Effects of Cooling Rate and Acid on Extracting Soluble Phosphorus from Slag with High P₂O₅ Content by Selective Leaching

Abstract

With the use of low-grade iron ores with high P content, slag with high P₂O₅ content generated after dephosphorization is considered a great potential source of P. Because of the solubility difference between the solid solution and matrix phase, it is possible to extract the P-rich solid solution selectively from slag by leaching. The soluble P obtained is suitable to produce phosphate fertilizers. To achieve selective leaching of P and increase its dissolution ratio, the effects of the cooling rate and acid on dissolution of the slag in aqueous solutions at pH 5 and 7 were investigated. This study found that during solidification, slow cooling facilitates coarsening of the solid solution and formation of the magnesioferrite phase. The solid solution was dissolved preferentially. At pH 7, the air-cooled slag showed the highest dissolution ratio of P. When the pH was decreased to 5, slag dissolution was significantly promoted. As the cooling rate decreased, the dissolution ratio of P increased. Slow cooling not only enhanced dissolution of the solid solution but also suppressed dissolution of the matrix phase. Citric acid performed better in promoting dissolution of slag. At pH 5, almost all of the solid solution was dissolved from the furnace-cooled slag. However, the dissolution ratio of Fe was also high. When nitric acid was used, 66.8% of the solid solution was dissolved, without dissolution of large amounts of the matrix phase. After leaching, the P₂O₅ content in the residue reduced and the Fe₂O₃ content increased.

1. Introduction

Steelmaking slag generated from hot metal dephosphorization is rich in valuable phosphorus (P). With the use of low-grade iron ores with high P content, the P content in hot metal will increase, resulting in the generation of slag with high P₂O₅ content.¹⁾ Because of the amounts of steelmaking slag produced and depletion of high-grade phosphate ores, slag with high P₂O₅ content is considered a great potential source of P, and development of an economical recovery process is highly important for the steelmaking industry. Dephosphorization slag is generally saturated with 2CaO·SiO₂, which acts as a sink for P and forms a P-rich 2CaO·SiO₂-3CaO·P₂O₅ (C₂S–C₃P) solid solution.^2,3) The high distribution ratio of P between the C₂S–C₃P solid solution and CaO–SiO₂–Fe_tO matrix phase indicates that P is enriched in the solid solution.⁴⁾ Therefore, P recovery actually focuses on separation of the C₂S–C₃P solid solution from other phases in slag.

On the basis of the difference between the C₂S–C₃P solid solution and other phases, extensive studies have investigated recovery of P from slag using different methods. Ono et al.⁵⁾ tried to separate the P-rich dicalcium silicate phase using the density difference and showed that the solid solution, which is less dense, floated up during solidification and separated from the other phases. Yokoyama et al.⁶⁾ and Kubo et al.⁷⁾ proposed a magnetic-separation method to recover P using the difference in the magnetic properties. Miki et al.⁸⁾ studied the separation of the solid 2CaO·SiO₂ phase and FeO-rich liquid phase by capillary action in sintered CaO. However, the recovery efficiency of these trials was inadequate and application to industry was not successful.

Leaching is an effective, energy-saving, and easily controlled method of extracting valuable elements from ores and is widely applied in non-ferrous metallurgy. Removing P from iron ores with a high P content by leaching has also been studied to improve the iron ore grade.⁹⁾ Patrick et al.¹⁰⁾ demonstrated that P-rich dicalcium silicates can be selectively removed from iron ore sinter. Teratoko et al.¹¹⁾ showed the solubility of solid solution in aqueous solutions by changing the ratio of C₂S to C₃P and proposed selective leaching of P from steelmaking slag using the solubility difference between the solid solution and matrix phase. In addition, they conducted experiments using slag that consisted of the solid solution and a matrix phase containing Fe oxide. Their results clarified that dissolution was strongly inhibited when FeO was used as the Fe oxide. Numata et al.¹²⁾ investigated the dissolution behavior of artificially made steelmaking slag of the CaO–SiO₂–Fe₂O₃–P₂O₅ system in aqueous solutions at a constant pH of 3 to 7, controlled using nitric acid (HNO₃) and clarified that the solid solution was dissolved selectively from slag. However, the dissolution ratio of P was not high enough under those conditions.

