| Vanadium-bearing black shale is an alternative resource of vanadium in China in addition to vanadium-titanium magnetite, which extensively exists in northwest and southern regions. The traditional sodium salt roasting-water leaching process had been obsoleted because of serious pollution and low metal recovery. Therefore, some improved processes such as calcium salt roasting, sodium salt and calcium salt roasting, and oxidization roasting were proposed. However, these processes were not applied to deal with black shale of different regions. Alternatively, acid leaching-solvent extraction of black shale has been considered for many years as a viable potion to produce pentoxide vanadium minimizing atmosphere pollution. But this process presents yet a problem concerning the low leaching kinetics of black shale. The problems mentioned above can be settled using pressure acid leaching of black shale. Based on a systematic technical study, this dissertation will addresses the mechanism during the pressure acid leaching process of black shale, investigating the decomposition behavior of vanadium-bearing minernal, the leachability of vanadium species presented in different valences and their leaching sequence, the kinetics mechanism of vanadium leaching, and the effect of Fe(Ⅲ) ion on the catalytic oxidation of low valence vanadium species. Listed below are the main results:(1) The occurrences of vanadium in black shale from Tongren region of Guizhou province was analyzed by means of many detection methods such as XRD, EPMA, SEM-EDS and BEI. The main phases in black shale are quartz, roscoelite and clay. Vanadium occurres with Al, Si and K, that is, vanadium presents in aluminosilicates. A mass of small grains, which are vanadium-bearing aluminosilicates and matrix, aggregate to form a large particle and there are many pore among the grains. The BEI of particles confirms that apart form large amounts of grains mutually gathers to produce a larger particle, and small amounts of grains distributes in the matrix. The phase composition of vanadium in black shale was analyzed using a sequential extraction procedure developed by Tessier et al. Results indicates that the distribution of vanadium in aluminosilicates, iron and manganese oxides, and organic matter are85.41%,6.23%and3.50%, respectively, and the distribution of vanadium in the form of adsorption is4.16%.(2) The potential-pH diagram for vanadium-water and the concentration-pHdiagrams for V(Ⅲ), V(Ⅳ) and V(Ⅴ) species were plotted at25℃. At lower pH value, V(Ⅲ), V(Ⅳ) and V(Ⅴ) ions are in the form of V3+, VO2+and VO2+, respectively. The stablity regions V3+, VO2+ and V02+ions moved to lower pH values with rising temperature. At the presence of Fe(III) ion, V3+can be oxidized into VO2+by VO2+, O2(aq) and Fe(III) ion. Yet, oxygen dissolved in solutions, O2(aq), plays a dominant role in the oxidation of V3+into VO2+and next VO2+and Fe(III) ion. Because the potential energy of O2(aq) oxidizing VO2+into VO2+is lower than that of VO2+oxidizing V3+into VO2+, vanadium in the pressure acid leaching liquor of black shale presents in the form of VO2+ion.(3) The leachability of different valences vanadium species and their leaching sequence were investigated. The results indicate that the leachability of the vanadium species is different. Comparing with low valence state of vanadium species (LVSVS), high valence state of vanadium species (HVSVS) could be easily leached. Therefore, the high content of the HVSVS in black shale is advantageous for the leaching of vanadium. At the initial stage of leaching and lower temperature, the dissolution of the HVSVS played a dominative role. The oxidizing atmosphere helps to extract all vanadium species.(4) The decomposition behavior of vanadium-bearing mineral in black shale was studied. The decomposition rate of the mineral is very low under atmosphere leaching condition, but it is fairly fast under pressure acid leaching. The rate is strongly affected by temperature and sulphuric acid concentration. At low temperature, agitation speed, liquid to solid ratio and particle size have a little influence on the rate. Conversely, the effects of these parameters on the rate increase gradually with rising temperature.(5) The kinetics of vanadium leaching from black shale was investigated under non-oxidizing and oxidizing conditions. Based on a previous study on the kinetics model (Grain model) for fluid-solid reactions in a porous solid, considering that the content of vanadium-bearing mineral (grain) in black shale is low, the model is amended by introducing grain grade (mass fraction). Under the non-oxidizing condition, the external diffusion resistance of particle reaches a minimal value when agitation speed is above200rpm. The leaching rate of vanadium increases with increasing temperature and sulphuric acid concentration and lowering particle size. Diffusion throughout the pores within particle controlled the leaching rate of vanadium and an apparent activation energy of56.09kJ/mol was obtained. Similarly, the leaching rate of vanadium increases as sulphuric acid concentration, temperature and oxygen partial pressure are increased under the oxidizing condition. The solubility of oxygen in leaching system was determined by a solubility model of oxygen developed by Tromans. The results show that the solubility of oxygen increases with an increase in temperature and oxygen partial pressure. A sufficient amount of dissolved oxygen, which can oxidize all LVSVS, is provided when the oxygen partial pressure reaches above7.3atm. The leaching rate of vanadium is still controlled by the pore diffusion and the apparent activation energy is41.57kJ/mol. Comparing with the apparent activation energies obtained under the non-oxidizing and oxidizing conditions, a difference of14.52kJ/mol was obtained, indicating that oxidizing atmosphere is advantageous for the leaching of vanadium.The leaching rate of vanadium from black shale is limited by the decomposition rate of vanadium-bearing mineral. However, the decomposition rate is controlled by the pore diffusion.(6) It found that Fe(Ⅲ) ion has a catalytic oxidation influence on the dissolution of the LVSVS. Adding6.0g/L Fe(Ⅲ) ion to leaching system, the dissolution percent of vanadium was increased by10%, but the oxidation-dissolution of the LVSVS was dominated by the oxygen dissolved in the sysytem. The catalysis of Fe(Ⅲ) ion decreases with increasing agitation speed and temperature, while increases with increasing oxygen partial pressure and lowering particle size, but is independent of sulphuric acid concentration. The dissolution process of vanadium catalyzed by Fe(Ⅲ) ion was controlled by the surface chemical reaction rate and apparent activity energy was found to be42.74kJ/mol. Comparing with the apparent activity energy of43.46kJ/mol obtained without Fe(Ⅲ) ion, a decrease of0.72kJ/mol in the apparent activity energy was observed, indicating that Fe(Ⅲ) ion has a catalysis for the dissolution of the LVSVS again.
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