# Dissolution of scheelite under soft acidic conditions via the formation of polyoxotungstate: kinetics and mechanism supported by small-angle X-ray scattering (SAXS) to identify and quantify H 3 PW 12 O 40

Valentin Legrand, Stéphanie Szenknect, Olivier Diat, Luc Girard, Pierre Bauduin

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Valentin Legrand, Stéphanie Szenknect, Olivier Diat, Luc Girard, Pierre Bauduin. Dissolution of scheelite under soft acidic conditions via the formation of polyoxotungstate: kinetics and mechanism supported by small-angle X-ray scattering (SAXS) to identify and quantify H 3 PW 12 O 40. Hydrometallurgy, 2025, pp.106538. 10.1016/j.hydromet.2025.106538. hal-05174755

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Dissolution of scheelite under soft acidic conditions
via the formation of polyoxotungstate: kinetics and
mechanism supported by small-angle X-ray scattering
(SAXS) to identify and quantify H3PW12O40

Valentin Legrand, Stéphanie Szenknect*, Olivier Diat, Luc Girard, Pierre Bauduin*

ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Marcoule, France

*Corresponding authors: stephanie.szenknect@cea.fr; pierre.bauduin@cea.fr

Abstract

The decomposition of scheelite in a synergistic H2SO4-H3PO4 mixture is an effective method for extracting tungsten from mining concentrates. The driving force behind the dissolution of scheelite in the H2SO4-H3PO4 mixture is the formation of a soluble Keggin-type polyoxotungstate, H3PW12O40, which prevents the formation of very low solubility tungstic acid. In this work, a multiparametric study of scheelite dissolution kinetics was carried out in a synergistic acid mixture. In particular, the independent contributions of temperature, acid concentration, and W:P molar ratio on scheelite dissolution kinetics and H3PW12O40 formation yield were evaluated. To this end, a method based on the use of small-angle X-ray scattering (SAXS) was developed to identify and quantify H3PW12O40 under different operating conditions. The results provide a better understanding of the stability range of H3PW12O40 in terms of H2SO4 concentration and stoichiometric W:P ratio. These findings led to the selection of optimized, soft-leaching conditions that ensure rapid dissolution of scheelite while avoiding surface passivation by the precipitation of secondary phases. A comparison with speciation calculations using thermodynamic data reported in the literature reveals an absence of a self-consistent thermodynamic dataset. Thus, measuring the concentration of H3PW12O40 in the leachate was necessary to optimize the dissolution conditions. From this perspective, SAXS appears to be a suitable quantitative method.

Keywords

Kinetics; Polyoxotungstate; Scheelite; Sulfuric and Phosphoric Acids; Lixiviation; SAXS
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# 1 Introduction

Tungsten, a transition metal recognized for its exceptional properties, such as an elevated melting point, remarkable hardness, and outstanding corrosion resistance, has garnered substantial interest in a variety of industrial applications. Its unique attributes make tungsten important for assembling high-performance composites, cutting devices, and electrical contacts, as well as developing fibers and other electronic components. The growing demand for tungsten, driven by its applications in the aerospace, automotive, hardware and defense industries, explains the economic importance of its extraction and handling (Koutsospyros et al., 2006; Lunk and Hartl, 2019). With global tungsten production facing difficulties linked to the security of production networks and ecological concerns, efficient and economical techniques for dissolving and extracting tungsten metals have become a major point of interest (Smes et al., 2023; Wang et al., 2022).

Scheelite (CaWO₄) and wolframite (Fe, Mn WO₄) are the most widely mined tungsten-bearing minerals (Brown and Pitfield, 2014; Yih and Wang, 1979). Extraction of tungsten from scheelite or wolframite currently involves the wet decomposition of ore concentrates using hydrometallurgy. Due to the excessive exploitation of high-grade tungsten minerals, scheelite often requires refinement before processing using hydrometallurgical unit operations. Since the 1970s (Vazquez, 1976), froth flotation has been an established technique for processing scheelite using various flotation reagents (Kupka and Rudolph, 2018). A collector, which is a specific type of surfactant, selectively binds to the target mineral particles, rendering their surfaces hydrophobic (Khan et al., 2022; Zheng et al., 2024). This process enables the mineral particles to attach themselves to air bubbles and rise to the surface of the pulp, effectively separating them from other minerals. One of the most widely used W extraction processes from ore concentrates is the decomposition of scheelite or wolframite using alkaline reagents, such as sodium hydroxide or sodium carbonate, in autoclaves to obtain sodium tungstate (Na₂WO₄) (Han et al., 2021; Liu et al., 2020; Zhao et al., 2011), which can then be purified and transformed into tungsten ammonium paratungstate (APT): (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O. In any case, traditional dissolution/precipitation processes often have limitations, such as high temperatures and pressures requiring autoclaves, and generate large volumes of byproducts like calcium hydroxide (Ca(OH)₂) or calcium carbonate (CaCO₃) that must be managed (Liu et al., 2020; Shen et al., 2019).

Alternative hydrometallurgical processes entail the direct dissolution of scheelite in tungstic acid (H₂WO₄) using acids such as HCl, HNO₃ or H₂SO₄ (Lei et al., 2022). The advantage of these acids is that they are widely available and reasonably inexpensive. They also require far less acid and a lower temperature to achieve full conversion than conventional alkaline decomposition. Nevertheless, HCl and HNO₃ acids decompose under the effect of temperature and produce gases that can strongly corrode infrastructures and are harmful to the environment. In view of these drawbacks, sulfuric acid (H₂SO₄) seems more appropriate for scheelite (CaWO₄) dissolution, producing two solid products with low water
solubility: gypsum (CaSO₄·2H₂O(s)) and tungstic acid (H₂WO₄(s)). However, the formation of CaSO₄·2H₂O(s) and H₂WO₄(s) on the surface of the scheelite grain, results in their passivation, which stops the reaction completely. Furthermore, the formation of tiny gypsum crystals closely mixed with the tungstic acid solid phase may complicate phase separation (Li and Zhao, 2016).

To solve these issues, Zhao and Li (Li and Zhao, 2016; Zhao and Li, 2010; Zhao and Li, 2014) have developed, more than ten years ago, an innovative methodology for the acid dissolution of scheelite via the synergistic combination of sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄), to obtain a soluble heteropolytungstic acid known as phosphotungstic acid. This method is interesting because the concentration of sulfuric acid (H₂SO₄), which primarily regulates the pH of the solution, significantly affects the concentration of the different phosphotungstate species that are present in the solution (Martins, 2014; Wesolowski et al., 1984). Specifically, it is possible to guarantee that the Keggin-type polyoxotungstate (POT), H₃PW₁₂O₄₀, remains stable when the sulfuric acid concentration is high enough to maintain the solution's pH below 1. Therefore, it is possible to ensure that the Keggin-type POT, which is characterized by the highest atomic ratio of 12:1 between tungsten (W) and phosphorus (P), emerges as the main species (Smith and Patrick, 2004). Meanwhile gypsum is obtained in the form of large crystals, which makes it easier to separate from the solution. As shown in equation (1), a minimal amount of phosphoric acid is sufficient to promote scheelite dissolution, leading to i) gypsum formation and ii) soluble POT generation, while simultaneously reducing the passivation of the scheelite surface by the H₂WO₄ layer (Lei et al., 2022; Li et al., 2024; Zhao and Li, 2010; Zhao and Li, 2014):

$$12\text{CaWO}_{4(\text{s})} + \text{H}_{3}\text{PO}_{4(\text{aq})} + 12\text{H}_{2}\text{SO}_{4(\text{aq})} + 12\text{H}_{2}\text{O} \rightleftharpoons \text{H}_{3}[\text{PW}_{12}\text{O}_{40}]_{(\text{aq})} + 12\text{CaSO}_{4} \cdot 2\text{H}_{2}\text{O}_{(\text{s})} \quad (1)$$

Thus, the driving force of the H₂SO₄-H₃PO₄ synergistic dissolution is the formation of soluble Keggin-type POT. This species can be identified by NMR spectroscopy (Smith and Patrick, 2004), and its domain of predominance is restricted to acidic conditions, typically at pH < 1.5. Although the process optimization requires to find leaching conditions that allow H₃PW₁₂O₄₀ formation, identification of this species in solution is rarely carried out in studies of the scheelite dissolution kinetics (Cao et al., 2020; Chen et al., 2019; Gürmen et al., 1999; Li et al., 2024; Li and Zhao, 2016). Recent research from 2015 has shown that Keggin-type polyoxometalates, specifically phosphotungstate and silicotungstate, exhibit strong sorption properties, referred to as superchaotropicity, on neutral surfactants (polyethoxylated and sugar-based), which are generally known for their weak interactions with ionic species (Naskar et al., 2015). These studies on the superchaotropic properties of polyoxometalates subsequently led to the development of a novel superchaotropic flotation concept, demonstrated on isopolyoxomolybdate species, enabling the selective separation of these polyoxo species from all other non-superchaotropic ionic species present in solution (Skorzewska et al., 2023).

