Overview: What is liquid-liquid extraction?

Liquid-liquid extraction is a separation process that relies on the miscibility of liquids to exchange a specific compound between two solvents. Liquid-liquid extractions typically operate by mixing an aqueous phase (liquid feed) containing the target solute with an organic phase (solvent). The solvent selected for the separation is ideally  immiscible or partly miscible with the liquid feed, but it has a strong affinity with the target compound [1]. Consequently, the organic phase is loaded with the compound aimed to be extracted. In order to achieve an effective separation, it is crucial to create a large interfacial area between the aqueous and organic phases [1]. The extracted feed solution is called the raffinate, and the solute-loaded organic stream is called the extract. A simple process flow diagram describing liquid-liquid extraction is shown below:

Criteria to choose the right solvent

One of the most relevant aspects to consider when conducting a liquid-liquid extraction is selecting the right solvent. The physical and chemical properties of the compound are reliable parameters that can tell whether a solvent will be efficient during the separation process. The following criteria can be considered as a guide for solvent selection [1]:

Distribution Coefficient: Having a high distribution coefficient entails that the solvent has a high affinity for the target compound. Consequently, the organic to aqueous phase ratio also becomes low.

Selectivity: A high selective for the target component will facilitate the operation and design of liquid-liquid separation processes since fewer equilibrium stages will be necessary.

Miscibility: The solvent chosen should be immiscible with the aqueous phase in the feed because the two phases (aqueous and organic) have to be contacted in large interfacial areas. However, the solvent only needs to mix with the target compound aimed at being separated.

Density: The aqueous and organic phases should present high-density differences for the separation process to be more efficient.

Cost: Cost represents a relevant aspect in any design decision. Therefore, it is necessary to make sure the solvent chosen is commercially available and has a reasonable price.

Designing criteria 

McCabe-Thiele method

LIquid-liquid extraction is used in several chemical industries. However, the foundation in its design is somewhat similar. A convenient place to start the design is to calculate the theoretical number of equilibrium stages for the separation. McCabe-Thiele diagrams can be used for this calculation. This graphical technique optimizes the conditions of an operation unit by identifying the optimal number of cells needed for an effective process and the flow rate ratio between aqueous and organic solutions. The McCabe-Thille method relies on conducting several experiments contacting the aqueous solution with the organic solution at different volumetric ratios. Then, the aqueous and organic phases are separated to obtain the target compound concentration in both phases. Once the concentrations for all the ratios tested are known, It is possible to produce a plot of the compound concentration in the aqueous phase vs. the compound concentration in the organic phase. Consequently, the data points collected allow to produce an operating line and the  McCabe-Thille diagram. Interpreting such diagrams allows to know the operating conditions for the pilot plants. The following picture can be used as a guide to interpreting the McCabe-Thille diagram:

The blue dots in the operating line come from data collected in experiments. This example shows that eight equilibrium stages are needed to design this liquid-liquid extraction process when the aqueous to organic phase is 2.2 to 1. This ratio comes from the slope of the equilibrium line. Furthermore, one aspect to consider as chemical engineer is coming up with the less number of equilibrium stages by changing the aqueous to organic ratio. However, it is also relevant to consider that such ratio has to be effective enough to see separation.

Co-current vs. Counter current operations

The two main patterns at which liquid-liquid extraction is conducted are co-current and counter-current operations. In a co-current setup, both the aqueous and organic phases flow in the same direction. In contrast, the feeds for the aqueous and organic phases enter the system at opposite ends in a counter-current setup [3]. Several experiments run on liquid-liquid extraction processes have proven counter-current flow to be more efficient since mass transfer becomes more effective using this setup. However, It is encouraged to perform mass balances on liquid-liquid extraction units to ensure the required separation is achieved [3]. The following diagrams show counter-current and co-current setups:

Advantages and disadvantages of liquid liquid extraction

Advantages [2]:

• This process can be used to separate azeotropic mixtures

• Physical properties and other parameters are flexible for the process to work

• The process can be used in several industries, including the separation of precious metals

• Liquid-liquid extraction entails a simple operation and apparatus in comparison with other separation methods

Disadvantages [2]:

• Emulsions can be formed in the solution because of the miscibility of the solvent used.

• Liquid-liquid extraction relies on the use of a third component. Therefore, this third component will have to be removed from the target compound in additional operations later in the process.

• De-entrainment of organic and aqueous flows is an issue that can be triggered in Liquid-liquid extraction.

Applications of liquid liquid extraction in the hydrometallurgical industry

Several industries rely on liquid-liquid extraction as a separation technique. Hydrometallurgy, for example, is a field where diverse solvent extraction processes occur to separate precious metals. The metals obtained by using liquid-liquid extraction are the ones soluble in adequate lixiviants such as sulphuric acid or ammonia. Additionally, extensive research has been conducted in the solvent extraction chemistry behind these metals [4].

Uranium is one of the metals that is extracted by using liquid-liquid extraction alongside other methods. This metal can be separated by forming an acid leach solution used as the aqueous feed for the extraction operation. However, the leaching process to obtain uranium is not selective. Therefore, other compounds such as soluble silica or molybdenum are also present in the solution [4]. As a consequence, these compounds will play a role when selecting a solvent. Generally, try-octyl and try-decyl amines are used as the organic phase for uranium extraction. Nevertheless, these two organic compounds are not soluble enough when molybdenum is present. Thus, trilaurylamines becomes a more suitable option. Additionally, aromatic diluents are added to the organic phase to improve solubility. Similarly, Isodecanol al at a concentration of 50% of the amine concentration also becomes part of the organic phase acting as a  modifier to increase phase separation [4]. Finally, it is worth mentioning that the circuit layout for uranium includes leaching, extraction, scrubbing, stripping, and other processes, as it is seen in the following process flow diagram [4]:

References