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

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 ZrOHCl 3(aq) 0 and NaZrOHCl 4(aq) 0 complexes, while dominant hafnium species are HfOHCl 3(aq) 0 , HfCl 4(aq) 0 , and NaHfOHCl 4(aq) 0 or NaHfCl 5(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.