Strategic Recovery of Titanium from Low-Grade Titanium-Bearing Blast Furnace Slag via Hydrothermal-Crystallization Coupling

Abstract

This study developed a hydrothermal-crystallization coupling strategy for selective titanium extraction from low-grade titanium-bearing blast furnace slag. Systematic parametric optimization revealed that an optimum titanium extraction efficiency of 92.3% was achieved under mild hydrothermal conditions. Phase evolution analysis demonstrated that the leaching residues comprised commercially valuable calcium oxalate hydrate and amorphous silica aggregates, while titanium primarily existed as stable Ti(OH)₂(C₂O₄)₂²⁻ complexes in the leachate. Subsequently, 99.4% of titanium in the leachate was precipitated through the hydrothermal decomposition method, and mixed-phase titanium oxides with a grade of 90.5% were obtained through alkaline washing. Comparative analysis highlights three notable advantages over conventional metallurgical processes: (1) selective extraction specificity for low-concentration titanium minerals, (2) process intensification through integrated hydrothermal-crystallization operations, and (3) environmental benignancy via reagent recyclability.

1. Introduction

As a critical transition metal, titanium demonstrates tri-domain functional dominance in advanced manufacturing [1,2]. In the aerospace field, the Ti-6Al-4V alloy exhibits exceptional specific strength and fatigue resistance, making it indispensable for operating hypersonic vehicle components [3,4,5]. In the metallurgical process field, as a multifunctional microalloying agent, metallic titanium enables grain refinement and precipitation strengthening, enhancing steel’s fatigue life [6,7]. In the chemical industries field, TiO₂ polymorphs serve as multiscale platforms for photocatalysis and energy storage [8,9,10].

As a strategic secondary resource generated during vanadium titano-magnetite smelting, titanium-bearing blast furnace slag demonstrates compositional heterogeneity requiring scientific categorization based on TiO₂ content, including low-grade titanium-bearing blast furnace slag (<10 wt% TiO₂), medium-grade titanium-bearing blast furnace slag (10–20 wt% TiO₂), and high-grade titanium-bearing blast furnace slag (>20 wt% TiO₂) [11,12]. Approximately 3 million tons of unusable low-titanium-bearing blast furnace slag were produced annually, resulting in significant strategic resource waste, environmental burdens, and land occupation [13]. Currently, extensive research has been conducted on extracting titanium from blast furnace slag via hydrometallurgical methods, with several representative approaches proposed as follows [14,15,16]:

The ammonium sulfate roasting–dilute acid leaching method for titanium extraction from water-quenched blast furnace slag was investigated. Thermodynamic analysis confirmed that titanyl sulfate intermediates formed during solid-phase sulfation, followed by selective dissolution in sulfuric acid. Under a roasting temperature of 380 °C, a roasting time of 90 min, an (NH₄)₂SO₄-to-blast-furnace-slag ratio of 2.5:1, and a sulfuric acid concentration of 1 mol/L, the titanium extraction efficiency reached 95.4% [17]. An alternative hydrometallurgical strategy utilizing concentrated sulfuric acid leaching was developed for titanium recovery from blast furnace slag. Through systematic process intensification under a leaching temperature of 160 °C, an acid-to-residue ratio of 1.5:1, and a sulfuric acid concentration of 50 wt%, a titanium extraction efficiency of 92.6% was achieved [18]. However, the aforementioned methods also confront dual challenges of elevated energy consumption and complex spent acid treatment [19]. More notably, the extraction of titanium from low-grade feedstocks remains critically underexplored, necessitating sustainable hydrometallurgical strategies that couple process intensification with three synergistic objectives, including maximizing extraction efficiency, minimizing energy consumption, and enabling closed-loop acid regeneration.

Our previous studies demonstrated a co-extraction efficiency of vanadium, titanium, and chromium of over 90% from vanadium slag through the oxalic acid hydrothermal leaching method [20,21,22]. Mechanistic investigations fundamentally revealed dual reaction pathways that involve spinel matrix deconstruction via Fe³⁺-oxalate chelation precipitation, followed by multi-metal coordination leaching through [VO(C₂O₄)₃]³⁻, [TiO(C₂O₄)₂]²⁻, and [Cr(C₂O₄)]⁺ complex formation. Crucially, we discovered calcium-mediated reaction redirection in low-grade titanium-bearing blast furnace slag, where Ca²⁺ preferentially coordinates with oxalate to form insoluble CaC₂O₄·H₂O, establishing selective dissolution channels that facilitate titanium leaching.

