Lösungskinetik von Gips und Anhydrit

Abstract

The dissolution kinetics of gypsum and anhydrite have been measured under various conditions. For gypsum an almost linear rate equation R=k:sub:s1:/sub:(1-c/c:sub:eq:/sub:):sup:n:sub:1:/sub::/sup: is valid, where R is the surface rate, n:sub:1:/sub: :img:=approx.gif:/img: 1 is the kinetics order, c is the total calcium concentration at the surface, and c:sub:eq:/sub: the equilibrium concentration with respect to gypsum. For the determination of the entire dissolution kinetics a batch set-up was used. This batch experiment reveals a dissolution rate equation R=k:sub:s1:/sub:(1-c/c:sub:eq:/sub:):sup:n:sub:1:/sub::/sup: which switch close to equilibrium to a nonlinear rate equation R=k:sub:s2:/sub:(1-c/c:sub:eq:/sub:):sup:n:sub:2:/sub::/sup: with n:sub:2:/sub: :img:=approx.gif:/img: 4.5. The experimentally observed dissolution rates from the batch experiment could be fitted by only minor variations with a mixed kinetics model. The rotating disk experiment on anhydrite reveals a surface controlled rate equation. For anhydrite the observed experimentally dissolution rates by a batch experiment are described by R=k:sub:s:/sub:(1-c/c:sub:eq:/sub:}):sup:n:/sup:, where k:sub:s:/sub: is the surface rate constant and n :img:=approx.gif:/img: 4.2 is the kinetic order. Furthermore, a method for the determination of the rate equation parameters was developed. Experimentally observed dissolution rates of minerals in aqueous solution are determined by surface reaction rates, mass transport by molecular diffusion through a diffusion boundarylayer (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.