MgO-saturated Slag Research Articles

Assessment of the dissolution rate and behaviour of raw dolomite and limestone with different calcination degrees in primary steelmaking slags

ABSTRACT Fluxes are added to steelmaking process to form basic slag and limit refractory lining consumption. Early formation of CaO- and MgO-saturated slag is critical for process productivity. To achieve this, flux must be dissolved in the melt in a limited process time, and the properties of a slag additive influence the fluxing efficiency. The dissolution behavior of samples with different qualities (i.e. raw dolomite, limestone, soft-burnt and hard-burnt quicklime and dolime) was investigated in the pre-melted model BOF and EAF slags at 1673 K and 1723 K. The disc-shaped sample was immersed into the melt and left there until slag solidification was reached. After a holding time between 5 and 20 minutes, the crucible with the sample were quenched with liquid nitrogen. The fused samples were analyzed by SEM method. Based on comparison of slag morphology and concentration changes in the slag, the dissolution behavior of the samples was examined.

Thermodynamic study on the formation of Mg(Al1-x,Fex)2O4 between single-crystal MgAl2O4 and FetO-containing slag at 1550°C

Herein, the effects of FetO content and basicity (CaO/Al2O3) on MgO solubility in calcium aluminate slag were evaluated. The experimental results revealed that the dependence of MgO solubility on basicity reversed across the equilibrium region as the phase changed from MgO to spinel. Meanwhile, a dense layer of Mg(Al1-x,Fex)2O4 solid solution formed at the interface between the single-crystal MgAl2O4 and FetO-containing MgO-saturated slag. Thermodynamic evaluation of the spinel solid solution formation indicated that the solid solubility of ferric ions (Fe3+) in the reaction layer was dominated by the redox equilibria of ferrous (Fe2+) and ferric ions in the slag. The standard Gibbs free energy change (ΔG°) of the formation of Mg(Al1-x,Fex)2O4 solid solution was derived considering both the energy change in mixing (ΔG°Mix) from the configurational change and the cation distribution owing to the inversion in the spinel, as well as a chemical reaction (ΔG°Rxn) with the FetO-containing slag. The dissolution mechanism via the substitution of cations, as inferred from these calculations, is discussed in detail.

The Effect of the MgO Content of Slag on its Penetrability into the Refractory Lining in a Ferrochrome Converter

The refining of ferrochrome with a ferrochrome converter permits the use of molten ferrochrome in an integrated stainless steel mill. During this process, the carbon and silicon contents of the ferrochrome are reduced using oxygen-blowing. To keep the slag liquid and maintain its compatibility with the employed refractory lining, the composition of the slag needs to be modified by slag former additions to compensate for the formation of SiO2. The objective of this work was to study the effect of the MgO content of slag on the penetration of the slag into the refractory lining. The reactivity between the slag and the refractory lining was studied by comparing the reactivity in high-temperature crucible experiments. The MgO and CaO stabilized slag was compared to MgO saturated slag which wears the magnesia lining less. The results of the experiments suggest that the refractory wear can be reduced by increasing the MgO content of the slag. The reduced refractory wear was verified in an experimental campaign conducted at Outokumpu Stainless Oy in Tornio, Finland. Consequently, it is concluded that modifying slag with dolomite lime can lead to significant economic benefits.

Calcium Transfer to Oxide Inclusions in Al-Killed Steel Without Calcium Treatment

Alumina inclusions generated from aluminum deoxidation may undergo several transformations depending on steelmaking conditions. Under reducing conditions, in contact with MgO-saturated slag, alumina inclusions are known to transform to spinel. However, there are uncertainties related to CaO pick-up by alumina-based inclusions. This work aims to provide some clarification of CaO pick-up by alumina inclusions through a series of experiments. First, the role of silicon in calcium transfer from slag was examined by conducting two similar experiments: one without any silicon addition and a second with 1 wt pct electronic grade silicon addition. Second, the role of crucibles was tested by conducting experiments in ZrO2 and CaO-3 pct ZrO2 crucibles. It was found that the addition of silicon significantly enhances the rate of CaO pick-up when there was no silica in the slag at the start of reaction. The effect of the crucible on CaO pick-up was found to be weaker than that of the 1 wt pct silicon addition.

