By Winfried Boenigk, D. Schnitzler, J. Stiegert, G. Buchebner, and S. Grassegger
BOF-vessels and electric arc furnaces are the major production facilities for the production of steel. A subsequent treatment of steel is usually done in ladles. For the lining of these furnaces and vessels magnesiacarbon bricks are the state of the art. Magnesiacarbon bricks are mainly composed of magnesia — sintered or fused — and flake graphite or carbon black. The bonding is usually achieved with pitch or resin. Customary resin binders are phenolic resins either as resols or as novolacs. The main arguments for these binders are their high carbon yield during pyrolysis, providing a high strength of the bricks during operation and a good workability during the production of the bricks. Additions of metallic additives, like Al, Mg, or Si, are common to improve the oxidation resistance of those products.
Compared to the pitch binder the resin binder in magnesiacarbon bricks has some disadvantages which originate from different properties of the coke formed during pyrolysis of pitch (“soft carbon”) or resin (“glassy carbon”). Due to its aromatic nature pitch forms a graphite-like carbon structure during its pyrolysis. On the contrary, resin coke is characterised by a low ordered nearly random carbon network. The special feature of the highly ordered pitch coke in contrast to resin coke is its low microporosity, low specific surface, high oxidation resistance, structural flexibility, and its non-brittle behaviour, respectively. These characteristics have great impact on the properties of pitch-bonded and resin-bonded magnesiacarbon bricks. The main differences on brick properties are the lower oxidation resistance, the higher brittleness and a lower hot strength of the resin-bonded type. The stress/strain behaviour reveals the much higher flexibility of the pitch-bonded magnesiacarbon brick in comparison to a resin-bonded type.
In practical application the pitch-bonded bricks perform up to 20% better than the resin-bonded types. One option to improve the oxidation resistance of resin-bonded bricks is the addition of metallic additives, but this again has a negative impact on the brittleness and therefore on the spalling of the bricks during operation.
To inquire about this paper, please contact Jens Stiegert, Director of Sales – Coal Tar Pitch.