Size Reduction Processes in Materials Engineering PDF

CHAPTER -2 SIZE REDUCTION  Size reduction What is size reduction? What are the different mechanisms used? When do I choose one type of one size reduction method over another? What type of systems is best suited for my application.  Size reduction  The t...

CHAPTER -2 SIZE REDUCTION  Size reduction What is size reduction? What are the different mechanisms used? When do I choose one type of one size reduction method over another? What type of systems is best suited for my application.  Size reduction  The term “size reduction” is applied to all the methods in which particles of solids are cut or broken in to smaller pieces.  Objective of size reduction:  To increase surface area. In most reaction involving solid particles, the rate is directly proportional to the area of contact. E.g. combustion of solid particles Leaching or extraction Drying Mixing of solids  To increase the reactivity of solids.  It permits separation of unwanted ingredients by mechanical method.  It reduce the bulk fibrous materials for easier handling and for waste disposal. Mechanism of size reduction  Impact:  produced by a single rigid force.  When a single lump of material is subjected to a sudden impact, it will break into few relatively large particles + intermediate particle sizes + number of fine particles.  When the energy of impact increases, the larger particles will further disintegrate while the size of finer particles will not much altered.  The size of fine particles is closely connected with the internal structure of the material but larger particles with the process by which Impact forces  Compression: particle disintegration by two rigid force.  used to course reduction of hard solids to give relatively few fines.  Shear: produced by a fluid or by particle-particle interaction.  Attrition: arising from particles scraping against one another or against a rigid surface.  Yields very fine products from non abrasive material. Rubbing and shear force  Size reduction  The process of size reduction consists of two parts: Opening up any small fissures (A long narrow depression in a surface) which are already present. Secondly forming new surface.  From the point of energy utilization, size reduction is a very inefficient process Between 0.1 and 2% of the energy supplied to the machine appears to effectively increase the surface  The efficiency of the process depends on: The magnitude of the load Force patterns Force in the internal structure  Factors Influencing Size Reduction operation:  Feed and product size  Nature of material Hardness - very hard materials are better in low speed or low contact machines. Structure - fibrous materials need tearing or cutting action. Moisture content - materials with 5 - 50% moisture does not flow easily and can be difficult to process. Stickiness - sticky materials need easily cleaned machines. Soapiness - if coefficient of friction is low crushing may be difficult. Explosives - need inert atmosphere. Hazardous to health - need good confinement. Closeness of distribution Friability Types of size reduction equipments  The principal type of size reduction equipments are:  Crusher  for coarse and fine material  Compression E.g. jaw crushers, crushing rolls  Grinders  for intermediate and fine material  Impact and attrition sometimes combined with compression E.g. hammer mill, attrition mill, rod mill, ball mill, roller mill  Ultrafine grinders  Attrition E.g. hammer mill with internal classification, Fluid energy mill  Cutting equipment E.g. knife cutters, slitters  Jaw crusher  Feed is admitted between two jaws, which are open at the top like V.  One of the jaws is fixed and vertical, while the other is the swinging jaw.  This jaw reciprocates in a horizontal plane and makes the angle of 20-30° with the fixed jaw.  Movable jaw is operated by an eccentric unit so as to impart great compressive force.  Solids which has to be broken is caught between the two jaws.  Large lumps of solid materials are caught between the upper parts of the jaws ; Subsequently broken and dropped into the narrower space below.  Broken pieces are further reduced next time when jaws come closer.  Jaw crusher is usually for crushing of hard and intermediate hard substances. Jaw crusher  The forces exerted on the particle are quite  F1 2 F2 cos   F2 F1 2 cos   larger than the force F1 as he horizontal force on the particles inside the jaw :  gets closer   Fh F2 sin   Fh  F1 2 tan  to 90o particle at position x, moment about the pivot point of the swinging jaw Fp * x Fh * L  Fp F1 * L tan   2 x  Ball mill  This machine is described as a grinding device, capable to crush and transform large hard materials into fine material.  