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# Carbothermal reduction of hematite involving biogenic carbon sourced from CO2

Matilde Bastide D'izard, Stéphane Abanades

► To cite this version:

Matilde Bastide D'izard, Stéphane Abanades. Carbothermal reduction of hematite involving biogenic carbon sourced from CO2. Chemical Engineering Journal Advances, 2025, 24, pp.100925. 10.1016/j.ceja.2025.100925. hal-05338195

HAL Id: hal-05338195

https://hal.science/hal-05338195v1

Submitted on 30 Oct 2025

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# Carbothermal reduction of hematite involving biogenic carbon sourced from CO₂

Matilde Bastide d'Izard, Stéphane Abanades*

<figure><img src="image_3.png" /></figure>

CNRS, Processes, Materials and Solar Energy Laboratory (PROMES), 7 rue du Four Solaire, 66120 Odeillo Font-Romeu, France

## ARTICLE INFO

**Keywords:**

- Ironmaking
- Solar metallurgy
- Direct reduced metallic oxide
- Carbothermal reduction
- Carbon-neutral process
- Bio-sourced carbon

## ABSTRACT

The reduction of hematite (Fe₂O₃) to metallic iron via a carbothermal process was investigated with the aim of using carbon derived from captured or recycled CO₂. This CO₂-sourced carbon can be obtained by pyrolysis either directly from biomass or methane (issued from biogas or via a synthetic methanation route, through the Sabatier reaction). The carbon can ultimately serve as the reducing agent in a carbon-neutral solar thermal process. A comparative study was conducted to assess the thermodynamic and kinetic behavior of Fe₂O₃ reduction using four distinct carbon materials: carbon black, activated charcoal, graphite, and biochar. The influence of heating rate and carbon-to-oxide molar ratio (C/Fe₂O₃) was analyzed. Thermogravimetric analysis was employed to examine the reaction kinetics and mechanisms, while independent experiments in a tubular reactor further validated the results. These techniques enabled the synthesis of metallic iron and provided valuable insights into the reaction sequence, gas evolution rates (CO, CO₂), and phase transformations under controlled conditions. The expected phase transformation sequence: Fe₂O₃ → Fe₃O₄ → FeO → Fe was evidenced by gas phase analysis. X-ray diffraction analysis of the solid products confirmed nearly complete conversion of Fe₂O₃ to Fe at a C/Fe₂O₃ molar ratio of 3.0 or higher. The heating rate did not significantly affect reduction efficiency, whereas a carbon excess promoted near-complete reduction. At a fixed C/Fe₂O₃ ratio of 4.5, conversion profiles showed slight variations based on the carbon type used. Graphite exhibited a sharp conversion increase at ~1110 °C, while the other reductants displayed more gradual and similar conversion profiles with stepwise release of CO₂ and CO according to a three-stage mechanism. Notably, a lower onset reduction temperature (~959 °C) was observed with biochar, indicating the potential of bio-sourced materials conversion for green ironmaking.

## 1. Introduction

The direct thermochemical reduction of metal oxides using concentrated solar energy offers a renewable and sustainable route for metal production [1]. The resulting metals can then serve as hydrogen vectors, enabling on-demand hydrogen production and long-term energy storage in solid form. This process is based on solar-driven thermochemical cycles in which metal oxides are reduced in high-temperature solar reactors to produce metals [2,3]. Concentrated solar energy is used as the source of high-temperature process heat, which offers various advantages. This eliminates CO₂ emissions associated with fossil fuels combustion for process heat supply, while enabling the storage of solar energy in chemical form through endothermic reactions, for the production of clean and transportable chemical fuels and materials.

In a typical two-step solar thermochemical cycle, the reduced metal

is further subjected to hydrolysis, generating hydrogen gas (H₂) while simultaneously regenerating the original oxide, thus closing the loop [2, 4,5]. CO₂ can also be used as an oxidant instead of steam, thus producing CO (forming syngas when mixed with H₂), as a promising route for the conversion and valorization of CO₂ emissions into synthetic fuels form concentrated sunlight. This dual functionality addresses two major energy transition challenges: (1) the decarbonized production of metals, and (2) the storage and release of green hydrogen (or syngas) in a reversible and efficient manner.

Previous thermodynamic analyses have examined the reaction equilibria associated with different reductants, in order to identify the most stable metallic compounds and chemical phases formed during the process [6]. Results indicate that carbon-based reduction pathways are generally more favourable than hydrogen-based routes, which often require a large excess of hydrogen to achieve acceptable yields. Notably,

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: stephane.abanades@promes.cnrs.fr (S. Abanades).</footnote>
for oxides such as Fe₂O₃, SnO₂, GeO₂, MgO, and ZnO, the use of excess hydrogen significantly enhances reduction performance, with near-complete conversion at moderate temperatures. In contrast, metals such as Al, Si, Ti, and V are more challenging to produce via hydrogen reduction due to the extremely high temperatures required. Only the complete reduction of NiO, Co₃O₄, and CuO to metals is predicted to occur without H₂ excess whatever the temperature, but the reverse step (steam oxidation of Ni, Co, and Cu) is however impossible. This justifies using carbon as a reductant instead of H₂ to provide more favorable conditions for oxides reduction to metals. Consequently, this study focuses on carbothermal reduction as a viable and energy-efficient method for sustainable metal production. Among the metal oxides studied, several candidates were identified as particularly suitable for carbothermal reduction based on theoretical evaluations, including ZnO, MgO, Ge₂O₃, SnO₂, MoO₃, and WO₃. Furthermore, the feasibility of solar carbothermal reduction of Al₂O₃, ZnO, SnO₂, WO₃, and MgO has already been experimentally studied and demonstrated [7–13]. However, Fe₂O₃ (hematite) deserves particular attention due to its abundance, low cost, environmentally safe compared to other metal oxides [14].

