Refining is the process of purification of a substance, usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly and quickly clog an engine with residues and byproducts. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal.
The refining of liquids is often accomplished by distillation or fractionation. Gases can be refined this way as well, by being cooled and/or compressed until they liquefy. Gases and liquids can also be refined by extraction with a selective solvent that dissolves away either the substance of interest, or the unwanted impurities.
Many solids can be refined by growing crystals in a solution of the impure material; the regular structure of the crystal tends to favor the desired material and exclude other kinds of particles.
Chemical reactions are often used to remove impurities of particular types.
The use of silicon and other semiconductors in electronics is highly dependent on precise control of impurities, and a number of special techniques have been developed, such as zone refining.
In formal methods, refinement is the verifiable transformation of an abstract (high-level) formal specification into a concrete (low-level) executable program.
Operation refinement converts a specification of an operation on a system into an implementable program (e.g., a procedure).
Retrenchment is an alternative technique when formal refinement is not possible.
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