MO Lecture 2 Gosu PDF

2. Size Reduction  Introduction to Size Reduction: Size refers to the physical dimensions of an object. Reduction involves decrement or the process of decreasing the size. Size reduction refers to the process of converting the object from physical dimension of higher order to another di...

2. Size Reduction  Introduction to Size Reduction: Size refers to the physical dimensions of an object. Reduction involves decrement or the process of decreasing the size. Size reduction refers to the process of converting the object from physical dimension of higher order to another dimension of small order. It is the operation performed to reduce the size of larger particles into smaller ones of the desired size and shape using external forces. Size reduction process is also termed as comminution or diminution or pulverization  OBJECTIVE OF SIZE REDUCTION To increase the surface area, because in most chemical reactions and some unit operations (drying, adsorption, leaching, etc.) involving solid particles, the reaction/transfer rate is directly proportional to the area of contact between the solid and the second phase, To produce solid particles of desired shape, size or size ranges, and specific surface To separate unwanted particles effectively To dispose solid wastes easily To mix solid particles more intimately To improve the handling (storage and transportation) characteristics.  Applications of Size Reduction Size reduction operation is carried out in coal washeries, ore processing industries, chemical industry, Paint industry, pharmaceutical industry, cement industry, and food processing industry. In the food industry - sugar, spices, grains, etc. In the cement industry - lime, aluminum, sand, and cement clinker. In the paint industry - pigments. In the fertilizer industry - raw phosphate rock must be reduced in size. Size Reduction: External Forces There are four basic ways to reduce the size of a material 1. Impact 2. Compression 3. Attrition 4. Cutting Most of the size-reduction equipment's employ a combination of all these size reduction methods  Compression Here, the particle is broken by two forces and the size reduction is done between two surfaces, with the work being done by one or both surfaces. Compression is chosen (i) if the material is hard and tough, (ii) if the material is abrasive, (iii) if the material is not sticky, (iv) where the finished product is to be relatively coarser in size, and (v) when the material will break cubically. Coarse reduction of hard solids to fines, Coarse reduction (NUTCRACKER) reduction to a size of up to 3 mm Impact Impact occurs when moving particles strikes against a stationary phase. In the same way, particles moving at high speed collide each other and produce smaller particles. Here, the particle is subjected to a single violent force. Feed: Brittle, hard, Abrasive, High moisture Medium reduction Product: fines, intermediate, some coarse Types of Impacts: gravity impact and dynamic impact Example: Hammer mill  Attrition It involves breaking down of the solid material by rubbing action scrubbing between two surfaces. It is generally necessary for fine grinding. Attrition consumes more power It is preferred for crushing the less abrasive materials such as pure limestone and coal Useful for the material is friable or not too abrasive Feed: Soft; Non-Abrasive Product: fines Example: Fluid energy mill.  Cutting The material is cut by means of sharp blade. It is useful for the commination of fibrous or waxy solids. Feed: Ductile, Fibrous Product: definite shape / definite size Example: Rotary knife Cutter Properties of solids For a particular size-reduction operation, the choice of machine to be used mainly depends on (i) the size and the quantity of material to be handled, and (ii) the nature of the product required. But the more important aspects about the feed material apart from its size and quantity are its properties such as hardness, toughness, stickiness, moisture content, friability, explosive nature, soapiness, crystallinity, and temperature sensitivity Hardness: The hardness of the material is its resistance to scratching/ deformation and it affects the power consumption and the wear on the grinding machine. It refers to the surface property of a material and is measured on the " Moh's scale" ranging from 1 to 10. Materials with a hardness below 3 are soft, between 3 and 7 are intermediate, and above 7 are considered hard. A very hard but brittle material may not pose significant problems in size reduction. Moh’s hardness number Toughness: Toughness is the resistance of a material to