Considering the recent advances in separation chemistry related to the formation of polyoxometalate species, it is essential to thoroughly understand the dissolution processes involving these species. The
102 aims and novel aspects of this study are as follow: (i) developing a multiparametric study of scheelite dissolution using a synergistic mixture of H₂SO₄ and H₃PO₄ under mild conditions; (ii) showing the independent contributions of several parameters, including temperature, pH, acid concentration, and W:P mole ratio, on H₃PW₁₂O₄₀ POT formation and scheelite dissolution rate; (iii) accurately determining the concentration of [PW₁₂O₄₀]³⁻ in the investigated dissolution conditions; (iv) developing a new method based on small angle x-ray scattering (SAXS) to track the formation of H₃PW₁₂O₄₀ and determine its concentration, and (v) calculating species distribution to model the behavior of tungsten under various conditions. This methodology allows the scheelite dissolution rate to be correlated with the formation yield of H₃PW₁₂O₄₀ and to identify a specific operational range that favors rapid tungsten release and POT formation.
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## 2 Materials & Methods

### 2.1 Scheelite sample and neoformed phases characterizations

114 Scheelite (CaWO₄) with a purity of 99 % was acquired from Sigma-Aldrich. The specific surface area (S<sub>BET</sub>) of the scheelite powder was quantified using a Micromeritics ASAP 2020 analyzer and the Brunauer-Emmett-Teller (BET) method. The analysis was conducted using nitrogen gas at 77 K. To remove surface contaminants, the sample was pre-treated by outgassing at 353 K for 24 hours. Three measurements were performed to ensure reproducibility, and the specific surface area was 0.35 ± 0.01 m² g⁻¹, which corresponds to scheelite grains of about 3 µm in diameter.
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The commercial scheelite powder prior to dissolution, as well as dissolution residues resulting from the precipitation of the secondary phase were characterized by Powder X-ray diffraction (PXRD). The patterns were recorded on a Bruker D8 Advance diffractometer in parallel beam geometry with Göbel Mirror configuration using Cu Kα1,2 radiation of λ<sub>average</sub> = 1.5418 Å and a position-sensitive LynxEye linear detector. The data were collected in θ/θ step-scan mode at room temperature, covering a 2θ range from 5° to 90° with a step size of 0.01°. Each sample required a collection time of around 2.5 hours.
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The initial scheelite sample as well as dissolution residues were observed using Scanning Electron Microscopy (SEM) with an FEI Quanta 200 ESEM FEG in backscattered electron (BSE) mode under vacuum conditions at an optimal working distance of about 12 mm and voltage of 15.0 kV. The purity of the initial scheelite sample was checked using Energy Dispersive X-ray Spectroscopy (EDS) coupled with SEM using the same operating conditions. The particle size distribution of the commercial scheelite sample was determined using SEM image analysis based on the FiJi software. The grain size distribution was obtained by measuring the diameters of approximately 1,200 grains.
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The X-ray diffraction (XRD) diagram, scanning electron microscopy (SEM) micrograph, grain size distribution, and energy-dispersive X-ray spectroscopy (EDS) spectrum of the commercial scheelite
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sample used in this work are shown in Supporting Information, Section I, Figure SI_1. As expected, the commercial scheelite sample was found to be single-phased (space group I41/a). No impurities were detected using EDS analysis. The grain size distribution by number followed a log-normal law and the median diameter of the scheelite grains was 5 µm.

## 2.2 Dissolution experiments

A multiparametric study of scheelite dissolution kinetics in a synergistic mixture of sulfuric and phosphoric acids was carried out and detailed hereafter. The impact of temperature, sulfuric acid and phosphoric acid concentrations on the scheelite dissolution rate were determined. For this purpose, high-purity sulfuric acid ($H_2SO_4$, 96 wt.%) and phosphoric acid ($H_3PO_4$, 85 wt.%) provided by Sigma-Aldrich were used. A 0.16 mol L⁻¹ (≈ 1 vol.%) $HNO_3$ solution was prepared by dilution of 69 wt.% $HNO_3$ Suprapur, for trace analysis (Merck). Deionized ultrapure water (resistivity: 18.2 MΩ cm) was generated using a Milli-Q ultrapure water system.

Polytetrafluoroethylene (PTFE) containers of 240 mL (Savillex®) were used to perform the dissolution experiments. Each dissolution reactor was equipped with a thermometer for accurate temperature control throughout the dissolution process. A water bath was used to ensure a safe and consistent method of temperature control during the heating phase. For each experiment, 100 mL of the acidic solution was charged into a PTFE reactor. The solution was then heated to the target temperature using a water bath. Upon reaching the desired temperature, 1 g of the scheelite sample was added to the reactor, and the solution was stirred. The corresponding surface area over volume ratio was kept constant at 3,500 m⁻¹. At predetermined reaction times, aliquots ranging from 100 µL to 800 µL were withdrawn from the reactor. Each aliquot sample was centrifuged at 11,000 rpm for 1 min 30 s in order to separate the solid and liquid phases. Then, 50 µL to 600 µL (depending on the required dilution factor for subsequent analysis) of the supernatant was collected and diluted with a 1 vol.% $HNO_3$ solution to reach a volume of 10 mL. Finally, an equal volume of the initial acidic solution was returned to the dissolution vessel after each solution sampling to maintain the original volume. The experimental conditions considered for all the dissolution tests are summarized in Table 1.
<table><caption>Table 1: Conditions used for the multiparametric study of scheelite dissolution as polyoxotungstates (POT).</caption><thead><tr><th colspan="5">Conditions<sup>a</sup></th><th colspan="3">Experimental results<sup>b</sup></th><th colspan="4">Speciation calculations<sup>c</sup></th></tr><tr><th>Dissolution test number</th><th>[H<sub>2</sub>SO<sub>4</sub>]<br>(mol L<sup>-1</sup>)</th><th>[H<sub>3</sub>PO<sub>4</sub>]<br>(mol L<sup>-1</sup>)</th><th>T<br>(°C)</th><th>W : P<br>ratio</th><th>R<sub>L</sub>(W)<br>(g m<sup>-2</sup> min<sup>-1</sup>)</th><th>R<sub>L</sub>(Ca)<br>(g m<sup>-2</sup> min<sup>-1</sup>)</th><th>POT yield<br>(%)</th><th>SI (Anhydrite)</th><th>SI (Gypsum)</th><th>SI (H<sub>2</sub>WO<sub>4(s)</sub>)</th><th>POT yield<br>(%)</th></tr></thead><tbody><tr><td>d1</td><td>3.5</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.108(4)</td><td>0.154(5)</td><td>32</td><td></td><td></td><td></td><td></td></tr><tr><td>d2</td><td>2.5</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.176(5)</td><td>0.172(5)</td><td>91</td><td>-0.3</td><td>-0.6</td><td>1.61</td><td>100</td></tr><tr><td>d3</td><td>1.5</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.173(5)</td><td>0.169(6)</td><td>84</td><td>-0.06</td><td>-0.28</td><td>1.63</td><td>100</td></tr><tr><td>d4</td><td>1</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.159(5)</td><td>0.158(6)</td><td>92</td><td>0.01</td><td>-0.19</td><td>1.65</td><td>100</td></tr><tr><td>d5</td><td>0.7</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.129(3)</td><td>0.141(4)</td><td>90</td><td></td><td></td><td></td><td></td></tr><tr><td>d6</td><td>0.5</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.110(3)</td><td>0.115(4)</td><td>88</td><td></td><td></td><td></td><td></td></tr><tr><td>d7</td><td>0.2</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.075(3)</td><td>0.077(3)</td><td>92</td><td></td><td></td><td></td><td></td></tr><tr><td>d8</td><td>0.1</td><td>0.3</td><td>60</td><td>12 : 100</td><td>0.056(2)</td><td>0.059(6)</td><td>94</td><td>0.19</td><td>0.02</td><td>1.66</td><td>100</td></tr><tr><td>d9</td><td>3.5</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.070(1)</td><td>0.091(4)</td><td>NF</td><td></td><td></td><td></td><td></td></tr><tr><td>d10</td><td>2.5</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.140(2)</td><td>0.140(4)</td><td>93</td><td>-0.16</td><td>-0.42</td><td>1.70</td><td>100</td></tr><tr><td>d11</td><td>1.5</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.200(3)</td><td>0.197(5)</td><td>90</td><td>-0.05</td><td>-0.27</td><td>1.72</td><td>100</td></tr><tr><td>d12</td><td>1</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.164(4)</td><td>0.165(4)</td><td>92</td><td>0.02</td><td>-0.18</td><td>1.73</td><td>100</td></tr><tr><td>d13</td><td>0.7</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.152(4)</td><td>0.155(4)</td><td>90</td><td></td><td></td><td></td><td></td></tr><tr><td>d14</td><td>0.5</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.129(3)</td><td>0.133(4)</td><td>90</td><td></td><td></td><td></td><td></td></tr><tr><td>d15</td><td>0.2</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.075(2)</td><td>0.076(2)</td><td>91</td><td></td><td></td><td></td><td></td></tr><tr><td>d16</td><td>0.1</td><td>0.03</td><td>60</td><td>12 : 10</td><td>0.055(2)</td><td>0.061(3)</td><td>92</td><td>0.21</td><td>0.04</td><td>1.75</td><td>100</td></tr><tr><td>d17</td><td>2.5</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.051(3)</td><td>0.096(6)</td><td>12</td><td></td><td></td><td></td><td></td></tr><tr><td>d18</td><td>1.5</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.105(2)</td><td>0.110(4)</td><td>88</td><td>-0.04</td><td>-0.26</td><td>1.92</td><td>100</td></tr><tr><td>d19</td><td>1</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.110(2)</td><td>0.116(3)</td><td>90</td><td>0.02</td><td>-0.17</td><td>1.93</td><td>100</td></tr><tr><td>d20</td><td>0.7</td><td>0.003</td><td>60</td><td>12 : 1</td><td>NM</td><td>NM</td><td>80</td><td></td><td></td><td></td><td></td></tr><tr><td>d21</td><td>0.5</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.100(1)</td><td>0.113(2)</td><td>91</td><td></td><td></td><td></td><td></td></tr><tr><td>d22</td><td>0.2</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.070(1)</td><td>0.072(1)</td><td>90</td><td></td><td></td><td></td><td></td></tr><tr><td>d23</td><td>0.1</td><td>0.003</td><td>60</td><td>12 : 1</td><td>0.044(1)</td><td>0.046(2)</td><td>96</td><td>0.21</td><td>0.05</td><td>1.92</td><td>100</td></tr><tr><td>d24</td><td>0.1</td><td>0.03</td><td>20</td><td>12 : 10</td><td>0.002(0)</td><td>0.002(1)</td><td>ND</td><td></td><td></td><td></td><td></td></tr><tr><td>d25</td><td>0.1</td><td>0.03</td><td>40</td><td>12 : 10</td><td>0.012(1)</td><td>0.015(1)</td><td>ND</td><td></td><td></td><td></td><td></td></tr><tr><td>d26</td><td>0.1</td><td>0.03</td><td>80</td><td>12 : 10</td><td>0.113(6)</td><td>0.124(6)</td><td>ND</td><td></td><td></td><td></td><td></td></tr></tbody></table>

