Regolith-hosted rare earth element deposits
(also known as ion-adsorption deposits) are
rare-earth element (REE) ores in decomposed
rocks that are formed by intense weathering
of REE-rich parental rocks (e.g. granite,
tuff etc.) in subtropical areas.
In these areas, rocks are intensely broken
and decomposed.
Then, REEs infiltrate downward with rain water
and they are concentrated along a deeper weathered
layer beneath the ground surface.Extraction
technology of the deposits has been evolving
over the last 50 years.
In the past, REEs were primarily extracted
in small amount as by-products in mines of
other metals or granitic sands at the beach.
However, in recent decades, the development
of the high-tech industries (e.g. aerospace
engineering, telecommunication etc.) leads
to high demand for REEs.
Hence, regolith-hosted rare earth element
deposits were recognised and extraction technologies
have been rapidly developed since the 1980s.Currently,
China dominates more than 95% of the global
REE production.
Regolith-hosted rare earth element deposits,
which contributes 35% of China's REE production,
are mainly found in South China.
== Global distribution ==
Regoliths are unconsolidated deposits of fragmented
and decomposed rocks and may include dust,
soil, broken rock, and other related materials.
They are the source of minerals and construction
materials and if they contain much biological
material are known as soils.
Most of the regolith-hosted rare earth mineral
deposits are found in South China, which currently
dominates more than 95% of global REE production.
There are two major types of deposit, namely
"light" rare earth elements (LREE) (i.e.
La, Ce, Pr and Nd) deposit and middle and
heavy rare earth elements (HREE) (i.e.
Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) deposit.
Both of these types are mainly found in Jiangxi,
Hunan, Guangdong and Fujian province.
The Zudong deposit in Jiangxi, the Datian
and the Xiawentian deposits are the major
HREE-mines in South China.
Meanwhile, LREE is dominated by the Heling
deposit and the Dingnan deposit in Jiangxi
Province.Meanwhile, exploration for this kind
of deposit are actively taking place across
the world.
Currently, some potential deposits have been
discovered in the US, Southeast Asia, Malawi,
Brazil and Madagascar.
== Geological overview ==
Regolith-hosted rare earth element deposits
are found along ridges in low-lying granitic
hills in South China.
The ore deposit can be profiled into four
layers based on its extent of weathering while
the orebody lies at lower layer of weathered
soil.
=== Geomorphology ===
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.Regionally,
the deposits are generally found in areas
with low topography, low hills lower than
500 m in elevation.
Moreover, as located in subtropical area,
South China has a warm and humid climate.
Therefore, the deposits are often densely-vegetated.Locally,
the deposits tend to form along ridges, rather
than valleys.
Thicker weathering crust, together with its
associated orebody is found along ridges while
its thickness decreases valley-wards or down
slope.
=== Ore-body profile ===
The whole weathering crust can range from
30 to 60m in thickness, depending on its local
structural, geomorphological, and hydrogeological
conditions.
In general, the deposit can be divided into
4 layers with accordance to its weathering
intensity.
Taking a 25-m-thick weathering crust as an
example, its weathering profile is as illustrated
as below:
REE enrichment generally occurs as a 5-to-10-meter-thick
zone between the completely weathered layer
and strongly weathered layer and it is targeted
for commercial mining.
Compared to other REE deposits, regolith-hosted
rare earth element deposits are substantially
low-grade (containing 0.05-0.3 wt.% extractable
REEs).
Nevertheless, due to its easy extraction method,
low processing costs and large abundance,
the orebodies are economic to be extracted.
== Genesis of orebody ==
The followings are some key aspects of ideas
in the genesis of regolith-hosted rare earth
element deposits.
Deposit-forming magma is sourced from either
LREE- or HREE-enriched granitic magma and
it is not necessarily related to any special
tectonic settings or geological time periods.
Then, LREE or HREE experiences first stage
enrichment through its own mechanism when
magma solidifies.
After the granite is brought to ground surface,
it experiences intense denudation and exhumation
in subtropic areas.
At this stage, REEs are further enriched,
which makes mining economical.
These processes are discussed in details below:
=== Magmatic origins ===
In terms of tectonic settings, no obvious
trend in tectonic setting that favours the
formation of regolith-hosted rare earth element
deposits is observed.
Geologists had long believed that among different
magma sources, magmas originated formed anorogenic
(non-mountain building) and anhydrous settings
(e.g. divergent plate boundaries), which consequently
form A-type granites, are REEs-enriched.
