Note: Descriptions are shown in the official language in which they were submitted.
104~i8Z6
This invention relates to a new and improved process
for producing aluminium metal having no more than about 5
weight percent of aluminium carbide under carbothermic conditions
from feed comprising an oxide of aluminium and carbon containing
compounds in a single furnace operation.
Even the most cursory inspection of the prior art relating
to the thermal production of aluminium will immediately indicate
that there has been much activity by many people in an attempt
to adequately define a process which would replace the
conventional electrolytic method of preparing aluminium. The
art has long been aware of the many theoretical advantages
which flow from the use of a thermal reduction process for the
production of aluminlum as opposed to an electrolytic method.
Unfortunately, there has been one drawback to the heretofore
suggested thermal processes for producing aluminium. That ~`
drawback has been that after all was said and done, the simple
fact remained that there was no way to produce significant
amounts of aluminium in a substantially pure state from a
thermal process.
It is to be immediately understood that the difficulty
in producing aluminium with respect to thermal processes does
not reside in the formation of the aluminium via reduction of the
alumina-bearing ore, but rather, in the recovery of the aluminium
in a substantially pure state. The patent ar~, as well as the
literature, is full of theories and explanations with respect
to various back reactions which can take place between aluminium
and the various carbon-containing compounds in the feed. The
sum total of all prior art efforts is simply that there is no
commercial process today for preparing aluminium other than by
an electrolytlc process.
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1045826
In general, it can be stated that the prior art
processes for the thermal production of aluminium can be divided
into two general categories, i.e. one wherein aluminium is
formed in the vapour state and the other being where aluminium
never reaches the vapour state, i.e. it is formed as liquid
aluminium.
With respect to the processes wherein aluminium is
produced in a vapour state, the major difficulty which was
encountered was due to the fact that aluminium vapour is
extremely reactive with carbon monoxide which is inherently
formed in the reaction, thereby producing aluminium-carbon
compounds. The patent and other literature contains many
proposals directed towards ways of minimising the reaction
of aluminium and carbon monoxide, but, in general, the
i5 solutions proposed heretofore have been impractical. One
solution to this general problem wherein aluminium is formed
- in the vapour state is disclosed and claimed in our United
States Patent 3,607,221. Although the process of this patent
- does result in the production of aluminium in a substantiallypure state, nevertheless, extremely high operating temperatures
are involved which leads to problems with respect to materials
of construction.
Those skilled in the art have long been aware that the
aforementioned difficulties with respect to back reaction of
- aluminium with carbon monoxide could be avoided if, indeed, the
aluminium i5 never in a vapour state. Thus, it is known that
if a process is carried out under conditions such that
aluminium is formed in the liquid state, then this liquid
aluminium will be relatively inert with respect to carbon
monoxide, thereby resulting in a process which should be free
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of back reaction products.
Although there are many processes disclosed in the prior
art wherein aluminium is produced in the liquid state, the
simple fact remains that none of these processes have met with
success with respect to initially producing aluminium containing
low percentages of aluminium carbide. The reason for the
failure of the prior art processes can be easily understood when
one considers the fact that aluminium carbide is solu~le in molten
aluminium and the solubility of aluminium carbide in aluminium
increases with increasing temperature. Aluminium carbide is
present in a carbothermic reduction process, owing to the fact
that it either is introduced as a reactant or is inherently
formed during the reduction reaction. This is because aluminium
is highly reactlve with carbon and certain aluminium-carbon - t
compounds to form alu~ m carbide. Thus, since the prior
art processes by necessity had to be carried out at elevated
- temperatures in order to form aluminium, the liquid aluminium
formed, although relatively unreactive with respect to the
carbon monoxide, nevertheless, does dissolve the aluminium i
I carbide which was also inherently present in the system, thereby
resulting in carbide-contaminated aluminium. It should be
pointed out that aluminium containing greater than about 5 weight
percent of carbide contamination is`extremely undesirable for
many reasons, including the fact that it sets up to a hard non- I
i flowable mass as the temperature is decreased slightly from ;
reaction temperature thereby resulting in severe practical
dlfficulties with respect to transferring it from one place to
another except at elevated temperatures. Additionally, lt must
~e realised that electrical energy has been spent to produce the
aluminium and if it is contaminated with more than about S
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104~Z6
weight percent of aluminium carbide, the additional energy
which must be used in subsequent recycle operations renders the
process non-competitive commercially with respect to power
consumption.
A whole body of prior art has arisen on various ways of
removing aluminium carbide in admixture with aluminium and there
have been many patents and literature articles on this subject.
