Tube mill versus ball mill: select the correct combined statements about product fineness, aspect ratio, and ball size in tube mills.

Introduction / Context:
Tube mills are a subtype of ball mills characterized by a much larger length-to-diameter ratio. They are widely used for finishing grinding (e.g., cement finish mills) where a finer product is required. The longer residence time and staged compartments allow the use of progressively smaller media to achieve the final fineness efficiently.

Given Data / Assumptions:

• Comparing conventional ball mills (L/D ≈ 1) with tube mills (L/D > 2).
• Grinding duty emphasizes finer product sizes.
• Media grading in tube mills typically shifts to smaller balls downstream.

Concept / Approach:
Greater length promotes classification of media sizes by compartment, enabling coarse breakage up front and fine grinding at the back. Smaller balls provide higher surface area and more points of contact, aiding finish grinding. Hence, tube mills produce finer products, are longer relative to diameter, and use smaller balls compared with standard short ball mills.

Step-by-Step Solution:
Identify product target: finer finish cement or minerals.Relate L/D and compartment design to residence time and media grading.Conclude that all three statements (a), (b), and (c) are accurate.

Verification / Alternative check:
Classic cement circuits specify multi-compartment tube mills with media gradation from larger to smaller sizes, confirming the practice and outcomes listed.

Why Other Options Are Wrong:
Any single statement alone is incomplete; the combined statement best represents the standard differences.

Common Pitfalls:
Assuming one ball size fits all compartments; failing to grade media undermines the benefits of tube mill geometry.

Final Answer:
all (a), (b) & (c).