An Integrated Hydrogen Metallurgy Route of Vanadium–Titanium Magnetite for Efficient Recovery of Fe, V, and Ti

Abstract

Vanadium–titanium magnetite is a strategically important resource for iron, vanadium, and titanium production, yet its utilization in conventional blast furnace–basic oxygen furnace routes is limited by the dilution of titanium into low-value slag. This study investigates an integrated process route combining pellet preparation, hydrogen-based shaft furnace reduction conducted in the temperature range of 800–1000 °C, and subsequent electric furnace smelting for efficient recovery of Fe, V, and Ti. Pellets prepared from 100 wt.% vanadium–titanium magnetite exhibited sufficient mechanical strength but showed poor reducibility and severe low-temperature reduction disintegration, rendering them unsuitable for hydrogen-based shaft furnace operation. To overcome these limitations, systematic ore blending was applied. An optimized pellet composition comprising 40 wt.% vanadium–titanium magnetite, 50 wt.% high-grade iron ore, and 10 wt.% titanium concentrate achieved reduction degrees above 90%, acceptable swelling and bonding behavior, and low reduction disintegration indices meeting industrial HYL requirements. Industrial trials in a hydrogen-based shaft furnace demonstrated stable operation and consistent product quality, producing direct reduced iron with controlled metallization and enrichment of titanium and vanadium. Subsequent electric furnace smelting achieved clear slag–metal separation, yielding hot metal with high iron and vanadium recovery and a TiO₂-rich slag containing approximately 45 wt.% TiO₂. Recovery rates of Fe, V, and Ti exceeded 90%, confirming the technical feasibility of the proposed process route.

1. Introduction

Vanadium and titanium are strategic metallic elements that play an essential role in advanced manufacturing sectors, including aerospace, national defense, and emerging energy technologies. In modern civil aviation, the use of titanium alloys in Chinese airliners is increasing: the COMAC C919 incorporates about 9.3 wt.% titanium alloy in its structural weight, exceeding the share seen in similarly classed Western aircraft such as the Boeing 737 and Airbus A320, while major international wide-body platforms like the Airbus A350 and Boeing 787 each integrate roughly 14–15 wt.% titanium in their structural mass. This trend suggests that future designs such as the COMAC C929 will continue to adopt high-performance titanium alloys at levels comparable to global benchmarks [1,2]. Beyond aerospace, vanadium demand is expanding rapidly toward energy-storage applications, with forecasts indicating that vanadium consumption in grid-scale energy storage may exceed that of the traditional steel industry within the next 3–5 years (especially vanadium redox flow batteries (VRFBs)). Despite rising demand, the vanadium–titanium industry is still largely focused on producing low-value iron products, while a significant fraction of vanadium and titanium is lost or downgraded during processing, resulting in an insufficient supply of high-value V- and Ti-based materials and inefficient resource utilization [3,4].

Vanadium–titanium magnetite (VTM) is a complex, multicomponent mineral resource characterized by the coexistence of iron, titanium, vanadium, and a range of associated valuable elements such as chromium, cobalt, scandium, nickel, gallium, and copper. Owing to this compositional complexity, VTM possesses exceptionally high comprehensive utilization potential but also presents significant metallurgical challenges. Over the past decade, various alternative processing routes beyond the conventional blast furnace–basic oxygen furnace (BF–BOF) process have been investigated, including coal- or gas-based direct reduction followed by electric furnace smelting, direct reduction–magnetic separation, and sodium roasting–direct reduction–electric furnace processes [5,6]. More recently, hydrogen-based reduction of VTM has attracted increasing attention at the laboratory scale, demonstrating potential advantages in reduction kinetics, energy efficiency, and vanadium–titanium enrichment behavior [7,8]. However, most reported studies remain limited to small-scale experiments or isolated process steps, often involving coal-based reduction or complex roasting pretreatments. Persistent challenges such as pellet degradation during reduction, insufficient control of titanium-bearing phase evolution, and weak integration between reduction and smelting stages have not yet been fully resolved. Moreover, although various reactor concepts (including rotary kilns, rotary furnaces, shaft furnaces, and fluidized beds) have been examined, systematic investigations that integrate pellet preparation, hydrogen-based shaft furnace reduction, and downstream electric smelting into a continuous and industrially validated process route remain scarce.

