A novel method for CO 2 injection direct smelting vanadium steel: Dephosphorization and vanadium retention

: A novel method for CO 2 injection direct smelting vanadium steel (CIDSVS) is proposed. Achieving selective oxidation of phosphorus is essential for the applicability of the suggested process. Under the guidance of thermodynamics, the mechanisms of CO 2 injection dephosphoriza-tion and vanadium retention were investigated with CO 2 ﬂ ow rate and dephosphorization slag composition as experimental variables. The results indicate that CO 2 as an oxygen source can remove 73.8% of phosphorus, while the oxidation rate of vanadium is 17.5%. The dephosphorization process can be divided into two stages: FeO-and CO 2 -dominated experimental processes. In the initial stage of slag feeding, [V] and [P] undergo fast oxidation, and the oxidation amount is positively correlated with the initial FeO content. The high basi-city (CaO/SiO 2 ratio) reduces the activity of V 2 O 3 in the slag and promotes the oxidation of [V]. Under the experimental conditions of 1,400°C, the optimal conditions were determined to be a CO 2 ﬂ ow rate of 1.5 mL·g − 1 ·min − 1 , a FeO content of 40%, and a basicity B of 2.5. Following the CIDSVS steel-making operation, 80% of the vanadium is retained, and the impurity elements ful ﬁ ll the speci ﬁ cations for steel. This method enhances vanadium utilization and is environmentally friendly.


