From a material handling standpoint agglomeration generally refers to the process of making larger particles (agglomerates) from smaller particles. It is used for duties as diverse as the formation of carbon black and, in the extreme, the production of scrap metal bales. Agglomerates can take the form of sheets or briquettes from an extruder or roll press process or pills and caplets from pill press operations. Other forms include prills and pearls.
Many reasons for agglomerating materials exist. These include: increasing a material’s bulk density, decreasing dust concentration during feeding, reducing segregation of material (and thus product uniformity), improving the flow rate of material and making an end product such as a briquette or pill.
A common method for agglomerating materials involves applying pressure to a bulk solid using a roll press or extruder. When creating agglomerates by applying pressure, it is important that the internal strength of the material is sufficient to maintain the integrity of the agglomerate after the pressure is released past the press or extruder. This requires improving the adhesion of particles, which demands an understanding of the mechanisms of adhesion. For presses these mechanisms include the interlocking of particles, welding of particles under pressure and temperature, chemical reaction and the use of binders to increase agglomerate strength .
Interlocking is particularly evident in agglomeration of large odd-shaped particles such as metal turnings. On a smaller scale, round hollow particles of materials like those used in nutraceuticals can break and interlock under pressure. Agglomerates formed by this mechanism can have high internal strength; however the strength can degrade shortly after agglomeration depending upon the tendency of the particle to return or spring back into its original particle size and shape. Metal turnings show this tendency, which can be overcome in some cases by applying pressure at a higher temperature.
Adding heat to pressure application can cause welding of particles in agglomerates. This is often helpful for materials like rubber and polyethylene flake that have a tendency to spring back. Such welding can greatly increase the integrity of the resulting agglomerate.
Binders — such as molasses, starch, water, kaolin clay and oil — are frequently and effectively used to raise the internal strength of agglomerates. Drying, heat curing and chemical reactions that might occur with a binder such as cement can further increase the internal strength .
Unsatisfactory agglomeration can arise because of problems both upstream of the roll press or extruder and during the feeding of newly agglomerated materials. Always keep in mind that upstream problems can critically affect the quality and uniformity of the finished product. Material flow into the feed system must remain consistent and uniform to avoid problems such as breakage of briquettes or unacceptable concentrations of active ingredients in pills.
Segregation in the roll press or extruder feed hopper can degrade the strength and quality of agglomerates. An excess of large particles can reduce the internal strength of briquettes while too many fines can lead to inconsistent flow rates into the feed-screw system, breakage due to confined high-pressure gas (capping) and fluctuations in viscosity of extruded materials. Segregation can result from the addition of some powdered binders. If the angles of repose of the binder and the main material differ significantly, segregation in the feed bin can occur. Permeability differences between the binder and main ingredient, combined with feed conditions into the bin, can lead to air-entrainment segregation as less-permeable fine particles are air-swept to the sides of the bin. If the main ingredient and binder are free flowing but dramatically dissimilar in size, sifting segregation can arise as the smaller particles fall between the larger particles and enter the feed system first.
In some cases the addition of binders will change a material’s properties enough to impede flow into the feed-screw system of a roll press or extruder. Tests have determined that adding just 1% water to agglomerate dust may change the flow rate of a material by an order of magnitude. A feed containing a binder may become sticky and cohesive. This may cause hang-ups in a feed system that works well without the binder .
Entrained air can pose problems in many aspects of the agglomeration process. For effective adhesion, particles must contact each other. Air trapped in material impedes particle-to-particle contact and prevents effective agglomeration. Under certain circumstances the air-entrained material will fluidize and flush through a system. Other situations prevent the flow of fine powder into the feed-screw system. Agglomerates with entrained air often break after compression.
Fine powders have the greatest porosity and ability to entrain air. These materials tend to have the least permeability and thus require ample time for air to escape the voids. Special consideration of these characteristics is necessary for effective press operation and agglomeration of fine particles.
Air entrainment begins as powder enters the hopper above the feed-screw system. Use of an anti-segregation letdown chute can decrease the distance the powder falls through the air and limit the amount of air entrained. Using a mass-flow hopper design (i.e., where all material is in motion as material is drained from the hopper) or preferably a first-in/first-out (FIFO) or plug-flow design will minimize segregation caused by differential flow patterns and increase the hopper usable capacity. This can allow the powder to sit in the hopper long enough to deaerate prior to entering the feed-screw system. The feed-screw system is integral in maintaining consistent flow to the roll press and should utilize both a rate-controlling horizontal screw and a vertical screw. A vent is required to release air pressure developed as the roll press compresses the powder .