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The kinetics of the dissolution of particulate sintered polycrystalline MgO under advanced kinetic regime conditions were investigated, with special regard to the effects of concentration of H+ ions, temperature and the type of the inorganic acid used (HCl, HNO3 and H2SO4). It was observed that the rate of chemical dissolution of MgO accelerated with increase in both concentration of H+ ions (from 10−4 to 10−2 M) and temperature (from 25 to 60 °C). The overall process of MgO dissolution was found to be controlled by the surface chemical reaction of MgO with H+ ions. The presented values of the reaction order for H+ ions are consistent, allowing for experimental error, with current kinetic theories and most other experiments. However, the rate-controlling step could not be assigned unambiguously, because the same value of the reaction order for H+ ions can be predicted by various theories for different rate-determining steps. Measured values of the reaction order for H+ ions are indicative that the rate is limited by the second protonation reaction, i.e. the slow reaction of surface hydroxide with a second proton. Though the reaction order for H+ ions seems not to be affected by the type of acid used, the apparent activation energy changes in the order HCl ≤ HNO3

Kinetics of the dissolution of particulate polycrystalline MgO in excess of hydrochloric acid were investigated, with special regard to the effect of activity of H + ions, temperature and particle size. The results are presented in terms of the shrinking particle model. The overall process is controlled by the chemical reaction of MgO with H + ions at the liquid–solid interface. The apparent activation energy was 58 kJ mol −1 . Acid concentration (from 0.5 M to 5.1 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H + ions (from −0.16 to −0.18). We conclude that the regime studied here, characterised by decrease in the dissolution rate of MgO with an increase in H + activity, occurs under far from mass-transfer control conditions in the solutions with low Mg 2+ :H + molar ratio.