MS-14-P-1496 FESEM in the investigations of interaction of molten iron with metallurgical coke

Metallurgical coke is a major energy source for blast furnace (BF) ironmaking. It also acts as a carburisation agent, a reductant, and a structural support [Andriopoulos et al., 2003]. It made of several blends of coal by heating their mix in coke batteries up to 1200^oC. The coke reacts with the droplets of molten iron in the bottom of a BF during ironmaking process.

The samples of metallurgical coke were drilled out of working BF. They were cut to slices of 5-7 mm in thickness and 25 mm in diameter, and after that they were polished according to a special procedure [Kekkonen & Gornostayev, 2011]. Investigations with the FESEM Zeiss ULTRA plus have found that the sizes of Fe-Si droplets are within a range of 0.1-3 mm (Fig. 1) in their longest dimention. The droplets vary in shape, contact angle and penetration degree into the BF coke matrix (Fig. 2). There are rounded (Fig. 1A – marked by arrow), elongated (Fig. 1A – larger droplet) and irregular droplets (Fig. 1D).

The shape and penetration degree of Fe-Si droplets may be interrelated, as the penetration depth can be the result of reaction between a certain Fe-Si droplet and the coke matrix. Since the amount of carbon dissolved in molten iron depends on the concentration of Si [Lacaze & Sundman, 1991; Kawanishi, Yoshikawa & Tanaka, 2009], droplets which are under-saturated with silicon may react better and penetrate deeper, forming irregular aggregates (Figs. 1D, 2E and 2F). These under-saturated droplets (Type 3) have small (<90^o) contact angle with the surface of coke. Whereas droplets saturated with Si (Type 1) may not react intensively with the matrix and retain more or less the round shape, as shown in Figs. 1A and 2A. These droplets have relatively large contact angle (>90^o) with the surface of coke. There is also intermediate type (Type 2) of the droplets, which has contact angle close to 90^o. They are partly submersed into the matrix and have semi-spherical shape (Figs. 1B, 1C and 2C). The contact area of Fe-Si droplets with the matrix is also different for all the types listed to above in respect of presence of mineral matter and graphite crystals. Type 1 has limited amount of both (Figs. 2A and 2B), whereas Type 3 has relatively large (above 100 µm) graphite crystals and noticeable amounts of mineral phases (Figs. 2E and 2F). Type 2 (Figs. 2C and 2D) has only single crystals of graphite (which are smaller than in Type 3) and it is somewhere in the middle in respect of these features.

The shape and relationships of Fe-Si droplets with the coke matrix reflect thermal and chemical history of coke-metal interaction under the BF conditions.