# High-pressure acid leaching of laterite ores: Effect of acid and solid content on Ni and Co yield under non-isothermal and isothermal conditions

Okechukwu Vincent Dickson, Thomas Deleau, Fabienne Espitalier, Christophe Coquelet, Julien Lombart, Philippe Accart

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Okechukwu Vincent Dickson, Thomas Deleau, Fabienne Espitalier, Christophe Coquelet, Julien Lombart, et al.. High-pressure acid leaching of laterite ores: Effect of acid and solid content on Ni and Co yield under non-isothermal and isothermal conditions. Hydrometallurgy, 2025, 238, pp.106576. 10.1016/j.hydromet.2025.106576. hal-05279563

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# High-pressure acid leaching of laterite ores: Effect of acid and solid content on Ni and Co yield under non-isothermal and isothermal conditions

Okechukwu Vincent Dickson<sup>a,b,*</sup>, Thomas Deleau<sup>b</sup>, Fabienne Espitalier<sup>b</sup>, Christophe Coquelet<sup>b,c</sup>, Julien Lombart<sup>a</sup>, Philippe Accart<sup>b</sup>

## ARTICLE INFO

### Keywords:

Hydrometallurgy

High-pressure acid leaching

Laterite ore processing

Nickel extraction

Solution purity

Optimisation

## ABSTRACT

The increasing demand for nickel (Ni) in electric vehicle batteries necessitates efficient extraction methods from laterite ores. High-Pressure Acid Leaching (HPAL) is a prominent hydrometallurgical technique known for its rapid kinetics and high (>90 %) metal dissolution. This study examines the influence of acid-to-ore (A/O) ratios (0.25, 0.35, and 0.45) and slurry solid contents (22, 26, and 30 wt.%) on Ni yield and selectivity under both non-isothermal (100 °C, 1.2 bar; 150 °C, 4.7 bar; 200 °C, 18.9 bar; 265 °C, 50.5 bar) and isothermal (265 °C) conditions.

Under non-isothermal conditions, Ni leaching efficiency improved with temperature, achieving a maximum yield of 94.67 ± 0.02 wt.% at 265 °C, compared to 50.86 ± 0.02 wt.% at 200 °C, using an A/O ratio of 0.45 and 26 wt.% solids over 60 min. Isothermal experiments at 265 °C revealed that higher A/O ratios enhanced Ni yield, reaching 94.45 ± 0.01 wt.% at 60 min with 26 wt.% solids and an A/O of 0.45. However, this increased yield was accompanied by higher dissolution of Fe (1.07 ± 0.12 wt.%) and Al (38.84 ± 0.07 wt.%), reducing selectivity to 0.73 ± 0.01. Conversely, an A/O ratio of 0.25 at 26 wt.% solids achieved a higher selectivity of 3.83 ± 0.05 but with a lower Ni yield of 79.14 ± 0.03 wt.%. Lower slurry solid content further improved Ni leaching; at 22 wt.% solids and an A/O of 0.45, Ni yield reached 95.87 ± 0.01 wt.% with a selectivity of 0.89 ± 0.02 at 60 min.

Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses of leached residues indicated that high temperatures facilitate the transformation of primary minerals like goethite and gibbsite into hematite and hydronium alunite. This transformation is more pronounced at higher A/O ratios, affecting the mineralogical composition of the post-leach solid residue.

## 1. Introduction

With growing emphasis on carbon neutrality, the electric vehicle (EV) market, a significant sector for clean energy adoption, is witnessing substantial growth in recent years. Notably, the global demand for electric cars has tripled between 2020 and 2023 (International Energy Agency, (IEA), 2024), significantly expanding the battery market and driving the need for increased mining of essential resources, as recycling alone cannot meet this growing demand (Zhang et al., 2025). The critical metals are essential for forming the cathode electrode, which significantly impacts the performance, durability, cost, and driving range of electric vehicles. Consequently, battery production depends on

the number of mines available, the concentration of the critical metals, the process deployed to recover these metals, and the dissolution of these metals with a view to their separation and purification. Of course, the most important point concerns the leaching of several key metals, including lithium, nickel, cobalt, and manganese. Among them, nickel (Ni) has emerged as a crucial element in this field (International Energy Agency, (IEA), 2024).

The mineralogy of nickel laterite deposits (even from the same region) can vary in nature (McDonald and Whittington, 2008). Nickel and cobalt do not exist in their pure metallic forms in nature but are instead found within various mineral matrices. They are commonly associated with iron oxides and silicate minerals in laterite ores, which form

<footnote><sup>a</sup> Prony Resources New Caledonia SAS, Usine du Grand Sud, Route de Kwa Neie-Prony, 98834 Yaté, NC, France</footnote>

<footnote><sup>b</sup> IMT Mines Albi, UMR CNRS 5302, RAPSODEE Centre, 81000 Albi, France</footnote>

<footnote><sup>c</sup> Mines Paris, PSL University, Centre for Energy Environment Processes (CEEP), 77300 Fontainebleau, France</footnote>

<footnote>* Corresponding author at: Prony Resources New Caledonia SAS, Usine du Grand Sud, Route de Kwa Neie-Prony, 98834 Yaté, NC, France.
E-mail address: Okechukwu_vincent.dickson@mines-albi.fr (O.V. Dickson).</footnote>
through the intense weathering of ultramafic rocks in tropical regions (Elias, 2002). These laterite deposits serve as major sources of nickel and cobalt, often containing significant concentrations of these metals (Butt and Cluzel, 2013; De Boissieu et al., 2018). The dissolution of these ores is essential to alter the mineral composition and extract Ni and Co.

Laterite deposits are extensively found in tropical regions worldwide, including New Caledonia, Australia, Cuba, Brazil, Colombia, Greece, the Philippines, Indonesia, and India (Butt and Cluzel, 2013). These ores typically contain about 1–1.5 wt.% nickel and 40–60 wt.% iron, primarily as goethite (Wells et al., 2009; Dublet et al., 2012). Notably, laterite ores possess a high moisture content, with up to 40 % comprising free and combined water (Wang, 2016). The specific chemical composition of laterite ores varies based on their geographic location and formation conditions (Whittington and Muir, 2000). Common components include (a) iron oxides, such as hematite (Fe₂O₃) and goethite (FeOOH) (Anand and Gilkes, 1987), as well as (b) aluminium oxides like gibbsite (Al(OH)₃) and boehmite (AlOOH) (Tardy et al., 1993). High-quality lateritic minerals are processed industrially through pyrometallurgical and hydrometallurgical operations, with the choice of technique depending on the ore's chemical and mineralogical characteristics (Mamyrbayeva et al., 2024). Hydrometallurgy is often preferred over pyrometallurgy for nickel leaching due to its lower energy consumption, reduced environmental impact and recovery of high-purity metals (Davis et al., 2023).

The hydrometallurgical leaching of nickel from lateritic ores is a critical process in meeting the global demand for these metals (Dalvi et al., 2004; McDonald and Whittington, 2008). Atmospheric pressure Acid Leaching (AAL) and High-Pressure Acid Leaching (HPAL) in continuous autoclaves have been extensively studied by researchers (McDonald and Whittington, 2008) for nickel leaching from lateritic ores. The main advantage of high-pressure (due to high operating temperature) hydrometallurgy over atmospheric-pressure hydrometallurgy lies in its fast kinetics, enhanced solubility of reagent gases, more stable residues and enhanced Ni selectivity (Stopić and Friedrich, 2011). However, the HPAL process used for these ores carries a significant environmental burden due to its high energy consumption, acid use, and greenhouse gas emissions (Faris et al., 2023; Manzoor et al., 2025). Therefore, alternative and more sustainable hydrometallurgical approaches with green chemicals are being actively explored to reduce the carbon footprint and environmental impact of nickel and cobalt production (Ni and Xiao, 2025). Nevertheless, until a large-scale, greener alternative becomes industrially viable, companies like Eramet and Prony Resources continue to operate HPAL facilities, highlighting the need to maximise nickel and cobalt recovery within existing processes to meet global demand (Lebrouhi et al., 2021).

The efficiency of Ni leaching in HPAL is strongly influenced by several key parameters, particularly the acid-to-ore (A/O) ratio and slurry solid concentration (Whittington and Muir, 2000). These factors

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** a: Schematic representation of the industrial-scale autoclave leaching process (Prony NC). HTF, MTF and LTF refer to High (200 °C), Medium (150 °C) and Low (92 °C) Flash Temperatures. (Adapted from High-Pressure Acid Leaching of Nickel Laterite Ores of Goro plant, New Caledonia: Chemistry and MOC Challenges IAPWS workshop presentation by Mohamed Buarzaiga, May 12, 2009).</figcaption></figure>

b: Pictorial representation of the batch experimental set-up and the autoclave tank used for the experiment (designed by Top Industrie® France) (Dickson et al., 2025).
play a crucial role in determining the extent of Ni dissolution and the selectivity of the leach solution, as higher acid concentrations can enhance Ni recovery but may also lead to increased dissolution of impurities such as Fe and Al (Chou et al., 1977; Stopić et al., 2004; Stopić and Friedrich, 2011). Studies have shown that optimising these parameters is essential to maximising Ni yield while maintaining solution purity (Krause et al., 1997; Whittington and Muir, 2000). Additionally, the transformation of primary minerals such as goethite and gibbsite into secondary phases like hematite and alunite can significantly impact the leaching process (Georgiou and Papangelakis, 1998). Understanding these effects is critical for improving HPAL efficiency and ensuring effective Ni recovery from laterite ores. Fig. 1a presents a schematic of the high-pressure acid leaching (HPAL) process for nickel laterite ores using an industrial (Prony Resources) autoclave system. The ore slurry, initially at ambient temperature, is fed into the autoclave and progressively heated through a series of flash tanks, LTF (~92 °C), MTF (~150 °C), and HTF (~200 °C). It then passes through a hot steam zone, raising the temperature further to the target range of 250–275 °C. At this elevated temperature, sulphuric acid is introduced to initiate the leaching process. The resulting post-leach slurry is subsequently cooled by passing back through the flash tanks, reducing its temperature to approximately 92 °C before proceeding to the neutralisation stage. Recycled hot water from the flash stages is returned to the system to enhance thermal efficiency.

Particle size is a critical factor influencing the leaching kinetics and nickel yield from laterite ores (Watling et al., 2011). Finer particles enhance the dissolution of nickel-bearing minerals (e.g., goethite, serpentine, garnierite), accelerating the leaching process (Feng et al., 2023). However, the increased surface area of finer ores also promotes the early co-dissolution of impurities (Fe, Al, Cr), which reduces nickel selectivity. Thus, while ore grinding may not significantly affect the overall extent of nickel extraction, it enables faster kinetics over shorter leaching times (Whittington and Muir, 2000).

