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Ball Mill For Grinding Lime Mortar. Ball Mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes. It works on the principle of impact: size reduction is done by impact as the balls drop from near the top of the shell. It is of welded steel construction, having an

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Ball Mill For Grinding Lime Mortar. Ball Mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes. It works on the principle of impact: size reduction is done by impact as the balls drop from near the top of the shell. It is of welded steel construction, having an

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Particle size reduction takes place by impact on the material with the tumbling grinding media and by abrasion between the media and the mill wall. Selection of Grinding Media : For a fresh charge, it is the most general practice to use three different ball sizes: 25% of large size, 50% of medium size & 25% small size

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A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker

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Particle size reduction takes place by impact on the material with the tumbling grinding media and by abrasion between the media and the mill wall. Selection of Grinding Media : For a fresh charge, it is the most general practice to use three different ball sizes: 25% of large size, 50% of medium size & 25% small size

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IV.1 Plate mills Plate mills are made of a cast iron base to which are attached two enclosed vertical grinding plates (see figure IV.3). One plate is fixed while the other is belt-driven from an electric motor (0.4 to 4 kW), or diesel engine (in the range of 11 to 19 kW). The moving plate rotates at a speed of approximately 600 rpm. Some models may, alternatively, be driven from a tractor engine. The grain is screw-fed from a conical hopper into the gap between the two plates. This gap may be adjusted to vary the fineness of the ground material. The grinding plates, approximately 25 cm in diameter, are made from hardened cast steel. They are grooved to aid the shearing (cutting and crushing) and grinding of the grain. Different plates, with a range of groove sizes, may be used for the production of meals of varying textures. The hourly output of plate mills depends upon the required fineness of the product and the variety and moisture content of the original grain. Electric plate mills have an output of approximately 67 kg per kW per hour. Thus, a plate mill equipped with a 4 kW electric motor may process approximately 270 kg of grain per hour. In parts of West Africa (e.g. Nigeria) and Central America, plate mills are used for the wet grinding of maize. For this purpose, plates with finer grooves than those used for dry milling are usually recommended by the manufacturer. A few developing countries produce plate mills with imported engines, while other countries import the fully equipped mills. Plates IV.1 to IV.4 illustrate a few plate mills supplied by a number of manufacturers from both developing and developed countries. Figure IV.3 Diagrammatic representation of a mechanical plate mill Plate IV.1 Superb plate mill Plate diameter: 270 mmPower required: 5 hpSpeed: 600 rpmOutput: 230-270 kg/hr Manufactured by: E.H. Bentall and Co. Ltd.,(United Kingdom) Source: ITDG (1976) Plate IV.2 Amuda flat plate mill No. 1 Spring mechanism allows the plate to open and avoids damage if any hard substance enters the machine. Shaker type feed mechanism can be easily regulated. Suitable for various grains. Manufactured by: Rajan Trading Co.(India) Source: ITDG (1976) Plate IV.3 Premier 127 plate grinding mill Output: 500 kg/hrPower required: 1-2 hp Manufactured by: R. Hunt and Co. Ltd.(United Kingdom) Source: FAO (1979) Plate IV.4 Diamant steel plate grinding mill Output: 500 to 1,100 kg per hourPower required: 5 to 15 hp Manufactured by: A.B.C. Hansen Co. A/S(Denmark) Source: ITDG (1976) IV.2 Hammer mills The hammer mills used in developing countries for maize dry milling are often imported from Europe or the United States. However, a growing number of these countries have started the manufacture of generally good quality hammer mills. The design and capacity of hammer mills vary between manufacturers. In general, they comprise a cast iron body through which passes a horizontal rotary shaft powered by an external energy source (see figure IV.4). The latter is usually an electric motor or diesel engine. Occasionally, power is obtained from a tractor engine. The capacity of the electric motor varies from 2 to 150 kW depending upon the size and model of the mill. A disc or discs, from which project short hammer-like plates, are attached to the end of the rotor shaft and enclosed in a metal casing. The hammer rotates at speeds of up to 3,600 rpm. They may be of the fixed or swinging type, and vary in number from 1 to 32. The fixed hammers are usually in the form of an iron casting whereas the swinging type are often made from heat-treated, 1.0 per cent chromium steel. A screen, mounted on a fixed circular support, surrounds