Pressure dependent kinetics of magnesium oxide carbothermal reduction

The rate of MgO carbothermal reduction was studied at temperatures from 1350 to 1650 °C and pressures from 0.1-100 kPa based on product gas analysis at near isothermal conditions. For all temperatures the initial rate of carbothermal reduction increased inversely with pressure, and between conversions of 20-35% a transition occurred after which the reaction rate was maximum at 10 kPa. Analysis of reacted pellets showed that the reaction stoichiometry, the ratio of C to MgO reacted, was less than unity and decreased with pressure indicating CO2 generation was more prevalent at elevated pressures. SEM imaging revealed the dissolution of C and MgO contact with conversion, and isoconversional analysis points to a change in the rate determining step between 1 and 10 kPa. The given experimental observations argue the importance of mass transfer and gaseous intermediates. A kinetic model is formulated based on a macroscopic species balance with CO2 as the reaction intermediate.