Experimental Study on Phosphorus Partitions Between Liquid Iron and Liquid Slags Based on DRI

Phosphorus partition between liquid iron and CaO-SiO2-MgO-FeOx slags have been investigated experimentally. Fe doped with Fe3P was equilibrated, at 1873 K, in a closed system, with a fully liquid, MgO saturated slag in a dense-sintered MgO crucible. Synthetic slags with low CaO/SiO2 ratio (1 to 1.32) and varying FeOx concentrations (10 to 30 wt pct) constituted the slag phase. P2O5 concentrations in the slag varied between 0.3 and 1.5 wt pct for added phosphorus concentrations of 0.06 to 0.316 wt pct. Phosphorus partition has been found to increase with increasing CaO/SiO2 ratios. Phosphorus partition increased with increasing oxygen potential over the investigated oxygen partial pressure range, $$ {\text{p}}_{{{\text{O}}_{ 2} }} = 1.4 \times 10^{ - 5} \;{\text{to}}\;4.8 \times 10^{ - 5} ({\text{Pa}}) $$. The present experimental result has been compared with literature data. The effect of slag basicity on the dephosphorization power of slags has been discussed based on this comparison. The minimum amount of slag to achieve sufficient dephosphorization using DRI has also been calculated and discussed.

Effect of Iron Redox Equilibrium on the Foaming Behavior of MgO-Saturated Slags

In this study, the foaming index of CaO-SiO2-FetO and CaO-SiO2-FetO-Al2O3 slags saturated with MgO was measured to understand the relationship between their foaming behavior and physical properties. The foaming index of MgO-saturated slags increases with the FetO content due to the redox equilibrium of FetO. Experimental results indicated that MgO-saturated slag has relatively high ferric ion concentration, and the foaming index increases due to the effect of ferric ion. Therefore, the foaming behavior of MgO-saturated slag is more reasonably explained by considering the effect of ferric ion on the estimation of slag properties such as viscosity, surface tension, and density. Specifically, the estimation of slag viscosity was additionally verified by NBO/T, and this is experimentally obtained through Raman spectroscopy.

In SituObservation of MgO Inclusions in Liquid Iron-Aluminum Alloys

This study showed that MgO inclusions can be stable in liquid iron with elevated Al and it illustrated an important role of vaporization in the evolution of inclusions. Previous studies have shown that dissolved Al reduces MgO from slags and refractories, leading to spinel (MgAl2O4) inclusions. The elevated Al content of newer steels raises the possibility that MgO inclusions can be stable. In this work, MgO inclusions were produced and observed in an Fe-Al alloy. The inclusions in the liquid alloy at 1873 K (1600 °C) were observed in situ with a Confocal Laser Scanning Microscope (CLSM). Two types of experiments were performed: one where only a metal sample was melted and the other where the sample was in contact with a liquid, MgO-saturated slag. When no slag was present, the MgO inclusions shrank and disappeared at 1873 K (1600 °C). No inclusions were observed in situ during cooling or in post-CLSM analysis. When the MgO-saturated slag was present, the inclusion sizes were essentially constant and MgO was observed on the surface of post-CLSM samples. Analysis of the results showed that MgO can be stable in 1873 K (1600 °C), but that its presence depends on the rate of removal of Mg due to vaporization and the supply of Mg due to slag/metal or refractory/metal reactions.

MgO Solubility in Steelmaking Slags

A predominantly liquid and MgO-saturated slag is preferred in EAF and BOF steelmaking. Fully liquid slag provides a better environment for faster mass transfer due to lower bulk viscosities and larger liquid slag volume and these help dephosphorization and desulfurization. Also, an MgO-saturated slag would be preferable in order to increase the lifetime of furnace refractory lining by reducing the extent of dissolution. This article will demonstrate the factors that would influence MgO saturation, which includes FeO, CaO, P2O5, and Al2O3 contents and temperature. In addition, this paper comments on the applicability and accuracy of FactSage prediction, which are compared to laboratory experiments. The results indicate that FactSage may underestimate MgO solubility by up to 2.5 wt pct at higher basicities while there is reasonable agreement with current measurements at lower basicities.

Effect of water vapour on distribution of phosphorus between liquid iron and MgO saturated slag relevant to flash ironmaking technology

As part of ongoing effort to develop a novel flash ironmaking process using reduction of iron ore concentrate by natural gas or hydrogen, equilibrium phosphorus distribution between MgO saturated slag and iron was determined in the temperature range of 1550–1650°C under three different gas atmospheres: H2/H2O, H2/H2O/CO/CO2 and CO/CO2. The distribution ratios of phosphorus LP were 8, 17 and 20 respectively under these atmospheres. In the flash process, over ∼25 wt-%P volatilises during ore concentrate downward flight in the reactor shaft in addition to avoiding the use of coke. Therefore, it is expected that the proposed method would produce iron with less phosphorus content than that of the blast furnace.

