Synthesis of Calcium Sulfite Tetrahydrate and Its Thermal Change

Abstract

Calcium sulfite tetrahydrate was synthesized from milk of lime containing 0.05 wt% sodium citrate and sulfur dioxide by semibatch heterogeneous reaction at 4-6° C. It was obtained as the transparent crystals with particle diameters of 3-60 gm (average diameter: 15 Atm). The chemical composition of the synthesized calcium sulfite tetrahydrate was CaO: 29.7 wt%, SO₂: 32.5 wt% and H₂O: 35.9 wt%. These values agreed well with the calculated values. In the powder X-ray diffraction pattern, the strong diffraction lines were observed at the dvalues(A) of 5.80, 3.64, 2.88 and 2.68 (Table 2). The thermal analysis of the tetrahydrate was carried out by DTA, TG and DSC under the condition of constant heating and cooling rates. The endothermic weight losses of 30.9 and 3.8 wt% were observed at temperature ranges of 30-69 and 363-390° C, respectively (Fig.1). It was considered that the former resulted from vaporization of 7/2 H₂O, and the latter from vaporization of 1/2 H₂O. It was revealed that the change of tetrahydrate to hemihydrate occurred at a temperature range of 34.3-55.5° C and absorbed the heat of about 17 kjimol(CaSO^2-.4 H₂0).