Whitepapers

Wetting powders such as fumed silica, guar, xanthan, alginates and others are some of the toughest challenges in mixing. When trying to mix them, they tend to float for hours on the surface of a liquid batch. Even with vigorous agitation, they remain on the slopes of a vortex and resist being drawn down into the batch. Once submerged, they form agglomerates that continue to resist being separated and dispersed.

Let's review the equipment being used today to wet out these powders, and try to determine which is best for you:

1. A tank using a propeller or turbine agitator.

The simplest way to add powders to a liquid batch is to pour them into an open vessel. A propeller or turbine mildly agitates the batch and creates a vortex that draws material into the liquid. This method remains popular, because it is simple and inexpensive. For many manufacturers this is adequate despite the very long mixing cycles required. But adding lightweight powders into an open vessel can present other problems. "Dusting," for example, brings particles into the air and creates an airborne hazard for employees. Open-vessel mixing also draws air down into the mix, and for many applications this makes another step necessary later on, during which the entrained air is removed.

2. A high-speed disperser speeds up mixing and increases shear.

A high-speed disperser is a common choice to shorten the cycle and achieve a better dispersion. Agitation is more vigorous than with either a propeller or a turbine, and a higher level of shear is applied to the mix. Like the prop and the turbine, the high-speed disperser suffers from the disadvantage of entraining air into the batch, and launching dust into the plant atmosphere. In addition, many thickeners tend to foam when they are agitated violently in the presence of air.

3. A rotor/stator mixer accelerates deagglomeration and produces a finer dispersion. A single-stage rotor/stator mixer breaks down agglomerates much faster than the high- speed

disperser. The close tolerances between the rotor and stator impart an intense shear on the materials. The rotor/stator mixer also draws material directly to the high shear zone. As the rotor expels mixed material through openings in the stator, new material from the surrounding batch is immediately drawn into the high shear generator. The batch rotor/stator mixer produces a finer dispersion than the high-speed disperser. But as with all batch systems, its efficiency is limited by the flow pattern created within the vessel. The wetting-out process is relatively slow, especially when contending with lightweight powders such as fumed silica.