Mechanical Operations
Filter aids like asbestos, kieselguhr, diatomaceous earth etc. are used to increase the porosity of the final filter cake & reducing the cake resistance during filtration. Filter aid is

Separated from the cake by dissolving solids or by burning it off
Precoated on the filter medium prior to filtration
All of these
Added to the feed slurry

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Mechanical Operations
Work index is defined as the

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

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