Extraction - Principles of Chemical Separations with Environmental Applications - page 127

Dispersed solids leaching is performed in a continuous mode in which the solids are
first dispersed into solvent by mechanical agitation. This mixed solids stream is then run
countercurrent to the solvent stream in a series of mixer-settlers as shown in Figure 5.1.
The design and estimation of the number of equilibrium-limited stages is similar to that
of liquid extraction, except that one additional stage is needed to disperse the solids in
the solvent. Solute concentrations will change significantly in this stage due to dilution
with solvent. An illustrative example is provided in a subsequent section of this chapter.
At the end of the process the leached solids residue is separated from the mixed solution
by settling or filtration. The solute can be recovered from the exiting liquid solvent stream
by evaporation or crystallization.
Stationary solid bed leaching is performed in a series of mixer-settlers similar to those
shown in Figure 5.1. This system is best for permeable solids, but can also be used to
treat reasonably semi-permeable solids if pressure is applied to force the solvent through
the beds. Each mixer-settler unit is a single tank with a perforated false bottom, which
both supports the solids and allows drainage of the solvent. The process is performed
in batch mode. Solids are loaded into each leaching tank, sprayed with solvent until the
solute concentration is adequately low, then excavated back out of the tank. Because of
the downtime required to pack and excavate the tanks, more than one is required in series
in a continuous process. Each vessel is analogous to an equilibrium-limited stage. The
solvent then passes through each vessel in series beginning with the tank containing the
solid with the lowest solute concentration (most completely extracted) and ending with the
least extracted tank. The solid in any one tank is stationary until it is completely leached.
The solvent system piping is arranged such that fresh solvent can be introduced to any tank
and spent solvent can be withdrawn from any tank. Countercurrent flow is maintained by
advancing the inlet and outlet tanks one at a time.
Another type of stationary bed extractor is shown in Figure 5.5. This Hildebrand extrac-
tor consists of a U-shaped screw conveyor with a separate auger, or screw, mechanism in
each section to transport the solids through the U. Each auger rotates at a different speed
to control compaction of the solids. Solids are fed into one end of the U and solvent into
the other to provide countercurrent flow.

5.6.1 Partially miscible extraction

The analysis of extraction is based on the two primary modes of contact: cross-flow and
countercurrent.

Cross-flow

Figure 5.6 is a schematic of a cross-flow cascade.
Mass balance :

O 0 + V 0 = M 1 = O 1 + V 1 (Stage 1) . (5.1)