Introduction
Vanadium steels are widely used in high-rise buildings, oil and gas pipelines, and large-span structures.Worldwide, vanadium consumption in the steel-making industry accounts for 92.9% of its total production [1].In recent decades, urban construction and manufacturing development have created a huge demand for vanadium steel.There is an immediate need to develop technology for the clean and efficient use of vanadium in the steelmaking process.
Vanadium-titanium magnetite is the main vanadiumbearing resource, contributing over 88% of vanadium production [2,3].Generally, the content of vanadium in the ore is less than 1%.To recover as much vanadium as possible, the ore is usually used as the feed of a blast furnace, where vanadium is co-reduced to molten metal.Then, oxygen is injected into the converter to obtain vanadium slag and 80-90% of V will be enriched in the vanadium slag to meet the needs of the vanadium extraction plant [4][5][6][7].
Roasting of vanadium slag followed by leaching produces V 2 O 5 products [8][9][10][11][12][13].The technique has been extensively investigated and provides a high vanadium recovery rate (85-90%).However, the roasting process requires a great deal of energy or generates toxic gases such as Cl 2 , SO 2 , and HCl.Significant quantities of hazardous wastewater and residues are produced by the leaching process [14], including ammonia-containing wastewater, Cr-containing wastewater, and hard-to-recover tailings.According to incomplete statistics, each ton of V 2 O 5 production will produce 30-50 tons of ammonia-containing wastewater (Na 2 SO 3 , (NH 4 ) 2 SO 4 , and NH 4 Cl) and 2-7 tons of difficult-to-recycle residues [8,15].Aluminum and iron particulates reduce V 2 O 5 at 1,800°C in an arc furnace to produce the V-Fe alloy [16].The procedure has a high energy consumption, and the waste residue contains 0.35% V 2 O 5 , which is a potentially dangerous solid waste.
When V-Fe is put into the refining furnace as an alloy additive, 10% or more of vanadium is lost in the form of oxidation or volatilization [17].In recent years, severe rules on high-strength steel have boosted demand for vanadium as a steel addition.Vanadium usage surged by 67.8% from 2011 to 2021, according to the International Vanadium Technical Committee (Vanitec), and Lee expects consumption to reach 130.1 kilotons by 2024 [1].The high demand for vanadium will further drive the vanadium-producing industry to pollute the environment.
After converter oxidation, vanadium in vanadiumcontaining molten iron is enriched in vanadium slag.V-Fe alloy is prepared by the reduction of V 2 O 5 obtained by roasting, leaching, and deposition of vanadium slag.Finally, in the form of V-Fe, vanadium participates in the alloying of steel.The utilization rate of vanadium decreases by degrees with the increase in the vanadium steel production process.About 40-50% of vanadium resources in vanadium-bearing hot metal are lost in the residue and effluent [17].In order to be environmentally friendly and improve the utilization efficiency of vanadium, the authors proposed a CO 2 injection direct smelting vanadium steel (CIDSVS) method.The vanadium-bearing hot metal is not used as the raw material for vanadium slag extraction; instead, the weak oxidizing gas CO 2 is used to carry out the impurity removal and steelmaking operations directly.The process flowchart is depicted in Figure 1.The CIDSVS method reduces vanadium extraction and subsequent production steps.The proposed approach can decrease the vanadium loss caused by frequent vanadium transfer during the manufacture of vanadium steel, reduce energy consumption and hazardous waste emissions, and reduce the environmental impact of vanadium steel manufacturing.
Phosphorus is the main harmful element in steel, which precipitates at the crystal boundary and reduces the impact resistance of steel.In general, [P] ≤ 0.045% is the limit for alloy steel (excluding low-phosphorus steel).Steel smelting has a long history, and the oxygen top-blown converter dephosphorization process is mature.The doubleconverter dephosphorization process can even produce [P] ≤ 0.01% of ultra-low phosphorus steel.However, selective oxidation of [V] and [P] cannot be achieved with traditional O 2 steelmaking.CO 2 is a weak oxidizing gas and is regarded as one of the culprits of the global greenhouse effect.As technology advances, CO 2 is increasingly incorporated into the steelmaking process as an auxiliary gas [18,19].The Boudouard reaction CO 2 (g) + [C] = 2CO(g) improves the gas stirring ability and has a good effect on suppressing the rapid increase in the furnace temperature [20][21][22].Consequently, some investigations [23][24][25] have achieved effective phosphorus removal by injecting CO 2 -O 2 mixed gas during dephosphorization.
The most important step in the CIDSVS technique is the selective oxidation of [V] and [P] in molten iron.This work focuses solely on the CO 2 dephosphorization process in the CIDSVS method.This is a crucial prerequisite for applying the proposed method.Using the unique chemical properties of CO 2 , the material flow of vanadium and phosphorus is altered, leaving vanadium in the metal phase while meeting the steel's impurity requirements.As the first investigation of short-process smelting of vanadium-containing hot metal, this study aims to employ CO 2 and alter slag composition to dephosphorization and retain vanadium.At the same time, the effects of varying CO 2 flux, slag FeO content, and basicity on the slag-metal distribution ratio of V and P were examined.According to the findings of the research, the process conditions for CO 2 injection dephosphorization for the CIDSVS method were obtained.
2 Materials and methods

Materials
The pig iron used in the experiment is a pre-desulfurized hot metal produced by Panzhihua Iron and Steel Co., Ltd.Table 1 lists the principal components.Dephosphorization slag is a CaO-SiO 2 -FeO ternary slag system prepared by uniformly mixing chemical reagents according to the target components listed in Table 2. Ferrous oxalate (FeC 2 O 4 ) was put into an Ar-filled resistance furnace at 800°C for 2 h to obtain FeO.The CO 2 utilized in the experiment has a purity of 99.999%, and the injection rate is detailed in Table 2.The thermodynamic verification experiment findings in Section 3.1 show that the dephosphorization results of the single slag method with an 8% slag quantity cannot match the steel impurity standards; therefore, in the experiments, we adopt a slag-metal mass ratio of 12%.The dephosphorization slag basicity is calculated by binary basicity (B), as shown in the following equation: The removal ratio η i of element i in the hot metal is calculated by equation (2).[i] 0 and [i] e represent the mass percent of element i in the hot metal at the initial point and the endpoint of dephosphorization stage, respectively:

