An extractant that is immiscible (or only partially miscible) with the solution to be separated (the feed solution) is added to the feed, creating two coexisting liquid phases. By exploiting differences in the solubility of various components in the original solvent versus the extractant (including solubility resulting from chemical reactions), the components are distributed unequally between the two phases; subsequent separation of the two liquid phases then achieves the separation of the components. For example, when an aqueous iodine solution is extracted with carbon tetrachloride, almost all the iodine transfers into the carbon tetrachloride, thereby separating the iodine from the bulk of the water.

The most fundamental operation is single-stage extraction. This involves bringing the feed solution and the extractant into intimate contact during mixing, allowing the component to be extracted to pass across the phase interface into the extractant until the distribution of the component between the two phases essentially reaches equilibrium. The mixture is then allowed to settle, separating into two liquid layers: The extract (derived from the extractant) and the raffinate (derived from the feed solution). At phase equilibrium in a single-stage extraction, the ratio of the concentrations of the extracted component B in the two phases-known as the distribution coefficient K-is defined as:
K = yB / xB
where yB and xB represent the concentrations of component B in the extract and the raffinate, respectively. When expressing concentrations, one must account for the various forms in which the component exists and perform calculations based on a consistent chemical formula.
If another component, D, in the feed solution is also extracted, the ratio of the distribution coefficient of component B to that of component D-representing the separation factor of B relative to D-is known as the selectivity coefficient α, defined as:
α = KB / KD = yB · xD / (xB · yD)
When α > 1, component B is preferentially extracted; α = 1 indicates that the two components are distributed identically between the two phases, meaning this extractant cannot be used to separate them. Single-stage extraction often yields a low extraction rate (the ratio of the amount of the extracted component in the extract to its initial amount in the feed) for a given component, frequently failing to meet process requirements. To improve the extraction rate, various methods can be employed: multistage cross-flow extraction, where the feed and the raffinate from each stage come into contact with fresh solvent, achieving a higher extraction rate but requiring a large amount of solvent and resulting in a low average concentration of the extracted component in the extract; and multistage counter-current extraction, where the feed and solvent enter from opposite ends of a cascade (or tray column) and flow counter-currently between stages, finally exiting as raffinate and extract from the respective opposite ends. Since both the feed and solvent undergo multiple extraction steps, this method achieves a higher extraction rate and a higher concentration of the extracted component in the extract, making it a common industrial process. Another common industrial method is continuous counter-current extraction, performed in differential-contact extraction columns (see extraction equipment), where the feed and solvent undergo mass transfer through contact while flowing counter-currently. Between the feed and solvent, the phase with the higher density is termed the heavy phase, and the one with the lower density is the light phase. The light phase enters at the bottom and overflows at the top, while the heavy phase is fed at the top and withdrawn at the bottom. During column operation, the liquid phase that fills the entire column is called the continuous phase, while the other liquid phase-usually dispersed as droplets within the continuous phase-is called the dispersed phase. The dispersed phase liquid is dispersed upon entering the column and coalesces into a separate layer before being withdrawn. The choice of which phase (feed or solvent) serves as the dispersed phase depends on both operational and process requirements. Additionally, reflux extraction and fractional extraction are methods capable of achieving higher degrees of separation.