Size Reduction PDF

1 1  Size Reduction (Comminution, Milling, grinding) Size reduction is the process of reducing large solid masses to smaller unit masses by mechanical means. ☻As the particle size is reduced, surface area is increased a factor that affects all the proper...

1 1  Size Reduction (Comminution, Milling, grinding) Size reduction is the process of reducing large solid masses to smaller unit masses by mechanical means. ☻As the particle size is reduced, surface area is increased a factor that affects all the properties of the particles. 2 1- Compression: material is crushed by pressure. (eg: Roller mill). 2- Impaction: the material is hit by an object moving at high speed or when moving particles strikes a stationary surface (e.g. Hammer mills). 3- Shear or cutting: The material is cut by means of sharp blades, )e.g., cutter mills(. 4- Attrition : The material is subjected to pressure but the surfaces are moving relative to each other or friction occur between two particles, )e.g.: Multiple drive roller mill). 5- Combined impact and attrition: (e.g.: Ball mill). 3 4 compression impaction cutting attrition 5 Size Reduction Equipment They are classified according to the particle size of the feed material into three classes: coarse, intermediate and fine crushers as shown in the following table: Coarse crushers Intermediate Fine crushers crushers Blake jaw crusher Hammer mill Ball mill Dodgejaw crusher Cutter mill Vibration mill Gyratory crusher Pin mill Fluid energy mill End runner mill Colloid mill 6 Edge runner mill  They are also classified according to the mechanism of size reduction into the following: Compression methods: e.g. Edge, end runner mill, and roller mill. Impact method: e.g. Hammer mill. Cutting method: e.g. Cutter mill. Attrition method: e.g. Multiple drive roller mill. Impact and attrition: e.g. Ball mill, pin mill and Fluid energy mill. vibration mill 7 A- Coarse crushers: Used when the feed particle size is from 40-150 mm or larger and to give products 5-50 mm particle diameter. Blake jaw crusher: It consists of a stationary jaw and a movable one. The maximum travel of the movable jaw is at the bottom. 8  Dodge crusher: It resembles the Blake jaw except that the movable jaw is fixed at the bottom. 9  The Gyratory crusher: It is constructed of a crushing head around it are concave crushing jaws. 10  Hammer mill There are two types : -The horizontal-shaft type. -The vertical-shaft type. widely used in the pharmaceutical industry. Intermediate as well as coarse feed can be used by simply changing hammer speeds and/or screen size and feed rates. The principle of operation : impact between the powder particles and rapidly moving hammers mounted on a rotor. 11  12  Screen is placed at the milling chamber outlet (at the bottom of the horizontal shaft mills and 360° around the vertical shaft mills). These screens serve to: - Retain the larger particles in the milling chamber for further size reduction. - Act in part as a classifier in allowing only certain size particles to pass out of the milling zone into the collector. 13  Hammers may take several shapes: - the two basic shapes are the stir-up and the bar, the bar-shaped hammers being used extensively in tablet granulating. Maximum mill versatility ‫ طالقة – كفائة‬is obtained by utilizing a hammer with a cutting blade on the trailing ‫ المتحرك_ الزائد‬edge of the bar. 14  Hammers may also be free-swinging. The free-swinging hammer has the advantage or disadvantage of increasing hammer-to-screen clearance if excessive buildup occurs within the mill. -This will minimize mill damage if the milled raw material fuses from heat, -or it may decrease output rate if the material is extremely difficult to mill. 15 16  air-swept hammer mill - in this type, air is forced through the milling chamber, entraining and carrying the smaller particles out of the milling chamber. - This air suspension of particles passes through a classifier, where the larger particles are further separated and recirculated back to the mill and the small particles are collected. - This type is useful where a small uniform particle size is required with heat-sensitive materials (the circulating air cools the particles & the mill). 17  Vertical-shaft mills - have feed inlets at the top (where powder enters the milling chamber perpendicular to the swing of the hammers). - have 360° surrounding screen, - normally have higher throughput potential than does horizontal- shaft equipment, in which the screen is mounted at the mill chamber outlet - There is some tendency to segregate the powder because the smallest particles exit through the screen near the top, whereas the larger particles, which have more mass, have a tendency to fall to the bottom of the mill before complete dispersion through the screen. the horizontal-shaft mills This is in contrast having the powder feed tangential to the hammer swing. the screen is mounted at the mill chamber outlet(at the bottom). 