At present, the BF–BOF long-process route remains the dominant industrial method for large-scale utilization of vanadium–titanium magnetite due to its high production capacity and technological maturity [9,10]. Nevertheless, this route suffers from inherent limitations. During blast furnace smelting, titanium is predominantly concentrated in the slag phase, mainly in the form of titanium oxides and complex titanate phases (e.g., TiO₂ and CaTiO₃), depending on slag basicity and oxygen potential. Increasing TiO₂ content leads to sharply elevated slag viscosity, which compromises furnace stability and results in titanium-bearing slags with low TiO₂ grade and poor reactivity, for which no mature large-scale extraction technology has yet been established. In addition, the vanadium recovery efficiency of the BF–BOF process is relatively low, with only about 67 wt.% of vanadium entering the hot metal, causing substantial losses of strategic resources [11,12,13,14,15]. To address these limitations, this work presents an integrated ore-to-materials optimization of vanadium–titanium magnetite pellet mixtures for a continuous hydrogen-based shaft furnace–electric furnace process with electric smelting separation. This study demonstrates efficient separation and recovery of iron, vanadium, and titanium and reports a kiloton-scale industrial test of hydrogen-based shaft furnace processing of vanadium–titanium magnetite.

2. Materials and Methods

2.1. Raw Materials

The Heishan vanadium-titanium magnetite (VTM) was obtained from the Heishan deposit (Chengde, China). The high-grade iron ore and titanium concentrate were supplied by HBIS Group Zhangxuan Technology Company Limited (Zhangjiakou, China). All three raw materials used in this study were taken from a single industrial batch to ensure compositional consistency and representativeness. Prior to use, the ores were homogenized and sampled following standard quartering procedures.

To meet the particle size requirements for pelletization, the VTM was subjected to fine grinding using a vibratory mill (MS-1, 1.5 kW, 380 V, 50 Hz, feed particle size < 12 mm). The grinding conditions were 1–5 min grinding time, speed 1440 rpm, charge of <150 g, with a specific grinding energy of about 160 kWh·t⁻¹, targeting a particle size distribution with D90 ≈ 40–100 μm. The high-grade iron ore and titanium concentrate were used as received, as their particle size distributions already satisfied the pelletization requirements.

Pellets were prepared using a disk pelletizer with a disk diameter of 1000 mm, an inclination angle of 45°, and a rotational speed of 25 rpm. During pelletizing, the moisture content of the green pellets was controlled at 8.5 wt.% by spraying deionized water. Bentonite was added at 1 wt.% as the binder. The resulting green pellets were screened to a size range of 10–16 mm, and pellets outside this range were recycled. Only pellets meeting the size and integrity criteria were selected for subsequent drying and induration experiments.

Except for the intentional variation in ore blending ratios, the pellet chemistry was kept constant throughout all experiments, with no additional fluxes added beyond bentonite, unless explicitly stated otherwise. This approach ensured that the observed differences in pellet performance and reduction behavior could be attributed primarily to ore composition and process conditions rather than changes in pellet chemistry.

2.2. Characterization Methods

The particle size distribution of ore powders was determined by sieve analysis using a standard vibrating sieve shaker. A representative dried sample of 200 g was sieved through a series of standard sieves ranging from 38 μm to 500 μm for 15 min. The results were expressed as a mass-based (volume-based) particle size distribution, calculated from the mass retained on each sieve. Each measurement was repeated three times, and the average values were reported. The analysis was conducted under dry conditions, without the use of dispersing agents, in accordance with common practice for pellet feed characterization.

The compressive strength of pellets was measured using a calibrated universal testing machine (WSD-20, Shandong Zhongyi Instrument Co., Ltd., Jinan, China) in accordance with GB/T 14201–2018 [16]. A total of 62 pellets were tested for each condition, with a loading rate of 30 mm·min⁻¹. The maximum compressive force sustained by each pellet before fracture was recorded, and the average value was reported as the representative compressive strength. Given the large number of tested pellets (n = 62), the reported average values are considered statistically representative.

The chemical compositions of ores, pellets, direct reduced iron (DRI), smelting slag, and hot metal were determined using standardized analytical techniques. Major oxide compositions were measured by X-ray fluorescence spectroscopy (XRF) using an Axios model spectrometer (PANalytical B.V., Almelo, The Netherlands), calibrated with certified reference materials. Trace and metallic elements were analyzed by ICP-OES (PerkinElmer Inc, Waltham, MA, USA) after acid digestion using the mixed acid digestion protocol.

Phase identification of ore powders and selected pellet samples was performed using X-ray diffraction (XRD) with an Empyrean diffractometer, employing Co Kα radiation (λ = 1.5406 Å) (MPDDY2094, PANalytical B.V., Almelo, The Netherlands). The diffraction patterns were collected over a 2θ range of 10–80°, with a step size of 0.08° and a scanning speed of 4.67°·min⁻¹. Phase analysis was conducted using qualitative methods based on standard diffraction databases.

The morphology, microstructure, and elemental distribution of pellets before and after reduction were examined using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) (Ultra Plus, Carl Zeiss AG, Oberkochen, Germany). The scanning angle range was 5° to 90°, and the scanning speed is 0.2° s⁻¹, with an accelerating voltage of 20 kV and a working distance of 10.6 mm. EDS analyses were performed as point scans depending on the objective of the observation.