In fact, finding alternative processes with low carbon footprint that could replace conventional metallurgical methods for producing metals (Fe, Al, Mg, and Si) is a significant challenge, especially given the high energy consumption and dependence on non-renewable resources of these traditional processes. Currently, the main route of steel production (71 %) is achieved by the reduction of iron ore in a blast furnace-basic oxygen furnace, and relies on energy-intensive processes using coal or coke. Globally, the steel metallurgy is responsible for 7 % of global greenhouse gas emissions (1.87 tCO₂ eq. per ton of steel produced). As a promising alternative for decarbonizing steelmaking, the direct solar thermal reduction of Fe₂O₃ to Fe emerges, combining high-temperature concentrated solar heat with bio-sourced reductants (biogas, biochar, biomass) or carbon-free reductants (green H₂ or NH₃) [7,15–17]. In parallel, carbothermal reduction can only be carbon-neutral if non-fossil carbon is used in the process [18,19]. Therefore, using biogenic carbon or carbon derived from CO₂ recycling is of major importance for warranting a long-term sustainable process. The integration of CO₂ reuse for green fuel and commodity synthesis has gained increasing attention [20]. Carbon can be obtained from CO₂ following two different main routes: conversion of bio-resources or synthetic approach. In the first approach, pyrolysis processes of biomass/waste or biogas/biomethane (in which CO₂ was preliminary stored through photosynthesis) yield solid carbon (biochar), which can be used as a biogenic reductant for a sustainable implementation of carbothermal reduction. In the latter approach, carbon can be derived from the CO₂ methanation process, which is based on Sabatier reaction: CO₂ + 4H₂ → CH₄ + 2H₂O [20]. This reaction combines green hydrogen and CO₂ in order to synthesize methane. Two main CO₂-to-methane conversion pathways exist: biological methanation, which operates at mild temperatures (37–65 °C) and pressures (1–15 bar) via anaerobic digestion of biomass [22], and catalytic methanation, which uses metal catalysts (e.g., Ni/Al₂O₃) at higher temperatures (200–550 °C) and pressures (up to 100 bar) for efficient conversion [23]. The resulting methane can then be thermally decomposed via pyrolysis to produce solid carbon and turqoise hydrogen: CH₄ → C + 2H₂ [24–27]. The produced H₂ can also be reused in the methanation process [28]. Therefore, when H₂ cannot be directly used as a reductant, it is possible to convert it into CH₄ by adding CO₂, for the synthesis of carbon as a more favourable reductant. Following the aforementioned pathways, the produced carbon is thus obtained from CO₂ (either atmospheric CO₂ captured from biomass crops, or industrial CO₂ sources). Then, the carbothermal process can be considered as carbon neutral.

This solid carbon, derived from renewable sources, serves as a reductant in the carbothermal reduction of metal oxides, thus eliminating direct net CO₂ emissions and supporting a clean metallurgical process. This paves the way toward the direct use of bio-based reductants such as solid biomass or waste residues for the production of

green iron using concentrated solar energy. This first step combines the use of two renewable resources (solar energy and biomass/biowaste feedstocks) to produce metals and a high-value H₂-rich syngas. The metals can be advantageously steam oxidized to produce H₂ on demand in an exothermal reaction or used as long-term hydrogen storage solid media, which could be transported or used for stationary applications. Ultimately, such solar pyrometallurgical cycling processes combining metal oxides reduction and bioresource conversion thus represent an attractive path for green hydrogen production and long-term storage.

Although numerous studies have investigated the carbothermal reduction of hematite, very few have specifically focused on the use of bio-based (bio-sourced) carbon materials. This represents a notable research gap—and a point of originality—particularly in the context of developing sustainable and carbon-neutral reductants for metallurgical applications.

The present study aims to conduct a comparative thermodynamic equilibrium and kinetic investigation of hematite reduction as a function of the temperature using four different types of carbon: Carbon Black from methane pyrolysis (CB), Activated Charcoal (AC), Graphite (G), and Biochar from biomass pyrolysis. In addition, the influence of the carbon-to-oxide molar ratio (C/Fe₂O₃) on the overall reduction efficiency is examined, with particular attention paid to the stepwise reduction pathway of iron oxides (Fe₂O₃ → Fe₃O₄ → FeO → Fe), in order to gain a deeper understanding of the involved reduction mechanisms.

The reduction behavior was analyzed using Thermogravimetric Analysis (TGA) supported by in situ gas analysis via Quadrupole Mass Spectrometry (QMS), followed by post-reaction solid products characterization via powder X-ray diffraction (XRD). This combined approach enables a detailed characterization of the gas-phase species released during the reaction and allows for a precise correlation with the phase evolution of the solid products.

To complement and further validate the TGA-based findings, a series of parallel experiments were conducted in a tubular furnace equipped with an online CO/CO₂ analyzer for gas products quantification. This independent methodology not only reinforces the reliability and reproducibility of the results but also provides an additional perspective on the reduction process under more practical conditions.

## 2. Thermodynamic analysis

The reduction of Fe₂O₃ using various reductants such as H₂, CO, a mixture of H₂ and CO, CH₄, biomass and solid carbon (graphite, coke) has been the subject of extensive study [29–36]. It has been evidenced that the conversion of hematite to metallic iron is highly temperature-dependent [37]. In the event of solid carbon being utilised as a reductant, the reduction of iron oxides is the consequence of both solid-solid and solid-gas mechanisms, each playing a significant role in the overall conversion [15]. It was shown that when the temperature is above 570 °C [32], the reduction of hematite occurs through a series of reactions: Fe₂O₃ → Fe₃O₄ → FeO → Fe [15,38,39].

To anticipate the dominant reaction pathways, the Gibbs free energy changes (ΔG°) for each of these reactions were calculated and plotted as a function of temperature, from 25 °C to 1500 °C, using HSC Chemistry 10.0 (Fig. 1).

Solid-solid mechanism:

$$3\text{Fe}_2\text{O}_3(\text{s}) + \text{C}(\text{s}) = 2\text{Fe}_3\text{O}_4(\text{s}) + \text{CO}(\text{g}) \quad (1)$$

$$\text{Fe}_3\text{O}_4(\text{s}) + \text{C}(\text{s}) = 3\text{FeO}(\text{s}) + \text{CO}(\text{g}) \quad (2)$$

$$\text{FeO}(\text{s}) + \text{C}(\text{s}) = \text{Fe}(\text{s}) + \text{CO}(\text{g}) \quad (3)$$

$$\text{Fe}_2\text{O}_3(\text{s}) + 1.5\text{C}(\text{s}) = 2\text{Fe}(\text{s}) + 1.5\text{CO}_2(\text{g}) \quad (4)$$

$$\text{Fe}_2\text{O}_3(\text{s}) + 3\text{C}(\text{s}) = 2\text{Fe}(\text{s}) + 3\text{CO}(\text{g}) \quad (5)$$

Solid-gas mechanism:
<figure><img src="image_4.png" /><figcaption>Fig. 1. Gibbs free enthalpy variation associated with possible reactions during Fe₂O₃ carbothermal reduction.</figcaption></figure>

$$3\text{Fe}_2\text{O}_3(\text{s}) + \text{CO}(\text{g}) = 2\text{Fe}_3\text{O}_4(\text{s}) + \text{CO}_2(\text{g}) \quad (6)$$

$$\text{Fe}_3\text{O}_4(\text{s}) + \text{CO}(\text{g}) = 3\text{FeO}(\text{s}) + \text{CO}_2(\text{g}) \quad (7)$$

$$\text{FeO}(\text{s}) + \text{CO}(\text{g}) = \text{Fe}(\text{s}) + \text{CO}_2(\text{g}) \quad (8)$$

$$\text{Fe}_2\text{O}_3(\text{s}) + 3\text{CO}(\text{g}) = 2\text{Fe}(\text{s}) + 3\text{CO}_2(\text{g}) \quad (9)$$

$$\text{C}(\text{s}) + \text{CO}_2(\text{g}) = 2\text{CO}(\text{g}) \quad (10)$$

At low temperature, below 600 °C, thermodynamics predicts that only reactions (6,8,9) are the most favored for hematite reduction with CO as reducing agent. Above 600–700 °C, all other reactions are spontaneous, whereas only reaction (8) becomes not favored. Reactions (6) and (9) tend to be favored whatever the temperature while reaction (1) is possible above 320 °C.

