Aqueous speciation of lead and tin chlorides in supercritical hydrothermal solutions

抄録

In order to obtain the stepwise formation constants of tri-chloro complexes of Pb²⁺ and Sn²⁺, experiments on the effect of NaCl concentration on the ion exchange equilibria in the systems PbS–ZnS–PbCl₂–ZnCl₂–H₂O and CaWO₄–SnWO₄–CaCl₂–SnCl₂–H₂O were carried out. The experiments were conducted at 500°, 600°, 700° and 800°C (800°C is only for Sri²⁺), 1 kbar and also at 600°C, 0.5 kbar using 2 m aqueous chloride solutions. In all the experiments, the Σm(Pb)/(Σm(Pb) + Σm(Zn)) ratio of the aqueous chloride solutions in equilibrium with both PbS and ZnS and the Σm(Ca)/(Σm(Ca) + Σm(Sn)) ratio of the aqueous chloride solutions in equilibrium with both CaWO₄ and SnWO₄ decrease with the increase of NaCl concentration. The experimental results were analyzed thermodynamically using the previously published dissociation constants for NaCl_aq, CaCl_2aq and ZnCl₃^-_aq. As a result, the stepwise formation constants (log K) for PbCl₃^-_aq (PbCl_2aq + Cl^-_aq = PbCl₃^-_aq) were obtained to be −0.1 at 500°C and 1 kbar, 0.9 at 600°C and 1 kbar, 1.4 at 700°C and 1 kbar, and 1.3 at 600°C and 0.5 kbar. The stepwise formation constants (log K) for SnCl₃^-_aq (SnCl_2aq + Cl^-_aq = SnCl₃^-_aq) were obtained to be 0.5 at 500°C and 1 kbar, 1.4 at 600°C and 1 kbar, 2.0 at 700°C and 1 kbar, 2.6 at 800°C and 1 kbar, and 1.9 at 600°C and 0.5 kbar. The stepwise formation constants (log K) of tri-chloro complexes of Pb²⁺, Sn²⁺, Fe²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Cd²⁺ vary with sixfold coordinated ionic radius showing a parabola-shaped curve with a maximum between Mn²⁺ and Cd²⁺. This may suggest that the best ionic radius for tri-chloro complex exists between Mn²⁺ and Cd²⁺.