Particle engineering by means of agglomeration processes

Particle engineering is all about the combined action of a base material, processing equipment and catalysts. The challenge is to form particles from this combination, which have the precise properties that contribute toward the successful application of the product by the end user. An important tool in this respect is the agglomeration process.

The role of a mechanical engineer such as Hosokawa Micron in Doetinchem involves much more than simply delivering machines and installations. The client is faced with the task of creating powders or grains that are suitable for the end user's objective. In the majority of cases, that product, whether in the form of powders or grains, must satisfy a series of requirements. The end user will need, for example, to dose, mix and dissolve the product. In that instance, the grains must flow freely, have a specific strength and particle size and must have an excellent wettability. It is absolutely essential that a client discusses the desired properties and functionalities of the product with the machine designer.

Particle engineering

The knowledge and experience of Hosokawa Micron enable the company to engineer the grains in such a way that they possess the desired physical properties. Sometimes, the compatibility of these properties is poor and it is necessary to seek an optimum balance. This may lead to a situation whereby, for example, a client is suggested to opt for an agglomeration, instead of for a pellet. In other instances, it is suggested that properties relating for example to the drying behaviour, porosity, specific particle surface, bulk density or compressibility be achieved with layered particles. Only when the most important functions of the end product have been formulated is it possible to examine the various processes that can be applied in order to form the desired particles and achieve the desired properties. This brings us to the stage in which a process is translated into machines and installations, which is yet another process that takes place in close consultation with the client.

Agglomeration

In many instances, the base materials used to form the product are poor-flowing powders. These poor flow properties are often a result of Van der Waals forces, electrostatic charges and coarse particle surfaces. An essential process, used specifically to improve the flow properties of such powders, is agglomeration. This process involves the interaction of the base material with the processing equipment and catalysts. The process often begins with a mixing phase, during which a liquid is fed into the base material. Liquid bridges then form between the product particles, as a result of which agglomerations form. The strength of the agglomerations can be improved by means of adding a binder. The liquid bridges in the agglomeration can be converted into solid bridges by drying. These solid bridges consist of the product itself, or of crystallised binder.

Oil-based binders

The choice of binder is extremely dependent on the application of the end product. Some binders are edible, whilst others are not. The strength of the binder varies somewhat. In the case of some binders, it is possible to easily crush or smear the agglomeration between the thumb and forefinger, whilst other binders on the contrary produce an extremely strong agglomeration. In the case of oil-based binders, the melting process is highly important, in relation to both food and non-food products. It may for instance be a requirement that the granules disintegrate at a temperature of 80°C, because the product is extruded at this temperature. Examples of oil-based binders include palm oil, cocoa butter, polyethylene glycol (PEG) and paraffin.

Water-soluble binders

In addition to oil-based binders, water-soluble binders are also applied. Examples of edible, water-soluble binders include molasses, CMC (CarboxyMethyl Cellulose) and modified starch. Molasses give rise to a strong, stable agglomeration. Agglomerations based on CMC and modified starch are weaker than this. Starch can be added to the base material both in powder form and as a solution. A non-edible, water-soluble binder is sodium silicate.

This binder is applied within the chemical and minerals industry. It produces solid granules that dissolve relatively slowly in water. Somewhat more costly water-soluble binders include PVP and PVP (polyvinyl pyrrolidone and polyvinyl acetate), which are used for the formation of strong agglomerations.

Principles

For the formation of agglomerations, various principles can be applied that correspond to different installations.

  • Cyclomix: In the case of shear agglomeration, the particles are rubbed against one another, giving rise to extreme shearing forces. A machine that operates according to this principle is the Cyclomix. Generally speaking, the agglomerations are dense and stable.
  • Flexomix: It is also possible to increase the particle size by allowing them to collide with one another intensively. A machine that makes use of the impact principle is the Flexomix. An agglomeration fluid is sprayed in a vertical, rapid product flow (20 - 50 m/s) that is immediately spread throughout the product. The Flexomix produces instant agglomerates with an open, raspberry-like structure, good solubility and good flow characteristics.
  • Bextruder: An agglomeration technology that is based on the exertion of pressure. The Bextruder is based on the extrusion principle. In this rolling machine, the particles are pressed together under high pressure. The extruded or compacted product has a specific shape and is dense.

Drying and cooling

The final stage of the agglomeration process may incorporate a drying stage (for water-soluble binders) or a cooling stage (for oil-based binders). The time at which drying or cooling takes place, the rate of drying or cooling and the drying or cooling intensity all bear a strong influence on the quality of the agglomeration and must therefore be determined with a view to the final application of the product.

Eng. Berthram Mak
Hosokawa Micron B.V.