as well as the accumulation of REEs. Phases of occurrence In regolith-hosted rare earth element deposits, rare earth elements ores do not exist as free ions. Instead, they physically adhere on clay minerals as clay-REE complex or chemically bond with REE-hosting minerals. Clay-REE Exchangeable phase (i.e. Clay-REE) accounts for 60-90% of the total REE content in the deposits. In this phase, REEs occur as mobile cations (i.e. REE3+), hydrated cations (i.e. [REE(H2O)n]3+or a part of positively charged complexes, which are adsorbed (physically adhered by weak electrostatic attraction) at sites of permanent negative charge on clay minerals (e.g. kaolinite, halloysite, illite etc.) Thus, REEs can be recovered and extracted easily by ion-exchange leaching with dilute electrolyte. REE-hosting minerals Mineral phases (i.e. REE-hosting minerals) account for 10-30% of the REE content in the deposits. REEs are incorporated in accessory minerals (i.e. Bastnäsite (REE)(CO3)F, Monazite (REE)PO4 and Xenotime (Y,REE)PO4) as a part of crystal lattice. As REEs are held by chemical bonds, alkaline bake or acid leach is required to decompose and extract REEs. Extraction techniques Chemical leaching is used to extract REEs in regolith-hosted REE deposits. By injecting leaching solution (lixiviant) to an orebody, REEs adhered to clay minerals are displaced by the ions of the leaching solution and dissolve into the leaching solution, which flows downward along the orebody. The equation below shows an example of ion-exchange reaction between REE-adhered clay mineral and lixiviant (metal sulphate). 2 Clay-REE + 3 M2SO4 -> 2 Clay-M3 + REE2(SO4)3 Since the discovery of this type of deposits in 1960s, leaching procedure has experienced three successive generations of technology, evolution in the use of leaching solution (lixiviant) and leaching techniques, which are summarised as follows: First generation leaching technology In the early 1970s, batch leaching using sodium chloride solution (NaCl) was carried out in the extraction of REEs. Firstly, REE-ores were extracted and sieved by open-pit mining. Then, they are leached in barrels with ~1M NaCl solution and precipitated with oxalic acid (C2H2O4). However, mining scale was highly limited by batch leaching (or bath leaching in late 1970s, using concrete pools instead of barrel) while high concentration of lixiviant could only produce low yield product with poor product quality (<70% of REE in concentration). These drawbacks surpassed the originals benefits of this kind of deposits (i.e. short processing time and extremely low costs). Second generation leaching technology In 1980s, batch and heap leach using ~0.3M