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Mineral Processing and Extractive Metallurgy Review
An International Journal
ISSN: 0882-7508 (Print) 1547-7401 (Online) Journal homepage: http://www.tandfonline.com/loi/gmpr20
Recovery of rare earth metals (REMs) from
primary raw material: sulphatization-leaching-
Zaure Karshigina, Zinesh Abisheva, Yelena Bochevskaya, Ata Akcil, Elmira
Sargelova, Bulat Sukurov & Igor Silachyov
To cite this article: Zaure Karshigina, Zinesh Abisheva, Yelena Bochevskaya, Ata Akcil, Elmira
Sargelova, Bulat Sukurov & Igor Silachyov (2018): Recovery of rare earth metals (REMs) from
primary raw material: sulphatization-leaching-precipitation-extraction, Mineral Processing and
Extractive Metallurgy Review, DOI: 10.1080/08827508.2018.1434778
To link to this article: https://doi.org/10.1080/08827508.2018.1434778
Published online: 20 Feb 2018.
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Recovery of rare earth metals (REMs) from primary raw material:
, Zinesh Abisheva
, Yelena Bochevskaya
, Ata Akcil
, Elmira Sargelova
, Bulat Sukurov
and Igor Silachyov
The Institute of Metallurgy and Ore Benefication, Almaty, Kazakhstan;
Mineral-Metal Recovery and Recycling (MMR&R) Research Group, Mineral
Processing Division, Department of Mining Engineering, Suleyman Demirel University, Isparta, Turkey;
The Institute of Nuclear Physics, Almaty,
The present study is devoted to the extraction of rare-earth metals (REMs) from a high-silica ore found in
one of the deposits in Kazakhstan. The ore was processed in a mixture with sulfuric acid at 200°C
(sulphatization) and subsequently followed by water leaching of the sulphate product (sinter), REMs
were extracted into the solution. Precipitation of the REMs-containing hydrate product from the sulfate
solution was carried out with sodium hydroxide. In order to obtain the REMs-containing nitrate solution,
the precipitate was dissolved in nitric acid. REMs solvent extraction from nitrate solutions was carried
out with tributyl phosphate (TBP).
High-silicon ore; rare-earth
The continuous development of high-tech industries has con-
tributed to the increase in demand for rare earth metals
(REMs). REMs are being used in areas such as in the produc-
tion of electronics, laser technology, superconductors and fuel
cells, communication and medical equipment, and others.
REMs are of immense importance in the so-called “green
technologies”. They are used in the production of wind tur-
bines, energy-saving light bulbs, as well as in the rising pro-
duction of electric and hybrid cars.
Over the past 50 years, the REMs production has increased
by 25-fold (from 5,000 to 125,000 tons per year) (Samsonov
and Semyagin 2014). According to estimates of the consulting
company, i.e. Industrial Mineral Company of Australia
(IMCOA), it will reach 200–240 thousand tons by 2020
(Editorial review 2013).
In recent years, the growing demand for rare earth
products has fascinated researchers toward complex com-
position of mineral raw materials. One such source of rare-
earth raw materials is the Kundybay deposit, located in
Northern Kazakhstan (Dzhafarov and Dzhafarov 2002).
Several research studies have been carried out in order to
enrich the REMs present in the ores of Kundybay deposit.
Alimzhanova et al. (2015) have reported that the ore of
Kundybay deposit is an alumina-containing ore of rare and
rare-earth metals with double persistence. The range of
alkali aluminosilicates were present in the form of kaolin,
nepheline to feldspar; organic matter-from relict hydrocar-
bons, micron oil to protein compounds, spinels, sulphides,
phosphates, carbonates and amorphous skeletal silica,
permeated with channels, cracks, voids, filled with organic
matter and REMs. The grinding of such fine-grained ores
to a standard particle size of 80%–90% grade less than 74
μm did not ensure the full distortion of the clusters con-
taining the minerals to be recovered. An increase in the
degree of grinding to 90% size fraction of minus 44 μmled
to a significant formation of sludge, for which an effective
physical or chemical method is yet to be developed.
Due to the very fine-grained formation and accumulation
of rare-earth metals in an empty rock, the traditional methods
of enrichment of rare-earths from weathering crust have been
ineffective (Ulasyuk and Kiseleva 1981; Shautenov et al. 2012).
Consequently, the metallurgical methods have been gaining
more interest for the processing of this ore.
Currently, the processing of bastnaesite ores serves as a
major source of production for REMs. In the first stage,
bastnaesite concentrates are roastedat temperatures of
400–800°C for the decomposition of fluorocarbonates
(Bolshakov 1976). In accordance with the recommended
roasting conditions, a porous sinter is obtained, which is
readily soluble in dilute sulfuric, nitric and hydrochloric
acids (Bolshakov 1976;Xing-Liangetal.,2013). A definite
problem in the processing of bastnaesite was the inability to
extract REMs fluorides after roasting. One way to solve the
problem was to conduct the conversion of REMs fluorides
to hydroxides by alkaline treatment of bastnaesite, which
was subsequently followed by hydrochloric acid leaching of
Gupta and Krishnamurthy 2005). Another option was
roasting in the presence of concentrated sulfuric acid at a
CONTACT Ata Akcil firstname.lastname@example.org;email@example.com Mineral-Metal Recovery and Recycling (MMR & R) Research Group, Mineral Processing
Division,Department of Mining Engineering, Suleyman Demirel University, TR32260 Isparta, Turkey.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmpr.
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW
© 2018 Taylor & Francis Group, LLC
temperature of 400–500ºСfor several hours, which is cur-
rently used in Bayan Obo. According to reports made by
Bolshakov (1976), the bastnaesite concentrate was mixed
with concentrated sulfuric acid at a ratio of S:L = 1:0.77,
then the mixture was kept at 100°C until the fluorine got
completely removed, and then the temperature was raised
to 650–750°C. The resulting sulfate sinter was leached with
water. Kul et al. (2008) carried out the decomposition
process at a lower temperature of 200°C.