Experimental apparatus and procedures
A graphite crucible containing a 200 g pig iron sample was placed in a vertical resistance furnace heated with the Mo-Si material.The experimental device is shown in Figure 2. The temperature was monitored by a B-type thermocouple, and the sample temperature was kept at 1,400°C for 30 min to guarantee sample consistency.Then, the uniformly mixed dephosphorization slag was put into the molten pool by a two-stage slag feeding method.In the first stage, 8% metal mass dephosphorization slag was fed into the molten pool, and, in the second stage, 4% metal mass dephosphorization slag was delivered.The reaction time of each stage was 15 min.The dephosphorization slag melts within 10-30 s, and then a corundum injection lance is Table 1: Initial composition of hot metals (mass%) [Fe] Mass% 4.42 0.31 0.11 0.17 0.05 0.12 0.013 94.62 inserted 10 mm below the liquid metal level to inject CO 2 gas into the molten pool.To prevent unwanted oxidation, Ar gas (2 L•min −1 ) was supplied during heating and experimentation.

Analysis methods
From the moment CO 2 is injected into the molten bath, a sample (1-2 g) is taken out with a quartz tube (OD 6 mm, ID 4 mm) every 180 s and quenched with water.The samples were dried and acid-solubilized for chemical composition analysis and the quantitative determination of [V] and [P] in metals using inductively coupled plasma optical emission spectrometry (ICP-OES; Prodigy Plus, USA).At the end of the experiment, the crucible was removed and placed in an air-isolated container to cool at room temperature, and the final slag was taken for analysis.An X-ray fluorescence analyzer (XRF, ZSX Primus IV; Rigaku Corporation, Tokyo, Japan) was used to qualitatively analyze the sample's chemical composition.The crystalline phase of samples was identified by an X-ray diffraction analyzer (XRD, D8 Advance, Bruker, Germany).XRD data were collected using a Cu-Kα radiation source in the range of 2θ = 10-90°with a scanning step of 0.04°•s −1 .The obtained data were compared with the Joint Committee on Powder Diffraction Standards International Center for Diffraction Data (JXPDS-ICDD) database to determine the crystal composition.The microstructure and chemical composition of the dephosphorization slag were observed with a scanning electron microscope (SEM, TESCAN MIRA LMS, Czech) and energy dispersive spectrometer (EDS, Xplore 30, UK) after cold inlay polishing.
3 Results and discussion

