Effect of Al₂O₃ on Enrichment of Phosphorus in Hot Metal Dephosphorization Slag

Abstract

The phosphorus enrichment behavior in dephosphorization slag was investigated by slag modification with Al₂O₃ in the present study. The mineral phase of the slag samples were measured by mineral phase microscope, SEM+EDS and XRD. The mechanism of Al₂O₃ modification was discussed according to FactSage calculation. The results show that phosphorus was mainly existed in nC₂S–C₃P solid solution and Al₂O₃ modification was beneficial to the enrichment of phosphorus. The content of phosphorus in phosphorus-rich phase was increased to 5.10% and 9.15% with 8% and 11% Al₂O₃ addition. With the slag temperature decreases, Ca₃(PO₄)₂ and Ca₂SiO₄ firstly precipitated and formed the nC₂P–C₃P solid solution. The addition of Al₂O₃ has a positive influence on the reaction of Al₂O₃ and nC₂S–C₃P solid solution. Al₂O₃ could react with the initially precipitated low phosphorus nC₂S–C₃P solid solution to produce higher phosphorus n′C₂S–C₃P solid solution (n′<n) and Ca₂Al₂SiO₇. When Al₂O₃ content was increased from 2% to 15%, the value of lnK decreased from 2.34 to 1.47.

1. Introduction

As the high-grade iron ore resource is limited and decreased significantly, high phosphorus and low-grade iron ores is exploited and utilized gradually in iron and steel industry. Using the high phosphorus iron ore can result in higher phosphorus content in hot metal. Usually, most phosphorus in the hot metal is oxidized and enriched into LD slag in subsequent steelmaking process. Therefore, the phosphorus content in the LD slag has a considerable increase from 1–3 wt.% P₂O₅ in ordinary LD slag to over 10 wt.%, resulting in high phosphorus LD slag.¹⁾ This slag is a direct source for producing phosphoric fertilizer.^{2,3,4,5,6,7,8,9,10,11,12)} However the P-containing phase in the high phosphorus LD slag is not easy to be enriched and grow into large grains under the present cooling condition in dephosphorization process. Phosphorus exists mainly as the form of n2CaO·SiO₂–3CaO·P₂O₅ (nC₂S–C₃P solid solution) in the slag whereas the content of phosphorus in the solid solution is low. It is required to enrich the phosphorus into the solid solution and increase its content in the slag,^{13,14,15,16,17,18)} which will enhance subsequent separation of phosphorus and thus be beneficial to phosphorus recovery.

A number of studies have been carried out to explore vanadium (V) enrichment by adding TiO₂, SiO₂ and Al₂O₃ into the V-bearing steelmaking slag. Li et al.^19,20) found that the growth of V-concentrating phase was promoted by the addition of TiO₂ and Al₂O₃, which led to the increase of vanadium concentration in the calcium phosphate-calcium vanadate solid solution in V-bearing slag. Wu et al.²¹⁾ used Al₂O₃ and SiO₂ to modify the distribution of vanadium in the industrial steel slag, and the results showed that the crystal grain of the V-enrichment phase had grown larger by this modification. Liang et al.²²⁾ clarified the possibility of enrichment of vanadium in LD slags with high V₂O₅ contents by adding SiO₂. The results indicated that the addition of SiO₂ changed the phase composition of the slags strongly, resulting in formation of two different vanadium enriched phases. One was a solid solution of 3CaO·V₂O₅ and 3CaO·P₂O₅ with 3% SiO₂ dissolved. The other had a higher SiO₂ content in the solid solution. Lin et al.²³⁾ used SiO₂ to modify the LD slag for enrichment of phosphorus by adding SiO₂ and then recycled phosphorus by magnetic separation. They identified that SiO₂ modification was favorable for enrichment of phosphorus and most phosphorus could be efficiently recycled. Inoue and Suito et al.²⁴⁾ explored the phosphorus partition behavior between 2CaO·SiO₂ particle and FeO_x–CaO–SiO₂ slags at 1300°C and 1560°C. They reported that the mass transfer of phosphorus from 2CaO·SiO₂ saturated slag to the 2CaO·SiO₂ particles is fast and a uniform CaO–SiO₂–P₂O₅ solid phase is formed within 5 seconds. Saito et al.²⁵⁾ studied the microscopic formation mechanisms of P₂O₅-containing phase at the interface between solid CaO and molten slag, the results indicated that the condensation of phosphorus as nC₂S–C₃P solid solution was controlled by P₂O₅ diffusion from bulk slag to reaction interface, not by absorption of P₂O₅ into 2CaO·SiO₂ particle. Ito et al.²⁶⁾ probed phosphorus distribution behavior between solid C₂S and CaO–SiO₂–FeO–Fe₂O₃ slags and Hirosawa et al.²⁷⁾ measured the partition of phosphorus between liquid slag and solid 2CaO·SiO₂ at hot metal pretreatment temperature. Pahlevani et al.²⁸⁾ measured the distribution of P₂O₅ between nC₂S–C₃P solid solution and liquid slags. Tsukihashi and co-workers et al.^29,30,31,32) studied reaction behavior of P₂O₅ at interface between solid C₂S containing B₂O₃ and CaO–SiO₂–FeO_x–P₂O₅ slags saturated with nC₂S–C₃P solid solution.

