Carbothermic reduction and prereduced charge for producing aluminum-silicon alloys

Disclosed is a method for the carbothermic reduction of aluminum oxide to form an aluminum alloy including producing silicon carbide by heating a first mix of carbon and silicon oxide in a combustion reactor to an elevated temperature sufficient to produce silicon carbide at an accelerated rate, the heating being provided by an in situ combustion with oxygen gas, and then admixing the silicon carbide with carbon and aluminum oxide to form a second mix and heating the second mix in a second reactor to an elevated metal-forming temperature sufficient to produce aluminum-silicon alloy. The prereduction step includes holding aluminum oxide substantially absent from the combustion reactor. The metal-forming step includes feeding silicon oxide in a preferred ratio with silicon carbide.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for the carbothermic reduction of 
aluminum oxide and silicon oxide to form an aluminum alloy wherein at 
least a portion of the heat required by the process is provided by an in 
situ combustion with oxygen gas such as in a blast furnace. 
The predominant commercial process today for producing aluminum metal is 
the Hall-Heroult process of electrolytically dissociating alumina 
dissolved in a fused cryolitic bath at a temperature less than about 
1000.degree. C. Many attempts have been made to displace this process and 
produce aluminum commercially by a direct thermal reduction process of 
aluminum oxide with carbon at sufficiently high temperatures according to 
a reaction written as: 
EQU Al.sub.2 O.sub.3 +3C.fwdarw.2Al+3CO. (1) 
However, such a process has presented a substantial technical challenge in 
that certain difficult processing obstacles must be overcome. For example, 
at the temperatures necessary for the direct thermal reduction of alumina 
to form aluminum, e.g., such as about 2050.degree. C., the aluminum 
volatilizes to a gas of aluminum metal or aluminum suboxide rather than 
forming as aluminum metal liquid which may be tapped from the process. For 
this reason, most attempts have incorporated an electrical furnace for the 
purpose of reducing the amount of volatile gaseous constituents in the 
system. 
Another problem found in attempts to reduce alumina thermally with carbon 
in the absence of other metals or their oxides shows up in substantial 
formations of aluminum carbide according to the reaction: 
EQU 2Al.sub.2 O.sub.3 +9C.fwdarw.Al.sub.4 C.sub.3 +6CO.uparw. (2) 
which proceeds favorably at or above 1800.degree. C. Other intermediate 
compounds also are formed such as oxycarbides by the reactions: 
EQU 4Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3 .fwdarw.3Al.sub.4 O.sub.4 C and (3) 
EQU Al.sub.4 O.sub.4 C+Al.sub.4 C.sub.3 .fwdarw.4Al.sub.2 OC. (4) 
These carbides and oxycarbides of aluminum readily form at temperatures 
lower than the temperatures required for significant thermal reduction to 
aluminum metal and represent a substantial slag-forming problem in any 
process intended to produce aluminum. A comprehensive overview of 
technical attempts to overcome the problems in achieving a process for the 
thermal reduction of alumina with carbon to form aluminum metal is found 
in Carbothermic Smelting of Aluminum, by P. T. Stroup, Transactions of the 
Metallurgical Society of AIME, April, 1964. 
An early attempt to produce aluminum alloy by carbothermic reduction and to 
avoid the volatilization problem is represented by the Cowles process, 
which probably is the first thermal process for the reduction of alumina 
with carbon that ever reached a commercial stage. The Cowles process used 
a collector metal of copper added to an alumina-carbon charge in an 
electric furnace to produce aluminum alloy. However, it was never found 
economically feasible to remove the copper collector metal from the 
aluminum alloy produced in the Cowles process. 
Thermodynamic calculations and experience have shown that all the major 
oxides in bauxite except zirconia are reduced by carbothermic smelting 
before alumina is reduced. In practice, however, the oxides do not behave 
as simply as predicted. Instead, intermediate compounds are formed such as 
carbides, oxycarbides, and volatile subcompounds. Nevertheless, it has 
been recognized that it would be propitious to use a collector metal for 
promoting the absorption of aluminum vapor set free at the high 
temperatures required for the reduction reaction, thus preventing loss of 
aluminum by volatilization and carbide formation, which collector metal 
could form a commercially desirable alloy with aluminum. Silicon would be 
one such desirable collector metal since silicon has a higher boiling 
point, i.e., 3280.degree. C., than copper (2560.degree. C.) as used 
previously in the Cowles process, and further since silicon oxide, 
combined with aluminum oxide, occurs in nature in almost unlimited 
quantities. It has been reported that aluminum-silicon alloys were 
produced commercially by carbothermic smelting in Germany during World War 
II at a power consumption of 14 to 16 kw hour per kilogram alloy. The 
German process used a molten salt bath containing cryolite to refine the 
furnace alloy and remove carbides, nitrides, oxides, calcium, and 
magnesium. 