the hammers. The maize grain must be sufficiently reduced in size to pass through the screen before it is discharged from the milling chamber. A range of screens is available for the production of a variety of grades of ground material. A conical hopper, fixed above the milling chamber, holds the whole grain which is gravity fed into the mill. Unlike the shearing action in the plate or stone mill, size reduction in a hammer mill occurs principally by impact as the grain hits the hammers, the metal of the screen, and the back wall and front casing of the mill. Impact also occurs between the grain itself. The grain is trapped and sheared between the hammer and the holes of the screen. The broken grain is retained in the milling chamber until its size is reduced sufficiently to allow its passage through the screen perforations. The output of ground material varies according to the capacity of the motor, the size of the perforations in the screen and the variety and moisture content of the maize. As a general guide, the output per kW per hour is approximately 74 kg for maize with a moisture content of 16 per cent and a screen with 3 mm holes. In the larger models (motor capacity greater than 5 kW), a cyclone discharges the ground material and cools both the mill and the product. In the smaller models (motor capacity less than 5 kW), the ground material is discharged by gravity from the base of the mill. Figure IV.4 Diagrammatic representation of a hammer mill Plates IV.5 to IV.8 illustrate various hammer mills manufactured in both developed and developing countries. Table IV.3 indicates the physical characteristics of a selected number of hammer mills. IV.3 Stone mills In a typical stone mill, a conical or pyramid-shaped hopper holds the whole grain which enters the milling chamber through a feed valve. In some models, a shaking device and a screen prevent large impurities from entering the milling chamber. The milling of the grain is achieved by the shearing action of the flat surface of two millstones which are identical in size and construction. One stone is fixed to the milling chamber door while the other is mounted on a rotating drive shaft connected to an external energy source (e.g. an electric motor, diesel engine, or tractor engine). Figure IV.5 illustrates the basic design of a stone mill. The grain from the hopper is fed, through the central hole in the rotating stone, into the gap between the two stones. As the rotating stone moves against the stationary stone, the grain is ground as it travels from the centre to the periphery of the stones. The two millstones may be set either horizontally with a vertical rotary shaft, or vertically with a horizontal rotary shaft. The vertical type is more common. It is shown in figure IV.5. The diameter of the millstones varies according to model type and size. Generally, because of the weight of the stones and the relative difficulty in supporting them in an upright position, vertical millstones are smaller in diameter (20 to 56 cm) than horizontal millstones (61 to 71 cm). There are exceptions, however; some manufacturers produce vertical millstones of 71 cm and 81 cm diameter, while some horizontal millstones are only 30 cm and 41 cm in diameter. In the horizontal type, the crushed grain is moved to the periphery of the stones by centrifugal forces, whereas gravity assists the movement of crushed grain between the vertical millstones. Table IV.3 Characteristics of selected hammer mills produced by manufacturers listed in Appendix I1 Manufacturer Model Type of hammers Number of hammers Use/maintenance Power (hp) Rotation (rpm) Suggested engine2 Output (kg/hr) Cyclone attachment ALVAN BLANCH ESSEXMAJOR Reversible 3-10 100-300 ETS CHAMPENOIS REQUIN 4 24 Reversible* 7.5 3,000 E 500-1,000 COMIA-FAO BNT 4000 7.5-8-10 4,000 E, H 75-150 DDD PRESIDENT MM 6 Re-useable** 3 3,000 E, 3 60-100 MM/F 9 Re-useable** 2.5-3 3,000 E, 2.5 Z2 12 Re-useable** 5.5 3,000 E, 5.5 200-300 C2 24 Re-useable** 7.5 3,000 E 250-500 B 30 Re-useable** 10 3,000 E 300-800 MP 7.5,10 E 200-500 ELECTRA BABY Swinging 6 Reversible* 4.5; 7; 7.5 6,000 E: 4.5-7.5D: 14; P: 7.5 150-700 MINI Swinging 6 Reversible* 2-3 3,000 P VS1 6,000 H: 14 Yes GONRAD T20 Swinging 20 Reversible* 4-8 5,000 E, H Yes T24 Swinging 24 Reversible* 16-20 3,000 E Yes LAW HBU4 Swinging 4 4; 7.5 E 100-600 EF 7.5 3,000 E 250 Yes Centaures Swinging Welding 12,15,25 3,000 E 500-2,000 Yes HPB Swinging 4 6-15 100-600 B15C 5.5 3,000 E: 4kW 150-250 NDUMEE ND20 12-25 4,000 ND30 16-100 3,600 GM40 25-100 2,000-2,600 PROMILL B2L Interchangeable 2.3.4,5.5 1,500-3,000 B4C Swinging 12 7.5,10,15,20,25 3,000 RENSON BM12/55 12 Reversible* 5.5 3,000 E 100-500 A5 15 5.5 2,800 E 300 B10 24 7.5 E 500 C15 36 10 E 700 SECA ARGOUD ALPIN Swinging 6 Reversible* 4-5.5 6,000 P: 7.5D: 14 150-800 STOUT 27 Swinging 24 4 faces 3,000 400-1,500 EUROP 76 4 faces 5.5 3,000 E 600 Yes MIRACLE 71 4 faces 7.5 3,000 E, D 250-400 SKJOLD SB Swinging 16 Reversible* 4-10 3,800 E, D: 11 300 Yes AM2 Swinging 12 10-13 3,800 E, D 120-250 Yes BM2 Swinging 12 Reversible* 7.5-10 2,900 400 Yes TIXIER REIXITBM Swinging 15, 18 Reversible 5.5-7.5 3,000 E 150-700 TOY BA Swinging 5.5; 7.5; 10 3,000 