Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to the carbothermic production
of aluminum from aluminum oxide and a carbonaceous material,
such as carbon, wherein said alumina and carbon are charged
to a reduction furnace operated under very specific conditions
so as to result in the production of aluminum contaminated
with no more than about 10 weight percent of aluminum carbide.
There has been much time and attention devoted by
the workers in the prior art in an attempt to produce aluminum
by a carbothermic process as opposed to the electrolytic process.
The potential advantages in the use of a carbothermic process
for the production of aluminum have been known for some time
and they have even assumed greater importance in recent times
due to the high energy costs which have been brought about at
least in part by the increased cost of fossil fuels. In spite
of all the potential advantages of a carbothermic process over
an electrolytic process, the simple fact remains that the
latter processes account for substantially all the aluminum
produced in the world today.
There have been many theori~s advanced by many
authors as to why it is not possible to produce substantially
pure aluminum from a carbothermic process. In recent years
there have been patents issued such as United States Patent
3,723,093 which proport to disclose process for the production
of aluminum of substantial purity via a carbothermic process.
In addition, United States Patent 3l607,221 also discloses a
process for the carbothermic production of substantially pure
aluminum utilizing very high temperatures.
In U.S. Patent ~o. 4,033,757, issued ~uly 5, 1977,
inventor Robert Milton Kibby, there is disclosed and claimed
a carbothermic process for the production of substantially
pure aluminum utilizing aluminum oxide and a carbonaceous
reductant and heating the charge in such a manner such that
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the aluminum formed by the reaction of the charge is maintained
substantially in the liquid state.
The majority of processes for the carbothermic
production of aluminum always result in the production of
aluminum contaminated with aluminum carbide and, in fact, for
reasons which are not completely understood the aluminum
carbide contamination is always in the range of approximately
20 weight percent or hi~her. There is a very severe practical
difficulty which arises in attempting to further purify
aluminum which is contaminated with aluminum carbide in
significant amounts due to the fact that the mixture becomes
non-pourable unless extremely high temperatures are maintained,
such that the problem of purifying the mass becomes extremely
complex. Thus, although the art is replete with various
techniques for the removal of aluminum carbide from a mixture
of the same with aluminum, the simple fact remains that such
processes cannot achieve their maximum potential unless a
very practical and readily available source of aluminum
contaminated with no more than about 10 weight percent of
aluminum carbide can be achieved. It should be noted that
the amount of aluminum carbide contamlnation bears a direct
relationship to the temperature which is employed, i.e. at the
normal reduction temperatures employed in a furnace the amount
of aluminum carbide which can dissolve in the formed aluminum
is about 20 weight percent or higher. It is not too surprising
that the majority of prior art processes resulted in the pro-
duction of high aluminum carbide containing products for the
simple reason that they utilized uniform heating such that the
majority of the charge was at a uniformly high temperature and,
therefor, it was possible to dissolve aluminum carbide in
appreciable amounts.
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The furnace operation is carried out such that an
intermittent type of heating is applied to the charge to be
reduced such that a portion of the charge is at r~action
temperature (about 2100C) but the majority of the charge is
not at reaction temperature at any given time. In afore-
mentioned U.S. Patent No. 49033,757 issued July 5, 1977, there
is described various procedures for applying intermittent heat
to a furnace charge, but said copending application requires
the maintaining of the aluminum formed by the charge substan-
tially in the liquid state. In the instant process, no suchrestraint is necessary and, in fact, provision is made for the
various vapors which are formed to pass through the unreacted
charge and to further react to produce aluminum, thereby
recovering the energy that was expended in the vaporization
reactions.
It is extremely important in carrying out the novel
process of this invention that the majority of the charge -
never reaches reaction temperature at any given time, such
that when the formed aluminum flows over the charge, the
charge is never at a temperature where i~ is possible to
dissolve more than about 10 weight percent of aluminum carbide.
