| Vanadium as a kind of strategic resource is widely used in many fields, and the vanadium production of China plays an important role in the world. Most of the vanadium used is produced from vanadium oxide containing slag (vanadium slag for short). The oxidation roasting process is the most widely used technology in vanadium extraction from vanadium slag. However, in the perspective of industrial ecology, there are still amounts of energy waste of molten vanadium slag in this process. At present, the study of heat recovery from molten vanadium slag has been reported rarely. From the perspective of energy conservation and emissions reduction, a novel process of vanadium extraction from vanadium slag in its molten state was proposed to use its heat energy, which can eliminate the roasting process in a rotary kiln or a multiple hearth furnace and shorten the whole vanadium extraction process chain. On the basis of the theories of previous studies about vanadium extraction using roasting method from vanadium slag, through lots of modern testing technologies and a great deal of theoretical analyses and experiments, correlative theories about direct oxidation of molten vanadium slag process and corresponding clinker leaching process were detailedly studied in this dissertation. Based on the research of vanadium extraction using calcium method, another new technology of dephosphorization during the process of vanadium extraction from vanadium-bearing hot metal was proposed to solve the dephosphorization problem of semisteel. The main contents of the thesis are as follows:Firstly, the melting point and viscosity of vanadium slag were investigated, especially in the presence of CaO and Na2CO3, which will lay a foundation for the further theoretical analysis and experimental research. Thermodynamic calcula-tions and analysis of the direct oxidation of molten vanadium slag process in the presence of CaO and Na2CO3were carried out to find out the possibility and tendency of the main vanadium extraction reactions and other side reactions. Then the formation mechanism of vanadate was proposed. The oxidation behavior of the molten vanadium slag was also discussed and the result showed that it was a restrictive link of the whole vanadium extraction process.Using Na2CO3as the additive, the direct oxidation of molten vanadium slag with pure oxygen was carried out in the laboratory scale. Pyrometallurgical experiments results showed that the two most important factors that affected the water-soluble sodium vanadate formation in vanadium slag were the addition amount of Na2CO3and the oxidation rate of vanadium, respectively. XRD analysis and water leaching experiments results indicated that the optimal amount of Na2CO3was added to25%of vanadium slag quality. The formation of sodium vanadate was confirmed by the XRD、SEM/EDS results. X-ray photoelectron spectroscopic (XPS) analysis was employed to determine the valence of vanadium in clinker and the relationship between the oxidation rate of vanadium and the recovery rate of vanadium was also discussed. The results showed that0.375m3O2was needed to make lkg vanadium slag fully oxidized, and the fully oxidized clinker was a necessary condition of high leaching rate of vanadium. On the basis of single factor experiments, the water leaching conditions, including leaching temperature, leaching time, the ratio of liquid to solid, stirring speed and so on, were studied. The vanadium leaching rate can reach above85%under the optimal condition:leaching temperature of95℃, leaching time of2min, the ratio of liquid to solid of3:1mL/g and stirring speed of150r/min.The direct oxidation of molten vanadium slag with pure oxygen was conducted in the laboratory scale selecting CaO as the additive. The oxidation rates of vanadium in all the clinker samples were above88%, indicating that the oxidation effect of vanadium slag was well. The formation of calcium vanadate in clinker samples was confirmed by the XRD、SEM/EDS results. The CaO addition content made the most important effect on the formation of vanadium containing phase. The main vanadium containing phase was MnV2O6when the CaO content in slag was low and it will gradually change to CaV2O6、Ca2V2O7and Ca3V2O8with the increase of CaO content. Excess CaO can react with SiO2or TiO2, which may not be of benefit to the vanadium recovery. Following with the pyrometall-urgical process, the acid leaching experiments of the clinker after CaO treatment were carried out. On the basis of single factor experiments, the acid leaching conditions such as particle size, acid concentration, the ratio of liquid to solid, leaching temperature, leaching time, stirring speed and so on were studied. The vanadium leaching rate can reach above90%under the optimal condition:particle size of<96μm, acid concentration of10%, the ratio of liquid to solid of5:1mL/g, leaching temperature of90℃, leaching time of30min, and stirring speed of200r/min. The kinetic model of the acid-clinker reaction was established based on unreacted shrinking core model. Kinetics study showed that the acid leaching process of vanadium slag clinker was governed by internal diffusion, with apparent activation energies calculated to be8.216kJ/mol.The calculation model of vanadium slag activity was established based on the coexistence theory of slag by which the effect of CaO addtion on extracting vanadium and removing phosphorus simultaneously from vanadium-containing hot metal was investigated. The results of slag-metal equilibrium experiment showed that the coexistence theory was more suitable for the prediction of vanadium distribution behavior between slag and metal, and it was found that there was a linear relation between the rate of vanadium extraction and that of phosphorus removal. The calculated results of the calculation model to vanadium slag indicated that the vanadium distribution ratio (Lv) and phosphorus distri-bution ratio (Lp) both increased with the increasing of the binary basicity, and the effect on Lp was more obvious, which means that vanadium extraction, under the condition of taking no consideration of temperature drop, was less affected by adding lime or limestone to make basic slag in the process of vanadium extraction. In addition, with the increasing of binary basicity, the activity and activity coefficient ratios of FeO and MnO increased and that of SiO2and TiO2decreased.Aiming at the dephosphorization problem of acidic leaching liquid of the clinker of vanadium slag with high contents of calcium and phosphorus, a carbonate leaching process was introduced in this study. The results of the leaching experiments dealing with the clinker of this type vanadium slag using Na2CO3, NaHCO3,(NH4)2CO3and NH4HCO3as the carbonate leaching agents indicated that the leaching rates of phosphors were less than11%in all experiments and the contents of other impurities such as Fe, Mn, Ti and Ca in the leaching solutions were low, respectively. The leaching rate of vanadium can reach above82%when Na2CO3and NaHCO3were chosen as the leaching agent, however, it was only about30%using (NH4)2CO3and NH4HCO3. Therefore, the carbonate leaching method can obtain the target of selective separation leaching between vanadium and phosphorus. The obtained vanadium-bearing solutions could be further treated to obtain V2O5product through subsequent procedures, that is, dephosphorization, acidic precipitation and calcination. The purity of V2O5product was more than98.5%. |
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