Physical Chemistry Study On Recovery Of Valuable Elements In Ti-bearing Electric Arc Furnace Molten Slag

Ti-bearing electric arc furnace molten slag (Ti-bearing EAF slag) which contains about47-51wt.%TiO2is obtained from vanadium-titanium magnetite concentrate through direct reduction reaction by rotary hearth furnace and smelting separation by electric furnace. Due to the high grade of TiO2and less impurities, comprehensive utilization of Ti-bearing EAF slag can be realized. At present, Ti-bearing EAF slag is mainly used for synthesis of TiO2pigment white by using traditional sulfate method, which not only produces low value products but also results in serious secondary pollution. In this thesis, a novel process was proposed based on alkali fusion-water leaching-acidolysis to prepare nano-structured TiO2photocatalyst and NaA zeolite. The recovery of valuable elements from Ti-bearing EAF slag was achieved with both environment and economic benefits.During the alkali fusion process, Ti-bearing EAF slag was treated by molten NaOH, during which the transformation process and mechanism of Ti-bearing phases and MgAl2O4phase were investigated systematically. In addition, the kinetics of the main phases transformation in the alkali fusion process were studied and the apparent activation energy of each phase was caculated. The results showed that the Ti-bearing phases in Ti-bearing EAF slag such as anosovite solid solution and Mg2TiO4could be easily converted to Na2TiO3with NaCl-type crystal structure, whilst the main impurity phase such as MgAl2O4was apt to be decomposed by molten NaOH to form NaAlO2and MgO. With increasing of roasting temperature and time, as well as decreasing of Ti-bearing EAF slag/NaOH mass ratio (Rslag/NaOH), the formed Na2TiO3could be partly changed to NaM02(M=Mg, Ti, Fe) with a-NaFeO2-type crystal structure, due to the coexisting metal ions like Mg2+and Fe3+in the slag be doped into or even substituted the Ti atoms of Na2TiO3to form NaMO2(M=Ti, Fe, Mg). Additionally, the decomposition kenitics of Ti-bearing phases and MgAl2O4phase by molten NaOH can be illustrated by the shrinking core model with chemical reaction controlled process, whose apperent activation energies were41.76kJ/mol and55.88kJ/mol, respectively.During the acidolysis process, the effect of different acidolysis conditions on the crystalline phases and morphologies of obtained nano-structured TiO2were investigated. The results indicated that when3g alkali fusion slag was used to acidolysis in200mL acid, rutile type TiO2with needle-like nanorod structures and anatase type TiO2with spherical nanoparticle structures were preferred to be obtained from HC1and H2SO4, respectively, while two types coexisted with needle-like nanorod structures in HNO3. Moreover, anatase type TiO2would be apt to change to rutile type TiO2with increasing of H2SO4concentration. While the crystalline phase and micro morphology of the obtained TiO2nanostructures would be apt to change from anatase type TiO2with spherical nanoparticle structures to rutile type TiO2with needle-like nanorod structures with decreasing of pH values and increasing of liquid to solid ratios. However, the influence of acidolysis duration on the crystalline phase was not obvious. The degree of crystallinity improved obviously and the diameter of crystallite size increased with the acidolysis time prolonging. The photocatalytic activities of the prepared nano-structured TiO2were evaluated for degradation of rhodamine B (RhB) solutions under visible light. It is shown that the photodegradation efficiency can reach91%in120min.During the water leaching process, the effect of water leaching conditions on the leaching efficiency of NaAlO2and Na2SiO3were discussed. Meanwhile, NaA zeolites were synthesized from the leaching solution of fused Ti-bearing EAF slag by hydrothermal method or reflux condensation method, respectively. The results indicated that the first leaching solution of fused Ti-bearing EAF slag with liquid to solid ratio of10:1at room temperature for60min was suitable for the preparation of zeolite. When the SiO2/Al2O3molar ratio was fixed at2.0and H2O/Na2O molar ratio ranged from60to100, NaA zeolite would be synthesized under appropriate reaction temperature and time by hydrothermal method or reflux method. Moreover, the prepared zeoite would be transformed from NaA zeolite to sodilate zeolite with the reaction temperature increasing, H2O/Na2O molar ratio decreasing or the reaction time prolonging. In addition, the adsorption properties for Cu2+by zeolites obtained at different temperatures were studied. It is shown that the optimum adsorption efficiency of70%was gained at150min for NaA zeolite synthesized at120℃for3h.Based on the aforementioned novel process, the distribution of main elements in each process was studied and the recovery efficiency was calculated. It is found that the valuable elements such as Ti, Al, Mg and Si in Ti-bearing EAF slag could be utilized comprehensively and the recovery efficiency were97.5%,87.0%,53.5%and26.3%, respectively.