In accordance with a broad aspect of the invention,
there is provided a carbothermic process for the production
of aluminum from an aluminum oxide which comprises (a) striking
an open electrical arc to a portion of the surface of a charge
comprising an aluminum oxide and at least one material selected
from the group consisting of carbon, aluminum compounds contain-
ing carbon, and mixtures thereof, with the proviso that only
a small portion of the charge in the reaction zone is heated
to reaction temperature while the majority of the charge in
the reaction zone and the majority of the metal product is
below reaction temperature at any given time' (b) causing any
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volatile products which are produced to pass upwardly through
the charge and to further react and, (c) causing the liquid
aluminum ~ormed under the reaction conditions to flow away
from the arc and over the non-reacted portions of the charge
to be collected, said liquid aluminum containing no more than
about 10 weight percent of aluminum carbide.
Figure 1 represents a furnace suitable for carrying
out the instant process. The charge comprising a prereduction
product containing A1404C plus additional carbon is introduced
into furnace 1 through charging port 2 so as to be placed in
the annular space formed between the side wall 3 of the
furnace 1 and the electrode casing 4 and deflector 13. The
charge rests upon a carbon hearth S and mixes with a pool of
aluminum containing about 10 weight percent A14C3 maintained
at about 1850 - 1950C. In this drawing the heat source
is a plasma jet 6 connected to a secondary power supply 7 to
transfer its arc from negative electrode 8 to the molten pool
9 The arc column is deflected by two or more sets of
magnetic circuits 10 which are phased to rotate the arc at
the desired frequency, for example, one cycle per second.
Insulation 11 protects the electrode from damage and the
product is removed via port 12.
As illustrated in Figure 1, a deflector 13 is
provided to prevent the charge from flowing and filling under
the heat source. The dimension of the bottom wall of deflector
13 is such that the angle of repose of the charge determines
the location 14 where the charge intersects the molten pool.
The diameter of this circle of intersection is preferably
equal to the diameter of the circle formed by the arc as it
migrates under the influence of the magnetic field. In this
manner, an open arc is assured.
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It will be appreciated that direct conduction of
electricity from any part of the electrode or plasma torch to
the charge is detrimental for at least two reasons. Firstly,
under such resistive heating, it is difficult to assure that
the temperature of the background charge and product pool are
maintained below 1950C when the reacting surface is taken to
the reaction temperature of about 2100-2300C. It is the
intermittent application of an open arc which permits the
desired dual temperature condition required for the production
of aluminum containing less than 10% A14C3. Secondly, charge
10 materials which are allowed to conduct to the electrode
assembly, tend to stick to the assembly and interfere with
the electrical performance of the electrode system.
Figure 2 represents a furnace suitable for carrying
out the instant process, wherein a carbon electrode is used
as a source of intermittently applied heat. The charge is
introduced into furnace 1 through charge port 2 so as to be
placed in the annular space formed between the sidewall 3 of
the furnace and an electrode shield casing 13. The sidewalls
and electrode shield casing may be of carbon. The electrode
shield casing is supported from the furnace roof 15 and is
electrically insulated from the electrode 16 by means of
sliding seal 17. The charge rests upon a carbon hearth 5 and
mixes with a pool of aluminum containing about 10 weight per-
cent A14C3 maintained at about 1~350-1950C. In this drawing
the heat source is the arc 19 emitted from electrode 16 and
striking the intersection 14 between the charge and the molten
pool 9. The power source is a full wave rectifier connected
to make the electrode 16 negative and the hearth 5 positive.
The arc column 19 must be smaller in diameter than the electro-
de 16 in order to be stable. Depending on the heat balanceof an arc column and the vapor pressure of aluminum in the
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area of the arc, each stable arc must carry currents suffi-
ciently high that one or more arcs migrate over the surface of
the electrode, tending to stay confined to the peripheral
corner of the electrode. Such arcs strike the charge to be
reduced and the adjacent metal intermittently, but randomly.
The electrode shield 13 prevents the charge from filling in
under and making electrical contact with the electrode, which
assures an open arc. Vapor products from the heating of
charge and product pool by arc 19 are passed through the
charge column between wall 3 and shield 13 where they react
to produce liquid aluminum and/or products which yield
aluminum by the heating action of the arc. The major portion
of the charge in the arc heating reaction zone and the vapor
reaction zone is held at 1850C or less. The aluminum product
containing 10% or less A14C3 is removed via port 12.
