Refractories: Refel

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 left to right). Sample height 30mm.

Fused 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

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

Reactant AZS test specimen as crucible lid }

Reaction area with exudate

Crucible

Unaffected test specimen

� Fig 2. After vapour test according to ASTM C987, the test crucible with phosphate containing alumina-chromia refractory as reactant (left) and exuded AZS test specimen with chrome discoloration. The difference between the reaction area and the unaffected, ‘dry’ AZS surface is remarkable. Crucible diameter is 35mm.

coordination.

Therefore, the intensity of exudation is correlated to the oxidation level of the fused cast AZS. Insufficiently oxidized low cost fused cast AZS show a substantially higher exudation level in comparison to western style – well oxidised – AZS.

Consequently, effective qualityenhancing measures include the use of selected raw materials with the lowest possible impurities and the implementation of controlled, stoichiometric oxidation with an oxygen lance during AZS manufacture.

Extrinsic reasons

Batch area

The exudation phenomenon is often influenced by batch carry-over. In the batch area, running down glassy phase is significantly present, which is dictated by the interaction with alkali. Batch dust and alkali, present in large quantities, are absorbed by the fused cast AZS. This alkali enrichment results in a decrease of the glassy phase’s viscosity and in higher exudation.

Molten glass volatile components

Changes of the AZS glassy phase composition occur due to the absorption of volatile components in the furnace atmosphere, evaporated – directly or by reaction – with combustion gases such as carbon monoxide (CO) and water (H2O). As a result, sodium oxide (Na2O) and sodium hydroxide (NaOH) are formed during melting in fossil-fuel fired sodalime-silica glass furnaces. Following alkali diffusion into the AZS, the glassy phase viscosity decreases.

Oxy-fuel fired furnaces

For combustion with high water vapour pressures and low gas volume flows, a high concentration of NaOH vapour will be obtained in soda-lime-silica glass furnaces. Observations in oxygen fired furnaces have shown that the NaOH vapour pressures in these furnaces can be up to 3-4 times higher compared to air fired glass furnaces. This results in higher alkali diffusion, coupled with all the aforementioned consequences of AZS glassy phase deterioration.

Phosphate binder

The aforementioned factors are well studied and covered by literature. However, various other refractories can have an influence on exudation or, more precisely, their combination with one another.

Upon investigation, the occasional combination of phosphate bonded refractories with glass defects has emerged. Based on this observation, vapour tests according to ASTM C987 were carried out, with phosphate bonded refractories as the reactant. The test results showed reactions with the sample AZS surface, such as forced glassy phase exudation (Fig 2).

Based on this, it can be concluded that at high temperatures the phosphate bond tends to lose phosphorus through volatilisation. The reactivity of phosphate with AZS glassy phase results in an additional portion of exudation in the glass furnace.

Inorganic bonding agents such as aluminium phosphates and phosphoric acid offer high strength at quite low temperatures and can be used to bridge the area between chemical bonding and ceramic sintering. Therefore, these as additives are often favoured in refractory formulations for alumina-chromia bricks and ramming mixes for superstructurecrown sealing which have to be suitable for high temperatures. However, as described above, the combination of evaporated phosphates with fused cast AZS refractories can apparently lead to unexpected reactions in the furnace.

In the future, this new finding should be taken into account when analysing glass defects and when choosing refractories during the planning of a new glass furnace.

Summary

The exudation from fused cast AZS refractory is correlated to a multitude of parameters. One factor in the occurrence and extent of exudation may be the conditions during the manufacture of AZS. All other influencing factors are related to the service conditions of the glass melting furnace.

The key finding is that, inside the refractory, under standard furnace working conditions, gas forming mechanisms are the main reason for the exudation of the fused cast AZS glassy phase. This natural exudation is an intrinsic behaviour and lasts only a limited amount of time. After the temperature and furnace working conditions have stabilised, the quantity of exuded glassy phase becomes stagnant and behaves normally.

The other kind of fused cast AZS glassy phase exudation has extrinsic reasons, which are related to furnace conditions, such as the maximum temperature the blocks are exposed to, airborne particles of batch, and a furnace atmosphere ripe with corrosive vapours, such as phosphorus.

The minimisation of the exudation phenomenon requires both the selection of high quality AZS refractory material and well-controlled day-to-day furnace operation. �

*Research and Development, Refel, San Vito al Tagliamento (PN), Italy www.refel.com