The rotary kilns used in the first half of the twentieth century were wet process kilns which were fed with raw mix in the form of a slurry.

Moisture contents were typically 40% by mass and although the wet process enabled the raw mix to be  omogenized

easily, it carried a very heavy fuel penalty as the water present had to be driven off in the kiln.

In the second half of the twentieth century significant advances were made which have culminated in the development of the precalciner dry process kiln.

In this type of kiln, the energy-consuming stage of decarbonating the limestone present in the raw mix is completed

before the feed enters the rotary kiln.

The precalcination of the feed brings many advantages, the most important of which is high kiln output from a relatively short and small-diameter rotary kiln.

Almost all new kilns installed since 1980 have been of this type. Figure 1.4 illustrates the main features of a precalciner kiln.

The raw materials are ground to a fineness, which will enable satisfactory combination to be achieved under normal operating conditions.

The required fineness depends on the nature of the raw materials but is typically in the range 10–30% retained on a 90 micron sieve.

The homogenized raw meal is introduced into the top of the preheater tower and passes downwards through a series of cyclones to the precalciner vessel.

The raw meal is suspended in the gas stream and heat exchange is rapid. In the precalciner vessel the meal is flash heated to ~900°C and although the material residence time in the vessel is only a few seconds, approximately 90% of limestone in the meal is decarbonated before entering the rotary kiln.

In the rotary kiln the feed is heated to ~1500°C and as a result of the tumbling action and the partial melting it is converted into the granular material known as clinker. Material residence time in the rotary kiln of a precalciner process is typically 30 minutes.

The clinker exits the rotary kiln at ~1200°C and is cooled to ~60°C in the cooler before going to storage and then being ground with gypsum (calcium sulfate) to produce cement. The air which cools the clinker is used as preheated combustion air thus improving the thermal efficiency of the process.

The calcium sulfate is added to control the initial hydration reactions of the cement and prevent rapid, or flash, setting.

If coal is the sole fuel in use, then a modern kiln will consume approximately 12 tonnes of coal for every 100 tonnes of clinker produced.

Approximately 60% of the fuel input will be burned in the precalciner vessel. The high fuel loading in the static precalciner vessel reduces the size of rotary kiln required for a given output and also reduces the consumption of refractories.

A wider range of fuel types (for example, tyre chips) can be burnt in the precalciner vessel than is possible in the rotary kiln.

Although kilns with daily clinker outputs of ~9000 tonnes are in production in Asia most modern precalciner kilns in operation in Europe have a production capability of between 3000 and 5000 tonnes per day.