A thermodynamic equilibrium analysis was conducted prior to the experimental work using HSC Chemistry v.10 and its database to predict and better understand the mechanisms involved in the carbothermal reduction of Fe₂O₃. These calculations are based on Gibbs free enthalpy minimization, assuming a closed system with ideal mixing behavior, and without heat/mass transfer and kinetic limitations. The reaction systems were simulated within a typical temperature range of 25 °C to 2300 °C and at a total pressure of 1 bar with a fixed amount of N₂ as gas diluent. The products with molar fractions below 10⁻³ were not represented.

The most thermodynamically stable compounds were computed under given conditions of temperature, pressure, and reactant quantities. Fig. 2.a presents the equilibrium phase composition of a Fe₂O₃ + 3C mixture as a function of temperature. Thermodynamic predictions suggest that, below 680 °C, Fe₂O₃ is initially converted into intermediate iron oxides such as Fe₃O₄ and FeO, as well as iron carbides (Fe₂C and Fe₃C). Above 1000 °C, the initial carbon is totally converted into CO, and Fe is stably produced in expected stoichiometric proportions.

Finally, the influence of carbon stoichiometry on the extent of conversion was assessed in order to determine the most favorable range of C/Fe₂O₃ molar ratio. To this end, the composition of the solid reacting mixture was evaluated as a function of the carbon content (C:Fe₂O₃ ratio) at a fixed temperature of 1200 °C and a pressure of 1 bar, as shown

in Fig. 2b. Excess carbon can promote the generation of CO gas via the Boudouard reaction (Reaction 10) at temperatures above ~700 °C, thereby facilitating hematite reduction. The results show that a C/Fe₂O₃ molar ratio of 3:1 is optimal to completely reduce Fe₂O₃, while the oxide is only partially reduced into other suboxides between 1 and 2.5 mol of carbon. Results also suggest that increasing the carbon amount beyond 3 provides no clear benefit, as this promotes the formation of iron carbide phases, such as Fe₂C and Fe₃C. Thus, the effect of carbon amount on the reaction mechanism was further elucidated in the experimental section of this study. The impact of adding sweep gas nitrogen (N₂) on the conversion extent was also studied at 750 °C. Adding N₂ sweep gas gradually increased the conversion of Fe₂O₃ until reaching completion with a large excess of N₂ (Fig. S1).

## 3. Experimental setup

### 3.1. Materials and methods

An experimental study was carried out to investigate the Fe₂O₃ carbothermal reduction reaction and to estimate the effect of the carbon type, as well as the composition of the reactant, namely C/Fe₂O₃ molar ratio (1.5; 3; 4.5), and heating rates on the reaction extent.

For the sample preparation, commercial Fe₂O₃ was used (purity ≥ 97 %, -325 mesh, bulk density: 0.34 g/cm³, purchased from VWR Chemicals, CAS N° 1309–37–1) and a variety of carbon sources: activated charcoal (Sigma-Aldrich Darco, particle size < 149 µm, 732 m².g⁻¹; denoted AC hereafter), graphite (Sigma-Aldrich, particle size < 20 µm, 77 m².g⁻¹; denoted G hereafter), carbon black (grade SB905, supplied by Asahi Carbon- Japan, 210 m².g⁻¹, average particle size = 15 nm; denoted CB hereafter), and biochar obtained by pyrolysis of soft wood at 700 °C (C: 90.21 wt %(d.b.), H: 1.83 wt %, O: 6.02 wt %, volatile matter: 6.66 wt %(d.b.), ash: 1.89 wt %(d.b.), total surface area: 162.3 m²/g).

Powder X-Ray diffraction (XRD) analysis was performed for phase characterization of the reduced samples using a Malvern-Panalytical Empyrean diffractometer (X-ray tube, tension = 40 kV, intensity = 30
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Equilibrium composition of the Fe₂O₃ + 3C system versus temperature at 1 bar, and (b) Equilibrium composition at 1200 °C as a function of the carbon content (C:Fe₂O₃ ratio) in the reactant mixture (1 mol of Fe₂O₃; P = 1 bar, 10 mol of N₂).</figcaption></figure>

mA, anode material = Co, radiation wavelengths: K-alpha1 = 1.78901 Å, K-alpha2 = 1.79290 Å, K-beta = 1.62083 Å). XRD measurements of 2θ symmetrical scans were made over an angular range from 10 to 85°, and the step size and counting time were fixed at 0.039° and 445 s, respectively. Two different complementary apparatuses, thermogravimetric analysis (TGA) and a tubular reactor, were used to study the Fe₂O₃ carbothermal reduction.

## 3.2. Thermogravimetric experiments

A thermogravimetric analysis (TGA using a Netzsch STA 449 Jupiter equipped with a SiC furnace and offering a resolution of 0.1 µg) coupled with differential scanning calorimetry (DSC) was carried out to quantify the reaction progress of the carbothermic reduction of Fe₂O₃ with the different carbon sources. The TGA operating principle is based on the measurement of the variation of the sample weight during thermal treatment, while qualitatively identifying the released gas species by evolved gas analysis (EGA) using quadrupole mass spectrometry (QMS 403C Aëolos, 1–300 amu).

A sample of approximately 20 mg, composed of a mixture of Fe₂O₃ and carbon (AC, G, CB, and Biochar), was placed inside an alumina

crucible (0.17 mL) and heated with a constant heating rate of 5, 10, or 20 °C.min⁻¹ until reaching the maximum desired temperature (1200 °C) in an argon atmosphere (99.999 % purity, flow rate of 0.04 NL.min⁻¹). The influence of the carbon excess on the reaction extent was investigated by varying the molar ratio of carbon to Fe₂O₃ (C/ Fe₂O₃) at 1.5 (under-stoichiometric ratio), 3 (stoichiometric), and 4.5 (50 % excess carbon). These tests aimed to study the effect of the stoichiometry of the sample (C/Fe₂O₃ mole ratio).

