Dissolution behaviour of valuable elements from the CaO–SiO 2 –Fe 2 O 3 glassy phase modified with Na 2 O and Al 2 O 3 in an acidic solution

Abstract

Dephosphorisation slag generally consists of a 2CaO·SiO₂–3CaO·P₂O₅ (C₂S–C₃P) solid solution, CaO–SiO₂–FeO_x glassy phase, and RO phase (FeO_x, MgO and MnO). Its components, such as CaO, FeO_x, MnO, and MgO, can be used as a soil conditioner in agriculture to improve acidic soil and supply nutrient elements. To promote its dissolution, CaO–SiO₂–Fe₂O₃ glassy phases with different Al₂O₃ and Na₂O contents were synthesised, and their dissolution behaviour and mechanism were investigated. The addition of Al₂O₃ significantly inhibited the dissolution of the glassy phase because it increased the degree of polymerisation of the silicate network, and the glassy phase structure became more stable as the Al₂O₃ content increased. When 3% Na₂O was added, the dissolution ratios of elements from the glassy phase were the highest, with the dissolution ratio of Si reaching 21.0% and the dissolution ratios of Ca and Fe reaching 21.6% and 17.7% at pH 5. The substitution of CaO with Na₂O significantly reduced the dissolution ratios of all elements. As the Na₂O content increased, Na₂O had a bigger influence on the silicate structure. Na₂O addition enhanced the proportion of $Q_{Si}^{2}$ units and formed additional bonds by interacting with non-bridging oxygens in the silicate tetrahedra, strengthening the stability of the glass network. Lowering pH increased the dissolution ratios of elements.

Keywords

Dephosphorisation slag leaching composition glassy phase silicate structure

Introduction

China Steel Association reported that the production of crude steel in China has reached about 1.029 billion tonnes in 2023, comprising nearly half of the global total.^1,2 A significant quantity of steelmaking slag is inevitably generated as a by-product, but only a small part of slag is used in road construction, cement admixture, and other applications,^3,4 resulting in a lower utilisation ratio. During the iron ore beneficiation or sintering process, P₂O₅ originated from iron ores is difficult to remove during the beneficiation or sintering process. In the blast furnace, it is reduced to phosphorus (P) and enters the molten iron. Therefore, lower-grade iron ore leads to higher P content in the molten iron, further affecting the following steelmaking process. To improve the removal of P from molten iron, dephosphorisation of molten iron in the converter has been employed in many steel plants, so large amounts of dephosphorisation slag were generated.^5,6 Dephosphorisation slag is composed of CaO, FeO_x, MnO, and MgO, making it effective in improving acidic soil conditions. The elements from slag, such as Ca, Si, P, and Fe ions, play a role in improving soil acidity and enhancing the bioavailability of soil phosphate.^7–9 In addition, the concentration of harmful elements in slag generally can meet the pollutant control standards for agricultural land, so it is suitable for agricultural use.^10,11 Europe, the United States, and other countries have conducted a lot of research and practice on the application of steelmaking slag in agriculture, such as the utilisation of steelmaking slag for the recovery of paddy fields damaged by Tsunami and slag fertiliser produced during the Thomas steelmaking process.^12,13

To achieve effective dephosphorisation in the steelmaking process, it is necessary to maintain high basicity, oxidation property, and strong agitation formed by oxygen blowing operation, which plays an important role in the oxidation of P. Upon the oxidation of P to form P₂O₅, P₂O₅ rapidly reacts with CaO present in slag to form the 3CaO·P₂O₅ phase. Subsequently, it further exists in the form of a 2CaO·SiO₂–3CaO·P₂O₅ solid solution (C₂S–C₃P).¹⁴ Certain amounts of Ca, Si, and P elements are enriched within this phase in the dephosphorisation slag. Metallic elements such as FeO_x and MgO are present in the RO phase. Moreover, a portion of Ca, Si, and Fe exists in the CaO–SiO₂–FeO_x glassy phase. Compared with converter steelmaking slag, dephosphorisation slag exhibits lower basicity (1.6–2.0), resulting in a higher proportion of glassy phase.¹⁵ In the pursuit of achieving the efficient utilisation of steelmaking slag in the agricultural and marine sectors, researchers have conducted extensive research to study the dissolution behaviour of dephosphorisation slag. Futatsuka et al.¹⁶ investigated the dissolution behaviour of valuable elements from the slag in seawater. It was verified that when the dephosphorisation slag and organic additives were added to the soil, the dissolution efficiency of Fe was better. Maruoka et al.¹⁷ developed a column device simulating paddy field soil during the flooding period to evaluate the role of converter steelmaking slag as a fertiliser. It was confirmed that the application of slag fertiliser had a significant impact on the pH of the soil and the dissolution of Ca and Fe elements, further improving the growth trend and yield of rice. To further study the effectiveness of steelmaking slag fertiliser in paddy fields, some researchers have carried out the application research of commercial slag fertiliser in paddy field,^18,19 found that the dissolution ratio of various elements was closely related to the proportion of mineral phase components. By adjusting the mineral composition, the modified slag fertiliser can be prepared to supply large amounts of Ca and Si to the paddy field.²⁰ However, the specific roles that each mineral phase plays in the dissolution process of various elements remain not fully understood at present. To supply Ca, Si, and Fe to paddy fields simultaneously, Koizumi et al.²¹ studied the dissolution behaviour of the Fe-containing glassy phase and found that the co-existence of the CaO–SiO₂–FeO glassy phase and the 2CaO·SiO₂ solid phase can effectively provide sufficient Ca, Si, and Fe elements to the soil. Since the Fe in slag exists in the form of FeO and Fe₂O₃, these studies did not clarify the influence of Fe₂O₃ enriched in the glassy phase on the dissolution behaviour of steelmaking slag. Moreover, they only focused on the dissolution of steelmaking slag in a weak nitric acid solution and did not investigate the dissolution behaviour in organic acids, which are closer to the soil environment.

