minerals

Review
Kenya’s Mineral Landscape: A Review of the Mining Status and
Potential Recovery of Strategic and Critical Metals through
Hydrometallurgical and Flotation Techniques
Nelson R. Kiprono * , Tomasz Smoliński , Marcin Rogowski, Irena Herdzik-Koniecko , Marcin Sudlitz
and Andrzej G. Chmielewski *

Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland;
t.smolinski@ichtj.waw.pl (T.S.); m.rogowski@ichtj.waw.pl (M.R.); i.herdzik@ichtj.waw.pl (I.H.-K.);
m.sudlitz@ichtj.waw.pl (M.S.)
* Correspondence: n.kiprono@ichtj.waw.pl (N.R.K.); a.chmielewski@ichtj.waw.pl (A.G.C.);
Tel.: +48-50-398-3882 (N.R.K.); +48-22-504-1205 (A.G.C.)

Abstract: Kenya is an East African country with the third-largest economy in sub-Saharan Africa. The
demand for metals and minerals continues to increase due to urbanization, population rise, and new
infrastructure growth in different countries. Kenya formally confirmed the discovery of oil and vari-
ous minerals in April 2013, launching itself as a new player in Africa’s rapidly expanding extractive
sector. This review paper highlights the mining status in Kenya and the role of hydrometallurgical
and flotation processes in the recovery of deficit metals from ores and mine wastes. The nation’s
2030 Vision is anticipated to benefit greatly from the proceeds from the sale of oil, gas, and valuable
metals. Because Kenya was originally mapped as an agricultural region, less mineral prospecting
was done in earlier times. The country’s mining industry is now dominated by the manufacture of
non-metallic goods, and it is largely neglected for minerals. One of the most serious problems for
the mining industry in Kenya is the production of tailings that hold strategic metals. The material is
Citation: Kiprono, N.R.; Smoliński, T.; already ground, which means the most energy-consuming process has been already applied, and
Rogowski, M.; Herdzik-Koniecko, I.; chemical engineering processes like leaching are more feasible at this point. Hydrometallurgical and
Sudlitz, M.; Chmielewski, A.G. flotation recovery of valuable metals from wastes, high and low-grade ores, or tailings is essential.
Kenya’s Mineral Landscape: A Review
The resources will be preserved, which ensures sustainability in the growth of the mining industry.
of the Mining Status and Potential
Recovery of Strategic and Critical
Keywords: mining; ores; tailings; hydrometallurgy; recovery; strategic metals; critical metals; flotation
Metals through Hydrometallurgical and
Flotation Techniques. Minerals 2024, 14,
21. https://doi.org/10.3390/
min14010021
1. Introduction
Academic Editors: Marinela Ivanova
Mining is the practice of extracting valuable minerals or other geological materials
Panayotova and Vladko Panayotov
from the Earth’s crust, for economic development. These materials can be found in seams,
Received: 27 October 2023 reefs, lodes, orebodies, etc. Sustainable growth satisfies existing demands without jeop-
Revised: 18 December 2023 ardizing the capacity of future generations to meet their needs. Justifiable development
Accepted: 22 December 2023 must be implemented in all human pursuits, and this requires responsible mining. It is
Published: 24 December 2023 no longer possible to extract raw materials just based on economic considerations; social
and environmental factors must also be taken into account. As a result, minimal environ-
mental effects, human health safety, full resource utilization, and waste-free mines should
all be goals of good practice standards for mining and processing mineral resources [1].
Copyright: © 2023 by the authors.
Mineral-rich nations make use of these resources to shift their economy in the direction
Licensee MDPI, Basel, Switzerland.
This article is an open access article
of sustainable development. Because of their abundant mineral resources, countries like
distributed under the terms and
South Africa, Australia, the United States, China, and Ghana, among others, are now
conditions of the Creative Commons industrialized [2]. The mining industry is anticipated to be the main engine of industrial
Attribution (CC BY) license (https:// expansion in low-income countries, particularly in Africa. These African nations include,
creativecommons.org/licenses/by/ among others, Kenya, Ghana, South Africa, Mali, and the D.R.C. (Democratic Republic of
4.0/). the Congo). Initiatives such as the Africa Mining Vision (AMV), a “pathway” developed by

Minerals 2024, 14, 21. https://doi.org/10.3390/min14010021 https://www.mdpi.com/journal/minerals

Minerals 2024, 14, 21 2 of 27

African states themselves, place the continent’s long-term and wide development goals at
the center of all policy formulation related to mineral exploitation. It was started to aid in
the economic transformation of Africa’s mineral-rich regions [3].
Since the Industrial Revolution, the use of energy resources (primarily fossil fuels)
has increased dramatically on a global scale [4]. In the coming decades, it is projected
that the global energy sector will experience a continuous shift towards renewable energy
alternatives [5]. With an expected rise of electricity demand of 150% from the year 2010 to
2050, renewable energy solutions, including photovoltaic power, hydroelectric power, and
wind power, are going to play a significant part in satisfying future global demands [6].
The need for key metals will increase substantially as a result of the broad acceptance of
technologies with low carbon footprints [5]. As millions of humans adopt contemporary
ways of life, the demand for vital metals is, in fact, on the rise [7]. Still, the loss of minerals
required for renewable energy technologies could make it more difficult to move from
a fossil fuel-based to eco-friendly substitutes. Kenya is one of the ideal places that can
contribute strategic and critical metals to the global economic chain. Therefore, this study
reviews Kenya’s present situation concerning strategic and crucial metals and discusses
techniques for recovering them through hydrometallurgical processes. The article also, in
its last section, introduces the concept of strategic and critical metal recovery using flotation
techniques. Proposals and prospects for the future application of the described technologies
in Kenya have also been given.

Strategic and Critical Metals

Strategic metals are resources required for vital use in times of crisis, but their ac-
quisition is uncertain in terms of quality, quantity, and timing and any justification for
their supply would require advanced planning [8]. When a metal is essential to the state’s
economic strategy, which includes its defense, energy, and security strategies, it is also said
to be strategic. However, metal could be seen as strategic for a certain business or set of
industries, including those in the automobile, renewable energy, aerospace, nuclear, ICT
(Information and Communication Technologies), and electronics sectors, and so on. Each
region has specific types of metal that have been determined to be crucial and strategically
important for their economies. Kenya classified Ti, gas, oil, Cu, diamond, Au, Nb, gypsum,
sand, and fluorspar, amongst others, as strategic and critical materials for Vision 2030 [8].
However, a region like the United Kingdom outlined Sb, Be, Cr, Au, Hf, In, Co, Ga, Ge,
platinum group metals (Pt, Ir, Pd, Os, Rh, Ru, Re), REEs (Rare Earth Elements), Ta, Ti, W, Li,
Mg, Ni, Nb, and V as strategic and critical materials.
Critical metals are those that are expensive, difficult to substitute for, geologically
rare, prone to potential supply constraints, and required for an economically significant
drive. These metals are significant due to their unique features. The possibility of supply
restrictions seems to be the most important aspect of these characteristics. Unlike in the
1950s, almost every metal in the periodic table is needed currently to produce different
products for society, Figure 1. A good percentage of these crucial metals were disregarded
and treated as mine waste fifty to sixty years ago [9].
 Minerals2023,
Minerals 14,x21
2024,13, FOR PEER REVIEW 3 of2827
3 of

Figure 1. A periodic table displaying the aspects of various types of metals and their economic status [10].
Figure 1. A periodic table displaying the aspects of various types of metals and their economic status [10].

Metals
MetalssuchsuchasasREEs,
REEs,Ge, Ge,Ta,
Ta,Se,Se,Sn,
Sn,In,
In,Ga,
Ga,andandTe Teare
aregeologically
geologicallysparse,
sparse,though
though
crucial
crucial [11]. Relatively low atomic weight La, Ce, Pr, Nd, and Pm are categorizedasaslight
[11]. Relatively low atomic weight La, Ce, Pr, Nd, and Pm are categorized light
REEs
REEs(LREEs)
(LREEs)while whileSm,
Sm,Tm,
Tm,Yb, Yb,Eu,Eu,Dy,Dy,Ho,
Ho,Gd,Gd,Tb,Tb,Er,
Er,and
andLuLuare
areclassified
classifiedasasheavy
heavy
REEs
REEs(HREEs) because ofoftheir
(HREEs) because theirhigh
high atomic
atomic weight.
weight. SinceSince
they they are widespread,
are widespread, even
even though
though it is rare to locate them in quantities large enough to permit
it is rare to locate them in quantities large enough to permit the production of profitable the production of
profitable minerals, these elements are referred to as REEs [12]. Due to their
minerals, these elements are referred to as REEs [12]. Due to their occurrence in ore deposits occurrence in
ore deposits comparable to those of lanthanides and exhibiting the
comparable to those of lanthanides and exhibiting the same chemical properties, Y and Sc same chemical prop-
erties, Y and Sc
are likewise are likewise
classified as REEsclassified
[13,14].as REEs [13,14].
When
Whencontrasted
contrastedtotothe theother
otherelements
elementsininthe theperiodic
periodictable,
table,REEs
REEsexhibit
exhibitdistinct
distinct
magnetic,
magnetic,electrical,
electrical,optical,
optical,luminescent,
luminescent,and andchemical
chemicalproperties
properties[15].
[15].Considering
Consideringthat that
the
themajority
majority of of REEs similar oxidation
REEs have similar oxidationstates
statesand
andatomic
atomic radii,
radii, they
they cancan substitute
substitute one
one another in a variety of crystal lattices. Due to this substitutional capacity,
another in a variety of crystal lattices. Due to this substitutional capacity, numerous REE numerous
REE occurrences
occurrences can can be found
be found in a in a single
single mineral,
mineral, whichwhich
leadsleads to a widespread
to a widespread distribu-of
distribution
REEs
tion of in
REEsthe in
Earth’s crust. crust.
the Earth’s They can They becan
found in a variety
be found of mineral
in a variety forms,forms,
of mineral including oxides,
including
carbonates,
oxides, phosphates,
carbonates, silicates,
phosphates, and halides.
silicates, Generally,
and halides. REEs are
Generally, REEshighareonhigh
the on
priority list
the pri-
of essential
ority metals in
list of essential manyinnations
metals many where
nationsthese
where metals
theseare in short
metals supply
are in short[13].
supply [13].

2.2.Mining
MiningSituation
SituationininKenya
Kenya
Kenya,located
Kenya, locatedininthe
theeastern
easternpart
partofofAfrica,
Africa,isisrichly
richlyendowed
endowedwithwithdifferent
differenttypes
typesofof
mineral resources. The Kenyan government assessed its mineral potential
mineral resources. The Kenyan government assessed its mineral potential and produced and produced
aaworking
workingdocument
document in in 1999,
1999, andand over
over 400 400 mineral
mineral occurrences
occurrences werewere identified.
identified. The
The gov-
government introduced regulations to drive this sector [16]. Vision 2030
ernment introduced regulations to drive this sector [16]. Vision 2030 recognizes the such recognizes the
such industry as one of the key drivers of the country’s economic development.
industry as one of the key drivers of the country’s economic development. Growth in this Growth
in this is
sphere sphere is expected
expected to increase
to increase the number
the number of jobs,ofthejobs, the income
income of theofworkers,
the workers, and
and the
the country’s GDP (Gross Domestic Product). The 2010 constitution created Counties and
country’s GDP (Gross Domestic Product). The 2010 constitution created Counties and
gave them the capacity to establish and carry out their development strategies. This new
gave them the capacity to establish and carry out their development strategies. This new
dispensation is forcing Counties to assess their resources and exploit them for their local
dispensation is forcing Counties to assess their resources and exploit them for their local
development. The resources vary from County to County and are highly dependent on
development. The resources vary from County to County and are highly dependent on
the topography, drainage, ecology, and climatic conditions of the County. The worldwide
the topography, drainage, ecology, and climatic conditions of the County. The worldwide
demand for minerals is projected to progressively rise at a mean rate of approximately
demand for minerals is projected to progressively rise at a mean rate of approximately 3%
Minerals 2024, 14, 21 4 of 27

