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A separation process is a method that converts a mixture or solution of chemical substances into two or more distinct product mixtures.  At least one product mixture 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, i.e. to identify the size of each fraction of a mixture is attributable to each component without attempting to harvest the fractions. The purpose of a separation may maybe preparative, i.e. to "prepare" fractions for input into processes that benefit when components are separated.
Some types of separation require complete purification of a certain component. An example is the production of aluminum metal from bauxite ore through electrolysis refining. In contrast, an incomplete separation process may specify an output to consist of a mixture instead of a single pure component. 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 cases of complete and incomplete separation, a series of separations may be 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.
- 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
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
- 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
- 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.
- 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
- Scrubbing, separation of particulates (solids) or gases from a gas stream using liquid.
- Sedimentation, separates using vocal density pressure differences
- Vapor-liquid separation, separates by gravity, based on the Souders-Brown equation
- Zone refining
- 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
- 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)
- Separation of Mixtures Using Different Techniques, instructions for performing classroom experiments
- Separation of Components of a Mixture, instructions for performing classroom experiments