scispace - formally typeset
Search or ask a question
Journal ArticleDOI

A review of the potential for rare-earth element resources from European red muds: Examples from Seydişehir, Turkey and Parnassus-Giona, Greece

01 Feb 2016-Mineralogical Magazine (Mineralogical Society)-Vol. 80, Iss: 1, pp 43-61
TL;DR: In this paper, the authors show that red mud from two case study sites, one in Greece and the other in Turkey, has an average of ∼1000 ppm total rare earth elements (REE) with an enrichment of light and heavy REE, respectively.
Abstract: Rare-earth elements (REE) are viewed as ‘critical metals’ due to a complex array of production and political issues, most notably a near monopoly in supply from China. Red mud, the waste product of the Bayer process that produces alumina from bauxite, represents a potential secondary resource of REE. Karst bauxite deposits represent the ideal source material for REE-enriched red mud as the conditions during formation of the bauxite allow for the retention of REE. The REE pass through the Bayer Process and are concentrated in the waste material. Millions of tonnes of red mud are currently stockpiled in onshore storage facilities across Europe, representing a potential REE resource. Red mud from two case study sites, one in Greece and the other in Turkey, has been found to contain an average of ∼1000 ppm total REE, with an enrichment of light over heavy REE. Although this is relatively low grade when compared with typical primary REE deposits (Mountain Pass and MountWeld up to 80,000 ppm), it is of interest because of the large volumes available, the cost benefits of reprocessing waste, and the low proportion of contained radioactive elements. This work shows that ∼12,000 tonnes of REE exist in red mud at the two case study areas alone, with much larger resources existing across Europe as a whole.

Summary (4 min read)

Introduction

  • RARE-EARTH elements (REE) are a collection of sixteen chemical elements, namely scandium, yttrium and fourteen of the fifteen naturallyoccurring lanthanides (excluding promethium); the former two are included as they occur with the latter in the same ore deposits and have similar properties (Cotton, 2006) .
  • Their unique properties make them essential for the hi-tech industry.
  • The last decade has brought a renewed concerted global drive towards REE research and development, led by the major end-users of rare-earth products, such as the European Union, USA and Japan, with the dual scope of finding new resources and improving processing/extraction technologies, as summarized by Adachi et al. (2010) .
  • Warm tropical and sub-tropical climates present ideal conditions for this process to occur (Sanematsu et al., 2013) .
  • Regardless of the low grades, ion-adsorption clays account for ∼35% of the China's total REE output and ∼80% of world's HREE production (Yang et al., 2013) .

Formation of weathered crust elution-deposited rare-earth ores (ion-adsorption clays)

  • The ion adsorption REE deposits were first discovered in 1969 in the Jiangxi Province (southern China) and declared a novel type of exogenous rare-earth ore (Chi and Tian, 2008) .
  • The formation of this ore type is due to physical, chemical and biological (microbially-assisted) weathering of REE-rich granitic and volcanic rocks under warm, humid, slightly acidic conditions in subtropical zones.
  • Up to 80-90% of the adsorbed REE are hosted by the strongly weathered layer (B), whereas <15% are found in the semi-weathered layer (C).

Nature of rare-earth elements in ion-adsorption ores

  • As explained above, the ion-adsorption ores contain clays with permanent negative surface charge, which is responsible for cation (such as REE) adsorption via electrostatic bonds (Meunier, 2005) .
  • These species have low occurrence in ores at the slightly acidic natural conditions and can be recovered only by acid leaching.
  • REE occur as soluble free cations/hydrated cations or part of positively-charged complexes in solution adsorbed species on clays, also known as (2) Exchangeable phase.
  • Depending on the nature of the original host rocks, other metals will become dissolved and carried downstream during the weathering, decomposition and alteration processes.

Overview of leaching technologies for the ionadsorption clays

  • Typically, the ores are leached with concentrated inorganic salt solutions of monovalent cations.
  • They related this behaviour to the lanthanide contraction in the ionic radii going from light to heavy REE.

The first-generation leaching technologybatch leaching with NaCl

  • In the early 1970s, the ores were processed by opencast mining, sieved and leached with ∼1 M NaCl in barrels, followed by oxalic acid precipitation.
  • The lixiviant is injected into the top of the pile at a solid to liquid (S:L) ratio of ∼0.25:1 and accumulates at the bottom in the collecting ditch.
  • This procedure is very well suited for the processing of very low-grade ores.
  • The in situ leaching technique is also currently applied in China for the recovery of residual REE from very low-grade ores and the tailings of older batch and heap leaching operations (Chi et al., 2014) .

