Hydrothermal synthesis of kaolinite: method and characterization of synthetic materials

Abstract A method of preparing spherical kaolinite was developed, using the most straight-forward procedure, reducing handling and preparation time to a minimum. The starting materials were amorphous aluminosilicate gels, prepared by cohydrolysis of tetraethyl orthosilicate and aluminum tri-isopropoxide in water, with Al 2 O 3 ( SiO 2 + Al 2 O 3 ) weight ratios of 0.316, 0.353, 0.402 and 0.462. The gels were treated with 0.1 M KOH solution, at 150, 175, 200, 225 and 250°C in Teflon coated aluminum bombs. The synthetic kaolinite was studied by XRD, DTA-TG and IR spectroscopy. These studies showed the formation of kaolinite during the hydrothermal treatment and showed the improvements of the characteristic parameters of the kaolinite [Hinckley's crystallinity index and (001) crystal size]. The SEM observations showed three kinds of particles in the samples: remaining gel, spherical kaolinite (0.25–0.35 μm) and small clusters of kaolinite crystals. TEM investigations showed that the clusters were formed by pseudo-hexagonal crystals of kaolinite. The behaviour of gels of different composition is similar. The 0.462 gel is an exception, because it displays a delay in kaolinite formation.