2.3. Hydrogen-Based Reduction Experimental Setups

2.3.1. Laboratory Reduction Furnace and Metallurgical Performance Testing

Laboratory hydrogen-based reduction experiments were conducted using a vertical tubular reduction furnace, designed in accordance with the testing principles defined in BS ISO 11258:2015 for the evaluation of iron ore pellets under reducing atmospheres [17]. A schematic diagram of the laboratory reduction reactor is provided in Figure 1, illustrating the principal components, including the furnace tube, sample holder, gas inlet and outlet, temperature control zone, and exhaust system. The schematic is intended to show the operational principle rather than detailed construction features.

The reduction disintegration index (RDI), reduction swelling, and reduction bonding tests were conducted according to standardized procedures [18] under controlled conditions. The typical reduction temperature for reducibility, reduction bonding, and reduction swelling tests was 1000 °C, while the low-temperature reduction disintegration test was conducted at 500 °C. All experiments were carried out under a reducing atmosphere of 65 vol.% H₂, 13 vol.% CO, 4 vol.% CO₂, and 18 vol.% N₂, supplied at a flow rate of 10 L·min⁻¹. The sample mass for each test was 500 g, and each experiment was repeated three times to ensure reproducibility.

The reduction degree, which characterizes the reducibility of the pellets, was calculated using Equation (1):

$$R_t=\left({\displaystyle \frac{0.11W_1}{0.43W_2}}+{\displaystyle \frac{m_1-m_t}{m_0\times 0.43W_2}}\times 100\right)\times 100\%$$

(1)

where R_t is the reduction degree, at time t, in (%); W₁ is the FeO mass fraction of the pellets before reduction (wt.%); W₂ is the total iron mass fraction of the pellets before reduction (wt.%); m₁ is the pellet mass before reduction (g); m_t is the pellet mass after reduction for time t (g); and m₀ is the mass of the oxidized pellet (g).

The furnace was heated from room temperature to the target reduction temperature at a controlled heating rate of 10 °C·min⁻¹. Once the target temperature was reached, the samples were held isothermally for 120 min, defined as the effective reduction time. After completion of the reduction stage, the samples were cooled by furnace cooling under inert gas protection (to room temperature) to avoid reoxidation.

The selected reduction temperature range of 950–1050 °C and gas composition were chosen to reflect the typical operating conditions of hydrogen-based shaft furnaces, enabling laboratory-scale evaluation of pellet reducibility, structural stability, and metallurgical performance under conditions relevant to industrial practice.

Pellet metallurgical performance was evaluated through reduction degree, reduction swelling, reduction bonding, and low-temperature reduction disintegration (RDI) tests. The corresponding indices were calculated following the definitions specified in the applied standards for direct comparison with HYL-recommended criteria, summarized in

Table 1. HYL recommended requirements for pellets used in hydrogen-based shaft furnaces.

Pellet Strength	Reduction Degree	Reduction Swelling Index	Reduction Bonding Index	IRD (+6.3)	IRD (−3.2)	IRD(Up)
2000 N·p−1	>90%	<15%	<20%	>80%	<10%	>60%

Note: The values are recommended by HYL; IRD(Up) represents the mass fraction of undamaged pellets.

[18,19].

2.3.2. Industrial Hydrogen Shaft Furnace Trial

The industrial hydrogen-based shaft furnace trial was conducted on a large-scale vertical shaft furnace reactor (effective furnace height 38 m) designed for continuous direct reduction of iron-bearing pellets, operating under elevated pressure and high gas throughput. A kiloton-level industrial trial was conducted on the 1.2 Mt·a⁻¹ hydrogen metallurgy demonstration line of HBIS Group. This industrial trial setup was designed to validate pellet performance, reduction behavior, and product quality under realistic hydrogen-based shaft furnace conditions, such as elevated operating pressure, high gas throughput, and continuous counter-current gas–solid flow. The trial therefore provides a direct link between laboratory-scale testing and full-scale industrial application.

The furnace operates in a counter-current mode, where solid pellets descend by gravity while reducing gas flows upward, enabling efficient gas–solid contact and heat utilization. A conceptual schematic of the industrial hydrogen shaft furnace is provided in Figure 2 to illustrate the principal zones, gas injection and extraction points, cooling gas circuit, and product discharge, thereby clarifying the scale transition from laboratory experiments to industrial operation.

The shaft furnace was operated at a reduction temperature range of approximately 800–1000 °C, with regenerated reducing gas entering the furnace at 1000–1050 °C under a pressure of 7–9 barg (gauge pressure). The internal furnace pressure was maintained in the range of 6–8 barg to ensure stable gas flow and burden permeability. The reducing gas consisted primarily of hydrogen-rich gas derived from coke oven gas (COG) after purification and regeneration, representing an industrially relevant hydrogen metallurgy configuration. Gas utilization was achieved through a closed-loop concept involving gas regeneration, cooling gas injection, and top-gas recycling, ensuring efficient use of reducing agents.

