Hydrothermal Crystallization of Ammonium-Saponite at 200 °C and Autogenous Water Pressure

Roland J. M. J. Vogels1, Johan Breukelaar2, J. Theo Kloprogge3, ‡, J. Ben H. Jansen4 and John W. Geus1
1 Department of Inorganic Chemistry, University of Utrecht, P.O. Box 80.083, 3508 TB Utrecht, The Netherlands
2 SRTCA (Shell Research B.V.), P.O. Box 38000, 1030 BN Amsterdam, The Netherlands
3 TNO—Institute of Applied Physics—TU Delft, Department of Inorganic Materials Chemistry, P.O. Box 595, 5600 AN Eindhoven, The Netherlands
4 Bowagemi, Prinses Beatrixlaan 20, 3972 AN Driebergen, The Netherlands
This paper is a joint contribution from the Debye Institute, Utrecht, and Shell Research B.V., Amsterdam, The Netherlands.
Current address: Hoevenbos 299, 2716 Zoetermeer, The Netherlands.

Abstract: The effects of reaction time (2 to 72 h) and NH4+/Al3+ molar ratio (1.6, 2.4 and 3.2) on the hydrothermal synthesis of ammonium-saponites are investigated. The gels are obtained by mixing powders, resulting in a stoichiometric composition, Mg3Si3.4Al0.6O10(OH)2, with aqueous ammonium solutions, with and without F, to result in initial NH4+/Al3+ molar ratios of 1.6, 2.4 and 3.2. The solid bulk products are characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM) combined with energy-dispersive X-ray (EDX) analysis. The cation exchange capacity (CEC) is determined with an ammonia selective electrode and the pH of the water from the first washing is measured. Ammonium-saponite is formed rapidly within 16 h. A higher NH4+/Al3+ molar ratio and the presence of F facilitate the crystallization of saponite. Small metastable amounts of bayerite, Al(OH)3, are present at low NH4+/Al3+ molar ratios; after short reaction times, they disappear. During the first 4 h, the pH decreases rapidly, then drops slowly to a constant level of approximately 4.6 after 60 h. With increasing reaction time, saponite crystallites grow in the ab directions of the individual sheets with almost no stacking to thicker flakes. The NH4+ CEC of the solid products increases strongly within the first 24 h. A maximum of 53.3 meq/100 g is observed. The saponite yield increases from approximately 25% after 2 h to almost 100% after 72 h.

Key Words: Cation Exchange Capacity • Saponite • Synthesis • X-ray Diffraction • X-ray Fluorescence

Clays and Clay Minerals; February 1997 v. 45; no. 1; p. 1-7; DOI: 10.1346/CCMN.1997.0450101
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)