a. The initial mass of scheelite was m₀ = 1.0 g for 100 mL of leaching solution (i.e. surface/volume ratio of 3,500 m⁻¹).

b. Experimental results include: dissolution rates obtained from the dissolution of scheelite samples, POT yield determined from SAXS measurements. NF: fitting with sphere model not applicable; ND: SAXS spectrum not measured; NM: dissolution rate not measured due to analytical issue

c. Speciation calculations include: saturation index (SI) of the solution with respect to anhydrite, gypsum and H₂WO₄(s) determined using PHREEQC when saturation phenomenon was suspected. The expected [H₃PW₁₂O₄₀] concentration for scheelite complete dissolution is 2.89 mmol L⁻¹, i.e. for 100% POT yield (see Supporting Information, Section II for details about the method of calculation of POT yield).
Tungsten concentration in the collected samples was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Spectro Arcos instrument. Emission intensities were monitored at four pre-selected wavelengths (λ = 209.860 nm, 218.920 nm, 239.709 nm, and 245.148 nm) to minimize spectral interferences. Similarly, calcium concentration was determined using ICP-AES at wavelengths of 183.801 nm, 184.006 nm, 315.887 nm, 317.933 nm, and 422.673 nm. Calibration of the ICP-AES was achieved using several standard solutions prepared by diluting certified 1000 mg L⁻¹ W and Ca standards (PlasmaCAL, SCP Science) in 0.16 mol L⁻¹ HNO₃. The elemental concentrations were determined as the average of triplicate measurements at each wavelength. The reported uncertainty corresponds to twice the standard deviation of all the elemental concentration measurements. Under these analytical conditions, the detection limits (LOD) of W and Ca reached 5.4 × 10⁻⁹ mol L⁻¹ and 2.5 × 10⁻⁹ mol L⁻¹, respectively.

The relative mass loss (Δmᵢ/m₀ in wt.%) and the normalized mass loss, N<sub>L(i,t)</sub> (in g m⁻²) were calculated from the elemental concentrations in solution. These calculations considered the initial specific surface area of the scheelite sample and the mass fraction of each element present in the solid sample, as described by the following two equations:

$$ \frac{\Delta m_{i(t)}}{m_0} = \frac{m_{i(t)} + \sum m_{i \text{ PE } (t-1)}}{f_i \times m_0} \cdot 100 = \frac{m_{i(t)} + \sum (C_i \times V_{\text{SP}} \times M_i)_{(t-1)}}{f_i \times m_0} \cdot 100 \quad (2) $$

$$ N_{L(i,t)} = \frac{\Delta m_{i(t)}}{f_i \times S_{(t)}} \quad (3) $$

where mᵢ(t) (g) corresponds to the mass of element i, dissolved in the solution at time t. The term Σmᵢ PE (t-1) (g) represents the sum of the masses of element i removed during sampling from t = 0 to t-1; Cᵢ (mol L⁻¹) is the elemental concentration in i; V<sub>SP</sub> (mL) is the sampling volume; Mᵢ (g mol⁻¹) is the molar mass of element i; fᵢ (g g⁻¹) represents the mass fraction of element i within the solid sample; m₀ denotes the initial mass of the scheelite sample introduced into the system; S<sub>(t)</sub> (m²) is the surface area of the sample calculated using the initial specific surface area of scheelite (S<sub>BET</sub> in m² g⁻¹). The current formulation of the normalized mass loss, N<sub>L(i,t)</sub> takes into account change in the reactive surface area of the scheelite sample, according to the following equation:

$$ N_{L(i,t)} = \frac{\Delta m_{i(t)}}{f_i \times S_{BET} \times (m_0 - \Delta m_{i(t-1)})} \quad (4) $$

The normalized dissolution rate per unit surface area, R<sub>L(i)</sub> (in g m⁻² min⁻¹), for element i corresponds to the time derivative of the normalized mass loss, N<sub>L(i)</sub>:

$$ R_{L(i)} = \frac{dN_{L(i,t)}}{dt} \quad (5) $$
The normalized dissolution rate was determined within the kinetic regime of the dissolution curve by linear regression of the N<sub>L(i, t)</sub> data.

## 2.3 Investigation of W speciation (POT) in solution

In order to identify the presence of polyoxotungstates (POT) in the dissolution medium and to give an estimate of the POT concentration, small-angle X-ray scattering (SAXS) measurements were conducted using Mo radiation with a wavelength of λ = 0.071 nm on a XENOCS-built instrument. The scattered X-ray beam was detected using a large online MAR Research scanner detector (diameter: 345 mm). An off-center detection configuration allowed for a broad q-range (0.2 to 35 nm⁻¹) investigation. A Xenocs 12:∞ multilayer mirror optimized for Mo radiation, coupled with two sets of scatterless FORVIS slits, provided a collimated X-ray beam with a size of 0.8 mm × 0.8 mm at the sample position. Initial data pre-processing was performed using PySAXS software, by plotting the scattered intensity, I(q), as a function of the magnitude of the scattering vector q, defined as $q = \frac{4\pi}{\lambda} \times \sin\frac{\theta}{2}$, where λ is the incident radiation wavelength and θ is the scattering angle. The samples were held in quartz capillaries (2 ± 0.2 mm). Standard corrections were applied, including background subtraction (empty cell and detector noise) and intensity normalization using a polyethylene (Lupolen™) film standard. The experimental resolution was ΔQ/Q = 0.05. Silver behenate in a sealed capillary was used for scattering vector calibration and absolute setting. All experiments were carried out at a controlled temperature of 22 ± 2 °C.

