Bonding Mechanisms in Agglomerates

Capillary bonds due to negative capillary pressure (suction) and interfacial forces  

Attractive forces between particles in the absence of liquid and solid 

Although measurement of the compressive strength of agglomerates is certainly more convenient for routine checking and quality control, the tensile strength is a more fundamental property, since in theory it can be 

A majority of the models incorporate what are essentially curve fitting parameters or functions. Some (C11, K12) are more pertinent to the pressed, briquetted, or tableted beds of particles rather than to granulated ensembles of particles, even though the distinction between the two kinds of pellets is necessarily somewhat arbitrary. 

Rumpf (R4) has derived an explicit relationship for the tensile strength as a function of porosity, coordination number, particle size, and bonding forces between the individual particles. The model is based on the following assumptions (1) particles are monosize spheres (2) fracture occurs through the particle-particle bonds only and their number in the cross section under stress is high (3) bonds are statistically distributed across the cross section and over all directions in space (4) particles are statistically distributed in the ensemble and hence in the cross section and (5) bond strength between the individual particles is normally distributed and a mean value can be used to represent each one. Rumpf s basic equation for the tensile strength is 

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Section III. Bonding Mechanisms in Agglomerates Adorjan, L. A., Theoretical prediction of strength of moist particulate materials, in Agglomeration 77 (K. V. S. Sastry, ed.), p. 130. Am. Inst. Min. Metall. Pet. Eng. Trans., New York, 1977. 

Rehydration Bonded Alumina. Rehydration bonded aluminas are agglomerates of activated alumina, which derive their strength from the rehydration bonding mechanism. Because more processing steps are involved in the manufacture, they are generally more expensive than activated aluminum hydroxides. On the other hand, rehydration bonded aluminas can be produced in a wider range of particle shape, surface area, and pore size distribution. 

Very often the bonding of wet agglomerates uses a combination of mechanisms III.l and III.2. In that case partial volumes exist which are completely filled with liquid while in others liquid bridges prevail. 

As will be shown in Sections 5.2 and 5.3, particle size of the particulate solids plays an important role in agglomeration. While the surface area of particles, the interface at which all binding mechanisms act, decreases with the second power of particle size, volume and, therefore also, mass, the most important particle properties which result in forces that challenge adhesion and cause separation of bonds, diminish with the third power of the particle size. If the particle size reaches a few pm or is in the nm range, the natural adhesion forces dominate and particles which contact each other or come into close proximity adhere to one another. This phenomenon can not be economically eliminated so that very fine particles always adhere and form loose agglomerates which may be desirable or undesirable (see Section 5.5). 

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