Assignment Uo.docx

State the purpose of size reduction The purpose of size reduction often meet stringent specifications regarding the size and sometimes the shape of the particles they contain, reducing the particles size increases the reactivity of solids, permits separation of unwanted ingredients by mechanical methods and reduces the bulk of fibrous materials for easier handling. Describe 4 common ways of solid broken: a. impact In crushing terminology, impact refers to the sharp, instantaneous collision of one moving object against another. Both objects may be moving, such as a baseball bat connecting with a fast ball, or one object may be motionless, such as a golf ball being driven off a tee. A sledge hammer and give coarse, medium, or fine products. b. attrition Attrition is a term applied to the reduction of materials by scrubbing it between two hard surfaces. Hammermills operate with close clearances between the hammers and the screen bars, and they reduce by attrition combined with shear and impact reduction. Though attrition consumes more power and increases wear on hammers and screen bars, it is practical for crushing less abrasive materials such as pure limestone and coal which yields very fine products from soft, nonabrasive materials. When a PCC Non-Reversible Hammermill is used for reduction, material is broken first by impact between hammers and material and then by shear and a scrubbing action (attrition) of material against screen bars. c. shear Shear consists of a trimming or cleaving action rather than the rubbing action associated with attrition. Shear is usually combined with other methods. For example, single-roll crushers employ shear together with impact and compression. A pair of shears exemplify these four types of action. Give a definite particle size and sometimes a definite shape with few or no fines. Reducing large feed by a combination of shear, impact and compression. PCC Hercules SingleRoll Crushers are noted for low headroom requirements and large capacity. d. compression As the name implies, crushing by compression is done between two surfaces, with the work being done by one or both surfaces. Jaw crushers using this method of compression are suitable for reducing extremely hard and abrasive rock. Nutcracker is used for coarse reduction of hard solids to give relatively few fines. As a mechanical reduction method, compression should be called for in these circumstances if the material is hard and tough, if the material is abrasive, if the material is not sticky and when the material will break cubically.

Describe the principle operation of crushers: a. jaw crushers The working principle of the jaw crushers is very simple. Powered by a diesel or gas motor, the jaw crusher brakes materials in a crushing chamber. The materials are pushed inside the chamber from the top opening and when crushed they are released through the bottom opening. The crushing power of the jaw crushers depends on the size of the chamber. The bigger the chamber, the more powerful the jaw crusher. The crushing process can be basically explained as a process where the materials are sandwiched between two jaw plates. The movable jaw plates squeeze and crush the materials into several small pieces. The main technical features of the jaw crushers are simple structure and convenient maintenance, stable performance and low operation cost, and even particle and large crushing ration. b. Gyratory crushers -In a gyratory crusher, a round moving crushing surface is located within a round hard shell which serves as the stationary surface. The crushing action is created by the closing the gap between the hard crushing surface attached to the spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric drive on the bottom of the spindle that causes the central vertical spindle to gyrate. c. Crushing rolls -Rolls crushers 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. These types of crushers are used in secondary or tertiary crushing applications. They are seeing a significant increase in use due to advances in their design and the improved liberation of minerals in the crushed product.

Describe the principle of grinders: a)hammer mills A hammer mill is a crusher that can grind, pulverize, and crush a wide range of materials. Hammer mill is defined as the device or the operator which is used to crush or milling of the agglomerates or large sized particles into small size particles with free flow properties depends upon the speed used in the mill. This crusher employs a rain of hammer blows to shatter and disintegrate the material. Hammer mills produce a finish product size that is dependent upon openings in perforated screens or grate bars, number, size and type of hammers, grinding plate setting and rotor speed.

b)rolling-compression mills : bowl mills Roller mill, a form of compression mill, consists of a single, double or triple cylindrical heavy wheel mounted horizontally that are stacked and enclosed in a steel frame; one of the roller is run by a motor and the others are rotated by friction as the material is drawn through the gap between the rollers. Roller mills use the process of stress (which is applied by the rotating wheels) and attrition in milling of solids in suspensions, pastes or ointments and some solid materials. The rollers rotate at different speeds and the material is sheared as it passes through the gap. Bowl Mill is driven by AC motor, coupled with flexible coupling to the worm shaft. The worm shaft drives worm gear, mounted on the Main Vertical Shaft. A bowl mounted on the top of the shaft rotates at a speed of 40 to 65 rpm. For a particular mill the speed is constant .Integral coolers are provided in the mill base with circulating water for removing heat from the oil. External lube oil systems are also used for cooling the oil in the gear box, which consists of a motor driven pump, cooler and filter. To prevent coal entering in to the Gear case, an air sealing system is provided. Hot primary air for drying and carrying pulverized coal, enters through insulated Mill side and Liner Assembly. Tramp Iron, pyrites and stones coming in the mill with raw coal move out of the bowl and being heavy, fall into the mill side and linear assembly. These are scraped and dumped in the pyrite hopper assembly through the Tramp Iron and Spout valve.