Kinetics and mechanism of ultrasound-enhanced citric acid leaching of chromium from steel slag

Abstract

Steel slag is a promising eco-material, yet its utilization remains constrained by chromium (Cr) levels exceeding regulatory limits in many countries. This study investigated the selective leaching of Cr from steel slag using an ultrasound-enhanced organic acid system. Among six carboxylic acids screened, including several bio-based options, citric acid exhibited the highest Cr extraction (63.3%) and Cr/Ca selectivity (2.86), attributed to its multidentate chelation capability. Under optimized ultrasonic conditions (0.8 mol L⁻¹ citric acid, 240 W, 30 °C, 10 min), Cr extraction reached 84.3%, reducing residual Cr from approximately 2189 mg kg⁻¹ to 281 mg kg⁻¹. Kinetic analysis indicated that conventional leaching followed a chemical reaction-controlled shrinking core model (E_a = 48.10 kJ mol⁻¹), whereas ultrasonic leaching exhibited mixed-control behavior with a markedly lower apparent activation energy (E_a = 18.02 kJ mol⁻¹), suggesting that cavitation reduced interfacial mass transfer resistance. Multi-scale residue characterization supported that ultrasound enhanced Cr accessibility through particle refinement, pore development, and disruption of the Ca–Si interlocking layer. A stability window was identified: prolonged reaction or elevated temperature promoted Si(OH)₄ accumulation and probable gel formation, impairing solid–liquid separation. Confining the process within this window allowed the primary Cr extraction to proceed before gel-dominated effects prevailed. Leaching toxicity tests (GB 5086.1–1997) suggested that treated residues met environmental safety standards. This work clarifies the apparent enhancement mechanism of ultrasound-assisted citric acid leaching and provides a feasible basis for the green utilization of Cr-bearing metallurgical slags.

1. Introduction

Steel slag is one of the most abundant solid by-products generated during iron and steelmaking, with an annual output exceeding 120 million tons in China alone [1], [2]. Owing to its complex mineral composition, high porosity, and enrichment in reactive elements such as Ca, Mg, Fe, and Si, steel slag exhibits favorable alkalinity regulation and slow-release characteristics [3], [4], making it a promising candidate for ecological applications. However, the presence of chromium (Cr) in certain steel slags remains a major obstacle, limiting their large-scale utilization. Although Cr is predominantly immobilized in the trivalent form (Cr³⁺), it may gradually oxidize into highly mobile and toxic hexavalent chromium (Cr(VI)) under weathering, alkaline oxidation, or microbial activity [5], [6], [7]. Under long-term leaching conditions, Cr(VI) can migrate into surrounding environments. As a result, increasingly stringent regulations in China and the European Union on total Cr and Cr(VI) contents restrict the direct use of untreated steel slag in agricultural and ecological engineering applications [8], [9], [10].

From a mineralogical perspective, steel slag is a typical multiphase metallurgical residue characterized by pronounced multiphase intergrowth [11], [12]. Chromium is preferentially enriched in highly stable spinel phases, such as FeCr₂O₄ and (Fe,Mg)(Cr,Al)₂O₄, which form complex composite interfaces with Ca–Si phases that severely limit Cr exposure and migration under conventional leaching conditions [13], [14], [15]. Previous studies have reported that the apparent activation energy for FeCr₂O₄ dissolution in CaO–MgO–Al₂O₃–SiO₂ slag systems can reach as high as 266 kJ·mol⁻¹, reflecting the exceptional stability of Fe–O and Cr–O bonds within the spinel structure [16], [17], [18]. These mineralogical and structural features fundamentally account for the poor leachability of Cr under mild conditions. In addition, during acidic leaching, dissolution of the Ca–Si matrix releases Si(OH)₄, and under poorly controlled conditions, subsequent silica polymerization may lead to gel formation, which can further impede solid–liquid separation and reduce leaching efficiency. However, the impact of this phenomenon in steel slag dechromization systems has received limited attention [19], [20], [21].

Currently, chromium removal from steel slag is mainly achieved through oxidative roasting followed by water leaching or by direct leaching using strong inorganic acids. Oxidative roasting converts Cr³⁺ into more soluble Cr(VI), resulting in high extraction efficiency but at the cost of substantial energy consumption, complex processing, and potential secondary pollution [22], [23]. Direct leaching with strong acids, such as sulfuric or hydrochloric acid, is operationally simple but suffers from poor selectivity, often leading to extensive dissolution of Ca and Si, which compromises the subsequent ecological utilization of steel slag [24], [25], [26]. These limitations have stimulated growing interest in alternative leaching strategies that can balance extraction efficiency, selectivity, and environmental compatibility.

In recent years, carboxylic acids have attracted increasing interest for selective leaching in multimetal systems due to their chelating capability and biodegradability [27], [28], [29]. Citric acid, as a multicarboxylate ligand, forms stable complexes with Cr³⁺ [30], [31], while other organic acids such as oxalic, itaconic, and crotonic acids have shown potential in the selective extraction of various metals [32]. Wang et al. recently employed crotonic acid for ultrasonic leaching of Zn from blast-furnace dust, achieving 92.79% extraction [33]. Meanwhile, ultrasonic enhancement has been widely applied in hydrometallurgical processes [34]. Acoustic cavitation induces localized high temperature and pressure, microjets, and interfacial disturbance, which can disrupt mineral surface layers, promote diffusion, and lower kinetic barriers [35], [36]. Despite these advances, the synergistic mechanism of ultrasound and carboxylic acids in attacking highly stable Cr-bearing spinel phases, particularly whether they can penetrate the Ca–Si matrix to act on FeCr₂O₄, has not yet been clearly established.

To address these kinetic limitations, ultrasound-assisted leaching offers a potential means to alleviate these structural constraints, particularly in systems where leaching is governed by interfacial accessibility rather than intrinsic chemical reactivity [37], [38]. Nevertheless, despite extensive studies on ultrasound-enhanced leaching of metallurgical wastes [36], [39], [40], the extent to which ultrasound can penetrate Ca–Si interlocking and influence the rate-controlling step during Cr extraction from steel slag remains insufficiently clarified. Clarifying this issue is essential for developing a mechanistically consistent interpretation of ultrasound-assisted weak-acid leaching in spinel-bearing slag systems.

Despite increasing interest in ultrasound-assisted leaching, two issues remain insufficiently clarified in the weak-acid extraction of chromium from steel slag: whether ultrasound changes the apparent kinetic regime rather than merely accelerating the same leaching pathway, and whether intensified dissolution of the Ca–Si matrix introduces an operability limit through silica gelation. The present study addresses both questions by combining kinetic analysis with mineralogical and microstructural characterization. In particular, it demonstrates an ultrasound-induced apparent transition from chemical reaction control to mixed control and identifies a gelation-constrained operating window that defines the boundary between efficient chromium extraction and process instability.