impact. Toughness indicates a material's ability to resist fracture or deformation. It is the reverse of friability or brittleness. A soft but tough material can cause more challenges in size reduction compared to a hard but brittle substance. Stickiness: Some materials may adhere to the grinding surface or screen meshes during size reduction, causing difficulties. Gummy substances, in particular, can be problematic in the size reduction process, especially it generate heat. Friability: The friability of the material is its tendency to fracture during normal handling. Generally, crystalline materials will break along well-defined planes. Moisture Content: Materials do not flow well if the moisture content is higher (more than 3 to 4%by weight). They tend to cake and clog the machine which reduces the crushing effectiveness. Too dry a condition can result in excessive dust. Softening Temperature: The heat generated during size reduction can result in loss of heat-sensitive components from solids. Softening or melting may also be important-leading to clogging. In some cases, cryogenic crushing may be necessary using liquid nitrogen or, dry ice, e.g., in milling of spices (e.g., stearic acid or drug-containing oils), to soften. Explosive Nature: Explosive materials must be ground under wet conditions or in the presence of an inert environment.  Factors affecting size Reduction process Apart from the properties of solids, certain factors affecting the size-reduction process in terms of capacity and the performance Presence of moisture and sticky materials in equipment's feed Presence of fines in the feed Segregation of feed particles in the crushing chamber Lack of feed control Wrong motor size Insufficient crusher discharge area Insufficient capacity of the crushers discharge conveyor Materials being extremely hard to crush Power consumption, and Selection of an appropriate crushing chamber. Energy and Power Consumption in Size Reduction Material feed is distorted and strained Energy is stored as mechanical energy of stress Additional force beyond ultimate strength Fracture occur New surfaces generated  Crushing Efficiency Crushing efficiency (ηc) is define by amount of surface energy created by crushing out of the total energy absorbed by the solid. Es= Surface energy per unit area, J/m2 Assp and Assf = Areas per unit mass of product and feed, respectively WA = Total energy absorbed by a unit mass of solid, J/kg  Mechanical Efficiency Mechanical efficiency (ηm) is defined by extent of energy absorbed by the solid from the energy input to the machine. W= Total energy/work input, J/kg WA = Total energy absorbed by a unit mass of solid, J/kg So, total energy input to the machine can be derived as Power Required for Continuous Operation If is the flow rate of solids to the machine then the power required, P by the machine is the product of the total energy input and the flow rate as: It can be understood that for higher flow rate and materials with high surface energy, the power consumption would be higher. The expressions for the specific surfaces of feed and product materials where, Φ , Φ = Sphericity of the feed and the product materials, f p Dvsf, Dvsp = Sauter mean diameter for the feed and the product, m and ρpf, ρpp = Density of the feed and the product materials, kg/m3 For the homogeneous materials, This relation tells us that the power requirement for crushing will be more for particles having higher surface energy and also for the higher flow rate  Laws of Comminution It is almost impossible to find out the accurate amount of energy requirement in order to effect size reduction of a given material. It is due to these reasons: There is a wide variation in the size and shape of particles both in the feed and product. Parts of input energy is wasted as heat and sound, which can not be determined exactly. There are three empirical laws have been proposed to relate the size reduction with the energy input to the machine. 1. Rittinger’s law (1867) 2. Kick’s law (1885) 3. Bond’s law (1952)  Rittinger’s law According to this law, the work required for size reduction is proportional to the new surface area created. P W E P W E For particles of constant sphericity and density, the work required will be Where, , =Sauter mean diameter for the feed and the product, and , is known as Rittinger’s constant. The inverse of Rittinger’s constant is known as Rittinger’s number. Rittinger s law is applicable mainly to that part of the process, where new surface is being created and holds most accurately for fine grinding where the increase in surface per unit mass of material is predominant. This law is applicable for feed size of less than 0.05 mm.  