|3. Mixing speed at loading
|4. Optimization of the loading of a
Loading a mixer should be the most trivial operation in the whole mixing cycle... but as often in bulk solids processing, it is not. Indeed, the way the components will be loaded in a mixer will have an effect on the performances of the mixer.
The important rule to apply is NEVER to load 1st in the mixer the small ingredients, like flavors, active principles, pigments... Indeed, those small ingredients can actually lie in dead area of the mixer and be difficult to agitate and to disperse, like the small gap in between the agitator and the mixer housing. If a good mix can be reached, it will be after a longer than required mixing time.
The best is to load in sandwich the small ingredients in between 2 layers of major ingredients. The distance towards each area of the mixer will be optimized. For efficient mixers, it is also generally acceptable to load the small ingredients on the top, which may have some interest to optimize the cycle time of the powder / bulk solids mixer.
Loading in layers with a symmetric spreading in the mixer is also recommended, particularly for tumblers for which the mixing time can be increased a lot if the materials are loaded by vertical sections .
For mixers with an agitator (ribbon, paddle, ploughshare...) it is recommended to run the mixer at low speed ("ATEX speed") when loading the mixer. It helps to even the product in the mixer, allows to optimize the load, make a kind of premix and avoids to stall the agitator if it were to start on the mixer already full of product.
The mixer should be equipped with a filter to allow for the air exchange required when introducing a large amount of solids to the mixer during loading. Loading should indeed be done as quick as possible to optimize the cycle and the flow of product should not be hampered by a slight overpressure in the mixer, in case a filter is not present, undersized or clogged.
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From the observations above, some recommendations can be drawn in order to improve the cycle time of a mixing operation by loading quicker and promoting a short mixing time :
|Dose gain in weight the ingredients in hoppers, ready to be discharged to the mixer||Reduce cycle time thanks to
|Load the mixer with symmetric
layers, consider loading small ingredients in "sandwich"
or on top of the mix, at the center for efficient mixers
DO NOT LOAD SMALL INGREDIENTS 1st
|Reduce cycle time thanks to mixing|
|Ensure the venting of the mixer is sufficient. Avoiding pressure buildup in the mixer when ingredients are discharged improves the flowrate||Reduce cycle time thanks to quicker loading|
|For mixers with an agitator, run the agitator at ATEX speed during loading||Reduce cycle time thanks to mixing|
Sources and other references
 Dealing with powder blending problems. Muzzio, Powder Bulk Engineering, 2018