Kinetic Study on Dissolution of CaO Particle into CaO–SiO₂–Al₂O₃ Molten Slag from Simulated Municipal Solid Wastes Melting

Abstract

For producing chemically stable slag from municipal solid wastes melting, the present work is concerned with a kinetic study on the dissolution of CaO into molten slag. Experiments for the dissolution of CaO were performed using a lab-scale electric furnace at the temperatures of 1,723 to 1,773 K under N₂ flow, where CaO particles of 0.3–0.355, 0.5–1.0, 2.0–2.36, 3.35–4.0 mm were allowed to melt in CaO–SiO₂–Al₂O₃ molten slag, which were prepared by mixing CaCO₃, SiO₂ and Al₂O₃ reagent powders at different weight fractions. Results showed that the amount of CaO dissolved in the molten slag increased with increasing melting time, and its rate increased with a decrease in the amount of CaO in the slag. The dissolution rate of CaO was faster with CaO particle of smaller size at higher melting temperatures. The rate expression for the dissolution of CaO particles into the molten slag was derived on the assumption that the dissolution rate of a CaO particle is controlled by the mass transfer of CaO through a boundary layer of 2CaO·SiO₂ which is produced around the surface of the CaO particle. Then, the rate of dissolution of CaO into the molten slag was evaluated by the apparent rate constant for CaO obtained. The apparent rate constant of the dissolution of CaO, k, was almost constant for the molten slag employed, of which the average k-value was 1.80×10⁻⁶ m/s for CaO particle sizes in the range of 0.3–0.355, 0.5–1.0, 2.0–2.36, and 3.35–4.0 mm. The time change in the ratio of CaO dissolved into the molten slag was estimated by calculation using the k-value thus obtained. As a result, the measured data were reasonably reproduced by the calculations in terms of the rate of CaO dissolution.