Features of vertical cellular corrosion of refractories by molten sodium-calcium silicate glass

Abstract

which is widely used in glass melting furnaces for the lining of the neck and other points of increased wear. The test temperature was 1475~ and the hold time of the specimens at the final temperature 72 h. To preserve the layer of glass in contact after completion of the test the refractories were removed from the molten glass and cooled rapidly. After the tests the specimens had acquired a typical wear profile of refractories in glass melting furnaces characterized by increased attack at the level of the surface of the molten glass with the formation of a meniscus. On the surface of the mullite and Bacor refractories facing downward there were noted signs of vertical cellular corrosion, cavities having the forms of cones at tips of which there are gas bubbles with a diameter from 1.0 to 2.5 mm. The interaction of the refractories with the molten glass occurs according to the traditional plan with the formation of sharply expressed zonality. Three zones are recorded in the specimens, the least changed, the transition, and the contact, which differ in thickness and structure at the level of the surface of the molten glass, the side faces, and the horizontal surface of the refractory facing downward. Mullite Refractory. After glass resistance testing a meniscus with a depth of 2-3 mm was formed on the side surfaces of the specimen. On the lower surface there are noted cavities filled with glass and containing gas bubbles penetrating 3-5 mm into the depth of the refractory. In mineral composition and structure the least changed and transition zones differ insignificantly. The main portion of the refractory is a microcrystalline substance in which very fine needles of mullite may be distinguished, The transition zone is distinguished by a higher degree of mullitization and an increase in the number of voids. The white contact zone consists of crystals of mullite and glassy phase. Under the microscope two subzones may be distinguished in it, the first, adjoining the transition zone~ in which crystals of mullite of irregular form up to 0.01 mm in size are noted in the glassy phase with a refractive index of 1.540 and the second, adjoining the glass in contact, in which there occurs recrystallizatio n the mullite with the formation of crystals of elongated prismatic form up to 0.05-0.15 mm in size uniformly distributed in the glassy phase with a refractive index of 1.530. In the second subzone there are newly formed crystals, B-alumina in small quantity and also nepheline and corundum identified by x-ray diffraction analysis. In the contact zone of the mullite refractory there was an increase in the content of Na, Ca, and Mg oxides with some decrease in A1 oxide (Table I). These changes are determined by diffusion of the alkaline and alkaline-earth elements from the molten glass into the depth of the specimen, which leads to the formation of reaction minerals in the contact zone of the refractory.