Lecture 3 mixing Flashcards
What is mixing
Two or more ingredients in a separate / rough mix are treated so each particle of any one ingredient is as nearly as possible adjacent to a particle of each of the other ingredients
What is the rationale for mixing
Content uniformity /
each sample represents overall mix,
correct dosing
Positive mixing:
Systems that spontaneously / completely / irreversibly mix e.g. gases or 2 miscible liquids
Negative mixing
Two-phase system that differ in density – separate unless agitates e.g. solid suspended in liquid
Neutral mixing
Neither mixing nor demixing occurs unless force acts on system e.g. solids with solids
Random mix
Best achievable mix for solid powder. Probability of finding any one particle at any one point is equal
Perfect mix:
Not possible for solid powder mixes (hypothetical).
Chess board pattern
Ordered Mix
Two components interact to form ordered units e.g. adhesion or coating
What is the scale of scrutiny
The amount of material within which the quality of mixing is important
e.g. 200 mg tablet SoS -200 mg
Sampling Procedure:
Minimum 10 samples using sampling thief, removed from different parts of mix
Mechanisms of mixing: just state them
convective/macro mixing
shear mixing
diffusive mixing
DESCRIBE CONVECTIVE MIXING
ransferring groups of adjacent particles from one location in mass to another > repeat e.g. planetary mix
DESCRIBE SHEAR MIXING
Shear – slip planes in mass / relative displacement of two regions
DESCRIBE DIFFUSIVE MIXING
Powder bed under flow creates void space into which other particles flow under gravity e.g. tumbling mixer
Powder segregation / demixing - reasons
Due to difference in particle size / shape / density/ agg Percolation / elutriation segregation
Mixing equipment
Double cone, rotating cube, twin shell (V) mixer, ribbon mixer, high-speed mixer granulator, fluidised bed mixer, agitator mixer
