Fluoride Evaporation of Low-Fluoride CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) Slag for Electroslag Remelting

To elucidate the behavior of fluoride evaporation in an electroslag remelting process, the non-isothermal evaporation of the low-fluoride CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) slag is studied using thermogravimetric analysis. The evaporation law of the melted slag is further verified using thermodynamic calculations. Fourier transformation infrared (FTIR) spectroscopy is used to evaluate the change in slag structure. It is discovered that the principal evaporating substances are CaF2, KF, and NaF, while the evaporation of MgF2, AlF3, and AlOF is less. KF evaporates absolutely in the early stage of the reaction, and CaF2 evaporates in a large proportion during the late reaction period. At 1500 °C, the order of vapor pressure is KF > CaF2. When K2O and Na2O are added to the residue sample at the same time, the evaporation ability of KF is stronger than that of CaF2 and NaF. As the K2O content increases from 0 to 8.3 wt%, evaporation increases from 0.76% to 1.21%. The evaporation rates of samples containing more K2O and those containing more Na2O are 1.48% and 1.32%, respectively. Under the same conditions, K2O has a greater effect on evaporation than Na2O. FTIR results show that the addition of K2O depolymerizes the network structure and that K2O can depolymerize the network structure better than Na2O.


Introduction
Electroslag remelting (ESR), a secondary refining technology combining refining and directional solidification, is widely used in the production of high-performance special steel and super alloys [1,2]. Considering the unique and outstanding advantages of ESR, such as uniform composition, effective removal of non-metallic inclusions, significant dephosphorization and desulfurization, and good surface quality, ESR has become an important technology in the metallurgical industry [3][4][5]. Slag plays an important role in the ESR process: (1) it acts as a heat source in the remelting process; (2) leads to efficient refining; (3) has a protective effect on the slag; and (4) during the solidification process of remelted metal, the ingot's surface develops a thin, homogeneous layer of slag that shields the mold from the high-temperature slag's direct impact while also making the ingot's surface smooth and simple to demold [6][7][8]. As the primary component of the electroslag system, the slag' surface tension and viscosity may be reduced using CaF 2 , and the slag's conductivity can be increased [9,10]. Around 40-70 wt% of conventional slag is made up of CaF 2 [11], which is the primary substance involved in fluoride evaporation during remelting. The evaporation of fluoride changes the components of the slag system, causing potential harm to health and safety, and gaseous substances released, such as HF, CaF 2 , AlF 3 , AlOF, MgF 2 and others, cause serious environmental pollution problems [12][13][14][15].

Thermodynamic Calculations
Factsage software (GTT Technologies, Aachen, Germany and Thermfact/CRCT, Montreal, QC, Canada) is a combination of two thermochemical software packages: FACT-Win and ChemSage. For this paper, the equilib module was used to calculate the weight of the evaporated species along with FToxid and FactPS databases. The selected temperature range was 650-1500 • C with an interval of 50 • C and a pressure of 1 atm. To assess the gaseous species evaporating from the slag at various temperatures and to quantify their weights, 100 g of each sample was used as the specimen.

XRD and FTIR Spectra Measurement
The pre-melted slag samples were used for X-ray diffraction (XRD, D8 Advance A25; Bruker AXS, Karlsruhe, Germany). As shown in Figure 1, no significant characteristic peaks were found in the samples, indicating that the slag sample was amorphous. Premelted amorphous slag was surveyed using a Fourier transform infrared spectroscope. FTIR spectroscopy (Nicolet iS5; Thermo Fisher Scientific, Waltham, MA, USA) was used to characterize the molten slag structure. Using a KBr detector with a spectral resolution of 4 cm −1 and a scan number of 32, FTIR transmission spectra in the 4000-400 cm −1 region were captured. Each 2 mg sample was combined with 200 mg of KBr in an agate mortar; the resulting mixture was then pressed into a clear flake measuring 13 mm in diameter.

