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\DOI{10.5802/crchim.347}
\datereceived{2023-12-15}
\daterevised{2024-05-31}
\datererevised{2024-09-04}
\dateaccepted{2024-09-27}
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%\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

\begin{document}

%\copyeditor{RB}

\begin{noXML}

\title{Filtering the unfilterable: tuning the mesostructure of
precipitating silica gels to improve the filterability of acidic
lixiviation slurries}

\alttitle{Filtrer l'infiltrable: fa\c{c}onner la mesostructure des gels
de silice pr\'{e}cipitants pendant la lixiviation acide de minerais
pour am\'{e}liorer leur filtrabilit\'{e}}

\author{\firstname{Christian} \lastname{Manfoumbi}}
\address{Laboratoire de G\'enie Chimique, Universit\'e de Toulouse,
CNRS, INPT, UPS, Toulouse, 31432, France}

\author{\firstname{Kevin} \lastname{Roger}\CDRorcid{0000-0001-7914-0951}\IsCorresp}
\addressSameAs{1}{Laboratoire de G\'enie Chimique, Universit\'e de
Toulouse, CNRS, INPT, UPS, Toulouse, 31432, France}
\email[K. Roger]{kevin.roger@cnrs.fr}

\author{\firstname{Martine} \lastname{Meireles}\CDRorcid{0000-0003-2112-0936}\IsCorresp}
\addressSameAs{1}{Laboratoire de G\'enie Chimique, Universit\'e de
Toulouse, CNRS, INPT, UPS, Toulouse, 31432, France}
\email[M. Meireles]{martine.meireles-masbernat@cnrs.fr}

\keywords{\kwd{Lixiviation}\kwd{Silica}\kwd{Precipitation}\kwd{Mesostructure}\kwd{Filterability}}

\altkeywords{\kwd{Lixiviation}\kwd{Silice}\kwd{Pr\'{e}cipitation}\kwd{M\'{e}sostructure}\kwd{Filtrabilit\'{e}}}

\begin{abstract}
Silica precipitation is a ubiquitous but deleterious phenomenon
occurring in many hydrometallurgical processes. Indeed, silicon is
often released during the dissolution of minerals under acidic leaching
conditions. Eventually, it precipitates into a hard-to-filter silica
gel, which has prompted some efforts to hinder silica precipitation
through pretreatment or extra dilution. However, these approaches are
usually either mineral-specific or costly. Here, we propose a
disruptive strategy based on controlling the gel's mesostructure and
therefore its filterability. We designed an alternative precipitation
pathway consisting of adding extra silicate ions but at basic pH.
Using small-angle X-ray scattering, we show that this pathway
transforms the network of polymeric silica (``polymer gel'') obtained
under highly acidic leaching conditions into a network of dense silica
particles (``particle gel''). This structural compaction at the
mesoscopic length scale cascades to the macroscale and leads to a
drastic improvement in filterability by two orders of magnitude.
Furthermore, we demonstrate that this method is generic by applying it
successfully to both a model and real ore\break systems.
\end{abstract}

\begin{altabstract}
La pr\'{e}cipitation de la silice est un ph\'{e}nom\`{e}ne
omnipr\'{e}sent mais d\'{e}l\'{e}t\`{e}re qui se produit dans de
nombreux proc\'{e}d\'{e}s  hydrom\'{e}tallurgiques. En effet, le
silicium est souvent lib\'{e}r\'{e} lors de la dissolution de
min\'{e}raux dans des conditions de lixiviation acide. Il finit par
pr\'{e}cipiter en un gel de silice difficile \`{a} filtrer, et
plusieurs m\'{e}thodes ont \'{e}t\'{e} con\c{c}ues pour tenter 
d'emp\^{e}cher la pr\'{e}cipitation de la silice par un
pr\'{e}traitement ou une dilution suppl\'{e}mentaire. Cependant, ces
approches sont g\'{e}n\'{e}ralement soit sp\'{e}cifiques au
min\'{e}ral, soit co\^{u}teuses. Nous proposons ici une strat\'{e}gie
innovante bas\'{e}e sur le contr\^{o}le de la m\'{e}sostructure du gel,
et donc de sa filtrabilit\'{e}. Nous avons con\c{c}u un chemin de
pr\'{e}cipitation alternative consistant \`{a} ajouter
d\'{e}lib\'{e}r\'{e}ment des ions silicates suppl\'{e}mentaires, mais
\`{a} un pH basique. En utilisant la diffusion des rayons X aux petits
angles, nous montrons que ce chemin  transforme le r\'{e}seau de silice
polym\'{e}rique (\og gel polym\`{e}re {}\fg) obtenu dans les conditions
de lixiviation tr\`{e}s acides en un r\'{e}seau de particules de silice
denses (\og gel de particules {}\fg). Ce compactage structurel \`{a}
l'\'{e}chelle m\'{e}soscopique se r\'{e}percute en cascade \`{a}
l'\'{e}chelle macroscopique et conduit \`{a} une am\'{e}lioration 
radicale de la filtrabilit\'{e} de deux ordres de grandeur. En outre,
nous d\'{e}montrons que cette m\'{e}thode est g\'{e}n\'{e}rique en
l'appliquant avec succ\`{e}s \`{a} un mod\`{e}le et \`{a} des
syst\`{e}mes de minerais r\'{e}els.
\end{altabstract}