In the previous experiments on the dissolution behaviour of Ca, Fe, and Si elements in the CaO–SiO₂–Fe₂O₃ glassy phase, it was found that the increase of basicity and Fe₂O₃ content (25%–35%) was conducive to the dissolution of the glassy phase. However, there exist other elements apart from Ca, Fe, and Si in the glassy phase of the actual dephosphorisation slag. Some researchers investigated the influence of Na₂O and Al₂O₃ on the dissolution behaviour of valuable elements and found that the addition of Na₂O promoted the enrichment of the P-rich phase in the dephosphorisation slag.²² The accumulation of the RO phase by Al₂O₃ addition was conducive to the dissolution of the P element,²³ but there were few studies on the influence of Na₂O and Al₂O₃ in the glassy phase on the dissolution behaviour. According to previous studies, several glassy phases with various Na₂O and Al₂O₃ contents were synthesised and leached in a citric acid solution to simulate the weakly acidic conditions found near plant roots. The dissolution ratios of Ca, Si, and Fe were examined, and the dissolution mechanisms were analysed from the perspective of silicate structure. This study aimed to establish a theoretical basis for agricultural applications and high-value utilisation of dephosphorisation slag.

Experimental method

Glassy phase synthesis

Reagent-grade CaCO₃, SiO₂, Fe₂O₃, Al₂O₃ and Na₂SiO₃ were used in this experiment. The reagents were mixed to synthesise glassy phases with different mass ratios of Al₂O₃ or Na₂O (0%, 3%, and 6%). To study the effect of the substitution of CaO with Na₂O as the basic component, glass samples with different mass ratios of Na₂O substitution (5%, 10%, and 15%) were synthesised. CaO was prepared by calcining CaCO₃ in an Al₂O₃ crucible at 1273 K for 10 h. Based on the target composition, 10 g of reagents were thoroughly mixed and heated to 1773 K in a Pt crucible under atmospheric conditions to form a homogeneous liquid phase. Following the melting process, the melt was quenched with water at 1773 K. The chemical composition of eight kinds of glassy phases is listed in Table 1. For the Na₂O-substituted series (C–S–F–5% Na, 10% Na, and 15% Na), the Fe₂O₃ content was fixed, and SiO₂ was adjusted to maintain a constant (CaO + Na₂O)/SiO₂ mass ratio, thereby isolating the effect of Na₂O substitution for CaO.

Table 1.

Composition of each synthesised glassy phase (mass%).

	CaO	SiO₂	Fe₂O₃	Al₂O₃	Na₂O	Basicity
CaO–SiO₂–Fe₂O₃ (C–S–F)	35.00	35.00	30.00	0.00	0	1.0
CaO–SiO₂–Fe₂O₃–3% Al₂O₃ (C–S–F–3% Al)	33.95	33.95	29.10	3.00	0	1.0
CaO–SiO₂–Fe₂O₃–6% Al₂O₃ (C–S–F–6% Al)	32.90	32.90	28.20	6.00	0	1.0
CaO–SiO₂–Fe₂O₃–3% Na₂O (C–S–F–3% Na)	33.95	33.95	29.10	0	3.00	1.1
CaO–SiO₂–Fe₂O₃–6% Na₂O (C–S–F–6% Na)	32.90	32.90	28.20	0	6.00	1.2
CaO–SiO₂–Fe₂O₃–5% Na₂O (C–S–F–5% Na)	27.50	32.50	35.00	0	5.00	1.0
CaO–SiO₂–Fe₂O₃–10% Na₂O (C–S–F–10% Na)	22.50	32.50	35.00	0	10.00	1.0
CaO–SiO₂–Fe₂O₃–15% Na₂O (C–S–F–15% Na)	17.50	32.50	35.00	0	15.00	1.0

The mineralogical compositions of synthesised samples were analysed by X-ray diffraction (XRD) and scanning electron microscope–energy dispersive X-ray spectrometer (SEM–EDS). Raman spectra were measured using a laser Raman microscope produced by JY Co. Ltd. The excitation laser used for analysis had a wavelength of 488 nm, a lens magnification of 50×, and a light source power of 100 MW. The glassy phases were scanned with a frequency range of 400–1300 cm⁻¹ and a wavenumber precision of 1 cm⁻¹. Two replicates were made for each measurement, and integration was performed to obtain the Raman spectra.

Leaching experiment

The primary research methodology and the configuration of the leaching experiment are illustrated in Figure 1. The synthesised glass phases were crushed and ground to particles with sizes <53 µm. One gram of powder was added to a Teflon beaker containing 300 mL of deionised water placed in an isothermal water bath (298 K). The aqueous solution was stirred at a rotational speed of 300 r/min using the magnetic stirrer. During dissolution, Ca²⁺ ions dissolved into the aqueous solution, which caused an increase in pH. The dilute citric acid (H₃C₆H₅O₇, 0.1 mol/L) was continuously injected through a peristaltic pump to keep the constant pH value of the aqueous solution, which simulated the weakly acidic environment of the plant root system. Previous studies²⁴ have demonstrated that almost all elements in the CaO–SiO₂–FeO glassy phase were dissolved within approximately 90 min. Consequently, the reaction time of 90 min is fixed to investigate the effect of Al₂O₃ and Na₂O addition on the dissolution behaviour of glassy phases. After 90 min, approximately 5 mL of leachate was sampled and filtered using a syringe filter (<45 µm).

