3.1.1. Principle and Operation Procedure
In a 10%HNO3 solution, the insoluble minerals present in steel slag include Fe3O4, refractory impurities such as quartz, corundum, and mullite, as well as an amorphous phase formed during the dissolution process. Initially, the amorphous phase is eliminated using a 20%NaOH solution. Subsequently, the TFe content of the sample is determined through chemical analysis after removing the crystalline phase to calculate the Fe3O4 content.
The determination process of Fe3O4 is divided into the following three steps:
Accurately weigh 1 g of sample S and place it in a beaker containing 80 mL of 10%HNO3 solution. Seal the beaker mouth with a plastic film and stir mechanically at 300 rpm for 3 h at a constant temperature of 30 °C. Filter the residue to obtain the sample residue, rinse it with distilled water several times, and dry it at 105 °C until constant weight.
The residue sample was mixed with 20%NaOH solution in a 100 mL beaker (Teflon) at a solid-to-liquid ratio of 1 g/4 mL. The beaker mouth was sealed with plastic film, and the mixture was mechanically stirred at 300 rpm at a constant temperature of 90 °C for 55~65 min. The filtered mixture was dried at 105 °C after being washed with deionized water at 75 °C several times. The weight of the residue was then measured.
The sample is fused with a mixture of sodium carbonate and boric acid after being dissolved in 10%HNO3 and 20%NaOH. The fused material is dissolved in hydrochloric acid to form a solution. First, ammonium chloride is added, followed by ammonia water, to precipitate the iron and aluminum elements in the solution. The precipitate is dissolved in hydrochloric acid to form a 250 mL solution. A volume of 100 mL of the solution is then taken and titrated with EDTA standard solution. The TFe content is calculated based on the volume of EDTA standard solution consumed. The content of the sample Fe3O4 is equal to the content of the Fe element in the sample multiplied by 1.38.
3.1.2. Feasibility Analysis
Sample S was dissolved in 10%HNO
3 solution at 30 °C for 3 h, and the residue sample obtained by extraction and filtration was labeled S
1, and the XRD pattern of S
1 was shown in
Figure 2. The sample S
1 contains two main mineral phases: a crystalline phase and an amorphous phase. The crystalline phase consists of mullite (3Al
2O
3∙2SiO
2), quartz (SiO
2), corundum (α-Al
2O
3), as well as refractory impurities mixed with Fe
3O
4 from the iron and steel smelting process, while other minerals in the steel slag are completely dissolved. This finding is consistent with the literature [
23]. The main chemical reaction equations during the dissolution of steel slag powder with a 10%HNO
3 solution are shown in Equations (1)–(5).
C
2S and C
3S react with HNO
3:
The main chemical composition of the RO phase is FeO and MgO, so the RO phase reacts with HNO
3:
In order to verify the preservation of Fe
3O
4 and refractory impurities in a 10%HNO
3 solution, four pure materials, including Fe
3O
4, quartz, corundum, and mullite, were dissolved in the aforementioned solution following the specified procedure. The residual rates of these materials were then determined and are presented in
Table 2. It is noteworthy that all four pure materials exhibit residual rates equal to or exceeding 99.98%, indicating that no reaction occurred between them and the 10%HNO
3 solution at a temperature of 30 °C.
In the diffraction angle range of 16~30°, a diffuse peak envelope is observed, which corresponds to the X-ray scattering caused by the presence of an amorphous phase. This phenomenon can be attributed to the decomposition of silicate minerals within the reaction structure with HNO
3, resulting in relatively compact aggregation of SiO
2 particles and subsequent local concentration leading to dense precipitates. Additionally, newly formed silica gel is generated during dissolution. The bonding structure of aluminate minerals is mainly composed of silico–oxygen bonds and alumino–oxygen bonds, which exist in the form of [AlO
4] tetrahedra or [AlO
6] coordination polyhedra [
23]. Under acidic conditions, the tetrahedral [AlO
4] or [AlO
6] species undergoes decomposition by H
+ ions, resulting in the formation of small molecular groups such as Al(OH)
4+. Eventually, these dissolved Al(OH)
4+ species precipitate to form amorphous Al(OH)
3. The amorphous Al(OH)
3, along with silica gel and refractory impurities, is filtered together with Fe
3O
4 into the residue. This phenomenon has been confirmed through studies on the dissolution mechanism of steel slag minerals under acidic conditions [
24,
25].
In the original method [
21], the beaker was placed on the magnetic pole surface of a strong magnet for physical separation. Due to the magnetic properties of Fe
3O
4, it would adhere to the magnet’s surface while non-magnetic impurities were suspended above the mixed liquid and then poured out through horizontal shaking by artificial means. However, during actual operation, amorphous phases adhered to Fe
3O
4 surfaces, making it difficult to separate refractory impurities from Fe
3O
4 particles, resulting in a high determination of Fe
3O
4 content. Therefore, further exploration and improvement of the method for determining Fe
3O
4 content is necessary.
