refining, any of various processes for separating impurities from crude or semifinished materials. It includes the finer processes of metallurgy, the fractional distillation of petroleum into its commercial products, and the purifying of cane, beet, and maple sugar and many other substances. The nature of the refining process depends on such factors as the type of material involved, the value of the end product, and the degree of purity necessary. The purification of a metal is based upon physical or chemical differences between the metal and its accompanying impurities, including density, melting point, magnetic properties, and reaction to certain chemicals. It may be accomplished by a continuation of a process used in separating the metal from its ore. Electrolysis is much used in the refining of various metals (e.g., copper); the open hearth process (see steel) and the basic oxygen process are used in the refining of iron; the amalgamation process and the cyanide process are chemical means of metal refining.
Refining (also called affining) is the process of purification of a substance. The term is 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 by products. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal (for which see Refining (metallurgy)).

The refining of liquids is often accomplished by distillation or fractionation. Gases can be refined in 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.

Types of materials that are usually refined:

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