Building upon this analytical framework, the present study systematically investigates titanium extraction and separation from blast furnace slag through two interconnected phases: (1) hydrothermal leaching behavior and mechanisms of titanium dissolution in slag matrices, and (2) hydrothermal separation behavior governing selective titanium recovery from complex leachates.

2. Materials and Methods

2.1. Materials

The low-grade titanium-bearing blast furnace slag utilized in this study was obtained from HBIS Group Co., Ltd. (Chengde, Hebei Province, China) as a water-quenched metallurgical byproduct. Particle size refinement to less than 200 mesh was achieved through planetary ball milling followed by mechanical sieving.

The elemental composition analysis was performed via a sequential X-ray fluorescence spectrometer (XRF, ZSX Primus III+, Rigaku Corporation, Tokyo, Japan).

Table 1. Chemical composition of blast furnace slag.

Compound	TiO2	CaO	SiO2	Al2O3	MgO	Fe2O3
Wt Pct	8.61	34.89	30.52	11.13	8.58	2.44

shows predominant CaO (34.89 wt%), SiO₂ (30.52 wt%), and TiO₂ (8.61 wt%) contents, which align with typical low-grade titanium-bearing blast furnace slag specifications. X-ray diffraction (XRD, Rigaku Ultimate IV, Rigaku Corporation, Tokyo, Japan) analysis employed Cu-Kα radiation in step-scan mode of 0.02°/step at 0.06 s/step. As shown in Figure 1, the characteristic halo pattern confirms amorphous silicate glass formation induced by rapid water quenching. The micromorphology and elemental distribution characteristics were quantitatively investigated using field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, AJSM-F100, JEOL Ltd., Akishima, Tokyo, Japan). Figure 2a shows a regular surface morphology with sharp edges, while Figure 2b reveals titanium-bearing phases with heterogeneous dispersion patterns, showing characteristic submicron-scale clusters embedded in the Ca-Si-Al-Mg matrix.

2.2. Procedure

The titanium extraction experiments from low-grade titanium-bearing blast furnace slag were performed in a hydrothermal reactor. A single-factor experimental study was conducted to evaluate the effects of leaching temperature (110–140 °C), oxalic acid concentration (15–30 wt%), and leaching time (90 min) on titanium extraction efficiency. The precursor mixture comprising 1.00 g of blast furnace slag, oxalic acid, and deionized water was sequentially charged into the Teflon-lined reactor. The aqueous phase volume was calculated based on a fixed 10:1 liquid–solid mass ratio, while the oxalic acid dosage was controlled through stoichiometric concentration adjustment. Then, the reactor was hermetically sealed followed by programming the reaction parameters of temperature, time, and agitation rate. Subsequent to hydrothermal treatment, phase separation was achieved through vacuum filtration coupled with aqueous washing cycles, yielding Ti-rich leachate and insoluble residues.

The titanium concentration in leachate was quantified via ICP-OES, with the extraction efficiency calculated using Equation (1). The oven-dried residues underwent multiscale characterization, including crystalline phase identification via XRD, functional group identification via FTIR, micromorphology identification via SEM, and aqueous speciation modeling via Visual MINTEQ.

$$\eta =\left({\displaystyle \frac{C\times V}{M}}\right)\times 100\%$$

(1)

where η represents the leaching efficiency (%), C represents the total titanium concentration in leachate (g/L), V represents the leachate volume (L), and M represents the initial titanium mass in low-grade titanium-bearing blast furnace slag (g).

The hydrothermal precipitation of titanium species from leachate was systematically performed in Teflon-lined reactors through the precise regulation of temperature, time, and agitation rate. Subsequent phase separation via vacuum filtration coupled with aqueous purification yielded titanium oxide precipitates.