Evaluation of leaching potential of various ion-adsorption ores

  • As new ion-adsorption REE deposits are being explored and discovered in the rest of the world, research on REE extraction from ores has expanded outside of China as well.
  • For the last six years, the University of Toronto has conducted systematic indepth studies on the leaching chemistry and optimum conditions for REE extraction from clay samples obtained from various geographical locations.
  • The final aim is to develop a fully contained optimized process for field implementation that minimizes the impact to the environment by providing options for efficient reagent use, maximized extraction and recycling/regeneration of the lixiviant (Cheuk et al., 2014) .

Batch leaching tests

  • The leach solutions were prepared using ACS reagent grade ammonium sulfate and deionized water.
  • The slurry was agitated via magnetic stirring then the mother liquor was separated by vacuum filtration.
  • The filter cake was washed by deionized water of pH 5 (2 × 100 ml), and the wash water was collected separately for analysis.
  • The resultant loaded solutions were diluted with 5% (vol.) nitric acid and analysed by ICP-OES to calculate the REE extractions.

Leaching results and discussion

  • Eight samples from three different geographical locations (Madagascar, Brazil and South-East Asia) were tested.
  • No specific pattern of preferential REE accumulation and distribution was observed, except that all ores seem to be rich in La, Y and Nd; although some similarities in terms of relative composition are observed within deposits originating from the same geographical areas (e.g. A1 through A5), there is no consistent trend.

Batch leaching

  • The ore samples listed in Table 1 were batch leached using the benchmarked procedure described above to investigate the terminal REE extraction levels (shown in Table 2 ) and TREE leaching kinetics, respectively (presented in Fig. 2 ).
  • From data in Table 2 it can be observed that all the minerals investigated are the ion-adsorption type, i.e. the lanthanides are physically adsorbed and can be easily recovered via a simple ionexchange leaching procedure, as described by Moldoveanu and Papangelakis (2012, 2013) .
  • The extraction levels vary between 40 to 80%, consistent with the predicted exchangeable REE percentage, as described by Chi and Tian (2008) .
  • In terms of extraction kinetics, all materials investigated showed a common trend of fast REE desorption which is the typical behaviour of the ionadsorption minerals.

Maximizing REE extraction

  • As the leaching process can be considered an ion exchange process at equilibrium, the authors investigated whether all the extractable REE are indeed recovered during the initial leaching stage.
  • One possible option to increase REE extraction from the clays is through multi-stage leaching using fresh lixiviant: i.e. the clays were leached, vacuum filtered, washed twice and re-leached with fresh solution for a total of three times, following the same base-line procedure; a L:S ratio of 2:1 was used for each leaching stage.
  • The ore A4 was selected for this experiment as it showed somehow lower TREE %E ( percent extractions) during the initial leaching step ( possibility that more could be extracted via repeated leaching; the results are presented in Fig. 3 .
  • Proper washing of leached material, however, plays an important role in maximizing the recovery of REE and the unspent lixiviant.
  • Figure 3 also shows the distribution of TREE recovery between the initial stage leachate, the first washing step and a second washing step for a single-stage leaching experiment.

Leachate loading

  • While maximum REE extraction is the primary objective of the leach process, it is important to note that the REE concentration of the resultant leachate impacts on the downstream circuit.
  • High REE concentration reduces the circuit size of the downstream precipitation process.
  • As the total amount of ammonium in solution is usually well in excess of the stoichiometric requirement to desorb REE, decreasing the L:S ratio has a minor impact on maximum extraction.
  • Figure 4 shows the total REE extractions expressed as %E, and the resultant total REE concentrations in the leachate expressed as [TREE] aq .
  • Additionally, as the L:S ratio decreased, agitation became increasingly more difficult due to increased slurry viscosity; slurries with L:S ratios below 0.5 were virtually impossible to agitate.

Column leaching studies

  • An alternative technique of increasing leachate loading and decreasing L/S ratio is column leaching, which simulates heap and/or in situ leaching processes presently practiced in the field (Chi et al., 2014) .
  • The column leaching tests were performed on the ore A4, according to the procedure described in the Experimental section.
  • It appears that increasing the column operation beyond 1.5 ml/g or 20 h would only bring minimal extraction improvement.
  • In order to completely elute all REE in the column and to ensure that the solid residue is free of lixiviant prior to disposal, column flushing with fresh water becomes necessaryand the results are shown in Fig. 6 .
  • For an overall comparison, Table 3 shows %E (TREE) for batch and column leaching modes, respectively.

