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\DOI{10.5802/crchim.327}
\datereceived{2023-11-06}
\daterevised{2024-04-10}
\datererevised{2024-06-28}
\dateaccepted{2024-07-18}
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\begin{noXML}

\makeatletter
\def\TITREspecial{\relax}
\def\cdr@specialtitle@english{GDR Prom\'eth\'ee - French Research Network on \textit{Hydrometallurgical Processes for Primary and Secondary Resources}}
\def\cdr@specialtitle@french{GDR Prom\'eth\'ee - Groupement de Recherche sur les Proc\'ed\'es Hydrom\'etallurgiques pour la Gestion Int\'egr\'ee des Ressources Primaires ...}
\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Reprocessing of waste produced during acid mine drainage lime
treatment by selective leaching}

\alttitle{Retraitement par lixiviation s\'{e}lective de d\'{e}chets
produits lors du chaulage de drainage minier acide}

\author{\firstname{J\'er\^ome} \lastname{Jacob}\CDRorcid{0000-0002-0586-0726}\IsCorresp}
\address{BRGM, F-45060 Orl\'eans, France}
\email[J. Jacob]{j.jacob@brgm.fr}

\author{\firstname{Douglas} \lastname{Pino-Herrera}\CDRorcid{0000-0002-9533-2889}}
\addressSameAs{1}{BRGM, F-45060 Orl\'eans, France}

\author{\firstname{Catherine} \lastname{Greffi\'e}}
\addressSameAs{1}{BRGM, F-45060 Orl\'eans, France}

\author{\firstname{Guillaume} \lastname{Wille}\CDRorcid{0000-0001-8460-8213}}
\addressSameAs{1}{BRGM, F-45060 Orl\'eans, France}

\author{\firstname{Emmanuelle} \lastname{Plat}\CDRorcid{0000-0002-7874-3643}}
\addressSameAs{1}{BRGM, F-45060 Orl\'eans, France}

\shortrunauthors

\keywords{\kwd{Acid mine drainage}\kwd{Lime}\kwd{Sludge}\kwd{Zinc}\kwd{Copper}\kwd{Recovery}\kwd{Leaching}}

\altkeywords{\kwd{Drainage minier acide}\kwd{Chaux}\kwd{Boue}\kwd{Zinc}\kwd{Cuivre}\kwd{Recyclage}\kwd{Lixiviation}}

\thanks{Department for Mine Safety and Risk Prevention of BRGM: 
French Geological Survey}

\begin{abstract}
Acid mine drainages (AMDs) are a major environmental problem, but they
can also contain valuable metals. A typical treatment of AMDs consists
in the neutralization and precipitation of metals by liming, which
produces a sludge mainly composed of iron oxyhydroxides and gypsum.
Depending on the AMD composition, the sludge can contain important
concentrations of valuable metals. This research work aimed at studying
acid leaching of copper and zinc present in the sludge
with the final goal of developing a process to recover these metals. 
Tests were carried out using sludge from a former mining site in
France. The first stage of this process involves a leaching stage to
selectively dissolve Cu, Zn, and Cd using H\textsubscript{2}SO\textsubscript{4}. This process
allows more than 80\% of Cu and Zn as well as 98\% of Cd in the sludge
to be leached under optimal operating conditions, which are between 5\%
and 15\% of solid content and between 150 and 200 g
H\textsubscript{2}SO\textsubscript{4}\textperiodcentered\text{kg}\textsuperscript{--1} at room temperature. Using more
acid marginally increases the dissolution rates but at the cost of much
higher acid consumption linked to the dissolution of iron and
therefore a loss of selectivity.
\end{abstract}

\begin{altabstract}
Les drainages miniers acides (DMA) posent un probl\`{e}me
environnemental, mais peuvent contenir des m\'{e}taux
d'int\'{e}r\^{e}t. Le traitement typique consiste en la neutralisation
et la pr\'{e}cipitation des m\'{e}taux par chaulage, produisant une
boue de pr\'{e}cipit\'{e}s d'hydroxydes de fer et de gypse. En fonction
de la composition de la DMA, la boue peut contenir d'importantes
concentrations de m\'{e}taux. Ce travail de recherche se concentre sur
le d\'{e}veloppement d'un processus de r\'{e}cup\'{e}ration du cuivre
et du zinc \`{a} partir de ces boues. Le proc\'{e}d\'{e} vise \`{a}
extraire non seulement les m\'{e}taux d'int\'{e}r\^{e}t, mais aussi
d'autres \'{e}l\'{e}ments qui d\'{e}passent les seuils
d'acceptabilit\'{e} (notamment le Cd). Les essais ont \'{e}t\'{e}
effectu\'{e}s sur des boues d'un ancien site minier en France. La
premi\`{e}re \'{e}tape comprend une \'{e}tape de lixiviation pour
dissoudre s\'{e}lectivement le cuivre, le zinc et le cadmium avec de
l'acide sulfurique. Ce proc\'{e}d\'{e} permet de lixivier plus de 80 \%
du Cu, du Zn et du Cd contenus dans les boues dans des conditions
op\'{e}ratoires optimales qui sont entre 5 et 10 \% de taux de solide
et entre 150 et 200 g de H\textsubscript{2}SO\textsuperscript{4}\textperiodcentered\text{kg}\textsuperscript{--1} \`{a}
temp\'{e}rature ambiante. L'utilisation d'une plus grande quantit\'{e}
d'acide augmente marginalement les taux de dissolution, au prix d'une
consommation d'acide beaucoup plus importante li\'{e}e \`{a} la
dissolution du fer et donc d'une perte de s\'{e}lectivit\'{e}.
\end{altabstract}

\maketitle
{\vspace*{.45pc}}

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\section{Introduction} \label{sec1}

It is estimated that 19,300~km of rivers and 72,000~ha of lakes and
aquatic reservoirs are severely affected by mining effluents
worldwide~\cite{1}. In addition to the economic damage they cause, acid
mine drainage (AMD) poses serious risks to human health. The
contamination of surface water and sediments by mining sites therefore
represents a major public health issue~\cite{2,3}. Like many other
European countries, France has a long tradition of mining dating back
to the Neolithic period~\cite{4}. In the 1980s, economic constraints
led to the end of mining in mainland France~\cite{5}. The map of
mineral deposits in France lists 1400 sites that have produced
ores~\cite{6}.

When mining waste or mining structures (galleries, mine shafts, and open
pits) containing sulfide minerals, such as pyrite (FeS$_{2}$) and
pyrrhotite (Fe$_{(1-x)}$S with $x &lt; 0.125$), come into contact with water
and oxygen, an oxidation reaction can occur that will lead to the
production of AMD according to Equation~(\ref{eq1}) in the case of
pyrite.
{\begin{equation} \label{eq1}
4\mathrm{FeS}_{2} + 15 \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}
\rightarrow 4 \mathrm{Fe}^{3+} + 4 \mathrm{H}^{+} + 8 \mathrm{SO}_{4}^{2-}
\end{equation}}\unskip
AMD can contain high concentrations of sulfate, Fe, Al, Mn, As, Cu,
Zn, and other metals and \mbox{metalloids}.\ Mine drainage can remain
acidic for a very long time. For example, the Montana Department of
Environmental Quality has validated a model which shows that acid
drainage from the Golden Sunlight mine (USA) will continue to flow for
thousands of years~\cite{7}. A recent study by Tabelin et~al.~\cite{8}
estimated that AMD generation in the relatively small tailings dam of a
closed Au--Ag--Cu--Co--Zn mine in Hokkaido, Japan, will continue for at
least 1000 years. One of the oldest documented mining regions in the
world, the Rio Tinto region in Spain, has been experiencing AMD-related
problems for around 5000 years~\cite{9}.\looseness=-1

If preventive measures such as handling plans, alkaline amendment, and
covers are not implemented before the oxidation reactions begin, AMD
production is difficult and even impossible to stop~\cite{10}. Besides
their potential impacts on the environment, AMD can also contain
valuable metals. In general, there is a positive correlation between
the acidity of the treated effluent and the concentration of the metals
in the water, particularly for Cu, Zn, Ni, and Mn, as explained by
Plumlee et~al.~\cite{11}. As the flows of AMD are difficult, if not
impossible, to control permanently, they need to be treated for a long
time. These systems represent a significant long-term cost to society,
to which must be added the storage of solid waste resulting from
these treatments that accumulates over time. Chemical neutralization
involves mixing AMD with alkaline materials (such as limestone,
magnesia, and lime) to precipitate most of the heavy metals. The 
AMD treatment by reaction with lime is described by Equation~(\ref{eq2}):
{\begin{equation} \label{eq2}
2\mathrm{H}^{+} + \mathrm{SO}_{4}^{2-} + \mathrm{Ca(OH)}_{2}
\rightarrow \mathrm{CaSO}_{4} \cdot 2\mathrm{H}_{2}\mathrm{O}
\end{equation}}\unskip
The increase in pH during lime treatment triggers the precipitation of
divalent and trivalent metals present in the AMD in the form of metal
oxy/hydroxides or sulfates, which are often poorly
crystallized~\mbox{\cite{12,13}}. Lime treatment is the method of
choice for mines as it is effective, easy to implement and operate, and
can be tailored to site-specific geochemical properties of
AMD~\cite{14}. It has been estimated that 90\% of AMDs are treated by
liming worldwide~\mbox{\cite{15,16}}. Although effective, this process
generates bulky sludge composed largely of gypsum
(CaSO$_{4}{\cdot}$2H$_{2}$O), which can be a source of secondary
impacts if it is not stored under optimal conditions.\ For example,
some heavy metals present in this sludge can be leached by rain
water and released into the environment.