Experimental studies support this observation. Büyükaçınci and Topkaya (2009) investigated limonitic and nontronitic laterite ores from the Gördes nickel deposit (Western Turkey) under agitation leaching at 95 °C for 24 h, considering variables such as time, acid concentration, temperature, and particle size. They reported that particle size had only a minor effect on overall metal extraction, with Ni yield reaching 96 % (nontronite) and 93.1 % (limonite) for –0.074 mm particles, compared to 95 % and 88.6 %, respectively, for –1.168 mm particles. The nickel-to-impurity ratio, however, was strongly affected. Finer particles (–0.074 mm) resulted in higher Fe extraction (81.8 % for nontronite and 81.4 % for limonite) compared to 78.5 % and 73.7 %, respectively, for –1.168 mm particles, due to increased surface area and additional nucleation sites for Fe and Al hydrolysis. Kaya and Topkaya (2011) similarly reported that for limonitic nickel laterite from the Gördes region, finer particles (≤ 38 µm) yielded higher Ni and Co extractions at 255 °C, 60 min, and acid/ore ratios of 0.275–0.35. Further evidence by Göveli (2006) using particle sizes of –1000 µm, –500 µm, –106 µm, and –38 µm (3 M HCl, 100 °C, 3 h) showed that Ni extraction peaked at 90.51 % for –38 µm particles, compared to 87.26 % for –1000 µm particles, owing to improved surface area and particle liberation.

Optimising leaching parameters is necessary to enhance metal recovery (%) and ensure the economic viability of leaching processes. Key parameters influencing leaching efficiency at the best identified leaching temperature (250 °C ≤ T ≤ 275 °C), fine particle size and sufficient agitation include the acid-to-ore ratio and the solids percentage in the slurry (Chou et al., 1977; Whittington and Muir, 2000).

The acid-to-ore ratio determines the availability of protons (H⁺) necessary for dissolving metal oxides, directly affecting the dissolution of Ni and Co. Similarly, the solids percentage in the slurry influences the mass transfer rates and the viscosity of the mixture, thereby impacting the leaching dynamics (MacCarthy et al., 2016). Previous studies have indicated that variations in acid concentration and slurry density can significantly affect the leaching outcomes (Georgiou and Papangelakis,

1998; Stopić et al., 2004), either increasing or decreasing Ni yield and the purity of the post-leach solution (leachate). Experiments conducted by Çoban and Baş (2024) with Ni-laterite ore from the Gördes district, Manisa, Türkiye, showed that nickel yield increased from approximately 70 % to over 94 % as the sulphuric acid addition rose from 250 to 450 kg per tonne of ore under typical HPAL conditions (isothermal 255 °C, 38 wt.% solids), while cobalt extraction remained consistently high (~93–96 %) due to its association with Mn-bearing phases that dissolve readily even at low acid levels. Extending the leaching duration from 30 to 90 min further enhanced Ni dissolution by promoting the goethite-to-hematite transformation, which liberates nickel from Fe-bearing minerals. However, excessive acid dosing and prolonged leaching can lead to higher reagent consumption, Fe and Cr co-dissolution, and accelerated autoclave scale formation, highlighting the importance of optimising both acid usage and residence time to achieve high Ni yield with minimal operational drawbacks (Çoban and Baş, 2024).

In laterite ore, nickel is commonly associated with iron-bearing minerals such as goethite (FeOOH), which require aggressive leaching conditions for effective dissolution. On the other hand, cobalt is often found in close association with manganese minerals such as asbolane (CoMn₂O₄(OH)₂·0.6H₂O), which is highly soluble in acidic solutions (Butt and Cluzel, 2013). The oxides of Ni and Co embedded within laterite ore matrix can be liberated by acid leaching (Whittington and Muir, 2000), primarily through Reactions (I) and (II) (Çoban and Baş, 2024).

$$NiO (s) + H_2SO_4 \rightleftharpoons NiSO_4 (aq) + H_2O \quad (I)$$

$$CoO (s) + H_2SO_4 \rightleftharpoons CoSO_4 (aq) + H_2O \quad (II)$$

Under high-pressure and high-temperature leaching conditions (>200 °C) in the autoclave, goethite undergoes hydrolysis, initially forming ferric sulphate in Reaction (III). This unstable intermediate quickly transforms into stable hematite (Fe₂O₃) as seen in Reaction (IV), a process that regenerates some of the acid consumed during leaching (Rubisov and Papangelakis, 2000; Çoban and Baş, 2024).

In some cases, soluble iron species (Fe₂(SO₄)₃) formed in Reaction (III) can undergo hydrolysis to yield basic ferric sulphates (FeOHSO₄) (Reaction (V)), which initially precipitate as unstable intermediates before rapidly transforming into stable hematite (Fe₂O₃) as seen in reaction (VI) (Rubisov and Papangelakis, 2000). Whether hematite forms directly, as implied by Reaction (IV), or via FeOHSO₄ as an intermediate during laterite ore leaching in HPAL remains uncertain (Rubisov and Papangelakis, 2000). According to Chou et al. (1977), high acidity promotes the precipitation of FeOHSO₄ while inhibiting its conversion to hematite. Ventruti et al. (2020) further reported that FeOHSO₄ partially decomposes to α-Fe₂(SO₄)₃ when heated at high temperatures ≥400 °C.

$$2FeOOH (s) + 3H_2SO_4 \rightleftharpoons Fe_2(SO_4)_3 (aq) + 4H_2O \quad (III)$$

$$Fe_2(SO_4)_3 (aq) + 3H_2O \rightleftharpoons Fe_2O_3 (s) + 3H_2SO_4 \quad (IV)$$

$$Fe_2(SO_4)_3 (aq) + 2H_2O \rightleftharpoons 2Fe(OH)SO_4 (s) + H_2SO_4 \quad (V)$$

$$2Fe(OH)SO_4 (s) + H_2O \rightleftharpoons Fe_2O_3 (s) + 2H_2SO_4 \quad (VI)$$

Similarly, aluminium dissolution follows a distinct path. The dominant phase, gibbsite (Al(OH)₃), transforms into boehmite (AlOOH) during preheating (Reaction (VII)). Subsequently, boehmite dissolves in sulphuric acid to form aluminium sulphate (Reaction (VIII)).

$$Al(OH)_3 (s) \rightleftharpoons AlOOH (s) + H_2O \quad (VII)$$

$$2AlOOH (s) + 3H_2SO_4 \rightleftharpoons Al_2(SO_4)_3 (aq) + 4H_2O \quad (VIII)$$

When the concentration of aluminium exceeds its solubility limit, hydrolysis of aluminium sulphate leads to the formation of hydronium alunite ((H₃O)Al₃(SO₄)₂(OH)₆) as seen in Reaction (IX) (Rubisov and
Papangelakis, 2000; Liu and Papangelakis, 2005).

$$3\text{Al}_2(\text{SO}_4)_3(\text{aq}) + 14\text{H}_2\text{O} \rightleftharpoons 2(\text{H}_3\text{O})\text{Al}_3(\text{SO}_4)_2(\text{OH})_6(\text{s}) + 5\text{H}_2\text{SO}_4 \quad (\text{IX})$$

Unlike the precipitation of iron, aluminium precipitation results in net acid consumption, which explains the need for high initial acid addition. Table S1 (Supplementary Material) presents the thermodynamic properties Gibbs free-energy (ΔG°), enthalpy (ΔH°), entropy (ΔS°), heat capacity (Cp), of Ni, Co, Al, and Fe species as well as the calculated equilibrium constants for their dissociation reactions in the autoclave at the investigated temperatures (100 °C, 150 °C, 200 °C, and 265 °C), determined using the Density model (Dickson et al., 2023).

In HPAL, temperature strongly influences the dissolution rate of nickel-bearing phases and the precipitation of iron and aluminium. Both non-isothermal (gradual heating) and isothermal (constant temperature) stages influence leaching performance; comparing these conditions provides insights into leaching kinetics, impurity dissolution, and overall nickel yield, offering a basis for optimising heating strategies and energy consumption (Pandey et al., 2023; Bana et al., 2025). However, comprehensive studies that systematically investigate the combined effects of acid-to-ore ratio and slurry solids percentage on leaching efficiency, product purity, free acid concentration, and pH levels are limited. Understanding the interplay between these parameters is needed for optimising leaching rates and achieving the desired product purity.

This study aims to fill this gap by conducting a series of leaching experiments with varying acid-to-ore ratios and slurry solids percentages at isothermal and non-isothermal temperatures. The objectives are to compare the leaching rates of Ni and Co, assess and compare the purity (selectivity) of the leachates obtained at different solid concentrations and A/O ratios, and evaluate the characteristics of the products after filtration (filtrate and residues). Additionally, the study aims to measure free acid concentrations and pH levels of the leached solution and analyse the solid samples (initial ore and leach residues) to determine their composition, morphology and phase structure. The findings are expected to help optimise leaching parameters to improve nickel yield and enhance process efficiency. The term Selectivity ([Ni, Co]/[Al + Fe]) used in this paper refers to the ability to preferentially extract nickel and cobalt over aluminium and iron in the HPAL leaching of laterite ore.

## 2. Experimental

### 2.1. Material and sample preparation

The laterite ore used for this experiment is a blend of limonite and saprolite ore collected from Prony Resources, New Caledonia, which is a French overseas territory in the southwest Pacific Ocean located adjacent to Australia (2020 km from Sydney) and New Zealand (1980 km from Auckland). The yellowish-brown laterite ore is mud-like when mixed with water and exhibits thixotropic flow behaviour. The elemental composition of the laterite ore is given in Table 1 in Section 3.

The dry powder of the laterite ore sample used for the experiments was passed through a mesh size of ≤160 µm to remove debris, sieve out the coarse fraction and ensure uniformity in the size fractions used for all the experimental trials. Initial slurry (ore + water) with varying solid (22, 26 and 30 wt.%) ore content was prepared, the total weight of the slurry and sulphuric acid used for the leaching experiments was at constant mass (2149.17 ± 0.1 g), with variations in the mass of acid, ore

and water depending on the acid-to-ore ratio for the desired experiment.

### 2.2. Equipment description and set leaching parameters

For the leaching experiment, an HPAL experimental set-up designed and commissioned by Top Industrie® on specifications provided by Prony Resources NC, France, as shown in Fig. 1b was used. The equipment consists of a 0.5 L acid injection tank, which allows the acid to be injected into the preheated slurry at a targeted temperature and a 2.8 L flat-bottom cylindrical autoclave tank made of titanium (grade 4) shell, which is thermostatically controlled. The autoclave tank is equipped with a baffle to improve the circulation flow pattern of the suspension, and a magnetically driven Mixell-TT axial flow-type impeller (diameter = 0.7 m) with 2 propellers was used to keep the solids in suspension.

Due to the limitations of the Zwietering correlation, which assumes a lower solid concentration than used in this study, the modified Zwietering equation proposed by Ayranci and Kresta (2014) was applied to determine the minimum stirring speed for the slurry. The minimum agitation speed required to suspend the slurry with the given impeller geometry was estimated to be 654 ± 65 rpm. This estimation considered the densest slurry condition with 30 wt.% solids (slurry density: 1.27 ± 0.01 g/cm³, ore density: 3.39 g/cm³, water density: 1.00 g/cm³) and a particle size of 300 µm for a single impeller. This speed corresponds to a power dissipation of 7.4 W/kg of solution.

During the experiment, the leached sample is collected into a 20 mL steel-made sampling cell where the temperature and pressure are quickly cooled by water circulating on the jacket before the sample is collected into a beaker.

