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 modern era, they are usually powered by electricity .
41-420: Traditionally, a Mill House is a working commercial property ( mill ) which also acts as a residence Mill House may refer to: Mill (grinding)#Types of grinding mills 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
82-418: A 28 MW (38,000 HP) motor. A SAG mill with a 44' (13.4m) diameter and a power of 35 MW (47,000 HP) has been designed. Attrition between grinding balls and ore particles 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 the mill, where it then falls off
123-466: 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 achieve a 30 to 50% lower specific energy consumption, although they are not as common as ball mills since they are
164-401: 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 greater than 50 MPa (7,000 PSI ). 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
205-413: A husk. This foundation isolated the building from vibrations coming from the stones and main gearing and also allowed for the easy re-leveling of the foundation to keep the millstones perfectly horizontal. The lower bedstone was placed in an inset in the husk with the upper runner stone above the level of the husk. American inventor Oliver Evans revolutionized the labor-intensive process of early mills at
246-408: 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. A VSI mill throws rock or ore particles against a wear plate by slinging them from
287-632: 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
328-412: A mechanical sieve to refine the flour, or turning a wooden drum to wind up a chain used to hoist sacks of grain to the top of the mill house. The distance between the stones can be varied to produce the grade of flour required; moving the stones closer together produces finer flour. This process, which may be automatic or controlled by the miller, is called tentering . The grain is lifted in sacks onto
369-499: 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)
410-460: 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. 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
451-463: A proportion on all grain processed in the community. Later, mills were supported by farming communities and the miller received the "miller's toll" in lieu of wages. Most towns and villages had their own mill so that local farmers could easily transport their grain there to be milled. These communities were dependent on their local mill as bread was a staple part of the diet. Classical mill designs are usually water-powered , though some are powered by
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#1732848691516492-415: 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 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
533-508: A spinning center that rotates on a vertical shaft. This type of mill uses the same principle as a VSI crusher . Flour mill A gristmill (also: grist mill , corn mill , flour mill , feed mill or feedmill ) grinds cereal grain into flour and middlings . The term can refer to either the grinding mechanism or the building that holds it. Grist is grain that has been separated from its chaff in preparation for grinding . The Greek geographer Strabo reports in his Geography
574-434: A system that allows the sequential milling of these grists, noting that "a mill, thus constructed, might grind grists in the day time, and do merchant-work at night." Over time, any small, older style flour mill became generally known as a gristmill (as a distinction from large factory flour mills). Modern mills typically use electricity or fossil fuels to spin heavy steel, or cast iron, serrated and flat rollers to separate
615-498: A very physically demanding job for workers, where the slave workers were considered little different from animals, the miseries of which were depicted in iconography and Apuleius ' The Golden Ass . The peak of Roman technology is probably the Barbegal aqueduct and mill where water with a 19-metre fall drove sixteen water wheels , giving a grinding capacity estimated at 28 tons per day. Water mills seem to have remained in use during
656-495: A water-powered grain-mill to have existed near the palace of king Mithradates VI Eupator at Cabira , Asia Minor , before 71 BC. The early mills had horizontal paddle wheels, an arrangement which later became known as the " Norse wheel ", as many were found in Scandinavia. The paddle wheel was attached to a shaft which was, in turn, attached to the centre of the millstone called the "runner stone". The turning force produced by
697-412: Is commonly used because it is inexpensive to obtain. A rotating drum causes friction and attrition between steel rods and ore particles. But 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
738-427: Is needed. Grinding degree is the ratio of the sizes from the grain disposition. There are several definitions for this characteristic value: [REDACTED] 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
779-581: 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. 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
820-438: The bran and germ from the endosperm . The endosperm is ground to create white flour, which may be recombined with the bran and germ to create whole grain or graham flour . The different milling techniques produce visibly different results, but can be made to produce nutritionally and functionally equivalent output. Stone-ground flour is preferred by many bakers and natural food advocates because of its texture, nutty flavour, and
861-413: The sack floor at the top of the mill on the hoist . The sacks are then emptied into bins, where the grain falls down through a hopper to the millstones on the stone floor below. The flow of grain is regulated by shaking it in a gently sloping trough (the slipper ) from which it falls into a hole in the center of the runner stone. The milled grain (flour) is collected as it emerges through the grooves in
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#1732848691516902-901: The belief that it is nutritionally superior and has a better baking quality than steel-roller-milled flour. It is claimed that, as the stones grind relatively slowly, the wheat germ is not exposed to the sort of excessive temperatures that could cause the fat from the germ portion to oxidize and become rancid, which would destroy some of the vitamin content. Stone-milled flour has been found to be relatively high in thiamin, compared to roller-milled flour, especially when milled from hard wheat. Gristmills only grind "clean" grains from which stalks and chaff have previously been removed, but historically some mills also housed equipment for threshing , sorting, and cleaning prior to grinding. Modern mills are usually "merchant mills" that are either privately owned and accept money or trade for milling grains or are owned by corporations that buy unmilled grain and then own
