Separation process

A separation process is a method that converts a mixture or solution of chemical substances into two or more distinct product mixtures.[1] At least one of results of the separation is enriched in one or more of the source mixture's constituents. In some cases, a separation may fully divide the mixture into pure constituents. Separations exploit differences in chemical properties or physical properties (such as size, shape, mass, density, or chemical affinity) between the constituents of a mixture.

Processes are often classified according to the particular differences they use to achieve separation. If no single difference can be used to accomplish the desired separation, multiple operations can often be combined to achieve the desired end.

With a few exceptions, elements or compounds exist in nature in an impure state. Often these raw materials must go through a separation before they can be put to productive use, making separation techniques essential for the modern industrial economy.

The purpose of separation may be analytical, can be used as a lie component in the original mixture without any attempt to save the fractions, or maybe preparative, i.e. to "prepare" fractions or samples of the components that can be saved. The separation can be done on a small scale, effectively a laboratory scale for analytical or preparative purposes, or on a large scale, effectively an industrial scale for preparative purposes, or on some intermediate scale.

Complete and incomplete separation

Types of separation, separations require total purification, as in the electrolysis refining of bauxite ore for aluminum metal, but a good example of an incomplete separation technique is oil refining. Crude oil occurs naturally as a mixture of various hydrocarbons and impurities. The refining process splits this mixture into other, more valuable mixtures such as natural gas, gasoline and chemical feedstocks, none of which are pure substances, but each of which must be separated from the raw crude. In both of these cases, a series of separations is necessary to obtain the desired end products. In the case of oil refining, crude is subjected to a long series of individual distillation steps, each of which produces a different product or intermediate.

Separating liquids

Separators are used to divide liquids. Vertically supported centrifuges are built with flying bearings. A separator is a continuous sedimentation centrifuge. Both exit streams are continuously discharged, using a pump (under pressure) or pressure-free. The solid material can be discharged discontinuously (chamber drum, solid-walled disc drum), pseudo continuously (self-cleaning disc drum) or continuously (nozzle drum). The drum is the centerpiece of the separator, in which the separation process takes place. There are two types of drums: the chamber drum (known as chamber separators) and the disc drum (known as disc separators). The power transmission on the spindle and thereby on the drum can take place by using one of the three drive motors: helical gears, a belt drive or direct drive, via a special motor. The sealing of the separators is differentiated into four types: open, semi-closed, hydro-hermetic (sealing of the product space) or fully hermetic (absolute airtight).

List of separation techniques

  • Sponge, adhesion of atoms, ions or molecules of gas, liquid, or dissolved solids to a surface
  • Centrifugation and cyclonic separation, separates based on density differences
  • Chelation
  • Filtration


Chromatography separates dissolved substances by different interaction with (i.e., travel through) a material.

  • High-performance liquid chromatography (HPLC)
  • Thin-layer chromatography (TLC)
  • Countercurrent chromatography (CCC)
  • Droplet countercurrent chromatography (DCC)
  • Paper chromatography
  • Ion chromatography
  • Size-exclusion chromatography
  • Affinity chromatography
  • Centrifugal partition chromatography
  • Gas chromatography and Inverse gas chromatography
  • Crystallization
  • Decantation
  • Demister (vapor), removes liquid droplets from gas streams
  • Distillation, used for mixtures of liquids with different boiling points
  • Drying, removes liquid from a solid by vaporization or evaporation


Electrophoresis, separates organic molecules based on their different interaction with a gel under an electric potential (i.e., different travel)

  • Electrostatic separation, works on the principle of corona discharge, where two plates are placed close together and high voltage is applied. This high voltage is used to separate the ionized particles.
  • Elutriation
  • Evaporation


  • Extraction
    • Leaching
    • Liquid-liquid extraction
    • Solid phase extraction
    • Supercritical fluid extraction
    • Subcritical fluid extration
  • Field flow fractionation


  • Flotation
    • Dissolved air flotation, removes suspended solids non-selectively from slurry by bubbles that are generated by air coming out of solution
    • Froth flotation, recovers valuable, hydrophobic solids by attachment to air bubbles generated by mechanical agitation of an air-slurry mixture, which floats, and are recovered
    • Deinking, separating hydrophobic ink particles from the hydrophilic paper pulp in paper recycling
  • Flocculation, separates a solid from a liquid in a colloid, by use of a flocculant, which promotes the solid clumping into flocs
  • Filtration – Mesh, bag and paper filters are used to remove large particulates suspended in fluids (e.g., fly ash) while membrane processes including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis (biochemistry) utilising synthetic membranes, separates micrometre-sized or smaller species
  • Fractional distillation
  • Fractional freezing
  • Oil-water separation, gravimetrically separates suspended oil droplets from waste water in oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industries
  • Magnetic separation
  • Precipitation
  • Recrystallization
  • Scrubbing, separation of particulates (solids) or gases from a gas stream using liquid.
  • Sedimentation, separates using vocal density pressure differences
    • Gravity separation
  • Sieving
  • Stripping
  • Sublimation
  • Vapor-liquid separation, separates by gravity, based on the Souders-Brown equation
  • Winnowing
  • Zone refining

See also

  • Analytical chemistry  Study of the separation, identification, and quantification of the chemical components of materials
  • High-performance liquid chromatography  Technique used in analytical chemistry
  • Unit operation
  • Filtration  Process that separates solids from fluids


  1. Wilson, Ian D.; Adlard, Edward R.; Cooke, Michael; et al., eds. (2000). Encyclopedia of separation science. San Diego: Academic Press. ISBN 978-0-12-226770-3.

Further reading

  • National Academies of Sciences, Engineering, and Medicine (2019). A Research Agenda for Transforming Separation Science (Report). Washington, DC: The National Academies Press. doi:10.17226/25421.CS1 maint: multiple names: authors list (link)

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