The principle of high-temperature treatment of a mix-
ture of ore material with concentrated acids is also used
for the processing of monazite concentrate. The ground
monazite concentrate is decomposed by using concen-
trated sulfuric acid in a volume of 230–250% of the stoi-
chiometric flow rate, at a temperature of 200–230°C for
2–4h(Zelikman1961; Habashi 2013). Monazite concen-
trate is also processed by an alkaline method. The decom-
position of the finely divided concentrate is carried out
with a solution of sodium hydroxide at a temperature of
140–170°C for about 3–4 h (Zelikman and Korshunov
The main commercially significant rich rare-earth
resources containing up to 60%–70% of REMs oxides, such
as bastnaesite and monazite have become limited and the
requirements for heavy rare earths has increased. Therefore,
sources of raw materials such as ion-adsorption ores are
gradually being involved in the production sphere.
Moldoveanu and Papangelakis (2012,2013)) studied the
extraction process of REMs adsorbed on clays by means of
an ion-exchange mechanism. At the same time, during the
leaching with solutions of sulfates and chlorides of monova-
lent cations (Li
) at room temperature,
sulfate and chloride salts based on Cs
the best results. The degree of REMs recovery was in the range
of 80%–90%. For further studies, a reagent (NH
chosen. Zhengyan et al. (2016) have shown the possibility of
extracting rare-earth metals from the weathering crust con-
taining 0.1% REMs and their separation from aluminum by
passing a mixture of reagents such as NH
Cl and NH
through the ore layer.
As it has been discussed above, the technological schemes
include acidic or alkaline treatment of REMs-containing raw
materials in the first stage, with the acid methods finding
more widespread use.
The resulting REMs-containing solutions are then further
processed using precipitation, ion exchange or solvent extrac-
tion methods. Jun et al. (2011) used three different methods
such as solvent extraction, sorption and membrane methods
to extract REMs from leachate of China weathering crusts,
while the extraction of REMs from the leach solution was
96%, 98.7% and 98%, respectively.
It is likely that processing of raw materials, simultaneously
accompanied by a large number of gangue components and a
relatively low content of recovered rare-earth metals would
lead to the formation of solutions in which the content of
impurities can exceed several times the content of REMs. For
such solutions, processing through the solvent extraction
method is most effective. The use of solvent extraction
method allows high levels of selectivity in order to separate
valuable components from such solutions and eventually con-
centrates the recoverable metals.
Solvent extraction of REMs present in nitric acid, hydro-
chloric acid or sulfuric acid solutions is carried out mostly by
using cation-exchange and neutral extractants (Zhang et al.
1982; Preston et al. 1996; McGill 1997; Morais and Ciminelli
2004; Rabie 2007; Joriani and Shahbazi 2012; Torkaman et al.
2015). Huang et al. (2006) treated bastnaesite ore and its
concentrate (China) with H
after roasting, which was
subsequently followed by leaching with water or dilute sulfu-
ric acid. Thorium and a major amount of cerium (IV) were
first extracted from the leach solution by solvent extraction
with di(2-ethylhexyl) phosphoric acid (D2EHPA) or (2-ethyl-
hexyl)-(2-ethylhexyl) phosphonium acid (P507). The raffinate
was then fed to the second solvent extraction circuit to pro-
duce individual REMs.
During REMs recovery by solvent extraction, neutral
extractants like tributyl phosphate (TBP) are most widely
used. Studies of solvent extraction process using TBP for
trivalent rare earths recovery from chloride and nitrate solu-
tions showed that the extractability of lanthanides increased
with increasing atomic number and the distribution coeffi-
cients were considerably lower in chloride solutions than in
nitrate solutions (Peppard et al. 1957a,1957b). The low
efficiency of TBP in chloride and sulfate media can be
explained by the formation of non-extractable chloride and
sulfate complexes of REMs. The solvent extraction of REMs
from nitrate solutions using TBP as an extractant has
received more attention over the years (Zelikman 1961;
Bolshakov 1976). However, in the methods used, the con-
centration of ΣREMs in the initial solution entering the
solvent extraction is usually more than 1 g/L. Therefore, it
is important to study the process of REMs solvent extraction
with TBP from nitrate solutions having a lower concentra-
tion of rare earths, while containing aluminum and iron in
Since the rare-earth ore of Kundybay deposit is practically
not amenable to enrichment, the use of directly hydrometal-
lurgical processing schemes is vital and requires more inves-
tigation. Though it is possible to obtain REMs-containing
solutions, however, they contain a large amount of impurities.
Therefore, further study is required to determine the possibi-
lity of processing such solutions, in which concentrations of
macro impurities are several times higher than the REMs
concentrations and to separate rare earths from the macro
The raw ore used in the present study was obtained from
Kundybay deposit located in Northern Kazakhstan. The ore
was milled to obtain a particle size of –0.1 mm.
For sulphatization, sulfuric acid H
with a concentra-
tion of 9 M (59.2%) was used. To obtain REMs-containing
precipitate, a solution of sodium hydroxide NaOH with con-
centration of 310 g/L (24.5%) was used. To dissolve the
REMs-containing precipitate, a solution of nitric acid HNO
2Z. KARSHIGINA ET AL.
was used. Calcium oxide with a concentration of 96% CaO
was used. Rare-earth metals were extracted using 3.67 М
(100%) TBP (C
X-ray diffraction (XRD) analyses of the ore and cake were
done using D8 ADVANCE (Bruker AXS GmbH) instrument
with cobalt anode, а-Cu emission.
SEM analysis of the ore and cake sample obtained after
sulfatization of Kundybay ore and water leaching of sul-
phate sinter was carried out using a scanning electron
microscope (SEM) with accelerating voltage of 20 kV
and an analyzer JEOL JXA-8230. Briquettes of the samples
were prepared prior to analysis (artificial polished
The experiments on high-temperature ore processing
with sulphuric acid (hereinafter referred to as sulphatiza-
tion) were conducted in a muffle furnace of brand SNOL
Leaching experiments were conducted in the thermo-
stated cell with a capacity of 600 ml, which was equipped
with a mechanical stirrer “OST basic”,providingafixed
number of revolutions i.e. 500 rpm (revolutions per minute).
Constant temperature was maintained using a thermostat
Solvent extraction experiments were carried out in separ-
ating funnel 20 ± 5°Сusing predetermined proportions of the
organic and aqueous phases. The contact time of the phases
was 5 min.
The resultant solutions and cakes were analyzed in order
to determine the content of aluminum, iron, and ∑REMs.
