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  • Agglomeration-A Basic Introduction

    In the modern food and chemical industries, there are ever increasing demands for controlling the performance of materials. Agglomeration is a viable approach to modifying bulk solid material to yield a more useable form with the desired properties. In this article, we hope to introduce the reader to the basic concepts of agglomeration, to introduce various agglomeration technology and to describe the properties of the materials delivered from each technology.

     

    Although agglomeration is a basis process that has been used for centuries, it still remains an art rather than an exact science. Perhaps this is due to the unpredictable behavior of bulk solids. In any case, we can define agglomeration as a process to transform finely divided particles into particles of larger size through the introduction of outside forces. In the most basic sense, these outside forces are manually delivered. Childhood activities such as making a snowball or a mud pie are examples of agglomeration processes. In industrial processes, these outside forces are delivered by some form of machine or a combination of machines.

     

    We see examples of agglomerated products every day. The most common examples are pharmaceutical tablets or bricks used for construction. Regardless of the final product, each is an example of building a larger final product starting with a finer material. Each has been subjected to a process that yields the properties specific to the application requirements.

     

    There are many reasons to consider agglomeration and a partial list is shown below:

     

    • Minimizing or eliminating dust
    • Improvingflowability
    • Improving storage and handling characteristics
    • Improving metering and dosing characteristics
    • Providing formulation stability, preventing segregation
    • Increasing or controlling bulk density
    • Offering defined shape, size, or weight
    • Providing solubility control, dispersibility
    • Improving appearance
    • Increasing product value

     

    The first four point to the poor handling properties of powders. Besides being dusty, powders do not flow well from packages or hoppers. They tend to lump or flow irregularly. The next five deal more with the final particle characteristics. Density or dispersibility can be controlled. A particle with specific size, shape or weight can be produced, such as the previously mentioned pharmaceutic tablet. Several ingredients can be blended and coagglomerated for product uniformity or consistency. But equally important, agglomerated materials often hold more value.

     

    Just as there are many reasons to consider agglomeration, you will see that there are several methods or processes available for size enlargement. A careful study of the available methods will lead to the best choice, but it is important to consider some things when choosing a process. First, in many cases, different agglomeration process will yield a final product. With that in mind, the consistency of the feed material should be consider, things such as particle size, moisture, consistent. Consider if your product can accept moisture addition or a binding additive. Operating cost should be balanced against capital costs. Most important, the final properties of your product should be well known and methods yielding these properties should be first considered.

    The numerous agglomeration methods can be divided into four major categories. First is pressure agglomeration, where fine powders are captured and subjects to high pressure. Under this category falls equipment such as roll briquetting or compaction presses, piston presses or tablet presses. The resulting products are granules, briquets, tablets or pucks. In tumbling or agitation agglomeration, powders and liquids are combined in a turbulent environment. Machines include pin or paddle mixers, drum or pan granulators or fluid beds. The products are known as granules or pellets.

     

    In extrusion, materials are forced through so called dies, which are simply holes in plates or rings. There are various types of extruders, high and low pressure. The pressure is controlled by the die length to diameter. Machines in this category are screw extruders, ram extruders, pellet mills, cooking or forming extruders. Products from this process are known as pellets, briquets, granules bars, and cubes.

     

    Thermal treatment is considered as an agglomeration process, but it is slightly different from the other methods in that it often involves a phase change. In this area are machines such as sintering grates, prilling towers, flaking drums, pastillators, spray dryers or melt extruders. In some the transformation comes from moisture removal. In others, material is cooled to transform a liquid to a solid. Typical products resulting from these processes are known as spheres, prills, pastilles, granules, nodules or beads.

     

    The various methods operate in different ways and produce different products. It is important to understand basic operation and process limitations. All have benefits and drawbacks and these must be considered in choosing the proper technology. Two methods will be looked at, Pressure agglomeration and Agitation or Wet agglomeration.

