Extraction - Principles of Chemical Separations with Environmental Applications - page 126

estimation of flowrates corresponding to different diameters. Both economic and mass
transfer considerations determine the “best choice” of many technical solutions.
The tower extractors described above rely purely upon density differences between
the two liquids and gravity flow to achieve good mixing and separation of the phases.
In some tower extractors, like the one shown in Figure 5.4, mechanical mixing is cre-
ated by agitators mounted on a central rotating shaft to enhance the mass transfer. Flat
disks disperse the liquids and impel them outward toward the tower walls, where stator
rings create “quiet” zones where the phases can separate. This design gives essentially
a cascade of mixer-settlers with greatly increased mixing and settling efficiencies. The
disadvantage of these designs is the problem of maintaining the internal moving parts
and the special construction materials that may be required for a corrosive or reactive
environment.

5.6 Leaching processes

As previously stated, leaching is another extraction process in which a liquid is used to
remove soluble matter from its mixture with an inert solid. With a few extra consider-
ations, the equilibrium analysis of leaching is the same as for liquid extraction. Several
assumptions are made in designing leaching processes. These can be rendered correct with
the proper choice of solvent. It is assumed that the solid is insoluble in the solvent (dirt
will not dissolve in water) and the flowrate of solids is essentially constant throughout
the process. The solid, on the other hand, is porous and will often retain a portion of the
solvent.
One way in which leaching differs is that, depending upon the solute and the solid
material from which it is leached, the solid may remain the same or change considerably
in form. For example, when impurities are leached from wood in the paper-making process,
the final wood product retains some solvent and becomes a pulpy, mushy mixture. Coffee
beans, on the other hand, are relatively unchanged when leached with hot water to make
coffee. The desirable end product determines whether or not this is significant. In coffee
making, the leftover ground beans are the waste product, so their final form is irrelevant.
In paper making, the wood pulp is the product, so a change is important.
There are two fundamental mechanical systems in which leaching can be performed,
dispersed solids leaching and stationary solid beds. The properties of the solid to be leached
determine which is applicable. When the solids form an open, permeable mass throughout
the leaching process, such that solvent can be reasonably percolated through an unagitated
bed of solids, stationary solid bed leaching is appropriate. This would be the case when
treating highly porous soils, such as grainy sands, that do not change form when exposed
to the leaching fluid. If the solids are either impermeable, such as dense clays, or if they
disintegrate into the solvent, the solids must then be dispersed into the solvent and then
later recovered from it. Under these conditions, dispersed solids leaching is suitable.