The main reasons for failure of a crucible lining are their through sintering and cracking, accompanied by the release of metal to the inductor. The weak link in the crucible lining is the slag belt, where the lining is richly saturated with SiO2, CaO, MnO, R2O. The MgO content in working areas is reduced to 20.97%, Fe2O3 reaches 8.2%. The results of chemical and mineralogical analyzes of the lining of the slag belt showed that refractories are very saturated with silicate melts, as a result of which the silicate content in the working area increases by 4 times and the highly refractory forsterite bond is replaced by a non-refractory monticellite. It is known that monticellite begins to corrode chrome spinelide grains at 1400 ° С, and the process intensifies with increasing temperature. When the temperature of the metallurgical process is 1600-1640 ° C with unlimited supply of lining with melts, destruction of aggregate aggregates, as well as individual grains of periclase and grains of chrome shinelide, is observed, resulting in a structure with corroded grains of periclase and chrome spinelide, separated silicate interlayers and sections of silicates instead of detrital structure. Such a structure is less wear-resistant in service and causes a high wear of the slag belt of the crucible due to reflow. As in the lining of the slag belt of arc furnaces, no recrystallization of periclase grains is observed in the slag belt of the crucible.
Image 1.8 – General view of the lining of the induction furnace
The wear of the crucible wall lining below the slag level is much less. The inflow of silicate melts to these areas of the lining is limited, as a result of which the composition and structure of the working zone change dramatically when the transition zone is almost the same as the transition zone with the slag belt. The disintegration of aggregate clusters of periclase is observed here, but at the same time the process of recrystallization of the grains of periclase is underway, as a result of which they increase in size to 0.3 mm (0.05 mm in the slag belt), the amount of secondary spinelide in the working zone increases to 5 -10%, and silicates are 10-15%, and closer to the surface their content decreases. Silicates are represented by monticellite and forsterite.
The furnace hearth operates in somewhat different (compared to the walls) temperature conditions, it is exposed to a longer exposure to the molten metal, i.e. the speed of the processes compared to the top of the crucible should increase. However, the contact of the bottom with the slag is limited (metal discharge, the beginning of melting), therefore the rates of solid-phase reactions should increase mainly. This is confirmed by mineralogical research data. It has been established that in the working zone of the hearth, the number of secondary spinelides of complex composition increases to 20–25%, the silicate content in the working zone does not exceed 5–15%.
The brick working layer in comparison with the monolithic is two times less saturated with SiO2, the content of Al2O3 is less in the working areas, the content of iron oxides (mainly iron oxide) increases to 70%, as the lining from the bottom to the slag belt increases, the smelting components increase. As in a monolithic crucible, in the transition zone of a crucible with a brick working layer, the processes of decomposition of periclase fragments, corrosion of chrome-spinelide grains, and formation of secondary spinelide are observed. In the hotter part of the zone (15-25 mm), a consolidation of the structure is observed due to metal penetration and some recrystallization of periclase grains. Silicates in the zone make up 8-10%. In the working area, the number of silicates is increased to 10-15%, periclase is in the form of individual grains, separated by silicates. The process of recrystallization of periclase grains is noted. Chromospinel fragments are almost completely recrystallized into small octahedral and cubic crystals. The connection between chromospinelide and periclase is often direct. The silicates in the working area are evenly distributed, represented by monticellite and forsterite with normal optical constants.
The increase in the amount of silicates in the lining leads to its wear and sintering. In the main lining, the role of the powdery buffer layer adjacent to the inductor increases. The presence of this layer is especially important when using materials with a high content of MgO, with increased thermal expansion in the temperature range of 1200-1400 ° C and prone to cracking due to shrinkage at a higher temperature, resulting in the passage of metal to the inductor.
Since the most melt-resistant phase is periclase, the recovery of the slag belt of the main crucibles is more expedient to carry out a mass of fused magnesite with a content of 95-96% of fraction mm.
1. Kaibicheva M.N. Lining electric furnaces. M .: Metallurgy, 1975, 280 p.
This article was taken from this resource.