A cement kiln is the world's largest moving manufacturing machine. Cement kilns are enormous cylindrical ovens, some as long as 1000 feet and as much as 24 feet in diameter. They rotate from 20 to 80 times an hour. The kilns are mounted at a slight incline, (about 1/2 inch per foot). The inside is lined with fire resistant brick. The kiln is fueled by powdered coal, oil, gas, or liquid waste-derived fuel in the burner end and solid waste-derived fuels in the center or calcining zone via Cadence's patented process.

HOW PORTLAND CEMENT IS MADE.
Raw materials such as limestone, clay and sand enter at the higher end of the kiln cylinder. The raw materials are completely dried by the heat as they slowly tumble down the tube toward the 3,500ºF. flame. In the hottest part of the kiln, about 25 percent of the raw materials melt. These melting materials mix with the remaining solids to form cement clinker, a gray lava-like material about the size of golf balls. The kiln discharges the clinker in an air cooler under the burner floor. After it has cooled, the clinker is ground into a fine powder. A small amount of gypsum is added to produce the mixture known as Portland Cement. Cement powder is stored in large silos to await packing and shipping. Cement is the world's most important building product.

A TYPICAL PRE HEATER CEMENT KILN

A TYPICAL PRECALCINER CEMENT KILN

Preheater and precalciner kilns are shorter than a long cement kiln and contain tall preheater towers that use the heat produced by the kiln to preheat the raw materials as they move through the various stages of the tower. These kilns reflect the latest in cement kiln design technology. Presently, about 25% of the kilns in the United States are either preheaters or precalciners. They are more fuel efficient than long kilns using up to 50% less energy. Waste-derived fuels can be introduced at either the burner end or through a special port at the rear with other raw materials to further increase fuel efficiency.