The discussion to this point has referred to prior attempts at the direct 
thermal reduction of alumina with carbon and other compounds incorporating 
electrical furnace heating as the sole energy source for the purpose of 
reducing volatilized components including those of aluminum or aluminum 
suboxide. These processes nevertheless have not overcome problems 
attributable to the formation of carbides and oxycarbides. Such problems 
include the formation of reactor-fouling agglomerations and degradation of 
any metal produced. Kibby, U.S. Pat. No. 4,033,757, U.S. Pat. No. 
4,216,010, and U.S. Pat. No. 4,334,917 illustrate the nature of such 
carbide formation problems and represent various attempts to minimize or 
cure the effect on aluminum formation. 
It has been recognized that a method of making aluminum-silicon alloy in a 
blast furnace would be commercially desirable by substituting a less 
expensive combustion heating for the electrical furnace. Frey et al, U.S. 
Pat. No. 3,661,561, disclose a process for producing aluminum-silicon 
alloy in a blast furnace using carbon, an alumina-silicon ore, and pure 
oxygen. According to the patent, oxygen reacts with carbon to form carbon 
monoxide gas to maintain temperatures in excess of 2050.degree. C. in the 
reaction zone of the furnace. Silicon carbide lumps are placed in the 
furnace bed to prevent aluminum carbide or silicon carbide forming with 
the carbon from the coke in sufficient quantity to be a processing 
problem. Assuming that the Frey et al process is operative to avoid the 
formation of carbide and oxycarbide slag in reactor-fouling amounts, Frey 
et al do not overcome the substantial problem of the formation of volatile 
components such as aluminum gas and aluminum suboxide gas which will form 
in the blast furnace disclosed to operate at temperatures in excess of 
2050.degree. C. moreover, Frey et al do not disclose the method for 
forming silicon carbide. 
The Atcheson process represents the principal commercial method for 
manufacturing silicon carbide from a mixture of sand and coke in an 
electrically resistance-heated batch-type operation. The Atcheson process 
is highly intensive in both labor and electrical energy. 
Enomoto, U.S. Pat. No. 4,162,167, discloses a continuous process for 
producing silicon carbide from silica and carbon by heating to a 
temperature of 1600.degree.-2100.degree. C. in an electrical furnace. 
Johansson, U.S. Pat. No. 4,269,620, discloses a process for producing 
silicon by reducing silicon oxide through an intermediate silicon carbide. 
Electrical energy is used to generate silicon suboxide gas which in a 
preheat zone reacts with carbon to form the silicon carbide. 
Bechtold and Cutler, "Reaction of Clay and Carbon to Form and Separate 
Al.sub.2 O.sub.3 and SiC," J. Am. Cer. Soc., May-June 1980, disclose 
producing alumina and silicon carbide from clay by carbon reduction 
proceeding through intermediates of CO and SiO. Bechtold et al employ 
temperatures up to 1505.degree. C. by an electrically heated furnace. 
Others have recognized the desirability of substituting a blast furnace 
energy source for electrical heat in the formation of the silicon carbide. 
Attempts also have been made to combine a staged silicon carbide formation 
with and as part of an attempt at carbothermically reducing alumina and 
silica with carbon. For example, Wood, U.S. Pat. No. 3,758,289, discloses 
the prereduction of an alumina-silica ore which is then thermally smelted 
in an electric arc furnace. No attempt is made in Wood to separate alumina 
from the alumina-silica silica ore prior to prereduction, and alumina 
thereby is present in the process disclosed to reduce the silica in the 
ore to silicon carbide. Prereduction is carried out at approximately 
1500.degree.-1800.degree. C., and preferably at a temperature in the range 
of 1600.degree.-1700.degree. C. 
Cochran, U.S. Pat. No. 4,053,303, discloses a process where the 
prereduction step of forming silicon carbide from alumina, silica, and 
carbon is carried out as a first stage in a multistage reactor. 
Prereduction to form silicon carbide is disclosed at a temperature in the 
range of 1500.degree.-1600.degree. C. The ore is processed through 
subsequent continuous stages, either in a blast furnace or electric 
furnace with the blast furnace technique being preferred because of 
economics, to form an aluminum-silicon alloy. 
Any attempt to substitute a blast furnace for an electrical furnace in an 
attempt to reduce an aluminum-silicon ore by carbothermic techniques must 
first overcome problems associated with the volatilization of the desired 
products, which volatilization is detrimentally encouraged by the gases 
formed in the blast furnace. 
