Rotary kiln applications: Rotary kilns are employed in which of the following industrial processes?

Introduction / Context:
Rotary kilns are long, rotating refractory-lined vessels that provide controlled high-temperature processing with continuous solids movement. They are ubiquitous in minerals and materials industries for thermal decomposition, phase transformation, and sintering operations.

Given Data / Assumptions:

• Limestone and dolomite calcination require elevated temperatures to drive off CO2 and produce quicklime/dolime.
• Portland cement manufacture relies on a rotary kiln to sinter raw meal into clinker.
• The question asks which applications use rotary kilns.

Concept / Approach:
Rotary kilns offer countercurrent/cocurrent gas–solid contact, residence time control, and robust handling of large throughputs. Both calcination and cement clinker formation are classic, large-scale rotary kiln applications, making them the correct combined choice.

Step-by-Step Solution:
Identify standard uses: lime/dolime calcination and cement clinker production.Confirm equipment match: both require sustained high temperatures and controlled residence times, well-suited to rotary kilns.Select “Both (a) and (b)”.

Verification / Alternative check:
Any industry overview of rotary kilns lists cement and lime among the top applications by installed capacity worldwide, confirming the selection.

Why Other Options Are Wrong:
Only one of the two: incomplete; both processes use rotary kilns.Neither (a) nor (b): clearly false.Only waste incineration: rotary kilns can incinerate wastes, but that is not the exclusive or primary application compared to lime/cement.

Common Pitfalls:
Assuming specialized furnaces are required for each process; in reality, the rotary kiln platform is versatile across cement, lime, magnesia, and more.

Final Answer:
Both (a) and (b)