GB759770A - Improvements in or relating to process for preparing crystalline magnesia and resulting product - Google Patents

Abstract

Periclase is formed by firing at at least 1300 DEG C. a mixture of a magnesium compound which yields periclase on firing, with an aluminium compound in an amount to provide up to 1.5 per cent of added aluminium, calculated as Al2O3 based on the fired product. The magnesian starting materials may be magnesium hydroxide, carbonate, basic carbonate, alcoholate, sulphate, chloride, magnesite, brucite or cryptocrystalline magnesia, the latter being prepared for example by firing a magnesium carbonate, basic carbonate or hydroxide at 1200 DEG C. for 45 minutes. The final periclase preferably contains at least 95 per cent magnesium oxide, and less than 2 per cent of each of calcium oxide and silica. The aluminium compound added may be alumina, aluminium hydroxide, sulphate, chloride, or other halide, phosphate or sodium or potassium aluminate and the compound may be added in finely divided form or preferably, as a solution or dispersion in a liquid suitably water. Aluminium bromide may be added in alcohol, carbon trisulphide or acetone. Part of the added aluminium compound may be replaced by chromium or iron compound, the total amount of additives, as R2O3 being up to 1.5 per cent of the fired product by weight. The mixture of magnesian materials and additives may be pressed before firing or fed directly to a kiln. If desired a two-stage firing may be employed, the first at below 1300 DEG C., preferably below 1200 DEG C. to form cryptocrystalline magnesia, and the second at at least 1300 DEG C. and preferably at 1500 to 1750 DEG C. The addition of the additives may precede either the first or second firing. Also there may be admixed with the cryptocrystalline magnesia up to 90 per cent of finely divided well crystallized periclase, after which the mass is pressed and fired at at least 1600 DEG C. to obtain periclase. Specifications 698,797, 711,908 and 758,089 are referred to.