One direction taken to reduce the volatility problem is found in Cochran et 
al, U.S. Pat. No. 4,299,619. Cochran et al disclose a process utilizing a 
two-zone reactor, wherein the first zone is heated to a reaction 
temperature of about 2050.degree. C. by the internal combustion of carbon 
and the second or lower zone is heated electrically to a temperature of 
about 2100.degree. C. Alumina and carbon are inserted to the upper zone 
and reacted at an elevated temperature to form CO and a first liquid of 
alumina and aluminum carbide. The first liquid is then transferred to a 
lower reaction zone beneath the upper reaction zone and heated to form CO 
and a second liquid of aluminum and carbon. Oxygen is added to preheat 
reactants in the upper zone and to maintain a desired reaction 
temperature. The lower zone is electrically heated by an electric 
resistance heater or alternative heat sources such as an electric arc or 
other heat sources not producing large volumes of gas. 
Kuwahara has filed disclosure Nos. 56-150141, 56-150142, and 56-150143 with 
the Japanese Patent Agency disclosing a smelting method of aluminum by 
reduction in a blast furnace using oxygen injecting tuyeres to achieve 
temperatures in the range of 2000.degree.-2100.degree. C. at the tuyere 
level of the blast furnace. An article entitled "Reductio ad aluminium, 
"Far Eastern Economic Review, June 16, 1982, at page 63, inexplicably 
refers to the Kuwahara process as charging aluminous ore briquettes into a 
blast furnace heated by an electric arc and the combustion of coke in the 
presence of oxygen in air to sustain temperatures of 2000.degree. C. 
Notwithstanding this inexplicable mention of the use of electric arc and 
the combustion of coke, the Kuwahara patent application disclosures 
nowhere suggest the use of a blast furnace heated by an electric arc. The 
Far Eastern Economic Review article must be characterized as far from an 
enabling disclosure. The Kuwahara process employs a molten lead spray 
splashed into the furnace at 1200.degree. C. to scrub and absorb molten 
metal product at the bottom of the furnace. 
Despite a considerable technical effort expended in the attempt to achieve 
a process for the production of aluminum and silicon alloy by the direct 
reduction of aluminum oxide and silicon oxide raw materials, processes 
disclosed to date have been unsuccessful in substituting combustion 
heating for the electrical furnace. Such a process for employing less 
expensive and more efficient combustion heating while overcoming the 
significant problems of product volatilization and reactor-fouling slag 
formation has been unavailable until now. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process has been discovered and 
is disclosed herein to provide a method for the carbothermic reduction of 
aluminum oxide to form an aluminum alloy including producing silicon 
carbide by heating a first mix of components including carbon and silicon 
oxide in a combustion heated reactor to an elevated temperature sufficient 
to produce silicon carbide at an accelerated rate, wherein the first mix 
heating is provided by an in situ combustion with oxygen gas; and then 
admixing silicon carbide with carbon and aluminum oxide to form a second 
mix and heating the second mix in a second reactor to an elevated 
metal-forming temperature sufficient to produce aluminum-silicon alloy. 
The process of the present invention encompasses such a method of 
carbothermic reduction which further includes holding aluminum oxide 
substantially absent from the prereduction step in the combustion reactor. 
The present invention further includes a process as stated above wherein 
the silicon oxide in proper proportions to silicon carbide is fed to the 
second reactor along with carbon and aluminum oxide and heating the second 
reactor so charged to an elevated metal-forming temperature to produce 
aluminum-silicon alloy.

DETAILED DESCRIPTION 
A combustion heated process for carbothermically smelting an aluminum oxide 
and silicon oxide to form an aluminum-silicon alloy cannot be visualized 
merely as an iron blast furnace-type reactor modified for higher 
temperature operation and injection of O.sub.2 instead of air. For 
reactions occurring in a packed bed of ore and coke with countercurrent 
flow of carbon monoxide gas generated by burning coke in the combustion 
zones located in front of oxygen injecting tuyeres, and generated as a 
product of reduction reactions, a number of significant differences exist 
between iron and aluminum-silicon production processes. 
Temperatures for aluminum-silicon alloy formation by the direct thermal 
reduction of alumina and silica are much higher than those in an iron 
smelting process, e.g., minimum temperatures of about 2000.degree. C. for 
aluminum-silicon alloy compared to about 1500.degree. C. for iron. Since 
less heat is available from combustion when the heat must be supplied at a 
higher temperature and since aluminum-silicon smelting is much more 
endothermic than that for iron, the fuel rate for aluminum-silicon 
smelting is expected to be much higher than for iron smelting. 