This invention is not concerned with the removal of
aluminium carbide from aluminium but is concerned with a
carbothermic process for the preparation of aluminium which never
has any substantial amount of alumlnium carbide present to begin
with. This invention accomplishes that which workers in the
art have long hoped could be done and what other learned people
have thought was theoretically impossible to do.
Tn fast, as will he seen from the actual working examples ;~
in this application, aluminium having an insignificant amount
of aluminium carbide admixed therewith has, indeed, been produced
in a significant amount by carbothermic reduction. -
` This invention is directed towards the carbothermic
3 reduction of alumlna-bearlng ores and it has certain -
characteristics in which it departs from the heretofore
practised processes. At the outset, it is noted that this
invention is basically a dual temperature process, i.e.
aluminium is formed at a first high temperature reaction zone
- , .
S and thereafter collected at a significantly lower temperature.
The lower temperature where aluminium is collected Ls chosen
such that it is physically impossible for aluminium carbide to
be dissolved by the aluminium since, as stated above, the
ability of aluminium to dissolve unreacted aluminium carbide ~ -
,0 is strictly a function of temperature. Thus, the aluminLum is
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produced at one temperature and is allowed to flow over cool
unreacted charge to a second zone maintained at substantially
lower temperature wherein the aluminium is physically incapable
of dissolv.ing aluminium carbide in appreciable amounts.
.. i~.
; An extremely important characteristic of the process of
` the invention resides in the way in which heat is applied to
~ the first zone. The process of this invention absolutely
j ` and positivelyrequires that only a small or minor portion of
f the charge be heated to the reaction temperature at any given time
while the majority of the charge must remain at a temperature
significantly less than the reaction temperature. This
requirement is absolutely contrary to any of the prior art
processes which have heretofore ~een practised. It should
~ become immediately apparent that if one is conducting a thermal
r' 15 operation, it appears almost logical that one of the main
objectives should be to heat the charge in the reaction zone to
the reaction temperature as quickly as possible and as uniformly
r i ~ :
as possible in order to ensure complete reaction. It is not !
surprising that prior art workers strove to accomplish just
that. It has now been discovered that if in fact such uniform
heating i6 carried out, aluminium in the pure state simply 1
will not be formed. The process of this invention does not i
utilise uniform heating of a charge but rather at the hottest
part of the reaction~zone, the ma~ority~of the charge is not
ae reaction temperature at any gi~en time and is deliberately
kept this way.
In fact, the uniform heating of large portions of the -~
-charge material to reaction temperature is perhaps the main !
reason why the prior~ art workers failed to produce su~stantially
pure aluminium in a carbothermic process which produced condensed
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~; 10458Z6
aluminium instead of an aluminium vapour. The reason appears
by hindsight to be that, if the entire charge is heated to
reaction temperature, then the aluminium which is formed and
which flows over the charge must contact carbon and aluminium
carbide inherently present in the furnace at elevated temperature
and a low aluminium carbide-containing product is not obtained in
a single furnace operation.
It has now been discovered that aluminium having only a
low aluminium carbide contamination can be produced in a single -
furnace operation, provided the charge material is heated
non-uniformly, i.e. substantially only the surface of the
charge is heated such that this surface-heated material produces
aluminium and then the aluminium is allowed to flow over the
non-reacted portion of the charge which is not at elevated
i temperaiur~, t1-1us rendering it sukstanti211y impossib]e for
the resulting condensed aluminium to dissolve aluminium carbide.
Another portion of the charge is then exposed to this high -;
degree of heat and the cycle is continued. -
As already mentioned, the process of this invention ,
) requires a two-temperature operation, a high temperature zone
wherein the reaction for the production of aluminium is driven
forward and a low temperature zone for transporting and collecting
aluminium while preventing it from substantially dissolving any
unreacted carbide. ' -
To the furnace is chargedan aluminium oxide-bearing
material in admixture with a carbon-containing compound which is
preferably aluminium carbide and/or carbon. Since it is
desired to produce substantially pure aluminium, the aluminium
oxide-bearing material is preferably alumina of a high degree
0 of purity, e.g. Bayer alumina, but it is to be understood that the
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104582~; `
r process of this invention is equally operable with impure
forms of alumina and aluminium oxycarbides in which case,
although the resulting product will still be free from carbide
contamlnation, it will contain the impurities normally present
' 5 in the alumina ores employed.
The ratio of aluminium oxide-bearing compounds to
carbon-containing compounds is preferably adjusted such that
the composite charge contains a 1:1 + 0.05 atomic ratio of
carbon to oxygen.
The process of this invention can be carried out at any
pressure higher than 0.1 atmosphere. It has been found that
at pressures below 0.1 atmosphere liquid aluminium is not made
any
t: under/practical set of conditions. On the other hand, it is
recognised from thermodynamic considerations that vaporisation
losses decrease as the pressure increases above 0.1 atmosphere.