Conclusions

  • The leaching performance of ion-adsorption REE deposits outside China have been demonstrated and a unified benchmark procedure for REE leaching from these types of ores has been established.
  • It was found that, regardless of variations in ore origin and REE content, all REE consistently reached peak extraction levels under ambient conditions with fast kinetics.
  • The final overall extractions were generally element-specific, i.e. not all REE reached similar recovery levels for a given ore, as shown in Table 2 .
  • It was found that decreasing the L:S ratio, leachate recycling and counter-current operation were all capable of increasing REE concentrations in the resultant leachate, however, at the expense of REE maximum extraction levels.
  • The water trapped in leached ore residues was found to contain significant amounts of REE and residual lixiviant necessitating significant washing for increasing REE recovery and environmental compliance.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

Deady, E., Mouchos, E., Goodenough, K., Williamson, B., & Wall, F.
(2016). A review of the potential for rare-earth element resources from
European red muds: Examples from Seydişehir, Turkey and
Parnassus-Giona, Greece.
Mineralogical Magazine
,
80
(1), 43-61.
https://doi.org/10.1180/minmag.2016.080.051
Publisher's PDF, also known as Version of record
Link to published version (if available):
10.1180/minmag.2016.080.051
Link to publication record in Explore Bristol Research
PDF-document
This is the final published version of the article (version of record). It first appeared online via INGENTA
CONNECT at
http://www.ingentaconnect.com/content/minsoc/mag/2016/00000080/00000001/art00005;jsessionid=6fi6rr6l3ee
qn.x-ic-live-03# . Please refer to any applicable terms of use of the publisher.
University of Bristol - Explore Bristol Research
General rights
This document is made available in accordance with publisher policies. Please cite only the
published version using the reference above. Full terms of use are available:
http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/

An overview of rare-earth recovery by ion-exchange leaching
from ion-adsorption clays of various origins
G. A. MOLDOVEANU AND V. G. PAPANGELAKIS
*
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto,
M5S 3E5, Canada
[Received 13 July 2015; Accepted 13 December 2015; Associate Editor: Kathryn Goodenough]
ABSTRACT
Continuous development of advanced technologies has created increasing demand for rare-earth elements
(REE), with global emphasis on identifying new alternate sources to ensure adequate supply. Ore deposits
containing physically adsorbed lanthanides are substantially lower grade than other REE deposit types;
however, the low mining and processing costs make them economically attractive as sources of REE.To
evaluate the commercial potential for the recoveryof REEs fromion-adsorption deposits in a systematic manner,
a standardized procedure for REE leaching was developed previously. Using this procudure it was found that,
regardless of variations in ore origin and REE content, all REE consistently reached peak extraction levels under
ambient conditions with fast kinetics. Various techniques to improve the REE extraction through process
variations were also investigated: it was found that decreasing the L:S ratio, re-using leachate on fresh ores and
counter-current leaching were all capable of increasing REE concentrations in the resultant leachate, albeit at the
expense of REE extraction levels. In addition, the water content trapped in the leached material was found to
contain significant amounts of REE and residual lixiviant requiring thorough washing of the solid residue.
K EYWORDS: rare-earth elements, ion-exchange leaching, ion-adsorption ores, lanthanide extraction,
clay minerals.
Introduction
R
ARE-EARTH elements (REE) are a collection of
sixteen chemical elements, namely scandium,
yttrium and fourteen of the fifteen naturally-
occurring lanthanides (excluding promethium);
the former two are included as they occur with
the latter in the same ore deposits and have similar
properties (Cotton, 2006). Their unique properties
make them essential for the hi-tech industry. They
are used in the manufacturing of high strength
permanent magnets, lasers, automotive catalytic
converters, fibre optics/superconductors and elec-
tronic devices (Gupta and Krishnamurthy, 2005).
They are grouped depending on the atomic number,
into light rare earth elements (LREE) La, Ce, Pr,
Nd, and into middle and heavy HREE Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. Because of the
ongoing development of hi-tech and security
applications, there is an increasing demand for
REE in the international markets, with emphasis on
identifying new resources to ensure adequate
supply for present and future use. In terms of ore
reserves and mineral resources, China dominates
the world with reserves estimated to be around 50%
of the total, followed by Australia, Russia, Canada
and Brazil, while completely leading and control-
ling the global production at 90% (Weng et al.,
2015). A review of rare-earth deposits of North
America by Castor (2008) concludes that world
reserves are sufficient to meet international demand
for most LREE, but the HREE such as dysprosium
will become scarce because the current source of
HREE is limited to ion-adsorption deposits in
China. Consequently, ion-adsorption clay deposits
in other parts of the world have gained interest as
sources of HREE. For the last 3 decades, R&D in
the field of REE in most of the Western world has
*E-mail: vladimiros.papangelakis@utoronto.ca
DOI: 10.1180/minmag.2016.080.051
© 2016 The Mineralogical Society
Mineralogical Magazine, February 2016, Vol. 80(1), pp. 6376