The no~el process of this invention advantageously
employs a prereduction step in which alumina is reacted with
a carbonaceous material in order to form an intermediate
product containing A1404C. In carrying out the prereduction
step, alumina and coke or coal char are blended and preferably
shaped into briquettes in the proportions of two moles of
alumina for three moles of carbon and the mixture is there-
after heated to a temperature of about 1900-1950C and more
preferably at about 1920C. The briquettes are preferably
converted to said A1404C in a shaft furnace which may be heated
electrically or by the combustion of additional carbon with
oxygen. The A1404C which is obtained by prereduction in a
separate furnace is thereafter mixed with additional coke or
coal char at a rate to yield a composite charge having a
carbon to oxygen mole ratio of about 1:1. Thus, for example,
each mole of A1404C in briquette form is mixed with three
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moles of carbon.
It is preferred that the charge to the reduction
furnace of figure 1 be a mixture of carbon and briquettes
containing A1404C. The briquettes should be formulated by
mixing an aluminum oxide and a carbon rich compound to yield,
upon complete reaction, the compound A1404C.
Complete reaction to form A1404C in the briquette is
desirable because this represents a significant degree of
reduction which can be performed ahead of the furnace of
figure 1, thereby relieving the intermittent open arc furnace
of all but the most critical reduction steps.
Although desirable, it is not necessary that this
reaction to form A14O4C be 100% complete before the briquettes
are added to the reduction furnace.
It is important, however, to have some A1404C in the
briquettes by the time the briquettes reach the reaction zone,
because the A1404C melts and provides a liquid surface in
contact with the carbon needed to reduce it to aluminum, thus
improving the reaction rates and ability of the reaction zone
to receive high heat fluxes without excessive vaporization.
The charge mixture is then fed into the annular space
formed between the side wall of the furnace and the electrode
casing, as is set forth in Figure 1, and an arc is applied to
the charge as illustrated in Figure 1 in such a manner that
the arc strikes the metal pool at the intersection of the
charge burden with the pool. The majority of the charged
burden not struck by the arc is maintained at a temperature
between 1650C and 1850C.
Typical of the reactions occurring in the furnace are
the following:
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(1) A1404C + 3C-~ 4 Al (~ ) + 4 CO
(2) A14O4C + 3C -~ 4 Al (g) + 4 CO
(3) A1404C + 3C-~ 2 A120 (g) + 2 CO + 2 C
(4) 4 Al (g) + 3C -~ A14C3
( ) A12O (g) + 2 CO-~ A1404C + C
(6) A14O4C + A14C3-~ 8 Al ( Q) + 4 CO
(7) 2 Al (g) + CO-~ A12OC
(8) Al (g) ~ Al (L)
The surface of the charge being struck by the arc
heats up until a surface temperature of about 2100C is achie-
ved. As the reactions 1, 2, and 3 proceed, heat is absorbed,
tending to stabilize the surface temperature at about 2100C.
Only when the heat flux exceeds the heat absorbed by
the reaction does the temperature exceed 2100C, causing high
vaporization rates. With the surfaces stabilized at about
2100C, about 20% of the aluminum is vaporized with the vapor
composition being about 50 mol % aluminum and 50 mol % A120.
The aluminum vapor reacts with carbon in the charge
to form A14C3 according to reaction 4. Some o~ the aluminum
condenses directly on surfaces of charge at 2100C as taught
in United States Patent 3,607,221. Aluminum can also react
with CO to form A12OC at temperatures below 2100C as in
reaction 7.
As the A12O passes up the charge column it reacts
with CO formed by reaction 1, 2, and 3 to form A1404C by
reaction 5.
Finally, as the A14O4C, A14C3, and A12OC are carried
- down to the reaction zone, reaction 6 proceeds and reaction 7
reverses to produce aluminum liquid, which flows to the
hearth pool over unreacted charge.
In this manner vapor reflux reactions within the
charge column recover the material and energy values from the
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vapors given off in the primary reaction zone.