Simultaneous Thermal Analysis Measurement
The non-isothermal evaporation behavior of the slag was evaluated using a simultaneous thermal analyzer (Netzsch STA449F3, Netzsch Instrument Inc., Selb, Germany). Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed on the slag. The temperature was determined using a B-index thermocouple under an argon atmosphere with a gas flow rate of 80 mL/min. For each TG-DSC measurement, approximately 50 mg of material was heated in a platinum crucible at a rate of 20 °C/min from 50 °C to 1500 °C for 1 min to remove bubbles and homogenize its chemical makeup. The platinum crucible dimensions were 5.5 mm inner diameter and 6.3 mm high.

Proportion of Fluorinated Gas Evaporation
To clarify the weight of the evaporation components and the proportions of the main fluorine gases released from the pre-melted slag, Factsage 8.0 software was used. It is clear from the calculations that the reaction begins at 650 °C, so the variation in the 650-1500 °C temperature range was studied. These calculated curves are visualized in Figure 2. These curves show the proportions of the main fluoride gases released in the pre-melted slag at different temperatures. The results show that when there is no K2O in the slag sample, CaF2 is the main evaporative substance, occupying an absolute advantage. When K2O is added, CaF2 and KF are the main evaporation species. KF begins to evaporate at 650 °C; in the temperature range of 650-900 °C, only the reaction of CaF2 and K2O to form KF occurs. At 900 °C, CaF2 tends to vaporize; when the temperature range is 1050-1500 °C, the evaporation ratio of CaF2 exceeds that of KF. The evaporation proportion of KF at each temperature point rises with the addition of K2O to the slag. MgF2 begins to evaporate at 900 °C; from 1350 °C, CaF2 reacts with Al2O3 to form AlF3 and AlOF. When compared with CaF2 and KF, the evaporation of MgF2, AlF3, and AlOF is negligible. When both K2O and Na2O are contained in the slag, CaF2, KF and NaF are the main evaporated substances. Both KF and NaF begin to evaporate from 650 °C. The evaporation of KF is gradually reduced, while the evaporation of NaF first increases and then decreases. In any case, at the beginning of the reaction, the vaporized gas is almost entirely KF; this is because K2O and CaF2 can react violently at a low temperature of 650 °C, while other oxides must reach a certain temperature in order to react violently.

Simultaneous Thermal Analysis Measurement
The non-isothermal evaporation behavior of the slag was evaluated using a simultaneous thermal analyzer (Netzsch STA449F3, Netzsch Instrument Inc., Selb, Germany). Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed on the slag. The temperature was determined using a B-index thermocouple under an argon atmosphere with a gas flow rate of 80 mL/min. For each TG-DSC measurement, approximately 50 mg of material was heated in a platinum crucible at a rate of 20 • C/min from 50 • C to 1500 • C for 1 min to remove bubbles and homogenize its chemical makeup. The platinum crucible dimensions were 5.5 mm inner diameter and 6.3 mm high.

Proportion of Fluorinated Gas Evaporation
To clarify the weight of the evaporation components and the proportions of the main fluorine gases released from the pre-melted slag, Factsage 8.0 software was used. It is clear from the calculations that the reaction begins at 650 • C, so the variation in the 650-1500 • C temperature range was studied. These calculated curves are visualized in Figure 2. These curves show the proportions of the main fluoride gases released in the pre-melted slag at different temperatures. The results show that when there is no K 2 O in the slag sample, CaF 2 is the main evaporative substance, occupying an absolute advantage. When K 2 O is added, CaF 2 and KF are the main evaporation species. KF begins to evaporate at 650 • C; in the temperature range of 650-900 • C, only the reaction of CaF 2 and K 2 O to form KF occurs. At 900 • C, CaF 2 tends to vaporize; when the temperature range is 1050-1500 • C, the evaporation ratio of CaF 2 exceeds that of KF. The evaporation proportion of KF at each temperature point rises with the addition of K 2 O to the slag. MgF 2 begins to evaporate at 900 • C; from 1350 • C, CaF 2 reacts with Al 2 O 3 to form AlF 3 and AlOF. When compared with CaF 2 and KF, the evaporation of MgF 2 , AlF 3 , and AlOF is negligible. When both K 2 O and Na 2 O are contained in the slag, CaF 2 , KF and NaF are the main evaporated substances. Both KF and NaF begin to evaporate from 650 • C. The evaporation of KF is gradually reduced, while the evaporation of NaF first increases and then decreases. In any case, at the beginning of the reaction, the vaporized gas is almost entirely KF; this is because K 2 O and CaF 2 can react violently at a low temperature of 650 • C, while other oxides must reach a certain temperature in order to react violently.