Figure 1.

Research methodology and the configuration of the leaching experiment.

The concentrations of various elements in the leachate were analysed using an Inductively Coupled Plasma Optical Emission Spectrometer. The remaining solution was filtered by vacuum filtration, and the undissolved slag sample was collected. The mineralogical composition of the residue was analysed by using SEM–EDS and XRD. To investigate the dissolution mechanism of glassy phases modified with Al₂O₃ and Na₂O, Raman spectroscopy was used to analyse the silicate structure of the glassy phase,²⁵ which can characterise the structural evolution of silicate with different Al₂O₃ and Na₂O contents. The microstructure of the residues was also investigated via Raman spectroscopy. According to the composition of leachate, equation (1) was used to calculate the dissolution ratio of each element from different samples:

R_{M} = \frac{C_{M} \cdot V}{m_{M}}

(1)

where R_M is the dissolution ratio of element M from the sample, C_M is the concentration of M after 90 min (mg/L), V is the final volume of leachate (L), and m_M is the mass of M in 1 g of sample (mg).

Results and discussion

Mineralogical composition

Four typical glass samples were selected, and their microstructure is shown in Figure 2(a)–(d). The morphology of C–S–F–5% Na and C–S–F–15% Na exhibits small pits, which may result from the continuous grinding and polishing during the SEM sample preparation. According to the EDS analysis, there was no apparent difference in chemical composition between the pit and the smooth part, as shown in Table 2, which further proved that this phenomenon is not caused by crystal phase precipitation. The samples were also confirmed as a glassy phase by XRD analysis, as shown in Figure 2(e). An amorphous hump was observed at approximately 30° in the glass phase. The intensity and position of the broad peaks exhibited slight variations among samples, potentially attributable to differences in the glass compositions within the SiO₄ tetrahedral framework.

Figure 2.

Appearance of the surface of glassy phases (a) C–S–F–3% Al, (b) C–S–F–6% Al, (c) C–S–F–5% Na, (d) C–S–F–15% Na, and (e) X-ray diffraction (XRD) patterns of each glass.

Table 2.

Average composition of each glassy phase (mass%).

	Point	O	Ca	Si	Fe	Al	Na
(a) C–S–F–3% Al	–	39.76	21.30	14.68	22.39	1.62	–
(b) C–S–F–6% Al	–	39.89	21.26	14.28	21.52	3.05	–
(c) C–S–F–5% Na	Point 1	40.42	18.12	15.70	22.38	–	3.38
	Point 2	39.12	18.49	15.95	22.96	–	3.49
	Point 3	39.09	17.75	16.68	22.78	–	3.69
(d) C–S–F–15% Na	Point 1	39.85	11.57	14.68	24.18	–	9.72
	Point 2	39.28	11.52	14.98	24.04	–	10.18
	Point 3	41.33	11.25	14.50	22.77	–	10.15

Effect of Al₂O₃ on dissolution behaviour of glassy phases

The dissolution ratio of each element from glassy phases with different Al₂O₃ contents at pH 5 is shown in Figure 3. For the original C–S–F glass, the dissolution ratios of Ca, Si, and Fe were 13.18%, 14.99%, and 14.51%, respectively. The dissolution of C–S–F–3% Al glass in the aqueous solution was inhibited, leading to a lower dissolution ratio of each element. Among them, Al₂O₃ addition has the least inhibitory effect on the dissolution of Fe elements, and its dissolution ratio decreased slightly, approximately 13.4%. When the Al₂O₃ content was further increased to 6%, the dissolution ratio of each element continued to decline, and those of Ca, Si, and Fe from C–S–F–6% Al were only 7.22%, 7.86%, and 9.95%, respectively. In particular, the Al₂O₃ that was added to the glass was not dissolved in large quantities; its dissolution ratio was around 5%. The leaching efficiency of the Fe element was higher than that of other elements. Therefore, to design a slag sample with high supply ability of Ca, Si, and Fe, its glassy phase should contain a lower Al₂O₃ content.

Figure 3.

Dissolution behaviour of each element from glassy phases with different Al₂O₃ contents.

Raman spectra were obtained in the range of 400–1300 cm⁻¹ to investigate the effect of Al₂O₃ addition on the glass structure. As shown in Figure 4(a), the spectra for the glasses are divided into two central wavenumber regions: 1200–800 cm⁻¹ and 800–600 cm⁻¹, corresponding to the Si–O–Si bonding and Si–O symmetric stretching vibrations,²⁶ respectively. Fe³⁺ occupies both tetrahedral and octahedral positions as an amphoteric cation. Peaks in the range of 550–650 cm⁻¹ are attributed to the symmetric stretching vibrations of Fe³⁺ in octahedral coordination, while peaks around 650–700 cm⁻¹ are associated with Fe³⁺ in tetrahedral coordination.²⁷

Figure 4.

Effects of Al₂O₃ content on the Raman spectrum of (a) glasses and (c) their residues, and the unit structure ratio of SiO₄ tetrahedra of (b) glasses and (d) their residues.

A broad Raman peak was observed in the 600–700 cm⁻¹ range for the original C–S–F glass. There was no distinct peak that appeared around 650 cm⁻¹ in the glasses with Al₂O₃ addition, which concentrated around 700 cm⁻¹. This could be attributed to the Fe³⁺ coordination state changing. An increasing Al₂O₃ content led to a reduction in [FeO₆]/[FeO₄],²⁸ with a shift in the Raman spectrum from 675 to 712 cm⁻¹. The Raman spectra of the glasses revealed a significant increase in peak intensity around 920 cm⁻¹ after the addition of Al₂O₃, accompanied by a shift in the peak position from 918 to 930 cm⁻¹. It indicated that Al₂O₃ addition induced local structural changes within the silicate network.

