Simultaneous desulfurization and dephosphorization reactions of molten iron by soda ash treatment

Desulfurization and dephosphorization reactions of molten iron by soda ash has been studied on laboratory heats of Fe-C, Fe-C-S, Fe-C-P, and Fe-C-S-P alloys at 1573 and 1623 K. The alloys were melted in helium gas flow and preheated soda ash was added; metal samples were taken at certain time intervals and analyzed for sulfur, phosphorus, and carbon. Evolved gas samples were also taken at certain time intervals and analyzed. The phosphorus and sulfur contents in metals decreased rapidly, reaching the lowest values two to four minutes after the soda ash addition. The degree of desulfurization was generally greater than that of dephosphorization, and both degrees were higher at lower reaction temperature. The major component of evolved gas was CO with small amounts of CO₂. Phosphorus appeared to form a stable phosphate compound with Na₂O, possibly 3Na₂O-P₂O₅, in the slag phase. Soda ash reacts with carbon resulting in decarburization of molten iron and vaporization of sodium; this reaction may cause the fading of soda ash and can be expressed as: Na₂CO₃(1) + (1 +x)C = (1 -xNa₂O(1) + 2xNa(g_ + (2 +xCO(g). For the phosphorus containing melt, the reaction can be expressed as: Na₂CO₃(l) +yC + 2x/3P =x(Na₂O · 1/3P₂O₅)(1) + (2 −y − 8x/3)Na₂O(l) + 2(−l + y + 5x/3)Na(g) + (1 +y)CO(g) and for the sulfur containing melt: Na₂O(l) +C +S = Na₂S(l) + CO(g). Katsumi Mori, Formerly Visiting Associate Research Scientist, University of Michigan, Ann Arbor, MI     

Desulfurization of bath smelter metal

The purpose of the present work was to determine the mechanism and optimal conditions for desulfurizing bath smelter metal with a CaO-CaF₂ flux. The minimum silicon (0.1 pct), or aluminum (0.3 pct), contents in the metal for optimal rates were determined. It was found that 8 to 10 pct CaF₂ at 1450 °C is required and that the rate below the CaO-CaF₂ eutectic temperature (1360 °C) is very slow. It is proposed that a liquid phase at the surface of the CaO particles is required, which is provided by the addition of CaF₂. The Si or Al is required to reduce the number of phases for the reaction from three, when carbon is controlling the oxygen potential, to two when Si or Al is; two-phase reactions are inherently faster than those involving three phases. For the optimal conditions, the rate is controlled by mass transfer of sulfur in the metal to the CaO-CaF₂ surface. A simple model for continuous desulfurization indicates 95 pct desulfurization can be achieved at high production rates for metal containing 0.10 to 0.15 pct Si using a CaO-10 pct CaF₂ flux at 1450 °C. Formerly Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University     

Phosphorus distribution between carbon-saturated iron at 1350 °C and lime-based slags containing Na2O and CaF2

The Corex process is capable of consistently producing hot metal with very low silicon contents (<0.1 pct), and as a consequence, its hot metal is ideally suited for the external removal of phosphorus. Various studies have shown that small additions of Na₂O significantly enhance the ability of lime-based slags to dephosphorize liquid iron. Additions of fluxes (such as CaF₂) may be required to ensure that the slags remain fluid during treatment. The aim of the present investigation was to study the dephosphorization capabilities of lime-based slags from the CaF₂-CaO-Na₂O-SiO₂ system. Phosphorus containing slag and carbon-saturated iron was equilibrated in carbon crucibles at 1350 °C under a carbon monoxide atmosphere. It was confirmed that additions of Na₂O increase the phosphate capacity of silicate and lime-based slags considerably. Additions of CaF₂ to Na₂O containing lime-based slags increase the activity coefficient of P₂O₅ and, therefore, decrease the phosphate capacity thereof. These slags have high phosphate capacities and low melting points, yielding them suitable as effective reagents for dephosphorization, and even desulphurization, of hot metal at relatively low temperatures. However, CaF₂ additions to these slags should be limited.     

