Potassium Tantalum Fluoride: Tantalite Fused with Bisulphate, Dissolved in Hydrofluoric Acid and Crystallised (Roscoe 1913)

Tantalum and niobium travel together in their ores and are parted by their double fluorides: potassium tantalum fluoride crystallises from strong hydrofluoric acid, the niobium salt stays in solution. A bisulphate fusion and an ammonium sulphide wash clean the ore first. Result: needles of potassium tantalum fluoride (K2TaF7), and from the mother liquor potassium niobium oxyfluoride. Source: Roscoe and Schorlemmer, A Treatise on Chemistry vol. 2 (1913), p. 948. Not given: absolute amounts and temperatures. Hydrofluoric acid burns deep and poisons through the skin: full protection, calcium gluconate gel at hand. Picture drawn by Youblob.