THE HYDRATION KINETICS OF MAGNESIUM OXIDE OBTAINED BY CALCINATION OF MAGNESITE (MgCO_3) OR SYNTHETIC NESQUEHONITE (MgCO_3-3H_2O)

摘要

The hydration kinetics of MgO, namely magnesia or magnesium oxide, obtained by calcination of magnesite MgCO_3 or synthetic MgCO_3-3H_2O has been studied. Nesquehonite (MgCO_3-3H_2O) was prepared by the reaction of MgCl_2 with Na_2CO_3 in a mixed-suspension mixed-product removal (MSMPR) crystallizer at 40°C. Three magnesium oxide samples were obtained by calcination of magnesite MgCO_3 or synthetic MgCO_3-3H_2O in muffle furnace at 900, 1100 and 1300°C for 3 h, respectively. The hydration experiments of MgO samples were investigated at 25, 35 and 45°C for 24 h in a stirred batch reactor. The hydration results indicate that the rate and extent of hydration of magnesia decreased remarkably with hydration temperature and increased with decreasing calcination temperature at a constant hydration temperature. The whole hydration reaction of magnesia was supposed to experience the promotive period and declining period. An empirical kinetic model (-In(1-α)=k_pt) and a modification of the classical shrinking core model for diffusion control (-[1-(1-α)~(1/3)]~2=k_pt+b) have been applied to fit the experimental data of hydration in two stages, respectively. The values of kinetic parameters (k _p and k_d) were obtained by regression analysis and the activation energy of the sample at 900°C was estimated to be 34.23 kJ/mol. The comparison between MgO from calcination of synthetic MgCO_3-3H_2O and a commercial MgO used in silicon steel production was carried out, indicating that special silicon steel grade MgO can be obtained by calcination of MgCO_3-3H_2O below 900°C.