Elsevier

Energy

Volume 32, Issue 12, December 2007, Pages 2420-2427
Energy

Carbothermal reduction of alumina: Thermochemical equilibrium calculations and experimental investigation

https://doi.org/10.1016/j.energy.2007.06.002Get rights and content

Abstract

The production of aluminum by the electrolytic Hall–Héroult process suffers from high energy requirements, the release of perfluorocarbons, and vast greenhouse gas emissions. The alternative carbothermic reduction of alumina, while significantly less energy-intensive, is complicated by the formation of aluminum carbide and oxycarbides. In the present work, the formation of Al, as well as Al2OC, Al4O4C, and Al4C3 was proven by experiments on mixtures of Al2O3 and activated carbon in an Ar atmosphere submitted to heat pulses by an induction furnace. Thermochemical equilibrium calculations indicate that the Al2O3-reduction using carbon as reducing agent is favored in the presence of limited amounts of oxygen. The temperature threshold for the onset of aluminum production is lowered, the formation of Al4C3 is decreased, and the yield of aluminum is improved. Significant further enhancement in the carbothermic reduction of Al2O3 is predicted by using CH4 as the reducing agent, again in the presence of limited amounts of oxygen. In this case, an important by-product is syngas, with a H2/CO molar ratio of about 2, suitable for methanol or Fischer–Tropsch syntheses. Under appropriate temperature and stoichiometry of reactants, the process can be designed to be thermo-neutral. Using alumina, methane, and oxygen as reagents, the co-production of aluminum with syngas, to be converted to methanol, predicts fuel savings of about 68% and CO2 emission avoidance of about 91%, vis-à-vis the conventional production of Al by electrolysis and of methanol by steam reforming of CH4. When using carbon (such as coke or petcoke) as reducing agent, fuel savings of 66% and CO2 emission avoidance of 15% are predicted. Preliminary evaluation for the proposed process indicates favorable economics, and the required high temperatures process heat is readily attainable using concentrated solar energy.

Introduction

Aluminum is currently produced industrially via the Hall–Héroult process by dissolving Al2O3 in fused NaF–AlF3 (cryolite) followed by direct current electrolysis, in which CO2 is discharged at a sacrificial carbon anode and Al is deposited at the bottom of the cell. The production of each kg of Al requires the consumption of 0.4–0.5 kg of the carbon anode [1]. The main drawbacks of the electrolytic production are its very high energy consumption (0.186 GJ/kg Al), the release of perfluorocarbons, and the high specific CO2-equiv emissions (7.42 kgCO2-equiv/kg Al) [2]. The greenhouse gas emission by the electrolytic Al production contributes 2.5% to the world anthropogenic CO2-equiv emissions [3]. Much effort has been spent to achieve the carbothermic reduction of Al2O3 to metallic Al. Using carbon or CH4 as reducing agents, the overall reactions can be represented by Al2O3+3C=2Al+3CO,ΔH298k0=1344.1kJmol-1.Al2O3+3CH4=2Al+3CO+6H2,ΔH298k0=1568.7kJmol-1.Reactions (1) and (2) are thermodynamically favorable at above 2320 and 1770 K, respectively [4]. However, both reactions are complicated by the formation of aluminum carbide, Al4C3, and of the oxycarbides Al2OC, and Al4O4C. At the ALCOA Corporation, a stack-type reactor was developed in which a charge of Al2O3 and C was inserted in a high-temperature upper reaction zone to form a liquid mixture of Al2O3 and Al4C3 that was then transferred to a lower reaction zone for the extraction of liquid Al. The total energy demand of 0.121 GJ/kg Al by this process for both electric energy and carbon consumption was thus significantly lower than that by the Hall–Héroult process. Replacement of the electrochemical process by carbothermic reduction of Al2O3 would decrease the total greenhouse gas emissions by at least 30% [5]. In spite of considerable effort, the carbothermic reduction of alumina to aluminum remains a formidable technical challenge, due to the high temperatures required, and to the formation of aluminum carbide and oxycarbide byproducts [2].

A differential thermal analysis method had been applied to study the aluminum–oxygen–carbon system at reduced pressures at 1700–2200 °C [6]. The results indicated that the direct reduction according to Eq. (1) did not occur. Instead, Al was proposed to be formed by the following steps occurring at progressively higher temperatures in the order listed, resulting in the overall reaction (1),2Al2O3+3C=Al4O4C+2CO,Al4O4C+6C=Al4C3+4CO,Al4O4C+Al4C3=8Al+4CO.The present work examines the thermodynamic constraints for achieving the carbothermic reduction of Al2O3 to Al by combining it with the exothermic partial oxidation of either methane to H2 and CO, or of carbon to CO, CH4+12O2=CO+2H2,ΔH298k0=-35.7kJmol-1,C+12O2=CO,ΔH298k0=-110.5kJmol-1.The conditions were determined for avoiding or minimizing the formation of Al4C3 and of the partial reduction byproducts, such as Al2O and AlO. The approach taken is analogous to that used for the carbothermic reduction of iron and zinc ores to the corresponding metals, and for the calcination of limestone, combined with the reforming/partial oxidation of CH4 [7], [8], [9], [10]. These thermodynamic constraints seem not to have been reported previously. In addition, experiments were performed to find conditions suitable for the application of concentrated solar energy to the production of aluminum.

Section snippets

Experimental tests

In the present work, the carbothermic reduction of Al2O3 mixed with activated carbon was examined initially by thermogravimetry coupled with gas chromatography of gaseous products, and by heating the above mixtures in an induction furnace.

Thermodynamic analysis

Thermochemical equilibrium calculations were performed using the CET85 and FactSage program codes [11], [12], assuming closed systems. Results were expressed as mole fractions against temperature, all at 1 bar pressure. Products with mole fractions of less than 10−5 were not considered. Reaction enthalphies were calculated using the data of the NIST chemistry web-book [13]. Substantial reduction of Al2O3 to Al was found to occur only above the melting point of Al, 933.5 K, and to be almost

Discussion

In laboratory scale experiments, using inductance furnace heating, the carbothermic reduction of Al2O3 by activated carbon was demonstrated to produce Al, together with Al2OC, Al4C3, and Al4O4C. Thermochemical equilibrium calculations were used to suggest further improved conditions for such reactions. Table 1 compares the predicted production of Al in three reaction systems at 2500 K. The carbothermic reduction of Al2O3 by carbon, both in the absence and presence of oxygen, would involve the

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