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\DOI{10.5802/crchim.311}
\datereceived{2023-12-14}
\daterevised{2024-03-01}
\dateaccepted{2024-04-11}
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\dateposted{2024-05-24}
\begin{document}

\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{Copper leaching with aqueous foams for the processing of
electronic waste}

\alttitle{Lixiviation du cuivre par des mousses aqueuses pour le
retraitement de d\'{e}chets \'{e}lectroniques}

\author{\firstname{Emilie} \lastname{R\'{e}gnier}}
\address{Institut de Chimie S\'{e}parative de Marcoule, ICSM, CEA,
CNRS, ENSCM, Univ~Montpellier, BP 17171, Marcoule, 30207
Bagnols-sur-C\`{e}ze, France}

\author{\firstname{Agathe} \lastname{Hubau}\CDRorcid{0000-0002-3896-7220}}
\address{BRGM, F-45060 Orl\'{e}ans, France}

\author{\firstname{Sol\`{e}ne} \lastname{Touz\'{e}}\CDRorcid{0000-0002-0840-4494}}
\addressSameAs{2}{BRGM, F-45060 Orl\'{e}ans, France}

\author{\firstname{Sylvain} \lastname{Guignot}\CDRorcid{0000-0001-6449-1337}}
\addressSameAs{2}{BRGM, F-45060 Orl\'{e}ans, France}

\author{\firstname{Daniel} \lastname{Meyer}}
\addressSameAs{1}{Institut de Chimie S\'{e}parative de Marcoule, ICSM, CEA,
CNRS, ENSCM, Univ~Montpellier, BP 17171, Marcoule, 30207
Bagnols-sur-C\`{e}ze, France}

\author{\firstname{Olivier} \lastname{Diat}\CDRorcid{0000-0003-3011-8168}}
\addressSameAs{1}{Institut de Chimie S\'{e}parative de Marcoule, ICSM, CEA,
CNRS, ENSCM, Univ~Montpellier, BP 17171, Marcoule, 30207
Bagnols-sur-C\`{e}ze, France}

\author{\firstname{Damien} \lastname{Bourgeois}\CDRorcid{0000-0002-0762-3476}\IsCorresp}
\addressSameAs{1}{Institut de Chimie S\'{e}parative de Marcoule, ICSM, CEA,
CNRS, ENSCM, Univ~Montpellier, BP 17171, Marcoule, 30207
Bagnols-sur-C\`{e}ze, France}
\email[D. Bourgeois]{damien.bourgeois@umontpellier.fr}
                     
\keywords{\kwd{Foams}
\kwd{Copper}
\kwd{Leaching}
\kwd{Shredding}}

\altkeywords{\kwd{Mousses}
\kwd{Cuivre}
\kwd{Lixiviation}
\kwd{Broyage}}

\thanks{ANR-17-CE08-0016 FOAMEX}

\shortrunauthors

\begin{abstract} 
Aqueous foams have been used in various hydrometallurgical processes,
and have recently emerged as an alternative technique for the leaching
of metallic copper, using hydrochloric acid and O\textsubscript{2} from
air as an oxidant. In the present study, we describe how this technique
has been successfully applied to the leaching of copper from waste
printed circuits boards (WPCBs). First of all, the use of HCl based
foams was successfully applied to the leaching of sole copper, and the
impact of the size of copper particles was studied. It was demonstrated
that use of mixtures of fine and large particles give excellent
results. Then, on ground WPCBs, a first classical leaching stage using
aqueous HCl solution was performed in order to remove maximum quantity
of reducible metals (aluminum, iron, zinc\,{\ldots}). Then, almost
quantitative copper leaching employing HCl based foam was performed.
Copper is almost exclusively leached during this stage, and the
complete mass balance in 5 major metals present in WPCBs (aluminum,
iron, zinc, nickel and copper) is detailed. Kinetics of the copper
leaching were greatly enhanced through addition of substoechiometric
quantity of cupric ions at the beginning of the process, so that
extensive grinding of the waste is not necessary.
\end{abstract} 

