News | January 22, 2004

A New Perspective on Powder Induction

Wetting out powders like fumed silica, TiO2, gums and thickeners is one of the toughest challenges our customers face – in virtually every industry we serve, from foods and pharmaceuticals to coatings and the CPI. In low-viscosity applications, the best approach usually involves a high-speed, high-shear rotor/stator mixer. In-line rotor/stator mixers, in particular, can quickly combine multiple streams in the high shear mixing chamber. But conventional high-shear mixers are much better at combining liquids than they are at mixing a powder into a liquid stream.

The problem is that in traditional rotor/stator mixers, shear rates and pumping capacity are inversely related. They can provide aggressive shear rates, but they are not able to draw a high volume of powders into the rotor/stator generator without the help of a pump and eductor upstream. The added equipment improves the mixer's ability to handle powders, but they only add to your maintenance headaches. The system is prone to clogging, and transfer rates are slow.

Simultaneous combination and mixing ? with no pump or eductor.
To envision a better mixing system, you need to distinguish between the point at which powders and liquids are simply combined and the point at which they are mixed under intense shear. When you combine powders and liquids in an eductor, then push them downstream to the mixer, the material will easily clog in the pipe. But when you combine and mix them at the same point, you eliminate this problem immediately. When powders and liquids are first combined in an eductor, then pushed downstream to the mixer, the system readily clogs. Throughput must be kept low, and a by-pass pipe should be installed as a precaution to carry the flow when the system clogs. This requires a rotor/stator generator that produces both intense shear and vigorous pumping action ? a theoretical hybrid that until recently had been considered impossible.

A new rotor/stator design developed in the Ross Test and Development Center generates a more intense vacuum in the high shear zone than traditional rotor/stator mixers were capable of producing. A feed tube adjacent to the back of the rotor provides a path to inject solids directly into the stream. Since the solids are combined and mixed into the flowing stream at the same point, the mixer is capable of handling extraordinarily large volumes of solids without clogging.

Results from customer applications in numerous industries confirm that the In-line Solid/Liquid Injection Manifold (SLIM) is capable of injecting powders such as calcium carbonate, fumed silica, pigments and other hard-to-disperse solids at remarkable rates. The system is available for tests in the Ross Test & Development Center.

Charles Ross and Son Company