Another charge formulation which can be used involves
blending alumina and aluminum carbide to have carbon and oxygen
in the atomic ratio 1:4. Briquettes of this composition are
mixed with sufficient coke or char to provide an overall charge
composition with carbon and oxygen in the atomic ratio 1:1,
This charge is used in place of A1404C and C and function in
a similar manner.
Vaporization products are the same as previously
described and the vapors react as heretofore described to
produce aluminum and recover the material and energy values
from the vapors given off in the primary reaction zone.
Under conditions of very careful control of furnace
operating conditions, it is possible to practice this invention
using a simple mixture of alumina and carbon with a mixture
weight ratio 74% alumina to 26% carbon. The difficulties with
such practice arise from the facts that (1) more C0 is evolved
per pound of aluminum produced where the arc strikes the charge,
increasing the quantity of vaporization products to be recove-
red in the charge column, (2) more energy has to be deliveredto the charge per pound of aluminum produced, which leads to
a greater quantity of ~7aporization products to be recovered
and greater difficulty in maintaining the dual temperature
condition necessary for the production of aluminum containing
10% or less of aluminum carbide.
The charge column has a certain capacity to absorb
heat from the back reactions of the vaporization products to
produce aluminum and products which can be made into aluminum
as they approach the arc heated reaction zone. But this
capacity is not unlimited, and when vapor product production
exceeds the capacity of the charge column to absorb heat to
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make useful products it becomes impossible to keep the heated
reaction zone down below the electrode system where it belongs.
Unreacted vapors break through the surface of the charge column
and cause what are known as "blow holes".
~ evertheless, this invention can be practiced at
reduced capacity and power levels with the charges of simple
mixtures of alumina and carbon in apparatus described in
relation to Figures 1 and 2, and such charges are considered
to be within the scope of this invention.
As haq heretofore been stated, if the unreacted
charge, over which the aluminum product flows, and the product
pool were to be maintained at a reaction temperature of 2100C,
the aluminum product would dissolve up to 20% A14C3. Such a
melt is extremely difficult to handle below 2100C and requires
expensive fluxing techniques to recover the aluminum.
In this operation, however, the molten pool and major
portion of the unreacted charge over which the product flows
are maintained at between 1850C and 1950C by virtue of the
fact that the arc strikes any portion of the charge less than
half the time and the heat flux to the furnace walls is high.
Under these conditions, only 10% or less of A14C3 is dissolved
in the aluminum produced.
The reaction between A1404C and A14C3 or carbon to
produce aluminum containing about 20% A14C3 is known and des-
cribed in USP 2,829,961. This patent also teaches that vapo-
rization products may be captured in a bed of alumina, or
carbon or aluminum carbide or mixtures thereof, which may be
arranged as a furnace cover over the fusion and melt. This
patent describes a furnace in which additional raw materials
are charged periodically through the top of the furnace, down
through the upper layer fusion of aluminum containing aluminum
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carbide and then into the fusion of Al404C where the reaction
takes place. The consequence o~ this arrangement is that the
product pool must be at the reaction temperature of about
2100C. Means are not described nor self-evident to keep the
product pool at a temperature below 1950C, nor to accomplish
the dual temperature operation necessary to produce aluminum
containing less than 10% Al4C3.
The critically essential features of the present
invention are that (1) the product pool is maintained below
1950C (which is at least 150C below the reaction temperature),
(2) the heat is applied intermittently to the surface of the
charge being reduced to aluminum (illustrated by the intermit-
tent application of an open arc), (3) the walls of the furnace
have their insulation adjusted to maintain the major portion of
the charge in the reduction zone below 1850C (especially that
portion of the charge over which aluminum flows to join the
product pool), and (4) the vaporization products are forced to
pass through the charge burden where they react to form liquid
aluminum containing ~ess than 10% Al4C3 or compounds which form
said aluminum when heated to reaction temperature (about 2100C).