The quantitative content of major elements and compounds
was determined by chemical methods of analysis. An
Optima 8300DV ICP atomic emission spectrometer was
used for the quantitative determination of rare-earth
metals. Samples of Kundybay deposit ore and hydrated
precipitates were also analyzed at the Institute of Nuclear
Physics (INP) by using a neutron activation analysis for
determining the content of REMs.
3. Results and discussion
3.1. Study of chemical and phase compositions of ore
The chemical composition of Kundybay deposit ore is pre-
sented in Tables 1 and 2. The ore contained a large amount of
silica and a smaller amount of aluminum and iron. The total
content of rare earth elements was 340 ppm, out of which, the
group of light REMs was 66% and the group of medium and
heavy REMs –34%.
The results of XRD analysis revealed that the ore from
Kundybay deposit mainly consisted of muscovite –KAl
, kaolinite –Al
. Apart from that, montmorillonite Na
O, clinochlor (Mg,Fe)
and goethite FeO(OH) were present in small amounts
According to data of Dzhafarov and Dzhafarov (2002)
and Podporina et al. (1980), rare earths in the weathering
crust are distributed among three groups of mineral
(1) REMs are found in clay minerals (kaolinite, goethite)
in the sorbed associated state, probably in the nodes
and internodes of their structural lattices in the form
of separate ions and small aggregates;
(2) REMs are concentrated in newly formed hypergenic
minerals –сhurchite, ittrorhabdophanite, yttrium and
neodymium bastnaesite and yttrium and neodymium
parisite, wherein the main mineral is yttrium dihy-
(3) REMs isomorphically occur in the composition of
residual endogenous minerals –apatite, garnet, alla-
nite and others.
In order to study the forms and nature of the REMs
distribution, the ore was investigated using a SEM. Photos
of the ore sample and spectra are shown in Figures 2–4.
Figure 2 (a, b) shows the photograph, map and energy
dispersive X-ray spectroscopy (EDS) analysis of the ore
sample with 800 times a magnified image. The photo and
map show that REMs compounds with grains size ranging
from 2–3to20–25 microns are most likely associated with
clay minerals such as montmorillonite Na
O and kaolinite Al
. The pattern of
potassium distribution on the map indicated a probable
intergrowth of REMs compounds in some places, with
mica-slabs of muscovite KAl
belongs to the group of layered silicates. The map indicated
the presence of silicon oxide, apparently in the form of
(OH) or hematite Fe
were also observed in the intergrowth, along with iron
mineral. The almost identical nature of distribution of
REMs and phosphorus indicated the presence of REMs
compounds in the form of phosphates. EDS analysis of a
specific area of the sample identified the presence of Ce, Nd
and Gd (Figure 2(b)).
Figure 2 (c,d) shows the photograph and the map of
another section of the ore sample with 600 times magnifica-
tion. The nature of distribution of REMs and phosphorus, as
observed in the map indicated the presence of rare earth
compounds in the form of phosphates. The snapshot showed
the phosphate grains of REMs occurring in size range of~10–
Table 1. Chemical composition of Kundybay deposit ore.
Content (wt %)
CaO MgO Na
59.05 19.76 7.25 0.65 1.25 0.46 1.55 0.76 0.034 0.016 8.85 0.37
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 3
30 μm, which appeared in close intergrowth inside the main
phase, consisting of muscovite.
The layered nature of the surface of the main phase in the
photograph also indicated the possible presence of mica,
which during the weathering process might have gradually
converted to montmorillonite and then to kaolinite
(Paffengolts et al. 1978). The presence of iron and magnesium
in the map (Figure 2c) indicated a possible presence of mag-
nesia-ferruginous character of mica (muscovite), which can
act as a source for clinochlore (Mg,Fe)
EDS analysis of the specific section of the sample indicated
that the REMs were mostly represented by lanthanum and
neodymium (Figure 2d).
Figure 3 shows the photos and results of EDS analysis of
the ore sample, with high REMs concentration i.e. (Figure 3a)
showing a 2000 times magnified image, (Figure 3b) showing
an area of 5 × 5 μm and a 4300 times magnified image.
Elemental EDS analysis of a specific section of the sample
revealed that REMs can be present both in the form of
phosphates (possibly, сhurchite and ittrorhabdophanite), and
in the form of carbonates or fluorocarbonates (possibly,
yttrium and neodymium bastnaesite). The presence of cal-
cium may indicate the presence of REMs-containing mineral
For a more accurate determination of REMs in the ore, the
area of the sample with a high REMs content (the largest grain
in the form of a light spot in the lower section of the photo of
Figure 2c) was analyzed using a more sensitive wave-disper-
sive spectroscopy (WDS) analysis. The spectra is shown in
Figure 4. WDS analysis of the ore sample was carried out
considering a specific area of diameter 10 μm. The following
rare earth elements were detected in the analyzed area:
yttrium (6.7%), lanthanum (15.6%), praseodymium (12.7%),
neodymium (21.8%), samarium (5.9%), gadolinium (9.2%)
and dysprosium (1.6%).
Using chemical and physical methods of analysis, it was
established that the ore of the Kundybay deposit consisted
of~60% of silicon oxide. The ore included aluminum and
silicon in various compound forms. SEM analysis showed
that rare earth elements were present in the ore, presumably
in the form of phosphates (major form) and also as carbo-
nates. REMs occur in close association with clay minerals,
mica, both on surface in “open”state and inside grains of
the main ore minerals in “closed”state.
Thus, the ore of Kundybay deposit can be an additional
source of raw materials for the production of REMs. The
different forms of REM compounds in the ore can have a
significant effect on the degree of REMs recovery during the
disruption of the ore matrix.
3.2. Selection of ore processing method
The main components of the rare-earth weathering crust
of Kundybay deposit are silicon, aluminum and iron. Since
the silicon content in the ore was about 60% (in terms of
its oxide), it was preferred to carry out an alkaline treat-
ment in the initial stage for transferring silicon into the
solution. According to phase composition studies of the
ore, most of the silicon was present in the form of com-
pounds that are difficult to decompose at atmospheric
pressure, such as quartz and muscovite. So it was prefer-
able to carry out the process in an autoclave. While leach-
ing the ore in an autoclave, under the following
conditions: temperature 220°C, C
= 310 g/L, ratio S:
L = 1:6, duration 3 h, recovery of silicon in the solution
Table 2. Rare-earth elements content in Kundybay deposit ore.