The mechanism for CO 2 dephosphorization
It is well known that dephosphorization requires low temperature, high oxidation, high basicity, and high fluidity.CO 2 is a weak oxidant, and its effect on dephosphorization is rarely reported.The Gibbs free energy change ΔG θ of the reaction of CO 2 , FeO, and O 2 with [V] and [P] was calculated with FactSage 8.1.For comparison purposes, 1 mol of O atoms is provided as a benchmark.The equation for the reaction is shown in Table 3, where ΔG θ is the standard Gibbs free energy change (J•mol −1 ), and T is the temperature (K).
As shown in Figure 3, ΔG θ values of dephosphorization and vanadium extraction reactions are negative in the temperature range of 1,523-1,973 K.The ΔG θ of the dephosphorization reaction is smaller than that of the vanadium oxidation reaction, which is independent of the oxygen source.The order of oxidizing power is O 2 > CO 2 > FeO.The dephosphorization and vanadium retention reaction in a CO 2 atmosphere were obtained by combining reactions (3) and ( 6), as expressed in reaction (9).
The molten iron can be regarded as a dilute solution, and the hypothetical mass of 1% is selected as the standard state.The Gibbs free energy change (ΔG) in reaction (9) can be expressed as • where R is the gas constant (= 8.314 J•mol −1 •K −1 ); f V and f P are the activity coefficients of V and P, respectively; [%V] and [%P] are the mass percentages of V and P, respectively, and According to Wagner's model [26], f V and f P can be calculated using equations (11) and (12) where the interaction coefficients of P and V can be taken as e e e 0.062, 0.015, 0.13 and −0.34; the e e and P V V P data are missing [27].Equation (10) shows that ΔG tends to decrease with the increase of V 2 O 3 activity, indicating that the dephosphorization and vanadium retention reaction (reaction 9) is closely related to [V], [P], and the activity of V 2 O 3 .Furthermore, based on the Gibbs free energy minimization principle, the thermodynamic equilibrium phase concentration changes of 100 g vanadium-containing pig −61,961 + 22.79T iron (Table 1) and 4.8 g CaO-2.4 g SiO 2 -0.8 g FeO-0.44 g CO 2 at different temperatures at one atmospheric pressure were calculated, as shown in Figure 4.
The dephosphorization process is highly dependent on low temperatures, and Figure 4 clearly shows that the equilibrium [P] content increases with temperature.The content of [V] in the hot metal remained relatively constant with the temperature change.Phosphorus was mainly fixed in the 2CaO•SiO 2 -3CaO•P 2 O 5 (C2S-C3P) solid solution phase and reached a maximum content at 1,400°C.This is consistent with the conclusion of Sun et al.'s 180-ton industrial dephosphorization experiment [28], and the C2S-C3P enrichment is the largest at 1,400°C.'s The industrial experiment of Tian et al. involving the dephosphorization of a 210-ton converter demonstrates that the dephosphorization rate diminishes with increasing temperature between 1,327 and 1,427°C [29].Zhou et al. found that with the increase in the dephosphorization temperature, dephosphorization increased first and then decreased; the optimum dephosphorization temperature was 1,400°C [30].
Further, to verify that CO 2 does provide assistance in the dephosphorization process, slag + CO 2 , slag + N 2, and slagonly experiments were carried out.As shown in Figure 5, [V] and [P] decrease first and then increase under the conditions of slag + N 2 and slag only.This is due to the strong reducing ability of the [C]-rich liquid iron and, as the reaction proceeds, the FeO activity at the interface decreases continuously and V 2 O 3 and P 2 O 5 are reduced into the iron water by [C].Under the condition of slag + CO 2 , no phosphorus reversion was observed, and [P] continued to decrease.Compared with the injection of N 2 , the dephosphorization rate increased from 6.7 to 36.7%.After proving that CO 2 can carry out dephosphorization and vanadium retention operations, the author investigated the CO 2 injection flow rate, slag FeO content, and basicity to expand the difference in oxidation behaviors of the two elements.The composition of the final slag and the contents of [V] and [P] in metals under different experimental conditions are shown in Table 4.

Reaction behaviors of [V] and [P] for
varying CO 2 flow rates    CO 2 injection direct smelting vanadium steel  5 exceeds 1.0 mL•g −1 •min −1 , the changing pattern of [P] tends to be the same, which indicates that the dephosphorization process is not controlled by the CO 2 flow rate at this time.
The thermodynamic feasibility of CO 2 dephosphorization is proved in Section 3.1.However, the realistic course of the reaction is influenced by kinetic factors.At high temperatures, the mass transfer rate is lower than the chemical reaction rate [6].The controlling step of the reaction process is the mass transfer of components (including the reactants and the oxidation products) in the hot metal and slag.At the interface of the blowing chamber, CO 2 is adsorbed to the liquid metal surface and reacts.The total interfacial reaction can occur in the following steps: [ ] [ ] ( ) Equation ( 14), the dissociation reaction of CO 2 , is considered to be the limiting step in the whole process.As illustrated in equation ( 17), Mannion and Fruehan apply the theories of absolute rate and microscopic reversibility to calculate the CO 2 dissociation rate [31]: where r is the rate of dissociation in mol•cm −2 •s −1 , n is the number of moles CO 2 per unit area, K p represents the  forward rate constant for the CO 2 reaction, and θ i is the proportion of vacancy covered by species i.The experimental temperature is constant at 1,400°C, K p is a constant, and the liquid-metal composition is uniform, so the dissociation rate r of CO 2 is a constant.However, the gas-liquid contact area increases with the CO 2 flow, resulting in an increase in the [O] production per unit of time.This increases the equilibrium constant in equation (15).The oxidation amount of [V] increased.Mukawa and Mizukami studied the effects of stirring energy and the oxygen supply rate on the dephosphorization rate of molten iron [32].The results showed that the dephosphorization rate was always controlled by the slagmetal interfacial mass transfer process in both molten iron and slag stages.In the present study, the lower CO 2 flow rate (1.0 mL•g −1 •min −1 ) provides limited [O] for the molten pool and cannot further promote the dephosphorization reaction.Moreover, the low-speed gas flow fails to provide good mass transfer conditions for the molten pool, so the dephosphorization rate is low.The experimental results show that the CO 2 flow rate at 1.5 and 2.5 mL•g −1 •min −1 provides better dephosphorization ability and kinetic conditions, and the dephosphorization rate can reach 71.8 and 79.1% (Figure 6(b)).

