Zr/Hf ratio in supercritical chloride fluids: Experimental study of zirconium and hafnium complexation at 450°C and 0.6–1 kbar

Abstract

The solubility method was used to study the Zr and Hf behavior in supercritical aqueous chloride fluids (m(HCl) = 0.5–1.5, m(NaCl) 1 and 2 mol kg−1 H2O) at 450°C and 0.6–1 kbar. The crystals of natural baddeleyite (Kovdor Deposit) served as a solid phase in all experiments. An increase in HCl and NaCl concentrations leads to the increase of Zr and Hf contents in solutions. Zirconium mainly occurs as ZrOHCl3(aq)0 and NaZrOHCl4(aq)0 complexes, while dominant hafnium species are HfOHCl3(aq)0, HfCl4(aq)0, and NaHfOHCl4(aq)0 or NaHfCl5(aq)0. An increase of acid concentration is accompanied by the decrease of Zr/Hf ratio (Hf enriches fluid), which results from difference in acid-base properties of these elements and may lead to their separation in natural processes.