Phase Transformation of Alumina, Silica and Iron Oxide during Carbothermic Reduction of Fly Ash for Ceramics Production

Abstract

Fly ash is a by-product from burning of coal. Utilization of fly ash by carbothermic reduction is an effective way to recover aluminum, silicon, and iron to enhance product-added value. This work is focused on the phase transformation of Al2O3, SiO2 and Fe2O3 during carbothermic reduction of fly ash in air. A comparative analysis of carbothermic reduction of fly ash in air and in nitrogen was made. Thermodynamics analysis was performed to illustrate the possible reactions for residue and condensate. X-ray diffraction (XRD), scanning electronic microscope (SEM), and energy dispersive spectrometry (EDS) were employed to characterize the phase composition, surface morphology, and microstructure of the reduced products. Results show that Fe3Si and Fe2Si appear sequentially with increasing of temperature. Al5O6N is an intermediate compound. Residue of Al9FeSi3, Al, and Si, and condensate of SiC, AlN and C are obtained. β-SiAlON was not found in the residue. Nitrogen is involved in the reduction of Al2O3 but not in the reduction of SiO2 and Fe2O3. Carbothermic reduction of fly ash in air did not behave the same as fly ash in nitrogen.

EFFECTS OF BORIC ACID DOSAGE AND REACTION TEMPERATURE ON THE PHASE COMPOSITION AND MICROSTRUCTURE OF Si C-B₄C COMPOSITE POWDERS SYNTHESISED BY THE CARBOTHERMAL REDUCTION METHOD

“…Only from the perspective of thermodynamic theoretical analysis, the reaction (1) could not occur [12]. However, based on the analysis of the actual situation in the present study, the reaction (1) could occur [23]. The reasons are as follows: first, the theoretical analysis of the thermodynamics was carried out under an assumed gas pressure of 0.1 MPa, but under these experimental conditions, the partial pressures of SiO and CO gas were far lower than the assumed gas pressure.…”