Effect of FeO content in slag on [V] and [P] in the hot metal
In order to more intuitively see the oxidation law of V and P by the change in FeO content in the initial slag, the [V] and [P] contents and time were plotted.As seen in Figure 7(a), the contents of [V] and [P] in the molten metal decreased rapidly within the initial 6 min following the addition of the dephosphorization slag, and the transition value increased with the increase of FeO content in the slag.With the increase of FeO content in the slag from 20 to 50%, the endpoint [P] decreased from 0.045 to 0.029% and the dephosphorization rate increased by 24.5%.The oxidation rate of [V] is the most prominent at 50% FeO, which is 28.4%.This demonstrates that an excessive increase in FeO promotes [V] oxidation, which is detrimental to vanadium retention operations.Studies have shown that the dephosphorization slag is a multiphase slag composed of a homogeneous glass phase (2CaO•SiO 2 ), a phosphorus-rich phase (C2S-C3P), and an iron-rich phase [34,35].At steelmaking temperature, C2S and C3P can form a solid solution in a wide composition range.As a result, the activity of P 2 O 5 in the slag decreases, which increases the driving force for the transformation of [P] to P 2 O 5 .As shown in Figure 8, the dephosphorization rate increases with basicity.
In the process of vanadium extraction, vanadium is enriched in the converter vanadium slag.The CaO content of the vanadium slag is generally less than 2.5%, and V 2 O 3 is mainly in the spinel phase.However, the content of CaO in the dephosphorization slag is more than 50%, so it is necessary to characterize the final slag by XRD and SEM.The XRD of the final slag is shown in Figure 9(a).Phosphorus and vanadium are mainly enriched in C3P, C2S-C3P, and CaV 2 O 4 phases.(V 2 O 3 ) is a neutral oxide, acidic in a basic slag system.The increasing alkalinity promotes the combination of V 2 O 3 and CaO to form CaV 2 O 4 , which further reduces the activity of V 2 O 3 in the slag and expands the equilibrium constant of reaction (6), causing a decrease in [V]. Figure 9(b) shows the SEM image of the final slag with B = 4.By EDS surface scanning analysis, it is shown that the slag is mainly composed of CaO, phosphorus-rich phase, and iron-rich phase.The vanadium and phosphorus in the slag are evenly distributed in the calcium-containing mineral phase and no vanadium-containing spinel phase is found.