\begin{altabstract} 
Les mousses aqueuses, d\'{e}j\`{a} utilis\'{e}es dans divers
proc\'{e}d\'{e}s hydrom\'{e}tallurgiques, ont r\'{e}cemment
\'{e}merg\'{e} comme technique alternative pour la lixiviation du
cuivre m\'{e}tallique, en utilisant l'acide chlorhydrique et le
dioxyg\`{e}ne O\textsubscript{2} de l'air comme oxydant. Dans la
pr\'{e}sente \'{e}tude, nous d\'{e}crivons comment cette technique a
\'{e}t\'{e} appliqu\'{e}e avec succ\`{e}s \`{a} la mise en solution du
cuivre contenu dans des d\'{e}chets de circuits imprim\'{e}s (CI). Tout
d'abord, l'utilisation de mousses \`{a} base d'acide
chlorhydrique HCl a \'{e}t\'{e} appliqu\'{e}e avec succ\`{e}s \`{a} la
lixiviation du seul cuivre. L'impact de la taille des particules de
cuivre a \'{e}t\'{e} \'{e}tudi\'{e} : Il a \'{e}t\'{e} d\'{e}montr\'{e}
que l'utilisation en m\'{e}langes de particules de petite et grosse
taille donne d'excellents r\'{e}sultats. Sur des CI broy\'{e}s, une
premi\`{e}re \'{e}tape de lixiviation classique \`{a} l'aide d'une
solution aqueuse de HCl a \'{e}t\'{e} effectu\'{e}e afin
d'\'{e}liminer le maximum de m\'{e}taux r\'{e}ductibles (aluminium,
fer, zinc\,{\ldots}). Lors d'une seconde \'{e}tape, une lixiviation
presque quantitative du cuivre utilisant une mousse \`{a} base de HCl a
\'{e}t\'{e} effectu\'{e}e. Le cuivre est presque exclusivement mis en
solution au cours de cette \'{e}tape, et le bilan massique complet de
cinq m\'{e}taux majeurs pr\'{e}sents dans les CI (aluminium, fer, zinc,
nickel et cuivre) est d\'{e}taill\'{e}. La cin\'{e}tique de la mise en
solution du cuivre peut \^{e}tre grandement am\'{e}lior\'{e}e par
l'ajout d'une quantit\'{e} substoechiom\'{e}trique d'ions
cuivre(II) au d\'{e}but du proc\'{e}d\'{e}, de sorte que le broyage
extensif des d\'{e}chets n'est pas n\'{e}cessaire.
\end{altabstract} 

\maketitle

\twocolumngrid

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\section{Introduction} \label{sec1}

Aqueous foams, which are composed of air bubbles separated by thin
liquid films stabilized by surfactant molecules, have been widely used
in hydrometallurgy for the concentration of various species: ions,
colloids, ore particles, microorganisms~\cite{1,2,3,4,5}\,\ldots At
industrial scale, foams are often employed to separate~mineral
particles in froth flotation processes~\cite{6}. Their use has been
extended to various extraction processes such as deinking of aqueous
solutions via flotation~\cite{7,8}, and in the nuclear industry for
extraction and concentration of uranium, in aggressive and strongly
acidic environments~\cite{9,10}. All these examples take benefit of the
peculiar features of liquid foams: These foams are mostly composed of
gas, so that less than 10\% of their volume fraction is water, even
less than 5\% in the case of dry foams. The resulting volume of aqueous
phase to be treated is considerably reduced. However, to be effective
in terms of extraction, the extraction must be carried out through the
liquid gas interfaces developed in the foam, and not by the transport
of the liquid in the aqueous films, as this fraction must be~minimized.
The gas can be inert ($\mathrm{N}_{2}$), simply air, containing
O$_{2}$, or also more reactive such as an O$_{3}$/O$_{2}$ mixture. The
use of an oxidizing gas brings additional reactivity to the solutes in
the aqueous phase. Foams also contain surface active agents, which have
detergency properties and can remove impurities from a surface and/or
activate it. The choice of the surfactant is key for the stability of
the foams, as the lifetime of the foams strongly depends on coalescence
of bubbles, facilitated by the drainage of liquid out of the
films~\cite{11}. Coalescence events lead e.g. to the detachment of
mineral particles from the interface and loss of the particles into the
initial slurry during flotation processes. Leaching of ores using foams
was proposed in the 70's~\cite{12}, however with scarce applications
so far to our knowledge. Recently, the possibility to use foam based on
diluted aqueous hydrochloric or sulfuric acid solutions for the
leaching of metallic copper or silver has been demonstrated~\cite{13}.
The perspectives opened by these model studies drove our attention to
the application of foams for the leaching of copper from electronic
waste (e-waste). 