The term "open arc" is used herein as intended to meàn
an arc from an electrode which is not in physical contact with
the charge to be reacted. The electrical power of the arc is
not narrowly critical since it is not the purpose of this pro-
cess to maintain substantially all of the aluminum formed in
the liquid state. However, it is desired that the electrical
power of the arc be such that it produces an electrical density
of greater than 50 kilowatts per square inch of charge struck
by the arc and more preferably, between lO0 and 200 kilowatts
per square inch of arc struck by the charge.
As has previously been pointed out9 it is also neces-
sary that the arc should be an intermittent arc, i.e. it should
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be on for a period of time and off for a period of time with
respect to a given area of charge. This type of operation is
referred to as an intermittent operation, and this expression
refers to the fact that a particular portion of the charge stock
is subjected to direct electrical heating via an open arc only
from 10-50 percent of the total time. Thus, by way of a con-
venient example the arc can be struck to a charge for a period
of time of one minute and then turned off for two minutes and
then be restruck for another minute, etc. In a preferred
embodiment of this invention it is desired that the arc be
applied for a period of time ranging from 1/120 to 90 seconds,
and thereafter be turned off for the appropriate period of time
such that the heating only occurs from 10-50 percent of the
total time.
It is to be immediately understood that the most pre-
ferred embodiment of this invention which will produce the
intermittent heating does not reside in turning the arc on or
off, but rather, as is illustrated in Figure 1, it is preferred
that the arc be left on continuously but moved over the surface
of the charge stock by either mechanical means or by the use
of a magnetic field such that the arc strikes a particular
portion of the charge between 10 and 50 percent of the total
time.
It is to be understood that althougX the invention
has been described with respect to the use of a plasma torch
it is also possible to carry out the process of this invention
using carbon electrodes. There are many techniques which can
-be employed with carbon electrodes in order to obtain the
intermittent type of heat above referred to. Thus, for example,
a bundle of electrodes can be combined in one pack e.g., six
electrodesS and each electrode separated electrically from the
other. Arcs can be made to progress from one electrode to
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another electrode by commutation of DC current or by electrical
commutation.
It is also possible to use a single carbon electrode
and achieve the same type intermittency. In this embodiment a
slowly rotating magnetic field transverse to the arc direction
can be applied to keep the arc spot regularly migrating around
the periphery of a single electrode.
It has been found to be advantageous to use an open DC
arc with the adjustable electrode negative with respect to the
charge. The reason for this is that the negative electrode
receives less of the heat while emitting electrons than the
anodic charge receives. Under the DC arc operation with the
movable electrode negative, the charge receives most of the
heat and the electrode remains cool enough to avoid excessive
volitilization of the carbon.
It is to be understood, however, that although the
invention is capable of being carried out with a carbon elec-
trode nevertheless, it is preferred to use a plasma torch as
an electrode as is set forth in Figure 1 since it would appear
that better furnace control can be obtained.
The open arc is viewed as desirable because the sur-
face temperature of the charge has an opportunity to decrease
rapidly upon arc interruption, thus permitting the majority of
the charge to remain at the required low temperature as a
result o~ heat transfer to the colder portion of the furnace
during periods of arc interruption~ ~
It is to be understood that carbon or graphite elec-
trodes of dimensions typical of present day arc furnaces
(40" diameter and larger) provide intermittent arcs axound the
periphery of the electrode when not directly conducting to the
furnace chaxge. The arcs migrate about the electrode in a
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random manner, which is not under the control of the operator,
for which reason we prefer to use electrode configuration or
plasma torches which permit control of where the arc strikes
the charge and when.
As long as provision is made to provide an open arc
from a conventional carbon or graphite electrode and avoid
conduction of electricity from the electrode to the charge, such
electrodes are considered to provide intermittent heating of
the type required to practice this invention.
Thus, in order for the novel process of this invention
to be effective, it is required that when the liquid aluminum
which is produced flows over an unreacted charge, it must be at
~ temperature below about 1900C and preferably at a temperature
ranging from 1850-1900C. On the other hand, after the con-
densed aluminum is removed from the unreacted charge or other
source of carbon then, quite obviously, it can be at any tem-
perature since there will be no unreacted charge or source of
carbon and therefore no aluminum carbide which can be dissolved
by the aluminum.
