3. Unit operation 1 Unit operation In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve bringing a physical change such as separation, crystallization, evaporation, filtration etc. For example, in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process. A process may have many unit operations to obtain the desired product. Historically, the different chemical industries were regarded as different industrial processes and with different principles. Arthur Dehon Little propounded the concept of "unit operations" to explain industrial chemistry processes in 1916. In 1923, William H.Walker, Warren K. Lewis and William H. McAdams wrote the book The Principles of Chemical Engineering and explained the variety of chemical industries have processes which follow the same physical laws. They summed-up these similar processes into unit operations. Each unit operation follows the same physical laws and may be used in all chemical industries. The unit operations form the fundamental principles of chemical engineering. An ore extraction process broken into its constituent unit operations (Quincy Mine, Hancock, MI ca. 1900) Chemical engineering unit operations consist of five classes: 1. Fluid flow processes, including fluids transportation, filtration, solids fluidization 2. Heat transfer processes, including evaporation, condensation 3. Mass transfer processes, including gas absorption, distillation, extraction, adsorption, drying 4. Thermodynamic processes, including gas liquefaction, refrigeration 5. Mechanical processes, including solids transportation, crushing and pulverization, screening and sieving Chemical engineering unit operations also fall in the following categories: • Combination (mixing) • Separation (distillation) • Reaction (chemical reaction) Chemical engineering unit operations and chemical engineering unit processing form the main principles of all kinds of chemical industries and are the foundation of designs of chemical plants, factories, and equipment used.
4. Unit operation 2 References  "The MIT Connection"http:/ / libraries. mit. edu/ archives/ exhibits/ adlittle/ mit-connection. html Retrieved March 6, 2010.  The Encyclopedia of Earth. "Walker, William H. http:/ / www. eoearth. org/ article/ Walker,_William_H. Accessed April 4, 2010. Compression (physical) Physical compression is the result of the subjection of a material or structure to compressive stress, which results in reduction of volume as compared to an uncompressed but otherwise identical state. The opposite of compression in a solid is tension. In any medium transmitting waves, the opposite of compression is rarefaction. In simple terms, compression is a pushing force. Explanation Compression has many implications in materials science, physics and structural engineering, for Compression test on a universal testing machine compression yields noticeable amounts of stress and tension. By inducing compression, mechanical properties such as compressive strength or modulus of elasticity, can be measured. Scientists and engineers may utilize compression machines to measure the resistance of materials and structures to compression. Compression machines range from very small table top systems to ones with over 53 MN capacity. In engines Internal combustion engines In internal combustion engines it is a necessary condition of economy to compress the explosive mixture before it is ignited: in the Otto cycle, for instance, the second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke. Steam engines The term is applied to the arrangement by which the exhaust valve of a steam engine is made to close, shutting a portion of the exhaust steam in the cylinder, before the stroke of the piston is quite complete. This steam being compressed as the stroke is completed, a cushion is formed against which the piston does work while its velocity is being rapidly reduced, and thus the stresses in the mechanism due to the inertia of the reciprocating parts are lessened. This compression, moreover, obviates the shock which would otherwise be caused by the admission of the fresh steam for the return stroke.
5. Compression (physical) 3 References  NIST, Large Scale Structure Testing Facility (http:/ / www. nist. gov/ bfrl/ facilities_instruments/ large_scale_struct_testing_fac. cfm), , retrieved 04-05-2010. • Beer, Ferdinand Pierre; Elwood Russell Johnston, John T. DeWolf (1992). Mechanics of Materials. McGraw-Hill Professional. ISBN 0-07-112939-1. Impact (mechanics) In mechanics, an impact is a high force or shock applied over a short time period when two or more bodies collide. Such a force or acceleration usually has a greater effect than a lower force applied over a proportionally longer time period of time. The effect depends critically on the relative velocity of the bodies to one another. At normal speeds, during a perfectly inelastic collision, an object struck by a projectile will deform, and this deformation will absorb most, or even all, of the force of the collision. Viewed from the conservation of energy perspective, the kinetic energy of the projectile is changed into heat and sound energy, as a result of the deformations and vibrations induced in the struck object. However, these deformations and vibrations cannot occur instantaneously. A high-velocity collision (an impact) does not provide sufficient time for these deformations and vibrations to occur. Thus, the struck material behaves as if it were more brittle than it is, and the majority of the applied force goes into fracturing the material. Or, another way to look at it is that materials actually are more brittle on short time scales than on long time scales: this is related to time-temperature superposition. Impact resistance will be decreased with an increase in the modulus of elasticity, which means that stiffer materials will have less impact resistance. Resilient materials will have better impact resistance. Different materials can behave in quite different ways in impact when compared with static loading conditions. Ductile materials like steel tend to become more brittle at high loading rates, and spalling may occur on the reverse side to the impact if penetration doesnt occur. The way in which the kinetic energy is distributed through the section is also important in determining its response. Projectiles apply a Hertzian contact stress at the point of impact to a solid body, with compression stresses under the point, but with bending loads a short distance away. Since most materials are weaker in tension than compression, this is the zone where cracks tend to form and grow. Applications A nail is pounded with a series of impacts, each by a single hammer blow. These high velocity impacts overcome the static friction between the nail and the substrate. A pile driver achieves the same end, although on a much larger scale, the method being commonly used during civil construction projects to make building and bridge foundations. An impact wrench is a device designed to impart torque impacts to bolts to tighten or loosen them. At normal speeds, the forces applied to the bolt would be dispersed, via friction, to the mating threads. However, at impact speeds, the forces act on the bolt to move it before they can be dispersed. In ballistics, bullets utilize impact forces to puncture surfaces that could otherwise resist substantial forces. A rubber sheet, for example, behaves more like glass at typical bullet speeds. That is, it fractures, and does not stretch or vibrate. A crane with a pile driver.
6. Impact (mechanics) 4 A 1/2" drive pistol-grip air impact wrench Accidents involving impact Road traffic accidents usually involve impact loading, such as when a car hits a traffic bollard, water hydrant or tree, the damage being localized to the impact zone. When vehicles collide, the damage is proportionate to the relative velocity of the vehicles, the damage increasing as the square of the velocity since it is the impact kinetic energy (1/2 mv2) which is the variable of importance. Much design effort is made to improve the impact resistance of cars so as to minimize user injury. It can be achieved in several ways: by enclosing the driver and passengers in a safety cell for example. The cell is A Chevrolet Malibu involved in a rollover crash reinforced so will survive in high speed crashes, and so protect the users. Parts of the body shell outside the cell are designed to crumple progressively, absorbing most of the kinetic energy which must be dissipated by the impact. Various impact test are used to assess the effects of high loading, both on products and standard slabs of material. The Charpy test and Izod test are two examples of standardized methods which are used widely for testing materials. Ball or projectile drop tests are used for assessing product impacts. The Columbia disaster was caused by impact damage when a chunk of polyurethane foam impacted the carbon fibre composite wing of the space shuttle. Although tests had been conducted before the disaster, the size of the chunks was much smaller than that which fell away from the booster rocket and hit the exposed wing.
7. Impact (mechanics) 5 References • Goldsmith, W, Impact; The Theory and Physical Behaviour of Colliding Solids, 2001, Dover Publications, ISBN 0-486-42004-3 • Poursartip, A, “Instrumented Impact Testing at High Velocities”, Journal of Composites Technology and Research, 1993, vol 15 issue 1, • Toropov, AI, “Dynamic Calibration of Impact Test Instruments”, Journal of Testing and Evaluation, 1998, vol 24, no 4 A mock-up of a space shuttle leading edge made with an RCC-panel taken from Discovery showing impact damage during a test Grinding (abrasive cutting) Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool. A wide variety of machines are used for grinding: • Hand-cranked knife-sharpening stones (grindstones) • Handheld power tools such as angle grinders and die grinders • Various kinds of expensive industrial machine tools called grinding machines • Bench grinders often found in residential garages and basements Sketch of how abrasive particles in a grinding Grinding practice is a large and diverse area of manufacturing and wheel remove material from a workpiece. toolmaking. It can produce very fine finishes and very accurate dimensions; yet in mass production contexts it can also rough out large volumes of metal quite rapidly. It is usually better suited to the machining of very hard materials than is "regular" machining (that is, cutting larger chips with cutting tools such as tool bits or milling cutters), and until recent decades it was the only practical way to machine such materials as hardened steels. Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shafts diameter by half a thousandth of an inch (thou) or 12.7 um. Grinding is a subset of cutting, as grinding is a true metal-cutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is often understood to refer to the macroscopic cutting operations, and grinding is often mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though, strictly speaking, grinding is a subset of cutting. Similar abrasive cutting processes are lapping and sanding.
