XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 603
INTRODUCTION
Minerals and metals have always been and will be the key
pillar of the economy and society. Their demand is projected
to remain robust in the future, increasing for the so-called
critical raw materials that are needed for the clean energy
infrastructure [1, 2]. For example, rare earth elements
(REE) are needed to manufacture magnets, batteries, cata-
lysts, and electronics. Cu is critical for electric energy trans-
mission and storage. The main industrial application of Zn
is anticorrosion protection, while Co is needed to produce
Li-ion batteries. However, mineral and metal resources are
limited, while their production is intrinsically invasive. In
addition, mineral and metal extraction industries are fac-
ing sustainability challenges posed by declining ore grades
combined with increasing regulatory pressure to minimize
environmental impact and improve water management.
One of the emerging strategies to address these chal-
lenges is to extract valuable elements from abundant depleted
secondary resources which include wastewater produced
by mineral and metal extraction [3, 4]. Abandoned mines
and tailings generate voluminous streams of mine and acid
mine drainage (AMD) water. Due to its acidity and the
high concentration of toxic heavy metals, this wastewater
kills the biota all around [5], which presents the second big-
gest environmental challenge after climate change [6]. The
industry acknowledges that “there is a problem with water
resources, that mines need to become more sustainable and
essentially become ‘water stewards,’ not only for the greater
good but because it does, and will continue to, directly and
negatively affect business”[7].
At the same time, mine water and AMD is a neglected
chemical trove. Apart from high concentrations (up to 50
g/L) of Ca, Mg, Na, Fe, Al, S, Cl, and Si ions, which are
produced by the dissolution of gangue minerals (silicates,
clays, and pyrite) [5, 8–13], it can contain mg/L amounts
of REE, Co, and Ni, along with up to several g/L of Cu
and Zn [3–5, 8–13]. When the large volumes of this water
are factored in, the amounts of carried valuable elements
are significant. For example, even at a flow rate as low as
2 m3/s, only one AMD stream pumps a mg/L element at
a rate of 170 kg/day or 62 t/year. These elements could be
marketed if there were an economically and environmen-
tally sustainable method of their extraction.
Such a separation method is yet to emerge. To get com-
mercialized, it should be able to economically extract target
dilute (100 mg/L) and ultradilute (1–10 mg/L) elements
from large aqueous streams that have a high level of total
dissolved solids, dispersed/suspended particles/colloids,
and dissolved organic molecules (e.g., residual flotation
reagents and natural organic matter). To reduce physical
footprint and allow scaling-up for on-site treatment, both
the uptake and discharge steps should be rapid and repeat-
able (e.g., sorbents should have fast regenerability and high
recyclability). Furthermore, the method should be capable
of efficiently removing toxic elements to levels below regu-
latory thresholds. Finally, it is preferable for the method to
be free from chemicals and sludge.
Existing separation methods have yet to meet the afore-
mentioned requirements. Even though chemical precipita-
tion and adsorption/ion exchange per se are very fast, the
recovery of precipitates or adsorbates is slow and requires
reagents (e.g., flocculants and mineral acids) and energy.
For example, the stirring-ageing time to grow sufficiently
large precipitates from a 0.5 mg/L REE coal-originated
AMD water is 24 h [14]. The settling of metal sulfide col-
loids formed at initial metal concentrations of 30–90 ppm
the chemical precipitation with HS– takes from 20 h to
several days [15]. Impressive selectivity can be obtained in
the Sc-La separation with titanium alkylphosphate-func-
tionalized mesoporous silica [16]. However, the adsorption
and desorption steps take 6 h each, while the desorption
step includes centrifuging, drying, redispersion, and acid
wash. Electro-deposition (electrowinning) is the most com-
mon method for the recovery of Cu and Zn from sulphate
solutions [17, 18]. However, this method is generally eco-
nomically attractive only at relatively high (30 g/L) con-
centrations [17], becoming impractical at concentrations
below 1 g/L [18].
Conventional approaches to improve metal extraction
rates include increasing the separating surface area and solu-
tion flow (advection) [19–21]. However, these approaches
significantly increase the materials and energy cost of the
separation process, while having their own limitations.
For example, cyclone electrowinning was recently devel-
oped and piloted for Te recovery [22]. However, this pro-
cess practically stops when reaching the Te concentration
of 0.1–1 mg/L. Electro-deposition on a high surface area
amidoxime-and graphene oxide-functionalized carbon is
highly efficient in removing heavy metals (Cu, Pb, and Cd)
from their ultradilute single-metal and mixed solutions [23,
24]. Specifically, an amidoxime-functionalized carbon elec-
trode in a two-electrode water-purification flow-through
cell at the cell voltage of 10 V drops concentrations of the
heavy metal from 100 ppb below 5 ppb at a water flow
rate of 5 mL min–1 [23]. Even more efficient is a graphene
oxide (GO)-functionalized carbon electrode operating at
the cell voltage of 20 V [24]. However, even though this
electro-deposition setup has economic advantages and the
electrodeposited metal can be recovered, there is no data on
the reuse of the regenerated electrodes.
