particle size reduction Flashcards
What is particle size reduction (comminution) important in:
Surface area,
absorption,
viscosity
mixing,
appearance,
stability
Small or large particles - which way to go?
smaller particles can have poor flow due to cohesion and adhesion
Surface hardness
size reduction is influenced by surface hardness
Draw mohs hardness
on whiteboard!!
An ideal milling equipment:
Would have a large capacity
Require a small power input per unit of product
Yield a product of the single size distribution desired
extent of size reduction is always related to
milling time
Milling equipment & Mechanisms of Particle Size Reduction
Cutting methods
Cutter mill
Compression methods
Mortar and pestle
End-runner mill
Edge-runner mill
Impact methods
Hammer mill
Vibration mill
Attrition methods
Roller mills
Combined impact and attrition methods
Ball mill
Fluid energy mill
Cutting methods - cutting mill - stationary and rotatory knives
Size reduction takes place with the aid of sharp blades
Principle: Cutting
Size reduction occurs by fracturing particles between stationary and rotating knives.
A screen is fitted to retain the material in the mill until a sufficient degree of size reduction is reached
size reduction range = 100 - 80,000 µm
Compression method -
We use the pestle and mortar
End runner mill —> this is a modified form of pestle and mortar
edge runner mill - the pestle is mounted horizontally and rotates against the powder bed
side reduction range 10 - 10,000 micrometres
Edge runner mill - in detail
There are two heavy wheels, either of stone or metal, connected by a shaft.
The wheels rotate at its axis in a shallow, circular pan.
The material is fed into the centre of the pan, and is worked outwards by the action of the wheels.
Scrapers are provided to scrape the material from the bottom of the vessel and feed it to the wheel, where it gets crushed to powder.
Size reduction is due to crushing and shearing
Impact methods - hammer mill
During milling, the hammers swing out radially from the rotating shaft.
The high velocity of the hammers cause brittle fracture to the particles. Small particles are less prone to fracture than larger particles.
Hammer mills produce powder with narrow size distributions.
Particles are retained within a mill by a screen which allows only adequately comminuted particles to pass through
Impact methods - vibration mill
Vibration mills are filled to approximately 80% total volume with porcelain or steel balls.
During milling, the whole body of the mill is vibrated, and size reduction occurs by repeated impaction.
Milled particles fall through a screen at the base of the mill.
The efficiency of vibratory milling is greater than for conventional ball milling.
Attrition methods - roller mills - explain
Size reduction takes place due to shearing of material in between the rollers
rollers roll in opposite directions
different speeds
1 - 500 microns
Combined impact and attrition methods - ball mill - speed of rotation - milling efficacy depends on:
Cascading: Low angular velocities: little movement of the balls, so that the size reduction is minimal (Fig a). Attrition dominates
Centrifugation: High angular velocities: The material and the balls rotate at the lining & there is no attrition or impaction. no size reduction occurs (Fig.b).
Cataracting: The ball reaches the top of the mill & falls on the material. Impact predominates in this process.
Combined impact and attrition methods - fluid energy mill
Air is injected at high pressure through nozzles at the bottom of the mill.
The high velocity air creates turbulence, into which solid particles are fed.
The kinetic energy of the air causes particles to collide with each other, generating sufficient momentum to fracture them.
Particle size reduction occurs through both impact and attrition.
Fine particles are carried out of the mill, while coarser particles remain inside for additional milling.
Combined impact and attrition methods - pin mill
Lower cylinder rotates at high speeds against the upper fixed cylinder.
Particle size reduction occurs by impaction with the pins and by attrition between pins as the particles travel outwards under the influence of centrifugal force.
particle properties
hard, tough, sticky, abrasive, friable