The quantification of POT using small-angle X-ray scattering (SAXS) was achieved by fitting the scattering spectra. The SASView software (http://www.sasview.org/) was used to fit experimental spectra using a form factor of a sphere characterized by a uniform scattering length density. This assumption was based on the resolved structure of the POT (Keggin, 1933; Pope and Müller, 1991), which is described in detail in Supporting Information, Section II. a. The 1-D scattering intensity was calculated according to following equation:

$$l(q)_{\text{sphe}} = \Phi \times V_m \times (\Delta\rho)^2 \times P(q, r) = \Phi \times V_m \times (\Delta\rho)^2 \times \left[ 3 \times \frac{\sin(qr) - qr \cos(qr)}{(qr)^3} \right]^2 + bkg \quad (6)$$

where Φ is the volume fraction, V<sub>m</sub> is the molecular volume of the scatterer, r (Å) is the radius of the sphere, q is the scattering vector and background (bkg, cm⁻¹) is the solvent scattering; Δρ is the difference of the scattering length densities (SLD) between those of the scatterer and the solvent. The POT-POT interactions were neglected, and no structure factor was considered, as the concentration range was low (<1 vol.%) and the salinity remains high under all tested conditions. This ensures that POT-POT electrostatic repulsions are fully screened (Malinenko et al., 2018). Quantification was performed based on the volume fraction determined by fitting with a sphere radius of 4.75 Å, a scattering length density (total scattering length per unit volume) for the POT anion of 70.93×10⁻¹⁰ cm⁻², and a SLD<sub>water</sub> of
9.43×10⁻¹⁰ cm⁻², over a range of scattering vectors q extending from 0.03 to 0.8 Å⁻¹ (see Supporting Information section II for more details on SLDs calculation and SAXS spectra fitting procedure). The goodness of fit was evaluated by minimizing Pearson's Chi² value (reported for each fitted spectrum in Table SI_6).

Solution-state ³¹P NMR spectra were acquired on a Bruker Avance400 spectrometer using a BBFO probe. Chemical shifts (δ) are given in ppm relative to a phosphoric acid internal standard. The low- and high-field limits of the spectrum were set at -50 and 50 ppm, respectively. The acquisition time was set to approximately 17 h for low-concentration samples. Deuterated dimethyl sulfoxide (DMSO-d₆) was used as the solvent for these measurements, which were performed at room temperature.

## 2.4 Multiparametric expression of the dissolution rate

A multiparametric equation for the kinetics of dissolution reactions was proposed by Lasaga (Lasaga, 1995) based on experimental results. The general form of the rate law is given by:

$$R_L(i) = k_0 \times e^{-Ea_i/RT} \times (H^+)^{n_H} \times g(I) \times \prod_j (E_j)^{n_j} \times f(\Delta_R G) \quad (7)$$

where $k_0$ (g m⁻² h⁻¹) is the rate constant, $Ea_i$ (kJ mol⁻¹) is the apparent activation energy of the overall reaction, R is the gas constant, T (K) is the temperature. ($E_j$) and ($H^+$) are the activities of $E_j$ and $H^+$ species in solution, respectively; $n_H$ and $n_j$ are the partial orders of the reaction related to the proton and to the $E_j$ species, respectively; $g(I)$ indicates a possible dependence of the overall rate on the ionic strength, I; $f(\Delta_R G)$ accounts for the variation of the overall rate with the deviation from equilibrium of the solution with a solid phase.

In a pure kinetic regime, the last term of eq. (7), $f(\Delta_R G)$ can be neglected and several authors have demonstrated the predominant role of acidity and temperature on the kinetics of dissolution of a large variety of minerals (Lasaga, 1984). Under acidic conditions, while keeping all experimental parameters constant, the normalized dissolution rate can be expressed as:

$$R_L(i) = k \times (H^+)^{n_H} \quad (8)$$

where k (g m⁻² min⁻¹) is the apparent dissolution rate constant at ($H^+$) = 1 and $n_H$ is the partial order related to the proton. The value of k depends on the fixed temperature, ionic strength and composition of the dissolution medium. Following eq. (8), k and $n_H$ can be calculated from the linear regression of log $R_L(i)$ versus log ($H^+$).

Keeping all parameters constant except for temperature, the dependence of the dissolution rate on temperature was described using the Arrhenius equation, eq. (9); $Ea_i$ represents the energy barrier that reactant molecules must overcome for a reaction to proceed under specific pH, ionic strength and composition of the dissolution medium.
$$R_L(i) = A \times e^{\frac{-Ea_i}{R \cdot T}} \qquad (9)$$

The apparent activation energy of dissolution was calculated by plotting ln R_L(i) with respect to the reciprocal temperature (1/T). The Ea_i value serves as a critical kinetic parameter that sheds light on the dominant mechanism governing the dissolution process. Typically, Ea_i values lower than 20 kJ mol⁻¹ characterize dissolution processes primarily controlled by aqueous diffusion step. Surface-controlled reaction mechanisms are generally associated with higher Ea_i values ranging from 40 to 140 kJ mol⁻¹ (Langmuir, 1997).

## 2.5 PHREEQC simulations

The molarities of the W, Ca, sulfate and phosphate species were calculated at the various temperatures from the initial concentration of the prepared mixture of sulfuric and phosphoric solutions using the geochemical speciation software PHREEQC-3 (Parkhurst and Appelo, 2013) associated with the modified database Thermoddem v1.10 from the French Geological Survey (Blanc et al., 2012). The molar to molal conversion was performed by PHREEQC-3 using the densities of the solutions at the corresponding temperature calculated from Novotny and Söhnel (Novotny and Sohnel, 1988). The corrections for ionic activities were performed using the SIT implemented in PHREEQC-3 and relevant Specific Interaction Theory (SIT) coefficient values. The value of ε(H⁺, HSO₄⁻) = 0.14 ± 0.05 was taken from PRODATA v1.5.3.1 thermodynamic database (Reiller and Descostes, 2020).

The thermodynamic database was completed with the following reactions to take into account the speciation of W in the synergistic mixture of sulfuric and phosphoric acids. Details concerning the calculations of the thermodynamic data used in this work are gathered in the Supporting Information, Section III:

$$\mathrm{HWO}_{4}^{\mathrm{r(aq)}}+\mathrm{H}^{+\mathrm{(aq)}} \rightleftharpoons \mathrm{H}_{2} \mathrm{WO}_{4(\mathrm{aq})}$$

$$\log K^{\circ}(298.15)=2.30 \text { and } \Delta_{R} H^{\circ}=-64.41 \text { kJ mol}^{-1} \qquad (10)$$

$$\mathrm{Ca}^{2+}_{\mathrm{(aq)}}+\mathrm{WO}_{4}^{2-}_{\mathrm{(aq)}} \rightleftharpoons \mathrm{CaWO}_{4(\mathrm{aq})}$$

$$\log K^{\circ}(298.15)=5.57 \text { and } \Delta_{R} H^{\circ}=-133.7 \text { kJ mol}^{-1} \qquad (11)$$

$$12\mathrm{WO}_{4}^{2-}(\mathrm{aq})+25\mathrm{H}^{+}(\mathrm{aq})+\mathrm{H}_{2}\mathrm{PO}_{4}^{\mathrm{r}}(\mathrm{aq}) \rightleftharpoons \mathrm{H}_{3}\mathrm{PW}_{12}\mathrm{O}_{40}(\mathrm{aq})+12\mathrm{H}_{2}\mathrm{O},$$

$$\log K^{\circ}(298.15)=171.25 \text { and } \Delta_{R} H^{\circ}=-1550.65 \text { kJ mol}^{-1} \qquad (12)$$

The PHREEQC-3 software was also used to calculate the solution saturation index (SI) with respect to the phases included in the database at the end of dissolution experiments. The SI values were estimated using the initial composition of the dissolution medium, the appropriate temperature, the concentrations of Ca, W at equilibrium and by considering equilibrium with air (P₂O₅ = 20 kPa and P₆O₂= 0.032 kPa). Among the phases of interest, gypsum and anhydrite were highly regarded, as these phases were found
293 to precipitate under certain conditions. The solubility constants of gypsum and anhydrite in the
294 Thermoddem v1.10 database are defined as follows:

$$
\begin{array}{l}
295 \quad \mathrm{CaSO_4} \cdot 2\mathrm{H_2O(s)} \rightleftharpoons \mathrm{Ca^{2+}(aq)} + \mathrm{SO_4^{2-}(aq)} + 2\mathrm{H_2O}, \\
296 \quad \log K_{S,0}^{\circ}(298.15) = -4.605 \text{ and } \Delta_R H^{\circ} = -1.054 \text{ kJ mol}^{-1} \qquad (13)
\end{array}
$$

$$
\begin{array}{l}
297 \quad \mathrm{CaSO}_{4(\mathrm{s})} \rightleftharpoons \mathrm{Ca}^{2+}_{(\mathrm{aq})} + \mathrm{SO}_{4}^{2-}_{(\mathrm{aq})}, \\
298 \quad \log K_{S,0}^{\circ}(298.15) = -4.436 \text{ and } \Delta_{\mathrm{R}}H^{\circ} = -17.940 \text{ kJ mol}^{-1} \quad (14)
\end{array}
$$

