Leaching occluded solution

Leaching is the removal of solutes from admixture with a solid by contracting it with a solvent. The solution phase sometimes is called the overflow, but here it will be called extract. The term underflow or raffinate is applied to the solid phase plus its entrained or occluded solution. 

Generally, the solids are not structurally homogeneous, but the solid and liquid nevertheless will be called phases and leaching will be treated as a two-phase, mass transfer process- The solid consists of a matrix of insoluble solids, the mare, and the occluded solution. It may also contain undissolved solute and a nonextractable secondary phase, for example, coffee oil in water-soaked coffee grounds. This secondary phase is treated as part of the mate. Dimensionless parameters that can affect solnta transfer include the solute equilibrium distribution coefficients, m and M tha Pick number, v the strippirg factor, a the Biot number, Bi and the Peclet number, Be. These parameters are defined more precisely in the Notation section. 

In homogeneous extractions, Tm = T at equilibrium. If Km < K, and die solute is not sorbed selectively by the marc, X t K. Consequently, M - 1/. M usually remains fairly constant during leaching processes, but m varies somewhat because the density of the occluded and external solutions change as x changes. 

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