Vapor Pressure
The makeup of the slag system has a direct impact on the evaporation of fluoride. When K2O and Na2O are added to the slag system, the following chemical reactions will occur, as shown in Equations (1)-(6).

Vapor Pressure
The makeup of the slag system has a direct impact on the evaporation of fluoride. When K 2 O and Na 2 O are added to the slag system, the following chemical reactions will occur, as shown in Equations (1)- (6).
Na 2 O (s) + CaF 2 (s) = 2 NaF (g) + CaO (s) The thermodynamics in the previous section calculated that CaF 2 , KF, and NaF are the main evaporation products, and the generated MgF 2 , AlF 3 and AlOF evaporation gases are lesser and negligible. Thus, Equations (1)-(3) are anticipated to play a major part in the evaporation process.
The propensity of the molecules in solution to separate from the slag and enter space is reflected in the vapor pressure of fluoride gas, a tendency also known as escape tendency. The equilibrium constants of chemical Equations (1)-(3) can be expressed as follows: Here, P i represents the vapor pressures (atm) of melt component i and a i is the activity of the melt component i in the molten slags. The FactSage 8.0 software was used to calculate the activities and equilibrium constants of CaF 2 , Na 2 O, and K 2 O with respect to the pure liquid standard state and CaO with respect to the pure solid standard state. According to calculations, the equilibrium constants for Equations (1)-(3) are 4.45 × 10 −4 [26], 1.10 × 10 5 [27], and 1.04 × 10 2 [28], respectively. Table 2 provides the data for the activity calculations.

CaF 2 KF NaF
By inputting the equilibrium constant values and activity data into Equations (7)-(9), the vapor pressures of CaF 2 , KF, and NaF are determined. The outcomes are shown in Tables 3 and 4.  When there are various vapor pressures in a gas mixture, the component with the greater vapor pressure will evaporate first. KF has a greater vapor pressure than CaF 2 at 1500 • C; this indicates that KF is more likely to evaporate at this temperature. When both K 2 O and Na 2 O are added to the slag sample, KF is always in the leading position, and its evaporation ability is stronger than that of CaF 2 and NaF.

TG and DSC Analysis of the Slag
Two experiments with the same conditions were carried out on K3 slag. Dotted lines depict the outcomes of the second experiment. The curves in Figure 3 almost overlap, indicating that the experiment is reproducible. TG and DSC experimental results of K 2 O content are summarized in Figure 4. It should be observed that when K 2 O content steadily increases from 0% to 8.3 wt%, evaporation increases. When the K 2 O content in the slag is 0, 2.3, 5.3, and 8.3 wt%, the evaporation rates of the slag are 0.76%, 0.93%, 1.09%, and 1.21%, respectively. The evaporation is greatest at 8.3 wt% of K 2 O and lowest without K 2 O. K 2 O plays a role in promoting evaporation in the slag system.  When there are various vapor pressures in a gas mixture, the component with the greater vapor pressure will evaporate first. KF has a greater vapor pressure than CaF2 at 1500 °C; this indicates that KF is more likely to evaporate at this temperature. When both K2O and Na2O are added to the slag sample, KF is always in the leading position, and its evaporation ability is stronger than that of CaF2 and NaF.