On one hand, these metals can be potentially harmful for human health
and for the environment, but, on the other hand, they can also be
economic and technologically valuable, particularly in the current
context of energy transition, as they can attain significant
concentrations in the sludge. For instance, analyses carried out on
lime treatment sludge from seven mines in Canada show that some metals
can be present at very high concentrations: up to 1.4\% of copper and
22\% of zinc~\cite{17,18}. There have been numerous studies focusing on
the recovery of metals directly from AMD using specific
processes~\cite{19,20}, but very few authors have studied the recovery
of metals in historical sludge dumps produced through conventional lime
treatment of AMD~\cite{21,22}.\looseness=-1

The work of Macias et~al.~\cite{18} is particularly noteworthy as well
as the patent by Ziemkiewicz et~al.~\cite{22} on the recovery of rare
earth elements (REEs) from AMD, which includes liming of the AMD and
subsequent treatment of the sludge although this process was designed
specifically to recover REEs~\cite{23}.

The recovery of valuable metals in AMD presents a significant challenge,
as fluxes of these metals are often too low. Inline metal recovery is
rarely economically viable due to both low water flow rates and/or low
metal concentrations. Consequently, this research work aims to address
the large-scale challenge posed by substantial sludge deposits that
have accumulated over decades instead of developing a process for
recovering metals directly from AMD or from the daily-produced sludge
by lime treatment. By targeting these existing deposits, we propose a
short treatment operation designed to recover valuable metals
efficiently, offering a novel solution to a long-standing environmental
and economic{\break} issue. 

In particular, this study covers the selective leaching of metals from
a specific AMD lime treatment sludge to recover Cu and Zn while
decreasing the amount of Cd in the sludge. For this purpose, sludge
from the lime treatment of AMD at a specific French site was
characterized and the optimal acid leaching operating conditions (solid
content, acid-to-solid ratio, temperature) were determined. Applying
optimal conditions means maximizing the leaching yields of Cu, Zn (for
economic viability), and Cd (for \mbox{environmental} concerns) while
minimizing the leaching of Fe and Al. 

The dissolution of Fe and Al consumes reagents without adding value,
increasing the operational costs of the process. Moreover, high
concentrations of Fe and Al in the leachate can impede the downstream
recovery processes. Compared to a pregnant leach solution (PLS) without
Fe and Al, the selective removal of these elements requires more
reagents, potentially different ones, and additional treatment steps
downstream.\ In particular, the solid/liquid separation of Al
precipitates is challenging. Finally, the presence of Fe and Al in the
PLS makes it more difficult to achieve the target purity of Cu and Zn
\mbox{products}~\cite{24,25}.

{\vspace*{-2pt}}

\section{Materials and methods} \label{sec2}

{\vspace*{-2pt}}
\subsection{Mine site and sludge deposit} \label{sec2.1}

{\vspace*{-2pt}}

The ore deposit from which the AMD is produced is a stockwork
containing barite (BaSO$_{4}$), galena (PbS), sphalerite (ZnS), and
chalcopyrite (CuFeS$_{2}$) in altered soda-dacitic lavas near the
Devonian Br\'{e}venne rift. It was covered by Triassic sediments (clay
and sandstone), which were mineralized in turn by hydrothermal fluids
during their deposit. Then, primary sulfides were oxidized allowing the
formation of secondary minerals of lead, zinc, and especially copper.
The mine was active from 1414 to 1842 for the extraction of copper, and
it is estimated that 15,000~t of Cu was produced during this period.
From 1842 to 1877, 9000~t of pyrite was mined for sulfuric acid
production in addition to the reprocessing of mining waste produced
during previous copper extraction (BRGM, internal communication).

AMD has been flowing from water adits since unknown but ancient times.
Its average characteristics and uncertainties (95\%
confidence intervals) calculated using 134 monthly samples from 1998 to
2012 are presented in Table~\ref{tab1}.  Metals were analysed by SGS company in
accordance with standard NF EN ISO~11885 and sulfate are in accordance with
standard NF T 90-40.
The AMD contains significant amounts of sulfate, iron, and zinc, but
also some aluminum and copper.

Lime treatment of AMD started in the middle of the 19th century and
still continues as of 2024. All the sludge resulting from more than 150
years of lime treatment was stockpiled on site. The size of the
sludge deposits was estimated with GIS software
using data from the bibliography including maps from different periods
and settling pond cleaning reports as well as an elevation model and
the depth of six sludge core samples drilled in 2002 (see
Section~\ref{sec2.2.1}). The total sludge volume is estimated at
77,000~m$^{3}$ or around 170,000~t according to the measure of sludge
density in the laboratory (2.2 ${\pm}$ 0.4~t${\cdot}$m$^{-3}$, $n =
3$).

\subsubsection{Core sample characterization} \label{sec2.1.1}

An aerial view of the sludge dump is shown on Figure~\ref{fig1},
including the position of six core samples drilled in order to obtain a
general characterization of the sludge accumulated on the site. 

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Sludge dump and core sampling locations (C1--C7)
on the mining site. For details of the core samples' depth and number
of analyses, refer to Table~\ref{tab2}.}
{\vspace*{-.25pc}}
\end{figure*}

\begin{table*}[t!]%tab1
\caption{\label{tab1}Chemical composition of AMD samples from 1998 to
2012 $(n = 134)^{*}$}
\begin{tabular}{ccccccc}
\thead
\parbox[t]{1.5cm}{\centering Flow rate ($\mathrm{m}^{3}{\cdot}\mathrm{h}^{-1}$)} &amp; pH 
&amp; \parbox[t]{1.2cm}{\centering Al ($\mathrm{mg}{\cdot}\mathrm{L}^{-1}$)} 
&amp; \parbox[t]{1.2cm}{\centering Cu ($\mathrm{mg}{\cdot}\mathrm{L}^{-1}$)} 
&amp; \parbox[t]{1.2cm}{\centering Fe ($\mathrm{mg}{\cdot}\mathrm{L}^{-1}$)}
&amp; \parbox[t]{1.2cm}{\centering SO$_{4}$ ($\mathrm{mg}{\cdot}\mathrm{L}^{-1}$)} 
&amp; \parbox[t]{1.2cm}{\centering Zn ($\mathrm{mg}{\cdot}\mathrm{L}^{-1}$)}\vspace*{2pt}\\
\endthead
7.2 &amp; 2.7 ${\pm}$ 0.1 &amp; 50 ${\pm}$ 5 &amp; 32 ${\pm}$ 7 &amp; 260 ${\pm}$ 40 &amp; 2300 ${\pm}$ 150 &amp; 210 ${\pm}$ 20
\botline
\end{tabular}
\tabnote{$^{*}n$ indicates the number of samples analyzed from 1998
to 2012.}
\end{table*}

The depth and number of analysis on each core sample are presented in
Table~\ref{tab2}. Sampling of core ``C5'' was planned but never
actually drilled, and therefore, no results of this sample are
available.

