Study On Mechanism Of Roasting And Vanadium Leaching For High Calcium Mica-type Vanadium-bearing Black Shale

Vanadium-bearing black shale is a unique vanadium resource in China, and the total reserve of vanadium-bearing black shale is abundant. It is widely distributed in many provinces of China. The typical techniques of vanadium extraction from vanadium-bearing black shale include:traditional high salt roasting-water leaching technique, direct acid leaching technique and blank oxidation roasting-acid leaching technique. High calcium mica-type vanadium-bearing black shale is a representative type of black shale in China, it is difficult to extract vanadium for the special occurrence state of vanadium in this type of balck shale. The mechanism of roasting and vanadium leaching for high calcium mica-type vanadium-bearing black shale, such as the effect of additives on the mica crystal lattice, the characteristics of the formation of calcium vanadate during roasting process, and the kinetics of leaching process of vanadium are not studied well.In orde to resolve these problems, this work, which was financially supported by the National Key Technologies R&D Program of China (No.2011BAB05B01), investigated the non-additive roasting and additive roasting processes for high calcium mica-type vanadium-bearing black shale in Hubei. Furthermore, the change of vanadium compounds in roasting process was studied, and the effect of roasting to mica crystal lattice, vanadium oxidation to vanadium leaching and the content of calcium to water leaching rate was also discussed. By using compound additives roasting, the high leaching rate of vanadium was achieved, and the acid consumption was reduced. The main conclusions are as follows:1. The leaching rate of aluminum from mica was higher by addtive (NaCl, Na2SO4) roasting than it by non-additive roasting. Accordingly, The additive roasting also can enhance the decompose of mica structure and improve and leaching rate of vanadium (LRV).2. The effect of non-additive roasting on the mineral structure and the LRV(1) The LRV was only24%when the raw black shale was leached by20%H2SO4for3h, but the LRV can reach70%after the raw ore was roasted at900℃. The optimum conditions of oxidation roasting were roasting temperature:900℃, roasting time:2h.(2) The dehydroxylation of mica occurs when the roasting temperature is above 830℃. The LRV increases apparently along with the dehydroxylation of mica, but the LRV decreases dramatically when the roasting temperature is too high and the vanadium-bearing materials is wrapped in newly formed aluminosilicate and sintered materials, which causes vanadium is hard to leach. It is less effective for vanadium leaching with dilute acid because mica is also found in the sample roasted at900℃with non-additive.3. The study on the compound additive roasting(1) The water leaching rate of vanadium (WLRV) is below30%using conventional high sodium roasting. By using compound additive roasting it can reduce the salt consumption and increase WLRV at the same time. The WLRV is45.01%, and the overall leaching rate can reach77.61%by roasting with6%NaCl and10%Na2S04(2) The premise of increasing the WLRV is increasing the content of pentavalent vanadium (V(V)), and the oxidation process of tetravalent vanadium is controlled by interface chemical reaction in the forepart, and by internal diffusion in later of the roasting process. The conclusions are as follows:1) Temperature is the key factor of the vanadium oxidation reaction.2) The reaction rate is in inverse proportion to particle size of reactant.3) When the oxygen concentration is fixed, The increase of the air velocity has no effect on vanadium oxidation.4) The compound additive can reduce activation energy barrier of interface chemical reaction, from173.230kJ/mol to104.690kJ/mol.5) The internal diffusion of oxygen is also important, the oxidation of V(IV) can be impeded if the materials is sintered.(3) The crystal phase in roasted sample by adding compound additive are K-Na feldspar and quartz. The more K-Na feldspar in roasted sample the more the LRV is. The K-Na feldspar grow well at the conditions of enough high temperature and additive content, but the LRV decreases dramatically when the temperature is too high, because the vanadium-bearing materials is wrapped in glass-like state material. So the appropriate temperature for roasting is850～900℃.(4) A mass of calcium vanadate will be generated when the raw ore is roasted with NaCl, and it is the reason for the lower WLRV due to calcium vanadate is insoluble in water. Through the roasting experiment using pure V2O5and CaCO3, some conclusions can be drawn as follows:1) the reaction products of excess content of CaCO3(mCaCO3:mV2O5>3) with V2O5was Ca3V2O8, and a part of Ca2V2O7will be generated at lower temperature.2) the calcium vanadate can not be impeded in the process of roasting with NaCl at the condition of excess CaCO3. The calcium vanadate can not be partly impeded in the process of roasting with Na2SO4additive, and a part of V2O5can form NaV03accordingly, hence, the Na2SO4in the compound additive can enhance the WLRV.(5) Chlorine can promote the decomposition of vanadium-bearing mica crystal lattice and the generation of metal vanadates.4. The study on the process of leaching vanadium from the raw ore and roasted sampleThe researches of leaching kinetics of raw ore, non-additive roasted sample and additive roasted samples show:(1) The apparent activation energy and apparent reaction order of the reaction direct acid (30%H2SO4, volume concentration) leaching vanadium from raw ore are48.63kJ/mol and1.2075, respectively. The kinetic results show that the reaction rate is controlled by chemical process. Leaching temperature affect the reaction rate apparently. When the acid concentration is below33%, the reaction rate of vanadium leaching is controlled by chemical process. Increasing the acid concentration accelerates the chemistry reaction rate. When the acid concentration is above33%, the leaching reaction is in internal diffusion control area and it is less effective to enhance leaching rate by increasing the acid concentration.(2) The apparent reaction order of the reaction that leaching vanadium from blank oxidation roasted sample is1.04335. It is lower than of direct acid leaching reaction. Non-additive roasting can reduce acid concentration when the reaction is transfered from chemical control to internal diffusion control. As acid concentration is above20%, the reaction is controlled by internal diffusion.(3) The process of leaching vanadium with acid from additive roasted sample is controlled by unsteady-state diffusion of liquid film. The process of leaching vanadium with acid (2%H2SO4) from water leached residue belongs to shrinking unreacted core model, and it is controlled by internal diffusion. Therefore, non-additive roasting and additive roasting both can reduce the apparent reaction order and the acid consumption.