Interfacial Reaction Between Magnesia Refractory and EAF Slag

Abstract

Magnesia -based refractory is generally used in an electric arc furnace (EAF) due to its relatively high corrosion resistance and strength at high temperatures. However, the magnesia refractory is attacked by EAF slag and thus the lining life continuously decreases. Thus, it is significant to identify the interfacial reaction between magnesia refractory and FeO-rich EAF slags. In the present study, the influence of FeO-rich slag on the corrosion behavior of MgO refractory was evaluated. The (Fe,Mg)O_ss layer was observed at the slag -refractory interface and its thickness increased with increasing content of FeO in the slag . The specific reaction phenomena and formation behavior of (Fe,Mg)O_ss layer were evaluated by thermochemical computing program, Factsage^TM7.0.

Introduction

In the steel shop, intense reactions occur at the interface of metal-slag -refractory phases during various operating vessels such as electric arc furnace (EAF), ladle furnace (LF), vacuum degasser (VD), etc. The refractory is attacked by complex reactions including gas-slag -metal multiphase reactions simultaneously. Thus, refractory must have superior resistance against thermal shock, mechanical abrasion, and chemical corrosion by basic slag under reducing and/or oxidizing atmosphere. Magnesia -based refractory is widely used in LF or EAF due to its relatively high corrosion resistance and strength at high temperatures. Therefore, it is significant to identify the reaction between magnesia -based refractory and various slag systems.

Several researchers investigated the complex reactions for the EAF slag -refractory in view of corrosion mechanism. Bygden et al. [1]. evaluated the interfacial reaction between CaO-xFeO-SiO₂ (x = 45–60wt%) slag and MgO refractory at 1473-1673 K and reported that (Mg,Fe)O_ss layer was formed at the slag /refractory interface and this reaction rate was controlled by diffusion process. Zhang et al. [2]. studied the effect of CaF₂ in the CaO-45FeO-xCaF₂-SiO₂ (x = 0–20wt%) slags on MgO refractory under static conditions at 1673 K. The thickness of the (Mg,Fe)O_ss layer increased with increasing content of CaF₂ up to 15wt%. Also, they found that the inter-diffusivity in the (Mg,Fe)O_ss layer was varied with the concentration of Mg²⁺ ions. However, there are only few studies about the interfacial reaction between “FeO”-rich EAF slag and MgO refractory . Therefore, the reaction between the CaO-SiO₂-Al₂O₃-xFeO-MgO-MnO (CaO /SiO₂ = 1.2, x = 20–50 wt%) slag and MgO refractory was investigated in this experiment. Moreover, the slag -refractory reaction was simulated by FactSage^TM 7.0 program and the results were compared with the experimental findings.

Experimental Procedure

The present experiments were carried out using an induction furnace . The quartz reaction chamber was evacuated before performing the experiments using a mechanical rotary pump, and then it was filled with highly purified Ar-3%H₂ gas by a mass flow controller. In order to simulate actual operation, the steel was placed in a high temperature sintered MgO crucible (50 mm ID, 60 mm OD, 100 mm HT). After reaching the target temperature (1823 K), the slag was added. The metal-slag -refractory reactions started and maintained for 1 h. The slag compositions used in the present experiments are listed in Table 1. Sampling was conducted at a specific time intervals (0, 5, 10, 30 and 60 min) and then slags were directly quenched. The composition of quenched slags was determined by XRF. Moreover, post-mortem MgO refractory was analyzed by FE-SEM and EDS .

