What is a grinding mill and common types of grinding mills

A grinding mill is generally a type of machine that breaks a material into smaller pieces. These mills can differ in terms of the types of materials they can grind, and how they grind them. The grinding mill is an industrial tool that reduces solid materials into a desired particle size and is vital in reducing solids, such as cement, into a more manageable and powdered form. It is also used to mix solids with liquids or change the gradient of animal feed and mills come in a variety of forms suited for different tasks and challenges. Some of the most common types include ball mill, rod mill, hammer mill, raymond mill and vertical mill.

Engineers and other professionals often refer to the process of grinding as a unit operation. This is a term used to describe a specific step in a process. For example, a material might be needed in a certain final form, such as a granule, in order to be applied for a specific task, or to make a specific product. The grinding mill is used to achieve this desired form.

Ball Mill

A ball mill uses centrifugal force and many metal balls to grind substances into a fine powder. It is a vital tool for regrinding work and is used mainly on materials such as cement, glass and chemical fertilizer. Its primary design is a large horizontal metal cylinder with two warehouses, or compartments, containing metal balls and scale-boarded, or grooved, edges. The cylinder is then rotated to create centrifugal force to lift the balls and drop them on the material, grinding them. The first warehouse is made to start the grinding process and when the material is made fine enough, it enters through specified holes into the second warehouse, which – with different specification balls and scale boards – grinds the material into a finer powder.

Ball Mills accept feed that range in size from 80% passing 20mm to fine feeds in regrind operations. Products may be as coarse as 0.5mm or as fine as 80% of 40 microns. In a ball mill the length may be less than, equal to, or greater than its diameter. A short length mill has a lower retention time and produces a coarser product. Longer mills have a longer retention practice and generally utilize closed circuit operation. Most Ball Mills operate with a reduction ration of 1:20 to 1:200.

Rod Mill

Like a ball mill, a rod mill is a large horizontal cylinder that uses centrifugal force to grind material. However, instead of balls, it uses long metal rods that are placed almost parallel to each other and the cylinder. The walls of the grinder are also scale-boarded, so when the rods tumble, they crush the material against them. A rod mill creates a relatively unified gradient, and is great for reducing very coarse material into a size manageable by ball mills.

Rod mills can accept feed up to approximately 2″ diameter and generally are selected to grind product to the – 4 mesh to – 35 mesh range. Grinding actions is by line contact between rods extending the length of the mill. Rods tumble and spin in roughly parallel alignment and in so doing simulate the crushing and grinding action from a series of roll crushers. Large particles spread the ends of rods and is so doing impart an additional action termed scissoring. These actions result in preferential grinding of coarse material and a minimum production of slimes and excessively large material termed tramp oversize. A relatively uniform sized product is produced. Due to the nature of the grinding action, rod mills can sometimes advantageously replace the fine crushing of damp or sticky material that tends to coat the faces of crushing surfaces. Rod mills can produce a final product on sand brick, lime, or coke breeze where the feed contains low moisture and is often ground dry. In the metal mining industry most rod mill applications involve wet grinding were material is reduced in size from crusher product size to a size suitable for ball mill feed. Rod milling in the size range utilized is more efficient than ball milling in that the desired product is obtained at a lower cost per ton.

Hammer Mill

A hammer mill is a machine whose purpose is to shred material into fine particles. They have many sorts of applications in many industries, including:

The concept is fairly straightforward. A hammermill is essentially a steel drum containing a vertical or horizontal cross-shaped rotor on which pivoting hammers are mounted. The hammers are free to swing on the ends of the cross. The rotor is spun at a high speed inside the drum while material is fed into a feed hopper. The material is impacted by the hammers on the ends of the rotating cross and thereby is shredded and expelled through screens in the drum.

Slow speed horizontal pallet grinder. Small grain hammer mills can be operated on household current. Large automobile shredders can use one or more 2000 horsepower (1.5 MW) diesel engines to power the hammer mill.

Raymond Mill

Raymond Mill (or raymond grinder) is a kind of grinding machine which is widely used to grind non-flammable and non-explosive materials like barite, calcite, potash feldspar, dolomite, gypsum, talcum, mica, marble, limestone, kaolin, clay, coal etc. The raw materials should be below scale 7 Moth’s hardness. The fineness of the finished product can be adjusted from 100 to 425 mesh (35micron) according to requirements.

Vertical Mill

In the vertical mill the spindle axis is vertically oriented. Milling cutters are held in the spindle and rotate on its axis. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. There are two subcategories of vertical mills: the bed mill and the turret mill.

- A turret mill has a stationary spindle and the table is moved both perpendicular and parallel to the spindle axis to accomplish cutting. The most common example of this type is the Bridgeport, described below. Turret mills often have a quill which allows the milling cutter to be raised and lowered in a manner similar to a drill press. This type of machine provides two methods of cutting in the vertical (Z) direction: by raising or lowering the quill, and by moving the knee.
- In the bed mill, however, the table moves only perpendicular to the spindle’s axis, while the spindle itself moves parallel to its own axis.

Turret mills are generally considered by some to be more versatile of the two designs. However, turret mills are only practical as long as the machine remains relatively small. As machine size increases, moving the knee up and down requires considerable effort and it also becomes difficult to reach the quill feed handle (if equipped). Therefore, larger milling machines are usually of the bed type.

Also of note is a lighter machine, called a mill-drill. It is quite popular with hobbyists, due to its small size and lower price. A mill-drill is similar to a small drill press but equipped with an X-Y table. These are frequently of lower quality than other types of machines.