\begin{table}%tab2
\caption{\label{tab2}Sludge depth on each core and number of chemical
analyses}
\begin{tabular}{cccc}
\thead
Core &amp; \parbox[t]{1.9cm}{\centering Core depth (m)} &amp;
\parbox[t]{1.8cm}{\centering Sludge depth (m)} &amp;
\parbox[t]{1.7cm}{\centering Number of analyses}\vspace*{2pt}\\
\endthead
C1 &amp; \08 &amp; \mn6 &amp; 6\\
C2 &amp; \08 &amp; ${&gt;}$8 &amp; 3\\
C3 &amp; \09 &amp; \mn3 &amp; 3\\
C4 &amp; \06 &amp; \mn4 &amp; 2\\
C6 &amp; 12 &amp; \mn8 &amp; 4\\
C7 &amp; 10 &amp; \mn8 &amp; 4
\botline
\end{tabular}
{\vspace*{-.7pc}}
\end{table}

22 samples allowed the calculation of a robust average and a good
estimate of the heterogeneity of this sludge deposit and not specifically the 
inductively coupled plasma atomic emission spectroscopy (ICP-AES). Carbonates were analyzed
by volumetric determination of mineral carbon (NF EN ISO~10693).
Mineralogy of the core sample was characterized by X-ray diffraction
(XRD, SIEMENS type D5000), scanning electron microscope (SEM, JEOL
6100), and transmission electron microscopy (TEM) with energy dispersive
spectroscopy (EDS) (Philips CM 120). Both SEM and TEM analyses were
employed to characterize our samples due to the wide range of crystal
sizes present. SEM was used to observe larger crystals such as gypsum
crystals that were up to 20~$\upmu$m in size. TEM was used to study
nanometric-sized mineral phases and to investigate possible carrier
phases for the metals Cu and Zn, including clean mineral phases as well
as substituted or adsorbed\unskip\break metals.
\vspace*{-.3pc}

\subsection{Leaching tests} \label{sec2.2}
\subsubsection{Characterization of sludge sample used in{\hfill\break}
leaching tests} \label{sec2.2.1}

For the leaching tests, six samples of superficial AMD sludge of a
total mass of 15~kg were collected from different points of the site in
2021. Sample preparation included drying in an oven at
40~{\textdegree}C for a period of 48~h to remove excess
moisture,{\break} disaggregating using a mortar, sieving at 2~mm, and
mixing and homogenization of the six samples. Representative subsamples
for characterization and leaching tests were then obtained using a
riffle splitter. The density of the dried sludge was measured by water
displacement. The chemical composition of the homogenized sample was
determined by ALS laboratory by different in-house methods. Fusion
decomposition was used for major elements such as Fe, Al, Si, and Ca,
while Cu, Cd, and Zn were analyzed using a four-acid digestion method
(HCl, HNO$_3$, HF, and HClO$_4$). Both analyses were followed by
\mbox{ICP-AES} and the loss on ignition test at 1000~{\textdegree}C.
Sulfate was determined from the difference between total sulfur (by
Leco furnace and infrared spectroscopy) and sulfide (by Leco furnace
and Na$_{2}$CO$_{3}$ leach). In selected experiments, metal contents of
solid samples were measured before and after leaching using a portable
X-ray fluorescence (pXRF) Niton XL3t 980 (Thermo Fisher) to perform
mass balance calculation. Given the semi-quantitative nature of pXRF
analysis, we implemented a stringent protocol to ensure that the data
utilized in our study provided accurate elemental concentrations and
enabled confident use in mass balance calculations. This protocol
includes validation against standard samples, repeated measurements,
and systematic verification of XRF spectra. They are inspired by the
methods and principles presented by several authors including Touz\'{e}
et~al.~\cite{26}, Lemi\`ere~\cite{27}, and
L\'{o}pez-N\'{u}\~{n}ez~\cite{28}.

SEM images were collected on TESCAN Mira3 XMU (low vacuum field
emission SEM) at BRGM, using a high voltage of 15~kV. EDS analyses were
performed on an EDAX APEX system equipped with a 70~mm$^{2}$ silicon
drift detector (SDD). Sample powders were dispersed on an aluminum stub
covered with a double-coated carbon conductive tape and coated with a
20~nm conductive carbon layer. TEM was conducted on JEOL ARM200CF
(double C$_s$ corrected cold FEG TEM/STEM) at the electron microscopy
platform MACLE (CNRS Orleans). EDS analyses were obtained on a JEOL
Centurio SDD detector. Images and analyses were performed at 200~kV.

\subsubsection{Experimental setup} \label{sec2.2.2}

Three different experimental setups were used to study the leaching of
Zn and Cu from the lime treatment sludge: 250~mL shake flasks, then
0.5~L stirred tank reactors (STRs), and finally 2~L STRs. The tests in
250~mL shake flasks were carried out only at low solid contents in order
to avoid mass transfer and solubility limitations. Tests with higher
solid contents necessitated the transition to STR, initially at 0.5~L and
then at 2~L to ensure a good solid suspension. The progression to a
larger volume was also driven by the need for sufficient sample
quantities for accurate residue analysis via pXRF.

\paragraph{Tests in shake flask} \label{sec2.2.2.1}

For preliminary leaching tests, 250~mL shake flasks were used at low
solid contents (${&lt;}$1.3\% mass/mass). The working volume of these tests
was fixed at 150~mL. Magnetic stirrers were added to the shake flasks,
and agitation was carried out using a magnetic stirring table at
150~rpm (Supplementary Figure~S1). Shake flasks were covered with
Parafilm$^{\text{\textregistered}}$ to avoid excessive evaporation. For
each experiment, the flasks were filled with 150~mL of deionized water
and the specific amount of solid required. Then, the pH was decreased by
adding H$_{2}$SO$_{4}$ to a value close to 3.0 ${\pm}$ 0.2. Throughout the
duration of each test, pH was maintained between 3.0 and 3.5 
by adding a solution of sulfuric acid
(1~mol${\cdot}$L$^{-1}$) dropwise when pH was higher than~3.5. 

This value was chosen so that the solution would be as acidic as
possible to leach a maximum of Cu and Zn without dissolving iron, as
Fe$^{3+}$ can begin to be significantly soluble below a pH of
3.0~\cite{29}.\ Samples were taken to determine metal concentrations,
and temperature, pH, conductivity, and redox potential ($E_h$) were
measured directly in the reactors at the initial time and at
approximately 0.25~h, 1~h, 2~h, 4~h, 6~h, and 24~h after the beginning
of the experiment. For metal concentration measurements, about 2~mL of
pulp was filtered at 0.45~${\upmu}$m, diluted 10 times \mbox{using} 1\%
HNO$_{3}$, and placed at 4~{\textdegree}C. The total amount of solids
passing through the filter was considered negligible since particle
size measurements of the sludge showed that only 0.82\% of particles
were smaller than 0.42~${\upmu}$m. Metal concentrations (Al, Cd, Cu,
Fe, and Zn) were then measured on the filtrate either by atomic
absorption spectroscopy (SpectrAA 220FS, Varian) or using a 4210
microwave plasma atomic emission spectroscope from Agilent\unskip\break
Technologies.

\paragraph{Tests in STRs} \label{sec2.2.2.2}

Leaching experiments were also performed in two types of reactors with
working volumes of 0.5~L (Supplementary Figure~S1) and 2~L. The
experimental setup was fully described by Guezennec et~al.~\cite{30}.
They were equipped with four baffles mounted 90{\textdegree} apart and
extended down to the base of the vessel for pulp mixing. The 0.5~L
reactor was equipped with an axial flow three-blade 45{\textdegree}
impeller, whereas the 2~L reactor (Figure~\ref{fig2}) had a dual
impeller system (axial/axial) with the same characteristics (three-blade
45{\textdegree} axial flow). The agitation speed was fixed at 600~rpm for
both systems. The temperature control system (LAUDA MC) circulated warm
water (22~{\textdegree}C, 40~{\textdegree}C, or 60~{\textdegree}C) in
the reactor jacket to maintain constant operating temperatures.
Experiments conducted above room temperature aimed to assess the
potential for reducing iron solubility and thereby enhance the
selectivity of the leaching process~\cite{31}.

\begin{figure}
{\vspace*{.3pc}}
\includegraphics{fig02}
\caption{\label{fig2}Photograph of the experimental setup using 2~L
reactors.}
\end{figure}

\looseness=-1
The experimental procedure was similar to that for shake
flasks but used the required working volume of deionized water
according to the reactor size (0.5~L or 2~L). However, no pH control
was performed in STR tests. The total amount of sulfuric acid required
for the experiment was added before the solid in the reactors. The goal
was to maintain an initially high acidic environment during the early
stages of the reaction. The monitoring of physicochemical
parameters and leachate sampling and analyses were performed as
described for shake-flask experiments (Section~\ref{sec2.2.2.1}). At
the end of some of the tests in 2~L reactors, the entire amount of pulp
was filtered using glass microfiber Whatman$^{\text{\textregistered}}$
filter grade GF/A (cut-off 0.8~$\upmu$m).

