DISSOLUTION RATES OF MINERALS AND THEIR RELATION TO SURFACE MORPHOLOGY

Abstract

Experimentally observed dissolution rates of minerals in an aqueous solution are determined by surface reaction rates, mass transport by molecular diffusion through a diffusion boundary layer (DBL) and the morphology of the mineral's surface. By solving the transport equation in the presence of a diffusion boundary layer for surfaces containing open pores their contribution to the observed dissolution rates can be quantified. Furthermore dissolution rates are calculated for fractal surfaces. A general solution is given. Two extremes are discussed. If the surface controlled rate constant k is small compared to the mass transport constant kt = D/# (# thickness of DBL, D coefficient of diffusion), the rates are surface controlled and the entire surface contributes to the observed dissolution rate. In this case rates must be normalized to the B.E.T.-surface area. When k # kt the observed rates are limited by diffusion and information on k cannot be obtained. In intermediate cases a careful analysis is required. If ink bottle pores are present their contribution to the observed rates can be neglected and rates must be normalized to the geometrical envelope surface area, although in such cases the B.E.T.-surface area can be much larger.