Modification of Desulfurization Slag for Hot Metal Bearing V-Ti and Industry Application

Abstract

In view of the high loss of iron during hot metal desulfurization treatment at Pangang Steel, the factors influencing slag skimming iron loss were analyzed thoroughly by thermodynamic calculation with the aid of FactSage. A desulfurization modifier containing Na₂O and Al₂O₃ was designed. An industrial verification test was conducted for the newly designed calcium-based agent. The test results indicate that adding 8% of the modifier to the passivating lime achieves the optimal modification effect on the desulfurization slag. After modifying the desulfurization slag, the consumption of magnesium powder for every 0.001% sulfur removed decreased from 0.0149 kg to 0.0136 kg, the iron loss during slag skimming reduced from 3.52% to 2.28%, and the average slag skimming time shortened by 1.5 min. These improvements significantly lowered production costs, enhanced desulfurization efficiency, and laid the foundation for the widespread application of the semi-steel silicon addition process.

1. Introduction

Pangang Steel utilizes the unique vanadium–titanium magnetite resources of the Panxi region for smelting, resulting in molten iron that contains vanadium and titanium, while having low levels of heat-generating elements such as silicon and manganese. This leads to lower molten iron temperatures and higher sulfur content. The carbon content of the semi-steel produced from vanadium-rich molten iron through converter refining fluctuates between 2.5% and 4.0%, with trace amounts of heat-generating slag-forming elements like silicon and manganese. Consequently, the steelmaking converter faces a severe shortage of thermal energy when refining the semi-steel, leading to slow slag formation. This situation compels Pangang Steel to adopt a carbon-increasing end-point control method during the production of medium and high-carbon steels [1,2,3,4,5,6,7,8,9,10,11,12]. This control method results in excessive deep blowing at the converter’s end, high oxidizing properties of the slag, and increases the refining costs while reducing the quality of the molten steel. To address the issue of insufficient thermal energy, Pangang Steel has previously attempted thermal compensation by adding temperature-raising agents such as coke powder, anthracite, ferrosilicon, and silicon carbide into the converter. Additionally, the exploration of dual-channel oxygen lancing to improve thermal efficiency and preheating scrap steel has not yielded satisfactory results [13,14,15].

Through years of dedicated investigations, Pangang Steel has developed a unique production process for the unique vanadium–titanium magnetite resources of the Panxi region to produce vanadium–titanium molten iron [16,17,18,19,20,21]. Recently, in order to solve the problem of insufficient heat sources for semi-steel smelting, a new process was developed wherein ferrosilicon is added to the semi-steel during the steelmaking process. This increases the silicon content in the semi-steel, thereby enhancing its chemical heat and promoting slag formation during the converter steelmaking process. The flow chart for vanadium–titanium ironmaking is illustrated in Figure 1. After the silicon addition of semi-steel, some silicon oxidation enters into the pretreatment desulphurization slag, the state of the desulphurization slag changes significantly, and the slag iron is not distinguished, which leads to the extension of slagging time and the increase in iron loss after the pretreatment desulphurization, which seriously restricts the popularization and application of the new process. To solve this problem, dilute slag agents such as ice crystal, broken glass, and fluorite are usually added to change the viscosity of the desulfurized slag to reduce the amount of iron in the slag [22,23,24,25,26,27]. There are few reports on metal iron and total iron in desulfurization slag at home and abroad. A foreign company added a 4% modifier to the desulfurization agent, reducing the metal iron in the desulfurization slag by 10 percentage points, but the modifier is costly [28]. Many domestic steel mills have added modified agents to the desulfurization process of molten iron, which has a certain iron reduction effect, but it is not significant. Most steel enterprises still have more than 30% of metal iron in sulfur slag [29,30,31,32,33].

In view of the particularity of Pangang semi-steel desulfurization, this paper designed the optimal modification agent with the help of FactSage 8.2 thermodynamic calculation software and modified the desulfurization slag through industrial experiments. The desulfurization slag state was further improved, effectively reducing the desulfurization slag content. The iron quantity and slag removal iron loss have laid the foundation for the comprehensive promotion and application of the new silicon-enhancing process.

