Non-Contact Reaction of MgO and Al at High Temperature and Under Vacuum
Authors: Hamawaki Yuzuki , Takenaka Toshihide 
Published: December 2024

The Pidgeon process is the major method for Mg metal production, but it has problems such as high CO2 emissions and energy consumption during production. Our Laboratory has been studying the Mg metal production from Mg waste by alumino-thermic reduction. Although it was shown that the alumino-thermic reduction seemed easier than the silico-thermic reduction, its reaction rate should be improved. The reaction mechanism of the alumino-thermic of MgO must be investigated in detail for the better Mg metal production by alumino-thermic reduction. In this study, the research focused on the non-contact reaction in alumino-thermic reduction of MgO has been performed, because it was said that the non-contact reaction dominated the silico-thermic reduction of MgO.
Porous rectangular blocks of MgO-CaO-Al (3:2:2 in molar) mixture and MgO-CaO (3:2 in molar) mixture were prepared from Mg(OH)2, Ca(OH)2 and Al powders. The MgO-CaO block was put on the MgO-CaO-Al block, and their direct contact was avoided by a thin spacer (t=0.05~1.0mm). The set of blocks was placed in an air-tight reaction tube with stainless foil liner inside as the MgO-CaO-Al block was below. The part of the vacuumed reaction tube where the specimen existed was gradually heated to 1150°C in a furnace, kept for 3h and cooled gradually. The reaction tube outside the furnace was always cooled sufficiently. The condensate on the stainless foil in the cool part, which was thought as the reaction product, was analyzed by XRD and SEM-EDX. The residues of the blocks were also analyzed, and the change in the bottom surface of the upper MgO-CaO block was essential in this study. The weight changes of the blocks were measured, and the reaction ratio was estimated by the weight change. The experiment using the MgO-CaO-Si instead of the MgO-CaO-Al block was performed as control experiment.
The elemental contents on the bottom surface of the upper block changed with the types of lower block as shown in Table 1. In the case that the MgO-CaO block was used as the lower block, the elemental contents on the bottom surface unchanged from those before experiment; the Al was hardly detected on it while the Mg content almost unchanged. Aluminum was detected on the bottom surface obviously when the MgO-CaO-Al block was used as the lower block. The Al content and Mg content on the bottom surface of upper block varied considerably, and a clear dependence on the distance between the blocks was not seen. The existence of Ca12Al14O33 and the decrease in Mg content on the bottom surface clearly implies that alumino-thermic reduction of MgO occurred even though Al was not contained in the upper block originally. Silicon was detected on the bottom surface of the upper MgO-CaO block when the MgO-CaO-Si block as the lower block. The Mg content on the bottom surface decreased, and Ca2SiO4 was detected. The degree of the decrease in the Mg content on the bottom surface was not significant, and strongly depended on the distance between the blocks. These results shows that a non-contact reaction also occurred in silico-thermic reduction of MgO, and its reduction rate was smaller than that by alumino-thermic reduction. Since a lower-valent silicon oxide was reported to mediate the silico-thermic reduction, it is considered that a lower-valent aluminum oxide participates in the non-contact reaction of the alumino-thermic reduction of MgO.

Figure 1