8. Grinding (abrasive cutting) 6 Processes Selecting which of the following grinding operations to be used is determined by the size, shape, features and the desired production rate. Surface grinding Surface grinding uses a rotating abrasive wheel to smooth the flat surface of metallic or nonmetallic materials to give them a more refined look or to attain a desired surface for a functional purpose. The tolerances that are normally achieved with grinding are ± 2 × 10−4 inches for a grinding a flat material, and ± 3 × 10−4 inches for a parallel surface (in metric units: 5 μm for flat material and 8 μm for parallel surface). The surface grinder is composed of an abrasive wheel, a workholding device known as a chuck, either electromagnetic or vacuum, and a reciprocating table. Typical workpiece materials include cast iron and minor steel. These Surface grinder two materials do not tend to clog the grinding wheel while being processed. Other materials are aluminum, stainless steel, brass and some plastics. Cylindrical grinding Cylindrical grinding (also called center-type grinding) is used in the removing the cylindrical surfaces and shoulders of the workpiece. The workpiece is mounted and rotated by a workpiece holder, also known as a grinding dog or center driver. Both the tool and the workpiece are rotated by separate motors and at different speeds. The axes of rotation tool can be adjusted to produce a variety of shapes. The five types of cylindrical grinding are: outside diameter (OD) grinding, inside diameter (ID) grinding, plunge grinding, creep feed grinding, and centerless grinding. A cylindrical grinder has a grinding (abrasive) wheel, two centers that hold the workpiece, and a chuck, grinding dog, or other mechanism to drive the machine. Most cylindrical grinding machines include a swivel to allow for the forming of tapered pieces. The wheel and workpiece move parallel to one another in both the radial and longitudinal directions. The abrasive wheel can have many shapes. Standard disk shaped wheels can be used to create a tapered or straight workpiece geometry while formed wheels are used to create a shaped workpiece. The process using a formed wheel creates less vibration than using a regular disk shaped wheel. Tolerances for cylindrical grinding are held within five ten-thousandths of an inch (+/- 0.0005) (metric: +/- 13 um) for diameter and one ten-thousandth of an inch(+/- 0.0001) (metric: 2.5 um) for roundness. Precision work can reach tolerances as high as fifty millionths of an inch (+/- 0.00005) (metric: 1.3 um) for diameter and ten millionths (+/- 0.00001) (metric: 0.25 um) for roundness. Surface finishes can range from 2 to 125 microinches (metric: 50 nm to 3 um), with typical finishes ranging from 8-32 microinches. (metric: 0.2 um to 0.8 um) Creep-feed grinding Creep-feed grinding (CFG) was invented in Germany in the late 1950s by Edmund and Gerhard Lang. Unlike normal grinding, which is used primarily to finish surfaces, CFG is used for high rates of material removal, competing with milling and turning as a manufacturing process choice. Depths of cut of up to 6 mm (0.25 inches) are used along with low workpiece speed. Surfaces with a softer-grade resin bond are used to keep workpiece temperature low and an improved surface finish up to 1.6 micrometres Rmax
9. Grinding (abrasive cutting) 7 With CFG it takes 117 sec to remove 1 in.3 of material, whereas precision grinding would take more than 200 sec to do the same. CFG has the disadvantage of a wheel that is constantly degrading, and requires high spindle power, 51 hp (38 kW), and is limited in the length of part it can machine. To address the problem of wheel sharpness, continuous-dress creep-feed grinding (CDCF) was developed in the 1970s. It dresses the wheel constantly during machining, keeping it in a state of specified sharpness. It takes only 17 sec. to remove 1 in3 of material, a huge gain in productivity. 38 hp (28 kW) spindle power is required, and runs at low to conventional spindle speeds. The limit on part length was erased. High-efficiency deep grinding (HEDG) uses plated superabrasive wheels, which never need dressing and last longer than other wheels. This reduces capital equipment investment costs. HEDG can be used on long part lengths, and removes material at a rate of 1 in3 in 83 sec. It requires high spindle power and high spindle speeds. Peel grinding, patented under the name of Quickpoint in 1985 by Erwin Junker Maschinenfabrik, GmbH in Nordrach, Germany, uses a tool with a with superabrasive nose and can machine cylindrical parts. VIPER (Very Impressive Performance Extreme Removal), 1999, is a process patented by Rolls-Royce and is used in aerospace manufacturing to produce turbine blades. It uses a continuously dressed aluminum oxide grinding wheel running at high speed. CNC-controlled nozzles apply refrigerated grinding fluid during the cut. VIPER is performed on equipment similar to a CNC machining center, and uses special wheels. Ultra-high speed grinding (UHSG) can run at speeds higher than 40,000 fpm (200 m/s), taking 41 sec to remove 1 in.3 of material, but is still in the R&D stage. It also requires high spindle power and high spindle speeds. Others Form grinding is a specialized type of cylindrical grinding where the grinding wheel has the exact shape of the final product. The grinding wheel does not traverse the workpiece. Internal grinding is used to grind the internal diameter of the workpiece. Tapered holes can be ground with the use of internal grinders that can swivel on the horizontal. Centerless grinding is when the workpiece is supported by a blade instead of by centers or chucks. Two wheels are used. The larger one is Centerless grinding used to grind the surface of the workpiece and the smaller wheel is used to regulate the axial movement of the workpiece. Types of centerless grinding include through-feed grinding, in-feed/plunge grinding, and internal centerless grinding. Pre-grinding When a new tool has been built and has been heat-treated, it is pre-ground before welding or hardfacing commences. This usually involves grinding the OD slightly higher than the finish grind OD to ensure the correct finish size. Electrochemical grinding is a type of grinding in which a positively charged workpiece in a conductive fluid is eroded by a negatively charged grinding wheel. The pieces from the workpiece are dissolved into the conductive fluid. Electrolytic in-process dressing (ELID) grinding is one of the most accurate grinding methods. In this ultra precision grinding technology the grinding wheel is dressed electrochemically and in-process to maintain the accuracy of the grinding. An ELID cell consists of a metal bonded grinding wheel, a cathode electrode, a pulsed DC power supply A schematic of ELID grinding and electrolyte. The wheel is connected to the positive terminal of the
10. Grinding (abrasive cutting) 8 DC power supply through a carbon brush whereas the electrode is connected to the negative pole of the power supply. Usually alkaline liquids are used as both electrolytes and coolant for grinding. A nozzle is used to inject the electrolyte into the gap between wheel and electrode. The gap is usually maintained to be approximately 0.1mm to 0.3 mm. During the grinding operation one side of the wheel takes part in the grinding operation whereas the other side of the wheel is being dressed by electrochemical reaction. The dissolution of the metallic bond material is caused by the dressing which in turns results continuous protrusion of new sharp grits. Grinding wheel A grinding wheel is an expendable wheel used for various grinding and abrasive machining operations. It is generally made from a matrix of coarse abrasive particles pressed and bonded together to form a solid, circular shape, various profiles and cross sections are available depending on the intended usage for the wheel. Grinding wheels may also be made from a solid steel or aluminium disc with particles bonded to the surface. Lubrication The use of fluids in a grinding process is necessary to cool and lubricate the wheel and workpiece as well as remove the chips produced in the grinding process. The most common grinding fluids are water-soluble chemical fluids, water-soluble oils, synthetic oils, and petroleum-based oils. It is imperative that the fluid be applied directly to the cutting area to prevent the fluid being blown away from the piece due to rapid rotation of the wheel. Work Material Cutting Fluid Application Aluminum Light duty oil Flood Brass Light duty oil Flood Cast Iron Heavy duty emulsifiable oil, light duty chemical oil, synthetic oil Flood Mild Steel Heavy duty water soluble oil Flood Stainless Steel Heavy duty emulsifiable oil, heavy duty chemical oil, synthetic oil Flood Plastics Water soluble oil, dry, heavy duty emulsifiable oil, dry, light duty chemical oil, synthetic oil Flood The workpiece Workholding methods The workpiece is manually clamped to a lathe dog, powered by the faceplate, that holds the piece in between two centers and rotates the piece. The piece and the grinding wheel rotate in opposite directions and small bits of the piece are removed as it passes along the grinding wheel. In some instances special drive centers may be used to allow the edges to be ground. The workholding method affects the production time as it changes set up times. Workpiece materials Typical workpiece materials include aluminum, brass, plastics, cast iron, mild steel, and stainless steel. Aluminum, brass and plastics can have poor to fair machinability characteristics for cylindrical grinding. Cast Iron and mild steel have very good characteristics for cylindrical grinding. Stainless steel is very difficult to grind due to its toughness and ability to work harden, but can be worked with the right grade of grinding wheels.
11. Grinding (abrasive cutting) 9 Workpiece geometry The final shape of a workpiece is the mirror image of the grinding wheel, with cylindrical wheels creating cylindrical pieces and formed wheels creating formed pieces. Typical sizes on workpieces range from .75 in. to 20 in. (metric: 18mm to 1 m) and .80 in. to 75 in. in length (metric: 2 cm to 4 m), although pieces between .25 in. and 60 in. in diameter (metric: 6 mm to 1.5 m) and .30 in. and 100 in. in length (metric: 8 mm to 2.5 m) can be ground. Resulting shapes can range from straight cylinders, straight edged conical shapes, or even crankshafts for engines that experience relatively low torque. Effects on Workpiece Materials Mechanical properties will change due to stresses put on the part during finishing. High grinding temperatures may cause a thin martensitic layer to form on the part, which will lead to reduced material strength from microcracks. Physical property changes include the possible loss of magnetic properties on ferromagnetic materials. Chemical property changes include an increased susceptibility to corrosion because of high surface stress. References  Stephenson, David. Metal Cutting Theory and Practice. 2nd. Boca Raton: CRC Press, 1997. 52-60.  Salmon, Stuart, "What is Abrasive Machining?," Manufacturing Engineering Feb. 2010, Society of Manufacturing Engineers.  Adithan & Gupta 2002, p. 129.  (http:/ / www. sciencedirect. com/ science/ article/ B6TGJ-4NJ0TF6-C/ 2/ 677965db64474d9ca41a35f207939171)), T. Saleh, M. Sazedur Rahman, H.S. Lim, M. Rahman, Development and performance evaluation of an ultra precision ELID grinding machine, Journal of Materials Processing Technology, Volumes 192-193, Pages 287-291. Bibliography • Adithan, M.; Gupta, A. B. (2002), Manufacturing Technology (http://books.google.com/ ?id=zeGOjAOZ-sMC), New Age International Publishers, ISBN 978-81-224-0817-1.
12. Mill (grinding) 10 Mill (grinding) Attrition Mill A tabletop hammer mill Other names Grinding mill Uses Grinding Related items Mortar and pestle Expeller Extruder A grinding mill is a unit operation 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), working animal (horse mill), wind (windmill) or water (watermill). Today they are also powered by electricity. 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
13. Mill (grinding) 11 • 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 Bonds grinding coefficient for different materials can be found in various literature. To calculate the KICKs and Rittingers coefficients following formulas can be used with the limits of Bonds 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.
14. Mill (grinding) 12 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. 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 grinds 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 Operation of a ball mill 0.0011% of the total worlds 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
15. Mill (grinding) 13 • 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 and ore particles. But note that 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 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. SAG mills use a ball charge of 8 to 21%. The largest SAG mill is 42 in diameter, powered by a 28 MW (38,000 HP) motor. A SAG mill with a diameter 44 in diamter has been designed with a power of 35 MW (47,000 HP). Attrition between grinding balls and ore particles causes grinding of finer particles. SAG mills are characterized by their large diameter and short length Principle of SAG Mill operation 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.
16. Mill (grinding) 14 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. High pressure grinding rolls The high pressure grinding rolls, 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. 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 are achieving 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. 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 (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.
