STAGES IN COUNTERCURREN

T LEACHING

The stages are numbered in the direction of flow of the solid.

The V phase is the liquid that overflows from stage to stage in a direction counter to that of the flow of the solid, dissolving solute as it moves from stage N to stage 1.

The L phase is the solid flowing from stage 1 to stage N.

concentrated solution

Exhausted solids

Exhausted solids leave stage N, and the concentrated solution is exiting the overflow of stage 1.

The flows V and L may be expressed in:

•mass per unit time•based on a definite flow or dry solute-free solid.

Also, in accordance with standard nomenclature, the terminal concentrations are as follows:

Solution on entering Solid xaSolution on leaving solid xbFresh solvent entering the system ybConcentrated solution leaving the system ya

As in absorption and distillation, the quantitative performance of a countercurrent system can be analyzed by utilizing an equilibrium line and an operating line.

Equilibrium In leaching, provided sufficient solvent is present to dissolve all the solute in the entering solid and there is no adsorption of solute by the solid, equilibrium is attained when the solute is completely dissolved and the concentration of the solution so formed is uniform. Such a condition may be obtained simply or with difficulty, depending on the structure of the solid. The equilibrium relationship is simply, xe=ye

Operating line The equation of the operating line is obtained by writing material balances for that portion of the cascade consisting of the first n units, as shown by the control surface indicated by the dashed lines

These balances are Total solution: Vn+1+La= Va+Ln

Solute: Vn+1yn+1+ LaXa= LnXn+ Vaya

Solving for yn+1 gives the operating- line equation:

Vn+1yn+1+ LaXa= LnXn+ Vaya