During reduction, the mass variation of the sample was measured and recorded continuously as a function of time. The mass loss of the sample was attributed to the release of both oxygen (from Fe₂O₃) and carbon in the form of CO (and CO₂). Particle conversion was then calculated by using the thermogravimetric mass variation as a function of time [Eq. (11)]:

$$X(t) = \frac{\Delta m \cdot M_{Fe_2O_3}}{3M_{CO} \cdot m_{i,Fe_2O_3}} \quad (11)$$

in which Δm is the sample mass change, MFe₂O₃ is the molecular weight of Fe₂O₃, mᵢ,Fe₂O₃ is the initial sample mass of Fe₂O₃, and MCO is the molecular weight of CO.
### 3.3. Tubular reactor experiments

The carbothermic reduction of $Fe_2O_3$ was also examined in a tubular furnace to consider the reaction with different carbon types (AC, G, CB, and Biochar) and to assess the evolution of gaseous products (CO, $CO_2$) generated during the progress of the reduction. Experiments were carried out under non-isothermal conditions during continuous sample heating. They aimed to determine the final iron oxide conversion, the temperature of reaction start, and to compare the reduction mechanism between the different carbon types. This method was thus complementary to TGA and aimed to validate the results.

The tubular reactor (50 mm inner diameter alumina tube, with a heated length of 180 mm) consisted of a horizontal electrically-heated tube furnace. In these experiments, a sample weight of approximately 0.5 g, composed of a mixture of $Fe_2O_3$ and carbon (AC, G, CB, or Biochar), was introduced in an alumina crucible and placed at the center of the tube. The sample was heated with a linear heating rate of 10 °C/min, in Ar atmosphere (0.5 NL/min) until reaching the targeted maximum temperature (1250 °C) measured by a type R thermocouple placed just above the sample. The Ar flow rate was regulated by a mass flow controller (Brooks Instruments, model 5850S).

This tubular reactor enabled precise quantification of the gas composition released during sample heating. Therefore, outlet gases mole fractions (CO–$CO_2$) were continuously recorded via online analysis using a non-dispersive infrared sensor (XSTREAM; full scale, 0 – 40 % $CO_2$ and 0 – 40 % CO; repeatability, ±1 % full scale) connected to the reactor outlet.

In these experiments, the final analysed gas consists of Ar (in an amount equal to Ar introduced at the inlet), as well as two additional gaseous products—CO and $CO_2$— formed during the reaction. The CO and $CO_2$ production rates were obtained with the results of the gas analysis [(Eq. (12))]:

$$F_i = F_T \cdot y_i \quad (12)$$

in which $F_T$ is the total outlet molar gas flow rate (mol s⁻¹), and $y_i$ the outlet molar fraction of species i (CO and $CO_2$) obtained from the gas analysis. Additionally, a global mass balance was used [Eq. (13)]:

$$F_T = F_{Ar} + F_{CO} + F_{CO_2} \quad (13)$$

in which $F_{Ar}$, $F_{CO}$, and $F_{CO_2}$ are the outlet molar flow rates (mol s⁻¹) of Ar, CO, and $CO_2$ respectively. Then, the outlet flowrates of CO and $CO_2$ were determined as follows [Eq. (14)]:

$$F_i = \frac{F_{Ar} \cdot y_i}{(1 - \sum y_i)} \quad (14)$$

Based on this information, the total chemical conversion, X(t), was calculated as the ratio of the amount of oxygen contained in CO and $CO_2$ produced at time t to the maximum amount of oxygen that could be released from the initial quantity of $Fe_2O_3$ in the sample [Eq. (15)]:

$$X(t) = \frac{M_{Fe_2O_3} \cdot \left( \int_0^t F_{CO} dt + 2 \int_0^t F_{CO_2} dt \right)}{3m_{i,Fe_2O_3}} \quad (15)$$

## 4. Results and discussion

### 4.1. Carbothermal reduction in TGA

#### 4.1.1. Typical reduction profile

Non-isothermal reduction experiments of hematite with various carbon sources were conducted to determine the characteristic temperatures associated with the sequential transformation steps in the reduction process. These experiments were performed using TGA-DSC thermal analysis, coupled with QMS for simultaneous real-time analysis of the evolved gases. QMS was directly connected to the TGA-DSC outlet, allowing for in-situ detection of evolved gaseous products. The

conversion rate was calculated using Eq. (11), in which the mass-loss was directly provided by TGA data.

Overall, the results reveal a clear sequence of conversion steps that appear to be consistent regardless of the carbon source used for the carbothermal reduction of $Fe_2O_3$. The characteristic gas profiles suggest that the underlying reaction mechanism remains similar across all carbon types. As an illustrative example, the reduction of hematite with activated carbon (AC) is presented in Fig. 3 using a molar ratio of 3:1 and a heating rate of 20 °C/min. This instance serves to identify and characterize the distinct phases involved throughout the reduction process. Moreover, a kinetic analysis was carried out to identify the most suitable reaction model and an activation energy of 187 kJ/mol was determined ($R^2 = 99 \%$), in agreement with [40] (as detailed in Supplementary Material).

The TGA data indicate that the conversion of the $Fe_2O_3 + 3C$ mixture proceeds through three distinct stages, corresponding to the sequential reduction of $Fe_2O_3$ to metallic Fe: hematite → magnetite → wüstite → iron. The first step starts with the mass reduction of the $Fe_2O_3 + 3C$ sample around 710 °C and is characterized by the formation of $CO_2$. Several hypotheses have been proposed to explain the presence of $CO_2$. First, the detected $CO_2$ could be attributed to combined reactions (1) and (6) [41]. This suggests that the CO produced by reaction (1) is immediately consumed in subsequent reaction (6), explaining why solid-gas reactions are generally faster than solid-solid reactions. This explains why only $CO_2$ is detected by the QMS during the initial conversion step. Another hypothesis is based on Ellingham diagrams, which indicate that above 600 °C, reaction (4) is favourable, and can also initiate the production of $CO_2$. This first phase illustrates a change in oxidation state, where $Fe_2O_3$ is partially converted to $Fe_3O_4$, with the loss of one oxygen atom out of the nine required for complete reduction to metallic iron, corresponding to a reduction degree of 11 %.

The second step of the conversion process is characterized by the initial production of CO and a subsequent increase in $CO_2$, both of which are detected by the QMS around 890 °C, with a $CO_2$ peak occurring at approximately 960 °C. Concurrently, the reduction degree increases from 11 % to 33 %, corresponding to the reduction of $Fe_2O_3$ in FeO (1 oxygen atom lost out of 3). The reactions involved for this conversion step can be described by reaction (2) followed by (7).

Interestingly, Boudouard reaction (10) begins before complete reduction of $Fe_3O_4$ to FeO is finished. This suggests that some of the wüstite reduced from $Fe_3O_4$ can be converted into metallic iron before $Fe_3O_4$ is fully reduced to wüstite [41].

Finally, the third step is characterized by two peaks of CO and $CO_2$ occurring in the same temperature range (around 1095 °C). During this stage, the production of CO becomes predominant over $CO_2$, with a CO/$CO_2$ ratio of 2.38. The conversion rate during this phase is the highest, and it is boosted by the Boudouard reaction which is strongly favoured in this temperature range, above 837 °C, according to the Gibbs free energy. This reaction helps to deplete the available carbon by generating a significant amount of CO. During this phase, the formation of metallic iron is made possible through the reduction of wüstite via reaction (3). The nucleation and growth of metallic iron particles enable carbon dissolution into the iron structure. The solubility of carbon depends on temperature (reaching a maximum around 1150 °C, at the eutectic point) and on the composition of the iron-carbon system [42,43]. In fact, the transformation of iron from ferrite ($Fe\alpha$), which dissolves little carbon, to austenite ($Fe\gamma$), which dissolves much more carbon, promotes this dissolution. This dissolved carbon plays a crucial role in further reducing the remaining wüstite (FeO) to produce metallic iron.