17. Mill (grinding) 15 Types of grinding mills • windmill, wind powered • watermill, water powered • horse mill, animal powered • treadwheel, human powered (archaic: "treadmill") • ship mill, floats near a river bank or bridge • arrastra, simple mill for grinding and pulverizing (typically) gold or silver ore. • roller mill, an equipment for the grinding or pulverizing of grain and other raw materials using cylinders • Stamp mill, 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 crushes apples to give cider • gristmill grinds grain into flour • oil mill, see expeller pressing, extrusion • paper mill produces paper • sawmill cuts timber • starch mill • steel mill manufactures steel • sugar mill (also called a sugar refinery) processes sugar beets or sugar cane into various finished products • textile mill (disambiguation) • silk mill, for silk • flax mill, for flax • cotton mill, for cotton • huller (also called a rice mill, or rice husker) is used to hull rice • powder mill produces gunpowder • Ball mill • Colloid mill • Conical mill • Disintegrator • Disk mill • Edge mill • Gristmill, also called flour mill or corn mill • Hammer mill • Jet mill • Mortar and pestle • Pellet mill • Planetary mill • Stirred mill • Vibratory mill • VSI mill • Wiley mill • Windmill
18. Mill (grinding) 16 References  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.  Mineral Beneficiation – The Third Theory of Comminution – Document Summary (http:/ / www. onemine. org/ search/ summary. cfm/ Mineral-Beneficiation--The-Third-Theory-of-Comminution?d=33A4C4B0EF40AA84EB2CE2F8607CDBC29A9E29D95F765C17CE2C0815BCC2006D18644& fullText=The Third Theory of Comminution& author=Fred Bond). Onemine.org. Retrieved on 2010-10-09.  Baron, M.; Chamayou, A.; Marchioro, L.; Raffi, J. (2005). "Radicalar probes to measure the action of energy on granular materials". Advanced Powder Technology 16 (3): 199. doi:10.1163/1568552053750242.  "ABB" (http:/ / www. abb. com/ cawp/ seitp202/ 8231666baa1c0a45c1257842002fe89b. aspx). ABB Communications. ABB Communications. .  Wills, B.A.. Mineral Processing Technology: An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery. 7th ed. Amsterdam ; Boston, MA. p. 157.  S. Strohmayr and W. Valery, Jr. SAG MILL CIRCUIT OPTIMISATION AT ERNEST HENRY MINING (http:/ / www. metso. com/ miningandconstruction/ mct_service. nsf/ WebWID/ WTB-120106-22576-29D77/ $File/ 057. pdf). metso.com  Andrew L. Mular; Doug N. Halbe; Derek J. Barratt (2002). Mineral Processing Plant Design, Practice, and Control: Proceedings (http:/ / books. google. com/ books?id=GibosO9NKWwC& pg=PA2369). SME. pp. 2369–. ISBN 978-0-87335-223-9. . Retrieved 26 October 2012.  Maarten van de Vijfeijken (October 2010). "Mills and GMDs" (http:/ / www04. abb. com/ global/ seitp/ seitp202. nsf/ 0/ bbe9642b2886578cc1257842003f2c64/ $file/ Mills+ and+ GMDs. pdf). International Mining: 30. .  Gearless mill drives (http:/ / www05. abb. com/ global/ scot/ scot244. nsf/ veritydisplay/ b5413bc88649e680c1257a5300514f7e/ $file/ Gearless mill drives_3BHS 490275 RevD_low. pdf). abb.com External links • Animation of Horizontal Grinder (http://www.vecoplanllc.com/technology/concepts/horizontal-grinder.asp) • Video of fine grinder in mining application (http://www.deswik.com/mills_deswik_video.asp) • Image of SAG mill during installation (http://www.fmt.biz/var/em_plain_site/storage/images/medien/ bilder/fmt/referenzen/stein-erde_und_zement/chelopech-mining-ead/20018-1-ger-DE/ Chelopech-Mining-EAD_overlay.jpg)
19. Sieve analysis 17 Sieve analysis A sieve analysis (or gradation test) is a practice or procedure used (commonly used in civil engineering) to assess the particle size distribution (also called gradation) of a granular material. The size distribution is often of critical importance to the way the material performs in use. A sieve analysis can be performed on any type of non-organic or organic granular materials including sands, crushed rock, clays, granite, feldspars, coal, soil, a wide range of manufactured powders, grain and seeds, down to a minimum size depending on the exact method. Being such a simple technique of particle sizing, it is probably the most common. Procedure A gradation test is performed on a sample of aggregate in a laboratory. A typical sieve analysis involves a nested column of sieves with wire mesh cloth (screen). See the separate Mesh (scale) page for details of sieve sizing. A representative weighed sample is poured into the top sieve which has the largest screen openings. Each lower sieve in the column has smaller openings than the one above. At the base is a round pan, called the receiver. The column is typically placed in a mechanical shaker. The shaker Sieves used for gradation test. shakes the column, usually for some fixed amount of time. After the shaking is complete the material on each sieve is weighed. The weight of the sample of each sieve is then divided by the total weight to give a percentage retained on each sieve. The size of the average particle on each sieve is then analysed to get a cut-off point or specific size range, which is then captured on a screen. The results of this test are used to describe the properties of the aggregate and to see if it is appropriate for various civil engineering purposes such as selecting the appropriate aggregate for concrete mixes and asphalt mixes as well as sizing of water production well screens. The results of this test are provided in graphical form to identify the type of gradation of the aggregate. The complete procedure for this test is outlined in the American Society for Testing and Materials (ASTM) C 136 and the American Association and State Highway and Transportation Officials (AASHTO) T 27 A suitable sieve size for the aggregate should be selected and placed in order of decreasing size, from top to bottom, in a mechanical sieve shaker. A pan should be placed underneath the nest of sieves to collect the aggregate that passes through the smallest. The entire nest is then agitated, and the material whose diameter is smaller than the mesh A mechanical shaker used for sieve analysis. opening pass through the sieves. After the aggregate reaches the pan, the amount of material retained in each sieve is then weighed.
20. Sieve analysis 18 Preparation In order to perform the test, a sample of the aggregate must be obtained from the source. To prepare the sample, the aggregate should be mixed thoroughly and be reduced to a suitable size for testing. The total weight of the sample is also required. Results The results are presented in a graph of percent passing versus the sieve size. On the graph the sieve size scale is logarithmic. To find the percent of aggregate passing through each sieve, first find the percent retained in each sieve. To do so, the following equation is used, %Retained = ×100% where WSieve is the weight of aggregate in the sieve and WTotal is the total weight of the aggregate. The next step is to find the cumulative percent of aggregate retained in each sieve. To do so, add up the total amount of aggregate that is retained in each sieve and the amount in the previous sieves. The cumulative percent passing of the aggregate is found by subtracting the percent retained from 100%. %Cumulative Passing = 100% - %Cumulative Retained. The values are then plotted on a graph with cumulative percent passing on the y axis and logarithmic sieve size on the x axis. There are two versions of the %Passing equations. the .45 power formula is presented on .45 power gradation chart, whereas the more simple %Passing is presented on a semi-log gradation chart. version of the percent passing graph is shown on .45 power chart and by using the .45 passing formula. .45 power percent passing formula % Passing = Pi = x100% Where: SieveLargest - Largest diameter sieve used in (mm). Aggregatemax_size - Largest piece of aggregate in the sample in (mm). Percent passing formula %Passing = x100% Where: WBelow - The total mass of the aggregate within the sieves below the current sieve, not including the current sieves aggregate. WTotal - The total mass of all of the aggregate in the sample.
21. Sieve analysis 19 Methods There are different methods for carrying out sieve analyses, depending on the material to be measured. Throw-action sieving Here a throwing motion acts on the sample. The vertical throwing motion is overlaid with a slight circular motion which results in distribution of the sample amount over the whole sieving surface. The particles are accelerated in the vertical direction (are thrown upwards). In the air they carry out free rotations and interact with the openings in the mesh of the sieve when they fall back. If the particles are smaller than the openings, they pass through the sieve. If they are larger, they are thrown upwards again. The rotating motion while suspended increases the probability that the particles present a different orientation to the mesh when they fall back again, and thus might eventually pass through the mesh. Modern sieve shakers work with an electro-magnetic drive which moves a spring-mass system and transfers the resulting oscillation to the sieve stack. Amplitude and sieving time are set digitally and are continuously observed by an integrated control-unit. Therefore sieving results are reproducible and precise (an important precondition for a significant analysis). Adjustment of parameters like amplitude and sieving time serves to optimize the sieving for different types of material. This method is the most common in the laboratory sector. Throw-action sieving Horizontal sieving In a horizontal sieve shaker the sieve stack moves in horizontal circles in a plane. Horizontal sieve shakers are preferably used for needle-shaped, flat, long or fibrous samples, as their horizontal orientation means that only a few disoriented particles enter the mesh and the sieve is not blocked so quickly. The large sieving area enables the sieving of large amounts of sample, for example as encountered in the particle-size analysis of construction materials and aggregates. Horizontal sieving
22. Sieve analysis 20 Tapping sieving A horizontal circular motion overlies a vertical motion which is created by a tapping impulse. These motional processes are characteristic of hand sieving and produce a higher degree of sieving for denser particles (e.g. abrasives) than throw-action sieve shakers. Sonic sieving The particles are lifted and forcibly dropped in a column of oscillating air at a frequency of thousands of cycles per minute. Sonic sievers are Tapping sieving able to handle much finer dry powders than woven mesh screens. Wet sieving Most sieve analyses are carried out dry. But there are some applications which can only be carried out by wet sieving. This is the case when the sample which has to be analysed is e.g. a suspension which must not be dried; or when the sample is a very fine powder which tends to agglomerate (mostly < 45 µm) – in a dry sieving process this tendency would lead to a clogging of the sieve meshes and this would make a further sieving process impossible. A wet sieving process is set up like a dry process: the sieve stack is clamped onto the sieve shaker and the sample is placed on the top sieve. Above the top sieve a water-spray nozzle is placed which supports the sieving process additionally to the sieving motion. The rinsing is carried out until the liquid which is discharged through the receiver is clear. Sample residues on the sieves have to be dried and weighed. When it comes to wet sieving it is very important not to change to sample in its volume (no swelling, dissolving or reaction with the liquid). Air Jet Sieving Air jet sieving machines are ideally suited for very fine powders which tend to agglomerate and cannot be separated by vibrational sieving. The reason for the effectiveness of this sieving method is based on two components: A rotating slotted nozzle inside the sieving chamber and a powerful industrial vacuum cleaner which is connected to the chamber. The vacuum cleaner generates a vacuum inside the sieving chamber and sucks in fresh air through the slotted nozzle. When passing the narrow slit of the nozzle the air stream is accelerated and blown against the sieve mesh, dispersing the particles. Above the mesh, the air jet is distributed over the complete sieve surface and is sucked in with low speed through the sieve mesh. Thus the finer particles are transported through the mesh openings into the vacuum cleaner.
23. Sieve analysis 21 Types of gradation Dense gradation A dense gradation refers to a sample that is approximately of equal amounts of various sizes of aggregate. By having a dense gradation, most of the air voids between the material are filled with particles. A dense gradation will result in an even curve on the gradation graph. Narrow gradation Also known as uniform gradation, a narrow gradation is a sample that has aggregate of approximately the same size. The curve on the gradation graph is very steep, and occupies a small range of the aggregate. Gap gradation A gap gradation refers to a sample with very little aggregate in the medium size range. This results in only coarse and fine Air jet sieving machine aggregate. The curve is horizontal in the medium size range on the gradation graph. Open gradation An open gradation refers an aggregate sample with very little fine aggregate particles. This results in many air voids, because there are no fine particles to fill them. On the gradation graph, it appears as a curve that is horizontal in the small size range. Rich gradation A rich gradation refers to a sample of aggregate with a high proportion of particles of small sizes. Limitations of sieve analysis Sieve analysis has, in general, been used for decades to monitor material quality based on particle size. For coarse material, sizes that range down to #100 mesh (150μm), a sieve analysis and particle size distribution is accurate and consistent. However, for material that is finer than 100 mesh, dry sieving can be significantly less accurate. This is because the mechanical energy required to make particles pass through an opening and the surface attraction effects between the particles themselves and between particles and the screen increase as the particle size decreases. Wet sieve analysis can be utilized where the material analyzed is not affected by the liquid - except to disperse it. Suspending the particles in a suitable liquid transports fine material through the sieve much more efficiently than shaking the dry material. Sieve analysis assumes that all particle will be round (spherical) or nearly so and will pass through the square openings when the particle diameter is less than the size of the square opening in the screen. For elongated and flat particles a sieve analysis will not yield reliable mass-based results, as the particle size reported will assume that the particles are spherical, where in fact an elongated particle might pass through the screen end-on, but would be prevented from doing so if it presented itself side-on.