A ball mill is a cylindrical machine , generally used for mashing and crushing paints, ceramic materials, ores and other hard materials. It copes with feed up to 50 mm in size.  Cylindrical or conical shell slowly rotating about a horizontal axis.  Half of its volume is filled with solid grinding balls.  usually, stainless steel or ceramic balls are used as grinding material.  When the ball mill rotates, the ball crush and grind the material inside.  The outlet is covered with a coarse screen to prevent the escape of the ball.  During grinding, the balls wear and are constantly replaced by new ones so that the mill contains balls of various ages and various sizes.  This is advantageous since the large balls deal effectively with the feed and the small ones are responsible for giving a fine product.  The efficiency of grinding increases with the hold-up in the mill, until the voids between the balls are filled. Further increase in the quantity then lowers the efficiency. Solomon Bogale Chemical Eng'g Department  Factors influencing the size of the product: 1. The rate of feed: With high rates of feed, less size reduction is effected since the material is in the mill for a shorter time. 2. The properties of the feed material: A smaller size reduction is obtained with a hard material. 3. Weight of balls: A heavy charge of balls produces a fine product. 4. higher density: Small balls facilitate the production of fine material although they do not deal so effectively with the larger particles in the feed.  The limiting size reduction obtained with a given size of balls is known as the free grinding limit. For most economical operation, the smallest possible balls should be used. 5. The slope of the mill  An increase in the slope of the mill increases the capacity of the plant because the retention time is reduced, although a coarser product is obtained. 6. Discharge freedom  Increasing the freedom of discharge of the product has the same effect as increasing the slope. In some mills, the product is discharged through openings in the lining. 7. The speed of rotation of the mill.  At low speeds of rotation, the balls simply roll over one another and little crushing action is obtained. Ball mill The critical speed is when the ball reaches the vertical top position of the 2 FZ FG  mg mR' C shell without g g  C being  free to fall. R' R as R ' R  rp  R  0.5c  0.75c Nc = = Where: Nc - rational critical speed r - radius of the mill less than the particle Advantage of ball mill:  Used wet or dry although wet grinding facilities the removal of the product.  The cost of installation and power are low.  The grinding medium is cheap.  Suitable for materials of all degrees of hardness.  May be used with an inert atmosphere and therefore can be used for the grinding of explosive materials. Rod mill Rod Mill  In the rod mill, high carbon steel rods about 50 mm diameter and extending the whole length of the mill are used in place of balls.  This mill gives a very uniform fine product and power consumption is low, although it is not suitable for very tough materials and the feed should not exceed about 25 mm in size.  It is particularly useful with sticky materials which would hold the rods together in aggregates, because the greater weight of the rods causes them to pull apart again. Hammer mill Hammer Mill  The hammer mill is an impact mill employing a high speed rotating disc, to which are fixed a number of hammer bars which are swung outwards by centrifugal force.  The material is fed in either at the top or at the center and it is thrown out centrifugally and crushed by being beaten between the hammer bar.  The material is beaten until it is small enough to fall through the screen which forms the lower portion of the casing.  The size of the product is regulated by the size of the screen and the speed of rotation.  The bars are readily replaced when they are worn out.  Hammer mill is suitable for crushing of Brittle and fibrous materials Hard materials Sticky materials Crystalline solids  It is advisable to employ positive pressure lubrication to the bearings in order to prevent the entry of dust. Hammer Mill  Advantage of hammer mill  Comparatively little space is required.  Worn out part can be replaced easily.  