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 D_s= 1.9 ± 0.5 × 10^–14 m² s^–1. However, at high temperatures (> 1160 K) when only CaSO₄ and CaS are produced, D_s falls to 2.3 × 10^–15 m² s^–1.