Phase Equilibrium for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ System for Dephosphorization of Hot Metal Pretreatment

Abstract

Recently, after the restriction of the use of CaF₂, dephosphorization process often generates large amount of slag, due to the neglect of refining functions of solid phases. Consequently, this brings environmental issues and influences refining. In order to improve the utilization efficiency of solid CaO and its compounds in the dephosphorization slag, multiphase flux refining has been proposed by considering the enrichment of phosphorus within the solid phases. As to provide theoretical fundamentals for both understanding on the reaction mechanism of phosphorus and practical slag control, phase relationship for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ system has been studied based on chemical equilibration technique with oxygen partial pressure of 10⁻¹⁰ atm at 1673 K. In current work, the liquidus saturated with P₂O₅-rich solid solution has been firstly deduced on the CaO–SiO₂–FeO ternary system, and the discussions on the relationship between solid solution and liquid phase has been proceeded. It has been found that the existence of Al₂O₃ enlarges the liquid phase area, but does not affect the composition of solid solution. On the other hand, the equilibrium solid phase has been confirmed as 2CaO·SiO₂–3CaO·P₂O₅ solid solution, while the ratio between both varies along with liquidus. As expected, the large equilibrium partition ratio of phosphorus between solid solution and liquid slag has also been found and discussed.

1. Introduction

In steelmaking process, dephosphorization often requires lime addition to achieve high slag basicity. As to accelerate the lime dissolution, fluorite has been used as a flux agent. But due to serious environmental issues, the application of it has been restricted. Consequently, much more lime is required to maintain high dephosphorization ability of molten slag. As a result, the slag amount increases and much solid phases remain after hot metal pretreatment. In order to promote the utilization efficiency of solid phases, the refining function of solid phases such as remained CaO and condensed 2CaO·SiO₂ solid solution has been reconsidered. Based on this perspective, it is expected that the lime consumption could be reduced. Afterwards, an innovative refining process for dephosphorization by using multiphase flux has been proposed.

Lots of efforts have been made to understand the multiphase flux system.^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19)} However, some thermodynamic studies, especially for the equilibrium phase relationship between solid phases and molten slag, yet remain insufficient.

In this study, the phase relationship for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ slag system has been studied experimentally with constant low oxygen partial pressure at hot metal pretreatment temperature. Components of CaO, SiO₂, FeO and P₂O₅ in the slag system stand for the basic dephosphorization slag, while Al₂O₃ which commonly exists in refining slag systems has been added due to the consideration of promoting the melting condition. Based on current experimental results and discussions, liquidus at high CaO/SiO₂ ratio region has been deduced, and also the solid phases equilibrated with it have been studied.

2. Experimental

Chemical equilibration technique^{20,21,22,23,24,25)} has been adopted to investigate the phase relationship, and the detail experimental procedures have been described in our previous work.²⁶⁾

Reagent grade of SiO₂, 3CaO·P₂O₅ and Al₂O₃, and synthesized CaO and FeO were used for preparing the initial oxide mixtures. The concentrations of CaO, SiO₂ and FeO were determined by using the phase diagram of the CaO–SiO₂–FeO_x ternary system, and the concentrations of P₂O₅ and Al₂O₃ were both set to be 5 mass% constantly according to practical refining condition. The compositions of initial oxide mixtures are shown in Table 1.

Table 1. Compositions of initial oxides mixture.

No.	FeO (mass%)	CaO (mass%)	SiO2 (mass%)	P2O5 (mass%)	Al2O3 (mass%)	CaO/SiO2 mole ratio
1	29.70	35.17	25.13	5.00	5.00	1.50
2	43.20	30.47	16.33	5.00	5.00	2.00
3	49.50	29.70	10.80	5.00	5.00	2.95
4	33.00	48.82	8.17	5.00	5.00	6.41
5	35.89	46.58	7.54	5.00	5.00	6.63
6	43.20	43.20	3.60	5.00	5.00	12.88
7	52.20	37.80	0.00	5.00	5.00	—
8	13.50	48.60	27.90	5.00	5.00	1.87
9	8.55	50.40	31.05	5.00	5.00	1.74
10	38.25	37.52	14.23	5.00	5.00	2.83
11	29.81	46.13	14.06	5.00	5.00	3.52
12	37.13	40.50	12.38	5.00	5.00	3.51
13	33.75	37.52	18.73	5.00	5.00	2.15
14	40.50	47.25	2.25	5.00	5.00	22.53
15	31.50	55.35	3.15	5.00	5.00	18.85

