The Twelve Principles of Circular Hydrometallurgy

Abstract

In this academic position paper, we propose the 12 Principles of a novel and more sustainable approach to hydrometallurgy that we call “circular hydrometallurgy.” The paper intends to set a basis for identifying future areas of research in the field of hydrometallurgy, while providing a “sustainability” benchmark for assessing existing processes and technological developments. Circular hydrometallurgy refers to the designing of energy-efficient and resource-efficient flowsheets or unit processes that consume the minimum quantities of reagents and result in minimum waste. The application of a circular approach involves new ways of thinking about how hydrometallurgy is applied for both primary and secondary resources. In either case, the emphasis must be on the regeneration and reuse of every reagent in the process. This refers not only to the acids and bases employed for leaching or pH control, but also any reducing agents, oxidizing agents, and other auxiliary reagents. Likewise, the consumption of water and energy must be reduced to an absolute minimum. To consolidate the concept of circular hydrometallurgical flowsheets, we present the 12 Principles that will boost sustainability: (1) regenerate reagents, (2) close water loops, (3) prevent waste, (4) maximize mass, energy, space, and time efficiency, (5) integrate materials and energy flows, (6) safely dispose of potentially harmful elements, (7) decrease activation energy, (8) electrify processes wherever possible, (9) use benign chemicals, (10) reduce chemical diversity, (11) implement real-time analysis and digital process control, and (12) combine circular hydrometallurgy with zero-waste mining. Although we realize that the choice of these principles is somewhat arbitrary and that other principles could be imagined or some principles could be merged, we are nevertheless convinced that the present framework of these 12 Principles, as put forward in this position paper, provides a powerful tool to show the direction of future research and innovation in hydrometallurgy, both in industry and in academia.
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Vol.:(0123456789)
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Journal of Sustainable Metallurgy
https://doi.org/10.1007/s40831-022-00636-3
OPINION ARTICLE
The Twelve Principles ofCircular Hydrometallurgy
KoenBinnemans1 · PeterTomJones2
Received: 1 July 2022 / Accepted: 21 November 2022
© The Author(s) 2022
Abstract
In this academic position paper, we propose the 12 Principles of a novel and more sustainable approach to hydrometallurgy
that we call “circular hydrometallurgy.” The paper intends to set a basis for identifying future areas of research in the field of
hydrometallurgy, while providing a “sustainability” benchmark for assessing existing processes and technological develop-
ments. Circular hydrometallurgy refers to the designing of energy-efficient and resource-efficient flowsheets or unit processes
that consume the minimum quantities of reagents and result in minimum waste. The application of a circular approach
involves new ways of thinking about how hydrometallurgy is applied for both primary and secondary resources. In either
case, the emphasis must be on the regeneration and reuse of every reagent in the process. This refers not only to the acids
and bases employed for leaching or pH control, but also any reducing agents, oxidizing agents, and other auxiliary reagents.
Likewise, the consumption of water and energy must be reduced to an absolute minimum. To consolidate the concept of
circular hydrometallurgical flowsheets, we present the 12 Principles that will boost sustainability: (1) regenerate reagents,
(2) close water loops, (3) prevent waste, (4) maximize mass, energy, space, and time efficiency, (5) integrate materials and
energy flows, (6) safely dispose of potentially harmful elements, (7) decrease activation energy, (8) electrify processes
wherever possible, (9) use benign chemicals, (10) reduce chemical diversity, (11) implement real-time analysis and digital
process control, and (12) combine circular hydrometallurgy with zero-waste mining. Although we realize that the choice of
these principles is somewhat arbitrary and that other principles could be imagined or some principles could be merged, we
are nevertheless convinced that the present framework of these 12 Principles, as put forward in this position paper, provides
a powerful tool to show the direction of future research and innovation in hydrometallurgy, both in industry and in academia.
The contributing editor for this article was Christina Meskers.
* Koen Binnemans
Koen.Binnemans@kuleuven.be
1 Department ofChemistry, KU Leuven, Celestijnenlaan
200F, Box2404, 3001Heverlee, Belgium
2 Department ofMaterials Engineering, KU Leuven,
Kasteelpark Arenberg 44, P.O. Box2450, 3001Heverlee,
Belgium

Journal of Sustainable Metallurgy
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Graphical Abstract
Keywords Circular economy· Extractive metallurgy· Hydrometallurgy· Solution chemistry· Sustainability
Introduction
Hydrometallurgy uses aqueous solutions to extract and refine
metals [1, 2]. Although a typical metallurgical flowsheet
tends to synergistically combine both pyrometallurgical
and hydrometallurgical unit operations, the final steps in the
purification and recovery of metals are nearly always hydro-
metallurgical. The transition to a climate-neutral society by
2050 relies heavily on the use of hydrometallurgy to extract
critical raw materials, like cobalt, nickel, and lithium for bat-
teries and rare-earth elements (REEs) for permanent mag-
nets in electric motors and wind turbines. However, we must
be careful to ensure that the processes applied to refine these
metals needed for clean-energy production do not undermine
our efforts by having an adverse environmental impact.
Large amounts of acids (H2SO4 or HCl), bases (CaO,
MgO, NH3, NaOH, or Na2CO3), and sulfides for precipita-
tion (Na2S, NaHS, H2S) are consumed in processes such as
leaching, the precipitation of iron and other impurities, the
generation of intermediates (e.g., mixed-hydroxide precipi-
tate (MHP) and mixed-sulfide precipitate (MSP)), and the
solvent-extraction (SX) and ion-exchange (IX) processes
required to separate and purify metals. By and large, conven-
tional hydrometallurgical flowsheets can be described as pre-
dominantly “linear,” in the sense that the reagents consumed
are not regenerated for subsequent reuse (although there are
examples where this is partially the case: cf., Section “Exam-
ples of (near) circular hydrometallurgical flowsheets” in the
online Supplementary Information). This can be problem-
atic because hydrometallurgical processes often give rise
to substantial amounts of both solid waste (e.g., gypsum,
goethite, or jarosite) and highly saline wastewater. Many
hydrometallurgical processes leave a large carbon footprint
because producing the reagents used in hydrometallurgy is
often energy intensive. Two of the major culprits are CaO
and MgO, which are obtained from carbonate raw materi-
als (CaCO3 and MgCO3) that require a high-temperature
calcination in kilns. It is estimated that between 978 and
1975kg of CO2 are emitted per ton of CaO produced [3].
The numbers for MgO production are similar. The familiar
Haber–Bosch and Solvay processes for ammonia production
and sodium carbonate synthesis are both classed as energy
intensive. Large amounts of energy can be consumed regen-
erating acids, a good example being the high-temperature
pyrohydrolysis process to regenerate HCl from metal chlo-
ride salts [4, 5]. The electrorefining and electrowinning pro-
cesses are also energy intensive [2], leading to large carbon
footprints when the energy is generated from fossil fuels. We
should also not underestimate the energy costs for transpor-
tation, e.g., chemicals from production facilities to mining
sites or hydrometallurgical plants.

Journal of Sustainable Metallurgy
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The past 10years have been blessed with a much greater
awareness of environmental issues in the extractive-metal-
lurgy sector. This period has seen the launch of the Journal
of Sustainable Metallurgy in 2015 [6] and an acknowledge-
ment that the total environmental impact of a metallurgi-
cal process can only be properly evaluated via a lifecycle
assessment (LCA) or, better still, via a multicriteria assess-
ment (MCA). Unfortunately, such studies do not provide the
metallurgist with practical guidelines as to how to improve
the sustainability of a metallurgical flowsheet. In the area of
environmentally friendly chemistry, the Twelve Principles of
Green Chemistry have proven their worth in making chemi-
cal syntheses safer and greener (TableS1) [7, 8]. However,
most of these principles are more closely associated with
the synthesis of organic compounds and are not relevant
to extractive metallurgy. The Twelve Principles of Green
Chemistry were reformulated for engineering practice as the
Twelve Principles of Green Engineering (TableS2) [9], and
the Nine Principles of Green Engineering of the Sandestin
Declaration (TableS3) [10]. Although some of these princi-
ples can be applied to extractive metallurgy, many others are
less relevant because they were devised for manufacturing.
At present, it is fair to say that such guidelines for extrac-
tive metallurgy are lacking. For this reason, we decided to
formulate a set of design principles adapted to the field of
hydrometallurgy that we hope will spur the development
of more sustainable hydrometallurgical processes [11, 12].
These guidelines contribute to the targets of Goal 12 of the
United Nations Sustainable Development Goals: Ensure sus-
tainable consumption and production patterns [13].
In this academic position paper, we are proposing a new
approach to sustainable metallurgy and more particularly to
sustainable hydrometallurgy. We refer to this new approach
as “circular hydrometallurgy”. The paper intends to set a
basis for identifying future areas of research in the field of
hydrometallurgy, while providing a “sustainability” bench-
mark for assessing existing processes and technological
developments. The Principles are applicable to flowsheets
for both the processing of ores and concentrates from pri-
mary mining as well as to the recycling of end-of-life, metal-
containing waste. They can also be applied to a wide variety
of metal-containing aqueous streams, such as spent elec-
trolytes from the electroplating industry. Finally, we will
show how circular hydrometallurgy can be applied to all
the metals that are processed via conventional hydrometal-
lurgy routes. As the focus of our discussions is on the unit
operations of hydrometallurgy (leaching, solution concentra-
tion and purification, and metal recovery from solution), the
unit operations of minerals processing (comminution, sizing,
physical separation methods, flotation, and drying) are not
considered in detail.
What Is Circular Hydrometallurgy?
Circular hydrometallurgy relates to the design of energy-
efficient and resource-efficient flowsheets or unit processes
that consume a minimum of reagents and produce as mini-
mum waste as possible. “Circularity” refers to the regen-
eration and reuse of all the waste-generating reagents from
the process. This not only includes the acids or bases used
for leaching and pH control, but also any reducing agents,
oxidizing agents, and other auxiliary reagents. In addition
to minimizing the consumption of chemical reagents, the
consumption of water must also be reduced. Ideally, there
should be no net consumption of reagents and no net con-
sumption of water, except for some evaporative losses. Obvi-
ously, “zero waste” is the ideal situation, but it will never
be achievable in practice. If we attempted to valorize all
the metals that are currently landfilled (often very diluted,
at ppm levels), the energy consumption would be prohibi-
tively high. Dilute streams are problematic because the
energy needed to recover the metal from such streams does
not increase linearly, but rather exponentially, with decreas-
ing metal concentrations. Moreover, there are unwanted ele-
ments like arsenic, cadmium, and mercury that need to be
safely disposed of. As a consequence, “near-zero waste” is
a much more realistic term [14, 15].
In existing hydrometallurgical flowsheets, partial circu-
larity is common (cf., Section “Examples of (near) circular
hydrometallurgical flowsheets” in the online Supplementary
Information). For instance, in solvent-extraction processes
the extractant is regenerated during the stripping of the metal
from the loaded organic phase. However, circular hydromet-
allurgy goes one step further and dictates the regeneration of
other reagents that are typically considered as consumables
in conventional linear flowsheets. For instance, during the
separation of cobalt and nickel by solvent extraction with
acidic extractants, large amounts of bases and acids are con-
sumed, respectively, for pH control and stripping [16]. As
a result, significant volumes of salt solutions are produced
(e.g., Na2SO4 solutions), which are typically discharged to
surface waters. Although attention is being paid to the water
balance in conventional flowsheets, the soluble salt balance,
the energy balance, and the input of reagents and output
of waste streams remain significant (Fig.1). It must also
be realized that the efficiency of chemical reagents is not
100%, due to the slow kinetics, impurities, side reactions,
and surface activity, and that reagent losses occur at every
stage, even though companies make great efforts to decrease
their consumption to restrain costs.
It is always possible to design a circular flowsheet by
regenerating the acids and bases that are consumed in the
process. However, every regeneration process requires
energy. In general, it is energetically more favorable to

