Reaction between gaseous sulfur dioxide and solid calcium oxide mechanism and kinetics

Abstract

Sulfur dioxide has been reacted with tiny (diameter 35–85 µm) particles of porous CaO in a thermogravimetric balance. Kinetic measurements, together with infrared studies of the products, indicate that two reactions, viz: CaO + SO2→ CaSO3 and CaO + SO2→ 3/4CaSO4+ 1/4CaS occur. In the presence of both SO2 and O2, when CaSO3 and CaS both oxidise to CaSO4, these two reactions of SO2 provide the rate-determining steps for the initial stages of reaction for such tiny particles. Thus, the conditions used in this study succeed in preventing the rates of either of these reactions being controlled by the diffusion of SO2. The rates of both reactions per unit surface area of CaO have the general form: k([SO2]–[SO2]e), where [SO2]e is the concentration of SO2 for the reaction concerned being at equilibrium and k is the rate constant. Measurements of initial rates indicate that k= 7.2 × 10–6 exp(–1443/T) m s–1 and 1.2 × 10–6 exp(–481/T) m s–1 for the two reactions, respectively, correct to 25%. However, CaSO3 is unstable above ca. 1123 K, so that only the slower (second) reaction occurs above ca. 1123 K. That the rates of these reactions are independent of the concentration of oxygen confirms that SO3 plays no part in them. Measurements for large extents of reaction showed that the diffusion coefficient of SO2 through the solid products of reaction (mainly CaSO3) was Ds= 1.9 ± 0.5 × 10–14 m2 s–1. However, at high temperatures (> 1160 K) when only CaSO4 and CaS are produced, Ds falls to 2.3 × 10–15 m2 s–1.