### 2.3. Effect of agitation speed on slurry homogenisation in the autoclave

A slurry containing 26 wt.% solids was heated to 50 °C and sampled over 85 min without acid addition. Samples were collected at 5, 15, 25, 35, 45, 55, 65, 75, and 85 min, starting 5 min after reaching the target temperature. The agitation speed was varied at each interval: 800, 700, 634, 600, 585, 585, 600, 634, 700, and 800 rpm, respectively. Approximately 1.5 g of the stirred slurry collected at different intervals was analysed using an infrared desiccator to ensure consistent solid content with the initial slurry. Additionally, 1 mL of the sampled slurry was taken for laser granulometry analysis using a Malvern Panalytical Mastersizer (MS3000) to assess the impact of agitation speed on particle size distribution. The MS3000 uses laser diffraction to measure particle size distribution from 0.1 to 3500 µm by analysing the scattered light as a laser passes through a dispersed particle sample. To determine the particle size of the slurry sample, a drop of the sample was injected into the granulometry chamber at 25 °C, with the refractive index of the particles and dispersion medium set accordingly. Measurements were conducted in wet dispersion mode using an MV hydro cell with water as the dispersant, at a stirring speed of 2000 rpm to prevent sedimentation. The particle refractive index was set to 1.52 and the absorption index to 0.1, with the water refractive index set to 1.33 and the analysed results obtained were recorded.

### 2.4. Leaching and characterisation methodology

Slurry samples were prepared by blending laterite ore with tap water to achieve a solid content of 22, 26 and 30 wt.%. For experiments in isothermal conditions, the slurry mixture was introduced into the

<table><caption>**Table 1**

Elemental concentration of the feed laterite ore sample.</caption><thead><tr><th>Elements</th><th>Ni</th><th>Co</th><th>Fe</th><th>Si</th><th>Al</th><th>Mg</th><th>Mn</th><th>Cr</th><th>Total</th></tr></thead><tbody><tr><td>C<sub>i</sub> (wt.%)</td><td>1.35</td><td>0.13</td><td>41.38</td><td>4.04</td><td>3.11</td><td>2.19</td><td>0.77</td><td>2.11</td><td>44.92</td></tr><tr><td>u(C<sub>i</sub>)</td><td>±0.09</td><td>±0.01</td><td>±1.48</td><td>±0.16</td><td>±0.25</td><td>±0.20</td><td>±0.01</td><td>±0.13</td><td>±2.28</td></tr></tbody></table>

C<sub>i</sub> is the initial elemental concentration in the laterite ore, while u(C<sub>i</sub>) is the error on the measured concentration in wt.% (from ICP-AES analysis).
autoclave reactor, ensuring the lid was securely fastened to prevent pressure leaks during the high-pressure leaching process. The system was then heated to the target temperature of 265 °C (50.5 bar) while being constantly stirred at 800 rpm. Upon reaching this temperature, sulphuric acid (98 wt.% purity at room temperature) was injected from the acid tank to initiate the leaching reaction. The reaction was monitored by collecting samples at predetermined intervals, with “0 min” defined as the point when the reactor, without acid addition, stabilised at the targeted temperature. The first sample post-acid addition was taken at 5 min, up to 180 min at different intervals.

To compare elemental leaching under non-isothermal and isothermal conditions, experiments were conducted under non-isothermal conditions by mixing sulphuric acid with the slurry at room temperature and progressively heating the autoclave to reach 100 °C, 150 °C, 200 °C, and 265 °C. These temperatures correspond to stabilised pressures of approximately 1.2, 4.7, 18.9, and 50.5 bar, respectively. At each target temperature, samples were collected upon reaching the set point (marked as “0 min”) and subsequently at 30, 60, and 120 min. The heating duration between each temperature increment was around 35 min.

At each sampling interval, the leached sample (mixture of solid residue and liquor) was collected into a 20 mL sampling cell and cooled to a temperature between 30 and 60 °C for a few minutes before collecting for filtration. A 1 mL sample of the post-leach slurry was set aside for granulometric analysis by a Mastersizer 3000 Malvern Panalytical laser diffractometer, while the remainder underwent filtration.

For solid/liquid separation post-leaching, the slurry was filtered using a vacuum pump and Whatman grade-40 filter paper on a Buchner funnel. Residual solids in the filtrate were removed using a 0.45-µm syringe filter. The solid residues on the filter paper were rinsed with distilled water to eliminate any remaining leach solution and reduce acidity. These residues were then dried at 60 °C for 24 h and stored. The density of the leachate (filtrate) was measured at ambient temperature. Fig. S1 shows a schematic representation of the leaching process and the products.

Following the standard operating conditions for characterisation (Minh et al., 2020), the elemental concentration of the collected post-leach filtrates and mineralised solid residues was measured by Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES) analysis. The identification of the mineral composition of the solid residues at each time was analysed by the Malvern Panalytical Empyrean X-ray diffractometer. The sample was placed in a horizontal holder in Bragg-Brentano geometry, with the X-ray tube and detector moving symmetrically. Copper radiation with a Kα wavelength of 1.543 Å was used. The device features a high-resolution 1Der detector and a PreFIX dCore optical module. During measurement, the sample holder rotates to improve counting and reduce orientation effects. Diffractograms were collected at 45 kV, 40 mA, over a 2θ range of 8° to 90°, with a step size of 0.017° and an acquisition time of 190 s per point. The morphology and elemental mapping of the solid samples were analysed with Quattro S SEM (Thermofisher) equipped with EDAX X-ray microanalysis (EDS-SDD ELEMENT detector 30 mm²).

The residual (free) acid of the post-leach filtrate collected at various times was determined by titrating 1 mL of the filtrate sample prepared in a solution of calcium-CDTA (calcium cyclohexane-1,2-diamine tetraacetate) to chelate the metal ions. The prepared solution was titrated against 0.1 M NaOH titrant using the Tiamo (902 Titrando) automatic titrator.

The elemental yield, $X_i$ (wt.%), in the filtrate and solid residues was obtained from Eq. (1):

$$X_i = \left[ \frac{C_{\text{filtrate}} \times V_{\text{filtrate}}}{(C_{\text{filtrate}} \times V_{\text{filtrate}}) + (m_{\text{residue}} \times w_{\text{residue}})} \right] \times 100 \equiv \frac{m_f}{m_f + m_r} \times 100 \quad (1)$$

where $X_i$ is the elemental yield of i (wt.%) at time t; $C_{\text{filtrate}}$ is the con-

centration (mg/L) of the element in the post-leach filtrate at time t, measured by ICP-AES; $V_{\text{filtrate}}$ is the volume (L) of the post-leach filtrate collected at time t computed from density and mass measurement; $w_{\text{residue}}$ is the total mass of post-leach solid residue collected at time t; $m_{\text{residue}}$ is the mass fraction of the element in the post-leach solid residue at t; $m_f$ and $m_r$ represent the mass of the filtrate and solid residues at t.

### 3. Results and discussion

Table 1 presents the elemental composition of the initial laterite ore studied in this work. The primary constituent is iron (Fe), accounting for 41.38 wt.%. The total elemental concentration sums to 55.08 wt.%, suggesting that nearly 45 wt.% of the sample remains unaccounted for. This difference is attributed to oxygen, bound water, and other undetected components such as carbonates and trace elements (Ca, Pb, K, Na, Ti).

The sulphuric acid molality corresponding to various acid-to-ore (A/O) ratios for different solid contents in this study is presented in Table 2.

A ratio of A/O = 0.35 g H₂SO₄/g dry ore represents the theoretical (baseline) acid dosage, calculated using a confidential correlation developed by Prony Resources (New Caledonia) under non-disclosure. This correlation is derived from the elemental composition of the feed ore (ICP results, Table 1) and follows the general structure of classical acid-consumption models as reported by Whittington and Muir (2000) in Eq. (2), considering the acid-consuming species Mg, Al, Ni, and Mn typical of New Caledonian laterites:

$$\text{wt.}\%\text{H}_2\text{SO}_4 = 4 + 6(\text{Mg}) + 2.4(\text{Al} - 0.8) + 3(\text{Ni} + \text{Co} + \text{Mn}) + 4\text{CO}_2 \quad (2)$$

where wt.% H₂SO₄ is the mass of “pure” sulphuric acid required on a dry ore basis, and Mg, Al, Ni, Co, Mn and CO₂ are the elemental concentrations (wt.%) in the feed ore (Whittington and Muir, 2000).

For sensitivity testing, A/O = 0.25 (≈32 wt.% below theoretical) and A/O = 0.45 (≈22 wt.% above theoretical) represent under- and overdosage scenarios, respectively, relative to the optimised HPAL leaching conditions variability that could occur in the continuous leaching process in Prony Resources.

#### 3.1. Effect of stirring speed (agitation) on homogenisation of suspension in the autoclave

Table 3 presents the cumulative percentile size fractions and mean diameter of the particle size distribution of the slurry used in this study, indicating that 90 % D[90] of the particles were ≤ 42.7 ± 0.7 µm. The surface area mean diameter (D[3;2]) reflects that the majority of the finer particles corresponded to 3.2 ± 0.1 µm, while the volume-mean diameter (D[4;3]) indicates that larger particles corresponded to 14.7 ± 0.2 µm.

Following the geometry of the tank and density of the slurry used in this work, the calculated minimum speed for a single blade of the Mixell-TT impeller to maintain slurry suspension is 654 ± 65 rpm (for the densest slurry, 30 wt.% solid). Operating below this speed may result in particle settling and inadequate homogenisation. To study the effect of agitation speed on the homogenisation of slurry, the slurry containing 26 wt.% solid was used for the test with a two-blade Mixell-TT axial flow-type impeller as depicted in Fig. 1b. Fig. S2(a) demonstrates consistent solid content (≈ 26 wt.%) in sampled slurry across agitation speeds ranging from 585 to 800 rpm, indicating uniform solid distribution without sedimentation.

Fig. S2(b) illustrates the effect of agitation speed on particle size distribution and slurry homogenisation. The size distribution remains consistent across stirring speeds (586–800 rpm). This suggests that higher speeds have minimal effect once the minimum agitation speed (585 rpm for a slurry with 26 wt.% solid) is reached. To ensure optimal slurry homogenisation and minimise the risk of settled scale or formation of denser mineral, especially with higher solid content, 800 rpm was
<table><caption>**Table 2**
Experimental Conditions: Variation of solid content, A/O ratios, and H₂SO₄ molality.</caption><thead><tr><th>Solid (wt.%)</th><th>22</th><th>22</th><th>22</th><th>26</th><th>26</th><th>26</th><th>30</th><th>30</th><th>30</th></tr></thead><tbody><tr><td>A/O ratio (g H₂SO₄ per g dry ore)</td><td>0.25</td><td>0.35</td><td>0.45</td><td>0.25</td><td>0.35</td><td>0.45</td><td>0.25</td><td>0.35</td><td>0.45</td></tr><tr><td>H₂SO₄ (mol/kg H₂O)</td><td>0.72</td><td>1.01</td><td>1.29</td><td>0.9</td><td>1.25</td><td>1.61</td><td>1.09</td><td>1.53</td><td>1.97</td></tr></tbody></table>

<table><caption>**Table 3**
Average particle size distribution of homogenous slurry sample (size ≤160 µm) before leaching.</caption><thead><tr><th rowspan="2">Cumulative percentile</th><th rowspan="2">D[10]<sup>a</sup></th><th rowspan="2">D[50]</th><th rowspan="2">D[70]</th><th rowspan="2">D[90]</th><th rowspan="2">D[97]</th><th>Sauter Mean Diameter<sup>b</sup></th><th>De Brouckere Mean Diameter<sup>c</sup></th></tr><tr><th>D[3;2]</th><th>D[4;3]</th></tr></thead><tbody><tr><td>Size (µm)</td><td>1.2<br>± 0.1</td><td>6.1<br>± 0.1</td><td>13.1<br>± 0.1</td><td>42.7<br>± 0.7</td><td>70.5<br>± 3.0</td><td>3.2<br>± 0.1</td><td>14.7<br>± 0.2</td></tr></tbody></table>

a 10 % of the total volume (or weight) of the sample consists of particles smaller than this size.

b Ratio of total volume/total surface area of particles.

c Volume-weighted mean diameter.

maintained throughout the leaching experiments, with an initial increase to 1000 rpm during acid injection for the first five minutes to enhance mixing before sampling began at 5 min.