     

    In pressure agglomeration, material is captured in a machine and high pressure is applied. The pressure is sufficiently high to result in particle crushing and extremely tight particle packing. The particle are in such close proximity that inter-particle forces, such as van der Waal forces, hold the product together. Because of this, agglomeration is possible without adding moisture or binder to the feed material. This makes the process economical in that post drying is not required. The products from this technology are hard, dense and can be large, in excess of 250mm. The products can have uniform size and shape, with good strength.

     

    Then why would someone select this technology. Consider the following as benefits of this process. First, the process requires no or minimal binder or water addition. This is important if your material can not be contaminated with a binder or if water addition is undesirable or prohibited. If a binder or moisture is required, the amount will be much lower than that required for other technologies. Larger product sizes possible. Unlike other technologies that build granules, products such as briquets can be made with pressure. These briquets can be uniform shape and size possible. High strength high density particles produced. Wider variations in feedstock are acceptable. Other technologies require fine feed materials. Pressure agglomeration affords lower processing costs in general.

     

    Of course, there are drawback. Most important to consider is that capital costs can be higher than other methods. High pressure requires robust machine construction. At the same time, high pressure results in wearing of the machine elements. Spare parts usage must be considered. High density products may not dissolve or break down. Some material cannot agglomerate with pressure alone (binder/water required). If binder must be added anyway, other technologies may be more attractive. Finally, the feed material must be “dry”. This does not mean zero moisture, but the feed should resemble a dry solid.

     

    Tumbling or agitation is commonly referred to as wet agglomeration. Fine powders are agitated with liquids to make granules. Various agglomerating devices are used. Pin or paddle mixers accept a continuous flow of powder and liquid. Rotors with paddles or pins cause liquid to solids interaction resulting in the growth of granules or balls. The size of the granules is dependent on the configuration and speed of the rotors. High speed horizontal shaft mixers will grow granules approximately 3mm. high speed vertical blade mixers will grow more porous granules up to about 1mm and slow speed pugmills will grow granules larger than 20mm. 

     

    Rotating drum, pan or cone granulators incorporate rotating machine bodies. Fine powders and liquids are continuously feed to the interior of the machine body. The rolling action results in the formation of balls. The ball size is depending on the machine type. Control on the pan granulator results in balls of relatively uniform size where the balls from a drum or cone granulator are variable in size.

     

    Products from this technology are medium to low in density and strength and offer good dispersibility. Often the particles are round or rounded and often they are uniform in size. There are things to consider when looking at wet agglomerationprocess. First, the feedstock must be fine powder. Because the process relies on particle growth by layering or clustering of particles, only fine feed particles give good final products. Next, a binder or moisture must be added to glue the individual solids particles together. In general, capacities can be high and these technologies are attractive for wetting a material for disposal.

     

    So why consider this technology? In general, capital costs are low, as are wear and maintenance. Rounded particles may be desired. The strength and density are acceptable for packaging, shipping and final use and quite often, the particles produced by these methods offer good dispersibility.

     

    There are drawbacks to wet agglomeration. First, the feed must be a fine powder. Therefore, premilling of the feed might be required. Often drying of the granules is required, resulting in higher operating costs. If a binder is not added, the product strength might be low. Finally, the technology offers little flexibility on particle shape.

     

    With this variety of agglomeration choices, how do you choose the best option? First, consider your feed material. Look at the particle size, moisture and consistency (powdery, fibrous, paste). Next, consider “contamination” Can you add water, binder or heat history? Most importantly, consider your product requirements, properties such as size, shape, hardness and functionality.

     

    In summary, it is important to remember that various agglomeration options exist for most feed materials. With all else equal, consider overall costs including investment and installed cost, maintenance/wear parts, utilities and labor. But, you will not be satisfied with any system that does not produce a product meeting your needs.

     

    Michael White is Director Business Development for Bepex International LLC. Bepex International is a US based company providing process systems for solids materials processing. Technologies include size reduction, thermal processing, agglomeration, mixing and liquids/solids separation. Bepex is represented in India by JLS Engineers & Traders (India) Pvt. Ltd., Pune (Tel:  +918888847943).