Hot Metal Desulfurization by CaO–SiO2–CaF2–Na2O Slag Saturated with MgO

The effect of CaF2 and Na2O on the sulfide capacity of the CaO–SiO2–CaF2–MgOsatd.(–Na2O) slags for hot metal desulfurization was investigated at 1623 and 1723 K by taking the role of MgO as an important refractory component into account. The liquidus line of the MgO-saturated system was similar to the CaO–SiO2–CaF2 ternary slag system. The silica content for the 2CaO·SiO2-saturation was slightly decreased with MgO saturation. Although the sulfide capacity at MgO-saturation is lower than that of the CaOsatd.–SiO2–CaF2 slag system, it is greater than the ternary slag system saturated with 2CaO·SiO2 phase. This suggests that MgO can increase the activity of free O2− ions under conditions of relatively low CaO activity. Near the composition of 3CaO·SiO2-saturation, MgO decreased the sulfide capacity of the slag because MgO is less basic than CaO in highly basic compositions. In addition, the addition of 5 mass% Na2O significantly increases the sulfide capacity of the MgO-saturated slag especially in the composition of low ‘CaO+CaF2’. This suggests that the input amounts of ‘CaO+CaF2’ can be reduced from 95 to 75 mass% for maintaining the sulfide capacity of 0.01 at 1623 K, which is important considering the reduction of CaO and CaF2 consumption in hot metal desulfurization process with MgO saturation.

Phosphorus equilibrium between hot metal and slag

In new smelting processes, such as HIsmelt, significant dephosphorisation occurs. Whereas there have been many studies of the phosphorus reaction in steelmaking, there have been few studies of the phosphorus equilibrium reaction relevant to smelting slags. A study of the phosphorus equilibrium reaction between liquid iron and MgO saturated slag with low basicity and low iron oxide content is reported. The phosphate capacity and phosphorus distribution ratio have been measured as a function of slag composition at 1823 K. Also, the phosphorus distribution ratio between carbon saturated liquid iron and slag at equilibrium has been calculated from experimental data. The phosphorus distribution ratio is in reasonable agreement with the industrial data for HIsmelt. The results indicate that significant phosphorus renewal can be achieved in smelting processes.

ニッケル合金とＦｅＯｘ‐ＭｇＯ‐ＳｉＯ２系ないしはＦｅＯｘ‐ＣａＯ‐ＭｇＯ‐ＳｉＯ２系スラグ間の相平衡

The phase equilibrium between the MgO saturated FeOx-MgO-SiO2 slag and Ni-Fe alloy was investigated in the oxygen pressure region from 3 × 10-5 and 3 × 10-2 Pa at 1,673, 1,773 and 1,873 K to analyze the nickel smelting processes. The effect of CaO addition on the phase relation between the same slag system and Ni-Fe or Ni-Cu-Fe alloy were also studied at 1,673 and 1,773 K. The following experimental results have been obtained in the present study. The MgO solubility in the FeOx-SiO2 slag decreases with decreasing SiO2 content and temperature and also by the addition of CaO. However, there is no significant difference between the MgO solubilities in the slag equilibrating with the Ni-Fe or Ni-Cu-Fe alloy. The distribution ratios of nickel and iron between the MgO saturated FeOx-MgO-SiO2 slag and Ni-Fe alloy phases at log pO2 = -8 and 1,773 K are almost 0.01 and 1, respectively. These values increase with increasing the oxygen pressure and decreasing temperature. These indicate that nickel condenses mainly in the metal phase while iron is distributed in both the slag and metal phases. The CaO addition to the MgO saturated slag depresses the distribution ratios of nickel and iron in the slag phase. Comparing the distribution ratio of nickel equilibrating with the Ni-Fe or Ni-Cu-Fe alloy system, the increase of nickel dissolution in the slag equilibrating with the metal phase containing copper is mainly attributed to the change in the activity coefficient of nickel in the alloy phase. The values of 3 and 7 are estimated as the activity coefficients of NiO(s) and CuO0.5(l) in the MgO saturated FeOx-MgO-SiO2 slag at 1,773 K. The activity coefficient values increase with the addition of CaO in the slag. Thus, the dissolution of nickel and copper in the slag has been thermodynamically evaluated to decrease with CaO addition.