Sillimanite (Al2O3·SiO2) neutral refractories: identify where they are generally NOT preferred for service among the following applications.

Introduction / Context:
Sillimanite refractories (which convert to mullite) are neutral and valued for good thermal shock resistance, low creep, and chemical stability. They find use across many high-temperature units. However, highly specialized furnaces demand materials with exceptional chemical compatibility to avoid product contamination.

Given Data / Assumptions:

• Sillimanite/mullite bricks are compared across typical furnace applications.
• Glass melting furnaces require extremely pure, silica-rich linings to prevent defects in glass.
• Neutral refractories are acceptable in many general thermal process units.

Concept / Approach:
Glass melting furnaces often employ fused silica, high-purity silica, or zircon-bearing refractories in contact zones to minimize contamination, blistering, and seeds. Alumino-silicate refractories such as sillimanite are more prone to react with glass melts, risking defects. By contrast, pottery furnaces, crucibles for many alloys, and gas retorts can tolerate neutral alumino-silicate linings without compromising product quality.

Step-by-Step Solution:
Identify application with the strictest chemical purity needs: glass melting furnace.Recognize that sillimanite linings can interact with glass melts.Therefore, sillimanite is generally not preferred in glass melting furnaces.

Verification / Alternative check:
Industry practice favors high-silica and zircon refractories (and specialty fused-cast materials) in glass tanks for contact with melts, reserving alumino-silicates for less critical zones or other furnace types.

Why Other Options Are Wrong:
Pottery furnace: neutral alumino-silicates are common and economical.Crucibles: many compositions exist; alumino-silicates are widely used depending on melt.Gas retorts: neutral refractories are serviceable.

Common Pitfalls:
Assuming one “best” refractory suits all furnaces; ignoring product purity constraints in glassmaking.

Final Answer:
Glass melting furnace