After conducting peak fitting analysis on the Raman spectra of glasses, the $Q_{Si}^{i}$ ratio of the SiO₄ tetrahedral structures was calculated by equation (2) based on the fitting results, the characteristic peak ranges corresponding to the structural units $Q_{Si}^{0}$ (SiO₄⁴⁻, monomer structure), $Q_{Si}^{1}$ (Si₂O₇⁶⁻, dimer structure), $Q_{Si}^{2}$ (SiO₃²⁻, chain structure), $Q_{Si}^{3}$ (Si₂O₅²⁻, sheet structure) and $Q_{Si}^{4}$ (SiO₂, three-dimensional-network structure) are 860, 910, 960, 1060, and 1130 cm⁻¹, respectively,^29,30

Q_{ratio}^{i} = \frac{S_{i}}{\sum_{n = 0}^{4} s_{n}}

(2)

where S_i and S_n represent the area of each peak within the Raman spectra post-waveform segmentation.

In the C–S–F glassy phase, the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units in silicate networks were higher than those in the glasses modified by Al₂O₃, indicating a higher proportion of non-bridging oxygens (NBOs) containing units in the silicate network, as shown in Figure 4(b). With increasing Al₂O₃ content, the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units decreased, while the proportion of $Q_{Si}^{2}$ units correspondingly increased. The addition of Al₂O₃ promoted the polymerisation of the silicate network, leading to a reduction in $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units with abundant NBOs. When the Al₂O₃ content reached 6%, the ratio of $Q_{Si}^{0}$ structural units dropped to 20.2%. Ye et al.³¹ used molecular dynamics simulations to study the variations in the structure and mechanical properties of sodium aluminosilicate glass with the Al₂O₃/Na₂O ratio and found that in the CaO–Al₂O₃–SiO₂ system, when the Al₂O₃ content was relatively high, Al was preferentially incorporated into $Q_{Si}^{4}$ units. Moreover, as the Al₂O₃ content increased, there was a rapid transformation from $Q_{Si}^{2}$ and $Q_{Si}^{3}$ units to $Q_{Si}^{4}$ units. However, this phenomenon was not clearly observed in the results of Raman spectroscopy and deconvolution calculations in this study. It is likely that the Al₂O₃ content was too low to exert a significant regular influence on the ratios of $Q_{Si}^{2}$ and $Q_{Si}^{3}$ structural units.

It is considered that the improvement in stability was closely related to structural changes within the silicate framework. This was confirmed by the corresponding dissolution ratio of each glass. The results showed that with the increase of Al₂O₃ addition in the glass, the dissolution ratio showed a significant downward trend. When the molar ratio of Al₂O₃/MO (MO represents basic oxides) was <1.0, the Al–O coordination existed as tetrahedra, similar to the [SiO₄] tetrahedral units. In this study, MO primarily presents as CaO, and the Al₂O₃ addition was relatively low, resulting in a lower molar ratio. Based on these results, it could be concluded that Al₂O₃ acted as a network former (NWF) in this glass, promoting the polymerisation of the network structure.

Figure 4(c) presents the Raman spectra of residues after leaching of glasses with different Al₂O₃ contents. The residue of C–S–F glass exhibited obvious changes, with significant peaks near 600–650 cm⁻¹ and 800–1000 cm⁻¹ absent and only a single peak at 699 cm⁻¹ detected. Compared to the residue of C–S–F glass, a consistent rightward shift in the characteristic Raman peaks near 700 cm⁻¹ was observed in the residue with Al₂O₃ addition, and the intensity of the peak near 920 cm⁻¹ increased slightly.

This result is also shown in the ratio of each structural unit, as shown in Figure 4(d). After the dissolution of the C–S–F glass, the proportions of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units in the silicate structure significantly decreased, while the proportion of $Q_{Si}^{4}$ structural units increased correspondingly. The short-range structural units in the solution were dissolved, leading to a relative increase in the ratio of other structural units. This also confirms the selective leaching of the $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units during the leaching process.

Effect of Na₂O on the dissolution behaviour of glassy phases

The dissolution behaviour of glasses with different Na₂O contents is shown in Figure 5. The dissolution ratios of various elements from C–S–F–3% Na glass increased compared to C–S–F glass. Among them, the dissolution ratios of Fe and Si elements increased significantly, reaching 21.6% and 21.0%, respectively. The dissolution ratio of Ca also increased, but the growth rate was lower than that of other elements, rising from 13.2% to 17.7%. The dissolution ratio of Na was lower than that of other elements, reaching 13.2%. However, when the Na₂O content continued to increase to 6%, the dissolution of the glass phase was not improved, and the dissolution ratios of all elements decreased significantly. Increasing Na₂O content had the most significant impact on the dissolution ratio of Si, which dropped from 21.0% to 5.1%. For the C–S–F–6% Na glass, the dissolution ratio of Fe was 8.8%, higher than that of other elements.

Figure 5.

Dissolution behaviour of each element from glassy phases with different Na₂O contents.

