Mill (grinding)

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Attrition Mill
Hammer mill open front full.jpg
A tabletop hammer mill
Other namesGrinding mill
UsesGrinding
Related items Mortar and pestle
Expeller
Extruder

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.

Contents

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:

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. [1] [2] [3]

In stirred mills, the Hukki relationship does not apply and instead, experimentation has to be performed to determine any relationship. [4]

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:

Instead of the value of d80 also d50 or other grain diameter can be used.
The specific surface area referring to volume Sv and the specific surface area referring to mass Sm can be found out through experiments.
The discharge die gap a of the grinding machine is used for the ground solid matter in this formula.

Simple Grinding Forces.png

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. [5]

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.

Operation of a ball mill Ball mill.gif
Operation of a ball mill

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, [6] 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: [7]

where

Buhrstone mill

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

High pressure grinding rolls

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 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 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 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.

Pebble mill

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.

Rod mill

A rotating drum causes friction and attrition between steel rods and ore particles.[ citation needed ] 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 the diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency. [8]

Principle of SAG Mill operation Principle of SAG Mill operation.jpg
Principle of SAG Mill operation

SAG mill

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%. [9] [10] The largest SAG mill is 42' (12.8m) in diameter, powered by a 28 MW (38,000 HP) motor. [11] A SAG mill with a 44' (13.4m) diameter and a power of 35 MW (47,000 HP) has been designed. [12]

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 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 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 (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.

Table top hammer mill Table top hammer mill.jpg
Table top hammer mill

Vertical shaft impactor mill (VSI mill)

A VSI mill throws rock or ore particles against a wear plate by slinging them from a spinning center that rotates on a vertical shaft. This type of mill uses the same principle as a VSI crusher.

Types of grinding mills

A windmill in Kuremaa, Jogeva County, Estonia Kuremaa moisa tuuleveski.jpg
A windmill in Kuremaa, Jõgeva County, Estonia
A watermill in Kuusamo, North Ostrobothnia, Finland Mill in the woods (31546576571).jpg
A watermill in Kuusamo, North Ostrobothnia, Finland

See also

Related Research Articles

A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, sand or rock dust.

<span class="mw-page-title-main">Ball mill</span> Machine used to grind or blend materials

A ball mill is a type of grinder used to grind or blend materials for use in mineral dressing processes, paints, pyrotechnics, ceramics, and selective laser sintering. It works on the principle of impact and attrition: size reduction is done by impact as the balls drop from near the top of the shell.

<span class="mw-page-title-main">Froth flotation</span> Process for selectively separating of hydrophobic materials from hydrophilic

Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, where it was one of the great enabling technologies of the 20th century. It has been described as "the single most important operation used for the recovery and upgrading of sulfide ores". The development of froth flotation has improved the recovery of valuable minerals, such as copper- and lead-bearing minerals. Along with mechanized mining, it has allowed the economic recovery of valuable metals from much lower-grade ore than previously.

<span class="mw-page-title-main">Grinding wheel</span> Abrasive cutting tool for grinders

Grinding wheels are wheels that contain abrasive compounds for grinding and abrasive machining operations. Such wheels are also used in grinding machines.

<span class="mw-page-title-main">Mineral processing</span> Process of separating commercially valuable minerals from their ores

Mineral processing is the process of separating commercially valuable minerals from their ores in the field of extractive metallurgy. Depending on the processes used in each instance, it is often referred to as ore dressing or ore milling.

A pulverizer or grinder is a mechanical device for the grinding of many different types of materials. For example, a pulverizer mill is used to pulverize coal for combustion in the steam-generating furnaces of coal power plants.

<span class="mw-page-title-main">Cement mill</span>

A cement mill is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical roller mills which are more effective than ball mills.

<span class="mw-page-title-main">Rawmill</span>

A raw mill is the equipment used to grind raw materials into "rawmix" during the manufacture of cement. Rawmix is then fed to a cement kiln, which transforms it into clinker, which is then ground to make cement in the cement mill. The raw milling stage of the process effectively defines the chemistry of the finished cement, and has a large effect upon the efficiency of the whole manufacturing process.

Geometallurgy relates to the practice of combining geology or geostatistics with metallurgy, or, more specifically, extractive metallurgy, to create a spatially or geologically based predictive model for mineral processing plants. It is used in the hard rock mining industry for risk management and mitigation during mineral processing plant design. It is also used, to a lesser extent, for production planning in more variable ore deposits.

<span class="mw-page-title-main">Grinding (abrasive cutting)</span> Machining process using a grinding wheel

Grinding is a type of abrasive machining process which uses a grinding wheel as cutting tool.

<span class="mw-page-title-main">Roller mill</span> Mill that uses cylindrical rollers to crush or grind various materials

Roller mills are mills that use cylindrical rollers, either in opposing pairs or against flat plates, to crush or grind various materials, such as grain, ore, gravel, plastic, and others. Roller grain mills are an alternative to traditional millstone arrangements in gristmills. Roller mills for rock complement other types of mills, such as ball mills and hammermills, in such industries as the mining and processing of ore and construction aggregate; cement milling; and recycling.

