Agglomeration
1. Fundamentals
One of the first difficulties met by any one attempting to give a survey of agglomeration is to define the term agglomeration comparing with other terms like granulation, size enlargement, balling, pelletization, tabletting, compaction, flocking, sintering etc. By usage these terms often include subsequent processing stages such as drying or firing in the overall application on the term.
In the broadest sense of the term agglomeration includes all processes in which fine particels, dispersed in either gas or liquid, aggregate to form a coarser product. The collection of particles that results is called an agglomerate or granule. Depending on the process, the size of the agglomerate is between 0.02 and 50 mm. In most cases, the preferred particle shape is spherical. In many processes the product is a cylindrical section such as a tablet or some other regular geometrical shape. Agglomeration is used in many industries, such as fertilizer production, irion ore, nuclear fuel, pulverized fuel ash, ceramics, lightweight aggregate production, carbon blacks, catalysts, pesticides and pharmaceutical products. Agglomeration has been a key technology in traditional engineering fields. At the same time agglomeration is now a key technology also in new developing hi-tech industries.
There are other reasons for the production and use of agglomerates:
1. Plant contamination and nuisance dusts in the workplace are reduced.
2. Hygiene is easier to maintain.
3. Air quality is easier to achieve and in some cases controls can be avoided.
4. The danger of dust explosions is reduced
5. With the increase in air and water pollution control regulations there is a need to
install dust and sludge removal equipment. As a result, there is a need to process and
dispose of the resulting fines. With agglomeration processes, perhaps with the addition
of a binder, dust and sludge can be recycled or disposed of without pollution.
6. There are many problems in the handling of fine particles. The flow properties and
consequently dosage control are poor. Agglomeration can eliminate these
disadvantages while retaining the desired particulate properties.
7. The chemical industry, with countless intermediate and final products, has a great
need for agglomeration technology. Some examples are the agglomeration of
simple or complex fertilizers to avoid segregation, agglomeration of fillers used in
plastics, to add exact amounts of iron oxide, zinc oxide, stearate or silicate to the
molten plastic, production of dustfree products, especially these with toxic or
corrosive materials. Size enlargement increases the commercial value because of
the improved physical properties. A typical example are detergents. Some materials,
such as fertilizers, pigments, pesticides and instant foods, are sold in
agglomerated form, but instantly decompose when added to liquids.
8. Many raw materials are only suitable for processing or end use as coarse particles.
These coarse solids have the desired strength and porosities, for example catalysts.
9. Ore beneficiation produces a large number of finelygrained products. Examples are
flotation concentrates from selective beneficiation processes, dusty products from
roasting, blast furnace byproducts, filter cakes and particulates from dust removal
equipment. Ores or ore mixtures must be agglomerated for smelting to obtain
optimal flow conditions in the roasting process.
10.The principal area of application in the ceramic industry is in the preparation of
pressed particles made of barium titanate and manganese-zinc, nickel-zinc and barium
ferrites. These materials are initially fine powders, which must be agglomerated to
obtain good dimensional replication in the pressed articles.
11. In addition, agglomeration can cause a delayed action, which is important for
pharmaceutical and agricultural applications. This allows good dosage or improves the
appearance of the product.
2. Agglomeration processes
The desired product properties determine which enlargement process is used. Agglomeration processes are classified by the principal mechanism by which the particles are made to come together. The selection of a specific process is only possible if the user clearly defines the properties required of the product.
Growth Agglomeration (Agitation Methods)
Fine particles are brought into contact with each other in a flowing system or in air when the concentration is higher. This is usually done in the presence of liquid and binders. The particle size enlargement occurs by coalescence or accregation (snowballing) based an capillary forces. In a few exceptional cases, the mayor cohesive force is the van der Waals force. Usually the agglomerates are spherical with diameters between 0.5 and 20 mm. Typical equipment types are inclined drums, cones, pans, paddle mixers and plowshare mixers. The maximum throughput is about 100 - 200 t/h for iron ore pellets and 50 t/h for fertilizers.