However, few studies have been conducted to explore the enrichment of phosphorus, especially the reaction mechanism of the enrichment, in dephosphorization slag by Al₂O₃ modification. Therefore, in the present study, the ultimate goal is to study the effect of Al₂O₃ on enrichment of phosphorus in hot metal dephosphorization slag (high phosphorus slag) and explore the mechanism of the slag modification. This study was expected to lay foundation for subsequent phosphorus recovery from the high phosphorus dephosphorization slag.

2. Experiments

2.1. Raw Materials

The slags used in this study were synthesized from chemical reagents of analytical grade and their chemical compositions are shown in Table 1. It should be noted that the No. 0 slag was the original slag; the No. 1 slag and No. 2 slag were slag samples modified by adding Al₂O₃. The slag binary basicity (CaO/SiO₂) was fixed at 2.1. FeO and P₂O₅ were replaced by ferrous oxalate and Ca₃(PO₄)₂, respectively, during the preparation of slag samples.

Table 1. Chemical composition of the No. 0 slag and modified slags, wt.%.

slag	CaO	SiO2	MgO	MnO	FeO	P2O5	Al2O3
No. 0	47.4	22.6	6	4	10	10	0
No. 1	42	20	6	4	10	10	8
No. 2	40	19	6	4	10	10	11

2.2. Experimental Procedure

To make sure the slag samples can be fully melted at 1450°C, testes of hemisphere melting points were performed firstly. The obtained results are listed in Table 2.

Table 2. Melting point of the No. 0 slag and modified slags, °C.

No. 0	No. 1	No. 2
1325	1340	1330

Experiments were carried out in a MoSi₂ resistant electric furnace with an accuracy of ±2°C. N₂ was used for protective gas during experiments. The mixture of chemical reagents was charged into a pure iron crucible and then placed inside in an alumina crucible, which was put a graphite crucible. The mixture was then heated to 1450°C and hold there for 30 min ensure the complete melt of the slag samples in the MoSi₂ resistant electric furnace. Thereafter, the mixture was cooled down to 1350°C at a cooling rate of 5°C/min and then hold for 2 hours to promote the precipitation of Ca₃(PO₄)₂. Then it was further cooled to 1250°C at a cooling rate of 5°C/min and finally quenched in air to the room temperature.

The morphologies of the prepared slag samples were observed by optical microscope and scanning electron microscope (SEM). For the analysis of phase composition, an energy dispersive spectrometer (EDS) was employed. The slags were ground to less than 48 μm (screened through 300-mesh). Mineralogical phases were determined by powder X-ray diffraction (XRD). During the XRD analysis, the diffraction patterns were measured in a 2 h range of 10–90° using Cu Ka radiation of 40 kV and 30 mA at a scan speed of 5°/min.

2.3. Factsage Calculation

Thermodynamic analysis of Al₂O₃ modification for the slag samples were calculated by FactSage software (version 6.3). Equilibrium phase compositions of slag samples at different temperatures during the cooling process were calculated in the equilibrium module. The principle of the calculation is based on minimization of Gibbs energy in the slag system.

