Performance Study of CaO-CaF₂- and CaO-Al₂O₃-SiO₂-Based High-Efficiency Desulfurizers

Abstract

In order to reduce the content of harmful impurity sulfur elements in steel to meet the quality requirements of high value-added steel, efficient desulfurization of RH vacuum degassing is essential. Based on the simplex lattice composition design method, the effects of typical compositions on liquidus temperature, sulfur capacity, melting temperature, the effects of typical compositions on liquidus temperature, sulfur capacity, melting temperature, viscosity, and desulfurization rate of CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers were studied by thermodynamic calculation, the melting temperature test, and the slag–steel contact experiment. The results show that in CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers, the changes in CaF₂, MgO, and Al₂O₃ contents has little effect on the equilibrium S content of molten steel at lower SiO₂ contents, whereas, at higher SiO₂ contents, the equilibrium S content of the molten steel is greatly increased when the CaF₂, MgO, and Al₂O₃ content is greater than a certain value. Meanwhile, the increase in CaF₂ and MgO content reduces the high-temperature viscosity and breaking temperature (corresponding to the turning point on the viscosity–temperature curve) to varying degrees, which results in a better slag fluidity and is favorable to the prevention of crusting. With the increase in Al₂O₃ and SiO₂ content, the breaking temperature of the CaO-CaF₂-based desulfurizer is significantly reduced, which is beneficial to preventing crust. However, when the breaking temperature of CaO-Al₂O₃-SiO₂-based desulfurizer increases, part of the slag system has solidified at 1400 °C, which is easy to lead to slag crust when the temperature drops. Comprehensively, for the CaO-CaF₂-based desulfurizer, CaO = 60 wt%, CaF₂ = 30 wt%, SiO₂ = 0–5 wt%, and add a small amount of Al₂O₃ and MgO, its desulfurization effect is significant. For the CaO-Al₂O₃-SiO₂-based desulfurizer, CaO = 39–57 wt%, Al₂O₃ = 20–35 wt%, SiO₂ = 10–15 wt%, MgO = 4 wt%, CaF₂ = 4–8 wt%, its desulfurization effect meets the demand, and it can reduce equipment erosion and environmental pollution.

1. Introduction

As steel enterprises enter a critical stage of transformation from quantitative growth to high-quality development, high-quality steel is facing more stringent requirements in response to the problems of some steel compositions with large fluctuations and high impurity content. Sulfur is a harmful impurity in steel, which mainly affects the hot workability, toughness and plasticity, formability, cold-working, and weldability of steel [1]. Therefore, it is crucial to strictly limit the sulfur content of steel for high value-added steel varieties. The RH vacuum desulfurization process is widely used in various enterprises, since it has the advantage of a high vacuum to reduce the oxygen activity in molten steel, and the desulfurization process avoids the top slag. Currently, the actual application of RH desulfurizers in steel enterprises mainly includes CaO-CaF₂, CaO-CaF₂-Al₂O₃, and CaO-Al₂O₃-SiO₂ slag systems [2].