Based on these considerations, this study develops a carboxylic acid–ultrasound synergistic system for selective chromium removal from steel slag. Six carboxylic acids were screened for their leaching performance toward Cr-bearing spinel phases, using Cr extraction efficiency and Cr/Ca selectivity as the primary evaluation criteria. The optimal leaching system was further investigated through kinetic analysis to compare the controlling steps of conventional leaching (RL) and ultrasound-assisted leaching (UL). Particle size distribution, pore structure evolution, and interfacial structural characterization were integrated to elucidate the mechanisms underlying the coupled effects of chelation and cavitation. In addition, the environmental safety of the leaching residues was evaluated in accordance with GB 5086.1–1997 standards to assess the feasibility of the treated steel slag for ecological applications. An overall schematic of the experimental workflow adopted in this study is shown in Fig. 1.

Fig. 1.

Schematic illustration of the ultrasonic leaching procedure.

2. Materials and methods

2.1. Raw materials and reagents

Steel slag samples were obtained from a steel plant in Hebei Province, China. After air drying, the slag was crushed and sieved to below 0.25 mm, then dried at 105 °C for 6 h and ground to pass a 200-mesh sieve. The samples were thoroughly homogenized and stored in sealed containers. No additional chemical pretreatment was applied apart from particle size standardization.

The bulk chemical composition of the steel slag was determined by X-ray fluorescence spectroscopy (XRF). Mineralogical phase composition was analyzed by X-ray diffraction (XRD) over a 2θ range of 10°–80°. To characterize the occurrence of Cr-bearing phases, automated mineralogical identification was performed using a Mineral Liberation Analyzer (MLA) [11], [41], coupled with energy-dispersive X-ray spectroscopy (EDS) to analyze the enrichment features of Cr-bearing spinel particles [42].

Citric acid, tartaric acid, crotonic acid, itaconic acid, acetic acid, and oxalic acid were of analytical grade (Sinopharm). Deionized water (resistivity >18 MΩ·cm) was used throughout the experiments.

2.2. Acid system screening experiments

To identify the optimal acid system for Cr removal from steel slag, six carboxylic acids: citric, tartaric, crotonic, itaconic, acetic, and oxalic, were screened at a uniform concentration of 1.0 mol L⁻¹ [43], [44]. Screening experiments were conducted under conventional magnetic stirring without ultrasonic irradiation, ensuring that differences among acid systems primarily reflected ligand properties. Experimental conditions: 5.0 g steel slag, S/L ratio 1:10 (g mL⁻¹), temperature 25 ± 2 °C, stirring speed 400 r/min, reaction time 60 min.

After reaction, the slurry was vacuum filtered to separate the solid and liquid phases. The filtrate was diluted and analyzed for Cr and Ca concentrations by inductively coupled plasma optical emission spectroscopy (ICP-OES). Cr extraction efficiency served as the primary evaluation criterion, while the Cr/Ca selectivity ratio (defined as the ratio of Cr extraction to Ca extraction) was calculated to assess the selective Cr removal capability of each acid system [45], [46]. The residue was washed with deionized water, dried at 105 °C, and used for phase and structural analysis.

2.3. Process parameter optimization experiments

Based on the acid screening results, the acid system exhibiting overall superior performance was selected as the primary leaching agent for subsequent ultrasound-assisted experiments. The effects of acid concentration, reaction temperature, solid-to-liquid ratio, and reaction time on chromium removal behavior were investigated through single-factor experiments.

For each experiment, 5.0 g of steel slag was placed into a 100 mL beaker, and a predetermined volume of the selected acid solution was added. The reaction was carried out under stirring in a thermostatic water bath. Upon reaching the preset reaction time, the slurry was immediately filtered, and the filtrate was collected for ICP analysis. The solid residues were used for subsequent phase and morphological characterization. The reaction temperature was generally controlled within the range of 25–60 °C.

Ultrasound-assisted experiments were performed using a probe-type ultrasonic generator (FS-600 N, Shanghai Shengxi) operating at a fixed frequency of 20 kHz and an adjustable power range of 100–600 W; the actual power used in this study was set between 100 and 400 W [36]. The ultrasonic probe (titanium alloy tip, diameter 16 mm) was vertically immersed to a depth of approximately 1.5 cm below the liquid surface, and ultrasound was applied in continuous mode throughout the reaction period [33], [34]. Except for replacing mechanical stirring with ultrasonic irradiation, all other experimental conditions were identical to those of conventional leaching. After reaction, the mixture was rapidly cooled and vacuum filtered.

The extraction efficiency was calculated according to Eq. (1):

$$\eta =\frac{C\times V}{m\times \omega }\times 100\%$$ (1)

where C is the element concentration in the filtrate (mg·L⁻¹), V is the filtrate volume (L), m is the slag mass (g), and ω is the mass fraction of the element in the slag (%). All experiments were performed in triplicate, and average values are reported.

2.4. Characterization methods

Phase composition and morphological changes of the samples before and after leaching under different conditions were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Particle size distribution was measured using a laser particle size analyser [47]. In addition, the Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution were obtained by nitrogen adsorption–desorption analysis [48], [49].

2.5. Leaching kinetics experiments

To investigate the effect of ultrasound on the rate-controlling step of Cr leaching, kinetic experiments were conducted under optimized conditions, comparing conventional leaching (RL) and ultrasound-assisted leaching (UL) modes. The Cr concentration in the filtrate was measured immediately after rapid vacuum filtration, and extraction efficiency was calculated.

The shrinking core model (SCM) was employed to analyze the apparent controlling steps of the leaching process [50], [51], including external diffusion control (Eq. (2)), chemical reaction control (Eq. (3)), and internal diffusion (mixed) control (Eq. (4)):

$$x=k_1·t$$ (2)

$$1-{(1-x)}^{1/3}=k_2·t$$ (3)

$$\frac{1}{3}\ln (1-x)+{(1-x)}^{-1/3}-1=k_3·t$$ (4)

where x is the fractional chromium extraction, t is the reaction time (min), and k₁, k₂, and k₃ are the apparent rate constants for external diffusion control, chemical reaction control, and internal diffusion (mixed) control, respectively. After identifying the best-fitting kinetic model, the apparent activation energy was calculated using the Arrhenius equation based on the corresponding apparent rate constants obtained at different temperatures (Eq. (5)) [52], [53]:

$$\ln (k)=\ln (A)-\frac{E_a}{R·T}$$ (5)

where k is the apparent rate constant, A is the pre-exponential factor, E_a is the apparent activation energy (kJ mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (K).

3. Results and discussion

3.1. Characterization of raw materials

The bulk chemical composition of the steel slag was determined by X-ray fluorescence spectroscopy (XRF) (Table 1). The slag was dominated by CaO, Fe₂O₃, and SiO₂, with minor amounts of Al₂O₃, MgO, TiO₂, and MnO; other impurities such as P₂O₅, Na₂O, and K₂O were below 1%. The combined mass fraction of CaO and MgO exceeded 40%, indicating high acid reactivity and potential suitability for ecological applications. The Cr₂O₃ content was 0.32%, corresponding to a total Cr concentration of approximately 2189 mg kg⁻¹, which exceeds the risk control value for agricultural land (1300 mg kg⁻¹ at pH > 7.5) specified in GB 15618–2018, potentially limiting its resource utilization.

