Some industrial wastes are shown to be useful in the production of mullite ceramics. These industrial wastes are rich in certain metal oxides such as silica (SiO2) and alumina (Al2O3). This gives wastes the potential to be used as a starting material source for mullite ceramics preparation. The purpose of this review paper is to compile and review various mullite ceramics preparation methods that utilized a variety of industrial wastes as starting materials. This review also describes the sintering temperatures and chemical additives used in the preparation and its effects. A comparison of both mechanical strength and thermal expansion of the reported mullite ceramics prepared from various industrial wastes were also addressed in this work.
Mullite, commonly denoted as 3Al2O3∙2SiO2, is an excellent ceramic material due to its extraordinary physical properties. It has a high melting point, low coefficient of thermal expansion, high strength at high-temperatures, and possesses both thermal shock and creep resistance [1]. These extraordinary thermal and mechanical properties enable the material to be used in applications such as refractories, kiln furniture, substrates for catalytic convertors, furnace tubes, and heat shields.
Mullite can be found only as scarce mineral at Mull Island, Scotland [2]. Due to its rare existence in nature, all the mullite ceramics used in industry are man-made. Much research has been done to prepare mullite ceramics using different precursors, starting either from industrial/laboratory grade chemical [3] or naturally occurring aluminosilicate minerals [4]. However, the cost of these starting materials is expensive, which are synthesized or mined beforehand. For years, researchers have been looking for economical alternatives to synthesize mullite ceramics. Hence, numerous mullite precursors derived from industrial wastes have been reported in the literature。 These industrial wastes have high content of useful silica and alumina, which are the essential chemical compounds needed to produce mullite ceramics. Other benefits of using these industrial wastes are the energy and cost saving if the wastes were diverted and reutilized as an engineering material. Furthermore, this could also help to reduce the environmental burden and enhance its economic benefit.
In order to investigate whether pure electroceramics waste could be used to synthesize mullite ceramics,the pure electroceramics waste mixed with alumina powders and the pure electroceramics waste as raw materials were compared.The effects of raw materials’composition and sintering temperature on the microstructure and physical properties of mullite ceramic were investigated. XRD and SEM were used to study the phase composition and microstructure.
The results show that the content of mullite is increased with raising sintering temperature, and at the same time the bulk density is escalated. The raw materials are the pure electroceramics waste, thus the sintering activity is greater, and the sintering process can be accelerated, and density is also increased. When the mullite is prepared only by the electroceramics waste, the bulk density and compressive strength are largest, the porosity is smallest, and the comprehensive physical properties will be the best
Driven by the need for low-cost and environmentally friendly alternatives, many research efforts have used a variety of industrial wastes as starting materials to produce mullite ceramics. The processing methods, sintering temperatures, and chemical additives have been reviewed. The traditional route processing method that involved mixing, pressing, and reaction sintering of the mullite precursor was the most commonly used method due to its simplicity and cost effectiveness. Although this method is able to produce porous mullite ceramics, the apparent porosities of the resultant mullite ceramic were reported to stay below 50%. On the other hand, freeze casting was shown to be able to produce highly porous mullite ceramic, with an apparent porosity of 67%, even at a very high sintering temperature of 1500 °C. A review of the sintering temperatures and different chemical additives used in the production of mullite was carried out. It is desirable to use a sintering temperature of above 1500 °C for mullite production, due to the higher reaction rate between Al2O3 and SiO2 in the precursor. However, excessive silica content associated with impurities in the precursor could lead to the sample deformation or meltdown during high-temperature sintering. As for the chemical additives, CaF2, H3BO3, Na2SO4, TiO2, AlF3, and MoO3 have been reported as an effective aid to lower sintering temperature while V2O5, Y2O3-doped ZrO2 and 3Y-PSZ can be used to promote densification for mullite ceramics. Doping with chemical additives such as AlF3, Na2SO4, NaH2PO4·2H2O, V2O5, and MgO assisted anisotropic growth of the mullite whiskers, which subsequently enhanced the physical strength and toughness of the mullite ceramics.
Post time: Aug-29-2023