Fly Ash Utilisation in Mullite Fabrication: Development of Novel Percolated Mullite

Pramod Koshy,Vicki Zhong,Naomi Ho,Sandor Alex Koszo,Charles Christopher Sorrell,Luisa Schreck,Erik J Severin

doi:10.3390/min11010084

Abstract

Fly ash is an aluminosilicate and the major by-product from coal combustion in power stations; its increasing volumes are major economic and environmental concerns, particularly since it is one of the largest mineral resources based on current estimates. Mullite (3Al2O3·2SiO2) is the only stable phase in the Al2O3-SiO2 system and is used in numerous applications owing to its high-temperature chemical and mechanical stabilities. Hence, fly ash offers a potential economical resource for mullite fabrication, which is confirmed by a review of the current literature. This review details the methodologies to utilise fly ash with different additives to fabricate what are described as porous interconnected mullite skeletons or dense mullite bodies of approximately stoichiometric compositions. However, studies of pure fly ash examined only high-Al2O3 forms and none of these works reported long-term, high-temperature, firing shrinkage data for these mullite bodies. In the present work, high-SiO2 fly ashes were used to fabricate percolated mullite, which is demonstrated by the absence of firing shrinkage upon long-term high-temperature soaking. The major glass component of the fly ash provides viscosities suitably high for shape retention but low enough for ionic diffusion and the minor mullite component provides the nucleating agent to grow mullite needles into a direct-bonded, single-crystal, continuous, needle network that prevents high-temperature deformation and isolates the residual glass in the triple points. These attributes confer outstanding long-term dimensional stability at temperatures exceeding 1500 °C, which is unprecedented for mullite-based compositions.

Highlights

Pure single-phase mullite exhibits short-term creep resistance at temperatures ≤1400 ◦ C and commercial mullites perform only to ≤1000 ◦ C [13]. This degradation in practice highlights the importance of the achievement of direct bonding between the mullite grains in contrast to chemically bonded refractories, the thermal properties of which are dominated by the glassy matrix [23]
Heating was done for a maximal time of only 4 h in all of these studies and none reported the long-term firing shrinkages. The combination of these factors represents the principal difference between these works and the present work, which reports the development of unique, dense, mullite microstructures revealed by firing shrinkage measurements and that are generated using high-SiO2 compositions and longterm heating
The present work considers previous work on the fabrication of mullite from fly ash and introduces a novel form of volumetrically percolated mullite with microstructures consisting of direct-bonded, single-crystal, continuous, needle networks, with the residual glass isolated in the triple points

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Unused fly ash generally is stored on-site at power stations in containers or in a slurry form in collection ponds [4]. Fly ash may contain remnants of unburnt carbon from the coal, which makes it appear grey to black in colour It may contain varying amounts of iron oxide, which can generate colours ranging from light to dark brown [4]. While SiO2 is the major component of the glassy spherical particles, undissolved α-quartz (SiO2 ) generally is the principal crystalline phase Another significant crystalline species that form during coal combustion is mullite (3Al2 O3 ·2SiO2 ) while lower levels of magnetite (Fe3 O4 ) and/or hematite (Fe2 O3 ) are observed commonly [7].

Orefractory

Percolated Mullite

Findings

Summary