Thermodynamic analysis of hot water leaching of sulphur from desulphurisation slag by Eh–pH diagrams of the Ca-S-H₂O system

ABSTRACT

Eh–pH diagrams of a Ca-S-H₂O system at different conditions were described with thermodynamic calculations to investigate the leaching of sulphur from desulphurisation slag. The influences of temperature, oxygen partial pressure and activity of the dissolved species on the hydrothermal leaching of sulphur were systematically studied. The results show that the leaching reactions occur spontaneously under normal conditions and that the sulphur leaches into the leachate as S²⁻, HS⁻, H₂S_(aq), S_(s) and ${\text{SO}}_4^{2-}$. At higher temperatures, HS⁻ converts into S²⁻ or H₂S_(aq). Additionally, H₂S_(aq), HS⁻, S²⁻ oxidise to elemental sulphur or ${\text{SO}}_4^{2-}$ more easily. Moreover, the sulphur in CaSO_4(s) would readily dissolve into the leachate as ${\text{HSO}}_4^{-}$ and ${\text{SO}}_4^{2-}$ ions. When the oxygen partial pressure increases, S²⁻ tends to be oxidised into ${\text{SO}}_4^{2-}$. However, the effect of the oxygen pressure on the equilibrium between O₂ and H₂O decreases when increasing the oxygen pressure. When the activity of the dissolved species is reduced, elemental sulphur converts into other ions in redox reactions. Simultaneously, the predominant area of Ca(OH)_2(s) decreases, which indicates that Ca(OH)_2(s) may dissolve into Ca²⁺ and increase the loss of calcium. Experiments were conducted to verify the thermodynamic analysis. The data confirm that the leaching of sulphur from desulphurisation slag by wet processes is feasible.

KEYWORDS:

• Desulphurisation slag
• Ca-S-H₂O system
• Eh–pH diagrams
• leaching of sulphur

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was supported by Natural Science Foundation of Hubei Province of China (No. 2014CFB806) and National Natural Science Foundation of China (No. 51604202).