PURPOSE: To remove effectively boron from light burned magnesia by bringing the magnesia into contact with an alkaline aqueous soln. and by separating and removing the liq. phase when the hydration of magnesium oxide attains to a prescribed rate.

CONSTITUTION: Magnesium hydroxide contg. boron produced from sea water is heat treated at ≥about 800°C. The resulting light burned magnesia is put in an aqueous soln. of ≥11pH such as about 0.1N aqueous soln. of NaOH, and they are stirred. When the conversion of magnesium oxide into magnesium hydroxide attains to ≥5%, preferably 30W55% rate, the liq. phase is separated by filtration and removed to obtain magnesium oxide and magnesium hydroxide having a low boron content.