slowed down due to the import of these elements
from China. Consequently, the development of
specialized extraction, refining and processing
technologies, including equipment and training of
engineering expertise, were allowed to lapse, thus
creating a dependence on Chinese supplies (Hurst,
2010). Starting in 2005, China the undisputed
leader in both REE mining and trade, started
restricting yearly export quotas for HREE, in order
to have enough resources for its own industries and
to gain control over the global market (Wübbeke,
2013). Consequently, the last decade has brought a
renewed concerted global drive towards REE
research and development, led by the major
end-users of rare-earth products, such as the
European Union, USA and Japan, with the dual
scope of finding new resources and improving
processing/extraction technologies, as summarized
by Adachi et al. (2010). Following negotiations
with the World Trade Organization (WTO),
China eliminated rare-earth oxides (REO) export
restrictions in 2014, causing a fall in the REE
prices in the international markets (Wang et al.,
2015).
Rare-earth elements are incorporated in acces-
sory minerals in various rocks, but the most
commercially significant sources, as reviewed by
Kanazawa and Kamitani (2006) and more recently
described in the comprehensive assessment by
Weng et al. (2015) are presented below:
(1) Bastnäsite, (REE)(CO
3
)F, is a fluorocarbonate
mineral containing 6575 wt.% light REO and
accounts for more than 80% of global REO
production. The two major sources in the world
for lanthanides are bastnäsite deposits at Mountain
Pass, California (USA, owned by Molycorp Inc.
devoted solely to REE production, and Bayan-Obo,
Inner Mongolia (China) mined primarily for iron
ore and REE as a by-product (also containing
monazite). In August 2015, due to a global decline
in REE prices, the rare-earth production at
Mountain Pass was suspended and the facility
was moved to Care and Maintenance, while
Molycorp Inc. filed restructuring plans which
included selling the Mountain Pass assets
(Molycorp News Releases, 2015a,b).
(2) Monazite, ( REE)PO
4
is a LREE phosphate
containing 5565 wt.% REO, associated with
granites and beach sands in Australia, Brazil and
India; the Mount Weld deposit in Western
Australia, owned by Lynas Corp., contains one of
the highest grade REE deposits in the world. Until
about 1965 monazite was the main REE source;
since then, the use of monazite has been
considerably reduced due to radioactivity caused
by thorium and radium.
(3) Xenotime (Y,REE)PO
4
is an yttrium-rich
phosphate containing 25 60 wt.% Y
2
O
3
and
other heavy REE. It is recovered mainly as a by-
product of mining for titanium, zirconium and tin
in Malaysia, Indonesia and Thailand.
(4) Weathered crust elution-deposited rare earth
ores (ion-adsorption ores) are aluminosilicate
minerals (e.g. kaolinite, illite and smectite)
containing 0.050.3 wt.% REEs physically
adsorbed at sites of permanent negative charge
(Chi and Tian, 2008). The ion-adsorption clay
deposits are the result of in situ weathering of host
rocks (mainly granitic), which, over geological
timescales, results in the formation of
aluminosilicate clays. Clay minerals are part of
the phyllosilicate class, containing layered
structures of shared octahedral aluminium and
tetrahedral silicon sheets, allowing water molecules
and hydrated cations to move in and out of the
interlayer spaces (Velde and Meunier, 2008). Very
commonly, isomorphous substitution of one cation
with another (of similar size but with lesser charge,
e.g. Al
3+
for Si
4+
or Mg
2+
for Al
3+
) within crystal
structures leads to a charge imbalance in silicate
clays, which accounts for the permanent negative
charge on clay particles, and thus the capability of
adsorbing lanthanide ions released/dissolved from
precursor REE-bearing minerals during weathering
(Meunier, 2005). Warm tropical and sub-tropical
climates present ideal conditions for this process to
occur (Sanematsu et al., 2013). The best example
of this formation process exists in Asia, where
many such deposits are known to exist, as
described by Bao and Zhao (2008), Murakami
and Ishihara (2008) and more recently by
Sanematsu et al . (2013). Regardless of the low
grades, ion-adsorption clays account for 35% of
the Chinas total REE output and 80% of worlds
HREE production (Yang et al., 2013). It is
estimated that the production of ion-adsorbed rare
earths will increase yearly by 1.7% and peak in
2024 at 45,793 t (Wang et al., 2015).
Carbonate and phosphate sources, of high grade,
are associated with elevated recovery costs due to
separation, beneficiation and need for aggressive
conditions to dissolve the REE. For example,
bastnäsite is generally leached with concentrated
H
2
SO
4
or HCl, whereas monazite/xenotime con-
centrates need to be baked either in 98% H
2
SO
4
or
70% NaOH to render the REE soluble (Gupta and
Krishnamurthy, 2005). According to Castor (2008)
other REE deposits in North America in addition to
64
G. A. MOLDOVEANU AND V. G. PAPANGELAKIS

