Phosphate Capacities of CaO–FeO–SiO₂–Al₂O₃/Na₂O/TiO₂ Slags

Definition of phosphate capacity

Phosphate capacity is an important indicator of the dephosphorization ability of slag. According to Wagner, dephosphorization can be expressed as shown in eq. (1). The phosphate capacity of the slag was defined by Wagner and expressed in eq. (2) [9].

Here, (%PO43−) is the weight percent of PO43− dissolved in the slag; PO2is the partial pressure of oxygen, and PP2is the partial pressure of phosphorus. To get the value of PP2, the eq. (3) is introduced [8]. Gγ0 is the standard Gibbs free energy of Pin γ−Fe, which can be calculated using eq. (5) [10]. GP20is the standard Gibbs free energy of PP2, whose values at 1300, 1350, and 1400°C are −250,446, −263,147, and −276,217 J/mol, respectively (extracted from FactSage 7.0). Then, by substituting eq. (4) into eq. (2), the logarithmic form of the phosphate capacity can be obtained as shown in eq. (6).

Here, MMPO43−/MMP is the ratio of the molar mass of phosphate in the slag and phosphorus in the metal, which is equal to 3.07. PO2 is determined by the activity of FeO from eqs. (7) and (8) [11, 12]. Where aFe=1 for the pure solid iron. L_P, which is expressed in eq. (9), is the phosphorus distribution ratio between the slag and metal at equilibrium, and it is experimentally determined. (%P) and [%P]  are the contents of phosphorus in the slag and the solid iron, respectively.

Calculation of the activity of FeO in CaO–FeO– SiO₂– Al₂O₃– Na₂O–TiO₂– P₂O₅ slag system

According to eq. (6), PO2and L_P are important factors for obtaining (CPO43−). PO2 is determined by the activity of FeO ((CPO43−)) using eqs. (7) and (8). L_P between the slag and solid iron foil can be obtained from slag–metal equilibrium experiments. Hence, (CPO43−) for slag needs to be first known for obtaining (CPO43−). In this study, (CPO43−) was calculated using the thermodynamic model of the ion and molecule coexistence theory (IMCT).

IMCT was originally developed to reflect the reaction ability of components in the metallurgical slag according to the defined mass action concentrations, N_i, of the structural units or ion couples in terms of the mass action law. The defined N_i in the structural unit or ion couples in the slag has been verified to be consistent with the reported activities of the components relative to the pure solid or liquid in the standard state. The basic hypothesis of the IMCT-N_i thermodynamic model for calculating the N_i of the structural units or ion couples in the metallurgical slag has been reported in detail elsewhere [13, 14]. The chemical formulae of the possibly formed complex molecules, their standard molar Gibbs free energies, and standard reaction equilibrium constants K_i were obtained from the database of the thermodynamic computing software FactSage and previous studies [14, 15, 16]. For the process of model establishment and calculation, please refer to the classical references on the IMCT thermodynamic model reported in detail elsewhere [13, 14, 15, 16].

To verify the accuracy of the calculation model based on the IMCT, values of (CPO43−) measured and those estimated from the model for different slag systems were compared (Figure 1). Espejo [17] and Ogura [18] have independently measured the activity of Fe_tO in the CaO–Al₂O₃–FeO_t and CaO–SiO₂–FeO_t slags at 1400°C. The results obtained from the calculation model of the IMCT were in reasonable agreement with the measured values of (CPO43−) (Figure 1), indicating that the model for calculating (CPO43−) is credible.

Figure 1:

Comparison of the calculated and observed values of (CPO43−).

Samples

The materials and chemical reagents included iron foil, CaO, Fe₃O₄, SiO₂, Al₂O₃, Na₂SiO₃, TiO₂, and P₂O₅. High-purity iron foil (0.1 mm thickness, mass Fe>99.999%) was used, and the content of P in the iron foil was less than 0.0001 mass%. FeO was produced by the reduction of Fe₃O₄ under CO (flow rate: 3 L/min) for 5 h at 900°C and subsequent quenching under pure argon. After cooling, the solid material was ground to a particle size of 200 µm. The sample was analyzed by X-ray diffraction (XRD). The XRD pattern confirmed that high-purity FeO is obtained. Na₂O in the form of Na₂SiO₃ was added to prevent the evaporation during the high-temperature process [3]. The P₂O₅ content was maintained constant in all experiments conducted in a group but was changed between experimental groups. CaO was dried in a drying box at 120°C for 24 h before use.

