Mechanism of Iron Powder to Enhance Solid-State Reduction of Chromite Ore

Abstract

This study investigated the solid-state reduction characteristics of natural chromite ore and the effect of iron powder on the solid-state reduction characteristics of natural chromite ore under isothermal conditions below 1200 °C. The enhancement mechanism of iron powder on the solid-state reduction of natural chromite ore was revealed using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS). The iron metallization rate of chromite ore exhibited a trend of increasing first and then decreasing with the addition of iron powder, and the optimal iron powder addition was determined to be 30%. The multi-step reaction gradually transforms into a single-step reaction with the increase in the dosage of iron powder. Iron powder facilitates the generation of a low-melting Fe-C alloy liquid phase and accelerates the speed of the solid-state reduction reaction of chromite ore and the disintegration of chromite spinel particles. When the iron powder dosage exceeds 30%, most of the multi-step reduction reaction of chromite ore is transformed into the single-step reduction reaction, which reduces the disintegration of chromite spinel particles and weakens the enhancement effect of iron powder on the solid-state reduction of chromite ore.

1. Introduction

Chromite ore is an important raw material for the production of ferrochrome in the metallurgical industry [1]. Approximately 90% of chromite ore is employed in the production of ferrochrome, and roughly 80% of the produced ferrochrome is utilized for the manufacture of stainless steel [2]. The steady expansion of the stainless steel industry has significantly driven up the demand for ferrochrome alloys [3]. Currently, submerged arc furnace (SAF) smelting is widely adopted in the world. However, SAF smelting is an energy-intensive process, with the average energy consumption being 3 to 5 times higher than that of the blast furnace ironmaking process [4,5]. Therefore, it is very important that enhanced measures are taken to reduce energy consumption of chromite smelting.

The solid-state reduction of chromite ore prior to entering the SAF smelting process is an effective strategy for reducing the energy consumption of the SAF smelting process [6,7]. Numerous studies have demonstrated that higher degrees of pre-reduction of chromite ore result in lower specific electricity consumption (SEC) [8]. The lowest specific electricity consumption (SEC) achieved through the solid-state reduction process of chromite ore is approximately 2.4 MWh/t FeCr, whereas the SEC for conventional SAF production ranges from 3.9 to 4.2 MWh/t FeCr [9]. Clearly, the solid-state reduction of chromite ore offers significant advantages in terms of SEC. Therefore, enhancing the solid-state reduction of chromite ore is crucial for reducing the energy consumption during chromite ore smelting. Many researchers have conducted extensive studies on measures to improve the solid-state reduction of chromite ore. These measures can be categorized into two groups: the pretreatment of chromite ore and the use of additives.

With regard to the pretreatment of chromite ore, Kleynhans et al. [10] demonstrated that pre-oxidizing treatment can enhance the reduction of chromite ore by promoting the release of Fe ions and inhibiting the release of chromium oxide (Cr₂O₃) from the spinel structure, thereby improving the overall reduction process. Pan et al. [11] further reported that pre-oxidation treatment facilitates the formation of a sesquioxide solid solution and generates cationic vacancies, both of which significantly increase the reactivity of chromite ore. Additionally, Apaydin et al. [12] showed that mechanical activation induces amorphization and structural disorder in chromite ore, creating more microcracks, disrupting the spinel structure, and consequently enhancing the solid-state reduction process of chromite ore.

