The nitrogen reaction between carbon saturated Iron and Na2O-SiO2 Slag: Part 1. Thermodynamics

The nitrogen partition ratio between Na₂O-SiO₂ slags and carbon saturated iron was measured for slags containing from 0.4 to 0.55 mole fraction of Na₂O and the temperature range 1200° to 1350 °C. The nitrogen is dissolved in the slag as the cyanide ion (CN⁻) and the partition ratio is proportional to the oxygen pressure to the −1/4 power as predicted for CN⁻ dissolution. The oxygen pressure for carbon saturated iron silicon alloys is controlled by the Si (metal)-SiO₂ (slag) equilibrium. The nitrogen partition ratio and the cyanide capacity increase with Na₂O content and temperature. Calculations indicate that Na₂O-SiO₂ slags will absorb three times more nitrogen than CaO-Al₂O₃ slags at the same basicity and temperature. Based on thermodynamic calculations it is estimated that for a typical Na₂CO₃ hot metal treatment, half of the nitrogen in the metal could possibly be removed. F.Tsukihashi is on leave of absense from Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo, Bunkyo, Tokyo 113, Japan     

The sulfur partition ratio and the sulfide capacity of Na2O-SiO2 slags at 1200 °C

The sulfur partition ratio between carbon-saturated iron and Na₂O-SiO₂ slags and the sulfide capacity of these slags have been measured at 1200 °C. The two measurements are consistent with each other and the results are compared with other investigations. These slags have higher sulfide capacities and partition ratios than equivalent CaO-based slags and are thus attractive desulfurizers. Both the sulfide capacity and the partition ratio increase with increasing Na₂O. The activity coefficient of Na₂S has been calculated; it also increases with increasing Na₂O. The solubility of sulfur in a slag of 0.4 mole fraction Na₂O is estimated to be 5 pct.     

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.     

Soda slag system for hot metal dephosphorization

Phosphorus partition ratios between Na₂O-SiO₂-PO_2.5 slags and carbon saturated iron have been measured as a function of slag composition at 1200 °C. To avoid excessive Na₂O loss by reaction with carbon, the initial slag-metal interfacial area was increased by using a powder mixture of slag and metal to reduce the time for the equilibrium. The results indicate that phosphorus partition ratios, L_p, are relatively high, about 20 for an equimolar slag (X _{Na 2 O}/X _{S 1 O 2} = 1) and are independent of P level in the slag. The phosphate capacity and the activity coefficients of PO_2.5 and FeO for these slags are calculated from the experimental results using available thermodynamic data. The oxygen potential at the slag-metal interface was found not to be controlled by Si in metal under an Ar atmosphere or by CO partial pressures in CO-Ar mixtures. Experimental results and thermodynamic calculations show thatP _{O 2} at the interface between Na₂O-SiO₂ slags and carbon saturated iron is controlled by the C-CO equilibrium where the CO pressure is determined by C-Na₂O equilibrium for which the CO pressure is close to 1 atmosphere in all cases.     

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.     

The sulfur partition ratio with Fe-CSAT melts and the sulfide capacity of CaO-SiO2-Na2O- (Ai2O3) slags

The sulfur partition ratio between slag and carbon saturated iron and the sulfide capacity of CaO-Na₂O-SiO₂ slags and a 48 pet CaO-45 pet Al₂O₃-7 pet SiO₂-(Na₂O) slag have been mea-sured at 1400 °C. The addition of Na₂O to a CaO-SiO₂ slag increases the sulfur partition ratio and the sulfide capacity; however, Na₂O at low concentrations has no measurable effect on the sulfide capacity of a CaO-Al₂O₃-SiO₂ slag. To convert the sulfur partition ratio to the sulfide capacity, the oxygen potential was calculated assuming equilibrium between iron in the alloy and FeO in the slag with the activity of FeO calculated via a regular solution model. The optical basicity may be used to correlate the data, but at high Na₂O contents the data do not adhere to the correlation previously developed for CaO-based slags. Formerly Graduate Student at Carnegie Mellon University     

Removal of Boron and Phosphorus from Silicon Using CaO-SiO2-Na2O-Al2O3 Flux

A combination of solvent refining and flux treatment was employed to remove boron and phosphorus from crude silicon to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper, and the alloy was subjected to refining by liquid CaO-SiO₂-Na₂O-Al₂O₃ slags at 1773 K (1500 °C). The distribution of B and P between the slags and the alloy was examined under a range of slag compositions, varying in CaO:SiO₂ and SiO₂:Al₂O₃ ratios and the amount of Na₂O. The results showed that both basicity and oxygen potential have a strong influence on the distributions of B and P. With silica affecting both parameters in these slags, a critical $ P_{{{\text{O}}_{2} }} $ could be identified that yields the highest impurity pick-up. The addition of Na₂O to the slag system was found to increase the distributions of boron and phosphorus. A thermodynamic evaluation of the system showed that alloying copper with MG-Si leads to substantial increase of boron distribution coefficient. The highest boron and phosphorus distribution coefficients are 47 and 1.1, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.3 times and 17 times mass of alloy would be required, respectively.     

