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More security, less process time required

Effective incorporation of powders into solvents

Numerous powders are processed in the production of plastics. These have a wide variety of functions: They are used as pigments or flame retardants, to increase conductivity or to reduce weight. In order to fulfill their function, these powders must be completely wetted and optimally dispersed.

ystral TDS-Induction Mixer The TDS suction mixer generates a vacuum in its mixing head, which it uses to suck powder directly under the surface of the liquid

Traditional processes for dispersing powders based on dissolvers or agitators quickly reach their limits. However, the safety risks associated with simply dissolving resins in the form of powders, granules or chips in solvents must also be effectively managed. It becomes particularly dangerous when a powder is dosed from above into a process container with solvent-containing media. As the process temperature is usually above the flash point, there is a risk of explosion due to the flammable gases and vapors above the liquid. Directly on the surface of the liquid, the solvent concentration is too high for ignition. Some distance away, the concentration is too low, but right in between is a zone in which the solvent concentration is between the upper and lower explosion limit. In this zone, we speak of an ignitable mixture. And this is precisely the critical zone through which the powder is poured. When the powder is added to a process container, it must therefore always pass through the zone in which the gas-air mixture is ignitable.

All that is needed to trigger an explosion is an ignition source with sufficient energy. This ignition source could be the flowing powder itself or the stirred liquid if it is not sufficiently electrically conductive. Even inerting the process container cannot reliably eliminate the risk. Powder itself contains a lot of air. In the case of heavy powders such as titanium dioxide and calcium carbonate, this is around 75 %, while light powders such as highly dispersed silica contain up to 98 % air. All other powders or resin granules are somewhere in between. And this air content renders inertization at least partially ineffective.
However, the risks of adding the powder to the solvent-containing liquid from above can be avoided very easily by not adding the powder from above or not adding it to the container at all.

Effective dispersion
These applications require an effective process for dispersing the powders: a system that can completely wet and optimally disperse powder particles in both liquid and viscous media and does not transport them through the solvent vapors above the liquid. TDS machines are systems with which powders are sucked into liquids using an internally generated vacuum, wetted and dispersed in the liquid without agglomerates. The machines are available in different versions; the inline version of this machine, the Conti-TDS, is particularly important for powder application in plastics production. With this machine, powders can be sucked in directly from the bag, big bag, powder hopper or silo without dust or loss and wetted in the liquid.

The Conti-TDS is installed outside the container and is connected to it via pipes in the circuit. No powder is poured onto the liquid surface, no bag has to be lifted onto the container, no dust sticks to the container wall, no powder is emptied onto the liquid surface in the presence of solvent vapors.

Powder and liquid enter the machine via completely separate paths and only come into contact with each other in the dispersion zone. There, dispersion takes place under massive shearing action and vacuum. The powder therefore does not come into contact with solvent vapors above the liquid; it is always sucked directly into the liquid. This makes the risk manageable. All particles are in flight in the powder, which is sucked in by suction conveying and flows at high speed. During vacuum conveying, the vacuum in the powder increases steadily from the point of addition to the zone of maximum vacuum. The distances between the individual particles increase accordingly along the way.

Directly in the liquid
The Conti-TDS generates its suction effect directly in the liquid. The maximum vacuum is exactly in the wetting and dispersing zone. The distances between the individual particles are greatest when they enter this zone. No additional conveying or fluidizing air is required for this effect, only the air present in the powder expands - and compresses again after dispersion. The particles are separated and completely wetted, pseudo-wetting is excluded. Dispersion qualities and solid concentrations can be produced that could never be achieved by stirring over the liquid surface or with dissolvers.

After powder application is complete, the powder inlet is closed. The Conti-TDS can then continue to be used as a normal inline disperser until the required particle size distribution, homogeneity or consistency is achieved. In this phase, the machine operates with a high throughput and many passages via the high-shear zone. Similar to a pump, the Conti-TDS conveys the liquid independently in a circle. The machine is often connected to two or more containers.

The Conti-TDS has another positive effect: it speeds up all processes. The greatest time savings are achieved when dissolving resin powders or granulates. These are completely wetted on first contact with the solvent in the shear field of the dispersion zone.

ystral Fachartikel Verfahrenstechnik

Magazine: Process engineering
Issue: 10/2019
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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