Refining

Abstract

In general, primary metals obtained through various processing routes are associated with some impurities. Ores, flux, and fuel are the main sources of impurities. As many physical, chemical, and mechanical properties of metals are influenced by the presence of impurities, it becomes essential to refine them for effective usage. A metal may be subjected to one or more refining methods depending on the chemical characteristics of the metal as well as those of the impurities. Broadly speaking, the objective of refining is to produce the metal as pure as possible and to recover precious metals such as gold and silver as by-products. On the other hand, steelmaking is an example of refining to produce steel with controlled amount of impurities. Therefore, purity is a relative term, which is referred to be based on the usage of the metal. One property may not be affected by the presence of a particular impurity but another property may be significantly influenced by the same. For example, density of copper does not change with traces of oxygen, whereas dissolved oxygen reduces the electrical conductivity of copper to a large extent. Similarly, the presence of traces of hafnium in zirconium poses problems in the application of the latter when used as fuel canning material in nuclear reactors. Low-neutron absorption cross-section makes zirconium (0.15 barn per atom) a valuable material for fuel cans. As the neutron absorption cross-section of the co-occurring sister element, hafnium, is very high (115 barn per atom) it must be restricted below 200 ppm in zirconium for nuclear applications. © 2021, The Minerals, Metals & Materials Society.