150-500 T1 12 Reversible* 5.5; 7.5; 8-10 E, P, D Yes SACM PM 73 6 6,000 E, H 100-200 BU 69 10 4,500 E, H 150-300 D. SECK Fixed 6 Reversible 12 3,200 E, D SISMAR (SISCOMA) E, P, D 250-360 1 Source: GRET (1983) 2 Letters in this column designate the following engines: E for electric engines, D for diesel engines, H for heat engines and P for petrol engines. The numbers designate the capacity of the engine in hp. * 4 faces ** 3 faces Figure IV.5 Diagrammatic representation of a mechanical stone mill with vertical grinding stones Plate IV.5 Kusinja hammer mill The Kusinja maize mill is designed to be powered by diesel motors of between 10-20 hp, and the milling capacity will vary according to the power source. With a 10 hp motor, the capacity would be 150 kg/hr, with a 20 hp motor, 400 kg/hr.Price: US$1,280 Manufactured by: Brown and Clapperton (Malawi) Source: Commonwealth Secretariat (1981) Plate IV.6 Atom hammer mill The Atom maize mill is a small size hammer mill designed to be powered by a 5-7 hp diesel engine. It is fitted with reversible hammers, screens and sealed bearings. The average capacity is about 180 kg per hour. Manufactured by: Brown and Clapperton (Malawi) Source: Commonwealth Secretariat (1981) Plate IV.7 Manik hammer mill Manik grinding mills are especially useful for grinding maize. The mills are manufactured in 4 sizes. The hammers are reversible, and can be used on 4 different faces before replacement. Output: 90 to 1,100 kg per hourPower required: 8 to 60 hpPrices: US$370 to US$690 Manufactured by: Manik Engineers (Tanzania) Source: Commonwealth Secretariat (1981) Plate IV.8 Ndume power-driven hammer mill The Ndume hammer mills are especially suitable for grinding maize into meal. There are 5 models: the ND20, ND30 and GM40. The hammers are reversible and replaceable. From the mill housing, a fan blows the meal up into an overhead screened hopper. The ND20 has the lowest capacity and can be driven from small power sources of 12-25 hp. The ND30 has double the capacity of the ND20, and is fitted with a special overhead screen which allows oversize meal particles to fall back into the mill for regrinding. The ND30 can be driven from small power sources of 16 hp. The GM40 is specially designed for power take-off from tractors. Outputs: 200 kg to 950 kg per hourPrices: US$690 to US$1,300 Manufactured by: Ndume Ltd. (Kenya) Source: Commonwealth Secretariat (1981) The capacities of electric motors used in stone mills vary between 0.4 kW and 15 kW according to mill capacity and the diameter of the millstones. The motor capacity governs, in turn, the speed of rotation of the millstones within an optimum range of 600 to 800 rpm. The smaller diameter stones rotate faster than those of larger diameter. Thus, in a typical horizontal mill, the optimum rotation speed may be reduced to 400 or 500 rpm for stone diameters exceeding 61 cm. The output of ground material depends upon the capacity of the motor, the speed of rotation, the diameter of the millstones, the variety of the grain and the desired fineness of the ground material. The average output of a vertical stone mill is 80 kg per kW per hour, while it may reach 107 kg per kW per hour in a horizontal mill equipped with large diameter stones. Thus, the average hourly output of stone mills varies between 33 kg and 1,600 kg per hour, depending on the motor capacity, the position (vertical or horizontal) and diameter of the millstones, the type of grain and the required fineness of the ground material. Millstones are made out of one of the following materials: - natural stones; - small pieces of natural stones embedded in a matrix of cement or other suitable material. Other ingredients, such as emery, may also be added in the matrix; and - artificial stones made of emery or carborundum, or a mixture of the above two materials embedded in a matrix of magnesium oxychloride cement. The carborundum may additionally be heat-treated or vitrified to increase its durability. All types of millstones are usually enclosed within a supporting and protecting metal band. They are grooved to allow the shearing of the grain, as well as to assist the movement of the latter to the stones' periphery. The casing of most stone mills is made out of cast iron although some models are made with a wooden frame. A large number of developing countries manufacture stone mills for local use or for export to neighbouring countries. In many cases, the motor of these mills is imported. Plates IV.9 to IV.14 illustrate various types of stone mills manufactured in developed and developing countries, while table IV.4 provides the characteristics of a selected number of mills. IV.4 Efficiency of plate, hammer and stone mills A comparison of the efficiency of the plate, hammer and stone mills shows that hammer mills are generally better suited than plate or stone mills for fine grinding. A plate mill generally consumes more power than a hammer mill during fine grinding, especially with grain at high initial moisture content. Plate mills would therefore seem to be more expensive to operate. A more efficient use of plate mills requires that the grain be pounded before milling: this is unnecessary with