Effect of dephosphorization conditions on [V] and [P] slag-metal distribution ratio
From the above discussion, CO 2 gas not only participates in the dephosphorization reaction but also has a low oxidation  rate of vanadium, which can be used as a dephosphorization oxygen source for smelting vanadium steel in the CIDSVS method.Further, in order to analyze the influence of different factors on dephosphorization and vanadium retention, based on the experimental data in Table 4, the slag-to-metal distribution ratios L V and L P of vanadium and phosphorus were calculated as follows: (18) The CIDSVS method pursues high L P and low L V , defines L P /L V to find the best process conditions in the feasible region, and high L P /L V represents a high dephosphorization and vanadium retention effect.Figure 10 shows the correlation obtained between L V /L P and experimental factors.
Figure 10(a) shows the relationship between the CO 2 flow and L P /L V , where L P /L V shows a maximum at 1.5 mL•g −1 •min −1 .
However, the correlation between the CO 2 flow and L P /L V was unsatisfactory.The curve for the current studied data shows a coefficient of determination (R 2 ) of 0.6159.In Figure 10(b), L P /L V shows a trend of increase to decrease with the increase of FeO.As discussed in Section 3.3, the low FeO content slag has poor fluidity and low dephosphorization efficiency.A high FeO content leads to a large amount of vanadium oxidation at the slag-metal interface, and a large amount of V 2 O 3 enters the slag phase.The results in Figure 10(c) show that the effect of dephosphorization and vanadium retention is better under low basicity operation.Figure 10(d) shows the correlation between L P and L V .L P increases with L V , indicating that a small amount of V 2 O 3 in the slag can increase dephosphorization efficiency.
In summary, the researchers concluded that a CO 2 flow rate of 1.5 mL•g −1 •min −1 can meet the dissolved oxygen required for dephosphorization and control the redox reaction of vanadium at equilibrium.A 40% FeO content in the initial slag and a basicity of 2.5 provide better kinetic  Further decarburization of steelmaking is needed after the dephosphorization of molten iron.According to previous studies [36], the oxidation order of carbon and vanadium reverses at 1,357°C, and carbon preferentially oxidizes vanadium when the smelting temperature is higher than 1,357°C; a decarburization rate of 95.6% was achieved at 1,550°C.Vanadium-bearing steel was obtained by decarburization of dephosphorization pig iron at 1,550°C, a CO 2 flow rate of 3 mL•g −1 •min −1 , and an injection time of 80 min.The main components are shown in Table 5.At this time, the impurity content satisfies the specifications for a particular vanadium steel, and after alloying, the desirable steel can be produced.
In comparison to the current process's yield of 50-60%, the CIDSVS method can retain 80% of the vanadium in steel.The resource utilization efficiency of vanadium has improved.According to the production route of vanadium extraction, the cost of processing per kilogram of vanadium slag is 75.23 RMB and does not include the treatment of wastewater and tailings [37].The CIDSVS process does not generate hazardous waste and can lower manufacturing costs by 4.8 kg of 50V-Fe per ton (about 55 kg of vanadium slag) of vanadium steel.Considering the low calorific value and long impurity removal time generated by CO 2 participating in the oxidation reaction, it is necessary to use electric energy to heat the system during the steelmaking process.The CIDSVS process has a good application prospect in the electric furnace steelmaking process [38] and has positive economic and environmental effects.4) The L P /L V parameters were defined and fitted with experimental factors.It was found that the L P /L V was well correlated with the FeO content and basicity.The optimum conditions in the feasible region are a CO 2 flow rate of 1.5 mL•g −1 •min −1 , FeO content of 40%, and basicity B of 2.5.Under the above conditions, the retention rate of vanadium is 82.5%, and the removal rate of phosphorus is 73.8%.After decarbonization, the [V], [P], and [C] percentages in the molten steel are, respectively, 0.248, 0.028, and 0.43%, which meet the requirements for special vanadium steels.This investigation provides the groundwork for the dephosphorization process of the CIDSVS method.

Figure 1 :
Figure 1: Traditional smelting process and the CIDSVS method flow chart of vanadium steel.

Figure 6 (
Figure 6(a) depicts the effects of varying CO 2 injection rates on [V] and [P].The oxidation rate of [V] increased with the CO 2 flow rate, and the oxidation amount of vanadium at the endpoint increased from 12.6 to 22.9%.When the CO 2 flow rate

Figure 4 :
Figure 4: The theoretical content of different components in an equilibrium system at different temperatures.

Figure 6 :
Figure 6: (a) The effect of varying CO 2 injection rates on [V] and [P] and (b) removal rate of [P] and [V].