TG and DSC Analysis of the Slag
Two experiments with the same conditions were carried out on K3 slag. Dotted lines depict the outcomes of the second experiment. The curves in Figure 3 almost overlap, indicating that the experiment is reproducible. TG and DSC experimental results of K2O content are summarized in Figure 4. It should be observed that when K2O content steadily increases from 0% to 8.3 wt%, evaporation increases. When the K2O content in the slag is 0, 2.3, 5.3, and 8.3 wt%, the evaporation rates of the slag are 0.76%, 0.93%, 1.09%, and 1.21%, respectively. The evaporation is greatest at 8.3 wt% of K2O and lowest without K2O. K2O plays a role in promoting evaporation in the slag system.  The thermogravimetric curves of K1, K2, and K3 show a sudden and violent weight loss at around 650 • C. This is likely due to the beginning of the reaction between K 2 O and CaF 2 to form KF; this can be verified using the thermodynamic calculations shown in Section 3.1. A second violent weight loss occurs at 1200-1500 • C. As can be seen in Figure 2, a large amount of CaF 2 gas is generated at this stage, as along with a small amount of KF, MgF 2 , AlF 3 , and AlOF. This process involves Equations (1), (2) and (4) The thermogravimetric curves of K1, K2, and K3 show a sudden and violent weight loss at around 650 °C. This is likely due to the beginning of the reaction between K2O and CaF2 to form KF; this can be verified using the thermodynamic calculations shown in Section 3.1. A second violent weight loss occurs at 1200-1500 °C. As can be seen in Figure 2, a large amount of CaF2 gas is generated at this stage, as along with a small amount of KF, MgF2, AlF3, and AlOF. This process involves Equations (1), (2) and (4)- (6).
The DSC curves in dashed lines are shown in Figure 4. In general, this process can be broken down into two phases: (I) the glass transition period, which corresponds to the exothermic peak, indicating the event that amorphous glassy phase transforms to crystalline state during the heating process, and (II) the melting period, which corresponds to two endothermic peaks [29]. The melting stage is primarily the evaporation behavior of fluoride. The first endothermic peak at this stage clearly shows that only A slag without K2O is not formed. This endothermic peak represents the continuous reaction of K2O and CaF2 to form KF; KF is still escaping during this process. The second endothermic peak in the melting stage represents the evaporation of CaF2 itself and the reaction of Al2O3 and MgO with CaF2. The endothermic peak of A slag shifts to a higher temperature, showing that A slag requires a higher temperature to react, thus indicating that K2O can reduce the melting temperature of the slag. Figure 5 presents the TG and DSC curves of samples K4 and K5. The evaporation rates of samples K4 and K5 are 1.48% and 1.32%, respectively. By comparing the TG diagrams of K4 and K5 slag, it can be understood that under the premise that the total amount of K2O and Na2O added is equal, when the amount of K2O added is more than Na2O, the amount of evaporation is relatively large, indicating that K2O has a greater impact on evaporation. The DSC curve's exothermic peak denotes the transition from the amorphous to the crystalline state. The transformation temperature and peak intensity of K5 slag are higher than those of K4 slag; this shows that the transformation driving force of K4 slag is larger. The DSC curves in dashed lines are shown in Figure 4. In general, this process can be broken down into two phases: (I) the glass transition period, which corresponds to the exothermic peak, indicating the event that amorphous glassy phase transforms to crystalline state during the heating process, and (II) the melting period, which corresponds to two endothermic peaks [29]. The melting stage is primarily the evaporation behavior of fluoride. The first endothermic peak at this stage clearly shows that only A slag without K 2 O is not formed. This endothermic peak represents the continuous reaction of K 2 O and CaF 2 to form KF; KF is still escaping during this process. The second endothermic peak in the melting stage represents the evaporation of CaF 2 itself and the reaction of Al 2 O 3 and MgO with CaF 2 . The endothermic peak of A slag shifts to a higher temperature, showing that A slag requires a higher temperature to react, thus indicating that K 2 O can reduce the melting temperature of the slag. Figure 5 presents the TG and DSC curves of samples K4 and K5. The evaporation rates of samples K4 and K5 are 1.48% and 1.32%, respectively. By comparing the TG diagrams of K4 and K5 slag, it can be understood that under the premise that the total amount of K 2 O and Na 2 O added is equal, when the amount of K 2 O added is more than Na 2 O, the amount of evaporation is relatively large, indicating that K 2 O has a greater impact on evaporation. The DSC curve's exothermic peak denotes the transition from the amorphous to the crystalline state. The transformation temperature and peak intensity of K5 slag are higher than those of K4 slag; this shows that the transformation driving force of K4 slag is larger.