## 3.2. Effect of temperature on laterite ore leaching – Non-isothermal conditions

Temperature is a critical parameter influencing the dissolution kinetics of nickel (Ni) and cobalt (Co) from laterite ores. Fig. 2 presents the elemental concentrations (g/L) of Ni, Co, Fe, and Al in the leachate under non-isothermal conditions ranging from 100 to 265 °C. The experiments were conducted with a slurry solid content of 26 wt.% and an initial mass ratio of acid-to-ore (A/O) of 0.25, 0.35, and 0.45. The leaching process lasted a total of 570 min, during which the temperature was increased stepwise every 30 min. The time labelled as -30 min corresponds to the period when the acid and slurry were mixed at room temperature (25 °C) inside the autoclave. The times 30, 120, 270, and

420 min mark when the mixture reached the target temperatures of 100, 150, 200, and 265 °C, respectively. These points, considered as the initial sampling times (0 min) for each temperature stage, are indicated by straight arrows in Fig. 2.

Table 4 shows the elemental yields (wt.%) of Ni, Co, Fe, and Al at 30 and 120 min of leaching under these non-isothermal conditions. High temperatures promote the decomposition of goethite and serpentine matrices, thereby enhancing the release of Ni and Co. As shown in Fig. 2, the yields of Ni and Co increased progressively with both temperature and leaching time, particularly at higher initial A/O ratios (0.35 and 0.45), exceeding 90 wt.% at 265 °C. This behaviour aligns with well-established HPAL leaching trends reported by Çoban and Baş (2024), where experiments at 255 °C, with an A/O ratio of 0.35 and 38 wt.% solids, achieved Ni and Co extractions of 88–94 % within 60–90 min of leaching.

Conversely, the yields of Al and Fe decreased sharply after reaching 265 °C, as detailed in Fig. 2 and Table 4. Iron (Fe) initially dissolved but

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Elemental concentration (g/L) of Ni, Co, Fe and Al in post-leach filtrate under non-isothermal conditions (100, 150, 200 and 265 °C) for 26 wt.% solid and initial acid-to-ore ratios of 0.25, 0.35, and 0.45, with 30-min intervals (sampling time) for a total leaching time of 570 min.</figcaption></figure>
<caption>**Table 4**</caption>

Elemental yield (wt.%) under non-isothermal leaching conditions (26 wt.% solids) at 30 and 120 min for different A/O ratios.

<table><thead><tr><th rowspan="2">Initial A/O ratios</th><th rowspan="2">Temperature (°C)</th><th colspan="2">time (min)</th><th colspan="4">Elemental yield (wt.%)</th></tr><tr><th>x</th><th>y</th><th>Ni</th><th>Co</th><th>Fe</th><th>Al</th></tr></thead><tbody><tr><td rowspan="6">0.25</td><td rowspan="3">100</td><td>30</td><td>30</td><td>20.10<br>± 0.01</td><td>31.82<br>± 1.85</td><td>5.19<br>± 0.01</td><td>21.07<br>± 0.09</td></tr><tr><td>120</td><td>120</td><td>20.66<br>± 0.01</td><td>31.71<br>± 1.03</td><td>4.94<br>± 0.01</td><td>25.28<br>± 0.10</td></tr><tr><td>30</td><td>180</td><td>24.85<br>± 0.01</td><td>33.41<br>± 0.38</td><td>4.47<br>± 0.01</td><td>24.76<br>± 0.07</td></tr><tr><td rowspan="3">150</td><td>120</td><td>270</td><td>29.15<br>± 0.01</td><td>38.35<br>± 0.46</td><td>3.97<br>± 0.01</td><td>31.24<br>± 0.10</td></tr><tr><td>30</td><td>330</td><td>33.88<br>± 0.01</td><td>84.36<br>± 1.06</td><td>0.52<br>± 0.01</td><td>28.49<br>± 0.11</td></tr><tr><td>120</td><td>420</td><td>33.80<br>± 0.01</td><td>92.79<br>± 1.09</td><td>0.40<br>± 0.01</td><td>22.78<br>± 0.10</td></tr><tr><td rowspan="6">0.35</td><td rowspan="3">200</td><td>30</td><td>480</td><td>84.13<br>± 0.03</td><td>98.75<br>± 0.99</td><td>0.27<br>± 0.01</td><td>5.21<br>± 0.05</td></tr><tr><td>120</td><td>570</td><td>90.67<br>± 0.02</td><td>97.11<br>± 0.69</td><td>0.18<br>± 0.01</td><td>3.91<br>± 0.03</td></tr><tr><td>30</td><td>30</td><td>27.77<br>± 0.01</td><td>31.93<br>± 0.53</td><td>7.50<br>± 0.01</td><td>23.75<br>± 0.10</td></tr><tr><td rowspan="3">150</td><td>120</td><td>120</td><td>29.15<br>± 0.01</td><td>31.35<br>± 0.46</td><td>6.97<br>± 0.01</td><td>31.24<br>± 0.10</td></tr><tr><td>30</td><td>180</td><td>30.84<br>± 0.01</td><td>35.19<br>± 0.54</td><td>4.38<br>± 0.01</td><td>41.92<br>± 0.11</td></tr><tr><td>120</td><td>270</td><td>33.12<br>± 0.01</td><td>39.18<br>± 0.44</td><td>2.66<br>± 0.01</td><td>43.66<br>± 0.10</td></tr><tr><td rowspan="6">0.45</td><td rowspan="3">200</td><td>30</td><td>330</td><td>44.49<br>± 0.01</td><td>89.22<br>± 0.80</td><td>1.47<br>± 0.01</td><td>83.44<br>± 0.14</td></tr><tr><td>120</td><td>420</td><td>52.59<br>± 0.01</td><td>90.68<br>± 0.79</td><td>1.14<br>± 0.01</td><td>80.21<br>± 0.14</td></tr><tr><td>30</td><td>480</td><td>95.08<br>± 0.02</td><td>94.37<br>± 0.71</td><td>0.26<br>± 0.01</td><td>60.49<br>± 0.34</td></tr><tr><td rowspan="3">100</td><td>120</td><td>570</td><td>95.50<br>± 0.02</td><td>94.52<br>± 0.80</td><td>0.19<br>± 0.01</td><td>54.43<br>± 0.45</td></tr><tr><td>30</td><td>30</td><td>32.20<br>± 0.01</td><td>31.44<br>± 0.93</td><td>18.02<br>± 0.01</td><td>32.51<br>± 0.11</td></tr><tr><td>120</td><td>120</td><td>31.97<br>± 0.01</td><td>31.66<br>± 0.85</td><td>17.09<br>± 0.01</td><td>35.88<br>± 0.10</td></tr><tr><td rowspan="6">200</td><td rowspan="3">150</td><td>30</td><td>30</td><td>37.15<br>± 0.01</td><td>45.92<br>± 0.82</td><td>13.39<br>± 0.01</td><td>51.09<br>± 0.10</td></tr><tr><td>120</td><td>120</td><td>37.75<br>± 0.01</td><td>59.64<br>± 0.65</td><td>10.00<br>± 0.01</td><td>53.40<br>± 0.08</td></tr><tr><td>30</td><td>180</td><td>50.34<br>± 0.02</td><td>79.82<br>± 0.98</td><td>5.11<br>± 0.01</td><td>85.56<br>± 0.12</td></tr><tr><td rowspan="3">265</td><td>120</td><td>270</td><td>69.92<br>± 0.02</td><td>82.36<br>± 0.77</td><td>3.58<br>± 0.01</td><td>84.74<br>± 0.10</td></tr><tr><td>30</td><td>330</td><td>94.33<br>± 0.02</td><td>95.12<br>± 0.69</td><td>0.64<br>± 0.01</td><td>67.31<br>± 0.10</td></tr><tr><td>120</td><td>420</td><td>93.70<br>± 0.02</td><td>93.59<br>± 0.61</td><td>0.52<br>± 0.01</td><td>60.74<br>± 0.10</td></tr></tbody></table>

In the “time (min)” column, ‘x’ represents the leaching duration at each specific temperature (100 to 265 °C), while ‘y’ indicates the total residence time of the laterite sample in the autoclave from the start (25 °C) up to the final set temperature (265 °C) under non-isothermal conditions.

precipitated sharply beyond 150 °C, consistent with hydrolysis and transformation to hematite (Reactions (IV) and (VI)). This decline in Fe concentration implies selective leaching favouring Ni and Co over Fe (Rodriguez and Wedderburn, 2007). Even though Ni is present in the goethite matrix, its release is slower than Fe (Fig. 2) and requires higher energy (higher temperature) for fast leaching.

Fig. 3 illustrates the phase evolution of laterite ore under non-isothermal leaching conditions at temperatures between 100 and 265 °C with an initial solid content of 26 wt.% and an acid-to-ore ratio (A/O) of 0.35. Table 5 shows the nomenclature of the letters presented in Fig. 3 and Fig. 10. A comparison of the initial laterite ore with the post-leach residues from 100 °C to 200 °C shows peak similarities. Baseline correction (background subtraction) was applied to the measured XRD results by fitting and removing the estimated background with Matlab® (Fig. S5). This process reduces noise, enhances peak

visibility for phase identification, and improves accuracy for further analysis, such as phase quantification. The baseline correction ensures that only the crystalline phase information remains in the final XRD pattern, leading to more precise material characterisation.

At low temperatures, partial goethite dissolution occurs, but Ni remains adsorbed or trapped in solid residues or intermediate phases (such as poorly crystalline iron hydroxides). The continuous and rapid drop of Fe concentration in the solution with time is due to the precipitation of iron-containing compounds (e.g., hematite or jarosite) as temperature increases, as seen in Fig. 3. This trend is similar to the Fe hydrolysis study carried out by Das and Li (2023), where the Fe concentration of the post-leach liquor increases with increasing acid dosage and temperature. Hematite formation during HPAL is strongly temperature-dependent and follows endothermic dehydration of goethite (Fan et al., 2006; Abbasi Gharaei et al., 2024). Similarly, the concentration of Al increases steadily with temperature and higher A/O ratios, reaching a peak before dropping at 265 °C, indicating precipitation as alunite at high temperatures (Fig. 3, Table 5). The steady rise in Al concentration, compared to Fe in Fig. 2, indicates that Al dissolution is less affected by temperature-induced precipitation compared to Fe (Whittington and Muir, 2000).