2. Experimental

In order to ensure the heat source for subsequent steelmaking, the silicon-increasing heat compensation process was adopted in the steel-making process of the vanadium-extracting converter, namely: ① The temperature of the semi-steel was 1360–1400 °C, the carbon content of the semi-steel was 3.40–3.60%, and the residual vanadium of the semi-steel is ≤0.035%. ② When tapping the steel, ferrosilicon (Chengdu Jiwei Internet of Things E-commerce Co., Ltd., Chengdu, China) was added to deoxidize and increase silicon when the steel was 1/3 tapped. The actual amount of ferrosilicon used when tapping the semi-steel was determined according to the carbon content of the semi-steel. The corresponding relationship between the carbon content of the semi-steel and the amount of ferrosilicon added when tapping the semi-steel is shown in

Table 1. Relationship between carbon content and added amount of ferrosilicon in semi-steel.

Carbon Content of Semi-Steel/%	<3.0	3.0–3.2	3.2–3.4	3.4–3.6	>3.6
Ferrosilicon dosage/kg·heat−1	300	200–300	100–200	100	0
Convert it to an increase in carbon/%	0.31	0.21–0.31	0.10–0.21	0.10	0

.

Because of the addition of ferrosilicon, the silicon content of semi-steel was increased to 0.10–0.15%. In the desulfurization process, some of the silicon oxidizes, leading to the SiO₂ content of the desulfurization slag increased by 4.59% during the subsequent desulfurization pretreatment (as shown in

Table 2. Composition of desulfurization slag/%.

Project	S	CaO	FeO	MgO	TotalFe	SiO2	Desulfurization Rate
Silicon-Increasing Process	4.83	32.54	12.58	9.79	41.75	5.79	96.36
Original Process	4.46	33.67	11.71	9.16	42.60	1.20	96.52

). Although this does not significantly impact the desulfurization rate, the state of the desulfurization slag is changed and severely affects the slag removal operation. As shown in Figure 2, it can be observed that the top of the desulfurization slag was severely powdered, and the interface of slag–metal was sticky and soft. Meanwhile, black solidified desulfurization slag continuously precipitated, resulting in the slag removal becoming more difficult, with the average sulfur recovery during BOF blowing increasing by 0.002–0.004% compared to the original process, along with an increase in iron loss during slag removal of 0.54 kg/t Fe. Thus, the purpose of this paper is to adjust the desulphurization slag, ensure the desulphurization rate, and improve the separation effect of slag and iron.

3. Results and Discussion

3.1. Original Desulphurization Slag

From Figure 3, we can see that the morphology of the original desulfurization slag is mainly composed of fine particles and dust. After adding silicon, the agglomeration of desulfurization slag increased, indicating that changes in the silicon content in the desulfurization slag increase the viscosity of the desulfurization slag, and changes in the interface tension make the slag and iron aggregate into blocks, making it more difficult to separate slag–iron.

Compared with the desulfurization of molten iron, the temperature of semi-steel is relatively low. In addition, the semi-steel magnesium composite desulfurization agent is only lime and magnesium powder. The MLA measurement results (as shown in Figure 4 and

Table 3. Phase composition of desulfurization slag/%.

Mineral Name	Metal Iron	Cao Solid Solution	Cao-Mgo Solid Solution	CaMg(SO4)2 Solid Solution	Iron Oxide	Ferrite	Plaster	Square Magnesium Stone	Calcium Manganese Olivine	CaSO4 Solid Solution	Other
Wt%	27.45	33.73	10.29	15.21	1.87	2.56	2.53	2.39	1.65	1.63	0.068

) show that desulphurization slag is mainly high-melting point oxides such as CaO, MgO, and Cao-Mgo solid solution. A large amount of iron is wrapped in the high-melting-point phase (as shown in Figure 5). So, the high melting point and low overheating degree of semi-steel contribute to this issue. Lime and magnesium oxide absorb heat within the ladle, which can locally cause the semi-steel to solidify and form a co-solidified shell with desulfurization slag. Lime can also co-solidify with the semi-steel; when the coalesced slag–iron rises to the surface of the ladle due to the rapid temperature drop, the semi-steel solidifies quickly and cannot return to the molten state. This portion of the semi-steel will be lost during the slag removal process.