18 19 advantages of the hammer mill: 1- Ease of setup and cleanup. 2- May operate at closed system to reduce dust. 3- Simple to install and operate. 4- Capable of grinding many different materials. 5- Small space requirements. 6- Operation is continuous. 7- Minimum contamination of the product with metallic contaminants. 8- Particle size of the product can be controlled by variation of rotor speed, hammer type, size and shape of mesh, location of the hopper. 20  Disadvantages of hammer mill: 1- Clogging of screens may occur if the rate of feed is not controlled well. 2- Heat bluid-up during milling that may affect thermolabile drugs. 3- Mill and screen wear with abrasive materials. Uses: - It is a versatile equipment. It can be used for any type of size reduction (except sticky materials) - example for milled materials : barks, leaves, roots, crystals, filter cake, ointment, slurry and wet granulation. 21  Cutter mill It consists of a series of knives attached to a horizontal rotor (rotating knives or flying knives) which act against a series of stationary knives attached to the mill casing. A screen is fitted in the base of the mill casing that acts to retain material in the mill until sufficient degree of size reduction has been affected. 22  23  Size reduction occurs by fracture of particles between the two sets of knives, which have a clearance of few millimeters. Uses: -producing a coarse degree of size reduction of dried granules prior to tableting. - size reduction of tablet batches requiring rework and of fibrous crude drugs such as roots, peel or parks (liquorice). 24  Pin mill It consists of two discs closely spaced with vertical pins of their inner faces. The upper disc is stationary while the lower disc is rotated at high speed. The material is fed in through a hopper to the center of the upper disc, and is thrown outwards by centrifugal action Milling occure by impaction with the pins and attrition as the particles travel outside under centrifugal force. 25  26  The mill gives a fairly uniform fine product with little dust. Control of the size of the product is affected by means of the speed and the spacing of the projections. Uses: extensively used with chemicals, fertilizers and other materials that are non-abrasive and easily broken. 27  Single drive roller mill It consists of 2 cylindrical rolls made of metal or stone mounted horizontally and rotate on their longitudinal axes. The feed is introduced from the top and undergoes size reduction by compression. The gap between them is adjusted to control the degree of size reduction. One of the rolls is driven by a motor while the other runs freely. Uses: Size reduction of seeds prior to extraction of their fixed oils. 35  End and Edge Runner Mill The two mills are mechanized form of mortar and pestle-type that act by compression. 29  End runner mill Consists of a rotating pan or mortar made of cast iron or porcelain. A heavy pestle is mounted vertically within the pan. Rotation of the pan → the pestle revolves → size reduction by compression due to the weight of the pestle and by shear. A scraper is used to redirect the material into the grinding zone. 30  31  The Edge runner mill Consists of 1 or 2 heavy granite or cast iron wheels or mullers mounted on a horizontal shaft and standing in a heavy pan. Either the muller or the pan is driven. The material is fed into center of the pan. Comminution occurs by compression due to: 1. weight of the mullers. 2. shear forces. Uses: They are suitable for wet grinding of very viscous materials (as ointments and pastes). 32 33  Multiple drive roller mill It consists of 2 or 3 rolls of metal or porcelain, mounted horizontally with a small adjustable gap between the rolls. The rolls rotate at different speeds so that the material is sheared as it passes through the gap. The material is transferred from the slower to the faster roll and finally it is removed by scraper. Uses: Size reduction of solids in suspensions, pastes, ointments. 34 C- Fine crushers: These machines take feed of 2-5 mm and give products of particle size as fine as 0.1 mm in diameter (about 200- mesh). Finer products down to 0.01 mm may be obtained from a colloid mill which takes a feed particle size of about 0.2 mm. 35  Ball mill The ball mill is a cylindrical vessel usually filled to about 30-50% its volume with a grinding media ]which can be varied in size, size distribution, shape(balls, rods..), and material of construction [. Rod mill use rods instead of balls. useful for sticky materials because the greater weight of the rods causes them to pull apart. The feed enters through one end of the mill, and milled material exits through ports at the opposite end of the chamber. Uses: suitable for potent drugs (podophyllum resin). 