Powder Mixing

Question 1

What is the definition of powder mixing?

Answer 1

Powder mixing is the process in which two or more powder components are treated so that each particle of one component lies as nearly as possible in contact with a particle of each additional component.

Question 2

What is the aim of powder mixing?

Answer 2

The aim is to achieve a homogeneous distribution of the individual components within the powder bulk.

Question 3

What are the components of a powder mix?

Answer 3

The components can include particles of different powders (e.g., drugs, excipients) and particle size fractions of the same powder.

Question 4

What is dry mixing?

Answer 4

Mixing of powders without the addition of a liquid phase

Question 5

What is Wet mixing?

Answer 5

Granulation - I.e powder mixed with a liquid binder

Question 6

What is Pre-Mixing?

Answer 6

Used for mixtures with less than 5 % w/w drug
- Mix then further deagglomerated by e.g sieving

Question 7

What is Post-mixing?

Answer 7

Addition of an external phase such as lubricant or gliding. Relative short mixing times (3-5 mins)

Question 8

Describe Tumbler Mixers

Answer 8

• For Free flowing powders
• Rotating vessels of various shapes e.g Y-cone, cylinder
• Rotation causes particles to tumble over each other on mixture surface

Question 9

What are the disadvantages of tumbler mixers?

Answer 9

• Prone to particle segregation, but fitting of internal impellers/prongs reduces segregation

Question 10

What are the advantages of tumbler mixers

Answer 10

• No particle attrition
• Useful for adding lubricants and gliding’s to granules

Question 11

Describe convective mixers?

Answer 11

• Mixer vessel is fixed not in rotation
• Internal impellers moves groups of particles from one location to another within the powder bulk

Question 12

What are the advantages of convective mixers

Answer 12

• Less fine particle segregation

Question 13

What are the disadvantages of Convective Mixers?

Answer 13

• Dead spaces, where powder hardly moved
• Adhesion to blades and inside surface of vessel
• Shear forces created an impeller surfaces can shatter powder particles
• Rarely used to dry powder mixing

Question 14

Describe Fluidised Bed Mixers

Answer 14

• Powder subjected to flowing gas stream
• Weight of particles counterbalanced by their ability to float in the air stream (buoyancy)
• Particle mobility is increased
• Turbulence
• Efficient and fast

Question 15

What are the economical and safety considerations?

Answer 15

• Energy Consumption
• Mixing time
• Continuous mixing or batch approach
• Time to fill, empty and clean
• Dust emission
• Explosion hazard due to electrostatic charging:
  • Surfaces involved
  • Mixer Speed
  • Relative Humidity of Environment

Question 16

What does diffusion mean in terms of mixing mechanisms?

Answer 16

• Random movement of individual particles in powder system, sometimes called ‘micromixing’

Question 17

What does convection mean in mixing mechanisms?

Answer 17

• Transfer of groups of particles, sometimes called ‘Macromaxing’

Question 18

What does shear mean in terms of mixing mechanisms

Answer 18

• Layers of particles move by sliding, so called ‘slip planes’

Question 19

What are three mechanisms incorporated into an efficient mixer?

Answer 19

• Simple tumbling mixers – free flowing powders
• Low shear blade/paddle mixers – moderately cohesive powders

e.g. Nautamixer – convective mixing (upward transport by screw,
diffusive mixing – particles percolate through mass

• High shear mixers – very cohesive, agglomerated powders