Reactivity of carbonates in superheated steam under atmospheric pressure

The effect of superheated steam on the decomposition of CaCO₃ and MnCO₃ and on the solid state reactions of ZnCO₃-FeOOH and MnCO₃-CaCO₃ was investigated. A newly developed apparatus for the experiments under 1 atm of pure water vapor was used. CaCO₃ decomposed at 800 °C in superheated steam to form the single phase of CaO. On the other hand, the decomposition was uncompleted in air. CaCO₃ transformed into CaO via Ca(OH)₂ in superheated steam. During the transformation of carbonate into hydroxide, the crystal lattice is temporarily disordered to make it active. So-called Hedvall effect would occur to make the decomposition temperature of CaCO₃ lower. MnCO₃ decomposed to form γ-Mn₂O₃ above 1000 °C in air, whereas γ-Mn₂O₃ was obtained at 800 °C in superheated steam. The solid state reaction in the steam was suppressed for the mixture of ZnCO₃ and FeOOH. This seemed to be due to the large difference in decomposition temperature between ZnCO₃ and FeOOH. MnCO₃ reacted with CaCO₃ to form CaMn ₂O₄ at 800 °C in superheated steam. However, a higher temperature of 1000 °C was required to cause the reaction in air. The low-temperature transformation of MnCO₃ and CaCO₃ in superheated steam would affect the reaction. It was concluded that the reactivity of carbonate in superheated steam was promoted by the Hedvall effect, which was caused by the formation of intermediate phase such as hydroxide.