Identification of Calcium Sulphoaluminate Formation between Alunite and Limestone

Abstract: This study was carried out to identify the conditions of formation of calcium sulphoaluminate (3CaO·3Al₂O₃·CaSO₄) by the sintering of a limestone (CaCO₃) and alunite [K₂SO₄·Al₂(SO₄)₃·4Al(OH)₃] mixture with the following reagents: K₂SO₄, CaCO₃, Al(OH)₃, CaSO₄·2H₂O, and SiO₂. When K₂SO₄, CaCO₃, Al(OH)₃, CaSO₄·2H₂O were mixed in molar ratios of 1:3:6:3 and sintered at 1,200~1,300 °C, only 3CaO·3Al₂O₃·CaSO₄ and calcium langbeinite (2CaSO₄·K₂SO₄) were generated. With an amount of CaO that is less than the stoichiometric molar ratio, 3CaO·3Al₂O₃·CaSO₄ was formed and anhydrite (CaSO₄) did not react and remained behind. With the amount of CaSO₄ that is less than the stoichiometric molar ratio, the amounts of 3CaO·3Al₂O₃·CaSO₄ and 2CaSO₄·K₂SO₄ decreased, and that of CaO·Al₂O₃ increased. In the K₂SO₄-CaO-Al₂O₃-CaSO₄-SiO₂ system, to stabilize the formation of 3CaO·3Al₂O₃·CaSO₄, 2CaSO₄·K₂SO₄, and β-2CaO·SiO₂, the molar ratios of CaO: Al₂O₃: CaSO₄ must be kept at 3:3:1 and that of CaO/SiO₂, over 2.0; otherwise, the generated amount of 3CaO·3Al₂O₃·CaSO₄ decreased and that of gehlenite (2CaO·Al₂O₃·SiO₂) with no hydration increased quantitatively. Therefore, if all SO₃(g) generated by the thermal decomposition of alunite reacts with CaCO₃ (or CaO, the thermal decomposition product of limestone) to form CaSO₄ in an alunite- limestone system, 1 mol of pure alunite reacts with 6 mol of limestone to form 1 mol of 3CaO·3Al₂O₃·CaSO₄ and 1 mol of 2CaSO₄·K₂SO₄.