Cement, Energy and Environment

in raw material grinding. The growing success of roller presses is primarily due to improved separator concepts. KHD Humboldt Wedag brought the V-Separator onto the market in 1994. This separator has meanwhile become an important module in grinding systems and more than 135 units have been sold. Such a system for raw material grinding (Figure 6) consists of one or two parallel roller presses, a downstream static V-Separator and a preceding high-efficiency separator. The two separators are connected by a pneumatic conveyor. In the V-Separator, the material is dried and the coarse fraction is removed. The coarse material is then returned via bucket elevator to the mill feed system for regrinding together with the fresh feed material. The fine material passes to the separator for finished product collection. Oversize material is also returned from there to the roller press. Throughput rates of up to 1000 tph can be achieved with this type of system. Particularly COMFLE.x® compact systems exceed expectations with regard to system simplicity. Polysius has implemented a similar concept, e.g. at TXI Oro Grande, 100 ~ - '$. 60 - • ! .. .c 40 lA 20 0 M echanical conveyors can be designed 25 to 40% smaller 6 Illustration of a Comflex® grinding system (KHD Humboldt Wedag) 59 with difference that both separators precede the roll er press. Clinker/cement grinding For cement grinding, normal mill capacities are between 100 and 200 tph. This means that even for medium- sized kiln lines of e.g. 5000 tpd , several mills usually have to be used. Cement grinding plants with capacities exceeding 300 tph have up to now only been installed at a handful of factories . Mill capacities depend greatly on the grindability of the Other Mill type • Withoutlocal Chinese m.ufcet clinker and of the intergrinding material, such as slag or limestone, as well as on the required cement fineness. The achieved mill capacities for different vertical mill sizes and cements are stated. Developments during recent years have shown that there are now only small differences in the cement quality produced by different cement mills. 7 Types of mill for clinker grinding (OneStone) Figure 7 shows that ball mills are still the type usually ordered for cement grinding applications, even though their market share has decreased from 59 per cent to 49 per cent. In contrast, vertical mills have significantly increased their share from 17 per cent to 39 per cent. The market shares of roller presses and horizontal mills have declined slightly. In view of the poor energy utilization of ball mills, one may wonder why they still account for almost half of all ordered cement mills. The reasons for this are complex. Firstly, ball mills are used in combination grinding systems and finish grinding systems because they are held 4