Thermodynamics show that carbon monoxide can reduce Fe.sub.2 O.sub.3, but 
CO cannot reduce SiO.sub.2 or Al.sub.2 O.sub.3. Direct reduction by carbon 
is required. For aluminum-silicon smelting, a ratio of CO.sub.2 /CO is 
about zero assuming negligible Boudouard reaction, while for iron smelting 
the C0.sub.2 /CO approximates one. Therefore, reduction reactions in the 
carbothermic reduction of aluminum oxide and silicon oxide alone result in 
greater gas volumes consisting of primarily CO. 
The reduction reactions to form aluminum and silicon from aluminum oxide 
and silicon oxide proceed by gaseous intermediates as opposed to a simple 
gas-solid reaction between CO or H.sub.2 and Fe.sub.2 O.sub.3 to produce 
Fe and CO.sub.2 or H.sub.2 O. The refluxing species Al, Al.sub.2 O, and 
SiO back react with CO at lower temperatures, forming deposits which cause 
reactor-fouling agglomerations resulting in bridging. The charge tends to 
become cenmented together and solids flow is held up. Although SiO may 
cause problems in a ferrosilicon or ferromanganese blast furnace, reflux 
of alkalis rather than suboxides can cause similar problems in a normal 
iron blast furnace. As reflux increases, the shaft behaves as a heat pipe 
which absorbs heat at high temperatures and liberates heat at low 
temperatures, resulting in increases in fuel rate and high off-gas 
temperatures. 
Excess carbon in contact with aluminum-silicon alloys at alloy-formation 
temperatures can cause rapid carbide formation which prevents recovery of 
the alloy. As to the iron blast furnace, however, liquid iron containing 
about 4.5% C is in equilibrium with carbon at normal blast furnace 
temperatures. The carbon content of aluminum-silicon alloys, on the other 
hand, is an anomalous function of composition and temperature. 
Thermodynamically, the standard free energy of oxide formation with respect 
to temperature indicates that carbon reduces alumina and silica at about 
2000.degree. C. and about 1540.degree. C., respectively. However, the 
presence of stable suboxides, oxycarbides, carbides, and vapors in a 
system of Al--Si--O--C at high temperatures must be recognized, and many 
species must be considered in a calculation of equilibrium products. Using 
recent Al.sub.2 O data and assuming an ideal solution behavior of any 
aluminum-silicon alloy produced, thermodynamic calculations indicate that 
the production of aluminum-silicon alloy from a raw material charge of 
alumina, silica, and carbon in a process heated exclusively by combustion 
heating in situ, i.e., such as in the form of a blast furnace, probably is 
not feasible. However, such a conclusion is based on assumptions and data 
having an uncertainty sufficiently large that technical feasibility cannot 
be ruled out on the basis of thermodynamic calculations alone. 
Nevertheless, it has been found from actual observation that severe 
reactor-fouling agglomerations and bridging problems occur when smelting 
aluminum oxide and silicon oxide ores together in one reactor. The 
problems are directly attributable to refluxing of the vapor as metal and 
suboxide species. It has been found that the substitution of in situ 
combustion heat for a portion of electrical heat at different levels in 
one reactor causes virtually insurmountable problems of reactor-fouling 
attributable to bridging and slag formation around the combustion heat 
zone. One reason this occurs is the fact that combustion heat produces a 
very high temperature in the zone of combustion, especially with 
combustion in an atmosphere rich in oxygen. These temperatures in the case 
of in situ combustion heat by combustion of carbon with essentially pure 
oxygen typically are in the range of 3500.degree.-4000.degree. C. Such 
high temperatures in a reactor combustion zone located to provide 
unilateral combustion heating and containing alumina, silica, and carbon 
have been found to cause considerable bridging and slagging problems which 
usually are substantial enough to shut down the reactor. 
In accordance with the present invention, a process has been discovered and 
is herein disclosed for providing combustion heat to be utilized for only 
a part of the high temperature heat required in the formation of 
aluminum-silicon alloy from aluminum oxide and silicon oxide by 
carbothermic direct reduction. Gaseous sweep rate, including CO sweep 
rate, through the metal-forming reactor is held sufficiently low to avoid 
transporting aluminum and silicon from the reaction zone. Not only can 
combustion heat be utilized at a temperature lower than alloy formation 
temperature, but also combustion heat is utilized in a reactor entirely 
separate from the alloy-forming reactor. Moreover, the process of the 
present invention has been found to provide surprising efficiency in terms 
of enhanced reaction rates. The present invention also provides a process 
for producing aluminum-silicon alloy in a reactor fed with a prereduced 
charge of silicon carbide and silicon oxide in a molar ratio within a 
defined range along with carbon and aluminum oxide to provide an 
unexpected improvement in the formation of aluminum-silicon alloy metal. 
