Thermogravimetric analysis of Ontario limestones and dolomites. II. Reactivity of sulfur dioxide with calcined samples

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Abstract
Thermogravimetric analysis was done for representative calcined samples of Ontario limestones and dolomites in a 5% SO2 controlled atmosphere. The SO2 absorption vs. temperature curves showed three distinct regions. (i) A unimolecular chemisorption of SO2 at room temperature. (ii) An intermediate temperature region between 200–450 °C where absorption of SO2 was due to increased rate of SO2 diffusion into the interior of CaO via grain boundaries. When the surfaces of these internal spaces were almost fully occupied the rate of SO2 absorption decreased. Further absorption was due to diffusion of SO2 into CaO lattice, which was a much slower process. (iii) A fast absorption region between 500–700 °C, where a disproportionation of CaSO3 into CaSO4 and CaS took place which effectively broke down the original CaO structure to expose fresh layers of CaO. The amount of SO2 absorption after 1 h at 745 °C corresponded to approximately 80% conversion of CaO to mainly CaSO4 and CaS and smaller amount CaSO3.Finally, the calculations showed that the reaction of SO2 with CaO was more exothermic than the reaction of SO2 with MgO, which accounted for the faster rate of the former.

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