By definition, the term “heating” implies disequilibrium because nothing happens when a heating system is at equilibrium. One of our main tools to manage disequilibrium during heating is refractories. These are materials resistant to chemical, thermal and physical damage, specifically chemical reaction, melting, erosion and wear. Refractories serve to contain the charge in a furnace without reacting with the furnace’s contents, to provide some thermal insulation and to absorb physical forces.
Refractories have been used for as long as humanity has built furnaces and ovens – large and small. However, the science of refractories is much younger. For example, recent use of phase diagrams has had a large impact on the reliability of refractories. In addition, an understanding of the detailed chemical and physical interaction of refractories with the furnace contents and of the role of impurities have greatly improved the longevity of refractory linings.
Refractories in Steelmaking
In converter steelmaking, slag splashing – developed at the then LTV Indiana Harbor plant in 1992 – coats the hot face of the refractory with some of the slag after steelmaking. The result has been very long lining lives – thousands of heats. The splashed slag can be seen as a type of “freeze lining,” in which solidified slag forms the working face of the lining. Freeze linings are used in several processes, including aluminum electrolysis cells, ilmenite smelting furnaces and some ferroalloy furnaces.
In electric-furnace steelmaking, careful control of the slag chemistry is needed to ensure that the slag is saturated with magnesium oxide to avoid attack of the refractory lining. Slight supersaturation of the slag with magnesium oxide facilitates slag foaming, which improves thermal efficiency and limits unwanted nitrogen pickup.
With some of my students in my lab at Carnegie Mellon University, I have studied several aspects of refractories, including freeze linings in ilmenite smelting furnaces, wear of tap-hole refractories in silicomanganese furnaces and reaction of liquid steel with oxide refractory. A recent chance observation showed that whiskers of magnesium oxide can be grown inside mixtures of magnesium oxide and carbon (commonly used in steelmaking refractories). Small slag droplets catalyzed whisker growth. Such whiskers have the potential to improve the strength and toughness of refractory bricks.
For more information on this research, please read “Modelling of an ilmenite-smelting DC arc furnace process” in Minerals Engineering or “Wear Mechanisms of Carbon-Based Refractory Materials in SiMn Tap-Holes—Part II: In Situ Observation of Chemical Reactions” in Metallurgical and Materials Transaction B.