Table 1

Experimental slag compositions (wt%)

	CaO	SiO₂	Al₂O₃	FeO	MgO	MnO
F-20	30.7	25.8	13.0	20.1	3.2	7.2
F-30	25.3	21.3	13.0	30.1	3.2	7.2
F-40	20.1	16.9	13.0	39.7	3.2	7.2
F-50	14.4	12.1	13.0	50.2	3.2	7.2

Results and Discussion

Interfacial Reaction Between “FeO”-Rich EAF Slag and MgO Refractory

From the analysis result of post-mortem MgO refractories using SEM-EDS , the formation of (Mg,Fe)O_{ss(solid_solution)} intermediate layer (IL) was confirmed at the slag -refractory interface as shown in Fig. 1. The IL thickness increased with increasing content of FeO in the slag . Also, it was confirmed that there was a concentration gradient of Mg and Fe within the IL through an EDS line scanning. At the initial stage of the slag -refractory reaction, the slag penetration and the dissolution of MgO from refractory were occurred at the same time. MgO, dissolved from refractory , reacts with the slag to form (Mg,Fe)O_ss at the interface. Simultaneously, slag penetrated through the micro-pores reacts with MgO grains to form (Mg,Fe)O_ss in the refractory . That is, (Fe,Mg)O_ss is generated at the interface by dissolved MgO and then connected monoxide IL was gradually formed.

../images/468727_1_En_52_Chapter/468727_1_En_52_Fig1_HTML.gif — Fig. 1
Back scattered electron image of *post*-*mortem* refractory samples as a function of FeO content in slag

The (Mg,Fe)O_ss monoxide saturation limit with different concentration of FeO in the slag , which was calculated by FactSage^TM 7.0, is shown in Fig. 2. The saturation limit of monoxide was predicted to decrease with increasing FeO in the slag . Dissolved MgO reacts more actively with FeO in liquid slag as the FeO content increases in the slag . It is consistent with the increase in the thickness of the layer formed at the interface as the FeO content increases in the slag (Fig. 1).

../images/468727_1_En_52_Chapter/468727_1_En_52_Fig2_HTML.gif — Fig. 2
MgO content in slag as a function of FeO content

The slag penetration into the refractory was observed, and the composition of penetrated slag was mainly CaO-SiO₂-Al₂O₃ system as shown in Fig. 3. The penetrated slag reacts with refractory to form (Mg,Fe)O_ss near the interface, resulting in a depletion of FeO content in the penetrated slag phase.

../images/468727_1_En_52_Chapter/468727_1_En_52_Fig3_HTML.gif — Fig. 3
The penetrated slag (mainly calcium silicate) into the refractory in the 50%FeO system

Simulation of Slag–Refractory Interfacial Reaction Using FactSage^TM 7.0

The following assumptions were made for the slag -refractory reaction simulation using the FactSage^TM 7.0. First, before the formation of layer network at the slag -refractory interface, the slag penetrates into the refractory . Second, the reaction between penetrated slag and refractory material is simulated. The slag -refractory interfacial reaction was calculated by varying the slag /refractory ratio. In the present calculation, FactPS and FToxide database were used and the oxygen partial pressure was assumed to p(O₂) = 10⁻¹⁰ atm. As the results, it was predicted that the monoxide is mainly composed of MgO and FeO and the spinel was also predicted to form in the 40–50% FeO systems as shown in Fig. 4a. The activity of MgO increases and that of FeO decreases from IL-slag interface to IL-refractory interface in (Mg,Fe)O_ss layer as shown in Fig. 4b. Hence, the experimental findings are well reproduced by the thermochemical calculations.

../images/468727_1_En_52_Chapter/468727_1_En_52_Fig4_HTML.gif — Fig. 4
a Phase fraction and b the activity of MgO and FeO in the (Mg,Fe)O_ss layer as a function of slag /refractory ratio

Conclusions

The interfacial reaction between FeO-rich EAF slag and MgO refractory at 1823 K was investigated. The thickness of (Mg,Fe)O_ss layer increased with increasing content of FeO in the slag . In addition, the slag penetration into the refractory was verified in the 50%FeO system by SEM-EDS analysis, which was occurred before the formation of network monoxide layer at the interface. Finally, a thermochemical simulation of the reaction at the slag -refractory interface using FactSage^TM 7.0 program also showed a good agreement with the experimental findings.