Structure and phase changes in thermally treated mixed hydroxides of Mg and Al-effect of compaction

Abstract

Mechanically mixed (I) and coprecipitated (II), hydroxides of magnesium and aluminium were investigated in both powder and compact forms in the temperature range 200–1000°C using DTG, DTA and X-ray techniques. Upon thermal treatment of the mechanically mixed hydroxide, the solid material gives rise to products which at first become poorly crystalline and then gradually pass into the crystalline state indicating a mixture of MgO, θ, δ and α—alumina together with a new spinel. This new spinel is stable in the temperature range 400–800°C—three of its distinct d-spacings are at 4.87, 3.86 and 3.74 Å. Coprecipitated hydroxides (II) proved to constitute a new species—probably a hydrated spinel. The spinel MgAl 2O 4 commences to form at a temperature as low as 200°C. Compaction with either 10 or 30 tons in. −2 decreases the crystallinity of products from (I) dehydrated ⩽500°C, whereas it favours crystallization for the products from II. Above ∼500°C, compaction has no appreciable effect on crystallization.