Agglomeration overview : Mechanisms of agglomeration of powders

Agglomeration processes are ubiquitous in the process industry, be it wanted agglomeration through a process, or unwanted agglomeration like for example caking. This page is aiming at explaining what are the physical phenomena that cause agglomeration of powder particles. Such an understanding is critical to ensure that a factory is operating properly processes like fluidized beds agglomerators, roller compactors, tablet presses, extrusion or sintering.

1. What is the agglomeration of powder particles ?

Agglomeration is the action to gather originally separated solids particles into a conglomerate, which is a mass of particles which adhere against each other.

The agglomeration process can be carried out thanks to different agglomeration technologies, the most common are the following :

• Disc agglomerators
• High shear mixes
• Tumblers
• Fluidized bed agglomerators
• Tablet presses
• Roller compactors
• Extrusion
• Sintering

Each of this technology is actually playing on a different mechanism to ensure that solid particles form an agglomerate solid enough to meet the target product properties.

2. Agglomeration binding mechanisms

The agglomeration of solids particles can be due to 5 different categories of binding mechanisms, for a specific agglomeration process, one or several of these mechanisms can be at play.

2.1 Solid bridges

Solid bridges are the stronger link that can be made in between particles during agglomeration as they become directly linked by solid.

Solid bridges can be created by the following phenomena :

• Sintering (the particles, sufficiently heated up, are "merging")
• Partial melting of the contact surfaced in between particles (different than the sintering principle described above)
• Chemical reactions
• Recrystalization, which often happens when water is present, dissolves the solid under certain conditions, and is then evaporated. It can also be done by using a solution with some colloidal material used to wet the particles of solids and which is afterwards evaporated leaving the colloidal particles create solid bridges.

2.2 Adhesion and cohesion forces

Those forces happen typically when a layer of a certain substance is in between 2 particles.

It is the mechanism of agglomeration used by viscous liquid binders which are are coming in between the particles and stick them together. The binder is added during the agglomeration process, depending on the quantity added, it can even turn into a matrix binder, which means that the whole space in between particles, not only a layer, is occupied by the binder.

The particles can also adsorb molecules at their surface. This very thin layer, for rather small particles, can play a role as 2 adsorption layers on 2 particles can attract and keep together the 2 hosts particles.

2.3 Surface tension and capillary forces : liquid bridges

The presence of a liquid, most often water, is one of the major causes of agglomeration of particles. Water at the surface of the solids particles can create liquid bridges thanks to free water, or capillary condensation. When water is filling the pores of the solids until the surface, a negative capillary pressure within the pore can develop, creating a force at the end of the liquid bridge and keeping the particles together.

Those liquid bridges happen at the "touch point" or coordination point in between 2 particles.

2.4 Attraction forces between solids

Those forces are significant at very small scale, in between molecules, at can be high at this scale, but as soon as the distance in between particles increases, those forces become negligible compared to the other mechanisms mentioned in this page. As they act at very close range in between particles, they are also mainly significant for very fine particles (few dozens microns, or even nano particles).

Attraction forces can be of molecular, electrical or magnetic origin. The forces involved are the following :

• Van der Waals forces
• Valence forces
• Non valence associations (hydrogen links for example)
• Electrostatic forces
• Magnetic forces

2.5 Interlocking

The interlocking phenomena, helping particles to stay together in a conglomerate, is related to the shape of the particles and how the mechanically block the movement of each other at the particle level. For example, fibers, long particles can play a role to interlock with some other particles and restrain their movement. It is also the case in compression of powder where some particles can break, deform, and "enroll" around other particles, strengthening the bond in between them.

3. Factors influencing agglomerations

Agglomeration processes are influenced by several key factors that determine their success and the quality of the final product. Understanding these factors is essential for optimizing agglomeration outcomes. The following table is gathering different properties that can affect particles agglomeration.

Factors	Impact on agglomeration
Particle Size	The size of the initial particles plays a critical role in agglomeration. Smaller particles tend to agglomerate more easily than larger ones due to their larger surface area available for bonding.
Moisture Content	The moisture content of the feed material significantly impacts agglomeration. Proper control of moisture levels is essential, as it affects the material's ability to form cohesive bonds.
Temperature	Temperature can influence the agglomeration process, especially in cases involving heat-sensitive materials. Elevated temperatures may be required for processes like sintering, while lower temperatures are suitable for cold agglomeration techniques.
Binder Properties	When binders are used, their type, concentration, and properties (e.g., viscosity, adhesion) are critical. Binders act as adhesives that hold particles together during agglomeration.
Process Parameters	Agglomeration equipment and process parameters, such as mixing intensity, residence time, and shear forces, affect the degree and quality of agglomeration. Proper adjustment of these parameters is essential for achieving desired results.
Material Characteristics	As shown in paragraph 2, the inherent characteristics of the materials being agglomerated, including their chemical composition, surface properties, and cohesion forces, influence the agglomeration behavior.
Binder Activation	In cases where binders are used, understanding how and when binders are activated (e.g., through heat, moisture, or chemical reactions) is crucial for successful agglomeration.

4. How to successfully control powder agglomeration processes

Achieving consistent and high-quality agglomerates is essential in many industries such as Pharma, or more generally chemicals. This section provides practical tips and guidelines to ensure successful agglomeration outcomes:

• Optimizing Moisture Control: Careful monitoring and control of moisture content in the feed material is critical. Implement precise moisture control systems to avoid over- or under-moistening.
• Binder Selection: Choose binders based on the desired properties of the agglomerates and the specific characteristics of the feed material. Conduct binder compatibility tests to ensure effectiveness.
• Equipment Maintenance: Regular maintenance of agglomeration equipment is crucial to prevent downtime and maintain process efficiency. Replace worn components and conduct inspections as needed.
• Process Monitoring: Implement real-time process monitoring and control systems to adjust parameters as necessary during agglomeration. This ensures consistent quality and minimizes waste.
• Particle Size Analysis: Perform particle size analysis to confirm that agglomerates meet size specifications. Adjust equipment settings to achieve the desired particle size distribution.
• Testing and Validation: Conduct thorough testing and validation of agglomeration processes to ensure they meet product quality standards and regulatory requirements.

5. Agglomeration applications

Agglomeration processes are important is many industries due to their ability to enhance material properties (flowability especially) and facilitate various production techniques.

• Pharmaceutical Industry
• Agglomeration processes are extensively used in pharmaceutical manufacturing to create cohesive granules from fine powders. These granules improve flowability, compressibility, and uniformity of pharmaceutical formulations, making them essential for tablet and capsule production.

• Food Production
• Agglomeration is employed in the food industry to create instant drink mixes, soups, and various powdered products. By agglomerating ingredients, manufacturers can achieve better solubility, dispersibility, and flavor release when these products are rehydrated.

• Chemical Manufacturing
• Agglomeration aids in the production of fertilizers, catalysts, and specialty chemicals. It allows for controlled release of active ingredients, improved handling, and reduced dustiness in chemical formulations.

• Metallurgy and Mining
• In the mining industry, agglomeration is used to agglomerate ore fines into larger, more manageable pellets or briquettes. This process improves the efficiency of heap leaching and smelting operations.

• Ceramics and Building Materials
• Agglomeration techniques are employed to create granules and pellets from ceramic powders and building materials. These agglomerated materials are easier to handle, transport, and mold into desired shapes.

• Materials Science
• Researchers utilize agglomeration processes to engineer new materials with tailored properties. This includes the development of advanced ceramics, composite materials, and nanomaterials.

Source

[Pietsch] Agglomeration Processes (Phenomena, Technologies, Equipment), Pietsch, Wiley-VCH, 2004