The effect of Na2O on dephosphorization by CaO-based steelmaking slags

The effect of Na₂O on the equilibrium phosphorous distribution ratio between slag and iron or iron alloys, L_P, has been measured for CaO-SiO₂, CaO-FeOr-SiO₂ (CaO or 2CaO·SiO₂ saturated), and CaO-Al₂-SiO₂ slags. The addition of Na₂O to CaO-SiO₂ slags significantly increases L_P and the phosphate capacity. A 25 pct CaO-25 pct Na₂O-SiO₂ slag has a distribution ratio nearly two orders of magnitude greater than a comparable binary 50 pct CaO-SiO₂ slag. For the CaO-saturated slags containing 40 wt pct FeO_T, the addition of 6 wt pct Na₂O increases L_P by a factor of 5. For the 2CaO·SiO₂-saturated CaO-FeO_T-SiO₂ slag, there is an optimum FeOr content (20 wt pct) for dephosphorization, and 10 wt pct Na₂O increases L_P by a factor of 2. For reducing slags typically used in ladle metallurgy for Al-killed steels (50 pct CaO-40 pct Al₂O₃-10 pct SiO₂), as little as 3 wt pct Na₂O increases L_P by a factor of 100. The present results indicate that small additions of Na₂O to conventional steelmaking slags can greatly improve dephosphorization. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Studies on the removal of phosphorus and sulfur from Fe-C and Fe-C-Mn melts at 1350°C

Laboratory studies have been performed on simultaneous dephosphorization and desulfurization of Si-free Fe-4.5 % C melts with [P]_o = 0.11 wt.% and [S]_o = 0.04 wt.% in MF induction furnaces at 1 350°C. In these investigations, CaO- or Na₂CO₃-based fluxes were used and the techniques of powder injection or single top slag addition were applied. The following results have been obtained:

• – The effectiveness of lime and soda-based fluxes with regard to dephosphorization is practically the same. But a lower sulfur level is attained when Na₂CO₃-based fluxes are used.
• – In the injection experiments, efficiencies of η_P = 80% for dephosphorization and η_s = 90% for desulfurization are easily reached at a powder consumption of 50 to 60 g/kg. But a further increase of the η values requires a remarkable increase in the amount of injected powder. Top slag addition instead of powder injection is less effective, in general.
• – Apparent rate constants k_[P] and k_[S] from 0.05 to 0.3 min^?1 have been determined in the initial stage of injection depending on the relative amount of injected flux. In the top slag experiments, the k_[P] and k_[S] values were practically constant at a level of 0.1 min^?1.

Furthermore, dephosphorization of molten Fe-C-Mn alloys at 1 350°C has been studied at variable Mn content. It is predicted from thermodynamic data and confirmed by experiments that dephosphorization lessens with increasing Mn content in the range from 0 to 15 wt.%.     

The rates of desulfurization of liquid iron by solid CaO and CaO-saturated liquid iron oxide at 1600‡c

The rates of desulfurization of Fe-O-S melts by CaO crucibles and by CaO-saturated liquid iron oxide have been measured at 1600 ‡C. It was found that irons containing 1.62 wt pct and 0.64 wt pct sulfur and 0.070 wt pct oxygen are desulfurized by a reaction with the containing CaO crucible which does not involve the formation of a CaS product layer. The rate of desulfurization reaction is controlled by diffusion of sulfur in the iron melt, and a value of 6.7 ±1.7 × 10^-5 cm² per second was obtained for the diffusion coefficient of sulfur in liquid iron. Iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen is not desulfurized by solid CaO. The rate of desulfurization of liquid iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen by CaO-saturated liquid iron oxide is significantly greater than that calculated on the assumption of diffusion control in the metal phase, and evidence is presented in support of speculation that the reaction rate is enhanced by Marangoni turbulence at the slag-metal interface. The addition of 4 wt pct CaF₂ to the CaO-saturated liquid iron oxide has no influence on the rate of desulfurization of the melt. A. Saelim formerly Lecturer, Faculty of Engineering, Prince of Songkla University, Thailand     

Desulfurization and deoxidation of Cu-S-O alloy in induction melting and solidification under argon and their rates of elimination in vacuum induction melting

