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Mixing Time / Blending time

Usual mixing time for ribbon blenders and other industrial dry mixers

How to improve mixing time ?

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Section summary
1. Mixing time
2. Typical mixing time of powder mixers
3. How to improve batch dry mixer mixing time

1. Mixing time

How can I determine mixing time in powder dry mixing?

The mixing time is defined as the time required to reach a desired homogeneity.

The following elements will influence the mixing time. As a general rule, mixers operators look for a mixing time as short as possible in order to increase the productivity of their line.

Table 1 : parameters influencing the mixing time

Operating parameter Influence on mixing time
Mixing volume A higher mixing volume will lead to higher mixing time
But doubling the volume, does not mean doubling the mixing time
For mixers at Fr>1, after a certain volume, mixing time will be almost constant
Note : a mixer should not be over-filled otherwise the mixing may even become impossible
Mixing speed A higher mixing speed usually gives a shorter mixing time, up to a certain speed at least
Influence on the powder should however be considered (powder breakage
Froude number Higher Fr number should give shorter mixing time
Powder breakage will be a concern when Fr >>1
Solids type The more the particle sizes of the mix consituents are different, the more difficult it will be to mix

The influence of the Froude number and the mixer volume are represented in the graph below :

Mixing time of solids blenders

Figure 1 : Mixing time = f(Fr,Volume) [Gericke]

Mixing time is also dependent on the sequence and place of filling the ingredients. In particular, the position of addition of micro ingredients is of prime importance. In case the micro ingredients are added on the side of a mixer, the mixing time can be greatly impacted. As a good practice, processes must be designed to have the small, minor and micro ingredients introduced in the middle of the mixer, in any case in the "active" area of the blender (the fluidization zone for instance).

2. Usual mixing time - What is the mixing time for usual powder blenders

The table below is presenting mixing time that can be expected from the most common types used in the industry. These are indicative values to have a rough idea when assessing a design or troubleshooting an existing system.

Table 2 : Typical mixing time

Mixer Mixing time range in min
Drum / Bin blenders
Diffusive mixers
5-15
Ribbon blenders 5
up to 10-15
Paddle mixers 0.5-3
Ploughshare mixers
Shear mixers
3-5

3. How to improve batch dry mixer mixing time ?

When facing an industrial batch powder mixer with a too long mixing time, the following points can be checked :

- Mixer speed : is the speed equal to the recommendation of the manufacturer ?

- Mixing volume : has it been checked that the mixer was not too full ?

- Loading of materials : are material loaded optimally ?

- Segregation : is it possible that the mixture is handled in a such way after mixing that it segregates ?

Source
Improving Batch Dry-Mixer operations, Thomas Lamotte, Powder Bulk Solids, July 2018


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