Rare-earth element Content (ppm) Rare-earth element Content (ppm)
La 62 Tb 2
Ce 86 Dy 11
Pr 16 Er 3
Nd 60 Tm 1
Sm 11 Yb 6
Eu 3 Lu 1
– muscovite; – kaolinite; –
uartz; – montmorillonite; – clinochlor; –
5 10 20 30 40 50 60
Figure 1. Diffractogram of the Kundybay deposit ore sample.
4Z. KARSHIGINA ET AL.
was~58% (Karshigina et al. 2016a). The low efficiency of
the process was probably due to the presence of about
together with silicon forms a slightly soluble sodium
hydroaluminosilicate, which leads to an incomplete trans-
fer of silicon into the solution. In addition, according to
reports made by Zelikman and Korshunov (1991), when
autoclave leaching occurs at temperatures of about 200°C,
rare-earth metals form hydroxides, which are hard to
solubilize and could create problems in the decomposition
of cakes for extraction of REMs. Therefore, preference was
given to the acidic methods for recovery of REMs from
According to Isaeva et al. (2015), the ores with clay miner-
als are associated with 13%–90.3% of REMs. Since a signifi-
cant part of the REMs in ore may be present in the adsorbed
state on clay minerals, therefore, the ion exchange mechanism
was preferred to extract them.
Aluminosilicate minerals capable of isomorphically sub-
stituting one cation of similar size with another, the later
Figure 2. Photo of ore sample made on SEM, maps and EDS analysis for section 1 (a, b) with 800 times and section 2 (c, d) with 600 times magnification.
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 5
having a less charge (e.g., Al
which leads to a charge imbalance in the structure of the
mineral and the occurrence of excessive negative charge
on its surface. This attracts cations, in particular cations of
the REMs group, toward the surface of minerals
(Moldoveanu and Papangelakis 2012,2013;Xiuliand
Junwei, 2015). Rare earth elements may be adsorbed on
aluminosilicate minerals such as kaolinite, montmorillo-
nite and muscovite. REMs, adsorbed on aluminosilicate
minerals, can be desorbed with subsequent transition to
solution. Xiuli and Junwei, (2015)used2%ammonium
nitrate solution and ammonium sulfate in a molar ratio
equal to 4:1, in the presence of 0.05% of ammonium
acetate (an inhibitor for aluminum) for leaching. When
leaching of the material was carried out by a percolation
column using the above-mentioned solution, the degree of
REMs recovery exceeded 80% and the degree of aluminum
inhibition was above 80%.
In order to extract the REMs from the weathering crust
of Kundybay deposit, the experiments were carried out
under similar conditions. A column was used, in which a
sponge was placed at the bottom of the column and then
the column was filled with the investigated ore. The flow
rate of the feed solution was maintained at 0.12 mL/min
for 13 h. However, the ΣREMs concentration in the solu-
tion after leaching was 0.01 mg/L, and the extraction was
0.004% (Abisheva et al. 2016). In another method,
Sharipov and Stryapkov (1985) proposed to extract the
REMs from the weathering crust without destroying the
crystal structure of the carrier minerals. Acid decathioni-
zation was carried out, which consisted of treating the ore
at low temperatures (20–60°C) with sulfuric acid (10%–
15% of stoichiometry), i.e. 80 g/L H
at a ratio of S:
L = 1:2. In this case, the REMs recovery into the solution
was more than 90%. However, the REMs recovery into the
solution was 34%–48%, while processing the Kundybay
ore under similar conditions (Karshigina et al. 2016a).
The methods of the ion-exchange mechanism or acidic
decathionization are more suitable when the REMs are
present almost entirely in the sorbed state with colloidal
minerals. As noted earlier, in Kundybay deposit ore, rare
earths are not only adsorbed on clay minerals, but also
enter as an isomorphic impurity into residual endogenous
minerals and form their own ore minerals. Therefore,
methods of disrupting the ore matrix which are acceptable
for all groups of REMs distribution are applicable.
Figure 2. (Continued).
6Z. KARSHIGINA ET AL.
Accordingly, sulfuric acid method for opening of the feed-
stock was of interest, which involved mixing with sulfuric acid
and aging at high temperatures, the process being called as
sulphatization. This process is widely used in aluminum metal-
lurgy for opening nepheline concentrates and kaolinites (Liner
1982), as well as in the metallurgy of rare-earth metals during the
processing of monazite concentrates, in which REMs are present
in the form of phosphates (Zelikman 1961). In the ore of the
Kundybay deposit, own minerals of REMs are represented
mainly by сhurchite(Y,Се)PO
O, and ittrorhabdophanite
O, yttrium and neodymium bastnaesite(Y,Nd)
and yttrium and neodymium parasite Ca(Y,Nd)
. During the sulphatization process own minerals of REMs
may interact with sulfuric acid by the following reactions:
Figure 2. (Continued).
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 7
Figure 2. (Continued).
8Z. KARSHIGINA ET AL.
Considering the phase and chemical composition of the ore,
in which the minerals of rare earths are closely intergrown
with muscovite, rutile, quartz and others, the use of the high-
temperature treatment method in a mixture with sulfuric acid
(sulphatization) to disintegrate the Kundybay deposit ore
seemed most appropriate (Abisheva et al. 2016).
In order to determine the possibility of disintegrating the main
ore minerals, calculations related to Gibbs energies of reactions for
interaction of muscovite, kaolinite and goethite with sulfuric acid
at 200°C were carried out using the computer program HSC 5
Chemistry of “Outokumpu Oy”(Finland) (Table 3). Rare earths
that are represented in the ore in the form of the above com-
pounds are not available in the database of the computer program
HSC 5 Chemistry. Rare earth compounds involved in reactions
(1–6) are hydroxyapatites and fluorocarbonates, in which calcium
is isomorphically replaced by the REMs. It is well known that
hydroxyapatite has ionic crystal lattice where Ca
ions can be
replaced by the REMs ions. The crystal lattice of bastnesite con-
sists of carbonate groups CO3
2, bound together by fluorine ions
and ions of rare-earth elements (REEs). Therefore, for the ther-
modynamic calculations, assumptions were made. The own aqu-
eous ions of REMs and their carbonate- and dihydrogen
phosphate aqueous ions, for which data are available in the
computer program, were also used as primary REMs-containing
components of phosphorous slag in addition to phosphates.
