X-ray Microprobe Investigation of Iron During a Simulated Silicon Feedstock Extraction Process

Elemental silicon is extracted through carbothermic reduction from silicon-bearing raw feedstock materials such as quartz and quartzites. We investigate the micron-scale distribution and valence state of iron, a deleterious impurity in several iron-sensitive applications, in hydrothermal quartz samples of industrial relevance during a laboratory-scale simulated reduction process. We use X-ray diffraction to inspect the quartz structural change and synchrotron-based microprobe techniques to monitor spatial distribution and oxidation state of iron. In the untreated quartz, most of the iron is embedded in foreign minerals, both as ferric (Fe³⁺, e.g., in muscovite) and ferrous (Fe²⁺, e.g., as in biotite) iron. Upon heating the quartz to 1273 K (1000 °C) under industrial-like conditions in a CO(g) environment, iron is found in ferrous (Fe²⁺) particles. At this temperature, its chemical state is influenced by mineral decomposition and melting processes, whereas at higher temperatures it is influenced by the silicate melts. As the quartz grains partially transform to cristobalite 1873 K (1600 °C), iron diffuses towards liquid–solid interfaces forming ferrous clusters. Silica is liquid at 2173 K (1900 °C) and the iron migrates towards the interfaces between gas phases and the silicate liquid.