\thanks{ERAMET Research, French State and R\'egion Occitanie (CPER IMATECBIO grant).}

\shortrunauthors

\maketitle

{\vspace*{1pc}}

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\end{noXML}

\section{Introduction}\label{sec1}

Rare earth elements (REEs) play a critical role in the global economy
due to their numerous applications. They are notably used in electronic
and steel industries, as well as permanent magnets, lasers, and
batteries. The ongoing development of cleaner energy sources and
transportation especially contributes to the sharp increase in REE
industrial uses~\cite{Alonso2012,DAVRIS2017115,BINNEMANS20131}. This
rising demand is still mostly met by increasing the extraction from
natural minerals ~\cite{BINNEMANS20131, TAN2015} despite ongoing
efforts in recycling REE, for instance in electronic equipment. Less
known and rarer, niobium (Nb) is also a key strategic element for
future technologies, and its demand is anticipated to increase by 700\% in
the next 25 years~\cite{Alonso2012} with about 10\% of the market
concerning high-purity Nb~\cite{Ribagnac2017LeachingON}. Strikingly,
about 99\% of Nb produced worldwide is extracted from pyrochlore-type
materials, which are refractory rocks that also contain REEs.  

In this context, the Mabouni\'e (Gabon) deposit discovered in 1986 is
an interesting example of a niobium-rich deposit that requires
innovative extraction strategies~\cite{Belair:2014vc}. Indeed,
hydrometallurgy is more suited than pyrometallurgy to extract both Nb
and REE, but the known fluoride acid route to perform the liquid--liquid
extraction is environmentally problematic. This situation stimulated
the development of an innovative hydrometallurgical process for the
leaching of niobium- and REE-bearing pyrochlore ores
and the subsequent separation of Nb from REE~\cite{DONATI,
Ribagnac2017LeachingON, BELTRAMI2015356, DEBLONDE2016180}. The upstream
part of this process produces a raw Nb concentrate through three main
steps. First, refractory pyrochlores bearing Nb, REEs, U, and Ta are
decomposed by making the non-magnetic ore  react with concentrated
sulfuric acid (96\% H\tsub{2}SO\tsub{4}), yielding a paste that is then
further roasted at 250--300~\textdegree C. 
Second, this roasted paste, also named
calcinated powder to differentiate it from the untreated ore, is leached with water so
that Nb, REE, and other elements from different mineral phases are
solubilized in the aqueous phase. As the paste contains a high amount of
sulfuric acid, this leaching step occurs at extremely acidic pH.
Third, Nb and Ta are selectively thermoprecipitated from REE and U.
In the downstream part, the raw Nb concentrate is\break purified.

However, the optimization of this complex process faced a ubiquitous
problem in many hydrometallurgy processes, which is the formation of
unfilterable silica gels. For instance, silica gel formation has been
reported in hydrometallurgical processes for the extraction of rare
earth from eudialyte mineral~\cite{DAVRIS2017115, RIVERA201882,
Vossenkaul:2017vx, BALINSKI2020105345}  or from bauxite residue 
\cite{RIVERA201882,BORRA2016151}. In hydrometallurgy, the occurrence of
silica precipitation and the formation of gelatinous solutions or cakes
is a serious drawback for the liquid/solid separation step with
deleterious consequences on extraction processes and filterability.
Apart from the problematic issue of clogging, the precipitate may blind
ore particles from dissolution and reduce leaching kinetics. For
instance, in the context of leaching calcium aluminate slags with
aqueous HCl, for Al extraction, Tsaousi et~al.\ state that avoiding
SiO\tsub{2} gelation is key to designing robust leaching\break
processes \cite{Tsaousi:2023vs}.

