Milling Flashcards
What is comminution
Particle size reduction
What does size reduction affect?
- Mean particle size
- Its distribution
Why do particles size need to be reduced?
- Improves the efficiency of pharmaceutical processes e.g. mixing or suspension production
What is crack propagation?
- High localised stress produces strain in the particles that cause bond rupture and propagation of crack through the regions which has discontinuities
- Takes place during particle size reduction
What is Griffith’s theory?
- Describes crack propagation as strain energy in specific region
- mean stress around crack is called crack multiplier
Griffith’s theory equation
Ok = 1+2(L/2r) Ok = crack multiplier L = length of crack r = radius of curvature of the tip of the crack
What material is able to propagate a crack?
Brittle
How do plastic materials react to crack propagation?
- Resist fracture at larger stress
- Allow strain relaxation without crack propagation
- Atoms or molecules slip over one another
- Requires energy
Mohs’ scale
Table of minerals
Diamond has Mohs hardness > 7
Talc has Mohs hardness < 3
Brinell
Developed a quantitative measurement of surface hardness
- Hard spherical indenter in contact with test surface and known constant load applied to the sphere
- Permanent deformation is measured
- Hardness calculated
How can you reduce the particle size of elastic materials?
Lowering the temperature to below glass transition (Tg)
How much of the energy affects particle size reduction?
Very small ( about 2% )
What happens to the rest of the energy in comminution?
Lost due to:
- elastic/plastic deformation
- interparticulate friction
- particle-machine wall friction
- heat, sound, vibration
What is Rittinger’s hypothesis?
- Relates energy used in comminution to new surface area produced
E=Kr(Sn-Si)
Kr= Rittinger’s constant
Sn= New SA
Si = Initial SA
What is Kick’s theory?
- Relates energy used in comminution to ratio of change in size
E = Kk log (di/dn) Kk = Kick's constant di = Initial particle diameter dn = New particle diameter
Cutting methods
- Cutter Mill
How does cutter mill work?
Series of knives attached to horizontal rotor which acts against a series of knives attached to mill casing
Size reduction occurs by fracture of particle between the two knives
Size reduction range of cutter mill
100-50000 mcm
- useful in producing coarse particle
Impact methods
Hammer mill
Vibrational mill
How does hammer mill work?
Consists of four or more hammers hinged onto central shaft enclosed in a rigid metal case
Hammers swing out radially from rotating central shaft at an angular velocity of 80 s-1
Strain rate is so high most particles undergo brittle fracture
Size reduction range of hammer mill
50-5000 mcm
How does vibrational mill work?
About 80% of mill filled with porcelain or steel balls and mill is vibrated
Size reduction range of vibrational mill?
1-1000 mcm
Compression methods
Runner mill
Roller mill
How does runner mill work?
Using a mortar and pestle
Size reduction range of runner mill?
20-10000 mcm
How does roller mill work?
Two cylindrical rollers mounted horizontally and rotated along their long axes
- one driven directly
- second driven by friction
- material drawn through gap between rollers
Size reduction range of roller mill
500-100000 mcm
Attrition method
Roller mill
How does roller mill of attrition method work?
- Two or three porcelain or metal rollers mounted horizontally with adjustable gap - upto 20 mcm small
- Rollers rotate at different speeds and material is sheared as it passes through the gaps transferring from slower to faster roller
- useful for suspensions, pastes and ointments
Size reduction range of roller mill of attrition method?
1-200 mcm
Combined impact and attrition method
Ball mill
Fluid energy mill
How does ball mill work?
Hollow cylinder rotates on its horizontal longtitudinal axes
- 30-50% of mill occupied with different diameter balls
- smaller ones form fine product
Size reduction range of ball mill
1-200 mcm
How does fluid energy mill work
Hollow toroid has a diameter of 20-200mm and air is injected as high pressure jet through the nozzle forming zones of turbulence which ensures high levels of particle-particle collisions
Particle size classifier ensures material is retained