3% annually [17]. That is likely to trigger more exploration and mining activities in the
resource-endowed regions of the country.
Because Kenya was originally mapped as an agricultural region, less mineral prospect-
ing was done in earlier times. The country’s mining industry is now dominated by the
manufacture of non-metallic goods, and it is largely neglected for minerals. For instance,
Kenya is the world’s third-largest supplier of soda ash and ranks seventh in terms of
fluorspar production. Additionally, Kenya formally confirmed the discovery of oil and
various minerals in April 2013, launching itself as a new player in Africa’s rapidly expand-
ing extractive sector [18]. The non-renewable resources mentioned above are, regrettably,
mainly found in places with high rates of poverty, ongoing droughts, instability, and
long-term marginalization by the government. Conflicts between local communities, inter-
national corporations, and governmental organizations are consequently high [19].
Fe ore, Ti, and Au are amongst the metallic minerals now produced in the country.
Kenya’s export data show that this industry is constantly expanding. Along with industrial
minerals like talc, gypsum, dolomite, and gemstones, Kenya’s mineral suite also contains
resources like Ag, Cu, and Zn, REEs, coal, limestone, soda ash, Mn ore, fluorspar (CaF2 ),
diatomite, chromite, Nb, and silica sand. Natural fluorspar (CaF2 ), largely mined in
Kenya, is typically associated with other minerals such as phosphates (PO4 3− ), quartz
(SiO2 ), celestite (SrSO4 ), galena (PbS), calcite (CaCO3 ), barite (BaSO4 ), and chalcopyrite
(CuFeS2 ) [20]. The most valuable REE resources in the world are also located in the coastal
region (Jombo Complex) at Mrima Hills, where Cortec Mining Kenya Limited estimated
that they could have an in-ground value of up to USD 62.4 billion [19,21,22]. Mrima Hill
is home to a carbonatite cluster. The laterite capping resulting from the weathering of the
hill’s igneous constituents has economic grades of REE and Nb mineralization [23].
In terms of Counties, Turkana County has oil, gypsum, graphite, Zr, Nb, Ba, Pb, Fe,
Te, Ag, Mn, Cu, and Ni ores, whereas Taita Taveta principally possesses Fe, Mn, Cu, and
quartz. While Kwale County is abundant in Zn, Pb, Cu, U, Ti, REEs, Nb, and oil, Kitui
County has significant amounts of Fe, coal, Cu, limestone, amethyst, and sapphire. Lamu
County is rich in Ti, gas, and oil, whereas Migori County has commercially exploitable
deposits of Zn, diamond, Au, Al, Fe, and Cu. Au generation in Migori and other regions
is a significant driver of the nation’s economic development and growth, contributing
approximately USD 10.3 million/year to the country’s economy [24]. Transmara and
Migori districts began to mine Au in the year 1930. However, industrial mining became
significant recently in the country due to the discovery of Au-bearing reefs in the Lolgorien
region. The local population and various enterprises that perform mining operations in
the area now rely heavily on these activities as one of their main sources of income [25].
Figure 2 shows some of the minerals that have been confirmed to be available for economic
purposes in Kenya.
Minerals 2023,
Minerals 2024, 14,
13, 21
x FOR PEER REVIEW 5 5of
of 28
27

Figure 2. Maps showing mineral occurrence in Kenya [26].

Figure 2. Maps showing mineral occurrence in Kenya [26].

For
For the
the nation
nation toto achieve
achieve the the objectives
objectives of
of Vision
Vision 2030,
2030, the
the Fe
Fe and
and steel
steel industry
industry will
will
play
play a significant role in the industrialization process [27]. Fe minerals are widely present
a significant role in the industrialization process [27]. Fe minerals are widely present
in
in western
western Kenya,
Kenya, especially
especially in in Nyanza.
Nyanza. Furthermore,
Furthermore, there
there are
are untapped
untapped reserves
reserves of
of Fe
Fe
ore in various
ore in various Kenyan
Kenyan Counties,
Counties, including
including Taita
Taita Taveta
Taveta (Manyatta),
(Manyatta), Siaya
Siaya (Samia),
(Samia), and
and
Taraka (Marimanti).Although
Taraka (Marimanti). Althoughcomplicated,
complicated, thethe process
process of making
of making steelsteel essentially
essentially in-
involves
volves melting Fe ore in a blast furnace along with coke. In some rural
melting Fe ore in a blast furnace along with coke. In some rural areas of Kenya and the areas of Kenya and
the neighbouring
neighbouring nations,
nations, the manufacturing
the manufacturing of sponge
of sponge Fe has
Fe has been been accomplished
accomplished by
by smelting
smelting
Fe ore usingFe ore usingand
charcoal charcoal
a forced andairadraught
forced air draught
from bellows from
madebellows made The
of goatskin. of goatskin.
majority
The
of themajority of the
ores were firstores wereand
crushed firstsent
crushed andtosent
straight straight
a blast to aespecially
furnace, blast furnace,
thoseespecially
with 50%
those
or morewith
Fe 50% or more
without Fe without
beneficiation. To beneficiation.
increase the FeTo increase
content andthe
getFethe
content and get
concentrate the
ready
concentrate ready for
for the blast furnace, thethe blast of
majority furnace, theores
extracted majority
nowadaysof extracted
go through ores nowadays[27].
beneficiation go
through beneficiation [27].
Minerals 2024, 14, 21 6 of 27

Some of the platinum group metals have also been reported in Kenya. In 14 petro-
logically distinct spinel peridotite xenoliths found in the Marsabit volcanic zone (Kenya
Rift), platinum group metals were identified [28]. However, their concentrations are very
low. Marsabit’s xenoliths have a total platinum group metals concentration that ranges
from 14 to 32 ppb. Unfortunately, such concentrations are too low to be recovered for
economic purposes.
Understanding the principles influencing element distribution in the Earth’s crust is
important. In particular, the role played by rocks and minerals in the uptake of certain met-
als from groundwaters requires comprehension of the movement and fixation of elements.
Scientists have researched the compositions of several hot water springs, related minerals,
rocks, and fossil bones from Kanjera and Kanam near Lake Victoria in Kenya. The location
selected for the study was ideal due to the presence of volcanic hot springs and sedimentary
deposits connected to Homa Mountain’s carbonatite complex, which is comparatively rich
in REEs [29]. The findings provided evidence of the absorption and mobility of REEs.
Bicarbonate and fluoride were found to be abundant in the spring waters of both locations,
and Kanjera’s Zr concentration was exceedingly high. All examined rock samples had
higher REE contents than chondrites. The rocks further showed higher concentrations
of light REEs (Nd, Sm, La, Ce, Eu, and Gd) than heavy REEs (Tb, Yb, Lu, Ho, and Tm).
The travertine specimen seemed to be moderately discoloured by Mn and/or Fe2 O3 or
hydroxide deposits. The acid-insoluble component of the travertine (5.08% by weight)
included less than 11% of each REE but more than 60% of the aggregate concentration of
some elements, including Cr, Hf, and Ta. The REEs, Th, and U were generally present in
fairly large quantities in the carbonate (acid-soluble) fraction. It was notable because such
levels were precipitated from percolating ground water. Contrarily, the trona sample only
contained detectable levels of the following elements: Ce, Sm, Sc, Cr Eu, Co, and Th. It
was concluded that groundwater–rock interaction has a significant impact on rock and
mineral composition [29].

2.1. Environmental Impacts of Mining Activities

Mining in Kenya and Africa at large is considered one of the main pillars of socio-
economic benefits. However, the same industry is known to be amongst the main sources of
pollutants. The influence of mining on environmental contamination can be both geogenic
(related to parent rocks) and anthropogenic, encompassing man-made activities such as
chemical use [30]. The discovery of mines and their exploitation was solely determined
by the economic aspect when the exploitation of natural resources began many years
ago. However, tailings production is one of the major issues facing the mining industry.
Tailings encompass the remains of the fine-grained (1 to 600 µm) ground rock after the
metals of value have been removed from the mined ore [31,32]. Such materials are very
reactive, attributable to their minute particle size, with the variation of their chemical
and physical properties depending on the ore type. Most of the tailings result during the
beneficiation process.
Nearly all the heavy metals in the ores exist as compounds with sulphide in their
structures. That contributes to the acid mine drainage (AMD) in the region, responsible
for the death of several fish in places like Lake Victoria and the surrounding rivers. AMD
is produced when sulphide-containing minerals such as pyrite are exposed to water and
oxygen, where oxidation of the minerals results in the generation of hazardous H2 SO4 acid.
AMD is one of the serious environmental challenges associated with the discharge of acidic,
metal, and sulphate-containing industrial wastewater into the surroundings [33].
Additionally, wastes produced by mining activities contain elevated concentrations of
metalloids and metals. The elements can be mobilized, infiltrating into the surface water
and groundwater [34]. A significant fraction of such elements in industrial wastes are non-
biodegradable and highly toxic. Chemical contamination occurs when the reagents added
by the mineral processing industries are released. For instance, cyanide or Hg for extracting
Au or H2 SO4 acids used in leaching Cu oxides can be supplemented under controllable
Minerals 2024, 14, 21 7 of 27

conditions but may leak into the environment if not properly handled. Alternatively,
chemical pollution can occur when naturally occurring minerals are oxidized [35]. This
is mainly predominant when sulphide minerals are involved while extracting Au. Other
metals released during artisanal Au mining in regions like Migori and related industries
include Cu, Cd, Ni, Zn, Pb, and As, all discharged into soil and water bodies [24].
Kenya is one of the countries in Africa that aims to intensify the extraction of REEs and
other metals from various sources. The intricate process of mining resources such as REEs
may result in negative effects on the environment and human health. The effects of mining
on the environment, nevertheless, rarely receive adequate attention in the majority of
developing nations. Deforestation, water contamination, soil erosion, changes in landscape
structure, and the destruction of wildlife habitats are a few of the environmental effects of
recovering REEs from ore. The effects of REEs are influenced by the type of rock around its
deposit, the availability of additional metals or substances within the rocks, the climate,
and the distance from streams and lakes to the mine.
Since agricultural land is lost due to the expansion of the mining sites, vegetation in
mining areas is also destroyed during ore extraction [36]. Moreover, mine closure results in
numerous wastes and the degradation of the land once minerals have been extracted to a
considerable extent and the remaining ones are no longer commercially viable for extraction.
Information for mitigating potentially hazardous environmental effects following the
closure of mining activities and preventing particulate matter emissions from mines should
be made available. This is critical, along with the responsible use and disposal of metal-
bearing technology devices [36]. Furthermore, treatment of tailings and the repurposing of
them will be crucial.

2.2. The Presence of Critical and Strategic Metals in the Mine Wastes
Valuable metals can be economically recovered from the tailings produced duringmin-
ing operations. When selecting the tailings for use in an alternate application, consideration
must be granted to the tailings’ physical and chemical characteristics [37]. In the dominant
geological circumstances within which they are found, the minerals that make up econom-
ically valuable ores are largely stable. Their chemical stability decreases when they are
exposed to the atmosphere. That explains the high reactivity of tailings. Despite containing
94% of the processed ore, these wastes still contain valuable metals [38]. Thus, from the per-
spective of protecting the environment and preserving raw materials, continued treatment
of these wastes is possible. Additionally, the material has already been pulverized, consum-
ing the high amount of energy in the process; hence, chemical engineering procedures such
as leaching become subsequently more practical at this stage [39].
Mine waste deposits are increasingly becoming dependable sources of critical and
strategic metals, though some of them could be regarded as low-grade ores based on the
current standards [40]. Closing the consumer cycles through the liberation of these metals
from complex and dilute waste streams will become significantly viable economically [41].
Access to mine deposits is easy because they are localized at the surface without consid-
erable overburden. The mineralogical composition of the tailings can vary depending on
the ores containing the metals of interest. Discarded tailings have been found to contain
significant amounts of critical metals such as Ga in bauxite residues or Ge or In in the
residues of Zn ores, among others. The processing of the tailings from the ponds and
waste-rock heaps of the Penouta mine in Spain led to the recovery of Ta and Nb [42].
However, the crucial influencing factors include the quantities or volumes of the critical
raw materials, their concentrations, quality of tailings, and the composition of the minerals
in the extractive wastes.
There have been reports of tailings in Kenyan mining industries, including the short-
lived Au mine in the Migori district. Both open-cut and subsurface mining techniques were
used. After that, the ore was pulverized and panned, leading to huge heaps of tailings
and waste rocks. Several elements like Ti, Cr, Rb, Sr, Y, Mn, Zn, As, Au, Pb, Fe, Co, Cu, Zr,
and Nb exist in various quantities in the Migori region [25,43]. Such minerals are crucial,
Minerals 2024, 14, 21 8 of 27

and they can be reprocessed for economic gain. For instance, through the application of
magnetic separation methods, valuable TiO2 can be extracted from the waste [44].
Western Kenya is another region endowed with minerals such as Au, which was
exploited by the British firm known as Rosterman Au Mine industries in the 1930s before
they exited the site in 1952. Since the firm’s exit, artisan-based miners have been extracting
Au deposits from the tailings of the abandoned mines. Alluvial mining is one of the
mining techniques used by artisan miners, which encompasses the excavation of the ore
from the slow-moving parts of River Isiukhu [45]. The tailings and the gravel of the
ore obtained are transported to appropriate locations for processing through panning or
sluicing. In the majority of the mining sites, the sluiced tailings are scavenged numerous
times before they are ultimately disposed of, to achieve recovery of the Au deposits lost in
the previous operations [32].

3. Recovery of the Strategic and Critical Metals from Various Sources

The numerous minerals that are already present in Kenya can be recovered using
a variety of techniques devised by academics from around the world. For the benefit
of the nation, Kenyan researchers and the mining sector can adopt and improve such
techniques. Compared to pyrometallurgical methods (roasting, smelting, calcining, and
thermal refining of the minerals), hydrometallurgical technologies (leaching, solvent extrac-
tion, precipitation, and ion exchange amongst others) offer a greater chance of efficiently
recovering very low concentrations of metals with less energy use. Pyrometallurgy has
been linked to low metal recoveries, increased air pollution, and huge consumption of
energy. Consequently, our review article places a strong emphasis on hydrometallurgy
because it is a better alternative.

