Mixer Cleaning How to Optimize Dry Cleaning Methods for Bulk Residues in Mixers

Editor: Dominik Stephan

Residues of bulk solids in mixers can be quite a nuisance if one device is used for several products. While rinsing mixer and screw with water is an easy approach to cleaning, it has certain disadvantages, most obvious the presence of moisture within the device. But also dry cleaning poses its challenges – several aspects have to be considered when laying out a dry cleaning procedure for powder mixers.

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Modern mixers, like Hosokawa’s Nauta mixers, are specially designed for easy (dry) cleaning. (Picture: Hosokawa Micron)
Modern mixers, like Hosokawa’s Nauta mixers, are specially designed for easy (dry) cleaning. (Picture: Hosokawa Micron)

With over 85 years of experience in supplying mixers to the market Hosokawa has created various options to facilitate dry cleaning. The idea behind this cleaning approach is to avoid water in the machine, in order to minimize the propagation of bacteria. In order to reduce cleaning efforts, large effects can be obtained from looking at the process design. The vessels used, material transfer steps, place and number of liquid additions should be taken into consideration here. The next step in creating a production line is to review the design of the mixer itself. Important features towards dry cleaning are:

  • Conical mixer design, using gravity for emptying the mixer. The inclined surface offers little opportunity for powder to form residues.
  • Central bottom outlet, without restrictions for a complete discharge of the mixer.
  • Mixer vessel made in one part, eliminating seams, using smooth surfaces.
  • In conical screw mixers, the mixing screw can be designed with open screw flights.

Cleaning Deposit Zones in Screw Mixers

As the above features can reduce the settling of product in the mixer, we now will have a look at the nature of these deposits. Three zones can be found in a screw-mixer, with respect to the nature of the deposits:

  • The surfaces which are above the product level of the filled mixer: Soilure of the walls in this area is mainly caused by dust, dispersed during filling but also during mixing and emptying. Therefore, such surfaces are generally fairly easy to clean.
  • The surface of the vessel below the filling level: Due to gravity, constant contact with the surface and the steady movement of the powder particles, the adherence is usually stronger in this area (liquids in the mixture will additionally increase adherence to the wall).
  • Surfaces on the mixing screw, especially the bottom side of the screw blades will show the toughest layers of product residues.

More about dry cleaning? Read also an interview Hendrik Freije, application engineer of Hosokawa Micron’s Food & Pharma team, about the Do’s and Dont’s of dry cleaning.