Determination of Desulphurising Ability for Ca–CaF₂ Flux Using Equilibration with Molten Silver at 1673 K in Closed Graphite Crucibles

Abstract

The present study aims to evaluate the desulphurising ability of Ca–CaF₂ flux. Desulphurisation equilibrium between molten CaS–CaF₂ flux and molten silver has been investigated using a crucible covered with a cap having a capillary. The relationship between calcium and sulphur concentrations in molten silver has been clarified for CaS saturated and unsaturated systems. Two thermodynamic properties have been determined, namely, the equilibrium constant for the dissolution reaction of CaS into molten silver and the activity coefficient product of calcium and sulphur in molten silver from the data for CaS saturated system, as follows:
CaS(s) = Ca(mass% in Ag) + S(mass% in Ag)
logK = –6.38
log(f_Caf_S) = –87.3(±12.3)√C (C < 0.032)
C = [%Ca]/M_Ca + [%S]/M_S
The activity coefficient of CaS in CaS–CaF₂ flux has been obtained as a function of CaS mole fraction in flux from the data for CaS unsaturated system:
logγ_CaS = 2.67(1–X_CaS)²–1.26 (X_CaS < 0.12)
Combination of these thermodynamic data leads to the distribution ratio of sulphur between molten Ca–CaF₂ flux and molten SUS316L stainless steel, 4.7×10⁶ at 1673 K. This value is extremely higher than that in conventional refining, indicating that metallic calcium based fluxes have extraordinary desulphurising ability.