Hydrothermal synthesis of high purity zeolite A from natural kaolin without calcination

• Published in: Microporous and mesoporous materials 2014-11, Vol.199, p.50-56
• Main Authors: Wang, Jing-Quan, Huang, Ya-Xi, Pan, Yuanming, Mi, Jin-Xiao
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Hydrothermal synthesis; Impurity removal; Ion-exchange; Kaolin; NaOH digestion; Surface physical chemistry; Zeolite A; Zeolites: preparations and properties
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2014.08.002

A new hydrothermal route has been developed for the synthesis of high purity zeolite A with controllable particle size and morphology, from natural kaolin without high temperature calcination. [Display omitted] •A new hydrothermal route to synthesize high-purity zeolite A from natural kaolin.•Optimization to ensure complete kaolinite dissolution and removal of impurities.•Zeolite A of controllable size and morphology, and enhanced cation-exchange capacity.•Potential extensions to use of low-grade kaolin and synthesis of other zeolites. Zeolite A is a commercially and environmentally important microporous material, which is commonly synthesized from calcined kaolin. One major drawback inherent to this approach is that the properties (i.e., purity, particle size, morphology, and cation-exchange capacity) of the final product depend largely on the quality of the raw kaolin ores. Herein we report on a new hydrothermal route for the synthesis of high purity zeolite A with controllable particle size and morphology, from natural kaolin without conventional high temperature calcination. This route starts with hydrothermal reaction of natural kaolin with NaOH to form hydrous sodium aluminosilicates (i.e. hydroxycancrinite and nepheline hydrate), which are then dissolved in dilute HCl. The resulting acid solution, after filtration to remove unreacted impurity minerals such as quartz and muscovite, is then adjusted to pH=7 to form an amorphous Si and Al gel, which is used for the hydrothermal synthesis of zeolite A. A comparative study of zeolite A samples synthesized from this new route and those from the conventional calcination method reveals that the former have more controllable size and morphology than the latter, giving rise to higher brightness and enhanced cation-exchange capacity. Another major advantage of this new method is that the common impurity minerals such as quartz, muscovite and feldspars in natural kaolin can be removed, giving rise to high purity zeolite A and possibly expanding this technique to the use of low-grade kaolin.