3.1. Leaching
Leaching is one of the crucial steps in the recovery of metals from different materials.
However, the process is non-selective; hence, there is a necessity to recover metals from
the pregnant solutions using other selective methods like solvent extraction, ion exchange
precipitation, etc. The reagents used in the leaching process vary. For extracting base metals
such as Cu, chemicals like inorganic acids can be used [46]. These agents are commonly used
with oxidants, which enhances the efficiency of the process. For precious metals such as
Au, the reagents used are halogens, cyanide, and thiocyanate. Cyanide is the most excellent
in the reclamation of precious metals from both secondary and mineral sources [47].
There are various types of leaching approaches, including bioleaching and chemical
leaching. Bioleaching is the biological solubilization of metals from wastes and minerals.
That means micro-organisms (for example, Acidithiobacillus ferrooxidans and Acidithiobacillus
thiooxidans) converting solid, insoluble metals and their compounds into extractable forms.
Nonetheless, chemical leaching involves the use of bases/acids, chelating agents, surfac-
tants, redox agents, or salts to liberate the metal from the solid phase to the aqueous phase.
Both organic and inorganic acids can be utilized for chemical leaching. EDTA (Ethylenedi-
aminetetraacetic acid) is one of the common chelating agents for various metals.However,
NaClO and H2 O2 are the prominent oxidizing agents employed to improve the recovery of
the metals from the solid phases [47].
Several technological methods use chemical leaching to process ores (in situ, heap,
and dump leaching). The in situ approach involves the application of a lixiviant (H2 SO4 ,
thiosulphates, thiourea, and cyanides) which is pumped directly into the channels in the
rocks or inactive mines. After some time, the resulting pregnant leach solution is pumped
to the surface and then taken for post-processing for metal recovery. The technique can be
used in the recovery of U and Cu [48]. Contrarily, in heap leaching, the piles are placed in
the open air on a previously prepared impermeable pad (plastic, clay). The largest grains
are at the base of the pile and the most pulverized at the top.The particle sizes of typical
ores range from 10 to 100 mm. The leaching agent is sprinkled on the upper surface of the
layer and percolates through it. An aqueous solution with dissolved metals is received
Minerals 2024, 14, 21 9 of 27

through the collecting channel [48]. Similar to this method is dump leaching, except that
the ore is taken directly from the mine and stacked without crushing. The method is used
for recovering Cu and precious metals and can be applied in extracting low-grade ores [49].

3.2. Solvent Extraction of the Common Transition Metals and Uranium

Given its capacity to cope with enormous quantities of diluted pregnant liquor of
aqueous solution, solvent extraction is a widely recognized hydrometallurgical procedure.
The technique is frequently used in the recovery of leached d-block, REEs, and precious
metals using extractants. The extractant is an organic ligand that is intended to coordinate
with the cations in a targeted manner [50]. This procedure is mostly accomplished by
vigorously agitating the two immiscible media, enabling the solute(s) to be transported
from one phase to the other in a controlled manner. Normally, there are two phases involved:
an organic phase, having the extractant dissolved in a suitable hydrocarbon diluent; and
an acidic aqueous medium, having the target metal ion(s) [51]. The pure extractant is
occasionally employed as an organic solvent. However, because of its high viscosity, pure
extractants are typically not used. For high product purity and large-scale manufacturing,
solvent extraction is a more practical technology that has gained widespread acceptance.
The above technique is incredibly practical in hydrometallurgy since it does not require
specialized equipment, and it uses inexpensive chemicals to extract metals from their
secondary and primary resources [52].
In recent years, there has been an evident expansion in the industrial application of
Ni and Co. This may be seen in two ways: the rise of the manufacture of such metals
globally, and on the other side, the presence of progressively greater quantities of different
wastes containing Co and Ni [51,53–55]. These metals’ unique characteristics, such as
their exceptionally high melting and boiling temperatures, hardness, thermal conductivity,
electrical and changing oxidation states, magnetism, and propensity for forming alloys
with additional elements, make their recovery and recycling intriguing at the moment. The
isolation of Ni and Co requires high precision due to their comparable physicochemical
characteristics. The most important step in the process is selecting an appropriate extractant,
which is typically dependent on the acidity and constitution of the leaching mixture [56]. By
utilizing diverse techniques, researchers have attempted to enhance the solvent extraction
approach. An efficient extractant for the separation of Ni and Co was reported to be Cyanex
272 [bis(2,4,4-trimethylpentyl) phosphinic acid]. Cyanex 272 and D2EHPA [Di-(2-Ethyl
Hexyl) phosphoric acid] can be used to separate Zn and Mn from Ni in the leach solution.
According to the observations, Cyanex 272 and D2EHPA produced a superior separation of
Ni and Co [56].
Kenya was ranked as the 10th highest exporter of Nb, Ta, V, and Zr ore worldwide
in 2021, with USD 44.1M in exports of these metals [57]. Nb, Ta, V, and Zr ore was rated
as Kenya’s 29th most exported good that year. Such critical metals can be recovered
through hydrometallurgical procedures. In recognition of its exceptional physicochemical
properties, V—often referred to as the “vitamin of modern industry”—is used extensively
in various industries. The two primary categories of V recovery methods are roasting
extraction and direct extraction [58]. The extraction of V4+ and Mn2+ was explored using
new technology for selectively separating and extracting them from a co-leaching mixture
of roasted stone coal and pyrolusite via solvent extraction. According to the findings,
99.13% of V4+ was recovered using three counter-current extraction steps with 5% (v/v)
EHEHPA (2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester) and an organic/aqueous
(O/A) ratio of 1:1 under a starting aqueous pH of 2.0. With three counter-current extraction
stages, 99.74% of Mn2+ was extracted by applying 40% (v/v) D2EHPA with a 60 mol%
saponification effectiveness at an O/A phase ratio of 2:1 and a preliminary aqueous pH
of 3.5. Following a two-stage counter-current experiment using 1 M H2 SO4 at a 2:3 O/A
phase ratio, Mn2+ was found to be completely scrubbed off [59].
A synergistic recovery method for Cu2+ and Zn2+ ions from chloride aqueous solutions
has also been developed. The selective extractant was obtained through a combination
Minerals 2024, 14, 21 10 of 27

of trioctylphosphine oxide (TOPO) and Aliquat 336. Employing 0.06 M Aliquat 336 and
0.025 M TOPO in kerosene, Zn2+ ions were effectively separated from Cu2+ ions [60].
Equivalent amounts of both phases (10 mL each) were physically mixed for 20 min during
the process at a fixed temperature (25 ± 2 ◦ C). The mass balance was then used to determine
the distribution ratio (D) of the cations measured by an atomic absorption spectrometer,
Equation (1).
[ Me]org
D= (1)
[ Me] aq
Equation (2) gives the extraction percentage (% E),

D
%E= × 100 (2)
D+1
where the concentrations of the metal ions in the aqueous and organic media are [Me]aq
and [Me]org , respectively.
D
S = M1 (3)
D M2
Equation (3) was used to obtain the selectivity coefficient of metal M1 over metal M2.
Low recovery of Cu2+ with TOPO was observed in the 0.1 to 2 M range. However, the
extraction efficiency of Zn2+ and Cu2+ from HCl solutions with Aliquat 336 was reported
to be quite high. In HCl solutions containing 0.05 M Aliquat 336, selectivity coefficients
of Zn2+ over Cu2+ were very low. The extraction of Cu2+ and Zn2+ to the organic phase
can be linked to results in the formation of their anionic complexes at greater chloride
concentrations. The Cl− ions’ impact increases the development of recoverable anionic
compounds such as ZnCl3 − , ZnCl4 2− , and CuCl4 2− species in the aqueous phase. That
increases the extraction of Zn and Cu. As a result, Aliquat 336 is highly effective at removing
these metal ions from the solution. This chemical functions as an anion exchanger. The
reaction between the Aliquat 336 extractant (R4 N+ Cl) and the metal can be represented as
Equation (4) [60]:
−
( MCl4 )2(aq ) + 2R4 NCl(org) ⇒ ( R4 N )2 ( MCl4 )(org) + 2Cl
−
(4)

The Institute of Nuclear Chemistry and Technology (INCT) in Warsaw, Poland, has
also created several technological schemes for recovering strategic metals from different
waste materials. The institute examined if Zn, Mn, and Mg could be separated from used
Zn electrolytes selectively, Figure 3. D2EHPA diluted in n-heptane was used to extract
these elements. The separation of the metallic compounds from the SO4 2− mixture having
0.89 g/dm3 of Mn, 15.9 g/dm3 of Zn, and 24.3 g/dm3 of Mg was evaluated. According
to the study’s findings, both for Zn and Mn, the extraction equilibrium was reached after
4 min, during which pH modulation regulated the extraction’s selectivity [10].
Cu floatation tailings have substantial amounts of Cu, plus other toxic elements such
as Pb, Zn, U, and Co. The regularly applied methods in the Cu mining industry often
result in great losses of the deficit and valuable metals in the streams of their tailings. The
formulation and implementation of hydrometallurgical advancements is a solution that is
viable for higher efficiency in recovering various elements [61].
A research investigation on the extraction of Cu and other metals was performed by
INCT [62]. An O/A ratio of 1:1 was used in the extraction procedure, and the extractant
used comprised a combination of benzoic acid (0.5 M), the aromatic solvent toluene, and the
aromatic amine p-toluidine (0.25 M). Cu2+ ions were removed using an aqueous sulphate
solution. With the help of sodium carbonate, the aqueous phase’s pH was kept between 3.6
and 3.8. Other metal ions, such as Ni2+ and Co2+ , could be separated once the Cu ions had
been separated. The procedure was carried out by shaking at 25 ◦ C in a mechanical shaker.
In the majority of cases, distribution equilibrium was reached with a shaking time below
60 min. Ion separation was continued until metal ions were recovered to a degree greater
 Minerals 2024, 14, 21 11 of 27

than 99%. The extraction of V, Mo, and U was studied using D2EHPA in toluene. The
concentrations considered in the research were 0.3, 0.2, and 0.1 M. The extraction processes
took place for 15 min under O/A ratio of 1:1. With 5% Na2 CO3 , the organic phase was
back-extracted for 15 min. A 100% extraction from the pregnant solution was confirmed by
a total transfer of Cu into the organic phase. The scholars were able to significantly speed
up the analytical work using the radiotracer approach, which was a very helpful research
tool. The radiotracer was essential in determining the ideal conditions that produced the
highest possible recovery efficiency for Cu. The findings could be utilized as a guide when
designing procedures for recovering Cu and other supporting components from flotation
tailings after Cu ores have been processed, as shown in Figure 4 [62]. The scheme represents
Minerals 2023, 13, x FOR PEER REVIEW 11 of 28
two options of hydrometallurgical process application: recovery of metals from flotation
tailings or low-grade Cu ore.

Figure 3. Extraction of Zn from electronic wastes.

Figure 3. Extraction of Zn from electronic wastes.

Cu floatation tailings have substantial amounts of Cu, plus other toxic elements such
as Pb, Zn, U, and Co. The regularly applied methods in the Cu mining industry often
result in great losses of the deficit and valuable metals in the streams of their tailings. The
formulation and implementation of hydrometallurgical advancements is a solution that is
viable for higher efficiency in recovering various elements [61].
A research investigation on the extraction of Cu and other metals was performed by
 very helpful research tool. The radiotracer was essential in determining the ideal condi-
tions that produced the highest possible recovery efficiency for Cu. The findings could be
utilized as a guide when designing procedures for recovering Cu and other supporting
components from flotation tailings after Cu ores have been processed, as shown in Figure
Minerals 2024, 14, 21 12 of 27 re-
4 [62]. The scheme represents two options of hydrometallurgical process application:
covery of metals from flotation tailings or low-grade Cu ore.

Figure
Figure 4.
4. Process for pyrometallurgical
Process for pyrometallurgicalrefining
refiningofofCuCu ore
ore and
and hydrometallurgical
hydrometallurgical recovery
recovery of metals
of metals
from flotation tailings or low-grade Cu ore [62].
from flotation tailings or low-grade Cu ore [62].