In accordance with the results of thermodynamic calcula-
tions, all reactions shown in Table 3 have negative Gibbs energy
T<0. This indicates thermodynamic probability for occur-
rence of all reactions. The reactions of the main ore minerals
(muscovite, kaolinite) with sulfuric acid under the calculated
conditions show the possibility of their good opening. The use
of aqueous ions, as components of REMs-containing minerals,
instead of substances in the solid state, leads to certain decrease
in the Gibbs energy. At the same time, for reactions with yttrium
and neodymium phosphates taken in solid state, the Gibbs
energy value remains negative and sulfatization process is ther-
3.3. Sulfuric acid opening the ore
Sulphatization of the ore was carried out under the following
conditions: the ratio S:L = 1:0.36; temperature –200°С; the
concentration of sulfuric acid was 9 M and the duration of the
process was 2 h. Sinter was stamped to a fineness of 0.5–1mm
and leached with water under the following conditions: S:
L = 1:2.5; temperature –90°С; the duration of the process
was 4 h. Concentrations and recovery of ΣREMs, aluminum
and iron obtained in the solution using the conditions indi-
cated above are presented in Table 4 (Karshigina et al. 2016b).
Figure 3. Photo of SEM and EDS analysis of the ore sample.
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 9
To estimate the thermodynamically stable states of REEs in the
obtained solutions, the Purbe diagrams for cerium, yttrium and
neodymium were constructed, which are shown in Figures 5–7.In
the diagrams, area of the steady state of water is represented by
dashed lines. The diagrams were constructed for the sulfates sinter
leaching temperature i.e. 90°C and the concentrations of the
corresponding REM and sulfate ion (in terms of elemental sulfur)
in the resulting solution. As can be seen from the diagrams, rare
earth elements were present in the form of RE
leaching of the sulphate sinter, while there were complex sulfate
2in the area of water stability.
pH of the solution after aqueous leaching of sulfate
sinter was 1.62, and redox potential was 0.73 V. It is
shown that in this area cerium and yttrium ions predomi-
nate in the form of RESOþ
4cations, and neodymium ions
in the form of anions REðSO4Þ
2. During further proces-
sing of sulfate solutions by solvent extraction, the use of
Figure 4. WDS analysis of the ore sample.
Table 3. Gibbs energy of reactions of interaction of REMs-containing components with sulfuric acid during ore sulfatization.
2FeO OH þ3H2SO4¼Fe2ðSO4Þ3þ4H2O−162.0
Table 4. Concentrations and recovery of metals in solution during water leaching of sulfate sinter.
Concentration in the solution Recovery into the solution (%)
∑REMs, mg/L Al
, g/L Fe
, g/L ∑REMs Al
99 25.7 9.4 83.9 33.9 24.8
10 Z. KARSHIGINA ET AL.
either a cation-exchange or an anion-exchange extractant
could have led to incomplete recovery of REMs into the
organic phase. Therefore, neutral extractants were pre-
ferred for the extraction of REMs. One of the effective
neutral extractants in the processing of REMs-containing
solutions is TBP (C
suitable for REMs recovery from nitrate solutions, as the
rare earths are present in sulfate solution in the form of
sulfate complex ions that are not extracted with TBP.
Therefore, it is first necessary to transfer the REMs into
After the water leaching of the sulphate sinter, a sili-
con-containing cake of the following composition was
obtained, %: 64–70 SiO
According to XRD analysis (Figure 8), the cake contained
Na) (Fe, Al,Mg)
), kaolinite Al
. The quantity of quartz had
increased in the cake due to decomposition of other
phase constituents of the ore and their transition into
solution in the form of soluble compounds. Reduction in
peak intensity of kaolinite and muscovite indicated that
most of them decomposed during sulfuric acid dissection.
The butlerite phase apparently resulted due to sulfatization
and hydrolysis of iron sulphate during aqueous leaching of
The cake sample was also analyzed using a SEM. The
results of the studies showed (Figure 9) that a zirconium
compound was present in the analyzed sample area, inside
the grain of quartz, possibly in the form of its orthosilicate.
Inside the grain of quartz, there was also a titanium
Ce(SO 4)2 (-a)
Figure 5. The Purbe diagram for Ce –S–H
O system at 90°С, pressure –0.1MPa: Сe concentration –2.03·10
O; S concentration –9.7·10
Y(O H) 3
YS O4 ( + a)
Y(S O 4 ) 2 (- a)
Y(+ 3 a)
Figure 6. The Purbe diagram for Y –S–H
O system at 90°С, pressure –0.1MPa: Y concentration –2.43·10
O; S concentration –9.7·10
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 11
compound, presumably in the form of its oxide. According to
SEM analysis, some REMs remained in the cake in “closed”
state inside the grains of un- decomposed minerals (Figures
10 and 11). Figure 10 shows that REMs compounds were
most presumably found in the form of carbonates and also
in the form of phosphates inside the grain of quartz with an
admixture of carbon, or possibly, organic substances.
Figure 11 shows that the expected carbonates and phosphates
of REMs were located inside the grain of titanium oxide.
As shown by the results of the analyses, a finer grinding of
the ore was necessary for a complete disintegration of the
minerals grains and better recovery of REMs. The cake con-
taining mainly silicon oxide in the form of quartz and alumi-
nosilicates may be suitable for the production of precipitated
silicon dioxide and the production of building materials.
3.4. Processing of the REMs-containing solution obtained
after sulfuric acid opening of the ore
In order to obtain REMs-containing nitrate solutions and
additionally concentrate them, studies including hydrolytic
precipitation of REMs from sulfate solutions of REMs-con-
taining hydrated products and further leaching of the latter
with nitric acid solutions were carried out.
