Resources and extractive metallurgy of scandium

Scandium is both widespread and widely dispersed in nature.

Sources of economic interest include both primary and secondary sources, and content is always low to very low.

Primary sources are either thortveitite ores, the only scandium mineral mined, or waste rock from wolframite or molybdenite mining operations. There are very few thortveitite mining operations worldwide, due to the scarcity of deposits. Mine tailings, on the other hand, are much more widespread. Secondary sources are either etching solutions from lateritic nickel-cobalt ores, uranium ores and rare earth ores, or solid residues from tungsten and titanium metallurgy or bauxite chemistry (Bayer process). In all these secondary sources, scandium is associated with numerous other elements (U, Th, TR, W, Sn, Fe, Ni, Co...). Except in the case of thortveitite ores, scandium thus appears as the by-product of one or more metals. Its recovery should therefore not interfere with that of the main metals.

The metallurgical processes used to bring scandium into solution are adapted to the different sources. From primary sources, mechanical preparation produces a concentrate to which pyro-hydro-metallurgical processes are then applied. From secondary sources, such pre-concentration is impossible. Hydro-metallurgical processes are applied directly, whether to treat etching solutions, solid metallurgical residues or chemical treatment sludges. In some cases, the chemical properties of scandium are close to those of the elements with which it is associated, making selective extraction difficult.

After solution, scandium is extracted by precipitation, following extraction of the main metals. Scandinia (Sc ₂ O ₃ ), obtained from the precipitate by thermal decomposition, is fluorinated after purification; this forms the intermediate compound ScF ₃ , from which scandium metal is extracted by calcio- or alumino-thermy, alumino-thermy having the advantage of directly obtaining the Al-Sc alloy.

The scandium economy is based on uses close to those of titanium, for alloys destined in particular for land, sea or air transport, specific sports equipment and welding. Other applications include lighting (halogen lamps (ScI ₃ )), lasers and aerospace alloys. The basic material for all these applications is scandinite, whose worldwide production is estimated at between 5 and 10 t/year in 2012.

As selective scandium recovery is difficult, only improved separation techniques will enable us to economically produce the quantities needed for new metallurgical...