However, the use of higher pressures re~uires equipment capable
of handling such pressures so it is evident that the choice of
; ~ . .
~` pressure above about 1.0 atmosphere involves an economic
balance between the energy lost because of vaporlsation and
the cost of equipment. In most cases, practical pressures for -
the instant process range~from a system pressure of O.S to 10
atmosphere, with from 1-5 atmospheres being preferred.
~' ~ It is also noted that an embodiment of this invention
, ~ which will be later described involves the use of a plasma torch.
In such a situation, the torch itself exerts a pressure
, depending on power density. Thus, it is recognised that the
~ pressure directly under the torch in the reaction site can be
; i
higher than the pressure away from the torch.
~ ~ It will be appreciated that the particular pressures and
; 30 temperatures utilised are mutually dependent on each other as
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~045826
is obvious from a rudimentary consideration of the laws of
thermodynamics. There exists :in the literature works of many
authors setting forth temperatures which are necessary for
various pressures. However, the exact temperatures which are
stated to be necessary for any given pressure vary depending
upon the author's interpretation of the thermodynamic data.
In general, however, a temperature of about 2500K is necessary
for operation at l atmosphere. Therefore, from a practical
point of view, it is difficult to specify the exact temperature
which is necessary to drive the reaction forward for any given
pressure. Additionally and perhaps more signficantly, such
a recitation of specific temperatures is of no practical
significance since in an actual furnace operation the instruments~
used to measure the temperature utilise optical principles and
the charge is hidden from view by the presence of electrodes.
The temperatures which are utilised in the process of this
invention can be described by stating that for any given
pressure a sufficient heat must be used to drive the reaction
forward but that too high a temperature will cause the aluminium
~: 20 produced to volatilise out of the furnace. However, from
a practical point of view the reduction of alumina-bearing
, ores to produce aluminium absorbs heat and the reaction itself
controls the temperature.
It has been found that irrespective of all the above-
stated principles with respect to temperature and pressure there
.... . . .
exists a very convenient manner for carrying out the process of
, this invention. It has been discovered that an accurate
. control of the reaction can be accomplished by striking an open
arc, as hereinafter defined, to the surface of the charge to be
reduced and by regulating the electrical density of the arc
striking the charge. It has been found that if the electrical
; density is maintained between lO and 50 kilowatts per square
.
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1045826
inch of charge struck by the arc, the reaction will proceed in
a desirable manner. When the arc density has passed its
minimum threshold value the very occurrence of the reaction
controls the temperature at higher arc densities because the
reaction absorbs heat.
By the expression "electrical density per square inch
of charge" is intended to mean measurement of the total
electrical power supplied to the arc ~i.e. amperes x volts)
divided by the area of charge struck by the arc. With respect -
,
to the use of a plasma torch, calculations of electrical density
ignore the internal power supply and only the transfer current
is taken into consideration.
A convenient method of measuring the total area struck -
by the arc is to use an optical instrument. The portion struck
by the arc will glow and its area can be measured. In the case
i where the surface of the charge is irregular, such area can be
measured by striking the arc to the bed of the furnace without
putting in any charge.
It has now further been discoverèd that one convenient
method for producing the type of heating which this invention
, employs resides in the use of an open arc wherein the
, adjustable electrode is negative with respect to the charge to
~i be reacted. The term "open arc" is used herein to mean an arc
from an electrode which is not in physical contact with the
, ~ 25 charge to be reacted. In this embodiment, a charge stock is
introduced into the furnace and an open arc is struck from a
suitable electrode such as a conventional graphlte electrode.
, If the electrical power of the arc is such that it produces an
electrical density of 10-50 kilowatts per square lnch of
charge struck by the arc and, more desirably, 25-35 kilowatts
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r
~ 10~58Z6
per square inch, then the surface of the charge will be
heated to the desired temperatures. However, this
condition alone does not ensure that the charge will not be
heated in a uniform manner. In order to accomplish this,
it has been found that the arc should be an intermittently
effective arc, i.e. it should be effective for a period of time
- and ineffective for a period of time with respect to a given
area of charge. This type of operation will hereinafter be
referred to as an intermittent operation and what is meant by
this expression is the fact that a particular portion of the
charge stock is subjected to direct electrical heating by an
open arc only from e.g. 10 to 50% of the total time. Thus,
by way of a convenient example, the arc can be struck to a
charge stock for a period of time of one minute then be turned
off for two minutes, then be restruck for another minute, and
so one. In the preferred embodiment of this invention, it
~ is desired that the arc be applied for a period of time ranging
g; from 1/120 to 90 seconds, and thereafter be turned off for the
appropriate periods of time such that heating only occurs fro~
10 to 50% of the total time.
