It is crucial when performing a mixing operation of bulk solids (powder, granules...) to reach a target homogeneity that makes the mixture good for sale or further processing. Engineers are often left wondering why a mix is not homogeneous enough : one of the key parameters to check is the size of the particle mixed.

1. Blend homogeneity

When performing a mixing operation the homogeneity at the end of the mixing time is the quality parameter that matters the most. Reaching the right mixing quality will allow to meet the properties that are required for the mix to perform well at a further process step, or directly with the customer. The homogeneity must be defined relatively to a sample size, chosen according to the mixture final usage, and through statistics by sampling several samples out of a mixer and determining for each sample a key measure that will represent the mixing quality (the concentration of a component for instance).

However, it happens that, despite a correct mixing time, a correct mixing speed, the blend homogeneity in the mixer, or after discharge from the mixer, is not satisfactory. In many cases, this is due to the difference in particle sizes in between the different components of the mix.

2. Influence of particle size on blend homogeneity

Having a mixture made of components having strongly different particle sizes makes it sensitive to segregration (demixing). Solids of different particle sizes have indeed the tendency to separate, with the small particles in one area and the big ones in another area.

There are indeed different mechanisms of segregations and all of them can be triggered by having particles of different sizes :

• Percolation : smaller particles can go through voids in between big particles typically when the mixture is vibrated. Inversely, this phenomena appears to make the coarse particles rise in the powder blend.
• Elutriation : when discharging powder from an equipment to a hopper for instance, air is displaced from the receiving hopper and goes up, it then meets the particles falling down, large particles are relatively unaffected, but the smaller ones will stay in suspension longer and thus will only deposit at the end of the discharge, leading to a higher concentration in small particles on top of the powder bed.
• Trajectory segregation : due to their size, bigger particles will tend to move further than small particles

As the difference in particle size is the root cause of most non homogeneity of bulk solid blends, one of the possibilities for industrials is to adapt the particle sizes of the ingredients of the recipes to reduce the distribution and thus reduce the risk of segregation.

3. Improving homogeneity by changing the size of the materials mixed

There are different possibilities to avoid segregation of particles in a solids blend :

• It can be possible to make sure the mixture is not free flowing, it is dramatically reducing the risks of segregation, especially for the next process operations, but it can cause severe processing issues if the mixture is just not flowing well out of hoppers.
• Another possibility is to perform a granulation (dry or wet) which consist in agglomerating the particles to form larger particles which have and keep a define composition. This method is very often used in pharma.
• The last possibility is to reduce the sizes of the particles so that the tendency to segregate is reduced. This reduction of particle sizes can be done either by sourcing different ingredients having a closer particle size distribution, by pre-grinding or by post-grinding

3.1 Specification of materials

The most effective, from a process and CAPEX point of view, is to adapt the specification of materials that are bought from a supplier. The PSD can for example be defined so that it matches the particle sizes of the other mix components. However, this approach is not always possible, and if possible it can come with a higher cost when buying the ingredient.

3.2 Pre-grinding

If not possible or economically sound to buy an ingredient with the right size, the alternative is to grind one or all the ingredients before the mixing step. Different equipment, depending on the capacity required and the size required, can be used, such as a nibbler, a cone mill or a universal mill.

This method however brings different operational complexities, 1st because a mill must be included in the process, which is not always possible depending on the layout, and because the mill itself brings different problematics to manage : difficulties in setting the mill, need of training for the operators, increased risk of dust explosion to manage through prevention and mitigation measures.

3.3 Post-grinding

If not possible to include a size reduction step before mixing, another possibility is to have the mill at the discharge of the mixer. It can be simpler from a layout point of view, although post grinding can lead to an overisized mill as all the components have to go through it, and post-grinding can also cause segregation if, for example, one of the components, finer, goes quicker out of the mill than the other ingredients. If a factory want to implement of post milling to reduce the segregation of the mixture, a careful study must then be carried out to make sure the mill is not actually causing additional de-mixing.