If the C₂S–C₃P solid solution can be efficiently separated from slag with a high dissolution ratio of P by acid leaching, this would notably affect the utilization of steelmaking slag as an alternative source of natural phosphate ores for production of phosphate fertilizer. In addition, the undissolved matrix phase with higher Fe_tO and lower P₂O₅ content can be reused as a flux in the steelmaking process.

We have investigated the dissolution behavior of a C₂S–C₃P solid solution with high P₂O₅ content at pH 5 and 7 and revealed the effects of acid and modification by Na₂SiO₃.¹³⁾ In this research, the dissolution ratio was greatly improved by using citric acid (H₃C₆H₅O₇), instead of nitric acid. However, the dissolution behavior of slag consisting of not only the solid solution but also other phases was not investigated.

In this study, to achieve selective leaching of P and increase its dissolution ratio, the effects of the cooling rate and choice of acid on the dissolution behavior of artificially made slag with a high P₂O₅ content were investigated at pH 5 and 7. It can be considered that the cooling rate significantly affects not only the size of the crystals but also the fraction of each mineralogical phase. For example, under rapid cooling, the liquid phase is solidified to form a glassy phase, but under slow cooling, various phases can be crystallized.^14,15) To investigate the effect of cooling on dissolution of slag, the leaching behaviors of slags at different cooling rates in citric acid and nitric acid solutions were examined.

2. Experimental Method

2.1. Synthesis of Slag with High P₂O₅ Content

Reagent-grade CaCO₃, SiO₂, Ca₃(PO₄)₂, Fe₂O₃, and MgO were used to synthesize CaO–SiO₂–Fe₂O₃ system slag. In this study, Fe₂O₃ was used as the Fe oxide because it has been clarified that FeO suppresses dissolution of the solid solution.¹¹⁾ First, to produce CaO, CaCO₃ was calcined at 1273 K for 10 h in air. Then, the obtained CaO was fully mixed with other reagents according to the slag composition (as shown in Table 1). The slag basicity (mass% CaO/mass% SiO₂) was set to 1.6, and 8.0 mass% of P₂O₅ was added to the slag. Considering the liquidus temperature of slag according to the CaO–SiO₂–Fe₂O₃ ternary phase diagram, the heat treatment pattern was determined, as shown in Fig. 1. During heating, the homogeneous liquid phase forms at 1823 K, and the C₂S–C₃P solid solution can precipitate during cooling to 1623 K. 10 g of mixture was placed in a Pt crucible and heated to 1823 K in air. After being held at 1823 K for 1 h, the slag was cooled to 1623 K at a cooling rate of 3 K/min and held at this temperature for 20 min. Next, one of three cooling methods was adopted: water quenching, air cooling, or furnace cooling. In water quenching, the slag was cooled quickly by water after being held at 1623 K. In air cooling, the crucible was placed on a refractory brick and cooled in air. In furnace cooling, slag was cooled to 1323 K by a furnace at a cooling rate of 5 K/min. In this case, the slag was withdrawn from the furnace at 1323 K, because the change in the mineralogical structural at temperatures below 1373 K is negligible.¹⁵⁾ These slags were analyzed by X-ray diffraction (XRD) analysis and electron probe microanalysis (EPMA).

Table 1. Composition of slag with high P₂O₅ content (mass%).

Composition	CaO	SiO2	P2O5	Fe2O3	MgO
Mass ratio (%)	37.0	23.0	8.0	29.0	3.0

Fig. 1.

Experimental conditions for synthesizing slag.

2.2. Leaching Experiments

The prepared slag was ground into fine particles smaller than 53 μm. Then, the slag (1 g) was added to a container that held 400 mL of deionized water. The aqueous solution was agitated by a rotating stirrer and kept at 298 K using an isothermal water bath. Further, acid was automatically added to the aqueous solution by a PC control system to maintain the pH value at 5 or 7, because dissolution of the slag increased the pH of the aqueous solution. Nitric acid (0.1 mol/L) and citric acid (0.1 mol/L) were selected as the leaching agent to control the pH. Leaching apparatus is the same as that used in previous research.¹³⁾ During leaching, about 5 mL of aqueous solution was sampled at adequate intervals within 120 min and filtrated using a syringe filter. The concentrations of each element in the sampled aqueous solution were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). After leaching, all the aqueous solution in the container was filtered to collect the residue. The obtained residue on the filter was dried and weighed. Then, its surface morphology and composition were observed by EPMA and XRD. The chemical composition of the residue was also determined by ICP-AES. In this study, leaching experiments under 12 conditions were performed to investigate the effects of the choice of acid and cooling rate on the dissolution of slag under different pH conditions, as shown in Table 2.