24. Sieve analysis 22 Properties Gradation affects many properties of an aggregate. It affects bulk density, physical stability and permeability. With careful selection of the gradation, it is possible to achieve high bulk density, high physical stability, and low permeability. This is important because in pavement design, a workable, stable mix with resistance to water is important. With an open gradation, the bulk density is relatively low, due to the lack of fine particles, the physical stability is moderate, and the permeability is quite high. With a rich gradation, the bulk density will also be low, the physical stability is low, and the permeability is also low. The gradation can be affected to achieve the desired properties for the particular engineering application. Engineering applications Gradation is usually specified for each engineering application it is used for. For example, foundations might only call for coarse aggregates, and therefore an open gradation is needed. Forecast Within the last years some methods for particle size distribution measurement were developed which work by means of laser diffraction or digital image processing. "Sieving" with digital image processing The scope of information conveyed by sieve analysis is relatively small. It does not allow for a clear statement concerning the exact size of a single particle; it is just classified within a size range which is determined by two sieve sizes ("a particle is smaller than sieve size x and greater than sieve size y"). And there is no additional information concerning other relevant properties like opacity or shape available. Devices which work with digital image processing enable to recall even this information and a lot more (surface analysis, etc.). The results can be fitted to sieve analysis so that a comparison between measurement results obtained with different methods is possible. References  p231 in "Characterisation of bulk solids" by Donald Mcglinchey, CRC Press, 2005.  ASTM International - Standards Worldwide. (2006). ASTM C136-06. http:/ / www. astm. org/ cgi-bin/ SoftCart. exe/ DATABASE. CART/ REDLINE_PAGES/ C136. htm?E+ mystore  AASHTO The Voice of Transportation. T0 27. (2006). http:/ / bookstore. transportation. org/ item_details. aspx?ID=659  Pavement Interactive. Gradation Test. (2007). http:/ / pavementinteractive. org/ index. php?title=Gradation_Test  M.S. Mamlouk and J.P. Zaniewski, Materials for Civil and Construction Engineers, Addison-Wesley, Menlo Park CA, 1999 External links • The Basic Principles of Sieve Analysis (http://www.retsch.com/products/sieving/dlDetails/1/file/5930/?L=)
25. Ball mill 23 Ball mill For the type of end mill, see Ball nose cutter. 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. Ball mill Description A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally Bench top ball mill consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on the principle of critical speed. The critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the Laboratory scale ball mill direction of the cylindrical device; thus causing no further grinding. Ball mills are used extensively in the Mechanical alloying process in which they are not only used for grinding but for cold welding as well, with the purpose of producing alloys from powders.
26. Ball mill 24 High-energy ball milling The ball mill is a key piece of equipment for grinding crushed materials, and it is widely used in production lines for powders such as cement, silicates, refractory material, fertilizer, glass ceramics, etc. as well as for ore dressing of both ferrous non-ferrous metals. The ball mill can grind various ores and other materials either wet or dry. There are two kinds of ball mill, grate type and Lead antimony grinding media with aluminium overfall type due to different ways of powder. discharging material. There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Key properties of grinding media are size, density, hardness, and composition. • Size: The smaller the media particles, the smaller the particle size of the final product. At the same time, the grinding media particles should be substantially larger than the largest pieces of material to be ground. • Density: The media should be denser than the material being ground. It becomes a problem if the grinding media floats on top of the material to be ground. A ball mill inside the Mayflower Mill near Silverton, Colorado. • Hardness: The grinding media needs to be durable enough to grind the material, but where possible should not be so tough that it also wears down the tumbler at a fast pace. • Composition: Various grinding applications have special requirements. Some of these requirements are based on the fact that some of the grinding media will be in the finished product. Others are based in how the media will react with the material being ground.
27. Ball mill 25 • Where the color of the finished product is important, the color and material of the grinding media must be considered. • Where low contamination is important, the grinding media may be selected for ease of separation from the finished product (i.e.: steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). An alternative to separation is to use media of the same material as the product being ground. • Flammable products have a tendency to become explosive in powder form. Steel media may spark, becoming an ignition source for these products. Either wet-grinding, or non-sparking media such as ceramic or lead must be selected. • Some media, such as iron, may react with corrosive materials. For this reason, stainless steel, ceramic, and flint grinding media may each be used when corrosive substances are present during grinding. The grinding chamber can also be filled with an inert shield gas that does not react with the material being ground, to prevent oxidation or explosive reactions that could occur with ambient air inside the mill. Varieties Aside from common ball mills there is a second type of ball mill called planetary ball mill. Planetary ball mills are smaller than common ball mills and mainly used in laboratories for grinding sample material down to very small sizes. A planetary ball mill consists of at least one grinding jar which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars (ratio: 1:-2 or 1:-1 or else). The grinding balls in the grinding jars are subjected to superimposed rotational movements, the so-called Coriolis forces. The difference in speeds between the balls and grinding jars produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill. History Devices for shaking materials along with hard balls might be old, but it was not until the industrial revolution and the invention of steam power that a machine could be built. It is reported to have been used for grinding flint for pottery in 1870. References  M. I. Florez-Zamora et al. Comparative study of Al-Ni-Mo alloys obtained by mechanical alloying in different ball mills (http:/ / www. ipme. ru/ e-journals/ RAMS/ no_31808/ martinez3. pdf) Rev. Adv. Mater. Sci. 18 (2008) 301  Mechanical Alloying Technology (http:/ / www. imp. mtu. edu/ webform/ index. htm), Institute of Materials Processing  Lynch, A., Rowland C (2005). The history of grinding (http:/ / books. google. com/ books?id=Kj7PSOqTZ3IC& printsec=frontcover). SME. ISBN 0-87335-238-6. .
28. Filtration 26 Filtration Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size and filter thickness). Filtration is also used to describe some biological processes, especially in water treatment and Diagram of simple filtration: oversize particles in the feed cannot pass through the sewage treatment in which undesirable lattice structure of the filter, while fluid and small particles pass through, becoming constituents are removed by absorption into filtrate. a biological film grown on or in the filter medium. Applications • Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas or a supercritical fluid. Depending on the application, either one or both of the components may be isolated. • Filtration, as a physical operation is very important in chemistry for the separation of materials of different chemical composition. A solvent is chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into the solution and pass through the filter, while the other will be retained. This is one of the most important techniques used by chemists to purify compounds. • Filtration is also important and widely used as one of the unit operations of chemical engineering. It may be simultaneously combined with other unit operations to process the feed stream, as in the biofilter, which is a combined filter and biological digestion device. • Filtration differs from sieving, where separation occurs at a single perforated layer (a sieve). In sieving, particles that are too big to pass through the holes of the sieve are retained (see particle size distribution). In filtration, a multilayer lattice retains those particles that are unable to follow the tortuous channels of the filter. Oversize particles may form a cake layer on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter (blinding). Commercially, the term filter is applied to membranes where the separation lattice is so thin that the surface becomes the main zone of particle separation, even though these products might be described as sieves. • Filtration differs from adsorption, where it is not the physical size of particles that causes separation but the effects of surface charge. Some adsorption devices containing activated charcoal and ion exchange resin are commercially called filters, although filtration is not their principal function. • Filtration differs from removal of magnetic contaminants from fluids with magnets (typically lubrication oil, coolants and fuel oils), because there is no filter medium. Commercial devices called "magnetic filters" are sold, but the name reflects their use, not their mode of operation. The remainder of this article focuses primarily on liquid filtration.
29. Filtration 27 Methods There are many different methods of filtration; all aim to attain the separation of substances. Separation is achieved by some form of interaction between the substance or objects to be removed and the filter. The substance that is to pass through the filter must be a fluid, i.e. a liquid or gas. Methods of filtration vary depending on the location of the targeted material, i.e. whether it is dissolved in the fluid phase or suspended as a solid. Filter media Two main types of filter media are employed in the chemical laboratory— surface filter, a solid sieve which traps the solid particles, with or without the aid of filter paper (e.g. Büchner funnel, Belt filter, Rotary vacuum-drum filter, Cross-flow filters, Screen filter), and a depth filter, a bed of granular material which retains the solid particles as it passes (e.g. sand filter). The first type allows the solid particles, i.e. the residue, to be collected intact; the second type does not permit this. However, the second type is less prone to clogging due to the greater surface area where the particles can be trapped. Also, when the solid particles are very fine, it is often cheaper and easier to discard the contaminated granules than to clean the solid sieve. Filter media can be cleaned by rinsing with solvents or detergents. Alternatively, in engineering applications, such as swimming pool water treatment plants, they may be cleaned by backwashing. Self-cleaning screen filters utilize point-of-suction backwashing to clean the screen without interrupting system flow. Achieving flow through the filter Fluids flow through a filter due to a difference in pressure — fluid flows from the high pressure side to the low pressure side of the filter, leaving some material behind. The simplest method to achieve this is by gravity and can be seen in the coffeemaker example. In the laboratory, pressure in the form of compressed air on the feed side (or vacuum on the filtrate side) may be applied to make the filtration process faster, though this may lead to clogging or the passage of fine particles. Alternatively, the liquid may flow through the filter by the force exerted by a pump, a method commonly used in industry when a reduced filtration time is important. In this case, the filter need not be mounted vertically. Filter aid Certain filter aids may be used to aid filtration. These are often incompressible diatomaceous earth, or kieselguhr, which is composed primarily of silica. Also used are wood cellulose and other inert porous solids such as the cheaper and safer perlite. These filter aids can be used in two different ways. They can be used as a precoat before the slurry is filtered. This will prevent gelatinous-type solids from plugging the filter medium and also give a clearer filtrate. They can also be added to the slurry before filtration. This increases the porosity of the cake and reduces resistance of the cake during filtration. In a rotary filter, the filter aid may be applied as a precoat; subsequently, thin slices of this layer are sliced off with the cake. The use of filter aids is usually limited to cases where the cake is discarded or where the precipitate can be chemically separated from the filter.
