Evaporation Mechanism of Sn and SnS from liquid Fe - Part I: Experiment and Adsorption of S on Reaction Site

Abstract

In order to evaluate feasibility of Sn-containing ferrous scrap recycling by evaporation of Sn, a number of liquid-gas experiments were carried out using an electromagnetic levitation melting technique. Rate of decrease of Sn concentration in liquid steel droplets by evaporation in Ar-H-2 gas mixture was determined at 1873 K (1600 degrees C). Evaporation rate of the Sn under various conditions (various flow rates of the gas mixture, initial S concentration, [pct Sn](0)) was examined using previously reported rate equations. Increasing flow rate increased the evaporation rate of Sn initially, but the rate became constant at higher flow rate, which indicates that the rate-controlling step is the chemical reaction at the liquid/gas interface. Increasing initial S concentration significantly increased the evaporation rate of Sn, which is in good agreement with previous understanding that Sn could be evaporated as SnS(g). It was found in the present study that neither a simple first-order reaction (rate proportional to [pct Sn]) nor a second-order reaction (rate proportional to [pct Sn] x [pct S]) could account for the Sn evaporation under a chemical-reaction-controlled regime. It is proposed in the present study that surface adsorption of S should be taken into account in order to interpret the evaporation rate of Sn in such a way that S blocks available sites for SnS evaporation on the liquid steel. The ideal Langmuir isotherm was applied in order to better represent evaporation rate constant k(SnS) as a function of [pct S] (0.06<[pct S](0)< 0.29). The obtained rate constant of a reaction Sn-i + S-i = SnSi (g), k(SnS)(R), is 2.57 x 10(-8) m(4) mol(-1) s(-1).