Study On Solid-Phase Synthesis And Mechanism Of Carbothermal Reduction Of Niobite

Niobium is one of rare metal in short on market in China. Niobium resource with 80% through China is deposited in Baiyunebo Ore, and the minerals concerned are niobite, fersmite and ilmenorutile. Niobite (FeNb₂O₆), usually the main phase composition in niobium concentrate, is of high value in economy. In present paper, pure niobite was synthesized in our laboratory, then subjected to carbothermic reduction. The reaction thermodynamics was analyzed, and the kinetics was investigated using isothermal and non-isothermal thermogravimetry and XRD techniques, the underlying mechanism was discussed.Thermodynamics study indicates that niobium oxide is reduced stage by stage, following the sequence of Nb₂O₅ to NbO₂, then to NbO, and to NbC finally.According to isothermal thermogravimetry for overall-range reduction reaction, the activation energy E=515.2 KJ/mol, the rate constant of reaction k is correlated with temperature T by the relationship lnk=36-61995/T, and the reaction kinetics follows the ZLT 3-D diffusion equation [(1-α)^-1/3-1]²= kt, whereαdenotes loss-in-weight rate. It indicates that CO gas diffusion cross the reaction product layer is the controlling step in carbothermic reduction of FeNb₂O₆.The combining of non-isothermal thermogravimetry with XRD techniques shows FeNb₂O₆ reduction consists of two stages: the first is at the temperature range of 1100-1240℃, FeNb₂O₆ is reduced by CO gass to Fe and NbO₂, the kinetics can be characterized by E=264.4KJ/mol, lnk=17.6-31799/T, andα+(1-α)ln(1-α)=kt. The second occurs at T=1250-1350℃, the first-stage product NbO₂ is reduced to NbC, correspondingly, E=482.7KJ/mol, lnk=17.6-31799/T, [(1-α)^-1/3-1]²=kt, in accord with the results obtained by isothermal thermogravimetry.