1. Mixing Volume ranges of common batch powder dry-mixers

Bulk Solids blenders can be ordered from manufacturers with quite a large range of sizes, from few liters to few or dozens of cubic meters. Solids blending is a volumetric process thus a volume should be specified when ordering a mixer, not a weight as the weight will vary with the bulk density of the solids. The table below shows the typical sizes of mixers that can be found on market.

Table 1 : Common solids mixers size range

2. Minimum and Maximum mixing volume batch powder dry-mixers

It is very important to understand the difference in total volume and useful volume for dry mixers.

Figure 1 : Mixer total and useful volume

Most suppliers are giving a total volume, but one should not attempt to fill it 100% ! Indeed, in order to have a good mixing there should ALWAYS be a free space inside the mixer, so that the solids can move, rearrange and thus mix.

The actual maximum volume that can be filled to get a good mix is actually the useful volume and represents only a certain percentage of the total volume.

The useful volume is changing from one mixer type to another, and should also be adapted to the material being mixed.

The table blow gives typical minimum and maximum - useful - mixing volume for powder and granulate blenders, it should be validated case by case by an homogeneity test.

Table 2 : Typical Min and Max mixing volume recommendation

Dry Mixer	Min mixing volume	Max mixing volume
Drum / Bin blenders Diffusive mixers	-	50-60%
Ribbon blenders	30%	70-80% Up to 90% may be possible
Paddle mixers	30-40%	70-80%
Ploughshare mixers Shear mixers	30-40%	70-80%

For mixers with agitators, a minimum volume is specified. Indeed, if the mixer is not enough filled, the agitator will not be able to pick up and move enough powder to have an efficient mixing.

3. Blender selection and sizing for powder / bulk solids

The selection of the blender is a function of many parameters : the technology, the impact of the technology on the product, how long the mixing takes, but also the volume of the mixer. A capacity calculation for the production line must be done and should typically define a batch size, in kg, as mass is the dimension of interest for sales.

It is then possible to size the mixer volume thanks to the bulk density of the mixture and the maximum filling rate of the mixer chosen.

For example, a ribbon blender must produce 275 kg of mix / batch so that the production line reaches the right capacity. The mixture loose bulk density if 0.6 kg/l and the maximum filling rate of a ribbon blender is assessed to be 80%

Batch size weight = 275 kg

Batch size volume = 275/0.6 = 459 l

Required total volume of the mixer = 459/0.8 = 573 l

The Engineer will then select a mixer whose total volume is >= 573 l.