the hammer mill. Cyclones fitted to the large hammer mills cool the mill parts and the ground material. Their provision in plate mills or stone mills is unusual. However, as an increase in the temperature of ground maize may impair its nutritional characteristics and shelf-life, manufacturers of stone mills recommend an optimum rotation speed of the millstones which should not be exceeded by more than 25 per cent. IV.5 Maintenance of plate, hammer and stone mills All types of mechanical grinders require regular maintenance if they are to perform the grinding operation efficiently at all times. All moving parts require lubrication on a regular basis (e.g. weekly). Most hammers, plates and millstones are reversible. Thus, they may be used for an extended period of time before sharpening, regrinding, dressing or replacement is necessary. Generally, the hammers need to be resharpened each week while the plates require regrinding every three to four weeks. Where excessive wear has taken place on some types of steel hammers, the tips can be returned to approximately their original dimensions by welding further metal. Using the correct materials, the new part may be made harder and thus more durable than the original. Natural stones wear more quickly than artificial stones and therefore need to be reversed or replaced more regularly. They are, however, cheaper to purchase. It should be emphasised that the life of the milling parts, whether hammers, plates or stones, will be prolonged if foreign matter of mineral origin (e.g. fragments of stone or metal or sand) are removed from the grain prior to milling. Table IV.4 Characteristics of selected stone mills produced by manufacturers listed in Appendix I1 Manufacturer Model Millstones Use/maintenance Power (hp) Rotation (rpm) Suggested engine2 Output (kg/hr)3 Material Diameter (mm) ABC HANSEN CO. DIAMANT Artificial stone 250-550 1 E or D6 to 30 FARMERS'FAVORITE Artificial stone 600,700 10 425 E or D6 to 30 600 BENTALL 200 L090SUPERB Cast steel 267 5 600 D: 11 250 ETS CHAMPENOIS CLB Cast steel 260 Reversible 4-6 850 E: 4, H: 4-6 60-180 NOVA Cast steel 160 Reversible 2.5-3 500-600 E: 3, H: 3 30 DIAMANT H4 Corundum 500 Re-sharpen 3-4 550-600 120 DIAMANT H6 Corundum 700 Re-sharpen 6-7 240 V.300 Bakelite or metal 300260 Replaceable 5.5-7.5 600-700 E 280-400 V.400 Vit. cor. 400 Replaceable 5.5-7.5 500-600 E, D, P 280-400 Metal 390 JUNIOR Hard cast steel 95 Reversible 0.5-7.5 100-125 E, H 25 COMIA FAO BA 318 Vitrified corundum 300 Non-interchangeable 4-6 750 E: 4, H: 5-6D, P 80 MB 317 Vitrified corundum Non-interchangeable 5-6 900 E: 5.5, H: 6 200 ECLIPSE B30 Corundum 300 4-6 700-750 150-350 DANDEKARMACHINE WORKS DS style Natural stone 160 6-8 E 250 DDD PRESIDENT Nr. 4/5/6/7 Natural stone 3-20 100-1,200 Nr. 4/5 GM Natural stone 5.5-7.5 R. HUNT & CO PREMIER 1A Steel 254 Reversible 4 600 D: 7 150 PREMIER 2A Steel 305 Reversible 6 600 D: 11 200 IRUSWERKE B/3/4/5/6 210-600 1.5-10 E 50-500 RM/2/3/4/5/6 210-600 2-10 E 50-500 RK/2/3/4/5/6 210-600 2-7.5 E 40-200 CR2/3/4/5/6 210-600 2-10 E, D 40-250 MOULIS(CRICKET) D4 Corundum 200-400 Non-revers. 2 E, P 100-400 RENSON & Cie LE MODERNE Corundum 300 4-5 400-600 E: 7.5,P; D: 7.5-11 200-300 AVIMAT Cast steel 90 .5 E: 5 60-120 SILEX 113 Corundum 6-8 E: 4 350-600 A320 Steel 320 4 750 600-1,000 SACM MF 75 6-10 800-1,000 E, H 250 SAMAP P220/380(horiz. stones) Stone 200 Interchang. 4 2,800 E 80-100 SECA ARGOUD C300 Corundum 300 2-5 350 E: 3 250-400 l/h D400 Corundum 400 4-8 350 E: 5 150-200 l/h B205 Vit. corund. 200 3-4 350 E: 3/4 100 l/h SICO GAUBERT JUNIOR 170 Cor./emery 200 1-2 450 E 50-200 SENIOR 170 Cor./emery 300 3 450 E 150-400 SIMON FRERES N2GCV Metal 218 1.5 85-200 NGC 51 Metal 250 2-4 200-400 SKJOLD KKE 16 6 650 D 300-400 TIXIER FRERES REIXIT M9CV Vit. corund. 250 3 700-800 150 REIXIT M10CV Vit. corund. 250 3 700-800 150-180 REIXIT M11CV Vit. corund. 300 5 700-800 200-300 REIXIT M12CV Vit. corund. 300 5 700-800 200-300 YAMAR Corundum 400-500 Corundum 600-700 1 Source: GRET (1983) 2 Letters in this column designate the following engines: E for electric engines, D for diesel engines, H for heat engines, and P for petrol engines. The numbers designate the capacity of the engines in hp. 3 Output in kg/hr unless otherwise stated. Plate IV.9 Horizontal stone mill Output: 120-150 kg per hourPower required: 3-4 hp Manufactured by: Etablissements Champenois (France) Source: FAO (1979) Plate IV.10 Stone mill fitted with natural stones Stone diameter: 400 mmOutput: 225-270 kg per hourPower required: 6-8 hp Manufactured by: Dandekar Brothers (India) Source: ITDG (1976) Plate IV.11 Modern stone mill Equipped with agitator feed from 35 litres capacity hopper.300 mm diameter grinding wheelsScrew adjustment for fineness of grinding.Output: 200-300 kg per hourPower required: 4-5 hp Manufactured by: Renson and Co. (France) Source: ITDG (1976) Plate IV.12 Kisan stone mill Stone mill for various grainsPower required: 1 hp electric motor Manufactured by: Kisan Krishi Yantra Udyog (India) Source: FAO (1979) Plate IV.13 R 2 grinding mill - Vertical millstones: 210 mm diam.- Power required: 2 hp- Output of fine grist: 100 kg/hr Manufactured by: Iruswerke Dusslingen (Federal Republic of Germany) Source: ITDG (1976) Plate IV.14 Diamant vertical stone mill - Special composition millstones- Constant feed rate from hopper to give uniform grinding- Millstone diameter: 300 mm to 500 mm- Output: 100 kg to 650 kg per hour- Power required: 2 hp to 10 hp Manufactured by: A.B.C. Hansen Co. A/S (Denmark) Source: ITDG (1976)

