The Importance of Slag Structure to Boron Removal from Silicon during the Refining Process: Insights from Raman and Nuclear Magnetic Resonance Spectroscopy Study

Slag structure plays an important role in determining the relative ease of boron removal from silicon. Correlation between slag structure and boron removal thermodynamics was experimentally studied by Raman and nuclear magnetic resonance (NMR) spectroscopy using CaO-SiO₂-Na₂O slags with different optical basicities (0.6 to 0.71). Optimization of slag depolymerization leads to efficient removal of boron. The extent of nonbridged oxygen content (NBO/T) and boron removal gradually increased with an increase in optical basicity from 0.6 to 0.66: B₂O₃ derived from boron oxidation captured nonbridging oxygens of Q ⁰(Si), Q ¹(Si), and Q ²(Si), and was incorporated into the silicate network in the form of Q ³(Si and B). When optical basicity increased to 0.71, NBO/T rapidly increased and boron removal decreased considerably. Quick depolymerization of Q ³(Si and B) deteriorated the stability of boron. Various structural forms of boron in the silicate network were successfully detected: the BO₃ trihedrons ^[3]B-3Si, ^[3]B-2Si-1NBO, and BO₃ (nonring), and the BO₄ tetrahedrons BO₄ (1B, 3Si) and BO₄ (0B, 4Si). BO₄ (1B, 3Si) was the main structure contributing to the increase of boron capacity; BO₃ (nonring), detected under higher optical basicity conditions, may cause deterioration of boron removal by suppressing its oxidation.