Additives have been shown to facilitate the reduction of chromite ore. However, the efficacy of different types of additives in promoting this reduction varies. Research has demonstrated that additives functioning as slagging and fluxing agents can lower the melting point of refractory components in chromite, reduce the diffusion resistance of reactants, and enhance heat and mass transfer conditions [13,14,15]. Furthermore, additives can catalyze the Boudouard reaction, thereby intensifying the reduction of chromite [16,17]. Additionally, oxide additives with large ionic radii can alter the spinel structure, improving the solid-phase diffusion capability of Cr and thus catalyzing the reduction reaction of chromite [8,18]. Although these additives demonstrate the potential to enhance the reduction of chromite, they still pose certain challenges in practical applications. For instance, although CaCO₃ reduces the Gibbs free energy of chromite reduction, its incorporation may compromise the compressive strength and wear resistance of chromite pelletized ores, thereby adversely affecting the stability and safety during smelting [18]. It also introduces some unnecessary impurity elements, which will increase the burden of subsequent separation and purification and affect product quality. Therefore, researchers have developed a new additive strategy by adding alloying elements or forming alloying phases in situ [19,20,21,22], such as nickel powder, iron powder, mill scale (FeOx), nickel laterite, etc. These additives promote the reduction of chromite ore mainly by forming low-melting-point alloys during the reduction process, lowering the temperature and apparent activation energy of the reduction reaction, and thus promoting the reduction of chromite ore. Therefore, the formation of low-melting-point alloy liquid phases is conducive to reducing the energy consumption in the smelting of ferrochrome alloys, which is of great significance for lowering the production cost of stainless steel and promoting the development of the steel industry.

Hu et al. [23] systematically investigated the influence of iron powder on the non-isothermal reduction process of synthetic chromite (FeCr₂O₄). Their findings demonstrated that iron powder serves as an effective catalyst for the reduction reaction of synthetic chromite, reducing the activation energy required for the reaction. Consequently, this leads to a decrease in the reduction temperature and an enhancement in the reaction rate. In general, iron oxide is more readily reduced compared to chromium oxide during the reduction process of natural chromite ore [24]. At temperatures lower than 1200 °C, only Cr₂O₃ is found in the reaction products, and chromium metal is not found [25]. If the iron oxide in the chromite ore can be reduced as much as possible in a short period of time and at a low reduction temperature (such as below 1200 °C), the energy consumption of the solid-state reduction of chromite ore will be reduced. Then, the iron metal can promote the reduction of chromium oxide during the subsequent smelting process. Therefore, it is worth studying the influencing mechanism of iron powder on the solid reduction of natural chromite ore under isothermal conditions below 1200 °C. The effect of iron powder on the carbothermic reduction of synthetic chromite under non-isothermal conditions has been revealed [23]. However, natural chromite ore is a spinel mineral containing Mg, Al, Fe, and Cr in varying proportions according to its formula (Mg, Fe²⁺)(Cr, Al, Fe³⁺)₂O₄ [2]. The relatively refractory components (i.e., Mg and Al in the form of oxides) remain as oxide(s) after reduction; there are significant differences between its reducing properties and those of synthetic chromite (FeCr₂O₄) [26]. In addition, Hu et al.’s research was conducted under non-isothermal conditions (1100–1700 °C). The isothermal reduction mechanism of natural chromite ore at temperatures below 1200 °C is still not clear. Therefore, it is necessary to study the effect of iron powder on the solid-state reduction of natural chromite ore under isothermal conditions.

In this investigation, the solid-state reduction characteristics of natural chromite ore and the effect of iron powder on the solid-state reduction characteristics of natural chromite ore under isothermal conditions below 1200 °C were studied. Moreover, the enhancing mechanism of iron powder on the solid-state reduction of natural chromite ore was revealed.

2. Materials and Methods

2.1. Raw Materials

The chromite concentrate used in this study was obtained from South Africa, and its chemical composition is shown in

Table 1. Chemical composition of chromite concentrate/wt%.

Raw Materials	TFe	FeO	Cr2O3	SiO2	CaO	Al2O3	MgO	S	P
Chromite concentrate	21.68	20.81	41.32	3.16	0.27	14.56	8.98	0.081	0.005

. The content of FeO and Cr₂O₃ in the chromite concentrate is 20.81% and 41.32%, respectively, with a Cr/Fe ratio of 1.99, indicating that the chromite concentrate belongs to low-grade ore. The content of the S and P elements is low, which is favorable for subsequent pyrometallurgical smelting. The percentage of chromite ore with a grinding particle size less than 0.074 mm reaches more than 85%, meeting the requirements for granulation, as shown in

Table 2. Size composition of chromite particle after grinding /%.

Raw Materials	0.074–0.15 mm	0.045–0.074 mm	−0.045 mm
Chromite concentrate	9.43	36.80	53.77

. The additive employed in this study was metallic iron powder with analytical grade purity, and 100% of the particles were smaller than 0.074 mm in size.

The fixed carbon content of the reductant used was 87.3%, with ash and volatile matter being 10.51% and 0.92%, respectively. The reductant had a low sulfur content and high drop strength, as shown in

Table 3. The composition and properties of the reductant/%.

Fixed Carbon	Ash	Volatile Matter	Falling Intensity	Sulfur	Abrasion
87.3	10.51	0.92	98.3	0.51	7.6

. The reductant was ground using a ball mill until the percentage of particles smaller than 0.074 mm reached 100%.

The XRD results of the raw ore can be analyzed to determine that the main mineral phases of chromite ore are spinel (Mg, Fe)(Cr, Fe, Al)₂O₄ and MgSiO₃. Both chromium and iron are primarily present in the spinel structure, as shown in Figure 1.

2.2. Experimental Methods

2.2.1. Briquetting

The chromite concentrate, carbon powder, and iron powder were mixed well in a mixer, moistened with an appropriate amount of water, and made into agglomerates (20 × 8 mm) using a DY-20 table-top electric tablet press. The agglomerates were then dried in a blast-drying oven at 105 °C for 6 h to remove excess moisture and subsequently subjected to reduction roasting.

2.2.2. Reduction Roasting

Reduction roasting experiments of agglomerates were carried out in a horizontal tube furnace with a diameter of 35 mm. To establish the reduction roasting experimental program, the agglomerates were placed in a porcelain boat and positioned in the constant temperature zone of the horizontal tube furnace as shown in Figure 2a. The heating and cooling rates for the reduction roasting experiment were set at 5 °C/min. To prevent the oxidization of the samples during the reduction roasting process, high-purity nitrogen was continuously fed into the furnace at a rate of 200 mL/min. The actual reduction roasting temperature profile is presented in Figure 2b, and the reduction conditions are listed in

Table 4. Reduction and roasting trial conditions.

Num.	Temperature/°C	Time/h	Carbon Dosage/%	Iron Powder Dosage/%
1	1150	2	5	0
2			10
3			15
4			20
5			25
6	1100	2	20	0
7	1125
8	1150
9	1175
10	1200
11	1175	0.5	20	0
12		1
13		1.5
14		2
15		2.5
16	1175	1.5	20	10
17				20
18				30
19				40
20				50
21				60

. The dosages of carbon powder and iron powder were determined based on the percentage of chromite ore. A single-variable approach was employed to investigate the effects of different conditions on the solid-state reduction of chromite, thereby determining the optimal reduction conditions.

2.2.3. Analysis and Characterization

After the reduction roasting trial was completed, some samples of agglomerates under each reduction condition were ground into powder and sieved through a 200-mesh sieve. The composition of the samples under various reduction conditions was systematically investigated. The content of metallic iron and total iron in the samples was quantitatively analyzed using the FeCl₃ titration method and the EDTA titration method, respectively. Based on these results, the iron metallization rate can be calculated using the following formula:

$${\mathsf{\eta }}_{(\mathrm{Fe})}={\displaystyle \frac{\mathrm{M}(\mathrm{Fe})-{\mathrm{M}}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})}{\mathrm{T}(\mathrm{Fe})-{\mathrm{T}}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})}}\ast 100\%$$

(1)

$${\mathrm{I}}_{(\mathrm{Fe})}={\displaystyle \frac{{\mathsf{\eta }}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})-{\mathsf{\eta }}_{\text{no-iron} \mathrm{powder}}(\mathrm{Fe})}{{\mathsf{\eta }}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})}}\ast 100\%$$

(2)

$\mathsf{\eta }(\mathrm{Fe})$ is the iron metallization rate of chromite ore, %.

