Rotary dryers are used to remove moisture from bulk materials through heating. This is done either directly or indirectly with heated air and/or gasses. They can consist of several drums (also called shells) or a single drum, with multi-drum rotary dryers enabling equipment to produce greater throughput in the same size space. Heating of rotary dryers with multiple drums is usually done directly via gas or oil burners. Some rotary dryers also have a combustion chamber positioned at the end of the feed, optimizing fuel usage to keep air temperatures the same throughout the drum. While their primary task is drying materials, some rotary dryers combine this process with shredding, separating, cooling, cleaning, or other processes.
Versatility of Rotary Dryers
Though rotary dryers are used primarily for chemical processing and mineral extraction, they are extraordinarily versatile machines. They’re also widely used in agriculture, ceramics, dairy, food, lumber, paper, rubber, and other industries.
Rotary dryers can be used for applications that include:
- Dehydrating corn gluten meal after wet milling.
- Drying rice directly from the paddy.
- Eliminating contaminants in soil.
- Extracting vegetable oils.
- Processing citrus peels and pulp for animal feed.
- Removing moisture during the processing of chemicals and minerals.
- Roasting cocoa beans, nuts, and sesame seeds.
- Mineral processing and lithium mining.
A rotary dryer’s job is essentially to dry out a variety of materials before, during, and after processing, including biosolids, concentrates, copper, fly ash, gypsum, limestone, manure, metal shavings and chips, plastics, potash, rubber, salts, sugars, and various ores.
How a Rotary Dryer Works
Rotary dryers are often used to process bulk materials and powders, fed continuously into the dryer’s drum, along with hot air. A rotary dryer’s drum rotates, using lifting flights to carry particles upwards. Near the drum’s top, particles come off the lifting flight and fall through a heated air stream.
The rotary dryer’s drum is at a slight incline, fed from the higher end of the drum and discharging from the bottom part. Hot air or another heated gas blows counter currently or concurrently, and as the drum rotates, material is pulled in the direction in which the drum is revolving. Once it reaches a certain angle, material falls back to the bottom of the drum, with most drying action happening as it falls through the heated air or gas.
Rotary Dryers: Direct vs. Indirect Heating
Heating in rotary dryers occurs either directly or indirectly. Direct drying happens when material comes into contact with heated air or gasses, whereas indirect drying happens due to contact with a drum that’s externally heated. Normally, a combustion chamber is found in directly heated rotary dryer designs, in order to stop material from coming into direct contact with the burner’s flame. Indirect drying is the less efficient means for removing moisture from material during processing, though this method is used for certain niche applications for which it performs better than direct rotary dryers.
A wide range of applications use rotary dryers that heat directly for processing bulk solids like minerals, mined ores, fertilizers and chemicals. In this direct configuration, material is heated via convection. For applications where materials must not make contact with heated gases, rotary dryers that utilize indirect heating should be used. Additionally, if a very narrow particle size distribution is necessary, directly drying the material would cause it to get trapped within the airflow. For this reason, adsorbents, catalysts and other specialty materials require rotary dryers that employ indirect drying methods.
Chemical Processing Applications for Rotary Dryers
Dry slurries and pastes use rotary dryers in mining activities to help chemically remove the ores from the rock they’re found, their robust nature and large capacities make them particularly useful for handling abrasive materials. They’re also used to dry and mix the chemicals used in making pharmaceuticals. Along with manure drying, rotary dryers can also be used to process fertilizers like DAP (di-ammonium phosphate) and NPK (nitrogen, phosphorus, and potassium) fertilizers of various ratios.
In chemical processing operations, rotary dryers help with:
- Augmenting handling, as dried chemical ingredients, are easier to convey.
- Forming material into specific textures, such as achieving maximal particle surface area in catalysts.
- Lowering transportation costs by removing liquids from materials.
- Meeting necessary requirements for post-processing of materials.
- Producing agglomerated chemical arrangements.
- Removing toxins in the material being processed
- Stabilizing and conserving solid materials.