The recovery of metals from e-waste, known as Urban
mining~\cite{14,15,16}, has drawn considerable attention during last
decade. The high content in copper, much higher than in natural ores,
has led to the set-up of pyrometallurgical and hydrometallurgical
processes dedicated to copper recovery~\cite{17}. Latter processes
always rely on a leaching stage, generally based on the use of a large
amount of a concentrated aqueous solution. In the case of copper, it is
well known that the sole action of H$^+$ in the acid is not
sufficient to leach metallic copper, and that an oxidant is
needed~\cite{18}. Metallic copper is thus generally leached using
nitric acid, ferric sulphate in sulfuric acid, or a mixture of
hydrochloric acid with hydrogen peroxide~\cite{19}. Electroleaching
can also be implemented, either directly on adequate waste which are
connected to the anode, or indirectly through the electrogeneration of
a reactive intermediate such as chlorine in chloride media~\cite{20}.
Latter two approaches require specific equipment. The use of O$_{2}$
as an oxidant suffers from slow gas--liquid transfer, so that the use
of a hydrochloric acid solution stirred under air is not sufficient for
copper leaching. Acceleration of gas--liquid transfer when employing a
foam proved to be sufficient to observe significant \mbox{leaching} of copper
in the above mentioned study~\cite{13}, which lets suggest that
further process development is possible using air as oxidant. It is
thus the purpose of this article to report our progress on the leaching
of copper from e-waste with hydrochloric acid-based foams.
Brij{\textregistered}S10 (polyoxyethylene(10) stearyl ether,
C$_{18}$H$_{37}$(OCH$_{2}$CH$_{2}$)$_{n}$OH, $n\sim10$) was employed to
generate the foam, as it is a standard neutral surfactant readily
available at industrial scale, which is known for its good foaming
ability and stability in acidic media~\cite{21}, and has been
previously studied in our laboratory~\cite{22}. The present study
resulted in a leaching strategy based on two stages, which enables
efficient recovery of base metals, with most copper almost
quantitatively leached during the second stage employing an acidic
foam.

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

Approximately 500 kg of waste printed circuits boards (WPCB) were
provided by a waste recovery company operating in France. Concentrated
nitric acid and concentrated hydrochloric acid were purchased from
Carlo Erba reagents. Surfactant Brij{\textregistered}S10
(polyoxyethylene(10) stearyl ether,
C$_{18}$H$_{37}$(OCH$_{2}$CH$_{2}$)$_{n}$OH, $n\sim10$) was purchased
from Sigma-Aldrich and used without further purification. Gas used was
filtered air provided through a compressor.

\begin{table*}[t!]
\caption{\label{tab1}Average composition of ground WPCB particles
according to their size}
\begin{tabular}{lcccccc}
\thead
\xmorerows{1}{Particles size}  & \multicolumn{6}{c}{Weight content$^{\mathrm{a}}$} \\\cline{2-7}
 & {Cu (\%)} & {Fe (\%)} & {Ni (\%)} & {Pb (\%)} & Zn (\%) & Co (mg/kg) \\
\endthead
${>}$2 mm & 13.0 & 57.1 & 0.99 & 0.14 & {2.28} & {485} \\
1--2 mm & 15.2 & 11.2 & 0.43 & 1.06 & {1.89} & \0{78} \\
500~$\upmu$m--1 mm & 25.2 & \08.3 & 0.50 & 1.74 & {1.95} & {551} \\
250--500~$\upmu$m & 24.0 & \07.7 & 0.50 & 1.58 & {1.40} & {845} \\
100--250~$\upmu$m & 18.8 & \09.8 & 0.40 & 1.50 & {1.40} & {818} \\
63--100~$\upmu$m & \05.5 & 19.3 & 0.46 & 1.55 & {1.53} & {557} \\
40--63~$\upmu$m & \02.7 & 15.2 & 0.32 & 1.00 & {1.07} & {240} \\
${<}$40~$\upmu$m & \02.5 & \09.3 & 0.35 & 1.53 & {1.33} & {162} \\
Total & 17.8 & 12.4 & 0.47 & 1.25 & {1.83} & {314} \\
0--750~$\upmu$m & 15.9 (1.8) & 14.8 (1.2) & 0.38 (0.07) & 1.3 (0.2) & 1.8 (0.3) & {392 (107)}
\botline
\end{tabular}
\tabnote{Note: ${}^{\mathrm{a}}$ The sum of contents in metals does
not reach 100\% as WPCB also contain organic polymers and inorganic
(glass) fibers.\vspace*{-2pt}}
\end{table*}

