Effects of Solution Chemistry on the Hydrothermal Synthesis of Kaolinite

Abstract

The hydrothermal synthesis of kaolinite was examined in the Al₂O₃-SiO₂-H₂O system to study inhibitory effects of additional ions on the formation of kaolinite. Syntheses were carried out with amorphous starting materials and salt solutions of various concentrations in Teflon pressure vessels at 220°C for 5 days. The reaction products were characterized by XRD, IR, DTA-TG, NMR and TEM. In all of the runs using solutions with cation concentrations less than 0.001 M, no significant effect on the formation of kaolinite was observed. The inhibitory effect of the univalent cations Li⁺, Na⁺ or K⁺ was less than that of divalent cations such as Mg²⁺ or Ca²⁺. The addition of trivalent Fe³⁺ or excess Al³⁺ ions interfered with the formation of kaolinite significantly. Sulfate and acetate solutions interfered with the formation of kaolinite more than chlorides and nitrates. No crystalline product was obtained using a 1.0 M basic solution of carbonate or hydroxide. The addition of the lithium ion to the system affected the crystallization of kaolinite only slightly. The use of 0.1 M LiCl and LiNO₃ solutions for the syntheses improved crystallization of kaolinite along the [001] direction.