size_2

Question 1

methods for size reduction

Answer 1

1. wet grinding
2. roll mill
3. hammer mill
4. cone mill
5. micronizing mill

Question 2

methods of size measurement

Answer 2

1. microscopy
2. sieving
3. electrozone sensing
4. laser diffraction
5. light scattering

Question 3

what is size reduction

Answer 3

mechanical process of reducing solid into smaller sub-division, also referred as comminution, pulverisation, grinding or milling

Question 4

reasons for size reduction

Answer 4

1. increase surface area for reaction
2. improve extraction of active principle
3. improve dispersibility in solution
4. allow better mixing or blending !!
5. enable rapid dissolution !!
6. preliminary process in preparation of products !!

Question 5

how does size affect dissolution

Answer 5

decrease size, increase SA, increase dissolution

Question 6

energy is loss by

Answer 6

• elastic deformation
• plastic deformation without fracture
• deformation to initial cracks
• deformation of machine parts
• inter-particle friction
• particle-wall friction
• heaty, sound, vibration

Question 7

milling process

Answer 7

particles fracture or abrade to give smaller particles:

1. crack opening by tensile spreading at crack tip
2. crack sliding by shear deformation parallel to crack direction
3. crack tearing by shear deformation perpendicular to crack direction

Question 8

consideration in size reduction

Answer 8

1. properties of material: thermolability, MP, flammability, deformation characteristics
2. fracture mechanics of particles: tensile strength, hardness
3. mechanical- type of equipment: impact, shear or pressure, material in contact with product (stainless steel, SS304, 316), temperature control

Question 9

size reduction mechanism

Answer 9

1. impact: particle concussion by a single force (hammer)
2. compression: particle disintegration by two rigid forces (nut cracker)
3. shear: produced by particle to particle interaction (scissors)
4. attrition: arising from particles scraping against one another or against a rigid surface (grinding stone)

Question 10

advantage of wet grinding

Answer 10

1. eliminate dust
2. easier to handle material
3. use less energy
4. increase mill capacity

Question 11

disadvantage of wet grinding

Answer 11

1. increase wear of grinding medium
2. not applicable to soluble materials
3. may need to dry products

Question 12

roll milling

Answer 12

grind paste, coarse crushing, peripheral velocity of rolls and clearance between rolls can be varied

best for soft material like creams with particulate

Question 13

types of hammer mill

Answer 13

1. rotor type/ rasping screen: lo energy calibrating mill
2. knife edge rotor: comminutor/ chopping mill- suitable for herbal cellulose breakdown
3. impact edge: pulverisation powder grinding

Question 14

which rotor speed is most effective

Answer 14

medium is most efficient and effective

• high speed: traps many small particles within the chamber
• low speed: very slow and inefficient, trapping mainly large particles

Question 15

advantages of hammer mill technology

Answer 15

1. several models available, vertical and horizontal designs
2. medium to high shear applications
3. blades and screens are interchangeable
4. suitable for very hard materials

Question 16

disadvantages of hammer mill technology

Answer 16

1. noisy and dusty
2. cannot plug feed (cannot put all the material in at once)
3. belt slip common due to load
4. high volume air generation, ventilation requirement
5. screen selection and installation complex, not scale
6. sifting required after milling
7. temperature rise due to friction

Question 17

operating principle of cone milling

Answer 17

• infeed falls into the conical screen chamber
• rotating impeller imparts a vortex flow pattern to the indeed material
• centrifugal acceleration forces particulates to the screen surface
• particles are continuously delivered to the action zone between the screen and impeller
• the particles are size reduced and instantly discharged through the screen openings

Question 18

micronising mills

Answer 18

1. vibratory ball mill
2. air jet mill
3. fluid energy mill

Question 19

Vibratory ball milling

Answer 19

• widely used for fine grinding
• extremely efficient with vibration, best wet
• balls: heavy stainless steel/ stoneware (ceramic)
• important factor: ball size and density
• cascading effect (long process time) or vibratory (vibro-energy- efficient by associated with rapid temperature rise; short process time or jacketed to remove heat)

Question 20

air jet mill (with classifier wheel)

Answer 20

• material gets into grinding chamber
• utilising high pressure air nozzle to direct particles into specific spot in the chamber
• particles accelerate and knock into each other
• classifier wheel generate high centrifugal force when spun at high speed
• if particles are small they will follow the escaping air into the classifier wheel out of the chamber

Question 21

major particle sizing methods

Answer 21

1. scales- vernier, mmicrometer (large particles >0.5mm)
2. sieves popular robust (>10mcm)
3. microscopy/ image analysis (5mcm-5mm)
4. laser diffraction (5mcm-5mm)
5. laser scattering (0.001mcm-5mcm)

Question 22

minor particle sizing methods

Answer 22

1. sedimentation- gravity, sedimentation (0.05 mcm-150mcm)
2. Scanning electron microscope SEM/ image analysis (nm-mcm)
3. atomic force microscope AFM/ image analysis nm-mcm)

Question 23

microscopy

Answer 23

direct visual examination and relatively cheap

• generally operator dependent and relatively few particles examined (real danger of non-representation)
• missing 10 micro meter particles = missing 1 thousand 1 micrometer particle

Question 24

Feret’s diameter

Answer 24

distance between two vertical lines tangent to the ends of the particle; greatest distance possible between any two points along the boundary of a region of interest

Question 25

Martin's diameter

Answer 25

length of horizontal line that appears to divide particles into halves

Question 26

sieves

Answer 26

low resolution method and usually only four to eight size fractions are provided, but good for quality control

Question 27

describe a size distribution plot

Answer 27

cumulative weight % frequency over sieve equivalent diameter; generally frequency increases with diameter

Question 28

how to find span

Answer 28

(D90-D10)/D50

Question 29

electrozone sensing

Answer 29

- developed originally for sizing blood cells - dispersed particles in an electrolyte as suspension - requires calibration standards that are expensive and can change size in distilled water and electrolyte - unaffected by optical properties, densities, colours and shapes of particles

Question 30

why are large particles not suitable for electrozone sensing

Answer 30

they sediment fast hence hard to size particles. below 2micrometer

Question 31

laser diffraction

Answer 31

more accurately called low angle light scattering LALLS, method of choice for particle characterisation and quality control