30. Filtration 28 Alternatives Filtration is a more efficient method for the separation of mixtures than decantation, but is much more time consuming. If very small amounts of solution are involved, most of the solution may be soaked up by the filter medium. An alternative to filtration is centrifugation — instead of filtering the mixture of solid and liquid particles, the mixture is centrifuged to force the (usually) denser solid to the bottom, where it often forms a firm cake. The liquid above can then be decanted. This method is especially useful for separating solids which do not filter well, such as gelatinous or fine particles. These solids can clog or pass through the filter, respectively. Examples Examples of filtration include • The coffee filter to keep the coffee separate from the grounds. • HEPA filters in air conditioning to remove particles from air. • Belt filters to extract precious metals in mining. • Horizontal plate filter, also known as Sparkler filter. • Furnaces use filtration to prevent the furnace elements from fouling with particulates. • Pneumatic conveying systems often employ filtration to stop or slow the flow of material that is transported, through the use of a baghouse. • In the laboratory, a Büchner funnel is often used, with a filter paper serving as the porous barrier. An experiment to prove the existence of microscopic organisms involves the comparison of water passed through unglazed porcelain and unfiltered water. When left in sealed containers the filtered water takes longer to go foul, demonstrating that very small items (such as bacteria) can be removed from fluids by filtration. In the kidney, renal filtration is the filtration of blood in the glomerulus, followed by selective reabsorbtion of many Filter flask (suction flask, with sintered glass filter containing substances essential for the body to maintain homeostasis. sample). Note the almost colourless filtrate in the receiver flask. References  Lecture notes, Postgraduate course on Filtration and Size separation at the Department of Chemical Engineering, University of Lougborough, England  Sterlitech (http:/ / www. sterlitech. com/ 37617/ Membrane-Disc-Filters. html)  How does a Brita water filter work FAQ (http:/ / www. brita. net/ uk/ faqs_household. html?L=1#10)  Eclipse Magnetics – Magnetic filter supplier (http:/ / www. eclipse-magnetics. co. uk/ product-categories/ magneticfiltration)
31. Filtration 29 External links • The Encyclopedia of Filters - Liquid Filtration (http://www.qfilter.com/Resource.aspx/DocumentDetail/16) An overview of the many types of liquid filtration. 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 (as in rock ore), so that pieces of different composition can be differentiated. Crushing is the process of transferring a force amplified by mechanical advantage through a material made of molecules that bond together more strongly, and resist deformation more, than those in the material being crushed do. Crushing devices hold material between two parallel or tangent solid surfaces, and apply sufficient force to bring the surfaces together to generate enough energy within the material being crushed so that its molecules separate from (fracturing), or change alignment in relation to (deformation), each other. The earliest crushers were hand-held stones, where the weight of the stone provided a boost to muscle power, used against a stone anvil. Querns and mortars are types of these crushing devices. Industrial use In industry, crushers are machines which use a metal surface to break or compress materials. Mining operations use crushers, commonly classified by the degree to which they fragment the starting material, with primary and secondary crushers handling coarse materials, and tertiary and quaternary crushers reducing ore particles to finer gradations. Each crusher is designed to work with a certain maximum size of raw material, and often delivers its output to a screening machine which sorts and directs the product for further processing. Typically, crushing stages are followed by milling stages if the materials need to be further reduced. Additionally rockbreakers are typically located next to a crusher to reduce oversize material too large for a crusher. Crushers are used to reduce particle size enough so that the material can be processed into finer particles in a grinder. A typical processing line at a mine might consist of a crusher followed by a SAG mill followed by a ball mill. In this context, the SAG mill and ball mill are considered grinders rather than crushers. In operation, the raw material (of various sizes) is usually delivered to the primary crushers hopper by dump trucks, excavators or wheeled front-end loaders. A feeder device such as an apron feeder, conveyor or vibrating grid controls the rate at which this material enters the crusher, and often contains a preliminary screening device which allows smaller material to bypass the crusher itself, thus improving efficiency. Primary crushing reduces the large pieces to a size which can be handled by the downstream machinery. Some crushers are mobile and can crush rocks as large as 60 inches. Primarily used in-pit at the mine face these units are able to move with the large infeed machines (mainly shovels) to increase the tonnage produced. In a mobile road operation, these crushed rocks are directly combined with concrete and asphalt which are then deposited on to a road surface. This removes the need for hauling over-sized material to a stationary crusher and then back to the road surface.
32. Crusher 30 Types of crushers Portable Close Circuit Cone Crushing Plant Cornish stamps used in the 19th A portable rock crusher from The entrance bin of a mine rock Mobile crusher century for breaking tin ore the early 20th century crusher The following table describes typical uses of commonly used crushers: Type Hardness Abrasion Moisture content Reduction Main use limit ratio Jaw crushers Soft to very hard No limit Dry to slightly wet, 3/1 to 5/1 Heavy mining, Quarried materials, not sticky sand & gravel, recycling Gyratory crushers Soft to very hard Abrasive Dry to slightly wet, 4/1 to 7/1 Heavy mining, Quarried materials not sticky Cone crushers Medium hard to Abrasive Dry or wet, not 3/1 to 5/1 Quarried materials, Sand & gravel very hard sticky Compound crusher Medium hard to Abrasive Dry or wet, not 3/1 to 5/1 Mine, Building Materials very hard sticky Horizontal shaft impactors Soft to medium Slightly Dry or wet, not 10/1 to 25/1 Quarried materials, sand & gravel, hard abrasive sticky recycling Vertical shaft impactors Medium hard to Slightly Dry or wet, not 6/1 to 8/1 Sand & gravel, recycling (shoe and anvil) very hard abrasive sticky Vertical shaft impactors Soft to very hard No limit Dry or wet, not 2/1 to 5/1 Quarried materials, sand & gravel (autogenous) sticky Mineral sizers Hard to soft Abrasive Dry or wet and sticky 2/1 to 5/1 Heavy mining
33. Crusher 31 Jaw crusher A jaw or toggle crusher consists of a set of vertical jaws, one jaw being fixed and the other being moved back and forth relative to it by a cam or pitman mechanism, acting as a class II lever, like a nutcracker. The jaws are farther apart at the top than at the bottom, forming a tapered chute so that the material is crushed progressively smaller and smaller as it travels downward until it is small enough to escape from the bottom opening. The movement of the jaw can be quite small, since complete crushing is not performed in one stroke. The inertia required to crush the material is provided by a weighted flywheel that moves a Operation of a jaw crusher shaft creating an eccentric motion that causes the closing of the gap. Single and double toggle jaw crushers are constructed of heavy duty fabricated plate frames with reinforcing ribs throughout. The crushers components are of high strength design to accept high power draw. Manganese steel is used for both fixed and movable jaw faces. Heavy flywheels allow crushing peaks on tough materials. Double Toggle jaw crushers may feature hydraulic toggle adjusting mechanisms. There are 3 types of jaw crushers according to the place the movable plate has been fixed around which position the rotates the movable jaw. 1. Blake crusher-fixed in the lower point 2. Dodge crusher-fixed in the upper point 3. Universal crusher-fixed in the midpoint Gyratory crusher A gyratory crusher is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head; both surfaces are typically lined with manganese steel surfaces. The inner cone has a slight circular movement, but does not rotate; the movement is generated by an eccentric arrangement. As with the jaw crusher, material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces. A gyratory crusher is one of the main types of primary crushers in a mine or ore processing plant. Gyratory crushers are designated in size either by the gape and mantle diameter or by the size of the receiving opening. Gyratory crushers can be used for primary or secondary crushing. The crushing action is caused by the closing of the gap between the mantle line (movable) mounted on the central vertical spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric on the Ruffner Red Ore Mine gyratory crusher bottom of the spindle that causes the central vertical spindle to gyrate. The vertical spindle is free to rotate around its own axis. The crusher illustrated is a short-shaft suspended spindle type, meaning that the main shaft is suspended at the top and that the eccentric is mounted above the gear. The short-shaft design has superseded the long-shaft design in which the eccentric is mounted below the gear.
34. Crusher 32 Cone crusher A cone crusher is similar in operation to a gyratory crusher, with less steepness in the crushing chamber and more of a parallel zone between crushing zones. A cone crusher breaks rock by squeezing the rock between an eccentrically gyrating spindle, which is covered by a wear resistant mantle, and the enclosing concave hopper, covered by a manganese concave or a bowl liner. As rock enters the top of the cone crusher, it becomes wedged and squeezed between the mantle and the bowl liner or concave. Large pieces of ore are broken once, and then fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough Cone Crusher to fall through the narrow opening at the bottom of the crusher. A cone crusher is suitable for crushing a variety of mid-hard and above mid-hard ores and rocks. It has the advantage of reliable construction, high productivity, easy adjustment and lower operational costs. The spring release system of a cone crusher acts an overload protection that allows tramp to pass through the crushing chamber without damage to the crusher. Impact crusher Impact crushers involve the use of impact rather than pressure to crush material. The material is contained within a cage, with openings on the bottom, end, or side of the desired size to allow pulverized material to escape. There are two types of impact crushers: horizontal shaft impactor and vertical shaft impactor. Horizontal shaft impactor (HSI) / Hammer mill The HSI crushers break rock by impacting the rock with hammers that are fixed upon the outer edge of a spinning rotor. HSI machines are sold in Stationary, trailer mounted and crawler mounted configurations. HSIs are used in recycling, hard rock and soft materials. In earlier years the practical use of HSI crushers is limited to soft materials and non abrasive materials, such as limestone, phosphate, gypsum, weathered shales, however improvements in metalurgy has changed the application of these machines.HS Vertical shaft impactor (VSI) VSI crushers use a different approach involving a high speed rotor with wear resistant tips and a crushing chamber designed to throw the rock against. The VSI crushers utilize velocity rather than surface force as the predominant force to break rock. In its natural state, rock has a jagged and uneven surface. Applying surface force (pressure) results in unpredictable and typically non-cubical resulting particles. Utilizing velocity rather than surface force allows the breaking force to be applied evenly both across the surface of the rock as well as through the mass of the rock. Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is thrown by a VSI Rotor against a solid anvil, it fractures and breaks along these fissures. Final particle size can be controlled by 1) the velocity at which the rock is thrown against the anvil and 2) the distance between the end of the Scheme of a VSI crusher with air-cushion support
35. Crusher 33 rotor and the impact point on the anvil. The product resulting from VSI Crushing is generally of a consistent cubical shape such as that required by modern SUPERPAVE highway asphalt applications. Using this method also allows materials with much higher abrasiveness to be crushed than is capable with an HSI and most other crushing methods. VSI crushers generally utilize a high speed spinning rotor at the center of the crushing chamber and an outer impact surface of either abrasive resistant metal anvils or crushed rock. Utilizing cast metal surfaces anvils is traditionally referred to as a "Shoe and Anvil VSI". Utilizing crushed rock on the outer walls of the crusher for new rock to be VSI crusher crushed against is traditionally referred to as "rock on rock VSI". VSI crushers can be used in static plant set-up or in mobile tracked equipment. Mineral sizers The basic concept of the mineral sizer is the use of two rotors with large teeth, on small diameter shafts, driven at a low speed by a direct high torque drive system. This design produces three major principles which all interact when breaking materials using sizer technology. The unique principles are the three-stage breaking action, the rotating screen effect, and the deep scroll tooth pattern. The three-stage breaking action: initially, the material is gripped by the leading faces of opposed rotor teeth. These subject the rock to multiple point loading, inducing stress into the material to exploit any natural weaknesses. At the second stage, material is broken in tension by being subjected to a three point loading, applied between the front tooth faces on one rotor, and rear tooth faces on the other rotor. Any lumps of material that still remain oversize, are broken as the rotors chop through the fixed teeth of the breaker bar, thereby achieving a three dimensional controlled product size. The rotating screen effect: The interlaced toothed rotor design allows free flowing undersize material to pass through the continuously changing gaps generated by the relatively slow moving shafts. The deep scroll tooth pattern: The deep scroll conveys the larger material to one end of the machine and helps to spread the feed across the full length of the rotors. This feature can also be used to reject oversize material from the machine. Technology For the most part advances in crusher design have moved slowly. Jaw crushers have remained virtually unchanged for sixty years. More reliability and higher production have been added to basic cone crusher designs that have also remained largely unchanged. Increases in rotating speed have provided the largest variation. For instance, a 48 inch (120 cm) cone crusher manufactured in 1960 may be able to produce 170 tons/h of crushed rock, whereas the same size crusher manufactured today may produce 300 tons/h. These production improvements come from speed increases and better crushing chamber designs. The largest advance in cone crusher reliability has been seen in the use of hydraulics to protect crushers from being damaged when uncrushable objects enter the crushing chamber. Foreign objects, such as steel, can cause extensive damage to a cone crusher, and additional costs in lost production. The advance of hydraulic relief systems has greatly reduced downtime and improved the life of these machines.
