How is a semiautogenous mill different from an autogenous mill?

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mineral processing Schematic diagram of a flotation separation cell.

In mineral processing, both autogenous and semiautogenous mills are types of cylinder mills used for grinding ore, but they differ in their use of grinding media.1

Autogenous mills: These operate entirely without external grinding bodies. Instead, the coarser pieces of the ore act as the grinding medium, effectively grinding themselves and the smaller fractions of the material.1
Semiautogenous mills: These are a common variation that incorporates a small amount of external grinding media—typically 5 to 10 percent metal spheres—to assist in the disintegration process.1

By adding these metal spheres, semiautogenous mills can improve grinding efficiency compared to purely autogenous systems.1