Milling part 2 2020
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The document discusses various milling techniques used in industrial pharmacy including impact, cutting, compression, attrition, and combined methods. It describes different types of mills like hammer mills, roller mills, and ball mills; and factors that influence particle size reduction like speed, screen size, number of balls, and feed rate. The goal of milling is to break down materials into the desired particle size range using techniques that minimize heat generation and avoid damaging the material being milled.
Milling part 2 2020
- 1. Industrial pharmacy I Lec 5: milling Pharmacist Hussein kadhim alkufi MSc in pharmaceutics Collage of thi-qar University of pharmacy
- 3. • The mechanical behavior of solid, which under stress are strained and deformed, is shown in the stress-strain curve. • The initial linear portion of the curve is defined by Hookes law ( stress is proportional to strain), and Youngs modulus (slope of linear portion) expresses the stiffness or softness in dynecm². • The stress-strain curve becomes nonlinear at the yield point, which is a measure of the resistance to permanent deformation. i.e is the point that cracks start to break the particles and persist beyond which for milling occuring
- 4. • Wit still greater stress, the region of irreversible plastic deformation is reached • The AUC represents the energy of fracture and is an approximate measure of the impact strength of the material. • Note 1. In all milling processes, it is a random matter if and, when a given particle is subjected to a sudden impact and is fractured, it yields a few relatively large particles and a number of fine particles, with relatively few particles of intermediate size. 2. So with time as smaller particles are formed we need more impact “ more energy is needed” for breaking up of particles as particle size decrease, but also because more energy is needed to initiate cracking. 3. The most efficient mills utilize less than 1 % of the energy input to fracture the particles
- 5. The rest of energy is dissipated in 1. Elastic deformation of unfractured particles 2. Transport of material within the milling chamber 3. Friction between particle 4. Friction between particle and mill 5. Heat 6. Vibration and noise 7. Inefficiency of transmission and motor
- 6. Energy of comminution The energy required to reduce the size of particles is inversely proportional to the size raised to some power. This may be expressed mathematically: Where dE is the amount of energy required to produce a change in size, dD, of unit mass of material, and where c and n are constants
- 7. Kicks hypothesis In this we have direct relationship between energy E and the reduction ratio D1D2
- 8. Rittinger hypothesis • In which we have direct relationship between the energy required and the increase in surface area
- 9. Method of milling Depending on the type of milling action or the principle of operation: we have different type 1. the cutting method 2. The pressure or compression method 3. Impact method 4. Attrition method 5. Combined impact and attrition method
- 10. These method are different in 1. The milling action or principle of action 2. The type of mill used or the milling medium 3. Particle size range. In µm or in mesh 4. Type of material to be milled Ex sticky or wetting, fibrous, friable, soft and abrasive material
- 11. Note: each milling equipment or mill consist of 3 basic part. 1. Feed chute, which delivers the material 2. Grinding mechanism, usually consisting of a rotor and stator for milling action 3. A discharge chute or chamber and sometime called selector. HW: the feeding and discharge speed or rate can effect the milling action and milling rate?
- 14. 1_Impact method a. Hammer mill b. Vibration mill
- 15. Hammer mill For this type the grinding medium is the number of swinging hammers fixed on a certain stator, this can be rotated in different rotation speed may reach to 10000 rpm in presence high speed rotter The material is retained until it is small enough to fall through the screen that form the lower portion of the casing. This type can be used at most in all drug except the abrasive material
- 18. Note: The particle size is controlled by 1_ select the speed of rotation 2_ the size and type of screen Q// Why speed of rotation hummer is crucial for particle size? HW
- 19. What is the effect of hammer speed and the thickness of screen on the particle size? If you have high speed rotation mostly we can get smaller particle size “ more decrease in particle size” because there is smaller angle under which the particles approach the screen and smaller screen, opening appear to the particles this will lead to increase the returning of material within the mill. If we have low speed “the reverse occur” the open appear larger and less reduction in particle size
- 20. Advantage: 1. Easy to setup and clean up 2. Minimal problems, wide range of particle size, small space is required Disadvantage a. Potential clogging of the screen b. The heat generated during milling may lead to degeneration or damage of the material to be milled B_ Vibration mill type
- 21. 2_ Cutting mill Size range 100 – 1000 µm Used for tough, fibrous materials and provide a successive cutting or shearing action rather than attrition impact The rotary knife cutter has a horizontal rotor with two to 12 knives spaced uniformly on its periphery turning from 200 to 900 rpm and a cylindric casting having several stationary knives attached to the milling casting
- 23. 3_ compression and compaction pressure • For small scale by use of mortal and pestle • For large scale by use of roller mill which consist of 2-5 cylindric smooth rollers operating at different speed. • One of rollers rotate directly but the second rotated by friction of another one. • This mill can give compaction action • Can be used for soft material • 100 – 10000 µm
- 24. 4_ Attrition method • Ex roller mill:. The material to be milled dispersion in susp paste or ointment. • The rollers used rotating at different speed so we have slower and faster roller • The material “after milling” can be removed by scrapper • Size range may reach to 1 µm
- 25. 5_ Combined impact and attrition method Ex: boll mill ( 1- 100 µm) ex pin mill (10 – 10000 µm) ex fluid energy mill (1- 10000 µm) boll similar vibration but differ in capacity, rotation speed so site of balls differ between them
- 26. Ball Ball milling is a size reduction technique that uses media in a rotating cylindrical chamber to mill materials to a fine powder.
- 28. The balls are of different size: 1_ the large one balls can give breaking down for the coarse particle 2_ small balls can make reduction for the fine particles Factors influencing the size of the product in the ball mill: 1. The rate of feed. 2. Weight of balls. 3. The diameter of the balls. 4. The slope of the mill. 5. Discharge freedom. 6. The speed.
- 29. Hw : 1_colloid mill 2_ special milling technique