Figure 6(a) presents the Raman spectra of glasses with Na₂O addition. As the Na₂O content increased, the central peak near 700 cm⁻¹ shifted to a higher frequency, and the Raman spectra in the range of 550–650 cm⁻¹ gradually became smoother, with no distinct peaks detected in this range for the C–S–F–6% Na glass. Around 920 cm⁻¹, the main peak shifts from 918 to 913 cm⁻¹. In the Raman spectrum of the C–S–F–3% Na glass, a broad peak was observed in this range, which was smoother than in other glasses. After Na₂O modification, the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units decreased, and this reduction became more pronounced with increasing Na₂O content, as shown in Figure 6(b). The decline in $Q_{Si}^{1}$ units was particularly notable. Conversely, the proportion of $Q_{Si}^{2}$ units increased correspondingly, reaching 34.9% in the C–S–F–6% Na glass. The addition of a basic component influences the dissolution behaviour of elements in the glasses. This depolymerisation of the silicate structure in glass can be described as follows³²:

MO = M^{2 +} + O^{2 -}

(2)

R_{2} O = 2 R^{+} + O^{2 -}

(3)

[S i_{2} O_{5}]^{2 -} (sheet) + O^{2 -} = [Si O_{3}]^{2 -} (chain)

(4)

[Si O_{3}]^{2 -} (chain) + O^{2 -} = [S i_{2} O_{7}]^{2 -} (dimer) + [Si O_{4}]^{4 -} (monomer)

(5)

Figure 6.

Effects of Na₂O content on the Raman spectrum of (a) glasses and (c) their residues, and unit structure ratio of SiO₄ tetrahedra of (b) glasses and (d) their residues.

where R⁺ is the cation of the basic oxides (CaO and Na₂O). It is well-known that Na₂O, acting as a network modifier (NWM), disrupts Si–O–Si bonds and introduces NBOs. Na₂O strengthens the glass network by binding to NBOs in silicate tetrahedra, forming additional bonds. This structural reinforcement limits the selective leaching of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units, resulting in low leaching ratios. This phenomenon is partially attributed to the ‘non-bridge’ effect of Na⁺ ions, which will be analysed in detail in the next section. It should be noted that while Na₂O typically depolymerises the silicate network as an NWM, the apparent stabilisation observed here at higher Na₂O contents is attributed to the formation of a Na-rich passivation layer on the glass surface during acid leaching, which physically limits further dissolution rather than inherently strengthening the bulk glass network.

Figure 6(c) displays the Raman spectra of the residue after leaching of these glasses in the acid solution. The peak associated with Si–O bond vibrations near 920 cm⁻¹ shifts to the right, from 916 to 929 cm⁻¹. In contrast, the Raman spectra of the C–S–F–6% Na glass showed only minor changes following leaching. Slight leftward shifts were observed in the peaks near 700 and 920 cm⁻¹, be less pronounced. The deconvolution analysis of the Raman spectra for the residues is presented in Figure 6(d). After leaching, the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units decreased, resulting in a relative increase in the ratios of other structural units. The addition of Na₂O weakened the influence of leaching on the ratios of various structural units in the silicate structure of the glassy phase, which is consistent with the results of element dissolution ratios in the glassy phase shown in Figure 5.

Effect of Na₂O substitution for CaO on dissolution behaviour of glassy phases

To further explore the influence of the Na₂O content on the dissolution behaviour of the C–S–F system glass, more glasses were synthesised in which the Na₂O substitution (5%, 10%, and 15%) for the basic component CaO in the glasses. The dissolution ratios calculated by equation (1) are shown in Figure 7. The dissolution ratios of various elements continuously decreased with the increase in the Na₂O substitution. In the C–S–F–15% Na glass, Ca, Si, and Na elements were hardly soluble, with dissolution ratios all around 1.0%. The dissolution ratio of Fe was still the highest in each glass, reaching 6.9% in the C–S–F–5% Na glass.

Figure 7.

Dissolution behaviour of each element from glassy phases with different Na₂O substitutions.

As the Na₂O substitution increased, a frequency elevation was observed, accompanied by a reduction in weak peaks within 500–600 cm⁻¹, resulting in a smoother spectrum, as shown in Figure 8(a). As the Na₂O substitution increased, the characteristic peak of Fe ions around 700 cm⁻¹ shifted from 675 to 725 cm⁻¹. When the Na₂O substitution rose from 0 to 10%, the Raman spectra exhibited a shift from 918 to 924 cm⁻¹. However, this increase in substitution did not significantly influence peak intensity. Similar to the Raman spectra of C–S–F–3% Na glass in Figure 6(a), the silicate structure predominantly consisted of short-range structural units ( $Q_{Si}^{0}$ , $Q_{Si}^{1}$ , and $Q_{Si}^{2}$ ), as shown in Figure 8(b). The 5%–10% Na₂O substitution did not result in significant changes in the ratios of structural units. Na⁺ ions act as NWM within the glass structure by binding to NBOs in Si–O bonds, effectively creating ‘bridges’. Na⁺ ions form a Na-rich protective layer on the glass surface,³³ slowing erosion of the internal structure by the leaching solution. This Na-rich layer passivates the surface, reducing both selective leaching and overall leaching efficiency. During leaching, reactions on the dissolution surface may produce an insoluble secondary mineral layer, such as a Na₂O- and SiO₂-rich film.²¹ This reaction layer limits solution access to internal $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units, further slowing the leaching process. In the glass with high Na₂O content, this passivation effect is particularly pronounced, further reducing the leaching rate of constituent elements. In the C–S–F–6% Na glass, the distribution of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units may be heterogeneous, with localised enrichment in deeper ranges. These areas are less accessible to the leaching solution, reducing the likelihood of contact with $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units and thereby lowering leaching efficiency despite the high overall concentration of these units. Additionally, the acidity of leachate may be insufficient to disrupt the structure of glass with high Na₂O content significantly. Consequently, although some $Q_{Si}^{0}$ and $Q_{Si}^{1}$ units may dissolve, the overall solubility remains limited.

Figure 8.

Effects of Na₂O content on the Raman spectrum of (a) glasses and (c) their residues, and the unit structure ratio of SiO₄ tetrahedra of (b) glasses and (d) their residues.

