As products become more specialized and the demand for quality rises, coatings are becoming increasingly more important in industries working with bulk solids. Whether intended to reduce dust, mitigate caking, or enhance performance, the ability to coat granules and other particles gives producers greater control over how their products look, behave, and perform.
Coating may be carried out using one of several devices. Oftentimes, and particularly in the fertilizer and soil amendment industry, the choice comes down to a coating drum or ribbon mixer. While both types offer an effective coating option, there are some settings where the coating drum provides a better approach to coating.
Here’s a look at these two types of coating equipment, and when it makes sense to choose a coating drum over a ribbon mixer.
About Coating Equipment
Coating equipment selection is just as important as the coating itself. The type of equipment chosen can have implications on critical aspects of the process and product, such as retention time, throughput, and even coating uniformity.
Both coating drums and ribbon mixers, also known as ribbon blenders, offer an effective coating solution in many different settings, but they each operate a little differently. In choosing between the two, it’s important to understand the fundamental differences between them.
Note: A spray system mounted above a belt conveyor is another common approach to coating. However, this produces inferior results due to the significantly uneven distribution of coating and for this reason, has not been included here. For more information on how coating drums outperform belt conveyors, see our article, Improving the Fertilizer Coating Process With a Coating Drum.
The Coating Drum
A coating drum is a type of rotary equipment. Granules or particles are fed into the drum, which is typically set on an incline to help move material through the unit. A spray system distributes the liquid coating over the bed of material as the drum rotates. The rotation of the drum causes the granules to tumble and roll in the bed as they move through the unit, causing the coating agent to evenly distribute throughout the bed onto each granule.
The Ribbon Mixer
A ribbon mixer is a type of horizontal mixer. Granules or particles are fed into one end of the mixing trough, and a centrally mounted agitator, typically in the design of helical “ribbon(s)” helps to move material through the unit while also promoting a mixing action to distribute the coating throughout.
When to Choose a Coating Drum
Coating drums boast several advantages that make them the preferred device in a variety of settings. Most notably, coating drums are chosen over ribbon mixers when:
A Long Retention Time is Needed
The most common reason for which a coating drum is selected over a ribbon mixer is when the process demands a long retention (also known as residence time) – the amount of time the material must be processed in the unit to reach the desired results. A longer retention time is sometimes needed to accommodate a chemical reaction or the absorption of an additive.
While in some settings the coating drum’s size may be considered a disadvantage, in many settings, it is its size that offers this distinct advantage; coating drums can offer a significantly longer retention time compared to ribbon mixers, often doing the work of what would otherwise require multiple ribbon mixers.
The Process Has a High Throughput
Also because of their size, coating drums can handle a much higher throughput compared to ribbon mixers, making them ideal in high-capacity settings such as potash processing plants.
The Product Requires Gentle Handling
Coating drums also offer the best coating solution when the material to be coated is fragile or requires gentle handling. This is because coating drums impart a gentle rolling action, in contrast to the forced “pushing” movement that occurs in the ribbon mixer. Impact forces such as those that occur in the ribbon mixer tend to break down fragile particles. Similarly, pinch points within the mixer can also lead to product breakdown.
As such, when processing any particles that are fragile or in a wet state where deformation could occur, a coating drum offers the best solution.
Maintenance is a Concern
Due to their low maintenance requirements, coating drums are also the device of choice in settings where downtime must be kept to a minimum.
Ribbon mixers tend to have higher maintenance and cleaning requirements than coating drums, requiring ribbon placement as needed.
Another advantage in terms of maintenance is that the maintenance required of a coating drum typically occurs on the drum’s exterior components, meaning operators usually have a visual indicator of when maintenance is necessary. In contrast, the ribbon mixer’s maintenance requirements occur on the unit’s interior and are therefore often not discovered until the unit has been shut down.
The Material is Heat Sensitive
Because a ribbon mixer uses mechanical action to distribute the coating and move material through the unit, frictional forces can generate a slight increase in temperature. In most cases this is not a concern, but when working with materials that are sensitive to heat, the coating drum is the preferred type of equipment to mitigate the potential for product degradation due to heat.
Confirming Coating Drum Feasibility
The coating of granules can be a highly specialized endeavor; diversity in both granule and coating characteristics creates an endless set of operating conditions required to reach specific product quality.
Those unsure of whether or not a coating drum would work for their intended operation can take advantage of testing facilities such as the FEECO Innovation Center. Here, producers can test coating feasibility with their specific coating and material to not only confirm proof of concept, but also to gather the process data necessary for commercial-scale equipment design and scale-up of the process.
The FEECO Innovation Center is unique in that it allows for coating drum testing in isolation, or as part of a larger agglomeration/granulation and drying process.
Conclusion
While coating drums and ribbon mixers are often compared side-by-side in coating equipment selection, recognizing their many differences is critical to selecting the most appropriate device for the application at hand. When a long retention time or gentle handling are needed, as well as when the process has a high throughput, can’t afford downtime, or the material is sensitive to heat, coating drums provide the best option.
FEECO engineers and manufactures custom coating drums to meet specific processing requirements. In addition to equipment design and parts and service support, we also offer both testing and tolling capabilities to support coating operations. For more information on our coating drums and services, contact us today!