Solid Cu-S-O alloy (about 150 g) containing 0.05 pct S and 0.04 pct O (by weight) was heated to 1473 or 1573 K under argon at 6.5 to 101 kPa. Desulfurization of 64 to 80 pct and deoxidation of 75 to 82 pct were observed. The extent did not depend on argon pressure, and this desulfurization and deoxidation probably occurred during melting. In the course of solidification and remelting of molten Cu-S-O alloys containing sulfur and oxygen at approximately the same concentration,i.e., 0.027 pct S and 0.026 pct O, and 0.0456 pct S and 0.0416 pct O under argon at 1.33 to 102 kPa, the percentage of sulfur removed was 49 to 75, the percentage of oxygen removed was 63 to 81, and the extent did not depend on argon pressure. The alloys containing more oxygen than sulfur before cooling,i.e., 0.0216 pct S and 0.081 pct O and 0.0185 pct S and 0.15 pct O, 73 to 90 pct of the sulfur was eliminated. Most desulfurization and deoxidation probably occurred in the course of solidification, because vigorous boiling was observed at this stage. The rates of removal of oxygen and sulfur from molten Cu-S-O alloys maintained at 1373, 1473, and 1573 K under vacuum were expressed by a first-order rate equation, and the dependence of their rate constants on the reciprocal of temperature was determined. In alloys in which pct S ≃pct O before evacuation and pct S ranges from 0.0202 to 0.042 and pct O ranges from 0.023 to 0.048, 79 to 96 pct desulfurization and 66 to 87 pct deoxidation were observed. For the alloys with pct O > pct S initially,i.e., 0.081 pct O and 0.0216 pct S, and 0.15 pct O and 0.0176 pct S, 85 to 94 pct desulfurization was observed and was close to the observed levels of desulfurization in solidification and remelting of the alloys with pct O > pct S under argon.     

Dynamics of the hot metal dephosphorization with Na2O slags

Dephosphorization reaction of hot metal by Na₂CO₃ has been studied experimentally to determine the reaction mechanism and thermodynamics. Most of the experiments were carried out at 1300 °C using Fe-C_sat.-Si-P-S alloys. The results indicate that the CO₂ gas released from Na₂CO₃ is important in the dephosphorization reaction as an oxidizer and increasing mass transfer by stirring the slag and metal. As the initial Si content in hot metal is increased, the degree of dephosphorization decreases significantly and the rephosphorization takes place earlier. The primary reason for the rephosphorization is that the activity of PO_2.5 increases in the slag because of the evaporation of Na₂O from the slag. The loss of Na₂O increases the activity coefficient of PO_2.5 and decreases the slag volume. At the later stage of Na₂CO₃ treatment, the reactions reach equilibrium with respect to phosphorus and sulfur, and the oxygen potential,P _o2, at the slag-metal interface is determined by the C-CO equilibrium (a _c=1 and 1 atm CO). The presence of sulfur in the metal increases the rate of the dephosphorization because of the electrochemical nature of the reaction; sulfur transfer to the slag accepts the electrons from phosphorus transfer.     

Optimization of Mold Flux for the Continuous Casting of Cr-Contained Steels

To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr₂O₃ but decrease with the addition of B₂O₃. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr₂O₃, and then further decreases to 25.3 pct with the addition of B₂O₃; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B₂O₃-1.10 pct MnO-2.10 pct Cr₂O₃ shows the best properties for the continuous casting of Cr-contained steels.     

Thermodynamic properties of the BaO-MnO flux system

Although a great number of works on BaO-bearing fluxes for refining Fe-Cr and Fe-Mn alloys have been carried out, there still remain several unresolved problems on using them in the refining process. The principal aim of the present study is to understand the thermodynamic properties of the BaO-MnO system, which has been shown to be very effective for dephosphorization of Fe-Mn alloys. The activity of manganese oxide in the BaO-MnO flux was measured at 1573 and 1673 K by equilibrating the flux, a Ag-Mn alloy, and a gas mixture of CO and CO₂ as functions of the flux composition and temperature. The influence of BaF₂, which is an effective additive for lowering the melting temperature of the flux, on the thermodynamics of the BaO-MnO system, including the solubility of MnO in the BaO-BaF₂ system, was also investigated.     

