-Types of separation, separations require total purification, as in the [[Hall–Héroult process|electrolysis refining]] of [[bauxite]] ore for [[aluminum]] metal, but a good example of an incomplete separation technique is [[petroleum|oil]] refining. Crude oil occurs naturally as a mixture of various [[hydrocarbon]]s and impurities. The refining process splits this mixture into other, more valuable mixtures such as [[natural gas]], [[gasoline]] and [[Oil#Chemical feedstock|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
+Some types of separation require complete purification of a certain component. An example is the production of [[aluminum]] metal from [[bauxite]] ore through [[Hall–Héroult process|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 [[petroleum|oil]] refining. Crude oil occurs naturally as a mixture of various [[hydrocarbon]]s and impurities. The refining process splits this mixture into other, more valuable mixtures such as [[natural gas]], [[gasoline]] and [[Oil#Chemical feedstock|chemical feedstocks]], none of which are pure substances, but each of which must be separated from the raw crude.