Solubility Products of Metal Sulfides in Molten Salts: Measurements and Calculations for Iron Sulfide in the Eutectic Composition

Abstract

An electrochemical titration technique is used to determine the solubilities of Li2S and the solubility products of FeS in the molten LiCl-KCl eutectic composition in the temperature range from 673 ° to 773 ° K. The solubility products of FeS are, on an ion fraction basis, KSP = 0.44 × 10-12. 2.3 × 10-12, and 1.3 × 10-11 at 673 °, 723 °, and 773 ° K, respectively. These values are in good agreement with values calculated a priori from an exact thermodynamic cycle; at 723 ° K, we deduce a value of -38.36 kcal mole-1 (or -160.50 kJ mole-1) for RT In KSP, as compared with an experimentally derived value of -38.50 kcal mole-1(or -161.11 kJ mole-1). During the operation of the cells at relatively high sulfide compositions, the complex phases Li2FeS2 (=X phase) and LiK-6Fe24S26Cl (=J phase) are formed, as established by x-ray and metallographic analyses. A discussion of their chemical stability in the electrolyte in the temperature range of this study is presented. A moderate change in the activities of LiCl and KCl can influence the formation of the J phase. © 1978, The Electrochemical Society, Inc. All rights reserved.