For each experiment, about 0.1 g of prepared oxide mixture were firstly loaded in a platinum crucible (height: 5 mm; diameter: 5 mm), and then it was heated to 1923 K for premelting with an atmosphere of argon gas. After 1 to 3 hours preservation at 1923 K, temperature decreased to 1673 K by 10 K/min, and during this period the CO–CO₂ gas mixture was introduced. The volume ratio between CO and CO₂ gases was maintained to be approximately 5:1 according to Eq. (1) to control the oxygen partial pressure of 10⁻¹⁰ atm which could reflect the practical thermodynamic condition within the multiphase flux system during dephosphorization. Therefore, the precise oxygen partial pressure for experiments was 9.24×10⁻¹¹ atm.   

$$\text{CO(g)}+{\text{1}/\text{2O}}_{\text{2}}\text{(g)}={\text{CO}}_{\text{2}}\text{(g)}$$(1)

  

$$\mathrm{\Delta }{G}^{°}=-281\mathrm{\hspace{0.17em} }000+85.23T\hspace{0.17em} \text{J}/\text{mol}$$²⁷⁾

Time to achieve chemical equilibration was 3 to 24 hours according to preliminary experiments. After equilibration, the crucible containing multiphase flux was quickly taken out and quenched by liquid nitrogen. Finally, Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS) was used to analyze the phase compositions.

In order to diminish the uncertainty of EDS analysis, quantity repetitions of analysis for the same phase and also area analysis were employed.

3. Results and Discussion

The compositions of the observed phases are listed in Table 2. By using the observed data of the elements, the corresponding oxide compositions were calculated. Since the valence of iron ions cannot be detected by EDS, and also due to the low Fe³⁺/Fe²⁺ ratio under current experimental conditions,²⁰⁾ only Fe²⁺ has been considered for presentation purpose.

Table 2. Compositions of observed phases.

No.	Phase	Compositions (mass%)
		Al2O3	SiO2	P2O5	CaO	FeO
1	Liquid	9.3	30.6	0.1	36.1	24.0
	2CaO·SiO2–3CaO·P2O5 solid solution	0.2	28.5	7.6	58.4	5.4
2	Liquid	9.4	13.9	0.0	23.0	53.7
	2CaO·SiO2–3CaO·P2O5 solid solution	0.5	28.4	6.4	59.4	5.3
3	Liquid	9.3	9.4	0.0	18.1	63.2
	2CaO·SiO2–3CaO·P2O5 solid solution	0.2	27.7	8.1	61.0	3.0
4	Liquid	9.8	4.3	0.2	24.2	61.6
	2CaO·SiO2–3CaO·P2O5 solid solution	1.0	20.4	21.3	56.2	1.1
5	Liquid	8.4	3.3	1.6	46.9	39.8
	2CaO·SiO2–3CaO·P2O5 solid solution	2.1	20.1	11.0	64.1	2.7
6	Liquid	6.2	2.7	2.0	44.7	44.4
	2CaO·SiO2–3CaO·P2O5 solid solution	1.4	15.9	18.5	62.2	2.0
7	Liquid	5.1	0.0	4.0	40.7	50.2
8	Liquid	12.9	31.2	1.3	42.1	12.5
	2CaO·SiO2–3CaO·P2O5 solid solution	1.2	28.6	6.7	60.5	3.0
9	Liquid	10.1	35.9	1.9	48.8	3.4
	2CaO·SiO2–3CaO·P2O5 solid solution	0.9	27.2	6.8	64.1	1.1
10	Liquid	10.3	8.8	0.2	18.2	62.4
	2CaO·SiO2–3CaO·P2O5 solid solution	0.4	23.6	11.6	62.3	2.0
11	Liquid	8.8	1.8	0.5	31.5	57.5
	2CaO·SiO2–3CaO·P2O5 solid solution	1.4	26.0	8.7	62.6	1.2
12	Liquid	9.7	2.8	0.2	26.9	60.5
	2CaO·SiO2–3CaO·P2O5 solid solution	0.5	23.8	11.2	63.2	1.3
13	Liquid	13.9	18.6	0.4	27.0	40.0
	2CaO·SiO2–3CaO·P2O5 solid solution	0.4	23.5	18.3	60.9	3.5
14	Liquid	7.6	3.1	5.8	49.1	34.5
	Solid CaO	0.0	0.1	0.0	96.0	3.8
15	Liquid	7.6	3.5	4.8	48.7	35.5
	2CaO·SiO2–3CaO·P2O5 solid solution	2.9	15.1	14.5	63.5	3.9
	Solid CaO	0.0	0.1	0.0	97.1	2.8

The equilibrium status shown in Table 2 has been determined by comparing current phase compositions to the well-studied ternary systems such as the CaO–SiO₂–FeO, CaO–SiO₂–Al₂O₃ and CaO–SiO₂–P₂O₅ systems.