299 Thus, the SI of the solution with respect to gypsum was calculated as:

$$
300 \qquad SI(\text{gypsum}) = \log \left( \frac{(Ca^{2+})(SO_4^{2-})(H_2O)^2}{K_{S,0}^{\circ}} \right) \qquad (15)
$$

301 The scheelite and tungstic acid thermodynamic data were calculated from the Δ<sub>f</sub>G° (298 K) and
302 Δ<sub>f</sub>H° (298 K) values provided by (Martins, 2014):

$$
\begin{array}{l}
303 \quad \mathrm{CaWO}_{4(\mathrm{s})} \rightleftharpoons \mathrm{Ca}^{2+}_{(\mathrm{aq})} + \mathrm{WO}_{4}^{2-}_{(\mathrm{aq})} \\
304 \quad \log K_{S,0}^{\circ}(298.15) = -12.36 \text{ and } \Delta_{\mathrm{R}}H^{\circ} = 133.7 \, \mathrm{kJ} \, \mathrm{mol}^{-1} \qquad (16)
\end{array}
$$

$$
\begin{array}{l}
305 \quad \mathrm{H_2WO}_{4(\mathrm{s})} \rightleftharpoons 2\mathrm{H}^{+}_{(\mathrm{aq})} + \mathrm{WO}_{4}^{2-}_{(\mathrm{aq})} \\
306 \quad \log K_{S,0}^{\circ}(298.15) = -16.14 \text{ and } \Delta_{\mathrm{R}}H^{\circ} = 141.86 \, \mathrm{kJ} \, \mathrm{mol}^{-1} \qquad (17)
\end{array}
$$

3 Results and discussion

308 **3.1 Typical shape of dissolution curve**

309 The dissolution kinetics of CaWO₄ was followed in various conditions summarized in Table 1. The
310 shapes of the obtained dissolution curves revealed a sequence of kinetic and thermodynamic stages, that
311 may or may not have been observed depending on the conditions. Figure 1 represents the evolution of
312 the normalized mass losses of tungsten and calcium obtained in 2.5 mol L⁻¹ H₂SO₄ and 0.3 mol L⁻¹ H₃PO₄
313 at 60°C (test d2). The complete sets of dissolution curves are presented in Supporting Information,
314 Section IV.
<figure><img src="image_3.png" /><figcaption>Figure 1: Evolution of the normalized mass loss (left y-axis) and of the relative mass loss (right y-axis) during the dissolution of scheelite in H₂SO₄ 2.5 mol L⁻¹, H₃PO₄ 0.3 mol L⁻¹ at 60°C (test d2), ● N_L(W) & ▼ N_L(Ca). The red zone indicates the kinetic regime in which the dissolution rate was determined. The blue zone indicates the existence of a saturation phenomenon.</figcaption></figure>

The first part of the curve (red zone in Figure 1) corresponds to the establishment of a steady state. During this step, the evolution of the N<sub>L</sub>(W) and N<sub>L</sub>(Ca) is linear and the dissolution rate remains constant, which characterizes a pure kinetic regime. Under the experimental conditions shown in Figure 1, this regime was maintained up to 90 % of relative mass loss. It is noteworthy that the scheelite dissolution is congruent during the pure kinetic regime. Then, a second kinetic regime occurs during which the evolution of N<sub>L</sub>(W) and N<sub>L</sub>(Ca) differs. Whereas the W release keeps increasing gradually until almost complete dissolution of the scheelite phase (i.e. Δm<sub>W</sub>/m<sub>0</sub> = 99 ± 1 wt.% in Figure 1), the normalized mass loss of calcium mass, N<sub>L</sub>(Ca) starts to decrease. This evolution reflects the potential precipitation of a Ca-bearing phase. Indeed, XRD analysis of the solid residues collected at the end of experiment (Figure 2-a) indicated the precipitation of gypsum. In addition, SEM image of the dissolution residues showed the formation of large needle-like crystals. Thus, in the second kinetic step (depicted in blue in Figure 1), the dissolution mechanism was influenced by saturation phenomena in solution, which could explain the decrease of the apparent dissolution rate R<sub>L</sub>(Ca). However, the calculations performed with PhreeqC revealed that the solution was close to equilibrium with anhydrite (SI = -0.3), but still slightly undersaturated (Table 1). Moreover, the fact that SI<sub>(anhydrite)</sub> is greater than SI<sub>(gypsum)</sub>, indicates that the phase that is anticipated to precipitate at 60°C is anhydrite. Nevertheless, the phase transformation from anhydrite to gypsum occurs at low temperatures (43°C) (Corti and Fernandezprini, 1984), and might take place during the cooling of the leachate at the end of experiment and before XRD analysis of the neoformed phase. More importantly, Figure 1 shows that precipitation of the secondary phase occurs at the end of the dissolution of scheelite, and thus does not strongly influence W
solubilization. Since speciation calculations showed that the solution remained slightly undersaturated with respect to anhydrite, it was suspected that this precipitation did not occur in the dissolution reactor at 60°C. Instead, it occurred in the solution samples that were removed from the reactor and centrifuged at room temperature prior to dilution and ICP-AES analysis of Ca elemental concentration. Indeed, for this experiment (test d2) the saturation index of the solution with respect to gypsum at 20°C was: SI(gypsum) = +0.1. This result is also a strong indication that the precipitation of gypsum occurred after the complete dissolution and was caused by the cooling of the solution that was oversaturated with respect to gypsum at room temperature. Comparison with scheelite dissolution rates obtained by Zhang et al. (2015) or Liu et al. (2016) in the HNO₃ - H₃PO₄ and HCl - H₃PO₄ mixtures, respectively, further reinforces this hypothesis. For instance, Zhang et al. (2015) showed that in a mixture of 3 mol L⁻¹ HNO₃ and 0.5 mol L⁻¹ H₃PO₄ and at 60°C, the relative mass loss of scheelite reached 75% in around 2 h. Under comparable conditions of acidity and temperature, and when gypsum or anhydrite precipitation is impossible, the dissolution rate of scheelite is therefore lower than that obtained in this work. Liu et al. (2016) determined that the relative mass loss of scheelite reached 99% in 20 min in 0.72 mol L⁻¹ HCl with an excess of phosphoric acid corresponding to W : P ratio of 12 : 1.7. The dissolution rate was thus slightly higher in HCl – H₃PO₄ but remained comparable. This comparison supports the hypothesis that CaSO₄·n H₂O precipitation does not occur in the dissolution reactor under the conditions used in this study, and does not slow down scheelite dissolution kinetics.

<figure><img src="image_4.png" /><figcaption>Figure 2: XRD pattern of the dissolution residue collected at the end of experiment performed in H₂SO₄ 2.5 mol L⁻¹, H₃PO₄ 0.3 mol L⁻¹ at 60°C (test d2). The stars indicate the characteristic peaks of gypsum (CaSO₄·2H₂O) extracted from CIF base ICDD-COD 5000039: a = 5,670 Å; b = 15,201 Å; c = 6,533 Å. Scanning electron microscopy (SEM) image of needle-shaped gypsum.</figcaption></figure>

359

360

361

362

363
Saturation phenomena did not influence the scheelite dissolution rates, R<sub>L</sub>(i) in the first kinetic step. Thus, reliable R<sub>L</sub>(i) values were determined from linear regression of the normalized mass loss data during the kinetic regime for all the experiments performed under various conditions. The results summarized in Table 1 revealed the respective impact of the different tested parameters.

## 3.2 Effect of H<sub>2</sub>SO<sub>4</sub> concentration

<figure><img src="image_5.png" /></figure>

<caption>Figure 3: Effect of H<sub>2</sub>SO<sub>4</sub> concentration ranging from 0.1 mol L<sup>-1</sup> to 2.5 mol L<sup>-1</sup> on the evolution of a) N<sub>L</sub>(W) and b) N<sub>L</sub>(Ca) during scheelite dissolution at 0.03 mol L<sup>-1</sup> H<sub>3</sub>PO<sub>4</sub> and 60°C. ■ H<sub>2</sub>SO<sub>4</sub> 2.5 mol L<sup>-1</sup> (test d10); ● H<sub>2</sub>SO<sub>4</sub> 1.5 mol L<sup>-1</sup> (test d11); ▲ H<sub>2</sub>SO<sub>4</sub> 1.0 mol L<sup>-1</sup> (test d12); ▼ H<sub>2</sub>SO<sub>4</sub> 0.1 mol L<sup>-1</sup> (test d16).</caption>
First of all, the impact of the sulfuric acid concentration on the dissolution kinetics of CaWO₄ (Figure 3 and Figure SI_4 to Figure SI_6 of Supporting Information for the complete set of experiments) was evaluated at a constant concentration of 0.03 mol L⁻¹ in H₃PO₄ and at 60°C (test **d9** – **d16** in Table 1). Based on the stoichiometry of reaction (1), these situations correspond to a molar excess of phosphorus to tungsten of 10 (i.e. W : P = 12 : 10). This large excess of phosphorus was chosen to promote POT formation and ensure that Keggin structure POT was the predominant W species in solution. For H₂SO₄ concentration increased from 0.1 mol L⁻¹ to 1.5 mol L⁻¹, the results depicted in Figure 3 and Table 1 show a positive impact of the sulfuric acid concentration on the scheelite dissolution rate. Nevertheless, this impact is low and the relative mass loss reaches 95 wt.% in less than 1.5 hours for [H₂SO₄] ≥ 0.7 mol L⁻¹. Then, increasing the H₂SO₄ concentration up to 2.5 mol L⁻¹ led to a slight but significant decrease in both R_L(W) and R_L(Ca). This behavior could be linked to a change in W speciation in solution, in particular the formation of tungstic acid (H₂WO₄(s)) is expected at high acidity and low temperature (Martins, 2014). These observations are in line with the literature, which reports an increase in the dissolution rate with the acidity of the medium, up to a certain threshold beyond which the rate remains constant or decreases (Chen et al., 2019).

