Frontier looking of rare-earth processed residue as sustainable thorium resources: An Insight into chemical composition and separation of thorium

Abstract

Rare-earth (RE) extraction activities in Malaysia produce an average of 75000 tonnes of Water Leach Purification (WLP) residue containing an estimated 106 tonnes of thorium that leads to negative perception and concern among the public especially related to the environmental and radiological aspects. The main challenge to separate thorium from the WLP residues is the formation of insoluble thorium pyrophosphate (ThP2O7) due to the treatment process during rare-earth extraction. Therefore, the objectives of this study are to verify the thorium composition in WLP residue as well as to investigate the possibility of separating thorium from WLP. The characterizations using FTIR, SEM-EDS, XRD, and XPS verified the presence of ThP2O7 along with other thorium phosphate compounds (Th(PO3)4) and (Th3(PO4)4) in WLP residue. The digestion study using sulphuric acid (H2SO4) at various molarity ranging from 3 to 18 mol/L showed that thorium in WLP can be dissolved in acid. The dissolution of thorium up to 50% was achieved at 6 mol/L H2SO4 and further increase to 90% with the increase of acid molarity at 150 °C within an hour of the leaching time. This research showed promising findings on the separating thorium from WLP residue through the acid digestion process that might open up the possibility of recycling rare-earth residue as sustainable thorium resources.