Refractories and Glass Defects

Glass defects such as bubbles, stones, knots and cords originate from many sources, including poor melting and/or refining of glass, as well as refractory quality and degradation. The published literature often cites alumina-zirconia-silica (AZS) refractory exudation as the leading cause of knot and cord defects. However, one study of the origin of knot defects in TV glass showed that AZS refractory exudation is only a short-term source of glass defects, while AZS refractory corrosion, especially in the superstructure, is a more potent and long-term source of defects.(1) Very little information exists in published literature on defect chemistry and frequency as a function of furnace age and AZS reuse, so researchers at Vesuvius Monofrax, Inc. attempted to correlate glass defect chemistry with AZS refractories of varying ages. They analyzed post-campaign AZS refractory blocks from three types of glass melting furnaces: a soda-lime container furnace, a soda-lime tubing furnace and a lead silicate TV funnel furnace. The results of this study are providing a better understanding of the link between AZS refractory corrosion and glass defects. Soda-Lime Container Furnace, 10-Year Campaign Figure 1. A 34% ZrO2 AZS superstructure (left and inset) and glass contact block (far right) after a 10-year campaign in a soda-lime container furnace. Figure 1 shows an AZS superstructure (left and inset) and a glass contact refractory block (right) taken from a soda-lime container furnace following a 10-year campaign. While the glass contact block reveals rounded edges from corrosion and a shiny surface due to glass adhering to the refractory, the superstructure refractory appears to be dry on the surface, has relatively sharp edges and shows a whitish crust on the entire exposed hot face. The glass contact block appears to have come from below the metal line. The holes in the blocks are from drilling core samples for characterization. Polished sections for microscopy were prepared from the core samples. The chemistry and microstructure were analyzed as a function of depth using SEM/EDS techniques. In addition, physical properties such as the bulk density and the apparent porosity were also measured as a function of depth. Figure 2. The matrix phase chemistry as a function of depth for both the glass contact and superstructure AZS refractory samples from the soda-lime container furnace. The photomicrographs show a corrosion depth of ~150 mm in the superstructure block compared to ~50 mm at the glass contact. The lower portion of Figure 2 shows SEM/BSE photomicrographs of the glass contact interface and superstructure hot face. The microstructure of both types of samples shows a near absence of the crystalline alumina phase. Both the superstructure and glass contact AZS samples were found to contain a nephelitic zone where the zirconia is no longer in solution in the glassy phase; instead, it exists as discrete zirconia crystals. This nephelitic zone extends to greater depth in the superstructure than in the glass contact interface. The chart on the upper portion of Figure 2 shows the matrix phase chemistry as a function of depth for both the glass contact and superstructure AZS refractory samples. Both samples exhibit similar changes in chemistry-i.e., an increase in the concentration of the alkali/alkaline earth species, an increase in the alumina concentration and a decrease in the concentration of silica. However, the superstructure refractory sample shows a greater depth of chemical change than the glass contact sample. An example of this is the depth of the nephelitic zone, mentioned above, which was found to be greater in the superstructure sample than in the glass contact sample. The results shown in Figure 2 suggest that the glass contact refractory corrosion lessens with time, while the superstructure corrosion continues over the life of the furnace campaign. To read the full article please [url="http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,152380,00.html"] click here [/url] By understanding how glass defects occur, manufacturers can take the appropriate steps to minimize these defects and improve glass quality. For more information about glass furnace refractories, contact Vesuvius Monofrax, Inc., 1870 New York Ave., Falconer, NY 14733-1797; (716) 483-7200; fax (716) 661-9296; e-mail amul_gupta@us.vesuvius.com; or visit http://www.monofrax.com.