Pellets with a size range of 8–16 mm were continuously charged into the shaft furnace. The direct reduced iron (DRI) product was discharged at the furnace bottom at a rate of approximately 50–75 t·h⁻¹, depending on operating conditions. Cooling gas was introduced in the lower section of the furnace to control DRI temperature prior to discharge, with cooling gas and COG inlet temperatures maintained at approximately 30–50 °C.

DRI samples were collected during steady-state operation periods, and the reported data represent average values over stable operating settings.

2.4. Electric Furnace Smelting Separation Setup

Electric furnace smelting–separation experiments were conducted using a laboratory-scale medium-frequency induction furnace equipped with a high-purity graphite crucible (inner diameter 100 mm, inner height 200 mm). The effective batch size per experiment was approximately 2–3 kg, corresponding to the laboratory crucible volume. The furnace was operated under an argon-protected atmosphere to prevent reoxidation during high-temperature smelting (Figure 3).

Metallized pellets produced from the optimized ore blend were crushed and homogeneously mixed prior to charging. Heating was achieved by electromagnetic induction. Thermal energy transferred to the charge through the graphite crucible by adjusting the furnace oscillation frequency and power input.

Optimal operational procedures were established and adopted following the smelting route described in detail in Ref. [20]. In the present study, which focuses on pellet design and hydrogen-based reduction behavior, only the optimized smelting condition is reported to evaluate phase separation and elemental recovery from the optimized pellet. Additional smelting experiments conducted under the same procedure are not discussed here to maintain the focus on the integrated process (the corresponding data are available from the authors upon reasonable request, as stated in the Data Availability Section). Carbon was added in the form of activated carbon powder with a particle size of approximately 100 μm and was premixed uniformly with the metallized pellets prior to charging. The carbon addition was defined using the C/O ratio, expressed as the molar ratio of fixed carbon to the total oxygen associated with reducible iron and vanadium oxides in the DRI. The C/O ratio was controlled at 1.10.

Slag basicity was adjusted by adding analytical-grade CaO as flux. Basicity was defined as CaO/SiO₂ = 0.50, calculated based on the total slag-forming oxide composition and controlled through pre-weighed flux additions.

Smelting experiments were conducted in a laboratory-scale induction furnace operated under manual, temperature-controlled conditions (rated power: 60 kW). The furnace reached the target temperature of 1600 °C within approximately 30 min, followed by a holding time of 30 min to ensure complete melting, carburization, deep reduction, and effective slag–metal separation. Phase separation occurred primarily through density-driven settling under molten conditions, with iron-rich metal collecting at the bottom of the crucible and titanium-rich slag floating above.

After completion of smelting, the furnace was allowed to cool under argon protection. The solidified slag and metal phases were then separated mechanically based on clear macroscopic and structural differences and prepared for subsequent chemical, phase, and microstructural analyses to evaluate elemental distribution and recovery.

3. Results and Discussion

3.1. Pellet Preparation and Properties of Vanadium–Titanium Magnetite

The chemical compositions of vanadium–titanium magnetite (VTM), high-grade iron ore, and titanium concentrate are summarized in

Table 2. Main chemical composition of different ore powders (wt.%).

Ore Type	TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	V2O5	Cr2O3
Vanadium–titanium magnetite	61.51	30.17	0.42	2.12	1.12	2.71	6.41	0.73	0.66
High-grade iron ore	69.25	28.67	0.23	2.43	0.42	0.59	0.15	0.09	<0.01
Titanium concentrate	36.12	28.85	1.29	5.29	0.88	1.36	42.47	0.15	<0.01

Note: The mass fractions of Pb, Zn, and As in the two ores are all below 0.01 wt.%.

. Compared with the high-grade iron ore, the VTM exhibits a lower total iron content (61.5 wt.% TFe) and a significantly higher TiO₂ content (6.4 wt.%), indicating high potential value for titanium recovery but also implying increased complexity for metallurgical processing. In conventional blast furnace operation, such elevated TiO₂ levels will increase slag viscosity and compromise process stability, already restricting the large-scale use of this ore.

The particle size distribution of the finely ground VTM is shown in Figure 4. Fine grinding significantly increases the fraction of particles smaller than 74 μm, which is essential for achieving adequate green pellet strength and, consequently, satisfactory fired pellet quality. In contrast, the use of coarse, unground ore as pellet feed generally leads to poor pelletizing and roasting behavior, resulting in oxidized pellets with low mechanical strength and limiting the allowable proportion of such material in the pellet feed [21].

