| Abundant vanadium-titanium bearing magnetite deposits around the Panzhihua-Xichang area in the southwestern part of China. At present, smelting the iron concentrate ores in blast furnace is the main process to use the resources at Panzhihua Iron and Steel Corporation. This process generates vanadium-bearing hot metal and high titanium-bearing blast furnace slag(Ti O2=20-30 mass %) in which 54 % titanium of vanadium-titanium bearing magnetite ores is present. Because of the difficulties of using this slag, more than 60 million tons of slag has been stacked in the air from the beginning of BF smelting, which not only brings environmental pollution but also wastes resources. How to extract the second titanium resources and use waste slag is a serious problem in China.The perovskite crystallizes readily and contains more than 50% Ti of the slag. So the green extractive process of titanium component such as perovskite-based Ti extraction process shows a bright prospect to extract the valuable titanium and reduce the environmental pollution caused by the stacking of slag. As a result, the study of the crystallization behavior, law and mechanism of perovskite has great value which is the main content of the present work.The crystallization behavior, law and mechanism of Ti-bearing slag were investigated by the theoretical thermodynamic calculation and in situ observation at high temperature. The isothermal phase composition of different titanium-bearing slags were calculated and predicted by the FACTSage software. The crystallization processes of different slags were observed in situ by using a confocal scanning laser microscope. The effect of heat treatment condition, cooling rate, slag composition, basicity and Ti O2 content on the crystallization of complex slag systems was investigated. Furthermore, the crystalline mechanism and crystal structure were analyzed by using the solidification principle and crystal structure theory to provide support on the crystallization of Ti-bearing phases in Ti-bearing BF slag.Theoretically, during continuous cooling, four phases were predicted to form: perovskite, titania-spinel, clinopyroxene and rutile in the 28.8Ca O-8Mg O-26.2Si O2--23 Ti O2-14Al2O3 slag. Their crystallization temperature were 1436℃、1325℃、1216 ℃ and 1122 ℃ respectively。During continuous cooling, the growth of perovskite in slag proceeded via the successive production of quasi-particles along straight lines, which further extended in certain directions. The morphology and structure of perovskite was found to vary as a function of the cooling rate. At the cooling rates of 10 and 30 K/min, the dendritic arms of perovskite crossed obliquely, while the arms were orthogonal at a cooling rate of 20 K/min, and hexagonal at cooling rates of 40 and 50 K/min. The three crystal morphologies thus obtained under different cooling rates respectively corresponded to the orthorhombic, cubic, and hexagonal crystal structures of perovskite. The observed change in the structure of perovskite could probably be attributed to the deficiency of O2-, when Ti2O3 was involved in the formation of perovskite.Under the condition of supercooling, the supercooling degree had obvious effect on the growth rate of perovskite. As the supercooling degree increasing, crystals grew faster and bigger. The columnar dendrite arrays of perovskite in liquid slag were clearly observed on line.Basicity had big influence on the crystallization temperature, Ti-bearing phases, crystalline volume, and crystallization behavior and crystal morphology. In theory, as the basicity increased from 0.5 to 1.3, the crystallization temperature of slag and perovskite increased, but the crystallization temperature of rutile kept stable. The increase of basicity made the rich titanium phase changed from rutile to perovskite. Basicity influenced the path of enrichment of titanium in slag. When the slag with basicity of 0.6 was cooled at 1100℃, Ti O2 was gathered in loveringite(Ca Ti21O38) which was tiny needle-like crystal and not easy to separate. In the slag with basicity of 0.8, Ti O2 was gathered in titanite(Ca Ti Si O5) which was irregular thin stripes crystal and was mutualistic with other phases.TiO2 content also had big influence on the crystallization temperature, Ti-bearing phases, crystalline volume, crystallization behavior and crystal morphology. With the Ti O2 content increasing, the crystallization temperature of slag increased first and then decreased. The amount of perovskite increased first and then decreased, and the amount of rutile kept increasing. When the slag Ca O-Si O2-Ti O2 was cooled at 1100℃, the crystallization temperature of slag decreased with the increasing Ti O2 content, and the primary phase changed from calcium silicate(Ca Si O3) to perovskite(Ca Ti O3).The phases formed by calcium and silicon changed from calcium silicate(Ca Si O3) with fine lamella shape to titanite(Ca3.01Si2.06O7.28) with flat bulk shape. When the Ti O2 content increased from 0 to 30% in pentabasic slag, the primary phase changed from melilite(Ca2(Al,Mg)[(Si,Al)Si O7]) to perovskite. The crystalline phase was melilite with flat tetragonal structure in the slag with 0% and 10% Ti O2. The crystalline phases were square clinopyroxene crystal(Ca Mg Si2O6) and dendritic perovskite in the slag with 20% Ti O2. The slag with 30% Ti O2 only precipitated perovskite.The three dimensional dendrite structure of perovskite were obtained. Furthermore, the growth mechanism of perovskite in melt was analyzed with the solidification theory. The initial slag could be considered as a binary component system that consisted of Ca Ti O3 and all the other species. The formation of perovskite required the diffusion of Ca O and Ti O2 to the solid/liquid interface and the rejection of the other species from the interface. The binary system phase diagram and schematic description of temperature field and concentration field at interface at the equilibrium were made. |
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