Notably, the Raman spectrum of the C–S–F–15% Na glass shows a distinct peak around 915 cm⁻¹ with greater intensity than that observed in other glasses modified by Na₂O. Using the Trimethylsilylation method, Yang et al.³⁴ investigated the structure of Na₂O–CaO–SiO₂ glasses and found that increasing the Na₂O/CaO ratio enhances the degree of polymerisation (DOP = NBO/T) in the glass structure. A higher Na₂O content reduces the ability of Ca⁺ ions to counteract the influence of Na⁺ ions. Therefore, as the Na₂O substitution increased, the larger the effect of Na₂O on the silicate structure, resulting in a higher DOP in the C–S–F–15% Na glass. The larger ionic radius and lower charge of Na⁺ facilitate its formation of ionic bonds with O²⁻, which occupy the voids in the glass network. This bonding increases the proportion of structural units containing a small amount of NBOs in the glassy phase. When the Ca²⁺ ions act as a network breaker, in order to use their double charge, they must form a link with the second NBOs separately. This is not the case with the Na⁺ ions, so silicates containing the M₂O tend to have a higher DOP than those containing the MO. The schematic diagram is shown in Figure 9.^³⁵

Figure 9.

Schematic diagram of bridging with Ca²⁺ ions and Na⁺ ions.

Figure 8(c) and (d) presents the Raman spectrum and peak fitting calculation results of each residue after leaching. As the Na₂O substitution in the glassy phase increased, the Raman peak near 920 cm⁻¹ shifted further left. The silicate structure in the residues of glassy phases with 5% and 10% Na₂O substitution still contained a significant proportion of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units. However, in the residue of the C–S–F–15% Na glass, a broad peak appeared in the range of 850–920 cm⁻¹, accompanied by a distinct peak near 712 cm⁻¹. Fe ions predominantly bond with most of the [SiO₄] tetrahedral structure, depleting the [SiO₄] tetrahedral network and minimising Si–O vibrational spectra in the melt structure. This results in the spectral intensity within the range of 800–1200 cm⁻¹ being lower than the [FeO₄] characteristic peak near 700 cm⁻¹ in the C–S–F–15% Na glass.³⁶

Effect of pH on dissolution behaviour of the glassy phase

To further investigate the effect of Al₂O₃ or Na₂O modification, the dissolution behaviour of the glassy phase was examined under different pH. The C–S–F–3% Al and C–S–F–5% Na glasses were selected for the analysis. The dissolution ratios of the elements from the C–S–F–3% Al glass under different pH conditions are shown in Figure 10(a). When the pH increased to 6, the glasses were almost not dissolved, with the dissolution ratios of Ca and Si being <0.5%. The dissolution ratio of Fe was higher than that of other elements, reaching 2.96%. As the pH decreased, the H⁺ concentration in the solution increased, and the glass dissolved easily. The dissolution ratios of the elements reached about 30% at pH 4, among which Si has the highest dissolution ratio of 31.87%.

Figure 10.

Dissolution behaviour of each element from glassy phases modified by (a) Al₂O₃ or (b) Na₂O with different pH.

Similarly, a lower pH led to an increase in the dissolution ratio of each element in the C–S–F–5% Na glass, as shown in Figure 10(b). When the pH dropped from 5 to 4, the dissolution ratio increased by approximately 85%, slightly higher than that of the C–S–F–3% Al glass. Although the reduction in pH significantly enhanced the dissolution of glasses, the dissolution ratio of each element was still low. The incomplete dissolution of the glass was likely attributed to the polymerisation effect of Al₂O₃ and Na₂O on the silicate structure in glass.

The Raman spectrum of the residue after leaching of the C–S–F–3% Al glass at different pH is shown in Figure 11(a). At pH 4, the spectrum of the residue was relatively smooth. Based on the structural unit ratios in Figure 11(b), the decreasing pH reduced the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units in the silicate structure. The significant change in peak intensity indicated the progressive breaking of Si–O–Al and Si–O–Si bonds under acidic conditions, leading to notable structural rearrangement. The decreasing pH significantly promoted the dissolution of the glass. However, the decrease in the ratio of each structural unit was lower than that in the residue of the C–S–F glass. In summary, the addition of Al₂O₃ significantly inhibited the dissolution of the glass. As the Al₂O₃ content increased, the stability of the silicate network structure was enhanced.

Figure 11.

Raman spectra and unit structure ratios of SiO₄ tetrahedra of residues from (a, b) C–S–F–3% Al and (c, d) C–S–F–5% Na glasses leached at different pH.

The Raman spectra of the residues obtained by dissolving the C–S–F–5% Na glass under different pH conditions are shown in Figure 11(c). Leaching at various pH values notably influences the vibrational spectra and the distribution of structural units. At pH 4, the central peak near 920 cm⁻¹ shifted to 933 cm⁻¹, with significantly lower intensity than other residues. The split-peak fitting results on the right side of Figure 11(d) reveal a clear decreasing trend in the ratios of $Q_{Si}^{0}$ and $Q_{Si}^{1}$ structural units with decreasing pH, confirming that the dissolution of short-range structural units accompanied leaching. This behaviour is consistent in both Al₂O₃- and Na₂O-modifying glasses, as observed under various pH conditions.

Assuming that only CaO acts as NWM in the Al₂O₃-modified glassy phases, while Fe₂O₃, SiO₂, and Al₂O₃ all serve as NWF, the average number of bridging oxygens (BOs) was calculated based on equations (6) and (7) using the glass slag composition and Raman spectroscopy results, respectively. The calculated results are presented in Figure 12.