36. Crusher 34 References  The MMD Group of Companies."MMD Sizers". The MMD Group of Companies, 2005, p 3. Pulverizer A pulverizer or grinder is a mechanical device for the grinding of many different types of materials. For example, they are used to pulverize coal for combustion in the steam-generating furnaces of fossil fuel power plants. Types of pulverizers Coal pulverizers may be classified by speed, as follows: • Low Speed • Medium Speed • High Speed Low Speed Ball and tube mills A ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods. A tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as slime (slurry). Both types of mill include liners that protect the cylindrical structure of the mill from wear. Thus the main wear parts in these mills are the balls themselves, and the liners. The balls are simply "consumed" by the wear process and must be re-stocked, whereas the liners must be periodically replaced. The ball and tube mills are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. Due to its shape only, people call it as Tube Mill and due to use of Grinding Balls for crushing, it is called Ball Mill. Hence, is the name Ball Tube Mill. These Mills are also designated as BBD-4772, Where- B – Broyer (Name of inventor). B – Boulet (French word for Balls). D – Direct firing. 47 – Diameter of shell (in Decimeters) i.e. 4.7m dia. 72 – Length of shell (in Decimeters) i.e. 7.2 m length By the name the grinding in the ball and tube mill is produced by rotating quantity of steel balls by their fall and lift due to rotation of tube. The ball charge may occupy one third to half of the total internal volume of the shell. The significant feature incorporated in the BBD mills is its double end operation, each end catering to one elevation of a boiler. The system facilitated entry of raw coal and outlet of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit. Mill Constructions And Details A ball Tube mill may be described as a cylinder made of steel plate having separate heads or trunion attached to the ends with the trunion resting on suitable bearings for supporting the machine. The trunion are hollow to allow for the introduction of discharge of the materials undergoing reduction in size. The mill shell is lined with chilled iron, carbon steel, manganese steel, High Chrome liners attached to shell body with counter sunk bolts.These liners are made in different shapes so that the counter inside surface of the mill is suited for requirement of application. The Shells are of three pieces. The Intermediate shell connects to the end shells by flange joints and the total length of shell is 7.2 m. The liners are fastened to the inner side of mill shell (cylindrical part) to protect the shell from the
37. Pulverizer 35 impact of steel balls. There are 600 nos. of liners of ten variants in each shell weighing 60.26 MT. The original lift value of the liners is 55 mm. and the minimum lift allowed is 20 mm. Working Primary air in the case of Tube Mill have dual function to perform. It is used as drying as well as transporting media and by regulating the same the Mill output is regulated. Governed by the pulverize fuel outlet temperature requirement the combination of cold air and hot air dampers are regulated to have proper primary air temperature. In addition to raising the coal temperature Inside the Mill for drying and better grinding the same air works carrying media for pulverized coal through annular space between fixed trunnion tube and rotating hot air tube on way to classifier. Coal-laden air passing through Double cone static classifiers with adjustable classifier vanes for segregation Into pulverized fuel of desired fineness and coarse particles continues its journey towards coal burners for combustion. Coarse particles rejected in classifier find their way back to mill for another cycle of grinding. In order to avoid excess sweeping of coal from Mill Only Part Of the primary air, directly proportional to the boiler load demand is passed through Mill. Further to ensure and maintain sufficient velocity of pulverized fuel and to avoid settling in P.F. pipes an additional quantity of primary air is fed in to mixing box on raw coat circuit. This by-pass air tapped from the primary air duct going in Mill makes appreciable contribution for drying of raw coal by flash drying effect in addition to picking pp the pulverized fuel from Mill outlet for its transportation towards classifiers. Tube mill output while responding to boiler load demand is controlled by regulating primary air-flow. Such regulation by sweeping away of pulverized fuel from Mill being very fast rather well comparable with oil firing response, needs coal level to be maintained in the Mill. Mill level control circuit sensing the decreased coat level in Mill increases the speed of raw coal feeder and vice avers. Maintaining the coal level in Mill offers built-in-capacity cushion of pulverized fuel to take care of short interruption in raw coal circuit. The mill is pressurized and the tightness is ensured by plenum chambers around the rotating trunnion filled with pressurized seal air. Bleeding seal air from plenum chamber to Mill provides air cushion between pulverized fuel in the Mill and the outside atmosphere. Inadequacy or absence of seal air will allow escape of pulverized fuel into atmosphere. On the other hand excess of seal air leaking into Mill will affect the Mill outlet temperature. As such the seal air is controlled by a local control damper by maintaining just sufficient differential pressure for sealing. Medium Speed Ring and ball mill This type of mill consists of two rings separated by a series of large balls, like a thrust bearing. The lower ring rotates, while the upper ring presses down on the balls via a set of spring and adjuster assemblies, or pressurised rams. The material to be pulverized is introduced into the center or side of the pulverizer (depending on the design). As the lower ring rotates, the balls to orbit between the upper and lower rings, and balls roll over the bed of coal on the lower ring. The pulverized material is carried out of the mill by the flow of air moving through it. The size of the pulverized particles released from the grinding section of the mill is determined by a classifier separator - if the coal is fine enough to be picked up by the air, it is carried through the classifier. Coarser particles return to be further pulverized. Vertical spindle roller mill Similar to the ring and ball mill, this mill uses large "tires" to crush the coal. These are usually found in utility plants. Raw coal is gravity-fed through a central feed pipe to the grinding table where it flows outwardly by centrifugal action and is ground between the rollers and table. Hot primary air for drying and coal transport enters the windbox plenum underneath the grinding table and flows upward through a swirl ring having multiple sloped nozzles surrounding the grinding table. The air mixes with and dries coal in the grinding zone and carries pulverized coal particles upward into a classifier.
38. Pulverizer 36 Fine pulverized coal exits the outlet section through multiple discharge coal pipes leading to the burners, while oversized coal particles are rejected and returned to the grinding zone for further grinding. Pyrites and extraneous dense impurity material fall through the nozzle ring and are plowed, by scraper blades attached to the grinding table, into the pyrites chamber to be removed. Mechanically, the vertical roller mill is categorized as an applied force mill. There are three grinding roller wheel assemblies in the mill grinding section, which are mounted on a loading frame via pivot point. The fixed-axis roller in each roller wheel assembly rotates on a segmentally-lined grinding table that is supported and driven by a planetary gear reducer direct-coupled to a motor. The grinding force for coal pulverization is applied by a loading frame. This frame is connected by vertical tension rods to three hydraulic cylinders secured to the mill foundation. All forces used in the pulverizing process are transmitted to the foundation via the gear reducer and loading elements. The pendulum movement of the roller wheels provides a freedom for wheels to move in a radial direction, which results in no radial loading against the mill housing during the pulverizing process. Depending on the required coal fineness, there are two types of classifier that may be selected for a vertical roller mill. The dynamic classifier, which consists of a stationary angled inlet vane assembly surrounding a rotating vane assembly or cage, is capable of producing micron fine pulverized coal with a narrow particle size distribution. In addition, adjusting the speed of the rotating cage can easily change the intensity of the centrifugal force field in the classification zone to achieve coal fineness control real-time to make immediate accommodation for a change in fuel or boiler load conditions. For the applications where a micron fine pulverized coal is not necessary, the static classifier, which consists of a cone equipped with adjustable vanes, is an option at a lower cost since it contains no moving parts. With adequate mill grinding capacity, a vertical mill equipped with a static classifier is capable of producing a coal fineness up to 99.5% or higher <50 mesh and 80% or higher <200 mesh, while one equipped with a dynamic classifier produces coal fineness levels of 100% <100 mesh and 95% <200 mesh, or better. In 1954 a Jet Pulverizer was developed in which operates like a Vertical Pulverizer only the item is pulverized by the high speed air action. For example forcing coal against coal.  Bowl mill Similar to the vertical roller mill, it also uses tires to crush coal. There are two types, a deep bowl mill, and a shallow bowl mill. High Speed Attrition Mill Rotor, Stationary Pegs Hammer Mill Used on farms for grinding grain and chaff for feed Demolition pulverizer An attachment fitted to an excavator. Commonly used in demolition work to break up large pieces of concrete.
