Pressure leaching of manganotantalite by sulfuric acid using ammonium fluoride as an assistant reagent

Manganotantalite is difficult to decompose by sulfuric acid alone. Even at sulfuric-acid mass concentrations of 60%, only 8.5% tantalum and 9.9% niobium leached. To obtain a high leaching efficiency of tantalum and niobium, ammonium fluoride is added as an assistant reagent. Over 93% tantalum and 96% niobium were leached by mixing − 74 μm manganotantalite with 50% sulfuric acid and ammonium fluoride, with heating for 2 h at 200 °C in a closed vessel. The ammonium fluoride/manganotantalite mass ratio was 0.8:1. A comparison of the novel technology that is proposed in this article with traditional hydrofluoric-acid technology shows that the mass concentration decreases significantly from 60 to 70% to 5.3%. Excess fluoride ions in solution can be recycled as the assistant reagent. Therefore, hardly any fluoride ions will be discharged as environmental pollutants. The local-density approximation was used for an adsorption-energy calculation of the fluoride ion on the MnNb2O6 (100) surface. The calculation results indicate that fluoride-ion adsorption on the MnNb2O6 (100) surface is a chemisorption process. The partial density-of-states analysis indicates that fluoride ions interact mainly with surface Nb atoms through a complicated orbital hybridization. The charge transfer from Nb to F confirms the ionic characteristics of the NbF bonds.