Column percolation leaching test for rare-earth processed residue: Standard laboratory column elution experiments towards development permanent disposal facility (PDF)

The rare-earth industry in Malaysia primarily produces two main by-products in vast quantities: Water Leach Purification (WLP) and Neutralisation Underflow (NUF) residue. The average total concentration of rare earth elements (TREE) in WLP and NUF was discovered to be 54.2 g/kg and 29.5 g/kg, respectively. The heavy metal (HM) concentration for both was below the Malaysian soil intervention threshold except for arsenic and lead. The average thorium (Th) concentration of WLP exceeded the 1 Bq/g regulatory threshold. Such Th concentration prompted the need for WLP residues to be stored in a permanent disposal facility (PDF). The proposed PDF intends to take advantage of the gypsum properties of the NUF by deploying it as a liner material. It was discovered that the repurposing of NUF increases the pH of a single composition of WLP from 3.6 to a less acidic condition. The three different layer mixes of composition ratio NUF to WLP (PC3, PC4, PC5) of the study lead the pH of the effluent to range between 5 and 6, as compared to the pH range of 9.8–10.2 for NUF composition solely. The optimum composition ratio of NUF to WLP proposed by the study for the PDF is 1:19 (PC5), as the leaching of Th, HM, and REE in this condition is shown to be insignificant.