FTIR Spectra of the Slag Sample
The FTIR spectra of the CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) slag system as a function of wavenumber is shown in Figure 6. The spectral bands can be divided into three ranges: 930-660 cm −1 , 660-550 cm −1 , and 550-450 cm −1 , which correspond to the asymmetric stretching vibration of the (AlOnF4−n)-tetrahedral complexes (n = 0-4), (AlO4)tetrahedra, and (AlO6)-octahedra, respectively. Moreover, the Ti-O stretching vibration is seen at a wave number of around 425 cm −1 [30].  The transmission band of the (AlOnF4−n)-tetrahedral complexes exists in the fluoroaluminate system. The existence of (AlOnF4−n)-tetrahedral complexes is based on the increased vibrational asymmetry caused by the coexistence of Al-F and Al-O bonds in the complexes [31]. As K2O content increases, the gravity center of the (AlOnF4−n)-tetrahedral complexes shifts to lower wavenumbers. This transition means that the distance between Al and O widens, that is, increasing the K2O content can depolymerize the network

FTIR Spectra of the Slag Sample
The FTIR spectra of the CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) slag system as a function of wavenumber is shown in Figure 6. The spectral bands can be divided into three ranges: 930-660 cm −1 , 660-550 cm −1 , and 550-450 cm −1 , which correspond to the asymmetric stretching vibration of the (AlOnF4−n)-tetrahedral complexes (n = 0-4), (AlO4)tetrahedra, and (AlO6)-octahedra, respectively. Moreover, the Ti-O stretching vibration is seen at a wave number of around 425 cm −1 [30].  The transmission band of the (AlOnF4−n)-tetrahedral complexes exists in the fluoroaluminate system. The existence of (AlOnF4−n)-tetrahedral complexes is based on the increased vibrational asymmetry caused by the coexistence of Al-F and Al-O bonds in the complexes [31]. As K2O content increases, the gravity center of the (AlOnF4−n)-tetrahedral complexes shifts to lower wavenumbers. This transition means that the distance between Al and O widens, that is, increasing the K2O content can depolymerize the network The transmission band of the (AlO n F 4−n )-tetrahedral complexes exists in the fluoroaluminate system. The existence of (AlO n F 4−n )-tetrahedral complexes is based on the increased vibrational asymmetry caused by the coexistence of Al-F and Al-O bonds in the complexes [31]. As K 2 O content increases, the gravity center of the (AlO n F 4−n )-tetrahedral complexes shifts to lower wavenumbers. This transition means that the distance between Al and O widens, that is, increasing the K 2 O content can depolymerize the network structure in the molten slag [32]. In other words, the degree of polymerization of (AlO n F 4−n )tetrahedral complexes decreases with the increase in K 2 O content. (AlO 4 )-tetrahedra behaves as network formers. At a wave number of about 625 cm −1 , a trough appears in the (AlO 4 )-tetrahedra; the trough is even smaller in the absence of K 2 O addition. The Al-O bond in the (AlO 4 )-tetrahedral structure has a gentler slope at 620 cm −1 ; this indicates a more complex network structure when K 2 O is not added. When K 2 O, a strong alkali oxide, is added, free oxygen ions (O 2− ) can be produced in the slag. O 2− can break the Al-O bond of the (AlO 4 )-tetrahedral structure, depolymerizing the network complex structure of aluminate into simpler structural units. The center of gravity of (AlO 6 )-octahedra shifts to a higher wave number, and the transmission peak intensity of (AlO 6 )-octahedra increases relatively. According to previous studies [33], this is because O 2− provided by K 2 O reacts with (AlO 4 )-tetrahedra to produce (AlO 6 )-octahedra, thus promoting the depolymerization of slag network structure. This is related to the Ti-O stretching vibration at about 420 cm −1 , and varying K 2 O content has a slight effect on the structure. This demonstrates that the Ti-O bond in the titanate structure is less affected by increasing the K 2 O level.
Through the above structural analysis, the addition of K 2 O to the slag significantly depolymerizes (AlO n F 4−n )-tetrahedral complexes, (AlO 4 )-tetrahedra, and (AlO 6 )-octahedra network structures, but has little effect on the stretching vibration of Ti-O. It further explains the TG curves of the previous section, where evaporation intensifies with increasing K 2 O content. Figure 7 compares the structural effects of Na 2 O and K 2 O on the slag. The spectrum of the current slag system shows four visible transmittance peaks, which corresponds to asymmetric stretching vibration of the (AlO n F 4−n )-tetrahedral complexes (n = 0-4) in the wavenumbers of 900-680 cm −1 , the (AlO 4 )-tetrahedra in the wavenumbers of 680-600 cm −1 , the (AlO 6 )-octahedra in the wavenumbers of 600-490 cm −1 , and the stretching vibration of Ti-O at 420 cm −1 .