Impact Crushers have a Reduction Ratio of approximately 15:1. An example of this ratio of reduction would be that a 30 input particle can be reduced to approximately a 2 discharge particle. Impact crushers tend to produce more fines that other types of crushers and are best suited to produce a dense graded type product or to provide feed material to a screening unit where the amount of fine product is important. Wear rates for an Impactor are increased when the feed material increase in abrasiveness

Crushers are classified according to the fineness with which they fragment the starting material. There are primary or secondary crushers (coarse finished products) and tertiary or quaternary crushers (fine particle finished products)

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IV.1 Plate mills Plate mills are made of a cast iron base to which are attached two enclosed vertical grinding plates (see figure IV.3). One plate is fixed while the other is belt-driven from an electric motor (0.4 to 4 kW), or diesel engine (in the range of 11 to 19 kW). The moving plate rotates at a speed of approximately 600 rpm. Some models may, alternatively, be driven from a tractor engine. The grain is screw-fed from a conical hopper into the gap between the two plates. This gap may be adjusted to vary the fineness of the ground material. The grinding plates, approximately 25 cm in diameter, are made from hardened cast steel. They are grooved to aid the shearing (cutting and crushing) and grinding of the grain. Different plates, with a range of groove sizes, may be used for the production of meals of varying textures. The hourly output of plate mills depends upon the required fineness of the product and the variety and moisture content of the original grain. Electric plate mills have an output of approximately 67 kg per kW per hour. Thus, a plate mill equipped with a 4 kW electric motor may process approximately 270 kg of grain per hour. In parts of West Africa (e.g. Nigeria) and Central America, plate mills are used for the wet grinding of maize. For this purpose, plates with finer grooves than those used for dry milling are usually recommended by the manufacturer. A few developing countries produce plate mills with imported engines, while other countries import the fully equipped mills. Plates IV.1 to IV.4 illustrate a few plate mills supplied by a number of manufacturers from both developing and developed countries. Figure IV.3 Diagrammatic representation of a mechanical plate mill Plate IV.1 Superb plate mill Plate diameter: 270 mmPower required: 5 hpSpeed: 600 rpmOutput: 230-270 kg/hr Manufactured by: E.H. Bentall and Co. Ltd.,(United Kingdom) Source: ITDG (1976) Plate IV.2 Amuda flat plate mill No. 1 Spring mechanism allows the plate to open and avoids damage if any hard substance enters the machine. Shaker type feed mechanism can be easily regulated. Suitable for various grains. Manufactured by: Rajan Trading Co.(India) Source: ITDG (1976) Plate IV.3 Premier 127 plate grinding mill Output: 500 kg/hrPower required: 1-2 hp Manufactured by: R. Hunt and Co. Ltd.(United Kingdom) Source: FAO (1979) Plate IV.4 Diamant steel plate grinding mill Output: 500 to 1,100 kg per hourPower required: 5 to 15 hp Manufactured by: A.B.C. Hansen Co. A/S(Denmark) Source: ITDG (1976) IV.2 Hammer mills The hammer mills used in developing countries for maize dry milling are often imported from Europe or the United States. However, a growing number of these countries have started the manufacture of generally good quality hammer mills. The design and capacity of hammer mills vary between manufacturers. In general, they comprise a cast iron body through which passes a horizontal rotary shaft powered by an external energy source (see figure IV.4). The latter is usually an electric motor or diesel engine. Occasionally, power is obtained from a tractor engine. The capacity of the electric motor varies from 2 to 150 kW depending upon the size and model of the mill. A disc or discs, from which project short hammer-like plates, are attached to the end of the rotor shaft and enclosed in a metal casing. The hammer rotates at speeds of up to 3,600 rpm. They may be of the fixed or swinging type, and vary in number from 1 to 32. The fixed hammers are usually in the form of an iron casting whereas the swinging type are often made from heat-treated, 1.0 per cent chromium steel. A screen, mounted on a fixed circular support, surrounds the hammers. The maize grain must be sufficiently reduced in size to pass through the screen before it is discharged from the milling chamber. A range of screens is available for the production of a variety of grades of ground material. A conical hopper, fixed above the milling chamber, holds the whole grain which is gravity fed into the mill. Unlike the shearing action in the plate or stone