Granulation Drums

A drum granulator works by tumbling material in the presence of a liquid binder to encourage granule formation and growth. Feed material and binder are constantly fed through the rotating drum. The rolling action, combined with the stickiness from the binder, causes fines to collect together into particles. These particles continue to pick up additional fines and binder as they tumble in the bed, causing them to densify and pick up additional layers in a phenomenon known as coalescence. Tumbling flights can be incorporated into the unit to increase agitation of the material bed. 

This way of forming agglomerates is a type of agitation agglomeration also known as wet granulation, balling, or pelletizing.

Generally speaking, drum granulators can process any bulk solid in the form of a dry, fine powder. This flexibility lends well to a host of industries, processing materials ranging from specialty chemicals and fertilizers, to minerals and ores, and everything in between. 

In some cases, the feedstock may require pretreatment in the form of crushing, grinding, or drying to be a suitable feed for the granulator and produce the characteristics desired in the end product. 

In choosing between a drum granulator and a disc pelletizer, the drum is often chosen because it offers a higher throughput than the disc pelletizer. The rotary drum also has the advantage of being a “closed” system, meaning that granulation occurs inside a contained environment, so fugitive material is limited. 

Granulation drums can be customized to nearly any size, accommodating capacities ranging from 500 lb/hr – 3500 TPH+. 

Rotary drum granulators are typically employed in a continuous setting, though they may function as a batch device in some R&D environments, such as the FEECO Innovation Center.

Retention time is one process parameter used to control the characteristics and quality of the material exiting the drum, so it can vary significantly depending on the process and product goals. In general, retention time in a granulator may fall anywhere between 30 seconds and 60 minutes. 

Rotary drum granulators are generally low maintenance, but some items can become worn, depending on the operating conditions, and particularly if the drum falls out of alignment. The most common wear items for a drum granulator typically include the liner (where applicable), tumbling flights (where applicable), and potentially trunnion wheels, tires, and thrust rollers.

Rotary drum alignment is an important aspect of drum maintenance to prevent wear of the tires and trunnion wheels. When a drum falls out of alignment, it puts added stress on all components, encouraging undue wear.  

Since the granulation process relies on a liquid binder to form and layer granules, the product exiting the drum is wet and must be dried unless it is moving immediately to a downstream process that does not require drying (such as in the case of iron ore balling, where pellets go to a grate kiln). 

Drying is typically carried out in a rotary dryer, which further rounds and “polishes” the granules as a result of the tumbling action that occurs as the drum rotates. 

Whether or not a granulation drum is the right fit for a given project depends on specific process and product goals, such as capacity, level of control over particle characteristics, product parameters, and more. 

When it is not clear if a granulator will be able to meet the project objectives, testing can be carried out in the FEECO Innovation Center to evaluate the process and identify the most suitable equipment configuration.

Granulation drums require a feeding and offtake system to convey material into and away from the drum. They also typically require screens and a recycle circuit to recover overs and unders and recycle them back into the process

Drum granulators are also typically followed by a rotary dryer and in some cases, may be preceded by a mixer for providing a homogeneous feedstock to the unit.

Rotary drum granulator design typically begins with testing to assess the feasibility of granulating the intended material and to gather key process data such as percent fill, retention time, drum speed and slope, spray system design, and more.

Once the data has been gathered, engineers work off of this to size the drum and incorporate any necessary features or modifications needed to produce the desired product at the intended capacity. 

A variety of data points are necessary for design. Along with a process description, the follow data is typically required:

Raw Feed Data

• Material and chemical composition
• Moisture content
• Temperature
• Bulk density and true density
• Material consistency (sticky, plastic, etc.)
• Material qualities (abrasive, heat sensitive, hazardous, etc.)

Product Data

• Desired particle size distribution (PSD – minimum and maximum)
• Expected bulk density
• Binder use and restrictions

Process/Equipment Data

• Preferred materials of construction
• Electrical controls (volts, hertz, phases)
• Temperature of ambient air in processing environemnt – minimum and maximum
• Humidity of ambient air in processing environment – minimum and maximum
• Elevation of plant
• Retention time (if known)

Depending on how much is known about the material, much of this data is often gathered through testing. 

Yes. Rotary drum granulators can be fitted with programmable logic controllers (PLCs) and motor control centers (MCCs) for automating the unit, adjusting the process from a human-machine interface (HMI), as well as collecting and trending valuable process data. 

Yes. Granulation drums offer ample opportunity for customization. FEECO tailors the mechanical components, internals, materials of construction, size, and more to optimize how the unit works with the material to produce the desired results efficiently. 

A variety of add-ons and customizations are available as well, including knockers, liners, spray systems, and more. 

While some processes don’t necessarily require a recycle circuit, the addition of one is always recommended, as it helps to make the process more efficient. 

In other cases, a recycle circuit is necessary for the operation to function on a continuous basis, relying on recycled fines to bring down the moisture content of the feedstock entering the process (a processing technique known as back mixing).