Table 1.

Chemical composition of the steel slag.

Oxide	CaO	Fe2O3	SiO2	Al2O3	MgO	MnO	Cr2O3	V2O5
Content (wt%)	42.75	24.18	16.49	6.71	4.06	1.83	0.32	0.34

XRD analysis (Fig. 2) revealed that the slag mainly consisted of β-Ca₂SiO₄ (larnite) and Ca₂Fe₂O₅ (brownmillerite), accompanied by minor amounts of Ca₁₂Al₁₄O₃₃ (mayenite), magnetite, and RO solid solution, reflecting the typical Ca–Si–Fe multiphase structure of steel slag. However, due to the low content and fine particle size of spinel phases, XRD was insufficient to reveal their textural relationships and degree of exposure.

Fig. 2.

XRD pattern of the steel slag.

To comprehensively characterize the occurrence of chromium in the steel slag, automated mineralogical analysis was performed using a Mineral Liberation Analyzer (MLA) [54]. Fig. 3a shows the backscattered electron (BSE) overview image of the slag, revealing particles with distinct brightness contrast. Fig. 3b presents the MLA-based mineral phase identification results. Quantitative analysis (Fig. 3c) indicated that the slag was mainly composed of β-Ca₂SiO₄ (larnite, 42.83%), Ca₂Fe₂O₅ (brownmillerite, 15.84%), magnetite/chromite (13.12%), Calcium aluminate (Ca₁₂Al₁₄O₃₃, 9.97%), and RO phase (4.50%).

Fig. 3.

MLA-based mineralogical characterization of the steel slag.

Elemental deportment analysis (Table 2) showed that 97.02% of Cr was hosted in Fe₃O₄–FeCr₂O₄ spinel solid solution, while virtually no quantifiable Cr was detected in β-Ca₂SiO₄, Ca₂Fe₂O₅, or RO phases (total < 3%). This showed that Cr in the slag does not exist as readily leachable Ca–Cr or Fe–Cr salts but is firmly locked within structurally dense and chemically stable spinel phases [55], [56].

Table 2.

Elemental distribution among major mineral phases determined by MLA.

Mineral Phase	Al (%)	Cr (%)	Fe (%)	Mg (%)	Mn (%)	Si (%)	Ti (%)	V (%)
RO phase	6.49	1.08	12.51	9.45	2.26	2.26	81.40	—
Magnetite / Chromite	4.86	97.02	39.01	48.57	62.07	0.40	—	—
β-Ca2SiO4	17.53	—	12.89	9.33	4.96	86.03	4.31	—
Ca2Fe2O5	7.17	—	22.12	10.62	16.35	2.79	—	—
Calcium aluminate	45.54	—	1.37	0.46	—	0.20	3.86	50.28
Anorthite	9.12	—	0.03	0.06	—	3.25	—	—
Diopside	6.14	—	0.19	8.92	—	3.30	8.46	—

High-magnification BSE imaging (Fig. 3d) further revealed significant intergrowth features among multiple phases. Cr-bearing magnetite/chromite particles, mostly 5–30 μm in size, were dispersed throughout the slag and exhibited complex interpenetrating contacts with β-Ca₂SiO₄ and Ca₂Fe₂O₅ phases, forming irregular and discontinuous interfaces. This resulted in a stable interlocking texture, with only a few particles exhibiting exposed surfaces.

Liberation degree analysis (Fig. 3e) quantified this observation: only 28.95% of Cr-bearing magnetite/chromite particles were highly liberated (exposed surface > 80%), while over 71.05% existed as binary or multiphase intergrowths with the Ca–Si matrix. Among these, 21.09% were binary intergrowths with β-Ca₂SiO₄, and 49.96% were more complex ternary or multiphase associations. In comparison, the RO phase exhibited a similar liberation behavior, with only 25.54% in the liberated state. This showed that Cr-bearing spinels are highly enclosed at the microstructural level, which is the key reason for the limited Cr mobility during subsequent leaching.

Furthermore, MLA particle size analysis showed that the D₈₀ and D₅₀ of the sample were 64.61 μm and 30.84 μm, respectively, indicating that the overall particle size already met the interfacial requirements for leaching experiments. Therefore, the primary limiting factor for Cr extraction is not particle size but rather the chemical stability and high degree of interlocking of the spinel phases. This mineralogical constraint provides a clear theoretical basis for the subsequent application of intensified leaching strategies, such as ultrasound–organic acid synergistic systems, to disrupt the interlocking structure and enhance Cr extraction.

SEM-EDS elemental mapping (Fig. 4) showed that the Ca–Si matrix and Fe–Cr-enriched phases exhibited a mutually interlocking composite particle structure rather than simple unidirectional interlocking. Ca and Si formed a continuous framework, with Fe-rich bright spots visible at corresponding positions, while Cr appeared only in some of the Fe-enriched regions. The circled areas in the figure clearly show that Cr hotspots almost completely overlapped with Fe hotspots, corresponding only to localized positions within the Ca–Si framework, with virtually no Cr signal detected in pure Ca–Si regions [57], [58]. This interlocking structure implies that Cr-bearing spinels are mainly embedded as fine particles within the Ca–Si framework with limited exposed interfaces, which is the key microstructural reason for the poor Cr leachability.

Fig. 4.

SEM-EDS elemental mapping of the steel slag.

To further elucidate the elemental distribution within particles, SEM-EDS line scan analysis was performed on a representative large particle (Fig. 5). Fig. 5a shows the BSE morphology of the large particle, with the yellow arrow indicating the line scan path; Fig. 5b presents the EDS spectrum of the area, showing Ca, Si, and O as the dominant elements, while Cr and Fe signals were discernible but relatively weak; Fig. 5c displays the elemental mapping results, visually illustrating the spatial distribution of Ca (cyan), Si (green), and Fe (red). The line scan profiles (Fig. 5d) revealed that along the particle cross-section, Ca and Si signals exhibited synchronized fluctuations, reflecting the continuous Ca–Si framework structure. Notably, Fe peaks appeared precisely in the valleys of Ca–Si signals, while the Cr signal, although weak, overlapped closely with Fe peaks. This feature further confirms that Cr-bearing spinels are embedded as fine particles within the Ca–Si matrix, exhibiting a complementary spatial distribution rather than a simple core–shell interlocking structure.

Fig. 5.

SEM-EDS of a representative large particle in the steel slag.

In summary, Cr in this steel slag is predominantly hosted as fine-grained spinel phases embedded within the Ca–Si matrix, providing the structural basis for the ultrasound-assisted leaching strategy developed below.