Removal of nitrogen from steel using novel fluxes

The solubility of nitrogen and the nitride capacity of CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₃-TiO₂ slags were measured at 1873 K using a gas-slag-metal equilibration technique with carbon-saturated iron and gas mixtures of CO and N₂. The nitride capacity increased with increasing the TiO₂ and BaO content and is significantly higher than the nitride capacity for normal ladle slags. The activity coefficient of TiO₂ in CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₂-TiO₂ systems were measured. This is necessary to know in order to estimate the possible pickup of titanium in the metal when an aluminum-killed steel is treated with these slags. Also, the activity coefficient of Ti in carbon-saturated iron was measured. The kinetics of the nitrogen reaction between slag and metal is influenced by the oxygen potential in the metal and is primarily controlled by liquid-phase mass transfer of nitrogen in the metal. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Chemical equilibria between silicon and slag melts

The equilibria between silicon and slags of the systems CaO-SiO₂, Na₂O-SiO₂, and CaO-SiO₂-Y with Y being A1₂O₃, MgO, TiO_x, B₂O₃, and Na₂O have been investigated in silica crucibles. The calcium content under silica-saturated CaO-SiO₂ slag is 262 parts per million (ppm) at 1500 °C. The aluminum and magnesium contents increase with increasing alumina or magnesium oxide contents, respectively, reaching about 1800 ppm Al at silica/mullite or about 390 ppm Mg at silica/protoenstatite saturation. Boron has a distribution ratio [B]/(B₂O₃) of 0.18. The sodium content under silica-saturated Na₂O-SiO₂ slag is 25 ppm at 1500 °C. In contrast, the titanium content of the silicon, if Y is TiO_x, and (Ti) is in the percent range, is highand varies with the titanium content of the slag according to [wt Pct Ti] = 2.7 √(wt pctTi). In other experiments, it is shown that metallurgical grade (MG) silicon can be purified from aluminum, magnesium, and calcium by treatment with suitable silicate slags.     

Dissolution Behavior of Rhodium in the Na2O-SiO2 and CaO-SiO2 Slags

To understand the behavior of rhodium during its recovery process, the dissolution behaviors of rhodium in Na₂O-SiO₂ and in CaO-SiO₂ slags at temperatures ranging from 1423 K to 1623 K (from 1150 °C to 1350 °C) and from 1773 K to 1873 K (from 1500 °C to 1600 °C), respectively, in an oxidizing atmosphere were investigated. The solubility of rhodium in the slags was found to increase with increasing oxygen partial pressure, temperature, and the basic oxide content. The correlation between the solubility of rhodium and the oxygen partial pressure suggested that rhodium dissolved into the slags as RhO_1.5. The dissolution of rhodium was slightly endothermic: the enthalpy change of the dissolution of solid rhodium was determined to be 50 ± 10 kJ/mol for the 50(mass pct)Na₂O-50SiO₂; and 188 ± 94 kJ/mol for the 56(mass pct)CaO-44SiO₂ slag systems. The increase in the solubility of rhodium with the basic oxide content indicated that rhodium exhibits acidic behavior in slags. The correlation between the solubility of rhodium and the sulfide capacity of the slags suggested that the ionic species of rhodium in slags is the rhodate ion, RhO ₂ ^? . The rhodate capacity of the slags was defined, and its application to estimate the possible rhodium content in various slag systems was proposed.     

Phosphate capacity of CaO-AI2O3 slags containing CaF2, BaO,Li2O,or Na2O

Phosphorus partition ratios between CaO-Al₂O₃ and CaO-Al₂O₃-CaF₂ fluxes and Fe-C_sat-P alloys have been measured as a function of slag composition at 1500 °C. The effects of additions of BaO, Li₂O, and Na₂O to the CaO-Al₂O₃-CaF₂ system on the phosphorus partition ratios at 1400 °C and 1300 °C have been measured. From the partition ratio, and assuming that the oxygen potential is controlled by C-CO equilibrium, the phosphate capacities of the fluxes were calculated. Also, the activities of Li₂O and Na₂O were measured as a function of slag composition at 1300 °C by equilibrating the flux and the metal with Pb-Li or Pb-Na alloy and CO in a graphite crucible. The results indicate that phosphorus partition ratios with carbon-saturated iron and the phosphate capacities for additions of more basic oxides decrease in the following order: Na₂Oτ;Li₂Oτ;BaO. The activities of Li₂O and Na₂O in calcium aluminate fluxes have large negative deviations from ideal behavior; the activity coefficients at infinite dilution are on the order of 0.05 and 10^-5, respectively. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Study on the foaming of CaO-SiO2-FeO slags: Part I. Foaming parameters and experimental results

