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

摘要

Sodium fluorohectorite Na_(0.6)[Mg_(2.4)Li_(0.6)]Si_4O_(10)F_2 was synthesized from the melt in an open glassy carbon crucible at 1265 ℃ under argon flow. A glass (Na_2O-Li_2O-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 ℃ up to 1265 ℃ 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/100g 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).