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The treatment o f finely dispersed particles in various kinds of aqueous, organic, emulsion or foam phases forms an important part of the processing route of many diverse chemicals, mineral and biological products. To understand the influence between the macroscopic bulk properties and microscopic particle behaviour is fundamental in understanding the governing parameters of a desired process, either being a separation technique, the formulation of a stable dispersed product (e. g. paint) or the kinetic control in a reaction vessel. As a result of different bulk medium conditions, in fact, particles can either experience strong attraction, forming larger agglomerates (unstable dispersion), or remain in a dispersed state (stable configuration).

Since the behaviour of fine particles is dominated by surface effects rather than by bulk properties [29], traditional separation techniques, such as filtration or particle settling, lead to poor rates of recovery without tackling the problem of selectivity, unless particles of the same species are agglomerated together (selectively agglomerated).

Particle-particle interaction in a liquid medium is more complicated than that in a gaseous phase. This is readily explained by the larger influence that electrical double layer and van der Waals forces have on the stability of suspended dispersions. The double layer effect, which w ill be extensively reviewed in section 3.3.3, is a specific interaction for particles submerged in a liquid. This latter interaction can be either attractive or repulsive depending on the surface charge of the particles and on the type and nature of any chemical species dissolved in the solution. Van der Waals attraction, on the contrary, is less sensitive to addition (in the solution) of chemical reagents and between particles of the same species is always attractive. In order to modify the relative influence of van der Waals and electrical double layer interactions, electrolytes or polymers are commonly used to promote selective agglomeration in the particle recovery processes.

A different way to favour particle agglomeration consists of adding small amounts of a second liquid, immiscible with the suspending medium and able to form liquid bridges between the particles. Under appropriate conditions of agitation, this method can lead to agglomerate formation in a similar way as has been described in section 2 . 1 for the

gaseous suspending medium. Differences with the gaseous phase methods yet exist and lie in the possibility of adding surface active agents (surfactants, see section 4.4) to the liquid bulk medium in order to promote the affinity of a desired species towards the binder and control selectively the agglomeration (selective recovery). Non­ agglomerated particles can still be recovered from a suspending medium if they remain attached to air bubbles, which are formed in the liquid, and are transported to the surface of the slurry.

Based on the previous methodologies, different processes have been developed as recovery techniques.

Coagulation and flocculation [30] are two processes largely and successfully employed for recovery and separation. Coagulation is the process in which dispersed particles are agglomerated by the addition of an electrolyte, which reduces the potential of the electrical double layer on the particles, thereby favouring the attractive van der Waals interaction to drive agglomerate formation. Flocculation, on the other hand, occurs when a low concentration of polymer is added to a dispersion of particles. Polymers are long hydrocarbon chains that adsorb on the particle surface, whilst segments of their molecule can protrude in the solution. Thereby, as particles approach one another the hydrocarbon chains might overlap and the bonds produced by these polymeric bridges result in adhesive forces that keep the particles agglomerated. Flotation is another separation technique widely used in the mineral industry. The water-ore slurry is conditioned with a frothing agent and sprayed with air to create a copious supply of bubbles. A collecting agent is also added to the slurry to increase the affinity of the desired mineral toward the air bubbles. The air bubbles, together with their attached mineral particles, rise to the surface of the pulp where they are removed from the system. According to the US Bureau of Mines, 293 flotation plants processed 485

million tons of ore in 1980 [31]. Flocculation, coagulation and flotation do not represent the focus of the work reported here and they w ill not be investigated further

Agglomeration of suspended particles promoted by addition o f an immiscible liquid binder is commonly referred to as spherical agglomeration, because of the final spherical geometry of the agglomerates. The separation method is based on differences in surface chemistry, which involves the selective adsorption of suitable surfactants and electrolytes onto the valuable particles to increase the affinity towards the liquid binder, but leaving the unwanted gangue dispersed in the aqueous pulp. In an aqueous suspending medium usually oil is used as binder collector and therefore the conditioning of the desired particles, through addition of surfactants able to increase the affinity toward the organic phase, is also called hydrophobization. When an oil phase is added to the aqueous medium, liquid bridges are preferentially formed between the hydrophobic species that can thus be agglomerated with a high degree of selectivity [29]

In the spherical agglomeration process, the agglomerates start to grow when sufficient binder liquid is added. The mechanism of growth starts with a flocculation regime during which particles form floes, loose open chains of particles held together by pendular liquid bridges. In the following growth step, the floes start to form pellets, closed chains of particles entrapping floes and liquid medium. The porosity of the floes decreases, as entrapped dispersion medium is squeezed out and eventually the pore space within the pellets is completely filled with binder liquid.

