Large scale melt synthesis in an open crucible of Na-fluorohectorite with superb charge homogeneity and particle size

Sodium fluorohectorite Na0.6[Mg2.4Li0.6]Si4O10F2 was synthesized from the melt in an open glassy carbon crucible at 1265 °C under argon flow. A glass (Na2O–Li2O–SiO2) precursor was used as a fluxing agent in order to maintain a low vapor pressure of volatile fluorides and sustain a low silica activity, which inhibits the formation of silicon fluoride gases and promotes the fluorine solubility in the melt. To minimize the loss of volatile fluorine compounds the pre-synthesized alkali silicate glass was heated together with additional raw materials needed in a high frequency induction furnace rapidly from 800 °C up to 1265 °C and then kept at this temperature for a short period of time (15 min). The synthesis method can easily be scaled to batches larger than 1 kg. The Na0.6-fluorohectorite obtained stands out by (i) phase purity as checked by X-ray powder diffraction (PXRD), (ii) a superb homogeneity of the charge density as demonstrated by stepwise hydration behavior followed by in-situ PXRD in a humidity chamber and the Lagaly method with alkylammonium exchange (Mermut and Lagaly, 2001), (iii) a high cation exchange capacity (CEC) of 136 meq/100 g as determined by the copper complex ([Cu(trien)]2+) method, and (iv) extreme lateral extensions with a median value of the particle size of 45 μm as measured by static laser light scattering (SLS) which was confirmed by scanning electron microscopy (SEM).