Mill (grinding)
Overview
 
A grinding mill is a unit operation
Unit operation
In chemical engineering and related fields, a unit operation is a basic step in a process.Unit operation involves bringing a physical change such as separation, crystallization, evaporation, filtration etc.. For example in milk processing, homogenization, pasteurization, chilling, and packaging are...

 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
Mortar and pestle
A mortar and pestle is a tool used to crush, grind, and mix solid substances . The pestle is a heavy bat-shaped object, the end of which is used for crushing and grinding. The mortar is a bowl, typically made of hard wood, ceramic or stone...

), working animal
Working animal
A working animal is an animal, usually domesticated, that is kept by humans and trained to perform tasks. They may be close members of the family, such as guide or service dogs, or they may be animals trained strictly to perform a job, such as logging elephants. They may also be used for milk, a...

 (horse mill
Horse mill
A horse mill is a mill that uses a horse as the power source. Any milling process can be powered in this way, but the most frequent use of animal power in horse mills was for grinding grain and pumping water. Other animals used for powering mills include dogs, donkeys and oxen. Engines powered by...

), wind (windmill
Windmill
A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...

) or water (watermill
Watermill
A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping .- History :...

). Today they are also powered by electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

.

The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces.
Encyclopedia
A grinding mill is a unit operation
Unit operation
In chemical engineering and related fields, a unit operation is a basic step in a process.Unit operation involves bringing a physical change such as separation, crystallization, evaporation, filtration etc.. For example in milk processing, homogenization, pasteurization, chilling, and packaging are...

 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
Mortar and pestle
A mortar and pestle is a tool used to crush, grind, and mix solid substances . The pestle is a heavy bat-shaped object, the end of which is used for crushing and grinding. The mortar is a bowl, typically made of hard wood, ceramic or stone...

), working animal
Working animal
A working animal is an animal, usually domesticated, that is kept by humans and trained to perform tasks. They may be close members of the family, such as guide or service dogs, or they may be animals trained strictly to perform a job, such as logging elephants. They may also be used for milk, a...

 (horse mill
Horse mill
A horse mill is a mill that uses a horse as the power source. Any milling process can be powered in this way, but the most frequent use of animal power in horse mills was for grinding grain and pumping water. Other animals used for powering mills include dogs, donkeys and oxen. Engines powered by...

), wind (windmill
Windmill
A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...

) or water (watermill
Watermill
A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping .- History :...

). Today they are also powered by electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

.

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

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.
  • Kick for d > 50 mm


  • Bond for 50 mm > d > 0.05 mm


  • Von Rittinger for d < 0.05 mm



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

A reliable value for the grain sizes dA and dE 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





with the limits of Bond's range: upper dBU = 50 mm and lower dBL = 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
Instead of the value of d80 also d50 or other grain diameter can be used.

  • Grinding degree referring to specific surface

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

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



Grinding machines

In materials processing a grinder is a machine for producing fine particle size reduction through attrition
Wear
In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....

 and compressive force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

s at the grain size level. See also crusher
Crusher
A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials , so that pieces...

 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.

Ball mill

A typical type of fine grinder is the ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...

. A slightly inclined or horizontal rotating cylinder is partially filled with ball
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

s, usually stone
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 or metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

, which grinds material to the necessary fineness by friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of 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
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 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, 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:
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

A rotating drum causes friction and attrition between steel rods
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...

 and ore particles. But note that the term 'rod mill' is also used as a synonym for a slitting mill
Slitting mill
The slitting mill was a watermill for slitting bars of iron into rods. The rods then were passed to nailers who made the rods into nails, by giving them a point and head....

, which makes rods of iron or other metal. Rod mills are less common than ball mills for grinding minerals.

Autogenous mill

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

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 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

A rotating drum causes friction and attrition between rock pebble
Pebble
A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...

s and ore particles. May be used where product contamination by iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 from steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 balls must be avoided.

High pressure grinding rolls

In high pressure grinding rolls, often referred to as HPGRs, the ore is fed between two rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 causes the rocks to fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

 into finer particles and also causes microfracturing at the grain size level. HPGRs are often considered as being more efficient than grinding mills, although they are not as common as grinding mills since they are a newer technology.