$\mathrm{M}(\mathrm{Fe})$ is the metal iron content of the reduced sample, %.

$\mathrm{T}(\mathrm{Fe})$ is the total iron content of the reduced sample, %.

${\mathrm{M}}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})$ represents the metal iron content contributed by iron powder, %.

${\mathrm{T}}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})$ represents the total iron content contributed by iron powder, %.

$\mathrm{I}(\mathrm{Fe})$ is the change in the iron metallization rate of the chromite ore with and without the addition of iron powder, %.

${\mathsf{\eta }}_{\mathrm{iron}\mathrm{powder}}(\mathrm{Fe})$ is the iron metallization rate of the chromite ore after the addition of iron powder, %.

${\mathsf{\eta }}_{\text{no-iron} \mathrm{powder}}(\mathrm{Fe})$ is the metallization rate of the chromite ore without iron powder addition, %.

The mineral-phase analysis of the reduced samples under different conditions was carried out by X-ray diffractometer (XRD, D8 Advance, Cu Kα) with a scanning angle of 5~90° and a scanning speed of 10°/min. In addition, the reduced samples were embedded in epoxy resin and then polished across the sections. A microscope and a scanning electron microscope (SEM, ZEISS Sigma 300, Carl Zeiss AG, Jena, Germany) were used to analyze the microstructure and mineralogy of the reduced samples, and an energy dispersive spectrometer (EDS, OXFORD Instrument, Abingdon, UK) was used to determine the mineral composition in the reduced samples. In addition, FactSage 8.0 was utilized to calculate the Gibbs free energy of the reduction reactions of chromite.

3. Results and Discussion

3.1. Thermodynamics of Chromite Solid-State Reduction

The initial temperatures of the reduction reactions for each component in chromite can be determined using thermodynamic data. Investigating the carbothermal reduction thermodynamics of pure FeO, Cr₂O₃, and FeCr₂O₄ provides valuable insights into understanding the carbothermal reduction thermodynamics of chromite [27]. The relationship between the standard Gibbs free energy of the reduction reactions involving carbon for these pure substances under standard atmospheric pressure and temperature is illustrated in Figure 3. It is evident that in the presence of solid carbon, a series of reduction and carburization reactions occur in FeCr₂O₄. The Gibbs free energy ∆G^θ < 0 at a reduction temperature of 1200 °C is the following reaction:

FeCr₂O₄ + 4C = Cr₂O₃ + Fe + 4CO

(3)

FeCr₂O₄ + 34C = 7Fe + 2Cr₇C₃ + 28CO

(4)

2/3Cr₂O₃ + 18/7C = 4/21Cr₇C₃ + CO

(5)

3Fe + C = Fe₃C

(6)

At this point, the Gibbs free energy ∆G^θ > 0 for reactions (7) and (8) indicates that the reaction cannot proceed spontaneously at this temperature condition.

FeCr₂O₄ + 4C = 2Cr + Fe + 4CO

(7)

Cr₂O₃ + 3C = 2Cr + 3CO

(8)

Therefore, at an initial reaction temperature of 1077 °C, FeCr₂O₄ is reduced to Cr₂O₃ and metallic iron, whereas the starting temperature for generating metallic chromium and metallic iron is 1258 °C [28,29].

When the reduction temperature is below 1200 °C, chromite is primarily reduced to metallic iron and ferrochromium carbide, with the amount of reduced metallic chromium being very small. These research findings offer theoretical guidance for this study. Since the primary objective of this study is to explore the effect of adding metallic iron on the reduction of iron oxides during the solid-state reduction process of chromite, the reduction temperature for subsequent solid-state reduction experiments will be set below 1200 °C.

