Cement is produced from raw materials such as limestone, chalk, shale, clay, and sand. These materials are quarried, crushed, finely ground, and blended to the correct chemical composition. Small quantities of iron ore, alumina, and other minerals may be added to adjust the raw material composition. The finely ground raw material is fed into a large kiln, where it is heated to high temperatures (about 1,500°C), which causes the raw material to react and form a hard nodular material called "clinker." Clinker is cooled and then ground with small amounts of gypsum and other minor additives to produce cement. The main steps in the cement manufacturing process are illustrated in Fig. 8.2.
The heart of the clinker production process is the kiln, which can be rotary or vertical shaft designs. Rotary kilns are commonly used in the U.S. and elsewhere.
These kilns are 6-8 m in diameter and 60 m to well over 100 m long. They are set at a slight incline and rotate at 1-3 rpm. The kiln is fired at the lower end and the feed materials move toward the flame as the kiln rotates. The materials reach temperatures between 1,400°C and 1,500°C in the kiln. Three steps occur with the raw material mixture during pyro-processing. First, all moisture is driven off from the materials. Then the calcium carbonate in limestone dissociates into CO2 and calcium oxide (free lime) during calcination. Finally, the lime and other minerals in the raw materials react to form calcium silicates and calcium aluminates, the main components of clinker.
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