The CaO-CaF₂ slag system has a high sulfur capacity and consumes the least desulfurizer under the same desulfurization task. Among which the desulfurization capacity is the strongest when CaF₂ is about 40 wt%, because CaF₂ is conducive to the destruction of desulfurization product solid phase CaS, thus improving the desulfurization kinetic conditions [3,4]. However, there are some problems in this slag system, such as erosion of refractory materials by CaF₂, dilution of CaO in slag [5], and SiF₄ polluting the environment [6]. Replacing CaF₂ with partial Al₂O₃ to form a CaO-CaF₂-Al₂O₃ slag system can alleviate the above problems [7], but due to the amphoteric properties of Al₂O₃, its influence on the performance of desulfurizer has not been determined. A small amount of Al₂O₃ will form 3CaO·Al₂O₃ or 12CaO·7Al₂O₃ with low melting point with CaO, thereby reducing the melting point and improving the desulfurization kinetic condition. However, when the content of Al₂O₃ exceeds a certain value, spinel and other substances with high melting point will be formed, resulting in poor fluidity of desulfurization slag [8]. In order to reduce the corrosion of CaF₂ in the RH desulfurizer on resistant materials, a low CaF₂-type CaO-Al₂O₃-based desulfurizer is the most ideal substitute for the CaO-CaF₂ slag system. By adding an appropriate amount of alkali metal or alkaline earth metal oxides [9,10] to the CaO-Al₂O₃ based desulfurizer, a three-high and one-low technical route with high alkalinity [11], high CaO activity, high melting speed, and low melting temperature can obtain a strong desulfurization ability. The desulfurization effect is better than that of 60 wt%CaO-40 wt%CaF₂ desulfurizer [12]. However, the low sulfur capacity and fluidity of CaO-Al₂O₃ slag desulfurizer have become main problems restricting its wide application. Adding appropriate amount of SiO₂ can significantly reduce melting point of CaO-Al₂O₃ slag desulfurizer, thus forming a CaO-Al₂O₃-SiO₂ slag system. By adding some compositions with high alkalinity and high sulfur capacity, such as MgO and BaO, the pre-melted CaO-Al₂O₃-SiO₂ desulfurizer can be made, which can achieve the goals of fast desulfurization speed, light erosion of vacuum tank, and high desulfurization rate [13]. And the desulfurizer of 60–65 wt%CaO, 25–30 wt% Al₂O₃, <10 wt% SiO₂ range with high sulfur capacity is suitable for aluminum deoxidized steel [14]. In addition, the CaO-Al₂O₃-SiO₂ slag system also has the advantages of having a low water content and not being easy to pulverize. Considering the influence of sulfur fraction ratio between slag and steel of desulfurizer, the melting rate, and residues in the vacuum chamber of the RH furnace, the appropriate Al₂O₃ mass fraction is 33–37 wt% [15].

In summary, there are many kinds of RH desulfurizers with different compositions, but their influence on the properties of the desulfurizer has not been analyzed in a systematic comparison. Therefore, the simplex–lattice composition design method was used to calculate the liquidus temperature and sulfur capacity of the desulfurizer by FactSage8.1 thermodynamic software based on the range of commonly used desulfurizer compositions, and the CaO-CaF₂ and CaO-Al₂O₃-SiO₂ desulfurizer slag systems with appropriate liquidus temperature and sulfur capacity were designed. Then, through the theoretical calculation of equilibrium S content of molten steel, the viscosity–temperature curve, combined with the slag–steel contact desulfurization test, the desulfurization capacity was investigated comprehensively, which provided theoretical guidance for the industrial application of the RH desulfurizer.

2. Research Methods

2.1. Thermodynamic Calculation

Since the value of experimental points is intuitive and convenient, and the results can be directly presented by means of isoline, the simplex–lattice method is the optimal composition design method under the condition of the minimum number of experimental points, which is adopted for the desulfurizer composition design in the current work. Based on domestic and foreign reports on the range of desulfurizer compositions, in order to maximize the study of the influence of each component on the physicochemical properties of slag samples, the change range and step size of each variable component, CaO, Al₂O₃, SiO₂, MgO, and CaF₂, are set in

Table 1. Variation range and step size of each component (wt%).

Component	CaO	CaF2	Al2O3	SiO2	MgO
Range	30–80	0–50	0–50	0–30	0–12
Step	5	5	5	5	3

. When a set of SiO₂ and MgO content values are fixed arbitrarily, the simplex–lattice test point composed of CaO, Al₂O₃, and CaF₂ is a cross-section of a four-dimensional space. The component range of this paper includes 35 sections with a total of 2059 test points. For the CaO-SiO₂-MgO-Al₂O₃-CaF₂ slag system, the liquidus temperature, sulfur capacity, and equilibrium sulfur content of molten steel were calculated with Equilib module, and the viscosity–temperature curve was calculated with Viscosity module in FactSage 8.1 thermodynamic software. Specifically, the liquidus temperatures of slag were calculated by selecting the FToxid database, and liquid was selected as precipitation target phase (P) in the products section. Based on the FactPS and FToxid databases, the mass fraction of S element in the slag phase, and the partial pressures of O₂ and S₂ in ambient atmosphere, can be obtained to calculate the sulfur capacity of slag. Moreover, the equilibrium sulfur content of molten steel was obtained with the slag–metal equilibrium calculation by selecting the FToxid and FSStel databases.