To facilitate the decomposition of mineral phases before leaching in
water solution, pretreatments by reaction with mild to high acid
concentration solutions as well as thermal steps are often implemented.
However, these strategies are restricted to certain ore types and are
typically ineffective for pyrochlore ores.  Although silica is initially
present in ores, its low solubility in aqueous solutions in both its
amorphous and crystalline solid states precludes its involvement in the
formation of unfilterable silica gels. Rather, silica gels form in situ
during the leaching step due to the release of silicon traces when
dissolving certain minerals, leading to their subsequent precipitation
as silica. Furthermore, at the high ionic strength of the leaching
solution, colloidal metastability is compromised due to charge
screening, which leads to aggregation and thus
gelation~\cite{Alexander:1954vo, ILER}. Gelation, a rheological
transition, stems from a structural transition, which is the formation
of a three-dimensional percolated network. Silica gels display a
multiscale structure that notably depends on pH, a key parameter
controlling its solubility and reactivity~\cite{ILER}. The
mesostructure that corresponds to supramolecular and colloidal length
scales (1--100~nm) is especially relevant to
filtration~\cite{RAGUEH20152}. 

In this article, we investigate strategies to avoid clogging during the
filtration step that follows the acidic leaching of the ore. We first
evaluate the extraction yield of the different elements over time. We
notably highlight the existence of a characteristic time for the
extraction of valuable elements and unveil silicon extraction from
dissolved mineral phases and its precipitation as silicates in
solution. We then discuss simple parametric approaches to hinder this
silica precipitation, such as decreasing the extraction time or
increasing the relative amount of leaching solution. We show that although
these approaches are efficient, they are costly, polluting, and
fragile. We then pivot to a different strategy that consists in rather
mastering the mesostructure of the forming silica gel since this
mesostructure underlies the overall filterability of the slurry. We
present a pathway consisting of deliberately adding more silicates, but
at a basic pH, before shifting back to acidic conditions to maintain
the solubility of valuable elements prior to filtration. We show that
this pathway indeed associates with a drastic densification of the
mesostructure, yielding a particle-like instead of a polymer-like
gel. \looseness=-1

{\vspace*{-4pt}}

\section{Results}\label{sec2}

{\vspace*{-2pt}}

\subsection{Extracting valuable elements through leaching: release and
precipitation of silicon into a clogging gel}\label{ssec21} 

{\vspace*{-2pt}}

The acidic leaching of an ore like that from the Mabouni\'e deposit
consists of dissolving minerals to solubilize in solution the elements
of interest, here rare earth, niobium, and tantalum. This operation is
performed at a given liquid/calcinated ratio ($L/C$), always expressed in
this article at the initial time since most of the solid quickly
dissolves. We characterized the calcinated
powder by X-ray diffraction, which shows it contains essentially mikasaite, rhomboclase,
koninckite (REE- and Nb-bearing phases), crandallite, quartz, baryte, and
anhydrite. Comparison with the residue after 4~h of leaching shows the
disappearance of rare-earth- and niobium-bearing phases (mikasaite,
rhomboclase, koninckite) during the leaching and the appearance of
sulfate phases. Therefore, while some of the ore is dissolved through
the leaching step, allowing the release in solution of elements of
interest, the rest of it remains as solids that must be filtered. Yet,
filtration is often hindered by the formation of a clogging gel. To
elucidate the nature of this gel, it is useful to monitor extraction
over time through an elemental analysis by inductively coupled plasma
(ICP). Figure~\ref{FIG1}A displays the extracted percentage of each
major mineral element in the supernatant, obtained by centrifugation of
the leached solution, measured by ICP at different times of leaching.
We first observe a sharp increase in the concentration of all elements,
indicating that some minerals dissolve and thus release elements in
solution. This correlates to the rapid dissolution of the calcinated ore,
with 77\% of its mass dissolved in solution after 30~min. An asymptotic
value is quickly reached within 1~h for all elements, with typical
values of 70\% for niobium and 60\% for tantalum. However,
there is a strong exception to this generic behavior in the case of
silicon. Indeed, following the increase in concentration, a strong
decrease is observed, indicating that silicon is somehow removed from
the solution. This indicates the precipitation of the dissolved silicon
into a new solid phase. It is worth noting that this reprecipitating
amount is only a very small fraction of the total silicon content in
the ore. QEMSCAN analyses indicate that silicon is mostly contained in
quartz, which does not dissolve during leaching. They also suggest the
formation of amorphous silica in small amounts. Thermodynamically,
silicon could not be simultaneously extracted from quartz and then
reprecipitated in the same solution as silicates. It must then
originate from mineral phases that dissolve during the leaching step
in which silicon can be present as traces, consistently with the small
percentages of extracted silica during leaching. 

