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Powder application in liquids

Dust, agglomerates, microfoam in the product: conventional processes for adding powders to liquids cause numerous problems. In contrast, the inline dispersing machine from a mixing and dispersion technology specialist relies on the principle of vacuum expansion and thus achieves reliable, dust-free and complete wetting and dispersion within a very short time.

The Conti-TDS machine directs liquid and powder on separate paths into the wetting zone 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.

Powder wetting under vacuum expansion

ystral process systems chemical polyamid A process system with the Conti-TDS powder wetting and dispersing machine can, for example, consist of two process containers and one powder container.

The weaknesses of these conventional powder wetting processes are avoided with the Conti-TDS inline dispersing machine developed by ystral. With the Conti-TDS, complete deagglomeration and wetting as well as substitution of air by liquid takes place within microseconds.

The inline dispersing machine is operated on one or more process containers. It conveys the liquid in a circuit and sucks the powder into the liquid from a bag, hopper, big bag, silo or container. To separate the powder particles, the Conti-TDS uses the principle of vacuum expansion: the air contained in the powder is expanded many times over, which greatly increases the distances between the particles. The particles are separated and fluidized without additional air. In the Conti-TDS, powder and liquid only come into contact with each other in the wetting chamber - under maximum vacuum and maximum turbulence. In the dispersing zone, the powder particles have the greatest possible distance from each other and can therefore be completely wetted and dispersed individually. The machine generates a liquid surface area of around 500,000 m²/min. This is more than is required for complete wetting.

The air previously contained in the powder is separated from the significantly heavier dispersion by the centrifugal effect of the high-speed rotor and coalesces into large air bubbles. These are then conveyed together with the liquid flow to the process container, where they can easily escape.

The economic advantages of the vacuum expansion method are enormous: in paint production, for example, costs can be reduced by more than 90 percent in this way, while resins can be dissolved in a fiftieth of the time - and with significantly improved product quality.

Article preview "Powder application in liquids"

Magazine: P A
Issue: 05/2022
Author: Dr. Hans-Joachim Jacob

About the author

Dr. Jacob is Senior Expert Process and Applications at ystral. Dr. Jacob, who studied mechanical engineering, joined the company in 1990 as a process engineer and has since been responsible for our key accounts worldwide. His professional passion is the mixing and dispersion of powders in liquids. During his long career, he has gained experience in handling thousands of powders from a wide range of industries and is happy to share his expertise in various technical articles, online seminars and lectures.

About ystral

With our vast knowledge and many years of experience in Process- and Application Engineering we offer targeted, customer-oriented solutions across industries - from lab equipment to production machines or plants. Together with you, we develop concepts and implementations for your individual applications, which mean mmediately realisable and quantifiable added value for you.

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