- THE MAGAZINE
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, contamination can result. Additionally, if the dryer cannot quickly and evenly evaporate the moisture content of the material, agglomeration and caking are almost inevitable, requiring subsequent grinding operations.
A new dryer* has been developed to eliminate these problems. A direct, gas-fired flash drying system, the dryer was designed to process a slurry of ultra-fine, ball-milled zirconium silicate containing 40% water to a free-flowing dry powder with final moisture content below 1%, and average particle size below 5 microns, at a production rate of 2,200 pounds per hour. Similar drying systems can also be used for other heat-sensitive or reactive ceramic products requiring fast, efficient drying.
Flash Dryer OperationThe new drying system is a recirculating loop flash dryer using a combination of temperature and pressure to simultaneously deagglomerate and dry a variety of materials. The system works by employing low-pressure air, steam or inert gas to continuously deagglomerate and convey materials through the drying circuit. The heated drying medium is introduced at high velocities into the drying chamber via a series of injection nozzles, creating a turbulent flow pattern. Drying begins instantaneously as the wet feed enters the dryer and encounters the turbulent gas stream. The turbulence creates particle-to-particle collisions to deagglomerate the feed and expose maximum surface area.
The increased surface area reduces drying time by maximizing heat transfer rates. During the drying process, material is conveyed through an integral centrifugal classification zone, where properly sized dry product is removed by the frictional drag of the exiting gases. Heavier, moist or agglomerated particles are recycled to the drying chamber.
Before the installation of the flash dryer, the zirconium silicate slurry was manually poured into trays, which were placed in vertical racks in drying ovens for up to 24 or 48 hours. The resulting “bricks” that formed in the trays were manually unloaded and ground in mechanical mills into discrete particles meeting product specifications. This process was not only time consuming and created a bottleneck in the process, but also required large amounts of labor. To solve this challenge, the loop flash dryer was designed to continuously dry and deagglomerate the slurry into a free-flowing powder. Only one operator is needed to occasionally check the system’s operation.
In the new system, the zirconium silicate slurry is fed by means of a slurry pump to a backmixer, where it is blended with recycled dry product to achieve a non-sticky, conditioned feed. Next, the feed enters the dryer through a rotary airlock. The material is then deagglomerated into discrete particles and dried to a final moisture content below 1%. Recirculating gases and deagglomeration ensure maximum thermal efficiency to achieve moisture specifications as low as 0.1%.
The end product is separated from the air stream in a baghouse and is transported by a screw conveyor to a collection station. An opening along the length of the screw conveyor, located above the hopper of a volumetric screw feeder, allows a portion of the collected product to recycle into the backmixer for feed conditioning.
Since the zirconium silicate is extremely abrasive, and maintaining a white color is critical for end product quality, the entire dryer is lined with 1 in. thick alumina tiles. This design not only greatly extends the life of the dryer, but also ensures a pure product. Automated system operation and high thermal transfer are designed to maximize product quality and drying efficiency.
Due to the high inlet temperatures, the dryer was designed with a separate hot air manifold section with expansion joints to accommodate thermal expansion.
BenefitsThe new dryer allows efficient drying at relatively low outlet temperatures due to the greater surface area and small particle size of the product. Slurries, sludges, filter cakes and moist solids containing up to 95% water or solvent can all be dried in a single system. The dryer produces deagglomerated products with high surface area and greater uniformity, eliminating the need for grinding after drying. Additionally, the dryer’s compact configuration requires much less space than conventional spray, fluid bed, or flash drying systems. With no internal moving parts, the dryer provides reliable operation with minimal maintenance.
For batch lots and materials that require cleaning between runs, the dryer can be designed to allow for fast, convenient access to all product contact surfaces. Continuous system operation and automatic conveying greatly reduce labor requirements.
Flash drying systems can be designed to accommodate a wide range of operational temperatures and drying gases to provide safe processing of all materials.