Furthermore, under certain experimental conditions (2.5, 1.5 and 0.1 mol L⁻¹ H₂SO₄), a decrease of the N_L(Ca) is observed at longer time during the second regime (Figure 3b and Figure SI_4b to Figure SI_6b of Supporting Information). The SI(anhydrite) values reported in Table 1 show that the solution became slightly oversaturated with respect to anhydrite for [H₂SO₄] ≤ 1 mol L⁻¹. Thus, at sulfuric acid concentrations higher than 1 mol L⁻¹, the precipitation of anhydrite is not expected from thermodynamic considerations. Nevertheless, the N_L(Ca) was found to decrease even at [H₂SO₄] = 2.5 mol L⁻¹ (test **d10**), but the formation of a solid phase was not observed at the time scale of the experiment. Again, an experimental bias is suspected to have caused the apparent decrease of the Ca concentration in solution. The precipitation of gypsum occurred certainly during the centrifugation of the solution samples at room temperature and a fraction of Ca was removed from solution before ICP-AES analysis.

## 3.3 Effect of H₃PO₄ concentration

Figure 4, presents the impact of the H₃PO₄ concentration on the scheelite dissolution kinetics in 2.5 mol L⁻¹ H₂SO₄. At this concentration, the dissolution rate is strongly influenced by the H₃PO₄ concentration. By increasing the phosphoric acid concentration from 0.003 to 0.3 mol L⁻¹, the activity of the protons in solution slightly increased, which had a positive impact on the dissolution rate value (increasing from 0.051 to 0.176 g m⁻² min⁻¹). Moreover, the activity of phosphate ions also increased, which promoted the formation of POT and thus the solubilization of W. This assumption is confirmed by the POT yield measured in solution by SAXS which increased from 12 wt.% to 96 wt.% when the H₃PO₄ concentration increased from 0.003 to 0.03 mol L⁻¹ in 2.5 mol L⁻¹ H₂SO₄ solution (Table 1, tests **d17** and **d10**, respectively). When the H₃PO₄ concentration is not high enough for the species
411 H₃PW₁₂O₄₀(aq) (test d17) to be predominant in solution, the formation of tungstic acid, characterized by
412 the appearance of a yellow gel, is observed (Figure 4a).

413 By considering the results presented in Figure 4 and Table 1, a concentration of 0.03 mol L⁻¹ in H₃PO₄,
414 which corresponds to a molar ratio W : P = 12:10, appears as a good compromise to ensure the rapid
415 and full dissolution of scheelite in 2.5 mol L⁻¹ H₂SO₄ without the influence of saturation phenomena.

<figure><img src="image_6.png" /><figcaption>Figure 4: Effect of H₃PO₄ concentration ranging from 0.003 to 0.3 mol L⁻¹ on the evolution of a) N<sub>L</sub>(W) and b) N<sub>L</sub>(Ca) during scheelite dissolution at 2.5 mol L⁻¹ H₂SO₄ and 60°C. ■ H₃PO₄ 0.3 mol L⁻¹ (test d2); ● H₃PO₄ 0.03 mol L⁻¹ (test d10); ▲ H₃PO₄ 0.003 mol L⁻¹ (test d17). The photo inserted in graph a) shows the presence of a yellow gel characteristic of tungstic acid formation.</figcaption></figure>

416

417

418

419

420

421

422

**3.4 Combined effect of sulfuric and phosphoric acids**

423

424

The normalized dissolution rates calculated from the experimental data and summarized in Table 1,
confirm the general trend of increasing dissolution rate with H₂SO₄ concentration regardless the H₃PO₄
concentration, up to a certain H₂SO₄ concentration, beyond which the dissolution rate decreases. The results presented in Figure 5 show a low impact of the H₃PO₄ concentration on the dissolution rate, especially at low acidity (typically for [H₂SO₄] ≤ 0.5 mol L⁻¹). However, the results presented in Figure 5 also show that a significant decrease in the rate occurred at lower H₂SO₄ concentrations when the H₃PO₄ concentration decreased, i.e., when the molar excess of phosphate decreased. For instance, at the stoichiometric ratio for reaction (1) (i.e., W : P = 12 : 1), the maximum dissolution rate of scheelite was observed at [H₂SO₄] = 1 mol L⁻¹ (test **d19**). This maximum was reached at [H₂SO₄] = 1.5 mol L⁻¹ (test **d11**) and at 2.5 mol L⁻¹ (test **d2**), at W : P = 12 : 10 and W : P = 12 : 100, respectively. In all cases, the conditions required to obtain the maximum dissolution rate led to a measured POT yield of 90 ± 1 %. The decrease in the dissolution rate of scheelite at elevated sulfuric acid concentrations is systematically associated with a decrease in the POT yield and the observation of a yellow precipitate of tungstic acid in solution (photos in Figure 5). This assumption was confirmed by SEM/EDS analysis of dissolution residues collected at the end of test **d17** ([H₂SO₄] = 2.5 mol L⁻¹ and W : P = 12 : 1). The elemental maps presented in Figure 6 show the presence of W rich and Ca depleted zones. These zones either correspond to isolated neoformed grains or deposits on the surface of altered scheelite grains. These deposits could act as a passivation layer on the scheelite surface grains lowering the dissolution rate. This observation highlights the synergy between the two acids in controlling the dissolution kinetics and the role of phosphate ions in promoting the formation of POT and consequently increasing the apparent solubility of tungsten in concentrated sulfuric acid solutions. However, the selected thermodynamic data “predict” tungstic acid precipitation under all conditions investigated, because the calculated SI was systematically positive (between 1.60 and 1.90 in Table 1), whereas it was only observed at H₂SO₄ concentrations above 1.5 mol L⁻¹ (i.e., for the least P-rich synergistic ratio). Unfortunately, thermodynamic calculations are insufficient to define the conditions favorable for the formation of tungstic acid in the synergistic mixture. There are two possible explanations for this failure: (1) tungstic acid precipitation is only observable above a SI value higher than 1.9, (2) the solubility data for H₂WO₄(s) need to be reassessed in the synergistic mixture of the two acids or lack of consistency with the POT formation constant estimated in this work using the available thermodynamic data.
<figure><img src="image_7.png" /></figure>

Figure 5: Effect of H₂SO₄ (Log aH⁺) concentration ranging from 0.1 mol L⁻¹ to 3.5 mol L⁻¹ on the evolution of Log R_L(W) during scheelite dissolution at 60°C for various concentration of H₃PO₄. ■ H₃PO₄ 0.3 mol L⁻¹ (tests d1-d8); ● H₃PO₄ 0.03 mol L⁻¹ (tests d9-d16); ▲ H₃PO₄ 0.003 mol L⁻¹ (tests d17-d23). The numbers indicate the POTs yield determined by SAXS. The photos show the presence of a yellow precipitate of tungstic acid when observed after centrifugation of the dissolution medium.

<figure><img src="image_8.png" /></figure>

Figure 6: SEM micrograph, W and Ca elemental maps obtained by EDS analysis of test d17 residues. Orange circles highlight the presence of Ca depleted phase.

