**1. Introduction**

In the blast furnace ironmaking, the properties of primary FeO-rich slag determine the process of carbon transfer to the freshly reduced solid iron, followed by separation of slag and metal into the independent liquid phases [1]. In some alternative ironmaking technologies, aimed to substitute blast furnace, an interaction between primary slag and solid iron also plays an essential role [2].

The schematic picture of the blast furnace is shown in **Figure 1**. In the lower part of the shaft of the blast furnace, reduction and heating processes result in forming of a cohesive zone where the layers of partially reduced softened iron ore material (sinter or pellet), impermeable for the ascending gas flow, are pressed between the gas-permeable layers of coke. The form and the thickness of the cohesive zone significantly affect all aspects of the blast furnace operation [4].

In the cohesive zone, chemical reactions take place mostly on the surface of softened material, notably on the lower part of the cohesive layer, where the primary slag, formed by interaction of iron monoxide (FeO) and the gangue compounds of iron ore material (predominantly, such as SiO2, CaO, Al2O3 and MgO), contacts with the hot reducing gas and solid coke [1]. Due to pressure of the materials in the shaft of the blast furnace, FeO-rich primary slag squeezes from the iron sponge (formed due to reduction of iron oxides) to the surface of the cohesive layer.

Initially, high oxidizing potential of FeO-rich primary slag prevents carbonization of the iron sponge. However, while the materials descend to the area of blast furnace

**Figure 1.** *Schematic picture of blast furnace [3].*

with elevated temperature and stronger reducing potential of gas, FeO is gradually reduced from the primary slag and oxidizing potential of the latter decreases. At certain point, change of physicochemical parameters of interaction among the solid iron, the primary slag, the gas atmosphere and the coke, results in very rapid carbonization and melting of iron at the lower part of the cohesive zone. After separation into the independent liquid phases, slag and metal start to drip through the active coke zone (a term applied to the area between the cohesive zone and coke combustion raceways) down to the hearth (cylindrical part at the bottom of blast furnace) where they settle into the layer of hot metal (pig iron) topped with the layer of slag. Pig iron and slag are periodically tapped from the blast furnace [1, 2, 4, 5].

Studies on the quenched blast furnaces, performed in the 1970s in Japan [6], revealed that dripping of metal in the lower part of the shaft of blast furnace starts after its carbonization to 0.8–1.0% C. In the belly (the widest region of the blast furnace below the shaft), the carbon content of metal droplets grows to around 2.0%; further, in the bosh (conic region of blast furnace between the belly and the hearth), it reaches around 4.0% (tapped pig iron contains 3.8–4.5% C). Same pattern was observed for the silicon transfer to metallic phase: In the softened cohesive layer, metal contains less than 0.03% Si, then, just after the onset of carbonization, Si content grows to 0.2%, reaching 2% or more at the top of the bosh-even if a conversion pig iron (a semi-product used to further produce steel) with less than 0.6% Si is produced (for the sake of brevity, the phenomena of partial oxidation of metal components near the raceways are not discussed in this chapter).

Along with the silicon, sulfur is another compound whose concentration in pig iron shall be controlled in blast furnace operation. Samplings performed by Volovik [7] on an operating blast furnace revealed that, although sulfur absorption by the iron ore sinter is observed yet while it descends from the blast furnace top to the middle of the shaft, the freshly reduced solid iron, being enveloped by the primary slag, does not significantly absorb sulfur. After the liquid metal is formed, its sulfur content

#### *Wetting of Solid Iron by Molten FeO-Containing Slag DOI: http://dx.doi.org/10.5772/intechopen.110701*

drastically increases, reaching 0.3–0.4% S as a maximum (a value then decreases to final content of around 0.03% S in the tapped iron due to desulfurization of metal by slag in the hearth).

The industrial data referred above show that, in the blast furnace, freshly reduced solid iron, enveloped by the primary slag, remains virtually unaffected by chemical reactions within a certain period of time while the materials descend. However, at a certain point, the interplay of complex heating and reduction processes triggers very rapid change in composition and physical state of the metallic phase. Therefore, knowledge of the processes of interaction between the liquid FeO-rich slag and the solid iron is very important for the definition of iron ore materials' composition-either for blast furnace cohesive zone optimization or for more energy-efficient operation of alternative ironmaking processes. However, our analysis reveals very few studies where interaction phenomena between solid iron and liquid primary slag are investigated (most studies focus on wetting phenomena at the interface of slag and metallic melts).

Iguchi et al. [8] studied wettability of solid iron by the slags in various CO2/CO atmospheres, revealing that the liquid slags with more than 30 mol% FeO content perfectly wet the solid iron surface. It was found that the contact angle increases with the increase in the basicity of slags under the constant oxygen pressure, while addition of Al2O3 and MgO has no significant effect.

Hino et al. [9] studied some parameters of interaction between the solid iron and liquid slags representing FeO-2CaO⋅SiO2⋅Al2O3(Gehlenite)-CaO⋅SiO2 system with the ratio of Gehlenite/(CaO⋅SiO2+ Gehlenite) = 0.3. Good wettability of iron by the studied slags was revealed, with the wetting angle decreasing in the range from 30° to 10° with the increased FeO content in the slag.

In both referred above studies, a sessile drop technique was applied with the temperature level fixed for all experiments-at 1350°C in [9] and at 1450°C in [8]. Noteworthy, these temperatures exceed the liquidus temperature for the slag studied systems and generally correspond to the conditions when the liquid slag phase is already separated from the iron phase-whether it is a lower boundary of cohesive zone in the blast furnace or, for example, an iron nugget, produced in the innovative ITmk3 ironmaking process [10]. In other words, by 1350°C a primary slag should be already long ago separated from the sponge iron, so the data available from the referred above studies [8, 9] are not very relevant to the conditions of sponge iron and primary slag interaction preceding their separation into the flowable phases.

Experimental approach with fixed temperatures, applied in [8, 9], allows for comparison of the wetting conditions in different slag systems. However, in reality, the temperature when primary slag and iron sponge separate into the independent liquid phases depends upon the reducing potential of gas, the reduction degree of the material and the gangue composition.

In our earlier study of the softening and melting properties of iron ore materials [11], it was shown that, in the viscous-plastic state under the load, an impermeable material is formed with the outer part coated by the slag relatively depleted in FeO due to the reduction, while its internal part contains iron sponge and FeO-rich slag. Under such conditions, the development of the physicochemical processes of iron oxides' reduction and carbon transfer to the metallic phase, resulting in slag and metal separation, is to a great extent determined by the slag properties. As far as FeO content in slag determines both fluidity and oxidizing potential of slag, it should play a predominant role in these processes. Kim et al. [12] also found that after the iron oxide in the slag is reduced, the separation of the final slag and the Fe–C melt takes place since the wettability between them decreases.

In the current research, we studied wetting of solid iron by the FeO-rich slag under the temperature conditions close to the temperature of complete melting for the given slag-conditions typical for the blast furnace cohesive zone, where interaction between the heterogeneous phases (liquid slag and solid sponge iron) takes place.