As has heretofore been stated, it is possible to use
an open arc in order to carry out the process of this invention
and although such open arc can be produced by using conventional
graphite electrodes in the manner previously described, a pre-
ferred embodiment of this invention resides in using plasma
torches in order to provide the open arc.
Although the use of graphite electrodes delivers heat
at an appropriate power density and provides a pressurized gas
effect which tends to move the aluminum produced at the surface
of the charge away from the charge, it does suffer from the
disadvantage in that it introduces a small amount of carbon to
the product. However, the use of graphite electrodes has a
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further practical operating disadvantage in that if the arc is
extinguished, the only practical way to re-establish an arc of
this power density is to lower the electrode until it touches
and makes electrical contact with the charge. This type of
action can lead to difficulties with the charge sticking to
the electrode. If too much charge is stuck to the electrode,
the electrical discharge properties of the electrode are
altered to the detriment of the overall operation. In order
to avoid such problems, careful control must be exercised over
the arc struck between a carbon or graphite electrode and the
charge.
The use of a plasma torch eliminates the above-
mentioned difficulties which can be experienced when using
conventional graphite electrodes in that, quite obviously, no
carbon is added to the product and the plasma jet has the
advantage that the arc can be established even though the jet
nozzle is completely removed from the vicinity of the charge.
Additionally, if the jet is extinguished for some reason, it
can be re-established without any physical part of the jet-
forming equipment being brought into contact with the charge.Still another advantage of the plasma jet is that in addition
to the normal tendency of the arc column to force the produced
aluminum away from the charge, the jet comprises additional
gas flow (which is an essential feature of the operation of
plasma jets) and this additional gas flow adds to the tendency
of the arc jet to remove the product aluminum away from the
site of the reaction such that it can cool rapidly and not
dissolve appreciable amounts of unreacted charge.
A still greater advantage can be obtained from the
use of plasma jets when additional circuits are provided
wherein a second power supply is connected between the cathode
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element of the plasma jet and the hearth so that the arc column
is drawn not from a negative electrode to the jet nozz~e, but
instead from the negative electrode to the hearth. In this
mode of operation 9 very little current flows to the nozzle,
Most of the current flows to the hearth. A very high heating
rate is established at the site of the reaction even though
the nozzle of the jet can be a substantial distance (for
example, 6-12 inches) away from the charge. This provides
ample opportunity for the charge to pass under the jet without
being struck by the casing of the jet apparatus.
If for some reason the transfer current, that is, the
current from the negative electrode of the jet to the hearth,
is interrupted, then the power supply of the internal jet
maintains the jet in normal plasma jet operation between the
negative electrode and the positive jet nozzle. This then
serves as a pilot light to re-establish the jet through the
second power supply to the hearth at any time, without having
to move the jet physically, relative to the hearth.
This starting and stopping of the transfer power
between the negative electrode and the hearth can be so rapid
as to occur as often as 60 cycles per second. In fact, one of
the preferred embodiments of the plasma jet application to
this invention is to use half-wave DC power (for example, 60
cycles half-wave DC) for the transfer power. In this way, for
one-half cycle, the transfer occurs with the interior electro-
de of the plasma torch negative and the hearth positive. When
the voltage of the alternating current supply reverses, rec-
tification blocks the transfer current from the hearth back
to the internal electrode of the jet.
It can be seen that with this type of half-wave
transfer between the internal electrode of the jet and the
hearth, the peak power delivered at the target area, mainly
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the site of reaction, is about four times the average power
delivered to the target area. The rate of heating by t~e
plasma jet to the charge is insignificant when the arc is not
transferred to the charge compared to when the arc is trans-
ferred. Therefore, on the half-cycle where the arc is not
transferred to the charge, the charge can be losing heat to
the relatively cool (for example 1600C) walls of the furnace.
It can be readily understood, therefore, that the very high
temperature required for the reaction only occurs in the very
thin layer where the jet is striking the charge and down into
the charge body and in the surrounding portions of the charge
the temperature is much lower. The high temperature zone is
only a small fraction of an inch thick when using half-wave
DC jet transfer.