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig01}
{\vspace*{2pt}}
\caption{\label{FIG1}(A) Extraction monitoring of each element in
solution over time, quantified by ICP and performed at $L/C = 1$.
Reaching a value of 1 means that the whole amount of the element
initially present in the calcinated ore was extracted in solution. All
elements reach an asymptotic value of the extraction yield (left
$Y$-axis) within 1~h except silicon, which displays a non-monotonic
behavior and much lower extraction yields (right $Y$-axis). Note that
after 30~min, 77\% of the calcinated mass is dissolved in solution. (B)
QEMSCAN field maps from residue samples collected after 0.5~h (top
left), 1.5~h (down left), and 24~h (down right) leaching time. Images
are field maps from $1\times 1$~mm thin polished sections with false
color composite images with different assigned mineral phases shown in
the legend panel. Quartz and silica are two phases that are difficult
to distinguish, but SiO\tsub{2} is identified on pixels for which the
Si content is lower than that expected for quartz. These pixels are
found either at the edge of quartz grains (map 0.5 h) or in small
isolated grains in the sulfate matrix (anhydrite).}
{\vspace*{3pt}}
\end{figure*}

\subsection{Precipitation of silica at acidic pH: filterability of
slurries}\label{ssec22} 

Since the formation of the silica gel is detrimental to filtration, the
simplest approach is to avoid or delay its formation, which corresponds
to tuning, respectively, thermodynamics or kinetics of silica
precipitation. Experimentally, the most straightforward way to play on
both is to increase the amount of the leaching solution compared to the
ore, which corresponds to increasing the liquid/calcinated ratio ($L/C$).
Silicon extraction and precipitation can then be measured over time by
sampling the reactor followed by an ICP analysis. Figure~\ref{L/C}A
displays the kinetics of silicon leaching and precipitation over time
for three liquid/solid ratios of respectively 2, 4, and 8.
Qualitatively, the same behavior is observed for all $L/C$ ratios and
mirrors the one observed for $L/C = 1$ displayed in
Figure~\ref{FIG1}A. First, the silica concentration increases as
silicon is released from the dissolved mineral phases bearing it as
traces, before decreasing at longer times as it precipitates as
amorphous silica in solution. However, the extent of this non-monotonic
variation decreases sharply upon increasing the $L/C$ ratio and thus
increasing the dilution of the ore in the leaching solution. At~the
highest ratio of 8, the decrease is very mild and occurs over several
hours, which is generally too long regarding operating conditions. 
In that case, silica is nearly at its solubility
and the excess precipitates very slowly due to a low supersaturation.
Therefore, the extent of precipitation is efficiently tuned by the $L/C$
ratio. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{L/C}(A) Kinetics of silicon leaching and precipitation
for three liquid-to-calcinated ratios $L/C$. The silica concentration is
determined by the molybdenum blue assay in the supernatant of the
leached slurry. Initially, silicon is extracted from dissolving mineral
phases in solution and eventually precipitates as silica, which
explains the non-monotonic variation of the silicon concentration in
solution over time. The larger the $L/C$ ratio, the smaller the extent of
precipitation. (B) Filtration kinetics for slurries obtained after 0.5~h
or 4~h of leaching for three $L/C$ ratios. The linear variation of $t/V$
with $V$ allows us to calculate the specific resistance to filtration,
$\alpha$ (kg/m\tsup{3}), according to Equation~(\ref{eq1}). The higher
the resistance, the less filterable the slurry.}
\end{figure*}

We then investigated the filterability of the corresponding slurries by
performing a dead-end filtration at constant pressure. We chose $L/C$
ratios of 4, 6, and 8 to echo the change in the extent of silica
precipitation evidenced in Figure~\ref{FIG1}A. We plot the
time-to-volume ratio ($t/V$) as a function of filtrate volume ($V$) and
observe a linear variation for all data. This is consistent with the
standard Darcy's filtration law. Indeed, neglecting the filter cloth
resistance and assuming cake incompressibility, we obtain
{\begin{equation}\label{eq1}
\dfrac{t}{V(t)} = \dfrac{\mu \alpha M }{A^2 \rmDelta P}V(t),
\end{equation}}\unskip
where $\mu$ is the viscosity of the fluid phase, $M$ is the mass of
solids deposited on the cloth per unit of filtrate volume, $A$ is the
filter cloth area, $\rmDelta P$ is the difference of applied pressure, and
$\alpha$ is the specific resistance of the cake. For each experiment, $M$
was calculated from the ratio of the mass of solid deposited to the
total volume of the filtrate, and the specific resistance of cake was thus
calculated from the above equation. Increasing $L/C$ leads to a
decrease in the specific resistance, which means that the filterability
is improved. We also see for $L/C = 4$ that the time at which
filtration is performed can also play a significant role in
filterability, which is related to the extent of silica precipitation.
There, the longer the time, the higher the specific resistance and thus
the less filterable the slurry. Dilution or reducing contact time to
limit precipitation or dissolution could thus be two strategies to
overcome the deleterious effect of silica released during the
extraction of valuable elements. Both strategies go hand in hand with a
low degree of supersaturation and thus limited precipitation. However,
none of them are applicable in practice. Indeed, to maximize the
extraction of valuable species, the extraction time has to be of at
least an hour and the time before filtration cannot be decreased too
much. Furthermore, using large values of $L/C$ ratios means a strong
increase in acidic effluents, which is detrimental from both
environmental and economic standpoints. Overall, the strategy is
to work at the highest possible $L/C$ and lowest time to extract sufficient
valuable elements but avoid extended silica precipitation. However,
this parametric approach is highly sensitive to any composition
change in the ore and seems too fragile to design a robust process. 
\looseness=-1