\subsubsection{Calculations} \label{sec2.2.3}

The solid content was calculated using Equation~(\ref{eq3}).
{\begin{equation}\label{eq3}
C_{\mathrm{solid}} = \frac{m_{\mathrm{dry~solid}}}{m_{\mathrm{pulp}}}
\times 100\%
\end{equation}}\unskip
where $C_{\mathrm{solid}}$ represents the percentage of the pulp that
is solid material, $m_{\mathrm{dry~solid}}$ (kg) is the mass of the
solid (the sludge) after drying and before leaching, and
$m_{\mathrm{pulp}}$ (kg) is the total mass of the pulp mixture,
including both the liquid and solid phases.

The acid-to-solid ratios were calculated according to Equation~(\ref{eq4}).
{\begin{equation} \label{eq4}
a_{\mathrm{ratio}} = \frac{m_{\mathrm{H}_{2}\mathrm{SO}_{4}}}
{m_{\mathrm{dry~solid}}}
\end{equation}}\unskip
where $a_{\mathrm{ratio}}$ (g${\cdot}$kg$^{-1}$) is the acid-to-solid
ratio, $m_{\mathrm{H}_{2}\mathrm{SO}_{4}}$ (g) is the total mass of
sulfuric acid added to the pulp, and $m_{\mathrm{dry~solid}}$ (kg) is
the mass of the sludge material after drying prior to the leaching
process.

When solid analysis was not possible (for tests with solid contents
lower than 1.3\%~m/m), leaching yields were calculated from analysis of
the liquid phase as shown in Equation~(\ref{eq5}).
{\begin{equation} \label{eq5}
\mathit{LY}_{\mathrm{liquid}} = \frac{C_{\mathrm{M},\mathrm{liquid}}{\cdot}
V_{\mathrm{liquid}}}{C_{\mathrm{M},\mathrm{solid},\mathrm{initial}}
{\cdot}m_{\mathrm{dry~solid}}}\times 100\%
\end{equation}}\unskip
where $\mathit{LY}_{\mathrm{liquid}}$ (\%) is the leaching yield of the metal
calculated from the mass of the element of
interest dissolved in the liquid phase,
$C_{\mathrm{M},\mathrm{liquid}}$ (mg${\cdot}$L$^{-1}$) is the
concentration of metal (M) in the liquid phase after the leaching
process and quantifies how much of the metal has been transferred into
the solution, $V_{\mathrm{liquid}}$ (L) is the total volume of the
liquid phase in the test, and
$C_{\mathrm{M},\mathrm{solid},\mathrm{initial}}$ (mg${\cdot}$kg$^{-1}$)
is the concentration of the metal (M) in the sludge before leaching.

When the compositions of the residual solid after leaching were
analyzed (for tests with solid contents higher than 2.9\%~m/m), leaching
yields were calculated both by Equation~(\ref{eq5}) and from the
analyses of the solids according to Equation~(\ref{eq6}).
{\begin{eqnarray}
&amp;&amp; \hspace*{-.45pc} 
\mathit{LY}_{\mathrm{solid}}
\nonumber\\&amp;&amp; 
= \frac{C_{\mathrm{M},\mathrm{solid},\mathrm{initial}}
{\cdot}m_{\mathrm{dry~solid}}-C_{\mathrm{M},\mathrm{solid},\mathrm{final}}
{\cdot}m_{\mathrm{leached~solid}}}{C_{\mathrm{M},\mathrm{solid},\mathrm{initial}}
{\cdot}m_{\mathrm{dry~solid}}}
\nonumber\\&amp;&amp; \quad 
\times\ 100\% \label{eq6}
\end{eqnarray}}\unskip
where $\mathit{LY}_{\mathrm{solid}}$ (\%) is the leaching yield of the metal
calculated from the solid phase,
$C_{\mathrm{M},\mathrm{solid},\mathrm{final}}$ (mg${\cdot}$kg$^{-1}$)
is the concentration of the metal (M) in the solid after leaching, and
$m_{\mathrm{leached~solid}}$ (kg) is the mass of the dry solid after
leaching.

Mass balances for the targeted metals were performed, and the
differences between the leaching yields calculated from solid and
liquid phases were always less than~10\%.

\section{Results} \label{sec3}
\subsection{Core sample characterization} \label{sec3.1}

The sludge is mainly composed of Ca, Fe, and SO$_{4}$ with significant
amounts of Cu and Zn, with about 1\% and 5\% respectively of the total
mass of the samples, as presented in Table~\ref{tab3}. Other elements
present in low amounts are Al and Si. It is important to highlight that
there is also a non-negligible amount of Cd, a heavy metal potentially
harmful for human health and the environment. Uncertainties calculated
as 95\% confidence intervals show that the concentration variabilities
are generally low despite the fact that samples included come from
different cores and several depths. This means that the sludge
accumulated on the dump can be considered fairly \mbox{homogeneous}.

\begin{table*}[t!]%tab3
\caption{\label{tab3}Average composition of sludge core samples and
the sludge used in leaching tests}
\tabcolsep13pt
\begin{tabular}{ccc}
\thead
Compound (\%) &amp; Core samples ($n^{*}=22$) &amp; Sludge used in leaching tests\\
\endthead
Al$_{2}$O$_{3}$ &amp; \0\03.3 ${\pm}$ 0.4$^{**}$ &amp; 3.5\\
CaO &amp; 24 ${\pm}$ 1\0 &amp; 17\\
Cd &amp; 0.011 ${\pm}$ 0.003 &amp; 0.010\\
$\mathrm{CO}_{3}^{2-}$ &amp; 0.7 ${\pm}$ 0.3 &amp; 1.3\\
Cu &amp; 1.3 ${\pm}$ 0.2 &amp; 1.1\\
Fe$_{2}$O$_{3}$ &amp; 17 ${\pm}$ 1\0 &amp; 16\\
SiO$_{2}$ &amp; 3.5 ${\pm}$ 0.8 &amp; 7\\
$\mathrm{SO}_{4}^{2-}$ &amp; 31.9 ${\pm}$ 0.6\0 &amp; 26\\
Zn &amp; 5 ${\pm}$ 1 &amp; 5\\
Mass loss at 105~{\textdegree}C &amp; 60 ${\pm}$ 2\0 &amp; 26
\botline
\end{tabular}
\xTabnote{${}^{*}$Number of samples analyzed. ${}^{**}$Uncertainties
correspond to 95\% confidence intervals.{\vspace*{-.3pc}}}
\end{table*}

The main minerals identified by XRD were gypsum
(CaSO$_{4}{\cdot}2$H$_{2}$O), quartz (SiO$_{2}$), an amorphous phase
(probable ferriferous, possibly ferrihydrite
(Fe$_{3-0.5y}$(OH)$_{y}{\cdot}n$H$_{2}$O) as commonly observed in
liming of AMD \cite{17,33}), feldspar
((Ba,Ca,Na,K,{\ubreak}NH$_{4}$)(Al,B,Si)$_{4}$O$_{8}$), illite
((K,H$_{3}$O)(Al,Mg,Fe)$_{2}$(Si,Al)$_{4}${\ubreak}O$_{10}$[(OH)$_{2}$,(H$_{2}$O)]),
barite (BaSO$_{4}$), calcite (CaCO$_{3}$), lepidocrocite
($\gamma$FeO(OH)), and jarosite
(KFe$_{3}$(SO$_{4}$)$_{2}${\ubreak}(OH)$_{6}$). The identification of
metal-bearing phases was made very difficult by the abundance of gypsum
and the high proportion of the amorphous phase. A detailed study of
these samples by EDS--SEM and XRD revealed the presence of Cu--Fe
crystallites with a ``desert rose'' morphology (Figure~\ref{fig3}),
identified by XRD as delafossite (CuFeO$_{2}$) and ferriferous tabular
crystallites with Zn. XRD diffractograms of a delafossite-rich sample
and a lepidocrocite-rich sample are presented in Supplementary
Figures~S2 and~S3. The only crystallized ferriferous phase identified
in this specific sample is lepidocrocite. There are also some
ferriferous aggregates with no trace of other metals and very abundant
gypsum that contains no trace of metallic elements. 
No zinciferous mineral phase was clearly identified by XRD.{\vspace*{-.3pc}}

\begin{figure*}
\includegraphics{fig03}
{\vspace*{-.2pc}}
\caption{\label{fig3}SEM microphotographs of (A)~iron-rich aggregates,
(B)~tabular iron-rich zinciferous rods, (C)~gypsum, and
(D)~delafossite, which is the focus of the red rectangle from~(C).}
{\vspace*{-.4pc}}
\end{figure*}

