Next Article in Journal
First-Principles Calculations of the Effect of Ta Content on the Properties of UNbMoHfTa High-Entropy Alloys
Previous Article in Journal
Investigation of Microstructural Evolution of Silicon Steel Weldment After Post-Weld Heat Treatment—Simulation and Experimental Study
Previous Article in Special Issue
Flow-Field Characterization of Kanbara Reactor Based on Numerical Simulations
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

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

1
School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan 243032, China
2
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Metals 2025, 15(5), 550; https://doi.org/10.3390/met15050550
Submission received: 24 March 2025 / Revised: 8 May 2025 / Accepted: 14 May 2025 / Published: 16 May 2025
(This article belongs to the Special Issue Green Super-Clean Steels)

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. 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 Cs is taken to analyze the influence law of each factor on lgCs.
1 2 S 2 + O 2 = S 2 + 1 2 O 2
l g C S = l o g C S = l o g S i n   s l a g × P O 2 P S 2 1 2
where S i n s l a g 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, 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, 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 Tm1 and Tm2, respectively. All the test results are shown in Table 4.
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.

Author Contributions

Conceptualization, R.C. and S.Q.; methodology, T.W.; software, T.W.; validation, R.C. and H.W.; formal analysis, T.W.; investigation, S.Q.; resources, S.Q.; data curation, R.C.; writing—original draft preparation, R.C.; writing—review and editing, S.Q., T.W. and H.W.; visualization, R.C.; supervision, H.W.; project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation China (No. 52374316) and the Key Project of Anhui Province’s Science and Technology Breakthrough Plan (No. 202423i08050049).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Feng, X.Y. Research and practice of optimization feeding technology and equipment for KR method. Metall. Equip. 2016, 3, 59–62. [Google Scholar]
  2. Sui, Y.F.; Li, C.Y.; Liu, P.; Xu, G.J. Effect of RH desulfurization on inclusions in non-oriented electrical steel. SteelMak 2019, 35, 34–38. [Google Scholar]
  3. Xu, C.S.; Tang, X. The kinetics of desulfurization of hot metal by CaO-CaF2 based fluxes. ISIJ Int. 1992, 32, 1081–1083. [Google Scholar]
  4. Choi, C.H.; Jo, S.K.; Kim, S.H.; Lee, K.R.; Kim, J.T. The effect of CaF2 on thermodynamics of CaO-CaF2-SiO2-(MgO) slags. Metall. Mater. Trans. B 2004, 35, 115–120. [Google Scholar] [CrossRef]
  5. Takahashi, K.; Utagawa, K.; Shibata, H.; Kitamura, S.Y.; Kikuchi, N.; Kishimoto, Y. Influence of solid CaO and liquid slag on hot metal desulfurization. ISIJ Int. 2012, 52, 10–17. [Google Scholar] [CrossRef]
  6. Liao, J.J.; Lai, Z.B.; Qing, Z.; Chou, S.T. Review of RH desulfurization. Nonferrous Met. Sci. Eng. 2013, 4, 74–80. [Google Scholar]
  7. Zhang, G.X. Experimental Study on Deep Desulfurization for Ultra-Low Carbon Steel in Pansteel. Master’s Thesis, Chongqing University, Chongqing, China, 16 November 2010. [Google Scholar]