Thus, it is evident that whether molten iron or semi-steel, it is essential to reduce the alkalinity of desulfurization slag and lower its melting temperature to improve the flow characteristics and rheological properties of the slag, thereby facilitating slag removal. Furthermore, only by decreasing the surface tension of the slag can we fundamentally reduce the viscosity of the desulfurization slag and minimize iron losses within the slag.

3.2. Suitable Silicon Content in Semi-Steel

As shown in Figure 6, the higher the silicon content in the semi-steel, the greater the viscosity of the semi-steel. Additionally, a lower molten iron temperature reduces the energy of atomic thermal motion, which decreases the activation energy required for particle movement. This may also increase the size of the viscous flow units that form clusters, thereby increasing viscosity. After the silicon content is increased in the semi-steel, the rise in viscosity affects the diffusion rate of sulfur within the semi-steel, leading to a reduction in desulfurization efficiency. Furthermore, it prolongs the time required for desulfurization slag, particularly fine powdery slag, to float to the liquid surface, resulting in the continuous appearance of fine slag after the desulfurization and slag removal processes in silicon-increased semi-steel. This leads to longer slag removal times and greater iron loss.

Considering the later heat supplement and slag-making requirements, the Si content level of 0.1% was determined for subsequent experiments.

3.3. Liquid-Phase Region Analysis of Slag System

Based on the characteristics of the desulfurization production process, the main components of desulfurization slag include CaO (primarily from passive lime), MgO (mainly from the oxidation of magnesium powder), FeO, Al₂O₃, and SiO₂ (mainly from the slag of previous processes). This composition results in a high melting point and a low number of low-melting-point substances in the desulfurization slag. During the desulfurization process, a portion of the molten iron mixes with the high-melting-point slag, leading to heat exchange. When the temperature of the molten iron drops below the melting point, it adheres to the slag, making it impossible to separate the slag from the iron and causing high iron losses. Therefore, three kinds of desulphurization slag with Al₂O₃ content of 0.08%, 2%, and 6.3% were designed according to the field application results, and 1.5%Na₂O was added to the base to calculate the change in the liquid phase of each desulphurization slag system. The main components of each slag system are shown in

Table 4. Main components of each slag system/%.

Items	CaO	MgO	Al2O3	FeO	Na2O	SiO2
Original slag	39.36	11.12	-	10.06	-	3.06
Original slag + Al2O3	36.66	10.34	4.02	9.40	-	2.96
Original slag + Na2O	38.42	10.84	-	9.85	0.96	2.96
Original slag + Al2O3 + Na2O	36.08	10.18	4.09	9.25	1.02	2.78

.

As shown in Figure 7 and Figure 8, when the slag contains only CaO, SiO₂, and FeO, the liquid-phase region at 1350 °C is relatively large. However, when MgO is present, the liquid-phase region decreases sharply. In the desulfurization process using magnesium as the primary desulfurizing agent, a large amount of MgO enters the slag, resulting in a minimal liquid-phase region in the desulfurization slag (see Figure 9), which leads to a higher iron content in the desulfurization slag.

To lower the melting point of desulfurization slag and improve the overall separation efficiency of slag and iron, it is essential to increase the liquid-phase region of the slag. From long-term practical experience, it is known that alkali metal oxides and chlorides can lower the melting point of the slag; however, they pose significant hazards to equipment and the environment. Given the high CaO content in desulfurization slag, one can consider adding Al₂O₃ to form low-melting-point compounds such as 12CaO·7Al₂O₃, thereby reducing the overall melting point of the slag. As illustrated in the melting point diagrams of slag systems with varying Al₂O₃ contents (Figure 10), it can be observed that as the Al₂O₃ content in the slag increases, the liquid-phase region tends to enlarge. Therefore, after adding Al₂O₃, introducing alkali metal oxides can further promote the expansion of the low-melting-point region.