~ It is to be immediately understood that there are other ;
!~ ways of achieving this intermittent heating rather than merely
turning the arc on and off. Thus, for example, a plurality ~:
of electrodes can be used over a wide surface area and each
electrode be turned on and off at appropriate intervals ~ -
~ within the guidelines above set forth. Alternatively, the
s electrode can be left on continuously but moved over the surface
of a charge stock by mechanical means such that the total amount
of time that the arc strikes a particular portion of the .
surface area is from 10 to 50~ of~the total time. In like
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~ -- 10458Z6 ~:
manner, the electrode can be left on continuously and the charge
moved in and out of the arc by mechanical means such that the
arc strikes a given area 10 to 50% of the total time.
It has been found to be advantageous to use an open
D.C. 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 èlectrons than the
anodic charge receives~ Under D.C. arc operation with a movable
negative electrode, the charge receives most of the heat and the
~ 10 electrode remains cool enough to avoid excessive volatilisation
¦ of carbon. This minimises the opportunity for hot carbon~ ~
~; vapo4r to come into contact with the condensed aluminium product ~-
where it could form aluminium carbide, which, in turn, would
readily go into solution in the aluminium product. ~ L
It has been found that a graphite electrode which is i
negative with respect to anodic aluminium can melt aluminium
without the addition of more than 0.3% A14C3 to the aluminium.
The open arc is regarded as desirable because the surface
temperature of the charge has an opportunity to decrease rapidly
upon arc interruption, thus permitting the majorlty of the charge
- to remain at the required low temperature as a result of heat
transfer to colder portions of the furnace during periods of
arc interruption. During the high temperature operation, above-
described, the carbon monoxide which is formed is removed from
the system while the aluminium is in a condensed state so that
for practical purposes substantially no aluminium compounds are
formed by back reaction. -
The second stage of the process of this inventionconsists in
removing the condensed aluminium at a temperature such that
substantial amounts of aluminium carbide simply cannot be
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~;
~! ' ' 104~8~6 ~
5'~ dissolved therein. The temperature of the second stage should
not exceed 1250C and preferably should be at a temperature
ranging from 670 to 1000C.
One technique for accomplishing this consists in
maintaining a liquid pool of aluminium inside the furnace and
floating the charge thereon, heating the charge in the manner
above described so as to form aluminium in the condensed
state and then allowing the formed aluminium to flow into the
liquid pool of aluminium which is maintained at the temperatures ;,2
above stated. As is well known in the art, a liquid metal
is an excellent conductor of heat and it will remove heat from
the arc and charge to areas where the heat can be lost through
, furnace walls, roof, and floor in a rapid manner thereby
ensuring the necessary temperature controls.
It is to be understood, however, that although the
maintaining of a liquid pool of aluminium is an effective manner
, , ~.
of ensuring the temperature control above described, there are
other ways of accomplishing the same, such that the maintaining
of a liquid pool of aluminium is not absolutely critical in the ¦ ~ ;
process of this invention.
It has been found, for example, that the action of an
open arc has a tendency to blow the aluminium which is formed
away from the unreacted charge such that the condensed aluminium r~
rapidly cools and when it passes over the unreacted charge it is
at a sufficiently low temperature not to dissolve appreciable
amounts of aluminium carbide.
The aluminlum formed can also be removed from the ,
unreacted charge simply by mechanical means. Thus, for
example, a sloping hearth can be used such that the condensed
aluminium immediately flows out of the reaction zone and
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~ 10458Z6
becomes cooled so that when it passes over the unreacted
charge it is at a temperature low enough to prevent dissolving
of appreciable amounts of aluminium carbide. Other techniques
accomplishing the same result include having a dual level
furnace hearth such that the upper and lower level of the
furnace hearth are connected by passages which are large
enough to allow aluminium to flow from the top level to the
bottom level, but small enough to prevent the charge from
passing from one level to the other. Thus, when the char~e
stock which is contained on the uppermost level is struck
by the arc and liquid aluminium is ormed, the condensed
aluminium flows through the charge into the second level
of the furnace hearth where there is no aluminium carbide for it
to contact.
Thus, in order for the process of this invention to be
effectlve, it is desirable that when the liquid aluminium which
is produced flows over an unreacted charge it must be at
a temperature below about 1250C and preferably at a temperature
ranging from 670-1000 C. On the other hand, after the condensed -~
aluminium is removed from the unreacted charge or other source
of carbon then it can be at any temperature since there will
be no unreacted charge or source of carbon and therefore no
al~minium carbide which can be dissolved by the aluminium.