The titanium concentration in leachate was quantified via ICP-OES, with precipitation efficiency calculated using Equation (2). The oven-dried products were characterized using crystalline phase identification via XRD and micromorphology identification via SEM.

$$\omega =\left(1-{\displaystyle \frac{C_2\times V_2}{C_1\times V_1}}\right)\times 100\%$$

(2)

where ω represents the precipitation efficiency (%), C₁ and C₂ represent the Ti⁴⁺ concentrations in leachate before and after precipitation (g/L), and V₁ and V₂ represent the corresponding leachate volumes (L). Our prior investigations on simultaneous vanadium, titanium, and chromium extraction from vanadium slag demonstrated analogous hydrometallurgical protocols [20].

3. Results and Discussion

3.1. Hydrothermal Titanium Extraction from Low-Grade Titanium-Bearing Blast Furnace

3.1.1. Thermodynamic Behavior Analysis

Mineralogical characterization revealed that titanium was dispersed within impurity-dominated phases of blast furnace slag. Given the critical role of slag microstructure disruption in enhancing Ti dissolution, the thermodynamic feasibility of oxalic-acid-mediated impurity precipitation was systematically evaluated via HSC Chemistry. As illustrated in Figure 3, the temperature-dependent Gibbs free energy change (ΔG^θ) profiles demonstrate the spontaneity boundaries for dominant reactions:

CaO + H₂C₂O₄ = CaC₂O₄·H₂O↓

(3)

As demonstrated in Figure 3, the reaction exhibited negative ΔG^θ values with progressive thermodynamic favorability at elevated temperatures of 0–100 °C. This temperature-dependent spontaneity confirms the enhanced oxalate-mediated complexation, which effectively disrupts the Ca-Ti-O network in low-grade titanium-bearing blast furnace slag to liberate Ti⁴⁺ species. Consequently, the temperature-dependent leaching mechanism was prioritized in parametric optimization studies.

3.1.2. Effect of Leaching Temperature

Systematic leaching experiments were conducted under controlled hydrothermal conditions with a temperature range of 110–140 °C, a leaching time of 90 min, an oxalic acid concentration of 20 wt%, a liquid-to-solid ratio of 10:1, and an agitation rate of 500 rpm.

As demonstrated in Figure 4, the Ti(IV) extraction efficiency from low-grade titanium-bearing blast furnace slag exhibited monotonic enhancement from 87.1% to 91.2% with elevated temperatures, followed by a 2.6% efficiency loss beyond the critical threshold of 120 °C. Elevated temperature enhances the leaching reaction, but excessively high temperatures may induce dissociation of the complexes. The above inversion originates from the hydrothermal decomposition of metastable Ti(IV)-oxalate complexes into TiO₂·× H₂O colloidal precipitates [23]. Consequently, 120 °C was identified as the thermodynamically optimum temperature.

3.1.3. Effect of Oxalic Acid Concentration

The effect of oxalic acid concentration on the extraction efficiency of titanium in the blast furnace is depicted in Figure 5. The concentration of oxalic acid changed from 15 to 30 wt%, while maintaining a leaching temperature of 120 °C, a leaching time of 90 min, a liquid–solid mass ratio of 8:1, and an agitation rate of 500 rpm.

Elevated oxalic acid concentrations enhanced titanium extraction efficiency in low-grade titanium-bearing blast furnace slag, reaching an optimal value of 92.3% at 25 wt%. However, this process triggered an exponential elevation in autogenous pressure in the reactor, which correlates with CO₂ generation from oxalate decomposition. Thermodynamic equilibrium analysis revealed that 25 wt% oxalic acid represents the critical threshold balancing metal recovery and equipment operational sustainability.

3.2. Mineral Phase Evolution in Leaching Residues

To investigate the phase transformation mechanisms of low-grade titanium-bearing blast furnace slag during hydrometallurgical treatment, time-dependent XRD patterns of residues were systematically analyzed under fixed conditions.

As revealed in Figure 6, the amorphous halo of low-grade titanium-bearing blast furnace slag dissipated within 5 min of leaching, concomitant with the emergence of crystalline CaC₂O₄·H₂O diffraction peaks. This phase evolution confirms the rapid ligand-exchange reaction between oxalic acid and Ca²⁺ from low-grade titanium-bearing blast furnace slag. The preferential calcium oxalate crystallization creates Ti(IV)-rich domains, thereby enhancing Ti(OH)₂(C₂O₄)₂²⁻ complexation.

