Regolith-hosted rare earth element deposits

[3] Then, REEs infiltrate downward with rain water and they are concentrated along a deeper weathered layer beneath the ground surface.

In the past, REEs were primarily extracted in small amount as by-products in mines of other metals or granitic sands at the beach.

[5] Hence, regolith-hosted rare earth element deposits were recognised and extraction technologies have been rapidly developed since the 1980s.

Most of the regolith-hosted rare earth mineral deposits are found in South China, which currently dominates more than 95% of global REE production.

(i.e. Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Both of these types are mainly found in Jiangxi, Hunan, Guangdong and Fujian province.

[9] Regolith-hosted rare earth element deposits are found along ridges in low-lying granitic hills in South China.

[11][12][13][3][14] The morphology of the deposits in South China (southern Jiangxi, southwestern Fujian, northern Guangdong and northwestern Guangxi in particular) is determined by both regional and local factors.

[15] The whole weathering crust can range from 30 to 60m in thickness,[16][14] depending on its local structural, geomorphological, and hydrogeological conditions.

– No relict granitic texture preserved Enriched at bottom (occasionally up to 20 m thick) – ≤ 30% clay minerals (i.e. kaolinite, sericite etc.)

– Relict granitic texture preserved Enriched at top – Abundant core-stones surrounded by regolith REE enrichment generally occurs as a 5-to-10-meter-thick zone between the completely weathered layer and strongly weathered layer[12] and it is targeted for commercial mining.

[19] After the granite is brought to ground surface, it experiences intense denudation and exhumation in subtropic areas.

Therefore, warm and humid climate, together with slightly acidic soil in subtropical zones favour the formation of regolith-hosted rare earth element deposits.

[1] In response to the mass removal, exhumation (an isostatic-uplifting process which deep-seated rock is brought to the land surface) occurs and hence, replenishes materials for on-going denudation.

[16][28] Thus, the dynamic equilibrium system between denudation and exhumation further facilitates the development of thicker weathering profile as well as the accumulation of REEs.

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,[30] which are adsorbed (physically adhered by weak electrostatic attraction) at sites of permanent negative charge on clay minerals[29] (e.g. kaolinite, halloysite, illite etc.)

[33] The equation below shows an example of ion-exchange reaction between REE-adhered clay mineral and lixiviant (metal sulphate).

[25] 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).

[25] These drawbacks surpassed the originals benefits of this kind of deposits (i.e. short processing time and extremely low costs).

REEs-bearing soil was mined from orebodies and piled up on a flat leak-proof layer with a collecting dish at the bottom.

[35] Due to stronger desorption capability of NH4+ compared with Na+,[13] the technology had an improved final product quality and a reduction in lixiviant consumption.

[25] 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.

Then, vertical leaching holes (0.8 m in diameter and 2 to 3 m apart)[13] 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).

Neodymium is used in the production of strong magnets in loudspeakers and computer hardware with a smaller size and better performance.

[39] Praseodymium metal has ultra-high strength and melting point so it is an important component in jet engines.

An overview of one of the regolith-hosted rare earth element deposits (a type of Rare Earth Element mine) in South China [ 1 ]
World distribution of regolith-hosted REE deposits [ 1 ]
Common landscape in South China: low-lying areas with densely vegetated low hills [ 10 ]
Weathering Profile of Regolith-hosted REE Deposits. (A) Humic layer. (B) Completely weathered layer. (C) Strongly weathered layer. (D) Weathering front. (E) Unweathered rock. [ 1 ]
Semi-weathered parental rock of regolith-hosted rare earth element deposits – REE-enriched granitoid
Evolutionary diagram showing the first stage of LREE-enrichment process by magma differentiation. [ 1 ]
Evolutionary diagram showing the first stage of HREE-enrichment process by auto- metasomatism . [ 1 ]
Abundant rainwater causes rapid infiltration and leaching of REEs to layer B and C. (Figure t 1 ) Moreover, secondary processes (i.e. weathering and erosion) remove upper layers of materials.(Figure t 2 ) In response to the ongoing removal of over-laying landmass, isostatic-uplifting process ( exhumation ) occurs continuously. [ 16 ] [ 28 ] (Figure t 3 ) Hence, the dynamic equilibrium system between denudation and exhumation further facilitates the accumulation of REEs. [ 1 ] (Figure t 4 )
Lixiviant (Leaching solution) is injected through plastics pipes, which are inserted into a leach hole.
A sample of Neodymium.