A kinetic and mechanistic investigation of the formation of calcium sulphate in reactions that may be of use in flue-gas desulphurization I. Oxidation of CaSO 3 or of CaS with O 2 or with SO 2

Abstract

A kinetic and microscopic study of the oxidation of anhydrous CaSO$\_{3}$ to CaSO$\_{4}$ in low pressures (< 700 N m$^{-2}$ O$\_{2}$) of oxygen between 823 and 973 K is reported. The work was undertaken to investigate further the chemistry and mechanism of this reaction because of its importance in environmental protection as a method of flue-gas desulphurization. The isothermal oxidation of CaSO$\_{3}$ is strongly deceleratory. Rate characteristics were satisfactorily described by kinetic expressions based on rate control by a diffusion process across a barrier product CaSO$\_{4}$ layer of progressively increasing thickness that develops between the CaSO$\_{3}$ and O$\_{2}$ reactants. Oxidation ceased when the thickness of this barrier reached about 0.05 $\mu $m, which corresponds to about 28% reaction of the fine reactant CaSO$\_{3}$ crystallites. This termination of reaction before completion is ascribed to the transformation of the initial ill-crystallized product into an impermeable layer of recrystallized CaSO$\_{4}$ that is coherent and strongly adheres to all reactant surfaces. After this partial oxidation, all surfaces of the solid are roughened by the development of irregularly disposed edges and corners of small (0.1-0.3 $\mu $m) product crystallites. The study included kinetic investigations of three related reactions (CaSO$\_{3}$ + SO$\_{2}$; CaS + O$\_{2}$ and CaS + SO$\_{2}$) that also yield CaSO$\_{4}$. These rate processes were satisfactorily represented by the same diffusion-controlled rate equation. Rate constants for both reactions with oxygen (+CaSO$\_{3}$ or CaS) agreed within experimental error. The two reactions in SO$\_{2}$ were relatively slower ($\times \frac{1}{6}$) but again rate constant magnitudes were the same. The calculated activation energy for all four reactions was 125 $\pm $ 8 kJ mol$^{-1}$ and all data were close to two parallel lines on the Arrhenius plot. These reactions, together with CaSO$\_{3}$ dissociation and disproportionation, occur in similar temperature intervals (above 800 K). We conclude that all involve similar formation and rupture steps of S-O bonds that results in facile transfer of oxygen atoms between the divalent anions involved (S$^{2-}$, SO$\_{3}^{2-}$ and SO$\_{4}^{2-}$), oxygen and SO$\_{2}$. The importance of this common step in the several chemical changes investigated here is discussed in the formulation of reaction mechanisms.