Effect of Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> Addition on Inclusions in High-Carbon Steel for Saw Wire

Inclusions cannot be sufficiently stretched to adapt extremely strict requirement of saw wire with only conventional inclusion softening art. In order to explore a potential new method to further enhance the deformability of inclusions, Na₂CO₃ addition should be comprehensively investigated due to the extremely low melting temperature of inclusions containing Na₂O. In the present study, an effective method of Na₂CO₃ addition was put forward and a presumable reaction mechanism between Na₂CO₃/steel/inclusion/slag was deduced by studying the effect of Fe/Na₂CO₃ (weight ratio), Na₂CO₃ addition amount and reaction time on inclusions using a graphite tube resistance furnace. The relations between Na₂O content, melting temperature, deformability and crystallization of inclusions were also briefly discussed. Through these studies, the deformability of inclusions was significantly improved on the whole.     

Inclusion modification in Al-killed valve spring steel by Na2CO3 addition

Hard inclusions with high melting temperatures such as Al₂O₃ (2054°C) and MgO·Al₂O₃ (2135°C) generate nozzle blockage problems during continuous casting of Al-killed valve spring steel and are very detrimental with respect to fatigue properties. In the present paper, inclusion modification in Al-killed valve spring steel by Na₂CO₃ addition was investigated in the laboratory using a graphite tube resistance furnace. The results show that inclusions with high melting temperature can be successfully modified into Na₂O-containing inclusions with lower melting temperatures by the addition of Na₂CO₃. The effectiveness of inclusion modification can be enhanced by increasing the Na₂CO₃ addition and/or decreasing the amount of Al. This suggests that Na₂CO₃ addition could possibly be a substitute for Ca treatment as a method for preventing nozzle blockage during continuous casting of Al-killed steel.     

The effect of surfactants on the interfacial rates of reaction of CO2 and CO with liquid iron oxide

The ¹⁴CO₂-CO isotope exchange technique has been used to measure the rates of dissociation of CO₂ on liquid iron oxide containing the surface active components P₂O₅ or Na₂O, principally at 1673 K. The apparent first-order rate constant is found to decrease monotonically with small additions of P₂O₅ up to a factor of about 4 at 3.5 mol pct. Vaporization losses prevented detailed studies of the effect of Na₂O, but it is shown that there is probably a twofold increase in the rate constant at a concentration of about 0.2 wt pct and a fivefold increase at a concentration between 0.5 and 1.6 wt pct. A smoothed surface potential model, based upon the Vol’kenshtein model for catalysis by semiconductors, is developed, and it is shown that the required surface potential changes due to the segregation of P₂O₅ and Na₂O are physically reasonable.     

Distribution Ratios of Phosphorus Between CaO-FeO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Na<Subscript>2</Subscript>O/TiO<Subscript>2</Subscript> Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO₂-Al₂O₃/Na₂O/TiO₂ slags and carbon-saturated iron (\( L_{\text{P}}^{\text{Fe-C}} \)) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na₂O and Al₂O₃, the liquid areas of the CaO-FeO-SiO₂ slag are enlarged significantly, with Al₂O₃ and Na₂O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the Na₂O content and decrease in the Al₂O₃ content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on \( L_{\text{P}}^{\text{Fe-C}} \) than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, \( \log L_{\text{P}}^{\text{Fe-C}} \) could be expressed as a function of the temperature and the slag component concentrations as follows:

$$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

     

Effect of Microwave Treatment Upon Processing Oolitic High Phosphorus Iron Ore for Phosphorus Removal

Influence of microwave treatment on the previously proposed phosphorus removal process of oolitic high phosphorus iron ore (gaseous reduction followed by melting separation) has been studied. Microwave treatment was carried out using a high-temperature microwave reactor (Model: MS-WH). Untreated ore fines and microwaved ore fines were then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA). Thereafter, experiments on the proposed phosphorus removal process were conducted to examine the effect of microwave treatment. Results show that microwave treatment could change the microstructure of the ore fines and has an intensification effect on its gaseous reduction by reducing gas internal resistance, increasing chemical reaction rate and postponing the occurrence of sintering. Results of gaseous reduction tests using tubular furnace indicate both microwave treatment and high reduction temperature high as 1273 K (1000 °C) are needed to totally break down the dense oolite and metallization rate of the ore fines treated using microwave power of 450 W could reach 90 pct under 1273 K (1000 °C) and for 2 hours. Results of melting separation tests of the reduced ore fines with a metallization rate of 90 pct show that, in addition to the melting conditions in our previous studies, introducing 3 pct Na₂CO₃ to the highly reduced ore fines is necessary, and metal recovery rate and phosphorus content of metal could reach 83 pct and 0.31 mass pct, respectively.     

