The Nauta® Conical Screw Mixer has a long-standing reputation in the solids industry. Since 1946, the concept has been recognized as an extremely reliable and highly effective mixer for a wide range of powders. Its inventor and manufacturer Hosokawa Micron has sold more than 10,000 units over the years and still regularly maintains numerous mixers that have been mixing powders satisfactorily for several decades. Although the mixing principle remains unchanged, today’s Nauta mixers are a world away from the older types according to Daan Goris, Team Manager Food at Hosokawa Micron.

“In the R&D center at our factory in Doetinchem we’re continually working on improvements and developing special options for a whole host of products and for the special requirements of various sectors, especially high-end food, pharma and chemicals,” explains Daan Goris. But the basic design of the Nauta mixer is always the same: a conical mixing vessel with a vertically rotating screw inside that is attached to a slowly rotating (orbital) arm which moves the mixing screw along the inner vessel wall.

One important innovation is that, nowadays, the mixing screw is only fixed to the arm at the top (cantilevered). This facilitates central discharge at the bottom of the mixing vessel. Goris: “The Nauta mixer is ideally suited for mixing a wide range of powders with very diverse properties.”

Hygienic

In recent years the Nauta mixer has been increasingly used for the production of high-end food products such as baby food and pre-mixes. High demands are placed on these in terms of mixing quality, hygiene and ease of cleaning, so the Nauta mixer has been significantly aligned with these needs over the past few years. Hosokawa is a member of the European Hygienic Engineering & Design Group (EHEDG) and many of the adaptations for food and pharma applications are related to hygienic design and ease of cleaning.

The standard food mixer now complies with all EHEDG requirements. In practice, this means that the interior of the mixer no longer contains any horizontal or vertical partitions or sharp corners and also that as few joins, flanges and seals as possible come into contact with the product. The mixing screw and the orbital arm are designed with special dynamic seals for optimum sanitation. Furthermore, the domed cover of the mixing vessel is seamlessly welded to the cone and has a smooth finish. Goris: “Thanks to this ‘sanitary’ design we comply not only with the EHEDG standards but also with the American 3A requirements. This means that an extremely low amount of residue (<0.05%) remains when the mixer is emptied. Hence, there is minimal risk of a subsequent batch becoming cross-contaminated.”

When making sanitary improvements, Hosokawa Micron’s starting point is to reduce the cleaning effort. After all, if something isn’t there, it doesn’t need to be cleaned. And in situations where cleaning is still necessary the mixer can be cleaned quickly and easily, either wet or dry. “There is a large hatch in the side of the mixing vessel for manual cleaning and visual inspection, plus the valve at the bottom of the mixing vessel has an inspection hatch. Needless to say, the valve also meets all the EHEDG requirements.”

Wet cleaning

In fact, Hosokawa is already another step ahead in terms of cleaning: the entire Nauta mixer can be supplied with a cleaning in place (CIP) system. This feature facilitates the fully automatic wet-cleaning and drying of the mixer. The discharge valve is fitted with a smart system which seals off the mixing vessel from the equipment beneath it. Goris: “You don’t want the cleaning water to end up in the packaging machine below, of course. That’s why we’ve designed a special diverter valve with an outlet. This hermetically seals off the discharge and enables the intake of dry air.”

Another recent development is a charging hopper for manual filling of the mixer. The unit is specially designed so that it can be connected to the CIP system.