Kinetics and mechanistic analysis of caustic magnesia hydration

Abstract

The kinetics of magnesia hydration to produce magnesium hydroxide is crucial for process design and control, and for the production of an Mg(OH) 2 powder with desirable particle morphology. In this study, highly pure magnesia has been hydrated in a batch reactor. The effects of the following variables were evaluated experimentally: temperature (308-363 K), reaction time (0.5-5 h), initial slurry density (1-25%wt) and particle size in the ranges −212 + 75 µm and −45 + 38 µm. Experimental data indicate increasing magnesia hydration rates with increasing temperature, as expected. In addition, it has been observed that the hydration of magnesia increases significantly up to about 4-5%wt initial slurry density, stabilising afterwards. On the other hand, the reaction was almost unaffected when magnesia with different particle sizes were hydrated because of similar specific surface areas involved. A reaction mechanism to explain the oxide dissolution and the hydroxide precipitation has been proposed, assuming no significant change in the initial solids size and dissolution rate as the controlling step. The calculated activation energy value of 62.3 kJ mol −1 corroborates the mechanism proposed in this study and compares well with values previously reported in the literature.

A novel approach for arsenic adsorbents regeneration using MgO

“…10 also support the extended MgO hydration and formation of Mg(OH) 2 during contact period. It should be pointed that such structural and compositional change occurs following a mechanism consisting of (i) water adsorption and diffusion, (ii) MgO dissolution and (iii) supersaturation, nucleation and growth of Mg(OH) 2 [23]. The precipitation of Mg(OH) 2 affects the observed morphology of the material as shown by the sharpening and homogenization of granules size after the regeneration cycle (Fig.…”

“…Although not clearly stated, the presented results in these studies, refer to equilibrium pH higher than 10, despite the fact that a neutral water pH was initially implemented. This comes out from the fact that MgO reacts with water to form Mg(OH) 2 , which presents a strong alkaline character [23]. Therefore, arsenic adsorption/disposal by MgO implies adsorption/disposal by MgO/Mg(OH) 2 .…”

Adição de cimento de aluminato de cálcio e seus efeitos na hidratação do óxido de magnésio

“…1b), pode ser observado que para a suspensão com MC, apenas o pico referente ao hidróxido de magnésio (Mg(OH) 2 ou brucita), em 2q = 38,5º, é observado após a hidratação, enquanto que para aquela com SM, brucita e periclásio (MgO, 2q = 43,2º) foram identificados. Esses resultados indicam que a MC apresenta uma maior velocidade de hidratação e são coerentes com outros trabalhos [7].…”

Neri Final Technical Report, De-Fc07-O5id14647, Optimization of Oxide Compounds for Advanced Inert Matrix Materials

“…19,26,[29][30][31][32][33] The activation energy for MgO hydration at temperature higher than 200 C has not been reported, yet.…”

“…The overall hydration process of MgO in liquid water can be summarized as follows [19][20][21][22] : (1) water molecules are adsorbed onto the MgO surface and dissociate into protons and hydroxyl ions, (2) the surface of MgO is reconstructed and dissolves into Mg 2+ and OH -, and (3) nucleation of Mg(OH) 2 occurs. The hydration of MgO by water vapor involves one more step which is the adsorption and condensation of initial water vapor on MgO surface to form a liquid layer, and the remaining steps are the same as in liquid water.…”