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

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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: Al 2 O 3 + SiO 2 → mullite, and Fe 2 O 3 → Fe 3 O 4 → FeO →Fe, followed by Fe+mullite→Fe 3 Si; SiO 2 + Fe → Fe 3 Si; Nosean → NaAlSiO 4 + Na 2 SiO 3 , followed by NaAlSiO 4 / Na 2 SiO 3 + Fe → Fe 3 Si.

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