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’ (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).

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