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 a1umina from the alumina-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.