Journal of Sustainable Metallurgy
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reduce the consumption of chemicals rather than regenerate
them, just as it is better to prevent waste (as much as pos-
sible) rather than to treat it or clean it up after it has been
created. To do this requires knowledge of the chemical reac-
tions involved and careful control of the process.
The idea of circularity in hydrometallurgy is not new.
About 100years ago, Prof. M.H. Caron stressed the impor-
tance of cyclic processes in hydrometallurgy in his inaugural
lecture at the Technische Hoogeschool Delft (now TU Delft)
in the Netherlands: “One of the most important requirements
that, in general, will be imposed on the lixiviant in hydro-
metallurgy is that it can be used in cycles without losing its
active properties or can be regenerated from its compounds
without great losses or costs.” (translated from Dutch) [17].
Unfortunately, the wide availability of cheap bulk chemicals
such as H2SO4 or CaO, and the relatively low costs associ-
ated with the landfilling of industrial process residues and
wastes have slowed the development of genuinely circular
metallurgical flowsheets. However, today’s mining and met-
allurgical engineers have the moral duty to minimize the
impact of primary mining and of the downstream process-
ing of the extracted metals on the environment. There is no
alternative to transforming linear flowsheets into circular
flowsheets. In Fig.1, a comparison is made between these
two approaches, as applied to the production of battery-
grade Co/Ni starting from Co/Ni concentrates.
The 12 Principles ofCircular
Hydrometallurgy
As guidelines for the design of circular hydrometallurgical
flowsheets, we are proposing an interrelated set of princi-
ples,: i.e., the 12 Principles of Circular Hydrometallurgy
(Table1). These principles are to help metallurgical engi-
neers achieve the goal of circularity in hydrometallurgy. As
such, they are practical guidelines, presented in the form of
Fig. 1 Comparison of a simplified linear (left) and a circular (right)
hydrometallurgical flowsheet for battery-grade cobalt and nickel pro-
duction. For the circular process, a conceptual flowsheet is shown,
with the re-introduction of protons to the system via hydrogen gas or
via SX-assisted carbonation with CO2. The acid is regenerated, and
the base-metal impurities are removed as metal carbonates. The dif-
ferent parts of the circular flowsheet will become more evident from
the discussions further in the text
Table 1 The 12 Principles of Circular Hydrometallurgy
1 Regenerate reagents
2 Close water loops
3 Prevent waste
4 Maximize mass, energy, space, and time efficiency
5 Integrate materials and energy flows
6 Safely dispose of potentially harmful elements
7 Decrease activation energy
8 Electrify processes wherever possible
9 Use benign chemicals
10 Reduce chemical diversity
11 Implement real-time analysis and digital process control
12 Combine circular hydrometallurgy with zero-waste mining

Journal of Sustainable Metallurgy
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imperatives, with each principle elaborated in more detail
below. The Principles of Circular Hydrometallurgy have
been numbered from 1 to 12. Although one could argue that
some principles are more important than others, their order
does not reflect a strict hierarchy. Some principles are more
general (e.g., Principle 1: Regenerate reagents), whereas oth-
ers are more specific (e.g., Principle 8: Electrify processes
wherever possible). The principles are not independent and
can often be combined to even more powerful principles, as
explained in a separate section.
Principle 1: Regenerate Reagents
Regenerating all the waste-producing reagents is the main
requirement when converting a linear flowsheet into a circu-
lar alternative. The regeneration must involve the minimum
energy input and the smallest consumption of auxiliary rea-
gents, with these reagents being regenerated whenever feasi-
ble. Furthermore, if we do not regenerate a reagent, we have
to cover the costs of introducing fresh reagents (additional
costs for logistics and warehousing) and waste treatment/
management. The most common operations in hydrometal-
lurgy that require the regeneration of reagents are leaching,
solvent extraction (SX) and ion exchange (IX). Reagents to
be generated are acids, bases, oxidizing and reducing agents,
as well as other auxiliary products.
Acids are the most common lixiviants in leaching opera-
tions. A comparison of the most-often-used acids can be
found in Table2. In addition to sulfuric, hydrochloric, and
nitric acid, also methanesulfonic acid (MSA) has been
added to the list. Methanesulfonic acid is an emerging acid
in hydrometallurgy [18, 19]. Protons are consumed either by
reaction of the acid with the ore minerals during the leaching
reaction or by reaction with a base. The latter neutralizes the
excess acid after leaching and increases the pH for solution
purification (e.g., removal of co-dissolved iron by precipita-
tion as a hydroxide). If the anions of the acids are kept in
the system, the acid can be regenerated by re-introducing
the consumed protons to the system. This re-introduction of
protons can be via (1) the direct reduction of dissolved metal
ions by hydrogen gas [20], (2) the oxidation of water to oxy-
gen gas at the anode during electrowinning (with release of
protons) [21], (3) the hydrolysis of highly valent metal ions
such as iron(III) [22], and (4) the formation of metal carbon-
ates through the introduction of CO2 to the solution [23].
Hence, protons originate either from hydrogen gas or from
water. CO2 only provides protons indirectly via its reaction
with water.
Sulfuric acid (H2SO4) is the main acid used in hydromet-
allurgical flowsheets. However, it is not so easy to regenerate
it from its sulfate salts by a low-temperature process. Na2SO4
is a problematic by-product that is formed by neutralization
of excess H2SO4 by NaOH. Na2SO4 has a high solubility
in water, so inhibiting its recovery from aqueous streams
and its subsequent storage in a stable solid form. Impure
Na2SO4 also has a low commercial value, and the discharge
of wastewater containing dissolved Na2SO4 into the environ-
ment is not always allowed. The salt splitting of Na2SO4 into
H2SO4 and NaOH is an attractive option because it solves
the waste issue, while simultaneously regenerating the acid
and base consumed in the process. Different electrochemi-
cal membrane processes have been developed to salt split
Na2SO4. These are often based on electrodialysis, such as
Bipolar Membrane Electrodialysis (BMED) [24, 25, 26].
The disadvantages of these methods are the slow kinetics
and that the expensive membranes are easily clogged and
fouled, requiring regular replacement.
Hydrochloric acid (HCl) is much easier to regenerate
than H2SO4, which is driving the development of chloride
hydrometallurgy. Pyrohydrolysis is applied in industry for
the regeneration of HCl [4, 5]. This process is typically car-
ried out in a spray roaster or a fluidized-bed reactor, where
the metal chlorides react with steam at high temperatures to
form the corresponding metal oxide and HCl gas. In theory,
pyrohydrolysis can be used to hydrolyze the chloride of any
multivalent metal ion to the corresponding oxide, but so far
it has been restricted in industry to MgCl2, FeCl2, and NiCl2.
Although pyrohydrolysis is an effective and well-proven
technology, the process is capital and energy intensive. Nev-
ertheless, there are major efforts to develop more energy-
efficient versions of the classic pyrohydrolysis process. At
the same time, low-temperature alternatives to pyrohydroly-
sis, which are more compatible with circular hydrometal-
lurgy, are also being developed [4, 27]. Examples include
sulfate crystallization and hydrolytic distillation. In elec-
trowinning processes for the recovery of metals from chlo-
ride electrolytes, HCl is regenerated indirectly by collecting
and burning chlorine gas evolved at the anode with hydrogen
gas. However, collecting the Cl2 gas at the anode during
electrowinning is not straightforward, and the additional step
of chlorine burning complicates the process. Therefore, the
direct electrolytic regeneration of HCl without an interme-
diate step to form Cl2 gas could be very beneficial to the
development of circular hydrometallurgical flowsheets [28,
29]. If water is oxidized at the anode with release of oxy-
gen gas, protons are formed simultaneously. These protons
migrate to the cathode where they combine with chloride
ions in solution to form hydrochloric acid. Recent advances
in the direct electrolytic splitting of seawater, with O2 gas
being produced at the anode, could initiate research into the
direct regeneration of HCl from hydrometallurgical solutions
[30]. The oxygen evolution reaction (OER) could success-
fully compete with the chlorine evolution reaction (CER) at
reasonably high current densities, if suitable anodes are used
[31]. Bipolar membrane electrodialysis (BMED) has been

