Major determinant of service life in magnesia-graphite slagline refractory lining in secondary steelmaking ladle furnace

Abstract

Magnesia-carbon (MgO-C) refractory bricks are used for slagline lining in secondary steelmaking to withstand the harsh basic slag and high temperature environment. In an operation using bottom purging ladle furnaces, preliminary study of residual bricks after lining failures shows less than 50 vol. % of the MgO-C bricks is typically used before the linings would fail. Slag basicity, high temperature oxidation and erosion are known as root causes of different degradation modes in this application. This work, presented in part herein, seeks to identify the dominant cause for this resource utilization inefficiency. Slag basicity excursions and thermal fluctuations versus service lives and residual thicknesses of each MgO-C brick after lining failure, were studied for five ladle linings. Optical and electron microscopy were also used in further post-mortem of the residual bricks. The operation data that showed thermal spikes above 1600 °C and acidic slags with basicity usually below the ideal of 1.67. Micrographs from the slag-refractory interfaces indicated effect of this operational extremes showing thermal shock cracking, slag penetration and evolution of phase due to the slag-refractory interactions. However, bricks evidencing these effects were not the most worn. Slag basicity attack and high temperature oxidations are diffusion controlled and did not proceed aggressively by themselves. Rather, least residual thicknesses were found in bricks from columns adjacent to the two purging gas entry points at the bottom of the ladle. Up to 86 % wear were obtained at such points, in contrast to 24 % for bricks farthest from this effect. Turbulence from the purging gas is thus responsible for localized and highest lining consumption rate that decimates lining life in these ladles. Without this purging turbulence, such as in induction stirring, it can be projected that the lining life can triple, with obvious economics and environmental advantages. While the MgO-C refractory is well able to withstand the chemical and thermal extremes of secondary steelmaking slag environment, bottom purging ladle furnace design deserves a review to improve refractory utilization.