\subsection{Grinding of e-waste}\label{sec21}

Batteries and aluminum heat sinks were first manually disassembled from
WPCB before the entire sample (485~kg) was shredded with an industrial
cutting mill to a particle size of less than 30~mm. The sample was
subsequently quartered with a rotary divider; one quarter (122~kg) of
the sample was then shredded anew, with an industrial cutting mill
equipped with a bottom sieve with perforation size of 10~mm. The ${-}$10~mm
sample was further divided with a riffle splitter to obtain sub-sample
masses of approximately 4~kg. Two of these fractions were gathered to
form a sample of 7.4~kg, which was then shredded in its entirety in a
lab knife mill (Retsch SM2000 with tungsten carbide grinding tools),
fitted with a bottom sieve with perforation size of 2~mm. The weights
of the disruptive materials and of the lost material amounted to less
than 9 and 1\% of the initial \mbox{materials} respectively. The former were
added to the final ground sample to produce the same composition as the
original WPCB materials. This sample was then dry-sieved up to 
$500~\upmu$m, and wet-sieved from $500~\upmu$m down to  ${-}40~\upmu$m.



\subsection{Elemental analysis of WPCB samples}\label{sec22}

Samples of weights varying between 4 and 6 g were taken from each of
the previously formed size fraction, and subjected to a hot,
atmospheric leaching in aqua regia, at 200~\textdegree C during 2 h.
After cooling, each leachate was filtered (filter porosity of 
$43~\upmu$m), and the resulting filtrate was diluted with weakly
concentrated nitric acid ($0.5~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$).
The concentrations of Fe, Cu, Ni, Zn, Pb and Co in the filtrates were
measured by flame AAS (VARIAN 220 FS). 


\subsection{Leaching using foam}\label{sec23}

In a 250~mL two-neck round bottom flask, was introduced the solid
(either Cu 40~$\upmu$m powder, Cu wire, Cu 1~mm beads, or samples of
ground WPCB detailed in Table~\ref{tab1}) to be leached (ca. 500~mg, weighed
with precision), followed by 10~mL of a 2~mM BrijS10 aqueous HCl
solution of fixed concentration (see text for details). Air was bubbled
during ca. 5~min using a fritted glass tube, at 5~mL/min flow, to grow
the foam to approximatively 100~mL (a mark was put on the outside of
the flask). The flask was then connected to a Buchi rotavapor, and
rotated at 100 rpm. At fixed time intervals, rotation was stopped and
the system was left to settle over few~min, an aliquot of the leaching
aqueous solution was taken (ca. 200~$\upmu$L using a glass Pasteur
pipette) and centrifuged during 1~min at 6000~rpm, and 100~$\upmu$L of
resulting solution were taken and diluted into a 1.0\% (w/w)
HCl/HNO$_{3}$ with HNO$_{3}$/HCl ${=}$ 9 (v/v) aqueous solution. Air was
also regularly reinjected, approximatively once per h, to regenerate
the foam as it progressively collapses. Concentrations of metals in the
aliquots were determined by inductively coupled plasma atomic emission
spectroscopy (ICP/AES, SPECTRO ARCOS ICP Spectrometer, AMETEK Materials
Analysis). Given concentrations are calculated as the means of three
replicates on three different wavelengths for each metal; relative
standard deviations were determined and lie between 1 and 4\%.

\subsection{Pre-treatment of PCB particles}\label{sec24}

About 5 g of WPCB weighed with precision were put in contact with 60~mL
(L/S ratio: 12~mL/g of solid) of an aqueous 3~M HCl solution in a glass
vial. The resulting mixture was heated to 60~\textdegree C and stirred for
3 h at room temperature (20~\textdegree C ${\pm}$ 2~\textdegree C). The
resulting mixture was then filtered on a B\"{u}chner set-up equipped
with a paper filter under vacuum. The \mbox{resulting} solid was used without
further treatment for subsequent leaching using foam, or stored in
glass vials under N$_{2}$ atmosphere.

\section{Results and discussion}\label{sec3}

\subsection{Grinding of e-waste}\label{sec31}

The collected WPCB were first of all shredded and ground. In order to
evaluate the optimal size for leaching stage, various fractions have
been sieved and collected (Table~\ref{tab1}).