In the last step, agglomerates reach a constant mean size that is presumably the size where a balance between adhesive and separating shearing forces has been reached (Figure 2-11). This growth mechanism resembles the coalescence mechanism seen in Figure 2-2a.

This growth mechanism shown in Figure 2-11 is reported by Bos and Heerens [32] who measured, using a light scattering technique, changes in agglomerate size with time for different types of solid dispersion, such as glass limestone and aluminium silicate, recovered from carbon tetrachloride using water/glycerol mixture as the binding agent.

flocculation Figure 2-11 agglomerate formation

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Schematic diagram of the spherical agglomeration process for the case of small liquid binder droplets.

A different growth mechanism can be predicted in the presence o f larger binder droplets, similar to the immersion mechanism shown in Figure 2-2b. In this situation particles migrate towards the organic phase driven by electrical interactions (van der Waals and electrical double layer), which is enhanced by addition o f appropriate surfactants and electrolytes. The growth can be predicted using the layering model proposed by Schaafsma et al [3] (see section 2.1.2), provided the agitation is not too strong.

The growth mechanism is also a function o f the liquid binder added. Capes and Darchovich [33] reviewed the method o f coal fines recovery in the process o f coal bonification (from ashes and sulphurs) using spherical agglomeration. Using a fine coal stream from a preparation plant containing coal particles all finer than 0.5 mm diameter, a variety o f types o f agglomerates were obtained as the amount o f oil binder was increased from less than 5% wt. up to 30% wt. In the lower range o f oil loading only a few pendular bridges were formed between the particles and an unconsolidated floe structure resulted. W ith larger amounts o f oil (in the 5-15% range) the chain-like floe structure was replaced by more compact three dimensional agglomerates. In this region the agglomerates grew in size and reached a peak strength and compaction near the saturated region (see Figure 2-9c), when the void space in the interior o f the agglomerates was just filled with oil. Beyond the 15% level agglomerates became soft

paste-like cohesive lumps in which the solids were essentially dispersed in the bridging liquid.

The first commercial applications based on the spherical agglomeration technique date back to the 1920s and were tailored to mineral industry processes [34]. However, despite the good performance achieved in industrial plants and the excellent results obtained in laboratory tests for different type of ore recovery (see section 4.4.3), the process has not yet received widespread commercialisation, a major drawback with respect to other recovery techniques (flotation, coagulation, flocculation) being the increased cost due to the oil binder.

Commercial and pilot plants are reported in the literature in the following areas. □ purification of fine coal from sulphur and ashes [34] [35]

□ recovery o f fine valuable minerals, either as a main process or integrated in a gross extraction process to recover mineral fines from waste gangue, increasing the total grade of recovery [29, 36, 37]

□ oil sand ore separation to recover bitumen [38]

Recent studies have been undertaken in order to apply spherical agglomeration to other industrial sectors, such as the deinking of recycled paper [39].

In the work reported here, liquid bridge adhesion between pairs of particles suspended in a liquid bulk medium and the electrical interaction (double layer and van der Waals) between particles and liquid binder have been investigated, both theoretically and experimentally, for the case o f particles of different surface energies and for different bulk medium solutions of electrolytes/surfactants, in order to gain a better understanding o f the basic phenomena regulating spherical agglomeration processes.

2,2.1 Spherical crystallization process

More recently, the spherical agglomeration technique has been used for the manufacture o f high value products, such as crystalline pharmaceutical drugs [40-42]. In this latter

application crystals o f drugs are agglomerated in a liquid medium in presence o f a solvent that promotes crystallization o f smaller crystals dispersed in the liquid giving rise to increase o f the drug size. This method is also known as spherical crystallization [43]. This technique, which presents many analogies with the spherical agglomeration process, is illustrated in Figure 2-12.