3. Results and Discussion

3.1. Phosphorus Enrichment Behavior

Figure 1 shows the micrographs of No. 0 slag and No. 1 slag. It can be seen that both No. 0 slag and No. 1 slag consisted of three mineralogical phases: white phase; dark grey phase and grey matrix phase. The grain size of the white phases in No. 0 slag and No. 1 slag were about 20 μm and 30–40 μm, respectively. Mineralogical phases in No. 0 slag were distributed dispersedly and it was not easy to be distinguished. However, the phases in No. 1 slag can be clarified apparently after the modification. The difference in specialties of mineralogical phases between No. 0 slag and No. 1 slag indicated that Al₂O₃ played an important role in changing the phase distribution.

Fig. 1.

Micrographs of the No. 0 slag (a) and No. 1 slag (b).

Figure 2 shows the SEM of the No. 0 slag and modified slags. It can be seen from Fig. 2 that the SEM image of the No. 1 slag was similar to that as shown in Fig. 1(b). The white phase was distributed in the grey matrix phase whereas another phase was difficult to be distinguished. Combined the EDS and XRD results (as shown in Table 3 and Fig. 3), the grey phase in No. 0 slag was a complicated phase of nC₂S–C₃P solid solution and Ca₃MgSi₂O₈ containing a little Fe and Mn, grey matrix phase was mainly composed of FeO–CaO–SiO₂ phases. The white phase was mainly consisted of iron oxides, manganese oxides and magnesium oxides; it can be defined as phosphorus-free manganese-enriched phase (white RO phase). The dark gray phase can be defined as FeO-free phosphorus-rich phase (nC₂S–C₃P solid solution). It should be noted that Fe in the slag mainly existed in the white phase while P existed in the nC₂S–C₃P solid solution.

Fig. 2.

SEM images of No. 0 slag (a), No. 1 slag (b) and No. 2 slag (c): (1-Matrix phase 2-RO phase 3-phosphorus-rich phase 4-other phase).

Table 3. EDS results of each phase in slag samples, wt.%.

Number	Main phase	O	Mg	Al	Si	P	Ca	Mn	Fe	Total
No. 0	Matrix phase-1	33.25	4.93	—	16.04	2.46	34.19	2.94	6.20	100
	RO phase-2	19.90	20.52	—	—	—	0.97	7.42	51.19	100
No. 1	Matrix phase-1	41.26	3.42	8.04	11.26	1.19	18.42	3.90	12.51	100
	RO phase-2	24.93	18.35	—	—	—	—	5.68	51.04	100
	P-rich phase-3	38.00	—	—	13.45	5.10	40.16	1.11	2.19	100
No. 2	Matrix phase-1	42.62	2.62	4.97	11.60	1.48	18.40	4.09	14.22	100
	RO phase-2	22.91	13.65	—	—	—	—	8.19	55.25	100
	P-rich phase-3	40.18	—	—	9.50	9.15	36.81	1.40	1.95	100
	Other phase-4	37	—	5.17	15.32	0.47	23.24	3.55	15.25	100

Fig. 3.

X-ray diffraction patterns of slag samples.

Different from No. 0 slag, three phases were clearly identified in the slag with 8% Al₂O₃ modification. These phases (as shown in Fig. 2(b)) were referred to matrix phase, phosphorus-rich phase and white RO phase, respectively. It can be seen from EDS and XRD results that the matrix phase was mainly composed of Ca₃MgSi₂O₈ and Ca₂Al₂SiO₇, and with a little Fe and Mn. The content of phosphorus in the matrix phase was decreased dramatically. The composition of white RO phase, solid solution of FeO, MnO and MgO, was almost unchanged. It is found that the phosphorus-rich phase had a high content of phosphorus. This indicated that the Al₂O₃ addition had an effect on the distribution of phosphorus.

When the Al₂O₃ content was increased to 11% in No. 2 slag, value of n in the nC₂S–C₃P solid solution was decreased. The Ca₃(PO₄)₂ was even appeared individually in the slag after modification. A new phase, which was likely to be Ca₂Al₂SiO₇ with small amounts of Fe and Mn, precipitated from the matrix phase. However, when the Al₂O₃ content was 8% in No. 1 slag, the Ca₂Al₂SiO₇ didn’t precipitate in the matrix phase. The condition for Ca₂Al₂SiO₇ precipitation needs further exploration.

