Estimated thermodynamic properties of NaFeS2 and erdite (NaFeS2:2H2O)

• First full set of thermodynamic properties for erdite (NaFeS2:2H2O) and NaFeS2.
• Combination of theoretical models and few but relevant experimental data.
• Coherency with CODATA recommended values and the Thermoddem database.
• Results useful for the management of by-products in the lead-recycling sector.
• New information for geologists about the occurrence of erdite in natural media.

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: NaFeS2 and erdite (NaFeS2:2H2O). The solubility product of erdite was measured at 20 °C in a batch reactor. It is −38.0 ± 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 °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 NaFeS2 and of the water of hydration for transforming NaFeS2 into erdite. At 25 °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 NaFeS2, 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.