The slag sample was formed by mixing CaO, FeO, SiO₂, P₂O₅, Al₂O₃, Na₂SiO₃, and TiO₂ in a porcelain mortar. After mixing well, the slagging agent was dried at 120°C for 24 h and then formed into cylindrical pellets using a presser.

Table 1 summarizes all the experimental results including the slag and metal compositions. For experiments Nos. 1–15, quaternary CaO–FeO–SiO₂–Al₂O₃/Na₂O/TiO₂ slag systems were used to verify the effects of Al₂O₃, Na₂O, and TiO₂ on the phosphate capacity of slag. For experiments Nos. 16–54, the six-component slag system CaO–FeO–SiO₂–Al₂O₃–Na₂O–TiO₂ was used, and the effects of the temperature, binary basicity, and FeO content were examined.

Apparatus and procedure

Figure 2 shows the schematic of the experimental apparatus used, which included a horizontal electric resistance furnace, a water-cooling system, and an associated purification plant for argon. The furnace equipped with MoSi₂ heating elements was controlled by a PID controller using a Pt–30%Rh/Pt–6%Rh thermocouple as the sensor, which was calibrated before use. The temperature of the furnace was controlled at 25–1700°C within ± 1°C. The equilibrium experiments were conducted under pure argon. The associated purification plant for argon consisted of allochroic silica gel for dehydration and magnesium and copper chips (heated to 500°C) for deoxidation.

Figure 2:

Schematic of the experiment setup (1-heating zone; 2-iron crucible; 3-thermocouple; 4-porcelain boat composed of Al₂O₃; 5-water-cooling device; 6-magnesium pieces; 7-copper pieces; 8-molecular sieve; 9-allochroic silica gel).

First, 20 g of the slagging pellets were added into an Armco iron crucible (mass Fe>99.8%, with an outer diameter of 27 mm, inner diameter of 25 mm, and a height of 31 mm). With the addition of the pellets in the crucible, 2 g of iron foil was cut into suitable pieces and placed in the gap between every two pellets. Each porcelain boat could hold five iron crucibles, which was placed in the heating zone and tied by a molybdenum wire. Then, argon (flow: 400 mL/min) was allowed to flow, and the furnace was switched on. In this study, the target temperatures were 1300°C, 1350°C, and 1400°C. The melt was equilibrated for 8–12 h. Previously, IM and MORIT [8] have determined that this equilibration time is sufficiently long to establish equilibrium. An equilibration time of 12 h was utilized herein. After equilibration, the porcelain boat was rapidly placed into the cooling zone under argon. The samples were quenched and removed from the furnace.

Analysis

After the iron pieces were cooled, the pieces were separated from the slag. Next, the slag was ground to a particle size of 200 µm and analyzed by X-ray fluorescence. As the presence of even a small amount of the slag in the iron phase can lead to significant errors in the phosphorus content, the iron foil pieces were carefully polished using a stainless-steel brush. Finally, the pieces were ultrasonically cleaned in a citric acid/acetone mixture and subsequently in deionized water. The content of P in the cleaned iron foil pieces was analyzed by the molybdenum blue colorimetric method. Then, L_P was determined, and (CPO43−) was calculated using the thermodynamic model of IMCT. Finally, CPO43−was determined using eq. (6).

Effect of temperature on (CPO43−)

Figure 3 shows the dependence of temperature on the phosphate capacity. Figure 3(a) shows the results obtained from experiment Nos. 36–40 and 50–54 in Table 1. For these experiments, the slag composition was similar. For similar slag compositions, (CPO43−) decreased with increasing temperature (Figure 3(a)). This result is in good agreement with the thermodynamic predictions for dephosphorization, which is exothermic and hence occurs at low temperatures. Wrampelmeyer [19] has measured and calculated the phosphate capacity of CaO–FeO_t–Al₂O₃ slag at 1550, 1600, and 1700°C. The results from his study indicate that the phosphate capacity decreases with increasing temperature, which is in agreement with that reported herein.

Figure 3:

Effect of temperature on (CPO43−): (a) Samples for Nos. 36–40 and 50–54 in Table 1 and (b) Samples for Nos. 16–54 in Table 1.

Figure 3(b) shows the dependence of the phosphate capacity on the temperature for the complete six-component slag system (Nos. 16–54 in Table 1). Although their composition significantly varied, (CPO43−) clearly decreased with increasing temperature, suggesting that temperature exerts a more significant effect on (CPO43−) compared to the other influencing factors. However, from Figure 3(b), the data encircled by a solid line (Nos. 23–25 in Table 1) revealed considerably lower phosphate capacities even at a low temperature of 1300°C because these samples exhibited low basicity (0.2≤R<0.8, R=(CaO%)/(SiO₂%)). This result suggested that an extremely low basicity (R<0.8) leads to an extremely weak phosphate fixing ability, related to the fact that extremely limited CaO is available for combination with P₂O₅ in the slag (as most of the CaO is assumed to combine with SiO₂).

