Powders can be free flowing, or on the contrary cohesive. Mixing free flowing powders is relatively easy while having a cohesive components in a mix, or mixing only cohesive powders can make it difficult to reach the right homogeneity. This page is explaining why cohesive powders are difficult to blend, which mixers are more appropriate for these kind of powders, and why cohesive mix are less sensitive to segregation after blending.

1. Cohesive powder behavior

Cohesive powder particles have the tendency to stick together. It happens often with powders having a very small size as it allows cohesive bonds such as Van der Waals forces, or electrostatic forces to be relatively high. It can also happen due to some humidity transfers creating solids bonds in between particles. This can also be due to the shape of the particles which can interlock in between each other.

It is a problem in mixing solids as the principle of mixing is to separate the particles of an ingredient of the mix to distribute them among the particles of the other components

The "cohesive" characteristic for a powder is rather subjective, there are also degrees of cohesiveness that can make difficult for a manufacturer to assess a priori if a powder will cause problem during mixing. The best advice here is to maintain a database of all the ingredients processed in a production line and for each of them to determine properties related to flowability. It can be simple, approximate methods, such as the angle of repose or the Carr / Hausner ratio, or more complex ones using a shear cell or a powder rheometer. From this database, a new ingredient can be easily compared to materials on which the factory has experienced and a if one of the parameter shows that it is more cohesive than materials used so far successfully, it can be flagged so that the operators be more careful during production and carry out additional tests to make sure the powder is well mixed.

2. Best mixer for cohesive powder blending

As explained above, the cohesive ingredients of a mixture tend to stay together, thus the mixer must be able to provide enough shear to break the agglomerates and distribute the powder homogeneously.

The most basic class of mixer, the diffusive mixers such as drum blenders, V mixers, bin blenders, double cone... are causing very low shear during mixing. In those mixers, the particles tend to roll at the surface of the powder bed during the rotation which is providing mixing over time. But in the case of cohesive powders this is particularly inefficient as the cohesive powder particles will just roll together. Those mixers should therefore not be used (at least in their most basic version - see below some possible improvement) when having cohesive components in the blend.

Mixes equipped with an agitator will fare much better in mixing cohesive powders. The agitator can indeed break the agglomerates. The efficiency will however depends on the amount of shear the mixer is able to apply. Convective mixers (ribbon blenders), fluidizing mixers (double shaft paddle mixer) will be able to process well mildly cohesive powders while high shear mixer (Lodige, centrifugal mixers) will be able to mix even very cohesive powders.

The following ranking, from less suitable to mixing cohesive powders to more suitable can be proposed :

Diffusive Mixers < Convective Mixers < Shear Mixers

Note that it is possible to enhance the capability of breaking cohesive powder agglomerate by adding some devices to mixers. V mixers can for example be fitted with an intensifier bar (an agitator positioned along the axis of rotation and which is running at high speed) or other kind of blenders can be equipped with choppers (typically running at 3000 rpm). These are equipment that are used in wet granulation to break agglomerate. However they bring additional cost, complexity and increase the difficulty to clean the equipment thus they should be considered as an ultimate fix rather than a base solution. The best is to chose directly the right kind of mixer.

3. An opportunity : reducing segregation after mixing

As seen above, having a cohesive powder in a formulation can cause issues at the mixing step, especially if the type of mixer available is inadequate. However, there is also an opportunity in mixing cohesive powders : the mixture will be less sensitive to segregation after mixing.

If it is possible to achieve the desired mixing quality, the mixture will then tend to stay closer to this state during the next processing steps (discharge, conveying, filling for example) compared to a free flowing powder.