Thermodynamic Simulation and Experimental Investigation of the Calciothermic Reduction of Metals from Manganese and Iron Niobates

The calciothermic reduction of metals from manganese (MnNb2O6) and iron (FeNb2O6) niobates, which are analogs of natural columbite (Fe, Mn)(Nb, Ta)2O6 contained in commercial rare metal ores, is studied. Thermodynamic simulation using the HSC Chemistry software package is used to calculate the change in the Gibbs free energy, the equilibrium constants, and the thermal effects of the interaction reactions of MnNb2O6, FeNb2O6, and Nb2O5 with calcium. The probability of formation of both niobium and its alloys with iron or manganese is found to be high. The changes in the equilibrium phase compositions of the oxide–calcium interaction products as functions of temperature (in the range 1000–2500°C) and the ratios of reagents are estimated. According to the calculations of the adiabatic temperature and the specific heat effect, the reduction of metals from FeNb2O6 is feasible under “out-of-furnace” process conditions, and the reduction from MnNb2O6 would require preheating a charge to 600–800°C. During continuous heating up to 1200°C, the temperatures of the thermal effects related to reagent transformations and phase formation in the interaction products are detected by combined thermogravimetry and differential thermal analysis. The complete reduction of manganese and iron niobates is found to occur with the participation of molten calcium at a molar niobate/calcium ratio of 6 with the formation of metallic niobium and its intermetallic compounds, NbMn2 or FeNb, respectively. At excess reducing agent, solid-phase products hinder the diffusion of the calcium melt to the reaction surface, which slows down the completion of the process and promotes the development of secondary interactions between intermediate oxides (NbO, Mn0.95O, Mn2.907O4) and calcium oxide. To complete the reduction process, it is necessary to increase the temperature or the heating time.