In recentyears,
In recent years,thetheINCT
INCT hashas
alsoalso
beenbeen researching
researching the potential
the potential for recovering
for recovering U and U
otherother
and elements created
elements as waste
created products
as waste duringduring
products the manufacture of Cu concentrate
the manufacture from
of Cu concentrate
Cu ore [61]. The utilization of the hydrometallurgical processes unlocks
from Cu ore [61]. The utilization of the hydrometallurgical processes unlocks the channels the channels for
retrieving all the metals present in the liquor. That can be achieved by using
for retrieving all the metals present in the liquor. That can be achieved by using a common a common
chemical engineering
chemical engineering unitunitprocess
processthatthatisisused
used toto
recover
recover U and
U and other metals
other [38].[38].
metals In situ
In situ
leach (ISL) processes, in addition to underground and open pit mining
leach (ISL) processes, in addition to underground and open pit mining techniques, are techniques, are
frequently used to extract minerals such as U. A small amount of U can be extracted as a
frequently used to extract minerals such as U. A small amount of U can be extracted as a
byproduct of processing Au and Cu ores or deposits of phosphate rock.
byproduct of processing Au and Cu ores or deposits of phosphate rock.
Research was carried out on the Polish Cu mining industry, with the prospect of re-
Research
covering U andwasrarecarried out on
metals from the Polish
industrial Cu [38].
wastes mining industry,
In the recoverywith
of U,the
theprospect
commonly of re-
covering U and rare metals from industrial wastes [38]. In the recovery
used method of precipitation was assessed. The hydrometallurgical recovery processes of of U, the com-
monly
U encompassed the application of a concentrated U mixture for the production of sphericalpro-
used method of precipitation was assessed. The hydrometallurgical recovery
cesses of of
particles U UO
encompassed the application of a concentrated U mixture for the production
2 by the complex sol–gel process (CSGP), Figure 5. The two hydroxyl sets of
of spherical
ascorbic acidsparticles of UO
were readily 2 by the
present, complexthe
including UO2 2+process
sol–gel (CSGP),
that permits Figure 5.
the creation of The
com-two
hydroxyl
plexes. The sets of ascorbic
study reportedacids were
that the readily present,
combination of bothincluding
extractionthe
andUO 2 that
2+
sol–gel permits the
processes
had a synergistic impact, which resulted in the design of a reliable scheme for the recovery
of U from Cu mining tailings and minerals. For both current and future generations of
nuclear reactors, such as pressurized heavy water or fast breeder reactors, a nuclear fuel
precursor can be produced using the direct approach of UO2 synthesis (with the potential
to eliminate the precipitation phase) [38].
 creation of complexes. The study reported that the combination of both extraction and
sol–gel processes had a synergistic impact, which resulted in the design of a reliable
scheme for the recovery of U from Cu mining tailings and minerals. For both current and
future generations of nuclear reactors, such as pressurized heavy water or fast breeder
Minerals 2024, 14, 21
reactors, a nuclear fuel precursor can be produced using the direct approach of UO132 syn-
of 27

thesis (with the potential to eliminate the precipitation phase) [38].

5. Scheme illustrating the hydrometallurgical procedures for extracting

Figure 5.
Figure extracting (UO
(UO22) from various
sources as described in the INCT complex sol–gel
sol–gel approach
approach [38].
[38].

3.3. Recovery
3.3. Recovery of
of the
the Platinum
Platinum Group
Group Metals
Metals
The use
The use of
of high-grade
high-grade platinum
platinum group
group metals
metals ore
ore has
has increased
increased due
due to
to their
their growing
growing
industrial demand. Low-grade ore is now more common due to the increasing shortage of
industrial demand. Low-grade ore is now more common due to the increasing shortage
high-grade platinum group metals ore. Unluckily, it is prohibitively expensive to extract
of high-grade platinum group metals ore. Unluckily, it is prohibitively expensive to ex-
them from low-grade ore, and doing so raises significant environmental issues. Pt is used
tract them from low-grade ore, and doing so raises significant environmental issues. Pt is
as a catalyst in fuel cells to effectively convert O2 and hydrogen into heat and electrical
used as a catalyst in fuel cells to effectively convert O2 and hydrogen into heat and electri-
power. The detection of low Pt levels in biological and environmental samples has been a
cal power. The detection of low Pt levels in biological and environmental samples has
serious concern with the introduction of Pt-containing catalysts in vehicle exhaust systems.
been a serious concern with the introduction of Pt-containing catalysts in vehicle exhaust
To track the buildup of Pt in the environment, precise and accurate background amounts
systems. To track the buildup of Pt in the environment, precise and accurate background
of Pt in a variety of substances are required. Since Pt is present in sediment samples in
such trace amounts, designing techniques to measure it is quite challenging. Developing
a process that includes digestion, preconcentration, separation, and Pt detection is an
exceptionally effective way to reach incredibly low detection quantities [63]. Utilizing
a 191 Pt radiotracer, the kinetics of the extraction of Pt4+ from an HCl solution applying
rubeanic acid in TBP, n-butyl alcohol-acetophenone and thenoyltrifluoroacetone (TPA),
were studied [63]. Investigating the effects of acidity, mixing duration, Pt quantity, and
back-extraction, the most favorable extraction outcomes for TBP and TPA were obtained
 ples in such trace amounts, designing techniques to measure it is quite challenging. De-
veloping a process that includes digestion, preconcentration, separation, and Pt detection
is an exceptionally effective way to reach incredibly low detection quantities [63]. Utilizing
a 191Pt radiotracer, the kinetics of the extraction of Pt4+ from an HCl solution applying
Minerals 2024, 14, 21 rubeanic acid in TBP, n-butyl alcohol-acetophenone and thenoyltrifluoroacetone (TPA), 14 of 27
were studied [63]. Investigating the effects of acidity, mixing duration, Pt quantity, and
back-extraction, the most favorable extraction outcomes for TBP and TPA were obtained
with
with 44 and
and 33 M
M HCl, respectively. INCT
HCl, respectively. INCTininWarsaw
Warsawalso
alsocreated
createda amethod
methodfor
forrecovering
recoveringPt
Pt
from the waste solution. Agua regia was applied as a leaching agent whereas NH4NH
from the waste solution. Agua regia was applied as a leaching agent whereas 4Cl
Cl was
was
usedused to precipitate
to precipitate Ptfurther
Pt for for further recovery
recovery process,
process, Figure
Figure 6. 6.

Figure 6. The recovery of Pt from the waste solution.

Figure 6. The recovery of Pt from the waste solution.
Apart from platinum group metals, precious metals like Au have drawn a lot of in-
Apart
terest [10]. from platinum
Researchers aregroup metals,toprecious
attempting use newmetals like Au
techniques inhave
both drawn a lot of and
identification in-
terest [10]. Researchers
quantitative analysis. Aare attempting
quantity to guse
of 0.02 of new
Au istechniques
often foundin in
both identification
a cell phone [10]. and
The
quantitative
development analysis.
and useAofquantity of hydrometallurgical
alternate 0.02 g of Au is oftenextraction
found in amethods
cell phone
has[10]. The
elevated
development
their significanceand for
usemany
of alternate
researchhydrometallurgical
institutions over theextraction
past fewmethods
decades.has elevated
Societal con-
cern regarding the environmental impacts of cyanidic Au extraction has increased. The
Department of Nuclear Methods of Process Engineering, INCT, Warsaw, carried out the
development of hydrometallurgical technologies for the separation of metals from waste.
The technological line for the solvent extraction of Au from a mixture of Mo-Ni waste was
established in Figure 7.
 their significance for many research institutions over the past few decades. Societal con-
cern regarding the environmental impacts of cyanidic Au extraction has increased. The
Department of Nuclear Methods of Process Engineering, INCT, Warsaw, carried out the
development of hydrometallurgical technologies for the separation of metals from waste.
Minerals 2024, 14, 21 The technological line for the solvent extraction of Au from a mixture of Mo-Ni waste
15 ofwas
27
established in Figure 7.

Figure7.7.Scheme
Figure SchemeofofAu
Aurecovery
recoveryfrom
frommolybdenum-nickel
molybdenum-nickelwaste.
waste.
3.4. Recovery of Rare Earth Elements
3.4. Recovery of Rare Earth Elements
Apatite, xenotime, and monazite are a few of the foremost popular minerals that
Apatite, xenotime, and monazite are a few of the foremost popular minerals that con-
contain REEs. The extraction and reuse of such minerals from various secondary re-
tain REEs. The extraction and reuse of such minerals from various secondary resources
sources has recently attracted a lot of interest. Considering their secondary origin, various
has recently attracted a lot of interest. Considering their secondary origin, various tech-
techniques have been carried out to separate them from phosphate or phosphogypsum
niques
rocks haveExtracting
[64]. been carried
themoutfrom
to separate them from
the aqueous phosphate
solution or phosphogypsum
(leachate) rocks
is a selective activity.
[64]. Extracting them from the aqueous solution (leachate) is a selective activity.
For instance, a mixture of minor actinides (MAs) and REEs was used for the extraction For in-
stance, a mixture of minor actinides (MAs) and REEs was used
′ ′ for the
studies. La was removed from MA by employing N,N,N ,N -tetrakis(4-propenyloxy-2- extraction studies.
La was removed from MA by employing
pyridylmethyl)ethylene-diamine (TPEN) N,N,N′,N′-tetrakis(4-propenyloxy-2-pyridylme-
extracting agent. The phase of separation and
thyl)ethylene-diamine (TPEN) extracting agent.toThe
recovery is intricate and delicate. It is essential phase ofthat
guarantee separation
each REE and recoveryisis
separation
highly effective and can be reclaimed at the final stage of the procedure. Knowledge of the
extraction characteristics, capacity, phase separation, solubility, selectivity, mass transfer,
and economic viability have all been used to develop feasible REE extraction methods [65].
Given their nearly non-volatile nature, low melting point, low flammability, thermal
stability, and high ionic conductivity, ionic liquids (ILs) are usually regarded as environ-
mentally friendly solvents and have attracted significant interest. In the extraction of both
organic and inorganic materials, ILs made from quaternary ammonium bases and organic
Minerals 2024, 14, 21 16 of 27

acid moieties are frequently utilized [66]. The benefits of ILs, such as high thermal stability
and designability, could potentially be introduced through the use of ILs in the solvent
extraction of REEs [67]. Numerous factors, such as the kind of acidic medium, diluent, the
temperature of the auxiliary agents, metal ions, contact time, organic-to-aqueous phase
ratio, pH, and extractant concentration, can have an impact on the process of solvent
extraction. Considering various operational conditions that depend on other factors, a
particular operational parameter may have various implications on the desired results [68].
It is challenging to selectively extract and purify REEs from a pregnant solution [69].
REEs’ distribution can also be explored by utilizing ILs grounded on phosphinic or
phosphoric acids and a metal ion. Due to coordination interactions with moieties of organic
and inorganic acids, REEs can produce complex anions (like other metals of side groups).
The separation of REEs utilizing an IL based on phosphoric acid and an organic base is
represented by Equation (5) [66]:

3RNH3+ A−
(org)
+
+ Me3(aq )
+ 3Cl(−aq) ↔ MeA3 ·3RNH3+ Cl(−org) (5)

Aliquat 336 can be treated with KNO3 to exchange Cl− with NO3 − which improves
selectivity over REEs, to reduce the viscosity of the extractant and establish a faster mass transfer.
Equation (6) can be used to represent reaction equilibria for A336[NO3] extraction [70].

Me3 +
( aq)
+ y( R3 N + CH3 ) NO3 − −
IL + 3NO3 ( aq) ⇌ ( R3 NCH3 ) Me ( NO3 )y+3 IL (6)

wherein Me3+ stands for the elemental cation, (R3 NCH3 )M(NO3 )IL for the IL and metal
cation complex, and (R3 N+ CH3 )NO3 − IL for A336[NO3 ]. IL stands for both the IL phase
and the aqueous phase. The equilibrium constant Keq is expressed as follows:
h i
( R3 NCH3 ) Me( NO3 )y+3 IL
Keq = 3
(7)
[ Me3 +
( aq)
][( R3 N + CH3 ) NO3 − y −
IL ] [ NO3 ( aq) ]

Equation (8) may be employed to express the reaction equilibrium for D2EHPA separation.

Me3+ + 3[ H A]2 ⇌ [ MeA3 ( H A)3 ] + 3H + (8)

whereby Me3+ stands for the metal cation, [HA] for the organic phase’s extractant, and
[MeA3 (HA)3 ] for the complex of the extractant and metal. The equilibrium constant Keq is
expressed as follows:
[ MeA3 ( H A)3 ][ H + ]3
Keq = (9)
[ Me3+ ][( H A)2 ]3
The separation of REE such as La can be accomplished using the IL [A336][CA-12]
in chloride media. Additionally, it was demonstrated that IL[A336][P507] was suited to
separate light REEs in chloride media and would work for the isolation of heavy REEs in
nitrate media. By applying IL [A336][P507], the Ce4+ and F− ions have been effectively
extracted from H2 SO4 solutions and separated [67]. The recovery of mid-heavy REEs in
H2 SO4 solution by [A336][P507] became the subject of a thorough examination. Accord-
ing to the findings, temperature, extractant concentration, and pH all boosted the rate
of REE extraction [67]. The results demonstrated that, as both the temperature and ex-
tractant concentration increased, so did the distribution ratios of REEs. The influence of
pH on the separation behaviors of Tb3+ by Cyanex 923, IL [A336][P507], TBP, and P350
(di-(1-methylheptyl)methyl phosphate) is important. Its recovery by Cyanex 923 and TBP
in systems of pH = 1.0 to 0.7 improved with rising acidity. At pH < 0, P350 showed a
comparable tendency as Cyanex 923 and TBP. Following a reduction in acidity, the Tb3+ was
extracted more effectively. IL [A336][P507] is more appropriate for the separation of REEs at
low acidity, as evidenced by the divergence in recovery performance [67].
Minerals 2024, 14, 21 17 of 27

