A influência do tratamento térmico em óxidos mistos de nióbio e tântalo nanoestruturados a partir da columbita

Abstract

In this work the study of heat treatment in a mixture of niobium and tantalum oxide extracted from columbite mineral obtained in Santa Cruz, RN. Initially the mineral was physically prepared for their composition and grain size were appropriate subsequent steps of extraction and heat treatment of oxides, passing thus the grinding step, screening and quartering for mixing of samples, followed by characterization by X-ray diffraction (XRD) and X-ray fluorescence (XRF). Following the mineral was leached for 8 hours at 70 °C for removal of iron and manganese present in their composition. To eliminate the tin which is present in high concentrations in the raw mineral, with percentages above 10% were conducted leaching for 8 hours at 50 °C and 5 hours at 90 °C in order to work with the thermal stability of cassiterite (SnO2), but significant changes in its percentage in the treated mineral were not verified. Thus, new leaching was performed for 30 min at 70 °C with a mixture of 5M HF and H2SO4 0.5M to solubilize the oxides of Nb and Ta, and so separate them persistent impurity in mineral cassiterite. Hydrated oxides of Nb and Ta were precipitated with NH4OH and characterized by fluorescence X-ray (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical microanalysis by dispersive (EDS). Then the oxides were calcined at 800 °C for 5h and variable heating rate, 900 ° C for 5h at 10 °C.min-1, and to 1000 °C at a fixed heating rate 10 °C.min-1 and variable time calcination, to be made monitoring the crystallization of oxides. Mixed oxides of Nb and Ta were characterized by fluorescence X-ray (XRF), diffraction X-ray (XRD), scanning electron microscopy (SEM), chemical microanalysis by energy dispersive (EDS) and specific surface area (BET ). The influence of the heating rate was found in testing at 800 °C. An increase in crystal size was observed when the rate was reduced from 10 °C.min-1 to 5 °C.min-1. There was a higher crystallinity and formation of peaks relating to the hexagonal phase shift to the orthorhombic phase of the oxides as the calcination temperature was increased from 800 °C to 900 °C. At a temperature of 1000 °C intermediate and low crystallinity peaks relating to the formation of the monoclinic phase of niobium pentoxide were observed. The change in 5h calcination time for 10h at this temperature caused a reduction in the niobium pentoxide to NbO2, evidenced by the analysis of EDS, and decrease in crystal size with consequent increase in surface area. The oxides calcined at 900 ° C showed the greatest surface area among those studied in this work.