Table 2. Leaching experiments at various pH values (pH).

Sample Acid	Slag A (water quenching)	Slag B (air cooling)	Slag C (furnace cooling)
Nitric acid (HNO3)	5, 7	5, 7	5, 7
Citric acid (H3C6H5O7)	5, 7	5, 7	5, 7

3. Results

3.1. Mineralogical Composition of Slag

Typical cross sections of slag with a high P₂O₅ content cooled at different cooling rates are shown in Fig. 2. Table 3 lists the average chemical composition of each phase. The water-quenched slag consisted mainly of two phases. The black phase, which contains about 30% P₂O₅, is the C₂S–C₃P solid solution. The other phase of the CaO–SiO₂–Fe₂O₃ system is considered to be the matrix phase. The high distribution ratio of P₂O₅ between the solid solution and matrix phase indicates that most of the P is concentrated in the solid solution. In addition to some large solid solution particles, many fine solid solution particles were also precipitated in the slag. When the cooling rate was decreased, another phase formed, and the slag consisted mainly of three phases. In the air-cooled slag, large amounts of small white particles that are rich in Fe₂O₃ and MgO were widely distributed in the matrix phase. Owing to the beam size (1 μm) limitation of EPMA, precise analysis of these white particles is difficult, and the analysis results may be influenced by the surrounding phase. Because the composition of the solid solution differed little from that observed in the water-quenched sample, the magnesioferrite phase is thought to be formed by decomposition of the matrix phase, which results in a decrease in the Fe₂O₃ content of the matrix phase. Three phases with large crystals are distinctly observed in the furnace-cooled slag, and fine solid solution particles did not exist. These observations indicate that slow cooling facilitates coarsening of the C₂S–C₃P solid solution and formation of the magnesioferrite phase.

Fig. 2.

Cross sections (×500) of the slags at different cooling rates.

Table 3. Average compositions of each phase in the slags at different cooling rates (mass%).

Phase	Slag	CaO	SiO2	P2O5	Fe2O3	MgO
(1) Matrix phase	Slag A	31.0	27.1	2.5	35.2	4.2
	Slag B	43.1	38.3	1.6	14.9	2.1
	Slag C	39.1	35.9	1.8	21.5	1.7
(2) Solid solution	Slag A	54.9	12.0	31.1	0.9	1.1
	Slag B	55.0	12.2	31.1	0.8	0.9
	Slag C	54.2	12.6	31.7	0.8	0.7
(3) Magnesioferrite	Slag B	8.8	7.5	0.4	74.8	8.5
	Slag C	1.2	0.1	0.0	88.1	10.6

The obtained XRD patterns are given in Fig. 3. The patterns reveal that each slag consists of the C₂S–C₃P solid solution phase. The magnesioferrite phase is also identified. Weak peaks of this phase were detected in the water-quenched slag, although the magnesioferrite phase was difficult to observe in the EPMA image. In addition, in the water-quenched slag, an amorphous phase apparently formed, as every peak was very weak. The degree of crystallinity was estimated using the intensity relations between the “crystalline peaks” and the “amorphous background”.¹⁶⁾ In the case of slow cooling, the peaks of the solid solution and magnesioferrite phase intensified, and the area of amorphous background decreased. This result indicates that as the cooling rate decreased, the crystallinity of the slag increased.

Fig. 3.

XRD patterns for the slags at different cooling rates.

On the basis of the above EPMA results, the mass fractions of each phase in the slags were calculated using Eqs. (1) and (2):   

$$\sum {(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}^i\times X^i={(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}_{Slag}$$(1)

  

$$\sum {\text{X}}^i=1$$(2)

The mass balance of each oxide can be written using Eq. (1), where Xⁱ is the mass fraction of phase i, (mass% MO)ⁱ is the content of oxide MO in phase i, and (mass% MO)_Slag is the content of oxide MO in the slag. The average mass fractions of each phase were calculated and are listed in Table 4. The mass fraction of the solid solution in the water-quenched slag is about 23%, and it changes little with decreasing cooling rate.

Table 4. Mass fractions of each phase and each oxide existing in each phase (%).

