Solids Mixing Principles

Industrial powder mixers mixing mechanisms

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Section summary
1. Introduction
2. Diffusive (or dispersion) mixing
3. Convective Mixing
4. Shear Mixing
5. Classification of mixers

Section summary

1. Introduction

2. Diffusive (or dispersion) mixing

3. Convective Mixing

4. Shear Mixing

5. Classification of mixers

1. Introduction

There are 3 types of mixing identified for bulk solids : diffusive mixing, convective mixing and shear mixing. Many design of industrial mixers are available and one can select a mixer based on any of these mechanisms or even a combination of those mechanisms. This page aims at detailing the mixing mechanisms and defining what kind of mixer fits in each category.

2. Diffusive (or dispersion) mixing

In diffusive mixing, the particles of solids move randomly, typically rolling down when their container is rotated. There is no forced pattern in the mixer, as such than the one that could be done by an agitator, but particles are moving locally one by one.

This type of mixing is typically long and sensitive to segregation.

Tumbler mixers, drum blenders, IBC blenders typically fall in this category, although their design can be complicated by the addition of an agitator (intensifier bar) that gives some convective or shear mixing.

3. Convective mixing

For convective mixing, the movement of solids is forced thanks to a mixing tool ("agitator") that is defining a pattern within the convective mixer. The mixing tool is moving large groups of particles, splitting the bulk of particles and thereby achieving mixing.

Mixing will be typically shorter than for diffusive mixers, although the actual mixing time will depend largely in the design of the agitator, process parameters and nature of the solids to be mixed.

Ribbon blenders, a very widespread dry mixer is working according to convective mixing, as well as paddle mixers, although paddle mixers are fluidizing the mixture, which constitute a kind of sub-type of convective mixing.

4. Shear mixing

In shear mixing, a mixing tool at high speed is used to make groups of particles slip in between each others. Such shearing force can be particularly efficient for granulation when the mixture has some undesirable agglomerates that should be broken down to achieve a good mix.

Ploughshare mixers, when operated at sufficiently high speed rely on convective AND shear mixing. To be noted that some deagglomerators can be fitted in other types of mixer to give them some shearing capabilities.

5. Classification of mixers

Fr	Mixing class	Mixer type	Pros	Cons
< 1	Diffusion	Type free fall mixers V Blenders Double Cone blenders Bin blenders Drums blenders	Very simple Low energy required Gentle mix No mixing elements in the equipment Reliability Access for cleaning	Long mixing Cannot achieve good mixing for powders of very different particle sizes Segregation effects can be experienced
< 1	Convection	Type thrust mixer Ribbon Blenders Screw Blenders	Achieve generally better mixing results than diffusion blenders Low energy inputs Generally less expensive than paddle or plough share mixers	Long mixing Mechanical complexity Access for cleaning Can damage product at long mixing time
> 1	Convection	Paddle Mixers Pneumatic mixers	Short mixing time Gentle mixing Low energy input Good access for cleaning (some design can be with extractible shafts For paddle mixers, exist in continuous mixing execution For padlle mixers, a liquid injection can be foreseen	Cost compared to diffusion tumblers / ribbon blenders If liquid injection, prone to agglomeration - then needs some additional mixing elements at higher shear For pneumatically generated fluid bed, attention must be given to risks of segregation due to fines "floating" at the top of the mixer
>> 1	Convection Shearing	Plough Share Mixers High shear mixing elements	Short mixing time Reduce risks of powder agglomeration Exist in continuous mixing execution	Higher powder breakage High energy input Cost