Compared to a conventional ball mill, a tube mill is characterised by which dimensional ratio and typical grinding behaviour?
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AHas a higher length/diameter ratio
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BProduces a coarser product at the same conditions
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CHas a higher diameter/length ratio
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DUses much larger balls as a rule
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EAlways operates at higher critical speed fraction
Answer
Correct Answer: Has a higher length/diameter ratio
Explanation
Introduction / Context:Tube mills are long, slender grinding mills; ball mills are relatively shorter. The dimensional ratio influences residence time, classification, and product fineness, especially in cement grinding.
Given Data / Assumptions:
- Tube mill = long cylinder, small diameter.
- Ball mill = shorter cylinder, larger diameter relative to length.
- Comparison is at similar operating conditions.
Concept / Approach:The key identifier is the higher L/D ratio for tube mills. This enables compartmentalisation and staged grinding media sizes for finer product. Statements about universally coarser product or larger balls are not defining characteristics and vary by design and duty.
Step-by-Step Solution:Recall definition: tube mill has higher L/D.Map consequence: longer residence enables fine grinding.Select higher length/diameter ratio.
Verification / Alternative check:Cement industry texts describe multi-compartment tube mills with L/D significantly greater than 2–3, confirming the distinction.
Why Other Options Are Wrong:Higher D/L: opposite of tube mill design.Coarser product/larger balls/higher speed: not general truths.
Common Pitfalls:Confusing tube mills with rod mills; both are long, but media and duty differ.
Final Answer:Has a higher length/diameter ratio