The phase composition of the ground VTM, determined by X-ray diffraction, is presented in Figure 5. The dominant phases include magnetite (Fe₃O₄), ilmenite (FeTiO₃), vanadium-bearing magnetite (Fe₂VO₄), and chromite (FeCr₂O₄). Titanium is primarily present in the form of FeTiO₃ solid solution, while vanadium is incorporated into iron oxide phases rather than forming discrete vanadium oxides.

To explore the material and metallurgical limitations of hydrogen-based shaft furnace operation, experiments were conducted using pellets prepared from 100 wt.% VTM. The results identify the key factors responsible for insufficient reducibility and severe low-temperature degradation, providing the basis for the ore blending strategy discussed in the following chapter.

Pellets prepared from 100 wt.% VTM exhibited stable and reproducible compressive strength, with an average value of 2332 N·p⁻¹. All tested pellets exceeded the minimum strength requirement of 2000 N·p⁻¹ recommended for hydrogen-based shaft furnace charging, indicating that mechanical strength and handling stability are not the limiting factors for the application of single-ore pellets.

Despite acceptable mechanical strength, the hydrogen-based reduction performance of 100 wt.% VTM pellets was found to be insufficient. Under typical shaft furnace–relevant conditions (1000 °C, H₂–CO–CO₂–N₂ atmosphere), the measured reduction degree reached only 88.61%, failing to meet the minimum 90% metallization requirement recommended by HYL (described in the Section 2.3.1, Table 1). This limited reducibility is primarily attributed to the presence of FeTiO₃, which is thermodynamically and kinetically more resistant to hydrogen reduction than simple iron oxides in the investigated temperature range of 950–1050 °C.

Furthermore, the pellets exhibited extremely severe low-temperature reduction disintegration. At 500 °C, the measured indices were IRD(+6.3) = 30.83%, IRD(−3.2) = 63.86%, and IRD(Up) = 28.34%, which deviate drastically from HYL-recommended criteria. The excessive generation of fines indicates a pronounced structural breakdown during the early stages of reduction, consistent with reported hydrogen-reduction behavior of vanadium–titanium magnetite pellets under similar conditions [22].

This behavior can be mechanistically explained by the lattice transformation–induced stress associated with titanium-bearing iron oxides. The presence of Ti in solid solution within hematite and magnetite promotes lattice distortion during reduction, generating higher internal stresses than those observed in Ti-free iron ores. These stresses accelerate crack initiation, pore coalescence, and ultimately pronounced pellet fragmentation [23].

The experimental results show that modification of vanadium–titanium magnetite by ore blending is required to achieve stable hydrogen-based shaft furnace operation. The implications of different blending strategies are examined in the following chapter.

3.2. Optimization of Pellet Mixture and Hydrogen-Based Reduction

3.2.1. Hydrogen Reduction Behavior of Blended Iron Ore Pellets

This chapter evaluates different blended pellet compositions to determine their suitability for hydrogen-based shaft furnace operation according to HYL criteria and their compatibility with subsequent electric furnace smelting.

Ore blending was carried out using vanadium–titanium magnetite (VTM), high-grade iron ore, and titanium concentrate. The blending ranges were selected to progressively dilute Ti-bearing phases responsible for poor reduction behavior while maintaining sufficient titanium input for enrichment in the smelting slag. High-grade ore was introduced to improve reducibility and structural stability, whereas titanium concentrate was added in controlled amounts to compensate for the reduction in TiO₂ content caused by dilution.

As shown in

Table 3. Hydrogen-based reduction metallurgical performance indicators of vanadium–titanium magnetite blended pellets.

No.	VTM/wt.%	High-Grade Ore/wt.%	Ti—Concentrate/wt.%	Pellet Strength/(N·p−1)	Reduction Degree/%	Reduction Swelling Index/%	Reduction Bonding Index/%	IRD (+6.3)/%	IRD (−3.2)/%	IRD (Up)/%
HYL criteria				>2000	>90	<15	<20	>80	<10	>60
1	70	30	0	2716	91.24	7.25	2.90	63.6	34.3	79.7
2	60	40	0	2948	92.47	8.63	2.90	77.5	21.4	90.3
3	50	50	0	3123	93.53	9.18	3.69	90.22	9.40	98.59
4	40	50	10	2823	90.32	6.62	4.23	89.03	9.60	98.14

, increasing the proportion of high-grade ore from 30 wt.% to 50 wt.% results in a systematic increase in pellet compressive strength, reflecting improved crystal bridging and reduced gangue interference, as widely reported for blended pellet systems with decreasing gangue content [9]. The addition of 10 wt.% titanium concentrate slightly decreases pellet strength due to the introduction of refractory Ti-bearing phases, but the resulting strength (2823 N·p⁻¹) remains well above the minimum requirement for shaft furnace charging.

The hydrogen-based reduction performance of blended pellets is summarized in Table 3, with all indices evaluated against HYL-recommended criteria. Compared with single-ore VTM pellets, blended pellets exhibit markedly improved reducibility and structural stability. At blending ratios of 50 wt.% VTM or lower, the reduction degree exceeds 90%, while low-temperature reduction disintegration indices fall within acceptable limits.