\subsection{Leaching tests} \label{sec3.2}
{\vspace*{-.15pc}}
\subsubsection{Characterization of sludge sample used in{\hfill\break}
leaching tests and residues} \label{sec3.2.1}

As can be observed in Table~\ref{tab3}, the composition of the sludge
used in the leaching tests is similar to that of the core samples.
Indeed, the contents of Fe, Cd, Cu, and Zn in the sludge used for the
leaching test fall within the 95\% confidence intervals of those found
in the core samples. The EDS--SEM{\break} mapping of sludge before
leaching is presented in Figure~\ref{fig4}. This SEM map illustrates
the elemental distribution within our sludge sample prior to leaching.
Notably, Ca and S are closely linked, forming large and well-defined
crystals indicative of gypsum. Conversely, the distribution of iron in
the sample appears diffuse, with no identifiable crystalline form at
this scale of observation. This suggests that the iron-bearing phases
are very finely grained and/or pseudo-amorphous~\cite{29,34}. Sulfur's
lack of association with iron suggests the minimal presence of iron
sulfate minerals, such as schwertmannite and jarosite, in this sample.
In contrast, Zn shows a strong association with Fe, which may be
representative of metal-substituted iron oxides or oxyhydroxides.
Interestingly, some regions rich in iron show no corresponding zinc
presence, pointing to a heterogeneous distribution of zinc within the
iron matrix. Cu is likely also associated with iron; however, its
concentration is too close to the quantification limit and the
bremsstrahlung background to allow a meaningful interpretation other
than indicating the absence of any Cu-rich minerals. Gypsum crystals
are completely devoid of zinc, whereas traces of copper are present.
The absence of distinct copper or zinc hotspots at this scale indicates
that there are no specific copper or zinc minerals present or if they
are, they exist in such low abundance that they are not
\mbox{detectable}.\looseness=-1

\begin{figure*}
\includegraphics{fig04}
{\vspace*{-.2pc}}
\caption{\label{fig4}EDS--SEM mapping of sludge before leaching.}
\end{figure*}

SEM mapping of residue after leaching (with an acid-to-solid ratio of
150~g${\cdot}$kg$^{-1}$ and with 0.7\% of solid content) was also
carried out. However, the concentrations of Zn and Cu after leaching
are too close to the quantification limit and the bremsstrahlung
background to allow a meaningful interpretation aside from the absence
of Zn-rich minerals at this scale. These results are presented in
Supplementary Material Figure~S4. 

The TEM images and EDS analyses of a gypsum crystal and iron-rich
aggregates are shown in Figure~\ref{fig5} and Table~\ref{tab4}.
They confirm the virtual absence of Cu and Zn in gypsum and their
strong association with iron. On the EDS analyses of the gypsum
crystals observed in the TEM, the stronger association of Cu with
gypsum is much less obvious (compared with the SEM map). At this much
smaller scale, the crystal form of the ferriferous phases is also
poorly defined, suggesting very fine-grained or pseudo-amorphous
minerals. 

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}TEM microphotograph and location of
semi-quantitative EDS analyses: (A)~gypsum crystal and (B)~iron-rich
aggregates.}
{\vspace*{.2pc}}
\end{figure*}

\begin{table*}[t!]%tab4
\caption{\label{tab4}TEM--EDS analysis of gypsum crystal and iron-rich
aggregates{\vspace*{-.3pc}}}
\begin{tabular}{lcccccccc}
\thead
&amp; Al (\%) &amp; Si (\%) &amp; S (\%) &amp; K (\%) &amp; Ca (\%) &amp; Fe (\%) &amp; Cu (\%) &amp; Zn (\%)\\
\endthead
(A) Whole area &amp; n.d &amp; n.d &amp; 46.59 &amp; n.d &amp; 52.52 &amp; 0.3 &amp; 0.1\0 &amp; \00.49\\
(B) 28 &amp; 8.37 &amp; 3.77 &amp; \05.33 &amp; 0.17 &amp; \00.91 &amp; 60.50 &amp; 7.40 &amp; 13.56\\
(B) 29 &amp; 7.77 &amp; 9.37 &amp; \02.38 &amp; 0.98 &amp; \01.30 &amp; 61.17 &amp; 5.63 &amp; 11.40
\botline
\end{tabular}
{\vspace*{-.4pc}}
\end{table*}

\subsubsection{Cu and Zn leaching behavior} \label{sec3.2.2}

Leaching tests of the sludge were performed first in shake flasks with
low solid contents and with \mbox{manual} pH control in order to obtain
basic information about the leaching kinetics. Preliminary tests
conducted at solid contents of 1.1\% and 0.75\% with pH levels of 5.4
and 4.0, respectively, indicated that a pH below 4.0 is necessary to
achieve substantial dissolution of target metals Zn and Cu. At these
conditions, leaching yields were 0\% for Cu and 24\% for Zn at pH~5.4
and 8\% for Cu and 37\% for Zn at pH~4.0. For this reason, it was
decided to perform experiments correcting manually the pH to 3.0 when
it was higher than~3.5.

Table~\ref{tab5} shows the operating conditions at which the leaching
tests in shake flasks were performed and the resulting Cu and Zn
leaching yields. Figure~\ref{fig6} shows the Zn and Cu concentrations
as well as the pH changes in time as a function of the leaching time
for both tests lasting 24~h (T1 and~T3). The pH increased between the
monitoring points at fixed time intervals as a result of the leaching
reactions, and it was necessary to manually decrease it with diluted
H$_{2}$SO$_{4}$.\ In these experiments, the pH \mbox{varied} from 2.7
to~4.2.\ It is important to note that the ``\mbox{final} pH'' shown in
Table~\ref{tab5} is the value \mbox{measured} at the last monitoring
point without manual pH{\break} adjustment.

\begin{table*}[t!]%tab5
\caption{\label{tab5}Operating conditions and results of shake-flask
leaching tests}
\begin{tabular}{cccccccc}
\thead
\xmorerows{1}{}
&amp; \morerows{1}{\parbox[t]{1.7cm}{\centering Leaching time (h)}} &amp;
\morerows{1}{\parbox[t]{1.7cm}{\centering Solid content (\%)}} &amp;
\morerows{1}{\parbox[t]{2.1cm}{\centering Acid-to-solid ratio (g${\cdot}$kg$^{-1}$)}} &amp;
\xmorerows{1}{Initial pH} &amp; \xmorerows{1}{Final pH} &amp; \multicolumn{2}{c}{$\mathit{LY}_{\mathrm{liquid}}$ (\%)}\\
\cline{7-8} &amp;&amp;&amp;&amp;&amp;&amp; Zn &amp; Cu\\
\endthead
T1 &amp; 24 &amp; 0.68 &amp; 106 &amp; 2.7 &amp; 3.6 &amp; 75 &amp; 66\\
T2 &amp; 5.3 &amp; 0.74 &amp; 102 &amp; 2.4 &amp; 3.5 &amp; 73 &amp; 67\\
T3 &amp; 24 &amp; 0.79 &amp; 118 &amp; 3.4 &amp; 3.6 &amp; 75 &amp; 70\\
T4 &amp; 5.3 &amp; 0.74 &amp; 120 &amp; 2.6 &amp; 3.5 &amp; 76 &amp; 73\\
T5 &amp; 5.3 &amp; 0.73 &amp; 125 &amp; 3.2 &amp; 3.5 &amp; 77 &amp; 71
\botline
\end{tabular}
{\vspace*{-.4pc}}
\end{table*}

\begin{figure*}
\includegraphics{fig06}
{\vspace*{-.2pc}}
\caption{\label{fig6}Concentrations of (A)~Zn and (B)~Cu and
(C)~leachate pH for tests with 24~h of duration in shake flasks.}
{\vspace*{-.3pc}}
\end{figure*}

The leaching curves follow a similar trend in both tests: (i)~a very
fast metal leaching during the first 0.25~h; (ii)~a fast decrease in
the leaching rate between 0.25~h and 4~h or 6~h; and (iii)~a
stabilization of the concentration at the end of the test. Moreover, it
can be noted that for test~T3, the concentrations of both Zn and Cu
increase with time and reach a plateau around
310~mg${\cdot}$L$^{-1}$ and 60~mg${\cdot}$L$^{-1}$, respectively, after
4~h. However, it is not possible to say whether the concentration
plateau was reached in test~T1 since no samples between 6~h and 24~h
were taken. Additionally, the final pH values of both tests are similar as
well as the final leaching yields for both metals. Moreover, the pH
curve shows that pH was manually decreased to slightly lower values in
T3 than in T1 during the monitoring points between 2~h and 4~h. This
resulted in a higher acid-to-solid ratio and a faster stabilization of
the concentration of metals \mbox{after} 6~h. These results imply that
the chemical reaction can be completed after only 6~h when the pH is
kept low (${&lt;}$3.5) since the beginning of the reaction. Extrapolating
this idea, adding more acid at the beginning of the reaction, and thus
decreasing the pH at early stages of the reaction, may help to reduce
the leaching time. Therefore, for the tests using STRs, it was
decided to add the total required amount of acid at the beginning
instead of controlling the pH. 