According to the on-site statistics of Xinyu Iron and Steel, the average content of elements in molten steel before RH desulfurization is Fe: 95.845 wt%, C: 0.05 wt%, Si: 4 wt%, Mn: 0.1 wt%, and S: 0.005 wt%. Combing the above steel composition, the influence of typical compositions of slag in Table 1 on the performance parameters was analyzed, which provided theoretical guidance for the design and application of the RH desulfurizer.

2.2. Melting Temperature Test

The melting temperature of desulfurizer is measured with the hemisphere melting point instrument, which consists of a furnace system, temperature measurement and control system, and image acquisition and display system. Analytical grade reagents were used to ensure accuracy (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and compositions of CaO and F⁻ were added in the form of CaCO₃ and CaF₂. The reagents were weighed according to the sample composition requirements and stirred evenly. Ground 30 g of experimental slag into powder less than 200 mesh and add anhydrous ethanol. The mixed sample is made into a 3 × 3 mm cylinder with a sampler and placed on a corundum liner to dry. The dried sample is put into the furnace, heated at 15 °C/min, and the change in the sample is observed. The system automatically records the hemisphere temperature of the slag sample, that is, the melting temperature. Each sample was tested more than three times until the difference between the two consecutive test results did not exceed 15 °C to minimize experimental error, and the mean value of three groups of similar data were selected as the final melting temperature result.

2.3. Slag–Steel Contact Experiment

In view of the on-site RH desulfurization process temperature is generally 1560–1620 °C, the slag–steel contact experiment temperature is set at 1600 °C in the laboratory. In addition, the slag-to-steel ratio was set at 1:10, which is basically consistent with the on-site parameters. The slag–steel contact experiment was carried out in a well-type resistance furnace with the power of 10 kW and a type B thermocouple, located in the constant temperature zone in the middle of the furnace tube for temperature control. The graphite crucible equipped with desulfurizer was placed in the induction furnace and heated up at 15 °C/min. After the desulfurizer was melted, it was poured out to cool down, and crushed to about 1~2 mm particle size for spare. Then, 210 g of steel sample taken on site was put into corundum crucible, coated with graphite crucible, and placed into the furnace at 1000 °C. With the furnace heating up, the temperature in the furnace reached 1600 °C and was kept warm for 10 min. A molten steel sample taken at this moment was set as the molten steel sample at time 0 min. About 21 g of the pre-melted desulfurizer was added into the crucible through a graphite tube, and the reaction was timed. When the reaction was 5 min and 10 min, the sample of molten steel was taken with a quartz dropper. The whole process was protected by Ar gas (flow rate: 0.5 L·min⁻¹, purity: 99.99%). The S content in molten steel samples was measured by the carbon-sulfur (CS) analyzer (LECO CS844, LECO Instruments (Hong Kong) Company, Hong Kong, China), and the melting temperature of desulfurizer after slag–steel reaction was measured by a hemisphere melting point instrument.

3. Results and Discussion

3.1. RH Desulfurizer Component Design

Both the melting point and the sulfur capacity of the desulfurizer have an impact on the desulfurization effect [16]. If the melting point of the desulfurizer is similar and the slag-forming rate is similar, the higher the sulfur capacity of the slag system, the higher the desulfurization rate. However, when the sulfur capacity and melting point of the slag system are both high, the desulfurization rate may not necessarily be high, so the low melting point and high sulfur capacity of the slag system are the guarantees of the desulfurization effect. It is difficult to measure the melting temperature and sulfur capacity of RH desulfurizer in large quantities, so FactSage thermodynamic calculations were used to analyze its liquidus temperature and sulfur capacity, which was used to initially design the RH desulfurizer compositions.