The copper content is strongly dependent on the size of the particles.
The large particles contain much more copper, which can be related to
the fact that (i)~copper is employed in coils of wires and printed
circuits, and (ii) copper is a ductile metal, difficult to shred into
tiny pieces. Thus, the leaching efficiency according to mean copper
particle size rapidly appeared as a key point to study~\cite{23}.

\subsection{Set up of experimental device for foam leaching}\label{sec32}

The design of solid--liquid--gas reactors has been the subject of
numerous chemical engineering studies, which will not be discussed
here. In the frame of this study, several approaches have been
envisioned to design a suitable tool for leaching experiments at the
laboratory scale. First of all, a classical column was tested, but it
rapidly appeared that it is complicated to have an homogeneous
suspension of particles in the foam. Dense particles, and especially
large copper particles, tend to fall to the bottom of the column and
thus in the liquid employed to generate the foam. Light particles are
rapidly expelled from the column at the front-end of the foam, and
hydrophobic particles accumulate at the periphery of the column, with
very poor contact with foam (see Figure~SI1). As it is
important to have a homogeneous dispersion of the solid in the foam we
turned to a rotating drum, which can be easily simulated with classical
laboratory apparatus and glassware (Figure~\ref{fig1}).

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Schematic description of the experimental
procedure employed for ground WPCB leaching using foams.}
\end{figure}

First set-up was based on a simple round bottom flask attached to a
mechanical stirrer inclined with an about 30\textdegree~angle, and
placed on a wooden support that would limit friction during rotation.
The set up was upgraded using a Buchi rotavapor, which limits friction
and enables to heat the flask at a \mbox{controlled} temperature using a water
bath if desired. A \mbox{two-neck} flask was employed to enable regular
sampling of the reaction mixture, and if necessary gas injection to
regenerate the collapsed foam. Leaching was performed at room
temperature: in these conditions, foam remains stable over more than 1
h without further gas injection. Experiments at 40~\textdegree C were
attempted in order to increase leaching rate, however, gas injection
was necessary after 15~min as the foam collapsed more quickly, so that
the effect of temperature has not been investigated. Such a laboratory
set-up is seen as a rough model for rotating drum devices, which have
been developed at large scale for leaching of various solids including
e-waste, as well as other classical operations such as filtration. In
our study we did not investigate the optimal volume of foam to employ,
as a small volume of liquid may lead to very large volumes of foams,
which could be an important drawback for further scaling-up or
industrial application. Each leaching experiment was repeated at least
three times, and the error bars displayed result from the mean results
over these repetitions. 

\subsection{Leaching of pure metallic copper}\label{sec33}

The first studies on pure metallic copper led to mitigated results
(Figure~\ref{fig2}). First of all, as expected, small particles (copper powder,
40~$\upmu$m average particle size) led to rapid dissolution. But larger
particles (copper beads, 1~mm diameter) led to sluggish Cu leaching.
Latter result could be explained by the fact that the beads sink and
stay at the bottom of the rotating flask. On the other hand, the small
\mbox{metallic} particles were well dispersed in the foam, 
\mbox{facilitating} the
contact to liquid (HCl) and gas (O$_{2}$ in air) necessary for the
reaction. However, results on Cu wire were quite disappointing, even if
the wires did not sink in the solution and were well dispersed in the
foam. These results will be discussed later in the paper. Similar
experiment conducted in solution without surfactant and with continuous
air bubbling (5~mL/min) led to 2 to 3 times lower Cu dissolution yield,
even after 5 h. In parallel, experiments performed with the surfactant
but without addition of air led to negligible dissolution. It is
reasonable to assume that the use of foams enables a better transfer of
gas into the liquid. As described before~\cite{13}, two physical
transfer reactions are important: (i) gas--liquid transfer of O$_{2}$,
and (ii) diffusion of dissolved O$_{2}$ and HCl in the liquid phase
towards the surface of the solid. These issues are easily answered
using liquid foams. The liquid fraction of foams was not properly
controlled in our system, in comparison with previous work~\cite{13},
only adjusted through regular bubbling in the foam to reach a ca. 10
fold volume of liquid initially employed. The concentration in
surfactant (2~mM) was fixed to ensure sufficient stability of the foam:
lower concentration led to progressive collapse of the foam, whereas
higher amount did not increase foam stability.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Evolution of Cu leaching using foams according to
the Cu source (foam prepared using 3~M HCl and 2~mM Brij S10, 10~mL of
liquid for 500~mg solid).}
\end{figure}

