Feasibility Study and Parameter Regulation of Magnesium Extraction from Serpentine by Vacuum Thermal Reduction with Al–Si Alloy

The traditional Pidgeon process for magnesium production suffers from high energy consumption, high carbon emissions, and large slag volume. To overcome these limitations, this study explored a novel process using serpentine as the magnesium source, with AlSi5 alloy and CaO serving as the reductant and slagging agent, respectively. When the amount of reductant was only sufficient to reduce MgO, the reduction degree of magnesium was very low because Al preferentially reduced SiO₂ over MgO. When the reductant amount was sufficient to reduce both MgO and SiO₂, the reduction degree could exceed 80 pct, confirming the feasibility of using serpentine as a magnesium source, although the reductant cost was relatively high. Under the conditions of 1150 °C and a CaO/MgO molar ratio of 0.3, the reduction degree reached 80.7 pct. However, when the CaO/MgO molar ratio was increased to 0.5, the reduction degree exhibited a “plateau,” which was attributed to part of the MgO being encapsulated in MgAl₂O₄ and Ca₂MgSi₂O₇ phases, making it unavailable for reduction. To reduce reductant consumption and utilize the reducing capability of Si generated from the reduction of SiO₂ by Al, the reductant amount was controlled to a level only sufficient to reduce SiO₂. However, under these conditions, even at 1200 °C and a CaO/MgO molar ratio of 0.8, the reduction degree was only 59.57 pct, due to the weak reducing ability of Si. Therefore, to improve the reduction degree, the reductant amount should be appropriately increased to achieve a balance between reduction degree and reductant consumption.