Mill

A tabletop hammer mill
Other names	Grinding mill
Uses	Grinding
Related items	Mortar and pestle Expeller Extruder

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 (mortar and pestle), working animal (horse mill), wind (windmill) or water (watermill). Today they are also powered by electricity.

The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the grinding 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:

• increase of the surface area of a solid
• manufacturing of a solid with a desired grain size
• pulping of resources

Grinding laws[link]

In spite of a great number of studies in the field of fracture schemes there is no formula known which connects the technical grinding work with grinding results. To calculate the needed grinding work against the grain size changing three half-empirical models are used. These can be related to the Hukki relationship between particle size and the energy required to break the particles. In stirred mills, the Hukki relationship does not apply and instead, experimentation has to be performed to determine any relationship.^[1]

• Kick for d > 50 mm

Failed to parse (Missing texvc executable; please see math/README to configure.): W_K=c_k*(ln (d_A) - ln (d_E))\,

• Bond^[2] for 50 mm > d > 0.05 mm

Failed to parse (Missing texvc executable; please see math/README to configure.): W_B= c_B*\left(\frac{1}{\sqrt d_E} - \frac{1}{\sqrt d_A}\right)\,

• Von Rittinger for d < 0.05 mm

Failed to parse (Missing texvc executable; please see math/README to configure.): W_R=c_R*\left(\frac{1}{d_E} - \frac{1}{d_A}\right)\,

with W as grinding work in kJ/kg, c as grinding coefficient, d_A as grain size of the source material and d_E as grain size of the ground material.

A reliable value for the grain sizes d_A and d_E is d₈₀. This value signifies that 80% (mass) of the solid matter has a smaller grain size. The Bond's grinding coefficient for different materials can be found in various literature. To calculate the KICK's and Rittinger's coefficients following formulas can be used

Failed to parse (Missing texvc executable; please see math/README to configure.): c_K=1.151*c_B*(d_{BU})^{-0.5}\,

Failed to parse (Missing texvc executable; please see math/README to configure.): c_R=0.5*c_B*(d_{BL})^{0.5}\,

with the limits of Bond's range: upper d_BU = 50 mm and lower d_BL = 0.05 mm.

To evaluate the grinding results the grain size disposition of the source material (1) and of the ground material (2) is needed. Grinding degree is the ratio of the sizes from the grain disposition. There are several definitions for this characteristic value:

• Grinding degree referring to grain size d₈₀

Failed to parse (Missing texvc executable; please see math/README to configure.): Z_d=\frac{d_{80,1}}{d_{80,2}}\,

Instead of the value of d₈₀ also d₅₀ or other grain diameter can be used.

• Grinding degree referring to specific surface

Failed to parse (Missing texvc executable; please see math/README to configure.): Z_S=\frac{S_{v,2}}{S_{v,1}}=\frac{S_{m,2}}{S_{m,1}}\,

The specific surface area referring to volume S_v and the specific surface area referring to mass S_m can be found out through experiments.

• Pretended grinding degree

Failed to parse (Missing texvc executable; please see math/README to configure.): Z_a=\frac{d_1}{a}\,

The discharge die gap a of the grinding machine is used for the ground solid matter in this formula.

File:Simple Grinding Forces.png

Grinding machines[link]

In materials processing a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. See also crusher for mechanisms producing larger particles. In general, grinding processes require a relatively large amount of energy; for this reason, an experimental method to measure the energy used locally during milling with different machines was recently proposed.^[3]

Operation of a ball mill

Ball mill[link]

A typical type of fine grinder is the ball mill. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls. Ball mills normally operate with an approximate ball charge of 30%. Ball mills are characterized by their smaller (comparatively) diameter and longer length, and often have a length 1.5 to 2.5 times the diameter. The feed is at one end of the cylinder and the discharge is at the other. Ball mills are commonly used in the manufacture of Portland cement and finer grinding stages of mineral processing. Industrial ball mills can be as large as 8.5 m (28 ft) in diameter with a 22 MW motor,^[4] drawing approximately 0.0011% of the total world's power (see List of countries by electricity consumption). However, small versions of ball mills can be found in laboratories where they are used for grinding sample material for quality assurance.