mill, size reduction in a hammer mill occurs principally by impact as the grain hits the hammers, the metal of the screen, and the back wall and front casing of the mill. Impact also occurs between the grain itself. The grain is trapped and sheared between the hammer and the holes of the screen. The broken grain is retained in the milling chamber until its size is reduced sufficiently to allow its passage through the screen perforations. The output of ground material varies according to the capacity of the motor, the size of the perforations in the screen and the variety and moisture content of the maize. As a general guide, the output per kW per hour is approximately 74 kg for maize with a moisture content of 16 per cent and a screen with 3 mm holes. In the larger models (motor capacity greater than 5 kW), a cyclone discharges the ground material and cools both the mill and the product. In the smaller models (motor capacity less than 5 kW), the ground material is discharged by gravity from the base of the mill. Figure IV.4 Diagrammatic representation of a hammer mill Plates IV.5 to IV.8 illustrate various hammer mills manufactured in both developed and developing countries. Table IV.3 indicates the physical characteristics of a selected number of hammer mills. IV.3 Stone mills In a typical stone mill, a conical or pyramid-shaped hopper holds the whole grain which enters the milling chamber through a feed valve. In some models, a shaking device and a screen prevent large impurities from entering the milling chamber. The milling of the grain is achieved by the shearing action of the flat surface of two millstones which are identical in size and construction. One stone is fixed to the milling chamber door while the other is mounted on a rotating drive shaft connected to an external energy source (e.g. an electric motor, diesel engine, or tractor engine). Figure IV.5 illustrates the basic design of a stone mill. The grain from the hopper is fed, through the central hole in the rotating stone, into the gap between the two stones. As the rotating stone moves against the stationary stone, the grain is ground as it travels from the centre to the periphery of the stones. The two millstones may be set either horizontally with a vertical rotary shaft, or vertically with a horizontal rotary shaft. The vertical type is more common. It is shown in figure IV.5. The diameter of the millstones varies according to model type and size. Generally, because of the weight of the stones and the relative difficulty in supporting them in an upright position, vertical millstones are smaller in diameter (20 to 56 cm) than horizontal millstones (61 to 71 cm). There are exceptions, however; some manufacturers produce vertical millstones of 71 cm and 81 cm diameter, while some horizontal millstones are only 30 cm and 41 cm in diameter. In the horizontal type, the crushed grain is moved to the periphery of the stones by centrifugal forces, whereas gravity assists the movement of crushed grain between the vertical millstones. Table IV.3 Characteristics of selected hammer mills produced by manufacturers listed in Appendix I1 Manufacturer Model Type of hammers Number of hammers Use/maintenance Power (hp) Rotation (rpm) Suggested engine2 Output (kg/hr) Cyclone attachment ALVAN BLANCH ESSEXMAJOR Reversible 3-10 100-300 ETS CHAMPENOIS REQUIN 4 24 Reversible* 7.5 3,000 E 500-1,000 COMIA-FAO BNT 4000 7.5-8-10 4,000 E, H 75-150 DDD PRESIDENT MM 6 Re-useable** 3 3,000 E, 3 60-100 MM/F 9 Re-useable** 2.5-3 3,000 E, 2.5 Z2 12 Re-useable** 5.5 3,000 E, 5.5 200-300 C2 24 Re-useable** 7.5 3,000 E 250-500 B 30 Re-useable** 10 3,000 E 300-800 MP 7.5,10 E 200-500 ELECTRA BABY Swinging 6 Reversible* 4.5; 7; 7.5 6,000 E: 4.5-7.5D: 14; P: 7.5 150-700 MINI Swinging 6 Reversible* 2-3 3,000 P VS1 6,000 H: 14 Yes GONRAD T20 Swinging 20 Reversible* 4-8 5,000 E, H Yes T24 Swinging 24 Reversible* 16-20 3,000 E Yes LAW HBU4 Swinging 4 4; 7.5 E 100-600 EF 7.5 3,000 E 250 Yes Centaures Swinging Welding 12,15,25 3,000 E 500-2,000 Yes HPB Swinging 4 6-15 100-600 B15C 5.5 3,000 E: 4kW 150-250 NDUMEE ND20 12-25 4,000 ND30 16-100 3,600 GM40 25-100 2,000-2,600 PROMILL B2L Interchangeable 2.3.4,5.5 1,500-3,000 B4C Swinging 12 7.5,10,15,20,25 3,000 RENSON BM12/55 12 Reversible* 5.5 3,000 E 100-500 A5 15 5.5 2,800 E 300 B10 24 7.5 E 500 C15 36 10 E 700 SECA ARGOUD ALPIN Swinging 6 Reversible* 4-5.5 6,000 P: 7.5D: 14 150-800 STOUT 27 Swinging 24 4 faces 3,000 400-1,500 EUROP 76 4 faces 5.5 3,000 E 600 Yes MIRACLE 71 4 faces 7.5 3,000 E, D 250-400 SKJOLD SB Swinging 16 Reversible* 4-10 3,800 E, D: 11 300 Yes AM2 Swinging 12 10-13 3,800 E, D 120-250 Yes BM2 Swinging 12 Reversible* 7.5-10 2,900 400 Yes TIXIER REIXITBM Swinging 15, 18 Reversible 5.5-7.5 3,000 E 150-700 TOY BA Swinging 5.5; 7.5; 10 3,000 150-500 T1 12 Reversible* 5.5; 7.5; 8-10 E, P, D Yes SACM PM 73 6 6,000 E, H 100-200 BU 69 10 4,500 E, H 150-300 D. SECK Fixed 6 Reversible 12 3,200 E, D SISMAR (SISCOMA) E, P, D 250-360 1 Source: GRET (1983) 2 Letters in this column designate the following engines: E for electric engines, D for diesel engines, H for heat engines and P for petrol engines. The numbers designate the capacity of the