\subsection{Precipitation of silicate solutions at acidic pH:
filterability}\label{ssec23} 

Since avoiding silica precipitation is not an acceptable option, we
turned to a different strategy consisting of mastering the
mesostructure of the silica gel, which determines its
filterability~\cite{RAGUEH20152}. However, investigating silica
precipitation in slurries obtained from the leaching of the real ore
is challenging as the system displays a complex composition. Yet, it is
possible as the first approach to start instead from silicate solutions
as the source of Si and study the precipitation under given
physicochemical conditions. Experiments were conducted under the same
highly acidic pH and temperature conditions as for leaching the ore and
filtered with the same protocol. This precipitation protocol is
therefore quite different from usual synthesis methods that operate at
much lower ionic strength. For instance, colloidal silica is typically
prepared at a pH of 10, yielding smooth and dense particles and
moderate ionic strength, ensuring colloidal stability through ionic
repulsions. Therefore, we expect to obtain quite different \mbox{structures}
at very low pH and high ionic strength, which can be investigated in
detail in this simpler model system with small-angle X-ray scattering
(SAXS). 

\begin{figure*}
\includegraphics{fig03}
\bcaption{\label{SAXS}\fontsize{9.8}{12}\selectfont
(A) Different silica precipitation paths in a
hydrometallurgical process. The conventional method is to remain at pH
0. The alternative methods tested are to increase pH to 9 and then add
or not silicates, before switching back to pH 0. Note that each step is
associated with a color code throughout the figure.}{2}{ (B) After each
step, a SAXS spectrum is acquired. Over time, a network forms in
solution as visualized by the increase in small wave vector intensity
$q$, which eventually leads to a structure factor decaying as $q^{-2}$
(grey to black curves). The extension of this slope down to larger wave
vectors and thus smaller distances indicates that the whole structure
can be seen as a polymeric silica gel. Increasing pH to 9 does not
change this structure (blue curve). However, adding silicate leads to
the compaction of the mesostructure at small distances, and the Porod
law in $q^{ -4}$ is recovered (green curve). This structure is
maintained when switching back pH to 0 (orange curve). In contrast,
when only the pH cycle is performed without adding silicates, the
structure changes only to a smaller extent (purple curve). Note that
scattering intensities are not on an absolute scale as the sampling is
not achievable at a controlled silica concentration. They were thus
scaled by arbitrary factors to highlight similarities and differences.
(C) Schemes of the mesostructures obtained depending on the pH cycle
used. (D) Filterability of the two different gels obtained (pH $=$\ 0
or pH cycle and silicate addition). The sticky gel obtained at pH 0 is
strongly clogging while the brittle gel obtained through the pH cycle
and silicate addition is much easier to filter. Small length scales
thus dictate the filterability, and compacting them is a profitable
process modification.}
{\vspace*{-3pt}}
\end{figure*}

\subsubsection{Precipitation of silicates at acidic pH: macro- and
mesostructures}\label{sssec231}

The SAXS is a powerful technique to probe
mesostructures and was deployed on aliquots sampled over time as silica
precipitates. After 60 min, we observe the formation of silica
nanoparticles with a finite size evidenced at low $q$ values by the
Guinier regime of the scattered intensity (grey curve,
Figure~\ref{SAXS}B). However, at high values and thus small
distances, we do not observe the Porod law, which states that for
well-defined interfaces, the scattered intensity should decrease as
$q^{-4}$ as is typically observed for colloidal silica prepared under
basic conditions~\cite{RAGUEH20152}. Instead, the scattered intensity
decreases as $q^{-2}$, which can notably correspond to polymeric
(so-called fractal) networks. Therefore, the acidic conditions under
which silica precipitates drastically impact its resulting
mesostructure. At~longer times, the $q^{-2}$ power law extends to lower
$q$ values, which highlights an increase in particle size and ultimately
the formation of a percolated network that leads rheologically to
gelation (black curve, Figure~\ref{SAXS}B). Interestingly, it means
that the silica gel obtained here is structurally a multiscale
polymeric network (``polymer gel''), rather than a network of dense
colloidal particles (``particle gel''). 