bastnäsite consist of the so-called hard-rock
peralkaline ores including zircon, titanate, niobate,
allanite, eudialyte, gadolinite; these deposits are
enriched in HREE but require harsh conditions to
break down the mineral matrix (e.g. caustic bake
followed by acid leaching); the processing of these
ores is directed mainly towards extraction of
niobium, tantalum and zirconium (Gupta and
Krishnamurthy, 2005).
The route followed by the European Union to
improve resource efficiency is via creation of
alternative sources through innovations in the
field of reuse and recycle of rare-earth wastes
such as magnets and polishing powders (ERECON,
2015). Although recycling from priority streams
such as fluorescent light bulbs and batteries is
presently feasible, and potential REE-rich sources
reaching end-of-life, such as hard disk drives, wind
turbines, magnets and automotive catalytic con-
verters can be considered for the near-future
processing sources, recycling rates at present are
still very low (<1%) and there are no large scale
commercially viable REE recycling operations
(Massari and Ruberti, 2013).
Ion-adsorption type deposits are substantially
lower grade than other types of lanthanide sources
(Kanazawa and Kamitani, 2006), nominally requir-
ing higher costs for REE extraction and recovery.
However, this disadvantage is largely offset by the
easier mining and processing costs, and the
relatively low content of radioactive elements
such as thorium and uranium (Murakami and
Ishihara, 2008). These deposits are mined by
open-pit methods and no ore beneficiation is
required. A simple leach using monovalent sulfate
or chloride salt solutions at ambient temperature
can produce a high-grade REO product, as
described by Chi and Tian (2008) and more
recently Moldoveanu and Papangelakis (2012,
2013). Because of their abundance in surface
layers in nature, ease of mining and processing,
these clays warrant a detailed study as important
sources of rare earths.
Forma tion of wea ther ed crust elution-deposited
rar e-earth ores (ion-adsorption clays)
The ion adsorption REE deposits were first
discovered in 1969 in the Jiangxi Province
(southern China) and declared a novel type of
exogenous rare-earth ore (Chi and Tian, 2008).
The formation of this ore type is due to physical,
chemical and biological (microbially-assisted)
weathering of REE-rich granitic and volcanic
rocks under warm, humid, slightly acidic condi-
tions in subtropical zones. According to Bao and
Zhao (2008), the weathering crusts are up to 30 m
deep and divided into four layers: (A) An upper
humic layer of quartz, organic matter and soil: 0
2 m thick, with very low/nil REE content; (B) a
strongly weathered layer enriched in REE:510 m
thick with kaolinite, halloysite, quartz and mica;
(C) a semi-weathered layer: 35 m thick with
kaolinite and sericite; (D) a weakly-weathered
bottom layer with the same mineral composition
as the host rock. Up to 8090% of the adsorbed
REE are hosted by the strongly weathered layer (B),
whereas <15% are found in the semi-weathered
layer (C). Depending on the nature of the original
host rocks, the general components of the weath-
ered ores are kaolinite, halloysite and muscovite,
with a typical composition (as wt.%) of 70%
SiO
2
, 15% Al
2
O
3
,35% K
2
O, 23% Fe
2
O
3
and
less than 0.5% of CaO, MgO and other elements
(Ishihara et al., 2008; Weng et al., 2015).
Considering the geological and climate conditions
for the formation of REE-bearing weather ed ores, there
is no reason to limit the occurrence of this type of
deposit within Chinese borders. While at the present
time China is the o nly country to a ctiv ely pursue and
developthistypeofresourcetocommerciallyproduce
REE, recent geological survey s (summarized by Weng
et al., 2015) ha ve led to the discove ry and inv estig ation
of similar ion-adsorption clay deposits in South
America (Rocha et al., 2013) and Africa (TRE
Pro ject, 2014) loca ted in the same w arm sub-tr opical
and tr opical weathering areas.
Nature of rare-earth elements in
ion-adsorption ores
As e xplained abov e, the ion-adsorption ores contain
clay s with permanent negat iv e surface charge, which
is responsible for cation (such as REE) adsorption via
electros ta tic bonds (Meunier, 2005).
According to Bradbury and Baeyens (2002) as
well as Piasecki and Sverjensky (2008), for acidic
and near-neutral conditions ( pH < 6.56.8), most of
the surface-adsorbed extractable lanthanides occur
as simple or hydrated cations such as clay-REE or
clay-REE(H
2
O)
n
species derived from straight-
forward cation-exchange reversible reactions at the
permanent negative charge sites on the clays
(physisorption); for pH > 7 the prevalent forms are
the irreversibly-fixed hydrolysed clay-O-REE
2+
species derived from permanent complexation
65
OVERVIEW OF LANTHANIDE RECOVERY FROM ION-ADSORPTION CLAYS