These results are consistent with those reported by Rao [44], who observed that at a reduction temperature of 877 °C, the solid-gas reaction became dominant due to a sufficient production of CO. This observation aligns with Rao's conclusions, which indicate that carbon gasification is the rate-limiting step of the overall process. In fact, a higher reduction rate is achieved when the temperature exceeds 1061 °C, thus favouring the conversion of hematite.
<figure><img src="image_6.png" /><figcaption>Fig. 3. (a) Conversion extent as a function of time and temperature in TGA, (b) normalized intensity of the main gas products (CO and CO₂) released from the reduction reaction and corresponding mass loss for Fe₂O₃ + 3AC.</figcaption></figure>

The effect of heating rate, carbon excess, and carbon type was then investigated in details. Table 1 summarizes the experimental runs and operating conditions in TGA. Reproducibility tests were successfully performed for several types of carbon and conditions in TGA experiments. This is the case for example for AC (runs #1 and #2, runs #3 and #4 in Table 1), G (runs #6 and #7), and biochar (runs #12 and #13).

### 4.1.2. Effect of the heating rate

The effect of the heating rate on the final conversion of the reaction was investigated. Fig. 4.a presents the results obtained at three different

heating rates (5, 10, and 20 °C/min) for a C/Fe₂O₃ ratio of 4.5. The data show that increasing the heating rate does not significantly modify the overall oxide conversion. Instead, it causes a temporal shift in the progression of the reduction reaction. As illustrated in Fig. 4.b, whatever the heating ramp applied, the reaction consistently reaches completion at 1200 °C under the conditions studied. The CO and CO₂ gases released during the reduction process keep a similar profile at any heating rate. These findings diverge from those reported by [40], who investigated the reduction of hematite using coke under both isothermal conditions (1423–1573 K) and non-isothermal regimes. In contrast to the present

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

Description of the operating conditions and results for TGA runs.</caption><thead><tr><th>Run #</th><th>Heat ramp (°C/min)</th><th>Apparatus</th><th>Carbon type</th><th>Mole Ratio (C/Fe<sub>2</sub>O<sub>3</sub>) (mol<sub>C</sub>/mol<sub>Fe<sub>2</sub>O<sub>3</sub></sub>)</th><th>Mass Ratio (g) (gC/gFe<sub>2</sub>O<sub>3</sub>)</th><th>Mass loss (%)</th><th>Onset (°C)</th><th>Peak (°C)</th><th>Maximum conversion</th></tr></thead><tbody><tr><td>1</td><td>20</td><td>TGA</td><td>AC</td><td>4.5 : 1</td><td>1.69/5</td><td>39.45</td><td>1015.1</td><td>1110</td><td>1.00</td></tr><tr><td>2</td><td>20</td><td>TGA/MS</td><td>AC</td><td>4.5 : 1</td><td>1.69/5</td><td>41.85</td><td>1008.8</td><td>1108.9</td><td>1.00</td></tr><tr><td>3</td><td>10</td><td>TGA</td><td>AC</td><td>4.5 : 1</td><td>1.69/5</td><td>46.41</td><td>984.4</td><td>1093</td><td>0.998</td></tr><tr><td>4</td><td>10</td><td>TGA/MS</td><td>AC</td><td>4.5 : 1</td><td>1.69/5</td><td>40.74</td><td>1004.0</td><td>1080</td><td>0.995</td></tr><tr><td>5</td><td>5</td><td>TGA/MS</td><td>AC</td><td>4.5 : 1</td><td>1.69/5</td><td>42.18</td><td>986.3</td><td>1062</td><td>0.999</td></tr><tr><td>6</td><td>20</td><td>TGA/MS</td><td>G</td><td>4.5 : 1</td><td>1.69/5</td><td>38.20</td><td>-</td><td>1117</td><td>1.00</td></tr><tr><td>7</td><td>20</td><td>TGA/MS</td><td>G</td><td>4.5 : 1</td><td>1.69/5</td><td>39.59</td><td>-</td><td>1107</td><td>1.00</td></tr><tr><td>8</td><td>20</td><td>TGA/MS</td><td>CB</td><td>4.5 : 1</td><td>1.69/5</td><td>39.26</td><td>1081.4</td><td>1137</td><td>0.999</td></tr><tr><td>9</td><td>20</td><td>TGA/MS</td><td>AC</td><td>3.0 : 1</td><td>1.127/5</td><td>38.80</td><td>1037.4</td><td>1134</td><td>0.997</td></tr><tr><td>10</td><td>20</td><td>TGA/MS</td><td>AC</td><td>1.5 : 1</td><td>0.563/5</td><td>25.24</td><td>1039.2</td><td>1118</td><td>0.650</td></tr><tr><td>11</td><td>10</td><td>TGA/MS</td><td>CB</td><td>4.5 : 1</td><td>1.69/5</td><td>40.14</td><td>1049.1</td><td>1114</td><td>0.999</td></tr><tr><td>12</td><td>20</td><td>TGA/MS</td><td>Biochar</td><td>4.5 : 1</td><td>1.69/5</td><td>43.14</td><td>959.1</td><td>1022.4</td><td>1.00</td></tr><tr><td>13</td><td>20</td><td>TGA/MS</td><td>Biochar</td><td>4.5 : 1</td><td>1.69/5</td><td>41.28</td><td>957.9</td><td>1022.4</td><td>1.00</td></tr></tbody></table>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Effect of the heating rates (5, 10, and 20 °C/min) on the Fe₂O₃ +4.5AC sample conversion (runs #1–5): (a) Conversion rate of Fe₂O₃ as a function of temperature, (b) Time evolution of the conversion rate of Fe₂O₃ and evolved gas species during carbothermal reduction (conversion profiles perfectly overlap for runs #1 and #2, and runs #3 and #4 carried out under the same conditions, confirming results repeatability).</figcaption></figure>

study, their results demonstrated that lower heating rates led to higher final degrees of reduction. For instance, the reduction degree of hematite decreased from 86.63 % to 71.61 % as the heating rate increased from 5 to 15 K/min. This observation contradicts the trends shown in the present work, where the final conversion is not significantly affected by the heating rate.