Another significant aspect of the process of this
$nvention is the fact that because an open arc is used, it is
possible to use a closed furnace rather than one which is
exposed to the atmosphere. The use of a closed furnace has
an additional benefit from an environmental point of view since
it minimises the gases which must be treated to remove the
pollutants therefrom in order to comply with environmental
- 14 -
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104,8Z6 ~
standards. The closed furnace also permits utilisation of the
fuel values of the carbon monoxide released by the process.
Thus, although it is not necessary to use a closed furnace in the
process of this invention in order to produce aluminium,
j nevertheless, the useof a closed furnace does provide added
economic advantages from the environmental and energy
conservation points of view, there~y improving the overall
attractiveness of the process.
As has already been mentioned, although an open arc
~` LO can be produced for the purposes of this invention by
` using conventional graphite electrodes in the manner previously `
¦ described, a preferred embodiment of this invention resides in
using plasma torches in order to provide the open arc. , - -
Although the use of graphite electrodes delivers heat :
i L5 at an appropriate power density and provides a pressurised gas
effect which tends to move the aluminium 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. As stated above, it has been found
!O that a graphite electrode which is negative with respect to -
anodic aluminium can melt aluminium without adding more than
~; about 0.3 weight percent of aluminium carbide to the aluminium.
However, the use of graphite electrodes has a further practical -
operating dlsadvantagein that if the arc is extinguished, the
!S only practical way to re-establish an arc of this power density
ls 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
r: much charge sticks to the electrode, the electrical discharge
properties of the electrodé are altered to the detriment of the
overall operation. In order to avoid such problems, careful
r ,
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10458Z6control must be exercised over an arc struck between a carbon
or graphite electrode and the charge. -
The use of a plasma torch a~oids these difficultiesexperienced with conventional grap~ite electrodes in that 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 part of the jet-
forming equipment being brought into physical 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 aluminium away from the charge, the jet comprises
additional gas flow (which is an essential feature of the
operatio~ of p~asma jetCs and thic additional gas flow adds
to the tendency of the arc jet to remove the product aluminium
away from the site of the reaction so 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 element of
the plasma jet and the hearth so that the arc column is drawn
not from a negative electrode to the jet nozzle, but instead from
the negative eLectrode to the hearth. In this mode of operation
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, 3-6 inches) away from the
charge. This provldes ample opportunity for the charge to pass
under the jet without being struck by the casing of the jet
apparatus.
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`~ 1045826
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 7
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 ,j
, - to use half-wave DC power (for example, 60 cycles half-wave
, DC) for the transfer power. In this way, for one-half cycle,
i the transfer occurs with the interior electrode of the pIasma
torch negative and the hearth positive. When the voltage of
the alternating current supply reverses, rectification 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 the site of
reaction, is about four times the average power delivered to the
target area. The rate of heating by the plasma jet to the
i charge is insignificantwhen the arc is not transferred to the ~
charge compared to when the arc is transferred. Therefore, on
the half-cycle where the arc is not transferred to the charge,
the charge can be radiating heat to the relatively cool (for
example, 1200C) walls of the furnace. It can`be readily
understood, therefore, that the very high temperature required j 0
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s~ for the reaction (2300C) only occurs ln the very thin
layer where the jet is striking the charge and down into the
charge body and in the surrounding portions of the charg~ 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 by the return of the voltage to zero
it must be re-lighted by some method which is not convenient with
~ a carbonor graphite electrode. -
!; In the accompanying drawings:
; t
Figure 1 is a vertlcal cross-section of a furnace
suitable for carrying out the present process;
~gu-c 2 _cpresent d-agra~matically the configuration of
a plasma arc equipped for DC transfer to the charge which can
be used for carrying out the present process; -
Figure 3 represents another configuration of a plasma
arc, which is equipped for half-wave DC transfer; and
Figure 4 shows diagrammatically a furnace suitable for
carrying out the process of the present invention utilising
the plasma arc configuration of Figure 2.
In both Figures 2 and 3, showing a plasma torch which
, can be used in the process of this invention, an orifice 14 is
provided in a plasma jet casing 15, and the cathode, or emitting -
electrode 16 of the plasma jet is insulated from the casing 15
by insulation 22. .
In a conventional plasma jet application, a power supply
19 supplies a negative voltage to the electrode 16 with ~
respect to the nozzle 14 and casing 15. Electrons are emitted -
- 18 ~
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- - : , -
~ ~04S82~>
't
from the tip of the electrode 16 and the force of the gas
between the nozzle and the electrode tip prevents direct
discharge between the electrode 16 and the nozzle. Instead,
the electrons flow out and then come back and attach to the
nozzle 14, leaving a pencil point-shaped jet which is
independent of any other anode surface. In other words, this
jet will exist and be maintained without having any other anode
surface around.
