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 – on the contrary to the disperse original substances – 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. In terms of ecology and user safety, the agglomerated product is superior to the initial particles: less dust means that the maximum allowable concentration levels at workplaces in production areas are easier to meet in a reliable manner.
Definition of agglomeration
This process can be used for a wide variety of applications. Nevertheless, or perhaps just because of that, 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 (granum, lat. grain) 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 (van der Waals and electrostatic forces) and indirect adherence mechanisms with product bridging (viscous and hardening binding agents, liquid bridges, cristal bridges, etc). These bonding mechanisms can be globally generated in three different manners: by introducing heat (fusion granulation), by mechanical compaction (press granulation) or by build-up granulation.
Build-up granulation process
Build-up granulation is an interaction between adherence and separating forces (mechanisms counteracting the sticking together of the particles) 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 – for example in case of longer storage.
Suitable 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 shovel mixers (in batch and continuous operation) and vertical high speed mixers, the liquid binding agent is introduced either via lances into the turbulent zone of the choppers (homogenisers) 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. The limit of size for agglomerates is approximately 3000 µm for build-up agglomeration in mixers, the size distribution is about 1:5 at d80.
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. Ploughshare shovels rotate as mixing elements in a horizontal, cylindrical drum close to the walls; the basic shape of the shovels was patented by the German manufacturer in 1949.
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. Depending on the product up to 175 kg/dm3 can be processed per hour.
Though the mixer is quite compact, it is designed for high and very high specific throughputs. Having a working volume of
5 to 3000 l, this machine meets the highest requirements in terms of profitability and availability.
Other solutions that can act as powerful agglomerators include the type KUM universal mixer and the type MGT mixing granulator, a vertical mixing system with low energy input and low temperature increase but high mixing power. Available in different sizes, the machines are characterised by a vertical, cylindrical mixing vessel and are 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.
The special shape of this element and its peripheral speed are coordinated in such a way that the mixing product is circulated as a vortex and thereby accelerated horizontally and vertically. A high speed chopper is placed above the mixing element, i.e. in the product vortex, directly in the turbulent area where agglomeration can proceed.
Gebr. Lödige Maschinenbau GmbH, Paderborn