Journal of Sustainable Metallurgy
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Table 2 Comparison of the main acids used in hydrometallurgy, with the inclusion of methanesulfonic acid (MSA), an “emerging acid”
Characteristic Sulfuric acid Hydrochloric acid Nitric acid Methanesulfonic acid
Commercial use Most commonly used acid in hydro-
metallurgy
Rather limited use in hydrometal-
lurgy (e.g., PGMs, Au) but high
potential
Limited use in hydrometallurgy; use-
ful for processing of sulfidic ores
and nickel laterites
Not used in hydrometallurgical
industry yet, but very promising.
Similarities with sulfuric acid but
much better solubility of salts
Solubility of its salts Sulfates are less soluble than salts of
other acids; poorly soluble double
salts
Chlorides are easily soluble in water,
with the exception of PbCl2 and
AgCl (dissolve via complex forma-
tion at high chloride concentra-
tions)
Nitrates are highly soluble in water Methanesulfonate salts are highly
soluble in water
Volatility Low volatility High volatility High volatility Low volatility
Oxidizing/reducing character Non-oxidizing in dilute solutions;
oxidizing as concentrated acid
Reducing acid (formation of Cl2);
HCl + Cl2 is strongly oxidizing
Oxidizing acid (formation of NOx) Non-oxidizing in dilute and concen-
trated forms
Corrosiveness Moderately corrosive for reactor
materials
Highly corrosive (stainless-steel reac-
tors cannot be used, but titanium
reactors can)
Less corrosive than HCl (stainless-
steel reactors can be used)
Very low corrosivity
Compatibility with electrowinning Electrowinning of numerous metals
in H2SO4 solutions is used industri-
ally and yields oxygen gas at anode
Electrowinning of metals in chloride
solutions is uncommon because it
is difficult to reduce chloro com-
plexes and release
Cl2 gas at anode (unless special
anodes are used)
Difficult to combine with electrowin-
ning
Excellent acid for electrowinning and
yields oxygen gas at anode. Used
industrially (e.g., tin electroplating)
Ease of regeneration (Principle 1) Difficult to regenerate Easier to regenerate than H2SO4, but
often energy intensive; regenera-
tion via metal sulfate precipitation
possible
Easier to regenerate than H2SO4;
regeneration via metal sulfate
precipitation possible
Limited studies available on regenera-
tion; regeneration via metal sulfate
precipitation possible
Smell Odorless Pungent smell Acrid odor Odorless
Biocompatibility Formed by oxidation of sulfide min-
erals by air or bacteria
Occurs in gastric acid (stomach acid) Present in nature in very minute
amounts
Part of the natural sulfur cycle
Cost (OPEX) Cheap Relatively cheap Moderate cost Moderate cost

Journal of Sustainable Metallurgy
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successfully used to generate concentrated HCl and NaOH
solutions from concentrated brines [32], and this method
could be extended to concentrated chloride solutions, pro-
vided that membranes with sufficiently long lifetimes can
be developed.
Nitric acid (HNO3) has been used for leaching much less
than H2SO4 or HCl, mainly because it is more expensive.
In the past, nitric acid processes were often considered as
environmentally unfriendly and unsafe. But this perception
is changing: HNO3 leaching is gaining traction in hydro-
metallurgy, for instance in the processing of nickel laterites
[33, 34, 35]. Metal nitrates are highly soluble in water. In
fact, there are no poorly soluble metal nitrates, and even
Pb(NO3)2 is relatively soluble. This makes it possible to
regenerate HNO3 via metal sulfate precipitation, for instance
by precipitating gypsum with the addition of a H2SO4 solu-
tion [36, 37, 38]. Iron can be recovered from HNO3 solutions
by the hydrolysis of Fe(NO3)3, leading to the formation of
hematite and the regeneration of HNO3 [39, 40].
When aqueous ammonia solution (NH3(aq) or NH4OH)
is used for alkaline leaching (of Co, Ni, Cu, or Zn), the
ammonia can be recovered from the leachate by boiling the
solution. This will volatilize NH3 gas and the dissolved met-
als are precipitated as hydroxides. If leaching is done by
a mixture of ammonia solution and ammonium carbonate
(Schnabel process) [41], the excess of NH3 is driven out by
steam heating and (NH4)2CO3 is thermally decomposed to
NH3 and CO2. The NH3 and CO2 gases are recovered and
recombined to regenerate the lixiviant.
In addition to acids or bases, oxidizing agents are often
used for leaching, especially in the processing of sulfidic
ores and concentrates. If these oxidizing agents are transi-
tion-metal salts, such as Fe2(SO4)3, FeCl3, and CuCl2, they
can be regenerated after the leaching step. Different methods
are available for the re-oxidization of Fe(II) to Fe(III), pref-
erably by (1) electrolytic oxidation, (2) the addition of air or
oxygen under pressure, (3) aeration at ambient temperature,
or (4) the use of bacteria in the presence of oxygen gas [42].
Regeneration of reagents is common in solvent extraction
(SX) and ion exchange (IX). For example, in SX, the sol-
vent is regenerated during the stripping of metals of interest
so that it can be reused for loading with metal in the next
extraction cycle. In the case of solvating extractants (e.g.,
TBP or Cyanex 923) and basic extractants (e.g., Aliquat 336
or Alamine 336), stripping can be achieved with water and
the metal is back extracted as a salt from the organic phase.
Metals extracted by an acidic extractant (e.g., D2EHPA
or Cyanex 272) are stripped from a loaded organic phase,
typically by using an aqueous solution of the acid. This acid
exchanges its proton for a metal bound to the extractant,
hence, regenerating the acidic extractant. Concerning the
regeneration of reagents, ion exchange raises similar issues
to solvent extraction. When cation exchangers are loaded
with metal ions, these metals can be removed from the
loaded resin by treatment with an acidic aqueous solution,
either in a batch reactor, in a continuous reactor or in a col-
umn, so that the metal ions are exchanged by protons and
the resin is regenerated in its acidic form (i.e., the H+ form).
Strongly basic anion exchangers are transformed into their
hydroxide form (OH−) by treating with an aqueous solution
of a strong base, typically a NaOH solution.
A good example to illustrate how reagents are gener-
ated in hydrometallurgical flowsheets is the solvent extrac-
tion–electrowinning (SX-EW) process in copper metallurgy,
sometimes referred to as the leach–solvent extraction–elec-
trowinning (L-SW-EW) process (more details are shown
in Section “Examples of (near) circular hydrometallurgical
flowsheets” in the online Supplementary Information). This
process represents the industrial benchmark for the extrac-
tion of copper from oxidic ores by leaching with dilute
H2SO4, because of its simplicity, elegance, and small envi-
ronmental footprint [43, 44, 45]. The process flowsheet is an
almost completely closed cycle and, in theory, only copper
oxide and electrical power are required to produce ultrapure
copper metal. The flowsheet is composed of three intercon-
nected closed circuits: the leaching (solubilization of Cu),
solvent extraction (up-concentration and purification of Cu
solution), and electrowinning (recovery of Cu metal) cir-
cuits. See Fig.2 for a schematic representation. The protons
that are consumed in the leaching step are regenerated in the
electrowinning circuit by oxidizing the water at the cath-
ode to oxygen gas, with the associated release of protons to
the solutions. Via the SX circuit, the protons are transferred
from the electrowinning circuit to the leaching circuit. The
only waste produced by the process is the barren rocks that
are left behind as tailings after the copper has been extracted
from the ore by heap leaching. Although these tailings are
visually not very appealing, they are physically and chemi-
cally stable and their storage does not pose environmental
risks. Gypsum is formed as a secondary mineral if the cop-
per deposit contains calcium-rich gangue minerals.
Although an SX-EW flowsheet for copper recovery and
refining appears fully circular at first sight, we must realize
that only the copper is recovered in the basic configuration
of the flowsheet and none of the valuable trace elements that
might be present in the copper ore (e.g., Au, Ag, Te, Co) are
valorized. The process is only of a closed-loop nature for
copper; the H2SO4 consumed in a reaction with impurities
in the ore should be compensated by addition of a make-up
acid.

Journal of Sustainable Metallurgy
1 3
Principle 2: Close Water Loops
Although water can be considered as a reagent (and can,
thus, be addressed in Principle 1), the omnipresence of water
in hydrometallurgical processes justifies a separate Principle
devoted specifically to water recycling. Water has different
functions in a hydrometallurgical plant [46]. Water is an
efficient way to transport particles within and between pro-
cesses as a pulp, and to mix particles. It can act as a solvent
for acid, bases, and other auxiliaries used in hydrometallur-
gical processes. It dissolves metal salts, and it is important
for washing of the solid residue after solid–liquid separation
in the leaching process. Ideally, no more water is consumed
than it is lost by evaporation or consumed by oxidation at
the anode during electrowinning (releasing H+ and O2 gas).
Closing water loops implies not only the recycling of process
water, but also the purification of water to avoid the buildup
of toxic elements or salts that might precipitate at undesired
stages of the process. When closing water loops, the plant
is technically isolated from any surrounding water systems
[47]. In a hydrometallurgical plant, it is essential to monitor
the water balance, which consists of the different input and
output streams, as well as the internal recycle streams. Water
balance analysis is essential for the design of the hydromet-
allurgical plant. The quality of the process water can have a
direct impact on the different unit processes, as well as on
the quality of the final products [48, 49].
This Principle is not limited to the recycling and reuse of
process water; it also implies limiting the use of water [50].
One approach to achieve that objective is to increase the
solid-to-liquid ratio in the leaching process. This not only
reduces the use of water and reagents, but also leads to more
concentrated leachates so that further downstream process-
ing is possible and metal recovery becomes more efficient.
However, there are limits to the maximum concentrations
of metals in solution, since exceeding the solubility product
will lead to precipitation or crystallization. Concurrently, we
can try to work with reagents that have concentrations which
are as high as possible, without creating safety issues (e.g.,
HCl, H2SO4, NaOH, NH3, and H2O2).
Mining and beneficiation consume vast quantities of
water, especially the flotation process. Flotation typically
takes place at 25 to 35% solids by mass [51]. A flotation
plant not reusing or recycling any water would, thus, require
1.9 to 3.0 m3 of water per ton of ore processed.
Although relatively small volumes of water are used in
hydrometallurgical flowsheets, and the environmental impact
of this water use is less severe than water use in the mining
industry, using less water should still be a major objective in
the development of circular flowsheets in hydrometallurgy.
Major efforts have been made to reduce the consumption
of water in the mining industry, and several of these tech-
niques are applicable to the hydrometallurgical industry as
well [47, 52].
After leaching, a solid–liquid separation step, such as
countercurrent decantation or filtration with cake washing,
is applied to the leach slurry to separate the solid residue
from the pregnant leach solution [53]. In general, most of
the water entering the system originates from washing of
the leach residues or other filter cakes. An optimum must be
found between the metal recovery and the cost to eliminate
and reuse the excess water. Filtration equipment needs to be
designed not only for efficient solid–liquid separation, but
also for efficient washing with the minimum consumption of
water. In some cases, a belt filter with countercurrent wash-
ing is the best option; in other cases, a membrane filter press
is a better choice. Efficient dewatering of the solid residue is
essential for closing the water loop [54, 55]. This dewatering
can include a drying step, but this is normally an energy-
intensive process. Emerging technologies for dewatering and
drying include (1) enhanced dewatering with flocculants,
surfactants, and super-absorbents [56, 57]; (2) electric-field-
assisted dewatering, also called electro-dewatering [58]; and
(3) microwave drying [59].
It is essential to avoid the buildup of impurities in the
water streams. Multivalent metal ions can be removed from
aqueous solutions by ion exchange if the metal concentra-
tions are low, with solvent extraction being more suitable
for higher concentrations (> 500ppm) [60]. A more mod-
ern approach to the treatment of hydrometallurgical efflu-
ents is the use of membrane techniques [26]. However, the
technological feasibility of membrane processes, which are
considered delicate, remains an issue in hydrometallurgy.
Fig. 2 Schematic flowsheet of the leach−solvent extraction−elec-
trowinning (L-SX-EW) process for recovery of copper from oxidic
ores. Adapted after ref. [43]

