Estudo termodinâmico da redução gás-sólido de óxidos de nióbio

Abstract

Most of niobium deposits worldwide are found in Brazil. This metal’s excellent physical and mechanical properties make it attractive to the industry. Niobium is commonly obtained through carbothermic and metalothermic reduction methods. The present work aimed to study the thermodynamics behind niobium oxides reduction by gaseous reducing agents, such as methane and carbon monoxide. The equilibrium constants of the representative independent reactions of each system were calculated and the temperatures at which the reactions are more feasible were selected. Equilibrium composition was calculated by the Gibbs energy minimization method in Microsoft Excel spreadsheets using the Solver supplement. Starting with NbO as the solid reactant, the reduction was investigated in different gas phase compositions (purê gas, 67.5%, 12.5% and 2.5% of a reductant/H2 mixture) and number of moles of the solid (0.5, 1. 1.2 and 2 mols). The optimum conditions found were then applied to Nb2O5 and NbO2. Results showed that CO is not a feasible choice for niobium oxide reductions. Reaction between NbO and CH4 tends to carbide formation but at lower methane compositions and higher temperatures it’s possible to obtain metallic niobium. Complete conversion of NbO in Nb is obtained at 2150 K under 2.5% CH4/H2 and 1.2 moles of NbO. In the same conditions NbO2 was completely cnverted in NbO at 2100 K. Nb2O5 is reduced to NbO2 under any conditions even at temperatures as low as 1100 K but the presence of H2 avoids methane decomposition. Thus, this work concludes that – from a thermodynamical point of view – CH4 is a promising reducing agent for niobium oxides.