Rotary drums play an essential role in chemical processing, where they serve not only as dryers but as coating drums, coolers, granulators, kilns, and other purposes. In fact, they play such an important part in many chemical processing plants that on the very rare occasions when they require maintenance, the whole plant needs to shut down. Because of their important role, rotary dryers for the industry require special designs that enable them to deal with factors common in chemical processing, such as abrasion, corrosion, and materials building up.
While chemical processing often involves handling abrasive materials, abrasion is further heightened by the rotary dryer’s drum continuous revolutions. For this reason, rotary dryers for the chemical processing industry often use abrasive-resistant materials, along with liners. Additionally, many equipment manufacturers suggest reinforcing areas that experience more wear, like discharge and feed chutes.
The most common issue when working with chemical compounds involves corrosion. Many chemicals corrode materials relentlessly, while others do so under certain conditions. Minimizing the effects of corrosion is a requirement for most rotary dryers designed for the chemical processing industry. Depending on the type of drum used, special alloys or superior grades of steel are often used within a rotary dryer’s construction. In cases where the material only becomes corrosive at when it has a high moisture content, stainless steel or similar corrosion-resistant alloys are used for the rotary dryer’s inlet section, while the remainder is built from carbon steel. Sometimes protective coatings made from acrylics, epoxy polymers, latex or urethane are used instead, or in addition to these other strategies. Non-heating drums for coating or granulating will generally use various types of liners to resist corrosion.
Regardless of the purpose for which it’s used, buildup of materials in a rotary dryer’s drum is often a concern, especially when working with hygroscopic or high-moisture materials. Material buildup can lead to serious issues during processing, depending on the application for which a dryer is used. This may increase the risk of fire, reduce the machine’s efficiency, impede material flow or even damage the drum internally as larger chunks break away.
There are, however, ways in which rotary dryer design can discourage this buildup. For non-heating drums, linings that also protect against abrasion and corrosion are effective, while cast iron plating is also sometimes utilized. Rotary dryers can also incorporate internal knockers into the drum design, which work to dislodge buildup as it rotates. In certain cases, rotary dryer construction may include polished stainless steel to help prevent sticking.
Another way to limit buildup involves reconfiguring airflow, especially for rotary dryers used as kilns. For material that tends to clump, fitting a trommel screen to the lower end of the drum where material discharges will help break up clumps as material exits. Flight design that expends less force can also mitigate material buildup while placing advancing flights close to the inlet allows the material to quickly leave the area where the material is discharged to discourage buildup.
Advantages of Using a Rotary Dryer
Though other types of dryers have been used to dry bulk materials – including fluid bed dryers and flash dryers – rotary dryers are still largely preferred for industrial processing.
Advantages of rotary dryers include:
- Customization: The wide range of designs, sizes, and materials from which rotary dryers can be made allows them to be easily customized to suit the material and processing objectives.
- Longevity: Due to their simple yet robust design, they can operate for decades without trouble when properly maintained.
- Polishing: When working with granulated materials, the rotary dryer’s rolling action helps round off edges on the particles, making them more “polished.”
- Reliability: With fewer moving parts and hardy construction, rotary dryers are known to be incredibly dependable machines.
- Throughput: The high capacity to process bulk solids makes them ideal for many industries, with output ranging from 1 to over 200 tons per hour.
- Versatility: Feedstock often varies when processing bulk solids, and the rotary dryer can tolerate variations in both feedstock and the conditions under which it’s processed; this makes it more capable of dealing with changes in moisture content, particle size distribution, and throughput.
Rotary Dryers by Heyl Patterson
Heyl Patterson Thermal Processing makes rotary dryers that are among the most versatile on the market. Used as workhorses in processing bulk and powdered solids, our rotary dryers can handle a wide range of materials, from liquid sludges to fine powders. Additionally, Heyl Patterson rotary dryers can be set for starting and finishing moisture content, retention time, air velocity, and temperatures for the product and the air used in drying.
Our rotary dryers are ideal for processing:
- Carbon fibers
- Inorganic Chemicals
- Pulp for paper
Our rotary dryers can also be customized for various cooling designs, including air and water cooling, countercurrent air sweeping, or internal and external water cooling. To learn more about our rotary dryers, contact the experts at Heyl Patterson.