39. Pulverizer 37 References  Coal Pulverising Mill Types, by Glenn Schumacher, 2010  "Jet Pulverizer." (http:/ / books. google. com/ books?id=Nd8DAAAAMBAJ& pg=PA156& dq=1954+ Popular+ Mechanics+ January& hl=en& sa=X& ei=Q3YzT6TaKu2o0AHgjvW_Ag& ved=0CDMQ6AEwATgK#v=onepage& q& f=true) Popular Mechanics, April 1954, p. 156. Bibliography • Schumacher, Glenn (201). Coal Pulverising Mill Types. ISBN 978-0-646-53759-7. Froth flotation Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries. Historically this was first used in the mining industry. History Initially, naturally occurring chemicals such as fatty acids and oils were used as flotation reagents in a large quantity to increase the hydrophobicity of the valuable minerals. Since then, the process has been adapted and applied to a wide variety of materials to be separated, and additional collector agents, including surfactants and synthetic compounds have been adopted for various applications. William Haynes in 1869 patented a process Diagram of a cylindrical froth flotation cell with camera and light used in image for separating sulfide and gangue minerals analysis of the froth surface. using oil and called it bulk-oil flotation. In 1885 Carrie Everson expanded upon this and patented a process calling for oil[s] but also an acid or a salt. The first successful commercial flotation process for mineral sulphides was invented by Frank Elmore who worked on the development with his brother, Stanley. The Glasdir copper mine at Llanelltyd, near Dolgellau in North Wales was bought in 1896 by the Elmore brothers in conjunction with their father, William. In 1897, the Elmore brothers installed the worlds first industrial size commercial flotation process for mineral beneficiation at the Glasdir mine. The process was not froth flotation but used oil to agglomerate (make balls of) pulverised sulphides and buoy them to the surface, and was patented in 1898 with a description of the process published in 1903 in the Engineering and Mining Journal. By this time they had recognized the importance of air bubbles in assisting the oil to carry away the mineral particles. The Elmores had formed a company known as the Ore Concentration Syndicate Ltd to promote the commercial use of the process worldwide. However developments elsewhere, particularly in Australia by Minerals Separation Ltd., led to decades of hard fought legal battles and litigations which, ultimately, were lost as the process was superseded by more advanced techniques. Charles Butters, beginning about 1899, and working with both the Elmores and Minerals Separations representative E.H. Nutter developed what was known to
40. Froth flotation 38 contemporaries as the "Butters Process".  The flotation process was independently invented in the early 1900s in Australia by Charles Vincent Potter and around the same time by Guillaume Daniel Delprat..  This process (developed circa 1902) did not use oil, but relied upon flotation by the generation of gas formed by the introduction of acid into the pulp. In 1902, Froment combined oil and gaseous flotation using a modification of the Potter-Delprat process. Another process was developed in 1902 by Cattermole, who emulsified the pulp with a small quantity of oil, subjected it to violent agitation, then slow stirring which coagulated the target minerals into nodules which were separated from the pulp by gravity. This was the basis of the Minerals Separation Ltd. process. By 1904, the MacQuisten process (a surface tension based method) was developed but this would not work when slimes were present. in 1912 Hyde modified the Minerals Separation Process and installed it in the Butte and Superior Mill in Basin, Montana.  John M. Callow, of General Engineering of Salt Lake City, had followed flotation from technical papers and the introduction in both the Butte and Superior Mill, and at Inspiration Copper in Arizona and determined that mechanical agitation was a drawback to the existing technology. Introducing a porous brick with compressed air, and a mechanical stirring mechanism, Callow applied for a patent in 1914. This method, known as Pneumatic Flotation, was recognized to revolutionize the process of flotation concentration. A detailed description of the history of flotation and this process can be found in Callows "Notes on Flotation" found in the Transactions of the American Institute of Mining Engineers; Vol 53-54, originally presented in New York in February 1916. The AIME presented Callow the James Douglas Gold Medal in 1926 for his contributions to the field of flotation. In the 1960s the froth flotation technique was adapted for deinking recycled paper. Industries Mining Froth flotation is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and waste gangue are increased through the use of surfactants and wetting agents. The selective separation of the minerals makes processing complex (that is, mixed) ores economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphates and coal are also Froth flotation to separate plastics, Argonne upgraded (purified) by flotation technology. National Laboratory Waste water treatment The flotation process is also widely used in industrial waste water treatment plants, where it removes fats, oil, grease and suspended solids from waste water. These units are called Dissolved air flotation (DAF) units. In particular, dissolved air flotation units are used in removing oil from the wastewater effluents of oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industrial facilities. Froth flotation cells to concentrate copper and nickel sulfide minerals, Falconbridge, Ontario.
41. Froth flotation 39 Paper recycling Froth flotation is one of the processes used to recover recycled paper. In the paper industry this step is called deinking or just flotation. The target is to release and remove the hydrophobic contaminants from the recycled paper. The contaminants are mostly printing ink and stickies. Normally the setup is a two stage system with 3,4 or 5 flotation cells in series. Principle of operation Froth flotation commences by comminution (that is, crushing and grinding), which is used to increase the surface area of the ore for subsequent processing and break the rocks into the desired mineral and gangue in a process known as liberation, which then has to be separated from the desired mineral. The ore is ground into a fine powder and mixed with water to form a slurry. The desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical. The particular chemical depends on which mineral is being refined. As an example, SEX is added as a collector in the selective flotation of galena and sphalerite, after the addition of other flotation reagents. This slurry (more properly called the pulp) of hydrophobic particles and hydrophilic particles is then introduced to a water bath which is aerated, creating bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed and the concentrate (con) is further refined. Science of flotation To be effective on a given ore slurry, the collectors are chosen based upon their selective wetting of the types of particles to be separated. A good collector will adsorb, physically or chemically, with one of the types of particles. This provides the thermodynamic requirement for the particles to bind to the surface of a bubble. The wetting activity of a surfactant on a particle can be quantified by measuring the contact angles that the liquid/bubble interface makes with it. Another important measure for attachment of bubbles to particles is induction time. The induction time is the time required for the particle and bubble to rupture the thin film separating the particle and bubble. This rupturing is achieved by the surface forces between the particle and bubble. The mechanisms for the bubble-particle attachment is very complex and consists of three steps, collision, attachment and detachment. The collision is achieved by particles being within the collision tube of a bubble and this is affected by the velocity of the bubble and radius of the bubble. The collision tube corresponds to the region in which a particle will collide with the bubble, with the perimeter of the collision tube corresponding to the grazing trajectory. The attachment of the particle to the bubble is controlled by the induction time of the particle and bubble. The particle and bubble need to bind and this occurs if the time in which the particle and bubble are in contact with each other is larger than the required induction time. This induction time is effected by the fluid viscosity, particle and bubble size and the forces between the particle and bubbles. The detachment of a particle and bubble occurs when the force exerted by the surface tension is exceeded by shear forces and gravitational forces. These forces are complex and vary within the cell. High shear will be experienced close to the impeller of a mechanical flotation cell and mostly gravitational force in the collection and cleaning zone of a flotation column. Significant issues of entrainment of fine particles occurs as these particles experience low collision efficiencies as well as sliming and degradation of the particle surfaces. Coarse particles show a low recovery of the valuable mineral due to the low liberation and high detachment efficiencies.
42. Froth flotation 40 Flotation equipment Flotation can be performed in rectangular or cylindrical mechanically agitated cells or tanks, flotation columns, Jameson cells or deinking flotation machines. Mechanical cells use a large mixer and diffuser mechanism at the bottom of the mixing tank to introduce air and provide mixing action. Flotation columns use air spargers to introduce air at the bottom of a tall column while introducing slurry above. The countercurrent motion of the slurry flowing down and the air flowing up provides mixing action. Mechanical cells generally have a Diagram of froth flotation cell. Numbered triangles show direction higher throughput rate, but produce material that is of of stream flow. A mixture of ore and water called pulp  enters lower quality, while flotation columns generally have a the cell from a conditioner, and flows to the bottom of the cell. Air low throughput rate but produce higher quality material.  or nitrogen is passed down a vertical impeller where shearing forces break the air stream into small bubbles. The mineral The Jameson cell uses neither impellers nor spargers, concentrate froth is collected from the top of the cell , while the instead combining the slurry with air in a downcomer pulp  flows to another cell. where high shear creates the turbulent conditions required for bubble particle contacting. Mechanics of flotation The following steps are followed, following grinding to liberate the mineral particles: 1. Reagent conditioning to achieve hydrophobic surface charges on the desired particles 2. Collection and upward transport by bubbles in an intimate contact with air or nitrogen 3. Formation of a stable froth on the surface of the flotation cell 4. Separation of the mineral laden froth from the bath (flotation cell) Simple flotation circuit for mineral concentration. Numbered triangles show direction of stream flow, Various flotation reagents are added to a mixture of ore and water (called pulp) in a conditioning tank. The flow rate and tank size are designed to give the minerals enough time to be activated. The conditioner pulp  is fed to a bank of rougher cells which remove most of the desired minerals as a concentrate. The rougher pulp  passes to a bank of scavenger cells where additional reagents may be added. The scavenger cell froth  is usually returned to the rougher cells for additional treatment, but in some cases may be sent to special cleaner cells. The scavenger pulp is usually barren enough to be discarded as tails. More complex flotation circuits have several sets of cleaner and re-cleaner cells, and intermediate re-grinding of pulp or concentrate.
43. Froth flotation 41 Chemicals of flotation Collectors Collectors either chemically bond (chemisorption) on a hydrophobic mineral surface, or adsorb onto the surface in the case of, for example, coal flotation through physisorption. Collectors increase the natural hydrophobicity of the surface, increasing the separability of the hydrophobic and hydrophilic particles. Xanthates • Potassium amyl xanthate (PAX) • Potassium isobutyl xanthate (PIBX) • Potassium ethyl xanthate (KEX) • Sodium isobutyl xanthate (SIBX) • Sodium isopropyl xanthate (SIPX) • Sodium ethyl xanthate (SEX) Dithiophosphates • Thiocarbamates • Xanthogen Formates • Thionocarbamates • Thiocarbanilide Palmatic acid Amines Frothers • Pine oil • Alcohols (methyl isobutyl carbinol (MIBC)) • Polyglycols • Polyoxyparafins| • Cresylic Acid (Xylenol) Modifiers pH modifiers such as: • Lime CaO • Soda ash Na2CO3 • Caustic soda NaOH • Acid H2SO4, HCl Cationic modifiers: • Ba2+, Ca2+, Cu+, Pb2+, Zn2+, Ag+ Anionic modifiers: • SiO32-, PO43-, CN-, CO32-, S2- Organic modifiers: • Dextrin, starch, glue, CMC
44. Froth flotation 42 Chemicals for deinking of recycled paper • pH control: sodium silicate and sodium hydroxide • Calcium ion source: hard water, lime or calcium chloride • Collector: fatty acid, fatty acid emulsion, fatty acid soap and/or organo-modified siloxane Specific ore applications Sulfide ores • Copper (see copper extraction) • Copper-Molybdenum • Lead-Zinc • Lead-Zinc-Iron • Copper-Lead-Zinc-Iron • Gold-Silver • Oxide Copper and Lead • Nickel • Nickel-Copper Nonsulfide ores • Fluorite • Tungsten • Lithium • Tantalum • Tin • Coal References  "Wales - The birthplace of Flotation" (http:/ / www. maelgwyn. com/ birthplaceflotation. html#top). . Retrieved 2010-01-13.  Rickard, Thomas A. (1922). Interviews with Mining Engineers. San Francisco: Mining and Scientific Press. pp. 119–131.  Osborne, Graeme (1981). "Guillaume Daniel Delprat" (http:/ / adb. anu. edu. au/ biography/ delprat-guillaume-daniel-5947). Australian Dictionary of Biography. Canberra: Australian National University. . Retrieved 7 June 2012.  "Historical Note" (http:/ / www. austehc. unimelb. edu. au/ guides/ mine/ historicalnote. htm). Minerals Separation Ltd. . Retrieved 2007-12-30.  Callow; 1916  Rickard, Thomas A. (1922). Interviews with Mining Engineers. San Francisco: Mining and Scientific Press. pp. 142.  Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons Ltd.. LCCN 67019834.  Voith EcoCell flotation plant http:/ / www. voithpaper. com/ applications/ productsearch/ files/ 594_VPR-PB-07-0001-GB-07. pdf  METHOD OF DEINKING. 2004 (published 05.02.2004).