Sulfur capacity is the main parameter of desulfurization ability of the desulfurizer, and it is an important basis for selecting a reasonable desulfurization slag system. In view of the fact that the FactSage database not only contains a large number of oxide data but also contains other dissolved compositions in slag such as S, F, and Cl, it can accurately calculate the sulfur capacity of multi-component slag system such as Al₂O₃-CaO-FeO-Fe₂O₃-MgO-SiO₂-TiO₂-Ti₂O₃ [17]. Therefore, based on FactSage, the following formula is used to calculate the sulfur capacity, Cs, of desulfurizer. For convenience, the logarithm of C_s is taken to analyze the influence law of each factor on lgC_s.

$${\displaystyle \frac{1}{2}}{S}_2+\left(O^{2-}\right)=\left(S^{2-}\right)+{\displaystyle \frac{1}{2}}{O}_2$$

(1)

$$lg{C}_S=log\left(C_S\right)=log\left[S_{in slag}\times {\left({\displaystyle \frac{P_{O_2}}{P_{S_2}}}\right)}^{{\displaystyle \frac{1}{2}}}\right]$$

(2)

where $S_{inslag}$ is the mass fraction of S element in slag phase, and $P_{O_2}$ and $P_{S_2}$ are the partial pressures of O₂ and S₂ in ambient atmosphere, respectively.

Based on the thermodynamic calculation of liquidus temperature and sulfur capacity, considering the desulfurization effect of RH desulfurizer and the prevention of crusting of desulfurization slag, the composition range of high sulfur capacity RH desulfurizer with suitable liquidus temperature was determined. The first design plan is to reduce the liquidus temperature to promote the desulfurization kinetic conditions. Considering the deviation between FacSage theoretical calculation and practice, if the liquidus temperature of the desulfurizer is 1650 °C, and the sulfur capacity of 1600 °C meets the lgCs > −2.0, the component range of CaO-Al₂O₃-SiO₂ slag system desulfurizer can be obtained. In the second design scheme, the composition range of CaO-CaF₂ slag desulfurizer can be obtained if the liquidus temperature and sulfur capacity are higher to improve the thermodynamic conditions of desulfurization, of which the liquidus temperature range is wider, and the sulfur capacity at 1600 °C meets lgCs > −0.3.

Taking the slag system cross-sections of MgO = 0 wt%, SiO₂ = 0 wt%, 5 wt%, 10 wt%, and 15 wt% as an example, the range of experimental points meeting the conditions is shown in Figure 1. Different colors in the figure are liquidus contour lines, 1650 lines indicate that the liquidus temperature is 1650 °C, and −2.0 and −0.3 indicate that the sulfur capacity at 1600 °C is lgCs = −2.0 and lgCs = −0.3, respectively. The range set by the 1650 and −2.0 lines is the component range of the CaO-Al₂O₃-SiO₂ slag system desulfurizer that satisfies the requirements, and the right of the −0.3 line is the component range of CaO-CaF₂ slag system desulfurizer that meets the requirements.

As can be seen from Figure 1, the range of eligible CaO-Al₂O₃-SiO₂-based desulfurizer fractions spans a wide range of CaO = 30–60 wt%, CaF₂ = 5–50 wt%, and Al₂O₃ = 0–40 wt% at different SiO₂ contents. With the increase in MgO content, the qualified region gradually decreases, and when MgO > 6 wt%, the lgCs = −2.0 sulfur capacity line no longer intersects with the liquidus temperature line at 1650 °C, that is, there is no qualified region anymore. On the other hand, the liquidus temperature and sulfur capacity of the CaO-CaF₂-based desulfurizer are relatively high, and the compositions are concentrated in the regions with low Al₂O₃, high CaO, and high CaF₂ content. With the increase in SiO₂ content, the eligible regions gradually become smaller, and the overall slag lgCs are less than −0.3 when SiO₂ > 5 wt%.

