CHEE2945 – Lecture 10
Particle size enlargement:
- Particle size enlargement is the process of forming bigger particles from
smaller particles without changing the chemical identity of the particles.
- It is important for the pharmaceutical, food, agricultural, mineral, and ceramic
industries.
- Particle size enlargement is used to increase:
o particle size,
o mass,
o volume.
Methods of size enlargement:
- Common methods for increasing particle size include:
o Granulation.
Agglomeration by agitation.
Tumbling, mixing, fluidisation.
o Confined compression.
Tableting, roll pressing, extrusion.
o Droplet formation.
Spray drying.
o Flocculation/coagulation.
Forces for particle size enlargement:
- Size enlargement is influenced by:
o Externally applied forces.
o Inter-particle forces.
- Common inter-particle forces are:
o Electrostatic forces.
o Van der Walls forces.
o Liquid bridge (capillary) forces.
o Solid bridge forces.
- Small particles can easily become charged due to tribocharging (charging
from friction), which can cause unwanted electrostatic forces.
- Typically, there is an increasing strength in the forces holding granules
together as you go from top to bottom of the list above.
Electrostatic forces:
, - Particles can acquire an electrostatic charge due to inter-particle collisions, or
collisions against equipment surfaces.
- This causes an electron transfer between materials.
- Can be attractive or repulsive.
- Does not require contact between particles.
- Acts over long distances.
Van der Walls forces:
- Universally attractive between particles of the same material.
- Inversely proportional to particle separation.
Liquid bridge forces:
- Liquid bridges (capillaries) form between the particles when the relative
humidity is high (above ~65%).
- This is an attractive force.
- It is proportional to surface tension, γ . Typically, the higher the surface
tension, the stronger the bridges.
- For a completely wetting surface, the liquid bridge force is the sum of the
capillary force, and the force due to surface tension.
- The force due to the liquid bridge can be controlled by changing particle size
or surface tension.
Solid bridge forces:
- Granulated particles via liquid bridges are usually not the end product, as:
o The liquid may be removed.
o A dry particle may be desired.
- A solid bridge may take three forms:
o Crystalline bridges.
o Liquid binder bridges.
o Solid binder bridges.
- Crystalline bridges occur when the material is soluble in the liquid that is used
to create the granules.
- The material crystallises as the liquid evaporates.
- Liquid binder bridges are formed when the liquid used to form the granules
contains a binder or glue.
- The glue takes effect upon evaporation of solvent.
- Solid binder bridges are formed using finely ground solid particulate as a
binder.
- It produces a solid cement to hold particles together.
Particle size enlargement:
- Particle size enlargement is the process of forming bigger particles from
smaller particles without changing the chemical identity of the particles.
- It is important for the pharmaceutical, food, agricultural, mineral, and ceramic
industries.
- Particle size enlargement is used to increase:
o particle size,
o mass,
o volume.
Methods of size enlargement:
- Common methods for increasing particle size include:
o Granulation.
Agglomeration by agitation.
Tumbling, mixing, fluidisation.
o Confined compression.
Tableting, roll pressing, extrusion.
o Droplet formation.
Spray drying.
o Flocculation/coagulation.
Forces for particle size enlargement:
- Size enlargement is influenced by:
o Externally applied forces.
o Inter-particle forces.
- Common inter-particle forces are:
o Electrostatic forces.
o Van der Walls forces.
o Liquid bridge (capillary) forces.
o Solid bridge forces.
- Small particles can easily become charged due to tribocharging (charging
from friction), which can cause unwanted electrostatic forces.
- Typically, there is an increasing strength in the forces holding granules
together as you go from top to bottom of the list above.
Electrostatic forces:
, - Particles can acquire an electrostatic charge due to inter-particle collisions, or
collisions against equipment surfaces.
- This causes an electron transfer between materials.
- Can be attractive or repulsive.
- Does not require contact between particles.
- Acts over long distances.
Van der Walls forces:
- Universally attractive between particles of the same material.
- Inversely proportional to particle separation.
Liquid bridge forces:
- Liquid bridges (capillaries) form between the particles when the relative
humidity is high (above ~65%).
- This is an attractive force.
- It is proportional to surface tension, γ . Typically, the higher the surface
tension, the stronger the bridges.
- For a completely wetting surface, the liquid bridge force is the sum of the
capillary force, and the force due to surface tension.
- The force due to the liquid bridge can be controlled by changing particle size
or surface tension.
Solid bridge forces:
- Granulated particles via liquid bridges are usually not the end product, as:
o The liquid may be removed.
o A dry particle may be desired.
- A solid bridge may take three forms:
o Crystalline bridges.
o Liquid binder bridges.
o Solid binder bridges.
- Crystalline bridges occur when the material is soluble in the liquid that is used
to create the granules.
- The material crystallises as the liquid evaporates.
- Liquid binder bridges are formed when the liquid used to form the granules
contains a binder or glue.
- The glue takes effect upon evaporation of solvent.
- Solid binder bridges are formed using finely ground solid particulate as a
binder.
- It produces a solid cement to hold particles together.
