Pick the wrong statement about insulating and special refractories used in furnace practice.

Introduction / Context:Terminology around refractory classes and properties (e.g., “superduty,” “dead-burnt,” “plumbago”) is frequently tested in metallurgical GK. Distinguishing correct statements from mislabeling avoids costly misapplications in furnace design.

Given Data / Assumptions:

• Insulating bricks derive low conductivity from high porosity.
• Graphite is historically termed plumbago.
• PCE (pyrometric cone equivalent) bands define duty classes for fireclay bricks.
• Dead-burnt magnesite denotes highly calcined MgO with minimal reactivity.

Concept / Approach:The misleading statement is the PCE band for “superduty” fireclay. Typical classifications place superduty significantly higher than 26–28 cones; values around the low-to-mid 30s (e.g., ~33–38) are cited for superduty, whereas 26–28 aligns more with medium-to-high duty. The other statements match standard usage: insulating bricks are processed to be light-weight; graphite ≡ plumbago; calcined magnesite is “dead-burnt”; and insulating silica bricks reduce conductivity via pores.

Step-by-Step Solution:Check each statement against conventional definitions.Identify PCE bands: 26–28 is not “superduty”.Confirm remaining terms are correctly matched to industry practice.Select statement (c) as the wrong one.

Verification / Alternative check:Fireclay classification tables list approximate PCE ranges: low duty (~19–23), medium (~23–29), high (~29–33), and superduty (~33–38), confirming the discrepancy.

Why Other Options Are Wrong:(a) Correct: light-weight refractories are compositionally similar but highly porous.(b) Correct: “plumbago” is a traditional name for graphite.(d) Correct: dead-burnt magnesite is the calcined, low-reactivity form.(e) Correct: air-filled porosity reduces heat transfer in insulating bricks.

Common Pitfalls:Assuming duty labels are interchangeable across manufacturers; while exact numbers vary, superduty is well above cone 28.

Final Answer:“Superduty” fireclay bricks correspond to a pyrometric cone equivalent (PCE) of about 26–28.