### 4.1.3. Effect of the reactant stoichiometry on the conversion extent

To assess the influence of carbon stoichiometry on the carbothermal reduction of Fe₂O₃, experiments were conducted with varying C/Fe₂O₃ molar ratios of 1.5, 3.0, and 4.5 at a heating rate of 20 °C/min (Fig. 5.a). The results indicate that complete conversion of Fe₂O₃ is attained at a molar ratio of 3.0 or higher, suggesting that this ratio represents the stoichiometric threshold required for full reduction under the given experimental conditions.
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Effect of carbon stoichiometry on the conversion extent of Fe₂O₃ using activated carbon as reductant: (a) Evolution of conversion extent as a function of time and temperature, along with the normalized intensity of the main gas products (CO and CO₂) released from the reduction reaction, for (b) Fe₂O₃ + 1.5AC (run #10), and (c) Fe₂O₃ + 3AC (run #9).</figcaption></figure>
In the sub-stoichiometric system (Fe₂O₃ + 1.5 C), the conversion extent is partial, reaching 65.0 % (Fig. 5.b). The initial stage of the reaction involves direct reduction of Fe₂O₃ by carbon, yielding CO₂ and metallic Fe via the reaction: Fe₂O₃ + 1.5C = 2Fe(s) + 1.5CO₂. This reaction is initiated from ~780 °C and is responsible for the observed mass loss of the oxide and the first CO₂ release.

Regarding the gaseous products, the formation of CO₂ starts around 800 °C and increases steadily to reach a concentration peak at 1031 °C. Concomitantly, the formation of CO starts around 970 °C, culminating in a distinct peak at 1147 °C. In fact, from ~1000 °C, as the temperature increases, the CO₂ produced during the first reaction involving solid-solid mechanism, reacts with solid C to form CO according to the Boudouard reaction (C + CO₂ = 2CO), which explains the peak of CO. A secondary increase in CO₂ concentration is evidenced by a peak at 1152 °C, suggesting subsequent reaction pathways involving CO according to Fe₂O₃ + 3CO → 2Fe + 3CO₂.

In contrast, in the Fe₂O₃ + 3C system (Fig. 5.c), characterized by a stoichiometric proportion of solid carbon, the first reactional mechanism occurring is related to solid-solid reaction, Fe₂O₃ + 3C = 2Fe(s) + 3CO. This CO produced then serves as the reducing agent in the solid-gas reaction Fe₂O₃ + 3CO = 2Fe(s) + 3CO₂, contributing to the initial CO₂ formation. It is noticeable that, when the molar ratio is increased from 1.5 to 3, the first CO₂ peak intensity is significantly reduced from 73 % to 38 % and its maximum peak is shifted to a lower temperature at 1001 °C. Therefore, a sub-stoichiometry of 1.5AC leads to a CO₂ peak of significantly higher relative intensity compared to 3AC or 4.5AC. This can be explained by the predominant role of reaction (4) at low temperature.

The Boudouard reaction can then generate CO, which reacts in the following solid-gas reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. This promotes a second peak of CO₂ at 1137 °C, subsequently leading to the formation of CO peak at exactly the same temperature.

In these stoichiometric conditions, the reduction of Fe₂O₃ is complete at 1200 °C, leading to full conversion to metallic iron.

#### 4.1.4. Effect of the carbon type

The selection of the carbonaceous reductant plays a pivotal role, as it determines the maximum temperature required to achieve complete conversion of hematite and significantly influences the relative quantities of CO and CO₂ evolved during the reaction [6]. The effect of the carbon type on the conversion profile is provided in Fig. S3.

It is first worth noting that the conversion and gas evolution profiles of AC and CB are similar, suggesting that comparable reaction mechanisms proceed in both cases, although a faster rate is observed for CB in the last stage of the reaction (Fig. S3). The main distinction lies in the temperature at which the reactions occur. For instance, in the Fe₂O₃ + 4.5AC system, the reduction onset occurs at approximately 1004 °C, with a CO evolution peak around 1110 °C. Regarding the Fe₂O₃ + 4.5CB system, Fig. 6 shows an onset at 1049 °C, with the CO peak reached at roughly 1137 °C. The DSC analysis (run #8) of Fe₂O₃ + 4.5CB at 20 °C/min reveals two distinct DSC peaks: the first peak corresponds to an enthalpy value of 58.4 J/g, and the second to 213.1 J/g. These peaks can be attributed to the last two stages of hematite conversion with carbon: Fe₃O₄ → FeO and FeO → Fe. For run #11, (Fe₂O₃ + 4.5CB at 10 °C/min), the first and second DSC peaks yield an enthalpy of 62.6 J/g and 299.4 J/g. The DSC peaks for AC are not significant, as the reactions are slower and thus their detection by DSC is tricky.

Regarding reduction with biochar (Fig. 7), the reaction temperature is significantly lowered with a peak rate at 1022.4 °C, thus about 100 °C lower compared to AC and CB. The repeatability of results is confirmed through two different runs under the same conditions in Fig. 7a and 7b. The corresponding DSC peaks amount to 63.1 J/g for the second reduction stage and 314.5 J/g for the third stage. The reduction mechanism appears to be unchanged, showing three consecutive and distinct release steps for CO₂ and two distinct peaks for CO. In addition, the release of H₂ is also evidenced from 700 °C, which may be attributed to decomposition of the volatile matter in biochar.

Previous experimental studies predicted that amorphous carbons, such as AC, CB, and Biochar, exhibit higher reactivity compared to those possessing a well-ordered crystalline structure exhibiting a limited

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Conversion extent as a function of time and temperature for Fe₂O₃ + 4.5CB, along with the normalized intensity of the main gas products (CO and CO₂) released from the reduction reaction, for two different heating rates at 10 °C/min (run #11) and 20 °C/min (run #8).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Conversion extent as a function of time and temperature for Fe₂O₃ + 4.5Biochar, along with the normalized intensity of the main gas products (CO, CO₂, and H₂) released from the reduction reaction, for two different trials at 20 °C/min: (a) run #12 and (b) run #13.</figcaption></figure>

number of active sites. Amorphous carbons like biochar contain defects and edge sites, while these defect-rich structures are more chemically reactive. Due to porous structure, carbon and Fe₂O₃ particles tend to better mix, thus enhancing particle dispersion and contact. This improves solid-state diffusion and reaction at interfaces, thereby favouring reduction at a lower temperature.

Compared to AC particle size (in the micrometer range), the CB particles presenting extremely small size (nanometer scale) with medium surface area, tend to promote solid-solid reactions, leading to increased production of carbon monoxide (CO). In contrast, AC, with much higher specific surface area and micropore area, favors solid-gas reaction mechanisms, which rather yields CO₂ [11,12]. In the case of biochar, the enhanced reactivity at lower temperatures can also be due to the presence of ash (1.89 wt %(d.b.) in the considered biochar), which may act as a catalyst for the reaction. In addition, biochar also contains a fraction of volatile matter (6.66 wt %(d.b.)) with the presence of H and O (H/C and O/C mole ratios=0.24 and 0.05, respectively), which can also have an effect on the reaction mechanism [45]. Therefore, the

enhanced kinetics for biochar is chiefly attributed to the presence of impurities, which however may have a detrimental effect on the produced iron purity [46]. As biochar properties highly depend on the biomass feedstock and synthesis process [47], the effect of the type of biochar and properties (e.g., ash content, volatile matter content, presence of metal impurities) should be investigated and compared in further studies to unravel their impact on the reduction rate.

