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). In 1980s, batch and heap leach using ~0.3M ammonium sulphate solution ((NH4)2SO4) was developed. REEs-bearing soil was mined from orebodies and piled up on a flat leak-proof layer with a collecting dish at the bottom. (NH4)2SO4 solution was then injected on top of the soil and allowed for leaching. After 100 to 320 hours, REE extraction (with purity up to 90%) was collected for final processing. Due to stronger desorption capability of NH4+ compared with Na+, the technology had an improved final product quality and a reduction in lixiviant consumption. Hence, it had been used as a primary model of REE leaching process in the following 30 years. In the last three decades, intense use of batch and heap leaching has posed a devastating and irreversible effect on the environment as well as the ecosystem in South China. Unregulated disposal of waste has also brought health problems to the residents near the mines. Thus, a compulsory "in-situ" leaching technology was implemented in 2011 to minimise aforementioned adverse effects. "In-situ" leaching technology requires comprehensive geological survey of local hydrogeological structure, rock joints pattern and ore characteristics in order to design a catchment area for the leaching process. Then, vertical leaching holes (0.8 m in diameter and 2 to 3 m apart) are drilled to reach the top of the REE-enriched layer (B) (1.5 to 3 m in depth) to allow injection of pressurised lixiviant (i.e. ~0.3M (NH4)2SO4). Finally, the REEs-loaded leaching solution is collected by recovery ponds at the bottom of orebody for final processing. Recently, researchers have been developing various techniques to increase the yield of leaching REE.