**1. Introduction**

In Korea, steel production in 2020 was 67.1 million tons. Approximately 31% (20.8 million tons) of this was produced by the electrical arc furnace (EAF) process. As approximately 169 kg of EAF slag is produced for each ton of crude steel, the estimated amount of EAF slag generated in 2020 in Korea was 3.5 million tons. With slags from blast furnaces and basic oxygen furnaces, most of the slag can be utilized as raw materials for road construction, backfill, or fertilizers [1]. However, owing to its high concentration of Fe (20–30 wt%), EAF slag is limited in applicability to value-added construction materials [2,3]. EAF slag is typically used as a roadbed or backfill material following an appropriate magnetic separation process [2]. Although several studies have demonstrated the applicability of EAF slag in concrete, road construction materials, and cement without preprocessing to reduce the FeO concentration [4–9], the total amount of EAF slag in these mixtures was limited to obtain the appropriate physical properties.

In order to utilize EAF slag in value-added construction materials and to recover valuable Fe from such slag, the reduction technique called the eco-slag process was proposed for EAF steelmaking [1–3]. Kim et al. [2] suggested a two-stage reduction process of Al reduction by Al dross and direct carbon reduction. In the first stage, Al dross consisting of 30 wt% Al and 70 wt% Al2O3 was added approximately 5 min before tapping the steel in the EAF steelmaking process. The addition of 100 kg of Al dross to 90 tons of steel reduced the total Fe content in the EAF slag from 21% to 15%. In the second stage, the tapped EAF

**Citation:** Kim, Y.; Min, D.-J. Viscosity and Structural Investigation of High-Concentration Al2O3 and MgO Slag System for FeO Reduction in Electric Arc Furnace Processing. *Metals* **2021**, *11*, 1169. https:// doi.org/10.3390/met11081169

Academic Editor: Dariush Azizi

Received: 30 June 2021 Accepted: 19 July 2021 Published: 23 July 2021

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slag was transferred to an induction furnace. The EAF slag was agitated using a graphite rod, and further reduction in FeO in the slag was simultaneously performed at 1773 K. After 60 min of operation, the final slag composition was achieved with approximately 5 wt% of FeO. By controlling the cooling rate of the slag, a magnetic and Fe-rich spinel phase forms alongside the amorphous material that was clearly separated [10]. Finally, a suitable slag composition was achieved through crushing and magnetic separation processes for use in ordinary Portland cement [2,3].

During the eco-slag process, the slag composition is significantly changed by adding Al dross and by reducing FeO. The change in slag composition affects the erosion of the EAF refractory. During the EAF process, MgO from the refractory is soluble in the molten slag. As refractory erosion can shorten the service life of the EAF system, MgO saturation in the EAF slag is maintained by the external addition of calcined dolomite or calcined magnesite. Previous studies have investigated the solubility of MgO in CaO–SiO2–FeO–Al2O3 systems [11–13]; these studies have shown that MgO solubility in the molten slag system is mainly affected by the equilibrated phase of the slag, such as magnesiowüstite ((Mg, Fe)O) or spinel (MgAl2O4). In addition, the change in the thermodynamically equilibrated phase affects the ionic state and slag structure of the network-forming oxide [11–13].

The MgO solubility and the viscosity of the EAF slag are mainly affected by changes in the equilibrium phase and its related slag structure. Recently, Lee and Min [14], who studied the activation energy of viscous flow in CaO–SiO2–FeO–Al2O3–MgO systems, reported an abnormal viscosity tendency as the equilibrium phase changed from melilite to di-calcium silicate. Viscosity is a dominant property related to operational conditions, including the slag foaming ability and tapping condition [15,16]. Therefore, understanding the rheological properties with variations in the FeO and Al2O3 compositions in the MgOsaturated condition is crucial for practical application of the eco-slag process. Although several studies have investigated the viscosity of molten EAF slag systems [17–21], slags with >10 wt% Al2O3 and MgO-saturated compositions have not been studied. In the present study, the viscosity in the high-MgO-concentration region was measured with variations in the FeO and Al2O3 contents of the slag, assuming a reduction in FeO by Al dross. In addition, the change in the slag structure was investigated to evaluate the effects of changes in the network structure of oxide melts on the rheological properties of the CaO–SiO2–FeO–Al2O3–MgO system using Raman spectroscopy and Fourier-transform infrared (FT-IR) spectroscopy.
