Identify the incorrect statement regarding raw materials and classifications: choose the option that is factually wrong for standard refractory knowledge.

Introduction / Context:
Refractory selection relies on accurate knowledge of raw materials, phase transformations, and service compatibility. This question asks you to spot the statement that conflicts with standard refractory practice and materials science.

Given Data / Assumptions:

• Silicon carbide can be synthesized in an Acheson furnace from silica sand and coke.
• Carbon refractories are stable in reducing atmospheres but oxidize in oxygen-rich environments.
• Mullite forms from alumino-silicate minerals (andalusite, kyanite, sillimanite) upon heating.
• Silica exists in amorphous (opal, fused silica) and crystalline (quartz, tridymite, cristobalite) forms.

Concept / Approach:
The clearly incorrect statement is that carbon refractories cannot be used in reducing furnaces. In fact, reducing atmospheres are precisely where carbon refractories excel; they avoid oxidation and display high refractoriness and thermal conductivity. The other statements align with accepted knowledge: SiC synthesis uses sand and coke; mullite derives from alumino-silicate minerals; and silica occurs in both amorphous and crystalline forms.

Step-by-Step Solution:
Evaluate each option against standard references.Identify the contradiction: carbon is suitable for reducing, not oxidizing, atmospheres.Mark option (b) as the incorrect statement.

Verification / Alternative check:
Industry practice employs carbon/graphite bricks in blast furnace hearths, cupolas, and other reducing environments, confirming that their proper domain is not oxidizing service.

Why Other Options Are Wrong (as choices for “incorrect”):
(a): Correct—SiC is made from sand and coke.(c): Correct—mullite forms from andalusite/kyanite/sillimanite on firing.(d): Correct—silica occurs in amorphous and crystalline forms.(e): Correct in general—many carbides show low wettability by certain slags.

Common Pitfalls:
Assuming carbon works universally; it rapidly oxidizes in air at high temperature.Equating raw-material synthesis routes with refractory plant feed choices.

Final Answer:
Carbon refractories cannot be used in furnaces operating under reducing conditions.