3.2. Screening of organic acid systems

To identify the most suitable organic acid system for selective Cr leaching from steel slag, six representative carboxylic acids (oxalic, acetic, tartaric, crotonic, citric, and itaconic acid) were screened under non-ultrasonic conditions. All experiments were conducted under identical conditions: acid concentration 1.0 mol L⁻¹, S/L ratio 1:10 (g mL⁻¹), temperature 25 °C, and reaction time 60 min, to obtain the intrinsic leaching characteristics of each acid system.

As shown in Fig. 6, the Cr leaching efficiencies varied significantly among the different organic acids. Citric acid exhibited the highest Cr extraction (63.3%), followed by itaconic acid (32.5%), tartaric acid (31.4%), and oxalic acid (30.2%). In contrast, crotonic acid and acetic acid showed relatively poor Cr leaching performance, with extraction efficiencies of only 15.1% and 1.6%, respectively [59].

Fig. 6.

Leaching efficiencies of Cr, Ca, and Fe from steel slag using different organic acids.

The superior performance of citric acid can be attributed to its unique molecular structure. As a tricarboxylic acid, citric acid possesses three carboxyl groups (–COOH) and one α-hydroxyl group (–OH), providing multidentate coordination capability to form stable chelates with Cr³⁺, thereby enhancing leaching efficiency. Furthermore, the triprotic dissociation characteristics of citric acid enable it to maintain strong proton-donating capacity and complexation activity over a wide pH range.

To further evaluate the selectivity of each acid system, the Cr/Ca leaching ratio (defined as the ratio of Cr extraction to Ca extraction) was calculated, which reflects the preferential extraction capability toward the target element Cr. Citric acid exhibited the highest Cr/Ca selectivity ratio (approximately 2.86), indicating its ability to preferentially extract Cr while retaining most of the Ca in the solid phase [27]. This characteristic is particularly important for the subsequent utilization of treated steel slag as ecological materials, as the retained Ca helps maintain the alkalinity regulation function and nutrient supply capacity of the slag. Citric acid was therefore selected as the optimal leaching agent for subsequent experiments [60].

3.3. Optimization of leaching process parameters

3.3.1. Effect of ultrasonic power

The effect of ultrasonic power on steel slag leaching behavior was first investigated under fixed conditions of citric acid concentration 0.8 mol L⁻¹, S/L ratio 1:20, and temperature 25 °C (Fig. 7). The results showed that at 20% power, the Cr extraction was approximately 40.2%, while the Ca extraction was only about 7%. As the power increased to 40%, the Cr extraction efficiency improved markedly (from 40.2% to 80.4%).

Fig. 7.

Effect of ultrasonic power on Cr and Ca leaching efficiency.

According to the MLA analysis in Section 3.1, over 71% of Cr-bearing spinels are tightly interlocked with the Ca–Si matrix, representing a typical refractory interlocking structure. The cavitation effect induced by ultrasound, particularly the collapse of transient cavitation bubbles and microjet impingement, can disrupt the interlocking interfaces, roughen particle surfaces, and reduce the diffusion boundary layer thickness, thereby overcoming the kinetic limitations imposed by this structure. Consequently, the Cr extraction increased rapidly with increasing power. In contrast, although Ca–silicate phases exhibit relatively high acid solubility, the dissolved Ca²⁺ rapidly forms sparingly soluble calcium citrate precipitates with citrate ions in the citric acid system, limiting the free Ca²⁺ concentration in solution. Since ultrasound primarily acts on disrupting the spinel interlocking structure with limited effect on the Ca precipitation–dissolution equilibrium, the apparent Ca extraction showed only a slight increase with power.

When the ultrasonic power was further increased to 50%–60%, the Cr extraction did not continue to improve but entered a plateau region. This is mainly attributed to excessive aggregation of cavitation bubbles at high power, where the dense bubble clusters generate acoustic shielding effects, preventing effective transmission of acoustic energy to the bulk reaction liquid and leading to saturation of the enhancement effect. Therefore, 40% was identified as the optimal working power for this system, achieving significant improvement in extraction efficiency while avoiding ineffective energy consumption [19].

3.3.2. Effect of citric acid concentration

The effect of citric acid concentration (0.4–1.0 mol L⁻¹) on Cr leaching behavior was investigated while keeping other conditions constant (Fig. 8). The results showed that Cr extraction increased significantly with increasing citric acid concentration, with the most pronounced improvement occurring in the 0.4–0.8 mol L⁻¹ range. At a concentration of 0.8 mol L⁻¹, the Cr extraction approached 80%.

Fig. 8.

Effect of citric acid concentration on Cr and Ca leaching efficiency.

When the citric acid concentration was further increased above 0.8 mol L⁻¹, no further significant improvement in Cr extraction was observed. Instead, the system gradually exhibited increased slurry viscosity and filtration resistance, indicating deterioration of solid–liquid separation performance. This result suggests that excessively high acid concentration may adversely affect the stability of the leaching system. Considering both Cr extraction efficiency and system stability, 0.8 mol L⁻¹ was identified as the optimal operating concentration for this system.

3.3.3. Effect of reaction temperature

The effect of reaction temperature (30–60 °C) on Cr leaching behavior was investigated under conditions of 40% ultrasonic power, 0.8 mol L⁻¹ citric acid, S/L ratio 1:20, and reaction time 10 min (Fig. 9). The results showed that temperature had a strong effect on Cr extraction efficiency. When the temperature increased from 20 °C to 30 °C, the Cr extraction improved from approximately 80.4% to 84.3%, reaching the highest value in this study.

Fig. 9.

Effect of reaction temperature on Cr and Ca leaching efficiency.

However, when the temperature was further increased to 40 °C, 50 °C, and 60 °C, the Cr extraction decreased rather than increased, accompanied by noticeably increased slurry viscosity and significantly reduced filtration rate, with the system exhibiting a pronounced gelation tendency. This behavior can be attributed to accelerated dissolution of the Ca–Si matrix at higher temperatures, which causes a rapid increase in the concentration of Si(OH)4 in solution. Once the concentration exceeds a critical supersaturation level, silica polycondensation and gel-like phase formation may occur [19], [20]. The resulting gel can encapsulate unreacted Cr-bearing particles and hinder contact between the leaching agent and mineral surfaces. It may also retain dissolved Cr species, leading to a lower apparent extraction.

Therefore, 30 °C was identified as the optimal reaction temperature for this system, at which both high Cr extraction efficiency and avoidance of adverse effects from silica gelation on system stability can be achieved [61].

3.3.4. Effect of reaction time

Reaction time experiments further revealed the window characteristics of this system (Fig. 10). Under optimal power (40%), concentration (0.8 mol L⁻¹), and temperature (30 °C) conditions, the leaching process showed an extremely rapid increase in the initial minutes: the Cr extraction had already reached a relatively high level at 5 min and approached the peak value (84.3%) at 10 min.

Fig. 10.

Effect of reaction time on Cr and Ca leaching efficiency.