It is an object of the present invention to provide a method of 
carbothermic reduction of aluminum oxide to form an aluminum alloy 
including producing silicon carbide by heating a mix of carbon and silicon 
oxide by in situ combustion with oxygen gas to an elevated temperature 
sufficient to produce silicon carbide at an accelerated rate, and then 
mixing the silicon carbide with carbon and aluminum oxide in a second 
reactor and heating to an elevated metal-forming temperature sufficient to 
produce aluminum-silicon alloy. 
It is an object of the present invention to substitute combustion heating 
for electrical heating while minimizing detrimental volatilization of 
metal and metal suboxide. 
It is an object of the present invention to substitute combustion heating 
for electrical heating in a process for the carbothermic reduction of 
aluminum oxide to aluminum while minimizing the formation of 
reactor-fouling bridging. 
These and other objects will become apparent from the drawing and from the 
detailed description which follows. 
Referring now to the FIGURE, a schematic diagram is depicted in which 
silicon oxide, such as silica in the form of quartz, and carbon such as in 
the form of briquettes of pitch and petroleum coke or lumps of 
metallurgical coke are fed to the top of combustion reactor 1 to form a 
gravity-fed moving bed. Sufficient carbon is fed to satisfy the reduction 
and heating requirements. Oxygen gas is injected through tuyere 2. By the 
time the mix of silicon oxide and carbon reaches tuyere 2 the silicon 
oxide will have been reduced to silicon carbide and coke converted to SiC 
by SiO in the gases rising in the reactor. Heat is provided in combustion 
zone 3 by an in situ combustion of a portion of the silicon carbide and 
unreacted coke not converted to SiC with oxygen injected through tuyere 2 
to form SiO gas and CO gas. By in situ combustion heating is meant a 
direct heating by the hot gases produced by combustion of SiC and carbon 
with oxygen, which combustion usually takes place in the reaction zone to 
be heated. SiO gas rises in the reactor. SiO gas and SiO.sub.2 in the 
charge react with carbon to form silicon carbide. Silicon carbide is 
formed in sufficient amount such that not all reacts with oxygen and a 
portion of the silicon carbide advances through the reactor and, after 
passing support grate 4, can be withdrawn at bottom 6 of reactor 1. Carbon 
monoxide gas exiting the top of reactor 1 can be refluxed (not shown) to 
reactor bottom 6 and passed in countercurrent heat exchange with silicon 
carbide product, thereby cooling silicon carbide and retaining heat to the 
reactor. 
The process can be balanced by controlling carbon and oxygen feed rates, 
the solids' discharge rate, and the temperature, so that all carbon is 
converted to silicon carbide above the combustion zone 4. Preferably 
reactor 1 is operated as a gravity-fed, moving bed reactor. For this 
reason the feed materials in solid form should have sufficient structural 
integrity and strength to hold up in such a moving bed. 
Silicon carbide withdrawn from reactor bottom 6 is mixed with aluminum 
oxide and carbon and charged to the top of reactor 7 which preferably is 
an electrically heated furnace. Reactor 7 may be heated by a submerged arc 
or alternative electrical methods provided that the heating means do not 
introduce substantial additional volumes of gas to the reactor. Electrodes 
8 provide heating by submerged arc. Aluminum-silicon alloy is tapped at 
port 9. 
Combustion reactor 1 is heated to a temperature higher than 1800.degree. C. 
It has been found that such a temperature provides for the production of 
silicon carbide at an accelerated rate. In this way the combustion reactor 
produces silicon carbide by heating a first mix of carbon and silicon 
oxide to a temperature greater than 1800.degree. C. to produce silicon 
carbide at an accelerated rate. More preferably, the combustion reactor is 
operated at a temperature exceeding 2000.degree. C. 
The metal-forming reaction in reactor 7 is conducted at a temperature in 
the range of about 2000.degree.-2400.degree. C. and preferably in the 
range of about 2000.degree.-2100.degree. C. to reduce aluminum vapor 
losses. 
Oxygen injected through tuyere 2 to combustion reactor 1 may be preheated 
to attain sufficiently high reaction temperatures with less oxygen gas and 
combustion of a smaller portion of the SiC and unreacted coke quantities 
introduced to the combustion zone 3. Tuyere 2 may be replaced by a burner 
and in such an embodiment, the combustion carbon may be injected through 
the burner. In the burner case, only the carbon for reduction in the form 
of coke briquettes or metallurgical coke is fed to the top of reactor 1 
along with quartz and combustion of SiC does not occur. 
