Investigation of continuous carbothermal reduction of magnesia by magnesium vapor condensation onto a moving bed of solid particles

Abstract

Magnesium metal production by carbothermic reduction (CTR) can reduce process emissions and energy requirements relative to silicothermic or electrolytic production, if high magnesium metal yields are achieved. A novel process was investigated where product magnesium gas condensed onto a moving bed of solid particles at high temperatures (≥300 °C) and in vacuum to minimize reversion and promote crystal growth. Using a half-system in which CTR product gases were artificially generated, magnesium continuously condensed onto steel, oxide, or carbide particles at yields >85% for PMg = 0.5 kPa. Steel particles exhibited the greatest bed retention and ease of separation, so this medium was used for scale-up to a full-system in which MgO CTR in a fixed bed gasifier at 1400 °C–1550 °C produced Mg(g) and CO. The initial reduction resulted in high Mg yield (>80%), but yield decreased as the reaction proceeded. The accumulation of Mg(s) and reversion product in the condenser promoted further reversion. The process yield could likely be improved by using high surface area particles to promote heat and mass transfer, allowing for shorter solid and gas residence times in the condenser. A finite volume model on the reactor tube and pellet bed described the kinetics, heat transfer, and mass transfer phenomena in the reduction reactor. The model predicted that the center of the bed was >50 °C cooler than the furnace, and the product gas pressures were near equilibrium. A wide and directly heated reactor could overcome these heat and mass transfer limitations.