Radiological assessment of bauxite residue processing to enable zero-waste valorisation and regulatory compliance

In this work, the presence of natural radionuclides in bauxite residue subjected to a neutralisation-leaching process and smelting-leaching process was investigated. The radionuclide concentrations of radium-226, radium-228 and thorium-228 were compared with the concentration of the same radionuclides in the untreated bauxite residue. It was found that neutralisation of bauxite residue at relatively high temperatures (120–150 °C) and high pressure (PCO2 = 30 bar) did not change the initial concentration of radionuclides. In fact, the concentration of radionuclides after an intensive neutralisation process remained below 500 Bq kg−1. The separation of iron from the rare-earth elements by reductive smelting permitted a significant decrease of iron concentration in the pregnant leach solution obtained after high pressure acid leaching. However, the radionuclide concentration in the slag was increased 1.5 times with respect to the initial concentration, although it did not exceed 800 Bq kg−1. Further processing of the slags by high pressure acid leaching led to the dissolution of several base metals, while a portion of the radionuclides remained unreacted in the solid fraction. Thus, a concentration of thorium-228 > 1000 Bq kg−1 was only observed after leaching a calcium oxide-based slag with hydrochloric acid. Meanwhile, in the leach liquors, the concentration of radionuclides was