Figure 11 and Figure 12 indicate that the liquidus region of the basic slag system CaO-SiO₂-Al₂O₃ is expanded with the increasing content of Na₂O from 1.5% at 1350 °C. Adding the 1.5% Na₂O to multiple systems, the melting point of the slag system decreased significantly.

Therefore, the formulation of the modifier can consider adding Al₂O₃ and Na₂O to adjust the state of the desulfurization slag. The amount of Na₂O should be kept below 2% to minimize its impact on equipment and the environment, while the content of Al₂O₃ can be optimized by adjusting the dosage of the modifier. For example, with the addition of 6.3% Al₂O₃ and 1.5% Na₂O (Figure 13), a noticeable expansion of the liquid-phase region is evident.

3.4. Analysis of Desulfurization Effect After Slag System Optimization

Thus, the main components of the desulfurization modifier specifically designed for the desulfurization station of Pangang Steel’s semi-steel magnesium desulfurization process should include CaO, Na₂O, and Al₂O₃. The desulfurizing modifier uses soda ash and dust from the electrolytic aluminum plant (as shown in

Table 5. Composition of dust removal ash in electrolytic aluminum plant/%.

F−	Na2O	MgO	Al2O3	SiO2	K2O	CaO	TiO2	Fe2O3	P2O5
2.0	1.6	2.1	79	6.9	2.0	0.8	0.3	0.6	0.1

) as raw materials and is added to the production process of passivated lime to ensure that the composition of passivated lime meets the requirements of

Table 6. Component requirements and actual components of modified passivation lime/%.

Items	CaO	Al2O3	Na2O	F−
Requirement	≥80	≥3	≥1.0	≥0.5
Real control	80.23	3.45	1.28	0.61

.

According to the theoretical calculation results, industrial validation tests with additive amounts of 6%, 8%, and 10% were designed to explore the optimal dosage of modifier. The desulfurization modifier was uniformly mixed with lime and then introduced into the mixing system for blending, grinding, and passivation, resulting in the improved passivated lime required for the experiments. The improved passivated lime was then injected for desulfurization using magnesium powder following the original injection process.

Among the commonly used indicators for evaluating desulfurization effectiveness, magnesium consumption per unit and iron loss are the most important, as they both represent the desulfurization capacity of the agent and reflect the desulfurization cost. Based on the results of the industrial verification (Figure 14), compared with the amount of other modified agents added, the desulfurization rate of 8% of the modified agent reached 88.01%, which was the highest, while the magnesium unit consumption was 0.0137 kg/t_Fe and the slag-stripping iron loss was 2.3%, which was the lowest. Therefore, adding an 8% modifier has a better comprehensive desulfurization effect. Therefore, the final component requirements for the passivated lime with an 8% modifier are listed in

Table 7. Composition of Modified passivated lime/%.

Component	CaO/%	Na2O	S	Calcination	<1 mm Particle Size/%	<0.075 mm Particle Size/%	<0.25 mm Particle Size/%
Requirement	≥85	≥1.0	≤0.10	≤5.0	100	≥80	≥98

, and the requirements for the modifier composition are detailed in

Table 8. Composition of modifying agent/%.

Component	CaO/%	Na2O	Al2O3	Calcination	<1 mm Particle Size/%	<0.075 mm Particle Size/%	<0.25 mm Particle Size/%
Requirement	≥40	≥15.0	≥40.0	≤5.0	100	≥80	≥98

.

Under the optimal modifier-adding conditions, the change in desulfurization slag composition is shown in

Table 9. Composition of desulfurization slag before and after optimization/wt%.

Items	S	CaO	SiO2	Fe2O3	FeO	MgO	TotalFe	MetalFe	Al2O3	Na2O	Sample Size
Original	4.77	35.07	2.07	0.75	9.64	11.37	33.63	24.79	0.60	-	20
After optimization	5.65	46.23	1.89	1.51	4.84	12.68	27.62	23.02	1.00	0.196	14

, and the desulfurization effect verified by industry is shown in

Table 10. Comparison of desulfurization rate and slag removal time.