Sample		Mass fraction	Mass fractions of each oxide
Phase	Slag		CaO	SiO2	P2O5	Fe2O3	MgO
Matrix phase	Slag A	77.5	66.1	88.7	21.5	99.3	93.0
	Slag B	47.9	56.9	78.5	9.3	25.1	27.9
	Slag C	57.8	63.1	87.2	12.0	43.6	32.1
Solid solution	Slag A	22.5	33.9	11.3	78.5	0.7	7.0
	Slag B	23.9	36.2	12.5	89.4	0.7	5.9
	Slag C	24.1	36.3	12.7	88.0	0.7	5.5
Magnesioferrite	Slag B	28.2	6.9	9.0	1.3	74.2	66.2
	Slag C	18.1	0.6	0.1	0.0	55.7	62.4

The mass fractions of each oxide existing in each phase were evaluated as:   

$$Y_{MO}^i=\frac{X^i\cdot {(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}^{\text{i}}}{{(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}_{Slag}}$$(3)

The results are also listed in Table 4, where, $Y_{MO}^i$ is the mass fraction of oxide MO in phase i. Although the mass fraction of the solid solution is not high, most of the P₂O₅ is distributed in the solid solution because of its high P₂O₅ content. As the cooling rate decreased, the mass fraction of P₂O₅ in the solid solution increased slightly. About 90% of the P is concentrated in the solid solution in the air-cooled and furnace-cooled slags. The cooling rate has little influence on the distribution of CaO and SiO₂, most of which are concentrated in the matrix phase. Fe₂O₃ and MgO are distributed mainly in the magnesioferrite phase.

3.2. Leaching Results

3.2.1. Dissolution Behavior of Slag at pH 7

The changes in concentration of P, Ca, and Fe in the aqueous solution with time are shown in Fig. 4. For all the elements in each case, the dissolution rates were initially fast, but after 60 min, the concentrations exhibited little change. When nitric acid was used as the leaching agent, the air-cooled slag had the highest P concentration in the aqueous solution, and the water-quenched and furnace-cooled slags showed similar content. The Ca concentration of the water-quenched and air-cooled slags was almost same, but that of the furnace-cooled slag was lower. In each slag, the Fe concentration was extremely low. In the citric acid solution, the concentrations of each element were higher than those in the nitric acid solution. The difference among the slags exhibited almost the same trend as that observed in the nitric acid solution. The air-cooled slag showed the highest concentrations of P and Ca. Fe was detected in the citric acid solution, although its content was only several mg/L.

Fig. 4.

Changes in the concentration of each element in nitric acid and citric acid solutions at pH 7.

On the basis of the above results, the dissolution ratios of each element from slag were calculated as:   

$$R_{\text{M}}=\frac{C_M\cdot V}{m_{\text{M}}}$$(4)

where R_M is the dissolution ratio of element M, C_M is the final concentration of element M (mg/L), V is the final volume of the aqueous solution (L), and m_M is the original mass of element M in the initial slag (mg). The calculated dissolution ratios of each element after 120 min are shown in Fig. 5. In the nitric acid solution, the dissolution ratios of Ca, Si, and Mg decreased with decreasing cooling rate, but that of P showed a different result. The dissolution ratio of P from the air-cooled slag was the highest. Fe did not dissolve in the nitric acid solution. When citric acid was used, the dissolution ratios of all the elements increased. Therefore, citric acid is considered an effective leaching agent to promote dissolution of slag. Similar to the case of leaching in the nitric acid solution, as the cooling rate decreased, the dissolution ratios of Si, Fe, and Mg decreased. However, the dissolution ratios of Ca and P from the air-cooled slag were the highest, reaching about 30% and 40%, respectively. Fe also dissolved from each slag but its dissolution ratio was low. In the case of furnace cooling, the dissolution ratio of P was far higher than that of other elements indicating that the P-rich solid solution was selectively dissolved compared with other phases. Overall, the air-cooled slag showed the best leaching result at pH 7, but with decreasing in cooling rate, the dissolution ratios of Ca, Si, and Mg decreased.

Fig. 5.

Dissolution ratios of each element from slag at pH 7.

