Note: Descriptions are shown in the official language in which they were submitted.
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"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
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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
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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
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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.