Effect of the slag basicity

Figure 4 shows the effect of the binary basicity on (CPO43−). Figure 4(a) summarizes the results from the two experimental groups, Nos. 21–25 and 41–44, in Table 1: Both groups showed similar compositions of FeO–Al₂O₃–Na₂O–TiO₂ but different (%CaO)/(%SiO₂) values. With increasing the binary basicity at 1300°C and 1350°C, (CPO43−) increased. Similar results have been reported by Im et al. [8] during their study on the phosphate capacity of CaO–FeO–SiO₂–P₂O₅ with basicities of 0.5–1.0 at 1300°C. However, a larger slope between (CPO43−) and basicity was observed at low temperature (1300°C) and low basicities (R=0.2–1.0, black rectangle solid points in Figure 4(a)). Moreover, (CPO43−) was 22.0 at a low basicity of 0.9 but at a low temperature (1300°C). This value is greater than that in all cases in which the basicity of slag ranges from 1.2 to 2.4 but at a high temperature (1350°C, (CPO43−)). This result indicated that under conditions of a slag basicity greater than 0.9, temperature considerably affects (CPO43−) compared to basicity.

Figure 4:

Dependence of (CPO43−) on the slag binary basicity: (a) Samples for Nos. 21–25 and 41–44 In Table 1 and (b) Samples for Nos. 16–54 in Table 1.

This result was further verified by the data shown in Figure 4(b), indicating the dependence of the binary basicity on (CPO43−) for all of the six-component slag samples under different compositions and temperatures (Nos. 16–54 in Table 1). Even the dependencies of the basicity were similar to those observed in Figure 5(a). The largest (CPO43−) was observed for R=1.0 at 1300°C (pink triangle solid points), while the lowest values were observed at R=0.8–1.2 at 1350°C (Nos. 50–54 in Table 1) despite the large variation in the basicity (R=0.2–2.4). This result indicated that temperature exerts a stronger effect on (CPO43−) compared to basicity and tends to be the most important influencing factor under the conditions utilized herein.

Figure 5:

Effect of Na₂O on (CPO43−): (a) Samples for Nos. 6–10 in Table 1 and (b) Samples for Nos. 6–10 and 16–54 in Table 1.

Effect of the content of Na₂O in slag

Figure 5(a) show the dependence of (CPO43−) on the content of Na₂O in the slag while simultaneously maintaining the temperature and other slag constituents constant (Nos. 6–10 in Table 1, quaternary slags). With increasing Na₂O content, (CPO43−) increased linearly. With the increase in the Na₂O content from 0.08% to 6.94%, (CPO43−) increased by approximately 0.44. The same trend has been reported by Diao and Li et al. [1, 2]. With the increase in the Na₂O content by 2.3% at 1350℃ in the former study, (CPO43−) increases by 0.416. With the increase in the Na₂O content by 4.07% at 1600℃ in the latter study, (CPO43−) increases by 0.31. In general, the comparison of the three studies showed that Na₂O significantly affects the phosphate capacity.

Figure 5(b) shows the dependence of the phosphate capacity on the Na₂O content for all of the Na₂O-containing six-component slag system samples (Nos. 6–10 and 16–54 in Table 1). As can be observed from the figure, the phosphate capacity was considerably affected ((CPO43−)=21.98, (CPO43−)=21.35) even with an almost constant content of Na₂O (Na₂O=3.06%, Na₂O=3.01%) because of the variations in the other factors such as the basicity and temperature. However, a clear trend was still observed with the exception of cases at temperatures less than 1400°C: (CPO43−) increased with increasing Na₂O content, indicating that the Na₂O content also significantly affects dephosphorization. On the other hand, the lowest (CPO43−) was still observed for experiments conducted at a high temperature of 1400°C despite the large variation in the Na₂O content (Na₂O%=0.08%–8.33%). Further, this result strongly suggested that temperature exerts the most significant effect. A high temperature of 1400°C considerably decreases the phosphate capacity of slag.

