MINERAL PROCESSING METHOD OF THE WEEK - GRINDING!!
In this process stage, the crushed material can be further disintegrated in a cylinder mill, which is a cylindrical container built to varying length-to-diameter ratios, mounted with the axis substantially horizontal, and partially filled with grinding bodies (e.g., flint stones, iron or steel balls) that are caused to tumble, under the influence of gravity, by revolving the container.
A special development is the autogenous or semi-autogenous mill. Autogenous mills operate without grinding bodies; instead, the coarser part of the ore simply grinds itself and the smaller fractions. To semi-autogenous mills (which have become widespread), 5 to 10 percent grinding bodies (usually metal spheres) are added - Encyclopaedia Britannica.