Because of the salting-out phenomenon found in the extraction of Lu3+ , the recovery
of REEs decreased as the concentration of Na2 SO4 increased. The process of extraction was
hindered by the binding of such cations with SO4 2− in the aqueous medium. However,
as the concentration of NaNO3 or NaCl increased, the yield of extraction rose as well.
The different behaviors of NO3 − , SO4 2− , and Cl− complexes could be caused by the ion
connection between anions and REE3+ , and the hydration effect. A decline in the recovery
of metal ions results from the generation of REESO4 + ions at high sulphate amounts. It
is understood that leaching agents for REEs ores of the ion-adsorbed class once included
NH4 NO3 , NaCl, NH4 Cl, and (NH4 )2 SO4 . NaCl was likewise the first leaching solvent to
be widely utilized. As the concentration of NaCl grew, so did the effectiveness of REEs’
extraction. The above extraction technique is more effective in extracting mid-heavy REEs
than the aqueous medium devoid of NaCl. Al3+ and La3+ experience low extractability
as a result, allowing this technique to be employed to retrieve mid-heavy REEs from the
pregnant solution. Following a rise in the concentration of NaNO3 , the distribution ratios
of all REEs rose, particularly for the mid-heavy REEs. The mixture with NaNO3 has a
substantially higher extractability than the one employing NaCl [67].
With heating, the precipitated form of Th and REEs such as Ce created after the recov-
ery process can be dissolved in 4 M HNO3 . Th can be obtained by extracting it with Aliquat
336 diluted in kerosene, stripping it with HCl solution while agitating, and precipitating it
as ammonium hydroxide. Ce3+ is, on the contrary, oxidized into its tetravalent form by the
incorporation of sodium bromate. It can then be recovered by Aliquat 336 in kerosene that
contains 1-octanol, stripped by shaking the organic solution with diluted HNO3 media, and
finally precipitated as hydroxide using ammonia. La, Nd, and Y-rich aqueous media can
be treated at 25 ◦ C by a synergistic TRPO-TOPO combination in kerosene with a 2:1 O/A
phase ratio. H2 SO4 can then be applied to strip the organic media in an equimolar ratio to
isolate the majority of the Y, after which, HCl is used as a stripping agent to retrieve Nd
and La [71].
In nitrate medium, REEs (Sm and Nd) were extracted from discarded fluorescent
lamps using Cyanex 923, which serves as a solvating extractant. Researchers noticed that,
although the co-extracted metals took longer than 15 min to reach a state of equilibrium,
the REEs did so in just one minute. For greater REE recovery, they recommended low
acidity (less than 1 M HNO3 ), one minute of reaction time in between the two phases, up to
1 M extractant concentration, stripping with 4 M HNO3 , and scrubbing by HNO3 or oxalic
acid. The REE metal ion was extracted from Nd-Fe-B magnet residue using solvent TBP
in IL (Aliquat 336). To improve the distribution coefficient throughout the REE recovery
from Ni metal hydride batteries employing chloride medium, [A336]NO3 − and Cyanex
923 were utilized [52].
To remove REEs and transition metals from Sm/Co and Nd/Fe magnets, Cyphos IL
101 was implemented. Scientists discovered that transition metals are efficiently extracted
by the IL, and the maximum distribution rates for Co and Fe, respectively, were attained
with 8.5 M and 9 M HCl. The separation coefficients for Sm/Co and Nd/Fe were discovered
to be 8 × 105 and 5 × 106 , respectively. Transition metals including Co, Mn, Fe, and Zn
could additionally be extracted using Cyphos 101 IL from an aqueous media that also
contains REEs and Ni with 8 M HCl. Following the initial extraction stage, Cyanex 923 and
Aliquat 336 nitrate were employed to recover the REEs from the raffinate. Because Aliquat
336 nitrate is more selective for REEs and can handle less Cyanex 923 molecules around
it than Cyphos 101, it is preferred over the latter. A336 chloride and A336 thiocyanate
were applied to separate Co and Mn. It was possible to recover La from Ni and Sm
from Co through the application of trihexyl (tetradecyl) phosphonium nitrate. REEs were
successfully recycled using this technique from used permanent magnets and rechargeable
batteries. Here, they added Na or ammonium nitrate to the aqueous media, and the
consequence of salting out caused the extraction to improve. They discovered that Sm
could be extracted as the pentakis complex [Sm(NO3 )5 ]3− and La as the hexakis complex
[La(NO3 )6 ]3− . H2 O was the medium used to do the stripping [52].
Minerals 2024, 14, 21 18 of 27

Synergistic extraction of REEs can offer the benefit of using reduced usage of the main
extracting agent, while still being able to achieve a high rate of separation and replenish the
extracting agent. TBP and D2EHPA are two of the most popular and commonly employed
of these in the commercial process for the extraction of REEs. Although TBP has a higher
loading capacity for REEs compared to D2EHPA, D2EHPA possesses a superior separation
coefficient for REEs. An assessment was conducted on the impact of the aqueous phase’s
concentration of NO3 − and H+ ions. Also, an evaluation was conducted on the kind and
number of extracting agents present in the organic phase, and on the extraction behavior
of La, Nd, Ce, and Y. An organic phase comprising 0.8 M of extracting agents with an
equal mole ratio of D2EHPA and TBP in kerosene was employed to test the influence of
the NO3 − ion concentration on the recovery of REEs. The concentration of H+ ions in
the aqueous media was set at 2 M (pH = −0.3). As the amounts of NO3 − in the aqueous
medium increased, the recovery of REE was reported to rise as well. In comparison to
the other examined REEs, Y exhibited a higher extraction efficiency. Therefore, when the
amount of NO3 − ions rises, so do the distribution ratios of REEs. A sharp increase in the
Y distribution ratio over 6 M NO3 − was observed. Because the REE3+ NO3− species were
removed following the recovery mechanism of REEs by TBP, which is determined according
to the reaction in Equation (10), a common ion effect could potentially be responsible for
the rise in extraction caused by an increase in NO3 − ions concentration [69].

REE3+ + 3NO3− + 3TBP ⇒ REE( NO3 )3 ( TBP)3 (10)

With an increase in the concentration of H+ in the aqueous media, the distribution

coefficient of REEs dropped. Relative to other REEs, Y is particularly susceptible to changes
in the concentration of H+ . The cation exchanger extracting agent D2EHPA’s recovery mech-
anism is capable of being used to illustrate how H+ affects REEs extraction. Below is the
equation showing the extraction mechanism of REEs from aqueous media using D2EHPA:

REE3+ + 3H2 A2 ⇒ REE( H A2 )3 + 3H + (11)

3.5. Precipitation
The extracted metals can be recovered in solid form by precipitation. This is realized
through the combination of selected ion(s) with an appropriate counter ion in adequate
concentrations to surpass the subsequent compound’s solubility product and produce a
supersaturated solution. Precipitation occurs at lower temperatures, low ion concentration,
a high degree of supersaturated solutions, and a minimal stirring of the solutions. The size
of the precipitate depends on the type of solvent and pH of the solutions. Metals from
sources such as acid mine drainage are normally precipitated as metal oxides, phosphates,
sulfides, hydroxides, and carbonates [72].
The precipitation technique is easy to control, simple, inexpensive, and applied in
industry. Double-salt precipitation, simple precipitation, and oxidative precipitation are
the techniques utilized in hydrometallurgy for secondary resources and primary ores. In
simple precipitation, precipitants such as OH− , C2 O4 2− , S2− , and CO3 2− are applied [73].
Generally, double-salt precipitation entails the use of salt with more than one anion or cation.
Double-salt precipitation is crucial in the recovery of REEs. When some compounds are
precipitated or eliminated from a solution by oxidation, the term "oxidative precipitation" is
generally used. In other cases, some metal ions can be precipitated by adding complexing
agents to form complexes when ligand ions or ligand molecules are available [73]. In
most cases, a complexing and precipitating agent can be used together for improved and
selective recovery of a metal such as Ni, Mn, Co, Cu, and Fe. For such metals, OH− ,
CO3 2− , and S2− precipitants combined with NH3 as a complexing agent are effective in
complexation–precipitation.
Minerals 2024, 14, 21 19 of 27

3.6. Ion Exchange

Ion exchange is among the separation techniques that play a big role in the recovery of
a wide range of metals with unique properties suitable for the current and future economy.
Due to intense mining activities, metals have significantly reduced in ores, and they exist in
low concentrations. The ion exchange technique is suitable for the recovery of such metals.
That is attributed to its environmental safety, selectivity, powerful resolving ability, high
efficiency, moderate cost, and suitability for combination with subsequent techniques in
the recovery of noble metals and other applications [74,75]. Ion exchangers with distinct
functional groups, due to their good kinetic properties, high mechanical strength, and
exchange capacity, increase their suitability in the concentration of microscopic amounts of
metals, extracting them in the presence of accompanying components.
Ion exchange resins acting like porous media are essential in separation industries
because of the elimination of thermal regeneration, a higher metal ion loading capacity,
higher loading rate, and being less prone to fouling by organic materials. The porous
property of the resin leads to enhanced adsorption rates because of the improved diffusion
of the ions. The metal recovery process’s efficiency is firmly reliant on the contact time,
pH of the solution, ion concentration, and ion coordination/exchange resin properties
such as structure and type of the ligand, crosslinking degree, and swelling [76]. Various
types of exchange resins are available commercially in the recovery of metals; for example,
Amberlite IR-120 can be used in separating Cu2+ , Zn2+ , and Cd2+ . Chelating ion exchange
resins such as Amberlite IRC 748 and Chelex 100 have aminodiacetic acid functional groups
to exchange Zn2+ and Cu2+ from aqueous solutions while macro-porous AMBER JET
1200 Na cation exchange resins are efficient in the separation of metals such as Ni2+ and
Pb2+ [77]. In the recovery of platinum group metals such as Pa2+ , Pt2+ , and Pt4+ from
HCl acid solutions, CYBBER and Purolite grades present high sorption capacity for the
anion exchange [74].

4. Recovery of Strategic and Critical Metals Using Flotation

Flotation is essentially a surface selectivity-based extraction method that separates
the hydrophilic portion of a material from the hydrophobic portion. To create a wide
range of separations, this technique selectively modifies the hydrophobicity of the min-
eral surfaces using several surfactant chemicals [27]. While gangue minerals—unwanted
minerals—remain in the water, the target minerals, which are frequently sulphide minerals,
are usually hydrophobic and will adhere to the air bubbles [78–82]. The subsequent froth
rich in minerals is subsequently scraped off the flotation cell’s surface, Figure 8.
If flotation techniques used in other regions are tailored to suit the current minerals,
they will be essential for the beneficiation of the minerals in Kenya. Research was conducted
to recover Cu and Co by treating the tailings from the flotation of Cu and Co oxidized
ores produced at the Kambove Concentrator (DRC). The study focused on determining
the dosage of chemicals needed to float the examined tailings and recover the maximum
amounts of Cu (44.80%) and Co (88.30%). Centered on the concentrate grade (3.31% Cu
and 2.22% Co) and the feasible extraction of the metals of interest, it was determined that
flotation was a desirable method for reprocessing the tailings [83]. The tailings from the
processing of Cu ores contain rocks that may be recycled as inexpensive raw materials.
The overconsumption of reagents due to the particle size distribution in a wide range of
slimes is still an obstacle. Such an issue should be taken into consideration for the most
effective technical and financial implementation of the aforementioned technique. While
the wastewater produced by the flotation of tailings can be reclaimed and fed back into the
Kambove Concentrator milling process, the tailings can be added to construction materials
as cemented pastes [83].
Slags are also known to have valuable metals, and instead of being an end waste,
they can serve as a secondary source of metals [84,85]. They come in a variety of forms, as
wastes from combustion processes or as the results of metallurgical operations. They have
been used in numerous fields as a resource material [85]. Furthermore, for certain purposes,
Minerals 2024, 14, 21 20 of 27

the characteristics of slags are as good or better than those of competing materials. From
nonferrous smelters, a range of nonferrous slags are generated.
Numerous investigations on metal recovery using nonferrous slags have already been
conducted. A large number of studies focused on recovering metals from Cu slags [85,86].
By considering variables, including the form of the processed ore, the kind of furnaces
employed, and the cooling techniques, the chemical composition of Cu slags from various
sources might range significantly. The main phases in crystalline Cu slag are often fayalite
and various silicates. Most of the time, Ni and Co are found as oxides where20the
Minerals 2023, 13, x FOR PEER REVIEW of Co
28
distribution in the slags is extremely homogeneous. On the other hand, many Cu slags
have diverse types of Cu minerals. They could take the shape of oxides, sulphides, or a
combination of the two. Investigations were conducted on the recovery of Cu and Co from
4. Recovery of Strategic and Critical Metals Using Flotation
ancient Cu slags collected from the Küre Plant basin of Turkey [86]. The slag was of the
Flotation
fayalitic kind,iswhich
essentially a surface
contained 1.24%selectivity-based
Cu, 53.16% Fe, and extraction
0.53% Co.method that separates
To retrieve the metal
the hydrophilic portion of a material from the hydrophobic portion. To create
values, two distinct paths were taken. The first method was leaching it after after roasting a wideit
range of separations, this technique selectively modifies the hydrophobicity of the
with pyrite (FeS2 ). The second method involved leaching, and roasting the flotation tailings mineral
surfaces
with FeSusing several surfactant chemicals [27]. While gangue minerals—unwanted min-
2 , followed by flotation of the slags. It was discovered that the second method
erals—remain
worked well for in treating
the water,thethe
Cutarget minerals,
slag. During thewhich are process,
flotation frequently sulphide
a Cu minerals,
concentrate with
are usually
around 11% hydrophobic
Cu was created,and will
andadhere
77% ofto the
the Cuairwas
bubbles [78–82].
recovered, The subsequent
leaving 93% of the froth
Co in
rich in minerals
the tailings [86].is subsequently scraped off the flotation cell's surface, Figure 8.

