A pellet mill, also known as a pellet press, is a type of mill

Mill (grinding)

A grinding 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. Historically mills were powered by hand , working animal , wind or water...

 or machine press

Machine press

A machine press, commonly shortened to press, is a machine tool that changes the shape of a workpiece.-Servomechanism:A servomechanism press, also known as a servo press or a electro press, is a press driven by an AC servo motor. The torque produced is converted to a linear force via a ball screw....

 used to create pellets

Pelletizing

Pelletizing is the process of compressing or molding a material into the shape of a pellet. A wide range of different materials are pelletized including chemicals, iron ore, animal compound feed, and more.- Pelletizing of iron ore :...

 from powdered material. Pellet mills are unlike grinding mills

Mill (grinding)

A grinding 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. Historically mills were powered by hand , working animal , wind or water...

, in that they combine small materials into a larger, homogeneous mass, rather than break large materials into smaller pieces.

Types

There are many types of pellet mills that can be generally grouped into large-scale and small-scale types.

Large-scale mills

There are two common types of large-scale pellet mills: flat die mills and ring die mills. Flat die mills use a flat die with slots. The powder is introduced to the top of the die and as the die rotates a roller presses the powder through the holes in the die. A cutter on the other side of the die cuts the exposed pellet free from the die. In the ring die there are radial slot throughout the die. Powder is fed into the inside of the die and spreaders evenly distribute the powder. Two rollers then compress the powder through the die holes. Two cutters are used to cut the pellets free from the outside of the die.

Large scale pellet mills are usually used to produce animal feed

Fodder

Fodder or animal feed is any agricultural foodstuff used specifically to feed domesticated livestock such as cattle, goats, sheep, horses, chickens and pigs. Most animal feed is from plants but some is of animal origin...

, wood pellets

Wood pellets

Wood pellets are a type of wood fuel, generally made from compacted sawdust or other wastes from sawmilling and other wood products manufacture, but also sometimes from sources such as whole-tree removal or tree tops and branches leftover after logging and which otherwise help replenish soil...

, and fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 pellets for use in a pellet stove

Pellet stove

A pellet stove is a stove that burns compressed wood or biomass pellets to create a source of heat for residential and sometimes industrial spaces...

.

Small-scale mills

Small-scale mills are usually variations of screw press

Screw press

A screw press is a type of machine press in which the ram is driven up and down by a screw. The screw shaft can be driven by a handle, or a wheel. It works by using a coarse screw to convert the rotation of the handle or drive-wheel into a small downward movement of greater force. The overhead...

es or hydraulic press

Hydraulic press

A hydraulic is a machine using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalenta mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795...

es. The same basic process is used for both types. A die, also known as a mold, holds the uncompressed powder in a shaped pocket. The pocket shape defined the final pellet shape. A platen

Platen

A platen is typically a flat metal plate pressed against a medium to cause an impression in letterpress printing...

 is attached to the end of the screw (in a screw press) or the ram (in a hydraulic press) which compresses the powder.

Some platens are heated to speed up the time it takes and improve the overall structure of the pellet. They may also have water ports for quick cooling between uses.

Applications

One of the more common applications is to produce KBr

KBR

KBR can stand for:* KBR - American engineering and construction company* Key-based routing* Royal Library of Belgium* The ISO 639 linguistic identifier code for the Ethiopian language Kaffa ....

 pellets which are used in infrared spectroscopy

Infrared spectroscopy

Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...

 applications.

Animal feed pellets are usually a mixture of dry powdered feedstock, such as flour

Flour

Flour is a powder which is made by grinding cereal grains, other seeds or roots . It is the main ingredient of bread, which is a staple food for many cultures, making the availability of adequate supplies of flour a major economic and political issue at various times throughout history...

, sawdust

Sawdust

Sawdust is a by-product of cutting lumber with a saw, composed of fine particles of wood. It can present a hazard in manufacturing industries, especially in terms of its flammability....

, or grass

Grass

Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

, and a wet ingredient, such as molasses

Molasses

Molasses is a viscous by-product of the processing of sugar cane, grapes or sugar beets into sugar. The word molasses comes from the Portuguese word melaço, which ultimately comes from mel, the Latin word for "honey". The quality of molasses depends on the maturity of the sugar cane or sugar beet,...

 or steam

Steam

Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

. Feedstocks for pellet mills can sometimes break down and then re-form, or polymerize

Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

, under the extreme heat and pressure of the pellet mill.