3.2.2. Metallurgical Performance of the Optimized Pellet Composition

The blend containing 40 wt.% VTM + 50 wt.% high-grade ore + 10 wt.% titanium concentrate achieves a balanced performance: adequate pellet strength, acceptable reduction degree, limited swelling and bonding, and IRD indices fully compliant with HYL requirements. This composition also yields DRI with elevated TiO₂ content (

Table 4. Chemical compositions of vanadium–titanium magnetite DRI with two blending ratios (wt.%).

wt.%			Metallization Rate	MFe	TFe	CaO	SiO2	MgO	Al2O3	TiO2	V2O3
VTM	High-Grade Ore	Ti—Concentrate
50	50	0	91.03	77.43	85.06	0.43	2.19	1.23	1.92	7.21	0.52
40	50	10	88.26	71.34	80.83	0.52	3.71	1.18	1.63	9.25	0.41

), ensuring compatibility with downstream titanium enrichment during smelting separation. These results demonstrate that ore blending effectively stabilizes reduction behavior without sacrificing titanium utilization potential.

The influence of reduction temperature on the reduction behavior of the selected blended pellets (40 wt.% VTM + 50 wt.% high-grade ore + 10 wt.% titanium concentrate) is shown in Figure 6a. Increasing the temperature from 950 °C to 1050 °C under a fixed isothermal reduction time of 120 min accelerates the reduction rate and increases the final reduction degree, reflecting enhanced gas–solid reaction kinetics under hydrogen-rich conditions.

To obtain comparable kinetic parameters, the experimental data were analyzed using an isothermal solid-state kinetic approach within a selected conversion range and fitted using the D3 (three-dimensional diffusion) model, yielding apparent rate constants k(T) for each temperature (Figure 6b). The resulting Arrhenius plot (Figure 6c) shows a linear relationship between lnk and 1/T, from which an apparent activation energy of 69.5 kJ·mol⁻¹ was derived. This value should be interpreted as engineering kinetic parameters, reflecting the combined effects of gas–solid reaction, diffusion through product layers, and microstructural evolution during reduction (rather than intrinsic chemical kinetics). Notably, the magnitude of the apparent activation energy is consistent with diffusion-influenced hydrogen reduction of iron oxides in porous pellets and supports the observed temperature sensitivity of reduction behavior discussed in the context of shaft furnace operation.

SEM observations (Figure 7) confirm that increasing reduction temperature promotes metallic iron growth and coalescence, consistent with the accelerated reduction kinetics and enhanced bonding observed at higher temperatures.

Reduction swelling indices at different temperatures are summarized in

Table 5. Reduction swelling index of blended pellets (40 wt.% VTM, 50 wt.% high-grade ore, 10 wt.% titanium concentrate) measured at different temperatures.

Reduction Temperature	950 °C	1000 °C	1050 °C
Maximum swelling index/%	8.18	11.22	12.86
Final swelling index/%	5.77	6.62	7.63

and illustrated in Figure 8. Both maximum and final swelling indices increase with temperature, indicating that accelerated reduction intensifies lattice transformation–induced stress and pore formation during the early stages of reduction, consistent with the established swelling mechanism of oxidized iron ore pellets under high reduction potential atmospheres [24].

SEM observations (Figure 9) show that increasing temperature enhances the formation of internal porosity and microcracks, mainly during the early transformation of iron oxide phases. However, the final swelling indices remain below 8% throughout the studied temperature range, confirming that pellet swelling stays within acceptable limits for shaft furnace operation.

Reduction bonding indices increase monotonically with temperature, as shown in Figure 10, reflecting enhanced metallic iron formation and diffusion at elevated temperatures. SEM analysis (Figure 11) confirms progressive growth, coalescence, and interconnection of metallic iron phases, leading to increased interparticle contact and bonding.

The combined swelling and bonding behavior indicates competing effects: higher temperatures promote faster reduction and stronger metallic bonding but also intensify swelling due to lattice transformation and pore evolution. Within the investigated temperature range of 950–1050 °C, these competing effects remain balanced, allowing stable hydrogen-based shaft furnace reduction of blended vanadium–titanium magnetite pellets.

3.3. Industrial Validation of Pellet Reduction in Hydrogen-Based Shaft Furnace

The industrial trial examines whether the optimized pellet composition and hydrogen-based reduction conditions developed at the laboratory scale can be transferred to shaft furnace operation without compromising operational stability or product quality.