An atmosphere rich in oxygen is preferred, e.g., over air, to achieve the 
high temperatures required in the combustion reactor. Furthermore, it has 
been found that the use of air produces undesirable nitride formation. For 
these reasons, it is preferred to use an atmosphere rich in oxygen gas 
containing at least about 90% by volume oxygen gas, and more preferably 
containing essentially pure oxygen gas, i.e., at least about 98% by volume 
oxygen gas. 
It has been found that the process of the present invention unexpectedly 
produces more metal when silicon oxide is fed to the metal-forming reactor 
along with the prereduced charge of silicon carbide and aluminum oxide and 
carbon. Moreover, it has been found that the process produces unexpected 
improvement in the quantity of metal produced when silicon carbide and 
silicon oxide are fed to the metal-forming reactor in a molar ratio of 
less than about 4/1. This ratio can be achieved by operating reactor 1 at 
a lower fuel rate so that less than 100% of the SiO.sub.2 is reduced to 
SiC. 
The silicon oxide used in the process for SiC production can be a silica 
such as quartz or sand, but preferably is quartz which has a larger 
material particle size than sand to prevent fluidization by the rising 
combustion gases. Quartz fed to the top of combustion reactor 1 preferably 
has a particle size in the range of about from 1/4 inch to 5/8 inch. 
Carbon can be fed to the combustion reactor in an amount ranging from about 
8 to 14 mols of carbon for each mol silica. The excess carbon above the 3 
mols required for reduction is used for combustion to heat the process and 
to enhance reaction rate. Below about 8 mols carbon to each mol silica, 
excess SiO.sub.2 occurs in the reactor product. On the other end of the 
range, i.e., above 14 mols carbon to each mol silica, excess C occurs in 
the reactor product or excess heat is produced and carbon is wasted. 
Moreover, carbon fed to the reactor in an amount ranging from about 10 to 
about 12 mols of carbon for each mol of silica is preferred. The preferred 
range provides minimum fuel rate, and an enhanced control of the product 
composition. 
Aluminum-silicon alloy is produced from the metal-forming reactor in a 
ratio in the range of about 40/60 to 70/30 by weight. Each extreme would 
result in very low metal yield. 
The mix of reactant charge fed to the metal-forming reactor preferably 
consists of lumps comprising a first lump of silicon carbide, a second 
lump of silica, and a third lump composed of finely divided alumina and 
carbon. The first lump of silicon carbide, the second lump of silica, and 
the third lump of finely divided alumina and carbon can have a particle 
size in the range of from about 1/4 inch to 5/8 inch. 
In the case for 100% prereduced charge, the mix fed to the metal-forming 
reactor should have a composition in the range of about from 46.6 to 63.3% 
aluminum oxide, about from 52.8 to 20.5% prereduced charge of silicon 
carbide, and from about 0.6 to about 16.2% carbon by weight. For less than 
fully prereduced charge, the mix fed to the metal-forming reactor should 
have a composition in the range of about from 40.8 to 60.9% aluminum 
oxide, about from 37.0 to 15.8% prereduced charge of silicon carbide, 
about from 13.9 to 5.9% silicon oxide, and from about 8.3 to about 17.4% 
carbon by weight. These ranges of charge composition for 100% prereduced 
and less than 100% mixes reflect the burden required to produce 
aluminum-silicon alloys in a ratio in the range of about 40/60 to 70/30 by 
weight respectively. 
As has already been mentioned, the silicon carbide and silicon oxide 
preferably are present in a molar ratio of less than about 4/1. This 
silicon carbide/silicon oxide molar ratio preferably is in the range of 
4/1 to 1/1. 
Further advantages of the process of the present invention will become 
apparent from the following examples. 
EXAMPLE 1 
Carbon and silica at a composition ratio, physical form, and particle size 
as indicated in Runs 1 and 2 in Table I were fed to a low bed isothermal 
batch reactor and heated to an elevated temperature sufficient to form 
silicon carbide. Reaction rates were determined. 
In this first example, pellets of coking coal and fused silica were 
calcined to give a coked SiO.sub.2 burden Pellets of two different 
C/SiO.sub.2 mol ratios were tested at 1700.degree. C. Reaction rates were 
determined for a C/SiO.sub.2 mol ratio of 3/1 (Run 1) representing just 
enough carbon for SiC formation and a ratio of 10/1 (Run 2) which 
represented sufficient carbon for supplying the heat for SiC formation by 
in situ combustion with oxygen injected through a tuyere. Data and results 
are shown in Table I. 
Pellets with C/SiO.sub.2 of 3/1 reacted at a slower rate than for the 
C/SiO.sub.2 10/1 pellets at the same temperature. 
