Introduction
Raw materials now enter the huge rotating furnace called the kiln—the heart of the cement-making process—a horizontally sloped steel cylinder, lined with firebrick, turning from about one to three revolutions per minute. The kiln is the world’s largest piece of moving industrial equipment.From the preheater, the raw material enters the kiln at the upper end. It slides and tumbles down the kiln through progressively hotter zones toward the flame. At the lower end of the kiln, fuels such as powdered coal and natural gas feed a flame that reaches 3,400°F (1,870°C)—one-third of the temperature of the sun’s surface. Here in the hottest part of the kiln, the raw materials reach about 2,700°F (1,480°C) and become partially molten.
The intense heat triggers chemical and physical changes. Expressed at its simplest, the series of chemical reactions converts the calcium and silicon oxides into calcium silicates, cement’s primary constituent. At the lower end of the kiln, the raw materials emerge as a new substance: red hot particles called clinker—grayish-black pellets, often the size of marbles.