reactions at the amphoteric surface hydroxyl groups
(chemisorption) (Chi and Tian, 2008).
Due to various weathering conditions (i.e. nature
of host rocks, water and soil pH, temperature,
pressure, redox conditions) there are three main
categories of REE present in the ion-adsorption
clays, as described by Chi et al. (2005) as follows.
(1) Colloid phase: REE deposited as insoluble
oxides or hydroxides or as part of colloidal
polymeric organometallic compounds. These
species have low occurrence in ores at the slightly
acidic natural conditions and can be recovered only
by acid leaching. (2) Exchangeable phase: REE
occur as soluble free cations/hydrated cations or
part of positively-charged complexes in solution
adsorbed species on clays. These species account
for 6090% of the total content of rare earths in ores
and can be recovered by ion-exchange leaching
with monovalent salts. (3) Mineral phase: REE are
part of solid fine particles with same mineral matrix
as the host rocks (REE part of the crystal lattice).
This phase usually accounts for the balance from
the ion-exchangeable phase towards the total rare-
earth content (TREE) content and can be recovered
only by decomposition of mineral phases by
alkaline bake or acid leach.
The vast majority of the ion-adsorption ores
present a negative cerium anomaly, as described
by Chi et al. (2005), Bao and Zhao (2008) and
Sanematsu et al. (2013), meaning that there is a
relative depletion in the normalized (usually to
chondritic concentration) concentration of Ce
compared to La and Pr. This is due to the fact
that, contrary to the majority of lanthanide
elements, which are usually adsorbed physically
as trivalent ions, Ce
3+
can be oxidized easily by
atmospheric oxygen (O
2
)toCe
4+
(Bard et al.,
1985), and precipitate as cerianite, CeO
2
.
Additionally, Ce
3+
can be oxidized to Ce
4+
during
adsorption on δ-MnO
2
, as described by Ohta and
Kawabe (2001). Consequently, these processes
facilitate a natural separation of Ce from the other
adsorbed trivalent lanthanides and lead to low
recovery of Ce by ion-exchange reactions.
Depending on the nature of the original host
rocks, other metals will become dissolved and
carried downstream during the weathering, decom-
position and alteration processes. The main impur-
ities associated with the ion-adsorption ores are
usually Al, Na, K, Mg, Ca, Mn, Zn and Fe. While
most base metals occur as part of the mineral matrix
and do not leach out during the mild ion-exchange
REE leaching conditions, a certain fraction of Al
(due to its trivalent state) and to a lesser extent Na, K,
Ca and Mg are adsorbed physically and become
liable to be dissolved during the process along with
the lanthanides, as reported by Chi and Tian (2008)
and Rocha et al.(2013).
Overview of leaching technologies for the ion-
adsorption clays
As described previously, the ion-adsorption clays
contain 0.05 to 0.3 wt.% REE, of which generally
more than 60% occur as physically adsorbed
species recoverable by simple ion-exchange leach-
ing (Chi and Tian, 2008; Chi et al., 2013, Tian
et al., 2013; Luo et al., 2014). Typically, the ores are
leached with concentrated inorganic salt solutions
of monovalent cations. During leaching, the
physisorbed REE are relatively easily and select-
ively desorbed and substituted on the substrate by
the monovalent ions and transferred into solution as
soluble sulfates or chlorides, following a theoretical
3:1 stoichiometry (equation 1). However, the actual
lixiviant usage generally exceeds the stoichiometric
requirements due to competing desorption of other
cations (such as Al) also adsorbed on clays.
Dissolved REE are usually selectively precipitated
with oxalic acid to form oxalates (equation 2) that
are subsequently converted to REO via roasting at
900°C according to equation 3. Finally, the mixed
REO are separated into individual REE by
dissolution in HCl and fractional solvent extraction.
2 ClayREE þ 3M
2
SO
4
! 2 ClayM
3
þ REE
2
SO
4
ðÞ
3
(1)
REE
2
SO
4
ðÞ
3
þ 3H
2
C
2
O
4
þ 10H
2
O
! REE
2
C
2
O
4
ðÞ
3
10H
2
O þ H
2
SO
4
(2)
REE
2
C
2
O
4
ðÞ
3
10H
2
O
! REE
2
O
3
þ 3CO þ 3CO
2
þ 10H
2
O (3)
Various investigations of the desorption of REE
from clays via ion-exchange leaching (Chi and Tian,
2008; Moldoveanu and Papangelakis, 2012, 2013)
indicated that, regardless of the initial content, not
all REE reached similar extraction levels (i.e. the
percentages of desorbed/recovered REE varied
widely). Coppin et al. (2002) reported that the
amount of trivalent lanthanide ions adsorbed on
smectite and kaolinite was inversely proportional to
the ionic radii and pointed to a fractionation during
selective sorption of lanthanides, with heavy
elements (i.e. higher atomic number: Tb to Lu)
being adsorbed stronger that the light ones (i.e La to
Gd). They related this behaviour to the lanthanide
66
G. A. MOLDOVEANU AND V. G. PAPANGELAKIS