TABLE I 
__________________________________________________________________________ 
Reaction Rate for Producing Prereduced SiC From Silica and Carbon 
Run 
(mol/mol)Carbon/SiO.sub.2 
Form SizeParticle 
(.degree.C.)TemperatureReduction 
(mins.)1500.degree. C.Time 
##STR1## 
__________________________________________________________________________ 
1 3/1 Pellet -3/8" + 6 mesh 
1700 215 3.4 
2 10/1 Pellet -3/8" + 6 mesh 
1700 92 6.6 
3 3/1 Lumps -10 + 20 mesh 
1775 373 2.2 
4 3/1 Lumps -10 + 20 mesh 
2150 95 8.8 
5 3/1 SiO.sub.2 + C 
activated coke 
1775 230 Failed 
powders 
bed -4 + 6 mesh 
through SiO 
generator 
6 3/1 SiO.sub.2 + C 
charcoal bed 
1775 150 Failed 
powders 
-4 + 6 mesh 
through SiO 
generator 
7 3/1 SiO.sub.2 + C 
charcoal bed 
2150 85 16.1 
powders 
-4 + 6 mesh 
through SiO 
generator 
__________________________________________________________________________ 
EXAMPLE 2 
A set of reactant materials was formed of lumps of quartzite and 
metallurgical coke having a particle size of -10+20 mesh (Tyler Series) 
and was reacted at temperatures of 1775.degree. C. (Run 3) and 
2150.degree. C. (Run 4). The reaction rates were determined in the reactor 
of Example 1 and are shown in Table I. 
A dramatically higher reaction rate was observed for SiC formation at the 
higher temperature. 
EXAMPLE 3 
SiO gas was produced through an SiO generator by reacting SiO.sub.2 and 
carbon powders in a 1 to 1 molar ratio in the bottom of an isothermal 
batch reactor. The SiO gas passed up a grate and was reacted to SiC in a 
bed of carbon. Two different bed carbons were tested, coke (Run 5) and 
charcoal (Run 6). See Table I. Activated coke and charcoal each were sized 
to "4+6 mesh. Silicon monoxide was passed up through the bed of carbon 
which filled the reactor. The bed contained 2 mols of carbon for each mol 
SiO generated per unit of operating time. SiO was provided through an SiO 
generator at a temperature of 1775.degree. C. (Run 5 and Run 6) and at 
2150.degree. C. (Run 7). 
The mixture of Run 5 reacted very slowly and eventually decreased to very 
low levels. A fused mass formed in the generator. Charcoal was used in the 
bed in Run 6 to raise the reactivity of the carbon, but the results were 
similar to coke. Run 7 was quite successful with the SiO.sub.2 and C raw 
materials being totally reacted and the bed containing only SiC. The 
reaction rate at 2150.degree. C. was dramatically high compared to the 
other runs at lower temperatures. 
EXAMPLE 4 
Alumina, silicon carbide, and petroleum coke were fed to a countercurrent 
shaft reactor operating as a metal-forming reactor having electrical 
induction heating. Silicon carbide produced in a separate step was crushed 
and ground and formed into a pellet with powdered activated alumina and 
coke. Activated alumina served as a binder for the pellets used in the 
run. A first run (Run 11, Table II) tested the metal production of 
aluminum-silicon alloy from a burden containing silicon carbide, alumina, 
and most of the required reduction carbon in one aggregate, i.e., in other 
words, a one lump burden. The burden was reacted for 160 minutes at a 
temperature of 2035.degree. C. 
A second one lump burden test was run using non-prereduced SiO.sub.2, i.e., 
the charge fed to the reactor consisted essentially of SiO.sub.2 with 
Al.sub.2 O.sub.3 and carbon. Results are shown as Run 12 in Table II. 
A third one lump burden test was run on a burden which had been partially 
reduced to 51.2% SiC. Results are shown as Run 13 in Table II. 
Substantially more metal and much less slag and carbide were produced in 
Run 13 using 51.2% prereduced SiO.sub.2 compared to the 100% prereduced 
charge of Run 11. The operation of this Run 13 produced a bed which was 
easily maintained above the hot zone to effect reflux. 
A bed of solids above the metal-producing zone was difficult to maintain in 
Run 11 except by an increase of 80% over the feed rate for a similar 
burden of SiO.sub.2 rather than SiC. The useful power input above the 
power required to supply the reactor heat losses had to be increased by 
about 90% to reach or maintain metal-producing temperature of 2035.degree. 
C. However, the product of Run 11 contained only 4% metal, with 44% 
carbide and 52% Al.sub.2 O.sub.3 indicating mostly slag rather than metal 
produced. The comparable Run 12 using fused silica rather than SiC 
produced normal quality metal with a minor amount of slag in the bottom of 
the ingot. 