3.1. Liquidus for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ System with P_O2 of 9.24×10^–11 atm at 1673 K

Since the liquid phase mainly contains CaO, SiO₂, FeO and nearly constant Al₂O₃, according to Table 2, it can be discussed by projecting on the CaO–SiO₂–FeO ternary section as shown in Fig. 1. Ignoring the variation in Al₂O₃ concentration, the solid line shown in Fig. 1 represents the liquidus for the CaO–SiO₂–FeO–P₂O₅–10mass%Al₂O₃ system with P_O2 of 9.24×10⁻¹¹ atm at 1673 K. This 10 mass% Al₂O₃ in liquidus is judged according to Fig. 2, which implies that mostly Al₂O₃ is enriched in liquid phase and its concentration fluctuates around 10 mass%. Fraction of solid phase to total slag in weight which was calculated by the mass balance of Al₂O₃ was in the range between 25 and 68%, mainly around 50%. The area fraction of solid phase to total slag calculated from SEM images was also around 50%.

Fig. 1.

Projections of equilibrium phases compositions on the CaO–SiO₂–FeO ternary section.

Fig. 2.

Distribution of Al₂O₃ between liquid slag and solid solution.

Comparing with the liquidus for the CaO–SiO₂–FeO_x system equilibrated with metallic iron,²⁸⁾ current measured liquid phase area enlarges a little. Although the oxygen partial pressure in current work is higher³⁾ and the condensed solid solution is different, both of which should lead to the shrinkage of liquid phase area and thus the slight enlargement occurs by the contribution of the existence of Al₂O₃ in liquid phase.

Comparing with the CaO–SiO₂–FeO–Fe₂O₃–5mass%Al₂O₃ system with P_O2 of 10⁻⁸ atm at 1573 K,²⁹⁾ the liquid phase area also enlarges, especially in the region where SiO₂ content is larger than 20 mass%. However, opposite trend appears near the inflection point of liquidus. The liquidus shifts towards the FeO apex a little, as shown as the solid line locating on the right side of the dash line at the region of FeO content between 60 and 80 mass% in Fig. 1, though current liquid phase contains larger Al₂O₃ content. This difference is due to the condensation of 2CaO·SiO₂–3CaO·P₂O₅, which consumes more CaO than 2CaO·SiO₂. On the other hand, this shrinkage of liquid phase area becomes noticeable only near the inflection point of liquidus where the FeO content reaches the largest value. This implies a stronger promotion of the condensation of P₂O₅-containing solid solution with larger FeO content in liquid phase.

The CaO primary region shrinks a lot comparing to the CaO–SiO₂–FeO_x system equilibrated with metallic iron, and the following two reasons can be considered. One is the enlargement of liquid phase area by the addition of Al₂O₃ as already mentioned. Another one could be that the 3CaO·SiO₂ solid solution has not been observed in current work as shown in Table 2, which means that the 3CaO·SiO₂ primary region no longer exists. Therefore, the confinement to the liquidus vanishes and the liquid phase could extend to a larger CaO content region.

Based on above discussion, the phase sections have been deduced as shown in Fig. 3. Because the projection of 2CaO·SiO₂–3CaO·P₂O₅ solid solution represents a composition range rather than a definite point, the composition of solid solution is expressed as the dash-dotted-dotted line with two arrows at both ends. On the other hand, due to the low solubility of FeO in solid solution, space has been kept between the boundaries of phase section 1 and the CaO–SiO₂ tie line.

Fig. 3.

Phase sections for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ system with oxygen partial pressure of 9.24×10⁻¹¹ atm at 1673 K on the CaO–SiO₂–FeO ternary section.

3.2. Effect of Al₂O₃ Addition on Liquidus

The effect of Al₂O₃ addition on the liquidus is shown in Fig. 4 by comparing with the CaO–SiO₂–FeO–5mass%P₂O₅ system equilibrated at the same experimental conditions.²⁶⁾ It has been found that the enlargement of liquid phase area becomes more distinct at larger FeO content region by the addition of Al₂O₃. Noting that the CaO–FeO and high T.Fe phases mentioned in the previous work²⁶⁾ has been assumed as those formed during quenching.