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Evolution of Cu wire leaching using foams upon
addition of 40~$\upmu$m Cu powder or 10~wt\% CuSO$_{4}$ (foam
prepared using 3~M HCl and 2~mM Brij S10, 10~mL of liquid for 500~mg
solid).}
\end{figure}

The kinetic profile also rose our attention. The initial Cu dissolution
rate is close to zero in the case of beads and wire, and appears to be
low in the case of powder, as if an initiation step was necessary in
the Cu dissolution mechanism. Furthermore, the sigmoidal profile
observed suggests an autocatalytic mechanism. Such an autocatalytic
mechanism has been previously reported in ammonia solutions~\cite{24}.
Depending on the oxygen (and ammonia) concentration, the autocatalytic
term was found to be predominent. To our knowledge, no similar
mechanism study on metallic copper has been performed in chloride
media, although the oxidizing features of cupric chloride are well
known, and have been employed for copper~minerals~\cite{25} or even
gold~\cite{26} leaching. In the present study, the occurrence of an
autocatalytic mechanism and the impact of cupric ions were clearly
proven by two key experiments (Figure~\ref{fig3}): First of all, addition of a
small quantity of Cu$^{2+}$ ions (10\% respective to totally Cu
content) led to a decrease by a factor of 2 of the duration needed for
complete Cu dissolution. More impressive was the gain in Cu dissolution
observed when starting from a 1/1 mixture of wire and powder: The
kinetics profile was very close to that observed with Cu powder alone
(see Figure~SI2). The remarkable increase in reaction kinetics can be
interpreted from a chemical point of view by dissolution of Cu metal by
Cu$^{2+}$ ions present in solution rather than by dissolved O$_{2}$ and
H$^{+}$, as latter reaction involves 3 reagents (Cu, O$_{2}$ and
H$^{+}$ ions). However, there is still a visible induction period,
suggesting that the surface state of the Cu metal is also important.
Physical activation of the Cu via e.g. Friction with the Cu particles
can be envisioned. \mbox{Latter} parameter was not further investigated, as
available reports in the literature suggest very complex
mechanisms~\cite{27}.


Altogether, these results suggest that the presence of copper particles
of various dimensions in the ground e-waste is a favorable factor for
efficient copper leaching, and we turned to leaching experiments on a
real sample of ground WPCB, of size lower than 750~$\upmu$m, obtained
after classical shredding and grinding operations of composition given
in Table~\ref{tab1}.

\subsection{Leaching of copper from ground e-waste}\label{sec34}

Copper leaching from ground WPCB with foam using the experimental
conditions described above led to rather poor results in comparison
with previous results obtained on pure metallic copper (Figure~\ref{fig4}). The
dissolution of Cu is (i) incomplete and (ii) much slower than in the case
of the mixture of Cu powder and Cu wire described in previous section
(and even slower than in the case of Cu wire alone). 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Evolution of Cu leaching from WPCB particles using
foams, with comparison with a mixture of Cu wire and Cu 40~$\upmu$m
powder (foam prepared using 3~M HCl and 2~mM Brij S10, 10~mL of liquid
for 500~mg solid).}
\end{figure}

E-waste are composed of numerous metals, with most of them expected to
be oxidized more easily than copper (Table~\ref{tab1}). The leaching of
metallic copper is based on an acidic oxidizing process, and in these
conditions, other metals such as Zn, Fe, Ni, Al, etc\,{\ldots} which are
more reducible than Cu are expected to be also leached~\cite{28}.
Consequently, it is reasonable to suppose that experiments led on
ground WPCB did not give satisfactory results regarding Cu leaching, as
these other metals were preferentially attacked, and Cu preserved.
After 5~h reaction time, the solution was still very acidic (control
using pH paper) and simple calculation showed that protons were still
in large excess: the limit in Cu leaching probably arises from kinetic
limitations, but we did not continue the experiment over a longer
period for practical reasons. Rather, a first treatment of ground WPCB
was set in place. Attack of the solid with aqueous HCl in solution led
to significant leaching of Al, and no leaching of Cu
(Table~\ref{tab2}). Leaching of other metals such as Fe, was moderate
(Table~\ref{tab2}). Al leaching kinetics were rapid (Figure~SI3), but
Fe leaching was much slower, and difficult to push to completion
(Table~SI1).

