A Study on the Correlation between Applied Electricity and Electrochemical Desulfurization of Liquid Iron By Molten Slag

Abstract

Based on the electrochemical character of the desulfurization of liquid iron by molten slag, a previous study reported that applying electricity could enhance the desulfurization. The authors have investigated the mechanism of electrochemical desulfurization and its practical feasibility in terms of reaction rate and electricity efficiency. Nevertheless, the quantitative relationship between electricity and desulfurization still needs to be comprehended. The present study attempted the correlation by carrying out a series of electrochemical desulfurization experiments and thermodynamic calculations. Experiments were designed in a two-step sequence: the first was the normal desulfurization of a liquid iron containing C and S by CaO-Al2O3-MgOsat. slags without applied electricity, and the second was the subsequent electrochemical desulfurization with various electric currents. A graphite disc was put on the surface of the slag and connected with Mo wire to a DC power supply to supply electricity to the system. All the experiments were conducted in the induction furnace at 1400 °C under a CO gas atmosphere. At predetermined times, a certain amount of the iron and slag was sampled and analyzed using the C/S combustion method (LECO CS844) for S contents in both phases. The results were interpreted by employing the Nernst equation to extract the potential difference ∆𝜙S for the electrochemical desulfurization. It was assumed that 1) ∆𝜙S was an electrochemical potential difference to enhance the electrochemical desulfurization, 2) potential differences due to the electrochemical reaction and others (the electric resistance over the slag, iron, etc.) are added to the overall voltage difference observed. It was found that applying electric current (I) to the liquid iron and the molten slag increased the final S distribution ratio between the two phases, the so-called L S (consequently ∆𝜙S increased). When the I was kept constant during each experiment, increasing C/A (= %CaO/%Al2O3) decreased R DeS (= ∆𝜙S / I). For a given C/A, R DeS was independent of the applied I within the range of the I, so the electric properties of the slag might influence it. A previous study reported that the electrical conductivity of a similar slag was increased by increasing the C/A of the slag. Therefore, increasing C/A in the present study increased the electrical conductivity of the slag (𝜎) and decreased the resistance of the slag. It was in the same direction as the R DeS. In the present study, the obtained ∆𝜙S was also predicted by applying the Nernst equation along with a thermodynamic model for liquid iron and sulfide dissolved in molten slag. The enhanced desulfurization by the electricity was interpreted as increasing activity of CaS in the slag and decreasing activity of S in the liquid iron. Accordingly, the predicted ∆𝜙S increased with increasing current under all slag conditions, which was in good agreement with the experimental data. The model is expected to predict the necessary current level for a desired electrochemical desulfurization.