Vanadium Extraction From Shale Based On The Selective Thermal Effect Of Microwave And Mechanism Study

China has the largest vanadium production output,consumption and supply in the world.In China,vanadium-bearing shale is a kind of unique and important vanadium resource,and the vanadium extraction from shale is a very improtant way to utilize vanadium resources.Most of the vanadium in shale exists stably in the structures of mica group minerals in the form of V（III）.High temperature roasting is usually used to destroy the structures of mica minerlas and oxidize the low-valent vanadium to high-valent soluble vanadium.Therefore,intensifying roasting is a key method to enhance the vanadium leaching efficiency.The microwave heating is researched and applied in many areas of drying,chemical etc.due to its special heating mechanism and characteristics.However,there is still a lack of study about the utilization of microwave heating in vanadium extraction form shale.In this study,the microwave heating was used in the roasting process of shale,the process and related mechanism of microwave roasting-acid leaching technique were intensively study.The main research contents and results are as follows:（1）Study of the microwave absorbing characteristics of shale and its constituent mineralsAccording to the difference of microwave absorbing characteristics,the consitituent minerals could be divided into three categories.The first category was the carbon and the pyrite with good microwave absorbing performance.They were the main microwave absorbing minerals and heat source in shale.The second category was the mica and the feldspar group minerals with poor microwave absorbing performance under the normal temperature.When affected by the high temperature,the structure of this kind mineral would deform.It caused the change of microwave absorbing property and enhanced the thermal effect of microwave on minerals.The third category was the quartz and the calcite which presented poor microwave performance during the whole roasting process.The difference of the microwave absorbing characteristics of constituent minerals is a essential prerequisite for the selective thermal effect of microwave on vanadium-bearing shale.Shale had good microwave absorbing property because of the carbon and pyrite.With the increase of temperature,the microwave absorbing property of shale showed a slightly rise after a sharp decline.When the temperature rised form normal temperature to 600°C,the contents of carbon and pyrite decreased rapidly causing a sharp decline of microwave absorbing property of shale.With the temperature continuing rising,there was a slightly rise of microwave absorbing property of shale due to the change of the microwave absorbing perfoemance of the mica and feldspar group minerals under high temperature.（2）Effects of the factors in microwave roasting-acid leaching process on the V leaching efficiency and the response surface optimizationIn the microwave roasting-acid leaching process,the effects of the single factors on the V leaching efficiency could be sorted as the concentration of H₂SO₄>the microwave roasting temperature>the leaching time>the microwave roasting time.The interactive effect of the concentration of H₂SO₄ and the microwave roasting temperature on V leching efficiency was significant.The microwave roasting temperature and the microwave roasting time,the concentration of H₂SO₄ and the leaching time had a certain degree of influence on V leaching efficiency.The fitting equation of V leaching efficiency（Y）versus the microwave roasting temperature（A）,the microwave roasting time（B）,the concentration of H₂SO₄（C）and the leaching time（D）was obtain using BBD（Box-Behnken Design）response surface experimental design method as follow:Y=-980.35392+2.06197A+5.20460B+11.03677C+11.21883D-4.06750E-003AB-6.86500E-003AC-1.63750E-003AD+0.023350BC+0.033750BD-0.17150CD-1.10839E-003A²-0.0460027B²-0.097507C²-0.51848D²The optimum parameters of the microwave roasting-acid leaching process were that the grain size of shale was 1-3 mm,the microwave inpute power was 1500 W,the microwave roasting temperature was 785°C,the microwave roasting time was 28 min,the H₂SO₄ concentration was 25vol%,the leaching temperature was 95°C,the leaching time was 6 h and the liquid to solid rate was 1.5 mL/g.Under the optimum conditions,V leaching efficiency was 93.44%.Compared with the conventional roasting-acid leaching process,when the vanadium leaching efficiency was similar,the roasting temperature was reduced by 115°C,the roasting time was shortened by about 32 min,H₂SO₄ concentration was reduced by 10 vol%and the leaching time was shorteded by 4 h.（3）Study of the promoting mechanism of microwave roasting on V leaching efficienySelective heating was the main reason for microwave roastinig to promote V leaching efficiency.On the view of macroscopic,microwave selectively heated the carbon and pyrite in V-bearing shale inhancing the reaction activity of these reducing substances.Meanwhile,selective heating resulted in thermal stress between minerals and lots of cracks and holes in roasted particles which promoted the mass transfer in oxidizing reaction.At the micro level,high temperature improved the microwave absorption characteristic of muscovite.Microwave roasting selectively promoted the structural aberration of Al-O（OH）octahedral layer of muscovite by improving the dihydroxylation reaction which enlarged the interlayer spacing of Al-O（OH）octahedral layer,reduced the binding energy of Al,V and improved the oxidation and dissolving of V.（4）Study of the mechanism of the“silicate minerals-based coating”in blank roasting process and its controlThere were mainly two stages during the formation process of silicate minerals-based coating.The first stage was the solid phase reaction stage.It was roughly from 850°C to 950°C where the vanadium leaching efficiency slightly decreased without obvious melting phenomenon in the roasted samples.In this stage,the Na-bearing feldspar minerals reacted with the V-bearing mica group minerals with the promotion of CaO to generate a new silicate mineral which was insoluble in acid solution.This new generated mineral could be expressed as Na₄（Al,V）₂Si₂O₉ and Na（Al,V）Si₂O₆.The second stage roughly exceeded 950°C where the vanadium leaching efficiency rapidly decreased with obvious melting in the roasted particle.In this stage,the new generated silicate mineral phases in the first stage preferentially melted,promoting the melting of roasted sample.Based on the formation mechanism of the silicate minerals-based coating and the outstanding performance of microwave roasting in the structure destruction of mica and feldspar group minerals,a method for weakening the silicate minerals-based coating by controlling the roasting atmosphere was proposed.In the heating and holding stages,N₂ was used to increase the formation temperatrure about 100°C.On one hand,it broadened the suitable temperature range which simplified the microwave roasting control process.On the other hand,it increased the effective temperature superior limit which was useful in promoting the structural aberration of V-bearing minerals.In the cooling stage,air was used to enhance the vanadim leacing efficiency by oxidizing the low-valence vanadium.The atmosphere control method could further strengthen the effect of microwave roasting of vanadium-bearing shale,enhancing the vanadium leaching efficiency.