Kick’s law This law states that the work required for crushing a given mass of material is constant for a given reduction ratio irrespective of the initial size. It is represented as: Where, , =Sauter mean diameter for the feed and the product, and is known as Kick’s constant. The reduction ratio is the ratio of initial particle size to final particle size. More accurate for coarse crushing where the surface area produced per unit mass is considerably less. This law is applicable for feed size of greater than 50 mm. Bond’s law This law states that the work required to form particles of size Dpp from a very large particle size is proportional to the square root of the surface to volume ratio Sp/Vp of the product. It is represented as: P Where, , = particle size of the feed and the product, respectively and is known as Bond’s constant. This law is applicable for feed size between 0.05 and 50 mm. The Bond s constant (Kb), is dependent on the type of machine used and, on the material, to be crushed. And it is found more accurately using Work Index Wi  Work Index Wi It is defined as the gross energy requirement in kilowatt-hours per short ton of feed to reduce a very large particle to a size at which 80 percentage of the product will pass through a 100-µm or 0.1 - mm screen. √Dpp Now, if P is in kW, m is in tons per hour, and (i) D is in μm then K = 10 pp b , and (ii) D is in mm then K = pp b = 0.3162 Thus, if 80% of feed particles pass through a Dpf millimeter screen and 80% of product particles pass through a millimeter screen, then W P The Bond work index provides a measure of how much energy is required to grind a sample of materials. For dry grinding the materials, these values are multiplied by 4/3. Hard Very Hard Differential form of laws  Classification of Size Reduction Equipments Mode of Size of feed and Operation Method by which a force is product applied Coarse Crusher Shear action of the Impact surrounding (Large feed size to medium 50-5 mm product Batch Operated size) Impact between Intermediate Compression Non-mechanical Crusher particles between two solid introduction of Continuous surfaces energy (50-5 mm feed size Operated to 5-0.1 mm product size) Rubbing the materials between Thermal Shock Impact at one two surfaces Fine surface Crusher/grinders Explosive (5-2 mm feed size shattering to ≅ 200-mesh) Crushing Ultrafine grinders (6 mm feed size to 50-1-µm) Cutting machines Grinding (definite size between 2-10 mm length)  Coarse crushers (Large feed size to 50-5 mm product size) Slow-speed machine for coarse reduction of large quantities of solids break large pieces of solid material into small lumps Primary crusher - accepts anything from mine & breaks into 150 - 250 mm Secondary crusher - reduces lumps into 5-6 mm main types: 1) Jaw crushers 2) Gyratory crushers 3) Smooth-roll crushers 4) Toothed-roll crushers Gyratory crusher Intermediate Crusher (50-5 mm feed size to 5-0.1 mm product size) For intermediate duty (from crushers to grinders for further reduction) Reduce crushed feed to powder Product from intermediate grinder might be 5-0.1 mm 1) Hammer mills & impactor 2) Roller mill 3) Cage mills 4) Granulator Fine Crusher/grinders (5-2 mm feed size to - mesh) Fine grinding which reduces the intermediate product to a finer size Product from fine grinder would pass a 200-mesh screen (74μm screen) commercial grinders: Reduce the solid size by impact, attrition Ball mill Pebble mill Rod mill Tube mill Attrition mill/Pulveriser  Ultrafine grinders: (6 mm feed size to 50-1-µm) Reduce solids to fine particles Accepts feed particles not larger than 6 mm. The product size is typically 1 to 50 μm. Reduce the solid size by attrition Examples 1. Classifying hammer mills, 2. Fluid Energy Mills 3. Agitated Mills 4. Colloid Mills. Cutting machines: (definite size between 2-10 mm length) Give particles of definite size and shape 2-10 mm in length Reduce the size by cutting, dicing and slitting. Examples Cutters, Knife, Scissors – produce cube, thin squares or diamonds.  Size Reduction Equipment Classification A. Crushers B. Grinder (intermediate and fine) C. Ultrafine grinder (coarse and fine) 1. Jaw crushers 1. Tumbling mills: rod mill, ball mill, 1. Fluid energy mill 2. Gyratory pebble mills, tube mills 2. Agitated mills crushers 2. Attrition mill 3. Hammer mills with 3. Crushing rolls 3. Hammer mill internal classification Compression Impact and Attrition, sometimes Attrition combined with compression McCabe, W.L., Smith, J.C., and Harriott, P., “Unit Operations of Chemical Engineering”, 6 th ed., McGraw Hill, 2001.  