Pressure dependent kinetics of magnesium oxide carbothermal reduction

Abstract

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.

Experimental carbothermal reduction of MgO at low pressure using concentrated solar energy

“…The diffusion of the gaseous compounds inside the reactor could participate to the reaction but the activation energy of this process is very high compared to those of the phase boundary and the nucleation (Table 3). Furthermore, Chubukov et al have recently demonstrated that solid-gas reaction are promoted only when the system pressure is higher, close to 10 4 Pa [22]. So, the reaction of Eq.…”

“…Fruehan et al have demonstrated that the reaction primarily occurred at the interface but suggested that the overall rate was controlled by the gaseous diffusion through a porous MgO layer that could be formed on the sample surface [21]. A model has been recently developed showing that the solid-solid reaction is the main mechanism controlling the first stage of the reaction whereas gassolid reactions could allow to complete the carbothermal reduction of MgO depending on the system pressure [22].…”

On the prevailing reaction pathways during magnesium production via carbothermic reduction of magnesium oxide under low pressures

“…Thermodynamic calculations indicate that the temperature required for the forward reaction (1) to proceed is dependent on the partial pressures of the products. 8,9,13,14 Reducing these from 50 to 0.5 kPa decreases the onset temperature of the forward reaction from ∼1750°C to ∼1350°C, thereby reducing heat losses and the energy required to preheat the reactants. In addition, it has been demonstrated that lower CO partial pressures drastically decrease the severity of the reverse reaction (1), which takes place during cooling of the product mixture.…”

“…In addition, it has been demonstrated that lower CO partial pressures drastically decrease the severity of the reverse reaction (1), which takes place during cooling of the product mixture. 14,15 These benefits thus present a strong incentive for investigating the CTR under vacuum, as they may compensate for the additional pumping work required to maintain low reaction pressures.…”

“…In spite of the extensive research dedicated to understanding the constituent steps of reaction (1), 14,16,17 at present there is no consensus on the prevailing mechanism of the carbothermic reduction. The reported findings gravitate around two hypotheses.…”