07/13/2012 | Autor / Editor: Dominik Seeger / Jörg Kempf
Whether in the chemical, pharmaceutical, cosmetics or food industry, the costs for raw materials are permanently increasing all over the world and in all fields. Manufacturing companies search for new processes and new possibilities to remain competitive and distinguish themselves from their competitors. In this regard dispersing plays an important role.
A large variety of products, such as lacquer and paint, glue and binders, baby food and beverages, detergents and shampoos, as well as drugs and creams are being produced based on a number of dispersions. A prevalent type of dispersion is the suspension, consisting of heterogeneous liquids with finely dispersed particles in it. The two components do not dissolve or combine chemically and will eventually sediment if left unattended.
For the production of suspensions, existing agglomerates and aggregates have to be crushed and reduced in size, distributed in a liquid, wetted and stabilised. A homogeneous distribution of these particles into all elements of the liquid volume is the target. Dispersing is mandatory for the quality of the suspension. In case powders (here the dispersed phase) are dispersed into a basic liquid (continuous phase), the complete and homogeneous wetting of the solids is the most important factor for the quality of the suspension.
A complete — respectively a colloidal — wetting of the raw materials very often provides the possibility to reduce the quantity of materials used in the process. There are different technical solutions to disperse or to produce a suspension.
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Mixer and Stirrer consisting of a drive, a shaft with respective seals and a mixing or stirring element. The stirring or mixing element may be executed as a simple propeller or equipped with a stator inside of which the propeller rotates (Jetstream mixer). These systems only distribute the existing particles in the liquid; a milling of the agglomerates and aggregates is hardly effected, as almost no shear energy is existing. These systems may be applied for dispersing of easy to wet, non-sticky and non-agglomerating raw materials and powders or to keep a suspension homogeneous.
The set-up of a dissolver is equal to the principal of the mixer and stirrer, but uses a toothed mixing disc instead for dispersing. This disc produces stronger impact effects. The processing aim in this case is to crush agglomerates and aggregates to achieve a finer suspension. The shear effect however is restricted and the shear gradient strongly depends on the viscosity of the liquid. The moment the liquid starts to rotate, the impact and shear effect is dramatically reduced.
Powder bulk products have their own dispersing characteristics. What seems simple in theory can actually be quite difficult... More on page 2!
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