Powder mixing: all about mixers and mixing processes
Powder mixing is distributing two or more dry materials evenly, so every portion has the same composition. The main mixer types are tumble mixers, ribbon blenders, paddle and ploughshare mixers and high-shear mixers. The choice depends on flow behaviour, particle size, density differences, product sensitivity and cleanability.
Key points
- Tumble mixers mix gently by rotating the whole vessel; suited to free-flowing, fragile products
- Ribbon, paddle and ploughshare mixers mix convectively with a rotating shaft; suited to cohesive powders and liquid addition
- High-shear mixers break agglomerates and are used for granulation
- Segregation after mixing is a real risk during transport and discharge
- Blend uniformity is measured with samples from several locations
Mixer types compared
| Type | Principle | Strong at | Less suited to |
|---|---|---|---|
| Tumble mixer (V-blender, bin blender) | Whole vessel rotates | Gentle mixing, easy cleaning, mixing in the transport container | Cohesive powders, liquid addition |
| Ribbon blender | Helical ribbons on a shaft | Large batches, free to moderately flowing | Very fragile products |
| Paddle or ploughshare mixer | Paddles throw the product through the vessel | Fast mixing, liquid addition, cohesive products | Very fragile particles |
| High-shear mixer | Fast impeller and chopper | Granulation, breaking agglomerates | Gentle mixing |
What to consider when choosing
- Product behaviour: free-flowing or cohesive, fragile or robust.
- Differences between components: large differences in particle size or density increase segregation risk.
- Minor components: a micro ingredient is often pre-mixed first.
- Cleanability and product changes: critical in pharma and food.
- Batch size and fill level: every mixer has an optimal fill range.
Preventing segregation
A good blend can segregate again during transport, free fall or silo filling. Limit drop heights, use closed conveying where possible and mix close to the point of use. See our powder transport solutions.
Measuring blend uniformity
Take samples at different locations in the mixer or end product and determine the spread of the content, usually as relative standard deviation. The limit depends on product and regulations. More on sampling: safe powder sampling.
Mixing in a closed process
For active ingredients or toxic powders, charging and discharging the mixer must be dust-free. This requires closed connections and a suitable containment level. Accurate charging starts with good dosing.
Frequently asked questions
How long should I mix?
This is determined per product with mixing trials. Longer is not always better; some blends segregate when mixed too long.
Which mixer is easiest to clean?
Usually a tumble mixer without an internal shaft, as few parts are in contact with the product.
Conclusion
The right mixer follows from product behaviour and cleanability requirements. Just as important is what happens before and after the mixer: dosing, conveying and dust-free transfer. Contact us for advice on your process.
Author: Joost van Velzen, Managing Director Hecht Technology B.V. Updated: .