The particle size of the solids determines the distance that solvent and solute must diffuse within the solid matrix.
THE SIZE AND SIZE DISTRIBUTION
OF THE SOLID PARTICLES
The
particle size of the solids determines the distance that solvent and solute
must diffuse within the solid matrix. Since this offers the major diffusional
resistance, reduction of the distance by comminution greatly increases the rate
of leaching, the concentration gradient being effectively increased. In
addition, the inverse relationship between particle size and surface area
prescribes an increase in the area of contact between the matrix and the
surrounding liquid. Transfer of solute at this boundary is therefore
facilitated. In leaching by immersion, a further advantage conferred by size
reduction is the ease with which finer particles are suspended. Finally,
extensive cell rupture occurs during grinding, allowing more direct contact
between solvent and solute and more rapid dissolution and diffusion.
Other
factors, however, operate against size reduction. Leaching by per-colation
demands the formation of a permeable bed. Low permeability will give low flow
rates and low rates of extraction. Permeability is a complex function of both
particle size and porosity, the former determining how a given void space is to
be disposed within the bed. The disposition of the void space will consist of
few channels of relatively large diameter, that is, a bed of high permeability,
if the particle size is large. In leaching by immersion, the difficulties of
separating solid and liquid increase as the particle size decreases.
The
opposition of the factors suggests an optimum particle size for any particular
extraction. This is determined to some extent by the physical nature of the
solids. A dense, woody structure would be extracted as a fine powder. An
example is given by the root of Ipecacuanha. A leafy structure, on the other
hand, would be more satisfactorily leached as a coarse powder.
Both
porosity and permeability are influenced by the particle size distri-bution. A
high porosity is secured if the distribution is narrow. Small particles may
otherwise fill the interstices created by the contact of larger particles.
After grinding, therefore, it is often necessary to classify the product and
remove undersize material. The undersize would then be bulked with the fines
from other batches and separately extracted. A further advantage arising from a
narrow size distribution is even packing and the creation of a regular system
of pores and waists. This promotes even movement of solvent and solution
through the bed.
In
some cases, size reduction may take a particular form. Seeds and beans are
often rolled or flaked to produce extensive cell rupture. In other processes,
the cell wall, although depressing the rate of extraction, may make the
extraction more selective by preventing the movement of unwanted materials of
high molecular weight. Here the size reduction must leave most cells intact.
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