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

Powder Mixing (main page)
# Types of mixers
# Mixing mechanisms
# Mixing time
# Mixing speed
# Mixing volume
# Mixing power
#Mixer loading # Discharge time
# Froude Number
# Coefficient of Variation
# Homogeneity
# Tracer
# Segregation / demixing
# Tip speed
# Sampling
# ATEX speed
# Mixer Bearing seals
# Mixer discharge valve

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 that gives some convective 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 when the mixture has some 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






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