Eq. (8) was used to describe the variation of R_L(W) with the activity of protons in the different mixtures of sulfuric and phosphoric acids. The variation of log R_L(W) with log (H⁺) was found to be linear in the region where the speciation of W is dominated by the H₃PW₁₂O₄₀ (aq) species (i.e., it represents more than 90 % of dissolved W). This experimental domain also corresponds to the absence of a saturation effect on the dissolution rate (i.e., f(ΔR G) ≈ 1 in eq. (7)). The fitted rate constant and the partial order of the dissolution reaction with respect to proton are given in Table 2. Regardless of the H₃PO₄ concentration, the parameter values are similar. This observation illustrates the low impact of the H₃PO₄
concentration on the dissolution rate of scheelite as far as the stoichiometric ratio for reaction (1) (i.e., W : P = 12 : 1) is reached. The fractional partial order of reaction with respect to protons indicates that the dissolution reaction is controlled by surface reactions involving the adsorption of protons at scheelite surface sites. A similar conclusion was drawn by Zhang et al. (2015) for a HNO₃ - H₃PO₄ mixture. These authors also demonstrated the positive impact of the nitric acid concentration on the dissolution rate and the low impact of the phosphoric acid concentration in the range of 0.1 to 0.75 mol L⁻¹. They obtained a fractional partial order of the reaction with respect to HNO₃ concentration of 0.23, which is thus slightly lower than the value obtained in this work. This comparison shows that the proton activity has a higher influence on the dissolution rate of scheelite in the synergistic mixture of sulfuric and phosphoric acids than in nitric and phosphoric acids. However, the impact of HCl concentration appears stronger in the study by Liu et al. (2016) with a partial order of reaction higher than 1. Nevertheless, in the range of W : P stoichiometric ratios investigated by these authors (from W : P = 12 : 1 to W : P = 12 : 3), the impact of phosphoric acid concentration was also found to be low. Liu et al. (2016) concluded that the formation of H₃PW₁₂O₄₀ inhibits the formation of tungstic acid in HCl solution, as far as W : P ≤ 12. This comparison with kinetic studies performed in different acidic mixtures reveals a similar scheelite dissolution mechanism and highlights the key role of phosphate ions compared to chloride, nitrate or sulfate in controlling W speciation.

<table><caption>Table 2: Rate constant and partial order of reaction with respect to proton determined for scheelite dissolution in a mixture of sulfuric and phosphoric acids obtained by linear regression of the log R<sub>L</sub>(W) data as a function of a<sub>H+</sub>.</caption><thead><tr><th>[H<sub>3</sub>PO<sub>4</sub>] (mol L<sup>-1</sup>)</th><th>n<sub>H</sub></th><th>Log k (g m<sup>-2</sup> min<sup>-1</sup>)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td><strong>0.3</strong></td><td>0.40(1)</td><td>-0.762(9)</td><td>0.9973</td></tr><tr><td><strong>0.03</strong></td><td>0.483(7)</td><td>-0.672(6)</td><td>0.9993</td></tr><tr><td><strong>0.003</strong></td><td>0.36(2)</td><td>-0.83(2)</td><td>0.9907</td></tr></tbody></table>

490

## 3.5 Effect of temperature

The impact of temperature on the dissolution kinetics of scheelite was investigated in 0.1 mol L⁻¹ H₂SO₄ at W : P molar ratio of 12 : 10. These soft conditions were chosen to avoid the precipitation of H₂WO₄ and maximize the impact of temperature. Figure 7a demonstrates the significant impact of temperature on the dissolution rate of scheelite determined during the kinetic step. The data presented in Table 2 show an increase by a factor 56 in the normalized dissolution rate when the temperature varied from 20°C to 80°C. For example, after 10 minutes of reaction, approximately 42 wt. % of CaWO₄ was dissolved at 80°C, compared to only 0.7 wt. % at 20°C. These results confirm the accelerating effect of temperature on scheelite dissolution kinetics.

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In order to quantify the influence of temperature on the dissolution reaction, the dissolution rate values were plotted in an Arrhenius diagram (Figure 7b). The slope of the linear regression was used to determine the apparent activation energy of the dissolution reaction, E_a = 58.3 ± 3.1 kJ mol⁻¹. The value obtained for the apparent activation energy is characteristic of a dissolution process controlled by an interfacial reaction (i.e. chemically controlled). Indeed, typical activation energy values for this type of reaction are between 40 and 140 kJ mol⁻¹ (Langmuir, 1997). The apparent activation energy obtained in this work is consistent with the value of E_a = 63.8 kJ mol⁻¹ obtained by Li and Zhao (2016) when dissolving a scheelite concentrate in a mixture of 2.5 mol L⁻¹ of sulfuric and 3.5 mol L⁻¹ phosphoric acid. The similarity between the two E_a values obtained in this work and by Li and Zhao (2016) shows that the impact of temperature is virtually independent of the acidity of the dissolution medium. A comparison with the work of Zhang et al. (2015) in HNO₃ - HPO₄ mixture is also interesting. Indeed, these authors obtained an activation energy of 52.9 kJ mol⁻¹ in 3 mol L⁻¹ HNO₃ and 0.5 mol L⁻¹ H₃PO₄, showing a lower temperature dependence of the scheelite dissolution rate than in the H₂SO₄ - H₃PO₄ mixture. Additionally, Liu et al. (2016) determined an apparent activation energy of reaction of 60.7 kJ mol⁻¹ in HCl - H₃PO₄ mixture, which shows a comparable impact of the temperature on the dissolution rate of scheelite in HCl as far as the W : P stoichiometric ratio is lower than 12 : 1. These observations indicate that the dissolution mechanism is similar in the 3 acidic mixtures. Whatever the acid used, in the presence of an appropriate phosphate concentration, the dissolution mechanism is controlled by a chemical reaction at the solid/solution interface rather than by a diffusion step caused by the formation of a passivation layer at the surface of scheelite grains.

520
<figure><img src="image_9.png" /><figcaption>Figure 7: a) Evolution of N<sub>L</sub>(W) during scheelite dissolution in 0.1 mol L<sup>-1</sup> H<sub>2</sub>SO<sub>4</sub> and 0.03 mol L<sup>-1</sup> H<sub>3</sub>PO<sub>4</sub> (R = 12/10) at various temperatures. ▲ 80°C (test d26); ● 60°C (test d23); ■ 40°C (test d25); ▼ 20°C (test d24); b) Variation of ln R<sub>L</sub>(i) with 1/T and determination of the apparent activation energy of the dissolution. ■ ln R<sub>L</sub>(W); ▲ ln R<sub>L</sub>(Ca); — linear fit.</figcaption></figure>

**3.6 Evidence of POT formation during leaching experiments**

The SAXS spectra were used in this work to investigate the formation of H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>, and a method was
developed to quantify it in the scheelite leaching solution (the complete description of the method is
presented in Supporting Information, section II). Figure 8 presents a typical SAXS spectra obtained after
scheelite dissolution in H₂SO₄ 0.1 mol L⁻¹ and H₃PO₄ 0.03 mol L⁻¹ at 60°C (test **d16**) and in H₂SO₄ 2.5 mol L⁻¹ and H₃PO₄ 0.003 mol L⁻¹ at 60°C (test **d17**) over a range of scattering vector, *q* from 0.03 to 3 Å⁻¹. The scheelite leachates were all analyzed by SAXS and are presented in Supporting Information, Figure SI_7 to Figure SI_12.

<figure><img src="image_10.png" /><figcaption>Figure 8: SAXS spectra of ■ water; ■ supernatant of sample **d16** corresponding to sheelite dissolution in H₂SO₄ 0.1 mol L⁻¹ and H₃PO₄ 0.03 mol L⁻¹ at 60°C; ■ supernatant of sample **d16** subtracted from the solvent (H₂SO₄ 0.1 mol L⁻¹ and H₃PO₄ 0.03 mol L⁻¹) signal; fit with a spherical model; ■ supernatant of the sample **d17** corresponding to sheelite dissolution in H₂SO₄ 2.5 mol L⁻¹ and H₃PO₄ 0.003 mol L⁻¹ at 60°C.</figcaption></figure>

541

Two ranges of *q* are investigated: a wide-angle X-ray scattering (WAXS) region located at *q* > 0.7 Å⁻¹ presenting scattering interference signals due to inter- and intra-atomic correlations, and a small-angle X-ray scattering (SAXS) region at lower *q* (< 0.7 Å⁻¹) presenting scattering interference signals due to inter- and intra-colloidal correlations. For instance, in the spectrum of water (black spectrum; i.e., first from bottom in Figure 8), a broad peak centered at 2 Å⁻¹ in the WAXS region corresponds primarily to strong pair distance correlations between oxygen atoms of neighboring water molecules. In the pink spectrum (i.e., second from the bottom of Figure 8) of the supernatant solution for the test **d17**, another broad and weaker peak, centered at 0.9 Å⁻¹, is detectable when the sulfate concentration exceeds 0.7 mol L⁻¹ (see Supporting Information, Figure SI_13). It can be attributed to the pair distance correlations between sulfate ions (Kinnibrugh and Fister, 2022). The SAXS region provides information on intra- and inter-particle correlations at length scales ranging from ten to more than hundreds of angstroms, which is typical of the colloidal domain, such as for example of nanoparticles and macromolecules. An
increase in the scattered intensity for $q < 0.7$ Å⁻¹ can be observed in the green (test **d16**) spectrum, i.e., fourth from the bottom in Figure 8 (and to a lesser extent in the **d17** pink spectrum), indicating the formation of nanometric species under these acidic conditions. This scattering pattern is typical of molecular solution of H₃PW₁₂O₄₀ POT (blue spectrum, i.e., third from bottom in Figure 8), which can be well fitted with a spherical model of radius rₚOT = 4.75 Å, in agreement with the reported size of H₃PW₁₂O₄₀ (Naskar et al., 2015) and with SLDₚOT = 70.93×10⁻¹⁰ cm⁻² about 8 times higher than the SLDsolvent = 9.48×10⁻¹⁰ cm⁻² (Supporting Information, Section II).