No practical way has been devised as yet to make a
simple carbon electrode perform on half-wave DC transfer.
Once the arc is extinguished due to the return of the voltage
to zero it must be relighted by some method which is not
convenient with a carbon or graphite electrode.
Additional details of plasma torch operation are
disclosed in U.S. Patent 4,033,757.
Quite obviously, after the aluminum is tapped from
the furnace, it can be subjected to purification techniques to
remove the aluminum carbide.
The following examples will illustrate the process of
this invention.
EXAMPLE 1
Alumina and coke or char are ground to pass 20%
through a 325 mesh and all through a 20 mesh screen and
30 blended to provide a mixture having 85 wt.% A12O3 and 15%C.
This mixture is mixed with, for example, polyvinyl alcohol
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- binder and formed into bri~uettes having a maximum dimension
of 2". The briquettes are heated in a shai~t furnace at 1950C
to convert at least half the alumina to A1404C. The briquettes
are analyzed for carbon and oxygen and blended with coke or
char to make a mixture having the overall weight proportions
12 C to 16 oxygen.
A furnace as described above and illustrated by
figure 1 is started by first heating the hearth by impingement
of the plasma jet transferred to the hearth until the hearth
10 achieves a temperature of about 1000C. The magnetic circuit
is turned on to rotate the arc at about 1 revolution per
minute and to deflect it to describe a circle of pre-determined
diameter corresponding to the expected intersection of the
charge with the molten pool. The plasma jet operates on an
internal voltage from the full wave rectifier of about 50 volts
and a half wave peak transfer voltage from negative electrode
to hearth of about 250 volts. Carbon chunks 2" - 4" in
dimension are placed on the hearth to a depth of 12" and molten
aluminum is poured into the furnace to a depth of 16". The
20 heating by the plasma jet is resumed until the molten pool
achieves a temperature of 1850C.
Charge is then added to fill the furnace to its
normal operating level. The plasma arc is caused to rotate
at 1 revolution per second and at the previously established
deflection by the control circuits of the magnet. Charge is
gently poked down periodically by conventional mechanical
means (not shown) within the furnace and new charge is added
as necessary to maintain the level in the charge column. Air
is excluded from the furnace by exhaust control keeping
30 pressure in the range 1 - 5 atmosphere. Torch power is
controlled to maintain the molten pool over the hearth in the
range 1850-1950C. Aluminum containing 10% or less A14C3
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is periodically tapped from port 12.
EXAMPLE II
Alumina and aluminum carbide are ground to pass 20%
through a 325 mesh and all through a 20 mesh screen and
blended to provide a mixture having 75.5 wt.% A1203 and
24.5 wt.% A14C3. The mixture is further blended with 2.5%
starch and cold pressed to form briquettes having a~maximum
dimension of 2". The briquettes are blended with coke or
char to make a mixture having the overall weight proportions
12 C to 16 oxygen.
A furnace as described above and illustrated in
figure 2 is started by adding coke 1" and down to a depth of
12"~directly under the electrode and surrounding this with
coke to 2" - 4" in dimension to a depth of 12 inches out to
the walls of the furnace. An arc is drawn from the ¢arbon
electrode to this coke on a schedule typical of the startup
of conventional furnaces for the production of silicon until
the hearth under the electrode has achieved a temperature of
1000C. Molten aluminum is then poured into the furnace to
a depth of 16". The arc is re-established to resume heating.
Sufficient charge is added tdepth of 6") to confine the heat
of the arc until the molten pool has reached a temperature of
1850C. Charge is thereafter added until the charge column
has reached its normal operating level. Power is controlled
to maintain the molten pool temperature in the range 1850C
to 1950C. Charge is gently poked down periodically by
conventional mechanical means (not shown) within the furnace
and new char~e is added as necessary to maintain the level of
the charge column. Air is excluded from the furnace by exhaust
control keeping pressure in the range 1-5 atmosphere. Aluminum
containing 10% or less A14C3 is periodically tapped from port
12.
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