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Process Description 7
2.2.2 Semi-wet processes
Semi-wet cement processing employs grate-kiln methods (Figure 2.3). In the semi-wet
cement manufacturing process the raw materials prepared by wet processing are first
mechanically dewatered - preferably with filter presses - and then fed in the form of
nodules to a drying unit. This drying unit may be the third compartment of a traveling grate
preheater. Nowadays, a dispersion dryer or impact dryer is preferably installed ahead of the
preheater or precalciner kiln. In cases where former wet kilns have been converted into
one-stage or two-stage preheater kilns on this principle, impact dryers have hitherto been
employed. Advantages of grate-kiln systems include:
- a controlled feed rate
- no flushing of materials into the kiln
- no segregation of raw materials due to differential shapes and densities
- avoidance of fluidization of the materials
- minimal dusting
- production of uniform clinker
- low energy requirement (70% of that required for modern long wet kiln), and
- ability to use higher-alkali feeds than many other processing techniques.
2.2.3 Semi-dry process
In the semi-dry process, nodules or pellets (approximately 12% water) formed from raw
meal with the aid of water are used. The traveling grate preheater kiln continues to be
available as a technically well developed pyro-processing unit. Kilns of this type, however,
suffer from some disadvantages inherent in the system, such as:
- relatively high initial cost and operating expenses associated with kiln outputs,
- specific quality requirements of the raw materials (grate process requires nodules to
be consistent in size and composition which is often very difficult to achieve),
- relatively high overall heat consumption (only the exhaust air from the cooler is
available for drying the materials during grinding),
- restrictions as to the use of low-grade fuels, and
- inability to apply precalcination.
Because of these drawbacks, this system has lost the weight it once possessed. However it
is reported that several of the newer installations in the United States do employ grate-kiln
methods (Sell, 1992). Depending upon the local conditions, in certain situations, they are
deemed preferable to preheater systems.