The evolution of the microstructure of silica bricks depends on two factors, temperature and melting operations. The longitudinal temperature gradient from the working surface to the cooling surface is roughly: 1600~300°C (without insulation) or 1600~1300°C (with insulation). The substantive impact is very small, generally speaking, it only affects the length and position of each section. If the firing temperature of the original silica brick is 1400°C, the main part that affects the change of the microstructure is the combination of phases in the band corresponding to the temperature gradient of 1400~1600°C. This temperature region includes the boundary of the stable phases of tridymite and cristobalite at 1470°C. Therefore, the silica bricks used in the dome can, in principle, be divided into 4 zones of cristobalite, tridymite, silicate-enriched zone and unchanged zone according to the degree of erosion.
From the analysis results in the above table, it can be seen that the SiO2 content of the original brick is about 94%, and the cristobalite zone is as high as 97.67%, an increase of nearly 3.7%; while the silicate enriched zone is only 90.53%, a decrease of about 3.5%. A satisfactory explanation can be obtained from the relative change of the total impurity content: this is that the impurities form a liquid phase and then transfer to the cold end, and enrich in the temperature region suitable for its crystallization.
Corresponding to the migration of chemical components, the microstructural changes of segment bands have the following regularity of phase transition.
The appearance of the cristobalite belt is white, because the low melting point liquid phase formed by the impurities in the original brick moves to the cold end of the brick, which relatively increases the SiO2 content of the belt, so the so-called “purification effect” has occurred. “Purification zone” cristobalite is a granular polycrystal cemented with glass phase, with a size of about 400-600 μm. The scanning electron microscope image shows a tetragonal double-cone aggregate morphology (Figure 1), and the crystals are often arranged in an orderly manner, indicating the orientation of the crystals. With the help of EDAX analysis, it is known that cristobalite single crystal has almost no solid solution of impurities, but a little glass phase can be seen between the polycrystals, and its composition range is: Na2O4%~7%, Al2O31%~2%, Fe2O33%~4% , SiO263%~65%, CaO18%~26%.
a. The appearance of the tridymite recrystallization zone is light yellow, the chemical composition is similar to that of the original brick, and the part close to the cristobalite zone has less impurities. Tridymite is columnar and spear-shaped twin crystals, and the crystal size can reach 1000-2000 μm (Figure 2). This single crystal that has grown up under high temperature for a long time (5 to 6 years) is analyzed by EDAX, except for containing a trace amount of Na+, no other elements can be detected. The above two zones contain only one stable crystalline phase that is compatible with the temperature of the segment zone, and the phase transitions in each zone basically reach equilibrium. However, at the boundary between the two zones, which is theoretically equivalent to around 1470°C, there are two phases coexisting. Here we can observe the process of tridymite gradually transforming into cristobalite, which is manifested by the splitting of tridymite crystal planes into agglomerated cristobalite. Since the phase transition is carried out along a certain crystallographic orientation of tridymite, these agglomerated cristobalites have definite optical orientations. Macroscopically, the boundary between the two zones is neat and clear. Measured on the microscopic scale, the two-phase coexistence region is still a very narrow and movable phase transition zone.
b. The liquid phase formed by cristobalite and tridymite recrystallization zone of silicate enrichment zone, containing various impurities, transfers to the cold end through the intercrystalline capillary channel, and enriches in the temperature region suitable for its crystallization, Silicate-rich bands formed, brown in appearance. The depth of liquid phase migration depends on the composition and its freezing point at that time. The common silicate crystal phase is pseudo wollastonite or ferropyroxene. The brick with internal phase has basically reached an equilibrium state. The quartz and metastable cristobalite in the original brick are all transformed into tridymite. The crystal size is different, and the size can grow from 100 μm to 2000 μm.
c. Unaltered zone This zone basically maintains the microstructure of the original brick. Since the roof of a glass melting furnace can last for several years, this prolonged heating provides sufficient conditions for phase transformation in the refractory material. In the continuous high-temperature operation of silica bricks, only the cristobalite belt is directly subjected to thermal load and chemical erosion. There are very few impurities in this zone and the viscosity of the glass phase is high, so its corrosion resistance is improved compared with the original brick, which is the basic reason why silica bricks are well suitable for use in glass melting furnaces.
In addition to the main phase, cristobalite and tridymite have almost no other crystalline phases, and only some glass phases exist between crystals, and their chemical composition tends to change gradually: the surface and inner layers of each zone, in terms of Na2O, CaO, etc. There are obvious differences; the main phase of the silicate enriched zone is pseudo wollastonite and its solid solution containing Fe2O3.
The mineralizers added in the production of silica bricks are basically calcium oxide (calcium hydroxide) and iron oxide-based raw materials. Due to uneven distribution and incomplete reaction, silicate (mostly calcium iron silicate) and glass phenomenon of coexistence. The glass phase can be separated, as listed in Table 3. In contrast, it can be seen that the glass phases of cristobalite and tridymite contain high SiO2; while the impurities are all reduced except for the enrichment of R2O, which is compatible with the working environment of alkali-rich steam.