HPGRs consist of a pair of horizontal 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 wheel
Wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

s 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.

Vertical shaft impactor mill (VSI mill)

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

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 balls 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.

Types of grinding mills

  • windmill
    Windmill
    A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...

    , wind powered
  • watermill
    Watermill
    A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping .- History :...

    , water powered
  • horse mill
    Horse mill
    A horse mill is a mill that uses a horse as the power source. Any milling process can be powered in this way, but the most frequent use of animal power in horse mills was for grinding grain and pumping water. Other animals used for powering mills include dogs, donkeys and oxen. Engines powered by...

    , animal powered
  • treadwheel
    Treadwheel
    A treadwheel is a form of animal engine typically powered by humans. It may resemble a water wheel in appearance, and can be worked either by a human treading paddles set into its circumference , or by a human or animal standing inside it .Uses of treadwheels included raising water, to power...

    , human powered (archaic: "treadmill")
  • ship mill
    Ship mill
    The ship-mill grinder today is a rare type of watermill. Its first recorded use dates back to mid-6th century AD Italy.- Technology :...

    , floats near a river bank or bridge
  • roller mill
    Roller mill
    Roller mills use cylindrical rollers to grind all sorts of materials rather than crushing them or grinding them between stones such as in a traditional grains mill. Roller mills are usually used for industrial grain milling and for grinding gravel into smaller rocks or pulverizing them...

    , an equipment for the grinding or pulverizing of grain and other raw materials using cylinders
  • Stamp mill
    Stamp mill
    A stamp mill is a type of mill machine that crushes material by pounding rather than grinding, either for further processing or for extraction of metallic ores. Breaking material down is a type of unit operation....

    , 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
      Cider mill
      A Cider mill refers to the location, structure, or machinery used to crushed apples into apple juice for use in making apple cider, applejack, hard cider, apple wine, pectin and other products derived from apples. The mills used to manufacture the juice products, ferment them, store them and ship...

       crushes apples to give cider
    • gristmill
      Gristmill
      The terms gristmill or grist mill can refer either to a building in which grain is ground into flour, or to the grinding mechanism itself.- Early history :...

       grinds grain into flour
    • oil mill, see expeller pressing, extrusion
      Extrusion
      Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...

    • paper mill
      Paper mill
      A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags and other ingredients using a Fourdrinier machine or other type of paper machine.- History :...

       produces paper
    • sawmill
      Sawmill
      A sawmill is a facility where logs are cut into boards.-Sawmill process:A sawmill's basic operation is much like those of hundreds of years ago; a log enters on one end and dimensional lumber exits on the other end....

       cuts timber
    • starch mill
      Starch mill
      A starch mill is a mill for making starch. The input is commonly a vegetable such as potato, casava or tapioca. This is pulped, pressed, dried and ground to form a starch powder which is then used as a food ingredient, glue or fabric treatment.-References:...

    • steel mill
      Steel mill
      A steel mill or steelworks is an industrial plant for the manufacture of steel.Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or...

       manufactures steel
    • sugar mill (also called a sugar refinery
      Sugar refinery
      A sugar refinery is a factory which refines raw sugar.Many cane sugar mills produce raw sugar, i.e. sugar with more colour and therefore more impurities than the white sugar which is normally consumed in households and used as an ingredient in soft drinks, cookies and so forth...

      ) processes sugar beets or sugar cane into various finished products
    • textile mill (disambiguation)
      • silk mill, for silk
      • flax mill
        Flax mill
        Flax mills are mills concerned with the manufacture of flax. The earliest mills were ones for spinning yarn for the linen industry.John Kendrew and Thomas Porthouse , both of Darlington developed the process from Richard Arkwright's water frame, and patented it in 1787...

        , for flax
      • cotton mill
        Cotton mill
        A cotton mill is a factory that houses spinning and weaving machinery. Typically built between 1775 and 1930, mills spun cotton which was an important product during the Industrial Revolution....

        , for cotton
    • huller
      Huller
      A rice huller or rice husker is an agricultural machine used to automate the process of removing the chaff and the outer husks of rice grain. Throughout history, there have been numerous techniques to hull rice. Traditionally, it would be pounded using some form of mortar and pestle. Machinery...

