Limestone (CaCO3) rotary kilns: chrome-magnesite linings are typically used in which zone of the kiln for best hot-face durability?

Introduction / Context:
Rotary kilns for limestone calcination experience varied thermal and chemical environments along their length. The refractory lining must be tailored to each zone to balance thermal shock resistance, chemical compatibility, and wear resistance.

Given Data / Assumptions:

• Feed decomposes to CaO + CO2 in the burning zone at high temperature.
• Chrome–magnesite (Cr2O3–MgO) refractories resist basic slags and alkali attack.
• Preheat and cooling zones see lower peak temperatures and different stresses.

Concept / Approach:
The burning (calcining) zone is the hottest and most chemically aggressive section, with dust abrasion and alkali/sulfate/vapour attack. Chrome–magnesite bricks are selected for their hot strength, basic slag resistance, and structural stability under these severe conditions. Other zones can often use fireclay/high-alumina materials where conditions are milder.

Step-by-Step Solution:
Identify zone with maximum temperature/chemical load → burning zone.Match material properties: Cr2O3–MgO offers hot strength and chemical resistance.Select “Burning (calcining) zone.”

Verification / Alternative check:
Typical kiln lining charts specify chrome–magnesite for burning zone duty due to alkali/sulfate attack and high radiative heat flux.

Why Other Options Are Wrong:
Preheating/cooling zones: lower severity; different refractories are economical.All zones equally/Only feed chute: not aligned with zonal design practice.

Common Pitfalls:
Using one refractory type throughout; zoning improves life and cost.

Final Answer:
Burning (calcining) zone