Dissolution kinetics: The nature of the particle attack of layered silicates in HF

The dissolution of layered silicates by hydrofluoric acid was studied with the aid of a slurry reactor. The nature of particle dissolution was delineat by measurements of reaction rates and by the examination of partially dissolved particles using a scanning electron microscope. The rates of particle d per unit mass were modeled as a function of the fraction x dissolved by an equation of the form [-r̄(x)]/[-r̄(x = 0)] = (1 - x) n−1. For the three layer silicates the parameter n takes on a value of 12, reflecting radial attack at the unit cell edges. The reactive edge surface area is considerably smaller than the total wetted particle surface area. In contrast, the two layer kaolinite structure dissolves at both the layer edges and the planar faces, and the parameter n varies from 0 to 1 as the particle dissolution proceeds to completion. Comparison of the rates of dissolution on the basis of the actual reactive surface areas show that layered silicates having the same octahedral sheet dissolve at the same rate. It is concluded that attack is preferential at surfaces where the octahedral sheets are exposed.