3.2. Solid-State Reduction Characteristics of Natural Chromite Ore

3.2.1. Carbon Dosage

The effect of different carbon dosages on the metallization rate of chromite ore is shown in Figure 4. The results indicate that with the increase in internal carbon dosages, the iron metallization rate exhibits a gradually increasing trend, particularly between 5% and 20%. After the carbon dosage is increased to 20%, the increase in the iron metallization rate slows down. Therefore, the optimal carbon dosage in this study is determined to be 20%. The amount of carbon is a critical factor influencing the outcomes of chromite solid-state reduction. An appropriate amount of carbon can enhance the metallization rate and reduction speed, thereby promoting the reduction reaction of chromite [6]. However, adding excessive carbon does not significantly improve reduction. The excessive input of carbonaceous reducing agents can facilitate the formation of stable carbide phases of metal elements, increase the technical complexity of the subsequent product separation process, and concurrently result in a rise in both energy consumption and carbon dioxide emissions [30]. Thus, it is essential to accurately control the amount of carbon added during experiments and production.

3.2.2. Reduction Temperature

Figure 5 demonstrates the influence of the reduction temperature on the rate of iron metallization in chromite ore. The results indicate that the iron metallization rate increased with the rise in the reduction temperature within the tested temperature range. Notably, a rapid increase in the metallization rate was observed between 1125 °C and 1175 °C, rising from 46.50% to 97.15%. At 1175 °C, the metallization rate approached stabilization, and further increases in temperature had negligible effects. Consequently, 1175 °C was determined as the optimal temperature for this study.

3.2.3. Reduction Duration

Figure 6 illustrates how the iron metallization rate is influenced by the reduction duration. As shown in Figure 6, under the same temperature conditions, with the extension of the reduction time, the metallization rate of iron exhibits an increasing trend. However, when the reduction duration exceeds 2 h, the growth trend of the iron metallization rate slows down. After 2 h of reduction at 1175 °C, the iron metallization rate of the sample reaches 97.15%. Therefore, a reduction duration of 2 h is recommended as optimal. To investigate the effect of iron powder addition on the solid-state reduction of chromite, a reduction time of 1.5 h was selected for subsequent investigations in this study.

3.3. Effect of Iron Powder Dosage on the Solid-State Reduction of Natural Chromite

The effects of iron powder additions on the iron metallization rate are shown in Figure 7.

With the addition of iron powder ranging from 0% to 60%, the iron metallization rate exhibited a trend of first increasing and then decreasing, with the highest point appearing at the 30% iron powder addition, where the iron metallization rate reached 96.33%. Compared with the chromite ore without added iron powder, the iron metallization rate of the sample with added iron powder increased by 5.50 percentage points. The results in Figure 7 indicate that the addition of iron powder can promote the solid-state reduction of chromite and improve the iron metallization rate; however, the improvement effect decreases with the excessive addition of iron powder. This suggests that adding a moderate amount of iron powder has a significant positive impact on enhancing the iron metallization rate of chromite. There is an optimal value for the addition of iron powder, and in this study, the 30% iron powder addition demonstrated the best performance. Beyond this point, the reduction promotion effect will weaken, and the reasons for this will be specifically discussed in Section 3.4.

3.4. Mechanism of the Enhanced Solid-State Reduction of Chromite by Iron Powder Addition

3.4.1. Microstructure Observation

Figure 8 presents a comparative analysis of metallic iron addition effects on the reduced microstructure evolution of chromite. The pristine chromite particles exhibit angular morphology with structural integrity, as evidenced in Figure 8a. In iron-free reduced samples (Figure 8b), metallic iron phases emerge along spinel particle peripheries accompanied by irregular intraparticle porosity while maintaining overall structural continuity without observable fragmentation. With 30% iron addition (Figure 8c), significant structural degradation occurs through spinel particle disintegration into smaller fragments. This fragmentation process facilitates continuous metallic iron precipitation along newly formed edges, accompanied by particle agglomeration—a clear indication of enhanced reduction kinetics under this condition. Notably, at 60% iron content (Figure 8d), the metallic iron phase achieves homogeneous distribution surrounding intact spinel particles, demonstrating that excessive additive levels suppress structural disintegration while maintaining phase stability.

