Temperature effects on coprecipitated Al, Fe-hydroxides during hydrothermal transformations

Abstract

Studies of the hydrothermal behaviour of coprecipitated A1, Fe-hydroxides with aluminium contents from 5 to 50 tool% reveal that the temperature of the hydrothermal process strongly affects the reciprocal influence of AI(III) and Fe(III) ions on the direction of transformation of the initially amorphous gels.It has been found by X-ray diffraction analysis and IR spectroscopic investigations that at 373 K, up tO 10 mol% A1, well-crystallized At-substituted hydrohematite is formed; at 15-25 mol% AI the crystallinity falls offand the products are amorphous; and at 27.5-50 mol% A1 the poorly-crystallized hydrohematite appears, accompanied by different crystalline aluminium trihydroxides, the equilibria of which depend essentially on the AI : Fe ratio and on the pH of the medium.The hydrothermal process carried out at 413 K yields the Al-substituted protohematite for samples with 15-50 mol% A1, whereas the unique crystalline phase of Al-hydroxide is a wellcrystallized boehmite.