Abstract

The refractory systems of the bosh, belly and stack of the blast furnace, probably, are the most critical in terms of their impact on the operating capability of the equipment. When selecting refractories for these zones, it is imperative to evaluate the wear mechanisms to be encountered, to identify the expected operation conditions which could affect the lining life and to evaluate all external factors which will impact refractory performance.

The main wear mechanisms that are encountered in the bosh, belly and stack are chemical attack by alkalis and zinc, carbon monoxide disintegration and oxidation, abrasion by the descending burden, thermal load and thermal shock. Silicon carbide linings are used in these zones of the blast furnace due to their higher resistance to chemical attack, abrasion and thermal shock than fireclay or high-alumina refractories. SiC refractories can have several different bond types which change the physical properties of the refractory. In general, silicon nitride (Si3N4) bonded SiC has proven to be preferred over various direct bonded, self-bonded or carbon silicon bonded materials. Although the SiC grains are stable when exposed to alkalis, this is not the case with bonding systems. Cermic bonding and even oxi-nitride are affected by potassium. SIALON (Si6-xAlOxN8-x) bonded materials have also been used for their improved alkali resistance.

In November 2015 Ternium Siderar Blast Furnace #1 was blown down after 5 years in operation (2010-2015). In order to evaluate the main wear mechanism encountered in the working lining of the middle shaft, it was performed a post mortem study of SiC bricks. These are Si3N4-bonded. The post mortem study included determination of the chemical and mineralogical composition, DT/ TG analysis and microstructural analysis by optical and electronic microscopy (SEM and EDS analyses).

The SiC bricks under study presented thermal and structural spalling. Chemical attack by alkali penetration was identified, mainly potassium oxide with the generation of new phases such as potassium hydrogen silicate. Cracks present in the material permit alkali condensation in the brick. Due to the thermal profile in brick thickness during operation, alkali content in the cold face is greater than in the hot face.