       (also called a rice mill, or rice husker) is used to hull rice
    • powder mill
      Powder mill
      The term powder mill is usually used for a mill that manufactures blackpowder, a type of gunpowder.A powder mill could be driven by wind or water power, and contained rollers for grinding the ingredients of gunpowder together, as well as presses and tumbling barrels and sieves for compacting,...

       produces gunpowder

  • Ball mill
    Ball mill
    A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...

  • Colloid mill
    Colloid mill
    A colloid mill is a machine that is used to reduce the particle size of a solid in suspension in a liquid, or to reduce the droplet size of a liquid suspended in another liquid. This is done by applying high levels of hydraulic shear to the process liquid. It is frequently used to increase the...

  • Conical mill
    Conical mill
    A conical mill is a machine used to reduce the size of material in a uniform manner. It is an alternative to the hammermill or other forms of grinding mills....

  • Disintegrator
    Disintegrator
    A disintegrator may refer to:* a fictional weapon as in disintegrator ray.* a real device such as an ultrasonic disintegrator that uses acoustic waves to accelerate chemical reactions or to break biological material and chemical compounds....

  • Disk mill
  • Edge mill
    Edge mill
    An edge mill or a chilian mill is a mill used for crushing or grinding in which stones roll around on their edges, on a level circular bed. It is used for milling ore and as an oil mill. Horse or oxen-driven versions were used in pre-industrial revolution America as tan bark mills to shred tree...

  • Gristmill
    Gristmill
    The terms gristmill or grist mill can refer either to a building in which grain is ground into flour, or to the grinding mechanism itself.- Early history :...

    , also called flour mill or corn mill
  • Hammer mill
  • Jet mill
  • Mortar and pestle
    Mortar and pestle
    A mortar and pestle is a tool used to crush, grind, and mix solid substances . The pestle is a heavy bat-shaped object, the end of which is used for crushing and grinding. The mortar is a bowl, typically made of hard wood, ceramic or stone...

  • Pellet mill
    Pellet mill
    A pellet mill, also known as a pellet press, is a type of mill or machine press used to create pellets from powdered material. Pellet mills are unlike grinding mills, in that they combine small materials into a larger, homogeneous mass, rather than break large materials into smaller...

  • Planetary mill
  • Stirred mill
  • Vibratory mill
  • VSI mill
    VSI mill
    VSI mill is the type of fine grinder which is used a free impact of material particles with a wear plate. Any hard or friable materials can be ground with low value of metal waste...

  • Wiley mill
    Wiley mill
    A Wiley Mill is a laboratory-scale machine for grinding materials to a powder and belongs to the family of cutting mills. The term "Wiley" refers to a specific group of grinding mills manufactured by Thomas Scientific and is their registered trademark. These mills prepare materials for analysis...

  • Windmill
    Windmill
    A windmill is a machine which converts the energy of wind into rotational energy by means of vanes called sails or blades. Originally windmills were developed for milling grain for food production. In the course of history the windmill was adapted to many other industrial uses. An important...



See also

  • Burr grinder
  • Coffee grinder
    Coffee preparation
    Coffee preparation is the process of turning coffee beans into a beverage. While the particular steps needed vary with the type of coffee desired and with the raw material being utilized, the process is composed of four basic steps; raw coffee beans must be roasted, the roasted coffee beans must...

  • Expeller
    Expeller
    Expeller pressing is a mechanical method for extracting oil from raw materials. The raw materials are squeezed under high pressure in a single step. When used for the extraction of food oils, typical raw materials are nuts, seeds and algae, which are supplied to the press in a continuous feed....

  • Extruder
  • Grist mill
  • Herb grinder
    Herb grinder
    A herb grinder is a cylindrical contraption with two halves that separate and have sharp teeth or pegs aligned in such a way that when both halves are turned, material inside—typically tobacco or marijuana—is shredded, resulting in a product that burns more evenly.Herb grinders can be used to...

  • Millstone
    Millstone
    Millstones or mill stones are used in windmills and watermills, including tide mills, for grinding wheat or other grains.The type of stone most suitable for making millstones is a siliceous rock called burrstone , an open-textured, porous but tough, fine-grained sandstone, or a silicified,...

  • Rock crusher


External links

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