Fig. 4.

Effect of Al₂O₃ addition on the liquidus with oxygen partial pressure of 9.24×10⁻¹¹ atm at 1673 K.

3.3. Condensed 2CaO·SiO₂–3CaO·P₂O₅ Solid Solution

In Fig. 5, the compositions of solid solutions have been projected on the CaO–SiO₂–P₂O₅ ternary section. It is observed that the compositions of condensed solid solutions locate within the 2CaO·SiO₂–3CaO·P₂O₅ stable region,²⁸⁾ rather than 3CaO·SiO₂ or 4CaO·P₂O₅ stable regions.

Fig. 5.

Projections of compositions of condensed solid solutions on the CaO–SiO₂–P₂O₅ ternary section.

Composition of solid solutions is close to the tie line between 2CaO·SiO₂ and 3CaO·P₂O₅ as shown in Fig. 5. Therefore, the solid solutions can be discussed by using the 2CaO·SiO₂–3CaO·P₂O₅ pseudo binary system³⁰⁾ as shown in Fig. 6. At 1673 K, only R type solid solution [(α-2CaO·SiO₂–α-3CaO·P₂O₅)_S.S.] equilibrates with liquid phase, and the effect of Al₂O₃ on the composition of solid solution has not been discovered.

Fig. 6.

Projections of compositions of condensed solid solutions on the 2CaO·SiO₂–3CaO·P₂O₅ pseudo binary system.

3.4. Phosphorus Partition between Solid Solution and Liquid Slag

The phosphorus partition between solid solution and liquid slag within multiphase flux system has been studied by many researchers.^7,31,32,33) In this study, the phosphorus partition has been considered with the equilibrium phase relationship. The phosphorus partition ratio between 2CaO·SiO₂–3CaO·P₂O₅ and liquid slag (see Sections 1 and 2 in Fig. 3) has been concerned. When liquid slag coexists with solid CaO, 2CaO·SiO₂–3CaO·P₂O₅ solid solution has not been observed. Therefore, the phosphorus partition ratio has not been calculated.

As shown in Fig. 7, current results agree well with the relationship observed by Ito et al.³¹⁾ Also, the dashed curve with arrows represents the composition change of liquid slag along the liquidus with decreasing SiO₂ concentration. As indicated by these arrows, the phosphorus partition ratio firstly increases and then decreases. The smallest value was observed when solid CaO, 2CaO·SiO₂–3CaO·P₂O₅ and liquid slag coexist. Comparing with the CaO–SiO₂–FeO–5mass%P₂O₅ system with the same experimental conditions,²⁶⁾ current results shift to the left side in Fig. 7, which reveals the movement of liquidus against the FeO apex after Al₂O₃ addition.

Fig. 7.

Relationship between phosphorus partition ratio and T.Fe content in liquid slag.

Fig. 8.

Relationship between phosphorus partition ratio and CaO content in liquid slag.

Similarly, the relationship between phosphorus partition ratio and CaO content in liquid phase is demonstrated in Fig. 8. Opposite to the effect of T.Fe content in liquid slag, the phosphorus partition ratio between solid solution and liquid slag decreases with increasing CaO content in liquid phase, which is consistent with the work of Pahlevani et al.³³⁾ The variation of liquid phase compositions with decreasing SiO₂ concentration along with liquidus is expressed as the dashed curve with arrows, which shows the similar trend shown in Fig. 7.

4. Conclusions

By using chemical equilibration technique, the phase relationship for the CaO–SiO₂–FeO–5mass%P₂O₅–5mass%Al₂O₃ with oxygen partial pressure of 9.24×10⁻¹¹ atm at 1673 K has been investigated. The conclusions are summarized as follows: the liquidus saturated with 2CaO·SiO₂–3CaO·P₂O₅ solid solution for the above slag system has been deduced. Solid solution almost consists of 2CaO·SiO₂ and 3CaO·P₂O₅, and the ratio between both varies with the liquid phase composition. Addition of Al₂O₃ enlarges the liquid phase area, while it does not affect the composition of solid solution since Al₂O₃ is mainly distributed to the liquid phase. At equilibrium, the large partition ratio of phosphorus between solid solution and liquid slag is found. However, this partition ratio does not reach larger value compared to that for the CaO–FeO–SiO₂ system with the concentration of Al₂O₃ in the liquid slag, because FeO content on the liquidus becomes smaller than that for the system without Al₂O₃.

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