How does Liquid-Liquid Extraction work?

Ola Akinsunmade

Ola Akinsunmade

Chemical Engineer

Table of Contents


Liquid-Liquid Extraction (LLE) has common applications in the Chemicals, Wastewater and Petrochemical industries for extracting valuable substances or removing contaminants from a feed stream.

 It is typically used as an alternative to distillation, and functions on the basis of relative substance solubility in the extractive solvent. It is also seen as a suitable replacement for distillation as an extraction process in the case of high energy requirements and process design economics (column sizing) [1]

picture showing the different stages of liquid liquid extraction
Liquid-Liquid Solvent Extraction

What is Liquid-Liquid Extraction?

Liquid-Liquid Extraction is a physical separation process involving the mass transfer of the desired solute typically from an aqueous carrier solvent phase to the organic extractant solvent phase. It is also sometimes a chemical process depending on the interactions between the solvent and feed stream (ion exchange between phases or formation of chemical intermediates).

Other examples of solvent extraction include Solid-Liquid Extraction, otherwise known as leaching. 

Stages of Liquid-Liquid Equilibrium

There are typically three stages involved in a liquid-liquid extraction mechanism:

  • Intimate contact between carrier solvent, solute (feed) and extractive solvent by dispersing one phase into the other as droplets
  • Separation of resulting phases (usually via gravitational settling)
  • Solute recovery from extract phase (usually via distillation)
How Does Liquid-Liquid Extraction Work?Single-Stage Process with Solvent Recovery
Single-Stage Process with Solvent Recovery (Coulson et al., 1998)

In industrial applications, the two resulting streams following the contact stage of the process are the extract (E) and the raffinate (R) streams. The extract stream, as indicated by its name, is rich in the extraction solvent and the desired components/solute. The raffinate stream is the residual liquid following the separation of phases. It is composed of a weaker concentration of the desired solute and the extraction solvent.

Operating Principle of Liquid-Liquid Extraction

Ultimately, LLE is driven by these three phenomena [2]:

  • Phase/Liquid-Liquid Equilibrium
  • Turbulent/Eddy Diffusion (created by mechanical dispersion of one phase into another via agitation or pumping)
  • Interfacial Area and Tension between liquid layers

Liquid-Liquid Equilibrium

The equilibrium phase is dictated by the chemical properties of the feed mixture although separation between phases is dictated by the relative affinity/degree of miscibility of the desired product to either solvent. 

The extraction solvent is typically non-polar(organic) whilst the extraction solvent is polar (aqueous). The concentration difference across both phases also determines the solute’s rate of mass transfer to the extracting solvent. 

As a system approaches a state of thermodynamic equilibrium, it seeks to minimize its chemical potential or Gibbs Free Energy, becoming more stable.

The two categories of miscibility (using the naming conventions outlined above) which dictate the equilibrium state of the liquid-liquid extraction system are:

  • Miscibility of solute (B) in extractive (C) and carrier solvent (A) in all proportions
  • Complete miscibility of solute (B) in carrier solvent (A) but limited miscibility in extractive solvent (C)

Ternary Diagrams/Liquid-Liquid Extraction Diagrams

The phase equilibrium of the components in liquid-liquid extraction is graphically represented by a ternary diagram.

picture showing how to read a ternary diagram
Interpreting a Liquid-Liquid Equilibrium Graph/Ternary Diagram

Here are a few important things to note about reading ternary diagrams:

  • The vertices of the triangle represent pure components 
  • The sides of the triangle represent binary mixtures of components 
  • The bases of the triangle represent 0% composition lines for the component on the opposing vertex
  • The composition of a single component in a binary mixture changes as you move along the base of the triangle
  • Points within the triangle represent a ratio of all three components. Concentrations of each component can be traced by reading the composition line that runs parallel to the base that represents its 0% composition. 
  • Ternary diagrams have different phases; single phase region and two phase
    • In the single phase region, all three components are perfectly miscible and are dissolved into a single liquid phase
    • Two phase region represents where the mixture splits into two immiscible liquid phases
  • The two regions of immiscibility are divided by the miscibility region which is the liquid-liquid equilibrium line
  • Raffinate and extract stream compositions lie on the miscibility boundary or liquid-liquid equilibrium line
  • Tie lines in the two phase region connect equilibrium points on the miscibility boundary
    • Tie lines converge to the Plait Point where both liquid phases (extract and raffinate) have the same solute composition. Extraction is impossible at this point.

These diagrams are also used to complete mass balances to determine the flow rates and compositions of the extract and raffinate streams as well as the number of theoretical stages required for extraction. Explanation of this methodology is out of the scope of this article.

Distribution/Partition Coefficient

The degree of separation in a LLE process is typically quantified by what is called the partition coefficient or distribution constant which is defined as a ratio of the solute (desired product) in the two phases (extract and raffinate).

picture of equation of partition or distribution coefficient


K’ = Distribution/Partition Coefficient


CE= Concentration of Solute in Extract/Organic phase

CR= Concentration of Solute in Raffinate/Aqueous phase

The ratio is only correctly applied if both the carrier and extraction solvent are immiscible at equilibrium and typically holds if the concentrations are small and no chemical reaction takes place.

Equipment Used In Liquid-Liquid Extraction

The three main types of vessels used in the continuous liquid-liquid extraction process include:

  1. Mixer settlers
  2. Tray/packed columns or
  3. Centrifugal rotators such as Podbielniak or Schiebel extractors

Industrial applications of liquid-liquid extraction are either batch or continuous processes. In In batch processes, the first contact and separation phases of the process occur within the same column.

picture of mixer settlers
Mixer Settler for Liquid-Liquid Extraction (De Dietrich Process Systems, 2022)

In a continuous process, contact between a feed stream containing the carrier and extraction solvent happens in multiple stages whereas the separation and recovery phases in another processing unit via gravitational settling and another extraction process such as distillation respectively.

Applications of Liquid-Liquid Extraction

In hydrometallurgy, liquid-liquid extraction is used to extract pure elements such as copper from their aqueous ores using organic solvents. In its petrochemical application, acidic gasses such as CO2 and H2S generated in fluid catalytic crackers are extracted from liquefied petroleum gas (LPG) using amine solvents [1].

How Do You Extract Liquid-Liquid?​Fluid Catalytic Cracking
Schematic Flow Diagram of a Fluid Catalytic Cracker (WikiCommons, 2008)

Another common application of liquid-liquid extraction is in the recovery of antibiotics such as penicillin from its aqueous fermentation broth using oxygenated organic solvents [1].


  1. R. H. Perry and D. W. Green, “Section 15: Liquid-Liquid Extraction and Other Liquid-Liquid Operations and Equipment,” in Perry’s Chemical Engineers’ handbook, New York etc.: McGraw-Hill, 2019. 
  2. J. M. Coulson and J. F. Richardson, “Liquid Liquid Extraction,” in Particle Technology and separation processes, Oxford: Pergamon Press, 1998. 
  3. D. D. P. Systems, “Extraction by de Dietrich Process Systems,” Extraction by De Dietrich Process Systems. [Online]. Available: [Accessed: 27-Apr-2022].

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