A mill is a device, often a structure, machine or kitchen appliance, that breaks solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an important unit operation in many processes. There are many different types of mills and many types of materials processed in them. Historically, mills were powered by hand or by animals (e.g., via a hand crank), working animal (e.g., horse mill), wind (windmill) or water (watermill). In the modern era, they are usually powered by electricity. The grinding of solid materials occurs through mechanical forces that break up the structure by overcoming 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. Milling also refers to the process of breaking down, separating, sizing, or classifying aggregate material (e.g. mining ore). For instance rock crushing or grinding to produce uniform aggregate size for construction purposes, or separation of rock, soil or aggregate material for the purposes of structural fill or land reclamation activities. Aggregate milling processes are also used to remove or separate contamination or moisture from aggregate or soil and to produce "dry fills" prior to transport or structural filling. 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. Mining engineers, Peter von Rittinger, Friedrich Kick and Fred Chester Bond independently produced equations to relate the needed grinding work to the grain size produced and a fourth engineer, R.T.Hukki suggested that these three equations might each describe a narrow range of grain sizes and proposed uniting them along a single curve describing what has come to be known as the Hukki relationship. In stirred mills, the Hukki relationship does not apply and instead, experimentation has to be performed to determine any relationship. 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
Z d = d 80 , 1 d 80 , 2 {\displaystyle 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
Z S = S v , 2 S v , 1 = S m , 2 S m , 1 {\displaystyle Z_{S}={\frac {S_{v,2}}{S_{v,1}}}={\frac {S_{m,2}}{S_{m,1}}}\,}
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
Z a = d 1 a {\displaystyle 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.
Grinding machines 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.
Autogenous mill Autogenous or autogenic 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.
Ball mill 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 grind 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, 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:
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