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.