Magnesiothermic Reduction of Natural Quartz

Abstract

In the current work, the metallothermic reduction of natural quartz by magnesium has been studied at 1373 K under different reaction conditions, i.e. quartz type, quartz particle size, Mg:SiO2 mole ratio and reaction time. The microstructure of reaction products was studied to illustrate the reaction progression through scanning and transmission electron microscopy techniques. X-ray diffraction analysis with Rietveld phase quantification was used to calculate the change in the amount of phases at different reaction conditions. The results showed that the Mg:SiO2 mole ratio strongly affects reaction mechanism and product characteristics such as phase content and microstructure. At lower Mg:SiO2 mole ratios, the reaction rate is fast at the beginning and the formation of a product layer consisting of different phases such as MgO, Si, Mg2Si, Mg2SiO4 and MgSiO3 around quartz particles limits the Mg diffusion. This phenomenon is more noticeable for larger quartz particle sizes where Mg should diffuse longer distance towards the quartz core to react with it. At higher Mg:SiO2 mole ratios, a significant amount of Si–Mg liquid alloy is formed during reaction where the high mobility of Mg in this liquid phase and cracking of quartz particles result in significantly higher reaction rate. Here the formation of intermediate phases is not significant and the products would be the mixture of MgO, Mg2Si, and either Si or Mg phases.

Kinetics of Magnesiothermic Reduction of Natural Quartz

“…Moreover, some pure Mg is found in this layer, shown as pink areas in the X-ray mapping of Mg, Figure 3 c. The formation of these layers is in accordance with the ternary phase diagram of Si–Mg–O that dictates the establishment of possible interfaces between different phases. These observations imply that the conversion of SiO 2 to Mg 2 Si happens through a series of intermediate reactions as discussed in detail in our previous work [ 40 ].…”

“…This shows that the amount of available Mg affects the reaction rate as is further discussed in Section 4.1 . The comparison of total Si formed at a Mg/SiO 2 mole ratio of 2 and 1173 K with results at 1373 K from our previous work [ 40 ] reveals that reaction rate is considerably higher at 1373 K than 1173 K.…”

“…This reaction consists of three steps: (1) mass transport of Mg from the bulk of Mg to the external surface of quartz particles, (2) diffusion of Mg through the product layer(s) towards the reaction zone and (3) chemical reaction at the reaction zones between silicate layers/SiO 2 and Mg. It was shown earlier that diffusion of Mg through the product layer toward the reaction zone is the rate limiting step [ 31 , 34 , 35 , 37 , 40 ].…”

“…At each reaction temperature, a series of experiments with varying reaction times from 10 to 240 min was performed. The experimental procedure to conduct the magnesiothermic reduction was described in detail in our previous publication [ 40 ].…”

“…In our previous work [ 40 ], it was demonstrated how the reaction conditions including Mg to SiO 2 mole ratio, SiO 2 particle size and quartz type affect the reduction of natural quartz. Complementary to our previous work, the aim of the current work was to investigate the reaction rate of the magnesiothermic reduction of natural quartz to produce either Si or Mg 2 Si at various temperatures.…”