*3.3. Characterization of Residue*

The XRD patterns of Slag 2 and its residue after leaching is shown in Figure 5. For Slag 2, the characteristic peaks associated with C2S–C3P solid solution and the broad peaks of non-crystalline phase were observed, confirming the existence of two mineral phases. Following leaching, the characteristic peaks of C2S–C3P solid solution all disappeared, and the broad peaks of non-crystalline phase intensified, illustrating that the P-concentrating phase was fully dissolved. Figure 6 shows the morphology of residue after the leaching of Slag 2. This residue consisted of some irregular particles with single mineral phase. As listed in Table 3, this mineral phase mainly containing CaO, SiO2, and Fe2O3 was similar to the matrix phase. It indicates that only the matrix phase remains in the residue, which is consistent with the XRD results. The average composition of residue is shown in Table 4. Compared with the composition in Table 1, the P2O5 content decreased from 10.5% to 4.3%, proving that the C2S–C3P solid solution was effectively separated, while the Fe2O3 content increased from 19.6% to 31.0%.

**Figure 5.** XRD patterns of Slag 2 and its residue after leaching.

**Figure 6.** (**a**,**b**) Morphology of residue surface of Slag 2 (EPMA analysis).


**Table 3.** Average composition of mineral phases on the residue surface (mass%).

**Table 4.** Chemical composition of the residue of Slag 2 (mass%).


Overall, in this process, the P-containing leachate can be used to extract calcium phosphates by chemical precipitation, and the residue consisting of CaO–SiO2–Fe2O3 system has lower P2O5 content, which can be used as a flux in the steelmaking process. These results will promote the comprehensive utilization of steelmaking slag and ease environmental burden.

#### **4. Discussion**

To achieve the selective leaching of P, the dissolution behavior and mechanism of P from quenched steelmaking slag should be understood. Several kinds of cations and anions exist in the leachate. Since phosphate ions can precipitate with metallic ions, it is necessary to consider the possibility of phosphate formation. In the aqueous solution, hydroxyapatite (Ca10(PO4)6(OH)2) and strengite (FePO4·2H2O) readily precipitate when Ca2+ and Fe3+ ions coexist with phosphate ions [20]. The precipitation of these phosphates plays a significant role in determining the P concentration in the leachate. Therefore, we investigated the concentration relationship between phosphate ions and metallic ions in the aqueous solution. There are several types of phosphate ions in the aqueous solution depending on pH, including PO4 <sup>3</sup>−, HPO4 <sup>2</sup>−, and H2PO4 − [21]. In thermodynamic calculation, these phosphates ions were considered, and the activity coefficients of ions were assumed to be 1 because their concentrations were relatively low. The solubility lines of FePO4·2H2O and Ca10(PO4)6(OH)2 were calculated using the reaction equilibrium constants of Equations (2)–(5) at pH 4, respectively [22,23].

$$\text{FePO}\_4\cdot2\text{H}\_2\text{O}=\text{Fe}^{3+}+\text{PO}\_4^{3-}+2\text{H}\_2\text{O}\text{log}\text{K}=-26.07\tag{2}$$

$$\text{Ca}\_2(\text{PO}\_4)\_6(\text{OH})\_2 + 2\text{H}^+ = 10\text{Ca}^{2+} + 6\text{PO}\_4^{3-} + 2\text{H}\_2\text{O} \\ \log\text{K} = -62.42 \tag{3}$$

$$\mathrm{HCO}\_{4}^{3-} + \mathrm{H}^{+} = \mathrm{HPO}\_{4}^{2-} \log \mathrm{K} = 12.36\tag{4}$$

$$\mathrm{^{1}PO\_{4}^{3-}} + 2\mathrm{H}^{+} = \mathrm{H\_{2}PO\_{4}^{-}} \\ \log\mathrm{K} = 19.56\tag{5}$$

Figure 7 shows the relationship between P and Fe concentrations in the aqueous solution and experimental results. It was found that the solubility of FePO4·2H2O was very low at pH 4 and large amounts of phosphate and Fe3+ ions could not coexist in the aqueous solution. The saturation concentration of P decreased with the increase in Fe concentration. The P concentration in the leachate was high while the Fe concentration was very low, near zero. The points of experimental result all located above the solubility line of FePO4·2H2O, indicating that the P and Fe concentrations were supersaturated and FePO4·2H2O could precipitate. During leaching, the C2S–C3P solid solution dissolved well, and the dissolution of the Fe-containing matrix phase was low, resulting in high concentrations of Ca, Si, and P. As approximately 1.0% Fe2O3 existed in the solid solution, some Fe was also dissolved. However, it was difficult for these Fe3+ ions to coexist with phosphate ions, and then Fe3+ ions precipitated in the form of FePO4·2H2O. Owing to a small quantity of Fe dissolved from slag, the formation of FePO4·2H2O had little influence on the decrease in P concentration in the leachate. To make phosphate ions exist stably in the leachate, the dissolution of Fe from slag should be suppressed as much as possible.

