With the arrival of rotary kilns, cement manufacturing processes became sharply defined according to the form in which the raw materials are fed to the kiln. Raw materials were either ground with addition of water, to form a slurry containing typically 30-45% water, or they were ground dry, to form a powder or "raw meal".
In the Wet Process, the kiln system is fed with liquid slurry, the water then being evaporated in the kiln.
In the Semi-Wet Process, raw material is prepared as a slurry, but a substantial proportion (50-80%) of the water is mechanically removed, usually by filtration, and the resulting "filter cake" is fed to the kiln system.
In the Dry Process, the kiln system is fed with dry raw meal powder.
In the Semi-Dry Process, a limited amount of water (10-15%) is added to dry raw meal so that it can be nodulised, and the damp nodules are fed to the kiln system.
The cement kiln is the heart of the cement making process: this is today almost a cliché, but was not always quite so true. In the modern cement plant, the kiln is the most expensive and technically complex part of the plant, and because it must be run all the time (unlike other sections of the plant) it effectively defines the output capacity of the plant. It stands out also in that, unlike the other processes involved, the “pyroprocessing” stage of cement manufacture is absolutely unique to the industry, and has devoted to it a complex and specialized multi-disciplinary technology all its own.