Spray Agglomeration (Spray Methods)
This is one of the most commonly used methods in the chemical, pharmaceutical, and food industries. Pumpable suspensions are atomized and the liquid is evaporated from the droplets by means of hot air, as a preliminary drying step. The first cohesive forces are the capillary forces, which are followed by crystal bridges at the contact points. The agglomerates are 20 - 500 mm. For chemical applications, throughputs of up to 50 t/h are possible.
Selective Agglomeration (Spherical Agglomeration)
The most recent agglomeration process, a second immiscible phase is added to the Suspension. This wets the solid phase and binds the particles together by means of capillary forces. As a results, rounded flocs or agglomerates form with diameters up to 5 mm. Selective agglomeration can be achieved for mixtures of solids. It is currently being studied for many substances. In the case of coal suspensions, throughputs up to and exceeding several tonnes can be achieved.
Pressure Agglomeration (Pressure Methods, Compaction)
Particles with only slight amounts of moisture are formed in tablets and briquettes in stamp presses, tabletpresses and roller presses. The principal binding force is van der Waals attraction. The agglomerates have uniform shapes and range in size from a few millimeters (pharmaceutical tablets) to decimeter size (fuels). In the case of smooth rollers, the resulting flakes are broken up into the desired size. The throughput for ores is roughly 100 t/h; for chemicals up to 30 t/h.
Sintering (Thermal Methods)
Fine particles are made into a paste by adding moisture and then processed in a horizontal sintering oven into sinter (burnt agglomerates). This is especially common in the mining and preparative industries. The final sintered product usually has an irregular shape and is usually coarser than other agglomerates. The mechanism of binding is the formation of solid bridges at the contact points. Sintering plants have throughputs up to 1000 t/h.
3. Binding Mechanisms and Adhesion Forces
The forces of cohesion between small particles are difficult to measure directly due to their very low magnitude and limitations resulting from small size. Nevertheless, a good understanding of the interaction between particles in an assembly is fundamental to size enlargement processes. For example, during the formation of agglomerates by agitation methods, relative bond strength determines growth mechanism as well as kinetics and influences agglomerate shape. In addition, bonding is important in determining the final properties of the product such as the ability to withstand handling during further processing, its rate of dissolution or reactivity, its density, etc.
Attractive forces can be calculated for model systems, such as smooth, fixed, ideal spheres. Even though the values calculated for these models apply only crudely for real systems, they do indicate the effects of important parameters an the agglomeration process. Models are essential for the understanding of the size enlargement process. To calculate the actual attractive forces between real particles is not currently possible because of their irregular shape and usually rough surfaces.
4. Field of research and terms of reference
Every aspect of the agglomeration process is in urgent need of further investigation but perhaps the most obvious aspects are:
1. Adhesion Force
The adhesion forces and the fundametals in forming aggregates as well as causing the strength of agglomerates.
There are only a few principles of possible adhesion mechanisms, but the forces of cohesion between small particles are difficult to measure directly due to their low magnitude and limitations resulting from small size. Therefore a more fundamental approach, based on the nature of the particle-particle interaction is needed, considering the influence of the form and the roughness of particles including the influence of the surrounding medium (air or water etc.).
2. Characterisation of Agglomerates
The objective of all processes utilizing agglomeration is to produce a product material with desired properties in optimum quantities. It is therefore most important to characterize the physical properties of agglomerates with comparible measuring systems. For this it is necessory to standardize the methods or to develop new measury systems.
3. Agglomeration Processes
Investigations of the mechanisms of agglomerate growth are required for understanding the agglomeration processes. The kinetics of agglomerate growth have been quite extensively investigated, but there is sufficient evidence for specific effects to indicate that much remains to be discovered in this field.
Perhaps with this knowledge it is possible to design the agglomeration equipments and to solve the scale-up problems.
4. Product Industries
The major developments in agglomeration processes have taken place over the past 200 years, related and specialized to the material of the special industries. Till now it was not possible to campore the experiences of the different industries.
There is a lack of experience exchange, which has to be forced in future.
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