Crushing and Grinding
- Weight percentages of material greater than 50% of the maximum size are about 50% for roll mills, 15% for tumbling mills and 5% for closed circuit ball mills.
- Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps of 14 in. diameter. Smooth rolls have reduction ratios of up to about 4. Speeds are 50-900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon passing through the rolls.
- Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing. A reduction ratio to 40 is feasible. Large units operate at 900 rpm, smaller ones up to 16,000 rpm. For fibrous materials, the screen has cutting edges.
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