Refractory selection insight: fireclay (acidic) bricks are generally not suitable for lining which of the following high-temperature units due to basic slag attack and severe chemical wear?

Introduction / Context:
Choosing the correct refractory for a furnace zone is primarily a question of chemical compatibility. Fireclay bricks are acidic alumino-silicate materials. They perform well against acidic slags and many flue environments but deteriorate quickly in basic slags rich in CaO, FeO, and alkalis. This question asks where fireclay bricks should not be used, focusing on a zone notorious for aggressive basic conditions: the bottom of a hot metal mixer.

Given Data / Assumptions:

• Fireclay refractories are acidic/near-neutral in behavior.
• Hot metal (molten iron) mixers experience basic slags and metal contact at high temperature.
• Cupolas and gas producers see combustion products and can employ silica or fireclay in certain regions.
• Open-hearth roofs are typically silica; some auxiliary areas may use fireclay.

Concept / Approach:
Acid-base refractory compatibility rule: acidic refractories are attacked by basic slags; basic refractories resist basic slags. The mixer bottom is a load-bearing, metal/slag-wetted zone under high temperature with basic slag chemistry. Basic bricks (e.g., magnesite, dolomite, magnesia-carbon) are preferred. Therefore, fireclay is unsuitable there.

Step-by-Step Solution:

Verification / Alternative check:
Metallurgical practice specifies basic refractories for mixing and steelmaking vessels. Failure analyses frequently show silica/fireclay dissolution where basic slags impinge, validating this selection.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring slag basicity; assuming temperature alone dictates brick choice; overlooking that different zones within the same unit need different chemistries.

Final Answer:
Bottom of hot metal mixer