In Fig. 3, the characteristic peaks corresponding to quartz (f), goethite (d, i, j, l, o), hematite (i, k), and talc (a) remain consistently present across these conditions. This suggests that these mineral phases are relatively stable and not easily dissolved or transformed at lower and intermediate temperatures. However, at 200 °C, slight differences begin to emerge. While the stable phases are still present, there is an increase in the intensity of certain peaks, such as hematite (i, k), indicating an increase in the proportion of mineral phases already present, such as hematite or the formation of secondary phases, such as hydronium alunite (x), which begin to appear due to supersaturation conditions at higher temperatures. This behaviour can be attributed to the higher temperature, enhancing the hydrolysis and precipitation of dissolved iron and aluminium species into crystalline oxides or sulphates (Miller, 2019).

A major difference is observed at 265 °C, where most of the initial phases, especially goethite (d, j) and gibbsite (c), are significantly diminished or absent, and new sharp peaks corresponding to crystalline oxides like hematite (l,k) and alunite minerals (x, y, z) become dominant. This change indicates extensive dissolution of the primary oxide minerals (goethite and gibbsite: Reactions (III) and (VII)) followed by the precipitation of stable phases (hematite and H-alunite: Reactions (IV), (VI), and (IX)) at elevated temperatures (265 °C) and prolonged reaction time. This transformation shows that higher temperature enhances dissolution rates but also promotes supersaturation and precipitation of complex minerals (Whittington and Muir, 2000), which become the dominant crystalline phases in the solid residue.

### 3.3. Ni and Co selectivity with temperature (non-isothermal conditions)

Higher A/O ratios enhance the elemental dissolution of Ni and Co as observed in Fig. 2, leading to higher concentrations in the leachate. This suggests that both increased temperature and higher acid dosage improve the leaching efficiency for Ni and Co. Higher acid dosage at a high A/O ratio facilitates the leaching of Fe and Al with time, while their precipitation significantly affects Ni and Co selectivity.

Fig. 4 illustrates the selectivity of Ni and Co in the post-leach filtrate under non-isothermal conditions for different A/O ratios and 26 wt.% slurry solid content. Across all three A/O ratios (0.25, 0.35, and 0.45), Ni and Co selectivity remained low at the early stages of leaching (≤150 °C) and began to increase markedly once the temperature exceeded 200 °C. This trend reflects the progressive breakdown of Ni- and Co-bearing minerals, such as goethite and serpentine ((Mg,Fe,Ni,Al,Zn,Mn)₂-3(Si, Al,Fe)₂O₅(OH)₄), with rising temperature, an observation supported in Fig. 3. This aligns with the study of Astuti et al. (2015), which shows that higher temperatures enhance the selectivity of Ni and Co, as faster
<figure><img src="image_4.png" /><figcaption>Fig. 3. X-ray diffraction patterns of initial ore and post-leach solid residues at different temperatures (Non-isothermal leach conditions).</figcaption></figure>

<table><caption>**Table 5**

Nomenclature of letters identifying the minerals in Fig. 3 and Fig. 10.</caption><thead><tr><th>Letters</th><th>Initial laterite ore & solid residue at 100–200 °C</th><th>Solid residues at 265 °C (5, 60 and 120 min)</th></tr></thead><tbody><tr><td>a</td><td>Talc, Nepouite</td><td>Talc, Nepouite</td></tr><tr><td>b</td><td>Nepouite, Lizardite</td><td>Nepouite, Lizardite</td></tr><tr><td>c</td><td>Gibbsite</td><td>Gibbsite, H-alunite, Chromite</td></tr><tr><td>d</td><td>Goethite</td><td>Goethite</td></tr><tr><td>e</td><td>Hematite</td><td>Hematite</td></tr><tr><td>f</td><td>Quartz</td><td>Quartz</td></tr><tr><td>g</td><td>Talc</td><td>Talc</td></tr><tr><td>h</td><td>Chromite, Magnetite</td><td>Chromite, Magnetite</td></tr><tr><td>i</td><td>Hematite, Goethite</td><td>Hematite, Goethite</td></tr><tr><td>j</td><td>Hematite, Goethite</td><td>Hematite, Goethite</td></tr><tr><td>k</td><td>Chromite, Magnetite, Hematite, Lizardite</td><td>Magnetite, Chromite, Hematite, Lizardite</td></tr><tr><td>l</td><td>Goethite</td><td>Goethite, Hematite</td></tr><tr><td>m</td><td>Goethite</td><td>Goethite</td></tr><tr><td>n</td><td>Goethite, Hematite</td><td>Goethite, Hematite</td></tr><tr><td>o</td><td>Goethite</td><td>Hematite</td></tr><tr><td>p</td><td>Goethite</td><td>Goethite</td></tr><tr><td>q</td><td>Goethite</td><td>Chromite, Magnetite, Hematite</td></tr><tr><td>r</td><td>Goethite</td><td>Hematite, Goethite</td></tr><tr><td>x</td><td>H-alunite; Hematite</td><td>H-alunite; Hematite</td></tr><tr><td>y</td><td>–</td><td>H-alunite</td></tr><tr><td>z</td><td>–</td><td>H-alunite</td></tr><tr><td colspan="3">H-alunite: Hydronium alunite</td></tr></tbody></table>

kinetics at elevated temperatures favour their dissolution over impurity elements such as Al and Fe.

Increasing the temperature from 200 to 265 °C significantly enhances the quality of the post-leach solution and drastically increases Ni and Co selectivity. This improvement is attributed to the accelerated precipitation of iron and aluminium cations in the solution and the reduced solubility of hematite and alunite, the primary solid products formed during leaching (Georgiou and Papangelakis, 1998).

A lower A/O ratio (0.25) resulted in a sharper increase in selectivity of Ni and Co, which became more pronounced at 265 °C. This is likely due to the low acid required to leach and subsequently coprecipitate Al and Fe impurities into the solution, due to the regeneration of acid (Reactions (IV) and (IX)) at higher temperatures. In contrast, higher acid dosages (A/O = 0.35 and 0.45) reduce the selectivity of Ni and Co, likely

due to concurrent dissolution of Fe and Al phases that initially suppress Ni-Co selectivity. Similar observations were reported in recent HPAL studies demonstrating that acid dose needs tight tuning: over-dosage improves total yield but degrades selectivity due to impurity co-dissolution, while under-dosage yields sharper selectivity but risks incomplete Ni/Co liberation (Çoban and Baş, 2024). The selective leaching of Ni over Al and Fe dominates in all cases, while Co follows a similar but slightly delayed pattern due to Co's lower concentration in the feed ore (Table 1 and Fig. 2). Section 3.4.2 presents Ni and Co selectivity from nine isothermal leaching experiments conducted at varying slurry solid contents and acid-to-ore (A/O) ratios, highlighting how the pH and residual/free acid of the leach solution influence Ni and Co purity during HPAL leaching.

### 3.4. Isothermal condition – Effect of A/O ratio and solid content on leaching at 265 °C

#### 3.4.1. Elemental yields

Overall, the elemental leaching profiles follow the same general trend regardless of the initial acid-to-ore (A/O) ratio and slurry solid content. However, the leaching rates and final yields of each element vary with changes in these parameters. Fig. 5 presents the leaching profiles of Ni, Co, Al, and Fe over time at 265 °C under different A/O ratios and solid concentrations. An increase in the A/O ratio consistently enhances the leaching of nickel, along with other impurities such as Fe and Al.

At 22 wt.% solid content, faster and higher nickel yield is achieved at higher A/O ratios, with Ni reaching its maximum leaching at 25 min for initial A/O = 0.35 (94.86 ± 0.01 wt.%) and initial A/O = 0.45 (97.08 ± 0.01 wt.%), remaining stable up to 180 min. In contrast, at A/O = 0.25, Ni leaching increases gradually, reaching its maximum (90.80 ± 0.01 wt.%) at 180 min. A similar trend is observed at 26 wt.% and 30 wt.% solids, although with a slight decrease in overall Ni yield as the solid content increases. This demonstrates that, at a constant A/O ratio, a more dilute slurry (lower solid content, 22 wt.%) favours higher nickel leaching. This observation is consistent with the findings of Georgiou and Papangelakis (1998), who reported that pulp density has an inverse relationship with nickel leaching rate. Fig. S6 summarises the nickel leaching results at 25, 60, and 180 min under isothermal conditions (265 °C), highlighting the effects of varying A/O ratios and solid
<figure><img src="image_5.png" /><figcaption>Fig. 4. Selectivity of Ni and Co in post-leach filtrate under non-isothermal conditions (100–265 °C) for 26 wt.% solid and initial A/O ratios of 0.25, 0.35, and 0.45.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Elemental yield (wt.%) at varying solid content (22, 26, 30 wt.%) and acid-to-ore ratios (0.25, 0.35, 0.45) at 265 °C, Isothermal condition.</figcaption></figure>

contents.

Comparing isothermal and non-isothermal conditions at 265 °C (Table 6), the final metal ion concentrations are similar despite the prolonged residence time under non-isothermal leaching. For example, at an initial A/O ratio of 0.35 and 60 min, Ni concentrations are 5.45 g/L (isothermal) and 5.52 g/L (non-isothermal), while Al was 1.72 g/L and 1.75 g/L, respectively. This emphasises that once the threshold temperature (> 250 °C) is achieved, temperature rather than total residence time dictates Ni and Co extraction efficiency, a trend corroborated by the works of Guo et al. (2011).

Similarly, the initial A/O ratio and solid content influence the leaching of aluminium (Al) and iron (Fe). As shown in Fig. 5, both Al and

Fe leaching yield increase with higher A/O ratios and higher solid contents, reaching their maximum concentrations at specific times depending on the experimental conditions, followed by a decrease in concentration in the post-leach filtrate. For Fe, this decline is more immediate and pronounced (as explained in Section 3.2 – non-isothermal condition), as the dissolution of iron is quickly followed by rapid precipitation of secondary iron phases. In contrast, Al concentrations remain relatively stable after an initial peak drop, except at higher solid content (30 wt.%), where a continuous decrease is observed. This is due to the higher initial leaching yield of Al at short reaction times under conditions of high solid content and high A/O ratio (0.45), leading to supersaturation and gradual precipitation. Alunite precipitates more
<caption>**Table 6**</caption>

Elemental concentration of Ni, Co, Fe and Al in post-leach filtrate under isothermal and non-isothermal conditions at 265 °C for 26 wt.% solid and initial A/O ratios of 0.25, 0.35, and 0.45.

