Dispersion Characteristics of Agglomerated and Non-Agglomerated Powdered Materials

Publication Reference: 
FRR-17-06
Author Last Name: 
Schubert
Authors: 
Prof. Dr.-lng. H. Schubert
Report Type: 
FRR
Research Area: 
Wet Systems
Publication Year: 
1998
Country: 
Germany

This research project aimed at working out a method for describing the dispersibility of an agglomerated material in stirred vessels and to apply this method to the improvement of the redispersing properties. Appropriate laboratory tests for describing the instant properties of the powder were also carried out, and a new testing method (dynamic wetting test) was developed. Furthermore a connection between the results from laboratory tests of the instant/dispersing properties and the large-scale dispersion of the product was established. Additional experiments were conducted with an unbaffled, partly or fully baffled vessel to investigate the influence of the flow field in the stirred vessel on the dispersion of powders in liquids.

It is difficult to establish a relation between the laboratory tests (static and dynamic wetting test) and the dispersing experiments in an agitated vessel. Since wetting tests do not take stirring power into account, they can not describe the processes taking place in the liquid. Therefore dynamic wetting tests were developed. It is possible anyway, to derive some trends from these tests. The results of the laboratory tests with lecithin, for example, are in good agreement with the experiments conducted using the stirred vessel.

The dispersion of powder can be divided in two steps: Submersion of powder and destruction of aggregates in the liquid. The wettability of a powder is an important powder property fo the immersion step. Although the specific stirring power that can be achieved in an unbaffled vessel is inferior to the specific stirring power for a baffled one, the improved ability to submerge the powder with help of the vortex formed by the rotating liquid proved to be of decisive importance. The powder can be wetted quickly if the critical height of wetting not exceeded so that lump formation in the liquid is avoided and a high power input is not necessary.

For difficult to disperse powders, however, this may be different. If such a material is to be dispersed, high specific stirring power is necessary and baffles have to be used. The use of smaller baffles close to the bottom of the vessel is a good compromise (partly baffled vessel). Liquid rotation and vortex formation in the upper part of the vessel is possible while the power input is increased by the small baffles compared to an unbaffled vessel.

Powder immersion in an unbaffled or partly baffled vessel is influenced by the vortex depth. At equal vortex depth, the course of a dispersion experiment appears to be independent of the stirrer type and size.

Experiments in a partly baffled and an unbaffled vessel are difficult to compare, but increasing vortex depth improves the immersion step in any case. Along with the vortex depth, the energy input is increased and aggregate dispersion is improved, Partly baffled vessels allow higher maximal energy input before gassing occurs and, for that reason, perform better at dispersing already submerged powder aggregates.

Scale-up experiments were conducted but further experiments are necessary because in stirred vessels scale-up rules cannot be used generally.

From the present point of view, any large-scale dispersing vessel should be unbaffled if possible or partly baftled if a higher specific stirring power is necessary and be equipped with the impeller type which allows the highest energy input without gas being entrained. If this criterion does not lead to a clear choice, the stirrer with the highest circumferential velocity should be taken since it will probably perform better at aa aregate destruction.