structure in the molten slag [32]. In other words, the degree of polymerization of (AlOnF4−n)-tetrahedral complexes decreases with the increase in K2O content. (AlO4)-tetrahedra behaves as network formers. At a wave number of about 625 cm −1 , a trough appears in the (AlO4)-tetrahedra; the trough is even smaller in the absence of K2O addition. The Al-O bond in the (AlO4)-tetrahedral structure has a gentler slope at 620 cm −1 ; this indicates a more complex network structure when K2O is not added. When K2O, a strong alkali oxide, is added, free oxygen ions (O 2− ) can be produced in the slag. O 2− can break the Al-O bond of the (AlO4)-tetrahedral structure, depolymerizing the network complex structure of aluminate into simpler structural units. The center of gravity of (AlO6)-octahedra shifts to a higher wave number, and the transmission peak intensity of (AlO6)-octahedra increases relatively. According to previous studies [33], this is because O 2− provided by K2O reacts with (AlO4)-tetrahedra to produce (AlO6)-octahedra, thus promoting the depolymerization of slag network structure. This is related to the Ti-O stretching vibration at about 420 cm −1 , and varying K2O content has a slight effect on the structure. This demonstrates that the Ti-O bond in the titanate structure is less affected by increasing the K2O level.
Through the above structural analysis, the addition of K2O to the slag significantly depolymerizes (AlOnF4−n)-tetrahedral complexes, (AlO4)-tetrahedra, and (AlO6)-octahedra network structures, but has little effect on the stretching vibration of Ti-O. It further explains the TG curves of the previous section, where evaporation intensifies with increasing K2O content. Figure 7 compares the structural effects of Na2O and K2O on the slag. The spectrum of the current slag system shows four visible transmittance peaks, which corresponds to asymmetric stretching vibration of the (AlOnF4−n)-tetrahedral complexes (n = 0-4) in the wavenumbers of 900-680 cm −1 , the (AlO4)-tetrahedra in the wavenumbers of 680-600 cm −1 , the (AlO6)-octahedra in the wavenumbers of 600-490 cm −1 , and the stretching vibration of Ti-O at 420 cm −1 .  It can be seen in Figure 7 that the (AlOnF4−n)-tetrahedral complexes' center of gravity changes to higher wavenumbers; this indicates that the addition of K2O may result in a lesser degree of polymerization of the network structure in the slag than that of Na2O. In the (AlO4)-tetrahedra wavenumber range, the K5 slag with a greater Na2O content (about 660 cm −1 ) exhibits a smaller slope, indicating that adding more Na2O will show a more complex network structure. The addition of K2O and Na2O has no obvious effect on the It can be seen in Figure 7 that the (AlO n F 4−n )-tetrahedral complexes' center of gravity changes to higher wavenumbers; this indicates that the addition of K 2 O may result in a lesser degree of polymerization of the network structure in the slag than that of Na 2 O. In the (AlO 4 )-tetrahedra wavenumber range, the K5 slag with a greater Na 2 O content (about 660 cm −1 ) exhibits a smaller slope, indicating that adding more Na 2 O will show a more complex network structure. The addition of K 2 O and Na 2 O has no obvious effect on the transmission band of the (AlO 6 )-octahedra. The stretching vibration of Ti-O around 420 cm −1 has no obvious change, indicating that the addition of K 2 O or Na 2 O has little effect on the Ti-O bond in the titanate structure.
Based on the aforementioned structural analysis, when the total amount of K 2 O and Na 2 O is the same, slag containing more K 2 O can better depolymerize (AlO n F 4−n )tetrahedral complexes and (AlO 4 )-tetrahedra network structures, and has little effect on (AlO 6 )-octahedra and Ti-O stretching vibration. This indicates that the K + provided by K 2 O is more advantageous than the Na + provided by Na 2 O. This can also explain why the evaporation of K 2 O is more intense for the same total amount of Na 2 O and K 2 O added.

