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Section summary |
---|
1. Long mixing
time : how to speed up the mixing time |
2. Homogeneity not
good enough : how to improve the homogeneity of a powder
mixture |
3. Powder Breakage
: how to reduce powder breakage in mixer |
4. Long discharge time : how to discharge a mixer faster |
5. Long cycle time : how to increase the capacity of the mixer |
Observation | Cause | Action |
The mixing time is too
long This reduces the capacity of the process |
Filling volume too high | All powder
mixers need to have a sufficiently free head space to
allow for a good mixing Ensure that the volume filled by the powder is not exceeding :
|
Too low speed | The mixer
speed is an important process parameter which has
different influences depending on the type of mixers
|
|
Incorrect filling procedure | The way to
fill a mixer has an importance on the mixing
performance and thus mixing time.
|
|
Too small quantity of one ingredients | Very small ingredients additions are possible in
convective or shear mixers like paddle mixers or ploughshare
mixers, some examples showed
good mixing at 0.1%. However it is likely to be too
low for rotating mixers where a preblending
step is advised in order to have an addition of
minimum 1% of the total load |
|
Particles properties too different | A mixture with solids having
significantly different particles sizes has a natural
tendency to
segregate. It is thus not possible to reach a certain
degree
of mixing even if the mixing time is increased
indefinitely. Before mixing bulk solids, assess the particles sizes of the ingredients to estimate the difficulty of the mixing. If the sizes are very different, assess if a mixing with a high variability is still acceptable for your application, or find a way to have more similar particle sizes (change supplier, milling...). In certain cases, applying a coating - fixing the small particles to the big ones - can be a method to obtain a very homogeneous mix |
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Observation | Cause | Action |
The degree of homogeneity is not enough for the application | Filling volume too high | Refer to the table above, each powder mixer must have a free head space to allow for a good mixing |
Too low speed | Refer to the table above, convective mixers must in general have a minimum speed in order to perform an appropriate mixing | |
Incorrect filling procedure | Refer to table above. In addition, check, for non rotating mixers, that the mixer is turning slowly during the addition of material, it helps to level the product and also make a small preblending during the loading. | |
Too small quantity of one ingredients | Refer to table above |
|
Particles properties too different | Refer to table above |
|
Segregation after mixing | The mixing step is usually not the last process step, thus the mixture needs to be handled further. Especially for mixture having components with significantly different particle size, a segregation can happen after the mixer, thus what appears as a bad mix is actually a good mix at the outlet of the mixer that has segregated. To avoid this phenomena, make sure the mixture is handled as less as possible and as gentle as possible : avoid long chutes, or equipment inducing vibration or elutriation. | |
Analysis incorrect | In some cases, the mixture is good enough but it is found in-homogeneous because of an issue in the analysis. Make sure the sampling is done properly, the sample size is respected and the analysis method follows the procedure. |
Observation | Cause | Action |
High powder breakage during mixing | Too high mixing speed | Especially for convective mixers, make sure the agitator speed is in line with manufacturer specification |
Too long mixing time | Optimize the mixing time to reduce the time the solids are submitted to shear in the mixer | |
Inaccurate type of mixer | Some mixers are, by design, more gentle than others.
|
Observation | Cause | Action |
The mixer takes too long to discharge | Agitator is not ON during discharge | Starting the agitator during discharge, for ribbon
blenders, paddle mixers, shear mixers... is helping the
discharge. However, the speed must be below the ATEX speed, and a balance
should be found in between no agitation and the ATEX speed.
Too slow will cause a long discharge time, too high will
actually prevent the powder to fall. The discharge should also be optimized in terms of quantity of powder left in the mixer. It is sometimes better to leave 5 kg in a big mixer, that will be mixed with the next batch, than to lose 1 minute trying to revover them - except at the end of production. |
Observation | Cause | Action |
Too long cycle
time |
Too long
dosing to mixer |
The cycle
time is made of several components, including the
loading of the mixer. If the loading of the mixer is also a
dosing step, it can lead to a long cycle time. Check if
possible to optimize the dosing speed or modify the process
to have Gain
in Weight dosing in hidden time, or parallel Loss In Weight dosing to the
mixer |