The results of the SEM and EDS analyses of the chromite-reduced samples with different iron powder additions are shown in Figure 9 and

Table 5. EDS results of SEM in Figure 9.

Powder Dosage/%	Area No.	Elemental Compositions/Mass pct.								Mineral Phase
		C	O	Mg	Al	Si	Ca	Cr	Fe
0%Fe	1	10.26	0.58	0.12	0.01	0.06	0	9.24	79.75	(Fe, Cr)7C3
	2	0.37	35.69	0.23	13.77	0	0.07	49.53	0.34	Sesquioxide
	3	0.26	35.82	11.68	9.69	0	0.03	39.48	3.30	Cr-rich spinel
	4	6.14	0.45	0	0.04	0.11	0.06	9.37	83.82	Fe-C-Cr alloy
30%Fe	5	5.91	0.22	0.03	0.02	0.10	0.03	9.16	84.55	Fe-C-Cr alloy
	6	0.36	34.97	0.16	13.77	0.04	0.08	49.97	0.66	Sesquioxide
	7	0.41	33.94	10.53	10.98	0	0.01	40.82	3.04	Cr-rich spinel
60%Fe	8	5.73	0.34	0	0.05	0.03	0	7.95	85.9	Fe-C-Cr alloy
	9	3.8	30.27	0.07	12.16	0	0	53.06	0.64	Sesquioxide
	10	3.94	30.56	8.21	11.34	0.06	0.01	41.13	4.75	Cr-rich spinel

. Figure 9a,b reveal the microstructural characteristics of chromite reduction without iron powder addition. The dark gray Cr-rich spinel phase (S) with chemical composition (Mg, Fe)(Cr, Al)₂O₄ exhibits spherical or rod-shaped Fe–C–Cr alloys (points 1, 5, 8) distributed on its surface. The XRD and EDS analyses demonstrate that the rod-shaped alloys evolved through the aggregation of spherical particles. Notably, residual Fe elements persist in the spinel phase, confirming incomplete reduction. Figure 9b shows abundant light-gray particles (point 2) in the peripheral and interior regions of the spinel grains, which are identified as (Cr, Al)₂O₃ sesquioxide formed during reduction. The formation of this phase not only induces structural cleavage in spinel but also significantly enhances the reducibility of chromite [31].

The addition of 30% iron powder induces significant structural degradation in chromite spinel, characterized by particle fragmentation and progressive size reduction. Disintegrated smaller spinel fragments undergo continued reduction to generate metallic iron, demonstrating that iron additives facilitate spinel matrix disintegration and enhance reduction kinetics. A comparative analysis with Figure 9b,d reveals enhanced continuity and increased thickness of the Fe-Cr-C alloy layers along the spinel peripheries, indicative of advanced reduction stages with substantial alloy precipitation. Zhao et al. [32] posit that metallic phases preferentially nucleate at pre-existing sesquioxide sites, attributable to a MgO deficiency in the (Cr, Al)₂O₃ phases, which lowers the thermodynamic barrier for iron reduction. This evidence collectively suggests that iron additives establish favorable kinetic conditions for metallic-phase nucleation and growth.

A significant change in the microstructure was observed when the iron powder addition was adjusted to 60% in the solid-state reduction experiments. As shown in Figure 9e,f, a large number of iron particles were densely arranged and closely packed around the spinel particles. This dense distribution of iron particles significantly affected the reaction process. The significant increase in iron particles drastically reduced the opportunity for contact between the carbon and spinel particles. In the direct reduction reaction, effective contact between carbon and spinel particles is key for the reaction to occur. However, the addition of excessive metallic iron particles inhibits the reaction by making it difficult for carbon particles to make full contact with spinel particles [33,34]. The spinel particles in the sample with 60% iron powder addition were larger and more structurally complete than those in the sample with the 30% iron powder addition. This suggests that excessive iron powder addition may contribute to spinel particle structural stability.