Effect of FeO content

Figure 6(a) (data from No. 16–20 and 45–49 in Table 1) shows the effect of the FeO content on (CPO43−), where the concentrations of all of the other slag constituents were nearly unchanged at a constant temperature. With the increase in the FeO content, (CPO43−) decreased. With the increase in the FeO content from 12.3% to 45.3% at R=1 (1300°C) and from 15.1% to 50.64% at R=2 (1350°C), (CPO43−) decreased by 0.69 and 0.64, respectively. Nakamura [20] and Wrampelmeyer [19] have calculated the phosphate capacities for the CaO_satd.–BaO–SiO₂–Fe_tO slag system at 1600℃ and the CaO–FeO_t–Al₂O₃ slag system at 1550℃, 1600℃, and 1700℃. The results of the two studies revealed that (CPO43−) linearly decreases with the increase in the FeO content. This result is consistent with those obtained from this study. The phosphate capacity reported by Nakamura is considerably higher than that reported by Wrampelmeyer because of the addition of strong alkaline substances such as BaO in the CaO_satd.–BaO–SiO₂–Fe_tO slag system.

Figure 6:

Effect of FeO on (CPO43−): (a) Samples for Nos. 16–20 and 45–49 in Table 1 and (b) samples for Nos. 6–10 and 16–54 in Table 1.

Figure 6(b) shows the dependence of the phosphate capacity on the FeO content with various slag components and temperatures (Nos. 16–54 in Table 1). With the exception of cases at a temperature of 1400°C, (CPO43−) decreased with the increase in the FeO content, even with a large fluctuation, because of the different parameters, e. g., variation in basicity and temperature. This result revealed that the FeO content in slag acts as another important factor and the high FeO content does not lead to the improvement in the phosphate capacity. Generally, the high FeO content leads to the increase in the dephosphorization efficiency. Actually, the dephosphorization of hot metal or liquid steel is divided into two steps: first, P in the metal phase was oxidized to P₂O₅ by the dissolved oxygen (eq. (10)), and then, P₂O₅ was fixed in the slag phase via the combination of basic materials such as CaO to form 3CaO· P₂O₅ and 4CaO· P₂O₅ (eq. (11)). A high content of FeO in slag promoted the performance of the first step via the increase in the oxygen activity of the metal phase to push the equilibrium of eq. (10) to the right side. However, the phosphate capacity theoretically reflects the phosphorus fixing ability of slag, and basic materials such as CaO play an important role for the fixation of phosphorus. The phosphate capacity increased with basicity. The high FeO content decreased the amount of the available CaO, thereby retarding the reaction in eq. (11). Hence, the phosphate capacity decreases with the increase in the FeO content.

Effects of the content of Al₂O₃ and TiO₂

Figure 7(a) and (b) show the effects of the Al₂O₃ content on (CPO43−) while simultaneously maintaining the remaining components of the slag and temperature constant (Nos. 1–5 in Table 1, quaternary slags) and under varying similar conditions (Nos. 1–5 and 16–54 in Table 1), respectively. With the increase in the Al₂O₃ content, (CPO43−) decreased. This result is in agreement with that reported by Diao and Li et al. (Figure 7(a)). [1, 2] According to the ionic theory of slags, (CPO43−) replaces (CPO43−) and then precipitates as Ca₃(PO₄)₂ during dephosphorization. At the same time, (CPO43−) is replaced by (CPO43−). At an extremely high Al₂O₃ content, the precipitation of (CPO43−) is suppressed, which adversely affects the fixing of (CPO43−) in slag. In this case, Al₂O₃ exhibited the characteristics of an acidic oxide [21]. Figure 7(c) shows the dependency of phosphate on the content of TiO₂ in the slag. The content of TiO₂ in the slag exerted a considerably weaker effect on (CPO43−) compared to other factors of temperature, basicity, and other slag components, e. g., FeO, Na₂O, and Al₂O₃ content.

Figure 7:

Effects of Al₂O₃ and TiO₂ on (CPO43−): (a) Samples for Nos. 1–5 in Table 1 and (b) samples for Nos. 1–5 and 16–54 in Table 1, and (c) samples for Nos. 11–54 in Table 1.

Regression analysis of (CPO43−)

Based on the data from Nos. 16–54 in Table 1, (CPO43−) was fitted as a function of various factors of temperature and the slag composition, i.e., of CaO, SiO₂, Al₂O₃, Na₂O, TiO₂, and FeO content using the Statistical Product and Service Solutions (SPSS) software [22], expressed in eq. (12). Figure 8 shows the calculation data using eq. (12) and the experimental results, which showed good agreement.

Figure 8:

Comparison of the observed values of (CPO43−) and the fitted curve.