Vertical roller mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, cements and ceramics. It is an energy efficient alternative for a ball mill.

Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of the Earth's crust. In industry, it is an important unit operation in mineral processing, ceramics, electronics, and other fields, accomplished with many types of mill. In dentistry, it is the result of mastication of food. In general medicine, it is one of the most traumatic forms of bone fracture.

The IsaMill is an energy-efficient mineral industry grinding mill that was jointly developed in the 1990s by Mount Isa Mines Limited and Netzsch Feinmahltechnik ("Netzsch"), a German manufacturer of bead mills. The IsaMill is primarily known for its ultrafine grinding applications in the mining industry, but is also being used as a more efficient means of coarse grinding. By the end of 2008, over 70% of the IsaMill’s installed capacity was for conventional regrinding or mainstream grinding applications, with target product sizes ranging from 25 to 60 µm.

<span class="mw-page-title-main">Fred Chester Bond</span> American mining engineer

Fred Chester Bond was an American mining engineer. A graduate of the Colorado School of Mines, he worked in the mining equipment and ore milling equipment business of Allis-Chalmers from 1930 to 1964.

Sepro Mineral Systems Corp. is a Canadian company founded in 1987 and headquartered in British Columbia, Canada. The outcome of the acquisition of Sepro Mineral Processing International by Falcon Concentrators in 2008, the company's key focus is the production of mineral processing equipment for the mining and aggregate industries. Sepro Mineral Systems Corp. also provides engineering and process design services. Products sold by Sepro include grinding mills, ore scrubbers, vibrating screens, centrifugal gravity concentrators, agglomeration drums, and dense media separators. The company is also a supplier of single source modular pre-designed and custom designed plants and circuits.

High-frequency vibrating screens are the most important screening machines primarily utilised in the mineral processing industry. They are used to separate feeds containing solid and crushed ores down to less than 200 μm in size, and are applicable to both perfectly wetted and dried feed. The frequency of the screen is mainly controlled by an electromagnetic vibrator which is mounted above and directly connected to the screening surface. Its high-frequency characteristics differentiate it from a normal vibrating screen. High-frequency vibrating screens usually operate at an inclined angle, traditionally varying between 0° and 25° and can go up to a maximum of 45°. They should operate with a low stroke and have a frequency ranging from 1500 to 9000 RPM. Frequency in High frequency screen can be fixed or variable. Variable High Frequency screen is more versatile to tackle varied material condition like particle size distribution, moisture and have higher efficiency due to incremental increase in frequency. G force plays important role in determining specific screening capacity of screen in terms of TPH per sqm. G force increases exponentially with frequency.

A grinder-mixer is a type of agricultural machine used to process livestock feed from grain. A grinder-mixer is a portable mill that combines the mixing and grinding operations.

Dry milling of grain is mainly utilized to manufacture feedstock into consumer and industrial based products. This process is widely associated with the development of new bio-based associated by-products. The milling process separates the grain into four distinct physical components: the germ, flour, fine grits, and coarse grits. The separated materials are then reduced into food products utilized for human and animal consumption.

References

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  2. A. Jankovic; H. Dundar; R. Mehta‡ (March 2010). "Relationships between comminution energy and product size for a magnetite ore" (PDF). The Journal of the Southern African Institute of Mining and Metallurgy. 110.
  3. Hukki, R.T. (1961). "Proposal for Solomonic settlement between theories of Rittinger, Kick, and Bond". scribd.com.
  4. Thomas, A; Filippov, L.O. (1999). "Fractures, fractals and breakage energy of mineral particles". International Journal of Mineral Processing. 57 (4): 285. doi:10.1016/S0301-7516(99)00029-0.
  5. Baron, M.; Chamayou, A.; Marchioro, L.; Raffi, J. (2005). "Radicalar probes to measure the action of energy on granular materials" (PDF). Advanced Powder Technology. 16 (3): 199. doi:10.1163/1568552053750242.
  6. "ABB". ABB Communications. ABB Communications.
  7. Mineral Beneficiation – The Third Theory of Comminution – Document Summary. Onemine.org. Retrieved on 2010-10-09.
  8. Wills, B.A. (2006). Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery. Vol. 7th ed. Amsterdam, Boston, MA. p. 157. ISBN   0750644508.
  9. Strohmayr, S. and Valery, W. Jr. SAG mill circuit optimisation at Ernest Henry Mining. ResearchGate.net
  10. Mular, Andrew L.; Halbe, Doug N.; Barratt, Derek J. (2002). Mineral Processing Plant Design, Practice, and Control: Proceedings. SME. pp. 2369–. ISBN   978-0-87335-223-9 . Retrieved 26 October 2012.
  11. van de Vijfeijken, Maarten (October 2010). "Mills and GMDs" (PDF). International Mining: 30.
  12. Gearless mill drives. abb.com