However, when the reaction time exceeded 10 min, the system exhibited obvious deterioration: the Cr extraction not only failed to continue increasing but actually decreased, and the filter residue mass increased significantly (from approximately 6.8 g to 8.6–9.0 g). Observation of the slurry revealed increased viscosity with a gel-like appearance, indicating that the system had entered the silica gelation stage.

Prolonged acid leaching caused continuous dissolution of the Ca–Si matrix, releasing large amounts of Si(OH)₄ into solution. When its concentration exceeded the critical supersaturation level, dehydration polycondensation occurred under acidic conditions (Eq. (6):

$$\equiv \text{Si - OH}+\text{HO - Si}\equiv \to \equiv \text{Si - O - Si}\equiv +{\text{H}}_2\text{O}$$ (6)

This was inferred to gradually form a three-dimensional silica gel network, which may retain dissolved Cr species and contribute to difficulties in solid–liquid separation [19]. Although ultrasound can significantly accelerate the reaction, the accelerated dissolution it brings may also cause silica to reach polycondensation conditions in a shorter time, pushing the system across the boundary between stability and instability. Therefore, the optimal reaction time is determined not by kinetics but by system stability, with 10 min being the optimal reaction time for this system.

3.3.5. Determination of the operating window

Based on the above single-factor experimental results, the optimal operating conditions determined in this study are: ultrasonic power 40% (240 W), citric acid concentration 0.8 mol L⁻¹, S/L ratio 1:20, reaction temperature 30 °C, and reaction time 10 min. Under these conditions, the Cr extraction reached 84.3%, and the system exhibited good operational stability [62].

To verify that the observed leaching enhancement requires the chemical action of citric acid rather than ultrasonic physical effects alone, a deionized-water ultrasonic control experiment was performed under the optimal ultrasonic conditions (240 W, 30 °C, 10 min, S/L = 1:20) without citric acid. Chromium extraction under water-only sonication was only 0.3 ± 0.1%, which was negligible compared with 84.3 ± 1.2% obtained in the citric acid system under the same ultrasonic conditions. This result confirms that ultrasound alone is insufficient for effective chromium extraction and that citric acid is essential for the high leaching efficiency observed in this study.

3.4. Leaching kinetics analysis

3.4.1. Kinetic analysis methodology

To elucidate the impact of ultrasound on the kinetic behavior of Cr leaching from steel slag, the kinetic datasets obtained under conventional leaching (RL) and ultrasound-assisted leaching (UL) were analyzed following the experimental procedure described in Section 2.5. As indicated in Section 3.1, chromium in the investigated steel slag is predominantly hosted by Cr-bearing spinel phases, occurring within Fe–Cr oxide assemblages (e.g., magnetite/chromite-related phases). The leaching of such phases represents a typical solid–liquid heterogeneous reaction, for which the apparent rate may be governed by interfacial chemical reaction, mass-transfer diffusion, and/or structural constraints caused by interlocking within the Ca–Si matrix.

The kinetic data for RL and UL at different temperatures were fitted using the shrinking core model (SCM). Three rate-controlling mechanisms were considered: diffusion control, interfacial chemical reaction control, and mixed control (diffusion + reaction), whose linearized forms are given in Eqs. (2), (3), (4) [62]. The dominant mechanism under each condition was identified by comparing the linear regression coefficient (R²) and the apparent rate constant extracted from each model. Temperature dependence of the apparent rate constants was further assessed using the Arrhenius relationship to provide an additional consistency check for the mechanism assignment.

3.4.2. Kinetic behavior under conventional leaching (RL)

RL kinetic tests were conducted at 20, 30, 40, and 50 °C under 0.8 mol L⁻¹ citric acid and S/L = 1:20, and the Cr extraction curves as a function of time are shown in Fig. 11a. The experimental data were subsequently linearized using the diffusion-control, chemical-reaction-control, and mixed-control SCM expressions (Fig. 11b–d).

Fig. 11.

Leaching kinetics and SCM fitting of Cr under conventional leaching (RL) conditions.

As evidenced by the fitting results, the chemical reaction control model provides consistently higher goodness-of-fit across all temperatures (R² = 0.9716–0.9991) compared with the diffusion-control model (R² = 0.9447–0.9932) and the mixed-control model (R² = 0.9234–0.9786). This showed that, within the investigated time scale and temperature range, Cr leaching under RL can be reasonably approximated as an apparent interfacial reaction-controlled process. This behavior is structurally consistent with the occurrence mode of Cr-bearing spinels in the slag: MLA results show that more than 71% of Cr-bearing spinels are intergrown and/or encapsulated by the Ca–Si matrix, leading to limited effective exposed interfaces. Consequently, leaching proceeds primarily at the few accessible reaction interfaces and exhibits pronounced temperature sensitivity characteristic of reaction-controlled kinetics.

To quantify the temperature dependence, the apparent rate constants derived from the chemical-reaction-control model were used to construct an Arrhenius plot (Fig. 11e). A good linear relationship between ln k and 1/T was obtained (R² = 0.9658), corresponding to an apparent activation energy of E_a(RL) = 48.10 kJ mol⁻¹ [63], [64].

The magnitude of E_a(RL) supports the conclusion that the RL process is predominantly governed by interfacial chemical reaction rather than purely diffusion-limited transport, consistent with the SCM fitting outcomes.

3.4.3. Kinetic behavior under ultrasound-assisted leaching (UL)

Compared with RL, the kinetic behavior under UL is influenced not only by intrinsic reaction and mass-transfer processes, but also by system structural stability. As discussed in Section 3.3, ultrasonic cavitation can markedly accelerate Cr leaching, while simultaneously promoting dissolution of the Ca–Si matrix and the release/accumulation of dissolved silica species (e.g., Si(OH)₄). Once temperature and/or leaching time exceeds a critical threshold, the system tends to deviate from a stable solid–liquid two-phase reaction state and may exhibit gelation, which compromises filtration and alters the apparent leaching response. In practice, noticeable gelation can emerge when the temperature exceeds 40 °C, becoming particularly severe at higher temperatures (e.g., 60 °C), implying that the SCM assumptions (constant particle geometry, stable reaction interface, and separable rate-controlling step) are no longer strictly satisfied outside the stable operating window.

Therefore, quantitative kinetic fitting for UL was restricted to the stable operating window, i.e., 20–30 °C (at 5 °C intervals) with reaction time ≤10 min, where the slurry maintained an operable two-phase state and Cr extraction versus time exhibited good reproducibility (Fig. 12a). The corresponding SCM linearizations are presented in Fig. 12b–d.

Fig. 12.

Leaching kinetics and SCM fitting of Cr under ultrasound-assisted leaching (UL) conditions.

