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.