INTRODUCTION
Minerals and metals have always been and will be the key
pillar of the economy and society. Their demand is projected
to remain robust in the future, increasing for the so-called
critical raw materials that are needed for the clean energy
infrastructure [1, 2]. For example, rare earth elements
(REE) are needed to manufacture magnets, batteries, cata-
lysts, and electronics. Cu is critical for electric energy trans-
mission and storage. The main industrial application of Zn
is anticorrosion protection, while Co is needed to produce
Li-ion batteries. However, mineral and metal resources are
limited, while their production is intrinsically invasive. In
addition, mineral and metal extraction industries are fac-
ing sustainability challenges posed by declining ore grades
combined with increasing regulatory pressure to minimize
environmental impact and improve water management.
One of the emerging strategies to address these chal-
lenges is to extract valuable elements from abundant depleted
secondary resources which include wastewater produced
by mineral and metal extraction [3, 4]. Abandoned mines
and tailings generate voluminous streams of mine and acid
mine drainage (AMD) water. Due to its acidity and the
high concentration of toxic heavy metals, this wastewater
kills the biota all around [5], which presents the second big-
gest environmental challenge after climate change [6]. The
industry acknowledges that “there is a problem with water
resources, that mines need to become more sustainable and
essentially become ‘water stewards,’ not only for the greater
good but because it does, and will continue to, directly and
negatively affect business”[7].
At the same time, mine water and AMD is a neglected
chemical trove. Apart from high concentrations (up to 50
g/L) of Ca, Mg, Na, Fe, Al, S, Cl, and Si ions, which are
produced by the dissolution of gangue minerals (silicates,
clays, and pyrite) [5, 8–13], it can contain mg/L amounts
of REE, Co, and Ni, along with up to several g/L of Cu
and Zn [3–5, 8–13]. When the large volumes of this water
are factored in, the amounts of carried valuable elements
are significant. For example, even at a flow rate as low as
2 m3/s, only one AMD stream pumps a mg/L element at
a rate of 170 kg/day or 62 t/year. These elements could be
marketed if there were an economically and environmen-
tally sustainable method of their extraction.
Such a separation method is yet to emerge. To get com-
mercialized, it should be able to economically extract target
dilute (100 mg/L) and ultradilute (1–10 mg/L) elements
from large aqueous streams that have a high level of total
dissolved solids, dispersed/suspended particles/colloids,
and dissolved organic molecules (e.g., residual flotation
reagents and natural organic matter). To reduce physical
footprint and allow scaling-up for on-site treatment, both
the uptake and discharge steps should be rapid and repeat-
able (e.g., sorbents should have fast regenerability and high
recyclability). Furthermore, the method should be capable
of efficiently removing toxic elements to levels below regu-
latory thresholds. Finally, it is preferable for the method to
be free from chemicals and sludge.
Existing separation methods have yet to meet the afore-
mentioned requirements. Even though chemical precipita-
tion and adsorption/ion exchange per se are very fast, the
recovery of precipitates or adsorbates is slow and requires
reagents (e.g., flocculants and mineral acids) and energy.
For example, the stirring-ageing time to grow sufficiently
large precipitates from a 0.5 mg/L REE coal-originated
AMD water is 24 h [14]. The settling of metal sulfide col-
loids formed at initial metal concentrations of 30–90 ppm
the chemical precipitation with HS– takes from 20 h to
several days [15]. Impressive selectivity can be obtained in
the Sc-La separation with titanium alkylphosphate-func-
tionalized mesoporous silica [16]. However, the adsorption
and desorption steps take 6 h each, while the desorption
step includes centrifuging, drying, redispersion, and acid
wash. Electro-deposition (electrowinning) is the most com-
mon method for the recovery of Cu and Zn from sulphate
solutions [17, 18]. However, this method is generally eco-
nomically attractive only at relatively high (30 g/L) con-
centrations [17], becoming impractical at concentrations
below 1 g/L [18].
Conventional approaches to improve metal extraction
rates include increasing the separating surface area and solu-
tion flow (advection) [19–21]. However, these approaches
significantly increase the materials and energy cost of the
separation process, while having their own limitations.
For example, cyclone electrowinning was recently devel-
oped and piloted for Te recovery [22]. However, this pro-
cess practically stops when reaching the Te concentration
of 0.1–1 mg/L. Electro-deposition on a high surface area
amidoxime-and graphene oxide-functionalized carbon is
highly efficient in removing heavy metals (Cu, Pb, and Cd)
from their ultradilute single-metal and mixed solutions [23,
24]. Specifically, an amidoxime-functionalized carbon elec-
trode in a two-electrode water-purification flow-through
cell at the cell voltage of 10 V drops concentrations of the
heavy metal from 100 ppb below 5 ppb at a water flow
rate of 5 mL min–1 [23]. Even more efficient is a graphene
oxide (GO)-functionalized carbon electrode operating at
the cell voltage of 20 V [24]. However, even though this
electro-deposition setup has economic advantages and the
electrodeposited metal can be recovered, there is no data on
the reuse of the regenerated electrodes.