Cement Rotary Kilns.

Rotary Kiln

The clinker mineralogical phase’s development in the rotary cement kiln is a key to properly specify the right refractory for every part of it. While some general assumptions can be made, no two kilns are alike. The rotary kiln is a chemical reactor where the process of converting the mixture of limestone, clay and other materials, results in clinker, the basic raw material of any Portland cement.

Thermal profile and kiln subdivisions

The rotary kiln thermal profile varies throughout its length, depending on the temperature and chemical reactions involved during the process (see in Table 1).

The rotary kiln can be subdivided into several zones or regions that are exposed not only to thermal and chemical wear but also to mechanical stresses. The influence of one or several of these factors, to minor or greater proportion determines the refractory lining type required for each zone:

• Decarbonation zone: from 300ºC to 1000°C (+)

This stage can occur either inside of the old wet process rotary kilns or in the preheater tower of modern units consisting of two steps: Firstly, between 300°C and 650°C where the raw meal heating occurs, accompanied by a dehydration reaction; Secondly, between 650°C and 1000°C, when the limestone decarbonation takes place generating CO2 and CaO.

The first step is characterized by the following aspects:

• Presence of raw meal (there are no new mineral phases development);
• Erosion (due to raw meal flow at high velocities);
• low temperature;
• Evaporation and dehydration (of water) chemically bonded to the raw material.

In this zone it is very important that the refractory products have the capability to protect the rotary kiln drive (good insulation degree) and good resistance to impacts of anomalous build-ups. Bricks with less than 45% Al2O3 content are suitable. Besides that, when alkaline salts are present, a vitreous glassy layer can develop with the alkali on the brick surface, preventing its propagation or later infiltration.

In the second stage of these reactions, the development of new mineralogical phases occurs:

- Formation of CaO and CO2;
- Formation of CA, C12A7 and C2S;
- Temperature variation;
- Alkali attack.

Usually, the use of bricks with a 70% Al2O3 content is recommended, which offers a high mechanical resistance, low porosity, and low thermal conductivity. However, the risk of eutectic reactions formations on the Al2O3-CaO- SiO2 , system and alkali resistance is a limiting factor.

• Upper transition zone: from 1000ºC to 1238°C (+)

It is the most unstable and difficult area for refractory specification. Although the temperature range varies from 1000°C to 1338°C, incidences of thermal overloads are frequent. This fact is linked on the flame shape, to the fuel type and to the design of the kiln main burner. Therefore, it is in this area where coating starts to develop as soon as first drops of liquid phase appear. Coating becomes very unstable if the operational conditions present high variability.