The reaction of kaolin with fluorides: I. Effect of neutral and acid sodium fluoride solutions

The reaction between kaolinite and neutral and acid sodium fluoride solutions was investigated at different temperatures and over the acid pH range. The stoicheiometric replacement of hydroxyls in the kaolinite crystal lattice by fluoride ions, as reported by earlier workers, was not confirmed. The release of hydroxyl ions into solution was due predominantly to the disruption of the kaolinite crystal lattice. In the presence of sodium ions and at pH < 7, sodium fluoro‐silicate and cryolite were found as solid phases. At pH > 7, only cryolite was found as a solid phase. Small, spherical, particles were observed in all cases. These particles were believed to be amorphous silica, formed as an intermediate phase in the disruption process.