In collaboration with their colleagues from University of Tartu, researchers from ProcESS/SIM² KU Leuven performed an in-depth radiological assessment of bauxite residue (BR) subjected to a neutralisation-leaching process, BR slags arising from the smelting of BR, and the leachates and solid residues obtained from high-pressure acid leaching process. The radiological exposure assessment for the specific BR studied yielded the conclusion that radiological properties would not be a limiting factor for industrial processes using this specific BR. The work, which was performed in the context of the EU MSCA-ETN REDMUD project, was published in the Journal of Cleaner Production.


[Featured image: Aughinish Alumina Refinery, Foynes, Ireland – 29th August, 2018: Aerial view of Aughinish Alumina Refinery on the Shannon River, Co Limerick. The largest alumina refinery in Europe. Note that the research in the present paper was performed on BR (and derived) samples from Mytilineos S.A. – Aluminium of Greece]

Bauxite residue (BR, also known as red mud) is a by-product generated during the production of alumina from bauxite ore by the Bayer process. It is mainly composed of compounds that have a low solubility in concentrated NaOH solution: iron and titanium minerals, undigested alumina minerals, sodium aluminium hydrosilicates and calcium compounds, and also contains REEs and some levels of naturally occurring radionuclides, whose concentration mainly depends on the origin of the bauxite ore. From these, potassium-40, uranium-238, thorium-232 and their decay products are the most important ones.

Ionising radiation

BR produced in Europe can be considered unlikely to cause elevated exposure to ionising radiation due to low radionuclide content and, they can often be exempted from radiological regulatory control by which the specific material does not pose a threat to humans. Although, few authors have reported a modification of BR’s radiological properties after metal separation (e.g., Al, Fe, Ti and REEs), until now, there was no systematic study on the radionuclide concentration in solid or liquid streams produced during the integration of smelting and acid leaching.

During conventional (direct) acid leaching of BR, the increase in dissolution of rare-earth elements (REEs) is associated with a high consumption of mineral acid and a substantial co-dissolution of Fe, which leads to low efficiencies in downstream processing (i.e., solvent extraction or ion exchange). Neutralisation of BR with CO2 was reported as a potential technology for reducing acid consumption in the acid leaching step for metal recovery from BR.

Although neutralisation of BR with CO2 gas did not enhance the extraction of REEs during acid leaching, it did help to reduce the alkalinity. The co-dissolution of Fe is detrimental, as it is difficult to separate it from REEs and Sc in particular, requiring a large quantity of reagents during downstream processing. Sc tends to be chemically associated with the Fe(III)- rich oxide lattice, which limits its complete dissolution. Therefore, Fe must be removed in advance in order to improve the extraction and selectivity of REEs.

Investigating natural radionuclides in BR

In this work, the presence of natural radionuclides in BR subjected to a neutralisation-leaching process, BR slags arising from the smelting of BR, and the leachates and solid residues obtained from high-pressure acid leaching process, were investigated. Thus, the radionuclide concentration of radium-226 (decay product of uranium-238), radium-228 and thorium-228 (decay products of thorium-232), were compared with the concentration of the same radionuclides in the untreated BR.

As highlighted in this work, the radiological exposure assessment for the specific BR studied yielded the conclusion that radiological properties would not be a limiting factor for industrial processes using this specific BR.

However, as many previously reported BR-related technologies have not benefited from such an assessment, it is highly important that the presence of natural radionuclides should be considered in future process technologies for metal recovery from BR.

Full reference paper

Andrei Goronovski, Rodolfo M. Rivera, Tom Van Gerven, Alan H. Tkaczyk, Radiological assessment of bauxite residue processing to enable zero-waste valorisation and regulatory compliance, Journal of Cleaner Production, 2020, https://doi.org/10.1016/j.jclepro.2020.125165.

Acknowledgements

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation pro-gramme (H2020/2014e2019) under Grant Agreement No. 636876 (MSCA-ETN REDMUD). This publication reflects only the authors’ view, exempting the European Union from any liability. Project website: http://www.etn.redmud.org. The authors thank Mytilineos S.A. – Aluminium of Greece for providing the BR samples.

Want to know more about the valorisation of bauxite residue?

Read this unique interview with Prof. Yiannis Pontikes: “The second [game-changing moment in my career] was while I was in the US for a research visit, where I realised that bauxite residue and copper slag mirror each other in terms of chemistry. The only real difference is that one is mostly crystalline with Fe3+, the other amorphous with Fe2+. I am simplifying things, but I am basically saying that if you thermally process bauxite residue, you end up with a copper slag. And that slag, is at least one of the cements of the future.”

Interview with Prof. Yiannis Ponitkes “Bauxite residue and copper slag mirror each other in terms of chemistry”

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