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. The unit operation which is most closely associated with the formation of hard agglomerates during powder processing is drying. However, the various mechanisms which lead to the formation of hard agglomerates during drying have not been previously systematically studied.
The effect of capillary pressure during drying on the strength of fine powder agglomerates was investigated. Silica and titania slurries were dried at different drying rates, using both spray drying and tray drying, and the strength and strength distribution of the dry agglomerates was quantitatively measured using a calibrated ultrasonic field. The slurry surface tension was varied by using aprotic and protic solvents of different surface tension and by using mixtures of water and n-propanol or n-butanol of varying composition. Particles of four different diameter (20 nm, 28 nm, 60 nm, and 500 nm) were used to vary the effective radius of curvature between the particles.
The average agglomerate strength was found to increase with increasing surface tension and/or decreasing particle radius. Alcohol washed samples had similar agglomerate strength to samples washed with aprotic solvents of similar surface tension. Based on FTIR, TGA, and 1% NMR results, it .was concluded that the role of alcohol washing, which is commonly employed in fine ceramic powder preparation to produce softer agglomerates, is that of primarily surface tension reduction and not particle surface esterification as previously presumed.
The effect of particle solubility and dissolution rate on agglomerate was studied by drying silica and titania particles from aqueous slurries with pH ranging from 2 to 12. The agglomerate strength and strength distribution was measured by a calibrated ultrasonic force and the strength was found to increase with increasing solubility and dissolution rate. Two different particle size silica powders (60 nm and 500 nm) were studied and smaller sized particles were found to form stronger agglomerates. The drying rate of the powders was varied by using both spray drying and tray drying and it was shown that slower drying leads to higher agglomerate strength.
The agglomerate strength of titania powder (insoluble in water) was found to be independent of the pH while the agglomerate strength of silica was found to depend on the PH. It was concluded that the solubility and dissolution rate are important parameters governing the strength of agglomerates.