Table 2. Chemical composition of leaching residues.

Chemical Composition	CaO	SiO2	Fe2O3	Al2O3	TiO2	MgO	Na2O	P2O5
Mass fraction (wt%)	69.81	26.92	0.20	1.29	0.31	0.94	0.15	0.01

indicates that the leaching residues predominantly contain 69.81% CaO and 26.92% SiO₂. As shown in Figure 7, FTIR analysis reveals characteristic absorption bands at 3483–3340 cm⁻¹ corresponding to O-H vibrations of Si-OH groups, confirming the hygroscopic nature of amorphous SiO₂. The characteristic doublet at 1616 cm⁻¹ (ν_as COO⁻) and 1320 cm⁻¹ (ν_s COO⁻) verifies the presence of calcium oxalate, while peaks at 771 cm⁻¹ (O-C=O out-of-plane bending) and 662 cm⁻¹ (O-C=O in-plane bending) validate its crystalline structure. The 519 cm⁻¹ vibration originates from Si-O bending modes. Additional peaks at 1093 cm⁻¹ (Si-O-Si asymmetric stretching) and 960 cm⁻¹ (Si-OH stretching) further demonstrate the amorphous SiO₂ framework. These results further demonstrate that the leaching residues are composed of calcium oxalate and amorphous silica, suggesting that the silicon transformation pathway during leaching follows Equation (4) [24,25,26].

≡Si-O-Si≡ + H₂C₂O₄ → ≡Si-OH···HO-Si≡ → amorphous SiO₂·nH₂O

(4)

As evidenced in Figure 8, the leached residue exhibited a homogeneous particulate morphology compared to the heterogeneous microstructure of raw blast furnace slag. Furthermore, SEM-EDS quantitative analysis confirmed the presence of trace titanium in the leached residues. As shown in Figure 8a,b, the localized concentrations in Areas A1 and A2 were 1.28 wt% and 1.06 wt%, respectively. The results further demonstrated that titanium in low-grade titanium-bearing blast furnace slag was efficiently leached via hydrothermal oxalic acid treatment.

3.3. Chemical Speciation of Key Elements

As shown in ionic concentrations in

Table 3. Composition analysis of leachate.

Concentration	Ti	Ca	Mg	Al	Mn	Fe	C2O42−
g/L	3.73	0.46	4.63	9.17	0.11	0.34	130.7

, the leachate was quantitatively determined by ICP-OES and UV-Vis, coupled with thermodynamic speciation modeling using Visual MINTEQ, which elucidates dominant complexes and their distribution ratios across pH gradients.

Given the scarcity of reliable thermodynamic parameters for titanium-oxalate complexes, the aqueous speciation of Ti⁴⁺ was systematically investigated through Visual MINTEQ modeling. Figure 9 delineates the pH-dependent speciation profiles, revealing Ti(OH)₄ and Ti(OH)³⁺ as predominant species. Under leachate pH conditions of 0–1.0, Ti(OH)₄ constitutes over 97% of dissolved titanium. Previous studies confirm that titanium ions undergo conversion to [Ti(OH)₂(C₂O₄)₂]²⁻ complexes in oxalic acid solutions, with the predominant reaction pathway expressed as follows [27,28]:

Ti(OH)₄ + HC₂O₄⁻ → Ti(OH)₃C₂O₄⁻ + H₂O

(5)

Ti(OH)₃C₂O₄⁻ + H⁺ → Ti(OH)₂C₂O₄ + H₂O

(6)

Ti(OH)₂C₂O₄ + C₂O₄²⁻ →Ti(OH)₂(C₂O₄)₂²⁻

(7)

The chemical speciation of dominant impurity ions in the leachate was simulated using Visual MINTEQ, where input parameters were rigorously calibrated against ICP-OES analytical data. pH-dependent speciation profiles quantifying ion-partitioning behavior are systematically presented in Figure 10.

The occurrence state of impurity ions in the leachate is shown in Figure 10a–e. In the leachate, Fe, Ca, Mg, Mn, and Al species exist in the forms of approximately 98% Fe²⁺, 99% Ca²⁺, 99% Mg²⁺, 98% Mn²⁺, 42% Al(C₂O4)₂⁻, 27% AlHC₂O₄²⁺, and 25% AlC₂O₄⁺, respectively. These quantitative speciation patterns provide guidance for developing selective titanium separation methods in downstream processes.