In conclusion, the evolution of the spinel structure during the reduction process can be deduced. The addition of 30% iron powder greatly disrupted the chromite spinel microstructure and increased the degree of particle cleavage, thereby increasing the reaction’s specific surface area. Additionally, the addition of iron powder provided favorable kinetic conditions for the nucleation and growth of newly reduced metallic iron, promoting the reduction reaction. However, adding 60% iron powder did not significantly disrupt the chromite spinel structure or reduce the particle size, thus weakening reduction promotion.

3.4.2. Mineral-Phase Evolution

The mineral-phase compositions of the original chromite and the samples with different iron powder additions were analyzed by XRD. The results are shown in Figure 10. In the original chromite, the main mineral-phase compositions are spinel ((Fe, Mg)(Cr, Fe, Al)₂O₄) and enstatite (MgSiO₃).

Comparing Figure 10a–d, it is evident that after the reduction of chromite ore, the spinel undergoes a transformation from pristine spinel (Fe, Mg)(Cr, Fe, Al)₂O₄ to Cr-rich spinel (Mg, Fe)(Cr, Al)₂O₄. This shift results in changes in the positions of the diffraction peaks due to alterations in the chemical composition. As depicted in Figure 10b, the raw spinel was reduced without the addition of iron powder, yielding Cr-rich spinel, sesquioxide (Cr, Al)₂O₃, (Fe, Cr)₇C₃, and Fe-C-Cr alloy. Upon the introduction of iron powder, the Fe-C-Cr alloy content increased, while the sesquioxide and (Fe, Cr)₇C₃ gradually decreased. When the iron powder content reached 30%, the sesquioxide remained relatively abundant, whereas the (Fe, Cr)₇C₃ content was comparatively lower. The increase in Fe-C-Cr alloy can primarily be attributed to the initial reaction between the added iron powder and solid carbon, forming Fe₃C. According to Figure 11, at temperatures exceeding 1148 °C (the experimental temperature of 1175 °C surpasses this threshold), a eutectic liquid phase forms through the interaction of Fe₃C and metallic iron. Consequently, newly formed metallic iron continues to dissolve in situ, reducing its reactivity. Additionally, iron particles serve as carbon carriers, enhancing direct reduction reactions and thereby promoting the reduction of chromite [35].

When the addition of iron powder reaches 60%, no carbide phase is detected in the XRD results, and the intensity of the sesquioxide peaks gradually decreases until it disappears, as shown in Figure 10d. The formation of sesquioxide leads to a change in the internal volume of the spinel, resulting in the cleavage of the spinel into smaller-grained spinel. However, with 60% iron powder, there is less sesquioxide, and therefore the spinel particles do not split into smaller ones, so the spinel particles are larger in size (as shown in Figure 9).

3.4.3. Reduction Process Analysis

The study analyzed the reduction process of chromite without added iron powder and found that this process was not a single-step but involved multiple consecutive chemical reaction stages. The schematic diagram of the reaction process for chromite with different iron powder additions is shown in Figure 12. Initially, the spinel structure reacts with carbon to form chromium-rich spinel and sesquioxide:

(Fe, Mg)(Cr, Fe, Al)₂O₄ +C → (Mg, Fe)(Cr, Al)₂O₄ + (Cr, Al)₂O₃ + (Fe, Cr)₇C₃

(9)

Subsequently, the iron oxide in the chromium-rich spinel reacts with carbon to form metallic iron:

(Mg, Fe)(Cr, Al)₂O₄ +C → (Mg, Fe)(Cr, Al)₂O₄ + Fe

(10)

The newly generated metallic iron and the composite carbide eventually form a Fe-C-Cr alloy:

Fe + (Fe, Cr)₇C₃ → Fe-C-Cr alloy

(11)

Meanwhile, chromium separates from the spinel structure in the form of (Cr, Al)₂O₃ rather than existing directly as a metal, which aligns with previous studies [36,37].

In addition, the multi-step reduction reaction of chromite gradually transforms into a single-step reduction reaction with the increase in the dosage of iron powder, as shown in the Figure 12.