\begin{table}[t!]
\caption{\label{tab2}Leaching yield of Al, Fe and Cu upon treatment of
ground WPCB with aqueous HCl of variable concentration during 5 h at
room temperature (5~g of ground WPCB, size $<$750~$\upmu$m, for 60~mL
aqueous HCl solution)}
\begin{tabular}{lcccc}
\thead
\xmorerows{1}{Metal}  & \multicolumn{4}{c}{HCl concentration (mol${\cdot}$L$^{-1}$)} \\\cline{2-5}
 & {0.1} &  {1} &  {3} &  {5} \\
\endthead
Cu &  ${<}$1\% &  ${<}$1\% &  \01\% &  \03\% \\
Fe &  \mn8\% &  15\% &  24\% &  22\% \\ 
Al & \mn5\% & 56\% & 92\% & 95\%
\botline
\end{tabular}
\end{table}

Such a pre-treatment of ground WPCB should thus improve the efficiency
of Cu leaching with the foam as less acid should be consumed by metals
more reducible than Cu such as Al. Also, the possibility to leach
sequentially different metals using the same acid is appealing: It can
be envisioned to leach reducible metals with HCl alone, then Cu with
HCl and $\mathrm{O}_{2}$ in the foam, leaving noble metals in the
residue for further treatment (not discussed in this article). Thus,
ground WPCB (size ${<}$750~$\upmu$m) was processed in two
stages: In a first stage, Al and some Fe were leached using a 3~M HCl
solution (L/S ratio: 12~mL/g of solid). This operation does not leach
significant Cu quantity (Table~\ref{tab2}). In a second stage, the
residue was further treated with an HCl based foam, and the Cu leaching
is almost complete  (Figure~\ref{fig5}).

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Influence of preliminary treatment of ground WPCB
with aqueous HCl (60~mL of 3~M aqueous HCl solution for 5 g of ground
PCB during 3 h at room temperature) on the outcome of Cu leaching using
foam (foam prepared using 3~M HCl and 2~mM Brij S10, 10~mL of liquid
for 500~mg solid).}
\end{figure}

Although the Cu leaching yield is almost quantitative ($97 \pm 3\%$),
the leaching rate remains low, and the improvement regarding the
untreated WPCB sample appears marginal. Therefore, we decided to have a
closer look at the impact of the fineness of the grinding. Two particle
fractions obtained after sieving and rich in Cu were studied
(Figure~\ref{fig6}). 

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Rate of Cu leaching from WPCB particles using
foams according to the particles size (foam prepared using 3~M HCl and
2~mM Brij S10, 10~mL of liquid for 500~mg solid).}
\vspace*{-5pt}
\end{figure}

As observed previously on metallic Cu alone, leaching is more rapid
when mean particle size is smaller. Quantitative Cu leaching was
observed on the 100--250~$\upmu$m fraction after 2~h, whereas the
reaction profile on the 500--1000~$\upmu$m particles fraction is
sluggish, and quite similar to the non-sieved (${<}$750~$\upmu$m) batch
previously employed. Also, addition of 10\% Cu$^{2+}$ ions
enabled to increase the leaching rate (Figure~\ref{fig6}). There seems
to be still a short \mbox{induction} time, shorter than that observed on Cu
wire (Figure~\ref{fig3}). This point was attributed to the fact that
during the pre-treatment, the very fine Cu particles should be
preferentially dissolved, as they are more reactive. Altogether these
results demonstrate that fine grinding of WPCB is not compulsory for
efficient Cu recovery using foams. A~minimum of grinding is necessary
to achieve liberation of all Cu contained inside components, but
excessive grinding is useless as leaching efficiency can be attained by
initial addition of a small quantity of Cu$^{2+}$ ions,
which will be recovered afterwards. Also, in a continuous process,
there should be always some Cu present in the leaching solution as
Cu$^{2+}$ ions. Furthermore, as stated beforehand, Cu is
difficult to grind into very fine particles, and most Cu is contained
in large particles: The 500--1000~$\upmu$m fraction is the richest in Cu
compared to other fractions. To our knowledge, the balance between cost
in energy (not evaluated here) to lower the mean particle size, and Cu
leaching efficiency has not been evaluated yet, whatever the leaching
strategy.

