| Vanadium-bearing shale is a typical vanadium resource in China. Extractingvanadium from vanadium-bearing shale is an important way for utilization ofvanadium resource. The vanadium in vanadium-bearing shale usually replaces Al(III)from dioctahedral structure as isomorphism in mica group minerals, which is hard todissolve by water, dilute acid and alkali. Hence, for destroying the mica structure andliberating vanadium, roasting at high temperature with additives is generallynecessary. Therefore, roasting is the key process for vanadium extraction, and theresearch on the roasting of vanadium-bearing shale, as well as its theoretical study, areof great significance to vanadium extraction from vanadium-bearing shale. In thisstudy, the theory related to the roasting process of vanadium-bearing shale, wasintensively investigated. The main research contents and results are as follows:(1) Technological mineralogy of vanadium-bearing shaleThe technological mineralogy of vanadium-bearing shale from Hubei proviencewas investigated by means of optical microscope analysis, chemical componentanalysis, mineral component analysis, vanadium occurrence state analysis anddetermination of calorific value. The results showed that the V2O5grade in the shale is0.71%, and the vanadium mainly existed in muscovite and illite. Hence, this shalebelongs to low-grade and mica type vanadium-bearing shale.(2) Process of decarbonization pre-roasting and property of decarbonized sampleThe thermogravimetric and differential scanning calorimetry (TG-DSC) analysisof the raw ore showed that the main reactions during pre-roasting were oxidation ofpyrite, combustion of carbon, and oxidation of vanadium. The sequence of thereactions was pyrite oxidation prior to carbon combustion, which was prior tovanadium oxidation.The X-ray diffraction (XRD), quantitative evaluation of minerals by scanningelectronic microscopy (QEMSCAN), and determination of vanadium valencedistribution were conducted to analyze the properties of decarbonized sample. Theresults indicated that the main mineral phases were quartz, muscovite and calcite, aswell as a spot of hematire and gypsum. Vanadium mainly existed as V(III) and V(IV),and the V(V) content is very low. During pre-roasting, the muscovite structure wasnot destroyed and vanadium was not liberated. Hence, the decarbonized sample needfurther roasting at high temperature with additive. (3) Effect of roasting factor on vanadium recovery of leachingThe effects of roasting temperature, roasting time and compound additive dosage on vanadium recovery of leaching were investigated. Meanwhile, the mechanisms roasting factors effecting vanadium recovery of leaching were also discussed from point of view of phase transformation, vanadium valence distribution, sintering, and melting behavior.(4) Thermodynamics and kinetics analysis of compound oxidation roastingBased on the Fick’s first law, the roasting kinetic equation of flaky mineral, controlled by diffusion, was established:Based on the definition of chemical reaction rate, the roasting kinetic equation of flaky mineral, controlled by chemical reaction, was established:The kinetics of roasting with and without compound additives, utilizing established models, were investigated respectively. The roasting was first controlled by chemical reaction, and it was immediately converted to diffusion control.(5) Phase transformation and vanadium behavior during compound roastingThe phase transformation during compound oxidation roasting was investigated by high temperature in situ method. The substance of roasting process is the process of vanadium-bearing muscovite converting to feldspar.Electro-probe microanalyzer (EPMA) was used to investigate the vanadium occurrence in decarbonized sample, water leaching residue, and acid leaching residue. The discipline of vanadium transitions during roasting was revealed. Most liberated vanadium reacts with sodium salt to generate water-soluble sodium vanadate, some reacts with calcite in shale to form water-insoluble calcium vanadate, and other liberated vanadium exists as free vanadium oxide. For some coarse muscovite grains, the reaction of muscovite converted to feldspar only occurs at the outer margin of muscovite grains, and the vanadium in the interior of muscovite grains is not liberated. This portion vanadium can not be leached out by water and dilute acid.The relationship among liberation, oxidation and conversion of vanadium was elaborated through crystal chemical theory and prue material analysis. During roasting, vanadium was liberated from muscovite as V(IV). Firtly, the vanadium bronze was generated, and then vanadium bronze was gradually converted to sodiummetavanadate.(6) Sintering behavior during compound additive roastingThe theory analysis of sintering behavior at different composition system wasresearched by thermodynamic calculation software FactSage. The effect ofcomposition on sintering behavior was also discussed, through investigating thechanges of apparent density, ash fusion point, and mineral phase content. Thevanadium in sintering sample existed as both chemical bonding and mechanicalpackage. The chemical bonding way is O-Al-V, in which vanadium was insoluble inwater and dilute acid.(7) Effect of roasting atmosphere on vanadium extractionThe decomposition characteristics of compound additives at different atmospherewere researched by TG-DSC analysis coupling with mass spectrometry (MS). Theresults showed that Cl2decomposed by NaCl and the released gas of Na2SO4is SO2.The effects of O2and Cl2concentration on roasting were studied, and the mechanismwas also investigated through pahse transformation, vanadium oxidation andvanadium volatilization. The results indicated that the oxygen concentration in air issufficient for vanadium oxidation. At the chlorine concentration of more than2%, thephenomenon of vanadium volatilization becomes obvious. In fact, during roastingwith compound additive in air, there is no vanadium loss.(8) Behaviors of impurity elements Ca and Fe during roasting and its controlThe behaviors of impurity element Ca and Fe during compound additive roastingof vanadium-bearing shale were investigated, through analyzing the bondingbehaviors of Ca, Fe with V and Ca, Fe with main phases in shale, respectively. Theresults showed that Ca in shale had an adverse effect on roasting. In system ofNaVO3-CaO, Ca would snatch V from Na, and generate water-insoluble calciumsodium vanadate. In system of NaCl-CaO-V2O5, CaO occupied a favorable position incompetition with NaCl. Fe had no effect on roasting, existing as hematite. In systemof NaVO3-Fe2O3, Fe would not snatch V from Na.Owing to the adverse effect of Ca, pre-treatment for concentrating vanadium andreducing impurity was proposed. It could improve the vanadium grade, reduce thecontent of impurities and improve roasting effect. |
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