Classifiers

In conventional dry roller mills, air is used to transport the particles from the grinding table to the classifier inlet and through the classifier, which is built into the mill housing. The fan and air transport system can take as much power as the mill, and this is shown in the power data (see Table 6.2) given for roller mills that grind limestone, coal, and clin­ker in the Morning Star Cement Hon Chong plant in Vietnam—the first plant designed to operate with only roller mills for grinding.

Very high airflows are needed to lift the coarser particles from the grinding table discharge to the separator, which is high above the table. This is an expensive process. Some mills are designed with an airflow that is too small to entrain the coarser particles, and these fall to the base of the machine from which they are lifted by an elevator to the separator. There is some indication that considerable savings in energy can be made by using a mechanical elevator to lift the particles rather than air blowing (Feige 1993), and this is being built into some dry-roller mill systems. Roller mills are now rated at 840 tph and can grind cement raw materials with feed moistures up to 25%. Table 6.3 shows how the throughput ratings of Loesche mills have increased over 70 years.

The comment by Brundiek (1989) that “The outlook for further use of roller grind­ing mills is encouraging from the technological aspect and extremely interesting from the economic aspect” points the way to the future.