The effects of hydrating agents on the hydration of industrial magnesium oxide

Abstract

Magnesium hydroxide, a stable flame retardant, can be obtained by mining or by the hydration of magnesium oxide. In this study, the effect of different hydrating agents on the pH of the hydrating solution, rate of hydration of MgO to Mg(OH)2 and product surface area were studied as a function of the temperature of hydration.

Ammonium chloride, magnesium acetate, magnesium nitrate, nitric acid, acetic acid, water, magnesium chloride, sodium acetate and hydrochloric acid were used as hydrating agents. The hydration experiments were carried out in a water bath between 30 - 80 oC for 30 minutes. Dried MgO samples were introduced to the hydrating solution and the slurry was stirred at a constant speed. At the end of each experiment, the slurry was vacuum filtered, washed with water, dried at 200 oC and hand ground. The products were then characterized by TGA, XRF, XRD and BET surface area analyses.

There was not a significant difference in the hydration behaviour of the hydrating agents up to 50 oC, where less than 10 % of magnesium hydroxide was formed. When compared to the hydration in water, all the hydrating agents with the exception of sodium acetate showed a significant increase in the degree of hydration. Sodium acetate formed the lowest amount of magnesium hydroxide, ranging between 1.2 and 12.2 % magnesium hydroxide. Hydrations performed in hydrochloric acid and magnesium nitrate formed the largest percentage (11.8 %) of magnesium hydroxide at 60 oC. Magnesium acetate, magnesium nitrate, magnesium chloride and hydrochloric acid seemed to be the most effective hydrating agents at 70 oC with the percentage magnesium hydroxide being formed ranging between 20.0 and 23.9 %. The amount of hydroxide formed doubled at 80 oC, with the largest percentage (56.7 %) formed from the hydration in magnesium acetate.

The hydration reaction seemed to be dependent upon the presence of Mg2+ and acetate ions. It seemed that magnesium oxide hydration is a dissolution-precipitation process controlled by the dissolution of magnesium oxide. The results have also indicated that the pH and temperature of the hydrating solution strongly influence the degree of hydration.