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|1. Why powders segregate ?|
|2. What are the mechanisms of segregation ?|
|3. Segregation in mixers|
|4. Good design practices to avoid demixing|
The difficulty of mixing is mainly due to differences in particle sizes. If the span in particles sizes within a mix is high, it will take longer time to reach the optimum CV homogeneity, and the value of the CV will be higher than for a mix with particles of similar sizes.
Conversely, a mixture made of particles of different dimensions will always tend to separate, grouping the particles of one size in one area, and the particles of another size in another area. This phenomenon is called segregation or demixing. It must be noted that all industrial mix will be made of particles of different dimensions, which means that all mix will be subjected to demixing in processing steps following the mixer.
Segregation is a function of of size, density, shape and resilience of the particles. But as mentioned above the primary factor is the size of the particle and more especially the difference of sizes between the particles of the mix . As a rule of thumb, segregation will be a problem for a ratio of particle sizes > 1.3.
Shape of particles is interesting : if particles have complex shapes leading to interlocking between each other, and therefore bad flow, will be difficult to mix. But once mixed, the interlocking effect will reduce the segregation compared to what happens with free flowing materials.
There are mainly 5 mechanisms leading to segregation  
|Percolation segregation||When the mixture is moved, the gaps in between large
particles open and small particles then can go below them.
Repetition of the movement allows the small particles to
concentrate at the bottom of the mix while coarse ones are
at the top.
Typical unit operations creating such demixing mechanism : filling and discharging a silo, tipping free fall of powder into a heap in a silo or a container
|Flotation segregation||Large particles float on the bed of of solids. It is due
to the vibration of the mixture that allows the small
particles to flow to spaces below the big particles.
Typical unit operations creating such issues : vibrating sieves (if not operated properly, with too much product on the sieve deck, the phenomena will be increased), silos with vibrating bottoms, vibrating tables of Big Bags
|Transport segregation||During transport in a gas flow, particles of different
sizes are subjected to different forces and will therefore
adopt different path leading to segregation.
It is the gas in a pneumatic conveying line for example where drag forces will not apply the same on big and small particles, leading to different transport speed. At the reception of a conveying line, in a silo for example, the inertia of the particles will depend on their size and they will adopt different trajectories leading to some segregation.
|Elutriation||This phenomena is a bit similar to the previous one but is to be considered in the case of free fall material : the fines (less than 50 microns typically) will stay in suspension in air longer than coarse particles and will finally fall on top of the powder or adhere to the pipe or hopper wall.|
|Agglomeration segregation||It can happen that some components form lumps. Those lumps
will create non homogeneity in the mix since locally they
will concentrate a lot of material of one case.
It can be the case for example when build-up of materials on the walls of a silo is released in a mixture. A typical example is for a food manufacturer to sell sachets which contains lumps, the customer then immediately see the inhomogeneity.
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Among the different types of mixers, the diffusive ones and the shear mixers will cause segregation for free flowing particles of different sizes.
Diffusive mixers are particularly sensitive to this phenomena since they will also be prone to segregation with cohesive powders, they can actually promote the aggregation of the powders.
Pneumatic fluidized beds will be, on their side, sensitive to density differences between particles.
As explained above, segregation phenomena will happen. The question is therefore here more to minimize the effects rather than totally avoid it.
In order to minimize demixing, if you have the possibility to design a new process, is to minimize the number of process steps from the mixer to the point of use (filler, or dosing to further processing). Position the mixer so that you can reduce the number of units operations, especially avoiding free fall (and even more inclined surfaces on which the powder can roll), mechanical transport, pneumatic conveying...
Some sources in literature  suggest to humidify the powder in order to fix particles in between each other : this is far from being applicable in all situations. If the particles sizes are drastically different, one should check if size reducing is a possibility (milling step of some components prior to mixture).
In general, funnel flow hoppers should be avoided. One tip is that the design of silos as mass flow will allow to re-mix, up to a certain extend, the component at the silo outlet (but the product is just poured out from the silo into a heap it will demix again)
Finally, the choice of the mixer is very important, some of the mixers will be able to overcome easier the tendency to separate of particles. It is the case of double shaft paddle mixers.
 Perry's Chemical Engineer's Handbook
 Principles of Powder Technology, Martin Rhodes et al, Wiley