echnology Brief

Increased demand, product improvements, change in raw material supplier, custom orders and many other situations can cause mixing processes to outgrow the original equipment. This bulletin offers some insights into the operation of high speed dispersers and how to address specific areas of improvement using alternative mixing solutions.

Mixing with a high speed disperser

The saw-tooth disperser is a standard workhorse used in the manufacture of chemicals, plastics, composites, inks, paints, adhesives and many other products. An economical and relatively simple piece of mixing equipment, its primary purpose is to incorporate powders into liquid and break down particle agglomerates to produce a fine stable dispersion.

Running at tip speeds of around 5,000 ft/min, the open disc blade of the high speed disperser is designed to create vigorous turbulent flow within a low viscosity batch. It generates a characteristic vortex into which dry ingredients can be added for quick wet-out. As the batch thickens or increases in volume, the blade speed may be adjusted to maintain the vortex and rate of material turnover. Most models allow the disperser to be raised and lowered during mixing to eliminate "stratification" or possible layering within the batch. At the end of the mix cycle, the level of dispersion in the finished product may be measured using a fineness of grind gauge, particle size analyzer or microscope.

Processing challenges

On its own, the disperser blade produces acceptable flow patterns for products up to around 50,000 centipoise (cP). Beyond this range, if more solids are added or viscosity continues to climb, product turnover will start to slow down and eventually, batch material around the periphery can become stagnant.

This becomes a serious concern not only in the sense of poor homogeneity but also because of the risk of product degradation due to overheating in areas around high speed blade. Another limiting factor to the high speed disperser's performance is shear input. If running the disperser blade at the maximum speed yields an equilibrium particle size distribution that is larger than desired, the material is usually transferred to a higher energy but low throughput device such as a media mill or three roll mill.