Liquid–liquid extraction

The liquid-liquid extraction ( LLE Abbreviated for liquid -liquid extraction) is a separation method that exploits the different solubility of substances in two immiscible solvents. Solvents, depending a hydrophilic phase are (mostly water) and a hydrophobic organic solvent. This type of separation is an often used method in organic laboratories for working up reaction mixtures.

Implementation

The solution with the component to be extracted is mixed with the extractant in a separating funnel with vigorous shaking. This leads to an increase in the phase boundary between the two solvents and thus to a better extraction of the component in the other phase. In the resting state form according to the different densities of two layers again. After separation of the two phases, the product may be recovered by evaporation of the solvent.

Mechanisms

According to Nernst's law of distribution, there is a distribution between the two phases, similar to the chemical equilibrium. Depending on the size of the equilibrium constant is a certain amount of substance more than the extraction agent and can be removed with this. Repeated addition of the extractant and again separating the desired product can be extracted almost completely.

Working Principle and Calculation

The liquid-liquid extraction consists of the solvate ( or more solvates ), located in the primary liquid solution, to be transmitted in another, non-miscible liquid ( solvent). The enriched with the solvate solvent is referred to as the extract, the diluted stock solution as raffinate.

The primary liquid and the solvent are brought into contact in order to effect the transfer of the solvate. The two liquid phases ( extract and raffinate) are static decantation ( mixer-settler ) or centrifugal force separately.

Mixer

In the mixing zone of the primary liquid solution is heated by means of a mechanical stirrer into intimate contact with the solvent, so that a good take Solvatübergang ¬ place. The mechanical stirrer is equipped with an electric motor which drives a mixer and pump turbine. This sucks the phases from the Settlers of the adjacent steps on, brings them into contact and the emulsion prepared in the mixer passes back into the settler.

Settler

Static settling between the two phases. Koaleszenzgitter facilitate separation of the emulsion formed in two phases ( heavy and light ). The transfer of the separated phases is done by overflowing over the weirs. The weir of the heavy phase is adjustable in height so that the position of the intermediate phase (difficult / easy) is positioned according to the density of each phase.

Centrifugal

In centrifugal extractors to extracting phase containing one or more solvates ( yellow on the diagram), put an immiscible solvent (blue on the scheme) with a different density, a mixing chamber, and which is located in the lower part of the device, supplied.

With the help of a rotating disk, the immiscible liquids are mixed to form a dispersion (green on the scheme). Depending on the interfacial tension between the liquids of different stirring discs are used. An effective mixing is an extremely high mass transfer surface area between the two fluids. The transfer of the solvate / solvates, is thereby facilitated.

One located in the lower part of the centrifuge drum turbine pumps and transfers the dispersion in the drum. Due to centrifugal force, the liquids are separated. The heavy phase (yellow) is thrown against the drum wall, the light phase (blue) is positioned in the central region of the drum.

The weir of the heavy phase stabilizes the position of the phase interface. Interchangeable weirs with different diameters allow a wide range of density ratio range to cover. The heavy phase flows out in the lower part of the outer casing of the centrifuge. The light phase passes by overflow in the top part of the outer housing.

The two liquids are discharged by gravity into the mind standing centrifugal extractor. For extraction methods which require several successive stages, the stage extractors are connected in series or set up as batteries. One unit per level. The two liquids in counter-current flow in the batteries.

Feed pumps between the extractors are superfluous. External connection lines as needed to allow liquids of an extractor in the other transmitted or derived from the process ( primary extraction, washing and re-extraction ). Optimum flexibility is hereby guaranteed.

Liquid-liquid microextraction

In the liquid phase micro- extraction (liquid - phase microextraction, LPME ) are an extraction of a small amount of sample to a drop (single -drop microextraction ) or a solvent -filled hollow fiber ( hollow -fiber LPME ). This method is mainly used for analysis of biological samples. Of advantage are the low sample volume, the high Vorkonzentrationsfaktor and easy sample preparation.

Liquid membrane permeation

The liquid membrane permeation is also used for the extraction of heavy metals from waste water traces. , For example, sulfuric acid is used in an oil phase which contains dissolved chelators emulsified and this emulsion is in turn emulsified in the wastewater. The heavy metals are dissolved by liquid -liquid extraction in the oil phase thereof, and also converted by the liquid-liquid extraction in the sulfuric acid. After separation of the oil phase, the acid -in- oil emulsion is broken in a high frequency AC field.