3.2.2. Dissolution Behavior of Slag at pH 5

Figure 6 shows the concentrations of each element as a function of time. The concentrations under each condition were much higher than those at pH 7 in Fig. 4. When nitric acid was used, the furnace-cooled slag had the highest P concentration. As the cooling rate increased, the P concentration decreased significantly. The Ca concentration of the furnace-cooled slag showed the lowest value, but the difference between the air-cooled and water-quenched slags were small. The Fe concentration was very low regardless of the cooling rate. Citric acid performed better in promoting dissolution of slag, resulting in higher concentrations of each element. The dissolution behavior of P was the same as that in the nitric acid solution. The Fe concentration reached a higher level. During leaching of the furnace-cooled slag, the dissolution rates of Ca and Fe were initially lower than that of P, but the Ca and Fe concentrations increased continuously during the leaching period. The result also shows that the P-rich solid solution dissolved preferentially in the initial period.

Fig. 6.

Changes in the concentration of each element in nitric acid and citric acid solutions at pH 5.

Figure 7 shows the calculated dissolution ratios of each element from slag after 120 min. A comparison with the results in Fig. 5 shows that dissolution of slag was significantly promoted by decreasing the pH. In the nitric acid solution, as the cooling rate decreased, the dissolution ratios of Ca, Si, and Mg decreased, but that of P showed the opposite result. The dissolution ratio of P from the furnace-cooled slag increased to 58.4%. Fe cannot dissolve in the nitric acid solution, even at pH 5. In the citric acid solution, most of the Ca, Si, and P were dissolved. The dissolution ratios of Ca and Si from the air-cooled slag were the highest, but the furnace-cooled slag showed the lowest value. The dissolution ratio of P increased with decreasing cooling rate. In the case of furnace cooling, 82.2% of the P was dissolved from the slag. However, the dissolution ratio of Fe reached 20%, and there is little difference in the dissolution ratios of Fe in each slag. In conclusion, slow cooling not only facilitates dissolution of P but also suppresses dissolution of Ca and Si. The furnace-cooled slag showed the best leaching result at pH 5. Better selective leaching of P occurred in the nitric acid solution because Fe cannot dissolve. Although a higher dissolution ratio of P was obtained by using citric acid, large amounts of other elements, especially Fe, can also dissolve.

Fig. 7.

Dissolution ratios of each element from slag at pH 5.

3.2.3. Residue after Leaching

The morphology and composition of the residue after leaching in the citric acid solution at pH 5 were analyzed, as shown in Fig. 8 and Table 5. Many small holes were observed on the residue surface of the water-quenched and air-cooled slags. The composition of the undissolved part (Area 1) was almost the same as that of the matrix phase before leaching, and the P₂O₅ content in these holes (Area 2) was lower. The C₂S–C₃P solid solution was not observed. Therefore, the dissolved area was considered to be the solid solution. However, the furnace-cooled slag exhibited some large holes on the residue surface, and only the magnesioferrite phase and matrix phase were observed. The small holes on the surface disappeared. This result showed that the P-rich solid solution aggregated and became coarser under slow cooling, and then could be selectively removed from slag by leaching.

Fig. 8.

Morphology of the residues after leaching in the citric acid solution at pH 5.

Table 5. Compositions of residue surface after leaching in the citric acid solution at pH 5 (mass%).

Residue		CaO	SiO2	P2O5	Fe2O3	MgO
Slag A	1	26.5	25.4	1.9	34.6	4.4
	2	25.8	16.1	1.4	34.7	0.8
Slag B	1	1.7	14.2	1.8	66.6	8.0
	2	29.8	21.6	2.1	23.8	1.5
Slag C	1	1.3	0.6	0.1	82.1	7.3
	2	38.8	32.8	1.9	17.9	1.4

4. Discussion

4.1. Effect of Acid

To better understand the effect of the choice of acid on selective leaching, the dissolution behavior of the furnace-cooled slag in each case is discussed, because it showed good leaching results. First, precipitation of the dissolved P during leaching is discussed. As discussed in previous research,¹³⁾ dissolved Ca²⁺ and H₂PO₄⁻ ions react easily and form Ca₁₀(PO₄)₆(OH)₂ (HAP) precipitate at a higher pH. This reaction and its equilibrium constant, which affect the concentrations of Ca and P in aqueous solutions, are given by Eq. (5):^17,18)   

$$\begin{array}{l}{\text{Ca}}_{10}{({\text{PO}}_4)}_6{\left(\text{OH}\right)}_2+14{\text{H}}^{+}=10{\text{Ca}}^{2+}+6{\text{H}}_2{\text{PO}}_4^{-}+2{\text{H}}_2\text{O}\\ logK=52.86\end{array}$$(5)