Figure 7 (
b) shows the theoretical viscosity of the slag under different FeO contents calculated using FactSage 8.1 and the content transition value of the target element in the metal for 0-6 min.With the increase of FeO, the viscosity of the initial slag decreased from 1.55 to 0.26 Pa•s, the Δ[V] increased from 0.017 to 0.039%, and the Δ[P] increased from 0.032 to 0.0499%.The FeO in slag shoulders the role of reducing the melting point, improving fluidity, and oxidizing impurities in the metal.The viscosity of the slag is the key characteristic that controls the molten slag's mass transfer and crystallization behavior.Considerable research has shown that FeO can depolymerize the Si-O tetrahedral network to reduce the degree of polymerization of the slag, resulting in lower viscosity of the CaO-SiO 2 -FeO-P 2 O 5 melt and improving the dephosphorization efficiency [33].But, excess FeO at the slag-metal interface causes reaction (7) to proceed positively, promoting [V] oxidation.Along the FeO consumed by [V], [P], and [C], the activity of FeO in the slag tends to balance and the oxidation process of the molten pool is controlled by CO 2 .

Figure 7 :
Figure 7: (a) Variation of [V] and [P] with time under different FeO contents and (b) viscosity and [V] and [P] content transition values.

Figure 8
Figure8shows the experimental results at different basicity levels.The dephosphorization rate increased from 71.8 to 76.1% with basicity, [V] showed a decreasing trend, and the oxidation rate increased from 15.9 to 25.6%.Studies have shown that the dephosphorization slag is a multiphase slag composed of a homogeneous glass phase (2CaO•SiO 2 ), a phosphorus-rich phase (C2S-C3P), and an iron-rich phase[34,35].At steelmaking temperature, C2S and C3P can form a solid solution in a wide composition range.As a result, the activity of P 2 O 5 in the slag decreases,

Figure 8 :
Figure 8: Removal rate and contents of [V] and [P] under different basicity levels.

Figure 9 :
Figure 9: (a) XRD results of the dephosphorization slag in the basicity range of 2-4 and (b) SEM images of the dephosphorization slag at B = 4.

Figure 10 :
Figure 10: Correlation between L P /L V and experimental factors: (a) CO 2 flow rate, (b) FeO content, (c) basicity, and (d) correlation between L P and L V .

( 1 )
According to thermodynamic calculations, the oxidation potency of the oxygen source for dephosphorization is as follows: O 2 > CO 2 > FeO.The dephosphorization reaction occurs before the vanadium oxidation reaction, and low temperatures are advantageous to the dephosphorization process.(2)The experimental procedure consists of two distinct phases: FeO-and CO 2 -dominated.The oxidation reactions of [V] and [P] were intense in the FeO-dominant stage, and the oxidation amount was positively correlated with the FeO content.The higher CO 2 flow rate increases the gas-liquid reaction area, resulting in an increase in the formation of dissolved oxygen per unit of time and an increase in [V] oxidation.(3) Excessive FeO content in the initial slag enhanced the oxidation at the slag-metal interface, and a high CaO/ SiO 2 ratio reduced the activity of V 2 O 3 in the slag.An appropriate FeO content and low CaO/SiO 2 are the keys to dephosphorization and vanadium retention.(

Table 3 :
Chemical reactions for the dephosphorization and vanadium oxidization and their corresponding Gibbs free energy calculated with FactSage 8.1 software

Table 4 :
Chemical compositions of slag and content of [V] and [P] in the metal at the end of the experiment

Table 5 :
Main components of hot metals at different stages and specific vanadium steel grade standard (mass%) conditions for the dephosphorization operation.Under the above experimental conditions, the vanadium retention rate was 82.5%, and the dephosphorization rate was 73.8%.The main composition of the dephosphorization iron is shown in Table5.The [C] content is elevated.The graphite crucible, which is selected to ensure the stability of the dephosphorization slag composition, has a certain influence on the experimental results.Due to the dissolution of graphite crucibles, C → [C], [C] in molten iron has been at a high content.The reactions [C] + (FeO) = Fe + CO and 5[C] + (P 2 O 5 ) = 5CO + 2[P] diminished the dephosphorization capacity of THE dephosphorization slag.The reducibility of molten iron in an industrial production environment will be lower than that in laboratory research, and the quantity of slag and FeO content required for dephosphorization can be reduced further.