Abstract: Unsteady state heterogeneous diffusion-reaction mold has been considered as a good tool for simulating fluid-solid reaction. Shrinking core model, two-stage model, and homogeneous modle, which have been widely used, are actually special cases in this category. Low recovery of vanadium from slags of high calcium contents through leaching with water was well noted. Leaching with solium carbonate and/or sodium bicarbonate solutions, however, will enable over 80% of vanadium to be recovered. Leaching kinetics and physical properties of the calcined products before and after leaching are experimentally investigated. Experimental results can be satisfactorily explained by the implementation of the unsteady state diffusion-reaction modle. The reaction of two solutes with two solid components, with the formation of a solid product which blocked part of the vanadium surface from entering into reaction, has been taken care of in the derivation of the model. The mold equations, which are a set of nonlinear partial differential equations, may be solved by emplaying the orthogonal collocation and the semi-implicit Runge-Kutta method. Parameters are estimated by the complex method.