Original Article

Reaction pathways in synergistic carbothermal reduction of fly ash and red mud for ferrosilicon alloy synthesis^⋆

¹ School of Metallurgy Engineering, Jiangsu University of Science and Technology, Zhangjiagang 215600, Jiangsu, PR China
² School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 September 2025
Accepted: 13 January 2026

Abstract

To enhance the utilization efficiency of industrial byproducts, a synergistic carbothermal reduction process was developed for the synthesis of ferrosilicon alloy from fly ash and red mud. The reaction mechanisms were systematically analyzed through thermodynamic calculations. The effects of key parameters, including the iron-to-silicon molar ratio (n_(Fe)/n_(Si)), carbon-to-oxygen molar ratio (n_(C)/n_(O)), roasting temperature, and holding time on ferrosilicon formation were investigated using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicate that the ferrosilicon phase emerges when the n_(Fe)/n_(Si) ratio decreases to 1.01 and remains detectable at 0.76. Within this range, an n_(C)/n_(O) ratio between 0.33 and 0.48 is required to ensure complete reduction. At a fixed holding time of 2 h, ferrosilicon formation commences at 1723 K with increasing roasting temperature from 1673 K to 1823 K. Lower temperatures hinder the complete reaction between metallic Fe and mullite. Although higher temperatures (up to 1823 K) do not induce significant phase transformations, they markedly reduce residual carbon content. At a constant temperature of 1723 K, extending the duration from 1 h to 2 h or longer promotes ferrosilicon formation, with a duration of 4 h substantially reducing the residual carbon, indicating a more complete reaction. The optimal synthesis conditions were determined as follows: n_(Fe)/n_(Si) = 0.76–1.01, n_(c)/n_(O) = 0.33, roasting temperature of 1723–1823 K, and holding time 2–4 h. In particular, satisfactory results were achieved either at 1823 K for 2 h or at 1723 K for 4 h. Combined thermodynamic and experimental analyses revealed the primary reaction pathway as follows: Al₂O₃ + SiO₂ → mullite, and Fe₂O₃ → Fe₃O₄ → FeO →Fe, followed by Fe+mullite→Fe₃Si; SiO₂ + Fe → Fe₃Si; Nosean → NaAlSiO₄ + Na₂SiO₃, followed by NaAlSiO₄ / Na₂SiO₃ + Fe → Fe₃Si.