The power predictions for ball mills typically use the following form of the Bond equation:^[2]

Failed to parse (Missing texvc executable; please see math/README to configure.): E= 10*Wi*\left(\frac{1}{\sqrt {P_{80}}} - \frac{1}{\sqrt {F_{80}}}\right)\,

where

• E is the energy (kilowatt-hours per metric or short ton)
• Wi is the work index measured in a laboratory ball mill (kilowatt-hours per metric or short ton)
• P₈₀ is the mill circuit product size in micrometers
• F₈₀ is the mill circuit feed size in micrometers.

Rod mill[link]

A rotating drum causes friction and attrition between steel rods and ore particles.^{[citation needed]} But note that the term 'rod mill' is also used as a synonym for a slitting mill, which makes rods of iron or other metal. Rod mills are less common than ball mills for grinding minerals.

The rods used in the mill, usually a high-carbon steel, can vary in both the length and the diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency^[5]

Autogenous mill[link]

Autogenous mills are so-called due to the self-grinding of the ore: a rotating drum throws larger rocks of ore in a cascading 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 below but does not use steel balls in the mill. Also known as ROM or "Run Of Mine" grinding.

SAG mill[link]

Principle of SAG Mill operation

SAG is an acronym for Semi-Autogenous Grinding. SAG mills are essentially autogenous mills, but utilize grinding balls to aid in grinding like in a ball mill. A SAG mill is generally used as a primary or first stage grinding solution. The SAG mills use a minimal ball charge of 6 to 15%. SAG mills can be as large as 42' in diameter, with up to a 28 MW (37549 HP) motor.

Attrition in the grinding balls causes grinding of finer particles. SAG mills are characterized by their large diameter and short length as compared to ball mills. The inside of the mill is lined with lifting plates to lift the material inside up and around the inside of the mill, where it then falls off the plates into the rest of the ore. SAG mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries.

Pebble mill[link]

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. Quartz or silica is commonly used because it is inexpensive to obtain.

High pressure grinding rolls[link]

The high pressure grinding rolls, often referred to as HPGRs or roller press, consists out of two rollers with the same dimensions, which are rotating against each other with the same circumferential speed. The special feeding of bulk material through a hopper leads to a material bed between the two rollers. The bearing units of one roller can move linearly and are pressed against the material bed by springs or hydraulic cylinders. The pressures in the material bed are bigger than 50 MPa. In general they achieve 100 to 300 MPa. By this the material bed is compacted to a solid volume portion of more than 80 %.

The roller press has a certain similarity to roller crushers and roller presses for the compacting of powders, but purpose, construction and operation mode are different.

Extreme pressure causes the particles inside of the compacted material bed to fracture into finer particles and also causes microfracturing at the grain size level. Compared to ball mills HPGRs are achieving a 30 to 50 % lower specific energy consumption, although they are not as common as ball mills since they are a newer technology.

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[link]

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

Vertical shaft impactor mill (VSI mill)[link]

Type of fine grinder which uses a free impact of rock or ore particles with a wear plate. High speed of the motion of particles is achieved with a rotating accelerator. This type of mill uses the same principle as VSI Crusher

Tower mill[link]

Tower mills, often called vertical mills, stirred mills or regrind mills, are a more efficient means of grinding material at smaller particle sizes, and can be used after ball mills in a grinding process. Like ball mills, grinding (steel) balls or pebbles are often added to stirred mills to help grind ore, however these mills contain a large screw mounted vertically to lift and grind material. In tower mills, there is no cascading action as in standard grinding mills. Stirred mills are also common for mixing quicklime (CaO) into a lime slurry. There are several advantages to the tower mill: low noise, efficient energy usage, and low operating costs.