In the citric acid solution, because of the presence of citrate ions (C₆H₅O₇³⁻), some Ca²⁺ combined with them and existed in the form of the Ca(C₆H₅O₇)⁻ complex, as described in Eq. (6):¹⁹⁾   

$${\text{C}}_6{\text{H}}_5{\text{O}}_7^{3-}+{\text{Ca}}^{2+}=\text{Ca}{\left({\text{C}}_6{\text{H}}_5{\text{O}}_7\right)}^{-}\hspace{1em}logK=3.22$$(6)

The remaining Ca²⁺, which was not involved in forming this complex, is considered to be free Ca²⁺ and its concentration can be calculated using Eq. (6). Figure 9 shows the leaching results for the P and free Ca concentrations and the solubility line of HAP calculated using Eq. (5). At pH 7, the observed points for the Ca and P concentrations were both located below the solubility line, indicating that the P concentration did not reach saturation. At pH 5, the solubility line moved to the high concentrations of Ca and P. Therefore, in these cases, as HAP could not be formed, the dissolved P from slag was measured in the aqueous solution without precipitates.

Fig. 9.

Solubility line of HAP and leaching results for the free Ca and P concentrations.

To evaluate the dissolution ratio from each phase, it is assumed that P dissolved only from the solid solution. The dissolution ratio of P from the solid solution is given by:   

$$R_P^{SS}=\frac{C_P\cdot V}{m_P^{SS}}$$(7)

where $R_P^{SS}$ is the dissolution ratio of P from the solid solution, C_P is the final P concentration (mg/L), and $m_P^{SS}$ is the original P mass in the solid solution (mg). The dissolution of the magnesioferrite phase could be ignored because its dissolution ratio was far lower than that of other phases.²⁰⁾ Therefore, in this study, we considered only dissolution of the solid solution and matrix phase.

On the basis of the above assumptions, the dissolution ratios of the other elements from the matrix phase ($R_M^{Matrix}$) can be calculated as:   

$$R_M^{Matrix}=\frac{R_M\times {(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}_{Slag}-R_M^{SS}\times {(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO})}_{SS}\times X^{SS}}{\left(\text{mass\%}\mathrm{\hspace{0.17em} }\text{MO}{)}_{matrix}\times X^{Matrix}\right)}$$(8)

where ($R_M^{SS}$) is the dissolution ratio of M from solid solution, which is equal to $R_P^{SS}$. The result is shown in Fig. 10. At pH 7, almost all of the dissolved Ca, Si, and Mg were supplied from the solid solution, and the dissolution ratio of the matrix phase was very low, which shows selective leaching of the solid solution. Compared with that in nitric acid, the dissolution ratio of the solid solution in the citric acid solution was higher, reaching 28.9%. However, this value is not high enough for phosphorus recovery. When the pH was decreased to 5, dissolution of both the solid solution and matrix phase was significantly promoted. In the nitric acid solution, 66.8% of the solid solution was dissolved, and the dissolution ratio of the matrix phase was less than 10%; in particular, the Fe cannot dissolve. In the citric acid solution, dissolution of each phase was further promoted. The dissolution ratio of the solid solution increased to 93.4%. Further, about half of the matrix phase was also dissolved, resulting in significant dissolution of Fe. Therefore, to obtain effective leaching of P, slag should be leached at pH 5. Using nitric acid can dissolve a large part of the solid solution without dissolving the matrix phase, so it showed better selective leaching, but the dissolution ratio of the solid solution was still inadequate. Citric acid performed better in increasing the dissolution ratio of the solid solution. However, it also dissolved a large amount of the matrix phase.

Fig. 10.

Dissolution ratios of each phase from the furnace-cooled slag in each case.