\subsubsection{Changing the precipitation pathway by adding silicate
ions at basic pH}\label{sssec232}

Following these results, we tried to change the mesostructure by tuning
pH during silica precipitation. For this purpose, we  increased the
pH during precipitation (blue arrow and curve in Figure~\ref{SAXS}).
The resulting SAXS curve at 72 min overlapped with the one obtained
after the precipitation of silica under acidic conditions after 240 min,
evidencing that the overall mesostructure was not modified but that
gelation proceeded faster. The pH was then returned to its original
acidic value to dissolve anew the elements of interest (purple arrow
and curve in Figure~\ref{SAXS}). The resulting SAXS data indicates an
increase in the exponent of the power law at high $q$ values, which
indicates slight densification of the network. To amplify this
modification, we devised an alternative pathway in which silicate is
deliberately added at basic pH (green arrow) before returning to acidic
pH (orange arrow). Strikingly, the two resulting spectra overlap
indicating that the mesostructure is then unchanged by the pH
variation. Furthermore, the power law then follows the Porod law
indicating that silica/water \mbox{interfaces} are now well defined and that
the network is denser. Adding silicates at basic pH thus allows for
drastically modifying the mesostructure, and can be described as a
consolidation of the existing polymeric network, yielding a
particle gel. This change in mesostructure cascades to the macroscale,
as shown in Figure~\ref{SAXS}D, which demonstrates that it leads to a
drastic improvement in the gel's filterability. Thus, our
counterintuitive strategy, which consists of deliberately adding at
the right pH the problematic species in the process, silica, is a
powerful means to solve the practical problem of filterability. 

\subsubsection{Applying the mesostructure shaping strategy to real
ore extraction}\label{sssec233}

To verify the feasibility of this strategy in the real system, we
applied the same procedure to the ore (see Figure~\ref{filter_ore}A).
We first checked that the pH cycle did not compromise extraction
yields, as the precipitation of some dissolved elements (Nb, REE, etc.)
at basic pH was in fact fully reversible when returning to the initial
acidic pH. Figure~\ref{filter_ore}B displays a decrease in
specific resistance by two orders of magnitude when tuning silica
precipitation through this pH cycle with a consolidation step,
similarly to the model system previously presented.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{filter_ore}(A) Conventional and alternative
precipitation pathways performed on the real ore. In the alternative
pathway, silicates are added at basic pH before shifting back the pH to
high acidic values. (B) Resulting filterability profiles for both gels
were obtained throughout the two pathways. The strong collapse of the
specific resistance when performing the silicate addition at the basic
pH pathway indicates a tremendous improvement in the filterability.
Note that the final liquid/solid ratio is 4. The scheme proposed is the
same as in Figure~\ref{SAXS} but includes insoluble mineral grains
(purple) in addition to the forming silica networks.}
\end{figure*}

In practice, the pulp thus becomes filterable. It is interesting to
stress that the overall procedure leads to a final liquid/solid ratio
of 4 and thus would indeed correspond to non-filterable conditions (see
Figure~\ref{L/C}B). We did not optimize this ratio, but it could be
further reduced if more concentrated sodium \mbox{hydroxide} and
silicate solutions are used. Overall, this method is thus more advantageous than
the dilution strategy tested and presented in Figure~\ref{L/C}B,
where a ratio of 8 would lead to comparable results but would require a much
larger volume and thus decreased profitability. We are confident that
this difference could be further increased through optimization,
whereas the dilution pathway is bounded by silicate solubility, an
equilibrium parameter. 

\section{Conclusion}\label{sec3}

In hydrometallurgy, the occurrence of silica precipitation is as
ubiquitous as it is deleterious for the design and operation of
leaching and downstream stages of the process. Indeed, the resulting
gel formation drastically hinders filterability. Although some
pretreatment steps have been proposed to restrain silica precipitation
through converting silica-bearing mineral phases into an insoluble
form, the so-called digestion steps, they are not universal and typically
do not apply to the pyrochlore ores we use in this study. Another
simpler way is to increase the liquid/solid ratio during extraction by
leaching, a dilution step, which eventually leads to operating under
conditions where silicates remain solubilized. However, they may
eventually precipitate later, which then hugely impacts the overall\break
profitability. 

Overall, it is challenging to fight thermodynamics when designing an
efficient process. In this work, we shifted our stance to accepting
silica precipitation into a gel but rather proposed that by carefully
tuning the structure of the gel in the 1--100~nm range, it could be made
filterable. Inspired by syntheses of filterable silica performed at
basic pH, our idea was to deliberately add the problematic species
rather than to remove it by using a pH shift. This may seem
counterintuitive, but this addition, when performed at basic pH, leads
to a compaction at the smallest length scales of the gel. Indeed, we
then transition from a multiscale polymeric network (``polymer gel'') to
a network of dense colloidal particles (``particle gel''). This
mesostructure change is readily observed in SAXS
as a change from a $q^{-2}$ to a $q^{-4}$ power law,
which is typically observed for smooth objects such as silica at basic
pH~\cite{RAGUEH20152}.