Within this stability-limited interval, the mixed-control model yields the highest fitting quality (R² = 0.9928–0.9989), outperforming both diffusion-control (R² = 0.9589–0.9676) and chemical-reaction-control models (R² = 0.9118–0.9268). This showed a mechanistic shift induced by ultrasound: cavitation-driven microjets and shock effects weaken the interlocking constraint of the Ca–Si matrix, increasing interface accessibility and enhancing interfacial reaction kinetics [65]; meanwhile, as reactive surfaces become more exposed and the reaction proceeds rapidly, intraparticle diffusion and mass transfer increasingly contribute to the overall rate, leading to an apparent diffusion–reaction mixed-control regime rather than a single-step reaction-controlled process.

The rate constants obtained from the mixed-control model at 20, 25, and 30 °C were used to construct an Arrhenius plot (Fig. 12e). A good linear relationship was obtained (R² = 0.9992), corresponding to an apparent activation energy of E_a(UL) = 18.02 kJ mol⁻¹.

The substantially lower E_a(UL) relative to RL suggests that ultrasound enhancement primarily reduces the apparent energy barrier through improved interfacial accessibility and mass-transfer conditions, rather than altering the intrinsic bond-breaking energetics of the Cr–O framework. The high linearity of the Arrhenius plot (R² = 0.9992) supports the validity of this activation energy within the investigated temperature range.

It should be noted that the shrinking core model was used here as an apparent macroscopic framework for interpreting kinetic trends rather than as a strict geometric description of particle evolution under ultrasonic cavitation. Under sonication, cavitation-induced fragmentation, surface erosion, and crack formation may violate the ideal assumptions of uniform particle shrinkage and a stable reaction front. Nevertheless, the good fitting quality obtained within the stable operating window suggests that the model remains useful for identifying the transition from reaction control to mixed control [34], [66]. Accordingly, the activation energies derived in this work should be interpreted as apparent kinetic parameters reflecting the combined influence of intrinsic reaction, mass transfer, and structural evolution under ultrasonic conditions.

3.5. Characterization of leaching residues

3.5.1. XRD and XRF analysis

To investigate the changes in mineral composition of steel slag during citric acid leaching, XRD analysis was performed on raw steel slag, conventional leaching residue (RL-residue), and ultrasound-assisted leaching residue (UL-residue), combined with XRF results of UL-residue for verification.

As shown in Fig. 13, raw steel slag mainly consisted of β-Ca₂SiO₄, Ca₂Fe₂O₅, and magnetite/chromite (Fe₃O₄/FeCr₂O₄) (see Section 3.1). After citric acid leaching, the diffraction patterns of both RL and UL residues underwent significant changes. Yellow shaded regions in the figure indicate key diffraction intervals for phase comparison.

Fig. 13.

XRD patterns of raw steel slag, RL-residue, and UL-residue.

The characteristic diffraction peaks of β-Ca₂SiO₄ (2θ = 32.3°, 32.7°) and Ca₂Fe₂O₅ (2θ = 33.6°) in both RL and UL residues were markedly weakened, indicating significant dissolution and structural disruption of the Ca–Si silicate matrix and Ca-ferrite phases. Additionally, in the mid-angle region of 2θ ≈ 40–43°, diffraction peaks of multiple iron-bearing phases were notably weakened. This region contains overlapping diffraction signals from magnetite, RO phase, and calcium aluminate, further confirming the dissolution of multiple phases. Meanwhile, the characteristic diffraction peak at 2θ≈61.3° associated with magnetite/chromite in raw slag was significantly attenuated to near-background levels in both residues, with this feature being particularly weak in UL-residue. These changes indicate substantial depletion of Cr-bearing spinel phases during leaching, which agrees with the observed color change of the residues from dark gray to lighter shades.

Furthermore, compared with raw steel slag, the leached residues exhibited newly emerged low-angle diffraction features. These peaks were compared with the reference pattern of calcium citrate tetrahydrate (PDF 00-025-1568), and the observed peak positions showed reasonable consistency with the reference peaks. The corresponding features were more pronounced in RL-residue and remained detectable, although weaker and broader, in UL-residue. These results provide supportive, but not definitive, evidence for the possible formation of calcium-citrate-related phases during citric acid leaching [67], [68].

XRF analysis results (Table 3) further verified these observations. Compared with raw steel slag, the Fe₂O₃ and Cr₂O₃ contents in UL-residue decreased by approximately 73% and 83%, respectively, indicating significant removal of magnetite/chromite phases during ultrasound-assisted citric acid leaching, while Ca remained at a relatively high proportion in the residue. The Mg content remained relatively stable before and after leaching, providing a material basis for subsequent functionalization and resource utilization.

Table 3.

Chemical composition of the UL-residue.

Oxide	CaO	Fe2O3	SiO2	Al2O3	MgO	MnO	Cr2O3	V2O5
Content (wt%)	32.62	6.61	4.86	2.60	3.94	0.481	0.041	0.043

3.5.2. Particle size distribution characterization

To analyze the effect of ultrasound on particle size characteristics of steel slag, the particle size distributions of raw steel slag, RL-residue, and UL-residue were measured (Fig. 14).

Fig. 14.

Particle size distribution characteristics of steel slag before and after leaching.

As shown in Fig. 14a, compared with raw steel slag, the cumulative particle size distribution curve of RL-residue shifted slightly toward smaller particle sizes, indicating that a certain degree of chemical erosion occurred on the particle surface during conventional leaching, but the effect on the overall particle size distribution was relatively limited. In contrast, the cumulative distribution curve of UL-residue exhibited a more significant leftward shift, with both D₅₀ and D₉₀ notably lower than those of RL-residue, and an increased curve slope, indicating that ultrasound caused further concentration and overall refinement of the particle size distribution.

Fig. 14b shows the volume distribution characteristics. The volume distribution peak of UL-residue shifted notably toward smaller particle sizes with a more concentrated peak shape, while RL-residue exhibited a broader distribution range and larger characteristic particle size. This result showed that particle refinement under ultrasonic conditions does not originate from intense mechanical crushing but is mainly related to the destruction of particle agglomerate structures, surface erosion, and deagglomeration processes caused by ultrasonic cavitation [33], [34], [35].

3.5.3. Specific surface area and pore structure analysis

To further reveal the effect of ultrasound on the internal structure of steel slag, N₂ adsorption–desorption tests were performed on raw steel slag and leaching residues.

As shown in Fig. 15, raw steel slag exhibited a moderate adsorption capacity, with a specific surface area of 30.11 m² g⁻¹ and total pore volume of 0.0800 cm³ g⁻¹, indicating a certain degree of pore structure development, possibly related to inter-particle voids and surface defects. After conventional leaching, the specific surface area (20.65 m² g⁻¹) and pore volume (0.0621 cm³ g⁻¹) of RL-residue decreased slightly, indicating that under stirring conditions, reaction mainly occurred on particle surfaces and near-surface regions, with limited penetration into particle interiors. The pore size distribution showed little change compared with raw steel slag, remaining dominated by mesopores. In marked contrast, UL-residue exhibited a dramatically reduced specific surface area of 0.87 m² g⁻¹ and total pore volume of 0.0051 cm³ g⁻¹. The average pore diameter increased from 10.63 nm (raw slag) and 12.02 nm (RL-residue) to 28.37 nm (UL-residue), further indicating substantial restructuring of the pore network (see Fig. 16).

