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Powder Mixing Troubleshooting

How to solve problems related to solids blending

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Section summary
1. What to do in case of too long mixing time
2. Control of dosing
3. How to design a Loss In Weight Feeder

1. Long mixing time : how to speed up the mixing time


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 :
Given as % of the total volume of the mixer

Too low speed The mixer speed is an important process parameter which has different influences depending on the type of mixers
  • Rotating mixers : a high rotation speed does not really help mixing, however, the mixing effect is actually based on a number of revolution, turning too low will simply increase the time required to reach the number of revolutions necessary to make a good mixing. Manufacturers recommendations are to be followed with most mixers having mixing speed in the 10-20 rpm range - more infos here
  • For convective mixers, a certain speed must be reached to have a good mixing, especially with double shaft paddle mixers which are fluidizing powder and for which the speed must be high enough to achieve this purpose. For double shaft paddle mixers, check the speed so that Fr is higher than 1.1. References, depending on the volume can be found here for double shaft paddle mixers and ribbon blenders.

Incorrect filling procedure The way to fill a mixer has an importance on the mixing performance and thus mixing time.
  • Rotating mixers like bin blenders, V blenders...etc... should be loaded by horizontal layers, and not vertical layers
  • Ribbon blenders or paddle mixers should be loaded with the small ingredients in the middle of the mixer in order to optimize the mixing time. Sandwich loading can also be a possibility to optimize the mixing time in some cases

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


2. Homogeneity not good enough : how to improve the homogeneity of a powder mixture

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.

3. Powder Breakage : how to reduce powder breakage in mixer

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.
  • Ribbon blenders or shear mixers will be causing medium to high breakage to the powder.
  • However, rotating mixers or double shaft paddle mixers, when operated properly, will be much more gentle leading to low level of breakage. The choice of the mixer must therefore be done also in accordance to the breakage level expected

4. Long discharge time : how to discharge a mixer faster

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 m trying to revover them - except at the end of production.

5. Long cycle time : how to increase the capacity of the mixer

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






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