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Alumina-mullite (AM) refractories are widely used as liners in gas turbines for power production, because of their peculiar properties, appropriate for the thermal insulation of combustion chambers, characterized by turbine inlet temperature around 1400 °C. The typical tiles are made with a mixture of alumina and mullite with different granulometries, including a coarse fraction. In this work the feasibility of recycling of ceramic wastes, which come from other industrial processes, into AM refractories was assessed. The effects of their addition on phase composition, microstructure and thermomechanical properties of AM refractories were investigated. MOR and Young’s modulus were determined at room temperature and up to 1500 °C by four point flexural tests; thermal shock resistance was evaluated by MOR measurements after quenching tests. The comparison with a typical AM refractory used as liners shows that thermomechanical properties and thermal shock resistance were not significantly compromised by ceramic waste additions up to 20%, and, on the contrary, were improved.
Refractory castables containing aluminamagnesia/self-forming spinel (MgAl2O4) are used in impact pads of steel ladles in steelmaking processes. In order to understand the wear mechanisms of refractory materials, several recipes were tested from a corrosion, slag resistance and thermal shock point of view. The results show that the corrosion is extremely limited at the interface slag/refractory for all cases. Nevertheless, for higher cement alumina content castables, the formation of micro cracks is observed in refractory castables into which slag deposit can penetrate. The slag reacts with alumina to form a new phase such as hibonite (CA6) and calcium dialuminate (CA2). The expansive reactions involving CA2 and CA6 lead to macro-cracks. Thus, the penetration of slag and steel are increased causing hot mechanical properties to degrade. For lower cement alumina castables, the formation of micro cracks is avoided by controlling volume expansion. Thus, the slag deposit reacts with alumina grains and the matrix at the slag/refractory interface to produce a monomineral layer of hibonite. In this way, the monomineral layer acts as a barrier and limits the penetration of slag and steel into the refractory lining.
The main target of this work is to prepare refractory magnesia- and alumina- based ceramics within the MgO-Al2O3-ZrO2-SiO2 system. These include periclase (MgO) and its composites with MgAl2O4-spinel and forsterite (MgSiO4)-zirconia (ZrO2) as well as corundum (α-Al2O3) and its composite with mullite (Al6Si2O13)-zirconia (ZrO2). These were processed from recycled pure and fine magnesium and aluminium hydroxide precipitates as well as raw zircon, ilmenite and rutile raw materials. Up to 5.0% lmenite and rutile as well as Mg(OH)2 , Al(OH)3 or MgO were used as doping materials to accelerate sintering rate of the periclase and corundum products. Also 5-20% of calcined alumina and raw zircon were mixed with the magnesium as well as aluminium hydroxides to prepare varieties of periclase-spinel, periclaseforsterite-zirconia as well as corundum-mullitezirconia composites, respectively. The corresponding batches were prepared, semi-dry pressed and fired at two-stages up to 1600 °C. The densification rate was followed by determining linear shrinkage, bulk density and apparent porosity. Phase composition as well as microstructure and microchemistry were investi-gated by XRD as well as EPMA techniques, respectively. The refractory quality was assessed by determining load-bearing capacity and volume stability up to1600 °C.
The mechano-chemical and microstructural evolution using magnesium-aluminium spinel in a MgO-CaZrO3 sintered refractory matrix acting as ceramic bonding was investigated. The microstructural study reveals MgAl2O4 spinel phase at contact points between MgO and CaZrO3 particles. In addition, the cold crushing strength of the MgO-CaZrO3 matrix increases with the addition of 2.5-3.5 wt% MgAl2O4 spinel, this being attributed to the strong bond formed between MgO and CaZrO3 due to the MgAl2O4 spinel phase aid. Finally, this refractory matrix exhibits a good thermal stability and excellent chemical resistance against cement raw meal.
A method has been developed to produce mullite from Trinidad porcellanite. Involving high temperature calcination followed by aqueous leaching of kalsilite with caustic soda, the process does not generate solid waste and is projected to yield some 0.65 kg of mullite mixed with silica and hematite per 1 kg of porcellanite. Depending on the concentration of the caustic soda used (in the range 200-500 g/l), the proportion of mullite, silica and hematite range between 49-53%, 42-44% and 5-7% respectively. The mullite agglomerated crystals are of mixed acicular and equiaxed morphology.
The susceptibility to staining of polished porcelain tile has been investigated by various authors for over a decade. However, the literature offers little information about the characteristics the finished product should present in terms of final porosity in order to prevent staining. This paper discusses the results of a comparative analysis of the final porosity of two types of commercial unglazed polished products showing dissimilar staining behaviors. The approach to this theme differs from that of other works reported in the literature in that it clearly identifies the main types of pores responsible for staining under the conditions evaluated here.
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