According to the analysis mentioned above, the nC₂S–C₃P solid solution was the main phosphorus-rich phase in the dephosphorization slag. The content of Al₂O₃ had great effect on both the formation of nC₂S–C₃P solid solution and the content of phosphorus in the solid solution. The content of phosphorus in the matrix phase was low with 2%. After Al₂O₃ modification, phases were changed dramatically. The phosphorus-rich phase had precipitated as an independent phase, in which the content of phosphorus was increased considerably. This may be that the increase in content of Al₂O₃ in the slag promoted the reaction of nC₂S–C₃P solid solution and Al₂O₃ during the early stage of crysllization of the solid solution, The reaction be expressed as follows, It is clarified that Al₂O₃ modifier was advantageous to the enrichment of phosphorus.   

$$\begin{array}{l} \left({\text{nC}}_{\text{2}}{\text{S-C}}_{\text{3}}\text{P}\right)+{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}\to \\ {\text{(n′C}}_{\text{2}}{\text{S-C}}_{\text{3}}\text{P)}+{\text{Ca}}_{\text{2}}{\text{Al}}_{\text{2}}{\text{SiO}}_{\text{7}}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\text{n′}<\text{n}\end{array}$$(1)

3.2. Al₂O₃ Modification Mechanism

The standard Gibbs free energies of Ca₂SiO₄, Ca₃(PO₄)₂, Ca₃MgSi₂O₈ and Ca₂Al₂SiO₇ were calculated in 400–1800°C by using thermodynamic package (Factsage version 6.3), the results are shown in Fig. 4. In this temperature range, the standard reaction Gibbs free energy changes were all smaller than zero, which indicated all the chemical reactions can occur. The order of standard Gibbs free energy was Ca₃(PO₄)₂<Ca₃MgSi₂O₈<Ca₂SiO₄<Ca₂Al₂SiO₇. Minimum of standard reaction Gibbs free energy change was the formation of Ca₃(PO₄)₂. It is believed that Ca₃(PO₄)₂ is more stable than Ca₂SiO₄. Therefore, CaO will react firstly with P₂O₅ forming the Ca₃(PO₄)₂. As shown in Figs. 1 and 3, Ca₃(PO₄)₂ and Ca₂SiO₄ precipitated and formed the nC₂S–C₃P solid solution.

Fig. 4.

The standard reaction Gibbs free energy changes as a function of temperature. (Online version in color.)

Figure 5 shows the calculated equilibrium phase composition of No. 0 slag at different temperatures. It can be seen that four chemical compounds, Ca₃(PO₄)₂, RO(FeO, MgO and MnO), Ca₃MgSi₂O₈ and Ca₂SiO₄, were precipitated during the cooling. At the primary crystallization stage, Ca₃(PO₄)₂ precipitated firstly and grew in the liquid slag. At the same time, the amount of liquid slag started decreasing. Ca₂SiO₄ was precipitated at 1365°C and the amount of Ca₃(PO₄)₂ reached maximum at 1350°C. This is the reason for holding slag samples at 1350°C for 2 h to promote the crystallization of Ca₃(PO₄)₂. According to XRD results of No. 0 slag as shown in Fig. 3, Ca₂SiO₄ and Ca₃(PO₄)₂ combined and formed nC₂S–C₃P solid solution. Both the RO phase (FeO, MgO and MnO) and Ca₃MgSi₂O₈ started to precipitate with the further decrease of temperature and had amounts of 15% and 40%, respectively.

Fig. 5.

Calculated equilibrium phase compositions of No. 0 slag during cooling. (Online version in color.)

Figures 6 and 7 show the equilibrium phase composition of No. 1 slag and No. 2 slag during the cooling, respectively. Similar to the No. 0 slag, Ca₃(PO₄)₂ precipitated firstly and grew in the liquid slag while the amount of liquid slag was decreased with decreasing temperature. It should be noted that a new compound (Ca₂Al₂SiO₇) was formed during the cooling process after Al₂O₃ modification. This further demonstrated that Al₂O₃ can react with nC₂S–C₃P solid solution at its early precipitation state. Amount of precipitated Ca₂Al₂SiO₇ was increased with increasing Al₂O₃ content. The content of phosphorus in n′C₂S–C₃P solid solution (n′<n) was also increased without precipitation of dicalcium silicate precipitation. The RO phase and Ca₃MgSi₂O₈ almost remained unchanged, which means the Al₂O₃ modification had little effect on formation of these two phases.