Types of grinding mills[link]

• windmill, wind powered
• watermill, water powered
• horse mill, animal powered
• treadwheel, human powered (archaic: "treadmill")
• ship mill, floats near a river bank or bridge
• arrastra, simple mill for grinding and pulverizing (typically) gold or silver ore.
• roller mill, an equipment for the grinding or pulverizing of grain and other raw materials using cylinders
• Stamp mill, a specialized machine for reducing ore to powder for further processing or for fracturing other materials
• a place of business for making articles of manufacture. The term mill was once in common use for a factory because many factories in the early stages of the Industrial Revolution were powered by a watermill, but nowadays it is only used in a few specific contexts; e.g.,
  • bark mill produces tanbark for tanneries
  • cider mill crushes apples to give cider
  • gristmill grinds grain into flour
  • oil mill, see expeller pressing, extrusion
  • paper mill produces paper
  • sawmill cuts timber
  • starch mill
  • steel mill manufactures steel
  • sugar mill (also called a sugar refinery) processes sugar beets or sugar cane into various finished products
  • textile mill (disambiguation)
    • silk mill, for silk
    • flax mill, for flax
    • cotton mill, for cotton
  • huller (also called a rice mill, or rice husker) is used to hull rice
  • powder mill produces gunpowder

See also[link]

References[link]

External links[link]

• Animation of Horizontal Grinder
• Video of fine grinder in mining application

Look up mill in Wiktionary, the free dictionary.

Wikisource has the text of the 1911 Encyclopædia Britannica article Mill.

Mill may refer to:

• Mill (grinding), an equipment for the grinding or pulverizing of grain and other raw materials using millstones
• Milling machine, metalworking machine that operates by rotating a cutting bit while the workpiece is moved against the cutter on an XY table.
• Mill (computing)
  • Arithmetic logic unit, used in the context of Charles Babbage's Analytical engine, a 19th century concept of a computer
  • an early term for the central processing unit of a digital computer, especially in early British machines; the term is still occasionally used to refer to the CPU resources consumed by a program
• Mill (currency), a denomination used by some currencies, the equivalent of a tenth of a cent or penny, or a thousandth of the currency unit
• Diploma mill or degree mill, an organization which awards academic degrees and diplomas with very little or no academic study and without recognition by official accrediting bodies
• Nine Men's Morris, a traditional board game; the term "mill" may also mean "three (playing pieces) in a row" within the game
• Windmill (breakdance move) or mill, a power move in breakdancing
• The Mill (post-production), a visual effects company
• a manual typewriter
• the standard author abbreviation Mill. that may be used to indicate botanist Philip Miller's work when citing a botanical name
• a slang term for any internal combustion engine used on a hot rod, or other sort of customised personal vehicle.[1]
• Millage, a property tax
• Wet mill, a device that steeps a substance in water to remove specific compounds
• Cutting mill, a device commonly used in laboratories for the preliminary size reduction of materials
• Certain kinds of factories, such as steel mill, sugar mill, and cotton mill

People named Mill:

• Andy Mill (born 1953), a skier
• Frank Mill (born 1958), a German football player (World Cup winner, Summer Olympics bronze medalist)
• Harriet Taylor Mill (1807–1858), a philosopher and women's rights advocate
• James Mill (1773–1836), a Scottish historian, economist and philosopher
• John Mill (theologian) (c. 1645–1707), an English theologian and author of Novum Testamentum Graecum
• John Stuart Mill (1806–1873), an influential classical liberal thinker and philosopher of the 19th century, son of James Mill

Mill in geography:

• Mill, Netherlands, a Dutch village
• Mill en Sint Hubert, a Dutch municipality

MILL may refer to:

• Major Indoor Lacrosse League, the American indoor lacrosse league, rebranded in 1997 as the National Lacrosse League

See also[link]

• Mil (disambiguation)
• Mille (disambiguation)
• Mills (disambiguation)
• Miller (disambiguation)
• Milling (disambiguation)

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