Practical 3 Flashcards
What is mixing
two or more ingredients in a separate or roughly mixed condition are treated
so that each particle of each of the other ingredients
Rationale of mixing
mixing is carried out to secure the uniformity of composition so small samples withdrawn from a bulk material represent the overall composition of the mixture
what is positive mixing
Applies to systems that would spontaneously, completely and irreversibly mix, such as two gases or two miscible liquids.
what is negative mixing
any two phase mixing in which the phases differ in density, will separate unless continuously agitated
e.g. solid suspended in liquid
what is neutral mixing
neither mixing nor remixing takes place
UNLESS system is acted on by a system of forces
what are the 3 types of mixing
Random mix - Random
Perfect mix – Gases / miscible liquids
Ordered mix – Adhesion / coating
what is the minimum number of samples that we can use
minimum = 10
what is the sampling procedure
use a sampling theif
samples must be removed from different part of the mixer (different depths middle and sides)
do not disturb the powder bed during sampling
UV spec to measure absorbance
what are the 3 mechanisms of mixing
connective
shear
diffusive
to achieve a good mix it is imperative that there is
inner particulate movement
what is convective mixing
transferring groups of adjacent particles from one location in the mass to another
what happens in convective mixing
the whole volume of material is continuously divided up and then mixed again after the portions have changed places
coarse scale dispersion (MACROMIXING)
what does connective mixing predominating in machines do
utilises a mixing element moving in a stationary container
planetary mixer, horizontal mixer
what is shear mixing and when does it occur
it is setting up slip planes within the mass
it occurs when a system of forces acting on the particles includes the formation of a slip plane = gives relative displacement of two regions
what is diffusive mixing
distributing particles over a freshly developing surface
what happens when a bed is forced to flow (diffusive mixing)
it will dilate (increase in volume) which creates the void space
particles fall under gravity through the voids created
what does diffusive mixing have the potential to produce
a random mix
what is powder segregation
components separate out
A mix changes from random to and as a result what happens
it may change from random to non random
as a result = fails uniformity of content test
when does segregation happen
during storage
transfer to filing machines
in the hopper of a tablet/capsule/satchet filling machine
what is an example of powder mixing equipment
tumbler mixers or blenders (non cohesive blending)
what do tumbling mixers operate via (the mechanism)
diffusive mechanism
what are tumbling mixers suitable for and why
free flowing powder and granules
because shear forces generated are usually insufficient to break up and any aggregates
which mix is the best for solids
random mix
