Drying is a necessary step in the processing of all ceramic products and components, whether it takes place as a separate step or is a preliminary step in the firing process. Ceramics are made from different combinations of materials that can be in slurry form or even more liquid, in the case of slip-cast ware at the start of the fabrication process. This is true of parts ranging from teacups to large refractory molds for melting aluminum to refractory insulation products. All ceramics need to be dried to drive off the large amounts of liquid that are required to make the product.
Problems can appear in many forms if drying is not done uniformly. Cracking due to shrinkage can be a major issue, especially in parts that vary in thickness. All ceramic products also shrink when fired. Drying and pre-firing ceramics help aid in the amount of shrinkage that occurs to the end product. This is very important in thermal processing; if ceramic insulation products are not dried or pre-fired, the resulting shrinkage could cause heat loss issues in a furnace, as well as problems with the end product when it shrinks as heated to the required temperature.
Catastrophic failure is a possibility if drying is not accomplished properly. Water content can turn to steam if heated too rapidly, destroying the product and the thermal processing equipment. Catastrophic results can occur to thermal processing equipment if large ceramic parts are heated too rapidly. This rapid heating can cause a steam explosion, such as what occurred several years ago in a southern U.S. refractories facility that makes large ceramic blocks and other industrial ceramic composites. A steam explosion resulted in explosive failure to the load, leading to damage to the furnace that was so severe that the furnace interior had to be completely replaced.
Another heat-related ceramic drying process failure occurred with a ceramic armor manufacturer located in the northeastern U.S. A large car-bottom furnace was loaded with ceramic plates that are manufactured from a lightweight ceramic product used for bulletproof armor and military helicopter seats and floor plates. The furnace experienced a thermocouple and process controller failure, which resulted in the furnace heating too rapidly until the over-temperature thermocouple engaged and shut the furnace down. The furnace sustained no damage. However, the product was no longer a series of ceramic plates and seats, but a pile of broken parts and rubble.
At the very least, improper drying can result in the end product cracking and weakening. Again, steam is the culprit and should be strictly controlled and monitored via temperature and relative humidity of the environment.
Controlling Humidity and Heat
The key factors in drying ceramics are humidity and heat. Both are necessary and must be controlled under strict guidelines to avoid both rapid evaporation and steam. Evaporation and steam can both be allies and enemies of the ceramic product being dried. The ultimate goal is to dry the ceramics as quickly as possible, especially in a production environment.
The heating, ventilating and air conditioning (HVAC) industry uses the following equation derived from the work of Willis Carrier, the man who invented air conditioning, to establish the maximum rate of evaporation from a pool of water:
lb water/sq ft of surface area/hr = k(1+200/V)(W0-WRH)
In theory, this is the maximum rate at which ceramics should be dried. Most ceramic-formed projects are placed in a controlled drying room, in which the temperature is set to ambient or elevated slightly, and the preliminary drying is done by humidity control. Ceramic products can stay in this environment for weeks or even months.
In practice, most manufacturers need to control all costs, and that means reducing inventory and maximizing throughput from their facilities while delivering high-quality products. Factors such as material composition, density and shape of the workpiece, moisture content, and trapped water all need to be considered when designing a proper drying program.
As noted previously, humidity and temperature are the variables that need to be controlled for successful drying. Whether drying takes place as a separate process or is a preliminary stage in a firing cycle, air movement can be key to uniform drying. During the early stages of drying, it is best for the air to be hot, humid and under a slight amount of pressure in order to flow through the load uniformly. Once the chamber is heated to temperature, forced convection fans inside the chamber move air through the load, and draft-inducing blowers pull ambient air through the chamber. As the temperature rises, the air loses some of its humidity, allowing the water content from the load to be taken up by the air. As the cycle progresses, more water content is absorbed and removed by the air flow until drying is complete and the product is ready for firing.
Case in Point
Platinum-alumina catalyst powders were successfully processed in a two-stage drying/firing procedure at a Mid-Atlantic plant using ceramic box saggers to contain the powder. The saggers were then loaded into tall, narrow alloy racks that rolled in and out of a pair of car bottom furnaces. The configuration of the alloy rack was designed to allow the saggers to be heated from the sides by the electric heating elements, and to allow for proper air flow around and through the load.
Two draft inducers at the roof pull ambient through the load at various points in the process cycle, especially during the initial stages of the drying profile, and at the end of the firing cycle to improve throughput. Not every ceramic product can be dried and fired in one contiguous cycle, but if it is a possibility, then it should be considered in order to lower production costs while maximizing output.
This article was originally posted on www.ceramicindustry.com.