Fig. 6.

Calculated equilibrium phase compositions of No. 1 slag during cooling. (Online version in color.)

Fig. 7.

Calculated equilibrium phase compositions of No. 2 slag during cooling. (Online version in color.)

With the temperature decreases, Al₂O₃ reacted with the initially precipitated nC₂S–C₃P solid solution to form high phosphorus n′C₂S–C₃P solid solution (n′<n) and Ca₂Al₂SiO₇. The reaction can be expressed as follows.   

$$\begin{array}{l}{\text{mCa}}_{\text{2}}{\text{SiO}}_{\text{4}}·{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_{\text{2}}+{\text{nAl}}_{\text{2}}{\text{O}}_{\text{3}}\\ ={\text{nCa}}_{\text{2}}{\text{Al}}_{\text{2}}{\text{SiO}}_{\text{7}}+{\text{yCa}}_{\text{2}}{\text{SiO}}_{\text{4}}·{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_{\text{2}}\end{array}$$(2)

  

$$\mathrm{\Delta }G=\mathrm{\Delta }{G}^{\theta }+RTln{K}_1$$(3)

  

$$K_1=\frac{a_{C{a}_3{(P{O}_4)}_2\cdot yC{a}_{2}Si{O}_4}\cdot a_{C{a}_{2}A{l}_{2}Si{O}_7}^n}{a{}_{C{a}_3{(P{O}_4)}_2\cdot mC{a}_{2}Si{O}_4}\cdot a_{A{l}_2{O}_3}^n}$$(4)

Where $a_{A{l}_2{O}_3}$ , $a_{mC{a}_{2}Si{O}_4\cdot C{a}_3{(P{O}_4)}_2}$ , $a_{C{a}_{2}A{l}_{2}Si{O}_7}$ and $a_{yC{a}_{2}Si{O}_4\cdot C{a}_3{(P{O}_4)}_2}$ are the activities of Al₂O₃, mCa₂SiO₄·Ca₃(PO₄)₂, Ca₂Al₂SiO₇ and yCa₂SiO₄·Ca₃(PO₄)₂, respectively. K₁ is the equilibrium constant of reaction (2). mCa₂SiO₄·Ca₃(PO₄)₂ and yCa₂SiO₄·Ca₃(PO₄)₂ were regarded as pure substances, their activities were 1.

According to reactions (2) and (3), the formation of both nC₂S–C₃P solid solution containing phosphorus and Ca₂Al₂SiO₇ were decided by the value of K₁. Figure 8 shows the relationship of the natural logarithms of activities of chemical compounds and Al₂O₃ content at 1350°C calculated by Factsage software. It can be seen from the Fig. 8 that the activity of Ca₂Al₂SiO₇ was increased with increasing Al₂O₃ content whereas lnK decreased. This indicated that reaction (2) was positive to generate high phosphorus solid solution and Ca₂Al₂SiO₇ with increasing Al₂O₃ content. The initially precipitated nC₂S–C₃P solid solution was placed by the high phosphorus solid solution with reaction (2), which explains the reason that phosphorus content in nC₂S–C₃P solid solution increases gradually with the adding Al₂O₃ into slag.

Fig. 8.

Relationship the natural logarithms of activities of chemical compounds and Al₂O₃ content at 1350°C. (Online version in color.)

When Al₂O₃ content in slag exceeded a certain value, nC₂S–C₃P solid solution containing phosphorous was formed by continuous reaction of Al₂O₃ and the initially precipitated nC₂S–C₃P solid solution, resulting in higher content of phosphorus in [(y–x)C₂S–C₃P] solid solution and complete consumption of the initially precipitated nC₂S–C₃P solid solution.   

$$\begin{array}{l}{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_{\text{2}}·{\text{yCa}}_{\text{2}}{\text{SiO}}_{\text{4}}+{\text{xAl}}_{\text{2}}{\text{O}}_{\text{3}}={\text{xCa}}_{\text{2}}{\text{Al}}_{\text{2}}{\text{SiO}}_{\text{7}}\\ +{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_{\text{2}}·\left(\text{y-x}\right){\text{Ca}}_{\text{2}}{\text{SiO}}_{\text{4}}\hspace{1em}\text{y}\ge \text{x}\end{array}$$(5)

