Drying Difficult Materials - No Caking, Please!
Ceramic-wax mixtures used in the manufacture of spark plugs must be dried at moderate temperatures to avoid melting the wax. Traditionally, such mixtures have been dried in batch tray dryers that require a great deal of hand labor to load and remove the trays. Furthermore, the batch tray dryers are prone to problems of caking and poor uniformity. A new dryer* has been designed to overcome these problems by intermittently transferring the material in a stagewise,** continuous operation.
The process involves wet blending of the ceramic mixture and wax, granulating and then drying. The dried mixture is molded and further shaped before going into a high temperature oven for sintering. The dryness of the mixture is crucial at this stage because any residual water will form steam that will cause cracks and fissures in the final product, reducing its strength and insulating properties.
A major supplier of automotive electrical parts has installed three of the new dryers for its ceramic-wax mixtures. Its most recent installation, the model LE-18, is 6 feet in diameter and approximately 10 feet in height. The complete package included an external electrical heater, fresh air supply fan with inlet filter and the interconnecting ductwork.
Dryer OperationInside the vertical housing is a stack of 18 circular shelves that rotate together. The shelves are divided into tray segments by radial slots, which allow the material to be transferred down to successive shelves and eventually discharged through the dryer bottom (see schematic). As wet material enters the dryer through a flange in the roof, it forms small piles on each tray segment. The rotating pile of material is transferred when it meets a stationary wiper blade and falls gently to the next tray. The fresh piles thus formed are leveled to a predetermined depth by another stationary blade.
Through this gentle action of piling and leveling, the ceramic mixture is dried without caking or agglomeration. Because it is a plug flow operation, the retention time can be determined accurately and adjusted by a variable speed drive on the trays. At the center of each tray a circular section is cut out, making room for the fans that are mounted vertically on an axial shaft.
The drying medium enters through a side mounted vertical manifold and leaves the unit through a roof exhaust. The fans mix the incoming hot air with the air inside the dryer and circulate the drying medium over each material bed. This method of heating allows the precise temperature control necessary to avoid melting of the binders. Furthermore, the multiple hot air inlets allow different temperature zones in the top and bottom of the dryer.
The new dryer takes a wet feed with 10% free moisture and discharges a uniform granular mixture with less than 0.3% moisture. Drying temperatures are carefully controlled to prevent melting the binder while drying in the shortest possible time. A belt conveyor carries the ceramic mixture to the top of the dryer, where it is fed continuously through a rotary airlock. The dried material is discharged through another airlock underneath the dryer directly into a vibratory separator, and then into totes for removal to other parts of the plant.
An interesting design modification for the new dryer is the “D-breeching” fitted to the roof. This attachment extends the roof in the area of the exhaust flange, thus minimizing turbulence and reducing dust carryover.
Process AdvantagesBy preventing caking during the drying stage, the dryer avoids the need for further processing before molding and sintering. The dryer is ideal for blended feeds because the components of the blend remain well mixed. The low vertical air velocity also results in lower dust carryover than most continuous dryers.
The dryer can easily handle unplanned changes in feed rate because the essential drying parameters of temperature, layer depth and transfer frequency remain unchanged.
Prior to implementing the new dryer, drying at the automotive parts facility was accomplished by manually spreading a bed of material on the drying trays and loading them into a static oven. The product suffered from non-uniformity due to overdrying at the top of the bed and underdrying at the bottom of the bed.
Abrasion is often a concern when using a continuous dryer for ceramic materials. The new dryer overcomes this problem with its gentle material handling.
Production staff at the automotive parts facility were initially skeptical of the new dryer until they saw one operating in a different part of the plant. They had an opportunity to run the ceramic-wax mixture through the dryer and were immediately impressed, not only with the product quality, but also the dryer’s ease of start-up and operation. They installed one dryer, and on the basis of its excellent operation have ordered two additional dryers for these ceramic mixtures.
Drying TestsTo establish the product quality available from the dryer with their new material, samples of the ceramic mixture were tested in the dryer manufacturer’s laboratory. An important consideration was that the dryer maintains the particle size from –28, +150 mesh so that re-agglomeration was not required. Furthermore, samples from the tests were assessed for bulk density and electrical resistance until the company was satisfied that the new dryer could provide them with the product they need. Other considerations in the selection of a dryer were its projected uptime, minimal wear and preventive maintenance, and the ease of access for cleaning the interior. Cleaning can be simplified by removing the side panels from the dryer for better access to the trays.
Efficient DryingThe new dryer offers reliability and low maintenance for the automotive parts facility. The dryer runs 24 hours per day, 7 days per week and is only shut down for scheduled maintenance when the entire plant is closed.
The dryer can also be used for other ceramic materials, especially where good temperature control and gentle handling are important. It offers many benefits for ceramic drying, especially when it can replace labor intensive and inefficient drying methods.
Editor's NoteAll equipment mentioned in this article was supplied by Wyssmont Co. Inc. For more information, contact the company at 1470 Bergen Blvd., Fort Lee, NJ 07024; (201) 947-4600; fax (201) 947-0324.
Footnotes*The Wyssmont TURBO-Dryer
**In a stagewise operation, the material spends a fixed time on each shelf before being transferred to the next. Each shelf represents a "stage."