Items	Sulfur Content Before Desulfurizationt/%	Sulfur Content After Desulfurizationt/%	Desulfurization Ratet/%	Slag Removal Time/ min
Original	0.0702	0.006	91.96	8.7
After optimization	0.0782	0.007	92.20	7.2

. It can be seen from the table that the desulfurization rate of the test furnace is basically the same when the sulfur content is increased, but the MFe and TFe in the desulfurization slag are 23.02% and 27.62%, which are reduced by 1.77% and 6.01%, respectively. The skimming time is reduced by 1.5 min, compared with the comparison, indicating that the separation effect of slag and iron has been significantly improved after slag adjustment.

Figure 15 and Figure 16 show the phase diagrams for quaternary and multi-component slag systems, representing the desulfurization slag before and after modification, respectively. After the addition of substances such as Al₂O₃ and Na₂O, the modified desulfurization slag exhibits a larger liquid-phase region compared to the original desulfurization slag, indicating a significant improvement in the melting characteristics of the slag. Additionally, there are substantial changes in the resulting materials. Under desulfurization temperature conditions, the original desulfurization slag primarily contains MgO, CaO, 3CaO·SiO₂, and 3CaO·MgO·2SiO₂ in the liquid phase. In contrast, the modified slag features more MgO and CaO participating in the formation of liquid-phase materials, while the solid phase contains not only MgO and CaO but also 3CaO·MgO·2Al₂O₃(s), 3CaO·MgO·2SiO₂, and 2CaO·SiO₂. The increase in the variety and composition of solid-phase materials, along with the addition of more types of liquid-phase substances (as shown in

Table 11. Type of substance in the multicomponent slag system.

Area	Substance Type
1	L, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s)
2	L, MgO(s), 3CaO.MgO.2Al2O3(s)
3	L, MgO(s), 3CaO.MgO.2Al2O3(s), 3CaO.MgO.2SiO2(s)
4	L, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s), 2CaO.SiO2(s)
5	L, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s), 2CaO.SiO2(s)
6	L, MgO(s), CaO(s), 2CaO.SiO2(s)
7	L1, L2, MgO(s), CaO(s), 2CaO.SiO2(s)
8	L1, L2, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s)
9	L1, L2, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s), 3CaO.MgO.2SiO2(s)
10	L, MgO(s), CaO(s), 3CaO.MgO.2Al2O3(s), 3CaO.MgO.2SiO2(s)

), facilitates the separation between slag and iron and promotes the rapid rise of solid-phase materials.

The interaction between desulfurization slag and semi-steel is illustrated in Figure 17. Without modification, the desulfurization agent is injected into the liquid semi-steel through nitrogen gas, coating the semi-steel and undergoing a desulfurization reaction. The desulfurization products primarily consist of calcium-magnesium oxides, with very little liquid slag present. The desulfurization slag absorbs heat from the semi-steel liquid, forming a solid shell that cools down further, resulting in slag-covered iron. In contrast, with the modified desulfurization agent, the coating and desulfurization reaction still occur on the surface of the semi-steel droplets, but the desulfurization products now include low-melting-point materials containing aluminum and sodium. When these low-melting-point materials are in sufficient quantity, they can effectively isolate the high-melting-point materials, preventing complete coverage of the semi-steel droplets and thus avoiding the formation of slag-covered iron. Therefore, the slag iron separation effect will be better.

The melting point of the desulfurization slag was measured by the HB-6 hemispherical point method melting point meter, and the imaging system obtained the melting of the slag. The melting states of desulfurization slags before and after modification at 1350 °C are shown in Figure 18, which shows that the melting point of the modified desulfurization slag is relatively lower, and its softening effect is superior to that of the unmodified desulfurization slag.