In order to verify the repeatability of the leaching tests, three
leaching tests in 0.5~L STR were carried out under very similar
operating conditions (shown in Table~\ref{tab6}). The concentrations of
Zn and Cu as well as the leachate pH are depicted in Figure~\ref{fig7}.
It is important to highlight that for these tests, the total amount of
acid necessary to reach the required acid-to-solid ratio in each reactor
was added at the \mbox{beginning} of the experiment. The leaching
behavior of the metals follows the same trend as in the shake-flask
tests previously presented, confirming that 6~h is sufficient to reach
an equilibrium in Zn and Cu concentrations under the conditions of the
test. One can note that the pH also follows a similar trend. The final
pH depends not only on the acid-to-solid ratio used but also on the
solid content. A notable pH difference was observed for R3 compared to
R1 and R2. Although the acid-to-solid ratio for R3
(153~g${\cdot}$kg$^{-1}$) was slightly lower than that for R1 and R2
(157~g${\cdot}$kg$^{-1}$), this difference seems insufficient to fully
explain the pH discrepancy. It is possible that minor variations in the
initial conditions or inhomogeneities within the sludge samples
influenced the pH results. The results show that even with slight
variations in the acid-to-solid ratio, the relative uncertainties of
the leaching yield are 2 and 3\% for Zn and Cu,\break respectively. 

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Concentrations of (A)~Zn and (B)~Cu and
(C)~leachate pH in 0.5~L STR tests.}
{\vspace*{.6pc}}
\end{figure*}

\begin{table*}[t!]%tab6
\caption{\label{tab6}Operating conditions and results of 0.5~L STR
leaching tests}
\begin{tabular}{ccccccc}
\thead
\xmorerows{1}{}
&amp; \xmorerows{1}{\parbox[t]{1.7cm}{\centering Solid content (\%)}} &amp;
\xmorerows{1}{\parbox[t]{2.2cm}{\centering Acid-to-solid ratio (g${\cdot}$kg$^{-1}$)}} &amp;
\xmorerows{1}{Initial pH} &amp;
\xmorerows{1}{Final pH} &amp;
\multicolumn{2}{c}{\parbox[t]{2.5cm}{\centering Leaching yield (\%~liquid phase)}}\\
\cline{6-7} &amp;&amp;&amp;&amp;&amp; Zn &amp; Cu\\
\endthead
{R1} &amp; 0.72 &amp; 157 &amp; 1.7 &amp; 3.0 &amp; 78 &amp; 80\\
{R2} &amp; 0.77 &amp; 157 &amp; 1.8 &amp; 2.9 &amp; 75 &amp; 78\\
{R3} &amp; 0.70 &amp; 153 &amp; 1.9 &amp; 3.6 &amp; 79 &amp; 74\vspace*{5pt}\\
 &amp;&amp;&amp;&amp; Average &amp; 77 &amp; 77\\
 &amp;&amp;&amp;&amp; Std. deviation &amp; \02 &amp; \03
 \botline
\end{tabular}
\end{table*}

\subsubsection{Effect of acid-to-solid ratio on leaching time and
efficiency} \label{sec3.2.3}

As shown in Table~\ref{tab5}, several acid-to-solid ratios were tested
in 250~mL shake-flask experiments. The results are depicted in
Figure~\ref{fig8}. One can note that higher acid-to-solid ratios
resulted in faster leaching rates during the first two hours. Moreover,
the maximum leaching yield plateau is reached between 1~h and 2~h for
both metals Zn and Cu, using an acid-to-solid ratio of
125~g${\cdot}$kg$^{-1}$ (T5). However, regardless of the acid-to-solid
ratio, we consistently observe incomplete dissolution (with a maximum
leaching yield of approximately 77\% for Zn and 75\% for Cu) in these
tests.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}(A)~Zn and (B)~Cu leaching yields
($\mathit{LY}_{\mathrm{liquid}}$) for tests in shake flasks.}
\end{figure*}

Experiments at different acid-to-solid ratios with low solid contents
(between 0.6\% and 1.3\%) were carried out in 250~mL shake flasks,
whereas 2~L reactors were used for higher acid-to-solid ratios with a
solid ratio of 0.7\%~m/m at reaction times of 6~h and 24~h. The results
are all depicted in Figure~\ref{fig9}. They show that the leaching
yield increased with acid-to-solid ratio up to about
250~g${\cdot}$kg$^{-1}$ for both Zn and Cu, regardless of the experimental
setup used. Further increases in the acid-to-solid ratio to
350~g${\cdot}$kg$^{-1}$ and 450~g${\cdot}$kg$^{-1}$ did not
significantly increase the maximum leaching yield obtained, which was
90\% for Cu and 84\% for Zn.

\begin{figure}
\includegraphics{fig09}
\caption{\label{fig9}Metal leaching yields ($\mathit{LY}_{\mathrm{liquid}}$) at
different acid-to-solid ratios in shake-flask tests and STRs for low
solid contents (from 0.7\% to 1.3\%~m/m).}
{\vspace*{-.5pc}}
\end{figure}

The Fe leaching also increases with acid-to-solid ratio, and it becomes
significant above 200~g${\cdot}$kg$^{-1}$. This may be due to the
dissolution of iron oxyhydroxides, such as ferrihydrite, caused by the
low pH values (${&lt;}$3.0) obtained at the beginning of the
\mbox{experiments} using high acid-to-solid ratios~\cite{29}. The
initial pH (before solid addition) and the final pH at different
acid-to-solid ratios in STRs for low solid contents (from 0.7\% to
1.3\%~m/m) are presented in Supplementary Material Figure~S5.
One can note that the higher the acid-to-solid ratio, the lower the
initial and final pH values. In the four reactors where Cd
concentrations were measured, the leaching efficiency ranged from 96\%
to 100\% for all acid-to-solid ratios tested (values of
200~g${\cdot}$kg$^{-1}$ or higher as presented in Supplementary
Material Table~S1).

Experiments in 2~L reactors with a solid content of 2.9\% at different
acid-to-solid ratios with a reaction time of 24~h were performed to
investigate the influence of acid-to-solid ratio on residue
composition and on metal leaching yields compared to leaching tests at
lower solid contents. Figure~\ref{fig10}A shows the resulting leaching
yields of these tests and Figure~\ref{fig10}B shows the resulting Zn and
Cu concentrations in the residues after leaching. An increase in 
acid-to-solid ratio from 200 to 450~g/kg resulted in an increase in 
dissolution rates of Cu from 86\% to 93\% and Zn from 82\% to 92\%.

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}(A)~Metal leaching yields ($\mathit{LY}_{\mathrm{solid}}$)
and (B)~metal contents in solid residue at different acid-to-solid ratios
with a solid content of 2.9\%~m/m.}
{\vspace*{.85pc}}
\end{figure*}

An augmentation of the acid-to-solid ratio to 650~g${\cdot}$kg$^{-1}$
did not have a visible impact on the leaching yield of Cu and Zn
compared to an \mbox{acid-to-solid} ratio of 450~g${\cdot}$kg$^{-1}$.
On the other hand, the Fe leaching yield increased significantly when
the acid-to-solid ratio increased from 200~g${\cdot}$kg$^{-1}$ to
450~g${\cdot}$kg$^{-1}$ and, as for Zn and Cu, it remained almost
unchanged with a further increase in this parameter to
650~g${\cdot}$kg$^{-1}$. The content in the leaching residue was
reduced from 5.2\% to 0.78\% for Zn and from 1.0\% to 0.22\% for Cu,
using an acid-to-solid ratio of 200~g${\cdot}$kg$^{-1}$. For the tests
using an acid-to-solid ratio of 650~g${\cdot}$kg$^{-1}$, the Zn content
in the leaching residue was 0.31\% while the Cu content was~0.08\%.
These results demonstrate the limited effect of increasing
significantly the acid-to-solid ratio on Zn and Cu leaching
efficiencies for values higher than 200~g${\cdot}$kg$^{-1}$.

