In chemistry and chemical engineering, a separation process, or a separation technique, or simply
a separation, is a method to achieve any mass transfer phenomenon that converts a mixture
of substances into two or more distinct product mixtures, at least one of which is enriched
in one or more of the mixture's constituents.
In some cases, a separation may fully divide the mixture into its pure constituents.
Separations are carried out based on differences in chemical properties, or physical properties
such as size, shape, mass, density, or chemical affinity, between the constituents of a mixture,
and are often classified according to the particular differences they use to achieve
separation.
In the case that no single difference can be used to accomplish a desired separation,
multiple operations will often be performed in combination to achieve the desired end.
Barring a few exceptions, almost every element or compound is naturally found in an impure
state.
Often these impure raw materials must be separated into their purified components before they
can be put to productive use, making separation techniques essential for the modern industrial
economy.
In some cases, these 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.
This article discusses separation techniques of any scale that completely or partially
separate components of a mixture from each other usually without substantial chemical
modification.
The purpose of such a separation may be analytical, i.e. to help analyze components in the original
mixture without any attempt to save the fractions, or may be 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.
Chromatography
In paper chromatography, the movement of each substance in the mixture depends on two factors:
solubility of the substance in the solvent and adsorption of the substance on the filter
paper.
The substance moves with the solvent easily if the substance is very soluble in the solvent,
and some solids can attract other substances and hold them on their surface.
This is called adsorption, and such solids are called adsorbents.
The substance does not move with the solvent easily if the substance in the mixture is
strongly absorbed by the filter paper.
Since neither substance has the same adsorption and solubility, each travels a different distance
along the filter paper—and the two separate.
Substances separated by chromatography need not be colored.
Colorless substances can be made visible by spraying the paper with a locating agent that
reacts with the colorless substances to produce a color.
Labs use chromatography to identify the substances in a mixture.
Hospital labs, for example, use the technique to determine if a patient has diabetes by
identifying sugar in urine.
Chromatography also identifies dyes used in food.
List of separation techniques Adsorption, 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 a material Crystallization
Decantation Demister, removes liquid droplets from gas
streams Distillation, used for mixtures of liquids
with different boiling points Drying, removes liquid from a solid by vaporisation
Electrophoresis, separates organic molecules based on their different interaction with
a gel under an electric potential Elutriation
Evaporation Extraction
Leaching Liquid-liquid extraction
Solid phase extraction
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 float, and are recovered
Deinking, separating hydrophobic ink particles from 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 while membrane processes including
microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis 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
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 Chemical engineering
High-performance liquid chromatography Unit operations
References
External links Separation of Mixtures Using Different Techniques,
instructions for performing classroom experiments Separation of Components of a Mixture, instructions
for performing classroom experiments