size_2 Flashcards
methods for size reduction
- wet grinding
- roll mill
- hammer mill
- cone mill
- micronizing mill
methods of size measurement
- microscopy
- sieving
- electrozone sensing
- laser diffraction
- light scattering
what is size reduction
mechanical process of reducing solid into smaller sub-division, also referred as comminution, pulverisation, grinding or milling
reasons for size reduction
- increase surface area for reaction
- improve extraction of active principle
- improve dispersibility in solution
- allow better mixing or blending !!
- enable rapid dissolution !!
- preliminary process in preparation of products !!
how does size affect dissolution
decrease size, increase SA, increase dissolution
energy is loss by
- elastic deformation
- plastic deformation without fracture
- deformation to initial cracks
- deformation of machine parts
- inter-particle friction
- particle-wall friction
- heaty, sound, vibration
milling process
particles fracture or abrade to give smaller particles:
- crack opening by tensile spreading at crack tip
- crack sliding by shear deformation parallel to crack direction
- crack tearing by shear deformation perpendicular to crack direction
consideration in size reduction
- properties of material: thermolability, MP, flammability, deformation characteristics
- fracture mechanics of particles: tensile strength, hardness
- mechanical- type of equipment: impact, shear or pressure, material in contact with product (stainless steel, SS304, 316), temperature control
size reduction mechanism
- impact: particle concussion by a single force (hammer)
- compression: particle disintegration by two rigid forces (nut cracker)
- shear: produced by particle to particle interaction (scissors)
- attrition: arising from particles scraping against one another or against a rigid surface (grinding stone)
advantage of wet grinding
- eliminate dust
- easier to handle material
- use less energy
- increase mill capacity
disadvantage of wet grinding
- increase wear of grinding medium
- not applicable to soluble materials
- may need to dry products
roll milling
grind paste, coarse crushing, peripheral velocity of rolls and clearance between rolls can be varied
best for soft material like creams with particulate
types of hammer mill
- rotor type/ rasping screen: lo energy calibrating mill
- knife edge rotor: comminutor/ chopping mill- suitable for herbal cellulose breakdown
- impact edge: pulverisation powder grinding
which rotor speed is most effective
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
advantages of hammer mill technology
- several models available, vertical and horizontal designs
- medium to high shear applications
- blades and screens are interchangeable
- suitable for very hard materials
disadvantages of hammer mill technology
- noisy and dusty
- cannot plug feed (cannot put all the material in at once)
- belt slip common due to load
- high volume air generation, ventilation requirement
- screen selection and installation complex, not scale
- sifting required after milling
- temperature rise due to friction
operating principle of cone milling
- 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
micronising mills
- vibratory ball mill
- air jet mill
- fluid energy mill
Vibratory ball milling
- 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)
air jet mill (with classifier wheel)
- 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
major particle sizing methods
- scales- vernier, mmicrometer (large particles >0.5mm)
- sieves popular robust (>10mcm)
- microscopy/ image analysis (5mcm-5mm)
- laser diffraction (5mcm-5mm)
- laser scattering (0.001mcm-5mcm)
minor particle sizing methods
- sedimentation- gravity, sedimentation (0.05 mcm-150mcm)
- Scanning electron microscope SEM/ image analysis (nm-mcm)
- atomic force microscope AFM/ image analysis nm-mcm)
microscopy
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
Feret’s diameter
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
Martin’s diameter
length of horizontal line that appears to divide particles into halves
sieves
low resolution method and usually only four to eight size fractions are provided, but good for quality control
describe a size distribution plot
cumulative weight % frequency over sieve equivalent diameter; generally frequency increases with diameter
how to find span
(D90-D10)/D50
electrozone sensing
- 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
why are large particles not suitable for electrozone sensing
they sediment fast hence hard to size particles. below 2micrometer
laser diffraction
more accurately called low angle light scattering LALLS, method of choice for particle characterisation and quality control