\subsubsection{Effect of solid content on leaching yield}
\label{sec3.2.4}

Another set of experiments was performed in which the acid-to-solid
ratio was fixed at 200~g${\cdot}$kg$^{-1}$ (Figure~\ref{fig11}) to
assess the influence of solid content on leaching yields. In this case,
an increase in solid content from 2.9\% to 4.5\% appeared to
enhance the leaching efficiency for both metals. Further increasing
this value to about 8.6\% did not significantly influence the leaching
yield. However, with a solid content of 16\%, a leaching efficiency
decrease is observed. Moreover, the variations in the leaching
efficiency of these metals were also reflected in the metal content in
the leaching residue.

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}(A)~Metal leaching yields ($\mathit{LY}_{\mathrm{solid}}$)
and (B)~metal contents in solid residue at different solid contents at
an acid-to-solid ratio of 200~g H$_{2}$SO$_{4}{\cdot}$kg$^{-1}$.}
\end{figure*}

The results of these experiments suggest that the leaching efficiency
of metals might have a positive correlation with the solid content up
to 4.5\%. However, further increases beyond 8.6\% values could have a
slightly negative effect on the leaching yields of Cu and Zn but a
more substantial effect for Fe. Given the small variations observed and
the limited number of data points (only two up to 4.5\%) without
replicates, these results should be interpreted with caution. Further
statistical confirmation with replicates and more data points would be
necessary to validate these findings.

Throughout the leaching experiments, the final calcium concentrations
were consistently measured between 440 and 570~mg${\cdot}$L$^{-1}$.
These concentrations are close to or moderately lower than the Ca
concentration of an aqueous solution in equilibrium with gypsum under
standard conditions. Consequently, the dissolution rates of calcium
were predominantly determined by the solid content, resulting in a wide
range of leaching yields from 4\% (at a solid content of~9.4\%) to 64\%
(at a solid content of~0.7\%) as presented in Supplementary Figure~S6.
There also appears to be a minor unknown phenomenon decreasing Ca
concentration at lower pH.

\subsubsection{Effect of temperature on leaching yield}
\label{sec3.2.5}

The effect of temperature on metal leaching efficiency was also
studied with an acid-to-solid ratio of 200~g${\cdot}$kg$^{-1}$, a solid
content of 2.9\%, and a reaction time of 24~h. Figure~\ref{fig12} shows
the Zn and Cu leaching yields as well as the Fe concentration in the
leachate at 22~{\textdegree}C, 40~{\textdegree}C, and 60~{\textdegree}C.
One can observe that by increasing the temperature from
22~{\textdegree}C to 40~{\textdegree}C, it was possible to increase
both Zn and Cu leaching yields by respectively 2 and 4 percentage
points (pp) at these conditions. However, no significant enhancement of
efficiency is observed if the temperature is further increased to
60~{\textdegree}C. It is interesting to observe that the Fe
concentration in the leachate decreases considerably as the temperature
increases. It~is known that Fe(III) can precipitate even at low pH
values in the form of jarosite (KFe$_{3}$(SO$_{4}$)$_{2}$(OH)$_{6}$) or
schwertmannite (Fe$_{8}$O$_{8}$(OH)$_{6}$(SO$_{4}$)${\cdot}n$H$_{2}$O)
and that this phenomenon is favored by the increase in{\break}
temperature~\cite{35}.

\begin{figure}
\includegraphics{fig12}
\caption{\label{fig12}Metal leaching yields ($\mathit{LY}_{\mathrm{solid}}$) at
different temperatures using 200~g H$_{2}$SO$_{4}$/kg and 2.9\%~m/m of
solids after 24~h.}
{\vspace*{-.6pc}}
\end{figure}

\section{Discussion} \label{sec4}
\subsection{Implication of mineralogy on metal leaching
behavior} \label{sec4.1}

A common result for all the leaching tests performed in this study is
the presence of a concentration plateau for Zn and Cu at the end of the
experiments, in general after 6~h, which corresponds to an incomplete
leaching of these metals. The parametric study aimed at improving the
leaching kinetics, which resulted in slightly higher leaching yields.
However, it is clear that there is a limitation regarding the maximum
leaching efficiency achieved at the conditions tested, which translates
into an incomplete removal of Cu and Zn from the sludge. Nevertheless,
introducing the entire acid quantity at the beginning of the reaction,
rather than having a constant pH in the system, improved the leaching
kinetics and yields for the same amount of acid. This implies that
having a higher acidic environment at the early stages of the reaction
is beneficial for metal leaching.\ Several hypotheses can explain these
observations. Our first hypothesis is that a small proportion of Cu and
Zn are embedded in the amorphous iron mineral. This is based on the
limited evidence of Zn- and Cu-rich mineral phases in the sludge
(excepting few delafossite crystals identified for Cu but which also
\mbox{contain}~Fe).

Figure~\ref{fig13} plots the correlations between Zn and Cu leaching
yields and Fe leaching yields. A positive linear correlation was found
for both Zn ($R^{2} = 0.9161$) and Cu ($R^{2} = 0.9025$) at
22~{\textdegree}C in 2~L reactors. Interestingly, for most experiments,
the increases in metal leaching yields over 85\% are observed for
conditions involving high acid-to-solid ratios
(${&gt;}$250~g${\cdot}$kg$^{-1}$) or moderate-to-high solid contents
(${&gt;}$2.9\%). These conditions imply a very low pH (${&lt;}$1.7) at the
beginning of the experiments, which caused the partial dissolution of
Fe compounds.

\begin{figure}
\includegraphics{fig13}
\caption{\label{fig13}Correlation between targeted metal leaching
yields and Fe leaching yields for experiments at 22~{\textdegree}C in
2~L reactors.}
\end{figure}

The maximum leaching yield obtained in this study was 94\% for Zn and
93\% for Cu at a solid content of 2.9\% and an acid-to-solid ratio of
650~g${\cdot}$kg$^{-1}$. However, when acid-to-solid ratios are higher
than 200~g${\cdot}$kg$^{-1}$, the improvement in both Cu and Zn leaching
efficiencies is limited since a significant part of the acid added reacts
with iron oxyhydroxides such as the ferrihydrite according to
Equation~(\ref{eq7})~\cite{34}:
{\begin{eqnarray}
&amp;&amp;
\mathrm{Fe}_{2}\mathrm{O}_{3-0.5y}(\mathrm{OH})_{y(s)} +
6 \mathrm{H}^{+}_{(\mathrm{aq})}
\nonumber\\&amp;&amp; 
\quad \rightarrow 2\mathrm{Fe}^{3+}_{(\mathrm{aq})} +
(3+0.5y)\mathrm{H}_{2}\mathrm{O}_{(\mathrm{liq})} \label{eq7}
\end{eqnarray}}\unskip
Here, log $K = 8.46$ for two-line ferrihydrite and 10.12 for six-line
ferrihydrite, both at standard conditions (25~{\textdegree}C and
1~bar). Fe$_{2}$O$_{3-0.5y}$(OH)$_{y}$ is the formula proposed by Yu
et~al.~\cite{34} as ferrihydrite has not only variable chemical
composition depending on the degree of hydration but also variable
crystallinity.