For condition **d16** (green spectrum in Figure 8), the SAXS fitting procedure gives a volume fraction Φ of 7.12×10⁻⁴, corresponding to 2.6 mmol L⁻¹ H₃PW₁₂O₄₀. From the complete dissolution of 1 g of scheelite in **d16** experiment, a concentration of 34.8 mmol L⁻¹ of tungsten was expected, which correspond to 2.89 mM of POT, a value rather close to the one determined using SAXS data. Thus, under the **d16** conditions, the measured POT yield reached 92 %. In **d17** experiment (pink spectrum in Figure 8), a pronounced scattered intensity at $q = 1$ Å⁻¹ is observed, linked to the high sulfate ion concentration (2.5 mol L⁻¹), whereas the formation of H₃PW₁₂O₄₀, which is characterized by an increase in the SAXS signal at $q < 0.8$ Å appears limited. Indeed, the fit of the **d17** SAXS spectrum led to a POT yield of only 12 %. This limited formation of POTs is related to the formation of H₂WO₄(s) under highly acidic conditions, as observed experimentally by the appearance of a yellow second phase, noted in Figure 4.

The formation of H₃PW₁₂O₄₀ in **d16** leaching solution was verified by ³¹P NMR spectroscopy (Figure 9). The observed chemical shift of -15.3 ppm is a key piece of evidence for identifying the α-PW₁₂O₄₀³⁻ which aligns with values previously reported in the scientific literature (Maksimovskaya and Maksimov, 2019). This confirmation of the molecular structure underscores the effectiveness of the SAXS method for detecting and analyzing H₃PW₁₂O₄₀ species. Furthermore, the absence of characteristic peaks associated with heteropolyacids or H₂WO₄(aq) formation supports the conclusion that the reaction conditions were optimized to selectively produce the desired compounds.

579
<figure><img src="image_11.png" /><figcaption>Figure 9: <sup>31</sup>P NMR spectra of H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> from the scheelite dissolution solution in 0.1 mol L<sup>-1</sup> H<sub>2</sub>SO<sub>4</sub> & 0.03 mol L<sup>-1</sup> H<sub>3</sub>PO<sub>4</sub> (test d16).</figcaption></figure>

# 4 Summary and Conclusions

This work involved a multiparametric investigation of scheelite dissolution kinetics in different combinations of sulfuric and phosphoric acids at different temperatures. For the first time, this study was linked to research on W speciation in leaching solutions. The concentration of Keggin-type POT (H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>), whose production is the primary factor driving the effective dissolution of scheelite in the synergistic mixture of phosphoric and sulfuric acids, was measured using SAXS methodology.

The results obtained allowed to determine the stability domain of H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> in a sulfuric acid medium. As far as the H<sub>3</sub>PO<sub>4</sub> concentration is sufficient to reach the stoichiometric molar ratio W : P = 12 : 1, H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> dominates the W speciation due to its high formation constants in solutions up to 1 mol L<sup>-1</sup> H<sub>2</sub>SO<sub>4</sub>. For higher H<sub>2</sub>SO<sub>4</sub> concentrations the formation of H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> is hindered mostly by the formation of tungstic acid. The H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> stability domain can be extended towards slightly more acidic conditions (i.e., 1.5 mol L<sup>-1</sup> H<sub>2</sub>SO<sub>4</sub>) for W : P = 12 : 10, and up to 2.5 mol L<sup>-1</sup> H<sub>2</sub>SO<sub>4</sub> for W : P = 12 : 100. However, the comparison with speciation calculations using thermodynamic data reported in the literature was not fully conclusive. Indeed, the standard formation constant of H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> estimated from literature data predicts the complete transformation of W in H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> over the full range of experimental conditions investigated in this work. This analysis indicates that until a comprehensive collection of trustworthy thermodynamic data is available, a straightforward and accessible method for determining the POT concentration in the leaching solution is required. Findings from this work show that SAXS is a convenient technique to address this requirement.
These results also show that either increasing the W : P stoichiometric ratio or the H₂SO₄ concentration does not lead to a significant increase in the dissolution rate of scheelite. Indeed, the highest dissolution rate was R_L(W) = 0.200 ± 0.003 g m⁻² min⁻¹ and it was obtained in a mixture of 1.5 mol L⁻¹ H₂SO₄ and 0.03 mol L⁻¹ H₃PO₄ (W : P = 12 : 10). The precipitation of tungstic acid, which lowers the overall rate of scheelite dissolution, limits the experimental domain for the pure kinetic regime at high acidity. Scheelite dissolved more slowly but remained efficient in the presence of phosphoric acid at H₂SO₄ concentrations as low as 0.1 M. The impact of saturation phenomena on the dissolution kinetics was prevented, even at low acidity levels. Actually, anhydrite formation is either minimal or happens after the full dissolution of scheelite in this range of acidic conditions and at 60°C. This result confirms that sulfuric acid is a suitable choice to perform scheelite acid leaching, with an efficiency comparable to nitric or hydrochloric acids, as long as phosphate ions are introduced in an appropriate proportion to control W speciation.

The resilience of POT formation under these acidic conditions indicates a robust process capable of withstanding low acid concentration fluctuations and remaining suitable for mild leaching conditions, which is important in the context of sustainable ore processing development. In 2022, Lei *et al.* (2022) showed that H₂SO₄ - H₃PO₄ synergistic decomposition method had economic and environmental advantages compared to the traditional sodium hydroxide or sodium carbonate decomposition method. Especially the energy consumption and the waste management costs are reduced. This work confirms this trend, as complete scheelite dissolution can occur at temperatures as low as 60°C, and the only by-product is gypsum, which is harmless and can be reused as a building material. Moreover, the precipitation of gypsum can be controlled after complete scheelite dissolution by simply lowering the temperature of the solution. This study also shows that the amount of phosphoric acid, the most expensive reagent used in this method (around 10 times more expensive than sulfuric acid), can be used at low concentrations. Indeed, a large excess over the synergistic ratio W : P = 12 : 1 did not significantly improve the dissolution kinetics and thus the process intensification. Nevertheless, the operating cost and environmental impact of the wet decomposition step is low compared to tungsten ore extraction and mineralogical pre-treatments (Ma *et al.*, 2017). In order to ascertain the real advantages of synergistic dissolution in a complete tungsten extraction and purification process, it will be necessary to test its effectiveness, particularly on real ores of varying quality, and to couple it with a selective W separation process. For this purpose, additional research will be investigated on the potential for separating and extracting POT from H₂SO₄ - H₃PO₄ acidic leachates by superchaotropic flotation, a technique previously explored with polyoxomolybdates (Skorzewska *et al.*, 2023) and also validated with H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ (Naskar *et al.*, 2015).

636

637 Acknowledgments
638 The authors would like to thank A. Jonchère, C. Pasquier, S. Maynadie, J. Lautru, G. Quintard and B.

639 Baus-Lagarde for their assistance with UV-Vis, SAXS, NMR, SEM, XRD and ICP measurements,

640 respectively. The authors are also grateful to M. Bonnemaison (E-Mines society) for providing fruitful

641 discussion on the process of W extraction from scheelite concentrates.

642

643 **Data statement**

644 Research data is available upon request. To request the data, contact the corresponding author of the

645 article.

646

647 **Funding sources**

648 This work was supported by the Université de Montpellier, Lab'Muse project PREDESTINE, ANR

649 ChaoPOM (ANR-22-CE92-0043) and the InnovTech project (PEPR Sous-sol, ANR-22-EXSS-0006).

650 The authors thank the International Research Project (IRP / CNRS program) entitled “NISHI” for the

651 financial support.

652

653 **Author contributions: CRediT**

654 V.L.: investigation ; methodology; formal analysis; writing – original draft

655 S. S.: conceptualization; validation; formal analysis; writing – original draft; writing – review & editing

656 O. D.: conceptualization ; supervision ; funding acquisition ; writing – review & editing

657 L. G.: conceptualization ; supervision ; validation; writing – review & editing

658 P.B.: conceptualization ; supervision ; formal analysis; validation; funding acquisition; writing – review

659 & editing

660

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