3.4.1. Hydrolytic precipitation of rare earths containing
products from sulfate solutions
It is well known that the pH precipitation of REMs depends
on the medium from which the precipitation is carried out
(Bolshakov 1976). The pH for the separation of REMs hydro-
xides from sulfate solutions is in the range of~6.5 to~8.5
Figure 7. The Purbe diagram for Nd –S–H
O system at 90°С, pressure –0.1MPa: Nd concentration –1.32·10
O; S concentration –9.7·10
– Quartz; – Butlerite; – Glauconite; – Anorthite; – Kaolinite; – Muscovite
2-Theta - Scale
810 20 30 40 50 60 70 80
Figure 8. Diffractogram of cake.
12 Z. KARSHIGINA ET AL.
Figure 9. Photo of SEM, maps and EDS analysis of cake sample.
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 13
(Zelikman 1961). As a result of the hydrolytic decomposition
of sulfuric acid solutions, the REMs can precipitate to form
hydroxides and basic sulfates.
During the hydrolytic precipitation process from acidic
solutions, insoluble iron and aluminum hydrate compounds
with a precipitation pH of less than six (for aluminum and
iron (III)) will simultaneously pass into the precipitate along
The precipitation process for hydrated precipitates from
sulfate solutions was carried out by using both solid and
dissolved form as a reagent-precipitant. During the precipita-
tion, the stock solution was heated in order to intensify the
process and produce more easily filtered precipitates. The
precipitation temperature was maintained at 50°C and at
higher temperatures, the precipitate could dissolve. The pro-
cess was carried out for 2 h with constant stirring.
To ensure the complete transfer of REMs to the hydrate
product, precipitation experiments were carried out at various
pH values. In order to carry out the study, sodium hydroxide
was used in the solid state and as a solution at a concentration
of 310 g/L (Table 5).
REMs were completely precipitated at pH = 7. Under these
conditions, aluminum and iron were also precipitated. The use
of NaOH precipitant, both as a solid and as a solution, was
effective. The precipitates obtained at pH = 7–8 contained
ΣREOs –0.08%–0.09%; Аl
3.4.2. Dissolution of the hydrate precipitate with nitric acid
and solvent extraction of REMs from nitrate solutions using
In order to obtain the REMs-containing nitric acid solution,
the hydrate precipitate was dissolved in nitric acid. The
amount of nitric acid or nitrate ions in solution can have a
significant effect on the efficiency required for further solvent
extraction of REMs with TBP.
Therefore, for better dissolution, as well as for improve-
ment of the parameters related to subsequent solvent extrac-
tion of REMs with TBP, the dependence of precipitate
dissolution on the concentration of nitric acid was studied.
The experiments were carried out while maintaining the fol-
lowing constant conditions: temperature –60ºС; S:L = 1:5;
duration –1 h; concentrations of HNO
–4, 5, 6, 7 and 8 M;
n = 500 rpm. The composition of the hydrate precipitate was:
; 17.5% Fe
; 0.0844% ∑REOs. The results of
the experiments are presented in Table 6.
The total conversion of the REMs into the solution pro-
ceeds at a nitric acid concentration of 8 M, in which alumi-
num and iron also dissolve well. It should be noted that
during the dissolution of the precipitate with 5 and 8 M
solutions, some evaporation loss took place, which
affected the concentrations of ΣREMs, aluminum and iron.
In case of nitrate media, rare-earth metals are extracted
with TBP from neutral and acidic solutions.
Figure 10. Photo of SEM and EDS analysis of cake section with an increase of 3,000.
14 Z. KARSHIGINA ET AL.
Solvent extraction of REMs nitrates from nitric acid solu-
tions using TBP has been described by the following equation:
3þ3TBP ¼RE NO3
Starting from equation (7),
where D is the equilibrium distribution coefficient, which is
the ratio of equilibrium concentrations of the extracted ele-
ment in the organic and aqueous phases.
To determine the optimum concentration of nitric acid,
a process of REMs solvent extraction from nitrate solu-
tions was carried out. This was done by dissolving the
precipitate with a nitric acid concentration of 5–8M.
Conditions for solvent extraction were: TBP concentration
3.67 M (100%); O:A ratio = 2:1; duration –5min;the
Figure 11. Photo of SEM and EDS analysis of cake section with an increase of 3,500.
Table 5. Effect of pH solution value on completeness of REMs, aluminum and iron recovery from sulfate solutions.
The precipitant was solid NaOH in granules The precipitant was a solution of 310 g/l of NaOH
Recovery into the precipitate, %
∑REMs Al Fe ∑REMs Al Fe
4 11.0 55.8 44.6 13.4 10.1 0.5
5 77.0 99.9 83.6 60.2 95.8 86.5
7 99.9 100 99.1 99.8 100 100
Table 6. Influence of nitric acid concentration on the dissolution of hydrate precipitate and recovery of metals in the solution.
Concentration in the solution Recovery into the solution, %
∑REMs, mg/L Al
, g/L Fe
, g/L ∑REMs Al
4 128.6 46.3 32.9 80.7 63.1 84.6
5 175.9 78.6 44.5 95.6 92.9 99.2
6 129.0 60.6 31.8 90.0 91.8 90.9
7 133.4 63.0 34.9 93.0 95.5 99.7
8 170.5 78.1 41.3 99.9 99.4 99.1
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 15
temperature was 20 ± 5°C. The conducted investigations
showed that aluminum and iron, despite their significant
concentrations in the aqueous phase, (as compared to the
concentration of ΣREMs) they were not extracted into the
organic phase. Therefore, the results of studies on solvent
extraction with TBP presented in Table 7 are only for
In solutions, aluminum and iron are present in the form
of their nitrate salts. In order to promote the formation of
extractable REMs compounds and thereby increase the dis-
tribution coefficient, solvent extraction is desirably carried
out in the presence of excess nitric acid or its salts prone to
hydration and acting as salting-out agents. Their action is
reduced by the suppression of hydration and dissociation of
compounds of REEs (the action of the ion of the same
name) in the solution. Hydration of the salting out agent’s
ions reduces the concentration of unbound water, which in
turn promotes the dehydration of REMs ions in the solution.
The ions of aluminum and iron in aqueous solutions are
highly susceptible to hydration. This property is one of the
reasons for the use of nitrate salts of aluminum and iron as
salting out agents when REMs are extracted from nitrate
solutions using TBP. According to the data of Bolshakov
(1976), the efficiency of salting out agents can be placed in
the following order:
As seen from Table 7, the highest results solvent extraction of
REMs by TBP were derived for solutions obtained by dissolving
5 and 8 M nitric acid and they were characterized by higher
concentrations of macro-components i.e. aluminum and iron.
