Fundamental Research Of Thermodynamic Optimization Of Fe-V-Si-O System And CO₂ Used In Vanadium-extraction Process

In recent years,the research on Fe-V-O system is a hot topic regarding vanadium-extraction in converter,smelting of high-strength low-alloy steel,corrosion mechanism of vanadium-based corrosion-resistant steel,treatment of vanadium-containing slag in steel mill and treatment of vanadium-containing waste in thermal power plants.All above industrial productions depend on an precise understanding of Fe-V-O system.At present,thermodynamic databases for Fe-V-O or Fe-V-Si-O systems are still lacking,and thermodynamic calculation software cannot be used to calculate thermodynamic problems involving Fe-V-O or Fe-V-Si-O systems.Besides,thermodynamic modeling methods containing two variable elements,Fe and V,are also urgently needed.In this paper,a series of studies on the thermodynamic optimization of Fe-V-O system were carried out,aiming to establish an accurate and widely applicable thermodynamic database.Besides,CO₂ has been proved to have the effect of controlling the oxygen potential of the converter and the temperature of the molten pool.It can be used as a cleaner coolant in the process of vanadium-extraction,and achieve the purpose of energy saving and emission reduction.The database could provide a theoretical guidance for the calculation of phase equilibrium and thermodynamic properties in vanadium-containing systems,and promote the new process of extracting vanadium from hot metal using CO₂.Firstly,this paper focuses on the thermodynamic modeling of the full-scale Fe₃O₄-FeV₂O₄ spinel solid-solution phase based on CALPHAD?CALculation of PHAse Diagram?technology,within the framework of compound energy formalism.According to the crystal field theory,the cation distribution between tetrahedral and octahedral sites were also simulated,and a structural model,(Fe²⁺,Fe³⁺,V³⁺)^T[Fe²⁺,Fe³⁺,V³⁺,Va]₂^OO₄,which conforming to the physical structure of spinel was established.Hence,the relationship between temperature or composition and the cation distribution can be calculated by the optimized thermodynamic parameters combined with the thermodynamic calculation software,FactSage.As well as thermodynamic properties such as cation distribution,Gibbs free energy,oxygen partial pressure,activity,and mixing free energy.All above theoretical calculations are consistent with the experimental data of other authors within the experimental error range.Secondly,comprehensive evaluation and thermodynamic optimization of a large number of experimental data regarding thermodynamic properties and phase equilibrium under a total pressure of 1 atm were carried out.The oxygen partial pressure ranged from air conditions?0.21 atm?to reducing-atmosphere conditions(10^-16atm).Thermodynamic parameters such as?H^o_{298 K},S^o_{298 K},Cp were rationally optimized for all stoichiometric compounds(FeVO₄,FeV₂O₆,FeV₆O₁₅ and Fe₂V₄O₁₃).The Braggs-Williams random mixing model was used to describe the wustite phase,corundum phase and VO₂ solid-solution phase,while the Modified Quasichemical Model was used to describe the slag?molten oxide?phase with all valence states of vanadium and iron(Fe²⁺,Fe³⁺,V²⁺,V³⁺,V⁴⁺and V⁵⁺)considered.All thermodynamic models used in this study were based on the physical structure of each phase?slag,spinel,wustite and corundum?.In this way,the configuration entropy of each phase could also be modeled in these thermodynamic models.The optimized parameters can be used with the Gibbs free energy minimization software,FactSage,to simultaneously reproduce all reliable thermodynamic data at different temperatures,compositions,and oxygen partial pressures.Besides,the thermodynamic properties under other conditions without experimental data could be reasonably and effectively predicted.The accuracy of the thermodynamic model was verified by the calculated values and the experimental values.As a result of the thermodynamic optimization,the database of Fe-V-Si-O system was developed.In addition,the predicting of higher-order systems,which based on the thermodynamic parameters of Fe-V-O system through geometric models“Kohler”and“Toop”,indicates the extensionality of the database.Afterwards,aimed to reduce carbon dioxide?CO₂?emissions and to utilize waste resources in steel plant,the present study investigated the use of CO₂ as a gas oxidant and coolant in the converter to increase productivity and energy efficiency.The thermodynamic analysis related to the vanadium-extraction process from hot metal was carried out through FactSage and the optimized vanadium-containing thermodynamic database.Experiments regarding CO₂-O₂ mixture injection of vanadium-extraction were also carried out.The results show that pure CO₂ could oxidize elements such as V,Fe and C in the hot metal pool,and the product CO will change the equilibrium condition of the gas phase,so that the oxidation of carbon can be delayed.In addition,CO₂absorbs heat when reacting with Fe and C,which is beneficial to the process of vanadium-extraction.The cooling capacity of CO₂ was calculated as 9426.6 kJ/m³.The top blowing method for CO₂-O₂ mixed injection was found the advantage of carbon retention.When the content of CO₂ is between 0 and 15 vol.%,the mechanism of CO₂in this process is mainly heat absorption and temperature control.,which is beneficial to vanadium-extracting and carbon-saving.Whereas the content of CO₂ is between 15 and25 vol.%,the mechanism is mainly weak oxidation,and the efficiency of vanadium-extracting is lower in this region.At the situation of 1300°C for the initial-temperature,the optimum ratio of CO₂ in the top-blown mixed gas was 15%,and the oxygen supply intensity was 1.5 m³/?t·min?.At the end of the experiment,the vanadium oxidation rate was 94.4%,and the oxidation rate of vanadium to carbon was the largest compared with other mixing methods,indicating that the ability to extract vanadium and save carbon was strongest.The bath temperature was reduced by about20°C compared to pure oxygen injection,and the O₂ consumption per ton of hot metal was reduced by 1 Nm³/ton and additional chemical energy has been recovered by the controlled injection of CO₂ into the bath.At the same time,CaCO₃ acted as a solid coolant and CO₂ carrier,and the cooling effect of the molten pool was also significant.The addition of CaCO₃ in the vanadium-extraction process from hot metal had little effect on the oxidation of carbon in the molten pool.With the increase of the additional amount,the oxidation rate of vanadium was slightly improved,and dephosphorization capacity of vanadium slag was greatly enhanced.When 1.3 wt.%of CaCO₃ was added,the P₂O₅ content in the vanadium slag increases from 0.12 wt.%to 0.32 wt.%,which is beneficial to industrial smelting of high phosphorus vanadium-titano-magnetite.The process can be followed by calcification roasting and acid leaching treatment for further green extraction process.