NOx Emissions from Cement Mfg - US Environmental Protection ...

dry process system. 5
The choice between the wet or dry process for cement
manufacturing often depends upon the moisture content in the raw
feed materials mined from quarries. If the moisture content of
the feed materials is already very high (15 to 20 percent), a wet
process may be attractive. The recent trend, however, has been
toward the dry process with preheater/precalciner systems. In
1990, about 25 million tons of clinker were produced by the wet
process with 55.8 million tons produced by a dry process. Within
the dry process category, 15.7 million tons were produced by
facilities equipped with a preheater system and 21.4 million tons
were produced by facilities equipped with a precalciner system.
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The different steps involved in the cement manufacturing
process are described in the following subsections.
3.3.3 Raw Materials and Kiln Feed Preparation
Oxides of calcium, silicon, aluminum, and iron comprise the
basic ingredients of cement. Because of the large requirement of
CaO, the plants are generally located near the source of the
calcareous material. The requisite silica and alumina may be
derived from a clay, shale, or overburden from a limestone
quarry. Such materials usually contain some of the required iron
oxide, but many plants need to supplement the iron with mill
scale, pyrite cinders, or iron ore. Silica may be supplemented
by adding sand to the raw mix, whereas alumina can be furnished
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by bauxites and alumina-rich flint clays. Industrial by-
products are becoming more widely used as raw materials for
cement, e.g., slags contain carbonate-free lime, as well as
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