The Role of Fluorine in Hydrothermal Transport of Tin Sn(IV) from Experimental Data

The solubility of SnO2(cr) (cassiterite) in H2O–HF ± NaF solutions was determined at 25 (1 bar) –250°C (Psat). Several experiments were carried out in HClO4 and NaOH solutions at 250°C. In HF-bearing solutions, the main Sn(IV) species are the hydroxofluoride complexes HSnO2F(aq) (Sn(OH)3F(aq)) and SnO2F2(aq) (Sn(OH)2F2(aq)), the latter predominating at 200°C at high HF concentrations up to 1 m [mol (kg H2O)–1] and being the main hydroxofluoride complex of Sn(IV) at t > 300°C. In alkaline fluoride solutions, the main complex is SnO2F– (Sn(OH)4F–). New experimental data were regressed together with literature data with the calculation of the standard thermodynamic properties and parameters of the HKF (Helgeson-Kirkham-Flowers) model equation of state for Sn(IV) hydroxofluoride complexes and SnO2(aq) (Sn(OH)4(aq)) complex. These parameters are suitable for modeling tin transport up to 500°C, 5000 bar with the possibility of extrapolation to higher PT parameters. The solubility of cassiterite in acidic fluoride fluids increases sharply with increasing temperature: an increase in temperature from 100 to 400°C leads to a more than tenfold increase in solubility, which at 400°C, 500 bar reaches 20 ppm at a total fluorine concentration of 1 wt %. Under reducing conditions, the dependence of solubility on temperature will be more pronounced due to the formation of Sn(II) complexes at supercritical temperatures.