Cement production introduction
Cement is one of the most essential materials (bonding agent) in the building industry. It can be produced in a simple process and at reasonable cost, the finished product is not harmful to the environment, and it can be recycled.
There has been a significant increase in the utilization of cement over the past few years. The demand is expected to increase further in the future as, especially in Asia and Latin America, infrastructure measures and housing will be intensified. There are double-digit growth rates in cement production mainly in China, but also in India, Indonesia, Malaysia and the Philippines. Several Middle East and African countries are also increasing their cement production.
FAG, as a rolling bearing manufacturer, has to prepare for the growing demand for bearings used in machines for cement production.
The most essential materials for cement production are:
• limestone (Ca0) ca. 65 %
• clay (Si02 ca. 20 %; AL203 ca- 5 % and Fe203 ca. 3 %)
• additives (small amounts)
Cement producer plants are usually erected near huge limestone deposits. If the geological structure of the limestone is not homogeneous, additives must be admixed during the preparation process.
Limestone deposits are virtually unexhaustible so that the availablity of this raw material for cement in the future is ensured.
Crushing limestone, alumina etc.
Limestone, alumina etc. are quarried or mined in quarries or pits and subsequently reduced to sizes ranging from 12 to 80 mm.
The following machines are mainly used for crushing these materials:
• Jaw crushers (see section 1.2.1)
• Cone crushers (see section 1.2.2)
• Hammer crushers (see section 1.2.3)
in addition, tertiary crushers can be used, in combination with vibrating screens, for crushing/grading.
The material broken in the crushers is taken to drying plants by truck or belt conveyors. In the drying process the moisture is reduced to approx. 8 to 15 %, depending on the further size reduction process. During the drying process, additives can already be admixed. The material is then put in intermediate storage.
State-of-the-art coarse grinding systems are usually vertical roller grinding mills (see section 1.2.4) or roller presses (see section 1.2.5). Due to their slighter energy consumption, these systems are gradually replacing ball tube mills (see section 1.2.6) which are still used, in combination with an impact mill or as independent units for coarse grinding. In the coarse-grinding process the final grain sizes of approx. 0.1 mm required for the further cement production process are produced.
Burning the raw meal/cooling the clinkers
In so-called cyclone preheating plants and succeeding rotary kilns the raw meal is continously heated from ambient temperature to approx. 1500 °C and discharged as granulated cement clinker.
The burning of raw meal to clinker constitutes a complex process. It consists of precisely defined steps and is constantly monitored. Large rotary kilns today can be approx. 6 m across and 100 m long, reaching throughputs of up to about 10,000 t per unit and day.
The energy carriers used for preheating and heating in preheating plants and for “burning” the material in rotary kilns are coal, oil and natural gas. Which of these fuels are ultimately used depends on their price and availability.
Where coal is used as energy carrier, coal crushers are frequently used in cement plants for size reduction and fine grinding. Generally, these coal crushers are smaller vertical rolling mills similar to those used for grinding raw meal. As a rule, several machines are run in parallel operation, grouped into units.
As the roller grinding mills have to cope with extremely adverse ambient conditions the bearings frequently fail due to wear so that there is a constant demand for new bearings. There is also a constant demand for bearings for beater wheel mills which are used for coal crushing.
Rotary kilns are usually radially supported in hydrostatic bearings. However, for some years now, support roller units with rolling bearings have also been used, fig. 22. The FAG product Programme includes such units. Axial support of the rotary kiln is always effected by support rollers (fig. 23) which in turn are supported in rolling bearings.