943-417: The diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency. SAG is an acronym for semi-autogenous grinding. SAG mills are autogenous mills that also use grinding balls like a ball mill. A SAG mill is usually a primary or first stage grinder. SAG mills use a ball charge of 8 to 21%. The largest SAG mill is 42' (12.8m) in diameter, powered by
984-634: The end of the eighteenth century when he automated the process of making flour. His inventions included the Elevator, wood or tin buckets on a vertical endless leather belt, used to move grain and flour vertically upward; the Conveyor, a wooden auger to move material horizontally; the Hopper Boy, a device for stirring and cooling the newly ground flour; the Drill, a horizontal elevator with flaps instead of buckets (similar to
1025-521: The following form of the Bond equation: where Another type of fine grinder commonly used is the French buhrstone mill, which is similar to old-fashioned flour mills . A high pressure grinding roll, 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
1066-472: The following purposes in engineering: 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
1107-569: 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
1148-535: The late 10th century onwards, there was an expansion of grist-milling in Northern Europe. In England, the Domesday survey of 1086 gives a precise count of England's water-powered flour mills: there were 5,624, or about one for every 300 inhabitants, and this was probably typical throughout western and southern Europe. From this time onward, water wheels began to be used for purposes other than grist milling. In England,
1189-495: The medieval Near East and North Africa , which were used for grinding grain and other seeds to produce meals . Gristmills in the Islamic world were powered by both water and wind. The first wind-powered gristmills were built in the 9th and 10th centuries in what are now Afghanistan, Pakistan and Iran. The Egyptian town of Bilbays had a grain-processing factory that produced an estimated 300 tons of flour and grain per day. From
1230-833: The number of mills in operation followed population growth, and peaked at around 17,000 by 1300. Limited extant examples of gristmills can be found in Europe from the High Middle Ages . An extant well-preserved waterwheel and gristmill on the Ebro River in Spain is associated with the Real Monasterio de Nuestra Senora de Rueda , built by the Cistercian monks in 1202. The Cistercians were known for their use of this technology in Western Europe in
1271-496: The old wheel mills. In most wheel-driven mills, a large gear-wheel called the pit wheel is mounted on the same axle as the water wheel and this drives a smaller gear-wheel, the wallower , on a main driveshaft running vertically from the bottom to the top of the building. This system of gearing ensures that the main shaft turns faster than the water wheel, which typically rotates at around 10 rpm . The millstones themselves turn at around 120 rpm . They are laid one on top of
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1312-414: The other. The bottom stone, called the bed , is fixed to the floor, while the top stone, the runner , is mounted on a separate spindle, driven by the main shaft. A wheel called the stone nut connects the runner's spindle to the main shaft, and this can be moved out of the way to disconnect the stone and stop it turning, leaving the main shaft turning to drive other machinery. This might include driving
1353-472: The period 1100 to 1350. Although the terms "gristmill" or "corn mill" can refer to any mill that grinds grain, the terms were used historically for a local mill where farmers brought their own grain and received ground meal or flour, minus a percentage called the "miller's toll". Early mills in England were almost always built by the local lord of the manor and had the exclusive right (the right of mulcture ) to
1394-526: The plates onto the rest of the ore charge. SAG mills are primarily used at gold, copper and platinum mines with applications also in the lead, zinc, silver, alumina and nickel industries. 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
1435-580: The post-Roman period. Manually operated mills utilizing a crank-and-connecting rod were used in the Western Han dynasty . There was an expansion of grist-milling in the Byzantine Empire and Sassanid Persia from the 3rd century AD onwards, and then the widespread expansion of large-scale factory milling installations across the Islamic world from the 8th century onwards. Geared gristmills were built in
1476-418: The process. Evans himself did not use the term gristmill to describe his automatic flour mill, which was purpose designed as a merchant mill (he used the more general term "water-mill"). In his book his only reference to "grist" (or "grists") is to the small batches of grain a farmer would bring in to have ground for himself (what would be generally called barter or custom milling). In his book, Evans describes
1517-416: The runner stone from the outer rim of the stones and is fed down a chute to be collected in sacks on the ground or meal floor. A similar process is used for grains such as wheat to make flour, and for maize to make corn meal . In order to prevent vibrations from the millstones shaking the building apart, they were usually placed on a separate timber foundation, not attached to the mill walls, known as
1558-595: The use of a conveyor but easier to build); and the Descender, an endless strap (leather or flannel) in a trough that is angled downward, the strap helps to move the ground flour in the trough. Most importantly, he integrated these into a single continuous process, the overall design later becoming known as the Automatic (or Automated) mill. In 1790 he received the third Federal patent for his process. In 1795 he published "The Young Mill-Wright and Miller’s Guide" which fully described
1599-454: The volume and speed of flow of the water also meant that the speed of rotation of the stone was highly variable and the optimum grinding speed could not always be maintained. Vertical wheels were in use in the Roman Empire by the end of the first century BC, and these were described by Vitruvius . The rotating mill is considered "one of the greatest discoveries of the human race". It was
1640-436: The water on the paddles was transferred directly to the runner stone, causing it to grind against a stationary " bed ", a stone of a similar size and shape. This simple arrangement required no gears , but had the disadvantage that the speed of rotation of the stone was dependent on the volume and flow of water available and was, therefore, only suitable for use in mountainous regions with fast-flowing streams. This dependence on
1681-410: The wind or by livestock. In a watermill a sluice gate is opened to allow water to flow onto, or under, a water wheel to make it turn. In most watermills the water wheel was mounted vertically, i.e., edge-on, in the water, but in some cases horizontally (the tub wheel and so-called Norse wheel ). Later designs incorporated horizontal steel or cast iron turbines and these were sometimes refitted into