EXPERIMENTAL ESTIMATION OF SULFUR SIGNIFI CANCE IN TIN TRANSFER BY SUPERCRITICAL FLUIDS

To study tin state in the sulfur-bearing supercritical solutions a ser ies of experiments has been carried out with herzenbergite SnS in wate r and HCl solutions, cassiterite and sulfur in water, cassiterite in N a2S solutions at 500-degrees-C and 1 kbar using ampoule technique unde r the hydrogen fugacity controlled by the buffer Ni-NiO. Herzenbergite has been found to dexompose with cassiterite formation under the give n conditions and comparatively high hydrogen partial pressure (1.74 ba r) corresponding to the Sn2+ ion-bearing complexes stability in soluti ons. Comparison of the data obtained and the previously published expe rimental results on the SnO2 dissolution in the fluids of different co mpositions points to the leading role of halogenide and hydroxo forms of Sn(II) and Sn(IV) and to a minor role of tin hydrosulfide complexes in sulfur-bearing solutions. The upper limit of the full dissociation constant pK0(SnHS2)0(0) 15.0 has been estimated based on comparison o f the experimental and model SnO2 solubility in the equilibrium SnO2(s ol) +2H2S(liq) +H-2(gas) = Sn(HS)2(0) + H2O. The constants of stepped dissociation of tin hydrosulfide complexes have calculated: at 500-deg rees-C, 1 kbar pK(SnHS+)0 8.7, and pK(Sn(HS)2)0(0) 6.3; at 25-degrees- C, 1 bar pK(SnHS+)0 congruent-to 3, and pK(SnHS2)0(0) congruent-to 2. The stability of tin hydrosulfide complexes decreasing during the hydr othermal experiment. A conclusion has been made that hydrogen sulfide can not play a significant role in tin transfer. Srtability fields of phases in the system SnO2-SnO-SnS-H2S-H2O-H-2 have been calculated. Th e calculations show that tin sulfides are precipitated in hydrothermal conditions under low temperatures, high reductive potential and high sulfur activity.