Figure 4 Main methods for major metals & REE extraction from BR by combining pyro- and hydro-metallurgical processes.
Such flowsheets result in more efficient leaching of the REE as the complex mineral matrix of the BR has been destroyed and significant amounts of base metals have been removed. At the same time such flowsheets allow for significant reduction of the final residue that needs to be disposed and may even achieve financially a near break-even status (Balomenos et al. 2016). On the other hand, the numerous processing steps and the back and forth between wet and dry materials make for a very energy and cost intensive processes. To achieve such processing in a sustainable (environmentally and financially) manner, a right balance between bulk applications with low value products (i.e. pig iron production) and low volume niche products (i.e. Scandium) needs to be found.

Conclusions

Bauxite Residue constitute a significant resource for REE and especially Scandium production. However, there is significant complexity in extracting these minor metal values form BR, as upon direct leaching with high Sc leaching yields, solutions dilute in REEs and rich in major metals (g/l) like Fe, Ca, Na, Al, etc are produced; effectively hampering downstream refining of the REE. To bypass such problems, processes with low Sc leaching yields (<30%) have to be considered, such as the ones demonstrated by RUSAL and MYTILINEOS. Combined pyro- and hydro- routes that aim to recover major metals like Fe, Al and Ti and at the same time up concentrate the REEs in slags could be more attractive as they would produce richer and cleaner REE leach solutions (allowing for high Sc leaching yields), while at the same time valorizing significant volumes of the BR itself. The goal of a near zero-waste and near break-even flowsheet for BR processing would be ideal for both the alumina and the REE industries. The realization of such a flowsheet, however, requires a high degree of industrial symbiosis and favorable logistics between different industries.