The hydration of magnesium oxide with different reactivities by water and magnesium acetate

Abstract

The use of magnesium hydroxide (Mg(OH)2) as a flame retardant and smoke-suppressor in polymeric materials has been of great interest recently. Because it contains no halogens or heavy metals, it is more environmentally friendly than the flame retardants based on antimony metals or halogenated compounds. Mg(OH)2 can be produced by the hydration of magnesium oxide (MgO), which is usually produced industrially from the calcination of the mineral magnesite (MgCO3). The thermal treatment of the calcination process dramatically affects the reactivity of the MgO formed. Reactivity of MgO refers to the extent and the rate of hydration thereof to Mg(OH)2. The aim of this study was to investigate the effect of calcination time and temperature on the reactivity of MgO, by studying the extent of its hydration to Mg(OH)2, using water and magnesium acetate as hydrating agents.

A thermogravimetric analysis (TGA) method was used to determine the degree of hydration of MgO to Mg(OH)2. The reactivity of MgO was determined by BET (Brunauer, Emmett and Teller) surface area analysis and a citric acid reactivity method. Other techniques used included XRD, XRF and particle size analysis by milling and sieving.

The product obtained from the hydration of MgO in magnesium acetate solutions contains mainly Mg(OH)2, but also some unreacted magnesium acetate. Magnesium acetate decomposition reaction takes place in the same temperature range as magnesium hydroxide, which complicates the quantitative TG analysis of the hydrated samples. As a result, a thermogravimetric method was developed to quantitatively determine the amounts of Mg(OH)2 and Mg(CH3COO)2 in a mixture thereof. The extent to which different experimental parameters (concentration of magnesium acetate, solid to liquid ratio and hydration time) influence the degree of hydration of MgO were evaluated using magnesium acetate as a hydrating agent. Magnesium acetate was found to enhance the degree of MgO hydration when compared to water. By increasing the hydration time, an increase in the percentage of Mg(OH)2 formed was observed.

In order to study the effect of calcining time and temperature on the hydration of the MgO, the MgO samples were then calcined at different time periods and at different temperatures. The results have shown that the calcination temperature is the main variable affecting the surface area and reactivity of MgO.

Lastly, an attempt was made to investigate the time for maximum hydration of MgO calcined at 650, 1000 and 1200oC. From the amounts of Mg(OH)2 obtained in magnesium acetate, it seems that the same maximum degree of hydration is obtained after different hydration times. A levelling effect that was independent of the calcination temperature of MgO was obtained for the hydrations performed in magnesium acetate. Although there was an increase in the percentage of Mg(OH)2 obtained from hydration of MgO in water, the levelling effect observed in magnesium acetate was not observed in water as a hydrating agent, and it seemed that the extent of MgO hydration in water was still increasing.

The results obtained in this study demonstrate that the calcination temperature can affect the reactivity of MgO considerably, and that by increasing the hydration time, the degree of hydration of MgO to Mg(OH)2 is enhanced dramatically.