DISSOLUTION OF SILICATE MINERALS BY HYDROFLUORIC-ACID

A slurry reactor was used to measure the rates of dissolution of silicates in hydrofluoric acid. Reaction rate laws are reported for the dissolution of kaolinite, sodium, montmorillonite, illite, pyrophyllite, muscovite, talc, and biotite. The measured rates for dissolution are attributed entirely to attack by by the HF molecules, rather than by F** minus or HF//2** minus ions. The rate of reaction is proportional to the concentration of HF molecules adsorbed at surface lattice bonds. For sodium montmorillonite, the magnitude of the HF adsorption constant K//A is significant, and the kinetic rate law is of the Langmuir-Hinshelwood form. However, for each of the other silicates studied, the Langmuir-Hinshelwood rate law reduces to a simple first-order rate law with respect to the concentration of molecular HF.