Journal of Sustainable Metallurgy
1 3
Impurities in end-of-pipe streams need to be reduced to very
low concentrations (often to ppb level) to avoid membrane
fouling. Impurities include not only multivalent metal ions,
but also fluoride and carbon compounds. Also, the genera-
tion of solutions with high acid concentrations by salt split-
ting is complicated due to the limited resistance of many
membrane types to highly acidic solutions. A method that is
particularly common for the removal of low concentrations
of metals from aqueous solutions, especially for potentially
harmful elements, is the use of adsorbents [61]. Another
method is the electrolytical removal of metals by means of
a fluidized-bed electrode (FBE) [62, 63]. This method can
be used to reduce the metal concentrations to below 1mg
L−1, but it can only be applied to metal ions that can be elec-
trodeposited from aqueous solutions. These include copper,
cobalt, nickel, zinc, cadmium, lead, and precious metals. For
more information on fluidized-bed electrodes, see Principle
8. To avoid the buildup of impurities in the aqueous stream,
the whole volume of the liquid stream can be treated or it
can be opted for one or more bleed streams that are treated
to eliminate difficult to remove impurities.
Low concentrations of metal-containing sulfate solutions
can be treated by the bacterial reduction of transition-metal
sulfates to their sulfides (e.g., ZnS and CuS) with sulfate-
reducing bacteria and the subsequent use of the biogenic
sulfide precipitate as a raw material. This is a waste-free
process to clean water from metal sulfates [64].
Principle 3: Prevent Waste
Waste generated by hydrometallurgical processes can be
divided into two main categories: (1) parts of the raw mate-
rials that do not find a use (e.g., tailings, leach residues,
soluble salts); (2) remains of reagents that are not recycled
(e.g., sulfates precipitated as (impure) gypsum, chlorides in
wastewater). The first category also includes elements that
are fundamentally unusable and often hazardous or toxic
that need safe disposal (see Principle 5). This category also
includes potentially usable elements, but of insufficient qual-
ity and/or quantity for economic valorization, such as resi-
dues rich in iron, calcium, or silicon.
All new hydrometallurgical processes need to be designed
to prevent waste as much as possible, while existing pro-
cesses must be adapted to produce less waste. An additional
issue is that with decreasing grades of metal ores, the feed
materials are becoming more complex and contain more
impurities. In particular, the increasing iron content in a feed
raises concerns. Consequently, it takes more reagents and
energy to process these low-grade raw materials into usable
products and metals.
The treatment of ores or concentrates by conventional
hydrometallurgy produces solid residues and liquid waste
solutions [65]. While slags produced by pyrometallurgy are
relatively stable when being stored outside, the residues left
after leaching are generally much less stable and usually
contain soluble components. The disposal of these residues
in tailing ponds might be hazardous because of the danger of
contaminating the surface and ground waters. Tailings-dam
failures can lead to environmental disasters and even to the
loss of human life, as illustrated by the 2010 Ajka alumina
refining plant accident in Hungary [66]. Furthermore, tailing
ponds often occupy large areas of land, which is especially
problematic in densely populated regions [67].
Leaching residues could find applications as raw mate-
rials to produce construction materials such as bricks, but
extensive testing is required to establish the immobilization
and non-leachability of traces of heavy metals. These leach-
ing studies must conform to regulations, but these regula-
tions are very country specific. Leachable heavy metals are
a showstopper for the valorization of industrial process resi-
dues, including metallurgical leaching residues. Even if the
material conforms to the regulations, there will be concerns
over the potential release of immobilized heavy metals if
these materials are to be recycled in the future (cf. second
life) [14].
Depending on the way the ore is treated, an element might
end up in a waste stream or it could be transformed into a
valuable resource. Take for instance sulfur, which is of key
relevance given the huge importance of sulfidic ores in pri-
mary mining and the related acid mine drainage problems
created by sulfur-rich tailings. The complete oxidization
of sulfide ions to sulfate might be followed by the removal
of the sulfate ions as gypsum, which could be used as raw
material in the construction industry. However, the gypsum
for this application must be of high purity, free of toxic trace
elements, and radioactive impurities. Unfortunately, the gyp-
sum precipitated from pregnant leach solutions is seldom of
an acceptable purity, so that the impure gypsum creates a
waste problem. However, sulfur from sulfidic ores can also
be removed in the form of elemental sulfur [68], which is a
useful raw material for the chemical and construction indus-
tries [69, 70]. At present, most sulfur is produced by the
desulfurization of crude oil. Given the transition away from
fossil fuels, it is likely that in the near future, the elemental
sulfur produced in oil refineries will not be sufficient to meet
the demands of the chemical industry. A strong acid can be
used to release H2S gas from sulfidic ores or concentrates.
Although H2S is a toxic gas, it can be accommodated by
the chemical industry, and intensive research activities are
directed towards more sustainable H2S control. In the chemi-
cal industry, the Claus process is widely used to remove
H2S from gas streams [71, 72] by catalytically converting
the H2S to elemental sulfur. The H2S released by the acid
leaching of sulfides can be collected and used in another part
of the flowsheet for sulfide precipitation, for instance in the

Journal of Sustainable Metallurgy
1 3
formation of mixed-sulfide precipitates (MSPs). It is also
possible to convert the sulfidic ores and concentrates directly
to elemental sulfur by selecting suitable oxidation condi-
tions. Careful control of the reaction conditions is required
to avoid partial oxidation of the sulfide to oxidation states
higher than the zero-valent state of elemental sulfur. The
quality of the sulfur might be challenging due to the metal
impurities.
Also waste generation by gaseous emissions must be con-
sidered in hydrometallurgical processes. Emission can be
due to formation of gaseous reaction products during the
leaching (e.g., H2S or CO2), or to volatile lixiviants (e.g.,
NH3 or HCl). Gaseous emission control is only possible
if the leaching operation is carried out in a closed reactor
such as an autoclave or if the gases can be collected via air
extraction points above an open vessel. It is very difficult
to have gaseous emission control in large-scale open leach-
ing operations, such as in the case of heap leaching. In the
previous paragraph, it was described how H2S formed by
non-oxidative acid leaching of sulfides can be treated and
transformed into elemental sulfur. Large volumes of CO2 are
generated during the leaching of carbonates with acids. For-
mation of CO2 during leaching can be avoided by switching
to leaching with aqueous ammonia solution or by mixtures
of ammonia solution and ammonium salts. In principle, it
is possible to develop hydrometallurgical processes with
negative CO2 emissions, i.e., processes that consume rather
than emit CO2, providing that the CO2 emitted during acid
leaching is captured and the divalent metal impurities in the
process streams are precipitated in the form of metal carbon-
ates: MCO3 (M = Ca, Mg, Mn, Fe). These carbonates are
environmentally friendly and are similar to natural carbonate
minerals. The carbonates can be precipitated by the addition
of Na2CO3, but this consumes valuable chemicals, and it
does not reintroduce protons to the system to close the pro-
ton loop (so no acids are regenerated). Therefore, we could
use CO2 as a precipitating agent (M = Ca, Mg, Mn, Fe) [73]:
This reaction will not go to completion when the gener-
ated HCl remains in the liquid phase because the equilibrium
in Eq.(1) is shifted to the left. The solution to this problem
is removal of the HCl from the aqueous solution by solvent
extraction to drive the equilibrium to the right (= SX-assisted
carbonation, see also Fig.1) [74, 75].
(1)
MCl2(aq)+CO2(aq)+H2O(aq)⇌MCO3↓+2HCl(aq).
Principle 4: Maximize Mass, Energy, Space,
andTime Eciency
Hydrometallurgical processes should be designed to maxi-
mize mass, energy, space, and time efficiency. This is also
one of the principles of green engineering [9]. This Principle
is not only important for the design of sustainable processes;
it also matters to the economics of a hydrometallurgical
plant, since it maximizes the recovery of valuable elements
or products at minimum cost.
Maximizing mass efficiency implies minimizing the con-
sumption of reagents. For instance, leach with an excess
of acid should not be done, which subsequently needs to
be neutralized by the addition of base. Maximizing mass
efficiency is closely related to Principle 3 (Prevent waste)
and is equivalent to the concept of “atom economy” in green
chemistry.
Maximizing energy efficiency can be achieved by process-
ing at room temperature or at slightly elevated temperatures,
and under atmospheric pressure. The heat generated in one
process is used to supply the heat to another process (see
also Principle 5). The pumping of solutions is limited as
much as possible, because these processes are very energy
intensive. Tailored agitators are used for efficient stirring of
solutions. This is essential for good process control and mass
transfer. Reactions are employed, which generate product
streams that are as pure as possible, so that the number of
separation steps and recycling streams can be minimized.
If possible, use selective reagents, or better still, specific
reagents. Unfortunately, most of the reagents used in hydro-
metallurgy are neither selective nor specific. But selectiv-
ity can be increased by determining the optimum process
conditions.
Maximizing space efficiency means processes should be
designed in such a way that they occupy the smallest possi-
ble space. This will inevitably lead to a reduced environmen-
tal footprint because smaller reactor volumes require less
heating or cooling, lower volumes of wastewater are gener-
ated, etc. An additional benefit of space efficiency is that
smaller reactor vessels, and, more generally, smaller plants
are required, which has a positive effect on the CAPEX of
the process. One obvious approach to maximizing space effi-
ciency is to work with concentrated rather than dilute solu-
tions. Also, the choice of the technology is important, for
instance mixer-settlers versus columns in solvent extraction.
Maximizing time efficiency means striving for faster reac-
tion kinetics (see also Principle 7). The longer a reaction
takes to proceed, the more time is required for the through-
put of the ore or concentrate through the flowsheet. This
leads to the inefficient use of infrastructure and impedes a
high space efficiency.

