Applications of Sardinian kaolins to the hydrothermal synthesis of zeolites and characterization of the resulting products

The kaolin deposit from Donigazza area (NW Sardinia, Italy) has been investigated to test its possible use as raw material for zeolite synthesis. The perspective is to draw a better exploitation of this mineral resource, as the deposits of NW Sardinia are mainly exploited to manufacture vitrified tiles and/or sanitary ware. Chemical and mineralogical investigations were performed by X-ray fluorescence (XRF), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). The Donigazza kaolin is mainly composed of kaolinite±dickite, and minor amount of opal-CT and quartz. The low iron content (likely as hydroxides), the high amount of kaolinite and amorphous silica, as well as the low amount of quartz encouraged investigation on the possible conversion of these rocks into zeolites. A representative sample was powdered, split, and hydrothermally treated with NaOH at 100 °C in sealed Teflon-lined stainless-steel vessels, under autogenous water vapor pressure. The vessels were placed in a thermostated oven equipped with a rotating device inside. Thirty experiments were performed by varying the concentration of NaOH solution (1, 2, 3, 4, and 5 mol L-1) and the reaction time (12, 24, 48, 72, 96, 168 hours); the liquid/solid ratio was 1:5. XRD analyses documented that different mineral species formed after kaolin hydrothermal alkali activation: NaP1 zeolite, NaP2 zeolite, gobbinsite, analcite, laumontite, chabasite, sodalite, hydroxy-sodalite, hydroxy-cancrinite, phillipsite. Amounts of residual kaolinite (from 0 to 70%) depend both on the NaOH concentration and on the reaction time (higher the time and NaOH, lower the amount) being undetectable at t ≥ 48 hours and NaOH ≥ 3 mol L-1. Consequently, relative abundances of zeolites (up to 91%) change significantly depending on the experimental conditions: as expected, NaOH strongly influences the abundance and mineral type but also time plays an important role thus indicating that a given mineral is in metastable condition. Microscopic observations documented the presence of idiomorphic crystals, whose dimensions vary from 100 nm up to 10 μm, as well as subspherical aggregates (up to 10 μm in size) of crystals (200-400 nm) having pseudo-octahedron shapes. Globular nanoparticles (usually < 50 nm), isolated or aggregated to form subspherical micrograins, are also present. It is likely that they are Al-Si gels. As expected, their amount, as well as the ratios among mineral species, depends on the length of the experiment and NaOH concentration. Results obtained in this experimental study document that kaolin deposits from NW Sardinia (Italy) are an excellent raw material for zeolite production.