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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.

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Santo, 35, 30160-030, Belo Horizonte -MG, Bmzil agnesium oxide or magnesia (MgO) is mostly produced by calcination of magnesite (MgCO,). Depending on the process-M ing temperature, caustic calcined magnesia (for temperatures <900°C) or dead-burned magnesia (temperatures >12OO0C) is obtained. Formed at relatively low temperatures, caustic magnesia is comparatively more porous and reactive than dead-burned magnesia, obtained at high temperatures. Caustic magnesia has many industrial applications, such as in agriculture, cattle feed, environmental control, manufacture of special cements and in many other specialty uses. The major consumer of the dead-burned magnesia (MgO) is the refractory industry. In both types of magnesia, the rate and extent of hydration are factors which affect the functional performance of magnesia. The hydration of refractory grade magnesia, for example, should be reduced in order to prevent the degradation of the brick. Conversely, hydration should be favored when the aim is the production of magnesium hydroxide.The hydration of MgO has been studied since the 1 gth century, a number investigations being reported in the 1960s (Layden and Brindley, 1963;Classon, 1963; Bratton and Brindley, 1965; Feitknecht and Braun, 1967;Smithson and Bakhshi, 1969). From these studies, it has been well established that the reaction mechanism comprises steps of magnesia dissolution with further magnesium hydroxide precipitation. The physical properties of the solid reagent were found to strongly influence the hydration reaction. In one of the early studies focusing the reaction kinetics, Smithson and Bakhshi (1 969) concluded that the shrinking core model for chemical control (this being derived from the value of the activation energy) was not adequate to describe the hydration of MgO samples In Equation (1) ro is the mean radius of the particle, a is the extent of conversion, t is the reaction time, to the inductive time and K is the ratio between the reaction constant and the particle density. The constant N "Author to whom correspondence may be addressed. E-mail address: sdrocha@, deq.uting. brThe kinetics of the hydration of magnesia to produce magnesium hydroxide is described by a kinetic model with no diffusive contribution, including an additional resistance to the reactive flux due to the transient variation of the porosity of the material during reaction. The proposed model has been applied to data from the literature and obtained by hydration of MgO samples with distinct physical characteristics. The model describes well the mechanism of hydration of powder and single crystal MgO in liquid and vapour water with temperatures varying from 35°C to 200°C.La cinetique d'hydratation de la magnesie pour produire de I'hydroxyde de magnesium est dkrite par un modhle cinetique sans contribution diffusive, incluant une resistance additionnelle au flux reactif en raison de la variation transitoire de la porosite du materiau lors de la reaction. Le Keywords: magnesia, magnesium hydroxide, magnesium oxide, kinet...

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