SiC ceramics production by spark plasma sintering from a coal fly ash residue after high-pressure NH4HSO4-and-H2SO4 leaching

Abstract

In this article, a novel method for SiC production from a residue of coal fly ash (CFA) after high-pressure leaching with a mixture of NH4HSO4 + H2SO4 is proposed. A thermodynamic analysis showed that SiC can be obtained at >1500 °C, and titanium carbide (TiC) and ferrosilicon (FeSi) can form as byproducts. Carbothermal reduction of the CFA residue with carbon in the range 1300–1500 °C with holding time 1–3 h was investigated. X-ray phase analysis (XRD) indicated that SiC forms at >1425 °C. Iron compounds in the form of Fe3Si and Fe5Si3 were detected by Mössbauer spectroscopy in a sample sintered at 1500 °C. The possibility of SiC production by a two-step procedure was demonstrated. First, a SiC powder was obtained from the CFA residue and carbon at 1500 °C, then SiC was mixed with yttrium aluminum garnet Y3Al5O12 and processed by spark plasma sintering at 1750–2000 °C, holding time 5 min, and pressure 50 MPa. Ceramic samples obtained at 1850 and 1900 °C with compressive strength of 350 ± 17.5 & 416 ± 20.8 MPa and Vickers hardness of 17.89 ± 0.89 & 15.97 ± 0.79 GPa, respectively, proved to be comparable to the best reported SiC + yttrium aluminum garnet ceramics. Thus, this study presents an original method for ceramics production from solid waste of coal-fired power plants; this approach will allow to create a comprehensive technology for utilizing 100 % of CFA.