<table><thead><tr><th rowspan="3">Initial A/O ratios</th><th rowspan="3">time (min)</th><th colspan="6">Elemental yield in leached filtrate (g/L)</th></tr><tr><th colspan="4">Isothermal (265 °C)</th><th colspan="4">Non-Isothermal (265 °C)</th></tr><tr><th>Ni</th><th>Co</th><th>Fe</th><th>Al</th><th>Ni</th><th>Co</th><th>Fe</th><th>Al</th></tr></thead><tbody><tr><td rowspan="2">0.25</td><td>60</td><td>4.08</td><td>0.33</td><td>0.35</td><td>0.71</td><td>4.06</td><td>0.32</td><td>0.35</td><td>0.53</td></tr><tr><td>120</td><td>4.65</td><td>0.36</td><td>0.27</td><td>0.57</td><td>5.46</td><td>0.42</td><td>0.33</td><td>0.56</td></tr><tr><td rowspan="2">0.35</td><td>60</td><td>5.45</td><td>0.40</td><td>0.62</td><td>1.72</td><td>5.52</td><td>0.39</td><td>0.44</td><td>1.75</td></tr><tr><td>120</td><td>5.34</td><td>0.39</td><td>0.46</td><td>1.43</td><td>5.32</td><td>0.38</td><td>0.36</td><td>1.54</td></tr><tr><td rowspan="2">0.45</td><td>60</td><td>5.14</td><td>0.39</td><td>1.98</td><td>5.03</td><td>5.35</td><td>0.40</td><td>1.21</td><td>5.85</td></tr><tr><td>120</td><td>5.15</td><td>0.40</td><td>1.51</td><td>5.12</td><td>5.53</td><td>0.42</td><td>1.12</td><td>5.65</td></tr><tr><td colspan="2"></td><th colspan="6">Uncertainty of elemental yield in leached filtrate (g/L) × 10<sup>-3</sup></th></tr><tr><td rowspan="2">0.25</td><td>60</td><td>12.80</td><td>1.90</td><td>0.10</td><td>11.80</td><td>10.10</td><td>3.30</td><td>5.10</td><td>10.80</td></tr><tr><td>120</td><td>14.50</td><td>2.10</td><td>0.10</td><td>12.50</td><td>11.10</td><td>2.90</td><td>4.50</td><td>9.60</td></tr><tr><td rowspan="2">0.35</td><td>60</td><td>14.90</td><td>2.00</td><td>0.40</td><td>15.90</td><td>10.00</td><td>2.70</td><td>4.20</td><td>9.00</td></tr><tr><td>120</td><td>15.80</td><td>2.20</td><td>0.50</td><td>15.80</td><td>11.70</td><td>3.20</td><td>4.90</td><td>10.40</td></tr><tr><td rowspan="2">0.45</td><td>60</td><td>15.20</td><td>2.20</td><td>0.80</td><td>35.40</td><td>10.50</td><td>2.80</td><td>4.30</td><td>9.30</td></tr><tr><td>120</td><td>15.10</td><td>2.20</td><td>0.70</td><td>35.80</td><td>11.00</td><td>3.00</td><td>4.60</td><td>9.80</td></tr></tbody></table>

In non-isothermal conditions at 265 °C, 60 and 120 min correspond to a total time of 480 and 570 min, respectively, as seen in Fig. 2.

slowly than hematite (Georgiou and Papangelakis, 1998), likely due to differences in their dissolution rates and solubility products. Furthermore, Al leaching is strongly favoured by both higher A/O ratios and higher solid contents, as evidenced by the sharp increase to 48.70 wt.% at 5 min for 30 wt.% solids and initial A/O = 0.45, compared to only 17.79 wt.% under the same time for 22 wt.% solids.

### 3.4.2. Solution purity with time at constant temperature

This section examines how leaching time, A/O ratio, slurry solid content, and the evolution of pH and free acid concentration govern solution purity under isothermal conditions. The purity of the post-leach solution is largely controlled by the dissolution and precipitation dynamics of Fe and Al (Miller, 2019), consistent with Reactions (III)–(IX). Nickel remains relatively stable in solution, and its selective leaching progressively increases as leaching proceeds, as shown in Fig. 6, coinciding with the gradual precipitation of Fe and Al (Fig. 5).

Higher A/O ratios maintain lower pH and higher residual acid, which sustains Fe and Al dissolution and temporarily suppresses Ni selectivity. The relative leaching efficiency of species i at a given A/O ratio and time t was calculated using Eq. (3). This parameter represents the percentage contribution of each A/O condition to the total dissolution of species i across all tested A/O ratios (0.25, 0.35, and 0.45) at the same time. The equation was employed to assess the influence of the A/O ratio on the purity of the post-leach solution by comparing the leaching rates of Ni, Al, and Fe across the different A/O conditions as seen in Fig. 7 (right):

$$L_{\text{effi}}\left(\frac{\text{A}}{\text{O}}, t\right) = 100 \left[ \frac{X_i\left(\frac{\text{A}}{0.25, 0.35, \text{or } 0.45}, t\right)}{X_i(0.25, t) + X_i(0.35, t) + X_i(0.25, t)} \right] \quad (3)$$

where i is the element (Ni, Co, Fe and Al); $L_{\text{effi}}(\frac{\text{A}}{\text{O}}, t)$ is the leaching efficiency of element 'i' at a given acid-to-ore ratio and time (e.g. A/O of 0.25 at t = 60 min) in wt.%; $X_i(\frac{\text{A}}{\text{O}}, t)$ is the elemental yield at a given acid-to-ore ratio and time.

The leaching data after 6 min in Fig. 7 (left, tabulated) illustrates that for 22 wt.% solid content, the Ni yield only increases slightly from 79.52 % to 95.87 % with increasing A/O ratio from A/O = 0.25 to 0.45. However, Fig. 7 (right) represents a modest increment in leaching efficiency of 6 % (from $L_{\text{effNi}}$ 29.4 wt.% at A/O = 0.25 to 35.5 wt.% at A/O = 0.45) in, while the increase for Fe and Al is much more pronounced. Both elements show steep rises in leaching efficiency, with Al and Fe increasing by 59 % (from $L_{\text{effAl}}$ 8.2 wt.% at A/O = 0.25 to 67.4 wt.% at A/O = 0.45) and ~ 50 % (from $L_{\text{effFe}}$ 13.7 wt.% at A/O = 0.25 to 63.4 wt. % at A/O = 0.45), respectively, under the same conditions (26 wt.% solid) as seen in Fig. 7 (right), leading to higher impurity levels in the leach solution.

The slurry solid content (22, 26, 30 wt.%) does not have a significant effect on the selectivity of Ni, as seen in Fig. 7a and b. However, solid content affects the available surface area for leaching, which can impact

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Selectivity of Ni and Co under isothermal conditions (265 °C) at various solid content and A/O ratios.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Elemental yield, Xᵢ (left) and leaching efficiency (right) of Ni and impurities (Al, Fe) using leaching data at 60 min for the experimental campaigns (initial A/O = 0.25, 0.35 and 0.45; solid content = 22, 26 and 30 wt.%).</figcaption></figure>

<figure><img src="image_9.png" /></figure>

both leaching rates and the distribution of dissolved elements (Peng et al., 2023). Higher solid content increases competition for the available acid, potentially reducing Ni selectivity relative to other elements. Additionally, interactions between the solid phase and the acidic solution can influence the overall reactivity and leaching dynamics. Fig. 6 shows minimal variation in Ni selectivity across different solid contents at constant initial A/O ratios, except at A/O = 0.25, where a difference emerges between 22 wt.% and higher solid contents (26 and 30 wt.%). At 25 min (A/O of 0.25), Ni selectivity decreases with the increase in solid content, from 4.67 ± 0.08 (for 22 wt.%) to 2.50 ± 0.03 (for 26 wt. %) and 3.33 ± 0.23 (for 30 wt.% solids). This decline is due to increased competition for acid at higher solid content and lower acid availability, reducing Ni's preferential leaching.

### 3.4.3. Free acid and pH measured at ambient temperature after leaching at 265 °C

As presented in Fig. 8, the free acid concentration drops sharply as H₂SO₄ reacts rapidly with easily soluble gangue minerals (oxides and hydroxides of Fe, Mg, and Al) while the pH rises correspondingly. This acid depletion is more pronounced at higher A/O ratios due to the greater availability of acid to react with reactive minerals (Table 6), which also contributes to reduced Ni selectivity. After the initial reaction, the dissolution rate slows as less reactive minerals (e.g. Al-bearing compounds) dominate and precipitate, reducing acid consumption, stabilising the pH and Ni, Co selectivity plateau. To conclude, the effect of slurry solid concentration is dependent on the acid concentration (A/

O ratio). Consequently, achieving optimal solution purity depends on balancing the A/O ratio with solid content to minimise Fe and Al dissolution while maintaining sufficient acid for effective Ni leaching.

### 3.4.4. Particle size distribution

Particle formation is a complex process that involves several steps: particle nucleation, crystal growth and possibly aggregation. The rate of each stage determines the final particle size distribution of the solid product (Lince et al., 2008). Given that the effect of solids is an extension of the acid-to-ore (A/O) ratio employed in the leaching process, and consistent behaviour is observed across 22, 26, and 30 wt.% solid content at their respective initial A/O ratios, only the data for 26 wt.% solid at A/O ratios of 0.25, 0.35, and 0.45 are presented in Fig. 9 for discussion. The particle size distribution of the post-leach solid residues, compared with the initial ore, indicates the formation of finer size fractions, as evidenced by a peak (size class) shift to the left. This can be attributed to the dissolution and possible precipitation of minerals driven by supersaturation. As the leaching progresses, these fine particles increase in volume, particularly at higher A/O ratios, highlighting the role of higher concentrations of acid in the dissolution of laterite ore and/or precipitation of secondary minerals (Ma et al., 2013).

Fig. S3 (supplementary file) captures the transformation of laterite ore before and after leaching at different temperatures, with the varying colour changes indicating transformation from yellowish-brown goethite (dominant mineral in the laterite ore) to deep reddish-brown, indicating mineralogical transformations to hematite at 265 °C.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Free acid and pH of the solution at different solid contents and initial A/O ratios under Isothermal conditions (265 °C), measured at ambient temperature.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 9. Particle size distribution of initial ore and post-leach solid residues (26 wt.% solid, initial A/O = 0.25, 0.35 and 0.45) after leaching at 265 °C, measured at ambient temperature.</figcaption></figure>

Thermodynamically, the leaching process involves the dissolution of nickel and cobalt-bearing phases in sulphuric acid while leaving behind iron-rich residues. At lower temperatures, goethite remains relatively stable, but as the temperature increases, it dehydrates to form hematite (Fe₂O₃), a process governed by Reactions (III) and (IV). This endothermic transformation is favoured at higher temperatures. Additionally, supersaturation effects influence precipitation behaviour, with increased temperature enhancing the solubility of certain species while promoting the precipitation of others, such as hematite, alunite, or kieserite (Liu and Papangelakis, 2005).

### 3.4.5. Morphology (SEM) and mineral identification (XRD) of the post-leach solid residues

The Scanning Electron Microscope (SEM) images in Figs. S8 and S9 reveal the structural evolution from the initial ore to post-leach solid residues. The initial laterite ore (feed sample) exhibits a fibrous morphology, showing the dominance of goethite (limonite ore) and lizardite (saprolite ore) with loosely packed particles, typical of untreated material. The feed sample displays a compacted structure with less-defined shapes as seen in Fig. S7(a). The elemental mapping (Fig. S7(b)) of the feed ore indicates the absence of sulphur and the presence of the elements shown in Table 1. Oxygen exhibits a higher intensity, highlighting its dominance in the laterite ore, likely due to the presence of oxygenated compounds such as goethite (FeOOH) and gibbsite (Al(OH)₃), among others.