An industrial trial was conducted using pellets prepared from the optimized ore blend (40 wt.% vanadium–titanium magnetite (VTM) + 50 wt.% high-grade ore + 10 wt.% titanium concentrate). During the trial period, shaft furnace operation remained stable, with no abnormal fluctuations in pressure drop or indications of burden permeability deterioration. Furnace operation proceeded smoothly throughout the test, indicating that the introduction of VTM-containing pellets did not disrupt gas flow, heat transfer, or overall furnace stability when compared with the reference operation (high-grade iron ore).

The chemical composition of the finished pellets used during the trial is summarized in

Table 6. Chemical composition of pellets (wt.%) used in industrial hydrogen-based shaft furnace in reference and trial operations.

Item	TFe	FeO	MgO	TiO2	SiO2	CaO	Al2O3	V2O5
Reference operation (iron ore)	66.83	0.66	0.42	0.48	2.47	0.37	0.50	0.17
Trial period (optimized ore blend)	62.31	1.32	0.86	6.58	2.08	0.63	1.36	0.38
Difference	−4.52	0.66	0.44	6.10	−0.39	0.26	0.86	0.21

. Relative to the reference pellets, the optimized pellets exhibit lower total iron content and higher concentrations of TiO₂ and V₂O₅, reflecting the intentional incorporation of vanadium–titanium magnetite and titanium concentrate. Corresponding changes in minor oxide components are consistent with the altered ore blend and remain within acceptable ranges for shaft furnace operation.

The main physical properties of the finished pellets are presented in

Table 7. Physical and mechanical properties of finished pellets applied in the industrial hydrogen-based shaft furnace.

Stage	Compressive Strength/(N·p−1)	Tumbler Index/%	Size Fraction 10–16 mm/%	Abrasion Index/%
	10–12.5 mm	12.5–16 mm	>16 mm
Reference operation	2831	3379	3885	96.9
Trial period	2635	2872	2928	94.2
Difference	−196	−507	−957	−2.7

. Although a moderate decrease in compressive strength and tumbler index is observed for the optimized pellets, all values remain well above the minimum requirements for hydrogen-based shaft furnace charging. The size distribution and abrasion index show no significant deterioration, confirming that the optimized pellets retain sufficient mechanical integrity for industrial handling and continuous operation.

The chemical composition and metallization rate of direct reduced iron (DRI) produced during the industrial trial are shown in

Table 8. Chemical composition of direct reduced iron (DRI) obtained under reference and trial operating conditions in the industrial hydrogen-based shaft furnace (wt.%).

Item	Metallization Rate	MFe	TFe	C	SiO2	Al2O3	MgO	CaO	TiO2	V2O3
Reference operation	94.67	84.81	89.58	2.71	2.98	0.70	0.59	0.56	0.58	0.21
Trial period	88.37	70.93	80.26	2.63	2.61	1.31	0.70	0.65	9.15	0.45
Difference	−6.30	−13.88	−9.32	−0.08	−0.37	0.61	0.11	0.09	8.57	0.24

. Compared with the reference operation, the optimized pellets yield DRI with a controlled reduction in metallization rate, accompanied by a pronounced enrichment of TiO₂ and V₂O₃. This compositional difference is consistent with the fundamental reduction behavior of vanadium–titanium magnetite in hydrogen-based shaft furnaces, where non-iron elements are thermodynamically difficult to reduce and therefore remain concentrated in the solid phase [25], enabling the intentional enrichment of titanium and vanadium for subsequent smelting separation.

Despite the lower metallization rate (approximately 88%), the trial-period DRI maintains adequate quality for electric furnace processing. The observed decrease in metallic iron content is offset by the strategic enrichment of valuable elements, ensuring that the DRI remains a suitable feedstock for downstream smelting while allowing improved overall resource utilization.

3.4. Electric Furnace Smelting Separation and Element Recovery

The final stage of the integrated hydrogen shaft furnace–electric furnace route is the melting separation stage, representing the key connection between hydrogen-based solid-state reduction and the generation of high-value intermediate products. Its primary objectives are to (i) separate iron-vanadium-rich metal from slag efficiently, (ii) concentrate titanium in the slag phase, and (iii) control the chemical form of titanium to ensure its accessibility for downstream utilization. In this context, the melting separation process must balance temperature, slag chemistry, and reduction potential to promote phase separation while avoiding excessive reduction of titanium oxides.

Based on preliminary optimization, melting separation experiments were conducted under a target temperature of 1600 °C, a holding time of 30 min, a slag basicity of CaO/SiO₂ = 0.50, and a C/O ratio of 1.10 [20]. These conditions provided stable melting behavior, low slag viscosity, and clear slag–metal stratification. Under these parameters, iron-rich metal accumulated at the bottom of the crucible, while titanium-bearing slag formed a continuous upper phase, enabling reliable post-solidification separation.