Now, with respect to Figure 2, if a second power supply
is connected between the electrode 16 and another electrically
conducting surface 18, and extra DC voltage (for example, 100
volts) from a supply 20 is connected through a switch 21, then
the arc transfers and instead of flowing between the electrode
16 and the nozzle 14, it now flows between the electrode 16 and
- 15 the target area 18. If a charge 17 is within the target area
of the transfer arc, it is heated rapidly and efficiently,
receiving most of the energy delivered in the arc. Under the
transfer mode of operation employing the second power supply
20, the charge is heated much more efficiently and more rapidly
than if the simple conventional plasma torch were to be brought
into the vicinity of the charge.
Figure 3 illustrates how a half-wave transfer can be
~ ¦ applied. Again, a power supply 19 maintains the arc whenever
,:, I
¦ the second power supply is not applying power between the
electrode 16 and the charge 17. When the AC voltage through a
i transformer 23 is in such a direction as to pass through a
; rectifier 24 to make the electrode 16 negative with respect to
the hearth 18 and charge 17, then on that half-cycle, current
will be transferred from the negative electrode 16 to the charge,
delivering heat to the charge. When the AC voltage through the
transformer 23 reverses such that it would make the electrode 16
~ . ' :'~:~
:' - 19 - ,
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.: ~04S~26
~; . .
' positive with respect to the hearth 18 then the rectifier
~ 24 blocks the passage of current and the arc transfer is
.,.,.
extinguished. The arc then reverts to the conventional plasma
jet mode, being drawn between the electrode l6 and the nozzle
14 and being maintained by the power supply 19.
As has been stated, the advantage of this type of arc i~
transfer resides in the fact that the charge surface is heated
to a temperature (for example, 2300C) sufficiently high to make
the reaction between alumina and carbon proceed with the
formation of aluminium in the condensed state and carbon
monoxide, but on the reverse half of the AC cycle where the
rectifier 24 blocks the passage of current the charge is not
heated and in fact radiates heat to surroundings which are in the
neighbourhood of 1200C. Therefore, the interior portion of
i the charge remains relatively cool, a condition whic~ iS
essential to avoid pick-up of carblde in the aluminium which
has been produced. Likewise, the surrounding charge particles- ~
which are not struck by the arc in its transfer mode are not
heated to a temperature sufficiently high to impart aluminium
carbide to the aluminium which rolls across them on its way
to the area of the furnace where the produced aluminium will be
held until tapping.
The following examples illustrate the practice of this
lnvention:
EXAMPLE i -
A furnace was constructed as shown in Figure 1 to permit
electric arc heating under a vacuum or controlled atmosphere.
furnace shell 1 of steel was fitted with a lid 2, sight
~ubes 3, and access tubes 4 and 5 (not used in this experiment). -
3 Casta~le refractory of bubble alumina filllng the space 6 and .
carbon flour 7 provided heat insulation. A graphite crucible
8 was connected to the positive terminal of a DC supply ~not
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1045826
shown) through a graphite rod 9. A negative electrode 10
of graphite was electrically insulated from the furnace lid 2
.
through an electrically non-conducting vacuum gland 11 and
was vertically adjustable by a screw mechanism 12. A vacuum
line (not shown) was connected from the furnace lid 2 through
; a bag filter to a vacuum pump in order to remove carbon
; monoxide.
Initially the furnace was heated by the application of an
' arc of 4 kw. power under a vacuum of 15 inch Hg below
) atmospheric i.e. about 1/2 atmosphere system pressure. The arc
struck an area estimated to be about 3/4 inch diameter on the
crucible at the location of the rod 9.
When the crucible had been warmed enough to show dull
red heat after arc terminations, 443 grams of molten aluminium
~ 5 were added. The arc was struck to this aluminium for several
t; minutes to bring its temperature up to about 1000C.
One pellet, weighing approximately 8 grams, of a mixture ~
in the weight ratio 58.5% A14C3 to 41.5~ metallurgical grade t
A1203, cold pressed with a binder of 5% starch, was floated
on top of the molten pool of aluminium, after the metal had been
skimmed. The pellet was positioned directly under the negative
electrode. ` - i
The system pressure was reduced to the range of 8-10 inch
Hg below atmospheric. An arc of 30V and 500A was-applied for
30 seconds, during which time the pellet was seen to react
and form aluminium, which coalesced with the starter pool.