Fig. 15.

N₂ adsorption–desorption isotherms of raw steel slag, RL-residue, and UL-residue.

Fig. 16.

Pore size distribution of raw steel slag, RL-residue, and UL-residue.

The dramatic reduction in pore volume and specific surface area, along with the shift in pore size toward macropores in UL-residue, suggests substantial pore-network alteration [33], [69], [70]. This observation is consistent with the conclusion in Section 3.4 that leaching kinetics under ultrasonic conditions shifted from interfacial chemical reaction control to a mixed diffusion–reaction control mode, suggesting that ultrasound-enhanced mass transfer enabled the leaching medium to penetrate and react within previously interlocked particle interior regions.

BJH pore size distribution analysis further indicates that the raw steel slag and RL-residue were still dominated by mesopores, whereas UL-residue exhibited a clear shift toward larger pores. Although part of the finer pore system was lost or merged during matrix disruption, the formation of larger and more accessible transport pathways is consistent with enhanced liquid penetration and interfacial renewal, thereby facilitating chromium leaching from previously interlocked internal regions.

3.5.4. SEM–EDS analysis

To directly visualize the effect of ultrasonic assistance on particle morphology and elemental distribution, SEM–EDS analyses were conducted on the residues obtained from conventional leaching (RL-residue) and ultrasound-assisted leaching (UL-residue), as shown in Fig. 17 and Fig. 18.

Fig. 17.

SEM-EDS analysis of RL-residue.

Fig. 18.

SEM-EDS analysis of UL-residue.

In RL-residue (Fig. 17), particle surfaces exhibited a certain degree of roughness and undulation. The EDS spectrum showed that in addition to the dominant Ca and O peaks, Cr and Fe peaks were still clearly discernible. Elemental mapping results indicated that Fe and Cr exhibited obvious localized enrichment regions in the residue (circled areas in the figure), with highly overlapping spatial distributions, suggesting that some Fe₃O₄/FeCr₂O₄ spinel phases were not completely removed under conventional leaching conditions.

By contrast, the UL-residue (Fig. 18) displays a markedly more homogeneous surface appearance at the microscale. The Cr and Fe signals in the EDS spectrum are significantly weakened to near-background levels. Elemental mapping shows that Ca remains uniformly distributed throughout the residue, while Fe appears only as sparse isolated spots and Cr is scarcely detectable. This observation showed that ultrasonic assistance enhances the removal of Cr-bearing spinel phases, which is in excellent agreement with the XRF results showing an 83% reduction in Cr₂O₃ content.

3.5.5. FTIR characterization of gelation-related features

FTIR spectra of raw slag, UL-residue obtained within the stable operating window (30 °C), and UL-residue obtained beyond the stable operating window (60 °C) are shown in Fig. 19. Compared with the raw slag, both ultrasonically treated residues exhibit stronger gelation-related spectral features. In particular, the 30 °C residue shows more pronounced Si–O–Si-related absorption, indicating that silica polymerization had already initiated under the milder condition. By contrast, the 60 °C residue displays broader hydroxyl/water-related bands in the 3200–3600 cm⁻¹ and ∼1630 cm⁻¹ regions, consistent with a more hydrated gel-like structure and the severe filtration deterioration observed experimentally. Both residues also exhibit carboxylate-related absorption bands that may be associated with calcium citrate-related species formed during leaching. These spectral differences support the interpretation that silica gelation in the present system is progressive rather than abrupt.

Fig. 19.

FTIR spectra of raw slag, UL-residue (30 °C), and UL-residue (60 °C).

The FTIR results, together with the gel-like slurry appearance, severe filtration difficulty, increased residue mass at extended leaching times, and the decline in apparent Cr extraction beyond the stable operating window, support the interpretation that silica gelation in the present system is a progressive process rather than an abrupt event. Previous studies have shown that silica polymerization in acidic systems can occur once dissolved silicic species exceed a critical threshold on the order of 100–150 mg L⁻¹ expressed as SiO₂ [61], [71]. Accordingly, silica gelation is interpreted here as a mechanistically supported inference rather than a directly proven conclusion.

3.5.6. Environmental safety assessment of leaching residues

After treatment under optimized conditions (citric acid 0.8 mol L⁻¹, ultrasonic power 240 W, 30 °C, reaction time 10 min), the total Cr content in the residue decreased from approximately 2189 mg kg⁻¹ in raw steel slag to 281 mg kg⁻¹ (determined by XRF), significantly below the risk control value of 1300 mg kg⁻¹ for agricultural land (at pH > 7.5) specified in GB 15618–2018.

To further evaluate the ecological application feasibility of steel slag after ultrasound–citric acid leaching treatment, Cr leaching toxicity tests were performed on the leaching residue according to GB 5086.1–1997 “Solid waste—Extraction procedure for leaching toxicity—Horizontal vibration method.” The results showed that the Cr leaching concentration from the treated residue was 2.1 mg L⁻¹, with no hexavalent chromium detected (Cr(VI) < detection limit), far below the Cr leaching limit (15 mg L⁻¹) and Cr(VI) limit (5 mg L⁻¹) specified in GB 5085.3–2007 “Identification standards for hazardous wastes—Identification for extraction toxicity.”

Based on total Cr reduction and leaching toxicity results, the treated residue exhibited low Cr leaching under the present test conditions, supporting its further assessment for controlled utilization.

3.6. Discussion

The leaching behavior of chromium from steel slag is strongly governed by its occurrence state and microstructural characteristics. XRD, MLA, and SEM–EDS results show that chromium is predominantly present as magnetite-associated spinel phases (Fe₃O₄/FeCr₂O₄), most of which are tightly embedded within the Ca–Si matrix. This dense intergrowth structure severely limits the accessibility of Cr-bearing phases, resulting in low chromium extraction efficiency under conventional leaching conditions, where the process is primarily controlled by interfacial chemical reactions.

In the citric acid system, chromium dissolution does not result from direct destruction of the spinel crystal structure. Instead, it is mainly constrained by the accessibility of reactive interfaces. As illustrated in Fig. 20, under conventional leaching conditions, protons and citrate ligands can react only at limited exposed surfaces, while the Ca–Si interlocking structure and the accumulation of relatively inert phases progressively increase mass-transfer resistance.

Fig. 20.

Schematic illustration of steel slag under conventional and ultrasonic-assisted conditions.

The introduction of an ultrasonic field enhances chromium leaching. Kinetic analysis showed that, within appropriate ultrasonic power and time ranges, the leaching process evolves from single interfacial reaction control toward mixed control involving both interfacial reaction and diffusion. This transition reflects a reduction in mass transfer resistance and an increase in effective reaction interfaces, rather than a change in the intrinsic reaction mechanism. The inherent limitations of applying the shrinking core model under ultrasonic conditions, and the interpretation of the derived apparent activation energies, are critically discussed in Section 3.4.3. The corresponding structural evolution is schematically illustrated in Fig. 20.

