Preparation of Calcium Aluminates by Solid State Reaction of CaSO₄-Al₂O₃ System and Reactivity for SO₂

Abstract

High temperature gas-flow DTA apparatus working up to 1500° C was used to investigate the reversible reaction among calcium sulfate, aluminum oxide, calcium aluminate, and 'air-S02 mixture, shown in the following equation;
For the forward reaction, the experiments were carried out by using- GaSO₄-77-Al203 and CaSO₄: -ce-Al203 mixtures (1: 1 in mole ratio) in flowing nitrogen (100 m//min) up to 1400° C. For the reverse reaction, the experiments were carried out by -using air-SO₂ gas mixture (1: 1 in mole ratio and the flow rate of 100 m//min) and calcium alurninate samples prepared from -CaSO₄-Al203 mixtures by heating them at 1300° C for 1 hr. Thermal decomposition of pure CaSO₄reaction of CaO with Al203 and that of Ca-0 with air-SO, mixture were also tesied. to compare them. with the reactions in CaSO₄-Al203 sy'ste'rng.
The results are summarized as follows: (1) In the CaSO₄-Al203 system, the decomposition of CaSO₄ (CaSO₄ Ca0 S08, 4H° (1373 K)=89.7 kcal/mol) and the formation of calcium aluminates (for example, CaO + Al203 CaO. Al203, .411° (1373 K)=.5 kcal/mol) occur simul7 taneously. The corresponding endothermic DTA peaks appear at 1140° C and 1190° C for CaSO₄ -n-Al203 and CaSO₄r-Al203 systems, respectively (Fig.1). (2) The decomposition of CaSO₄ is promoted by Al203; the decomposition occurs at lower temperature and the rate is higher when Al203 is present.72-Al203 was found to be more effective than a-Al203 (Fig.1 and 2). (3) CaO formed by the decomposition of CaSO₄ shows higher', reactivity than commercial CaO for the formation of calcium aluminate. Various types of aluminates, CaO. Al203, 12 Ca().7 Al203, 3 CaO.5 Al203, 3 CaO Al20 CaO.6 Al203 and x Ca0. y Al203 are formed and the kind of aluminates formed seemed to depend on the reactivity of CaO and/or Al203, as well as upon the composition of system under the working state. A new unknown aluminate x-CaO Al203 is formed for which x is assumed to be smaller than y (Table 1 and Fig.3 and 5). (4) Calcium aluminates are more reactive than commercial CaO for air-SO₂ mixture and the starting reaction temperature with SO₂ is lower for the aluminates prepared from 7-Al203 than that from ce-Al203 (Fig.4 and 6). (5) Calcium aluminates react with air-SO₂ mixture to produce CaSO₄ and 77-Al208, and the aluminates with values of m/n 1 are active (Fig.5 and 7). (6) The above results were confirmed by -the consecutive DTA experiments carried out in air-SO₂ mixture and in N2 (Fig.6).