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Mills

Mill: Is a unit operation designed to break a solid material into smaller pieces.

There are many different types of grinding mills and many types of materials processed in them. The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the frinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape.

Grinding may serve the following purposes in engineering:

      Magnification of the surface area of a solid
      Manufacturing of a solid with a desired grain size
      Pulping of resources.

Ball Mill: A Ball Mill is a type of grinder used to grind materials into extremely fine powder for use in paints, pyrotechnics, and ceramics. A Ball Mill is a typical type of fine grinder. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, whcih grinders material to the necessary fineness by friction and impact with the tumbling balls. The feed is at one end of the clyinder and the discharge is at the other. Ball Mills are commonly used in the manufacture of Partland Cement.
Rod Mill: A rotating drum causes friction and attrition between steel rods and ore particles. But note that the term 'rod mill' is also used as a synonym for a slitting mill, which makes rods of iron or other metals.

SAG Mill: Semi-Autogenous Grinding Mill: Applies to mills that utilise steel balls in addition to large rocks for grinding. The SAG Mills use a minimal balll charge of 6 to 15%. A rotating drum throws large rocks and stell balls in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. SAG mills are characterised by their large diameter and short length. The inside of the mill is lined with lifting plates to lift the material inside up and around the insde of the mill, where it then falls off the plates and falls back down. SAG Mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries.

Autogenous Mill: A rotating drum throw large rocks in a cataracting motion which causes impact breakage of larger rocks in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. It is similar in operation to a SAG Mill as described above but does not use steel balls in the mill. Attrition in the charge causes grinding of finer particles. Also known as ROM or "Run Of Mine" grinding.

Pebble Mill A rotating drum causes friction and attrition between rock pebbles and ore particles. May be used where product contamination by iron from steel balls must be avoided.

High Pressure Grinding Rolls: The ore is fed between rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes microgracturing at the grain size level. It consists of a pair of vertical cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.

Buhrstone Mill: Another type of fine grinder commonly used is the buhrstone mill, which is similar to old-fashioned flour mills.

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