Abstract
Bauxite residue, the byproduct of alumina production, may potentially be a valuable source of strategically important metals, e.g. Gallium. Ga is considered critical element for the EU. To ensure adequate supply of Ga for the future, secondary sources such as bauxite residue should be exploited with efficient extraction methods. Therefore, in this study, mineral acids (H2SO4, HCl, and HNO3) and an organic acid (H2C2O4—oxalic acid), were evaluated for their efficiencies to extract Ga from bauxite residue. Using H2C2O4, the highest Ga leaching efficiencies were achieved, compared to other acids. The achieved leaching experimental results were considered for the construction of a design of experiment (DOE) model to achieve optimal conditions for Ga extraction using H2C2O4. These values were validated by experiments which resulted in ~ 94% accuracy. In the second part of the study, using pure Ga solution, the adsorption of Ga onto zeolite HY was studied. The effects of adsorbent dosage, temperature, and contact time on the adsorption of Ga from solution by zeolite HY were studied. The obtained adsorption experimental results were used to construct a DOE model to achieve optimal conditions for Ga adsorption on to zeolite HY. The DOE-achieved optimal conditions were evaluated by experiments in pure Ga solution, which resulted in an efficiency of ~ 99.4 %. In the third stage, the bauxite residue was leached in H2C2O4 under the optimal DOE conditions which resulted in 71% efficiency; thus the resulting bauxite residue solution was subjected to adsorption using zeolite HY under the optimal DOE conditions achieved. The Ga adsorption onto the zeolite was only 16% compared to the Ga adsorption of 99.4 % under the pure Ga solution, thus, representing the influences of the other leachates in the solution, which are minimizing the Ga adsorption onto the zeolite HY and providing an opportunity for future studies on the different mechanisms involved.
Highlights
Recent studies by major economies [1,2,3,4] have identified strategically important elements, called critical raw materials (CRMs), for their economy
The elemental composition of the bauxite residue was dominated by Fe (~ 43.3%), Al (~ 16.5%), Si (~ 9.4%), Ti (~ 8.9%), Ca (~ 6.2%) and Na (~ 6.0%) oxides as detected by X-ray fluorescence (XRF) (SI, Table 2); the Ga concentration was 114.5 ± 5.2 mg/ kg which was analyzed by ICP-OES (SI, Table 3)
Increasing the H2C2O4 concentration from 0.05 to 3 M increased extraction efficiencies of Ga from 3% (~ 3.6 mg/kg) to 44% (~ 50.9 mg/kg) at 24 h, 60 °C, 100 g/L slurry concentration and 250 rpm. 41% (~ 44.1 mg/kg) Ga extraction efficiency was already achieved using 1 M H2C2O4, 1 M H2C2O4 was chosen for the remaining leaching experiments
Summary
Recent studies by major economies [1,2,3,4] have identified strategically important elements, called critical raw materials (CRMs), for their economy. These are defined as materials with a high supply risk and an above average. Biological Sciences Department, University of Limerick, Limerick, Ireland. Chemical Sciences Department, University of Limerick, Limerick, Ireland. A potential secondary source of CRM, can contain considerable amounts of strategically important metals depending on its geologic origin and processing [6, 7]. Fewer efforts have been made to use the bauxite residue as a secondary source of other CRMs such as V and Ga [18, 21, 22]
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