Developments and operational experience with vertical roller mills in the cement industry

Vertical roller mills are the preferred means of producing cement raw meal in the cement industry. The basic reasons for the widespread use of these m ills are the combined execution of several process steps within a single ma chine, the low electrical power consumption and the utilization of large qu antities of process exhaust gas at low temperatures. Great efforts have bee n made in past years to reduce capital and production costs. Capital costs have been reduced through on-line operation of mills and rotary kilns and b y dispensing with large-capacity raw meal storage systems; electrical power consumption has been reduced by using high efficiency classifiers and mate rial circulating systems which are external to the mill. There continues to be a trend towards large production units. Mill manufacturers must therefo re adjust to the demand for even larger machines, for which reliable dimens ioning of the mechanical components is essential. The finite element method (FEM) provides a powerful procedure for the numerical solution of strength problems. Since the start of the 80s vertical roller mills have also been used in the cement industry for combined or separate grinding of cement cli nker and cement additions. The electrical power consumption with these mill s was reduced by 20 to 40 % compared with grinding systems with ball mills. Where there are large differences in grindability between the individual c omponents of the mire the quality of the end product can be increased and t he power consumption reduced still further by separate grinding and subsequ ent mixing of the additions. Wear protection is very important in cement gr inding. Composite armoured plates with hardface welding, cast Ni Hard IV an d chrome steel, and silicon carbide ceramics have proved particularly succe ssful as wear protection materials. When granulated blast-furnace slag is p rocessed the abrasiveness can, also be reduced by primary measures. The gri ndability of the particular mill feed forms the basis for dimensioning an i ndustrial grinding plant.