(385f) The Mechanism and Kinetics of Magnesium Oxide Carbothermic Reduction

Compared to the commercially established production routes (FeSi reduction of MgO and electrolysis of MgCl₂), the carbothermic reduction of MgO has been considered as an attractive candidate for production of Mg due to its potentially lower energy consumption and CO₂ emissions. In this process MgO-C blends are heated to temperatures that, depending on pressure, range from 1350°C (at 10 mbar) to 1750°C (at 1 bar), which results in production of gaseous Mg and CO. As at lower temperatures the products recombine readily, rapid quench of the product gas is needed to increase the yield of Mg. To identify the key parameters that control the reduction rate, we investigated the prevailing pathway and kinetics of Mg production at 1400°C and total pressure of 10 mbar. Accurate kinetic data has been acquired utilizing a novel experimental method that precluded confounding the product gas composition by the partial product recombination in the quencher. By varying reactant molar ratio, morphology, and physical proximity, we have conclusively elucidated the rate-controlling steps along the course of reaction. Our findings imply that the Mg production rate is initially controlled by the removal of the O₂, originating from thermal dissociation of MgO, via reaction with C. The further progress of reaction is hindered by a gradual sintering of MgO that increases the overall O₂ transport resistance, thereby decreasing the Mg production rate. In this stage the Mg is mainly produced by the MgO reduction with CO yielding CO₂ as intermediate, which is then removed from the reaction site via reaction with C that replenishes the CO to sustain the further MgO reduction.