  8. Yan, P.C.; Guo, X.L.; Huang, S.G. Desulphurisation of stainless steel by using CaO-Al2O3 based slags during secondary metallurgy. ISIJ Int. 2013, 53, 459–467. [Google Scholar] [CrossRef]
  9. Gao, X.M.; Sun, L.J.; Li, G.J.; Zeng, J.H.; Zhang, M.; He, S.P.; Wang, Q. Effect of constituents of BaO, Na2O bearing refining slag series on sulfur distribution ratio. SteelMak 2008, 24, 33–36. [Google Scholar]
  10. Taniguchi, Y.; Sano, N.; Seetharaman, S. Sulphide capacities of CaO-Al2O3-SiO2-MgO-MnO slags in the temperature range 1673–1773 K. ISIJ Int. 2009, 49, 156–163. [Google Scholar] [CrossRef]
  11. Etsuro, S.; Takashi, N.; Takeshi, N.; Mitsunori, E.; Hideyuki, I.; Tomio, T. Evaluation of desulfurization flux in CaO-Al2O3-BaO-Ce2O-MgO system. Steel Res. Int. 2004, 54, 308–313. [Google Scholar]
  12. Hao, X.; Wang, X.; Wang, W. Study on desulfurization ability of CaO-Al2O3-SiO2 and CaO-CaF2 slags at 1600 °C. Steel Res. Int. 2016, 86, 1455–1460. [Google Scholar] [CrossRef]
  13. Yan, P.C.; Huang, S.G.; Joris, V.D. Desulphurisation and inclusion behaviour of stainless steel refining by using CaO-Al2O3 based slag at low sulphur levels. ISIJ Int. 2014, 54, 72–81. [Google Scholar] [CrossRef]
  14. Yu, X.B.; Shi, Q.L.; Chen, Q.Q.; Huang, C.H.; Li, J.Z.; Wang, Y.; Guo, T.Y. Development of new type desulfurizer for RH vacuum treatment. SteelMak 2006, 22, 51–54. [Google Scholar]
  15. Ai, L.Q.; Cai, K.K. Desulfurization technology for molten steel in RH treatment process. SteelMak 2001, 17, 53–57. [Google Scholar]
  16. Qi, J.H.; Xue, Z.L.; Gao, J.B. Study on the melting point of desulfurization refining slag. Res. Iron Steel. 2007, 35, 9–11. [Google Scholar]
  17. Kang, Y.B.; Pelton, A.D. Thermodynamic model and database for sulfides dissolved in molten oxide slags. Metall. Mater. Trans. B. 2009, 40, 979–994. [Google Scholar] [CrossRef]
  18. Zhang, C.; Wu, T.; Lei, J.; Wang, H.C.; Wang, Q. First-principles calculation of CaO-Al2O3-CaF2 slag. Metall. Mater. Trans. B 2024, 55, 105–113. [Google Scholar] [CrossRef]
  19. Jeong, T.S.; Park, J.H. Effect of fluorspar and industrial wastes (red mud and ferromanganese slag) on desulfurization efficiency of molten steel. Metall. Mater. Trans. B 2020, 51, 2309–2320. [Google Scholar] [CrossRef]
Figure 1. Thermodynamic calculated liquidus temperature and lgCs at 1600 °C of CaO-Al2O3-SiO2 based desulfurizer when MgO = 0 wt%. (a) SiO2 = 0 wt%. (b) SiO2 = 5 wt%. (c) SiO2 = 10 wt%. (d) SiO2 = 15 wt%.
Figure 1. Thermodynamic calculated liquidus temperature and lgCs at 1600 °C of CaO-Al2O3-SiO2 based desulfurizer when MgO = 0 wt%. (a) SiO2 = 0 wt%. (b) SiO2 = 5 wt%. (c) SiO2 = 10 wt%. (d) SiO2 = 15 wt%.
Metals 15 00550 g001
Figure 2. Effect of MgO content on the equilibrium S content of molten steel by thermodynamic calculation.
Figure 2. Effect of MgO content on the equilibrium S content of molten steel by thermodynamic calculation.
Metals 15 00550 g002
Figure 3. Effect of CaF2 content on the equilibrium S content of molten steel by thermodynamic calculation. (a) CaO-CaF2-based desulfurizer, (b) CaO-Al2O3-SiO2-based desulfurizer.
Figure 3. Effect of CaF2 content on the equilibrium S content of molten steel by thermodynamic calculation. (a) CaO-CaF2-based desulfurizer, (b) CaO-Al2O3-SiO2-based desulfurizer.
Metals 15 00550 g003
