Introduction

GRINDERS INTRODUCTION A grinding mill is a unit operation designed to break a solid material into smaller pieces. In general, milling refers to the size reduction and separation operations used for processing of food grains into edible form by removing and separating the inedible and undesirable portions from them. Milling may involve cleaning/separation, husking, sorting, polishing, grinding etc (5). 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 (6).

GRINDING MAY SERVE THE FOLLOWING PURPOSES IN ENGINEERING • magnification of the surface area of a solid • manufacturing of a solid with a desired grain size • pulping of resources (6)

PRINCIPLE Observation and research finding indicate that earliest type of grinding mill is called a quern-stone. Two heavy stones which fit together. The top rotates and the corn (etc) is trapped between the two stones. The faces usually have small groves carved into then, and these function a lot like the teeth on a file. The corn gets "stuck", and other side rotates, and files away the free surface of the stuck grain. There are LOTS of different patterns for these groves. Different ones are probably better for different grains (2). If the grain are approximately a sphere (but isn't) and isn't perfectly spread over the grinding faces. The upper stone will be supported by thin layer of grain which works a little bit like

ball-bearings. But because the grains aren't perfect spheres, when they rotate a few will rotate and try to raise the upper stone, increasing the pressure on the grain to such an extent that the husk is likely to crack. If the upper stone is being turned fairly fast, the pressure gets bigger (3).

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THE CHARACTERISTIC OF GRINDING OPERATION IS AFFECTED BY THE FOLLOWING GRAIN PARAMETERS: Type of cereal grain Variety Moisture content Extent of hydro thermal treatment given to the grain Mechanical properties. (8)

TYPES OF GRINDING MILLS: Ball mill Colloid mill Conical mill Disk mill Edge mill Grist mill, also called flour mill or corn mill Hammer mill Wiley mill (8) 1. 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. 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 also at lower temp. ball mill is used for grinding of spices(2).

2.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 very similar to ball mills, except they use long rods for Rod mills are very similar to ball mills, except they use long rods for grinding media. The rods grind the ore by tumbling within the the mill, similar to the grinding balls in a ball mill. To prevent the conditions leading to rod charge tangling, the length to diameter ratio is maintained at 1.4 to 1.6. Rod mills accept feed up to about 50 mm (2 in.) and produce a product in the size range of 3000 to 270 mm (–4 to –35 mesh). Grinding action is by line contact between the rods extending the length of the mill. Rods tumble and spin in roughly parallel alignment simulating a series of roll crushers. This results in preferential grinding of coarse material and minimizes production of slimes. The rods grind the ore by tumbling within the mill, similar to the grinding balls in a ball mill. The rod mill, a tumbling mill characterized by the use of rods as grinding media, grinds ores, coal/ coke, and other materials for both wet and dry applications (3,4).

3.SAG MILL: SAG is an acronym for Semi-Autogenous Grinding, and applies to mills that utilize steel balls in addition to large rocks for grinding. The SAG mills use a minimal ball charge of 6 to 15%.

Principle of SAG Mill operation A rotating drum throws large rocks and steel balls in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. SAG mills are characterized by their large diameter and short length. The inside of the mill is lined with lifting plates to lift the material inside up and around the inside of the mill, where it then falls off the plates and falls back down. SAG mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries .In food industry SAG mills are used for reducing the size of sugar to fine powder (2,5,8).

4. HAMMER MILL : A hammermill is a machine whose purpose is to shred material into fine particles. They have many sorts of applications in many industries, including: • Milling grain. • Ethanol plants (corn). • Grinding used shipping pallets for mulch. • Sawmills, size reduction of trim scrap and planer shavings into boiler fuel or mulch.

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A farm machine, which mills grain into coarse flour to be fed to livestock.

Desktop hammer mill used for preparing growth media in a lab. • Shredding paper. • Shredding scrap automobiles (see automotive shredder residue). • In waste management (8) • Fruit juice production

5. DISC MILL: A disc mill, is a type of crusher that can be used to grind, cut, shear, shred, fiberize, pulverize, granulate, crack, rub, curl, fluff, twist, hull, blend, or refine. It works in a similar manner to the ancient Burstone mill in that the feedstock is fed between opposing discs or plates. The discs may be grooved, serrated, or spiked (3). Applications SINGLE DISC MILL DOUBLE DISC MILL three stages of the wet milling of field corn, manufacture of peanut butter, processing nut shells, ammonium nitrate, urea, producing chemical slurries and recycled paper

bark, barley, borax, brake lining scrap, brass chips, sodium hydroxide, chemical salts, coconut shells, copper powder, cork, cottonseed hulls, pharmaceuticals, feathers, hops, leather,

slurries, and chromium metal.

grinding oilseed cakes, phosphates, rice, rosin, sawdust, and seeds.