Compared with nitric acid, citric acid is an excellent chelating agent and can form a soluble organometallic complex with Ca²⁺ and Fe³⁺ ions.²¹⁾ The mechanism of slag dissolution in the citric acid solution can be explained as follows: H⁺ ions first displace metal cations from mineral phase, and thus C₆H₅O₇³⁻ ions sequester metals into soluble metal-ligand complexes by chelation.²²⁾ The formation of complexes at the mineral surface shifts the electron density toward the metal ion, which destabilizes the M–O lattice bonds and facilitates detachment of metal ions into the solution.²³⁾ This process can effectively destroy the crystal lattice and promote dissolution of the mineral phase. Therefore, under the same pH, citric acid showed a stronger ability to dissolve slag, and a higher dissolution ratio was obtained. When the pH was decreased to 5, the quantity of H⁺ ions increased, which caused more effective lattice destruction. Therefore, dissolution of the slag was significantly promoted. At a lower pH, citric acid was more active than other acids because of a combination of an abundance of H⁺ ions and complex formation.²²⁾ This could explain why the improvement in slag dissolution in the citric acid solution was larger at pH 5 than at pH 7.

Figure 11 shows the XRD patterns for the residues of the furnace-cooled slag after leaching. The peaks of the C₂S–C₃P solid solution still appeared after leaching at pH 7, indicating that sufficient solid solution could not be removed, and the magnesioferrite phase remained in the residue. At pH 5, the peaks of the C₂S–C₃P solid solution weakened and even disappeared. In particular, after leaching in the citric acid solution, only the peaks of the magnesioferrite phase were observed, indicating that the P-rich solid solution was fully separated. These results are consistent with the above analysis. In addition, the average composition of these residues was also determined by ICP analysis, as shown in Table 6. Owing to the lower dissolution ratio at pH 7, the P₂O₅ content in the residue decreased slightly, and the residue composition differs little from that of the original slag. At pH 5, the P₂O₅ content was reduced significantly with a corresponding increase in the Fe₂O₃ content of the residue. When citric acid was used, the P₂O₅ content decreased from 8.0% to 1.0%, and the Fe₂O₃ content increased from 29.0% to 54.5%.

Fig. 11.

XRD patterns for the residues after the furnace-cooled slag was leached.

Table 6. Compositions of the residues after the furnace-cooled slag was leached (mass%).

pH	Acid	CaO	SiO2	P2O5	Fe2O3	MgO
pH=7	Nitric acid	35.8	22.8	6.5	31.8	3.1
	Citric acid	35.5	22.6	6.1	32.8	3.2
pH=5	Nitric acid	30.9	24.6	2.9	38.1	3.6
	Citric acid	20.6	18.2	1.0	54.5	5.5

Finally, the ratios of the residue and the dissolved mass, which was calculated using the dissolution ratio, were estimated after leaching. The results are shown in Fig. 12 and compared with the phase fractions of the initial slag. The total of the dissolved mass and residue was less than the initial mass. The difference may result from the loss during sampling of the aqueous solutions. At pH 7, most of the slag remained in the residue after leaching. The dissolved mass ratio was lower than the mass fraction of the solid solution, indicating that only part of the solid solution dissolved. When the pH was decreased to 5, the dissolved mass ratio improved significantly. In the nitric acid solution, the dissolution ratio reached about 20%, which is slightly lower than the mass fraction of the solid solution, indicating that a large part of the solid solution dissolved. However, a portion of the solid solution still existed in the residue. In the citric acid solution, the dissolution ratio of the slag increased to 51%. This value is approximately equal to the sum of the total mass of the solid solution and half of the matrix phase. This results shows that citric acid acts so strongly that it dissolves much of the matrix phase, which is consistent with the above calculations. Therefore, in the further it is necessary to investigate methods of increasing the dissolution ratio of P in nitric acid solution and suppressing dissolution of the matrix phase by citric acid solution.

Fig. 12.

Mass ratios of residue and dissolved part, compared with the initial mass fractions of the furnace-cooled slag.

4.2. Effect of Cooling Rate

To estimate the effect of the cooling rate on selective leaching, the dissolution behavior of each phase from the slags at different cooling rates at pH 5 is discussed. Using the same method as described in Eqs. (7) and (8), the dissolution ratios of each phase from the water-quenched and air-cooled slags were calculated. These calculated results are listed in Table 7.

Table 7. Dissolution ratios of each phase from the different slags at pH 5 (%).

