Agglomeration, Its Types and Suggestions for Reducing Undesired Agglomeration

Pressure Agglomeration

The process of forming particles into desired shapes by applying pressure at various grades is called pressure agglomeration. There are several methods for it; such as Compaction, Briquetting, Tabletting, Molding and Extrusion (feed pelleting belongs to this category).

Agglomeration and shaping are achieved by the pressure forces that push the product into the hole and the friction forces that occur as the product passes through the hole. The level of force applied during the densification and shaping phase is the most determining factor in pressure agglomeration. When the pressure is applied, particles in the pile begin to get closer to each other (compaction). When the pressure is increased further, brittle particles begin to crumble and soft particles start to deform (plasticization). Two factors determine the speed of compaction and the capacity of the agglomeration machine.

1. Compressed residual gas (air) in the pores
2. Elastic spring-back

Some reasons for the selection of pressure agglomeration: Larger feed particle size, high initial strength, dry or hot processing, no or little binder, no post treatment, processing of elastic materials, automatic operation, easy clean-out, quick turn-over.

Product characteristics in pressure agglomeration: Specific shape, large pieces, specific mass, high density, low porosity, high final strength, long shelf life, amenable to the production of near net-shape parts.

Material density can be changed by adjusting various parameters in pressure agglomeration. For example, density increases with stronger pressing force. In cylindrical compactors, density decreases with larger diameters (even if length/diameter ratio remains the same). Density also decreases a bit with a fluid addition. When lubrication is applied to the pressing tools/die, density decreases.



                                                     Modern Pellet Mill

Size enlargement by agglomeration is also highly preferred in animal feed industry. Some of many reasons for this are listed below.

• Conversion of feed components with different grain size, weight, moisture absorption ability and other physical conditions into a stable feed formulation
• Prevention of dust related losses
• Better product flow, storage, transportation
• Prevention of segregation/dispersion of feed components
• Preventing animals from choosing the feed, making it more accepted by animals
• Ability to prepare animal type-specific feed (chicken, fish, etc.)
• Prevention of clumping and accumulation
• Prevention of losses due to oxidation or other reactions during storage
• Better collection of added fluids during feeding
• In some cases, better dissolution / dispersion in liquid
• Possibility to contain a wide variety of waste materials including nutritious liquids
• Increased availability of nutrients for digestion after conditioning and pelleting
• Increased starch value of feed
• In pelleting, lower moisture requirement for agglomeration after conditioning.;thus, lower energy requirement for drying process
• For pet food, pleasant appearance of the product for the buyer
• Increased production process economy
• Higher profit margin

Desired or Undesired Agglomeration in the Process

Although agglomeration is generally a desired and targeted process, it may occur unintentionally in some cases. The steps of feed production process are examined below.

• SEPARATION: Undesirable at screening/sieving, undesirable or sometimes desirable at classifying/sorting
• MIXING: Undesirable or sometimes desirable at dry mixing, undesirable or desirable at wet mixing
• GRINDING: Undesirable at dry grinding and wet grinding
• CONVEYING: Undesirable or sometimes desirable at mechanical conveying, undesirable or sometimes desirable at vibratory conveying, undesirable at pneumatic conveying
• STORAGE: Undesired at silos and hoppers

Suggestions for Reducing Undesired Agglomeration

1- SEPARATION:

• Remove or "scalp-off" fines immediately at the source.
• Destroy agglomerates mechanically (rubber balls, shear, impact, brushes, air jets, ultrasound, etc).
• Modify amplitude and/or frequency.
• Remove moisture from material and/or gas (air) environment.
• Heat (direct resistance / inductive).
• Modify wetting angle and/or surface tension of liquid (surfactants).
• Disperse particulate solids in suitable liquid (wet separation).
• Use dispersion aids (chemicals, mechanical [e.g. stirring, ultrasound]).

2- MIXING:

• For dry or moist mixing, apply shear or frictional stresses (tumbling mass, mixing tools, special accessories such as shredders, baffles).
• For wet mixing, use shearing (stirring) or dispersion agents. Also, utilize controlled flocculation (addition of electrolyte) that allows repeatable re-dispersion by shaking.

3- GRINDING:

• Remove fines and/or moisture.
• Coating of the inner walls of mills and crushers (elastic or non-stick).
• Lower reduction ratio (size reduction amount in grinding). Use multiple grinding steps for that.
• Use cryogenic milling (making use of material brittleness at low temperatures).

4- TRANSPORTATION:

• Remove fines and/or moisture.
• Make equipment, chutes and pipes from electrically conducting materials (avoid plastics).
• Finish inner walls smooth or with coating (e.g. Teflon).

5- STORAGE:

• Remove fines and/or moisture.
• Cool materials to ambient conditions prior to storage.
• Insulate and/or heat walls.
• Ground (electrically conductive) hoppers, silos and storage containers.
• Smooth and/or coat (e.g. with Teflon) all inner walls.
• Use steep (as close to vertical) walls, especially towards discharge openings.
• Employ mechanical bridge breakers (vibration, shock, pulsed air jet, etc.)
• Apply load (overburden) relieving means (e.g. "Chinese" hats, baffles).
• Carry-out conditioning / curing prior to storage.