A mild conditions synthesis route to produce hydrosodalite from kaolinite, compatible with extrusion processing

Abstract

Hydrosodalites are a family of zeolitic materials which have a diverse range of possible applications such as water desalination. Typical synthesis methods are relatively complex, using hydrothermal production and pre-processing and it is desirable to use lower energy and more cost-effective processing routes. For the first time, a low temperature, non-hydrothermal synthesis procedure for hydrosodalites, compatible with extrusion processing, is demonstrated. Kaolinite precursor, without calcination, was activated with a sodium hydroxide solution and formed at a workability consistent with extrusion. The cured samples were characterised using a range of advanced analytical techniques including PXRD, SEM, TGA, 27Al and 29Si-MAS-NMR, and FTIR to confirm and quantify conversion of the precursor to product phases. The synthesis consistently formed a 8:2:2 basic hydroxysodalite phase and the reaction was shown to follow a largely linear relationship with Na:Al until full conversion to the hydrosodalite phase was approached. The hydrosodalite became more ordered for Na:Al ≥ 1. There is good agreement between quantitative measurements made using PXRD, TGA and 29Si-MAS-NMR methods, providing confidence in the results. It has been shown that it is possible to synthesise hydrosodalite materials in a consistent and predictable manner, using non-hydrothermal methods, at the viscosity used for extrusion processing. This novel processing route could reduce production costs, production impacts and open up new applications for this important family of materials.