The principle of alkaline dissolution for dissolving the amorphous phase is that OH
- can effectively destroy the amorphous phase structure formed under acidic conditions, causing it to decompose rapidly and completely. Sodium hydroxide solution shows good selective dissolution in mineral phase separation processes [
26,
27]. Many researchers have reported the dissolution mechanism of amorphous phases in NaOH solution [
28,
29], and the literature has verified through experiments that most of the amorphous phases in fly ash are dissolved in a 20%NaOH solution, with the SiO
2 leaching rate reaching a maximum of 42.13% [
30].
The sample S
1 obtained after 10%HNO
3 treatment was dissolved in 20%NaOH solution to eliminate the influence of amorphous relative Fe
3O
4 content determination. Among them, the reaction equation of NaOH solution with silica gel and amorphous Al(OH)
3 is as follows:
The change of mineral phase types of the sample after dissolution by 20%NaOH solution at 90 °C for 1 h is shown in
Figure 3. By comparing the phase composition of the sample after dissolution of 10%HNO
3 solution and 10%HNO
3-20%NaOH solution, it is found that the peak envelope in the range of 16~30° disappears, and only the diffraction peak of crystal minerals is found in the figure.
In order to ensure that the NaOH solution does not affect mineral phases other than the crystal phase, four pure materials, including Fe
3O
4, quartz, corundum, and mullite, were dissolved in a 20%NaOH solution following the aforementioned procedure. The residual rates of these materials were determined and are presented in
Table 3. The dissolution rates of all four pure materials are found to be less than 0.03%. At a constant temperature of 90 °C, the 20%NaOH solution selectively dissolves only the amorphous phase in the residue; Fe
3O
4 does not react with the NaOH solution, while mullite can react and dissolve in dilute alkali solutions above 160 °C [
30]. Corundum possesses a stable crystal structure and typically remains unreactive towards acids and bases at both room temperature and high temperatures. Quartz is also stable under normal pressure at 90 °C and exhibits low reactivity towards NaOH solution.
According to the mass ratio of m(Fe3O4):m(SiO2):m(α-Al2O3):m(3Al2O3·2SiO2) = 7:1:1:2, the mixed samples of four pure materials were prepared. The mixed samples were dissolved by 20%NaOH, and the quality of the mixed samples basically did not change before and after dissolution. It is proven that there is no mutual interference between mineral components in the process of determination. Therefore, it can be determined that the amorphous phase is the only soluble component of NaOH.
Chemical analysis can calculate the content of mineral phases by determining the element content. For example, the
f-CaO content in steel slag can be determined by the ethylene glycol-EDTA chemical analysis method. According to the principle of
f-CaO reacting with ethylene glycol, calcium glycolate is generated, and the calcium glycolate content is determined by the consumption of EDTA, thereby allowing the
f-CaO content in the slag to be measured [
31]. Similarly, the
f-MgO content is mainly determined by the ethylene glycol-iodine ethanol method and the ammonium nitrate-ethanol method [
32,
33]. However, the titration methods used for the two mineral phases above are for the determination of the total Ca and Mg content in slag, which leads to an overestimation of the calculated mineral phase content. In this study, the impurity mineral phases in the steel slag treated with 10%HNO
3-20%NaOH were all oxides of silicon and aluminum and did not contain other Fe-containing minerals. Therefore, the Fe
3O
4 content was calculated by determining the TFe content of the treated sample through chemical analysis, which was more accurate than other chemical analysis methods for determining the mineral phase content.
In order to validate the feasibility of the chemical analysis method, the sample with the amorphous phase removed was selected as the research object, labeled S
1′, and the impurity chemical composition of sample S
1′ was studied by XRD and SEM-EDS analysis. In order to enrich refractory impurities, wet magnetic separation was performed on sample S
1′, and a comparison was made between the intensity of diffraction peaks in XRD patterns obtained from raw materials before magnetic separation and tailings after magnetic separation (
Figure 4). The intensity of the diffraction peak for Fe
3O
4 and refractory impurities shows a significant difference. Specifically, after magnetic separation, there is a decrease in the diffraction peak (35.44°PDF#97-015-9925) corresponding to the Fe
3O
4 mineral phase in the tailings compared with that of raw materials. On the contrary, there is an evident increase in diffraction peaks associated with quartz, corundum, and mullite mineral phases. This indicates that wet magnetic separation greatly affects Fe
3O
4 separation. Additionally, from
Figure 4, it can be observed that refractory material impurities become more concentrated in tailings with higher relative content; notably absent are any discernible diffraction peaks related to other iron-containing minerals besides Fe
3O
4.
