Precipitation of niobic acid from potassium niobate in sulphuric acid solution: A route to niobium oxide

Abstract

This study investigates the precipitation of niobic acid (Nb2O5.nH2O) from potassium niobate solution with addition of sulphuric acid, and its subsequent thermal conversion into niobium oxide (Nb2O5). Thermodynamic modelling with PHREEQC and OLI Studio was used to predict the optimal conditions for niobic acid formation. Precipitation with different H2SO4 concentrations produced amorphous niobic acids similar to commercial reference, with impurity removal optimized through a washing protocol. X-ray fluorescence (XRF) analysis indicated that precipitation using 0.50 mol/L H2SO4 and a solid-to-liquid ratio of 20 g/L yielded the highest niobium content (68.93 wt.%). Characterization revealed that residual potassium promoted the formation of crystalline potassium niobium oxide phases (K2Nb8O21, K2Nb14O36) during calcination, particularly in samples with ~2 wt.% potassium, which exhibited rod-like agglomerates (~30 μm). Precipitate washing at low solid-to-liquid ratios significantly reduced potassium content in the product. A decrease from 60 to 20 g/L reduced the potassium concentration by a factor of three (to 0.06 wt.%), eliminating potassium niobium oxide phases. The pure Nb2O5 phase, after the 20 g/L-washing, was confirmed by X-ray diffraction (XRD) and transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDS). These results emphasize the importance of supersaturation control, calcination conditions, and purification protocols in producing high-purity niobium oxide from secondary niobium sources.