[China Aluminum Network] China has proved that the reserves of high-alumina bauxite is 2.5 billion tons, accounting for 2.4% of the world's total reserves, and it is one of the countries with more reserves in the world. At present, the production of high-alumina bauxite clinker in China is still dominated by calcined natural blocks. Compared to high-alumina bauxite clinker, bauxite homogenization material adopts technologies such as homogenization and purification, and the production process is more energy-saving and environmentally friendly, and at the same time, the stability of product quality can be ensured. These two types of alumina raw materials have their own advantages and disadvantages. The correct understanding of the differences between the two types of alumina materials, the rational use of their respective advantages, and the development of high-quality synthetic materials are the problems to be solved in the study of alumina homogeneous materials. In this regard, the researchers through the in-depth analysis of the performance of the two raw materials and the microstructure of the crystal phase and crystal development to reveal the differences between them, so as to more reasonably understand and apply the alumina homogeneous material.
Comparative experiments provide scientific basis for analysis. There are two kinds of raw materials used in the experiment: one is high-alumina bauxite clinker, which is the clinker obtained after the mined bauxite ore is processed into a shaft kiln after calcined at 1550°C~1600°C. . The other type is alumina soil homogenization material, which is the clinker obtained by processing ore dressing, crushing, grinding, granulation, shaping, drying and calcining (1560°C~1600°C). X-ray fluorescence spectrometer was used to analyze the chemical composition of high-alumina bauxite clinker and bauxite homogenized material. The volume density and apparent porosity were measured by GB/T2997-2000. The phase composition was analyzed by X-ray diffractometer and the results were confirmed by scanning electron microscope. Microstructure analysis was performed using a transmission electron microscope to analyze the transmitted light microstructure.
The observation and analysis confirmed the physical and chemical properties of the alumina material. The appearance of high-alumina bauxite clinker is pale gray and yellowish-white, heavy and hard; the appearance of alumina homogeneous material is uniformly dark grey. From the results of physical and chemical properties, it can be seen that the chemical composition of the two materials is not very different, and the Al2O3 mass fraction is more than 80%. According to the ratio of Al2O3 content and SiO2 content, the two phases of the raw materials are theoretically corundum. Phase and mullite phase; the difference is that the density of alumina homogeneous material is greater than that of high alumina alumina clinker.
Phase analysis. From the XRD patterns of the high-alumina bauxite clinker and the alumina homogenized material, it can be seen that the angular positions of the diffraction peaks of the two alumina materials are basically the same, and the main crystal phases are corundum phases, and the sub-crystalline phases are all Stone phase, but the peak intensity of the same diffraction peak angle is slightly different.
Microstructure analysis. The researchers first observed the microstructures of the two raw materials at different magnifications. Through scanning electron microscope photographs of the two alumina materials at low magnification, it was found that the crystal distribution of the high-alumina bauxite clinker was uneven, and directional burning was also observed. The cracks were formed; however, the grain size and distribution of the alumina homogeneous material were relatively uniform, and small and uniform closed pores were also observed. In comparison with low magnification, the grain size and pore size of the alumina homogeneous material are larger than that of the high alumina alumina clinker, and the porosity of the high alumina alumina clinker cannot be clearly seen at this multiple.
Through the further magnification (800 times) observation of the scanning electron micrographs of the two alumina materials, it can be seen that the bauxite clinker is heterogeneous: the upper part of the figure is denser and the grains are smaller; The dark areas have more pores and mainly open pores with larger grains. EDS analysis of each part of the enlarged photo shows that most of the locations are corundum phases, indicating that this area is the enrichment area of ​​the corundum phase; the white bright spot area is calcium titanate and other substances. Through further observation, it can be seen that the structure of the alumina homogeneous material is relatively dense and the crystal distribution is relatively uniform. The pores contained in the crystal are mainly closed pores, which reduces the water absorption of the material. The glassy phase (white part) containing Ti and Fe in the alumina homogenizing material is obviously more than the high alumina bauxite clinker, and is evenly distributed between the grains and the grains. As the glass phase promotes sintering at high temperatures, the grains of the alumina homogeneous material grow significantly larger and the structure is denser. From the EDS analysis, the composition of the glass phase is mainly TiO2, Fe2O3, CaO, MgO, KO2, and the like.