In view of the fact that the main source of SiO₂ in the desulfurizer is impurities in the raw material, although SiO₂ can reduce the viscosity of the desulfurizer and improve its fluidity when it is increased within a certain range, thus contributing to the improvement in the desulfurization kinetic conditions, an increase in the content of SiO₂ will reduce the alkalinity of the desulfurizer and the activity of CaO, which is unfavorable for desulfurization. Therefore, the content of SiO₂ in the desulfurizer should be reduced as far as possible, so as to achieve the purpose of desulfurization under high alkalinity conditions. CaO in the desulfurizer is directly involved in the desulfurization reaction as the reactant of the desulfurization reaction, and increasing its content is conducive to improving the basicity of the slag and also conducive to the desulfurization reaction. Therefore, in order to ensure a good desulfurization effect, the content of CaO in the desulfurizer is required to be in the range of 40–60 wt%. However, in RH desulfurization, the high content of CaO cannot be fully melted, which is not conducive to desulfurization, so that it is necessary to add other compositions to reduce the melting point of the desulfurizer. It is generally believed that CaF₂ does not participate in the desulfurization reaction and only acts as a diluent [18]. However, it is believed that the role of CaF₂ is as follows: F⁻ reduces the activity of Ca²⁺ in CaO-containing slag, which can improve the solubility of Ca²⁺ in CaF₂-containing slag; F⁻ reacts with network silicate to increase a small amount of O²⁻, so the addition of CaF₂ is conducive to desulfurization when the basic component content is certain [19]. From the perspective of thermodynamics and slag ion theory, MgO can also provide O²⁻, and the desulfurization capacity is slightly lower than CaO. When the content of MgO increases, the alkalinity of the desulfurizer can be significantly increased, and the desulfurization ability of the desulfurizer can be enhanced thermodynamically. In addition, a certain amount of MgO can reduce the erosion of slag on resistant materials and extend the life of refractory materials. However, the melting point of MgO is very high, which can reach 2852 °C, so too high MgO in the slag will significantly improve the melting point and viscosity of the slag, which is very unfavorable to the fluidity of the slag, thus reducing the desulfurization ability of the slag. Therefore, the MgO content in RH desulfurizer is designed to range from 0 to 8 wt%. Al₂O₃ is a component commonly used in desulfurizer, and Al₂O₃ and CaO tend to generate compounds. On the one hand, they can reduce the melting point of the desulfurizer, making it have better fluidity and improving the desulfurization kinetic conditions. On the other hand, they will bind some free oxygen ions and affect the desulfurization ability of slag. Therefore, at present, there is no consistent conclusion on the behavior of Al₂O₃ in the desulfurizer, so the content of Al₂O₃ in CaO-Al₂O₃-SiO₂ slag desulfurizer in this paper is designed to be between 15 and 40 wt%.

In summary, based on liquidus temperature and lgCs diagram of desulfurizer, the compositions of CaO-Al₂O₃-SiO₂ and CaO-CaF₂ slag desulfurizer were designed, and the effects of typical component contents on the properties of desulfurizer were investigated. The specific compositions of RH desulfurizer are shown in

Table 2. RH desulfurizer component (wt%).

Desulfurizer.	CaO	SiO2	CaF2	Al2O3	MgO
CaO-CaF2 based	51~66	0, 5	30	4	0~10
	47~77	0, 5	15~40	4	4
	51~66	0, 5	30	0~10	4
CaO-Al2O3-SiO2 based	41~56	10, 15	4	30	0~10
	41~56	10, 15	0~10	30	4
	37~67	10, 15	4	15~40	4

, where the residual is CaO.

3.2. Influence of RH Desulfurizer Compositions on S Content of Molten Steel

The RH treatment temperature is 1560–1620 °C, and the S content in the molten steel should be reduced to 0.002 wt%. In this paper, the slag-to-steel ratio was set at 1:10. FactSage calculation was used to investigate the reaction of RH desulfurizer with molten steel with initial sulfur content of 0.005 wt% at 1600 °C, and the S content of molten steel after reaction equilibrium was obtained. The comparison with the target molten steel S content of 0.002 wt% was used to evaluate the desulfurization efficiency of the desulfurizer. The effects of various factors on desulfurization efficiency were obtained.

The influence of MgO content in CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers on the equilibrium S content of molten steel is shown in Figure 2. It can be seen that when SiO₂ in different desulfurizers is less than 10 wt%, the change in MgO content in desulfurizers has little effect on the equilibrium S content of molten steel, and the desulfurizer has good desulfurization performance. The slag with SiO₂ = 15 wt%, MgO > 4 wt% will significantly increase the equilibrium S content of molten steel, which is because the increase in MgO content will reduce the sulfur capacity of desulfurizer under high SiO₂ conditions. In view of the fact that a certain amount of MgO can reduce the erosion of slag on resistant materials and extend the life of refractory materials, a small amount of MgO should be added to the RH desulfurizer to neither reduce its desulfurization capacity nor reduce the erosion of resistant materials.

