Powder application in liquids

With the Conti-TDS powder wetting and dispersing machine, even powders that are difficult to wet, dusty or sticky can be dispersed agglomerate-free

Powder handling creates dust. This is not the only problem. In the vast majority of cases, powders are agglomerated - and the finer a powder is, the more it tends to agglomerate. The causes of agglomerates are manifold. In addition to physical and electrostatic forces (Van der Waals, Coulomb, sintering bridges), there are numerous other causes for the formation of agglomerates in everyday production. For example, temperature increases above the glass transition temperature during transportation cause particles to fuse together (caking), while cool storage can lead to the temperature falling below the dew point, to condensation within the powder and thus to liquid bridges between the particles.

In order to achieve the best possible dispersion result, existing agglomerates must be broken down immediately during powder application and the formation of new agglomerates during powder application in liquids must be avoided from the outset. Otherwise, these agglomerates have to be broken down afterwards by prolonged stirring and time-consuming post-dispersion - often with negative consequences for the product quality: the texture of yoghurt is destroyed and additional proteins and stabilizers are required, the viscosity of shampoos decreases and additional thickeners have to be used. Polymers are destroyed and resins or binders are overheated. In addition, post-dispersion costs time and energy and unnecessarily blocks the process containers.

The powder particles must therefore be separated before they come into contact with the liquid and each particle must be completely wetted individually. The particle surface to be wetted during powder application is huge. It is between one and one thousand square meters per gram of powder. A 25 kg bag of powder can therefore have a particle surface area to be wetted of between 25,000 km² and 25 km². In addition to this outer surface, porous particles such as silica gels also have an inner surface - and this must also be completely wetted.

Powders also contain a lot of air. Even heavy powders such as titanium dioxide contain more than 75 percent air by volume. With light powders, the proportion can be over 95 percent - and this air must be completely replaced by liquid and separated. The air must not be dispersed with the powder, as this leads to microfoam.

Problems with conventional powder wetting processes

Conventional methods of adding powder to liquids using agitators, injectors or inline mixers mainly produce undesirable agglomerates. The powder particles do not come into contact with the liquid individually, but as a compact bulk. The liquid surface available to the powder for wetting is orders of magnitude smaller than the powder surface to be wetted. This leads to stable, partially wetted agglomerates that are difficult to break down.

When powder is added to an open container from above, these problems become particularly obvious: partially wetted lumps form on the surface of the liquid. In the worst case they float on the surface, in the best case they sink. Dust above the liquid leads to adhesions on moist surfaces, powder crusts and soiling on the container wall, container lid, agitator shaft and all installations in the container. These later crumble into the product and reduce the quality. If an extraction system is used to prevent dust, this results in an uncontrolled amount of powder being lost in the filters. In addition, mixing in an open container causes the formation of air pockets, which introduce additional air into the liquid.

Suction conveyors, which transport the powder into the container with little dust, still generate a lot of dust above the liquid. The wetting problems in the container are therefore not avoided, but rather intensified.

Agglomerates also inevitably occur in vacuum process containers because the particles are not separated before contact with the liquid and the liquid surface is far too small for complete wetting. At the same time, tumbling occurs and there is a risk that some of the powder will be sucked off by the vacuum pump without being wetted and thus lost.
Even in the case of injectors with an upstream or downstream pump, the liquid surface area is far from sufficient. A dispersing machine is often installed downstream of such a system in order to break down the agglomerates introduced. However, this disperses the air contained in the powder particularly finely, which significantly impairs the dispersing effect and produces stable microfoam.