  

$$\mathrm{\Delta }G=\mathrm{\Delta }{G}^{\theta }+RTln{K}_2$$(6)

  

$$K_2=\frac{a_{C{a}_3{(P{O}_4)}_2\cdot (y-x)C{a}_{2}Si{O}_4}\cdot a_{C{a}_{2}A{l}_{2}Si{O}_7}^x}{a{}_{C{a}_3{(P{O}_4)}_2\cdot yC{a}_{2}Si{O}_4}\cdot a_{A{l}_2{O}_3}^x}\hspace{1em}\text{y}\ge \text{x}$$(7)

Where $a_{A{l}_2{O}_3}$ , $a_{C{a}_{2}A{l}_{2}Si{O}_7}$ , $a_{(y-x)C{a}_{2}Si{O}_4\cdot C{a}_3{(P{O}_4)}_2}$ and $a_{yC{a}_{2}Si{O}_4\cdot C{a}_3{(P{O}_4)}_2}$ are the activities of Al₂O₃, Ca₂Al₂SiO₇, (y–x)Ca₂SiO₄·Ca₃(PO₄)₂ and yCa₂SiO₄·Ca₃(PO₄)₂, respectively. K₂ is the equilibrium constant of reaction (5). Both (y–x)Ca₂SiO₄·Ca₃(PO₄)₂ and yCa₂SiO₄·Ca₃(PO₄)₂ were regarded as pure substances and their activities were 1.

Thermodynamic of reaction (5) is decided by K₂, which has important influence on generation of nC₂S–C₃P solid solution containing phosphorus and enrichment grade of phosphorus in the phase. Figure 8 shows K value reduces when Al₂O₃ content in slag continues to increase, which is positive to increase thermodynamics of reaction, and makes the initially precipitated low phosphorus solid solution gradually reduce. Low phosphorus solid solution is concentrated gradually to become higher phosphorus n′C₂S–C₃P solid solution and even Ca₃(PO₄)₂ (Ca₂SiO₄ is exhausted in the solid solution), that have been proved by XRD and SEM/EDS test results.

Above all, in cooling process, Al₂O₃ in slag reacts with the initially precipitated low phosphorus nC₂S–C₃P solid solution, products are high phosphorus n′C₂S–C₃P solid solution and Ca₂Al₂SiO₇, until the initially precipitated low phosphorus nC₂S–C₃P solid solution disappear. With further increasing Al₂O₃ content, the high phosphorus n′C₂S–C₃P solid solution continues to react with Al₂O₃, product gradually become higher phosphorus [(y–x)C₂S–C₃P]solid solution and even Ca₃(PO₄)₂, phosphorus grade in phosphorus-rich phase increases. Al₂O₃ modification experiments for dephosphorization slag provide theoretical and technological foundation for better phosphorus separation from slag.

4. Conclusion

(1) Both the original slag and modified slags consisted of white manganese-enriched phase, dark gray phosphorus-rich phase and grey phosphorus-free matrix phase. The phases in the original slag distributed dispersedly and were not easy to be distinguished while phases in modified slags were clarified clearly. Phosphorus was mainly existed in nC₂S–C₃P solid solution.

(2) The content of phosphorus in the matrix phase was low as 2.46%. The content of phosphorus in phosphorus-rich phase was increased significantly with increasing Al₂O₃ content. Phosphorus content in the phosphorus-rich phase reached 5.10% with 8% Al₂O₃ addition. When the Al₂O₃ content was increased to 11%, the phosphorus content in phosphorus-rich phase reached 9.15%.

(3) With temperature decreasing, Ca₃(PO₄)₂ and Ca₂SiO₄ firstly precipitated and formed the nC₂P–C₃P solid solution in the slag. Al₂O₃ could react with the initially precipitated low phosphorus nC₂S–C₃P solid solution to produce higher phosphorus n′C₂S–C₃P solid solution and Ca₂Al₂SiO₇. When Ca₂SiO₄ is exhausted, the product is even Ca₃(PO₄)₂. When Al₂O₃ content was increased from 2% to 15%, the value of lnK decreased from 2.34 to 1.47.

Acknowledgements

Financial supports from Fundamental Research Funds for the Central Universities (Project No. CDJZR14130001) is greatly appreciated.

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