Silica refractories: which of the following statements about silica bricks is correct?
-
AThey crack when subjected to a sudden change of temperature (poor thermal-shock resistance).
-
BThey cannot be used in the dome of hot-blast stoves.
-
CThey have lower thermal conductivity than fireclay bricks.
-
DAll of the statements (a), (b), and (c) are correct.
-
EThey always exhibit excellent thermal-shock resistance.
Answer
Correct Answer: They crack when subjected to a sudden change of temperature (poor thermal-shock resistance).
Explanation
Introduction / Context:Silica bricks are widely used where high refractoriness and load-bearing at elevated temperatures are required, such as in coke ovens and hot-blast stoves. However, they have well-known limitations, particularly poor thermal-shock resistance. Selecting the correct statement requires recalling both their strengths and weaknesses.
Given Data / Assumptions:
- Silica bricks are rich in SiO2 (often > 93%).
- Service areas include coke oven walls, regenerators, and hot-blast stove crowns.
- Thermal conductivity relative to fireclay depends on temperature but is typically not lower at high temperatures.
Concept / Approach:Silica bricks possess high refractoriness and good creep resistance at high temperatures, yet they are susceptible to spalling under rapid heating or cooling, i.e., poor thermal-shock resistance. Contrary to option (b), they are indeed used in hot-blast stove domes. Regarding (c), silica's thermal conductivity is not generally lower than that of fireclay bricks at service temperatures; in many data sets, it is higher. Hence only statement (a) is the accurate characterization among the options provided.
Step-by-Step Solution:Review applications: silica is used in stoves and coke ovens → (b) is incorrect.Compare thermal-shock behavior: silica spalls easily → (a) is correct.Compare conductivity: silica is not lower than fireclay at high T → (c) is incorrect.Therefore, only (a) stands as correct.
Verification / Alternative check:Property tables list low thermal-shock indices for silica and extensive use in stove crowns, confirming (a) and refuting (b) and (c) for typical service conditions.
Why Other Options Are Wrong:(b) Misstates a common application; silica bricks are suitable for stove crowns.(c) Misrepresents relative conductivity; silica is not characteristically lower than fireclay at high temperatures.(d) Cannot be true because (b) and (c) are false.(e) Opposite of reality; thermal-shock resistance is a weakness.
Common Pitfalls:Assuming high refractoriness implies good thermal-shock resistance—properties are independent.Overgeneralizing conductivity at room temperature to high-temperature service.
Final Answer:They crack when subjected to a sudden change of temperature (poor thermal-shock resistance).