New thermodynamic data on the Hg-OS system: With application to the thermal processing of mercury containing wastes

A modified transpiration reactor was used to measure the composition of the equilibrium gas phase formed above compounds in the Hg-O-S system at high temperatures (600 to 900K). A thermodynamic database ({Delta}G{degrees}{sub f}, {Delta}H{degrees}{sub f}, {Delta}S{degrees}{sub f}) for HgO, HgS, HgSO{sub 4}, Hg{sub 2}SO{sub 4}, HgSO{sub 4}-HgO, and HgSO{sub 4}{circ}2HgO was developed based on the experimental results. Prior to this study, only estimated data were available in the literature for the sulfates and oxysulfates at temperatures above 298K. A series of vapor pressure / stability diagrams were constructed from 473 to 973K with isobars of Hg(g), HgO(g), S{sub 2}(g), and SO{sub 3}(g). These diagrams were found to be significantly different than those available previously and do provide insight to the phase relations of importance during the thermal processing of Hg-containing wastes. The thermodynamic results were used to conduct a series of Gibbs energy minimization calculations for a thermal processor / afterburner system for the treatment of HgS-containing wastes. The retention of Hg in the discharge calcine was examined as a function of temperature and excess air. Hg was found to report to the calcine as HgSO{sub 4}, Hg{sub 2}SO{sub 4}, or HgSO{sub 4}-HgO depending on the process conditions, and Hg retention occurs at temperatures as high as 930K. The precipitation of Hg-containing phases from cooled afterburner discharge gas starts at approximately 900K and the cooling of the gas an additional 30K (to 870K) results in over 50% of the Hg being precipitated as HgSO{sub 4}-HgO. It was also determined that for small amounts of Hg present in sulfide concentrates during roasting, the Hg will be retained as HgSO{sub 4} or HgSO{sub 4}-HgO at temperatures as high as 870K. 8 refs., 30 figs., 3 tabs.