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

3.3.5 Finish Grinding
The cooled clinker is conveyed to clinker storage or mixed
with 4 to 6 percent gypsum and introduced directly into the
finish mills. These are large, steel cylinders [2 to 5 m (7 to
16 ft) in dia] containing a charge of steel balls, and rotating
at about 15 to 20 rpm. The clinker and gypsum are ground to a
fine, homogeneous powder. About 85 to 96 percent of the product
is in particles less than 44 µm in diameter. These objectives
may be accomplished by two different mill systems. In open-
circuit milling, the material passes directly through the mill
without any separation. A wide particle size distribution range
is usually obtained with substantial amounts of very fine and
rather coarse particles. Open circuit grinding is, however,
rarely practiced in U.S. cement plants. In closed-circuit
grinding, the mill product is carried to a cyclonic air separator
in which the coarse particles are rejected from the product and
returned to the mill for further grinding.
3.3.6 Quality Control
Beginning at the quarry operation, quality of the end
product is maintained by adjustments of composition, burning
conditions, and finish grinding. Control checks are made for
fineness of materials, chemical composition, and uniformity.
Clinker burning is monitored by the liter weight test weighing a
portion of sized clinker, a free lime test, or checked by
microscopic evaluation of the crystalline structure of the
clinker compounds. Samples may be analyzed by X-ray
fluorescence, atomic absorption, or flame photometry. Standard
cement samples are available from the National Institute of
Standards and Technology. Fineness of the cement is most
commonly measured by the air permeability method. Finally,
standardized performance tests are conducted on the finished
cement.
3.3.7 Emission Control Systems
Most cement plants are equipped with particulate collection
devices to remove cement kiln dust (CKD) from the kiln exhaust
gases as well as clinker cooler gases. Several small dust
collectors are also installed at various dust emission points
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