engine in hp. * 4 faces ** 3 faces Figure IV.5 Diagrammatic representation of a mechanical stone mill with vertical grinding stones Plate IV.5 Kusinja hammer mill The Kusinja maize mill is designed to be powered by diesel motors of between 10-20 hp, and the milling capacity will vary according to the power source. With a 10 hp motor, the capacity would be 150 kg/hr, with a 20 hp motor, 400 kg/hr.Price: US$1,280 Manufactured by: Brown and Clapperton (Malawi) Source: Commonwealth Secretariat (1981) Plate IV.6 Atom hammer mill The Atom maize mill is a small size hammer mill designed to be powered by a 5-7 hp diesel engine. It is fitted with reversible hammers, screens and sealed bearings. The average capacity is about 180 kg per hour. Manufactured by: Brown and Clapperton (Malawi) Source: Commonwealth Secretariat (1981) Plate IV.7 Manik hammer mill Manik grinding mills are especially useful for grinding maize. The mills are manufactured in 4 sizes. The hammers are reversible, and can be used on 4 different faces before replacement. Output: 90 to 1,100 kg per hourPower required: 8 to 60 hpPrices: US$370 to US$690 Manufactured by: Manik Engineers (Tanzania) Source: Commonwealth Secretariat (1981) Plate IV.8 Ndume power-driven hammer mill The Ndume hammer mills are especially suitable for grinding maize into meal. There are 5 models: the ND20, ND30 and GM40. The hammers are reversible and replaceable. From the mill housing, a fan blows the meal up into an overhead screened hopper. The ND20 has the lowest capacity and can be driven from small power sources of 12-25 hp. The ND30 has double the capacity of the ND20, and is fitted with a special overhead screen which allows oversize meal particles to fall back into the mill for regrinding. The ND30 can be driven from small power sources of 16 hp. The GM40 is specially designed for power take-off from tractors. Outputs: 200 kg to 950 kg per hourPrices: US$690 to US$1,300 Manufactured by: Ndume Ltd. (Kenya) Source: Commonwealth Secretariat (1981) The capacities of electric motors used in stone mills vary between 0.4 kW and 15 kW according to mill capacity and the diameter of the millstones. The motor capacity governs, in turn, the speed of rotation of the millstones within an optimum range of 600 to 800 rpm. The smaller diameter stones rotate faster than those of larger diameter. Thus, in a typical horizontal mill, the optimum rotation speed may be reduced to 400 or 500 rpm for stone diameters exceeding 61 cm. The output of ground material depends upon the capacity of the motor, the speed of rotation, the diameter of the millstones, the variety of the grain and the desired fineness of the ground material. The average output of a vertical stone mill is 80 kg per kW per hour, while it may reach 107 kg per kW per hour in a horizontal mill equipped with large diameter stones. Thus, the average hourly output of stone mills varies between 33 kg and 1,600 kg per hour, depending on the motor capacity, the position (vertical or horizontal) and diameter of the millstones, the type of grain and the required fineness of the ground material. Millstones are made out of one of the following materials: - natural stones; - small pieces of natural stones embedded in a matrix of cement or other suitable material. Other ingredients, such as emery, may also be added in the matrix; and - artificial stones made of emery or carborundum, or a mixture of the above two materials embedded in a matrix of magnesium oxychloride cement. The carborundum may additionally be heat-treated or vitrified to increase its durability. All types of millstones are usually enclosed within a supporting and protecting metal band. They are grooved to allow the shearing of the grain, as well as to assist the movement of the latter to the stones' periphery. The casing of most stone mills is made out of cast iron although some models are made with a wooden frame. A large number of developing countries manufacture stone mills for local use or for export to neighbouring countries. In many cases, the motor of these mills is imported. Plates IV.9 to IV.14 illustrate various types of stone mills manufactured in developed and developing countries, while table IV.4 provides the characteristics of a selected number of mills. IV.4 Efficiency of plate, hammer and stone mills A comparison of the efficiency of the plate, hammer and stone mills shows that hammer mills are generally better suited than plate or stone mills for fine grinding. A plate mill generally consumes more power than a hammer mill during fine grinding, especially with grain at high initial moisture content. Plate mills would therefore seem to be more expensive to operate. A more efficient use of plate mills requires that the grain be pounded before milling: this is unnecessary with the hammer mill. Cyclones fitted to the large hammer mills cool the mill parts and the ground material. Their provision in plate mills or stone mills is unusual. However, as an increase in the temperature of ground maize may impair its nutritional characteristics and shelf-life, manufacturers of stone mills recommend an optimum rotation speed of the millstones which should not be exceeded by more than 