by-product

Substance formed incidentally during the production process of some other substance; for example, slag is a by-product of the production of iron in a blast furnace. The by-product is incidental to the main product. For industrial processes to be economical, by-products must be recycled or used in other ways as far as possible; in this example, slag is used for making roads. Another common example occurs in the petrochemical industry, where crude oil is refined principally to produce petroleum, with a number of by-products being produced, including creosote and paraffin.

Often, a poisonous by-product is removed by transforming it into another substance, which although less harmful is often still inconvenient. For example, the sulphur dioxide produced as a by-product of electricity generation can be removed from the smoke stack using flue-gas desulphurization. This process produces large amounts of gypsum, some of which can be used in the building industry.