We have demonstrated that this strategy, devised on a model system, can
be successfully implemented in the extraction of real ores. It is worth
stressing that this strategy is generic and not very sensitive to the
exact ore used as silica precipitation is mostly controlled by the pH
value. Overall, rather than avoiding silica precipitation, we have
controlled it at the relevant length scale for filterability, which
paves the way for disruptive process modifications in
hydrometallurgical processes involving silica precipitation and
filtration steps.

\section{Materials and methods}\label{sec4}
\subsection{Leaching of calcinated powder}\label{ssec41}

Calcinated powder was  provided by ERAMET. It originates from
the Mabouni\'e upstream hydrometallurgical process that combines
consecutive sulfation and roasting of pristine ore, mostly aimed at
converting the pyrochlore mineralogical phase containing the value
elements into metallic sulfates to facilitate their leaching. The
conditions used to obtain the calcinated powder are described by
Beltrami and co-workers in several
publications~\cite{BELTRAMI2015356,DEBLONDE2016180,Ribagnac2017LeachingON}.
The mass composition in terms of the main chemical elements in
calcinated powder is given in Table~\ref{table1}.

%tab1
\begin{table*}
\caption{\label{table1}Mass composition of the different main elements
in the calcinated powder}
\fontsize{10}{10.5}\selectfont
\begin{tabular}{cccccccccc}
\tbody
Element & Al & Ca & Ce & Fe & Nb & P & S & Si & Ti \\
Mass percentage & 4 & 1.5 & 0.4 & 10.8 & 1.1 & 2.3 & 17.3 & 4.4 & 1.3
\botline
\end{tabular}
{\vspace*{6pt}}
\end{table*}

Leaching of the calcinated powder was performed in an insulated glass
reactor equipped with a three-blade impeller, a pH electrode, and a
thermometer equipped with a Pt100 probe. Deionized water was used
throughout all the experiments. First, the reactor was filled with
500~ml of deionized water. Then a given mass of calcinated solid
particles was introduced. This given mass defines the liquid-to-solid
ratio: a liquid-to-solid ratio equal to 1 was obtained by mixing 500~g
of calcine powder with 500~ml of water. Right after the calcinated
powder addition, the mixture was continuously agitated at 400~rpm. The
leaching experiments were conducted at a temperature of 90~\textdegree
C and for contact times between 30 and 1500 min, starting from the time
of powder addition. To ensure the solubilization of valuable elements
such as niobium and tantalum from metallic sulfates, leaching has to be
performed below pH 2. This condition is easily met without any acid
addition thanks to the initial acidity of the calcinated powder. For
liquid-to-solid ratios ($L/C$) ranging from 1 to 4, the solution
acidity is equivalent to 1 mol/l H\tsup{+} in the leaching solution
just after the addition of calcinated powder. \looseness=-1

In the first series of experiments, the chemical compositions of both
solid and liquid phases were monitored over time from 0.5~h to 24~h.
First, slurries were collected from the reactor at the end of the
experiment and diluted by a factor 2 with deionized water before its
centrifugation at 3000~g for 15 min. The yielded sediment  was
washed twice with 5~mL of deionized water per gram of solid and
subsequently dried at 80~\textdegree C for 48~h. The resulting powder
was analyzed by both X-ray diffraction and scanning electron
microscopy (QEMSCAN). Diffraction patterns were obtained using a
PANalytical X'Pert Pro diffractometer (Cu K radiation, 1.5406~\AA) and
identified with the HighScore Software.  SEM-QEMSCAN analyses were
carried out using an FER Quanta 650 F SEM platform. Separately, the
liquid phase obtained after centrifugation was analyzed by inductively
coupled plasma atomic emission spectrometry for the compounds
with low atomic weight (Al, Ca, Fe, P, S, Si, Ti) and by inductively
coupled plasma atomic mass spectrometry for the compounds with
high atomic weight (Ce, Nb, Ta, Th, U).  \looseness=-1

In the second series of experiments, we focused on the kinetics of silica
dissolution/precipitation for liquid-to-solid ($L/C$) ratios of 2, 4,
and 8. The same leaching protocol as described above was carried out,
but this time, aliquots of slurries were sampled from the reactor at
different contact times from a few minutes to several hours. The collected
aliquots were diluted by a factor 2  by adding deionized water and then
filtered (0.45~$\upmu$m cutoff) to remove undissolved particles. The
filtrates were analyzed by adapting a silicomolybdic acid
spectrophotometric method, which is predominantly used for
environmental or industrial waters, in order to measure the
concentration of silica precursors in the liquid phase at different
contact times~\cite{Alexander:1953us,Coradin:2004tk,Nagul:2015un,Truesdale:1976wj}.