When the amount of added iron powder reaches 30%, the reduction reaction of chromite is mainly the multi-step reduction reaction and the decomposition extent of the chromite spinel increases, as shown in Figure 12b. During the reduction process, metallic iron particles initially come into contact with carbon, forming cementite (Fe₃C). Under conditions of high temperature and high carbon concentration, the cementite further reacts to form a eutectic liquid phase with metallic iron. The presence of this liquid phase plays a critical role in the reduction process: Firstly, it accelerates the dissolution of nascent iron, enabling newly formed metallic iron to rapidly integrate into the liquid phase. This reduces the activity of nascent iron and promotes the reduction reaction. Secondly, the liquid phase facilitates the diffusion and dispersion of carbon atoms, thereby inhibiting carbon aggregation on the surface of spinel particles and suppressing carbide formation [38,39]. These findings indicate that cementite accelerates the multi-step reduction reaction of chromite ore and then induces particle fracture when the amount of added iron powder reaches 30%.

However, when the addition amount of iron powder further increases (e.g., 60%), as shown in Figure 12c, the reduction mechanism of chromite changes, and the reaction shifts from being mainly multi-step superimposed to being primarily a single-step reaction. The reaction equation is as follows:

(Fe, Mg)(Cr, Fe, Al)₂O₄ + C + Fe→ (Mg, Fe)(Cr, Al)₂O₄ + Fe-C-Cr alloy

(12)

The sesquioxide produced by the decomposition of the spinel gradually disappears, the microstructure changes little, and the spinel particle size remains intact; thus, the promoting effect on reduction decreases.

To sum up, when no iron powder is added, the solid-state reduction of chromite proceeds via multi-step reactions, with intermediate products including sesquioxide (Cr, Al)₂O₃ and complex carbides (Fe, Cr)₇C₃. The volume change associated with the formation of sesquioxide facilitates the cracking of chromite spinel particles. The incorporation of metallic iron promotes the formation of a low-melting-point liquid phase of Fe-C alloy. This liquid phase suppresses the activity of newly formed iron, enhances the solid-state reduction reaction of chromite, accelerates the cracking of chromite spinel particles, increases the surface area for the reduction reaction, improves the kinetic conditions of the reaction, reduces the apparent activation energy of the reduction process, and expedites the formation of metallic iron. As the amount of iron powder added increases, the solid-state reduction of chromite gradually shifts from multi-step reactions to single-step reactions. Since single-step reactions do not produce intermediate products such as sesquioxide, no significant volume changes occur in chromite spinel and thus chromite spinel particles do not fragment into smaller particles. When the addition of iron powder exceeds 30%, the volume of chromite spinel particles progressively increases, leading to a reduction in the surface area available for the reduction reaction. Consequently, the reinforcing effect of metallic iron on the solid-state reduction of chromite diminishes.

4. Conclusions

(1)

The optimal iron metallization rate of 97.15% was achieved under the following optimized parameters: 20 wt% carbon dosage, reduction temperature maintained at 1175 °C, and a duration of 2 h.

(2)

The addition of iron powder can enhance the solid-state reduction of chromite ore, and the enhance effect first increases and then decreases with the increase in the iron powder dosage. The iron metallization rate increases from 91.31% to 96.33 with the increase in the iron powder dosage from 0% to 30% under the conditions of reducing at 1175 °C for 1.5 h.

(3)

The multi-step reduction reaction gradually transforms into a single-step reduction reaction with the increase in the dosage of iron powder. Iron powder promotes the formation of a low-melting-point iron–carbon alloy liquid phase, which reduces the activity of nascent metallic iron and accelerates the speed of the solid-state reduction reaction of chromite ore and the disintegration of chromite spinel particles. However, excessive iron powder addition (>30%) shifts the most multi-step reduction reaction of chromite ore to a single-step reduction reaction, which diminishes the fragmentation degree of chromium spinel particles and weakens the enhancement effect of iron powder on the solid-state reduction of chromite ore.