Study On Smelting Molybdenium And Vanadium Steel By Direct Alloying With The Self-reduction Of MoO₃and V₂O₅

The method of direct alloying with oxide ores is a new process in steelmaking, whichdevelops from the direct reduction and smelting reduction process. The traditional method ofalloying in steelmaking is using ferroalloys made from oxide ores, while the method of directalloying takes the oxide ores as alloy additions. The alloy elements will be reduced out and thendissolve into the molten steel after the oxide ores are added into the furnace or ladle. Theferroalloy production process can be omitted in the method of direct alloying, for which thereduction process of oxides and steel alloying process can be completed in the same metallurgicalreactor. Thus, it has so many advantages for the new method, such as process simplification,energy saving, environmental improvement and so on.The basic theory about smelting molybdenum and vanadium steel by direct alloying withmolybdenum oxide (MoO3) and vanadium oxide (V2O5) has been systematically studied. It mainlycontains the thermodynamic principle of direct alloying with MoO3and V2O5, the method toinhibit high-temperature volatilization of MoO3and V2O5, and the self-reduction reactionmechanism of MoO3and V2O5. And on this basis, the direct alloying experiments with theself-reduction briquettes of MoO3and V2O5have been conducted in the medium frequencyinduction furnace. The optimum compositions of the self-reduction briquettes have been studied.On the basis of laboratory experiments, an industrial test has been carried out and a relativelygood result achieved.(1) Based on the basic thermodynamic data, the standard Gibbs free energy of reducingmolybdenum oxides and vanadium oxides by common reductants (C, Si, Al, SiC, etc.) can beobtained. The standard Gibbs free energy at different temperatures has been drawn on thethermodynamic state diagram. Analytical results indicate that the solid reaction can happenbetween MoO3or V2O5and the reductants (C, Si, Al, SiC, etc.) at low temperatures. Based on thecoexistence theory of slag structure, the activity model of the CaO-MgO-FeO-SiO2-Al2O3-MoO2system has been established, as well as the CaO-MgO-FeO-SiO2-Al2O3-V2O3system. The activityof low valance molybdenum oxide (MoO2) among the slag has been calculated in LF refiningconditions, as well as that of low valance molybdenum oxide (V2O3). After that, the likelihood ofreducing the trace amount of low valance molybdenum oxide and low valance molybdenum oxideby [C] and [Si] in molten steel has been confirmed.(2) It is found that both of MoO3and V2O5have low melting points and results in an easy volatilization at high temperatures. The effect of addition agents (FeSi, CaO or MgO) ondecreasing the volatilization of MoO3and V2O5were investigated by TG-DTA. The resultsindicated that the reduction of MoO3to low valance molybdenum oxide by FeSi could minimizethe volatilization loss, as well as V2O5. It can decrease the volatilization of MoO3when addingbasic oxides (CaO, MgO, etc.), which contributes to the formation of non-volatile molybdates.The volatilization loss of pure MoO3is94.27%at1400oC, while that of MoO3in MoO3-FeSi is25.7%, and that is about4.5%in MoO3-CaO and MoO3-MgO. The volatilization loss of pureV2O5is3.39%at1500oC, while that in V2O5-FeSi is0.41%.(3) Self-reducing experiments of MoO3and V2O5have been performed in a carbon tubefurnace. At900oC, MoO3could be reduced to MoO2by FeSi. At1300oC, MoSi2and FeMo wereobserved in the reducing products. It was favor to the silicothermic reduction for adding CaO (orMgO), that was because CaO (or MgO) could react with the reduction product SiO2into CaSiO3(or MgSiO3, Mg2SiO4), and the volatilization of MoO3could be inhibited to~2%. Adding CaF2again to form a low melting slag resulted in good slag metal separation. At600oC, V2O5could bereduced to VO2by FeSi. With the increase of temperature, V2O5could be reduced to low valancemolybdenum oxides, and FeV could be found at1500oC. With the existion of MgO and CaF2, theslag has melted at1500oC, most of FeSiV2and FeV have been found in the products. It indicatesthat it was favor to the reducing process and resulted in good metal slag separation, which couldhelp Mo diffuse into steel melt during the direct alloying process.(4) The self-reducing briquettes of MoO3(or V2O5) with FeSi, MgO and CaF2had beenprepared and added into a medium frequency induction furnace for direct alloying experiments.The results indicated that the self-reducing briquettes could melt within6min after being addedinto the molten steel. The alloying elements could be reduced from MoO3(or V2O5), and thendissolve into the molten steel. With the optimum composition of self-reducing briquettes, the yieldof Mo could be more than95%, while the yield of V could reach above94%.(5) The industrial scale tests on direct alloying with self-reducing briquettes of MoO3(orV2O5) has been carried out in a60t BOF–LF refining process. The technical process was that theself-reducing briquette was added into the ladle during converter tapping for direct alloying. Asthe silicothermic reduction process is exothermal, moreover with the help of CaF2in theself-reducing briquette, the slag melted well. When the ladle reached to LF refining station,samples were taken from the steel so as to analyze the contents of Mo (or V). The average yield ofMo during direct alloying was97.69%, and the average yield of V during direct alloying was95.77%. The results of inclusion appraisal and flaw detection indicated that the finished products meet the required quality standard.(6) A model on calculating economic benefit can be established by comparing the cost ofdirect alloying with M0O3(or V2O5) self-reducing briquettes or alloying with ferromolybdenum(or ferrovanadium), and then a visualization system for calculating the economic benefit can beedited by using Visual Basic. Wth the help of the calculation model，the economic benefit couldbe conveniently forecasted when making steel increase0.1%Mo (or0.1%V) during the directalloying process.