TABLE II 
______________________________________ 
Metal Production and Prereduced Burden 
Run 11 12 13 
______________________________________ 
SiO.sub.2 /Al.sub.2 O.sub.3 Wt. Ratio 
.63 .61 .56 
Prereduced Charge 
100% SiC None 51.2% SiC 
(molar) (100% SiO.sub.2) 
Wt. % Fe.sub.2 O.sub.3 
-- -- 1.5 
Time &gt;2000.degree. C. minutes 
160 180 260 
Time of CO Evolution 
250 255 320 
minutes 
T.sub.max .degree.C. 
2035 2030 2050 
Average Product 
Analysis wt. basis 
% Al 39.0 60.9 67.5 
% Si 23.9 29.6 23.2 
% Fe .3 .05 2.5 
% Ti .05 .05 1.4 
% O 24.7 1.3 .9 
% C 12.7 5.9 4.9 
Al Yield gm 297 1089 1037 
Si Yield gm 631 543 362 
##STR2## .24 .65 .47 
______________________________________ 
EXAMPLE 5 
The metal-forming reaction step was further investigated in a pilot 
submerged arc reactor with prereduced burden. Run 21 consisted of a 
clay/alumina/metallurgical coke pellet which had been prereduced to a 
level of 75% SiC. Results and data are shown in Table III. 
Metal was produced and tapped from the reactor, but it was evident from the 
poor cavity formation that the submerged arc was not operated properly. 
EXAMPLE 6 
Runs 22 and 23 in the submerged arc pilot reactor used a pellet made from 
activated alumina, SiC, and metallurgical coke. Results and data are shown 
in Table III. 
The runs simulated a 100% prereduction of silica to SiC. Only slag was 
removed from the unit. 
TABLE III 
______________________________________ 
Metal Production and Prereduced Burdens 
Run 21 22 23 
______________________________________ 
Submerged Arc 
Initial Volts 
64 28 65 
Amps 1650 1800 1800 
Final Volts 
80 32 75 
Amps 1100 1550 1400 
Pellet Alumina-clay-met. 
Activated Activated 
coke alumina alumina 
SiC-met. coke 
SiC-met. coke 
Prereduction 
75 100 100 
(molar %) 
Si/Al .53 .48 .48 
Total Run Time 
3 hrs. 20 min. 
3 hrs. 2 hrs. 18 min. 
Total Metal 
4790 g None None 
Tapped (2785 g slag) 
______________________________________ 
Surprisingly, it appears from experimental observation that no fully 
prereduced burdens could be processed to form aluminum-silicon alloy in a 
submerged arc furnace, yet partially prereduced burdens having varying 
SiC/SiO.sub.2 ratios along with unprereduced burdens resulted in 
successful metal production. An explanation, though the scope of the 
claims of the present invention should not be limited thereby, 
hypothetically is found in the concept that the conversion of silicon 
oxide to silicon carbide has a critically important role in the rate of 
aluminum carbide and oxycarbide slag formation. If the reaction of silicon 
oxide to silicon carbide is delayed until the temperature of the moving 
bed approaches 1900.degree.-2000.degree. C., silicon carbide formation 
will compete with the slag production reactions for the available heat and 
reduction carbon at that place in the moving bed. Since the endothermic 
heats of reaction for forming SiC, Al.sub.4 O.sub.4 C, and Al-Si alloy are 
roughly in the ratios of 35%: 15%:50%, the heat transfer limitations 
apparently affect these reactions. If half the SiC and all the Al.sub.4 
O.sub.4 C were forming at the same temperature, an equal competition would 
exist for the available heat to sustain the formation of each and slow 
down slag formation. If heat and reactants are supplied at ratios 
sufficient to make metal, and slag production must precede metal 
production, the latter could only occur if the feed rate of reactants 
balanced the rate of metal production. 
In the case of a metal-forming step using a prereduced charge of silicon 
carbide in lieu of silicon oxide along with alumina and carbon in a 
separate reactor, the lack of a silicon carbide reaction in the heat sink 
which it causes has important implications on the temperature profile in 
the bed. In this way, a preferred charge to the metal-forming reactor 
includes a molar ratio of silica to prereduced charge of silicon carbide 
in the range of from about 1/4 to about 1/1. The higher limit is important 
such that a portion of that charge can be prereduced in a combustion 
reactor thereby providing an economical process which is less expensive 
than the use of 100% electrical energy. The lower limit importantly must 
be observed in order to avoid reactor-fouling slag formation. In other 
terms, the silica introduced to the metal-forming reactor should be 
prereduced to silicon carbide in an amount at least 50% and no more than 
80%. A preferred range for such prereduction of silica to silicon carbide 
includes a range of from about 50% to 75% of the silica to be introduced 
as prereduced charge of silicon carbide.