In order to understand the effect of slag composition on foaming in iron and steelmaking processes, slag foaming was quantitatively studied for CaO-SiO₂-FeO slags in the temperature range of 1250 °C to 1400 °C. It was found that slag foaming could be characterized by a foaming index (Σ), which is equal to the retention or traveling time of the gas in the slag, and the average foam life ( τ). The effects of P₂O₅, S, MgO, and CaF₂ on foaming were studied. As expected, slag foaming increased with increasing viscosity and decreasing surface tension. It was found that suspended second-phase solid particles such as CaO, 2CaO SiO₂, and MgO stabilized the foam and had a larger effect on foaming than changes in viscosity and surface tension for the slags studied. Kimihisa Ito, Research Associate, formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Thermodynamics of Arsenic in FeOx-CaO-SiO<Subscript>2</Subscript> Slags

The distribution of arsenic between calcium ferrite slag and liquid silver (wt pct As in slag/ wt pct As in liquid silver) with 22 wt pct CaO and between iron silicate slag with 24 wt pct SiO₂ and calcium iron silicate slags was measured at 1573 K (1300 °C) under a controlled CO-CO₂-Ar atmosphere. For the calcium ferrite slags, a broad range of oxygen partial pressure (10^–11 to 0.21 atm) was covered, whereas for the silicate slags, the oxygen partial pressure was varied from 10^–9 to 3.1 × 10^–7 atm. The measured relations between the distribution ratio of As and the oxygen partial pressure indicates that the oxidation state of arsenic in these slags is predominantly As³⁺ or AsO_1.5. The measured distribution ratio of arsenic between the calcium ferrite slag and the liquid silver was about an order of magnitude higher than that of the iron silicate slag. In addition, an increasing concentration of SiO₂ in the calcium-ferrite-based melts resulted in decreases in the distribution of arsenic into the slag. Through the use of measured equilibrium data on the arsenic content of the metal and slag in conjunction with the composition dependent on the activity of arsenic in the metal, the activity of AsO_1.5 in the slags was deduced. These activity data on AsO_1.5 show a negative deviation from the ideal behavior in these slags.     

Thermodynamic behavior of phosphorus in CaO-CaF2-SiO2 and CaO-Na2O-SiO2 systems

In order to understand the thermodynamic behavior of phosphorus, such as the polymerization of phosphate ions in slag melts, the influence of phosphorus content of slag melts on the partition of phosphorus between slags and carbon-saturated iron or silver was investigated for the CaO-CaF₂-SiO₂ and CaO-Na₂O-SiO₂ systems at 1573 and 1473 K, respectively. The predominant species changes from PO ₄ ³ to P₂O₇ ^4-approximately at 2 mass pct of phosphorus in slag melts, which is defined as the critical phosphorus content, for the CaO-CaF₂-SiO₂ system which is doubly saturated with CaO and 3CaO · SiO₂. The critical phosphorus content was found to be about 1 mass pct for the CaO-CaF₂-SiO₂ melts saturated with 2CaO · SiO₂. On the other hand, in the case of the 20CaO-35Na₂O-45SiO₂ system, PO₄/^3- is the predominant species until the slag becomes saturated with Ca₃(PO₄)₂. No effect of sulfur on the phosphorus partition ratio was observed for the CaO-CaF₂-SiO₂ system. Formerly Undergraduate Student, Department of Metallurgy, The University of Tokyo.     

Reduction rate of SiO2 in slag by carbon- saturated iron

The reduction rate of SiO₂ from CaO-SiO₂-Al₂O₃ and CaO-SiO₂-Al₂O₃-TiO₂ slags by carbon-saturated iron melts was investigated over the temperature range 1350 °C to 1600 °C under an argon atmosphere. It was found that the reduction rate of silicon increased with in-creasing temperature and decreased with increasing ratio of CaO/SiO₂ in these slags. A kinetic analysis of the experimental results developed on the basis of the two film theory showed that the silicon transport rate from slag to metal phase was controlled by the rate of chemical reaction at the slag-metal interface. The rate constants obtained for the reaction were 10 g m^-2 s^-1 at 1550 °C. The apparent activation energy was 238.0 kJ mol^-1.     

