The pyrometallurgical term refining refers to the selective removal of impurities from bulk metal, normally in the liquid state.
The impurity that is removed is normally transferred to either a liquid phase immiscible in the bulk metal or to a gaseous phase.
Impurity removal is required either because the impurities are detrimental to the physical and mechanical properties of the metal being produced, or the impurities have high intrinsic value.
Impurities can be removed by several means.
Without addition of chemical reagents as in preferential volatilization of impurities, and impurity segregation during phase separation from a liquid during cooling
With chemical reagent additions, by preferential reactions of impurity with the added reagent, as in, for example, oxidation, sulfidation, chlorination and reactions with a second immiscible metal.
Engh, T. (1992) Principles of Metal Refining, Oxford University Press