The influence of CaF₂ content in CaO-CaF₂- and CaO-Al₂ O₃-SiO₂-based desulfurizers on the equilibrium S content of molten steel is shown in Figure 3. The results show that with the increase in CaF₂ content in the desulfurizer when SiO₂ ≤ 10 wt%, the equilibrium S content of molten steel is close to 0. When SiO₂ = 15 wt%, the equilibrium S content increases significantly, but it can still meet the target of less than 0.002 wt%, which is attributed to the fact that under the condition of high SiO₂, increasing CaF₂ can significantly reduce the sulfur capacity of desulfurizer. It can be seen that CaO-CaF₂-based desulfurizer has strong desulfurization ability, while the CaO-Al₂O₃-SiO₂-based desulfurizer should choose a component range with SiO₂ content less than 15 wt% and a low CaF₂ content.

The influence of Al₂O₃ content in CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers on the equilibrium S content of molten steel is shown in Figure 4. As can be seen from the figure, when the content of SiO₂ is 0 wt% and 5 wt%, the change in Al₂O₃ content in the desulfurizer has no effect on the equilibrium S content of molten steel, and the desulfurizer has good desulfurization performance. When SiO₂ content is 10 wt%, 15 wt%, and Al₂O₃ ≥ 25 wt%, the equilibrium S content of molten steel is significantly increased, that is, the desulfurization performance of RH desulfurizer becomes worse, but it can still meet the target desulfurization requirements. This may be because of the amphoteric property of Al₂O₃. When the content of SiO₂ is high, Al₂O₃ acts as an alkaline oxide in the range of 15–25 wt%, and the increase in its content will not worsen the desulfurization effect, but when the content of Al₂O₃ is further increased, its nature changes to acidic, significantly reducing the sulfur capacity of the desulfurizer, making the desulfurization effect worse. Therefore, the RH desulfurizer should choose a low SiO₂ and low Al₂O₃ component range.

3.3. Analysis of Viscosity—Temperature Curve of RH Desulfurizer

The viscosity of the desulfurizer is an important factor affecting the slag–steel interface desulfurization reaction, and the mass transfer rate in the liquid phase is inversely proportional to its viscosity. In the process of desulfurization, if the viscosity of the desulfurizer is too large, it deteriorates the kinetic conditions, resulting in desulfurization difficulties. In the temperature range of the RH treatment process, a certain temperature drop makes the viscosity increase, so that the slag crusts, further worsening the desulfurization conditions. Reducing the viscosity of the desulfurizer can improve its fluidity, which in turn can reduce the average diameter of the emulsion droplet, increase the slag-gold contact area, and promote desulfurization. However, if the viscosity is too small, the permeability of the desulfurizer to the refractory increases, which will cause the loss of the refractory. Therefore, the viscosity of RH desulfurizer is required to be moderate.

Since the amount of RH top slag is relatively small and the composition is unknown, the influence of RH top slag on the viscosity of desulfurizer is not considered. In addition, because the RH treatment temperature is as high as 1560–1620 °C, it is difficult to accurately measure the viscosity of desulfurizer in this temperature range under laboratory conditions. The FactSage viscosity model, which is based on the quasi-lattice viscosity model, has become the most accurate and widely used viscosity model after continuous revision and expansion. Therefore, FactSage thermodynamic calculation is used to analyze the viscosity change in the RH desulfurizer within a certain temperature range, so as to obtain the range of desulfurizer compositions whose fluidity still meets the demand within the process temperature drop range.

The influence of MgO content on the viscosity of CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers is shown in Figure 5. For CaO-CaF₂-based desulfurizers shown in Figure 5a,b, the increase in MgO content has almost no effect on the viscosity within the temperature drop range, indicating that adding an appropriate amount of MgO can prolong the life of resistant materials without deteriorating the fluidity of the desulfurizer. For the CaO-Al₂O₃-SiO₂-based desulfurizer shown in Figure 5c,d, the breaking temperature on the viscosity–temperature curve decreases when the MgO content increases in the range of 0–4 wt%, indicating that the increase in MgO content within this range can effectively prevent the slag from encrusting tendency. However, when the MgO content further increased to 8 wt%, the breaking temperature increased slightly, indicating that high MgO content would cause the viscosity of desulfurizer to increase sharply with the decrease in temperature, thus worsening the desulfurization kinetic conditions and reducing the desulfurization ability of desulfurizer.

