Hot-face insulating linings made from high-purity alumina–fused mullite: in which operating environments are they especially selected?

Introduction / Context:
High-purity alumina–fused mullite hot-face insulating bricks are specialty refractories that combine low thermal conductivity with excellent refractoriness and chemical stability. The question targets when such premium materials are chosen over ordinary insulating firebrick.

Given Data / Assumptions:

• Service may include extreme temperatures.
• Reducing atmospheres may exist.
• Contamination by Fe or Si can be detrimental to the product or lining.

Concept / Approach:
Alumina–mullite materials maintain strength at high temperature, resist many slags and atmospheres, and avoid introducing extraneous silica or iron—important in high-purity processes, reduction atmospheres, and where contamination control is critical. Therefore, they are selected when all three difficulties may arise simultaneously: very high temperature, reducing conditions, and sensitivity to Fe/Si.

Step-by-Step Solution:
Match temperature capability: alumina–mullite withstands high heat.Check atmosphere: good performance in many reducing environments.Check purity constraints: low Fe/Si contamination risk.Conclude broad suitability: all (a), (b), and (c).

Verification / Alternative check:
Supplier catalogs list these bricks for demanding, clean, and reducing services in petrochemical and specialty ceramics.

Why Other Options Are Wrong:
Single conditions (a) or (b) or (c) alone are true but incomplete.Low-temperature dryers do not justify such premium linings.

Common Pitfalls:
Using general-purpose IFB where purity/strength at temperature is crucial.

Final Answer:
All (a), (b) and (c)