EXECUTIVE SUMMARY
Agglomeration is an inherent problem in almost all industrially relevent powder processing techniques. Succesful dispersion of powders often requires the total elimination of agglomerates. To achieve this, it is important to understand the nature of, and to ascertain the properties of, these agglomerates. Of particular importance is the strength of the interparticle bonds, (between primary particle units forming the agglomerate), in relation to the powder processing procedure used for their generation.
In this IFPRI-funded project, a characterization technique has been developed to quantitatively determine agglomerate strength distributions in bulk powders by exposing a sample dispersion to a calibrated ultrasonic field and following the ensuing changes via particle size analysis. This technique may be used throughout a powder processing procedure to determine the source (onset) of agglomeration, as well as an aid in the elimination of these agglomerates by helping the user to understand the nature of the interparticle bonds.
The primary advantage of this technique (over other agglomerate strength determination techniques) is that it classifies agglomerate strength with a distribution rather than a single value, giving the user a more complete understanding of the system under consideration, as single value agglomerate strengths describe only the magnitude of agglomeration and not the extent. Furthermore, this is a ‘wet’ testing technique that allows agglomerate strengths to be determined under conditions similar to process conditions, which can be of great significance as powders often display different physical and/or chemical properties in and out of a liquid. Also, the simplicity of the testing procedure lends itself to automated, in-situ analysis (currently under development), whereby agglomeration in a system can be continuously monitored. Thus changes in the extent/magnitude of agglomeration (in response to modifications in the processing procedure) can be observed, and also monitored in a time dependent fashion.
The validity of this approach has been demonstrated for model silica agglomerates of known bond strength distribution and primary particle size prepared by heat treating ordered, sub-micron silica spheres at various temperatures. The use of this approach to find at what step agglomerates form in industrial processing is illustrated by measuring agglomerate strength during various titania processing schemes. As an example, the type of washing (ethanol versus water) and the dry ing conditions (temperature) caused large differences in the quantity and strength of hard agglomerates formed from the same titania precursor powder.
A numerical scheme has been developed for analyzing the change in the particle size distribution of a powder dispersion during ultrasonic breakdown and determining the actual mechanism of particle breakdown (erosion vs. fracture). This type of analysis also allows a specific particle size group (and thus particle strength group) to be followed throughout the degradation process. When applied to industrial processes, this approach should provide design information to minimize agglomerate formation during processing and for selecting successful dispersion and size reduction strategies.