The influence of CaF₂ content on the viscosity of CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers is shown in Figure 6. It can be seen that for the CaO-CaF₂-based desulfurizer, the increase in CaF₂ content in the range of 15–35 wt% resulted in a decrease in high-temperature viscosity and, hence, better slag fluidity. With the decrease in temperature, the viscosity of desulfurizing agent increases more when CaF₂ is 15 wt%, so it is easy to appear crust under a certain temperature drop. However, the breaking temperature on the viscosity–temperature curve is lower than 1350 °C when CaF₂ is increased to 25 wt% and 35 wt%, so the crust tendency decreases significantly. For the CaO-Al₂O₃-SiO₂ slag system, the high temperature viscosity changes little with the increase of CaF₂ content, that is, the high temperature fluidity of desulfurizer is good. With the decrease in temperature, the breaking temperature on the viscosity–temperature curve is close to 1400 °C, indicating that the subsequent process temperature should not be lower than 1400 °C, otherwise the desulfurizer will solidify. Meanwhile, the increase in CaF₂ content can reduce the breaking temperature slightly, which is beneficial to preventing crust formation.

Al₂O₃ has amphoteric properties, and its effect on the slag structure varies according to its nature. In an acidic environment, Al₂O₃ acts as a network modifier, and the increase in its content will depolymerize the network structure and reduce the viscosity of slag. In an alkaline environment, Al₂O₃ acts as a network former and participates in the construction of network structure to form an Al-O network structure, thus increasing the viscosity of slag. As can be seen from Figure 7, for the CaO-CaF₂-based desulfurizer, a small increase in Al₂O₃ content significantly reduces the breaking temperature, which is beneficial for preventing crusting. However, for the CaO-Al₂O₃-SiO₂-based desulfurizer, the increase in Al₂O₃ content increases the breaking temperature, and some slag system has solidified at 1400 °C, which easily leads to slag crust when the temperature drops.

In addition, as shown in Figure 5, Figure 6 and Figure 7, for the CaO-CaF₂-based desulfurizer, an increase of 5 wt% SiO₂ can significantly reduce the breaking temperature, which is beneficial to improving the fluidity of the desulfurizer and preventing slag crusting. For the CaO-Al₂O₃-SiO₂-based desulfurizer, the increase of SiO₂ from 10 wt% to 15 wt% can significantly increase the breaking temperature, indicating that the content of SiO₂ in the desulfurizer should be kept in a low content range.

3.4. Desulfurization Experiment of RH Desulfurizer

Based on the above performance analysis of desulfurizer, the preferred compositions were selected as shown in

Table 3. Composition of RH desulfurizer for slag-steel experiment (wt%).

Number		CaO	SiO2	MgO	Al2O3	CaF2
Enterprise RH desulfurizer		64.08	4.92	2.66	0.46	27.88
CaO-CaF2-based desulfurizer	1	62	0	4	4	30
	2	57	5	4	4	30
CaO-Al2O3-SiO2-based desulfurizer	3	52	10	4	30	4
	4	48	10	4	30	8
	5	48	10	8	30	4
	6	47	10	4	35	4
	7	57	10	4	25	4
	8	43	15	4	30	8
	9	39	15	8	30	8
	10	53	15	4	20	8

, and the slag–steel balance experiment was conducted by comparing the desulfurizer currently used in the enterprise. The sulfur content in the molten steel at 0 min, 5 min, and 10 min of the slag–steel reaction is S₀, S₅, and S₁₀, respectively, and the melting temperature of the desulfurization slag before and after the slag-steel reaction is T_m1 and T_m2, respectively. All the test results are shown in

Table 4. Sulfur content in molten steel (wt%) and melting temperature of desulfurization slag (°C).

Number		S0	S5	S10	Tm1	Tm2
Enterprise RH desulfurizer		0.039	0.003	<0.002	1450	1457
CaO-CaF2-based desulfurizer	1	0.025	<0.002		1465	1460
	2	0.035	<0.002		1458	1424
CaO-Al2O3-SiO2-based desulfurizer	3	0.022	<0.002		1371	1355
	4	0.025	<0.002		1364	1348
	5	0.028	0.007	<0.002	1389	1372
	6	0.017	<0.002		1368	1354
	7	0.017	<0.002		1396	1386
	8	0.025	<0.002		1353	1358
	9	0.023	0.005	<0.002	1344	1328
	10	0.034	<0.002		1360	1350

.