In contrast, G exhibits a distinct behaviour from AC and CB. It is characterized by a different gas evolution pattern and higher transition temperatures. This suggests that alternative reaction mechanisms govern the reduction of Fe₂O₃ when graphite is employed as the reductant. This mechanism yielding the steep production of CO occurs at a defined transition temperature suggesting direct reduction of iron oxide with solid carbon (solid-solid mechanism). This result is supported by the reproducibility of the experiment, which has been confirmed through two repeated trials (Fig. 8). They show that from ~ 1114 °C, a peak concentration of CO emerges suddenly whilst the Fe₂O₃ conversion extent simultaneously increases by +67 % between 1104 °C-1114 °C.
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Conversion extent as a function of time and temperature for Fe₂O₃ + 4.5 G, along with the normalized intensity of the main gas products (CO and CO₂) released from the reduction reaction, for two different trials at 20 °C/min: (a) run #6 and (b) run #7.</figcaption></figure>

The faster reaction rate leads to a single sharp endothermic DSC peak, yielding an enthalpy of 634 and 638 J/g for runs #6 and #7, respectively.

Kinetics of Boudouard reaction has been investigated separately with TGA (Fig. 9), to compare the effect of different sources of carbon (AC, CB, G). The highest reaction rate above 950 °C was achieved with CB, followed by AC and G.

In comparison, reaction with graphite was slower than with the other carbon types. Graphite has an organized crystalline structure made of graphene layers, with stronger bonds between carbon atoms, resulting in a higher activation energy of 273 kJ/mol (Fig. S2), thus requiring higher temperatures for its conversion.

The reaction with CB (amorphous carbon) shows a lower activation energy (252 kJ/mol) than with graphite. This can be explained by the disordered arrangement of carbon atoms, which creates more accessible active sites. This type of carbon typically reacts more quickly with reactive gases like CO₂ at lower temperatures.

Activated carbon (AC) shows the lowest activation energy (213 kJ/mol). This behaviour is because AC is a porous material with a large specific surface area, which induces a high sorption capacity with CO₂.

## 4.2. Carbothermal reduction in tubular reactor

A quantitative analysis of the conversion of Fe₂O₃ to metallic iron was carried out concurrently with gas-phase monitoring. In addition to TGA, a tubular furnace was employed to verify the reproducibility of the carbothermal reduction results using an independent method. The data obtained from the tubular reactor are summarized in Table 2 and Fig. 10, and are consistent with the trends observed in TGA measurements.

In agreement with the results obtained from TGA, the tubular furnace experiments reveal that AC and CB follow similar reaction mechanisms. This is evidenced by the onset of Fe₂O₃ reduction, which begins at approximately 650 °C for both AC and CB systems (Fig. 11.a,b). The maximum reduction rate coincides with the peak in CO evolution, corresponding to the inflection point of the reaction curve. This occurs at 1073 °C for the AC system and 1112 °C for the CB system. At these respective temperatures, the CO flow rates are 8.88 NmL/min for AC and 16.51 NmL/min for CB. The total volumes of CO produced over the course of the reaction are nearly identical: 98.4 NmL for AC and 98.5 NmL for CB. Furthermore, both reductants exhibit comparable CO/CO₂
<figure><img src="image_13.png" /><figcaption>Fig. 9. Kinetics of Boudouard reaction depending on the source of carbon (AC, CB, G) with 20 % of CO₂.</figcaption></figure>

<table><caption>**Table 2**

Description of the operating conditions and results for tubular reactor runs.</caption><thead><tr><th>Run #</th><th>Heat ramp (K/min)</th><th>Argon Flow rate (mL/min)</th><th>System</th><th>Starting of conversion (°C)</th><th>Temperature of conversion peak (°C)</th><th>Peak CO Flow rate (NmL/min)</th><th>Peak CO₂ Flow rate (NmL/min)</th><th>Total CO amount (NmL)</th><th>Total CO₂ amount (NmL)</th><th>Ratio CO/CO₂</th></tr></thead><tbody><tr><td>1</td><td>10</td><td>500</td><td>Fe₂O₃ + 4.5AC</td><td>650</td><td>1073</td><td>8.88</td><td>2.04</td><td>98.4</td><td>41.3</td><td>2.38</td></tr><tr><td>1'</td><td>10</td><td>200</td><td>Fe₂O₃ + 4.5AC</td><td>647</td><td>1064</td><td>8.80</td><td>2.28</td><td>96.2</td><td>44.9</td><td>2.14</td></tr><tr><td>2</td><td>10</td><td>500</td><td>Fe₂O₃ + 4.5CB</td><td>650</td><td>1112</td><td>16.51</td><td>3.33</td><td>98.5</td><td>42.4</td><td>2.32</td></tr><tr><td>2'</td><td>10</td><td>200</td><td>Fe₂O₃ + 4.5CB</td><td>636</td><td>1087</td><td>12.89</td><td>3.00</td><td>91.6</td><td>43.3</td><td>2.11</td></tr><tr><td>3</td><td>10</td><td>500</td><td>Fe₂O₃ + 4.5G</td><td>720</td><td>1093</td><td>117.00</td><td>17.81</td><td>102.5</td><td>35.4</td><td>2.89</td></tr><tr><td>4</td><td>10</td><td>500</td><td>Fe₂O₃ + 4.5Biochar</td><td>646</td><td>981</td><td>15.28</td><td>3.11</td><td>99.5</td><td>35</td><td>2.83</td></tr></tbody></table>

ratios, measured at 2.38 for AC and 2.32 for CB, indicating similar gas-phase reaction during the reduction process. The same experiments were repeated with AC and CB (runs #1' and #2') with a lower Ar flowrate (0.2 NL/min), without any significant effect on the reduction rate (Fig. S6).

Fig. 11.d shows that the complete conversion of hematite using biochar (Fe₂O₃ + 4.5Biochar) is achieved more rapidly than with the other carbonaceous reductants, as also observed in TGA (Fig. S3). The onset of Fe₂O₃ reduction is predicted at approximately 646 °C, and full conversion is reached at 981 °C. The CO flow rate is measured at 15.28 NmL·min⁻¹ at this temperature, with a corresponding CO/CO₂ ratio of 2.83.

In contrast, a steep increase of the conversion corresponding to CO production is observed for Fe₂O₃ + 4.5 G, closely aligned with TGA results. The onset of reduction for the Fe₂O₃ + 4.5 G system (Fig. 11.c) occurs at around 720 °C and the maximum conversion rate is reached at 1093 °C, coinciding with the sharp peak CO evolution, which reaches 117 NmL/min. Among all tested reductants, graphite produces the highest total volume of CO released during the reaction, estimated at 102.5 NmL. It also yields the highest CO/CO₂ ratio, measured at 2.89, suggesting the involvement of additional reaction mechanisms that promote enhanced CO formation in this system.

