Synthesis, hydrothermal stability and thermal reaction behavior of nepheline hydrate i (NH I)

Abstract

The field of formation of nepheline hydrate I (NH I) was investigated under mild hydrothermal conditions (473 K for 5 d) using low temperature reaction of kaolinite. Therefore alkaline transformation in 2-, 4-, and 6 molar NaOH solutions was performed to investigate the influence of sodium hydroxide on the phase formation process controlled by kinetics under mild reaction conditions. As already known from high alkaline reactions, a co-crystallisation of sodalite and cancrinite was observed using the 6 molar solution. In contrast nepheline hydrate I could be synthesized during kaolinite transformation in 4 - 2 molar solutions. Beside working out a simple preparation method of NH I an investigation of hydrothermal stability as well as thermal dehydration behaviour of NH I was the further aim of this experimental study. Our results show, that NH I exhibits a high degree of hydrothermal stability. During 11 experiments of hydrothermal treatment at 473 K (each for 20 hours and with exchange of water by fresh water) no observable destruction of the structure occurred. An IR-heating measurement up to 873 K resulted mainly in the total dehydration of NH I and in intensity loss of framework absorption bands, but not in the destruction of the aluminosilicate framework structure. After the sample was cooled down under water excluding conditions a dehydrated structure was found, but under open conditions the sample totally rehydrates to NH I again. X-ray powder diffraction of NH I, heated for 24 h at 383 K indicate that dehydration reaction starts with formation of “Barrers species T', i.e . nepheline hydrate with low water content (0.5 H 2 O), compared with NH I.