In view of the causes of the occurrence of the bauxite deposits in China and the geological conditions of the deposits, the components of the ore are often different in the same ore and in the same section. At 400 °C ~ 1200 °C, bauxite and kaolinite in the dehydration reaction occurred in succession, boehmite dehydration to form corundum pseudo-phase, kaolinite decomposed into mullite and free SiO2. At 1200°C, secondary mullite begins to form from the corundum pseudo-phase formed by dehydration of the boehmite and the free SiO2 decomposed from kaolinite. Observing the high-alumina bauxite clinker at higher multiples (2000 times), it was found that the corundum phase concentrates in the place where the diaspore is enriched, and the mullite phase concentrates in the place where kaolinite is enriched, or corundum and mullite. Closely intertwined. This kind of structure can support the material to withstand the applied load and high temperature, and the content of glass phase is less, so its high temperature performance is superior. When the alumina homogeneous material is observed at the same magnification, it is impossible to clearly distinguish the individual phases and the interlaced areas, but the entire microstructure becomes uniform and uniform, and the glass phase content is high. .
Due to the presence of more homogeneous materials (silicate glass phase) in the alumina homogeneous material, the structural details cannot be clearly seen under orthogonal polarization. Under transmission electron microscopy, the transmitted light photographs were taken with single polarized light, and the inhomogeneity of the high-alumina bauxite clinker can be seen: the size and distribution of the corundum crystals are not uniform, and the content of the glass phase is small. The uniformity of the alumina homogenized material can also be seen from the transmitted light photos: the size and distribution of the corundum crystals are relatively uniform, and the glass phase content is more and evenly distributed.
Subsequently, the researchers also observed the difference in the microstructure of the two raw materials from the perspective of the glass phase through the observation of the TEM images (single polarized light, 400 times) of the two alumina materials. Taking the distribution of impurity component TiO2 in the two alumina materials as an example to illustrate. The distribution of TiO2 in the high-alumina bauxite clinker also showed regional enrichment in the same way as corundum. At high temperature, the enriched TiO2 reacts with Al2O3 to form aluminum titanate. Only a small amount of dispersed TiO2 will enter the corundum lattice to form a very small amount of glass phase. When subjected to high temperature and external stress, the dense corundum phase The support function shows good high temperature performance, such as anti-erosion, erosion resistance, high load softening temperature, and small creep. However, in the alumina homogenized material, the distribution of TiO2 is relatively uniform, and it is easier to enter into the corundum lattice to form a large number of glass phases at high temperature, distributed among the corundum grains, so that the intercrystalline bond strength is weakened, and it easily occurs at high temperatures. Slip, which affects its high temperature performance.
Application Analysis. If the alumina homogenizing material is used as a raw material in the form of granules, or Si3N4, SiC or other non-oxide refractory materials are added in more isolated phases, the effect of the high temperature performance of the glass relative to the system can be reduced, thereby increasing the alumina content. The use of chemical materials. According to this idea, the researchers studied the application of high-alumina bauxite clinker and bauxite homogenizer in magnesium-aluminum-carbon materials.
The researchers used high-alumina bauxite clinker and bauxite homogenized material as bauxite aggregates, and made two groups of magnesia-alumina carbon bricks according to the above-mentioned thinking, and compared the difference in normal temperature and high temperature strength of the two groups of bricks. Through observation and study, it was found that the high temperature performance of magnesia-alumina carbon brick made from alumina homogeneous material was not weakened, and its high temperature flexural strength was higher than that of brick made of high bauxite clinker. This confirms the use value and prospect of homogenized materials in bauxite and non-oxide composites.
In summary, the density of high-alumina bauxite clinker is not uniform, the bulk density is low (3.32g/cm3), the porosity is high (4.19%), and mostly open pores with high water absorption. However, in the densely packed corundum phase or where the corundum phase and the mullite phase are closely intertwined, they can withstand the applied loads and high temperatures, and the glass phase content is small, so the high temperature performance is superior.
The alumina homogeneous material has a dense and uniform structure with a high bulk density (3.42g/cm3) and a low porosity (0.84%), mostly small and uniform closed pores with low water absorption. Due to the homogenization process, the composition and structure of the homogenized material are more uniform, but a large number of evenly distributed glass phases are filled between the crystal grains, which weakens the intercrystalline bonding force and weakens the high temperature performance of the homogenized material to some extent. However, the use of alumina as a homogeneous material in the form of granules, or addition of Si3N4, SiC or other non-oxide refractories in more isolated phases, can reduce the effect of glass on the high temperature performance of the system and increase its use value. . Therefore, the alumina homogeneous material is more suitable for the production of alumina and non-oxide composites.
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