- With the arc off, the furnace was returned to atmospheric
pressure with argon flowing through the sight tubes, the sight
glasses 13 were removed and two additional pellets were
O floated on the metal pool directly under the negative electrode
i after the pool was skimmed to expose unoxidised melt. The
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` temperature of the pool was about 1100C.
The sight glasses were replaced, the system pressure
was reduced to 8-10 inch Hg below atmospheric and an arc of 15
kw.was again struck to cover the intersections of the pellet with
the metal pool. The arc target area was estimated to be about
3~4" diameter. The arc was applied for 60 second, during which
time aluminium was formed on the exposed surface of the pellet and
at the intersection of the pellet with the starter pool until i.
most of the pellet was consumed, the aluminium produced
coalescing with the molten pool. -
This cyclical process was repeated until 83 grams of
charge had been reacted. At no time was the arc applied for
more than 90 seconds. The delay time between arcs, because of
~; the furnace charging operation, was from 2 to 5 minutes between
i each arc application. The molten pool was maintained at a
temperature between 1000C and 1250C during the run. The 3
system pressure varied between 4 and 10 inch Hg below atmospheric. -
- After solidifying, the metal was removed. It weighed
478 grams, indicating a yield of 35 grams of aluminium from a
charge of 83 grams of A1203/A14C3 mix. The surface of the molten
pool directly under the arc was undisturbed at the end of the
experiment. Three chunks of this undisturbed metal were analysed
for A14C3 content and were found to have respective analyses
of 0.48 wt. ~, 0.48 wt. % and 0.28 wt. % of A14C3. The '
aluminium metal produced was of extraordinary purity compared :
with metal prepared by prior art single furnace operations.
EXAMPLE 2 -
;~ The furnace was the same as in Example 1 except that the
vacuum was drawn from the access tube 4 instead of the lid.
This was done to keep the sight glasses clear during the run.
244 grams of charge of composition 61.2 wt. % A14C3 and
38.8 wt. % A1203 (compacted to pellets without starch additionj
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` 1045826
were reacted in 33 cyclesO No arc application exceeded 60
seconds. The minimum delay time was two minutes between arc
application. The arc intensity was about 12.5 kw. in the first
27 cycles. The cumulative time of arc application was 0.459
i hours. The elapsed time of the run was 1.8 hours. The rule
was adopted that the maximum time of any arc application would
be 60 seconds but the operator would terminate the arc before
this time if the charge had completely reacted.
The surface of the molten pool was at temperatures between ,
L0 823C and 1180C for all but the last six arc applications.
In some of the last six operations, the power intensity was
~ raised to the range 21-22 kw and apparently unreacted charges
ç from previous cycles were more completely reacted. The highest ' ~;
temperature of the product pool observed after terminating these
higher power applications was 1320C.
The system pressure during the run ranged from 6 to
-11 inch Hg below atmospheric. - ¦
The starting aluminium pool weighed 515 grams. The ¦
total metal recovered was 617 grams, giving a net recovery of,
' 20 metal produced of 102 grams. - .
Analysis of the aluminium produced showed that it contained
only 2 weight percent of aluminium carbide. I ¦
EXAME'LE 3 . ¦
This example illustrates the practice of the invention '
without the requirement that the charge be engaged with a liquid
pool of aluminium. This example employs the furnace described
with reference to Fig. 4 using the plasma arc described in Fig. ,
2.
~,; The furnace comprises a gas-tight shell 25, a rotating
ç 30 electrically-conductive hearth of graphite 18, a connecting
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)458Z6
;
post 26 which engages the hearth 18 and conducts current to
brushes 27 which go to the positive terminal of the DC power
supply 20. The negative terminal of the power supply 20 is
connected to the internal electrode 16 of the plasma arc torch.
terminal
The positive/of the power supply 19 connects to the casing 15
of the plasma torch. Also rotating with the hearth 18 is a
sub-hearth 28, composed of an alumina refractory which is of a
shape to receive the metal produced by the reaction of the
charge under the plasma jet. The hearth rotates at
0 approximately 0.2 revolutions per minute. The casing 15 is
offset from the centre of the hearth so that as the hearth 7
rotates, the charge passes under the arc, permitting intermittent
application of the arc to the charge.
Briquettes of charge are made from compositions
comprising alumina, aluminium carbide, carbon, furnace condensate,
and other carbon-aluminium compounds derived from the process
with the sole proviso that the composite analysis of the charge
has a carbon-to-oxygen atomic ratio of l:l.
In this example, the charge consists of a furnace
'0 condensate having the following analysis: `
aluminium 108 pounds
oxygen 32 pounds
carbon 12 pounds
to which are added 204 pounds of alumina and 84 pounds of carbon.