Ultrasonic cavitation and vibration are considered to weaken the interlocking effect of the Ca–Si matrix, promoting particle deagglomeration, crack formation, and interface renewal, which can facilitate proton transport and dissolution product diffusion. Consequently, Cr-bearing phases that were previously inaccessible may gradually become exposed and participate in the leaching reaction. This interpretation is further supported by the macroscopic evolution of leaching residues, where the gradual disappearance of dark regions and overall color transition from dark gray to light gray indicate continuous dissolution and removal of Fe–Cr-enriched magnetite phases, while Ca shows relative retention due to dissolution–reprecipitation behavior.

However, the enhancement effect of ultrasound is not unlimited and is closely related to the structural evolution of the Ca–Si matrix. As shown in Fig. 21, continuous dissolution of Ca–Si phases releases dissolved silicic species into solution, which may subsequently undergo condensation and progressive polymerization, eventually leading to gel-like silica-rich products [61]. During the pre-gelation stage, highly open reaction interfaces favor efficient chromium leaching. As gelation progresses, the formation of a continuous silica-rich gel-like layer increases mass transfer resistance and attenuates the interface renewal effect induced by ultrasound, thereby limiting further leaching enhancement [36], [61], [69]. Additional support for the proposed gelation mechanism is provided by the FTIR results in Section 3.5.5, which indicate progressive evolution of silica-related spectral features from the stable operating window to the gel-dominated condition. Together with the commonly reported silica polymerization threshold in acidic systems, these observations support a gelation-constrained operating window for ultrasonic leaching.

Fig. 21.

Proposed structural evolution of Ca–Si phases and silica gel formation.

From an energy perspective, the ultrasonic treatment operated at 240 W for 10 min, corresponding to a total input of approximately 0.144 MJ per batch (100 mL slurry containing 5.0 g slag) and an apparent volumetric power density of 2.4 kW L⁻¹. Although the local power density is high at laboratory scale, the shorter treatment time resulted in substantially lower total energy input than that of conventional stirred leaching under similar chemical conditions, which required 60 min with a combined water-bath and stirrer input of approximately 1.5–2.3 MJ per batch while achieving lower Cr extraction. In addition, ultrasound-assisted leaching differs from mechanical activation routes in that it couples particle refinement and interfacial renewal directly with the leaching process [36], [72], [73]. Industrial implementation would more likely rely on bath-type or flow-through sonoreactors operating at lower volumetric power densities over larger processing volumes [73], and a hybrid strategy combining short ultrasonic pre-treatment with subsequent conventional leaching may represent a practical direction for scale-up [69], [70].

Following leaching, the filtrate contains dissolved Cr(III), Fe(II/III), and Ca(II), predominantly present as citrate complexes. A practical downstream route involves pH adjustment to moderately alkaline conditions (e.g., pH 8–9), which promotes precipitation of a chromium-enriched Fe–Cr hydroxide phase [74]. The resulting hydroxide product may be directed toward recovery or stabilization, thereby reducing the risk of secondary hazardous liquid waste. Meanwhile, the separated leach residue may be considered for safer downstream utilization after appropriate environmental evaluation. Although citrate complexation may increase the apparent stability of dissolved metal species, the exclusive presence of Cr(III), rather than Cr(VI), in the citric acid system represents an inherent environmental advantage. A conceptual process flowsheet illustrating the leaching–separation–recovery sequence is presented in Fig. 22.

Fig. 22.

Conceptual downstream treatment route for chromium-bearing filtrate generated during ultrasonic citric acid leaching of steel slag.

The present experiments were conducted using a probe-type sonicator operating at a fixed resonance frequency of 20 kHz, and the influence of frequency could therefore not be independently evaluated. Low-frequency ultrasound is generally associated with stronger cavitation collapse and more pronounced mechanical disruption, which is consistent with the matrix-breaking effect observed in this study [75]. However, intermediate frequencies may provide a different balance among cavitation intensity, spatial uniformity, and process stability.

Future work incorporating frequency-dependent investigation, more rigorous kinetic modeling, and direct quantification of dissolved silicon species may further clarify the coupling among cavitation intensity, gelation tendency, and apparent kinetic behavior under tunable ultrasonic conditions [76], [77]. Such efforts would mainly serve to refine the present mechanistic interpretation rather than alter the overall conclusions of this study [78], [79].

4. Conclusions

This study developed an ultrasound-assisted citric acid leaching system for the selective removal of chromium from steel slag, in which Cr-bearing spinel phases (FeCr₂O₄/Fe₃O₄) are encapsulated within a Ca–Si matrix. The main conclusions are as follows:

• (1)Among the six carboxylic acids screened, citric acid exhibited the highest Cr extraction efficiency (63.3%) and Cr/Ca selectivity (2.86) under conventional stirring conditions, owing to its multidentate chelation capability. Under the optimized ultrasonic conditions (0.8 mol L⁻¹ citric acid, 240 W, 30 °C, 10 min, S/L = 1:20), the Cr extraction increased to 84.3%, reducing the residual Cr content from approximately 2189 to 281 mg kg⁻¹.
• (2)Kinetic analysis indicated an ultrasound-induced shift in the apparent rate-controlling regime. Conventional leaching followed a chemical reaction-controlled shrinking core model (E_a = 48.10 kJ mol⁻¹), whereas ultrasound-assisted leaching exhibited mixed control (E_a = 18.02 kJ mol⁻¹). This result suggests that cavitation mainly reduced interfacial mass-transfer resistance, rather than fundamentally altering the intrinsic bond-breaking step. Consistent with this interpretation, residue characterization by XRD, BET, and SEM–EDS showed particle refinement, pore development, and partial disruption of the Ca–Si interlocking structure, all of which improved Cr accessibility.
• (3)A gelation-constrained operating window was identified. Prolonged reaction time or elevated temperature was associated with the accumulation of silicic species and the progressive development of gel-like silica, which in turn impaired both mass transfer and solid–liquid separation. The effectiveness of ultrasonic enhancement therefore lies in accelerating Cr extraction within this stability window, before gel-dominated conditions become prevalent.
• (4)Leaching toxicity tests (GB 5086.1–1997) showed that the treated residue exhibited Cr leaching concentrations far below the regulatory limit, supporting its potential for safer downstream utilization.

CRediT authorship contribution statement

Zhiheng Nie: Writing – review & editing, Writing – original draft, Validation, Supervision, Software, Methodology, Conceptualization. Fusheng Niu: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Data curation, Conceptualization. Jinxia Zhang: Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition, Data curation. Xinwei Wang: Visualization, Project administration, Methodology, Formal analysis. Qi Chen: Supervision, Formal analysis, Data curation.

Declaration of competing interest

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