Refractories
An update on fused cast AZS exudation Dr Roland Heidrich* discusses how, under standard furnace working conditions, gas forming mechanisms are the main reason for the exudation of the fused cast AZS glassy phase.
� Fig 1. Fused cast AZS refractory samples after exudation testing. The three samples show each 1.2%, 2.4%, and 5% (volume percent) glass phase exudation (from
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used cast alumina-zirconia-silica (AZS) continues to be the most popular choice of refractory material for glass melting furnaces. However, AZS refractories exhibit a phenomenon called ‘exudation’, which is the migration of the in-situ glassy phase onto the refractory surface (Fig 1). All fused cast AZS refractories exhibit this exudation phenomenon, the intensity of which depends on various factors in both the AZS’ quality and the parameters of the subsequent glass melting process. Exudation can either be a natural and temporary behaviour during the initial heating-up of the furnace, or it can be a long-lasting and pathologic property related to furnace conditions. Exudation can be a source of concern for the glass manufacturer because the expelled viscous phase is capable of rundown over the tuckstones, eventually making its way into the glass melt, potentially causing glassy inhomogeneity – resulting in striae, cords, or knots. This is because the chemistry of the exudate is different to the chemistry of the glass being melted in the furnace.
Intrinsic causes of exudation Zirconia transformation
Upon heating, a reversible phase transition from monoclinic to tetragonal zirconia takes place between 900°C and 1200°C. For fused cast AZS with 33 weight percent zirconia, this will result in a volume shrinkage of approximately 1.3%. Upon subsequent cooling, this transition between crystalline structures results in a thermal induced expansion hysteresis loop. Due to this repeated expansion and shrinkage a pumping action occurs, expelling the glassy phase from the AZS. Chemical composition of the glassy phase The chemical composition of the aluminosilicate glassy phase contributes significantly to its viscosity and to the exudation it causes. To suppress the formation of mullite, the dosage of sodium oxide is crucial. However, the viscosity depends strictly on the network modifier content. Therefore, accurate and real-time control of the chemical composition is essential to ensure strict compliance with the raw material recipes. With a careful balanced alkali content, exudation only starts occurring at over 1500°C and with only minor glassy phase rundown.
Oxidation of reduced species Fused cast refractories are molten in an electric arc furnace, working with graphite electrodes. The chemical effect of the carbon monoxide produced in the electric arc generates a reducing atmosphere during the fusion process. During its first heat-up, the glass furnace is exposed to ambient atmosphere, resulting in changes of oxidation state of reduced species inside the fused cast AZS. This reaction is responsible for pushing the melt phase out of the AZS. The amount and type of reduced species present in the glassy phase are crucial for the consequent exudation behaviour. The oxidation of suboxide, nitride and carbide contamination is responsible for a single exudation event. Meanwhile, the presence of polyvalent oxides – typical raw material impurities like iron and titanium – can create a multiple exudation events. There is also a second effect here: under reducing conditions, Al3+ and Fe3+ are partly present in sixfold coordination, thus decreasing the viscosity and causing the segregation of the melt phase to also become easier. In an oxidised glassy phase, both ions usually occur in tetrahedral Continued>>
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left to right). Sample height 30mm.
41 Glass International February 2022
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