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Leaching
Extraction of soluble constituent from a solid by means of a
solvent
The method used for the extraction is determined by the
proportion of soluble constituent present, its distribution
throughout the solid, the nature of the solid and the particle size
Rate of extraction falls, as the process proceeds
Process can be considered into three parts
Change of phase of the solute as it dissolves in the solvent Its diffusion through the solvent in the pores of the solid to the outside of the particle The transfer of the solute from the solution in contact with the particles to the main bulk of the solution
Any one of these three processes may be responsible for
limiting the extraction rate, the first process usually occurs so rapidly that it has a negligible effect on the overall rate
Factors Influencing the Rate of
Extraction
Factors which are responsible for limiting the extraction rate
If diffusion of the solute through the porous structure of the
residual solid is the controlling factor, the material should be of
small size so that the distance the solute has to travel is small
If the diffusion of the solute from the surface of the particles to
the bulk of the solution is the controlling factor, a high degree
of agitation of the fluid is required
Particle Size
Interfacial area Smaller length of diffusion Separation of particle from the liquid and drainage of the solid residue are made more difficult May wedge in the interstices of the larger particles and impede the flow of the solvent
Mass Transfer in Leaching
Operations
V is the total volume of a batch process solution, and is assumed to remain constant
Mass Transfer in Leaching
Operations
In most cases the interfacial area will tend to increase during the extraction
When the soluble material forms a very high proportion of the total solid, complete disintegration of the particles may occur
Although this results in an increase in the interfacial area, the rate of extraction will probably be reduced because the free flow of the solvent will be impeded and the effective value of b will be increased
Equipments for Leaching
The rate of extraction will, in general, be a function of the
relative velocity between the liquid and the solid
In some plants the solid is stationary and the liquid flows
through the bed of particles
In some continuous plants the solid and liquid move counter-
currently
Extraction from Cellular Materials
With seeds such soya beans, containing only about 15 % of oil, solvent extraction is often used because mechanical methods are not very efficient
The upper section is filled with the charge of seeds which is sprayed with fresh solvent via a distributor
The solvent percolates through the bed of solids and drains into the lower compartment where, together with any water extracted from the seeds, it is continuously boiled off by means of a steam coil
The vapors are passed to an external condenser, and the mixed liquid is passed to a separating box from which the solvent is continuously fed back to the plant and water is run to waste
Batch Plant for Extraction of Oil from Seeds
Leaching of Coarse Solids
A simple batch plant used for coarse solids consists of a cylindrical vessel in which the solid rest on a perforated support
The solvent is sprayed over the solids and, after extraction is complete, the residue is allowed to drain
If the solid contains a high proportion of solute such that it disintegrates, it is treated with solvent in a tank and the solution decanted
In a simple countercurrent system, the solid is contained in a no of tanks and the solvent flows through each
First vessel contains solid which is almost completely extracted and the last contains fresh solid
After some time, first tank is disconnected and fresh is introduced at other end
Gravity or pressure driven solvent flow
Heat before it enters each tank
This system involves frequent interruption while recharging, counter flow is not obtained within the units themselves
Leaching of Coarse Solids
Counter current system
No of these units may be connected in series
Leaching of Fine Solids
Fine solids offer too high a resistance to flow
Particles of less than about 200-mesh (0.075 mm) may be maintained in suspension with only a small amount of agitation
As the total surface area is large, extraction can be effected in a reasonable time
Because of the low settling velocity of the particles and their large surface, the subsequent separation and washing operation are more difficult for fine materials than with coarse solids
Agitation may be achieved either by the use of a mechanical stirrer or by means of compressed air
Counter Current Washing
n washing thickeners arranged for counter current washing
L is the amount of solute in the overflow, w’s are the
corresponding quantities of solution
S and W are the amount of solute and solution in the underflow
Counter Current Washing
Counter Current Washing
Sn/ S 0 is the fraction of the solute fed to the washing system
which remains associated with the washed solids. If in given
case it is required that this fraction should not exceed a value f,
the minimum number of washing thickeners required is given by
Counter Current Washing
In terms of the total amount of solution entering and leaving each thickener