\subsection{Leaching of major metals from WPCB during the entire process}\label{sec35}

Based on these results, we examined the outcome of five major metals
(Cu, Fe, Ni, Zn and Al) during the two-stage leaching sequence: first
stage is a leaching in solution using a 3~M aqueous HCl solution (S/L
ratio 1~L/83~g) during 5~h at room temperature, and second stage is a
leaching in foam using again a 3~M aqueous HCl solution containing 2~mM
Brij S10 (S/L ratio 1~L/50~g) during 5~h at room temperature. Repartition
of these 5 metals between first leachate, second leachate and solid
residue is given in Figure~\ref{fig7}.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Repartition of major metals contained in ground
WPCB according to the leaching stage and remaining solid residue (1st
stage: 3~M HCl solution, S/L ratio 1~L/83~g, 5~h; 2nd stage: foam prepared
using 3~M HCl and 2~mM Brij S10, S/L ratio 1~L/50~g).}
\end{figure}

As expected, most Al is leached during the first stage. Remaining Al is
found in the residue, probably as oxide incorporated in the glass
fibers that reinforces the epoxy plastic support. Then, almost all the
Cu is leached during the second stage, with a non-significant loss in
the solid residue. The outcome of Fe, Zn and Ni is more intriguing as
these metals are mostly leached during the second stage, although the
acid quantity obtained in the first stage largely exceeds the amount
required for complete dissolution. Passivation of metallic Zn with a
resistant oxide surface can be invoked, but this argument may not stand
for Fe, which passivation by oxide is inefficient. The subsequent
isolation of copper from the foam leachate after foam collapse has not
been investigated. A neutral surfactant employed, with a very low
amount (2~mM, i.e.\ 1.4~g${\cdot}$L$^{-1}$). Therefore, \mbox{interferences}
with classical precipitation or electrodeposition processes are not
anticipated. However, it is not clear whether these processes will be
affected by easy foam formation, or if the final purity of copper will
be altered. Finally, the reuse of the surfactant will require further
dedicated studies. 

\section{Conclusion}\label{sec4}

Foams appear as an interesting alternative to classical aqueous
solutions for the leaching of Cu from ground WPCB. By simply adding a
small quantity of readily available surfactant Brij S10 (2~mM, i.e. ca.
1.4 g/L) into a dilute aqueous HCl solution, efficient Cu leaching was
observed after injection of air into the solution and formation of the
foam. The foam is stable over extended period and increases gas--liquid
transfer, so that $\mathrm{O}_{2}$ contained in air can act as Cu
oxidant. This approach is different from classical use of foams in
hydrometallurgy (particular and ionic flotation, foam fractionation).
Foams are used as a biphasic fluid medium to replace aqueous solutions
employed for leaching,~minimizing the amount of liquid employed. In
continuation to our simple laboratory setup, our intention is to
develop a pilot reactor (ca. 1~L scale), fully characterized in terms of
gas--liquid transfer, with adequate gas injection and samplings systems.
The efficiency of Cu leaching by the foam was improved by performing a
first leaching in HCl solution (without surfactant), and a 2-stage
strategy that enables complete Al removing, as well as part of Fe, Zn
and Ni, was devised. The final leachate contains Cu in a concentration
close to 10 g/L, and its processing using existing techniques such as
precipitation can be easily envisioned. Furthermore, during this study
we evidenced that Cu in e-waste accumulates in fraction of larger size
during grinding, and that the average size of the fraction with the
highest Cu content (500--1000~$\upmu$m) is well suited to leaching with
foams. Leaching is complete after 5~h, and the kinetics can be improved
through addition of 10\% Cu$^{2+}$ ions to the leaching medium. Thus,
extensive milling of the waste, which can be quite energy consuming, is
not necessary. This study also revealed a particular behavior of Fe:
although this metal was expected to be easily removed during first
stage of leaching using dilute HCl aqueous solution without foam (i.e.
without $\mathrm{O}_{2}$ as an oxidant), it proved to be reluctant to
dissolution. On the contrary, during the leaching of Cu with the foam,
Fe was easily dissolved along with Cu. The mutual interactions between
both metals during leaching with HCl/$\mathrm{O}_{2}$ has not been
studied to our knowledge. Detailed studies dedicated to the
understanding of the mechanism of both Cu dissolution by
HCl/$\mathrm{O}_{2}$ in foam and relationship with Fe are underway and
will be reported in due course.\looseness=-1

\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}
We would like to acknowledge ANR-17-CE08-0016 FOAMEX. 

\section*{Acknowledgements}
We would also
like to acknowledge \mbox{C. Monteux}, N. Pantoustier, P. Bauduin and P.
Perrin for fruitful discussions, and B. Baus-Lagarde for assistance
during the ICP-OES analyses.

\CDRGrant[ANR]{ANR-17-CE08-0016}

\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-311-suppl.pdf}}

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