The energy required is relatively smaller than for most size reduction equipments.  Can be built from a small size handling to large size.  Soft minerals and fibrous materials such as bones, wood, asbestos can be broken efficiently.  Strain free particles are produced that are not sensitive to cracking or breaking.  Roller Mill  The roller mill consists of a pair of rollers that rotate at different speed ( E.g. 3:1 ratio) and in opposite direction.  Since the rollers rotate at different speeds, size reduction is effected by a combination of compressive and shear forces.  As in crushing rolls, one of the rollers is held in a fixed bearing whereas the other has an adjustable spring –loaded bearing.  The clearance between two rollers can be adjusted according to the size of feed and the required size of product.  The roller mill is extensively used in the flour milling industry and for the manufacture of pigments for paints.  FR  FT cos 2  FR  FN sin  2  FT cos 2   FN sin  2  FT sin  2   FN cos 2  FT  tan 2   FN  0.3 0.35   30 o  39 o DR  DP  cos 2  Roller mill DR  DF  Attrition Mill  Attrition mill consist of disc mill with toothed conically arranged discs.  Integrated sound adsorber.  Arrangement of the grinding gaps outside the mill achieved by a central adjusting thread. Air classifying Mills  In air classifying mills, materials enter to the mill pneumatically or volumetric feed device.  The size of material is reduced by impact hammer then presented to classifier wheel.  Important variable in air classifier operation are:  Feed rate  Airflow through the mill  Configuration of grinding and impact surface i.e. smooth or deflection liner.  Peripheral speed and type of hammer  Speed and size of classifier wheel  Jet mill:  Jet mill is an ultrafine grinder using much higher kinetic impact energy than mechanical grinding.  The high velocity fluid carries the particles, resulting in collisions with other particles and/or surfaces in the grind chamber.  Common types of jet mills are 1. Spiral jet mill 2. Fluidized bed jet mill 3. Loop jet mill 4. Target jet mill Spiral Jet Mills  Use particle to particle collision and 50% mechanical impact for size reduction.  Feed material introduce into the mill through feed nozzle.  Primary Parameters affecting particle size are:  Feed rate  Nozzle size and angle  Grinding air pressure  Gas flow rate Spiral Jet Mills  Benefit  Simple design and operation  No moving parts  Easy to clean and inspect  Limitation  Lack of particle size control  Oversized feed particles can block inlet  Not good for hard, abrasive materials or heat sensitive material  noisy Fluidized Bed Jet Mills  Feed enters from top or side or bottom, not through jet nozzle.  Use particle to particle impact as the mechanism for size reduction.  High pressure compressed gas is used to accelerate particles to a central focal point where impact and size reduction take place.  Primary parameters  Use dynamic air classifier to affecting particle size are: control top particle size.  Classifier speed  Grinding air  Internal recycle of course pressure particles.  Grinding bed level Fluidized Bed Jet Mills  Benefit  Milling very hard, abrasive materials is possible  Improved and consistence product quality  Contamination free process  Excellent particle size control  Very stable, quite operation  Ability to process heat sensitive and sticky materials  No limitation on feed size.  Reduce energy cost  Limitation  Processing of small sample  Residual material in mill bed. Fluidized Bed vs. Spiral Jet Mills Milling classification Milling classification Methods of operating size reduction equipment's  Open circuit grinding  Closed circuit grinding  Open circuit grinding  In many mills the feed is broken into particles of satisfactory size by passing it once through the mill.  When no attempt is made to return oversize particles to the machine for further reduction , the mill is said to be operating in open circuit.  It require excessive amounts of power ; for much energy is wasted in regrinding particles that are already fine enough.  Closed circuit grinding  It is the term applied to the action of a mill and separator connected so that oversize particles are returned to the mill. Methods of size reduction Open circuit Closed circuit grinding grinding  Energy for size reduction  The most important parameter in size reduction is the power consumption which decides the energy efficiency of the size reduction equipment.  As little as 1 % of the applied energy may be used for size reduction, the other energy lost.  