Acid	Phase	Sample	CaO	SiO2	P2O5	Fe2O3	MgO
Nitric acid	Matrix phase	Slag A	47.8	27.4	0.0	0.0	19.0
		Slag B	43.4	21.7	0.0	0.0	20.3
		Slag C	10.5	4.4	0.0	0.0	3.4
	Solid solution	Slag A	34.6
		Slag B	53.4
		Slag C	66.8
Citric acid	Matrix phase	Slag A	69.8	58.6	0.0	20.7	37.6
		Slag B	95.9	76.8	0.0	64.9	85.6
		Slag C	57.8	48.6	0.0	45.4	59.3
	Solid solution	Slag A	83.2
		Slag B	87.5
		Slag C	93.4

The water-quenched and air-cooled slags showed large amounts of Ca, Si, and Mg in the matrix phase were dissolved. In the nitric acid solution, as the cooling rate decreased, the dissolution ratio of the matrix phase decreased. In contrast, dissolution of the solid solution was significantly promoted. In the case of furnace cooling, the dissolution ratio of the solid solution increased from 34.6% to 66.8%, and only less than 10% of the matrix phase was dissolved. When citric acid was used, a similar result was also observed. As the cooling rate decreased, the dissolution ratio of the solid solution increased from 83.2% to 93.4%. However, the air-cooled slag showed the highest dissolution ratio of the matrix phase. The reason is not clear. In conclusion, decreasing the cooling rate not only suppressed dissolution of the matrix phase but also promoted dissolution of the solid solution, which is beneficial for selective leaching of P.

A study on the effect of the cooling rate on slag dissolution clarified that some elements were easier to dissolve from amorphous slag than from crystalline slag.^24,25) The reason is thought to be that the silicate structure in the amorphous phase collapsed easily during acid leaching, but crystalline phase had better chemical stability.²⁴⁾ Therefore, in the present study, the different dissolution behavior of matrix phase is attributed to the crystalline state of the matrix phase. Under rapid cooling, slag did not have enough time to crystallize, and the amorphous matrix phase formed, which resulted in a higher dissolution ratio of the matrix phase. As the cooling rate decreased, the matrix phase became more stable. Therefore, a lower dissolution ratio of the matrix phase was obtained.

In each slag, there is little change in the mass fraction and composition of the solid solution, but the slags show different dissolution behavior. This difference is attributed to the size of the solid solution particles. As shown in Figs. 2 and 8, in addition to large solid solution particles, there are many fine solid solution particles in the water-quenched and air-cooled slags. Because some fine solid solution particles were wrapped in the matrix phase, they cannot be exposed to the aqueous solution, which inhibits their dissolution. However, under furnace cooling, the solid solution particles aggregated and became coarser. Large solid solution particles have more opportunities to contact the aqueous solution, so they have a higher dissolution ratio. Overall, to achieve selective leaching of P, the slag should be cooled slowly during solidification to coarsen the solid solution particles and make the matrix phase more stable.

5. Conclusions

To achieve selective leaching of P and increase its dissolution ratio, the effects of the cooling rate and choice of acid on dissolution of slag with a high P₂O₅ content in aqueous solutions at pH 5 and 7 were investigated. The following conclusions can be drawn:

(1) During solidification, slow cooling facilitates coarsening of the C₂S–C₃P solid solution and formation of the magnesioferrite phase, but it has little effect on the distribution ratio of P₂O₅ between the solid solution and matrix phase.

(2) The C₂S–C₃P solid solution was dissolved preferentially compared with other phases. At pH 7, the air-cooled slag showed the highest dissolution ratio of P. When the pH was decreased to 5, slag dissolution was significantly promoted. As the cooling rate decreased, the dissolution ratio of P increased. Slow cooling not only enhanced the dissolution ratio of the P-rich solid solution but also suppressed dissolution of the matrix phase, which was beneficial for selective leaching of P.

(3) Citric acid performed better in promoting dissolution of slag. At pH 5, almost all of the solid solution was dissolved from the furnace-cooled slag. However, about half of the matrix phase was also dissolved, resulting in a higher dissolution ratio of Fe.

(4) In the nitric acid solution, 66.8% of the solid solution was dissolved from the furnace-cooled slag at pH 5, without dissolution of large amounts of the matrix phase. Better selective leaching of P was obtained, but the dissolution ratio of P was still inadequate.

(5) After removal of the P-rich solid solution by selective leaching, the P₂O₅ content in the residue was reduced significantly, with a corresponding increase in the Fe₂O₃ content.

Acknowledgement

The present study was partly supported by JFE 21st Century Foundation. The authors would like to express a deep appreciation to the JFE 21st Century Foundation and Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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