!5 The composite final charge has an atomic ratio of carbon to {
oxygen of 1:1 and analysed as follows:
aluminium 216 pounds
oxygen 128 pounds
carbon 96 pounds
~0 The final charge is formed into briquettes, or pellets.
. , - . '.
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~ 104~8Z6
These charge pellets 17 are introduced through a feed
chute 29 to the hearth 18 (Fig. 4). The power supplies 19
and 20 are activated and the torch casing is brought to within
about six inches of the hearth so that the arc is seen to
transfer from the torch to the charge pellets. In other
words, the arc strikes the charge pellets in a broadening
arc pattern as distinguished from the narrowing arc pattern
which would be seen if the jet were operating in a simple
plasma mode with no arc transfer.
As the hearth rotates and the pellets pass under the
charge, they are seen to react to form a bright liquid surface
on each pellet struck by the arc. This liquid flows over the
curb of the hearth 18 down into the receiving reservoir formed
between the he~rth 18 and the hearth 28, to form a viscous
i product mas_ 30. The carbon ~noxide evolved from the reaction
is removed from the furnace through a tube 31. The furnace
operates at substantially 1 atmospher~e pressure. The hearth
18 is controlled to approximately 1000C. Approximately 85%
of the aluminium content added to the furnace in charge pellets
17 is recovered in the viscous mass 30, the remainder being lost
through vaporisation and entrained with the carbon monoxide
escaping at vent 31. This entrained aluminium is captured as
furnace condensate by simple cooling and filtration, and this
furnace co~densate is returned to the charge preparation
operation to be recycled with new charge. -
An analysis of the viscous mass 30 indicates a composition
of aluminium containing 3% aluminium carbide. After sub-
hearth 28 is filled with products of the reaction of the plasma
` upon the charge 17, the plasma jet is moved to strike the melt ~ ~ -
0 30 and the furnace casing is opened at 32 to admit alr. Furnace
~
~ - 25 ~
- . ~
~ 1~45826
rotation is continued, the mass 30 is returned to a fluid
' condition by the action of the arc transferred from the plasma
' torch, and some air is entrained with this arc jet, providing an
effective decarbonisation action such that after two to three
revolutions of the hearth the carbide content has been reduced
to the level where the melt in 30 will flow at approximately
! gooc. A residue comprising alumina, aluminium carbide, and
aluminium is skimmed from the pool 30 and removed from the
furnace to be recycled as part of the composition of the charge
i .
pellets along with the furnace condensate. Approximately 60%
of the weight in the product 30 is recovered as pourable
aluminium containing less than 0.2~ aluminium carbide. This
melt is poured out of the furnace by tilting the furnace in a
; conventional manner. The torch is then returned to its position
for action upon charge pellets 17 which are introduced to
continue the operation.
EXAMPLE 4
~ It is to be understood that it is not necessary toS perform the decarbonisation reaction within the furnace as set
forth in Example 3, supra. As an alternative, the following
practice can be used.
While the furnace is in operation as described in
Example 3 producing melt 30 from charge pellets 17, a secondary ;
torch (not shown) is provided to keep the melt 30 in a fluid
condition without the admission of air to the furnace. When a
product 30 fills the chamber provided for it, it is then tapped
,
t at an ele~ated temperature, e.g. about 1800C, to a container
outside the furnace. This product tapped directly from the
furnace contains less than 5 wt. ~ of aluminium carbide.
From the above examples, it can be seen that the process
~' of this invention is applicable to compounds of aluminium and
';
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- 1~)45826 . I
oxygen other than A1203. Thus, the expression "an aluminium
oxide" is intended to include any compound of oxygen and
aluminium, e.g. aluminium tetraoxycarbide. "Aluminium-carbon
containing compounds" are intended to include aluminium carbide.
The only requirement of the feed is that the atomic ratio of
carbon to oxygen is 1:1 + 0.05.
In this connection, it is also noted that although the .
furnace charge can consist of alumina and carbon, such is not
preferred. It is known that when alumina and carbon react, at
least one of the intermediate products is an aluminium carbon
compound such as.aluminium carbide. The optimum conditions
for producing aluminium carbon compounds are not necessarily the
. same as those for producing aluminium. Therefore, if carbon .
alone is to be used as the reductant, it is preferred that the
S process be conducted in two separate steps. The ~irst step would ~ .
involve the reaction of alumina and carbon to form aluminium-
carbon containing compounds as is known in the art, and the secon~
- . _.
. step would involve charging the product of the first step .
together with additional aluminium oxide and carbon such that .
O the feed has an atomic ratio of carbon to oxide of 1:1 + 0.5. .
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