Refractories for Glass Making
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Introduction Glass melting has changed very little in gênerai principles since the earliest times, still being produced in fireclay pots or crucibles—even up to the présent day. In Europe, experiments to melt glasses in tank furnaces began about 1700 A.D.,1 but this became an important form of glass manufacture after Siemens introduced the regenerative furnace in 1870.2 This design was the basis for the development of modem furnaces and there is still a considérable similarity to the original. Until the late 1920s the glass contact refractories used in tank furnaces were based on fireclay or sandstone3,4 blocks. About this time important changes began when sillimanite5 and fusion-cast mullite refractories became available.6 However, because of the higher cost of fusion-cast refractories the introduction of thèse materials was delayed and they did not corne into gênerai use for lining the glass melting tank until the late 1940s. The high performance of tank furnaces today is related to a number of factors such as improved furnace design and régénération, but the most significant has been an improved melting rate brought about by the use of higher températures. This has only been achievable as a resuit of the improved quality of fusion-cast and other refractory materials, such as those used in the furnace superstructure and regenerators. Garstang7 showed that there has been a steady increase in melting températures in the container glass industry. In data going back to 1920, there has been an increase from about 1300°C to some 1590°C. Bondarev 8 showed that the increase in production achieved by using higher températures reduces the spécifie consumption of fuel. According to his data from flat glass tank furnaces, productivity increases 4% for every 10CC
MRS BULLETIN/NOVEMBER1989
increase in melting température. This resuit would seem to suggest that melting températures would continue to rise. However, in laboratory tests Botvinkin9 suggests that températures above 1600°C do not cause a commensurate increase in melting rate. Further, the need for fuel efficiency and the requirement to limit the development of NOx chimney émissions are also factors that tend to reduce further increases in melting température. Since 1970 the top melting température of container tank furnaces has stabilized at about 1580-1590°C, but there has still been a steady increase in melting efficiency. The use of better quality refractories accompanied by higher levels of insulation has been a significant factor in this improvement (see Table I). Tank furnaces hâve been fueled by coal, producer gas, oil, electricity and natural gas. The choice of fuel has largely been decided by économies, and consequently progress in the introduction of electric melting has been slow. Technically electric melting can hâve many advantages — the heat is dissipated directly into the melt, volatilization of the batch chemicals can be considerably reduced, and chimney émission problems eliminated. However, glass manufacture is a very compétitive business and the
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