Citations
More filters
Journal ArticleDOI
TL;DR: In this paper, the balance of the individual rare earth elements (REE) in each deposit type and how that matches demand is considered, and some of the issues associated with developing these deposits are discussed.
Abstract: The rare earth elements (REE) have attracted much attention in recent years, being viewed as critical metals because of China’s domination of their supply chain. This is despite the fact that REE enrichments are known to exist in a wide range of settings, and have been the subject of much recent exploration. Although the REE are often referred to as a single group, in practice each individual element has a specific set of end-uses, and so demand varies between them. Future demand growth to 2026 is likely to be mainly linked to the use of NdFeB magnets, particularly in hybrid and electric vehicles and wind turbines, and in erbium-doped glass fiber for communications. Supply of lanthanum and cerium is forecast to exceed demand. There are several different types of natural (primary) REE resources, including those formed by high-temperature geological processes (carbonatites, alkaline rocks, vein and skarn deposits) and those formed by low-temperature processes (placers, laterites, bauxites and ion-adsorption clays). In this paper, we consider the balance of the individual REE in each deposit type and how that matches demand, and look at some of the issues associated with developing these deposits. This assessment and overview indicate that while each type of REE deposit has different advantages and disadvantages, light rare earth-enriched ion adsorption types appear to have the best match to future REE needs. Production of REE as by-products from, for example, bauxite or phosphate, is potentially the most rapid way to produce additional REE. There are still significant technical and economic challenges to be overcome to create substantial REE supply chains outside China.

319 citations


Cites background from "A review of the potential for rare-..."

  • ...significant REE resources (Wang et al. 2010; Boni et al. 2013; Deady et al. 2016), and research is...

    [...]

  • ...These bauxites and associated red mud waste products thus have the potential to contain significant REE resources (Wang et al. 2010; Boni et al. 2013; Deady et al. 2016), and research is ongoing to develop processes for extraction of REE (Borra et al. 2015)....

    [...]

  • ...Bauxites are widely mined across the globe for the extraction of aluminum, producing a waste material (red mud) with moderate REE enrichment, of the order of 1000 ppm (Deady et al. 2016)....

    [...]