Q_{Cal .}^{ave .} = 4 - \frac{2 M_{CaO}}{M_{{SiO}_{2}} + M_{{Fe}_{2} O_{3}} + M_{{Al}_{2} O_{3}}}

(6)

Q_{Raman}^{ave .} = \frac{\sum S_{J} \times J}{\sum S_{J}}

(7)

Figure 12.

Relations between the average number of bridging oxygens in silicate tetrahedral and different Al₂O₃ or Na₂O contents.

Although the calculated results of the two methods for BO numbers showed minor deviations, their overall trends were consistent, validating the previous hypothesis that Fe₂O₃ tends to act as NWF in the glassy phase structure, thereby further inhibiting dissolution behaviour. After Al₂O₃ modification, the number of BOs in the structure exhibited an upward trend, indicating enhanced structural stability and resulting in lower dissolution ratios, which aligns with the results in Figure 3. Additionally, the addition of Na₂O did not significantly reduce the number of BOs in the structure, suggesting that Na₂O modification cannot promote network depolymerisation.

Through the study of the dissolution behaviour of glassy phase samples with the addition of Na₂O and Al₂O₃, it was found that the Al₂O₃ addition promoted the stability of the silicate structure in the glassy phase, thereby inhibiting its dissolution in dilute acid solutions. After adding a small amount (3%) of the Na₂O content, the dissolution ratios of various elements in the glassy phase increased slightly. However, when the Na₂O addition increased to 6%, the dissolution ratios decreased significantly. Whether adding Na₂O while keeping the contents of other components constant or Na₂O substitution for CaO would result in the formation of a more stable CaO–SiO₂–Fe₂O₃–Na₂O structure in the slag sample, which inhibited the dissolution reaction.

Conclusion

This study synthesised CaO–SiO₂–Fe₂O₃ glassy phases with different Al₂O₃ and Na₂O contents to explore their dissolution behaviour and structural mechanism, providing theoretical support for the high-value agricultural utilisation of dephosphorisation slag.

Al₂O₃ acts as an NWF in the glassy phase, which elevates the DOP of the silicate network and enhances the structural stability of the glassy phase; the dissolution ratios of Ca, Si, Fe and other elements decrease continuously with the increase of Al₂O₃ content.

Na₂O exerts a dual regulatory effect on the dissolution of the glassy phase: 3% Na₂O addition moderately promotes the dissolution of each element, while further increasing Na₂O content to 6% or substituting Na₂O for CaO significantly inhibits dissolution. Na₂O interacts with NBOs in silicate tetrahedra to form additional bonds, strengthening the glass network and reducing element leaching efficiency.

Fe₂O₃ tends to act as an NWF in the glassy phase structure, which further inhibits the dissolution behaviour of the glassy phase; the calculation of the BO number verifies the structural stability regulation mechanism of Al₂O₃ and Na₂O on the glassy phase.

Solution pH is a key factor regulating the dissolution of modified glassy phases; the dissolution ratios of all elements increase significantly with the decrease of pH, and the dissolution is almost inhibited when pH rises to 6.

Footnotes

ORCID iD

Chuanming Du

CRediT authorship contribution statement

Jialin Jiang: Investigation, formal analysis and writing – original draft, reviewing and editing. Chuanming Du: Conceptualisation, investigation, supervision, resources and project administration. Xinyi Huo: Investigation, formal analysis and supervision. Yibo Wei: Investigation and formal analysis.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (52474433) and the Liaoning Province Science and Technology Plan Joint Program (Key Research and Development Program Project) (2023JH2/101800058).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

References

Yang

. Tracing China’s steel use from steel flows in the production system to steel footprints in the consumption system. Renew Sust Energ Rev 2023; 172: 113040.

Jiang

, et al. FePO₄ preparation and residue recycling by co-processing of steelmaking slag and steel pickling waste liquor: comprehensive utilization of metallurgical wastes. CEJ 2023; 478: 147491.

Fan

, et al. Acid-leaching separation of phosphorus from the BOF slag modified with Al₂O₃. J Environ Chem Eng 2022; 10: 108394.

Guo

Bao

Wang

. Steel slag in China: treatment, recycling, and management. Waste Manage 2018; 78: 318–330.

Jiang

, et al. Efficient extraction of phosphate from dephosphorization slag by hydrochloric acid leaching. J Cleaner Prod 2022; 332: 130087.

, et al. Effect of BOF slag modification on the dissolution behavior of phosphorus from practical dephosphorization slag. Metal Mater Trans B 2022b; 53: 3635–3647.

Zhao

, et al. Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash. J Hazard Mater 2015; 283: 423–431.

Teratoko

Maruoka

Shibata

, et al. Dissolution behavior of dicalcium silicate and tricalcium phosphate solid solution and other phases of steelmaking slag in an aqueous solution. High Temp Mater Process 2012; 31: 329–338.

Yokoyama

Kubo

Mori

, et al. Separation and recovery of phosphorus from steelmaking slags with the aid of a strong magnetic field. ISIJ Int 2007; 47: 1541–1548.

10.

Gwon

Khan

Alam

, et al. Environmental risk assessment of steelmaking slags and the potential use of LD slag in mitigating methane emissions and the grain arsenic level in rice (Oryza sativa L.). J Hazard Mater 2018; 353: 236–243.

11.

Wang

Cai

. Steel slag as an iron fertilizer for corn growth and soil improvement in a pot experiment. Pedosphere 2006; 16: 519–524.

12.

Thomas

Iss

Modeling of the continuous casting of steel past, present and future

Pheonix, AZ: Iron & Steel Soc.,. 59th electric furnace conference/19th process technology conference. Electric furnace conference-proceedings, 2001, Dec., pp.3–30. https://doi.org/10.1007/s11663-002-0063-9.