This is because lower degree of partial melting
in this tectonic setting favours the enrichment
of the REEs, which are incompatible and tend
to melt preferentially.
However, from field observations, A-type granite
is not outstandingly REEs-enriched (in Total
REE %). Instead, it is similar to I-type granites
(sourced from magma of partially-melted igneous
rocks) and S-type granites (sourced from magma
of partially-melted sedimentary rocks), which
are originated from orogenic (mountain-building)
settings (e.g. convergent plate boundaries).In
terms of geological times, these REEs-enriched
granites which formed evenly over a wide geological
time period (i.e. from Ordovician to Cretaceous),
showing that these deposits are not formed
in special environments related to any major
geological events.
=== Magmatic-hydrothermal processes ===
In general, parental rocks of regolith-hosted
rare earth element deposits are felsic igneous
rocks (e.g. granite, rhyolite, rhyolitic tuff
etc.), which are associated with granitic
magmatism and volcanism in subduction system.
During magma crystallisation, LREE and HREE
are primarily enriched in granitoids through
two separate mechanisms.
LREE enrichment: LREE-enriched granitoids
are formed by magma differentiation, which
progressively fractionates magma composition
into chemically-distinctive layers during
its cooling process.
As REEs are incompatible elements (less preferred
to incorporate into the structure of solidifying
crystals), they remain as melt in magma chamber
until the last stage of cooling.
Therefore, the last and the uppermost fraction
of granitoid is highly REEs-enriched.
HREE enrichment: HREE-enriched granitoids
are formed by auto-metasomatism.
It is a process of chemical alterations of
recently crystallised felsic magma by the
left-over hydrothermal fluid (e.g. water,
CO2 etc.) at the later stage of magma crystallisation.
During the chemical alteration, through various
chemical reactions with hydrothermal fluids,
HREEs are then introduced into secondary minerals
along veinlets.
=== Secondary processes ===
Secondary process (i.e. weathering) is essential
in further enrichment of HREE-/LREE-rich granitoid.
It turns the grannitoid to an economically-extractable
orebody.
Therefore, warm and humid climate, together
with slightly acidic soil in subtropical zones
favour the formation of regolith-hosted rare
earth element deposits.
A combination of intense chemical, physical
and microbiological weathering allows the
removal of REEs in upper, more acidic completely
weathered layer, downward migration through
rainwater and eventually, deposition (and
concentration) at lower, less acidic moderately
weathered layer.
(REE forms a more stable complex in soil with
higher pH).In addition, intense weathering
in subtropical areas (i.e.
South China) continuously removes significant
volume of overlying materials from the in-situ
weathering system, which is a process called
denudation.
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.
Thus, the dynamic equilibrium system between
denudation and exhumation further facilitates
the development of thicker weathering profile
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
M
2
SO
4
⟶
2
Clay
−
M
3
+
REE
2
(
SO
4
)
3
{\displaystyle {\ce {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
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.
==== Modern mining methods (The third generation)
====
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.
==== Current research and development (Bioleaching)
====
Recently, researchers have been developing
various techniques to increase the yield of
leaching REE.
Bioleaching, a technique where REEs are solubilised
by microbial activities or by-products of
microbial metabolism, is actively studied
as a greener alternative to the current method,
which has been serious pollution to the environment.
In terms of extraction effectiveness, some
studies have reported that the recovery of
REE by bioleaching could vary from less than
1% to nearly 90%.
Thus, further understanding of the bioleaching
mechanism is required before it is commercially
practised.
== Applications of Rare Earth Elements ==
Rare earth elements, the products of regolith-hosted
REE deposits, are the fundamental building
blocks of many daily-life high-tech products.
Some of the examples and their applications
are provided as follows.
Neodymium is used in the production of strong
magnets in loudspeakers and computer hardware
with a smaller size and better performance.
Moreover, together with its excellent durability,
neodymium is widely applied in wind turbines
and hybrid vehicles.Praseodymium metal has
ultra-high strength and melting point so it
is an important component in jet engines.
Praseodymium is used in a special type of
glass, for the manufacture of visors to protect
welders and glassmakers.Scandium is used in
building the framework of aircraft or spacecraft
to increase strength.
It is also used in high-intensity street lamps.Cerium
is used in catalytic converters in vehicles
due to its high chemical stability under high
temperature.
More importantly, it is responsible for the
chemical reactions in the converter.Gadolinium
compounds are the active component in various
MRI contrast agents.
For more applications of other rare earth
elements, check "Rare-earth element#List".
== See also ==
Rare-earth element
Adsorption
Leaching