Journal ArticleDOI
TL;DR: A review of the disposal and storage of bauxite residue from the late nineteenth century and how the environmental aspects of storage and disposal have changed can be found in this paper, where the success of a soil-free approach in Jamaica is discussed.
Abstract: The paper serves to briefly review the disposal and storage of bauxite residue from the late nineteenth century and discusses how the environmental aspects of storage and disposal have changed The paper describes some of the remediation/rehabilitation trends and describes the success of a soil-free approach in Jamaica The paper further discusses the development of uses for bauxite residue over the same period In spite of over a century of effort looking for uses, over 1200 patents and hundreds of technically successful trials, less than 4 million tonnes of the 150 million tonnes of bauxite residue produced annually is used in a productive way A large proportion of material that is used is in China and driven by government pressure This paper discusses the barriers and why the technical successes do not always translate into large-scale uses The most successful large-scale uses are reviewed and include cement production, raw material for iron and steel manufacture, manufacture of building materials, landfill capping, road construction, and soil amelioration Some of the more recent promising developments are also presented

289 citations

Journal ArticleDOI
TL;DR: Experimental results and density functional theory calculations reveal that the crucial role of single Er atoms in promoting photocatalytic CO2RR performance is revealed.
Abstract: The solar-driven photocatalytic reduction of CO2 (CO2 RR) into chemical fuels is a promising route to enrich energy supplies and mitigate CO2 emissions. However, low catalytic efficiency and poor selectivity, especially in a pure-water system, hinder the development of photocatalytic CO2 RR owing to the lack of effective catalysts. Herein, we report a novel atom-confinement and coordination (ACC) strategy to achieve the synthesis of rare-earth single erbium (Er) atoms supported on carbon nitride nanotubes (Er1 /CN-NT) with a tunable dispersion density of single atoms. Er1 /CN-NT is a highly efficient and robust photocatalyst that exhibits outstanding CO2 RR performance in a pure-water system. Experimental results and density functional theory calculations reveal the crucial role of single Er atoms in promoting photocatalytic CO2 RR.

263 citations

Journal ArticleDOI
TL;DR: In this article, the extraction of rare earth elements from bauxite residue by dry digestion method followed by water leaching was investigated, and it was shown that at ambient temperatures, silica dissolution increases with increasing acid concentration, which leads to the formation of silica gel.

111 citations


Cites background from "A review of the potential for rare-..."

  • ...It has been estimated that the annual global production of bauxite residue exceeds 150 million tonnes (Deady et al., 2016; Evans, 2016) and, according to numbers from the year 2007, about 2.7× 109 tonnes have been already accumulated in tailing ponds, dry stacking and other dry disposal methods…...

    [...]

  • ...It has been estimated that the annual global production of bauxite residue exceeds 150 million tonnes (Deady et al., 2016; Evans, 2016) and, according to numbers from the year 2007, about 2....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a combination of micro-analytical techniques was used to reveal the modes of occurrence of scandium (Sc) in bauxite residue, where Sc is mainly hosted in hematite where Sc3+ probably substitutes Fe3+.

78 citations


Cites background from "A review of the potential for rare-..."

  • ...By 2015, the volume of bauxite residue accumulated in Greece was estimated to be about 5 Mt, resulting from the yearly output of 0.7 Mt (Anagnostou, 2010; Deady et al., 2016)....

    [...]

  • ...A recent review and a case study of the deposit, with an emphasis on REEs occurrence, is compiled by Deady et al. (2016)....

    [...]

References
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.

10,830 citations

Journal ArticleDOI
TL;DR: The Karakaya marginal sea was already closed by earliest Jurassic times because early Jurassic sediments unconformably overlie its deformed lithologies as discussed by the authors, and it was closed by collision of the Bitlis-Poturge fragment with Arabia.

2,899 citations

Journal ArticleDOI
06 Aug 1966-Nature
TL;DR: In this article, the authors reported that the resulting densities in the lower mantle are in good agreement with shock-wave measurements on rocks having FeO contents in the range 10 ± 2% by weight.
Abstract: RECENTLY, Birch1 reported data on the density and composition of the mantle and core. He wrote: “The resulting densities in the lower mantle are found to be in good agreement with shock-wave measurements on rocks having FeO contents in the range 10 ± 2% by weight … except for iron oxide, the chemical composition of the mantle is indeterminate. The density of the outer core is lower than that of iron by about 10%”.

2,659 citations

Journal ArticleDOI
TL;DR: The state of the art in preprocessing of End-of-life materials containing rare-earth elements (REEs) and the final recovery is discussed in detail in this article, where the relevance of Life Cycle Assessment (LCA) for REE recycling is emphasized.

1,718 citations

Book
01 Jan 1984

1,626 citations

Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "An overview of rare-earth recovery by ion-exchange leaching from ion-adsorption clays of various origins" ?

In this paper, a standardized procedure for rare-earth leaching was developed to evaluate the commercial potential for the recovery of rare earth elements from ion-adsorptiondeposits in a systematic manner.