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Solvent extraction is a common form of chemical
extraction using organic solvent as the extractant. It is commonly used in
combination with other technologies, such as solidification/stabilization,
precipitation and electrowinning.
Solvent extraction is a selective separation procedure for isolating and
concentrating a valuables metals from an aqueous solutions with the aid of an
organic solution. In the procedure the aqueous solution containing the metal
of interest, often at a low concentration and together with other dissolved
substances, is mixed (extraction) with an organic solvent containing a
reagent. The metal of interest reacts with the reagent to form a chemical
compound, which is more soluble in the organic than in the aqueous solution.
As a consequence, the substance of interest is transferred to the organic
solution
Subsequently, in order to recover the extracted substance, the organic
solution is mixed (stripping) with an aqueous solution whose composition is
such that the chemical compound between the metal and the reagent is split
and, thus, the metal is recovered in the "new" aqueous solution, in a pure
form. The concentration of the metal in the "new" aqueous solution may be
increased, often to 10-100 times that of the original aqueous solution,
through adjustment of the liquid flow rates. The organic solution is returned
for further extraction, either directly or after a fraction of it has been
cleansed of impurities.
McCabe Thiele Diagrams
In general terms the use of McCabe Thiele Diagrams enables the metallurgist or engineer to calculate the number of stages required, or alternatively, to predict the performance of a given set of conditions. Phase separation times are critical as they have a direct bearing on the size of the settler required and on the overall process efficiency.
Metal loading refers to the solvent phase and is a function of the
solution properties, namely pH. Competing ions will also have an effect
on the metal loading. The significance of the determination of loading
is in the definition of optimum plant operating conditions in terms of
inventory of organic, O/A ratio and recirculation rates. The rate of which the
metal is transferred from one phase to another (kinetics) can be a significant
factor in the design and operation of the plant, whether in extraction or in
stripping.
Continuous Counter
Current test
The basic equipment to perform
a continuous mixing and coalescence process on a technical scale is called a
mixer-settler. It is often used in laboratories as an ideal tool for basic
system design of continuous solvent extraction processes because it offers reproducable phase flow and contact times.
The device comprises a
continuously fed and stirred mixing compartment (see sketch below) and a
gravity settler compartment where the liquids are allowed to separate. At the
end of the settler two individual weirs care for good separation of the liquid
bulk phases. The flow capacity of a mixer-settler is reached when the emulsion
phase dispersion band overflows the light phase weir or underflows the heavy
phase weir.
In technical mixer-settler
devices the volume of the settler compartment is often 10-fold the size of the
mixer compartment to provide sufficient settling time even for systems with
low coalescence rates (in case of high viscosity and/or low density and/or low
interfacial tension).
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