Electric furnace smelting of the vanadium–titanium DRI produced under shaft furnace conditions resulted in clear and stable slag–metal stratification. After smelting and controlled cooling, the molten products separated into a lower metallic phase and an upper slag phase with a well-defined interface (Figure 12). No visible metallic iron entrainment or dispersion within the slag was observed, indicating effective density-driven separation and good slag fluidity under the selected operating conditions.

The chemical compositions of the smelting slag and hot metal are summarized in

Table 9. Composition of smelting slag.

Component	TFe	V	SiO2	CaO	MgO	Al2O3	TiO2
Content/wt.%	2.45	<0.05	14.78	10.02	6.35	11.80	45.12

and

Table 10. Composition of smelted metal phase.

Component	TFe	Ti	C	V	Si
Content/wt.%	95.36	0.16	3.67	0.24	0.19

, respectively. The hot metal is characterized by a high total iron content (95.36 wt.%) and contains the majority of vanadium, confirming preferential partitioning of Fe and V into the metallic phase during smelting. In contrast, titanium is strongly enriched in the slag phase, with the TiO₂ content reaching 45.12 wt.%, which exceeds the typical threshold required for downstream titanium extraction processes. The low iron content of the slag (2.45 wt.% TFe) further indicates efficient metal–slag separation and minimal iron loss.

X-ray diffraction analysis of the slag obtained under optimized conditions revealed that titanium was predominantly present in the form of MgTi₂O₅, accompanied by secondary phases such as MgAl₂O₄, Mg₂SiO₄, and minor complex silicates (Figure 13). Although the slag contains a measurable amount of CaO, no discrete Ca-bearing crystalline phase was resolved by XRD, indicating that calcium is predominantly accommodated within an amorphous or low-crystallinity silicate matrix and/or distributed among complex Ca–Mg–Al–Si silicates with overlapping diffraction features. Notably, no dominant reflections corresponding to calcium titanate (CaTiO₃) or titanium carbide (TiC) were detected. The absence of CaTiO₃ demonstrates that the selected slag basicity effectively suppresses the formation of calcium-bound titanates, which are known to exhibit poor solubility and limited reactivity in downstream titanium extraction processes. Similarly, the lack of TiC confirms that the reduction potential was adequately controlled, preventing over-reduction of titanium oxides that would otherwise increase slag viscosity and deteriorate slag–metal separation.

SEM observations showed that the slag possessed a well-developed microstructure characterized by discrete, elongated Ti-rich phases embedded within a silicate–aluminate matrix (Figure 14). Metallic iron was essentially absent from the slag, indicating efficient separation and favorable slag fluidity during smelting.

Based on mass balance calculations using the measured compositions of the smelted products, the recovery rates of the major valuable elements were determined. The recovery rate of iron reached 96.19%, while vanadium and titanium (expressed as TiO₂) achieved recovery rates of 90.02% and 92.85%, respectively. The results confirm that the hydrogen shaft furnace–electric furnace route enables efficient and selective recovery of iron, vanadium, and titanium, completing the technical validation of the proposed integrated process.

4. Conclusions

This study demonstrates a technically viable process route that integrates pellet design, hydrogen-based direct reduction conducted in the temperature range of 800–1000 °C, and subsequent electric furnace smelting at approximately 1600 °C, enabling high-value utilization of vanadium–titanium magnetite. Pellets prepared from 100 wt.% vanadium–titanium magnetite exhibit adequate mechanical strength but suffer from insufficient reducibility and severe low-temperature reduction disintegration, making them unsuitable for direct hydrogen-based shaft furnace operation. The proposed approach provides a practical foundation for industrial deployment of hydrogen metallurgy in complex iron ores while enabling selective recovery of Ti and V, two valuable strategic elements.

Through systematic ore blending, a pellet composition of 40 wt.% vanadium–titanium magnetite, 50 wt.% high-grade iron ore, and 10 wt.% titanium concentrate was identified that satisfies hydrogen-based shaft furnace requirements. The blended pellets exhibit stable mechanical properties, acceptable reducibility, controlled swelling and bonding behavior, and low-temperature reduction disintegration indices fully compliant with industrial HYL criteria. The influence of reduction temperature on reduction degree, swelling, bonding, and microstructural evolution indicates that balanced metallurgical performance can be achieved under hydrogen-rich conditions without compromising structural stability.

Industrial trials show that the optimized pellets operate stably in a hydrogen-based shaft furnace, with no adverse effects on furnace stability, burden permeability, or continuous operation. The resulting direct reduced iron has a deliberately lower metallization level, enabling enrichment of titanium and vanadium while remaining suitable for electric furnace processing.

Electric furnace smelting of the vanadium–titanium DRI results in clear and stable slag–metal separation, with preferential partitioning of iron and vanadium into the metallic phase and strong enrichment of titanium in the slag. High recovery rates of Fe, V, and Ti exceeding 90% demonstrate efficient resource utilization and effective element separation, yielding a TiO₂-rich slag suitable for downstream titanium extraction.