Simplifying Materials Drying

April 1, 2002
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Flash dryers can accommodate a wide range of temperatures and drying gases, providing quick and safe processing for a number of materials.

Flash dryers are designed to deagglomerate a wide variety of materials to produce dry, discrete particles.
Particle size and purity are of the utmost importance for many ceramic materials, but keeping wet powders and slurries from agglomerating and clumping during drying can be a challenge. Often, further size reduction and classification operations are necessary to ensure the quality of the finished material—but these operations add more energy, more equipment and more time to the overall manufacturing process, making it difficult for manufacturers to streamline their operations.

Flash dryers can solve these problems. They are designed to deagglomerate a wide variety of materials to produce a dry, discrete particle. Additional benefits are short residence times which prevent thermal degradation of products, and with no moving parts, the dryers require minimal maintenance and ensure that product purity is maintained.

Flash dryers can accommodate a wide range of temperatures and drying gases, providing safe processing of all types of materials. Slurries, sludges, filter cakes and moist solids containing up to 95% water or solvent can be dried and deagglomerated in a single system, eliminating the need for additional grinding after drying. Additionally, through continuous operation and automatic conveying, flash dryers can also help manufacturers reduce their labor requirements.

Figure 1. Flash dryers typically employ a recirculating loop that combines air temperature and pressure to simultaneously deagglomerate and dry a variety of materials.

The Flash Drying Process

Typically, flash dryers employ a recirculating loop that combines air temperature and pressure to simultaneously deagglomerate and dry a variety of materials (see Figure 1). The system works by using low-pressure air, steam or inert gas to continuously deagglomerate and convey materials through the drying circuit.

A heated drying medium (up to 1300F) is introduced at high velocities into the drying chamber from a dryer manifold and through a series of high velocity nozzles, creating a turbulent flow pattern. Material is then fed from the screw feeder into the high-velocity gas stream that circulates within the torus. Drying begins as soon as the feed enters the chamber and encounters the turbulent gas stream. This creates particle-to-particle collisions that break apart the material and expose maximum surface area. Increased surface area reduces drying time by maximizing the heat transfer rates.

During the drying process, material is conveyed to a centrifugal classification zone, where the properly sized dry product is removed by the frictional drag of the exiting gases. Heavier, moist or agglomerated particles are kept within the unit and recycled to the drying chamber.

The airflow can be set manually by adjusting a blast gate at the discharge of the inlet blower. Fine-tuning and control over the drying process is further achieved by setting the drying chamber discharge air temperature to automatically control the firing rate of the air heater. Adjusting the variable-speed drive on the screw feeder manually controls the feed rate of the material.

The dried product can be separated from the air stream by a cyclone, while exhaust gases (containing the fines) are passed through a dust collector or scrubber. If a cyclone is not utilized, the end product can be separated from the air stream directly in a baghouse, where it is transported by a screw conveyor to a collection station. An opening along the length of the volumetric screw conveyor, located above the hopper, can allow all or a portion of the collected end product to be recycled into a backmixer for feed conditioning.

On applications with high inlet temperatures, flash dryers can also be designed with a separate hot air manifold section (with expansions joints) to accommodate thermal expansion.

Solving Drying Problems

Because they can be designed to operate with a wide range of temperatures, operating pressures and atmospheres, flash dryers can provide fast, efficient drying for a variety of materials used in ceramic manufacturing. Following are several examples of materials that are usually very difficult to dry, but that can be dried easily with flash dryers.

Zirconium Silicate. When producing matte finish ceramic glazes, the purity and particle size of the raw material are of the utmost importance. Drying is a crucial step in achieving these qualities. If the dryer components are not designed to handle abrasive materials, such as zirconium silicate, contamination can result. Subsequently, if the dryer cannot quickly and evenly evaporate the moisture content of the material, then agglomeration, mass clumping and caking are almost inevitable, and additional grinding operations are required.

A direct, gas-fired flash drying system can eliminate these problems. One such system* was designed to process a slurry of ultra-fine, ball-milled zirconium silicate containing 40% water and convert the material to a free-flowing dry powder with final moisture contents below 1%. Average particle sizes are typically below 5 microns, and production rates of 2200 lbs per hour are well within the system’s capacity.

