Study On The Reduction And Volatilization Mechanism Of Valuable Elements In Vacuum Carbon Thermal Reduction Of Steel Slag

The rapid development of the electric furnace steelmaking industry has increased the amount of slag produced.How to use steel slag in an efficient and environmentally friendly way is a strong guarantee to achieve the goal of"double carbon".The RO phase（mainly Fe O,Mg O and Mn O in a continuous solid solution）in the slag is the main reason for its low cementing activity and the difficulty of large-scale utilization.Based on this,this thesis proposes to use vacuum carbon thermal reduction to recover Fe,Mg and Mn valuable group elements in steel slag to decompose the RO phase and form slag with high gelling activity at the same time,with the following main conclusions:Theoretical calculations show that the reduction rates of Fe,Mg and Mn increase with the increase of carbon allocation and then remain constant,reaching a maximum at 16%carbon.Lowering the pressure can lower the reaction starting temperature and promote the reaction.The boiling points of the reduced metals are V>Si>Fe>Cr>P>Al>Mn>S>Ca>Mg.With the increase of Si O₂ addition in the slag,low melting point phases such as Ca₂Al Si₂O₇ and Ca₃Si₂O₇ appear in the system,which can reduce the melting point of the material,improve the material fluidity and promote the reaction.The results of vacuum carbon-thermal reduction experiments show that:the weight loss rate and volatility of magnesium and manganese increase with the increase of carbon and temperature;when the temperature exceeds 1450℃,the generation of Ca₃Mg（Si O₄）₂ increases and the volatility of magnesium decreases;the addition of Si O₂makes the system form low melting point phase,which is beneficial to the reaction and intensifies the volatility of manganese,and at the same time,the reduction of alkalinity is conducive to Promote the self-powdering of steel slag;when adding excessive Si O₂,a large amount of Ca₄Mg_1.42Al_1.02Si_3.48O₁₄ is generated,which makes the volatilization rate of magnesium decrease significantly.Extending the reaction time,the volatilization rate of magnesium and manganese increases,while the weight loss rate of the system also increases,which is favorable to the aggregation and growth of iron particles,but the pulverization rate of steel slag will slightly decrease.In a comprehensive analysis,the optimal conditions for the reduction of valent group elements in vacuum carbon thermal reduction of steel slag are:T=1450°C,t=60min,C=16%,R=1.0.The mechanism of vacuum carbothermal reduction of steel slag in electric furnaces was derived from non-isothermal kinetic studies and vacuum carbothermal reduction experiments.As the temperature rises to the required reaction conditions,the RO phase first diffuses to the reaction interface with the solid carbon,and at the solid-solid reaction interface the RO phase decomposes to produce iron,magnesium and manganese monomers,which are easily volatilized as gases under vacuum.After the addition of Si O₂,the low melting point phases Ca₂Si O₄,Ca₂Al₂Si O₇ and Ca₃Mg（Si O₄）₂are generated in the steel slag,which makes the liquid phase in the system and strengthens the reaction kinetics.In addition,Ca₂Si O₄ undergoes a crystallographic transformation fromβ-Ca₂Si O₄ toγ-Ca₂Si O₄ during the cooling process with the furnace,a process accompanied by volume expansion,which leads to the self-powdering of the steel slag.The apparent activation energy of the reduction process is36.49 k J/mol.The apparent activation energy of the magnesium volatilization process is 160.48 k J/mol.