Besides, lower initial pH values obtained at high acid-to-solid ratios
can also increase the solubility of the gypsum present in significant
quantities in the sludge. Gypsum dissolved according to
Equation~(\ref{eq8}) with a solubility product in water
($K_{\mathrm{sp}}$) of $2.58 \times 10^{-5}$~\cite{36}:
{\begin{equation} \label{eq8}
\mathrm{CaSO}_{4} \cdot 2\mathrm{H}_{2}\mathrm{O} \rightarrow
\mathrm{Ca}^{2+} + \mathrm{SO}_{4}^{2-} + 2\mathrm{H}_{2}\mathrm{O}
\end{equation}}\unskip
At pH lower than 1.9, the species $\mathrm{HSO}_{4}^{-}$ may become predominant
over $\mathrm{SO}_{4}^{2-}$~\cite{37}. Thus, the increase in sulfate
concentration in the leachate caused by the gypsum dissolution may act
as a buffer, according to Equation~(\ref{eq9}), with a $\mathrm{p}K_a$
of~1.9.
{\begin{equation} \label{eq9}
\mathrm{SO}_{4}^{2-} + \mathrm{H}^{+} \leftrightarrow
\mathrm{HSO}_{4}^{-}
\end{equation}}\unskip
Therefore, at high acid-to-solid ratios and very low pH values, free
protons are not only consumed in the leaching of Cu and Zn but also in
other reactions involving the dissolution of other metals such as Fe
and Ca. The increase in acid-to-solid ratios to values higher than
about 200~g${\cdot}$kg$^{-1}$ may decrease the selectivity of the
process towards Zn and Cu \mbox{recovery}, and it can be even
considered as an inefficient way of enhancing the leaching efficiencies
of the targeted metals. The limitation on metal dissolution despite the
increase in acid-to-solid ratio could then be explained by the presence
of non-soluble Cu and Zn minerals in sulfuric acidic media at the pH
tested in this study. As previously detailed, the study of sludge
mineralogy revealed the presence of Cu--Fe crystallites, identified as
delafossite (CuFeO$_{2}$), as well as ferriferous tabular crystallites
with Zn (probably $\gamma$FeO(OH)). Therefore, the remaining Cu and Zn
may be trapped in these less soluble Fe minerals present in the sludge.
The dissolution reaction of lepidocrocite~\cite{29} is presented in
Equation~(\ref{eq10}).
{\begin{equation} \label{eq10}
\gamma\mathrm{FeOOH}_{(s)} + 3\mathrm{H}^{+}_{(\mathrm{aq})} \rightarrow
\mathrm{Fe}^{3+}_{(\mathrm{aq})} + 2\mathrm{H}_{2}\mathrm{O}_{(\mathrm{liq})}
\end{equation}}\unskip
Here, log $K = 1.4$ at standard conditions (25~{\textdegree}C and
1~bar). Indeed, by comparing the solubility products of the iron
minerals in this study at standard conditions (similar to the
conditions of experiments performed), it is possible to observe that
there is a difference of about 2.9 orders of magnitude for the product,
dissolved Fe$^{3+}$, between two-line ferrihydrite and lepidocrocite.
The difference is even higher for the better crystallized six-line
ferrihydrite. This implies a lower solubility of the lepidocrocite,
which may potentially contain residual Zn and Cu at similar conditions
of pH, solution composition, and ionic strength.

Due to the absence of specific literature detailing the acid leaching
reaction of delafossite with sulfuric acid but knowing that in
delafossite, iron is trivalent while copper is monovalent~\cite{38}, we
have proposed a plausible chemical reaction balanced
stoichiometrically according to Equation~(\ref{eq11}).
{\begin{equation} \label{eq11}
\mathrm{CuFeO}_{2(s)} + 4\mathrm{H}^{+}_{(\mathrm{aq})} \rightarrow
\mathrm{Cu}^{+}_{(\mathrm{aq})} + \mathrm{Fe}^{3+}_{(\mathrm{aq})} +
2\mathrm{H}_{2}\mathrm{O}_{(\mathrm{liq})}
\end{equation}}\unskip
In their investigation of copper smelter flue dust containing
delafossite, Vitkova et~al.~\cite{39} subjected the material to
leaching with HNO$_{3}$, adjusting the pH to~3.0. They found that
approximately 60\% of copper remained undissolved, and delafossite was
present in all residue samples post-leaching. Complementing this,
Khalid et~al.~\cite{40} observed incomplete copper dissolution from
delafossite-containing converter slag when using 1~M H$_{2}$SO$_{4}$.
These studies imply that delafossite exhibits limited solubility in
acidic environments, thereby supporting the \mbox{hypothesis} of the
presence of undissolved delafossite in our residues.

\subsection{Implications in a potential industrial{\hfill\break}
process for the valorization of sludge} \label{sec4.2}

The results of this study indicate the feasibility of selectively
leaching Zn, Cu, and Cd from lime treatment sludge, particularly when
compared to Fe, which is a predominant component of sludge. This is
supported by the leaching tests in 2~L reactors at an acid-to-solid
ratio of 175~g${\cdot}$kg$^{-1}$, which demonstrated leaching yields of
84\% for Cu and 79\% for Zn and complete leaching of Cd while keeping
the leaching yield for Fe below~1\% as detailed in
Figure~\ref{fig9}. 

The rise in temperature from 22~{\textdegree}C to 40~{\textdegree}C
can also be a means to augment both Zn and Cu dissolution. It also
decreases the Fe concentration in the leachate due to the precipitation
of Fe oxyhydroxysulfates, which is favored at higher temperatures.
Nonetheless, this would imply heating the reactors, which will
influence the operating costs. Given the little gain regarding Zn and
Cu leaching yields at 40~{\textdegree}C compared to ambient
temperature, the implementation of heated reactors with temperature
control might not be viable from an economic and environmental point of
view. Nevertheless, this option should be further studied.

On the other hand, experiments show that it is possible that a high
solid content in the reactor (16\%) caused solid--liquid mass transfer
limitations, which produced lower leaching yields than those found at
lower solid contents. High solid contents can lead to increased
viscosity and reduced mixing efficiency, hampering mass transfer
between the solid phase and the leachate. This can limit the
accessibility of the leachate to Cu and Zn minerals on the
particles. Another factor that could explain this is the decreased
ratio of surface area to volume: as the solid content increases, the
available surface area for reaction per unit volume of the solution
decreases, leading to lower reaction rates. An optimum solid content at
which metal leaching is optimal should exist, probably between 5\%
and 15\%~m/m. In any case, more information is needed on this point to
define whether the efficiency limitation at high solid contents is
linked to the kinetics of the chemical reactions involved in 
mineral dissolution or is related to the hydrodynamic conditions of
the reactor.

Nonetheless, at 16\% solid content and an acid-to-solid ratio of
200~g${\cdot}$kg$^{-1}$, the leachate contains 8.2~g${\cdot}$L$^{-1}$
Zn, 1.5~g${\cdot}$L$^{-1}$ Cu, and 1.7~g${\cdot}$L$^{-1}$ Fe. These
concentrations of Cu and Zn are sufficient for an economically viable
downstream metal recovery process to be envisaged~\cite{24,41}. Iron
concentrations at this level are obviously an issue given the objective
of selective leaching (as explained in Section~\ref{sec1}), but as
shown in Figure~\ref{fig9}, a limited decrease in acid-to-solid ratio
significantly decreases iron leaching. Nevertheless, the optimum
operating conditions for leaching will have to be investigated in
reactors fed in continuous mode.

\section{Conclusion} \label{sec5}

Sludges produced from the lime treatment of AMD can sometimes represent
a significant deposit of valuable metals. On the site presented in this
study, the deposit of anthropogenic ore is estimated at 150~kt
containing 1.3\% Cu and 5.1\% Zn. These contents of Cu and Zn are
higher than those found in many deposits being mined today. The
selective leaching process applied in this study allowed the leaching
of more than 80\% of Cu and Zn from the sludge under optimal operating
conditions, which are defined between 5\%~m/m and 15\%~m/m of solid
content with an acid-to-solid ratio between 150 and
200~g${\cdot}$kg$^{-1}$ at room temperature (22~{\textdegree}C). Using
more acid marginally increases the dissolution rates at the cost of
much higher acid consumption linked to the dissolution of iron and
therefore a loss of selectivity. Increasing the temperature to
40~{\textdegree}C improves the dissolution of Cu and Zn and decreases
that of Fe, but its potential economic impact has not yet been
estimated. The results of this work will serve to further develop the
process by adding selective precipitation and purification steps.
Moreover, the data acquired during this study will be used to carry out
a preliminary economic assessment, including operating expenses,
capital expenses, and return on{\break} investment. 

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

This work has been funded by the Department for Mine Safety and
Risk Prevention of BRGM: French Geological Survey.

\section*{Acknowledgments}

The authors would like to thank the former and current project team
members: Maxime Vignon, Rachel Seillier, Rany Ajami, Benoit Henry,
J\'{e}r\'{e}my Engevin, Camille Becquet, Mohamed Djemil, St\'{e}phane
Touzelet, and Marine Respaut. This project has benefited from the
expertise and facilities of Platform MACLE-CVL, which was
co-funded by the European Union and Centre-Val de Loire Region (FEDER).

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-327-suppl.pdf}}

\printbibliography
\refinput{crchim20230968-reference.tex}

\end{document}
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