13.

Kimio

. Steelmaking slag for fertilizer usage. Nippon Steel Sumitomo Metal Tech Rep 2015; 109: 130–136.

14.

Fix

Heymann

Heinke

. Subsolidus relations in the system 2CaO·SiO₂–3CaO·P₂O₅. J Am Ceram Soc 1969; 52: 346–347.

15.

Kong

, et al. Dissolution behavior of nutrient elements from dephosphorization slag in the weakly acidic solution. J Mater Cyc Waste Manage 2022; 24: 259–267.

16.

Futatsuka

Shitogiden

Miki

, et al. Dissolution behavior of nutrition elements from steelmaking slag into seawater. ISIJ Int 2004; 44: 753–761.

17.

Maruoka

, et al. Improvement of desalted paddy soil by the application of fertilizer made of steelmaking slag (recovery of a paddy field damaged by the tsunami using fertilizer made of steelmaking slag-1). Tetsu-to-Hagane 2015; 101: 445–456.

18.

Furukawa

Inubushi

. Feasible suppression technique of methane emission from paddy soil by iron amendment. Nutr Cycl Agroecosys 2002; 64: 193–201.

19.

Tozsin

Oztas

. Utilization of steel slag as a soil amendment and mineral fertilizer in agriculture: a review. J Agr Sci-Tarim Bili 2023; 29: 906–913.

20.

Gao

Maruoka

Kim

, et al. Dissolution behavior of nutrient elements from fertilizer made of steelmaking slag, in an irrigated paddy field environment. J Sust Metall 2015; 1: 304–313.

21.

Koizumi

Gao

Sukenaga

, et al. Influence of glass structure on the dissolution behavior of Fe from glassy slag of CaO–SiO₂–FeO_x system. J Sust Metall 2021; 7: 583–596.

22.

Gao

Ueda

, et al. Effect of Na₂O addition on phosphorus dissolution from steelmaking slag with high P₂O₅ content. J Sust Metall 2017; 3: 671–682.

23.

Kim

Park

, et al. A study on the effect of Na₂O on the viscosity for ironmaking slags. Steel Res Int 2010; 81: 17–24.

24.

Jiang

Wang

, et al. Dissolution behavior of valuable elements from the glassy phase of CaO–SiO₂–FeO_x system in weakly acidic solution. JOM 2025; 77: 1–15.

25.

Tian

Wang

Zhao

, et al. A Raman and multinuclear ²⁹Si, ²⁷Al, and ¹⁹F NMR study on the structural roles of CaF₂ in SiO₂–CaO–Al₂O₃-based welding fluxes. Metall Mater Trans B 2022; 53: 232–241.

26.

Mysen

Virgo

Seifert

. Relationships between properties and structure of aluminosilicate melts. Am Mineral 1985; 70: 88–105.

27.

Wang

Zhang

Zhao

, et al. Structural transformation of molten CaO–SiO₂–Al₂O₃–Fe_xO slags during secondary refining of steels. ISIJ Int 2020; 60: 220–225.

28.

Lucazeau

Sergent

Pagnier

, et al. Raman spectra of apatites: La_10−xSi_6−y(Al,Fe)_yO_26±δ. J Raman Spectrosc 2007; 38: 21–33.

29.

Mead

Mountjoy

. A molecular dynamics study of the atomic structure of (CaO)_x(SiO2)_1−x glasses. J Phys Chem B 2006; 110: 14273–14278.

30.

McMillan

. Structural studies of silicate glasses and melts—applications and limitations of Raman spectroscopy. Am Miner 1984; 69: 622–644.

31.

Smedskjaer

, et al. Structure and properties of sodium aluminosilicate glasses from molecular dynamics simulations. J Chem Phys 2013; 139: 044507.

32.

Hayashi

Hori

Susa

, et al. Oxidation states and coordination structures of iron ions in silicate melts during relaxation process and at equilibrium. Phys Chem Glasses 2000; 41: 49–54.

33.

Lentz

. Silicate minerals as sources of trimethylsilyl silicates and silicate structure analysis of sodium silicate solutions. Inorg Chem 1964; 3: 574–579.

34.

Yang

Chen

. Study on the distribution of SiO₄⁴⁻ tetrahedral polymerization in silicate glasses—III. Polymerization structure of Na₂O–CaO–SiO₂ glasses. J Chin Ceram Soc 1989; 2: 165–172.

35.

Mills

Yuan

, et al. Review of the factors affecting the thermophysical properties of silicate slags. High Temp Mater Process 2012; 31: 301–321.

36.

Gao

Ueda

, et al. Recovery of phosphorus from modified steelmaking slag with high P₂O₅ content via leaching and precipitation. ISIJ Int 2018; 58: 833–841.

Dissolution behaviour of valuable elements from the CaO–SiO 2 –Fe 2 O 3 glassy phase modified with Na 2 O and Al 2 O 3 in an acidic solution

Abstract

Keywords

Introduction

Experimental method

Glassy phase synthesis

Leaching experiment

Results and discussion

Mineralogical composition

Effect of Al2O3 on dissolution behaviour of glassy phases

Effect of Na2O on the dissolution behaviour of glassy phases

Effect of Na2O substitution for CaO on dissolution behaviour of glassy phases

Effect of pH on dissolution behaviour of the glassy phase

Conclusion

Footnotes

ORCID iD

CRediT authorship contribution statement

Funding

Declaration of conflicting interests

Data availability

References

Effect of Al₂O₃ on dissolution behaviour of glassy phases

Effect of Na₂O on the dissolution behaviour of glassy phases

Effect of Na₂O substitution for CaO on dissolution behaviour of glassy phases