Figure
Figure8.8.AAsketch
sketchof
ofthe
thefroth
frothflotation
flotationprocess.
process.

IfDue
flotation
to the techniques
small amounts used
of in other regions
platinum are tailored
group metals to suitores
in primary the(less
current
thanminerals,
10 g/ton)
they will be essential for the beneficiation of the minerals in Kenya. Research
and the complex processes involved, their extraction from primary ores is costly. Ores was (such
con-
ducted to recover
as Merensky ReefCuoreand Co by
grades, treating
which the tailings
comprise from
3 to 8 g/t the flotation
platinum group ofmetals
Cu andtied Cotooxi-
Ni,
dized
Cu, asores produced
sulphides) areat the Kambove
mined, ground, Concentrator (DRC).
processed through The study
gravity focusedand
separation, on floated.
deter-
mining the dosagewhich
Their concentrate, of chemicals needed
is generally toover
200 to float2000
the g/t
examined
platinum tailings
group and recover
metals the
alongside
maximum
0.4% to 2.8%amounts
Cr2 O3 , of Cu (44.80%)
undergoes and Co
meltdown at(88.30%).
excessive Centered
temperatureson the ◦ C). Thereafter,
concentrate
(>1500 grade
(3.31% Cupurified
they are and 2.22% Co) and
through the feasible extraction
hydrometallurgical of the[87].
processes metals of interest,froth
Traditionally, it was deter-
flotation
mined that flotation was a desirable method for reprocessing the tailings [83]. The tailings
from the processing of Cu ores contain rocks that may be recycled as inexpensive raw
materials. The overconsumption of reagents due to the particle size distribution in a wide
range of slimes is still an obstacle. Such an issue should be taken into consideration for the
most effective technical and financial implementation of the aforementioned technique.
Minerals 2024, 14, 21 21 of 27

has been used to extract platinum group metals from pure (unweathered) sulphide ores.
This method usually yields recoveries of over 85% for Pt. The efficient concentration of such
metals through flotation in virgin sulphide ores is dependent upon the presence of highly
floating base metal sulphide minerals, such as pyrrhotite, chalcopyrite, and pentlandite.
Nevertheless, as an alternate source for keeping up their production, the rapid depletion
of sulphide platinum group metal-bearing minerals has sparked a lot of interest from
scientists. The focus has been on investigation of their recovery from near-surface oxidized
metal ores.
Besides platinum group metals, Au can be extracted using the flotation technique [25,43].
Au floats easily, and it is possible to separate free Au from sulfide-containing ore via
selective flotation. A product that is directly smeltable could potentially be produced by
removing the floating Au from the sulphides. According to research, selective flotation
of Cu sulphides could be observed from FeS2 ores at pH values higher than 11 [88]. The
behavior of the free Au in these circumstances was less obvious. Additional collectors
(monothiophosphates) could be included for increased Au recovery because they are
known to be selective Au accumulators [88,89]. Researchers looked into how several
operating factors, such as pH, collector additions, and grind size, affected the efficacy of
Au flotation [88]. With a variety of collectors, Au was recovered selectively against FeS2 at
elevated pH levels. Nevertheless, in the flotation tests, there was no specificity towards
chalcopyrite. Granular Au drifted towards the flotation tail, but fine Au floated more
readily. That is because the flotation of Au is highly dependent on some parameters like
the type of the collectors, size, structures, and the mineral containing Au, amongst others.
Numerous studies have also been conducted to examine the dynamics of flotation
of REEs in terms of pulp chemistry, froth stability, mineral particle size distribution, and
collector–mineral surface interactions [90]. The flotation effectiveness of REEs is measured
by the flotation rate test, which considers concentrate grade, mass pull, and recovery. The
flotation kinetics of the mine is defined as the fluctuation in the recovery of metals over
time. The outcomes are utilized to characterize a mineral’s flotation behavior in specific
flotation conditions. The flotation rate of the REEs depends on the length of flotation,
pulp pH, the kind and dose of the collector, the extent of surface–collector contact, and
the mineral species. The principal materials that are the primary industrial sources of REE
minerals are monazite (REE)PO4 (55 to 60% RE oxides), xenotime (REE)PO4 (55 to 60%
RE oxides), and bastnaesite [REE(CO3 )]F (70 to 75% RE oxides). Flotation is considered
to be one of the most essential methods for separating REEs from related minerals and
producing concentrates. The resulting material often contains 60 to 70% mixed REEs, due
to the intricate and fine properties of the REE ore [91].
The assessment was done on the flotation of rare earth oxide (REO) in monazite by
combining hematite (Fe2 O3 ) and quartz (SiO2 ) with hydroxamic acid [90]. The results of
the micro-flotation experiments carried out on individual minerals demonstrated that each
model mineral’s flotation response depends on pH. The optimal flotation extraction of
quartz happened at pH = 3, while that of hematite and monazite was attained at pH = 7. As
predicted, adding more hydroxamic acid increased each mineral’s flotation yield. When the
dose of hydroxamic acid was increased, the specificity coefficients of monazite over quartz
remained lower than those of monazite against hematite. In a mixed minerals flotation,
it was anticipated that a higher percentage of Fe2 O3 would be associated with REO in
flotation concentrates than SiO2 . The outcome suggested that depressants are necessary to
accomplish selective beneficiation and extraction of REO. Further results on heterogeneous
minerals separation experiments demonstrated that, in the absence of depressants, the
flotation of REO from SiO2 and Fe2 O3 was unselective. It was reported that 96% Fe2 O3 , 99%
REO, and 80% SiO2 recoveries of the concentrate were produced by a 2000 g/t hydroxamic
acid collector. Whenever 4000 g/t starch was employed, the extraction effectiveness of REO
rose from its low average of 6.42% devoid of depressants to 44.78%. Matching recoveries
of 93% REO, 38% SiO2 , and 81% Fe2 O3 were observed. Contrarily, when Na2 SiO3 was
utilized, the resulting concentrates had greater gangue contents (SiO2 > 60% and Fe2 O3
Minerals 2024, 14, 21 22 of 27

> 90%). That resulted in lower REO separation efficiency. When hydroxamic acid was
present, starch provided a better REO upgrade. Nevertheless, Na2 SiO3 was a better choice
when REO recovery was the only factor considered. Furthermore, a mixture of starch and
Na2 SiO3 demonstrated considerable recoveries [90].
The beneficiation reaction of gangue minerals and REE in the tailings differs noticeably.
The separation techniques of froth flotation, wet magnetics, and gravity led to widely differ-
ing REE upgrades and recoveries. A flotation study was conducted on the ore by applying
sodium oleate acting as a collector. To determine if there was a possibility of extracting
and enriching REE minerals in saprolite ore, a study examined three distinct treatment
configurations: the relative impacts of pulp pH, depressants, and de-sliming. Most REE
minerals (>50%) could be extracted using flotation methods on raw feed. However, the
technique was not selective, given that clay minerals and silicate gangue also found their
way into the flotation concentrate together with the REE minerals. The specificity of the
flotation of REE minerals was enhanced by de-sliming before flotation, using a mixture
of depressants (Na2 SiO3 and starch). That resulted in concentrates that had total REO
grades of 5.87% and 4.22%. Recovery yields of 45% and 50% at pulp pH = 9 and 10.5 were
obtained. Because of their fine-to-ultrafine properties, clay gauge and silicate minerals
were collected through the synergistic process of surface activation and entrainment. A
random grade–recovery relationship was found when all test results were compared. It
indicated that there could be a need for more flotation tests where its process limitations
can be looked into. Optimization is also necessary to further maximize both REE recovery
and grade. Additionally, the possibility of employing magnetic separation was raised [92].
Apart from the physical methods, ILs have been seen as revolutionary in the flotation of
REEs. Tetraethylammonium mono-(2-ethylhexyl)2-ethylhexyl phosphonate ([N2222][EHEHP])
is an IL that has been studied earlier for REE solvent extraction and revealed to be selective
and efficient. For the first time, [N2222][EHEHP] was assessed as a collector in bastnäsite
flotation, which is the main deposit for producing REEs [93]. The findings were contrasted with
two typical gangue minerals found in quartz and hematite. Hematite exhibited an enhanced
collectability of [N2222][EHEHP] over bastnäsite, which significantly recovers at pH = 5 with an
increased dosage of IL (500 g/t). It implies that these minerals’ extraction mechanisms could
not be the same. Based on the Fourier transform infrared spectroscopy (FT-IR) data, [EHEHP]
moiety adsorption on bastnäsite was verified. It seemed to be most effective in slightly acidic pH
conditions. The micro-flotation data, which indicated that bastnäsite recovery was maximum
(~50%) at pH = 5, was consistent with this. Furthermore, micro-flotation indicated that a very
high dose of IL must be administered to directly float bastnäsite. Protons may interact with
[EHEHP]− and obstruct interactions between REE ions and [EHEHP]− , which explains why
bastnäsite flotation recovery is lower at pH = 3. According to FT-IR data, the adsorption of the
[EHEHP]− moiety on hematite is evident in acidic conditions, which matches the micro-flotation
recovery pattern. Furthermore, hematite may be more recoverable because of its better yield
compared to bastnäsite. The little recovery for quartz at both dosages across the pH range under
test signified the absence of reagent adsorption. By using magnetic separation to examine the
concentrates and tails, it was discovered that higher collector dosages might result in bastnäsite
recovery capacities over 90% and an optimal conversion ratio of 1.7 [93].

5. Proposals and Prospects for Future Application of the Hydrometallurgical and

Flotation Techniques
In 2016, Kenya implemented the Mining Act meant to modernize and expand the
mining industry. Even though the Act was improved from the previous legislation, output
in the mining sector remains poor. However, the new regime announced the removal of
the moratorium in 2023. The Government emphasized the importance of creating a good
business atmosphere for stakeholders and investors. Kenya’s future development of the
mining sector will be influenced by environmental impact and the adoption of sustainable
mining techniques. Given the rising number of international mining firms promising
to decarbonize and achieve net zero goals, the mining sector should develop practical
Minerals 2024, 14, 21 23 of 27

approaches to tackle environmental issues in their operations. The approach will compel
investors to apply environmentally justifiable techniques for metal recovery.
As Kenya seeks to diversify its mining practices, hydrometallurgical and flotation
procedures emerge as a compelling alternative. These innovative approaches will not only
address the constraints posed by low-grade ores but also align with global sustainability
goals. Through investment in research and regulatory and infrastructure adaptations,
Kenya can pave the way for a dynamic and responsible mining sector. That will contribute
to both economic development and environmental preservation.
The metal recovery techniques outlined in this article can easily be applied to the
needs of the Kenyan mining industry. The development of advanced leaching technologies
can enhance the efficiency and selectivity of metal extraction. Continuous research and
investment in these technologies can optimize the recovery of strategic and critical metals.
One of the challenges in Kenya’s mining industry is the presence of low-grade ore deposits.
Hydrometallurgy offers a viable solution, enabling the cost-effective extraction of metals
from ores that were previously considered economically unviable. Such an option be-
comes crucial in times of depleted mineral reserves for important metals, when alternative
sources must be explored. Beyond primary ores, hydrometallurgy provides an avenue for
recovering metals from secondary sources. With the growing demand for recycling and a
circular economy, Kenya can leverage hydrometallurgical procedures to extract valuable
metals from secondary sources, contributing to resource sustainability. The mining research
institutions in Kenya will need to also initiate progressive efforts to enhance the efficacy
of the flotation process. That could encompass the implementation of advanced control
systems or the formulation of improved surfactants.
There will be a need for the establishment and demonstration of pilot plants to exhibit
the effectiveness and feasibility of the described methods. That can be reinforced by intro-
ducing capacity-building programs and training dynamism, to equip local experts with the
knowledge required to operate flotation and hydrometallurgical processes. Collaboration
with research institutions, both domestically and internationally, can fast-track the devel-
opment of competent and environmentally friendly metal-recovery processes tailored to
the country’s mineral resources. In addition, regulatory frameworks ought to be adapted
to address the drawbacks and opportunities associated with chemical-based techniques
for metal recovery. Clear guidelines, environmental impact assessments, and monitoring
mechanisms should be established to foster responsible mining practices.

6. Conclusions
The vast unexploited mineral resources in Kenya present a major opportunity for
economic development. Nonetheless, the environmental impacts of mining in the region
cannot be neglected. Mineral extraction processes require a well-adjusted approach that
considers both sustainability and economic returns. The adoption of modern flotation
and hydrometallurgical methods emerges as an important solution to efficiently extract
strategic and critical metals while reducing environmental effects. For full balance in society
of a sustainable and new world, we must not forget about people. Skillful management
of natural resources and investment in local communities should guarantee stable and
peaceful development. The mining industry and government stakeholders must adopt
sustainable technologies, while prioritizing responsible mining practices. All-inclusive and
collaborative steps are necessary for harnessing Kenya’s potential mineral wealth, while
protecting the delicate balance of its environment.