Methods of Feeding Two methods of feeding material to a crusher are possible, Free crushing: Feeding the material at a comparatively low rate so that the product can readily escape. Residence time in the machine is sort. And then production of appreciable quantities of undersize material is avoided. Choke Feeding Machine is kept full of material and discharge of product is impeded so that the material remains in the crusher for a longer period right. Higher degree of crushing The energy consumption would be very high because of accumulated product inside the machine. Used only for small amounts of material when it is desired to complete the whole of size reduction in one operation Coarse crushers (Large feed size to 50-5 mm product size) Jaw crushers Two jaw - V opening One – Fixed jaw – anvil jaw; Other – moving jaw – swinging jaw Angle b/w jaws 20o – 30o Jaws are flat or slightly bulged Crushed at upper portion & then dropped and re-crushed at narrow end. Jaw open and close 250-400 times per minute Eccentric drive a pitman which is connected to toggles among which connected to moving jaw Capacity: 1200 ton/hr Distance between the two jaw plates at the feed opening is known as gape  The jaws themselves are usually constructed from cast steel and are fitted with replaceable liners, made from manganese steel, or "Ni-hard", a Ni-Cr alloyed cast iron  PITMAN Single toggle jaw crusher Double toggle jaw crusher (two shafts) escape. Less uniformity in product size Single Toggle Jaw Crusher Double Toggle Jaw Crusher Dodge crusher It has a large opening at the top enables it to take large feed. Material is crushed at upper portion, then dropped and recrushed at narrow end. The constant opening at the discharge end gives the crusher an annoying tendency to clog. This type of crusher is effective for hard and abrasive materials as the crushing force is concentrated on a single point. More uniformity in the product size. Capacity of Jaw Crusher The theoretical capacity of a jaw crusher is Industrial Applications Crushers are used 1.Heavy duty mining 2.Cement industry 3.Fertilizers industry 4.Plaster of Paris Plant  Advantages of Jaw Crusher Simple construction makes for easy maintenance Stable performance High crushing ratio Low operational cost High capacity Adjustable discharge port It costs less energy than other crushers and it causes less environmental problems by making less noise and less dust. Demerits: Jaw Crushers Very expensive The crushing action is intermittent due to which heavy foundations required. It is impossible to stop jaw crusher in emergency due to heavy flywheel. Restarting with choked machine is impossible. Flat shape particles can pass through jaws uncrushed. Product size is in between 20-50 mm Reduction ratio is 4:1 to 6:1  Product size is in between 20-50 mm Suitable for all hard materials, abrasive materials, b not for soft and porous materials  The smooth mantle design is an ideal choice for high abrasive applications Features of Gyratory: High capacity per investment dollar. Handles thin, slab-like material better than a jaw crusher. Feeding is simpler. Foundation costs are lower than for jaw crusher Easier installation than JC since more compact design and crushing stresses more even. Features of Jaw Crusher: 1. Large receiving opening per investment dollar. 2. Shape of opening favors large block shaped feed better than a gyratory. 3. Handles dirty or sticky feed better, without buildup, 4. Maintenance is easier. 5. Can be easily split into sections, which makes it easier to carry it underground. ROLL CRUSHER The history of roll crushers is more than 200 years old but in recent years, lost their popularity over jaw and gyratory crushers due to their poor wear characteristics with hard rocks. Depending on the number of rolls employed, roll crushers are of two types 1. Single-roll crushers 2. Double-roll crushers. The single-roll crusher is one of the oldest and the simplest crushers which are mainly used for primary crushing, whereas double-roll crushers are used for secondary crushing.  Single-roll crushers Single-roll crushers employ three different methods of size reduction -impact, shear, and compression Single-roll crushers have a roll assembly consisting of a roll shaft and a fabricated roll shell with integral fixed teeth breaker plate Feed Material Entry: 1. Feed material enters the crusher through the feed hopper. 2. The material comes into contact with the teeth of the revolving roll. 3. Initial breakage occurs due to impact with the teeth. Crushing Chamber : Roll 1.The rotation of the roll moves the material into the crushing chamber. 