Disc mills are relatively expensive to run and maintain, and are not used where ball mills or hammermills produce the desired results at a lower cost (8). Mechanism Substances are crushed between the edge of a thick, spinning disk and something else. Some mills cover the edge of the disk in blades to chop up incoming matter rather than crush it. 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 (3).

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6.AUTOGENOUS MILL: A rotating drum throws large rocks in a cataracting 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 above but does not use steel balls in the mill. Attrition in the charge causes grinding of finer particles. Also known as ROM or "Run Of Mine" grinding. Autogenous mills are available for both wet and dry grinding. The diameter of Autogenous mills is normally two to three times the length. The ore charge is usually 25 to 35% of the mill volume.Autogenous mills have grate discharges to retain the coarse grinding media in the mill (8).

7.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 (3,8).

8.BUHRSTONE MILL: Another type of fine grinder commonly used is the buhrstone mill, which is similar to old-fashioned flour mills (8).

9. HIGH PRESSURE GRINDING ROLLS: The ore is fed between two rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes microfracturing at the grain size level. It consists of a pair of horizontal cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. 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 (2,8) .

10. PELLET MILL: pellet mill is a type of mill used to create cylindrical pellets from a mixture of dry powdered feedstock, such as flour, sawdust, or grass, and a wet ingredient, such as molasses or steam. The pellets are made by compacting the mash or meal into many small holes in a die. The die is usually round and the pellets are pushed from the inside out. Pellet mills are used in the production of animal feeds, and of wood and grass fuel pellets for use in a pellet stove. Pellet mills are unlike grinding mills, in that they combine small materials into a larger, homogeneous mass, rather than break large materials into smaller pieces. In this way, pellet mills are similar to extruders. Feedstocks for pellet mills can sometimes break down and then re-form, or polymerize, under the extreme heat and pressure of the pellet mill. Pressures in the die can reach up to 25,000 psi (2,3,8).

11. GRIST MILL gristmill or grist mill is a building where grain is ground into flour, or the grinding mechanism itself. In many countries these are referred to as corn mills or flour mills. Gristmills only grind clean grains, that is, grain from which stalks and chaff have previously been removed, but some mills also housed equipment for threshing, sorting, and cleaning prior to grinding. Gristmills also grind corn into meal. Gristmills only grind clean grains, that is, grain from which stalks and chaff have previously been removed, but some mills also housed equipment for threshing, sorting, and cleaning prior to grinding. Gristmills also grind corn into meal

(Grain mill with bevel gears) Although gristmill can refer to any mill that grinds grain, the term historically was used to refer to a local mill where farmers brought their own grain and received the flour from it, minus the "miller's toll"(8).

12. WILEY MILL

The Wiley mill is a laboratory-scale machine for grinding materials to a powder. This mill prepares materials for analysis with minimal moisture loss. Well-dried samples are preferred. In this grinding mill, the material is loaded cut into crude pieces or lumps and loaded into a hopper. From the hopper, the material drops by gravity into the path of a set of rotating hard tool steel blades (driven by an electric motor).

The revolving knives work against stationary knives and the resulting powder is forced through a steel screen. The powdered material then drops into a waiting collection vessel underneath. The Wiley mill is commonly used in agriculture and soil science laboratories (8).

REFRENCES: 1. Lynch, A., Rowland C., The history of grinding,SME, 2005 2. Gimpel, J., The Medieval Machine, Gollanz, 1976, Chapter 3. ^ Donald Routledge Hill (1996), "Engineering", p. 781, in (Rashed &

Morelon 1996, pp. 751-95) 4. ^ Donald Routledge Hill, "Mechanical Engineering in the Medieval Near

5. 6. 7.

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East", Scientific American, May 1991, pp. 64-69 (cf. Donald Routledge Hill, Mechanical Engineering) Unit operation in food processing – R.L EARLE Post harvest technologywww.googleimages.com www.wikipedia.org