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

Qingchun Yu,Yong Deng,Yuebin Feng,Ziyong Li

doi:10.3390/met11081165

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.

Highlights

Al9 FeSi3, Al, and Si, and condensate of SiC, AlN and C are obtained. β-SiAlON was not found in the residue
Fly ash emerges as a by-product mainly from the combustion of raw coal in thermal power plants
The diffraction peaks of silica and mullite existed at 1573 K and 1673 K, but iron oxide did not, which means reduction of iron oxide occurred

Summary

Introduction

Al9 FeSi3 , Al, and Si, and condensate of SiC, AlN and C are obtained. β-SiAlON was not found in the residue. Carbothermic reduction of fly ash in air did not behave the same as fly ash in nitrogen. Fly ash contains mostly aluminosilicate mineral [2,3] and small amounts of iron oxide. Fly ash can be considered a mineral resource if the potential metal values are exploited [4,5,6,7]. Carbothermic reduction of fly ash to recover aluminum, silicon and iron can increase extra value of products and reduce environmental pollution. It was found that reduction of alumina using carbon as a reductant was favored in the presence of limited amounts of oxygen [8]. Alumina [9] decomposes into Al-containing gases and oxygen, carbon reacts with oxygen to reduce oxygen partial pressure.

Methods

Results

Conclusion