Due to improved product attributes, the agglomeration of powdered or fine-grained substances is continuing to gain in importance. Although this classical process still places special demand on the machines used in particular for high-quality products.
In many industries and applications, powdered or fine-grained substances are hardly suitable as intermediates or end products. They are prone to create dust, to stick or to segregate. Agglomeration is therefore a process alternative which consists in bringing together disperse substances to build agglomerates. These show nearly the same specific surfaces. At the same time, they have the advantage in offering much improved product properties like better flowability, simpler dosability, optimised transportability as well as a dust-free handling.
Definition
This process can be used for a wide variety of applications. Nevertheless, there are always terminological discrepancies when dealing with the basic operation of “agglomeration”. “Agglomeration” is often confused with the comprehensive term “granulation”. However, “granulation” only refers to the form of the end product but does not indicate how the granulate has been achieved.
This is different in the case of agglomeration: Agglomeration consists in bringing together disperse substances to form bigger particles. This implies bonding mechanisms between the particles whereby one can differentiate two sorts of bonding: direct adherence mechanisms without product bridging and indirect adherence mechanisms with product bridging. These bonding mechanisms can be globally generated in three different manners: by introducing heat, by mechanical compaction or by build-up granulation.
Process
Build-up granulation is an interaction between adherence and separating forces generated by a mutual interaction of particles, agglomeration seeds and agglomerates. The selection principle will apply in this case i.e. the weak bondings are broken and only the stronger ones remain.
The achieved firmness depends therefore on the strength of the bonding forces, the intensity of the separating forces and time subjected to these forces. The quantity of binding agent generates the product bridging between the particles. By increasing the liquid quantity in the pore space between the particles, one can go through different states of saturation leading to different bonding strengths. In any case the firmness of the agglomerates is an important requirement to preserve the product properties in the long run.
Suitable machines and mixing systems
The success of agglomeration in a mixer depends on three factors: the construction of the mixing system, its operating parameters and the method of introduction of the liquid binding agent. In case of horizontal shovel mixers and vertical high speed mixers, the liquid binding agent is introduced either via lances into the turbulent zone of the choppers or directly into the product area. In case of ringlayer mixers, the binding agent will be introduced via nozzles into the annular layer generated by the mixing tools. The desired product form and quality will determine the necessary retention time.
As a global leader in machines and systems for industrial mixing and associated process technologies, Lödige Process Technology develops application-specific solutions for a wide variety of agglomerates with top product quality. These solutions include Ploughshare® Mixers for batch operation and for continuous operation using the mechanically generated fluid bed system introduced to mixing technology by Lödige.
Ringlayer Mixer and vertical high shear mixer
A further machine suitable for build-up agglomeration is the high speed, continuous Ringlayer Mixer CoriMix®. The high peripheral speed of the mixing tools generates a centrifugal force forming a concentric annular layer of product. The product moves through the mixing chamber in a plug-like flow manner under constant energy input. The high speed difference causes intensive mixing within the product layer and this creates particularly short residence times. Having a working volume of 5 to 3000 litres, this machine meets the highest requirements in terms of profitability and availability.
Another solutions that can act as powerful agglomerator is the type MGT Mixing Granulator, a vertical mixing system. The machine is characterized by a vertical, cylindrical mixing vessel and is operated in a batchwise manner. A three-bladed mixing element rotates close to the wall and with minimum clearance to the bottom of the vessel. A high speed chopper is placed above the mixing element, in the product vortex, directly in the turbulent area where agglomeration can proceed.
Conclusion
Agglomeration processes can considerably improve the product properties provided that the firmness of the product is sustainable. That is why in the field of build-up agglomeration, the selected machinery will have to ensure highest mixing quality taking into consideration the individual product and application.