36  37 38  Continuous closed circuit milling is arranged using the continuous mill with a gas or liquid classifying system. The classifier removes the smallest particles, while the larger particles are recycled to the mill inlet for further size reduction. Wet grinding with a liquid can also be carried out, in a very similar manner. 39  Factors affecting the size reduction process in a ball mill - At low speed: the balls will slide or roll over each other and negligible size reduction occurs. - At high speed : no grinding occurs because the balls will be thrown to the wall by centrifugal force. 40 - Sothe optimum speed at which maximum size reduction is obtained is at about 2/3 of the speed at which centrifugation just occur (Critical speed). - At the optimum speed: the balls are carried to the top of the mill and then fall in cascade across the diameter of the mill with the result of maximum size reduction due to impact of the particles by the balls and attrition between balls. 41  Amount of grinding medium (balls) which should not exceed 30-50% of the mill volume. oo much feed will lead to cushioning. -Too little feed will lead to loss of efficiency and abrasion. A fine product is obtained using small balls. - If Mixed balls charged, small ones deal with small particles while large ones act on the large sized feed. But this would interfere with the ball movement. Increasing weight of balls increases efficiency of the mill. facilitate milling if caking of the material may occur. -Surfactants may be added to prevent particle flocculation. 42  Advantages of ball mill: 1- Used for dry and wet grinding. 2- Used for batch and continuous. 3- Low labor cost. 4- control of dust and toxic hazards. 5- Can be sealed in case of sterile products. 6- Capable of grinding a wide variety of materials. Disadvantages of ball mill: 1- Possibility of contamination due to excessive wear of balls and casing. 2- Can not be used for soft and sticky materials (use Rod mill). 3- Very noisy machine. 43  Hardinge Mill It is similar to ball mill but the cylinder has a conical end towards the discharge opening. Accordingly, balls segregate, the largest collecting in the cylinder portion, and those decreasing in size towards the apex. As a result the coarser grinding is carried out by the large balls, the particle size decreasing as the material moves towards the smallest balls. Product is finer and more uniform than normal ball mill. 44  45 46  Vibration mill It is similar to ball mill but a vibration motion is used instead of rotation. The body of the mill is supported on powerful springs and caused to vibrate in a vertical direction. It allows: (1) fast grinding (1/10 the time). (2) higher capacity. (3) size range is closer. (4) minimum energy requirement. 47  48  Fluid energy mill (micronizer) It consists of a A jet of air is injected tangentially at high pressure through nozzles fitted tangentially at the bottom of the mill. Material is introduced into the stream of air and as a result of the high degree of turbulence, particle-particle collision is high enough to produce size reduction by impact and attrition. A classifier is incorporated at the outlet of the mill so that particles are retained until sufficiently fine. 49  50  51 Advantages of micronizer: 1-Thermo-labile substances can be milled because of the cooling effect of the expanding gases. 2-Inert gases can be used to minimize the oxidation. 3-Soft materials such as waxes may be milled by pre- chilling in liquid nitrogen to make them brittle. 4-The method is effective when particle size reduction is required to bring the slightly soluble drugs to a large surface area, and hence, enhance their dissolution and bioavailability. 52  Colloid mill Colloidal suspensions, emulsions and solid dispersions are produced by means of colloid mills. The mill consists of a flat rotor and stator. The material to be treated is fed to the center of the rotor and passes outwards under the action of centrifugal forces. The faces of the rotor and stator may be rough or smooth. The clearance between the rotor and stator is regulated by means of a micrometer adjusting device. The rotor is usually cooled by means of a water jacket. The material is always re-circulated until the desired degree of homogeneity is obtained. 53 54