Efthymios Balomenos1, Panagiotis
Davris1, Dimitrios Panias1, Ioannis
Paspaliaris1
Laboratory of Metallurgy, National Technical University of Athens,
Zografos Campus, Greece
Email: thymis@metal.ntua.gr
Bauxite Residue
‘Bauxite Residue’ (BR) refers to the insoluble solid material,
generated during the extraction of alumina
(Al2O3) from Bauxite ore using the Bayer
process. When bauxite ore is treated with caustic soda, the aluminium
hydroxides/oxides contained within, are solubilized, with approximately
50% of the bauxite mass being transferred to the liquid phase, while
the remaining solid fraction constitutes the -bauxite- residue.
Active lime is usually added during digestion to control and reduce
caustic soda and alumina losses from the formation of desilication
products.. The solid-liquid separation after ore digestion takes place
in thickeners and washers, resulting in the formation of a red-colored
bauxite residue slurry (approx. 50% solids) which was previously termed
‘red mud’. Nowadays many plants use, as a final step of slurry
treatment, high pressure filtration (the most efficient method of alkali
recovery), in which the bauxite residue slurry is pressed to remove the
maximum of remaining liquor and produce a compact filtercake with a
relative humidity of 25-30%.
It is estimated that for each ton of alumina produced 1.0- 1.5 tons of
solid residue (on a dry basis) is generated depending on the initial
bauxite ore grade and alumina extraction efficiency (Evans 2016).
Bauxite residue consists of of various metal oxides of Fe, Al, Ti, Si,
Ca, Na, V, Ga (depending on the initial chemical composition of the
bauxite ore) along with inclusions of unwashed sodium aluminate
solution.
As the global demand for primary aluminium metal increases so will the
BR production, currently in excess of 150 million tons per year
worldwide (Power et al. 2011). This is generated at more than
100 active alumina refining plants worldwide. In addition, there are at
least another 50 closed legacy sites, so the combined stockpile of
bauxite residue at active and legacy sites is estimated at three
thousand million tons. ( World Aluminium and the European
Aluminium Association, 2015)
The primary aluminium industry has always focused on discovering
potential applications for BR utilization. The vast amount of research
and studies on BR utilisation is justified by more than 734 patents
since 1964. Possible applications can broadly be broken down into
various categories, such as cement and building materials production,
iron production, trace element (Ga, REE, V,etc.) recovery, use as soil
amelioration, landfill capping, acid mine drainage treatment and others
(Evans 2016). The recent REE crisis fueled significant research effort
in recovering the REE that are found in some BRs in concentrations
between 1 - 2 kg of total REE / t of BR. Given the large quantity of the
annual BR production, the total amount of contained REE becomes
significant and could cover part of the global REE demand (Balomenos et
al., 2017a). Furthermore, while the treatment of BR for the recovery of
REEs does not solve the BR deposition problem, as the volume of the
waste remains practically unaffected, it does help in the economic
viability of holistic processing flowsheet seeking to achieve near
zero-waste through multiple processing steps (Balomenos et al., 2017b).
Rare Earths in Bauxite
Residue
The bauxite ore is one of the factors that affect the concentration of
REE in bauxite Residue. Bauxites are classified in three categories,
lateritic bauxites (88%), karstic bauxites (11,5%) and Tikhvin type
bauxites (0,5%) (Bardossy, 1982; Bárdossy and Aleva, 1990). Karstic
bauxites are mainly found in Europe, Jamaica, Russia and China. The
karstic bauxites contain higher concentrations of REE than the lateritic
bauxites. REE are detected in bauxite ore as fluorcarbonate or phosphate
minerals which are very similar to the main industrial minerals of REE
(bastnaesite - monazite) (Li et al., 2013; Mouchos et al., 2017;
Ochsenkühn-Petropulu 1995; Vind et al., 2018a). It has also been
reported that in the Bayer process REE end up in the BR in 2:1 ratio,
compared to the initial bauxite ore. (Derevyankin et al., 1981;
Ochsenkühn-Petropulu et al., 1994; Wagh and Pinnock, 1987).
The worldwide typical concentration of REE in BR is 800-2500 mg/kg and
is related to the initial bauxite ore and the operating conditions of
the Bayer process (Deady et al., 2018). Recent research shows that REE
in BR can be found in secondary mineral phases produced by the Bayer
process, known as the desilication product (DSP). DSP is the result of
the silicon removal from the aluminate solution during the leaching of
the bauxite ore, as silicon is major pollutant for the final alumina
product. Presence of REE in the DSP can be attributed to REE from the
bauxite ore being dissolved in the Bayer process; these REE are
incorporated into the newly formed DSP mineral matrix that contains a
mixture of Fe, Ti, Si, Al Ca and Na ions (Vind et al., 2018a).
Scandium (Sc) often differs from the other REE behavior. Especially in
lateritic bauxites and their corresponding BR, it is often correlated
with iron and titanium and zircon minerals (Vind et al., 2018a) (Liu et
al., 2018; Zhang et al., 2017), which for the most part are unaffected
by the Bayer process. This is also confirmed by the laterite deposits in
Australia and the Greek BR (Chassé et al., 2016) where the main mineral,
with high concentration of Sc is goethite (Vind et al., 2018b). However,
there are cases of BR ,where Sc is found to be related to larger extent
to the soluble Al-bearing minerals, as is reported by Russian
researchers (Suss et al., 2018).
Published chemical analysis and leaching studies of bauxite residue
focus on the concentration of Sc, because Sc represents 95% of REE’s
financial values found in BR (case of Greek Bauxite, reported by
Ochsenkühn-Petropoulou et al., 2002). Table 1 presents the Sc
concentration in different Bauxite Residues worldwide as reported in
literature.
Table 1 Concentration of Scandium
in Different Bauxite Residues