Mills are the key equipment for grinding after the crushing process, which are widely used in the manufacture industries, such as cement, silicate, new building material, refractory material, fertilizer, ferrous metal, nonferrous metal and glass ceramics and can be used for the dry and wet grinding for all kinds of ores and other grind-able materials.

Grinding of materials in a tumbling mill with the presence of metallic balls or other media dates back to the late 1800's. Since that time, more than 8,000 grinding mills have been designed and manufactured.

Ball Mill sizes range from 5 ft. x 8 ft. with 75 HP to 26 ft. x 41 ft. with 20,000 HP. Larger ball mills are available with dual pinion or ring motor drives. Variations of the standard designs are available to suit any appropriate application. These may include but not be limited to: grate discharge, peripheral discharge, dry grinding, special length to diameter ratio, high temperature milling operations, or pebble milling.

A ball mill grinds ores and other materials to a typical product size of 35 mesh or finer.

In the discharge end of this mill, a slotted full diameter or partial diameter grate with discharge lifter assembly conveys the material to the discharge trunnion opening. The grate serves to retain balls, tramp, and coarse material. A gradient created between the feed opening and the discharge slots eases the flow of material.

A trunnion overflow mill has an open discharge end. Material flows through the trunnion as a result of the pulp gradient between the feed and discharge openings. A reverse helix in the discharge trunnion liner retains balls in the mill.

Ball mills operate either in open or closed circuit.

Open circuit operation is best utilized when:

·        another stage of grinding follows the mill;

·        reduction ratio is small;

·        feed material is already fine and one pass through the mill produces the desired results;

·        product size control is not critical and oversize material can be tolerated in the product.

Where size control of the final product is important, closed circuit grinding is the most efficient method of operation. The production of a higher quality product at a lower cost justifies the additional capital investment.

Common wet grinding flowsheets include:

·        open circuit;

·        closed circuit with hydro cyclones;

·        closed circuit with vibrating screens;

·        two stage variations of the above utilizing two mills in series.