Milling Flashcards
(35 cards)
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