The dryer was designed to be recirculating, continuously drying and deagglomerating the slurry into a free-flowing powder in a closed-loop operation. In conventional zirconium silicate drying, the material slurry had to be manually poured into trays, which were placed on vertical racks and into drying ovens for up to 48 hours. The resulting bricks were then manually loaded and ground in mechanical mills to meet product specifications. With the flash dryer, no prior brick making is needed and no mechanical milling is required, saving both time and energy. Additionally, only a single operator is needed to check the system’s operation, providing labor savings as well.

As the material is dried and deagglomerated into discrete particles, the process ensures maximum thermal efficiency—even with moisture specifications as low as 0.1%. Since zirconium silicate is extremely abrasive, and maintaining a white color is critical for end-product quality, this particular flash dryer was lined with 1-in. alumina tiles—a design option that greatly extends the life of the dryer while ensuring product purity.

Talc Powder. Dry talc is a slippery, soft material (Mohs scale hardness of 1) that has two surfaces in its structure—one is hydrophobic and organophilic, and the other is hydrophilic and organophobic. Although talc is not heat-sensitive, wet talc cake will retain moisture. Clumped or agglomerated talc particles will stick together if steps are not taken to deagglomerate during drying.

After months of testing and evaluating various talc-processing dryers, one company determined that the most energy-efficient dryer was a toroidal (or doughnut)-shaped, closed-loop flash dryer. The closed-loop dryer was found to be twice as efficient as the straight-through dryer. In addition, the new unit dried and completely deagglomerated the product. Size reduction was also achieved through the high circulation velocity within the small, closed-loop body. The size of the unit also meant it would require less space compared to other processing equipment.

Installation of the new toroidal-shaped flash dryer system increased talc production capacity at this facility by about 20% due to better thermal efficiency. Using a specially designed heat recovery system also meant that less operator attention was required. With the flash dryer, the operation is now more stable with smaller temperature fluctuations.

Iron Oxides. Rotary kiln dryers are often used for drying and calcining iron oxides. However, this process can cause moisture to be trapped in the material, turning it into hard, lumpy masses. These masses can get very large in size, and additional milling or screening procedures are typically required to deagglomerate the material once it is dry. Additionally, when material is calcined in a rotary kiln, there is often a delayed response to any process changes.

Flash dryers, on the other hand, produce discrete, fine particles without the need for additional milling. A two-unit flash drying system is typically used, with the dryers arranged in a series. The first dryer is used to process the wet, yellow oxide cake with initial moisture of 40% to a deagglomerated yellow powder with 10% bound moisture. The second dryer calcines the yellow powder into a fine, red powder containing less than 1% water. Drying temperatures run from an inlet of 800F to an outlet of 210F, while calcining temperatures run from an inlet of 1300F to an outlet of 750F. When calcining in a flash dryer, any process changes are immediately visible in the material, so no time is wasted on trial and error.

Silicates. Silicates can be very difficult to dry. They are also very abrasive and tend to quickly wear down moving parts in conventional dryers. With a flash dryer, high pressures are used to achieve size reduction in addition to deagglomeration during the drying process. Wet caked silicate having an initial moisture content of 35 to 65% can easily be reduced to 5 and 20%. In fact, final particle sizes of 95% at < 10 microns can easily be achieved. Additionally, the systems have no moving parts, so maintenance is very low. And ceramic or metallurgical coatings can be used to provide even more abrasion resistance to the dryers’ internal surfaces.

Other ceramic materials can also be efficiently dried in a flash dryer, including alumina slurries. The systems have also been proven in drying such difficult materials such as food, municipal sludge and agricultural chemicals.

Flash Drying Benefits

Drying difficult materials is a key part of many ceramic manufacturing operations—but it doesn’t have to be time-consuming and problematic. By using a flash dryer, manufacturers can eliminate the need for additional milling by simultaneously drying and deagglomerating their material into fine, free-flowing, discrete powder. Flash dryers also prevent thermal degradation of the product through shorter heat cycles, minimize maintenance requirements due to the lack of internal moving parts, and maintain product purity by using air deagglomeration instead of mechanical methods.

For More Information

For more information about flash drying, contact Fluid Energy Processing & Equipment Co., Corporate Sales & Marketing, 4300 Bethlehem Pike, Telford, PA 18969; (215) 721-8990; fax (215) 721-2355; e-mail a href="mailto:lflahart@fluidenergype.com">lflahart@fluidenergype.com; or visit www.fluidenergype.com.

*The ThermaJet™ system, designed by Fluid Energy Processing and Equipment Co., Telford, Pa.

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