Magnesia-aluminium spinel
Magnesia-aluminium spinel is one of the main raw materials for spinel-based refractories. Magnesia-aluminium spinel can be divided into two main categories, namely, sintered spinel and electrofused spinel.
01 Chemical Composition and Density of Spinel
Because spinel exists over a wide solid-solution range and can coexist with magnesite and corundum, spinel or spinel-based composites exhibit different densities, crystallite sizes, and sinterability depending on their chemical compositions.
Table 1 shows the physical properties and mineral compositions of sintered spinels prepared from Al₂O₃ and MgO as raw materials with different MgO/Al₂O₃ ratios, after heat treatment at 1900 °C for 30 minutes. These spinels were finely crushed, molded, and sintered at 1800 °C for 30 minutes to determine their bulk density and porosity, and the results are presented in Fig. 1. Magnesium-rich spinel exhibits lower porosity and higher bulk density, which can be attributed to a decrease in the rate of grain boundary migration.
02 Solid Solution and Segregation of Spinel
Spinel solid solutions, which include regions coexisting with magnesite or corundum, exist over a wide compositional range. The lattice constant of the solid solution is inversely proportional to the Al₂O₃ content, as shown in Figure 2a. Spinel can crystallize at temperatures below the synthesis temperature, and Al₂O₃ may precipitate from the solid solution as corundum under two-phase equilibrium conditions.
The Al₂O₃ content of spinel can be easily calculated from the lattice constant of its solid solution. Spinel belongs to the cubic crystal system, and the lattice constant of this system can be determined from the following formula based on X-ray diffraction data:
For magnesium–aluminium spinel, the strongest diffraction peak corresponds to the (311) plane, for which h2+k2+l2=11. The value of dhkl can be obtained from X-ray diffraction data, from which the lattice constant a can be calculated. The relationship between the Al₂O₃ content of magnesium–aluminium spinel and its lattice constant is shown in Fig. 2b; therefore, the Al₂O₃ content can be determined when the lattice constant is known.
The segregation of spinel solid solutions can also be reflected in changes in the thermal expansion coefficient of spinel after repeated heating. As shown in Table 1, spinel samples were heated to 1500 °C and then cooled, a process repeated three times. In the MgO-rich region, the change in the thermal expansion coefficient of spinel is very small. However, in Al₂O₃-rich spinel, especially samples 80 and 90, the thermal expansion coefficient increases due to the precipitation of corundum. This phenomenon should be taken into account when determining the composition ratio for synthetic spinel.
03 Mineral Composition and Microstructural Characteristics
Synthetic magnesium–aluminium spinel has spinel as the main crystalline phase. Depending on the composition, magnesium-rich spinel may also contain magnesite, while aluminium-rich spinel may contain corundum. Figure 3 shows the X-ray diffraction line profiles of three types of high-purity sintered spinel with Al₂O₃ contents of 66%, 76%, and 90%, respectively. Small amounts of β-Al₂O₃ (visible in the X-ray diffraction peaks) were detected in sintered spinel with 94% Al₂O₃ and in electrofused spinel with 94% Al₂O₃. β-Al₂O₃ in solid solution with spinel can cause volumetric swelling, which may deteriorate the sintering process. However, the presence of a very small amount of β-Al₂O₃ does not have a significant effect on the slagging resistance of spinel.
Figure 4 shows SEM images of the three spinels described above. The grain boundaries of Mg-rich or Al-rich spinel, which usually coexist with magnesite or corundum, are less distinct, whereas in specimens with compositions close to stoichiometric spinel, the grain boundaries associated with grain growth are difficult to discern. The crystallite size of spinel in sintered spinel is around 20 μm, with Mg-rich spinel exhibiting smaller crystallites than Al-rich spinel. Electron microprobe analysis indicates that regions of free MgO are present within the spinel matrix, and impurities such as CaO are also observed, separated from the bonded spinel crystals.
04 Thermal Expansion of Spinel
Figure 5 shows the thermal expansion curves of sintered and electrofused spinel with different Al₂O₃ contents. The expansion rate of electrofused spinel increases with increasing Al₂O₃ content, reaching a maximum around 1300 °C, and then decreases at higher temperatures. Sintered spinel exhibits similar behavior, but the decrease in expansion rate above 1300 °C is larger than that of electrofused spinel. In general, for both fused and sintered spinel, the lower the MgO content, the smaller the decrease in expansion rate after reaching the maximum.
05 Thermal Shock Resistance of Spinel
Good thermal shock stability is a major characteristic of spinel refractories. Based on the thermal expansion behavior of spinel, it can be seen that spinel reaches its maximum expansion rate at around 1300 °C. The measurement of the damage strength of spinel particles before and after heating provides a way to evaluate their resistance to thermal shock. Table 2 presents the destructive strength of several types of spinel, compared with fused alumina and sintered alumina. In general, the single-grain damage strength of sintered spinel before and after thermal shock is higher than that of fused spinel. This difference is related to their microstructural characteristics, with the fused product being glassy and the sintered product being dense.
06 Slag Resistance of Spinel
Corundum–spinel castables made from commercially available spinel with different Al₂O₃ contents were tested in a gas-fired rotary slag test furnace (synthetic slag composition: CaO 48%, SiO₂ 12%, Fe₂O₃ 16%, Al₂O₃ 13%, MnO 6%, MgO 5%), and the results are shown in Figure 6. With increasing Al₂O₃ content (calculated from lattice constants) in spinel, the slag erosion resistance of the castables increases, while slag penetration resistance decreases. As shown in Figure 6, the maximum difference in slag erosion among castables made from spinels with different Al₂O₃ contents is about 3.5%, and the maximum difference in slag penetration is about 2.1%. Results from the high-frequency slag test furnace indicate that castables made from spinels with 70%, 90%, and 95% Al₂O₃ show very little difference in slag resistance. The study also showed that the wear resistance of castables made from spinel containing 89–90% Al₂O₃ and 94% Al₂O₃ is very similar. Spinel with 85% Al₂O₃ content is considered the most suitable raw material for the manufacture of castables.
07 Hydration of Magnesium–Aluminium Spinel
Magnesia–aluminium spinel sand can hydrate upon contact with water vapor. In the coexistence region of spinel and magnesite, a high MgO/Al₂O₃ ratio leads to an increased amount of magnesite, which accelerates the hydration reaction and can cause the particles to collapse. Therefore, during production and storage, magnesium–aluminium spinel must be kept in a dry warehouse and protected from moisture during transport.
Methods used to prevent the hydration of alkaline refractory raw materials can also be applied to magnesium–aluminium spinel. According to Japanese research, the addition of 0.1–0.3% organosilicon compounds can limit the weight gain of magnesium-rich spinel sand to approximately 0.1%, whereas without additives, the weight gain can reach 1.8%.