Energy consumed in process > Energy required to fracture particles.  The lost energy is consumed in:  Producing elastic deformation of the particles before fracture occurs.  Producing inelastic deformation which results in size reduction.  Causing elastic distortion of the equipment.  Friction between particles, and between particles and the machine.  Noise, heat and vibration in the plant.   Reduction ratio: is the ratio of the largest size in feed to the largest size in the product. dF RR  dP where : RR  reduction ratio d F  average feed particle size d P  average product particle size Draw back: Kick’s law: the work required for  The energy required to reduce shing a given quantity of material 100mm to 50mm is the same to onstant for a given reduction ratio that required for reducing 1mm espective of original size. to 0.5mm. E  dF   High energy is required to K k ln  reduce finer particles. m  dP   Can be applied for coarse where : E is energy consumed per unit mass. crushing K k is Kick ' s cons tan t 2.Rittinger’s law: the work required for size reduction is directly proportional to the new surface created. E  Applicable for intermediate and m ' ' K R S P  S F  Coarse size reduction where : E is energy consumed per unit mass S F' & S P' are specific surface areas per unit mass of feed and product K R is Rittinger ' s cons tan t E   S P'  S F'  m Rittinger ' s number E 6K R  1 1   1 1     '  K R  m S d ' dS', F   d dS', F   S ,P   S', P  where : d S ' , P is specific area per unit volume equivalent diameter of the product d S ' , F is specific area per unit volume equivalent diameter of the feed Overall energy efficiency =  3.Bond’s law: The total work useful in breakage that has been applied to a given weight of material is inversely proportional to the square root of the average particle size of the product  and the feed. E 1 1  K b   wi m  dP d F  is calculated Kb based on work index  where : K b is Bond ' s cons tan t Work index : is defined as the gross energy (in kWh/tonne of feed) necessary to reduce a very large feed to such a size that 80% of product particles will pass through a 0.1 mm screen E If d F  , d P 0.1mm and wi (in kWh / tonne) the m n Kb wi   K b 0.3162 wi 0.1 E  1 1  0.3162 wi    m  d 80, P d 80, F    For dry grinding, Applicable for fine size reduction multiply this values by EXAMPLE S SOLVED PROBLEMS A grinder is to be used (which is 8% efficient) to handle 10 tonnes/hr of siliceous ore (specific gravity = 2.65). The feed and product analysis are given below: The grinder operates on a 24 hour basis for 300 days per year. Electricity costs 0.7Birr per kWh. If the work index is 13.57 kWh/tonne, then calculate the annual energy cost. Screen Size [mm] Feed mass fraction Product mass fraction -3.327+2.362 0.143 0.0 -2.362+1.651 0.211 0.0 -1.651+1.168 0.230 0.0 -1.168+0.833 0.186 0.098 -0.833+0.589 0.120 0.234 -0.589+0.417 0.076 0.277 -0.417+0.295 0.034 0.149 -0.295+0.208 0.0 0.101 -0.208+0.147 0.0 0.068 -0.147+0.104 0.0 0.044 -0.104 0.0 0.029 olution: nergy per unit mass based on Bond’s law  1 1  E 0.3162 wi      80, P  80, F    From the cumulative distribution plot, the diameters from which 80% of particles are less for the product and the feed can be read.  80, P 0.7 mm ,  80, F 2.13mm  1 1  E 0.3162 *13.57 *    2.1816 kWh tonne  0.7 2.13  energy required  E * m F 2.1816 kWh tonne * 10 tonne hr 21.816 kWh hr energy sup plied E * m F  0.08 272.696 kWh hr Annual power consumption 272.696 kWh hr * 24 hr day * 300 day year 1,963,411 kWh year Annual energy cos t 1,963,411 kWh year * 0.7 Birr kWh 1,374,389 Birr year  Undersize Oversi ze Screen (xF) or (xP) or Σ(Dmi Σ(Dmi Σ(Dmi Σ(Dmi fmax fmin fi Size [mm] (Dmi)F (Dmi)P )F )P )F )P - - 2.36 2.844 3.327+2.36 3.32 3.327 2.362 0.143 0 1 1 0.143 0 2 5 2 7 - - 1.65 2.006 2.362+1.65 2.36 2.362 1.651 0.211 0 0.857 1 0.354 0 1 5 1 2 - - 1.16 1.409 1.651+1.16 1.65 1.651 1.168 0.23 0 0.646 1 0.584 0 8 5 8 1 - - 0.83 1.000 1.168+0.83 1.16 1.168 0.833 0.186 0.098 0.416 1 0.77 0.098 3 5 3 8 - - 0.58 0.833+0.58 0.83 0.833 0.589 0.711 0.12 0.234 0.23 0.902 0.89 0.332 9 9 3 - - 0.41 0.589+0.41 0.58 0.589 0.417 0.503 0.076 0.277 0.11 0.668 0.966 0.609 7 7 9 - - 0.29 0.417+0.29 0.41 0.417 0.295 0.356 0.034 0.149 0.034 0.391 1 0.758 5 5 7 - - 0.20 0.251 0.295+0.20 0.29 0.295 0.208 0 0.101 0 0.242 1 0.859  1.2 1 0.8 cumulative mass fraction 0.6 Feed-undersize Product-undersize Feed-oversize Product-oversize 0.4 0.2 0 0 0.5 1 1.5 2 2.5 for oversize fmin for undersize fmax END