Figure 4. Effect of Al2O3 content on the equilibrium S content of molten steel by thermodynamic calculation. (a) CaO-CaF2-based desulfurizer, (b) CaO-Al2O3-SiO2-based desulfurizer.
Figure 4. Effect of Al2O3 content on the equilibrium S content of molten steel by thermodynamic calculation. (a) CaO-CaF2-based desulfurizer, (b) CaO-Al2O3-SiO2-based desulfurizer.
Metals 15 00550 g004
Figure 5. Effect of MgO content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Figure 5. Effect of MgO content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Metals 15 00550 g005aMetals 15 00550 g005b
Figure 6. Effect of CaF2 content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Figure 6. Effect of CaF2 content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Metals 15 00550 g006aMetals 15 00550 g006b
Figure 7. Effect of Al2O3 content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Figure 7. Effect of Al2O3 content on viscosity of desulfurizer by FactSage calculation. (a) SiO2 = 0 wt%, (b) SiO2 = 5 wt%, (c) SiO2 = 10 wt%, and (d) SiO2 = 15 wt%.
Metals 15 00550 g007
Table 1. Variation range and step size of each component (wt%).
Table 1. Variation range and step size of each component (wt%).
ComponentCaOCaF2Al2O3SiO2MgO
Range30–800–500–500–300–12
Step55553
Table 2. RH desulfurizer component (wt%).
Table 2. RH desulfurizer component (wt%).
Desulfurizer.CaOSiO2CaF2Al2O3MgO
CaO-CaF2 based51~660, 53040~10
47~770, 515~4044
51~660, 5300~104
CaO-Al2O3-SiO2 based41~5610, 154300~10
41~5610, 150~10304
37~6710, 15415~404
Table 3. Composition of RH desulfurizer for slag-steel experiment (wt%).
Table 3. Composition of RH desulfurizer for slag-steel experiment (wt%).
NumberCaOSiO2MgOAl2O3CaF2
Enterprise RH desulfurizer64.084.922.660.4627.88
CaO-CaF2-based desulfurizer16204430
25754430
CaO-Al2O3-SiO2-based desulfurizer352104304
448104308
548108304
647104354
757104254
843154308
939158308
1053154208
Table 4. Sulfur content in molten steel (wt%) and melting temperature of desulfurization slag (°C).
Table 4. Sulfur content in molten steel (wt%) and melting temperature of desulfurization slag (°C).
NumberS0S5S10Tm1Tm2
Enterprise RH desulfurizer0.0390.003<0.00214501457
CaO-CaF2-based desulfurizer10.025<0.002 14651460
20.035<0.002 14581424
CaO-Al2O3-SiO2-based desulfurizer30.022<0.002 13711355
40.025<0.002 13641348
50.0280.007<0.00213891372
60.017<0.002 13681354
70.017<0.002 13961386
80.025<0.002 13531358
90.0230.005<0.00213441328
100.034<0.002 13601350
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Cao, R.; Qiu, S.; Wu, T.; Wang, H. Performance Study of CaO-CaF2- and CaO-Al2O3-SiO2-Based High-Efficiency Desulfurizers. Metals 2025, 15, 550. https://doi.org/10.3390/met15050550

AMA Style

Cao R, Qiu S, Wu T, Wang H. Performance Study of CaO-CaF2- and CaO-Al2O3-SiO2-Based High-Efficiency Desulfurizers. Metals. 2025; 15(5):550. https://doi.org/10.3390/met15050550

Chicago/Turabian Style

Cao, Ruihong, Shengtao Qiu, Ting Wu, and Haijun Wang. 2025. "Performance Study of CaO-CaF2- and CaO-Al2O3-SiO2-Based High-Efficiency Desulfurizers" Metals 15, no. 5: 550. https://doi.org/10.3390/met15050550

APA Style

Cao, R., Qiu, S., Wu, T., & Wang, H. (2025). Performance Study of CaO-CaF2- and CaO-Al2O3-SiO2-Based High-Efficiency Desulfurizers. Metals, 15(5), 550. https://doi.org/10.3390/met15050550

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop