The Solubility of Sodium Sulfate and the Reduction of Aqueous Sulfate by Magnetite under Near-Critical Conditions

The solubility of Na2SO4 (s) (thenardite) and the interactions between magnetiteand aqueous Na2SO4 near the critical point of water have been determined in azirconium-alloy flow reactor at temperatures 350°C ≤ t ≤ 375°C and isobaricpressures 190 ≤ p ≤ 305 bar. The experimental solubility data are describedwell as a function of temperature and solvent density ρ1 byln x(Na2SO4, aq.) = −10.47 − 27550/T +(4805/T) ln ρ1.The interaction between magnetite and Na2SO4 (aq.) was examined from 250 to370°C at molalities near the saturation composition of Na2SO4 (s). While no solidreaction products were observed, HS− (aq.) was observed to form above 350°Cby sulfate reduction, as a product of the reaction8 Fe3O4(s) + Na2SO4 (aq.) + H2O(l)= 12 Fe2O3 (s) + NaHS (aq.) + NaOH (aq.).The reduction reaction appears to be controlled by surface reaction kinetics, ata level well below the equilibrium molality of HS− (aq.). Metallic iron reactedwith Na2SO4 (aq.) in a similar fashion at temperatures above 350°C, to yieldhigher molalities of HS− (aq.).