Estimated thermodynamic properties of NaFeS sub(2) and erdite (NaFeS sub(2):2H sub(2)O)

The main by-products of a lead-recovery process from waste are scoriaceous sodium-iron sulphides, whose storage is problematic due to their great reactivity with air and water. The objective of this work was to complete thermodynamic databases with a totally new set of properties for the two major components: NaFeS sub(2) and erdite (NaFeS sub(2):2H sub(2)O). The solubility product of erdite was measured at 20 degree C in a batch reactor. It is -38.0 plus or minus 0.3 when the reaction is written as a function of the predominant species at the experimental conditions: NaFeS2:2H2O+0.5H2O=HFeO2-+1.875HS-+0.125SO4=+Na++2.125H+. The resulting standard Gibbs free energy of formation of erdite from elements is -809.76 kJ/mol at 25 degree C and 1 bar. An original calculation procedure, relying on a combination of different theoretical models, then allowed the determination of the standard Gibbs free energies of formation of NaFeS sub(2) and of the water of hydration for transforming NaFeS sub(2) into erdite. At 25 degree C and 1 bar, they are -322.85 and -243.46 kJ/mol respectively. The other standard thermodynamic properties (enthalpy, entropy, and heat capacity) of erdite are -922.04 kJ/mol, 232.6 J/(mol K) and 175.7 J/(mol K), respectively. For NaFeS sub(2), they are -331.33 kJ/mol, 114.3 J/(mol K) and 97.7 J/(mol K), respectively. For the water of hydration they are -295.36 kJ/mol, 59.2 J/(mol K) and 39.0 J/(mol K), respectively.