Journal of Sustainable Metallurgy
1 3
To maximize efficiency, the hydrometallurgical indus-
try can look to the good practices of the chemical industry.
Continuous processes are to be preferred over batch pro-
cesses. Reactions in closed systems allow a better control
of emissions and effluents. Recent findings have empha-
sized the benefits of process intensification, making it pos-
sible to reduce the physical size of chemical plants [76].
Although solvent-extraction processes are typically run in a
continuous mode, and ion exchange can be continuous, we
are convinced that there remain opportunities in hydromet-
allurgy for flow technology. It is important to realize that
the different techniques that are used for solution purifica-
tion or for separation have an optimum concentration range
(Fig.3) [60]. For instance, solvent extraction is the method
of choice for concentrated solutions, whereas ion exchange
is preferred for dilute solutions (although there is some over-
lap). Of course, a separation technique can be used outside
its optimum concentration range, but it will be less efficient.
In many cases, it is preferable to purify process solu-
tions by solvent extraction or ion exchange, rather than by
precipitation. When compounds are precipitated from solu-
tions containing high concentrations of impurities, there
is a major risk of co-precipitating impurities, either by the
incorporation of impurity ions in the crystal structure of
the precipitated solids, by the occlusion of small droplets
of the mother liquor in the solid, or by adsorption of metal
ions on the surface of the precipitate. At first sight, this
might appear advantageous because potentially harmful ele-
ments can be removed together with the main impurities,
for example, during the combined removal of arsenic and
iron from leachates. However, as discussed later (Principle
6), this approach leads to contaminated solids that are dif-
ficult to valorize. The purification of solutions by multiple
precipitations and re-dissolutions implies a large number of
solid–liquid separations, and these are less attractive from
an engineering point of view than liquid–liquid separations.
If possible, the pregnant leach solution should be purified by
unit process operations that do not require precipitation steps
(e.g., solvent extraction or ion exchange) and the formation
of solids should be restricted to the recovery step, where the
final product is recovered from a purified process solution
by precipitation or crystallization.
An example of how the efficiency of hydrometallurgical
processes can be enhanced is by pressure hydrogen strip-
ping of the metal from the loaded organic phase after sol-
vent extraction (Fig.4) [77]. In a conventional flowsheet
that involves solvent extraction with an acidic extractant,
the metal is stripped from the loaded organic phase by an
acid and the metal is recovered from the aqueous stripping
solution by, for instance, electrowinning. The replacement of
the conventional aqueous strip solution by pressure hydro-
gen stripping could provide a way to combine stripping and
metal recovery (as a metal powder) into a single-process
step. The technique could be applied to copper, cobalt,
nickel, and precious metals. Hence, the flowsheet could be
simplified by hydrogen stripping.
Fig. 3 Solute concentration
ranges for separation technolo-
gies. Adapted from ref. [60]

Journal of Sustainable Metallurgy
1 3
Principle 5: Integrate Materials andEnergy
Flows
The design of hydrometallurgical processes must include
integration and interconnectivity with available energy and
materials flows. On a lab scale, there is a tendency to focus
on a single unit process, rather than the flowsheet, making
it inefficient in terms of energy and materials use. As an
example, the optimization of one unit process might result
in the suboptimal performance of other unit operations in
the same flowsheet [50]. A metallurgical flowsheet must be
developed with a holistic, integrated approach. Moreover,
several metallurgical flowsheets include both pyrometallur-
gical and hydrometallurgical unit processes. The heat gener-
ated in pyrometallurgical processes or exothermic hydromet-
allurgical process steps can be recovered and used during
other stages in the flowsheet to increase the reaction rate or
drive an endothermic process. Thus, energy integration can
be described as the use of hot streams to heat cold streams,
and vice versa, before other energy sources are used to do
the heating or cooling. This results in cost savings, increased
throughput and a reduction in emissions and environmental
impact. An example can be found in zinc metallurgy, where
the hot water from the absorbers in the sulfuric acid plant
is used to preheat the spent acid from the electrowinning
operation that is going to the leaching step. The heat gener-
ated in the roasting step can be used to generate the process
steam. The combined use of hydrometallurgical and pyro-
metallurgical processes is often a smart approach to optimiz-
ing energy efficiency.
The Verbund system of the German chemical company
BASF is an example of efficient value chains that extend
from basic chemicals to high-value-added fine chemicals
and materials [78]. In addition, the by-products of one plant
are often used as the starting materials of another. In this
system, chemical processes consume less energy, produce
higher product yields, and conserve resources. In this way,
BASF saves on raw materials and energy, minimizes emis-
sions, cuts logistics costs and fosters synergies. Such a Ver-
bund approach can be extended to the metallurgical industry.
An example is Umicore’s integrated smelter-refinery facility
for recycling metals at Hoboken, close to Antwerp in Bel-
gium [79, 80]. This facility contains different hydrometallur-
gical flowsheets that are coupled, such as a copper-leach and
electrowinning facility, a lead refinery, a precious metals’
refinery and a special metals’ refinery.
Principle 6: Safely Dispose ofPotentially
Harmful Elements
The primary mining of ores not only provides access to tar-
geted metals, but also to unwanted elements for which there
are very few or no applications, or others that are poten-
tially harmful elements (PHEs). These are also known as
“elements of concern” [15], and the best known of these
elements are arsenic, mercury, cadmium, thallium, thorium,
and uranium. The presence of these PHEs in metal ores is
the main reason why the ideal of zero-waste metallurgy is
impossible to achieve and that near-zero-waste metallurgy
is a more practical target [81].
Most of the PHEs are included as minor elements in the
ore minerals (up to a few thousand ppm), but they can also
be present as major components, such as arsenic in nickeline
Fig. 4 Conventional solvent-
extraction flowsheet with
stripping followed by metal
recovery from the aqueous
phase compared with a con-
ceptual flowsheet with pressure
hydrogen stripping from the
loaded organic phase. Adapted
from ref. [77]

Journal of Sustainable Metallurgy
1 3
(NiAs), enargite (Cu3AsS4), or tennantite (Cu12As4S13).
Similarly, most rare-earth ores contain radioactive thorium,
up to 15 wt% ThO2 [82]. PHEs can also be an issue in urban
mining; for instance, the mercury present in end-of-life fluo-
rescent lamps [83].
As we cannot avoid co-extracting these PHEs during
mining and subsequent downstream processes, we must find
sustainable solutions. The economic dimension of the safe
disposal of PHEs should not be underestimated as they are
“penalty elements” and additional charges are demanded for
their disposal. It is important not to dilute PHEs in large-
volume waste streams such as jarosite, goethite, or gyp-
sum. By doing this, we simply create enormous volumes
of contaminated streams that cannot find applications and
that might have a negative environmental impact. The PHEs
are better concentrated in solid compounds so that much
smaller volumes of these PHEs need to be managed, which
is much easier. The most obvious approach to mitigating the
PHE problem is to incorporate these elements into highly
insoluble compounds that have a long-term stability [84].
Moreover, these insoluble compounds must be straightfor-
ward to form, i.e., avoid cumbersome syntheses and strive
for economic feasibility. Because a compound with zero
solubility does not exist, a realistic compromise must be
identified to confine the PHEs so that they can be stored
safely and will not contaminate ground-water reservoirs. A
good knowledge of the mineralogy of PHEs helps identify
promising mineral phases for immobilization. Furthermore,
synthetic inorganic compounds could be used as well in host
matrices. A recent development for the immobilization of
PHEs is encapsulation technology [85, 86], a method in
which a PHE in the form of either small particles or larger
chunks is isolated within an inert compound with a high
structural integrity. This minimizes the surface area of the
hazardous waste exposed to water in the environment, so
that the dissolution of the hazardous element is minimized
as well. The immobilized PHEs could be landfilled in moni-
tored industrial landfills or stored underground, for instance,
in abandoned mine galleries. Different adsorbents have been
developed for the adsorption of PHEs from wastewater, often
based on activated carbon [87].
When working with arsenic-containing solutions in
hydrometallurgy, we must be aware of the risk of generat-
ing highly toxic arsine (AsH3) gas under reducing condi-
tions [88]. In general, AsH3 is formed whenever hydrogen
is produced in the presence of arsenic-containing solutions.
This can happen during the cementation of cadmium by
zinc powder, or in electrometallurgical processes; in both
cases, hydrogen gas can be formed in a side reaction. All
arsenic compounds are toxic, but AsH3 poses massive chal-
lenges. Several fatal accidents have occurred, which means
that AsH3 levels must be monitored and control measures
must be taken if there is a risk of AsH3 formation [89]. For
instance, during cementation with zinc powder, the condi-
tions should be maintained above pH 3. Cadmium is typi-
cally only cemented from solutions that are largely free of
arsenic. Fortunately, AsH3 formation is often kinetically
suppressed.
Principle 7: Decrease Activation Energy
Because hydrometallurgical processes are carried out at
ambient (or slightly elevated) temperatures compared to
the much higher temperatures used in pyrometallurgy, their
reaction rates are in general much slower [90]. Slow kinetics
are a major issue for the leaching of solid materials because
heterogeneous reactions are involved that can only take place
at the liquid/solid interface. The reaction rates can then be
slowed down even further if the surface of the solid material
is passivated by the formation of an insoluble surface layer.
Slow reaction rates can cause bottlenecks in hydrometal-
lurgical processes, especially if we want to maximize time
efficiency (Principle 4).
To increase the reaction rate of a thermodynamically
favorable reaction with slow kinetics at moderate tempera-
tures, we can try to decrease the activation energy using a
catalyst. Although catalytic reactions are omnipresent in the
chemical industry, they are not as common in hydromet-
allurgy. The best-known examples are the silver-catalyzed
leaching reactions of chalcopyrite [91, 92, 93]. Likewise,
catalysis is used in zinc metallurgy: in the Goethite Process,
we need about 1g L−1 of Cu(II) ions as a homogeneous
catalyst to efficiently oxidize the Fe(II) to Fe(III) ions with
oxygen in the hydrolysis step [94].
The use of catalysts is even less common in solvent
extraction. This is because solvent extraction using acidic,
basic, and solvating extractants is rapid, and chemical equi-
librium is attained within a few minutes. However, the reac-
tions of chelating extractants are often slow, and in these
cases, catalysts are added [95]. Catalysts in solvent extrac-
tion are often called “accelerators.”
The authors are convinced that there is still a lot of
untapped potential in the field of application of catalysts to
chemical reactions relevant to hydrometallurgy, especially
in the case of oxidation and reduction reactions. A glance at
the older literature on redox titrations in analytical chemistry
shows that many inorganic redox reactions can be catalyzed
by different inorganic ions. For instance, the iodide ion has
often been used as catalyst in cerimetry (redox titrations
with Ce4+ ions) and also silver ions are known to catalyze
cerium(IV)-mediated oxidation reactions [96].
A further example of this Principle is found in the elec-
trowinning of metals. The most common anode reaction is
oxygen formation by oxidation of water (oxygen evolution
reaction, OER). The high overpotential for the OER leads to

