Grinding, drying, and classifying in one machine
Depending on its deposit and composition, the cement raw material may contain a more or less big portion of water. Especially with high feed moistures, drying is of particular importance because the existing surface moisture has to be evaporated almost down to zero in the raw mill. The thermal energy required for this purpose is made available in the form of hot gases which are drawn through the mill. To handle the large gas flow needed when high moisture rates are involved, a bigger mill housing can be provided. Such measure might also be taken in case the available gases have a relatively low temperature, for example when they come from a heat recovery system.
Almost any plant for the grinding of cement raw material is a combination of machines specially adapted to the particular physical properties of the material to be ground. As our solutions are designed to suit the customers’ requirements, we can guarantee that our mills work efficiently without any trouble.
Details on raw material
Cement raw material is a mixture of different minerals. The mixture must contain a defined ratio of calcium oxide, quartz, aluminum oxide, and iron oxide as required for clinker production. Calcium oxide is contained in various types of limestone but also in marl, shelly limestone, or chalk. To add aluminum oxide to the mixture, clay is used as a rule. Alternatively, bauxite is used but less often. Marl which can be used as a component includes calcium oxide, aluminum oxide, and quartz. The latter is also contained in clay. The required percentage may as well be ensured by adding sand. The iron oxide component is ensured via the iron ore included in the cement raw material mixture.
Grinding of cement raw material – material separation in cyclones
The cement raw material mixtures are ground, dried, and classified in the Pfeiffer vertical roller mill. For drying the material, hot gases are directed into the mill. Product fineness can be adjusted within a wide range (0.063 to 0.2 mm). The ground and dried product is separated from the process gas in a cyclone collecting system which is followed by the mill fan. Downstream of the mill fan, the gas volume flow is divided: part of it flows back to the mill, the remainder is directed through a dust collection unit and then evacuated from the system. The mill is equipped with an external material recirculation allowing the reduction of pressure loss in the mill. It can also be used for emptying the mill in case of maintenance.