Rossetti, Damiano; (2002) Direct investigation of liquid bridges in relation to the mechanisms of particle agglomeration in gaseous and liquid media. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Particle agglomeration processes occur in a wide variety of important industrial applications, either intentionally, for product formulation purposes (e.g. pharmaceutical granulation) or unintentionally, as in the caking of powder in storage silos. Examples of agglomerated products are: fertilisers, ceramics, catalysts, pesticides, minerals, pharmaceuticals, foods (particularly 'instant' products) and detergents, where the common objective is to improve the handling and ease of use of the product. Agglomeration can also be used as a separation technique. This is often carried out in solid/liquid suspensions, where either the desired or undesired particle species are encouraged to agglomerate by the addition of surfactants and electrolytes, so that they can then be separated from the remaining gangue. The process is referred to as spherical agglomeration, because of the final spherical geometry of the formed agglomerates; in this case the liquid binder is chosen to be immiscible with the suspending medium. Spherical agglomeration is used in the separation of minerals and valuable ores as small as 10 m in diameter with a high grade of recovery, as well as in the manufacture of speciality chemical products. In both agglomeration and spherical agglomeration processes the presence of particles of different wetting behaviour can create selective agglomeration of some particles at the expense of others and this phenomenon may be either beneficial, as in a selective recovery process, or undesired in the agglomeration of a formulated product. Moreover, in liquid media both the particles and the liquid binder assume a charge that can be altered by the addition of surfactants. Such conditions can largely modify the particle-to-binder interaction before any liquid bridge is formed. This thesis reports on the extensive experimental programme undertaken to study the physiochemical properties of liquid bridges formed between pairs of particles of similar/dissimilar surface energy that are either surrounded by air or are submerged in a liquid medium. The objective was to elucidate the fundamental mechanisms governing the initial stages of agglomeration in relation to the wetting behaviour exhibited by the particles and, for the case of a suspending liquid medium, the conditions (addition of surfactant/electrolyte) that may improve the affinity of the particle toward the liquid binder. The experimental work on liquid bridges was carried out with a unique Micro-Force Balance (MFB), which was developed in previous work to measure liquid bridge forces in gaseous media. The present work describes the continued development of the measurement technique and its adaptation to the measurement and observation of interparticle forces in liquid media. The principal parameters investigated were the geometry (in both gaseous and liquid bulk media), the strength and the energy (in the liquid bulk medium) of a liquid bridge during separation of particles exhibiting similar or dissimilar surface energies. In the case of the experiments carried out in a liquid medium, the influence that surfactants have on the particle wetting behaviour and on the adhesiveness of the liquid bridge was also investigated. The force exerted by a liquid bridge was largely influenced by the wetting behaviour of the liquid on the particle. Liquids that welted both particles well produced the highest forces. This phenomenon may explain the reason why during agglomeration of different species particles with higher wettability can stay together whilst those with lower affinity toward the binder may separate again and be segregated under the influence of agitation. The affinity of particles toward the liquid binder and its relation with the presence of surfactant/electrolytes in the bulk medium was investigated in a set of separate experiments using an Atomic Force Microscope (AFM). The latter work was undertaken at the University of Maine, USA and was aimed at determining the mutual effects that liquid bridge adhesion forces and the DLVO forces have on the mechanism of particle agglomeration in a liquid medium. In addition a feasibility study was conducted on behalf of a pharmaceutical company, Merck, Sharp and Dohme Ltd., to determine whether the MFB technique (in the gaseous bulk medium) could identify differences in behaviour between a paracetamol crystal and two different polymeric binders. Fresh paracetamol crystals were contacted and retracted from the two binders and differences in the amount of the liquid binder retained by the crystal were observed, which can be correlated to the mechanism of liquid binder distribution among particles during the process mixing. The success of the study demonstrated the flexibility and usefulness of the MFB approach to the investigation of real systems.

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