1.
When the slag contains both K 2 O and Na 2 O, the main evaporating substances are CaF 2 , KF, and NaF. In comparison, MgF 2 , AlF 3 , and AlOF seldom ever evaporate. KF and NaF begin to evaporate at 650 • C. NaF increases and subsequently decreases as the temperature rises, while KF decreases as temperature rises. At 900 • C, CaF 2 tends to evaporate, and the evaporation intensifies with the increase of temperature. At the beginning of the reaction, KF dominates absolutely, while CaF 2 dominates when it exceeds 1050 • C.

2.
The vapor pressure of KF is stronger than that of CaF 2 at 1500 • C. When K 2 O and Na 2 O are added to the residue sample at the same time, the evaporation ability of KF is stronger than CaF 2 and NaF. 3.
Evaporation increases from 0.76% to 1.21% when K 2 O content rises from 0% to 8.3 wt%. The evaporation rates of samples K4 and K5 are 1.48% and 1.32%, respectively. Under the premise that the total amount of K 2 O and Na 2 O added is equal, when the amount of K 2 O added is greater than Na 2 O, the evaporation rate is relatively large, indicating that K 2 O has a significant influence on evaporation.

4.
FTIR results show that with the addition of K 2 O, the (AlO n F 4−n )-tetrahedral complexes, (AlO 4 )-tetrahedra, and (AlO 6 )-octahedra network structures are depolymerized; this has little effect on the stretching vibration of Ti-O. Comparing the effects of Na 2 O and K 2 O addition under the same conditions, it is found that the slag with higher K 2 O content can better depolymerize the (AlO n F 4−n )-tetrahedral complexes and (AlO 4 )-tetrahedra network structures, and has little effect on the (AlO 6 )-octahedra and Ti-O stretching vibration.

5.
Although the addition of a small amount of alkali metals can promote the partial evaporation of slag, it can also change the melting characteristics of the ESR slag system, including viscosity and melting temperature, which are conducive to the melting of slag system. The applicability of these characteristics in industry should be considered comprehensively.