2.The chamber is formed between the breaker plate and the roll itself. 3.Material compression and shearing occur in this chamber. Material Compression and Shearing: 1. The turning roll compresses the material against the stationary breaker plate. 2. The teeth on the roll shear the material, further breaking it down. Sized Material Discharge: 1. Properly sized materials fall directly out through the discharge end. 2. The discharge end is fully open, without screen bars. Product discharge 3. No re-crushing of sized materials occurs, reducing power demand and fines. Product sizes ranging from 75 to Adjustable Product Size: 300 mm depend on machine size 1. The clearance between the breaker plate and the roll determines product size. 2. Product size can be adjusted from outside the machine using a shim arrangement. Applications: Single-roll crushers Petroleum Coke Coal Limestone Phosphate Rock  Advantages of Single Roll Crusher: Simplicity: Single roll crushers are relatively simple in design and operation, making them easy to maintain and operate. Compact Design: They have a compact design, which makes them suitable for installations with limited space. Uniform Product Size: Single roll crushers provide consistent and controlled product size due to their one-roll design. Economical: They are cost-effective and require less maintenance compared to some other crushing equipment. Versatility: Single roll crushers can handle a wide range of materials and sizes, making them versatile for various applications.  Disadvantages of Single Roll Crusher Limited Feed Size: Single roll crushers are not suitable for large feed sizes and may struggle with oversized materials. High Wear and Tear: The single roll's surface is subject to high wear and tear, which can result in higher maintenance and replacement costs. Limited Reduction Ratio: limited reduction ratios compared to other types of crushers. Limited Control: The adjustable clearance between the roll and the breaker plate may not provide as precise control over the final product size as other crushers. The material fed to the machine is protected by spring loading  Crushing Roll Double roll coal, limestone, clay limestone, gypsum, and iron ore Single Roll Crusher provides up to a 6:1 ratio of reduction and double roll crusher 4:1. Features and benefits of toothed roller crusher: Good Performance with Moisture Control: Suitable for materials with moisture content up to 15%. Even Discharging Size: Ensures consistent product size without blocking. Environmental Protection: Full-sealed design prevents dust pollution. Meets environmental standards. Sample Collection Convenient sample collection for analysis. Efficient Maintenance: Removable shell for easy maintenance and inspection. Attractive Appearance: Total electro-static coating for a polished look. Easy to clean and maintain appearance. Work Safety: All moving parts have safeguards for work safety. Applications: Double Tooth Roller Crushing of Oil Seeds Coal Phosphate Rocks Abrasive Materials Lime Limestone Petroleum Coke Explosive Materials In Gunpowder Industries.  Roll Crusher Capacity The capacity of roll crushers can be calculated using the theoretical formula: Q (theoretical) = 188.5 N D W s d N: Speed of rolls (rpm); D: Roll diameter (m); W: Roll width (m); s: Density of feed material (kg/m³); d: Distance between the rolls (m). In practice, several factors influence the actual capacity of roll crushers: Voids between Particles: Considering the spaces between particles. Loss of Speed: Accounting for speed loss when gripping the feed. Feed Characteristics: The nature of the material being processed. Machine Design: Roll size, configuration, and operating conditions. Actual Capacity ≈ 0.25 * Theoretical Capacity https://www.sciencedirect.com/topics/engineering/roll-crusher  Advantages of Roll Crushers High tonnages up to 12000 MTPH. Compact in size and therefore reduced installation cost. Cannot mobile: lower capital cost than any other crusher. Large reduction ratio in relation to tonnages.  Disadvantage of Roll Crushers Low reduction ratio. Required a high-performance electrical system as there are peak power loads Can not use economically for low tonnages. Will not handle trash metal. Selection of crushing rolls While selecting the rolls for a certain duty, it is necessary to know (i) size of feed, (ii) size of the product, and (iii) Amount of material to be handled. Angle of Nip The angle of nip is the angle formed by the tangents to the roll faces at a point of contact with a particle to be crushed. Angle of Bite :