluminium droplets of settle upwardly as a surface layer in said laterally displaced chamber and introducing current into the molten slag in such laterally displaced chamber via a submerged upper electrode.

4. A process according to claim 1, 2 or 3 further comprising cooling said upper and lower electrodes to a temperature at which they are essentially non-reactive with the slag.