Reduction of Stable Metal Oxides
28 August 2007
Carbothermal reduction of stable metal oxides such as MnO, Cr2O3, TiO2 and Al2O3 requires high temperatures. Equilibrium temperatures for conversion of these oxides to carbides are presented in Table 1.
Table 1: Standard Gibbs free energy changes and equilibrium temperatures for carbothermal reduction reactions.
Standard Gibbs free
energy change, kJ
|MnO + 10/7C = 1/7Mn7C3 + CO||DG° = 257.75 – 0.1598T||1340|
|3/2Cr2O3 + 13/2C = Cr3C2 + 9/2CO||DG° = 1,078.19 – 0.778T||1113|
|TiO2 + 3C = TiC + 2CO||DG° = 371.84 – 0.2541T||1190|
|2Al2O3 + 9C = Al4C3 + 6CO||DG° = 2,397.76 – 1.056T||1998|
Obviously, to have high productivity, industrial processes are run at much higher temperatures than those listed in Table 1. At these temperatures, ores are liquid, and reduction occurs from molten slag, in which activity of an oxide is low, while carbon monoxide pressure at the reaction interface has to be above 1 atm to provide conditions for a gas bubble nucleation.
Thus, ferromanganese is produced at about 1500°C; temperature in the ferrochromium production is above 1700°C. Carbothermal reduction of titania to titanium carbide is also implemented at high temperatures, 1700-2100°C. High temperature for carbothermal reduction of alumina is a major obstacle in development of carbothermal technology for aluminium production.
Needless to say that high temperature processes are energy demanding and have high maintenance cost. Production efficiency can be improved if the reduction temperature is lowered. This can be achieved by changing the equilibrium conditions and increasing the reduction rate. One way in this direction is to use methane-containing gas, which offers more favourable thermodynamics in comparison with solid carbonaceous materials. Reduction reactions with methane-containing gas, their standard Gibbs free energy changes and equilibrium temperatures are presented in Table 2.
Table 2: Standard Gibbs free energy changes and equilibrium temperatures for reduction reactions by methane-containing gas.
Standard Gibbs free
energy change, kJ
|MnO + 10/7CH4 = 1/7 Mn7C3 + CO + 20/7 H2||DG° = 377.68 – 0.3144T||928|
|Cr2O3 + 13/3CH4 = 2/3Cr3C2 + 3CO + 26/3H2||DG° = 1,097.52 – 0.9898T||836|
|TiO2 + 3CH4 = TiC + 2CO + 6H2||DG° = 649.35 – 0.5890T||829|
|Al2O3 + 9/2CH4 = 1/2Al4C3 + 3CO + 9H2||DG° = 1,615.43 – 1.0308T||1294|
Reaction Standard Gibbs free energy change, kJ Equilibrium temperature, °C
MnO + 10/7CH4 = 1/7 Mn7C3 + CO + 20/7 H2 DG° = 377.68 – 0.3144T 928
Cr2O3 + 13/3CH4 = 2/3Cr3C2 + 3CO + 26/3H2 DG° = 1,097.52 – 0.9898T 836
TiO2 + 3CH4 = TiC + 2CO + 6H2 DG° = 649.35 – 0.5890T 829
Al2O3 + 9/2CH4 = 1/2Al4C3 + 3CO + 9H2 DG° = 1,615.43 – 1.0308T 1294
Reduction temperatures under standard conditions are much lower than in carbothermal reduction with solid carbon. In addition to thermodynamic advantage, the gas-solid reaction has a higher rate in comparison with solid-solid carbon-oxide reaction.
Another way to decrease the temperature of carbothermal reduction of stable oxides is decreasing the CO partial pressure and/or increasing mass transfer in the gas phase. This can be achieved by running reduction processes in inert atmosphere or in hydrogen, which is also involved in the reduction reactions.
It is well recognised that carbothermal reduction of metal oxides in the solid state goes through the gas phase. Reduction of oxide MO to carbide MC can be presented by Reactions (1) and (2):
MO + 2CO = MC + CO2 (1)
CO2 + C = 2CO (2)
In these reactions, carbon and oxygen are transferred between solid phases by CO and CO2, respectively. Gas phase also plays important role when reduction proceeds with formation of metal or metal oxide vapour, as in the case of alumina reduction. Gas species are directly involved in the carbothermal reactions; the composition of the gas phase may affect the reaction rate. This is illustrated by Figure 1 which presents reduction curves for manganese oxide obtained in hydrogen, helium and argon. Reduction is much faster in hydrogen than in helium, and faster in helium than in argon.
Fig 1: Reduction curves of MnO2 at 1275°C in H2, He and Ar
We study the kinetics and mechanisms of carbothermal reduction and reduction by methane containing gas to make the reduction processes more efficient.
Sheikh A. Rezan
Mohammad A. R. Dewan