Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
2001-08-27
2002-11-05
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S674000
Reexamination Certificate
active
06475260
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a carbothermic process for the direct thermal reduction of aluminum oxide to form aluminum metal.
2. Background
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° C. Many attempts have been made to replace 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 equation (1).
Al
2
O
3
+3C→2Al+3CO Eq. (1)
Aluminum may be produced by the carbothermic direct thermal reduction of alumina, e.g., in an open or submerged-arc electric or slag resistance heating furnace. The scientific principles involved in the chemistry and thermodynamics of the reactions are now fairly well understood (C. N. Cochran, Metal-Slag-Gas Reactions and Processes, Electrochem. Soc., Princeton, N.J. 1975, pp. 299-316; K. motzfeldt and B. Sandberg, Light Metals 1979, A I M E, New York, N.Y. 1979, Vol. 1 pp. 411-428, and references cited therein). Nonetheless, no commercial process based on these principles has been established.
INTRODUCTION TO THE INVENTION
The carbothermic direct thermal reduction process involves reacting an aluminum oxide containing compound with a reductant which is usually carbon, aluminum carbide, or a mixture thereof in an electric furnace to reduce the aluminum oxide to metallic aluminum. Although the reaction on first impression would appear to be a simple one, i.e., the reduction of aluminum oxide to aluminum, substantially pure aluminum is not obtained via conventional carbothermic processes and, in fact, the product tapped from the furnace is aluminum contaminated with aluminum carbide. The amount of contamination with aluminum carbide varies depending on the particular carbothermic process which is carried out, but, in general, conventional carbothermic processes result in the production of aluminum contaminated by 10-30% by weight of aluminum carbide.
The carbothermic direct thermal reduction 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° 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.
In attempts to reduce alumina thermally with carbon in the absence of other metals or their oxides, substantial amounts of aluminum carbide are produced according to the reaction written as equation (2).
2Al
2
O
3
+9C→Al
4
C
3
+6CO Eq. (2)
Equation (2) proceeds favorably at or above 1800° C. Other intermediate compounds also are formed such as oxycarbides by the reactions written as equation (3) and equation (4).
4Al
2
O
3
+Al
4
C
3
→3Al
4
O
4
C Eq. (3)
Al
4
O
4
C+Al
4
C
3
→4Al
2
OC Eq. (4)
The reduction of alumina by carbon, when carried out under reduced pressure, proceeds with aluminum oxycarbide and aluminum carbide as intermediate products written as equation (5) and equation (6).
2Al
2
O
3
+3C→Al
4
O
4
C+2CO Eq. (5)
Al
4
O
4
C+6C→Al
4
C
3
+4CO Eq. (6)
Below 1900° C., all reactants and products except CO are solids. To attain an equilibrium gas pressure of 1 atm, however, temperatures of around 2000° C. are required, the reaction mixture is partially molten, and the simple equations (5) and (6) are no longer directly applicable. Likewise, the final, metal-producing step might be written as equation (7).
Al
4
O
4
C+Al
4
C
3
→8Al(l)+4CO Eq. (7)
The equilibrium gas pressure for this reaction reaches 1 atm at about 2100° C. In a reduction furnace operated under atmospheric pressure, the reaction zone must be maintained at a temperature at least sufficient to give the equilibrium pressure of CO equal to 1 atm. Allowing for some over-pressure to drive the reaction means a temperature of about 2150° C. At this temperature, the system includes solid carbon plus two liquids, an oxide-carbide melt and a metallic melt or metal melt. Equation (7) is not applicable, and the metal-producing reaction may be written schematically as equation (8).
(oxide−carbide melt)+C(s)→(metal melt)+CO Eq. (8)
Concurrent with the production of carbon monoxide and condensed products, volatile aluminum-bearing species Al
2
O(g) and Al(g) also will be formed. In the first steps of the reaction, formally described by equation (5) and equation (6), the equilibrium pressures of Al
2
O and Al amount to only a few percent of the equilibrium pressure of CO. In the final step, represented by equation (7) or equation (8), the proportions of Al
2
O and Al in the equilibrium gas are higher, but not excessive. It has been shown, however, that the reaction between alumina and carbon proceeds via a mechanism involving a gas phase with a high proportion of Al
2
O and Al, and, as a consequence, the losses by volatilization will be higher than those expected from the equilibria. Further, the metallic melt has a lower density than that of the oxide-carbide melt and thus the metallic melt floats on top of the oxide-carbide melt. The CO gas evolved by reaction (8) must pass through the metal melt, which further increases losses by volatilization.
Volatilization of Al and Al
2
O from the hot zone does not necessarily lead to metal loss. In a submerged-arc furnace, the reaction gas passes upwards through layers of colder charge, where the metal-bearing vapors may condense, at the same time preheating the charge. With a high fraction of metal vapors in the gas, however, the charge runs too hot, and losses by volatilization occur.
A primary difficulty in the carbothermic production of aluminum is caused by the substantial solubility of carbon in the metal at reaction temperature, about 20 atom % C. when the metallic melt is in equilibrium with solid carbon. When the melt is cooled, the carbon precipitates as aluminum carbide as written in equation (9).
(12Al+3C, molten mixture)→Al
4
C
3
(s)+8Al(l) Eq. (9)
About one-third of the metal value is precipitated as carbide. This necessitates a subsequent separation step, and recycling of the aluminum carbide, which is a disadvantage to the economy of the process.
Another difficulty in the carbothermic reduction of alumina in a submerged-arc furnace relates to the energy input and heat transfer. The metallic melt floats on top and will be directly underneath the electrodes. Because of the high electrical conductivity of the metal, the resistance in the furnace circuit will be low, and difficulties are experienced in maintaining an adequate energy input to the furnace. Further, the heat generation will take place predominantly on the surface of the metal, leading to very high metal temperature and substantial evaporation. To the extent this metal is condensed in the charge above the melt, it runs right back into the hot zone and is re-evaporated. The net result of this cyclic process of vaporization and condensation is that a large fraction of the generated heat is transferred upwards in the furnace, instead of being conducted downwards to the oxide-carbide melt where the heat is needed for the endothermic reaction (8).
The 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 metal.
All the major oxides in bauxite except zirconia are reduced by carbothermi
Alcoa Inc.
Andrews Melvyn
Glantz Douglas G.
Klepac Glenn E.
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