Distribution of vanadium between SiO2 rich slags and carbon saturated liquid iron

In order to determine optimal slag compositions for the extraction of vanadium from hot metal to SiO₂ rich slags, vanadium distributions were determined between slags of the following systems: FeO*–SiO₂ [(%V) ã 1.7] FeO*–SiO₂(sat)-CaO,Na₂O,MgO,Al₂O₃,TiO₂ [(%V) ã 1.7] FeO*–SiO₂–VO_n [(%FeO*)/(%SiO₂) = 1.5–1.7] and carbon saturated liquid iron at 1 300°C using the solid iron foil technique. The distribution increased with FeO* content in the binary FeO*–SiO₂ system, while additions of CaO, Na₂O, MgO and Al₂O₃ to FeO*–SiO₂ based slags under SiO₂ saturation caused the distribution to decrease. A slight decrease in distribution was also observed with increasing vanadium oxide content in FeO*–SiO₂ based slags having a constant (%FeO*)/(%SiO₂) ratio. The highest vanadium distributions were found in SiO₂ saturated slags with high TiO₂ contents. Vanadium valencies in the slags were determined by a wet analytic titration technique and the results showed that V^III+ is predominant. It was suggested that the predominating ionic species of vanadium in SiO₂ saturated slags are V³⁺ and VO⁺ while a change towards VO^?₂ may occur for FeO rich slags.     

The slag-metal equilibrium in tin smelting

An equilibrium study was undertaken to investigate the effect of the CaO/SiO₂ and Fe/SiO₂ ratios and the SnO and Al₂O₃ contents of slags on the distribution of Fe and Sn between slag and metal in tin smelting. The experiments were performed at 1200 °C by equilibrating Sn-Fe alloys with silicate slags under reducing conditions in closed crucibles. The slag and metal analyses were used to calculate the γ_SnO/γ_FeO ratio in the slags and a multiple-linear regression on these values indicated that, in the range of slag compositions investigated, γ_SnO/γ_FeO is a function only of the CaO/SiO₂ ratio. At 1200 °C, γ_SnO/γ_FeO varies from about 1.1 for CaO-free slags to 3.6 for slags in which the CaO/SiO₂ ratio is 1.0. In practical applications, the slag-metal equilibrium in tin smelting is usually discussed in terms of the variation of the distribution coefficient,k, with the Fe content of the metal, wherek is defined ask = [pct Sn]/[pct Fe] · (pct Fe)/(pct Sn). An equation fork was derived in terms of the atom fraction of iron in the metal, the γ_SnO/γ_FeO in the slag, and the temperature. This equation was used to construct graphs ofk as a function of the iron content over the slag compositions and at temperatures which cover the range of tin smelting practice.     

Sulfide capacity of CaO-CaF2-SiO2 slags

The sulfide capacityC _S ^2- = (pct S^2-) · (P _{O 2}/P _{S 2})^1/2) of CaO-CaF₂-SiO₂ slags saturated with CaO, 3CaO · SiO₂ or 2CaOSiO₂ was determined at 1200 °C, 1250 °C, 1300 °C, and 1350 °C by equilibrating molten slag, molten silver, and CO-CO₂ gas mixtures. Higher sulfide capacities were obtained for CaO-saturated slags. A drastic decrease was observed in those values when the ratio pct CaO/pct SiO₂ is less than 2. The sulfur partition between carbon-saturated iron melts and presently investigated slags was calculated by using the sulfide capacities obtained and the activity coefficient of sulfur in carbon-saturated iron, which was also experimentally determined. For slags saturated with CaO, partitions of sulfur as high as 10,000 were obtained at 1300 °C and 1350 °C. Correlations between the sulfide capacity and other basicity indexes such as carbonate capacity and theoretical optical basicity were also discussed. Formerly with the Department of Metallurgy, The University of Tokyo.     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

The reduction of zinc from slags by an iron-carbon melt

An experimental investigation was undertaken to study the mechanism of reduction of zinc from slags in the presence of a carbon-saturated iron melt. Batch tests were performed at 1400 °C, and the variation of the zinc and iron concentration in the slag during reduction was determined by sampling the slag at intervals during the test. In graphite crucibles, zinc in slags containing iron was reduced faster than zinc in iron-free slags, both when an iron bath was present and when it was absent. Zinc was reduced faster from slags containing iron when an iron bath was present than when an iron bath was absent. The dominant mechanism of reduction of zinc from slags containing iron appears to be the reaction of Zn²⁺ ions with Fe²⁺ ions to form zinc vapor and Fe³⁺ ions. When an iron bath is present, the Fe³⁺ ions are reduced back to Fe²⁺ predominantly by reaction with iron from the bath. Mass transfer of Fe³⁺ ions in the slag appears to be the rate-controlling step. Reduction of iron from slag by carbon occurred in parallel with the reduction of zinc, and whether there was a net increase or decrease of iron in the slag depended on the relative rates of production and consumption of iron. Lead and copper in the slag were reduced to low levels. The lead volatilized and the copper dissolved in the alloy.