Desiliconization and decarburization behavior of molten Fe-C-Si(-S) alloy with CO<Subscript>2</Subscript> and O<Subscript>2</Subscript>

One of the most important problems in the steelmaking process is an increase of the disposal slag mainly discharged from the dephosphorization process. In order to reduce the quantity of the disposal slag, the complete removal of silicon from molten pig iron is considered very effective before the dephosphorization in the pretreatment process. From this point of view, the desiliconization and the decarburization behavior of Fe-C-Si alloy with CO₂ and O₂ has been investigated in the present work. It is thermodynamically calculated that silicon should be oxidized in preference to carbon over 0.60 mass pct Si under the condition of s_SiO2=a _C=1 at 1573 K and is experimentally confirmed that silicon is only oxidized under the condition in actual. Even under the competitive region of desiliconizing and decarbonizing, under 0.60 mass pct Si, silicon is found to be oxidized down to about 0.1 mass pct Si in preference. The overall rate constants for the desiliconization and the decarburization are derived, and the value for the desiliconization is one order of magnitude larger than that for the decarburization. The influence of sulfur is also examined, and the retarding effect is not observed on the oxidation reactions.     

The kinetics of dephosphorization of carbon-saturated iron using an oxidizing slag

The kinetics of dephosphorization of carbon-saturated iron by oxidizing slags were studied at 1330 °C. Nine slag compositions were investigated in the systems CaO-Fe₂O₃-SiO₂-CaF₂ and CaO-Fe₂O₃-SiO₂-CaCl₂. Increasing Fe₂O₃ up to 50 pct was found to increase the rate and extent of dephosphorization, whereas further increases were found to decrease the rate and extent of dephosphorization. This was explained in terms of two competing effects on the driving force, where increased levels of iron oxide increase the oxygen potential for dephosphorization, hence the driving force, but simultaneously dilute the basic components in the slag, lowering the driving force for dephosphorization. CaF₂ and CaCl₂ were found to decrease the rate and extent of dephosphorization at levels higher than 12 pct. The rate of dephosphorization was found to be first order with respect to phosphorous in the metal and was controlled by mass transport in the slag. The oxygen potential at the slag/metal interface was controlled by the FeO activity in the slag. When the kinetic results were analyzed to take account of different driving forces, Fe₂O₃, CaF₂ and CaCl₂ were all found to increase the mass transfer coefficient of phosphorous in the slag, and a quantitative relationship has been demonstrated between these mass transfer coefficients and the slag viscosity for each system studied.     

On the interfacial rate of reaction of CO2 with a calcium ferrite melt

Measurements of the rate of dissociation of CO₂ have been made by the¹⁴CO₂-CO isotope exchange technique on calcium ferrite melts with Ca/Fe = 0.30 at 1300 °C. Studies have also been made of the interfacial rates of oxidation of calcium ferrite melts with an average CaO content of 19.45 wt pct (CaJFe ≃0.33) in CO₂-CO atmospheres at 1362 °C. It is shown that the rates of oxidation are consistent with the rates of isotope exchange, indicating a common rate determining step. Measurements of the equilibrium Fe³⁺/Fe²⁺ ratio as a function of the CO₂/CO ratio for 19.3 wt pct CaO melts at 1360 °C and for 28.7 and 18.6 wt pct CaO melts at 1300 °C are found to be in close agreement with the deductions of Takeda, Nakazawa, and Yazawa. Combination of the equilibrium data with the results of the isotope exchange studies indicate that the apparent first order rate constant for the dissociation of CO₂ is inversely proportional to the square of the Fe³⁺JFe²⁺ ratio of the melt, as has been previously found for liquid iron oxide, lime-saturated calcium ferrites, silicasaturated iron silicates, and an equimolar “FeO”-CaO-SiO₂ melt.