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

Sodium fluorohectorite Na(0.6)[Mg(2.4)Li(0.6)]Si(4)O(10)F(2) was synthesized from the melt in an open glassy carbon crucible at 1265 degrees C under argon flow. A glass (Na(2)O-Li(2)O-SiO(2)) 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 degrees C up to 1265 degrees 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 Na(0.6)-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 mu m as measured by static laser light scattering (SLS) which was confirmed by scanning electron microscopy (SEM). (C) 2009 Elsevier B.V. All rights reserved.