3.4. Hydrothermal Recovery of Titanium from Titanium-Rich Leachate

Prior research demonstrates that titanium-oxalate complexes can be effectively transformed into titanium oxides via hydrothermal crystallization processes [23]. In the present study, targeted recovery experiments were conducted under reaction temperatures of 140–170 °C, a time of 90 min, and an agitation rate of 500 rpm.

Figure 11 delineates the temperature-dependent titanium precipitation behavior in the leachate. The hydrothermal crystallization efficiency exhibited strong thermal sensitivity, escalating from 58.3% to 99.4% as the temperature increased from 140 °C to 170 °C. Notably, near-complete titanium recovery was achieved at 170 °C within 90 min, demonstrating the viability of this thermally driven phase-separation strategy for selective titanium extraction.

Figure 12 presents the XRD characterization of hydrothermal decomposition products, dominated by quasi-spherical mixed titanium oxides (TiO₂, Ti₃O₅, Ti₇O₁₃) with potential amorphous phases. As shown in Figure 13, complementary SEM-EDS analysis further verified the presence of colloidal amorphous silica adhering to TiO₂ surfaces, which was undetected by XRD. The phase transformation process of titanium ions in the leachate can be approximately described as

[Ti(OH)₂(C₂O₄)₂]²⁻ → Ti(OH)₄·nH₂O → Ti-O-Ti polycondensation → Crystalline oxides

(8)

To verify the chemical composition of the amorphous substances, XRF analysis of the hydrothermal decomposition products was conducted. As shown in

Table 4. Chemical composition of decomposition products.

Chemical Composition	TiO2	SiO2	Fe2O3	Al2O3	K2O	CaO	V2O5	P2O5
Mass fraction (wt%)	52.44	44.03	0.84	0.71	0.36	0.23	0.18	0.10

, the results indicated that the products primarily contained 52.44% TiO₂ and 44.03 wt% SiO₂. Based on the characteristic that amorphous silica is easily soluble in sodium hydroxide solution, under reaction conditions of 8% sodium hydroxide solution, a washing time of 45 min, and a stirring speed of 500 rpm, the grade of TiO₂ was enhanced to 90.5%. Notably, the titanium oxides demonstrate significant industrial relevance as key precursors for commercial titanium white pigments.

4. Conclusions

This study proposes an innovative process for the selective extraction and separation of titanium from low-grade titanium-bearing blast furnace slag, and demonstrates a streamlined process flow, high extraction efficiency, significant energy conservation, and eco-friendly operational characteristics. The final process flowsheet for titanium recovery from low-titanium blast furnace slag is shown in Figure 14.

(1)

The hydrothermal leaching method enables efficient titanium extraction from low-grade titanium-bearing blast furnace slag. Under a hydrothermal temperature of 120 °C, a time of 90 min, an oxalic acid concentration of 25 wt%, and an agitation rate of 500 rpm, the leaching process achieved an optimal titanium extraction efficiency of 92.3%, with titanium predominantly existing as the [Ti(OH)₂(C₂O₄)₂]²⁻ complex species.

(2)

The resulting leaching residue was primarily composed of calcium oxalate and amorphous silica, indicating an effective separation of titanium from the Ca-Si-Al-Mg matrix. The Ti, Fe, Ca, Mg, Mn, and Al species in the leachate mainly existed in the forms of Ti(OH)₂(C₂O₄)₂²⁻, Fe²⁺, Mg²⁺, Mn²⁺, Ca²⁺, Al(C₂O₄)₂⁻, AlHC₂O₄²⁺, and AlC₂O₄⁺, respectively.

(3)

The hydrothermal decomposition method enables complete titanium separation from leachates through thermally driven crystallization. Under a hydrothermal temperature of 170 °C for 90 min, the [Ti(OH)₂(C₂O₄)₂]²⁻ complex underwent decomposition, achieving 99.4% titanium precipitation as mixed-phase composites of TiO₂, Ti₃O₅, and Ti₇O₁₃. Finally, the grade of TiO₂ in the product was enhanced to 90.5% through alkaline washing.