Higher concentrations of aluminum and iron salts (Table 7)
apparently, had a salting out effect and increased the solvent
extraction efficiency of the REMs, thereby, increasing the dis-
tribution coefficient. The concentration of Al(NO
in the aqu-
eous phase of the solvent extraction process (Table 7) was below
2.5 M, which is desirable for an effective salting out effect.
present in the solutions could also partici-
pate in the binding of water molecules. The amounts of Al(NO
moles for the first and last experiments of
Table 7 (solvent extraction of REMs from solutions, obtained
after precipitate dissolution by 5 and 8 M HNO
imum and were 2.1 M and 2.05 M respectively. Despite the fact
that these values are somewhat lower than 2.5 M,~90% and
higher REMs were extracted into the organic phase.
A concentration of 8M was considered as the most
optimal concentration of nitric acid, at which almost com-
plete dissolution of the precipitate occurred and the trans-
fer of REMs into nitric acid solution took place. From the
resulting solution, 90.5% of REMs were extracted into the
From the above formula (9), it can be seen that calcium
nitrate can also be used as a salting out agent. Therefore, it
was interesting to study the effect of presence of calcium
nitrate in aqueous phase on the solvent extraction of REMs.
3.4.3. Dissolution of hydrate precipitate with nitric acid in
the presence of calcium oxide and recovery of REMs from
solutions by solvent extraction with TBP
In some cases it is recommended to add calcium nitrate in the
solution before REMs solvent extraction with TBP. According
to Valkov (2014), during processing of apatite, a hydrate-
phosphate precipitate of REMs was obtained, which was dis-
solved in nitric acid to get a solution containing 40–60 g/dm
. In this case, calcium nitrate melt was added in solu-
tion to obtain a concentration of 800–1000 g/dm
Subsequently, the solution was directed for solvent extraction
In order to study the effect of calcium salt in nitrate solution
on REMs solvent extraction with TBP, experiments were carried
out to dissolve the REMs-containing hydrate precipitate together
with calcium oxide in nitric acid. From the resulting solutions,
REMs were recovered by solvent extraction with TBP.
Experiments for dissolution of hydrate precipitate were
carried out under the following conditions: temperature –
60°C; concentration of HNO
–8 M; S:L = 1:5; duration –
1 h; n = 500 rpm. The composition of the hydrate precipitate
was: 29.3% Al
; 20.8% Fe
; 0.1927% ∑REMs oxides.
During dissolution, 5%–20% of CaO from the total amount
of the solid phase was added. The composition of the solu-
tions obtained is shown in Table 8.
In connection with an increase in the proportion of cal-
cium oxide and a decrease in the fraction of the hydrated
precipitate, the concentration of calcium in the solution
increased, and the concentrations of aluminum, iron and
Experiments on the solvent extraction of ΣREMs from the
resulting solutions were carried out under the following con-
ditions: concentration of TBP 3.67 M (100%); ratio of O:
A = 1:2 and 1:8; duration –5 min; the temperature was
20 ± 5°C. The results of the studies related to the effect of
Table 7. The effect of the HNO
initial concentration on the solvent extraction of rare earth metals by tributyl phosphate during the dissolution of the hydrate
concentration for precipitate
∑REMs concentration in the
∑REMs recovery into the organic
5 1.54 0.56 11.7 7.0 93.3
6 1.19 0.40 19.4 2.8 85.0
7 1.23 0.44 19.7 2.9 85.2
8 1.53 0.52 16.2 4.8 90.5
16 Z. KARSHIGINA ET AL.
metal nitrates concentration in initial aqueous phase on REMs
recovery into the organic phase are shown in Figure 12.
With an increase in the concentration of calcium nitrate
from 0.08 to 0.43 M in the initial aqueous solution, REMs
recovery into the organic phase decreased from 62.8% to
54.5% with O:A = 1:2 and from 24% to 17.4% with O:
A = 1:8. It should be noted that at the same time concentra-
tions of aluminum and iron salts decreased, which could have
also affected the solvent extraction of REMs. The ability
toward hydration of REMs ions, aluminum, iron and calcium
To evaluate the process of hydration of ions, models of M.
Born, K.P. Mishchenko, A.M. Sukhotin and others exist. When
calculating according to the equation proposed by K.P.
Mishchenko, it is assumed that the dominant role for the change
of enthalpy of ions during hydration is played by the ion-dipole
interaction (Voldman and Zelikman 2003):
ΔHhydration ¼ NAnzeμ
–Avogadro number; n –number of water molecules
in the layer nearest to the ion; z –the value of ion charge
(without regard to its sign); e –electron charge; μ–dipole
moment of water; ε
–electric constant; r
–ion radius; r
the effective radius of water molecule, assumed to be 0.193 nm; β
–correction associated with asymmetry of position of dipole
moment in water molecule: the distance between the centers of
dipole and cation is larger, while dipole and anion is smaller than
the sum r
by the value β= 0.025 nm.
aluminum, iron, and calcium ions calculated by
Mishchenko method are given in Table 9.Table 9 also
shows the Gibbs energy of hydration of ions calculated by
Table 8. Composition of solutions obtained after dissolution of the hydrate
precipitate together with calcium oxide.
∑REMs, mg/L Al
, g/L Fe
, g/L CaO, g/L
233.0 41.1 31.9 4.6
223.2 38.0 28.5 9.1
209.5 33.5 27.9 16.3
177.8 28.6 22.3 24.3
54 56 58 60 62 64
Metal nitrate concentration in solution, M
REMs recovery in organic phase (%)
Al(NO3)3 Fe(NO3)3 Ca(NO3)2
17 18 19 20 21 22 23 24 25
Metal nitrate concentration in solution, M
REMs recovery in organic phase (%)
Al(NO3)3 Fe(NO3)3 Ca(NO3)2
Figure 12. Influence of concentration of aluminum, iron and calcium nitrates on REMs recovery into the organic phase: (a) ratio of O:A = 1:2; (b) ratio of O:A = 1:8.
MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 17
Table 9 shows that the hydration of REMs cations is higher
than that of calcium and lower than that of aluminum and
iron. That is, hydration of REMs compounds is more easily
suppressed in presence of aluminum and iron cations. This, in
turn, promotes the formation of extractable REMs com-
pounds –RE (NO
Hydration is the formation of an ionic solution of certain
composition from ions in the gaseous state and water. The
hydration reaction of ions can be represented in the following
Since the Gibbs energies for the hydration of aluminum and
iron cations have more negative values as compared to REMs,
the reactions (12) and (13) were thermodynamically more
probable than the hydration reaction of REMs (14). With an
increase in concentration of aluminum and iron nitrates in
solution and with hydration of their salt ions, concentration
of unbound water decreased. This might have led to suppres-
sion of reaction (14) and its realization in the opposite direc-
tion with dehydration of REMs ions:
Excess nitric acid or increased concentration of nitrate ion in
the aqueous phase contributed to an increase in the distribu-
tion coefficient D, suppression of REMs nitrates dissociation
and formation of the extractable compound:
3þ3TBP ¼RE NO3
According to Valkov (2014), if calcium nitrate is used as a
salting out agent, in order to carry out effective solvent
extraction of REMs with TBP, a much larger amount of salt-
ing out agent would be required to ensure the binding of a
sufficiently large number of water molecules.
Based on the results of the research, a technological scheme
for processing ore from Kundybay deposit is shown
According to the scheme, the ore of Kundybay deposit was
decomposed by sulfuric acid at 200°C. Subsequently, the sulfate
sinter was leached with water to obtain a REMs-containing
sulfate solution and a silicon-containing cake. From the resulting
sulfate solution, REMs were precipitated with sodium hydroxide
as part of a hydrated precipitate. The precipitate was dissolved in
nitric acid. The resulting solution, containing REMs nitrates, was
processed by a TBP solvent extraction method. During the
solvent extraction, aluminum and iron remained in the raffinate.
From the resulting loaded organic phase, it was possible to strip
the REMs into an aqueous solution.
According to the results of the conducted study, it was estab-
lished that the ore of Kundybay deposit was represented by
; kaolinite Al
, and also montmorillonite Na
O; clinochlorine (Mg,Fe)
goethite FeO(OH). Analysis using a SEM showed that REMs
were present mainly in the form of phosphates and, possibly,
in small amounts in the form of carbonates. Rare earths were
associated with clay minerals, as well as occurred in close
intergrowth with muscovite, quartz, oxides of iron and tita-
nium, both on the surface and inside the mineral grains.
When high-temperature treatment of the ore was per-
formed in a mixture with sulfuric acid at 200°C and further
water leaching was carried out for the obtained sulfate sinter,
the REMs recovery into the solution was~84%. After the
precipitation of REMs-containing precipitate from the sulfate
solution, the influence of nitric acid concentration (in the
range of 4–8M HNO
) on its dissolution was studied, with
8M being concluded as the most optimal concentration. The
results related to solvent extraction of REMs from nitric acid
solutions (with a low concentration of rare earths and a high
content of macro-components of aluminum and iron) using
TBP showed that rare earths can be effectively extracted into
the loaded organic phase. The presence of macro-quantities of
aluminum and iron did not interfere, but on the contrary, had
a salting out effect and even facilitated the solvent extraction
of REMs into the organic phase. Replacement of some part of
aluminum and iron to calcium in nitrate REMs-containing
solutions did not improve the solvent extraction of REMs
with TBP and on the contrary, lowered its results.
List of symbols
∑REMs –total rare-earth metals
∑REOs - the sum of rare earth metal oxides
S:L –solid-to-liquid ratio
rpm –revolutions per minute
LOI –loss on ignition
O:A –ratio of volumes of organic and aqueous phases
SEM –Scanning Electron Microscope
EDS–energy dispersive X-ray spectroscopy
WDS –Wave-dispersive spectroscopy
●Based on the results of XRD analysis, the ore from the
Kundybay deposit mainly consists of muscovite KAl
, kaolinite Al
●When high-silicon REMs-containing ore was processed
with sulfuric acid at 200°C and subsequent water leaching
Table 9. Enthalpy ΔHo
hydration and Gibbs energy ΔGo
hydration of ions hydration in
infinitely dilute solutions at 25°C (Voldman and Zelikman 2003).
hydration, kJ/mol ΔGo
18 Z. KARSHIGINA ET AL.
of the sulphate product, REMs was extracted into the solu-
tion to ~84%.
●Small amounts rare-earth metals can be separated from
the related macroimpurities of aluminium and iron by
tributyl phosphate solvent extraction.
●Macrocomponents of aluminum and iron salts present
in the aqueous phase promote the extraction of the
REMs into the loaded organic phase, being salting out.
●The silicon containing cake can be suitable for the
production of precipitated silicon dioxide.
The study has been completed by means of the grant №1524/GF4
received from the Ministry of Education and Science of the Republic
of Kazakhstan. This collaborative research was based on the results
of bilateral contacts of the Institute of Metallurgy and Ore
Benefication, Almaty, Kazakhstan and the Mineral-Metal Recovery
and Recycling Research Group, Mineral Processing Division,
Department of Mining Engineering, Suleyman Demirel University,
The authors report no conflicts of interest. The authors alone are
responsible for the content and writing of the article.
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Kundybay depositore (-0.1 mm)
SULPHATIZATION (200°C; S:L=1:0.36; 2 h)
Sinter (0.5-1 mm)
LEACHING (90°C;S:L=1:2.5; 4 h)
REMs- containingsolution: 99 mg/L REMs;
25.7 g/L Al2O3;
9.4 g/L Fe2O3
PRECIPITATION (50°C; until pH 7-8; 2 h)
Cake After flushing
Precipitate: 0.1-0.2 % ∑REMs oxides;
29-33 % Al2O3; 18-21 % Fe2O3
8 M HNO3
For the production
DISSOLUTION (60°C;S:L=1:5; 1 h)
100 % TBP
SOLVENT EXTRACTION (O:A=2:1)
Raffinate: 16.2 mg/L ∑REMs;
78.1 g/L Al2O3;
41.3 g/L Fe2O3
On stripping and obtaining
of REMs concentrate
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