In the blast furnace hearth, a solidified 'skull' does not adhere to the carbon (graphite) blocks upon cooling primarily because of which material property?

Introduction / Context:
Hearth linings in blast furnaces commonly use carbon blocks for their chemical compatibility and thermal properties. After shutdowns or localized cooling, metal/slag may freeze and form a “skull.” Whether this adheres to the lining depends on interfacial wetting and chemistry as well as temperature gradients.

Given Data / Assumptions:

• Lining material: carbon/graphite blocks.
• Phases contacting the lining: molten iron and slag.
• Cooling causes temporary solidification (skull formation).

Concept / Approach:

Wetting describes how a liquid spreads on a solid surface. Poor wetting reduces adhesion of the solidified layer. Carbon surfaces are not readily wetted by molten iron and many slags, particularly in the presence of reducing conditions. Thus, even when a skull forms, it tends to separate rather than bond strongly to the carbon lining.

Step-by-Step Solution:

Verification / Alternative check:

Operational experience and literature on hearth wear note the benefit of carbon’s non-wetting behavior in limiting skull adherence and chemical attack.

Why Other Options Are Wrong:

High thermal conductivity: Influences heat flow but not direct adhesion. Crushing strength: A mechanical bulk property unrelated to interfacial bonding. None of these: Incorrect because non-wetting is the key factor.

Common Pitfalls:

Confusing bulk mechanical properties with surface/interfacial phenomena that govern adhesion and skull formation.

Final Answer:

Non-wetting characteristic of carbon by iron/slag