Sulfur determination in carbon-saturated iron by solid-state electrochemical sensor

In the frame of the continuous efforts to thein situ monitoring of the chemical composition of melts in steelmaking, the solid-state electrochemical probe (−)Pt/Nb-NbO/Caβ″-alumina-CaS/S, Fe (1)/C/Pt (+) was developed and tested to measure the sulfur content in carbon-saturated iron. The solid electrolyte used is calcium β″-Alumina, which behaves as a good ionic conductor by calcium vacancies at steelmaking temperatures. The calcium chemical-potential gradient is fixed at the electrolyte interfaces, on the left side, by keeping constant the oxygen chemical potential through the Nb-NbO coexistence mixture and, on the right side, by the activity of dissolved sulfurvia the solubility equilibrium of CaS. A theoretical equation of the probe electromotive force (emf), as a function of the dissolved concentration of sulfur in the bath, was derived. From this equation, which agrees well with the experimental results, it is clearly shown that the emf of the probe can be approximately linear against log ([S]/pct) in small intervals of sulfur concentration. The experimental emf s were taken up in the sulfur concentration range from 13 to 140 ppm. In this interval, the average slope of emfvs log ([S]/pct) curve is about −50 mV/decade. The effect of dissolved oxygen is discussed because it generates a mixed potential related to the CaS instability. The lower sulfur concentration limit, that can be detected, depends on the concentration of the oxygen dissolved in the melt because CaS tends to react with it. Some characteristic quantities of the theoretical equation were chosen as fitting parameters to fit the equation itself through the experimental emfvs log ([S]/pct) data. By this procedure, it was also found that the CO supersaturation in the bath was 10.2 bar.