Mechanical Operations
Work index is defined as the

Energy needed to crush one tonne of feed to 200 microns
Gross energy (kWh/ton of feed) needed to reduce very large feed to such a size that 80% of the product passes through a 100 micron screen
Energy (kWh/ton of feed) needed to crush small feed to such a size that 80% of the product passes a 200 mesh screen
Energy needed to crush one ton of feed to 100 microns

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Mechanical Operations
Higher is the mesh number, smaller will be the aperture size of the screen. It means that the aperture size of a 200 mesh screen will be smaller than that of 20 mesh screen. This is valid for

American standard screens (ASTM and Tyler standard screens)
All of these
British standard screens
German standard screens (DIN 1171) etc

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