According to the experimental results, the melting temperature of the CaO-CaF₂-based desulfurizer is high, which is close to that of existing desulfurizer used by enterprises, while the melting temperature of the CaO-Al₂O₃-SiO₂-based desulfurizer is lower than 1400 °C, which is conducive to improving the kinetic conditions of desulfurization. In addition, the desulfurization effect of the CaO-CaF₂-based desulfurizer is remarkable, and the sulfur content in molten steel can be reduced to the target sulfur content of 0.002 wt% after 5 min reaction, which is better than the desulfurizer currently used in enterprises. The desulfurization effect of the CaO-Al₂O₃-SiO₂-based desulfurizer is slightly worse, but in addition to the desulfurizer with MgO content of 8 wt%, other compositions of the desulfurizer can also reduce the target sulfur content in steel to less than 0.002 wt% within 5 min.

In summary, for the CaO-CaF₂-based desulfurizer, CaO = 60 wt%, CaF₂ = 30 wt%, SiO₂ = 0–5 wt%, and add a small amount of Al₂O₃ and MgO, the desulfurization effect is significant. For the CaO-Al₂O₃-SiO₂ slag desulfurizer, CaO = 39–57 wt%, Al₂O₃ = 20–35 wt%, SiO₂ = 10–15 wt%, MgO = 4 wt%, CaF₂ = 4–8 wt%, its desulfurization effect meets the demand, and this desulfurizer can reduce equipment erosion and environmental pollution.

4. Conclusions

(1)

By comprehensive thermodynamic calculation of liquidus temperature and sulfur capacity of the desulfurizer, it is obtained that the CaO-CaF₂-based desulfurizer compositions with high liquidus temperature and sulfur capacity are concentrated in areas with low Al₂O₃ content and high CaO and CaF₂ content, and the CaO-Al₂O₃-SiO₂-based desulfurizer compositions with low liquidus temperature and sulfur capacity have a wide range of CaO = 30–60 wt%, CaF₂ = 5–50 wt%, Al₂O₃ = 0–40 wt%, and MgO ≤ 6 wt%.

(2)

In CaO-CaF₂- and CaO-Al₂O₃-SiO₂ based-desulfurizers, when the content of SiO₂ is low, the change in CaF₂, MgO, and Al₂O₃ contents has little effect on the equilibrium S content of molten steel, whereas, when the content of SiO₂ is high, the equilibrium S content of molten steel increased significantly when the content of CaF₂, MgO, and Al₂O₃ are greater than a certain value.

(3)

In CaO-CaF₂- and CaO-Al₂O₃-SiO₂- based desulfurizers, the increase in CaF₂ and MgO content reduces the high-temperature viscosity and breaking temperature to varying degrees, so that the slag system has better fluidity and is beneficial to prevent crust formation. With the increase in Al₂O₃ and SiO₂ contents, the breaking temperature of the CaO-CaF₂-based desulfurizer decreases significantly, which is beneficial to preventing crust. However, when the breaking temperature of the CaO-Al₂O₃-SiO₂-based desulfurizer increases, part of the slag has solidified at 1400 °C, which is prone to leading to slag crust when the temperature drops.

(4)

The melting temperatures of CaO-CaF₂- and CaO-Al₂O₃-SiO₂-based desulfurizers in the present work is lower than the RH desulfurization process temperature, so the desulfurization kinetics conditions will not be worsened. In summary, for the CaO-CaF₂-based desulfurizer, CaO = 60 wt%, CaF₂ = 30 wt%, SiO₂ = 0–5 wt%, with a small amount of Al₂O₃ and MgO added, the desulfurization effect is significant. For the CaO-Al₂O₃-SiO₂-based desulfurizer, CaO = 39–57 wt%, Al₂O₃ = 20–35 wt%, SiO₂ = 10–15 wt%, MgO = 4 wt%, and CaF₂ = 4–8 wt%, its desulfurization effect meets the demand, and this desulfurizer can reduce equipment erosion and environmental pollution.