Clean magnesium production using concentrated solar heat in a high-temperature cavity-type thermochemical reactor

Abstract

The synthesis of magnesium from the corresponding oxide via a solar carbo-thermal and methano-thermal reduction process using high-temperature concentrated solar heat was investigated. The reduction of magnesium oxide (MgO) was experimentally demonstrated in a directly-irradiated prototype solar reactor at reduced pressure and temperature up to ∼1650 °C. The solar reactor was successfully operated with a variety of reducing agents (carbon and CH4) in batch and continuous modes under atmospheric and low pressure conditions (0.1–0.9 bar), thus representing the first process demonstration of MgO carbothermal reduction with continuous reactant injection in vacuum condition. A parametric study regarding operating pressure, carbon feedstock type, and C/MgO molar ratio was conducted to emphasize their effect on products yield (Mg and CO) and solar reactor performance. MgO conversion, reduction rate, and CO yield increased with decreasing pressure, in agreement with thermodynamic analysis. Utilizing activated charcoal as reducing agent showed the highest MgO conversion and CO yield. High MgO conversion over 99% was demonstrated with maximal CO yield up to 24.59 mmol/gMgO, closely approaching theoretical maximum value (24.81 mmol/gMgO). Employing methane as a reducing agent was also shown to be an alternative option to produce Mg, although methane cracking occurred simultaneously at the elevated reaction temperature. Mg recovery in the outlet products was identified as one of the most critical process challenges because of the pyrophoric property of the produced nanopowder and its strong oxidation reactivity with air.