Figure 19 shows the XRD comparison of desulfurization slag before and after modification. It can be seen from the figure that the desulfurization slag mainly contains Fe, CaS, CaO, and MgO phases, and no calcium aluminate phase is found, which may be because the amount of aluminum ash added is too low. The formation of the CaS phase indicates that FeS in semi-steel reacts with CaO in desulfurization slag. The diffraction peak strength of Fe decreases with the addition of aluminum ash, and the addition of aluminum ash is beneficial to the reduction in Fe content in the slag. The diffraction peak intensity of CaS in sample 20805759 and sample 20805757 is higher than that in the desulphurization slag, which indicates that the addition of aluminum ash can improve the desulphurization rate of desulphurization slag. The reasons for the improvement of the desulfurization effect are as follows: in the process of desulfurization, after the formation of the outer CaS, its diffusion in the lime particles is very slow, resulting in a sharp decline in the reaction rate, coupled with the short residence time of the desulfurizer in the hot metal during the test, once the CaS reaction layer is formed, it is difficult for the internal CaO to continue to participate in the reaction. After adding aluminum ash, Al₂O₃ in aluminum ash reacts with CaO to form calcium aluminate with a low melting point, forming a liquid reaction layer, so that sulfur diffuses easily into the interior of the CaO particles. Therefore, adding a certain amount of Al₂O₃ to the desulfurizer is conducive to speeding up the speed of the desulfurization reaction, increasing the desulfurization amount and desulfurization rate of the desulfurizer, and improving the utilization rate of the lime particles. At the same time, the low-melting-point slag has better fluidity and lower viscosity, which can inhibit the inclusion of metal Fe in the desulphurization slag, and then reduce the content of TFe in the slag.

According to the analysis results of the modified desulphurization slag phase (as shown in Figure 20 and

Table 12. Mass percentage of each phase.

Spectrum	Number	O	Na	Mg	Al	Si	Ca	Ti	Mn	Fe
Mangan-magnesio olivine	1	31.15	37.12	23.47				7.56
Calcium manganese pyroxene	2	31.17	0.81	2.89	12.57	18.66	18.59	3.51	10.89	0.91
Manganmagnesio olivine	3	21.56	38.02	23.67		0.69			6.06
Calcium magnesium pyroxene	4	31.59	5.64	11.17	17.19	22.81	5.9		5.7
Calcium magnesium pyroxene	5	31.15	5.7	11.58	17.23	22.83	4.82		6.75
Manganmagnesio olivine	6	31.06	37.68	23.58		0.77			6.91
Manganmagnesio olivine	7	31.32	38.01	23.52	11.77	21.11
Mean		31.28	0.81	21.53	11.77	23.49	9.52	4.75	7.24	0.91
Sigma		0.21	0	17.59	0.72	3.07	11.03	1.2	1.72

), in addition to the phase formed by CaO and MgO, there are also phases containing Na and Al in the desulphurization slag, which further proves that the modifier plays a regulating role in the desulphurization slag phase and supports the realization of the desulphurization efficiency and the iron reduction effect of the desulphurization slag.

4. Conclusions

• In response to the issue of high iron loss during the pretreatment desulfurization slagging process in the semi-steel silicon enhancement new technology, a desulfurization modifier containing Na₂O and Al₂O₃ was designed using FactSage thermodynamic calculation software. Industrial tests have verified that adding an 8% modifier to passivated lime can achieve optimal magnesium desulfurization agent consumption and iron loss indicators.
• After modifying the desulfurization slag, the magnesium powder consumption per 0.001% sulfur removal decreased from 0.0149 kg to 0.0136 kg, while the iron loss during slagging reduced from 3.52% to 2.28%. The average slagging time was shortened by 1.5 min, significantly lowering production costs and improving the efficiency of pretreatment desulfurization.
• By modifying desulphurization slag, good industrial verification results were obtained, and the research results have been widely applied in the Pangang production site. However, further improving the reaction efficiency of slag–metal and improving the separation effect of slag and metal will be an important research direction in the future desulfurization process of vanadium and titanium hot metal pretreatment.
• The successful application of desulfurization slag modification technology has laid the foundation for the comprehensive promotion and application of the semi-steel silicon enhancement process, effectively alleviating the long-time historical problem of an insufficient heat sources for semi-steel smelting, and effectively supporting the efficient utilization of vanadium–titanium magnetite in the Panxi area of China.