Journal of Sustainable Metallurgy
1 3
a much higher energy consumption for electrowinning than
predicted by thermodynamic calculations. This overpotential
is responsible for about 20 to 25% of the energy costs of zinc
electrowinning, so that a decrease of the overpotential can
lead to a significant reduction in energy consumption. Such
a decrease in overpotential can be achieved by the develop-
ment of new anode materials [21].
Besides adding a catalyst, the activation energy for leach-
ing can also be lowered by the activation of the solids prior
to the leaching. A lot of academic research has been devoted
to the mechanical activation of minerals prior to leaching by
means of a planetary ball mill [97, 98]. Although this can
yield very good leaching efficiencies, planetary ball mills
cannot be scaled up easily to an industrial scale. Therefore,
an IsaMill is recommended [99], although the energies
involved are much smaller. Milling will result in smaller
particle sizes and, hence, in a larger specific surface area,
while the freshly exposed surfaces facilitate leaching. High-
energy milling can also induce defects in the crystal lattice
or even amorphization, rendering the solids more suscepti-
ble to attack by the lixiviant. However, milling remains an
energy-intensive process [100, 101].
Ultrasound-assisted leaching is becoming increasingly
popular in hydrometallurgy. The technique is known to
enhance reactions at the solid–liquid interface. It has also
been found to help reduce the consumption of reaction metal
during cementation, e.g., the amount of zinc powder needed
to purify the ZnSO4 electrolyte in zinc metallurgy [102].
Ultrasound removes the metals deposited on the zinc sur-
face, exposing clean metal surfaces to the solution. Robust
industrial ultrasonicators for use in hydrometallurgy are
commercially available, so that ultrasound-assisted leach-
ing or cementation is feasible on an industrial scale and not
only on lab scale.
Principle 8: Electrify Processes Wherever
Possible
The most sustainable way to conduct oxidation and reduc-
tion reactions in hydrometallurgy is via electrochemistry,
as this approach does not introduce ionic impurities into the
solution. Alternatively, the reducing and oxidizing reagents
can be regenerated electrochemically in a separate step. Far-
reaching electrification of hydrometallurgical processes can
facilitate circular hydrometallurgical flowsheets and render
hydrometallurgy more sustainable. The necessary condition
is, however, the availability of ample quantities of green
electricity.
The use of electricity has a long tradition in hydromet-
allurgy. The electrowinning and electrorefining of metals
that can be electrodeposited from aqueous solutions (such as
copper, zinc, nickel, cobalt, and precious metals) have been
applied in the industry for more than a century. Large R&D
efforts are attempting to make electrorefining and electrow-
inning more sustainable, and more energy efficient. Since
electrorefining and electrowinning consume substantial
amounts of energy, small improvements in energy efficiency
result in significant energy savings. A detailed discussion
of these developments is beyond the scope of this paper.
However, one important research domain that should be
mentioned here is the design of new anodes that promote
the oxygen evolution reaction (OER) over the chlorine evo-
lution reaction (CER). The ability to suppress the evolution
of chlorine gas is important for the electrowinning of metals
from chloride electrolytes and is a missing link in chloride
hydrometallurgy [28].
The conventional approach to electrowinning with paral-
lel-plate electrodes immersed in an electrolyte tank, works
well to recover metals from concentrated electrolytes, which
feature rapid mass transfer and where high current densi-
ties can be achieved. Conversely, this approach is much
less efficient for the recovery of metals from dilute aque-
ous solutions. Performance issues for dilute solutions can be
mitigated by switching from two-dimensional (2D) to three-
dimensional (3D) electrodes [103, 104]. Different types of
3D electrodes have been developed, each with their own
pros and cons. Examples include porous electrodes, screen,
and grid electrodes, packed-bed electrodes, circulating-bed
electrodes, and fluidized-bed electrodes. Among these 3D
electrodes, the fluidized-bed electrode (FBE) has particular
promise to efficiently recover metals from (1) dilute aqueous
solutions, such as dilute leach solutions or process solutions
from hydrometallurgical processes, (2) metal-contaminated
waste waters, and (3) natural metal-bearing aqueous solu-
tions such as acid mine drainage (AMD). A FBE consists
of a bed of electrically conductive particles fluidized by an
upward flow of electrolyte (Fig.5). The entire bed behaves
as an electrode with a high surface area. The surface area of
a FBE is several orders of magnitude larger than that of con-
ventional electrodes, which means excellent efficiency and
high space–time yields. FBEs are not a recent development;
they were fist devised in the 1960s [105, 106] and were used
in several pilot studies of hydrometallurgical processes in
the 1970s [107]. That being said, interest in FBEs faded
after 1990, although there seems to be a revival of research
activities in this field at the moment [62, 108].
The oxidative leaching of ores can be replaced by the
more sustainable process of electrochemical leaching (elec-
trochemical dissolution), where the material of interest is
dissolved at the anode. This process is similar to anode dis-
solution in an electrorefining process, with the difference
being that the metal of interest is present as a compound
rather than in its elemental state. The anodic dissolution
reaction can be coupled with cathodic deposition of the dis-
solved metal, but other reduction reactions might take place

Journal of Sustainable Metallurgy
1 3
as well. An obvious condition for the application of anodic
dissolution is that the material to be leached has sufficient
electrical conductivity. This is the case for sulfidic minerals
that show semiconducting properties. In general, the sulfide
ions are transformed into elemental sulfur, but they can be
further oxidized to sulfate ions by a more positive potential.
The advantage of electrochemical leaching is the simulta-
neous recovery of elemental metal and sulfur in a single
leaching step [109].
Oxidizing and reducing agents used in leaching processes
can also be synthesized electrochemically. The most obvious
compound that can be produced electrochemically is chlo-
rine gas, which is formed at the anode during the electrowin-
ning of metals from chloride electrolytes. Chlorine gas for
leaching can also be electro-generated intentionally [110],
whereas H2O2 can be produced electrochemically from water
and oxygen [111].
Principle 9: Use Benign Chemicals
Although it is evident that no dangerous, harmful, or highly
toxic chemicals should be used in hydrometallurgical pro-
cesses, this is less straightforward than it might seem. First,
the toxicity is highly concentration dependent. Second, the
metals need to be solubilized before they can be further puri-
fied in an aqueous solution. However, several ore minerals
are very resistant to attack by chemicals and high concentra-
tions of strong acids or bases are needed to decompose the
ore minerals and bring their metal content into the solution.
We must distinguish between processes that involve
direct contact with the environment, such as in situ leaching
(ISL), heap leaching, or dump leaching, and processes that
are carried out in chemical reactors, such as vat leaching or
autoclave leaching. It is much easier to control the release
of potentially harmful chemicals to the environment from
reactor-based operations, although attention needs to be
paid to wastewater treatment, and accidental spills cannot
be excluded. Hence, the requirements for chemicals to be
used in open reaction systems are much stricter.
A rule of thumb is that a chemical that comes into contact
with the environment should have no more than a transient
effect on it. This means that natural or biodegradable chemi-
cals should be used as lixiviants. Examples include H2SO4,
HCl, HNO3, methanesulfonic acid (MSA), NaOH, ammonia,
and natural organic acids like citric acid or lactic acid. MSA
is an unusual member of this list (see also Table2). It is an
example of a strong organic acid that can be considered as a
green chemical [18]. This acid has a low toxicity, is readily
biodegradable, and is part of the natural sulfur cycle, but
its salts are far more soluble in water than sulfates and it
is commercially available at low cost. The high solubility
of methanesulfonate salts can assist in reducing water con-
sumption and maximizing space efficiency.
Poorly biodegradable or persistent chemicals should be
avoided. An example is ethylenediaminetetraacetic acid
(EDTA). Although EDTA is often used for the remediation
of soils contaminated by lead and other heavy metals, it is
poorly biodegradable [112]. Likewise, fluorinated chemicals
such as perfluorooctane sulfonates (PFOS) must be avoided
because they are highly persistent in nature and can survive
in ecological systems for decades, if not centuries. There
is no place in circular hydrometallurgy for these ‘’forever
chemicals’’ [113, 114]. Furthermore, we must realize that
many ionic liquids that are considered as green solvents for
ionometallurgy often contain fluorinated anions such as
bis(trifluoromethylsulfonyl)imide (vide infra). A good exam-
ple is betainium bis(trifluoromethylsulfonyl)imide, [HBet]
[Tf2N], which is a powerful lixiviant for oxide minerals, but
which combines persistency with a relatively high solubility
in water [115, 116].
Different solvent-selection guides have been developed
by industry to classify organic solvents according to how
sustainable (“green”) they are and how safe they are to work
Fig. 5 Schematic diagram of
two common types of fluidized-
bed electrode (FBE) reactors:
the “plane-parallel design” reac-
tor (left) and the “side-by-side
design” reactor. Adapted from
ref. [62]

Journal of Sustainable Metallurgy
1 3
with [117]. It is important to realize that these solvent-selec-
tion guides were drawn up for the chemical and pharma-
ceutical industries and not for activities in extractive metal-
lurgy. Nevertheless, some conclusions have general validity.
Avoidable solvents are those that (1) have a very low flash
point, (2) easily form explosive peroxides, (3) are ozone-
depleting substances, (4) have other environmental issues,
or (5) are toxic or carcinogenic. For a detailed discussion
about the use of (green solvents) in extractive metallurgy,
the reader is referred to our review paper on solvometal-
lurgy [118].
The preferred oxidizing agents are air and oxygen gas
because these are cheap and yield clean oxidation reactions,
since they do not introduce impurities into the solution. A
disadvantage is that pressures above atmospheric are needed
for rapid reactions so that the use of autoclaves is required.
In the case of the oxidative leaching of sulfidic concentrates,
there is a danger of the over-oxidation of sulfides to sulfates.
Another example of a benign oxidizing agent is hydrogen
peroxide (H2O2), since it forms water as a reaction product
[119, 120]. A disadvantage of H2O2 is its tendency to dis-
proportionate into oxygen gas and water, especially in the
presence of redox-active metals. The handling and transport
of H2O2, as well as its availability, might be an issue [121].
Likewise, H2O2 is more expensive than some other oxidants.
When strongly oxidizing conditions are required and
pressurized oxygen gas does not suffice, the use of ozone
(O3) is an option [122]. Ozone has been used to leach differ-
ent types of refractory sulfide minerals, such as chalcopyrite
[123] and even pyrite [124, 125]. The ozone treatment of
refractory gold ores is a sustainable alternative to a roasting
pre-treatment step [126, 127].
Special attention must be paid to hydrogen gas (H2),
which is a key chemical for closing hydrometallurgical
flowsheets because the reduction of metal ions by hydrogen
gas re-introduces protons to the system [128, 20]. Hydrogen
gas is a clean reducing agent and is not toxic, although there
are fire and explosion risks when mixed with air. We should
also remember that hydrogen is only a sustainable chemical
when its synthesis is sustainable as well.
For an assessment of how benign a chemical is, its non-
corrosivity should be included as a criterion. Non-corrosive
chemicals are not only safer to use, and they also impose
fewer requirements on the construction materials for reac-
tors, pipes, and other pieces of equipment that come into
contact with the chemical reagents.
Finally, the sustainability of a reagent in hydrometallurgy
should not only be assessed during its use and recycling
phase. The sustainability of its production must also be fully
considered (vide supra).
Principle 10: Reduce Chemical Diversity
The fact that an infinite number of molecules can be
designed and synthesized by chemists might give the wrong
impression that the use of new, more sustainable chemicals
is the key to the development of circular hydrometallurgical
flowsheets. However, the history of development of extract-
ants for solvent extraction clearly shows that the discovery
of new reagents is not necessarily the way forward. A wide
range of new extractants, designed with selectivity for one
particular metal or for a group of metals, have been devel-
oped over the years. Many of these extractants that were
once considered as silver-bullet solutions, have been dis-
continued, due to stability issues and/or too small markets.
Examples include Cyanex 301 and 302 [129, 130].
What is more, there are strict legislations for the intro-
duction of new chemicals on the market. In Europe, the
need to register new chemicals under the so-called REACH
(Registration, Evaluation, Authorization and restrictions of
Chemicals) regulations makes the commercialization of new
chemicals cumbersome [131, 132]. The very expensive and
time-consuming environmental and toxicological studies
that are required by REACH or their equivalents elsewhere,
undoubtedly impose a formidable obstacle to the adoption of
new chemicals in the hydrometallurgical industry. REACH
is also an impediment to the use of a known chemical to be
applied in a new process, as even in that case a company
needs to go through the registration procedure. As corrobo-
rated by industrial experts, the use of widely available and
well-known reagents is highly recommended, since this
greatly facilitates the development and implementation of a
new flowsheet. Design engineers prefer well-known chemi-
cals over novel compounds because of the easily available
data and the mitigation of safety risks and process-perfor-
mance risks. The same holds for health, safety, and environ-
ment (HSE) advisors and the authorities who deliver the
required safety and environmental permits to authorize the
building and startup of a new plant.
Therefore, rather than spending excessive time and effort
on developing new, exotic reagents and solvents, it makes
more sense to fully exploit the possibilities of existing chem-
icals. We can even take this a step further and ask which of
the chemicals that are presently used in hydrometallurgy
could be made obsolete in the future. Apart from reduc-
ing the amount of chemicals consumed in hydrometallurgi-
cal flowsheets, we should also aim to reduce the number of
chemicals used. In other words, reducing the diversity in
chemicals rather than promoting it. We already know that
existing chemicals can be combined in new ways to obtain
synergistic effects, for example, extractants [133]. How-
ever, the smarter use of existing chemicals is possible only
if their chemical and thermodynamic properties, as well as

Journal of Sustainable Metallurgy
1 3
the chemical reactions involved, are well understood. This
emphasizes the importance of metallurgical chemistry in
the development of circular hydrometallurgical flowsheets.
Principle 11: Implement Real‑Time Analysis
andDigital Process Control
Real-time analysis and digital process control are required
for resilient flowsheets that can cope with fluctuations in
feed composition and maximize efficiency. Reliable and
quickly available analytical data on various aspects of the
process streams, such as metal concentrations, pH, den-
sity, etc. are essential inputs for flowsheet modeling and for
process control. Analytical measurements should not only
monitor the major and minor elements, but the impurities
and trace elements as well.
In practice, on-line and in-line process analyses are com-
patible with the requirements of digital process control. Peri-
odic sampling in combination with off-line sample analysis
is too slow. The dead time between sampling, analysis, and
the adjustment of the process parameters might result in inef-
ficient conditions in the process circuits. The effective use
of real-time, on-line/in-line analysis eliminates the excess
lag time between sampling and adjustments to the process
parameters in the circuit. Process parameters such as tem-
perature, pressure, pH, oxidation–reduction potential (ORP),
mass density, and viscosity are routinely measured on-line/
in-line with robust, dedicated probes, and sensors. Although
the real-time analysis of hydrometallurgical streams has long
been hampered by the lack of instrumental methods and
sensor development for on-line/in-line analysis of metals,
this has been changing rapidly in recent years. Neverthe-
less, real-time on-line/in-line monitoring of concentrations
of metal ions in complex process solutions still presents a
formidable challenge. This is true for the major components,
and even more so for the trace elements. One-size-fits-all
solutions do not exist for process analysis; different analyti-
cal techniques have to be used depending on the concentra-
tion ranges of the metals and the salt content of the aqueous
and organic process solutions.
The most useful and powerful method for real-time meas-
urements of metal content is without doubt X-ray fluores-
cence (XRF), especially in the form of energy-dispersive
X-ray fluorescence (EDXRF) [134]. XRF has been widely
been used in the mining and mineral-processing industries
for real-time on-line measurements [135, 136, 137]. The
technique is also routinely used by the metallurgical industry
to measure metal concentrations in solutions, but these are
typically not real-time on-line measurements, but rather off-
line analyses. On-line XRF would be a very interesting tech-
nique for monitoring hydrometallurgical processes, but on-
line XRF analyzers for solutions are not routinely available
yet. In hydrometallurgy, inductively coupled plasma optical
emission spectrometry and mass spectrometry (ICP-OES
and ICP-MS) are often the analytical methods of choice for
the off-line analysis of complex liquid process streams, but
it is not straightforward to adapt these techniques to on-line
applications.
If the metal ion absorbs electromagnetic radiation in
the UV–VIS-NIR region of the electromagnetic spectrum,
optical absorption spectroscopy can be used for the on-line
monitoring of solvent extraction [138]. However, optical
absorption spectroscopy is less suitable for the concentrated,
complex process streams found in hydrometallurgy, because
of the too strong light absorption and many overlapping
absorption bands. If the solvent-extraction process involves
anions with characteristic vibration bands, FTIR, or Raman
spectroscopy can be used for the on-line monitoring of the
process [138].
In these days of rapidly developing artificial intelligence
(AI), it is not surprising that the different branches of AI,
such as expert systems, fuzzy logic, neural networks, and
machine learning, could find applications in efficient process
control for hydrometallurgy. However, the implementation
of AI methods in hydrometallurgy is slower than in related
industrial domains, such as minerals processing [139]. Still,
AI methods have been applied to leaching processes [140],
solvent extraction [141, 142], and electrowinning [143, 144].
Decision and optimization methods can be used to decide
between alternative hydrometallurgical unit processes [145,
146], while the digitalization of a hydrometallurgical pro-
cess can be seen as part of a broader narrative, i.e., the digi-
talization of the circular economy [147, 148].
Principle 12: Combine Circular
Hydrometallurgy withZero‑Waste Mining
Metallurgy has always been closely connected with min-
ing and mineral processing (beneficiation), and these ties
will become even closer in the future with the ongoing
paradigm shift in the mining industry towards “zero-waste,
multi-metal/mineral mining” or “responsible mining” [149,
150]. The multi-metal/mineral aspect implies that not only
the primary metal of interest is recovered (e.g., lithium in
hard-rock pegmatite ore) but also the accessory metals (e.g.,
niobium, tantalum and tin) and the industrial minerals (e.g.,
quartz, feldspar) that are present in the same ore body (cf.
Horizon Europe EXCEED project).
Of all the different approaches to zero-waste mining, the
most relevant to hydrometallurgy is “Invisible Zero-Waste
Mining,” i.e., the concept of a mining with no impact on
the surface [151]. This can be achieved with the technique
of in situ leaching (ISL), also known as in situ recovery
(ISR), or solution mining. This technique is well established

Journal of Sustainable Metallurgy
1 3
in the uranium mining industry [152, 153]. The technol-
ogy has proven its usefulness for the extraction of soluble
salts (e.g., potassium salts) and copper. It is likely that its
importance will increase in the future and that ISL will be
extended to other metals [154]. It is very useful for deep ore
deposits, although ISL cannot be used if the hydrogeology
or the permeation of the rocks is unfavorable. For example,
there is always the environmental concern of contamination
of ground water by lixiviants. Bacterial ISL is a promis-
ing alternative for leaching with chemical lixiviants, while
experience gained in heap leaching by bacteria (bioleaching)
could be very helpful in the further development of bacterial
ISL. Given the fact that ISL results in dilute aqueous streams
from which the metals must be recovered, the close link
between ISL and hydrometallurgy is obvious, ion exchange
(IX) is the recommended technique to recover the dissolved
metals, with the ion-exchange resins either packed in a col-
umn, or added directly to the solution (resin-in-solution,
RIS). Alternatively, adsorbents such as activated carbon
can be used.
The recovery of metals from dilute aqueous mining
streams will become more important in the future, and not
only in combination with ISL. Several aqueous streams exist
that could be treated with hydrometallurgical methods, for
the removal of toxic elements, but also for the recovery of
valuable elements. Typical examples are acid mine drain-
age (AMD) and other effluents from abandoned mines
[155, 156], effluents of mine tailings and landfills, process
waters from the metallurgical industry, and effluents from
the electroplating industry. Here, water purification could be
combined with the recovery of metals [64]. Small, modular,
mobile hydrometallurgical plants can be designed to recover
metals from aqueous streams at remote locations [157, 158].
In this way, metal recovery could be combined with the
decontamination of soils contaminated by heavy metals.
Combining Dierent Principles
Although there is no strict hierarchy among the principles
and their numbering is somewhat arbitrary, Principle 1
(Regenerate reagents) is by far the most important principle
because no circular hydrometallurgical flowsheets can be
designed without regeneration of reagents. The 12 Principles
are not independent, and they can be combined to even more
powerful overarching principles. Hence, there can be syner-
gies between the principles. Principle 4 (Maximize mass,
energy, space, and time efficiency) is related to several other
principles. Integration of materials and energy flows (Prin-
ciple 5) leads to higher efficiencies (Principle 4). More effi-
cient processes with a lower reagent consumption (Principle
4) result in less need for regeneration of reagents (Principle
1). Real-time analysis and digital process control (Principle
11) induce more efficient processes (Principle 4), while
combination of circular hydrometallurgy with zero-waste
mining (Principle 12) is a form of process intensification
(Principle 4) as it allows to largely omit mineral-processing
operations, linking mining directly to hydrometallurgy. Less
minerals processing also translates into a substantial energy
saving since size reduction by milling and grinding is highly
energy intensive (energy efficiency is addressed in Principle
4). By decreasing the activation energy of hydrometallurgi-
cal processes (Principle 7), mass, energy, space, and time
efficiency can be maximized (Principle 4). Concentration of
potentially hazardous elements and their encapsulation in an
inorganic host matrix (Principle 6) represent a form of waste
prevention (Principle 3). Reduction of the chemical diver-
sity (Principle 10) facilitates the regeneration of reagents
(Principle 1). Many methods for regeneration of reagents
(Principle 1) constitute electrochemical methods, thereby
boosting Principle 6 (Electrify processes wherever possible).
What is more, there are also key interactions between
the different principles. Optimization of a process towards a
certain principle may lead to a worse performance in terms
of another principle. For instance, for the regeneration of
reagents (Principle 1), the consumption of acids should be
minimized. This can be achieved by carefully controlling
the pH using a pH–stat and adding acid at the same rate as
it is consumed by the leaching reactions [159], so that only
a small excess of acid is consumed with respect to the stoi-
chiometric demands of the target mineral. This is feasible
only for compounds that easily dissolve in dilute acids, such
as copper and zinc oxides and carbonates. However, lower
acid concentrations have a negative effect on the kinetics of
the leaching reactions. Slower kinetics are bad for space and
time efficiency (Principle 4). To maximize the driving force
of leaching, but with a minimum of acid consumption, mul-
tistage countercurrent leaching could be used, although this
leaching technique cannot be applied to all types of feed. For
instance, clay-rich feeds are problematic because they gen-
erate suspensions with solid–liquid separations that are too
slow. One could also opt for leaching with an excess of acid,
followed by recycling of the residual acid (i.e., the excess
acid remaining after leaching). For this acid recycling, sol-
vent extraction is the preferred technique because it is pos-
sible to select solvents in such a way that the extracted acid
can be stripped thermally with hot water instead of stripping
with the use of a base [160, 161]. These examples, therefore,
corroborate that there are no easy solutions or shortcuts to
circular hydrometallurgy: a comprehensive, system-level
approach towards the 12 Principles is key.

Journal of Sustainable Metallurgy
1 3
Implementation ofCircular
Hydrometallurgical Flowsheets
The metallurgical sector is rather conservative, as large capi-
tal investments are required to build new factories. Thus,
it would be naïve to assume that metallurgical companies
would suddenly embrace new disruptive technologies with-
out proven long-term viability. Moreover, every project is
unique; each project must be studied individually, and many
factors must be considered, e.g., location, logistics, availabil-
ity of suitable labor force, local environmental legislation,
and governmental incentives.
A major advantage of metallurgical processes is that
they have a modular structure, making it possible to plug
in various unit operations as part of existing flowsheets.
Circular hydrometallurgy can be introduced by adapting or
optimizing existing flowsheets, rather than by developing
new flowsheets from scratch. We could compare the tran-
sition from linear to circular hydrometallurgy to the reno-
vation and energy optimization of an existing building. In
most cases, an older building is not razed to the ground and
rebuilt according to the latest insight into energy efficiency
(although this is an ideal situation to strive for). However,
it is a good starting point to perform a thorough analysis of
the existing building and to first address those issues where
the greatest gains can be achieved. In a similar way, existing
hydrometallurgical flowsheets should be scrutinized using
state-of-the-art lifecycle assessment (LCA) and multicrite-
ria assessment (MCA) methodologies [162, 163]. These can
reveal the problematic hotspots of the flowsheet in terms
of energy, water, and chemical consumption. Those parts
of the flowsheets with the largest environmental (and eco-
nomic) impacts can be addressed first, by plugging in new
unit operations or replacing older unit operations.
When evaluating or comparing the sustainability of dif-
ferent processes or flowsheets (see 4 case-studies of (near)
circular hydrometallurgical flowsheets in the online Supple-
mentary Information: TableS4, Figs. S1-S4), it is necessary
to include the type of final product. A process that leads to
a product that does not need much refining is preferred to
a process that generates an intermediate product that still
needs several refining stages. Producing an intermediate
product in a circular flowsheet, which ultimately ends up
in a non-circular process for further treatment, is not that
circular.
Researchers working in academia are inclined to underes-
timate the importance of the economics of hydrometallurgi-
cal processes. We should not only assess the technological
feasibility and the sustainability metrics for a new unit oper-
ation of a flowsheet but also perform an economic assess-
ment. For LCA and MCA studies, a detailed flowsheet with
mass balances and energy balances is required. A simplified
conceptual flowsheet is not sufficient. The challenge is that
if we want to close all the loops, the CAPEX and OPEX will
skyrocket in a way that the process would only be profit-
able after many years of operation. The question arises as
to whether there will be enough feedstock for the kind of
raw materials the process was designed for. Often, however,
there are not enough materials close to the industrial site. In
principle, raw materials can be transported to the site from
another location, but transportation is expensive and has a
large CO2 footprint.
Flowsheet modeling is important for the design and
operation of hydrometallurgical plants. As such, flowsheet
modeling is an essential tool in circular hydrometallurgy.
While initial bench-scale and pilot-scale studies can pro-
vide valuable information about the chemistry of the system,
the required long-term testing in a pilot plant is notoriously
expensive. Therefore, the use of flowsheet modeling to assist
in project evaluation is becoming more widespread. Simula-
tion software can be used for process development to study
alternatives, assess feasibility and preliminary economics,
and interpret pilot-plant data. Hardware and flowsheets can
be optimized to estimate equipment and operating costs and
to investigate feedstock flexibility. During plant operation,
modeling helps reduce chemical reagent and energy use, to
increase yield, to control pollution, and to analyze troubled
operations. Commercial modeling packages (e.g., METSIM,
Aspen Plus, OLI Systems Stream Analyzer, HSC Chemis-
try) are continuously being updated, as new computational
techniques are being developed and our knowledge of unit
operations is improved.
The design of an efficient hydrometallurgical process
requires expert knowledge of the mineralogy of the ores or
ore concentrates, and not just knowledge of the chemical
composition [90]. As hydrometallurgy is often applied to
low-grade or complex ores, the feed materials are more com-
plex in terms of mineralogy and chemical composition than
the feed materials for pyrometallurgical processes. In pyro-
metallurgy, the spatial complexity of the feed materials is of
little importance. While knowledge of the chemical compo-
sition provides an indication of (1) the minimum amount of
reagent that is required to solubilize the targeted metals and
(2) which impurity elements can be expected in the leach
solution, the mineralogy is of vital importance for the selec-
tion of the reagents and the most suitable process conditions.
For instance, knowledge of the mineralogy of the metal ores
and of the gangue minerals might make it possible to select
leaching conditions with a minimum co-dissolution of the
gangue. Of course, efficient hydrometallurgical flowsheets
also require the mineralogical and chemical compositions
of the feed to remain relatively constant over time. Careful
process control is required to compensate for fluctuations
in the feed composition, while simultaneously keeping the
process stable.

Journal of Sustainable Metallurgy
1 3
An enormous saving in terms of consuming chemicals
and energy can be achieved by avoiding what is termed
“over-purification” of the final product in a hydrometallur-
gical flowsheet. In theory, it is possible to purify metals to
any desired purity, but higher purity means a higher price.
Hence, before designing a flowsheet for the purification of
a metal, we should ask what level of purity is required to
deliver the properties we need.
Conclusions
In this academic position paper, we have outlined our vision
of how hydrometallurgical flowsheets could be redesigned
to lower reagent consumption, reduce waste, and increase
energy efficiency. By and large, traditional flowsheets can
still be considered as (predominantly) linear flowsheets,
whereas the next-generation flowsheets should be (near) cir-
cular. Circular flowsheets are at the heart of circular hydro-
metallurgy (see also Section “Examples of (near) circular
hydrometallurgical flowsheets” in the online Supplementary
Information). At the limit, no reagents are consumed, with
the exception of some unavoidable losses due to the Second
Law of Thermodynamics. Added to this, the energy must
come from renewable resources.
To provide a compass that can guide the metallurgical
engineer in developing circular flowsheets in hydrometal-
lurgy, a set of design rules has been provided, i.e., the 12
Principles of Circular Hydrometallurgy. Although we real-
ize that the choice of these principles is somewhat arbitrary
and that other principles could be imagined, we are never-
theless convinced that these principles make powerful tools
to show the direction of future research and innovation in
hydrometallurgy, also in academia. This is especially the
case if these qualitative rules are combined with quantitative
assessments of the materials and energy balances for newly
developed hydrometallurgical flowsheets. Because only by
applying sustainability metrics (e.g., LCA and MCA) it is
possible to objectively compare different options for new
processes, for both primary mining and recycling flowsheets.
A circular use of chemicals in a primary-mining-based pro-
cess might beat a non-circular use in a concurrent recycling
flowsheet. The question is what we should do if we develop
a very elegant circular flowsheet that is not economical to
build. Which of the principles can be sacrificed first? Is there
a hierarchy to these principles? There is no easy answer.
Only a system-level approach will work. What is key is that,
instead of introducing more complexity, we should aim for
minimalism and simplicity, so that the hydrometallurgy of
the future evolves to a form of low-energy-input, circular
hydrometallurgy.
Research in hydrometallurgy should not focus too much
on the development of new reagents and solvents, but rather
on the smarter use of existing reagents, or even the elimina-
tion of as many chemicals as possible. The unnecessary use
of chemicals must also be avoided. We are convinced that
genuine breakthroughs in hydrometallurgy will come from
a deeper understanding of hydrometallurgical processes at a
molecular level. Knowledge of the interactions at a molecu-
lar level, the chemical speciation, and the redox chemistry
of process solutions is essential. The development of analyti-
cal techniques for the in-line and on-line detection of ionic
and molecular species is important. Experiments should be
complemented with computational chemistry methods and
with thermodynamic models for multiphase, multicompo-
nent systems.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s40831- 022- 00636-3.
Acknowledgements The authors thank the Industrial Research Fund
(IOF) and the LRD Division RARE3 of KU Leuven for financial
support. For this paper, the authors had numerous discussions with
industry experts (including Rene Wiersma, Charles Geenen, Mark
Saxon, Justin Salminen, Jo Vandenbroucke, Mohammad Khoshkhoo,
Jan Luyten, and Joris Roosen) who provided valuable feedback on the
first draft of this manuscript. The authors are also indebted to particu-
lar SOLVOMET researchers (Rayco Lommelen, Willem Vereycken,
Brecht Dewulf, Dzenita Avdibegovic, Stijn Raiguel, Thomas Abo Atia,
and Lieven Machiels) for their much-appreciated inputs and sugges-
tions. The authors acknowledge Paul McGuiness for the text editing
and the layout/redrawing of the figures.
Declarations
Conflict of interest No conflicts of interest to declare.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
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the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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