The SEM images depict the mineral phase of raw materials before magnetic separation (
Figure 5a) and tailings after magnetic separation (
Figure 5b). The chemical composition analysis conducted using EDS is presented in
Table 4. From
Figure 5a, it can be observed that Fe
3O
4 is densely distributed throughout as bright white plates or small particles within a grayish-white base color. However,
Figure 5b shows a black base color with a significant decrease in bright white mineral particles. This indicates that some Fe
3O
4 present in the tailings samples has been separated as a concentrate during the magnetic separation process, leading to reduced content within the tailings.
The refractory impurities of quartz and mullite are distributed more uniformly in the visual field. Under SEM, quartz appears as dark gray slab-shaped structures, while mullite exhibits a light gray distribution in small block or elliptical granular forms. Corundum is observed as light gray strips with relatively lower content distribution. According to the SEM image analysis, the refractory impurities in the tailings are enriched, and the types of impurity mineral phases do not change significantly. No other mineral phases are detected except quartz, corundum, and mullite, which is consistent with the XRD analysis results. Based on the chemical composition of these three refractory impurities’ mineral phases, it can be concluded that only a small amount of Fe2O3 exists in solid solution within these minerals (approximately 0.043 mol). Therefore, it is determined that these mineral phases are mixed into the steelmaking process or during stacking and transportation as refractory impurities rather than inherent impurities within the steel slag itself. Most of the Fe elements exist in the form of Fe3O4 mineral phases. Estimating Fe3O4 content by measuring TFe content is considered feasible.
The improved method was used to determine the Fe
3O
4 content in sample S, which was found to be 5.16%. To verify the applicability of the method to steel slag samples and the stability of the data, the following linear extrapolation mathematical model was used for calculation and verification. Steel slag powder was mixed with Fe
3O
4 sample (purity 99.7%) in a ratio of 9:1. The theoretical calculated value is 5.16% × 0.9 + 99.70% × 0.1 = 14.61%. When repeating the experiment three times, the actual average value determined was 14.54%. As shown in the
Figure 6, the maximum error between the measured and theoretically calculated values is less than 0.50% for all three determinations. The measured values are stable and consistent with the calculated values, which proves that this method is applicable to the determination of Fe
3O
4 content in steel slag.
3.1.3. Parameter Selection
In order to design an appropriate process flow to achieve complete dissolution of the amorphous phase, it is crucial to clarify the dissolution characteristics of the amorphous phase under different conditions. The reaction temperature and reaction time are important operating parameters in the alkaline solution process. Research in reference [
34,
35] shows that when the reaction temperature is high (60 °C and 75 °C), the reaction degree of the amorphous phase of fly ash can reach the theoretical maximum value after about 336 h of reaction, while at lower temperatures (20 °C and 40 °C), the reaction degree can only reach 14.1% and 39.1%. Prolongation of the reaction time may promote the production of silicoaluminate and reduce the dissolution of the amorphous phase. In this paper, based on the existing literature, the experiment was conducted in the range of high temperatures, 60~90 °C, so that the dissolution rate was relatively improved so as to achieve the purpose of completely dissolving the amorphous phase in a short time.
To eliminate the influence of the amorphous phase on the determination of Fe
3O
4 content, the amorphous phase was dissolved in 20%NaOH solution. Sample S
1 is the residue sample obtained after the dissolution of 10%HNO
3 solution. Under the condition of an alkaline dissolution time of 1 h, the residual rate of sample S
1 is determined in the range of 60–100 °C, and the optimal alkaline dissolution temperature is determined. As can be seen from
Table 5, when it is set at 90 °C, the dissolution amount is basically stable, and the amorphous phase of the sample is completely dissolved, as detected by XRD. The maximum solubility has been reached.
Figure 7 shows the change of sample residual rate after sample S
1′ reacts in 20%NaOH solution at 90 °C for 20~90 min. It can be seen from the figure that the residual rate decreases with the increase of time within 65 min and reaches 69.73% with the increase of reaction time at 20 min. The residue rate decreased by 5.58% and reached the minimum residue rate of 64.15% at 55 min~65 min. However, after 65 min, the residue rate increased slightly. In order to analyze the reasons for the trend change, XRD analysis was conducted on the solid samples obtained at the above time points.
Figure 8 shows the XRD patterns of the products obtained with 20%NaOH solution at 90 °C at different reaction times. When the reaction time reached 20 min, the peak envelope uplift area collapsed obviously, and most of the amorphous phase reacted with NaOH into the solution. When at 55–65 min, the residual rate was stable and reached the maximum, and the amorphous phase was completely dissolved in 20%NaOH solution. When the reaction time reached 70 min, the diffraction peak of the aluminosilicate phase appeared. This is because, with the increase of alkali dissolution time after the amorphous phase is completely dissolved, the content of SiO
32− and AlO
2− in the solution increases and further combines to form aluminosilicate [
30], which increases the quality of the solid sample. In order to ensure that the amorphous phase is fully dissolved and prevent the secondary reaction from generating new impurity minerals, 55 min to 65 min is selected as the alkali dissolution time.