Author Contributions: Conceptualization, editing, and writing review, N.R.K.; conceptualization

and editing, I.H.-K.; conceptualization and editing, M.R.; conceptualization and editing, T.S.; concep-
tualization and editing, M.S.; editing and conceptualization, A.G.C. All authors have read and agreed
to the published version of the manuscript.
Funding: TC Project RER1023: Harmonizing Implementation of Radiotracer and Sealed Sources Tech-
niques for Efficient Use of Natural Resources and Environmental Monitoring ME-RER1023-2105581.
Minerals 2024, 14, 21 24 of 27

Data Availability Statement: Not applicable.

Acknowledgments: The task was completed with assistance from the Institute of Nuclear Chemistry
and Technology’s research activities.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Haneklaus, N.; Sun, Y.; Bol, R.; Lottermoser, B.; Schnug, E. To extract, or not to extract uranium from phosphate rock, that is the
question. Environ. Sci. Technol. 2017, 51, 753–754. [CrossRef] [PubMed]
2. Ascher, W. Why Governments Waste Natural Resources: Policy Failures in Developing Countries; JHU Press: Baltimore, MD, USA, 1999.
3. Hilson, G. The Africa Mining Vision: A manifesto for more inclusive extractive industry-led development? Can. J. Dev. Stud./Rev.
Can. d’études Dév. 2020, 41, 417–431. [CrossRef]
4. Kim, J.; Guillaume, B.; Chung, J.; Hwang, Y. Critical and precious materials consumption and requirement in wind energy system
in the EU 27. Appl. Energy 2015, 139, 327–334. [CrossRef]
5. Grandell, L.; Lehtilä, A.; Kivinen, M.; Koljonen, T.; Kihlman, S.; Lauri, L.S. Role of critical metals in the future markets of clean
energy technologies. Renew. Energy 2016, 95, 53–62. [CrossRef]
6. Moss, R.L.; Tzimas, E.; Kara, H.; Willis, P.; Kooroshy, J. Critical Metals in Strategic Energy Technologies: Assessing rare metals as
supply-chain bottlenecks in low-carbon energy technologies. In JRC-Scientific and Strategic Reports, European Commission Joint
Research Centre Institute for Energy and Transport; EU Publications: Luxembourg, 2011. [CrossRef]
7. Zhang, S.; Ding, Y.; Liu, B.; Chang, C.-c. Supply and demand of some critical metals and present status of their recycling in WEEE.
Waste Manag. 2017, 65, 113–127. [CrossRef]
8. Chakhmouradian, A.R.; Smith, M.P.; Kynicky, J. From “strategic” tungsten to “green” neodymium: A century of critical metals at
a glance. Ore Geol. Rev. 2015, 64, 455–458. [CrossRef]
9. Žibret, G.; Lemiere, B.; Mendez, A.-M.; Cormio, C.; Sinnett, D.; Cleall, P.; Szabó, K.; Carvalho, M.T. National Mineral Waste
Databases as an Information Source for Assessing Material Recovery Potential from Mine Waste, Tailings and Metallurgical Waste.
Minerals 2020, 10, 446. [CrossRef]
10. Kiprono, N.R.; Smolinski, T.R.; Rogowski, M.; Chmielewski, A.G. The State of Critical and Strategic Metals Recovery and the Role
of Nuclear Techniques in the Separation Technologies Development: Review. Separations 2023, 10, 112. [CrossRef]
11. Ayres, R.U.; Peiró, L.T. Material efficiency: Rare and critical metals. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2013,
371, 20110563. [CrossRef]
12. Purwadi, I.; van der Werff HM, A.; Lievens, C. Targeting rare earth element bearing mine tailings on Bangka Island, Indonesia,
with Sentinel-2 MSI. Int. J. Appl. Earth Obs. Geoinf. 2020, 88, 102055. [CrossRef]
13. Wang, L.; Liang, T. Geochemical fractions of rare earth elements in soil around a mine tailing in Baotou, China. Sci. Rep. 2015,
5, 12483. [CrossRef] [PubMed]
14. Campbell, G.A. Rare earth metals: A strategic concern. Miner. Econ. 2014, 27, 21–31. [CrossRef]
15. Fleming, P.; Orrego, P.; Pinilla, F. Recovery of Rare Earth Elements Present in Mining Tails, by Leaching with Nitric and
Hydrochloric Solutions. World J. Nucl. Sci. Technol. 2021, 11, 1–16. [CrossRef]
16. Government of the Republic of Kenya. Mining and Mineral Policy. 2016. Available online: https://www.idlo.int/sites/default/
files/pdfs/highlights/Kenya%20Mining%20Policy%20Popular%20Version-LowRes.pdf (accessed on 17 August 2023).
17. Zhang, W.; Zhu, Z.; Cheng, C.Y. A literature review of titanium metallurgical processes. Hydrometallurgy 2011, 108, 177–188.
[CrossRef]
18. Mwaura, F. An audit of environmental impact assessments for mining projects in Kenya. J. S. Afr. Inst. Min. Metall. 2019,
119, 485–493. [CrossRef] [PubMed]
19. Opongo, E. Knowledge and Policy Gaps in Extractive Industries in Kenya. SSRN Electron. J. 2017, 12, 1–138. [CrossRef]
20. Barmao, K.J.; Cherutoi, J.K.; Mitei, C.Y.; Were, M.L.L.; Kiprop, A.; Achieng’, O.G. Assessment of Fluoride and selected heavy
metals in food chain around Fluorspar mining Plant, Kenya. Greener J. Environ. Manag. Public Saf. 2019, 8, 15–24. [CrossRef]
21. Kipsang, R.B. Economic and Job Creation Economic and Job Creation Potential of Artisanal and Small-Scale Mining in Taita
Taveta County, Kenya. Natural Resources Management for Sustainable Development in Kenya Extractive Industry, UNDP. 2014.
Available online: https://www.jkuat.ac.ke/departments/mining/wp-content/uploads/2017/10/Small-Scale-Mining-n-Taita-
Taveta-County-Kenya.pdf (accessed on 12 October 2023).
22. Kaniu, I.; Iain, G.D.; Hudson, K.A. Radiological Mapping of the Alkaline Intrusive Complex of Jombo, South Coastal Kenya by
In-Situ Gamma-Ray Spectrometry. 2016. Available online: https://meetingorganizer.copernicus.org/EGU2016/EGU2016-17917-
1.pdf (accessed on 21 August 2023).
23. African Natural Resources Centre. Rare Earth Elements (REE). Value Chain Analysis for Mineral Based Industrialization in Africa.
2021. Available online: https://www.afdb.org/fr/documents/rare-earth-elements-ree-value-chain-analysis-mineral-based-
industrialization-africa (accessed on 21 August 2023).
24. Odumo, B.O.; Nanos, N.; Carbonell, G.; Torrijos, M.; Patel, J.P.; Rodríguez Martín, J.A. Artisanal gold-mining in a rural
environment: Land degradation in Kenya. Land Degrad. Dev. 2018, 29, 3285–3293. [CrossRef]
Minerals 2024, 14, 21 25 of 27

25. Ogola, J.S.; Mitullah, W.V.; Omulo, M.A. Impact of Gold Mining on the Environment and Human health: A Case Study in the
Migori Gold Belt, Kenya. Environ. Geochem. Health 2002, 24, 141–158. [CrossRef]
26. Government of Kenya. Kenya Mining Investment Handbook. 2016. Available online: https://www.tralac.org/documents/
resources/by-country/kenya/1928-kenya-mining-investment-handbook-2016/file.html (accessed on 21 August 2023).
27. Bett, A.K.; Maranga, S.M. Considerations for Beneficiation of Low Grade Iron Ore for Steel Making in Kenya. In Proceedings
of the Sustainable Research and Innovation Conference 2014, Nairobi, Kenya, 3–4 May 2012; pp. 263–267. Available online:
https://sri.jkuat.ac.ke/jkuatsri/index.php/sri/article/view/481 (accessed on 12 October 2023).
28. Bourdon, E.; Kalt, A.; Meisel, T.; Kaeser, B.; Bourdon, E.; Kalt, A.; Meisel, T.; Kaeser, B. Platinum-group element abundances in
xenoliths from Marsabit Volcanic field (Kenya Rift). AGUFM 2006, 2006, V31D-0609. Available online: https://ui.adsabs.harvard.
edu/abs/2006AGUFM.V31D0609B/abstract (accessed on 24 August 2023).
29. Henderson, P.; Pickford, M.; Williams, C.T. A geochemical study of rocks and spring waters at Kanam and Kanjera, Kenya, and
the implications concerning element mobility and uptake. J. Afr. Earth Sci. 1987, 6, 221–227. [CrossRef]
30. Compaore, W.F.; Dumoulin, A.; Rousseau DP, L. Gold Mine Impact on Soil Quality, Youga, Southern Burkina Faso, West Africa.
Water Air Soil Pollut. 2019, 230, 207. [CrossRef]
31. Ceniceros-Gómez, A.E.; Macías-Macías, K.Y.; de la Cruz-Moreno, J.E.; Gutiérrez-Ruiz, M.E.; Martínez-Jardines, L.G. Charac-
terization of mining tailings in México for the possible recovery of strategic elements. J. S. Am. Earth Sci. 2018, 88, 72–79.
[CrossRef]
32. Antwi-Agyei, P.; Hogarh, J.N.; Foli, G. Trace elements contamination of soils around gold mine tailings dams at Obuasi, Ghana.
Afr. J. Environ. Sci. Technol. 2009, 3, 353–359. Available online: https://www.ajol.info/index.php/ajest/article/view/56263
(accessed on 12 April 2023).
33. Matinde, E.; Simate, G.S.; Ndlovu, S. Mining and metallurgical wastes: A review of recycling and re-use practices. J. S. Afr. Inst.
Min. Metall. 2018, 118, 825–844. [CrossRef]
34. Bakatula, E.N.; Cukrowska, E.M.; Straker, C.J.; Weiersbye, I.M.; Tutu, H.; Rüde, T.R.; Freund, A.; Wolkersdorfer, C. Biosorption
of heavy metals from gold-mine wastewaters by Penicillium simplicissimum immobilised on zeolite: Kinetic, equilibrium and
thermodynamic study. In Mine Water–Managing the Challenges; Rüde, T., Freund, A., Wolkersdorfer, C., Eds.; International Mine
Water Association: Aachen, Germany, 2011; pp. 271–272.
35. Bridge, G. Contested terrain: Mining and the environment. Annu. Rev. Environ. Resour. 2004, 29, 205–259. [CrossRef]
36. Oladipo, H.J.; Tajudeen, Y.A.; Taiwo, E.O.; Muili, A.O.; Yusuf, R.O.; Jimoh, S.A.; Oladipo, M.K.; Oladunjoye, I.O.; Egbewande,
O.M.; Sodiq, Y.I.; et al. Global Environmental Health Impacts of Rare Earth Metals: Insights for Research and Policy Making in
Africa. Challenges 2023, 14, 20. [CrossRef]
37. Ndlovu, S.; Simate, G.S.; Matinde, E. Waste Production and Utilization in the Metal Extraction Industry. In Waste Production and
Utilization in the Metal Extraction Industry; CRC Press: Boca Raton, FL, USA, 2017; pp. 1–512. [CrossRef]
38. Chmielewski, A.G.; Wawszczak, D.; Brykała, M. Possibility of uranium and rare metal recovery in the Polish copper mining
industry. Hydrometallurgy 2016, 159, 12–18. [CrossRef]
39. Kiegiel, K.; Gajda, D.; Zakrzewska-Kołtuniewicz, G. Recovery of uranium and other valuable metals from substrates and waste
from copper and phosphate industries. Sep. Sci. Technol. 2020, 55, 2099–2107. [CrossRef]
40. Tunsu, C.; Menard, Y.; Eriksen, D.Ø.; Ekberg, C.; Petranikova, M. Recovery of critical materials from mine tailings: A comparative
study of the solvent extraction of rare earths using acidic, solvating and mixed extractant systems. J. Clean. Prod. 2019, 218, 425–437.
[CrossRef]
41. Zhuang, W.Q.; Fitts, J.P.; Ajo-Franklin, C.M.; Maes, S.; Alvarez-Cohen, L.; Hennebel, T. Recovery of critical metals using
biometallurgy. Curr. Opin. Biotechnol. 2015, 33, 327–335. [CrossRef] [PubMed]
42. Blengini, G.A.; Mathieux, F.; Mancini, L.; Nyberg, M.; Cavaco Viegas, H.; Salminen, J.; Garbarino, E.; Orveillion, G.; Saveyn, H.
Recovery of Critical and Other Raw Materials from Mining Waste and Landfills: State of Play on Existing Practices; EU Publications:
Luxembourg, 2019. [CrossRef]
43. Ngure, V.; Kinuthia, G. Health risk implications of lead, cadmium, zinc, and nickel for consumers of food items in Migori Gold
mines, Kenya. J. Geochem. Explor. 2020, 209, 106430. [CrossRef]
44. Belardi, G.; Piga, L.; Quaresima, S.; Shehu, N. Application of physical separation methods for the upgrading of titanium dioxide
contained in a fine waste. Int. J. Miner. Process. 1998, 53, 145–156. [CrossRef]
45. Kyalo, M.N.; Munyerere, I.F.; Rop, B.; Maranga, S.M. Scouring abandoned mines in search for elusive metal (gold) in Kakamega’s
Rosterman area-A case study in Kenya. In Proceedings of the Sustainable Research and Innovation Conference 2015, Nairobi,
Kenya, 6–8 May 2015; pp. 362–366.
46. Richardson, J.F.; Harker, J.H.; Backhurst, J.R. Coulson and Richardson’s Chemical Engineering, 5th ed.; Butterworth-Heinemann:
Oxford, UK, 2002; Volume 2.
47. Bertuol, D.A.; Amado, F.R.; Cruz, E.D.; Tanabe, E.H. Metal recovery using supercritical carbon dioxide. In Green Sustainable
Process for Chemical and Environmental Engineering and Science: Supercritical Carbon Dioxide as Green Solvent; Elsevier: Amsterdam,
The Netherlands, 2020; pp. 85–103. [CrossRef]
48. Kumar, P.A.; Vengatasalam, R. Mineral Beneficiation by Heap Leaching Technique in Mining. Procedia Earth Planet. Sci. 2015,
11, 140–148. [CrossRef]
Minerals 2024, 14, 21 26 of 27

