Simplified synthesis of Na-4-mica from kaolinite and cation- exchange properties

The synthetic process of a high-charge-density sodium fluorophlogopite mica, Na-4-mica (with an ideal chemical composition of Na₄Mg₆Al₄Si₄O₂₀F₄·nH₂O), using kaolinite as a raw material, has been investigated with respect to developing a simplified procedure and controlling crystal size. A fine and highly crystalline phase of the hydrated Na-4-mica, which is essential for practical applications, could be easily prepared from a mixture of kaolinite (or metakaolin) and magnesium nitrate using NaF flux at temperatures below 800°C. An increased mass of NaF flux in the simplified process at 800°C yielded somewhat larger and well-dispersed Na-4-mica crystals with hexagonal and plate-like shapes of 2-3 μm in size. Cation- exchange equilibria was studied for alkaline earth metal ions of Sr, Ba, and Ca, as well as transition metal ions of Cu and Pb, at room temperature using the hydrated Na-4-mica synthesized by the simplified process. The order of selectivity for the alkaline earth metal ions was Ba²⁺ >> Sr²⁺ > Ca²⁺, and the cation- exchange capacities were 197, 83, and 77 mequiv (100 g)^-1, respectively.