Rawmill

A typical problem would be to make an intimate mixture of 75% chalk and 25% clay, and burn this to produce an ”artificial cement".

Both Louis Vicat and James Frost used this technique in the early 19th century, and it remained the only way of making rawmix for Portland cement until 1890.

A modification of the technique used by the early industry was "double-burning", in which a hard limestone would be burned and slaked before combining with clay slurry.

Rawmixes are formulated to contain a correctly balanced chemistry for the production of calcium silicates (alite and belite) and fluxes (aluminate and ferrite) in the kiln.

Chemical analysis data in cement manufacture are expressed in terms of oxides, and the most important of these in rawmix design are SiO2, Al2O3, Fe2O3 and CaO.

Because the major oxide required is CaO, the most prevalent rawmix component is limestone, while the others are mostly contributed by clay or shale.

However, cement raw materials are for the most part dug from the Earth's crust and contain most of the elements in the periodic table in some amount.

It consists of a large bowl (up to 15 m in diameter) into which the crushed (to less than 250 mm) raw materials are tipped along with a stream of water.

Relatively hard minerals (such as flint) in the mix, are more or less untouched by the grinding process, and settle out in the base of the mill, from where they are periodically dug out.

It holds spherical, cylindrical or rod-like grinding media of size 15–100 mm that may be steel or a variety of ceramic materials, and occupy 20–30% of the mill volume.

The various mineral components of the rawmix are fed to the mill at a constant rate along with water, and the slurry runs from the outlet end.

Clearly, a thinner slurry is easily obtained by adding more water, but at the expense of high energy consumption for its subsequent removal.

This results in a large amount of excessively fine soft material, which "cushions" the grinding of the harder mineral.

Dry rawmills are the normal technology installed today, allowing minimization of energy consumption and CO2 emissions.

On the other hand, it is much easier to dry a fine material than a coarse one, because large particles hold moisture deep in their structure.

A high velocity of hot gas flow is maintained close to the dish so that fine particles are swept away as soon as they are produced.

The gas flow carries the fines into an integral air separator, which returns larger particles to the grinding path.

The remaining dusty gas is usually returned to the main kiln dust control equipment for cleaning.

Hammer mills (or "crusher driers") swept with hot kiln exhaust gases have limited application where a soft, wet raw material is being ground.