Cement rotary kilns—burning zone selection: which refractory brick type is commonly used in the burning zone of a cement rotary kiln where the clinker is hottest and most aggressive?

Introduction / Context:
The burning zone of a cement rotary kiln experiences the highest temperatures, severe thermal cycling, and intense chemical attack from basic, lime-rich clinker. Refractory selection here is critical to campaign life and kiln stability. Although many modern kilns use basic magnesia-spinel or magnesia-chrome systems, in question banks where only oxide choices like alumina, fireclay, thoria, or silicon carbide are offered, the best fit among these is high-alumina bricks for their superior hot strength over fireclay and their practicality compared with exotic or unsuitable options.

Given Data / Assumptions:

• Choices are limited to the listed brick families.
• Service includes high temperature, thermal shock, and basic clinker chemistry.
• We prioritize the best match among given options, not an exhaustive industry survey.

Concept / Approach:
Fireclay bricks lack the hot strength and basic slag resistance needed in the hottest zone. Silicon carbide is rarely used in rotary kiln burning zones due to oxidation and joining issues. Thoria is an exotic, radioactive oxide—impractical and not used in cement kilns. High-alumina bricks, while not as resistant to basic clinker as magnesia-spinel, provide a significantly better combination of refractoriness and mechanical integrity than fireclay and are the most appropriate choice among the provided options.

Step-by-Step Solution:
Eliminate fireclay: insufficient RUL and corrosion resistance for burning zone.Eliminate thoria: impractical and not industrially used here.Eliminate silicon carbide: oxidation/compatibility concerns in cement kilns.Select high-alumina as the most viable option provided.

Verification / Alternative check:
Textbook discussions note historical use of high-alumina bricks in certain burning zones before widespread adoption of basic linings; among the listed options, high-alumina remains the most credible practical choice.

Why Other Options Are Wrong:
Fireclay: lowest hot strength of the group.Thoria: not a commercial lining material for cement kilns.Silicon carbide: unsuitable under oxidizing, clinker-laden conditions.Silica: omitted in options list here but, if considered, would also be unsuitable due to basic attack.

Common Pitfalls:
Assuming the question lists industry’s modern best practice; real burning zones today often use magnesia-spinel systems.Confusing transition zone materials with burning zone linings.

Final Answer:
High alumina