Some aspects of calcium sulphite reduction with carbon monoxide

The non-isothermal reductive decomposition reaction of crystalline CaSO₃·0.5H₂O and anhydrous CaSO₃ under CO, CO₂ and CO-CO₂ gas mixtures at the two heating rates of 25°C min⁻¹ and 6°C min⁻¹ up to 900°C, respectively, was investigated employing thermogravimetric analysis, X-ray powder diffraction, IR spectroscopy and continuous SO₂ measurement by on-line fluorescence spectroscopy. During the heating process the reaction involved a multistage weight loss, in both cases, with a corresponding multistage SO₂ release. The multistage weight loss and the corresponding SO₂ release were presumed to be the consequence of different sequential reactions taking place along with the direct reduction of CaSO₃ to CaS, including the formation and decomposition of CaCO₃ and CaSO₄, respectively. The corresponding SO₂ evolution was found to be considerably lower at slow heating rates. In the presence of additional CO₂, the SO₂ release was increased and the highest SO₂ concentration was found for a feed-gas mixture of 70% CO₂ and 30% CO, and also in pure CO₂. The CaCO₃ product also increased with increasing CO₂ concentration. The possible reaction pathway is discussed.