25 per cent. IV.5 Maintenance of plate, hammer and stone mills All types of mechanical grinders require regular maintenance if they are to perform the grinding operation efficiently at all times. All moving parts require lubrication on a regular basis (e.g. weekly). Most hammers, plates and millstones are reversible. Thus, they may be used for an extended period of time before sharpening, regrinding, dressing or replacement is necessary. Generally, the hammers need to be resharpened each week while the plates require regrinding every three to four weeks. Where excessive wear has taken place on some types of steel hammers, the tips can be returned to approximately their original dimensions by welding further metal. Using the correct materials, the new part may be made harder and thus more durable than the original. Natural stones wear more quickly than artificial stones and therefore need to be reversed or replaced more regularly. They are, however, cheaper to purchase. It should be emphasised that the life of the milling parts, whether hammers, plates or stones, will be prolonged if foreign matter of mineral origin (e.g. fragments of stone or metal or sand) are removed from the grain prior to milling. Table IV.4 Characteristics of selected stone mills produced by manufacturers listed in Appendix I1 Manufacturer Model Millstones Use/maintenance Power (hp) Rotation (rpm) Suggested engine2 Output (kg/hr)3 Material Diameter (mm) ABC HANSEN CO. DIAMANT Artificial stone 250-550 1 E or D6 to 30 FARMERS'FAVORITE Artificial stone 600,700 10 425 E or D6 to 30 600 BENTALL 200 L090SUPERB Cast steel 267 5 600 D: 11 250 ETS CHAMPENOIS CLB Cast steel 260 Reversible 4-6 850 E: 4, H: 4-6 60-180 NOVA Cast steel 160 Reversible 2.5-3 500-600 E: 3, H: 3 30 DIAMANT H4 Corundum 500 Re-sharpen 3-4 550-600 120 DIAMANT H6 Corundum 700 Re-sharpen 6-7 240 V.300 Bakelite or metal 300260 Replaceable 5.5-7.5 600-700 E 280-400 V.400 Vit. cor. 400 Replaceable 5.5-7.5 500-600 E, D, P 280-400 Metal 390 JUNIOR Hard cast steel 95 Reversible 0.5-7.5 100-125 E, H 25 COMIA FAO BA 318 Vitrified corundum 300 Non-interchangeable 4-6 750 E: 4, H: 5-6D, P 80 MB 317 Vitrified corundum Non-interchangeable 5-6 900 E: 5.5, H: 6 200 ECLIPSE B30 Corundum 300 4-6 700-750 150-350 DANDEKARMACHINE WORKS DS style Natural stone 160 6-8 E 250 DDD PRESIDENT Nr. 4/5/6/7 Natural stone 3-20 100-1,200 Nr. 4/5 GM Natural stone 5.5-7.5 R. HUNT & CO PREMIER 1A Steel 254 Reversible 4 600 D: 7 150 PREMIER 2A Steel 305 Reversible 6 600 D: 11 200 IRUSWERKE B/3/4/5/6 210-600 1.5-10 E 50-500 RM/2/3/4/5/6 210-600 2-10 E 50-500 RK/2/3/4/5/6 210-600 2-7.5 E 40-200 CR2/3/4/5/6 210-600 2-10 E, D 40-250 MOULIS(CRICKET) D4 Corundum 200-400 Non-revers. 2 E, P 100-400 RENSON & Cie LE MODERNE Corundum 300 4-5 400-600 E: 7.5,P; D: 7.5-11 200-300 AVIMAT Cast steel 90 .5 E: 5 60-120 SILEX 113 Corundum 6-8 E: 4 350-600 A320 Steel 320 4 750 600-1,000 SACM MF 75 6-10 800-1,000 E, H 250 SAMAP P220/380(horiz. stones) Stone 200 Interchang. 4 2,800 E 80-100 SECA ARGOUD C300 Corundum 300 2-5 350 E: 3 250-400 l/h D400 Corundum 400 4-8 350 E: 5 150-200 l/h B205 Vit. corund. 200 3-4 350 E: 3/4 100 l/h SICO GAUBERT JUNIOR 170 Cor./emery 200 1-2 450 E 50-200 SENIOR 170 Cor./emery 300 3 450 E 150-400 SIMON FRERES N2GCV Metal 218 1.5 85-200 NGC 51 Metal 250 2-4 200-400 SKJOLD KKE 16 6 650 D 300-400 TIXIER FRERES REIXIT M9CV Vit. corund. 250 3 700-800 150 REIXIT M10CV Vit. corund. 250 3 700-800 150-180 REIXIT M11CV Vit. corund. 300 5 700-800 200-300 REIXIT M12CV Vit. corund. 300 5 700-800 200-300 YAMAR Corundum 400-500 Corundum 600-700 1 Source: GRET (1983) 2 Letters in this column designate the following engines: E for electric engines, D for diesel engines, H for heat engines, and P for petrol engines. The numbers designate the capacity of the engines in hp. 3 Output in kg/hr unless otherwise stated. Plate IV.9 Horizontal stone mill Output: 120-150 kg per hourPower required: 3-4 hp Manufactured by: Etablissements Champenois (France) Source: FAO (1979) Plate IV.10 Stone mill fitted with natural stones Stone diameter: 400 mmOutput: 225-270 kg per hourPower required: 6-8 hp Manufactured by: Dandekar Brothers (India) Source: ITDG (1976) Plate IV.11 Modern stone mill Equipped with agitator feed from 35 litres capacity hopper.300 mm diameter grinding wheelsScrew adjustment for fineness of grinding.Output: 200-300 kg per hourPower required: 4-5 hp Manufactured by: Renson and Co. (France) Source: ITDG (1976) Plate IV.12 Kisan stone mill Stone mill for various grainsPower required: 1 hp electric motor Manufactured by: Kisan Krishi Yantra Udyog (India) Source: FAO (1979) Plate IV.13 R 2 grinding mill - Vertical millstones: 210 mm diam.- Power required: 2 hp- Output of fine grist: 100 kg/hr Manufactured by: Iruswerke Dusslingen (Federal Republic of Germany) Source: ITDG (1976) Plate IV.14 Diamant vertical stone mill - Special composition millstones- Constant feed rate from hopper to give uniform grinding- Millstone diameter: 300 mm to 500 mm- Output: 100 kg to 650 kg per hour- Power required: 2 hp to 10 hp Manufactured by: A.B.C. Hansen Co. A/S (Denmark) Source: ITDG (1976)

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