45. Mechanical screening 43 Mechanical screening Mechanical screening, often just called screening, is the practice of taking granulated ore material and separating it into multiple grades by particle size. This practice occurs in a variety of industries such as mining and mineral processing, agriculture, pharmaceutical, food, plastics, and recycling. General categories Screening falls under two general categories: dry screening and wet screening. From these categories, screening separates a flow of material into grades, these grades are then either further processed to an intermediary product or a finished product. Additionally the machines can be categorised into moving screen and static screen machines, as well as by whether the screens are horizontal or inclined. Applications The mining and mineral processing industry uses screening for a variety of processing applications. For example, after mining the minerals, the material is transported to a primary crusher. Before crushing large boulder are scalped on a shaker with 0.25 in (6.4 mm) thick shielding screening. Further down stream after crushing the material can pass through screens with openings or slots that continue to become smaller. Finally, screening is used to make a final separation to produce saleable products based on a grade or a size range. Process A screening machine consist of a drive that induces vibration, a screen cloth that causes particle separation, and a deck which holds the screen cloth and the drive and is the mode of transport for the vibration. There are physical factors that makes screening practical. For example, vibration, g force, bed density, and material shape all facilitate the rate or cut. Electrostatic forces can also hinder screening efficiency in way of water attraction causing sticking or plugging, or very dry material generate a charge that causes it to attract to the screen itself. As with any industrial process there is a group of terms that identify and define what screening is. Terms like blinding, contamination, frequency, amplitude, and others describe the basic characteristics of screening, and those characteristics in turn shape the overall method of dry or wet screening. In addition, the way a deck is vibrated differentiates screens. Different types of motion have their advantages and disadvantages. In addition cloth types also have their different properties that lead to advantages and disadvantages. Finally, there are issues and problems associated with screening. Screen tearing, contamination, blinding, and dampening all affect screening efficiency.
46. Mechanical screening 44 Physical principles • Vibration - either sinusoidal vibration or gyratory vibration. • Sinusoidal Vibration occurs at an angled plane relative to the horizontal. The vibration is in a wave pattern determined by frequency and amplitude. • Gyratory Vibration occurs at near level plane at low angles in a reciprocating side to side motion. • Gravity - This physical interaction is after material is thrown from the screen causing it to fall to a lower level. Gravity also pulls the particles through the screen cloth. • Density - The density of the material relates to material stratification. • Electrostatic Force - This force applies to screening when particles are extremely dry or is wet. Screening terminology Like any mechanical and physical entity there are scientific, industrial, and layman terminology. The following is a partial list of terms that are associated with mechanical screening. • Amplitude - This is a measurement of the screen cloth as it vertically peaks to its tallest height and troughs to its lowest point. Measured in multiples of the acceleration constant g (g-force). • Acceleration - Applied Acceleration to the screen mesh in order to overcome the van der waal forces • Blinding - When material plugs into the open slots of the screen cloth and inhibits overflowing material from falling through. • Brushing - This procedure is performed by an operator who uses a brush to brush over the screen cloth to dislodged blinded opening. • Cloth, screening cloth - it is the material defined by mesh size, which can be made of any type of material such steel, stainless steel, rubber compounds, brass, etc. • Contamination - This is unwanted material in a given grade. This occurs when there is oversize or fine size material relative to the cut or grade. Another type of contamination is foreign body contamination. • Oversize contamination occurs when there is a hole in the screen such that the hole is larger than the mesh size of the screen. Other instances where oversize occurs is material overflow falling into the grade from overhead, or there is the wrong mesh size screen in place. • Fines contamination is when large sections of the screen cloth is blinded over, and material flowing over the screen does not fall through. The fines are then retained in the grade. • Foreign body contamination is unwanted material that differs from the virgin material going over and through the screen. It can be anything ranging from tree twigs, grass, metal slag to other mineral types and composition. This contamination occurs when there is a hole in the scalping screen or a foreign materials mineralogy or chemical composition differs from the virgin material. • Deck - a deck is frame or apparatus that holds the screen cloth in place. It also contains the screening drive. It can contain multiple sections as the material travels from the feed end to the discharge end. Multiple decks are screen decks placed in a configuration where there are a series of decks attached vertically and lean at the same angle as it preceding and exceeding decks. Multiple decks are often referred to as single deck, double deck, triple deck, etc. • Frequency - Measured in hertz (Hz) or revolutions per minute (RPM). Frequency is the number of times the screen cloth sinusoidally peaks and troughs within a second. As for a gyratory screening motion it is the number of revolutions the screens or screen deck takes in a time interval, such as revolution per minute (RPM). • Gradation, grading - Also called "cut" or "cutting." Given a feed material in an initial state, the material can be defined to a have a particle size distribution. Grading is removing the maximum size material and minimum size material by way of mesh selection. • Shaker - A generic term that refers to the whole assembly of any type mechanical screening machine.
47. Mechanical screening 45 • Stratification - This phenomenon occurs as vibration is passed through a bed of material. This causes coarse (larger) material to rise and finer (smaller) material to descend within the bed. The material in contact with screen cloth either falls through a slot or blinds the slot or contacts the cloth material and is thrown from the cloth to fall to the next lower level. • Mesh - Mesh refers to the number of open slots per linear inch. Mesh is arranged in multiple configuration. Mesh can be a square pattern, long-slotted rectangular pattern, circular pattern, or diamond pattern. • Scalp, scalping - this is the very first cut of the incoming material with the sum of all its grades. Scalping refers to removing the largest size particles. This includes enormously large particles relative to the other particles sizes. Scalping also cleans the incoming material from foreign body contamination such as twigs, trash, glass, or other unwanted oversize material. Types of mechanical screening There are a number of types of mechanical screening equipment that cause segregation. These types are based on the motion of the machine through its motor drive. • Circle-throw vibrating equipment - This type of equipment has an eccentric shaft that causes the frame of the shaker to lurch at a given angle. This lurching action literally throws the material forward and up. As the machine returns to its base state the material falls by gravity to physically lower level. This type of screening is used also in mining operations for large material with sizes that range from six inches to +20 mesh. • High frequency vibrating equipment - This type of equipment drives the screen cloth only. Unlike above the frame of the equipment is fixed and only the screen vibrates. However, this equipment is similar to the above such that it still throws material off of it and allows the particles to cascade down the screen cloth. These screens are for sizes smaller than 1/8 of an inch to +150 mesh. • Gyratory equipment - This type of equipment differs from the above two such that the machine gyrates in a circular motion at a near level plane at low angles. The drive is an eccentric gear box or eccentric weights. • Trommel screens - Does not require vibrations, instead, material is fed in to a horizontal rotating drum with screen panels around the diameter of the drum. Circle-throw vibrating equipment Circle-throw vibrating equipment is a shaker or a series of shakers as to where the drive causes the whole structure to move. The structure extends to a maximum throw or length and then contracts to a base state. A pattern of springs are situated below the structure to where there is vibration and shock absorption as the structure returns to the base state. This type of equipment is used for very large particles, sizes that range from pebble size on up to boulder size material. It is also designed for high volume output. As a scalper, this shaker will allow oversize material to pass over and fall into a crusher such a cone crusher, jaw crusher, or hammer mill. The material that passes the screen by-passes the crusher and is conveyed and combined with the crush material. Also this equipment is used in washing processes, as material passes under spray bars, finer material and foreign material is washed through the screen. This is one example of wet screening.
48. Mechanical screening 46 High frequency vibrating equipment High frequency vibrating equipment is a shaker whose frame is fixed and the drive vibrates only the screen cloth. High frequency vibration equipment is for particles that are in this particle size range of an 1/8 in (3 mm) down to a +150 mesh. These shakers usually make a secondary cut for further processing or make a finished product cut. These shakers are usually set at a steep angle relative to the horizontal level plane. Angles range from 25 to 45 degrees relative to the horizontal level plane. Gyratory equipment This type of equipment has an eccentric drive or weights that causes the shaker to travel in an orbital path. The material rolls over the screen and falls with the induction of gravity and directional shifts. Rubber balls and trays provide an additional mechanical means to cause the material to fall through. The balls also provide a throwing action for the material to find an open slot to fall through. The shaker is set a shallow angle relative to the horizontal level plane. Usually, no more than 2 to 5 degrees relative to the horizontal level plane. These types of shakers are used for very clean cuts. Generally, a final material cut will not contain any oversize or any fines contamination. These shakers are designed for the highest attainable quality at the cost of a reduced feed rate. Trommel Screens Trommel screens have a rotating drum with screen panels around the diameter of the drum and is on a shall angle. The feed material always sits at the bottom of the drum and as it rotates, always comes in to contact with clean screen. The oversize travels to the end of the drum as it does not pass through the screen, while the undersize passes through the screen in to a launder below. References  http:/ / www. rotex. com/ 02applications/ applications. aspx  Woven Wire Mesh Glossary of Terms (http:/ / www. screentg. com/ wiremesh. htm)  Woven wire (http:/ / www. wovenwire. com/ products. htm)  Soil Gradation (http:/ / tpub. com/ content/ engine/ 14081/ css/ 14081_454. htm)  Screening (http:/ / www. metsominerals. com/ inetMinerals/ mm_segments. nsf/ WebWID/ WTB-041223-2256F-10C45?OpenDocument)  The Complete Wire Mesh Glossary of Terms (http:/ / www. wovenwire. com/ reference/ glossary. htm)  WS Tyler » F-Class (http:/ / www. wstyler. on. ca/ 85. html)  RHEWUM WA- The original (http:/ / www. rhewum. comon)  Engelsmann Separators and Screeners (http:/ / www. engelsmann. com)  Sweco - Vibratory Screener, Sifters, Separators, Round Screen, Vibratory Separator (http:/ / www. sweco. com/ round. html)