Ceramics - Incredible Refractory Materials
First we will start with the definition of refractories and ceramics. Refractories and ceramics are non-metallic materials capable of maintaining physical and chemical stability at high temperatures. Refractories in modern practice are usually ceramic in nature, and are used in a wide variety of primary, secondary and tertiary industries.
Wherever an industrial process involves heat in excess of 700 to 800 degrees Fahrenheit (roughly), one will find refractory material in place, either as a lining or forming the process vessel itself. Some common process vessels using refractories are; boiler combustion chambers, furnaces like the one in the foundry, incinerators, many emission control scrubbers, rotary kilns and so on. The list is by know means exhaustive. For example, Launch Pads 39A and 39B at the Kennedy Space Center are refractory lined. The shuttles themselves are lined with ceramic tiles to protect them from the heat of re-entry into earth’s atmosphere, these tiles are! unique to the shuttle, but are non-metallic and heat resistant.
The Making of Refractory Materials
The first step in processing ceramics is crushing of the raw materials. Crushing is usually done in a ball mill, either wet or dry. Wet crushing is more effective because it keeps the particles together and prevents the suspension of fine particles in air. The ground particles are then mixed with additives, the functions of which are one or more of the following:
1. Binder for the ceramic particles
2. Lubrication for mold release and to reduce internal friction between particles
3. Wetting agent to improve mixing
4. Plasticizer to make the mix more plastic and formable.
5. Various agents to control foaming and sintering.
6. De-flocculent to make ceramic-water suspension. De-flocculention changes the electrical charges on the clay particles so that they repel instead of attract each other.
Next, it’s time to begin the casting process. The shaping process for refractories are casting plastic forming and pressing. The most common casting process is slip casting. The slip is poured into a porous mold made usually of plaster of paris. Then inverted and the remaining suspension is poured out for making hollow object much like slush casting. The part is then trimmed the mold opened and the part removed.
The second process of shaping ceramics is plastic forming. We have various methods of plastic forming such as extrusion, injection molding and jiggering. Plastic forming tends to orient the layered structure of clays along the direction of material flow.