Difficulty: Easy
Correct Answer: all (a), (b) and (c).
Explanation:
Introduction / Context:
Power consumption in tumbling mills is a central design and operating consideration, impacting throughput, grinding efficiency, and operating cost. Several controllable variables influence the draw and the breakage environment inside the mill.
Given Data / Assumptions:
Concept / Approach:
Mill power ∝ ρ_bulk * N^3 * D^5 (dimensionally), modified by charge filling, lifter geometry, slurry holdup, and frictional losses. Operating too far below or above the optimum speed reduces cascading/cataracting efficiency. Ball load alters the number and energy of impacts. Slurry/ore density changes drag and impact cushioning, shifting the breakage regime and net power.
Step-by-Step Solution:
Speed (a): controls motion regime; near 70–85% of critical is typical for efficient grinding.Ball load (b): affects collision frequency/energy; there is an optimum filling fraction.Material density (c): modifies holdup, viscosity (if wet), and energy dissipation.Hence, all listed factors matter → select “all (a), (b) and (c).”
Verification / Alternative check:
Plant surveys and Morrell/Bond-based models confirm sensitivity of kWh/t to speed, filling, and slurry properties.
Why Other Options Are Wrong:
Selecting any single factor ignores strong interactions among variables that govern collision dynamics.
Common Pitfalls:
Running at high speed assuming higher power means better grinding; beyond optimum speed, balls centrifuge and breakage worsens.
Final Answer:
all (a), (b) and (c).
Discussion & Comments