The post-leach solid residue is dominated by the presence of the botryoidal shape, which refers to a mineral's form composed of many rounded, grape-like segments. This texture occurs when tiny crystal nuclei, like specks of dust or sand, act as growth points for minerals, causing fibrous or acicular crystals to grow outwards radially and form spherical masses that often overlap and fuse together, compared to the fibrous structure of the initial laterite feed ore (Fig. S8(a)). This botryoidal shape indicates the presence of iron oxide, likely dominated by hematite, formed from the transformation of the primary mineral (goethite) during the acidic leaching of laterite ore.

The post-leach solid residue exhibits significant structural modifications with a well-defined shape appearing less compact than the feed ore as seen in Fig. S8(a). The particles show more uniformity in size, and the presence of voids (black region) suggests the removal of soluble phases like nickel and other metals from the initial ore.

Unlike the initial ore (Fig. S7), sulphur is detected in the elemental mapping of the post-leach residue (Fig. S8(b)), which is evidently due to the sulphuric acid used in the leaching process, which modified the structure of the ore. A higher intensity of oxygen, iron and aluminium, as

shown in Fig. S8(b), further confirms the dominance of oxygenated groups and Fe, Al-bearing minerals, such as hematite and alunite, in the post-leach residue.

Since the same peak patterns are observed across all solid residues at 265 °C, regardless of solid concentration or acid-to-ore ratio (with only changes in peak intensity), the residue obtained at 30 wt.% solid content (shown in Fig. 10) is selected for detailed discussion.

The X-ray diffraction (XRD) analysis in Fig. 10 further highlights the progressive dissolution of primary minerals present in the initial ore (first diffractogram line in Fig. 3), including goethite, gibbsite, and lizardite and the simultaneous formation or intensification of secondary phases such as alunite and hematite. Under isothermal conditions at 265 °C, the XRD patterns of the solid residues remain compositionally consistent over time, with variations primarily observed in peak intensities.

Fig. 10 also shows the phase evolution of laterite ore leached at 265 °C with varying initial acid-to-ore ratios (A/O = 0.25 and 0.45) and reaction times (5 min and 60 min). The nomenclature of the letters representing each mineral is shown in Table 5. At both acid dosages and short reaction time (5 min), the diffraction patterns display dominant peaks of primary phases such as quartz (f), goethite (d), and hematite (k, l), alongside minor sulphate phases (hydronium alunite (z)) beginning to appear. This indicates that at the early stage of leaching, the dissolution of major minerals is incomplete, and only slight precipitation of secondary sulphate phases occurs. Additionally, the observed decrease in the peak intensity of goethite (d) and a corresponding increase in the intensity of hematite (k) in the solid residues at A/O = 0.45, compared to A/O = 0.25, indicates that higher acid availability accelerates the dissolution of goethite and promotes the precipitation of hematite and other secondary phases, even at short reaction times (Reactions (III)–(VI)).

The difference between A/O = 0.25 and A/O = 0.45 becomes more pronounced at 60 min. At A/O of 0.25 and 60 min, there is a significant reduction in the intensity of goethite (d), while sharp peaks of alunite (x, z) and hematite (k, l) are strongly evident. This confirms that over time, even with lower acid dosage, dissolution proceeds and leads to the formation of stable secondary minerals due to supersaturation. However, with a higher acid-to-ore ratio (A/O = 0.45) and 60 min, the changes are more drastic. The primary oxide phases are almost completely absent, and there is a very strong development of sharp, well-defined peaks of alunite and hematite. The more intense peaks indicate larger crystalline domains and higher crystallinity. The stronger acidity enhances the dissolution of iron-, aluminium-, and magnesium-bearing minerals, increasing metal ion concentration in solution, and
<figure><img src="image_12.png" /><figcaption>Fig. 10. X-ray diffraction patterns of post-leach solid residues at 265 °C at different times and initial A/O (Isothermal leach conditions).</figcaption></figure>

facilitating precipitation. The presence of well-formed crystals in the leached residue and the proposed structural transformation observed during sulphuric acid leaching at temperature are consistent with the observations reported by Wang (2016).

## 4. Conclusions

This study evaluated the effects of acid-to-ore ratio (A/O) and slurry solid content on the extraction of nickel and cobalt from Prony Resources, New Caledonian laterite ore under HPAL conditions. Experiments were conducted under both isothermal (265 °C) and non-isothermal (100–265 °C) leaching conditions to compare the influence of temperature evolution and time on metal yield. The results demonstrate that temperature exerts a far greater influence than leaching duration alone.

Under non-isothermal conditions, the slurry was heated sequentially from 100 °C → 150 °C → 200 °C over 7 h, yet Ni and Co extraction remained low, as the activation energy required to break the laterite mineral matrix was not reached. Significant dissolution only occurred when the autoclave reached 265 °C, achieving 94.67 wt.% Ni yield at 60 min, corresponding to 8 h in non-isothermal conditions (A/O = 0.45, 26 wt.% solids). In comparison, isothermal leaching at 265 °C produced a similar Ni yield of 94.45 wt.% under the same conditions in just 60 min. This illustrates that prolonged leaching at sub-optimal temperatures contributes minimally to final Ni and Co yield, and high, stable temperatures are critical for liberating metals from Fe-rich phases such as goethite.

While the final concentrations of Ni and Co are comparable between isothermal and non-isothermal operations, the heating profile primarily affects leaching kinetics and early-stage selectivity, which is highly relevant for industrial HPAL, where gradual heat-up is unavoidable.

Key findings operating at the industrially optimised isothermal temperature (265 °C) include:

1. Solid content strongly influences metal yield; at 60 min (A/O of 0.45), lower solids (22 wt.%) achieved Ni and Co yields of 95.87 wt. % and 53.52 wt.%, respectively, while higher solids (30 wt.%) reduced yields to 90.60 wt.% and 51.32 wt.%, due to increased slurry viscosity and reduced ion mobility.

2. Acid concentration (A/O) enhances Ni dissolution, with a maximum yield of 97.08 wt.% at A/O = 0.45 and 25 min. However, higher acid levels also increase Fe and Al co-dissolution, lowering selectivity and product purity. An optimal balance tailored to the ore composition is

required to achieve a trade-off between maximising Ni and Co yield and maintaining high selectivity in the leached solution.

3. Selectivity optimisation occurs at moderate acid levels (A/O = 0.35), where Ni yield remained high (92.90 wt.%, using leaching time at 60 min with 26 wt.% solid content) while Fe and Al dissolution decreased sharply, improving the Ni-to-impurity ratio.

4. Post-leach residue analysis (XRD/SEM) confirmed that higher A/O ratios and prolonged leaching promote the transformation of goethite and gibbsite to hematite and hydronium alunite, correlating with enhanced Ni release.

Overall, this work highlights the industrial importance of balancing acid dosage and solid loading to maximise Ni yield, maintain selectivity, and manage impurity dissolution. These findings provide practical guidance for HPAL operations aiming to improve metal yield efficiency and downstream processing performance.

### CRediT authorship contribution statement

**Okechukwu Vincent Dickson:** Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Thomas Deleau:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Conceptualization. **Fabienne Espitalier:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Conceptualization. **Christophe Coquelet:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Conceptualization. **Julien Lombart:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition. **Philippe Accart:** Validation, Investigation.

### Funding

This work was funded by the Association Nationale Recherche Technologie (ANRT-CIFRE), France, under grant [number 2021/1702].

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
## Acknowledgements

The authors wish to thank Prony Resources, New Caledonia, Chaire MINAUMET – Mines Paris – PSL, and the Association Nationale Recherche Technologie (ANRT-CIFRE), France, and for supporting this study. We are also grateful to RAPSODEE Research Centre https://rapsodee.imt-mines-albi.fr for providing a conducive laboratory to carry out this research. Special thanks to Veronique Nallet for the X-ray diffraction analysis & treatment, and Christine Rolland for the Scanning Electron Microscope & cartography analysis.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.hydromet.2025.106576.

## References

Abbasi Gharaei, A., Rezai, B., Hamidian Shormasti, H., 2024. Environmental friendly approach: atmospheric and high-pressure acid leaching studies, low-grade laterites nickel processing. J. Min. Environ. 15 (4), 1591–1606. https://doi.org/10.22044/jme.2024.14230.2656.

Anand, R.R., Gilkes, R.J., 1987. Iron oxides in lateritic soils from Western Australia. J. Soil Sci. 38 (4), 607–622. https://doi.org/10.1111/j.1365-2389.1987.tb02158.x.

Astuti, W., Hirajima, T., Sasaki, K., Okibe, N., 2015. Kinetics of nickel extraction from Indonesian saprolitic ore by citric acid leaching under atmospheric pressure. Min. Metall. Explor. 32 (3), 176–185. https://doi.org/10.1007/BF03402286.

Ayranci, I., Kresta, S.M., 2014. Critical analysis of Zwietering correlation for solids suspension in stirred tanks. Chem. Eng. Res. Des. 92 (3), 413–422. https://doi.org/10.1016/j.chedr.2013.09.005.

Bana, I.S., Wan, W., Shen, L., 2025. Optimization of high-pressure acid leaching for nickel and cobalt recovery from ultralow-grade laterite ores. JOM 77 (7), 5192–5204. https://doi.org/10.1007/s11837-025-07402-2.

Butt, C.W., Cluzel, D., 2013. Nickel laterite ore deposits: weathered serpentinites. Elements 9, 123–128. https://doi.org/10.2113/gselements.9.2.123. Special Issue "Serpentines".

Büyükakinci, E., Topkaya, Y.A., 2009. Extraction of nickel from lateritic ores at atmospheric pressure with agitation leaching. Hydrometallurgy 97 (1), 33–38. https://doi.org/10.1016/j.hydromet.2008.12.014.

Chou, E.C., Queneau, P.B., Rickard, R.S., 1977. Sulfuric acid pressure leaching of nickeliferous limonites. Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 8 (4), 547–554. https://doi.org/10.1007/BF02669329.

Çoban, B., Baş, A.D., 2024. HPAL of a lateritic nickel ore: an investigation on the relationship between nickel and cobalt extraction and acid consumption. Miner. Eng. 218, 109014. https://doi.org/10.1016/j.mineng.2024.109014.

Dalvi, A., Bacon, W., Osborne, R., 2004. The past and the future of nickel laterites. In: Proceedings of the PDAC 2004 International Convention, Trade Show and Investors Exchange, Toronto, ON, Canada, 7–10 March 2004. pp. 1–27.

Das, G.K., Li, J., 2023. Iron removal as goethite from synthetic laterite leach solutions. ACS Omega 8 (13), 11931–11940. https://doi.org/10.1021/acsomega.2c07595.

Davis, K., Demopoulos, P., G., 2023. Hydrometallurgical recycling technologies for NMC Li-ion battery cathodes: current industrial practice and new R&D trends. RSC Sustain. 1 (8), 1932–1951. https://doi.org/10.1039/D3SU00142C.

