Direct precipitation of niobium and tantalum from alkaline solutions using calcium-bearing reagents

The recovery of Nb(V) and Ta(V) from alkaline solutions was investigated using calcium-bearing reagents (CaCl2(aq), Ca(CH3COO)+(aq), Ca(NTA)−(aq), Ca(EDTA)2 −(aq), CaCO3(s) and Ca(OH)2(s)) at 25 °C. It was found that hexaniobate and hexatantalate ions (HxNb6O19x − 8 and HxTa6O19x − 8; 0 ≤ x ≤ 3) can be quantitatively precipitated with calcium chloride, calcium acetate or calcium hydroxide when the ratio (Ca/M)initial is at least 0.4 mol/mol (M = Nb, Ta). Precipitation yields higher than 95% are obtained for solutions that contain, as low as, 3 ∗ 10− 4 mol/L of Nb and 1 ∗ 10− 4 mol/L of Ta. Moreover, the proposed precipitation method can be operated in a wide pH range and consumes two times less reactant than the classical way for recovering Nb and Ta which consists of neutralizing their alkaline solutions with a mineral acid. The amorphous calcium niobate concentrate was characterized by elemental analysis (ICP-OES), thermogravimetric analysis (TGA-MS) and Raman spectroscopy and the solid phase K3Ca2.2(H0.6Nb6O19)·nH2O was identified under specific conditions.