The rate of MgO carbothermal reduction was studied at temperatures 7 from 1350-1650°C and pressures from 0.1-100kPa based on product gas 8 analysis at near isothermal conditions. For all temperatures the initial 9 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 10kPa. 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 CO 2 generation was more prevalent at elevated pressures. SEM imaging revealed the dissolution of C and MgO contact with conversion, andisoconversional analysis points to a change inthe rate determining step between 1 and 10kPa. 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 CO 2 as the reaction intermediate.

A novel method is presented to study magnesium metal vapor condensation/oxidation in CO and CO 2 atmosphere at reduced pressures. Mg(s) was evaporated and mixed with an equimolar amount of CO or CO 2 at 1000°C after which the gaseous mixture flowed through an air cooled tubular condenser. Measurements of the axial temperature profile, calculation of partial pressures and analysis of deposits within the condenser allowed for identification of deposition/condensation onset temperatures, supersaturation ratio and reaction mechanism. In the presence of CO 2 , rapid oxidation of Mg(g) has been observed. In the presence of CO, no Mg(g) oxidation was found above 950°C. Mg(g) oxidation observed at lower temperatures is believed to be initiated by CO disproportionation. The proposed mechanism is able to explain the increase in Mg metal yield with decreasing CO partial pressure. At a CO partial pressure <3 mbar, high Mg metal mass yields of >90w% were found. The presented method is applicable to the study of a variety of metal vapor/oxidizer combinations e.g. of interest in metal and solar fuel production.

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Catalysis, milling, vacuum operation, and their interactions were studied as methods for increasing the rate of carbothermal reduction (CTR) of hard-burned and soft-burned MgO powders. For CTR at 1550 °C and 10 kPa, pellets made with soft-burned MgO reached 90% conversion in 30 min without milling and achieved the same conversion in 5 min with 120 min of milling. Crystallite attrition of soft-burned MgO was not observed, and the increased reactivity was attributed to mixing and aggregate attrition. Catalytic additives improved the reactivity in the CTR of hard-burned MgO but decreased the rate of CTR for soft-burned MgO. Additives were shown to catalyze the gas–solid reaction pathway, and thermogravimetric analysis revealed that the rate of carbon oxidation by CO2 was approximately 2 orders of magnitude higher than that of MgO reduction by CO; thus, the latter reaction was rate-limiting to the gas–solid reaction pathway.

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