De Boissieu, F., Sevin, B., Cudahy, T., Mangeas, M., Chevrel, S., Ong, C., Rodger, A., Maurizot, P., Laukamp, C., Lau, I., Touraivane, T., Cluzel, D., Despinoy, M., 2018. Regolith-geology mapping with support vector machine: a case study over weathered Ni-bearing peridotites, New Caledonia. Int. J. Appl. Earth Obs. Geoinf. 64, 377–385. https://doi.org/10.1016/j.jag.2017.05.012.

Dickson, O.V., Deleau, T., Coquelet, C., Espitalier, F., Lombart, J., Tardy, A., Lachaize, F., 2023. Speciation and reaction equilibrium constant modelling of aqueous hydrometallurgical systems at elevated temperatures: a review. Chem. Thermodyn. Therm. Anal. 11, 100117. https://doi.org/10.1016/j.ctta.2023.100117.

Dickson, O.V., Deleau, T., Espitalier, F., Coquelet, C., Lombart, J., 2025. Multilevel factorial optimisation of acid-to-ore ratio and solids concentration in hydrometallurgical recovery of nickel from laterite ore. Res. Eng. 27, 106744. https://doi.org/10.1016/j.rineng.2025.106744.

Dublet, G., Juillot, F., Morin, G., Fritsch, E., Fandeur, D., Ona-Nguema, G., Brown, G.E., 2012. Ni speciation in a new Caledonian lateritic regolith: a quantitative X-ray absorption spectroscopy investigation. Geochim. Cosmochim. Acta 95, 119–133. https://doi.org/10.1016/j.gca.2012.07.030.

Elias, M., 2002. Nickel laterite deposits – geological overview, resources and exploitation. In: Giant Ore Deposits: Characteristics, Genesis and Exploration, 4, pp. 205–220. CODES Centre Of Ore Deposit Research, Special Publication.

Fan, H., Song, B., Li, Q., 2006. Thermal behavior of goethite during transformation to hematite. Mater. Chem. Phys. 98 (1), 148–153. https://doi.org/10.1016/j.matchemphys.2005.09.005.

Faris, N., Powneby, M.I., Bruckard, W.J., Chen, M., 2023. The direct leaching of nickel Sulfide flotation concentrates – a historic and state-of-the-art review part I: piloted processes and commercial operations. Miner. Process. Extr. Metall. Rev. 44 (6), 407–435. https://doi.org/10.1080/08827508.2022.2070617.

Feng, Z., Li, B., Wei, Y., Zhou, S., 2023. The aggregation and growth of ferronickel particles during the reduction of laterite nickel ore. J. Sustain. Metall. 9 (3), 1099–1113. https://doi.org/10.1007/s40831-023-00689-y.

Georgiou, D., Papangelakis, V.G., 1998. Sulphuric acid pressure leaching of a limonitic laterite: chemistry and kinetics. Hydrometallurgy 49 (1), 23–46. https://doi.org/10.1016/S0304-386X(98)00023-1.

Göveli, A., 2006. Nickel Extraction from Górdes Laterites by Hydrochloric Acid Leaching. master thesis. Middle East Technical University. https://open.metu.edu.tr/handle/11511/15968.

Guo, X., Shi, W., Li, D., Tian, Q., 2011. Leaching behavior of metals from limonitic laterite ore by high pressure acid leaching. Trans. Nonferrous Metals Soc. China 21 (1), 191–195. https://doi.org/10.1016/S1003-6326(11)06098-5.

International Energy Agency, (IEA), 2024. Global EV Outlook 2024: Moving towards Increased Affordability. IEA. https://www.iea.org/reports/global-ev-outlook-2024#overview.

Kaya, Ş., Topkaya, Y.A., 2011. High pressure acid leaching of a refractory lateritic nickel ore. Miner. Eng. 24 (11), 1188–1197. https://doi.org/10.1016/j.mineng.2011.05.004.

Krause, E., Singhal, A., Blakey, B.C., Papangelakis, V.G., Georgiou, D., 1997. Hydrometallurgy and refining of nickel and cobalt. In: Cooper, W.C., Mihaylov, Y. (Eds.), Proc. Nickel-Cobalt. 97 Int. Symp. 1, Canadian Institute of Mining, Metallurgy and Petroleum, pp. 441–458.

Lebrouhi, B.E., Khattari, Y., Lamrani, B., Maaroufi, M., Zeraouli, Y., Kouskou, T., 2021. Key challenges for a large-scale development of battery electric vehicles: a comprehensive review. J Energy Storage 44, 103273. https://doi.org/10.1016/j.est.2021.103273.

Lince, F., Marchisio, D.L., Barresi, A.A., 2008. Strategies to control the particle size distribution of poly-epsilon-caprolactone nanoparticles for pharmaceutical applications. J Colloid Interface Sci. 322 (2), 505–515. https://doi.org/10.1016/j.jcis.2008.03.033.

Liu, H., Papangelakis, V.G., 2005. Chemical modeling of high temperature aqueous processes. Hydrometallurgy 79 (1), 48–61. https://doi.org/10.1016/j.hydromet.2003.10.014.

Ma, B., Wang, C., Yang, W., Yang, B., Zhang, Y., 2013. Selective pressure leaching of Fe (II)-rich limonitic laterite ores from Indonesia using nitric acid. Miner. Eng. 45, 151–158. https://doi.org/10.1016/j.mineng.2013.02.009.

MacCarthy, J., Nosrati, A., Skinner, W., Addai-Mensah, J., 2016. Atmospheric acid leaching mechanisms and kinetics and rheological studies of a low grade saprolitic nickel laterite ore. Hydrometallurgy 160, 26–37. https://doi.org/10.1016/j.hydromet.2015.11.004.

Mamrybayeva, K.K., Kuandykova, A.N., Chepushtanova, T.A., Merkibayev, Y.S., 2024. Review of technology for hydrometallurgical processing of lateritic nickel ores over the past 20 years in the world. Non-Ferrous Met. 1, 13–21. https://doi.org/10.17580/nfnm.2024.01.03.

Manzoor, U., Mujica Roncery, L., Raabe, D., Souza Filho, I.R., 2025. Sustainable nickel enabled by hydrogen-based reduction. Nature 641 (8062), 365–373. https://doi.org/10.1038/s41586-025-08901-7.

McDonald, R.G., Whittington, B.I., 2008. Atmospheric acid leaching of nickel laterites review: part I. Sulphuric acid technologies. Hydrometallurgy 91 (1), 35–55. https://doi.org/10.1016/j.hydromet.2007.11.009.

Miller, M., 2019. The role of dynamic factors in autoclave scaling. In: 24th Annual Conference Proceedings, ALTA, Melbourne, Australia. Nickel-Cobalt-Copper Conference, pp. 494–529.

Minh, D.P., Accart, P., Boachon, C., Galvet, R., Chesnaud, A., del Confetto, S., Dirion, J.-L., Dong, J., Ephraim, A., Haurie, L., Lyczko, N., Munirathinam, R., Nzihou, A., Patry, S., Rolland, C., Millan, L.M.R., Roques, L., Sane, A.R., Sani, R., White, C.E., 2020. Generic and Advanced Characterization Techniques, Vol. 1. Springer International Publishing, p. 31. https://doi.org/10.1007/978-3-030-35020-8-2.

Ni, S., Xiao, C., 2025. Innovative applications of deep eutectic solvents in hydrometallurgy: progress, challenges, and opportunities. In: Deep Eutectic Solvents, vol. 1504. American Chemical Society, pp. 291–329. https://doi.org/10.1021/bk-2025-1504.ch013.

Pandey, N., Tripathy, S.K., Patra, S.K., Jha, G., 2023. Recent Progress in hydrometallurgical processing of nickel lateritic ore. Trans. Indian Inst. Metals 76 (1), 11–30. https://doi.org/10.1007/s12666-022-02706-2.

Peng, X., Shi, L., Qu, T., Yang, Z., Lin, L., Xie, G., Xu, B., 2023. Kinetics of Ni and co recovery via oxygen-enriched pressure leaching from waste lithium-ion batteries. Separations 10 (2). https://doi.org/10.3390/separations10020064. Article 2.

Rodríguez, M., Wedderburn, B.J., 2007. Hematite Precipitation at Elevated Temperature and Pressure (World Intellectual Property Organization Patent No. WO2007095689A1). https://patents.google.com/patent/WO2007095689A1/en.

Rubisov, D.H., Papangelakis, V.G., 2000. Sulphuric acid pressure leaching of laterites—a comprehensive model of a continuous autoclave. Hydrometallurgy 58 (2). https://doi.org/10.1016/S0304-386X(00)00092-X. Article 2.

Stopić, S., Friedrich, B., 2011. Pressure hydrometallurgy: a new chance to non-polluting processes. Mil. Tech. Cour. 59, 29–44. https://doi.org/10.5937/votjehg1103029S.

Stopić, S., Friedrich, B., Anastasijevic, N., 2004. Kinetics of high pressure leaching of nickel lateritic ores. In: Pressure Hydrometallurgy: Proceedings of the International Conference on the Use of Pressure Vessels for Metal Extraction and Recovery. 34th Annual Hydrometallurgy Meeting of CIM, Banff, Alberta, Canada.

Tardy, Y., Boeglin, J.-L., Novikoff, A., Roquin, C., 1993. Petrological and geochemical classification of laterites. Clays Control. Environ. 10, 481–486.

Ventruti, G., Della Ventura, G., Gomez, M.A., Capitani, G., Sbroscia, M., Sodo, A., 2020. High-temperature study of basic ferric sulfate, FeOHSO4. Phys. Chem. Miner. 47 (10), 43. https://doi.org/10.1007/s00269-020-01113-7.
Wang, G.C., 2016. 3—Nonferrous metal extraction and nonferrous slags. In: Wang, G.C. (Ed.), The Utilization of Slag in Civil Infrastructure Construction. Woodhead Publishing, pp. 35–61. https://doi.org/10.1016/B978-0-08-100381-7.00003-3.

Watling, H.R., Elliot, A.D., Fletcher, H.M., Robinson, D.J., Sully, D.M., 2011. Ore mineralogy of nickel laterites: controls on processing characteristics under simulated heap-leach conditions. Aust. J. Earth Sci. 58 (7), 725–744. https://doi.org/10.1080/08120099.2011.602986.

Wells, M., Ramanaidou, E., Verrall, M., Tessarolo, C., 2009. Mineralogy and crystal chemistry of ‘garnierites’ in the Goro lateritic nickel deposit, New Caledonia. Eur. J. Mineral. 21. https://doi.org/10.1127/0935-1221/2009/0021-1910.

Whittington, B.I., Muir, D., 2000. Pressure acid leaching of nickel laterites: a review. Miner. Process. Extr. Metall. Rev. 21 (6), 527–599. https://doi.org/10.1080/08827500008914177.

Zhang, Z., Zhang, W., Zhang, Z., Chen, X., 2025. Nickel extraction from nickel laterites: processes, resources, environment and cost. China Geol. 8 (1), 187–213. https://doi.org/10.31035/cg2024124.