REACTION BETWEEN GASEOUS SULFUR-DIOXIDE AND SOLID CALCIUM-OXIDE - MECHANISM AND KINETICS

Sulfur dioxide has been reacted with tiny (diameter 35-85 mu m) partic les of porous CaO in a thermogravimetric balance. Kinetic measurements , together with infrared studies of the products, indicate that two re actions, viz: CaO + SO2 --> CaSO3 and CaO + SO2 --> 3/4CaSO(4) + 1/4Ca S occur. In the presence of both SO2 and O-2, when CaSO3 and CaS both oxidise to CaSO4, these two reactions of SO, provide the rate-determin ing steps for the initial stages of reaction for such tiny particles. Thus, the conditions used in this study succeed in preventing the rate s of either of these reactions being controlled by the diffusion of SO ,. The rates of both reactions per unit surface area of CaO have the g eneral form: k([SO2] - [SO2](e)), where [SO2](e) is the concentration of SO2 for the reaction concerned being at equilibrium and k is the ra te constant. Measurements of initial rates indicate that k = 7.2 x 10( -6) exp(-1443/T) m s(-1) and 1.2 x 10(-6) exp(-481/T) m s(-1) for the two reactions, respectively, correct to 25%. However, CaSO3 is unstabl e above ca. 1123 K, so that only the slower (second) reaction occurs a bove ca. 1123 K. That the rates of these reactions are independent of the concentration of oxygen confirms that SO, plays no part in them. M easurements for large extents of reaction showed that the diffusion co efficient of SO, through the solid products of reaction (mainly CaSO,) was D-s = 1.9 +/- 0.5 x 10(-14)m(2) s(-1). However, at high temperatu res (>1160 K) when only CaSO4 and CaS are produced, D-s falls to 2.3 x 10(-15) m(2) s(-1).