\subsection{Filtration of slurries}\label{ssec42} 

Filtration experiments were conducted with a laboratory filtration cell
(Choquenet, France) and filter cloths from Sefar Fyltis (France). The
volume of the cell was 102~cm\tsup{3} and the filtration area was
38.5~cm\tsup{2}.  The hydraulic resistance of filter cloths was a
hundred times less than the hydraulic resistance of the cakes formed
during the filtration experiments, so the resistance of the medium was
neglected. The cell was fed with the slurry collected from the leaching
reactor and circulating from a guard reservoir maintained at a constant
pressure by a regulated air supply. Filtration tests were undertaken
just after the slurry reached room temperature and operated at a
constant operating fluid pressure of 700~kPa and a temperature of
25~\textdegree C.  During the filtration, solid particles present in
the slurry accumulate on the filter to form a cake. The experiment was
terminated when the cake filled the entire space of the cell or when
fluid permeation out of the cell was no longer observed. After the
filtration, the wet cake was recovered and the mass of solids in the
cake was measured by thermogravimetric analysis (Mettler Toledo,
Viroflay, France). This mass included the contribution of both solid
particles and salts formed during leaching. The mass of salts was
determined by measuring the conductivity of a dry pellet redispersed in
Milli-Q water. The mass of solid particles in the cake was therefore
estimated from the difference between the mass of solids and salt mass.
Because we were interested in the permeability of the filtration cakes,
we gravimetrically determined the flow rate by recording the mass of
fluid permeating out of the cell on an electronic balance at regular
time intervals.  The cake-specific resistances were directly determined
from the variation of the permeating volume $V$ with filtration time
$t$. Filtration experiments were repeated three times to check the
reproducibility of the filtration data.

\subsection{Precipitation of silicate solutions}\label{ssec43}

Precipitation experiments were carried at $T=90$~\textdegree C by
quickly injecting 100~mL of a sodium silicate solution (pH $=$\ 11,
[SiO\tsub{2}] $=$\ 130~g/L, density 1.39~g/mL) in a reactor filled with
900~ml of an acidic solution (sulfuric acid medium at  [H\tsup{+}] $=$\
1~mol/L). The initial \mbox{silicate} concentration is thus of 13~g/L. During
precipitation, samples are collected and ``quenched'' at pH 2 with a
soda solution at 1~mol/L to stop the reaction as described in the
literature by Iler and \mbox{Alexander.} The arrested samples are analyzed by
SAXS on the Xeuss 2.0 platform available\break
in-house. 

The alternative pathway consists of increasing pH after 1~h, from pH 0
to 9 by adding 250~mL of a 8~mol/L sodium hydroxide solution at 
20.8~mL/min. A consolidation step is then performed by slowly injecting
166~mL of a sodium silicate solution (70~g/L), at 4~mL/min, while
maintaining pH at a set value of 9 by adding 40~mL of a sulfuric
solution (1~mol/L). After this consolidation step, the pH was decreased
back to 0 by adding 100~mL of a sulfuric acid solution at 8~mol/L. 

\subsection{Monosilicic acid concentration in solution using
molybdenum blue methods and ultraviolet--visible light spectroscopy}\label{ssec44}

To titrate the amount of monosilicic acid in the solution, we used a
spectroscopic method proposed by Nagul et~al.\ that uses the molybdenum
blue reaction~\cite{Nagul:2015un}. While ICP measures the whole silicon
content in solution, this method is more specific to monomeric silica.
This method was calibrated using a standardized silica stock solution
(99.9\% Si(OH)\tsub{4} monomer, VWR). The monitoring was performed at
800~nm, which corresponds to the absorption by the blue silicomolybdate
complex.

\subsection{Small-angle X-ray scattering}\label{ssec45}

SAXS experiments were performed on a Xeuss 2.0 laboratory instrument
from Xenocs. Samples were injected in disposable quartz capillaries
(1.5~mm inner diameter). Azimuthal averaging was performed after mask
subtraction, yielding one-dimensional spectra after normalization by
transmissions. Since the sampling is heterogeneous, we did not display
intensities on absolute scales but rather scaled them by arbitrary
factors to highlight scaling laws over the $q$ range. 

\section*{Declaration of interests}

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

\section*{Acknowledgments}

ERAMET Research is thanked for its financial support of the PhD grant
to CM. Simon Blancher and C\'eline Rodriguez are acknowledged for
performing the mineral characterization. Dominique \mbox{Mannoni} and Amandine
Chabaneix are acknowledged for performing the ICP characterization.
Bruno Courtaud, Philippe Ribagnac, and Denis Beltrami are acknowledged
for stimulating discussions on this topic. The research federation
FERMAT (Universit\'e de Toulouse, France) is acknowledged for providing
access to the Xeuss 2.0 SAXS instrument (Xenocs) through the CPER
IMATECBIO grant with support from Pierre Roblin.

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