THE ROLE OF MIGRATION IN SHRINKING-CORE LEACHING REACTIONS CONTROLLEDBY PORE TRANSPORT

The conventional shrinking-core model for pore diffusion through produ ct layers formed during leaching reactions has been generalized to inc lude electrical migration as well. This has involved solving the stead y-state diffusion-migration transport equations for all soluble reacti ng and nonreacting species in the system in conjunction with the elect roneutrality requirement and the appropriate boundary condition for a given rate-controlling regime. The resulting expression for transport- limiting conditions has the identical form 1 - 2/3 alpha - (1 - alpha) (2/3) = k(d)t as the conventional one, except that k(d) is now a funct ion of the valences and bulk concentrations of ail species and the eff ective diffusion coefficients of all reacting species. The analysis sh ows that as long as the diffusion coefficients differ, migrational tra nsport will occur and that the greater this difference the greater is its importance. For conditions typical of some practical leaching syst ems, it is estimated that migrational flux can be considerable, reachi ng as high as 80 pet of the diffusional flux. Other comparisons of the generalized and standard models are made and the resulting errors in neglecting migration are discussed.