Hydrofluoride decomposition of natural materials including zirconium-containing minerals

Abstract

Recently, interest in ammonium hydrogen difluoride (NH₄HF₂) as a versatile fluorinating agent for the decomposition of natural materials resumed. It is considered to be a new and more efficient than hydrofluoric acid (HF) reagent in analytical chemistry. Thermodynamically possible fluorination reactions with NH₄HF₂ are exothermic and proceed even at room temperature with the entropy reserve. The fluorination products are of high symmetry phases (tetragonal or cubic) with partial substitution of fluoride ion for oxide (or hydroxide). The fluorination of refractory silicate zircon (ZrSiO₄) is kinetically hindered, and its complete decomposition requires the use of a Teflon autoclave at 200^oC. The fluorination products are cubic (NH₄)₃Zr(OH)_xF_7-x (x ≤ 0.3) and tetragonal double salt (NH₄)₃SiF₇, which can be separated due to incongruent sublimation of (NH₄)₂SiF₆. The mechanism of the latter process is proposed.