**Figure 7.** Relationship between P and Fe concentrations in the aqueous solution and experimental results.

Because the Ca concentration was higher than other elements in the leachate, the possibility of Ca10(PO4)6(OH)2 formation was evaluated. As shown in Figure 8, it was difficult for the precipitation of calcium phosphates to occur in the aqueous solution at pH 4 unless concentrations on Ca and P were high. In this study, the maximum P concentration was 211.8 mg/L and it was far lower than the saturation concentration. The points of P and Ca concentrations were located far below the solubility line of Ca10(PO4)6(OH)2, illustrating that Ca concentration has no effect on phosphate precipitation. The P concentration in the leachate primarily depended on the Fe concentration. Under this condition, most of the P dissolved from slag existed stably in the leachate, proving that selective leaching of P from quenched steelmaking slag with high P2O5 content was possible, even in the hydrochloric acid solution.

To evaluate P-selective leaching from each slag, we compared the P distribution ratio in the C2S–C3P solid solution with the P dissolution ratio from slag. The mass fractions of solid solution and matrix phase were first calculated using Equations (6) and (7), where *α* and *β* are the mass fraction of solid solution and matrix phase, respectively, *w<sup>α</sup>* XO and *<sup>w</sup><sup>β</sup>* XO are the XO content in the solid solution and matrix phase, respectively. The mass fraction was defined as the average results of each oxide. Then, the P distribution ratio in the solid solution (*D*) was calculated using Equation (8).

$$w\_{\rm XO} = \alpha w\_{\rm XO}^a + \beta w\_{\rm XO}^\beta \tag{6}$$

$$
\alpha + \beta = 1 \tag{7}
$$

$$D = \frac{\alpha w\_{\text{P}\_2\text{Og}}^{\alpha}}{w\_{\text{P}\_2\text{Og}}} \tag{8}$$

**Figure 8.** Relationship between P and Ca concentrations in the aqueous solution and experimental results.

As shown in Figure 9, most of the P in slag was distributed in the C2S–C3P solid solution. With the P2O5 content increased, the P distribution ratio in the solid solution decreased, suggesting that the slag with lower P2O5 content facilitated P enrichment. 70.8% of P in Slag 4 containing 16.3% P2O5 was concentrated in the solid solution. Increasing Fe2O3 content was not beneficial for P enrichment. A higher slag basicity resulted in a higher P distribution ratio in the solid solution. For Slag 7 with low basicity, only half of the P in slag was distributed in the solid solution, which made it difficult to achieve P-selective leaching. In each case, the P dissolution ratio from slag was a little lower than the P distribution ratio in the solid solution. Its variation tendency was the same as that of P distribution ratio. One reason for this was considered to be that a small part of P-concentrating solid solution was not dissolved. The other reason was that a small amount of P dissolved from slag precipitated with Fe3+ ions, resulting in a little decrease in P dissolution ratio. Although the P dissolution ratio was not very high, the effective dissolution of P-concentrating solid solution from quenched steelmaking slag was achieved under this condition, similar with the furnace-cooled slags as reported in previous studies [16–18].

To better understand selective leaching, we weighted the mass of remained residue and compared that with the mass fraction of matrix phase. As shown in Figure 10, a large amount of slag was not dissolved under this condition, and the mass fraction of residue was almost equal to that of matrix phase in each slag. This proved that the vast majority of P-concentrating solid solution was dissolved, and the dissolution of matrix phase was difficult, which was consistent with the residue analysis. For Slag 8 with high basicity, the mass fraction of solid solution was high, and then a large amount of slag was dissolved. In summary, the P dissolution ratio from slag was mainly determined by the P enrichment in the C2S–C3P solid solution and phosphate precipitation in the leachate. The P-concentrating solid solution was effectively dissolved and separated from quenched steelmaking slag when hydrochloric acid was used as leaching agent.

**Figure 9.** P distribution ratio in the solid solution compared with P dissolution ratio from slag.

**Figure 10.** Mass fraction of matrix phase in each slag compared with that of residue.

#### **5. Conclusions**

To separate P from steelmaking slag using a simple and low-cost method, selective leaching of C2S–C3P solid solution was adopted, and hydrochloric acid was selected as leaching agent. In this study, the dissolution behavior of quenched steelmaking slags with different composition in the acidic solution was investigated. The results obtained are summarized below:


**Author Contributions:** Conceptualization, C.-M.D. and N.-N.L.; methodology, C.-M.D.; investigation, H.K. and Y.-H.Y.; resources, N.-N.L. and C.-M.D.; writing—original draft preparation, N.-N.L.; writing—review and editing, C.-M.D., H.K., and Y.-H.Y.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was Funded by the Open Project Program of Key Laboratory of Metallurgical Emission Reduction & Resources Recycling (Anhui University of Technology), Ministry of Education (No. JKF20-01), Fundamental Research Funds for the Central Universities (No. N2025005), the National Natural Science Foundation of China (No. 52074004), and Director Fund of Anhui Province Key Laboratory of Metallurgical Engineering & Resources Recycling (Anhui University of Technology).

**Data Availability Statement:** Data are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

