Synthesis of Na–X zeolites from tripolaceous deposits (Crotone, Italy) and volcanic zeolitised rocks (Vico volcano, Italy)

Abstract

Synthesis of zeolitic minerals (Na–X and HS) by the use of natural materials (naturally zeolitised alkaline volcanic rocks and “Tripoli”) has been achieved. Synthesis was conducted at hydrothermal conditions (80 °C) by the use of alkaline silicates (Na x Si y O z ) and alkaline aluminates (Na x Al y O z ). Chemical treatments were made on “Tripoli” rocks (mainly constituted by opaline silica) coming from Crotone Basin (southern Italy) in order to obtain Na 2 SiO 3 · nH 2O. Volcanic pyroclastic rocks (naturally zeolitised to chabazite and phillipsite) from the Vico Administrative Province (central Italy) were treated to obtain the NaAlO 2 solution. Both products were mixed to gain the gel NaAlSi x O y · nH 2O necessary for the zeolitic synthesis. Mineralogical–crystallographical–chemical and textural–physical characterisation showed that Na–X zeolite synthesis begins after 5 h and reaches its crystallisation climax at 18 h, with a broad field of existence (about 500 h) of the Na–X phase. Then Na–X is substituted by HS (Hydroxysodalite) zeolite. The characterisation shows that microtextural (scanning electron microscope images) and crystallographic values as well as other physical properties (specific surface, density) are comparable to the reputed ones for commercial zeolite. The thermal behaviour (TG-DTA) and IR response also provide consistent evidence of good achievement of Na–X zeolite synthesis. Finally, the wide (in terms of time) range of stability of Na–X zeolite seems to allow transfer to an industrial production scale.