49. Thenepalli, T.; Chilakala, R.; Habte, L.; Tuan, L.Q.; Kim, C.S. A brief note on the heap leaching technologies for the recovery of
valuable metals. Sustainability 2019, 11, 3347. [CrossRef]
50. Lommelen, R.; Vander Hoogerstraete, T.; Onghena, B.; Billard, I.; Binnemans, K. Model for Metal Extraction from Chloride Media
with Basic Extractants: A Coordination Chemistry Approach. Inorg. Chem. 2019, 58, 12289–12301. [CrossRef] [PubMed]
51. Alvial-Hein, G.; Mahandra, H.; Ghahreman, A. Separation and recovery of cobalt and nickel from end of life products via solvent
extraction technique: A review. J. Clean. Prod. 2021, 297, 126592. [CrossRef]
52. Swain, N.; Mishra, S. A review on the recovery and separation of rare earths and transition metals from secondary resources. J.
Clean. Prod. 2019, 220, 884–898. [CrossRef]
53. Gajda, B.; Mariusz, B.B. The Effect of Tributyl Phosphate on the Extraction of Nickel(II) And Cobalt(II) ions with Di(2-
Ethylhexyl)Phosphoric Acid. Physicochem. Probl. Miner. Process. 2007, 41, 145–152.
54. Huang, Y.; Guo, H.; Zhang, C.; Liu, B.; Wang, L.; Peng, W.; Cao, Y.; Song, X.; Zhu, X. A novel method for the separation of zinc
and cobalt from hazardous zinc–cobalt slag via an alkaline glycine solution. Sep. Purif. Technol. 2021, 273, 119009. [CrossRef]
55. Choubey, P.K.; Dinkar, O.S.; Panda, R.; Kumari, A.; Jha, M.K.; Pathak, D.D. Selective extraction and separation of Li, Co and Mn
from leach liquor of discarded lithium ion batteries (LIBs). Waste Manag. 2021, 121, 452–457. [CrossRef] [PubMed]
56. Wang, L.Y.; Lee, M.S.; Wang, L.Y.; Lee, M.S. Separation of Co(II) and Ni(II) from chloride leach solution of nickel laterite ore by
solvent extraction with Cyanex 301. IJMP 2017, 166, 45–52. [CrossRef]
57. Niobium, Tantalum, Vanadium and Zirconium Ore in Kenya. The Observatory of Economic Complexity. 2023. Available online:
https://oec.world/en/profile/bilateral-product/niobium-tantalum-vanadium-and-zirconium-ore/reporter/ken (accessed on
12 October 2023).
58. Peng, H.; Zhang, C.; Hao, Z.; Jiang, S.; Guo, J.; Huang, H.; Li, B. Vanadium recovery by glycine precipitation. Environ. Chem. Lett.
2022, 20, 1569–1575. [CrossRef]
59. Cai, Z.; Feng, Y.; Li, H.; Zhou, Y. Selective separation and extraction of vanadium(IV) and manganese(II) from co-leaching solution
of roasted stone coal and pyrolusite via solvent extraction. Ind. Eng. Chem. Res. 2013, 52, 13768–13776. [CrossRef]
60. Pospiech, B. Synergistic Solvent Extraction and Transport of Zn(II) and Cu(II) across Polymer Inclusion Membranes with a
Mixture of TOPO and Aliquat 336. Sep. Sci. Technol. 2014, 49, 1706–1712. [CrossRef]
61. Rogowski, M.; Smolinski, T.; Pyszynska, M.; Brykala, M.; Chmielewski, A.G. Studies on hydrometallurgical processes using
nuclear techniques to be applied in copper industry. II. Application of radiotracers in copper leaching from flotation tailings.
Nukleonika 2018, 63, 131–137. [CrossRef]
62. Smolinski, T.; Wawszczak, D.; Deptula, A.; Lada, W.; Olczak, T.; Rogowski, M.; Pyszynska, M.; Chmielewski, A.G. Solvent
extraction of Cu, Mo, V, and U from leach solutions of copper ore and flotation tailings. J. Radioanal. Nucl. Chem. 2017, 314, 69–75.
[CrossRef]
63. Parent, M.; Cornelis, R.; Dams, R. Investigation of extraction and back-extraction behaviour of platinum (IV) with rubeanic acid
in tributyl phosphate, with tributyl phosphate and with thenoyltrifuoroacetone in n-butyl alcohol-acetophenone by means of
platinum-191 radiotracer for platinum-enrichment purposes. Anal. Chim. Acta 1993, 281, 153–160. [CrossRef]
64. Abbass, M.K.; Jalhoom, M.G.; Kadhim, A.M. Extraction of Rare Earth Elements from Iraqi Phosphate Ore by Using of Tributyl
Phosphate. Eng. Technol. J. 2020, 38, 240–245. [CrossRef]
65. Hidayah, N.N.; Abidin, S.Z. The evolution of mineral processing in extraction of rare earth elements using solid-liquid extraction
over liquid-liquid extraction: A review. Miner. Eng. 2017, 112, 103–113. [CrossRef]
66. Fedorova, M.I.; Levina, A.V. Application of ionic liquid based on Aliquat 336 and D2EHPA in the extraction of transition metals.
IOP Conf. Ser. Mater. Sci. Eng. 2022, 1212, 012021. [CrossRef]
67. Shen, L.; Chen, J.; Chen, L.; Liu, C.; Zhang, D.; Zhang, Y.; Su, W.; Deng, Y. Extraction of mid-heavy rare earth metal ions from
sulphuric acid media by ionic liquid [A336][P507]. Hydrometallurgy 2016, 161, 152–159. [CrossRef]
68. Gorzin, H.; Ghaemi, A.; Hemmati, A.; Maleki, A. Studies on effective interaction parameters in extraction of Pr and Nd using
Aliquat 336 from NdFeB magnet-leaching solution: Multiple response optimizations by desirability function. J. Mol. Liq. 2021,
324, 115123. [CrossRef]
69. Ferdowsi, A.; Yoozbashizadeh, H. Solvent Extraction of Rare Earth Elements from a Nitric Acid Leach Solution of Apatite by
Mixtures of Tributyl Phosphate and Di-(2-ethylhexyl) Phosphoric Acid. Metall. Mater. Trans. B Process Metall. Mater. Process. Sci.
2017, 48, 3380–3387. [CrossRef]
70. Jürjo, S.; Siinor, L.; Siimenson, C.; Paiste, P.; Lust, E. Two-Step Solvent Extraction of Radioactive Elements and Rare Earths from
Estonian Phosphorite Ore Using Nitrated Aliquat 336 and Bis(2-ethylhexyl) Phosphate. Minerals 2021, 11, 388. [CrossRef]
71. El-Nadi, Y.A. Solvent Extraction and Its Applications on Ore Processing and Recovery of Metals: Classical Approach. Sep. Purif.
Rev. 2016, 46, 195–215. [CrossRef]
72. Blais, J.F.; Djedidi, Z.; Cheikh, R.B.; Tyagi, R.D.; Mercier, G. Metals precipitation from effluents: Review. Pract. Period. Hazard.
Toxic Radioact. Waste Manag. 2008, 12, 135–149. [CrossRef]
73. Nie, Z.R.; Ma, L.W.; Xi, X.L. “Complexation-precipitation” metal separation method system and its application in secondary
resources. Rare Met. 2014, 33, 369–378. [CrossRef]
74. Kononova, O.N.; Duba, E.V.; Shnaider, N.I.; Pozdnyakov, I.A. Ion exchange extraction of platinum (IV) and palladium (II) from
hydrochloric acid solutions. Russ. J. Appl. Chem. 2017, 90, 1239–1245. [CrossRef]
Minerals 2024, 14, 21 27 of 27

75. Cummins, P.M.; Dowling, O.; O’Connor, B.F. Ion-exchange chromatography: Basic principles and application to the partial
purification of soluble mammalian prolyl oligopeptidase. Methods Mol. Biol. 2011, 681, 215–228. [CrossRef]
76. Rodriguez-Freire, L.; Gonzalez-Estrella, J.; Li, G. Technologies for fractionation of wastewater and resource recovery. In Wastewater
Treatment Residues as Resources for Biorefinery Products and Biofuels; Elsevier: Amsterdam, The Netherlands, 2019; pp. 329–354.
[CrossRef]
77. Zewail, T.M.; Yousef, N.S. Kinetic study of heavy metal ions removal by ion exchange in batch conical air spouted bed. Alex. Eng.
J. 2015, 54, 83–90. [CrossRef]
78. Walkowiak, W. Mechanism of Selective Ion Flotation. 1. Selective Flotation of Transition Metal Cations. Sep. Sci. Technol. 1991,
26, 559–568. [CrossRef]
79. Ejtemaei, M.; Gharabaghi, M.; Irannajad, M. A review of zinc oxide mineral beneficiation using flotation method. Adv. Colloid
Interface Sci. 2014, 206, 68–78. [CrossRef] [PubMed]
80. Önal, G.; Bulut, G.; Gül, A.; Kangal, O.; Perek, K.T.; Arslan, F. Flotation of Aladağ oxide lead–zinc ores. Miner. Eng. 2005,
18, 279–282. [CrossRef]
81. Crundwell, F.K.; du Preez, N.B.; Knights BD, H. Production of cobalt from copper-cobalt ores on the African Copperbelt—An
overview. Miner. Eng. 2020, 156, 106450. [CrossRef]
82. Kar, B.; Sahoo, H.; Rath, S.S.; Das, B. Investigations on different starches as depressants for iron ore flotation. Miner. Eng. 2013,
49, 1–6. [CrossRef]
83. Lutandula, M.S.; Maloba, B. Recovery of cobalt and copper through reprocessing of tailings from flotation of oxidised ores. J.
Environ. Chem. Eng. 2013, 1, 1085–1090. [CrossRef]
84. Shen, H.; Forssberg, E. An overview of recovery of metals from slags. Waste Manag. 2003, 23, 933–949. [CrossRef]
85. Zuo, Z.; Feng, Y.; Dong, X.; Luo, S.; Ren, D.; Wang, W.; Wu, Y.; Yu, Q.; Lin, H.; Lin, X. Advances in recovery of valuable metals
and waste heat from copper slag. Fuel Process. Technol. 2022, 235, 107361. [CrossRef]
86. Bulut, G.; Perek, K.T.; Gui, A.; Arslan, F.; Onal, G. Recovery of metal values from copper slags by flotation and roasting with
pyrite. Min. Metall. Explor. 2007, 14, 13–18. [CrossRef]
87. Yakoumis, I.; Panou, M.; Moschovi, A.M.; Panias, D. Recovery of platinum group metals from spent automotive catalysts: A
review. Clean. Eng. Technol. 2021, 3, 100112. [CrossRef]
88. Forrest, K.; Yan, D.; Dunne, R. Optimisation of gold recovery by selective gold flotation for copper-gold-pyrite ores. Miner. Eng.
2001, 14, 227–241. [CrossRef]
89. Acarkan, N.; Bulut, G.; Gül, A.; Kangal, O.; Karakaş, F.; Kökkiliç, O.; Önal, G. The effect of collector’s type on gold and silver
flotation in a complex ore. Sep. Sci. Technol. 2011, 46, 283–289. [CrossRef]
90. Abaka-Wood, G.B.; Addai-Mensah, J.; Skinner, W. A study of selective flotation recovery of rare earth oxides from hematite and
quartz using hydroxamic acid as a collector. Adv. Powder Technol. 2018, 29, 1886–1899. [CrossRef]
91. Chelgani, S.C.; Rudolph, M.; Leistner, T.; Gutzmer, J.; Peuker, U.A.; Chelgani, S.C.; Rudolph, M.; Leistner, T.; Gutzmer, J.; Peuker,
U.A. A review of rare earth minerals flotation: Monazite and xenotime. IJMST 2015, 25, 877–883. [CrossRef]
92. Abaka-Wood, G.B.; Johnson, B.; Addai-Mensah, J.; Skinner, W. Recovery of Rare Earth Elements Minerals in Complex Low-Grade
Saprolite Ore by Froth Flotation. Minerals 2022, 12, 1138. [CrossRef]
93. Li, R.; Marion, C.; Espiritu ER, L.; Multani, R.; Sun, X.; Waters, K.E. Investigating the use of an ionic liquid for rare earth mineral
flotation. J. Rare Earths 2021, 39, 866–874. [CrossRef]

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