However, some discrepancies in temperature readings with TGA were observed, likely due to differences in the accuracy of the measurement systems. The TGA-DSC setup, coupled with a quadrupole mass spectrometer (QMS), allows for direct in situ analysis of the reaction gases. In this system, the thermocouple is located immediately beneath the crucible, providing accurate temperature measurements. In contrast, the thermocouple in the tubular furnace is positioned several mm above the sample, rather than in direct thermal contact. This placement leads to less precise temperature control and may account for the slight deviations observed when comparing the two systems. The materials recovered after carbothermal reduction were characterized by SEM/EDX (Figs. S7-S14), which revealed the formation of a porous iron microstructure in the case of AC, whereas the product was composed of large iron particles covered by carbon uniformly dispersed over the particle surface for the other reductants.

### 4.3. Phase characterization of the iron products

The various materials obtained after the reduction reactions were collected and analysed using powder X-ray diffraction (XRD) to identify the main phases formed and confirm the production of metallic iron. The raw hematite material was also characterized for comparison purpose.
<figure><img src="image_14.png" /><figcaption>Fig. 10. Comparative study of reduction with different types of carbon (AC, CB, G, and Biochar) as a function of temperature in tubular reactor.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 11. Conversion extent as a function of time and temperature in tubular furnace, along with the main gas products (CO and CO₂) released from the reduction reaction, for (a) Fe₂O₃ + 4.5AC, (b) Fe₂O₃ + 4.5CB, (c) Fe₂O₃ + 4.5G, and (d) Fe₂O₃ + 4.5Biochar, at a heating rate of 10 °C/min.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 12.** X-ray diffraction patterns of Fe₂O₃ reduced with different types of carbon in (a) TGA and (b) tubular furnace.</figcaption></figure>

Overall, the results of the non-isothermal reductions conducted in TGA (Fig. 12a) and in the tubular furnace (Fig. 12b) indicate that complete conversion occurred in nearly all cases, as the primary phase identified was metallic Fe (PDF 01-089-7194). In addition, some samples reduced with excess carbon, such as (Fe₂O₃ + 4.5G) and (Fe₂O₃ + 4.5Biochar) systems, also exhibit residual carbon (PDF 01-089-8487), while possible traces of carbon are evidenced in other systems as (Fe₂O₃ + 4.5CB) and (Fe₂O₃ + 4.5AC) (Fig. 12b).

Notably, in the sub-stoichiometric carbon system (Fe₂O₃ + 1.5AC) and stoichiometric conditions, the presence of Fe₀.₉₇₁₂O (wüstite) phase (PDF 01-074-1884) was identified (Fig. 12a). The detection of wüstite suggests that carbon deficiency led to incomplete reduction, corroborating the partial conversion results discussed in Section 4.1.3 (Fig. 5).

Phase analysis confirms the successful production of metallic iron, thus supporting the feasibility of a fully realized direct reduced iron (DRI) process.

## 5. Conclusion

Thermodynamic equilibrium provided a preliminary overview of favorable reaction steps and further showed that a C/Fe₂O₃ molar ratio over 3 was required to reach reduction completion. The carbothermic reduction of hematite (Fe₂O₃) using various carbonaceous reductants (activated carbon (AC), graphite (G), carbon black (CB), and biochar) was successfully achieved using TGA coupled with DSC, and further validated through independent experiments conducted in a tubular furnace. This combined experimental approach enabled the synthesis of metallic iron and provided a detailed understanding of the reaction mechanism, gas evolution rates (CO, CO₂), and conversion behaviour under both controlled laboratory and more practical conditions. Products analysis confirmed the feasibility of complete reduction of Fe₂O₃ in stoichiometric conditions, with characteristic peaks of metallic iron.

The reduction mechanism followed the expected transformation sequence: Fe₂O₃ → Fe₃O₄ → FeO → Fe, whatever the carbon used. This
process was accompanied by three distinct CO₂ evolution peaks, likely associated with solid-solid reactions and the production of CO, which was subsequently consumed in slower gas-solid reduction steps. The first CO₂ peak corresponded to an ~11 % conversion of Fe₂O₃ to Fe₃O₄, the second to a ~33 % conversion for Fe₂O₃ conversion to FeO. CO evolution began before the completion of FeO formation and became predominant at the third CO₂ peak, indicating the activation of the Boudouard reaction (C + CO₂ → 2CO). This reaction plays a crucial role in the final reduction step (FeO → Fe), and is considered the rate-limiting stage, requiring an excess of carbon to maintain a sufficient CO atmosphere. The inflection point of the overall reaction curve coincided with the maximum CO evolution, indicating the highest reduction rate. The comparison between TGA and tubular furnace data showed slight discrepancies, with higher onset temperatures observed in TGA. This can be attributed to the higher precision and in situ measurement capabilities of TGA compared to the furnace setup. The conversion profiles revealed that the heating rate had no significant influence on the final degree of reduction. Besides, the carbon-to-hematite molar ratio played a critical role: sub-stoichiometric conditions led to incomplete reduction and the formation of intermediate phases such as wüstite (FeO), as confirmed by XRD. In contrast, a carbon excess favoured nearly complete reduction, while traces of residual carbon were detected in some cases. At a fixed molar ratio Fe₂O₃/C = 4.5, the conversion profiles showed slight variations depending on the type of carbon used. Graphite exhibited a distinct conversion pattern characterized by a sharp increase due to CO production within a narrow temperature window (~1110 °C), while the other reductants (activated carbon, carbon black, and biochar) showed more gradual and similar conversion profiles with stepwise release of CO₂ and CO. Notably, biochar enabled reduction to occur at a lower onset temperature (~959 °C), highlighting its potential for more energy-efficient processing.

Finally, XRD analysis of the solid residues confirmed the complete conversion of hematite to metallic iron for all the carbon types considered, with characteristic Fe diffraction peaks and, in some instances, residual carbon. The trends observed in the tubular furnace experiments were consistent with those obtained via TGA, further validating the experimental approach and supporting the robustness of the findings.

This pyrometallurgical process is an attractive approach for carbon-neutral iron production using carbon derived from captured or recycled CO₂, paving the way toward sustainable and eco-friendly solar processes.

### CRediT authorship contribution statement

**Matilde Bastide d'Izard:** Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Stéphane Abanades:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### 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.

### Acknowledgement

This work was funded by Region Occitanie in France (Défi clé Hydrogène Vert - RHyO – HYDROMET project). Funding of ANR (METASOL project, contract N° ANR-20-CE05–0008-02) is also gratefully acknowledged.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cej.a.2025.100925.

### Data availability

Data will be made available on request.

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