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Mill (grinding)
 
 
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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. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process....
 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 substances. The pestle is a heavy stick whose end is used for pounding and grinding, and the mortar is a bowl....
), working animal
Working animal

A working animal is an animal that is kept by humans and trained to perform tasks. They may be close members of the family, such as guide dogs, or domestications such as logging elephants....
, wind (windmill
Windmill

A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails....
) or water (watermill
Watermill

A watermill is a structure that uses a water wheel or water turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping ....
). Today they are also powered by electricity
Electricity

Electricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction....
.

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






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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. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process....
 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 substances. The pestle is a heavy stick whose end is used for pounding and grinding, and the mortar is a bowl....
), working animal
Working animal

A working animal is an animal that is kept by humans and trained to perform tasks. They may be close members of the family, such as guide dogs, or domestications such as logging elephants....
, wind (windmill
Windmill

A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails....
) or water (watermill
Watermill

A watermill is a structure that uses a water wheel or water turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping ....
). Today they are also powered by electricity
Electricity

Electricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction....
.

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:
  • magnification 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:

  • KICK for d > 50 mm


  • BOND for 50 mm > d > 0.05 mm


  • 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.

Grinding degree

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
Attrition

Attrition may refer to:*Physical wear*Loss of personnel by retirement*Attrition , the loss of participants during an experiment*Attrition , the loss of tooth structure by mechanical forces from opposing teeth...
 and compressive
Physical compression

Physical compression is the result of the subjection of a material to compressive stress, resulting in reduction of volume. The opposite of compression is tension ....
 force
Force

In physics, a force is that which can cause an object with mass to change its velocity. Force has both Euclidean_vector#Length of a vector and Direction , making it a Vector quantity....
s at the grain size level. See also crusher
Crusher

A crusher is a machine designed to reduce large solid material objects into a smaller volume, or smaller pieces. 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 of different composition can...
 for mechanisms producing larger particles.

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 paints, pyrotechnics, and ceramics....
. A slightly inclined or horizontal rotating cylinder is partially filled with ball
Sphere

A sphere is a symmetrical geometrical object. In non-mathematical usage, the term is used to refer either to a round ball or to its two-dimensional surface....
s, usually stone
Rock (geology)

In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock....
 or metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
, which grinds material to the necessary fineness by friction
Friction

File:Friction alt.svgFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact....
 and impact with the tumbling balls. 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....
.

Rod mill

A rotating drum causes friction and attrition between steel rods
Cylinder (geometry)

A cylinder is one of the most curvilinear basic geometric shapes: the surface formed by the points at a fixed distance from a given straight line, the axis of the 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 nail , by giving them a point and head....
, which makes rods of iron or other metal.

SAG mill

SAG is an acronym for Semi-Autogenous Grinding, and applies to mills that utilize steel balls in addition to large rocks for grinding. The SAG mills use a minimal ball charge of 6 to 15%.

A rotating drum throws large rocks and steel 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 characterized 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 inside 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 throws large 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 pebble
Pebble

A pebble is a clastic rocks of rock with a particle size of 4 to 64 millimeters based on the Krumbein phi scale of sedimentology. It is generally considered to be larger than gravel and smaller than cobble....
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. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 from steel
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective 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

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 to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
 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.

It consists 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 circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load , or performing labour in machines....
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.

Types of grinding mills

  • Ball mill
    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....
  • 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....
  • 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 Mill ....
  • 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....
  • Grist mill, also called flour mill or corn mill
  • Hammer mill
  • Mortar and pestle
    Mortar and pestle

    A mortar and pestle is a tool used to crush, grind, and mix substances. The pestle is a heavy stick whose end is used for pounding and grinding, and the mortar is a bowl....
  • Pellet mill
    Pellet Mill

    A pellet mill is a type of Mill used to create cylindrical pellets from a mixture of dry powdered feedstock, such as flour, sawdust, or grass, and a wet ingredient, such as molasses or steam....
  • Wiley mill
    Wiley mill

    The Wiley mill is a laboratory-scale machine for grinding materials to a Powder . This mill prepares materials for analysis with minimal moisture loss....
  • Windmill
    Windmill

    A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails....


See also

  • 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 then be ground, the ground coffee m...
  • 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....
  • 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 placed inside is effectively shredded....
  • 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 buhrstone , an open-textured, porous but tough, fine-grained sandstone, or a silicified, fossiliferous limestone....
  • Rock crusher
    Rock crusher

    A rock crusher is a machine designed to take large Rock s and reduce them to smaller rocks, gravel, or rock dust. Rock crushers produce Construction aggregate and ready-to-process mining ores, as well as rock fill material for landscaping and erosion control....


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