Al2O3-SiO2 stones in glass: a study of dissolution processes

Solid inclusions called "stones" are quite a common problem for the hollow glass industry. Their presence is particularly dangerous when these defects create stresses in glassware, thus degrading its mechanical performance. The possible origins of stones can be summarized as follows: contamination from cullet and/or raw materials or refractories of the glass furnace. The analysis of stones allows their nature to be identified, to understand how and why they form and consequently, to take corrective action to avoid or reduce their formation. The microstructural and physico-chemical complexities of industrial Al2O3-rich stones were investigated by SEM-EDS. The partial transformation of the original material, recrystallisation, secondary and vitreous phases were characterised. The different behaviours of Na+ and K+ ions in the dissolution mechanism and in the formation of the secondary and vitreous phases originating from the glass interaction is emphasised. In order to understand the diffusion and dissolution processes and to assist their modelling, laboratory tests were performed with appropriate times and temperatures to reproduce the stones experimentally. Fragments of Al2O3-rich ceramic and refractory materials were immersed in melts containing the same molar amount of alkali but with different Na2O to K2O ratios. The experimental results enabled identification of different roles for K+ and Na+ ions in the dissolution process; the special role of even small amounts of potassium is emphasised.