The Relative Movement Imposed on the Solids and the Liquid

THE RELATIVE MOVEMENT IMPOSED ON THE SOLIDS AND THE LIQUID

The major and controlling resistance to the diffusion of the solute to the bulk solution is normally found in the cell matrix. Increase in the rate of movement of the solution past the surface will not, therefore, greatly affect the rate of extraction. This is in marked contrast to the processes of dissolution and crys-tallization. Nevertheless, movement is imposed on the menstruum in both the general methods described earlier.

In the percolation of a liquid through a bed of solids, mass transfer of the solute from the surfaces of the solid to the liquid in the interstices of the bed takes place by molecular diffusion and by natural convection arising from the density changes created by dissolution. Although these processes are slow, they are much quicker than mass transfer in the matrix under the same differences in concentration. Concentration gradients in the liquid outside the particles are, therefore, very low. At any point in the bed, the introduction of dilute solution from above and the loss of concentrated solution to below decrease the inter-stitial concentration by dilution or displacement. This effect can be considered simply to decrease the solute concentration at the junction of solid and solution, thus imposing a favorable concentration gradient within the matrix.

Similarly, the agitation of the slurry in leaching by immersion is not pri-marily to decrease the thickness of the boundary layer at the surface and its diffusional resistance. Rather, agitation serves only to keep the particles in suspension and to equalize the solute concentration throughout the liquid. If the particles settle, the solute must diffuse through the stagnant fluid filling the interstices of the bed. High diffusional resistance is created, and the rate of extraction is depressed.

THE PHYSICAL PROPERTIES OF THE SOLVENT

The relevant properties of the solvent are low viscosity and free solution of wanted constituents. These and other aspects of the solvent have already been discussed.