A rotary kiln is a device that supplies tremendous amounts of heat in order to change the chemical composition of an object. It is made up of a strong reinforced steel outer shell that is coated with a heat-resistant inner lining, support rollers and a drive gear to keep the contents in a continuous rotating motion and internal heat exchangers capable of producing temperatures well over 2732 degrees Fahrenheit (1500 degrees Celsius). Rotary kilns will sit slightly at an angle so that the inner contents will be sifted downwards toward the heat source and allow for any evaporative gasses to escape from the top during the process through sealed ductwork. The contents are then sorted and ejected through an opening in the lower end of the device that automatically sorts processed materials and waste into separate bins. Other common terms to describe such a device may be a rotary kiln incinerator or a rotary kiln dryer.
Perhaps one of the most common uses for this technology is the creation of a rotary cement kiln, which grinds limestone, clay, and shale down to small bits of rock and transforms them into a usable cement mixture that is ready to be either packaged or immediately used. During this process, the materials are exposed to varying temperatures that will evaporate any water present and transform the limestone and clay back to their original oxide states. As the internal heat increases up to 2642 degrees F (1450 degrees C), the rocks that were transformed into belite and calcium oxide melt together to form the composition alite. Due to the extreme heat associated with this process, the resulting lumps of alite are no larger than 0.39 inches (10 mm) across and are easily handled once the cooling process is completed.
The rotary kiln design allows for many similar types of solids to be broken down into more readily usable materials, and it is commonly used on alumina, titanium dioxide and lime. Metakaolin, iron ore pellets, and many other metallic and non-metallic components may also be broken down by a rotary kiln. Refractory material, or a solid whose composition does not possess metal, but still remains suitable for construction, is also created using this method and ultimately installed inside rotary kilns and other furnaces as an insulator to protect the outer shells from the extreme internal temperatures.