Cement, Energy and Environment

(Figure 12) that are used for finish grinding. For the grinding of Portland Cement with 95 per cent clinker, mill capacities of more than 350 tph can be achieved, although the usual mills sizes offered by most vendors only permit outputs of around 200 tph (Figure 13). With the aim of achieving higher capacities a -~ trend towards four or more grinding rollers is discernable, as 11 Sepal® HR separator for Polycom finish grinding plant (Polysius) this keeps roller dimensions small Finish grinding plants equipped with roller presses achieve a specific energy requirement of 21 kWh/t, the roller press being responsible for 15.5 kWh/t or approx. 79 per cent and the auxiliary units, separator, separator fan and bucket elevator accounting for 21 per cent. The application of vertical mills for cement grinding is currently regarded as a real technological breakthrough. However, this does not refer to so-called pregrinders, but to vertical mills with integral separator 13 Vertikal mill for clinker grinding at Ras-AI-Khaimah in VAE (Loesche) ~~----------~----------------~ 2500 1100 noo 3100 noo 3500 3700 3500 4100 •3oo 4500 Blolne••fue (cni>tp 14 Specific energy requirement as a function of product fineness (FLSmidth) and thus minimizes the dynamic 12 OK mill at California Portland Cement in the USA (FLSmidth) loadings and vibrations in the mill. The energy savings achieved with vertical mills in comparison to ball mills are primarily a function of the required fineness of grinding (Figure 14). All vertical mills provide the advantage of quick product type changeover because the short material residence times in the mill enable the attainment of quality and capacity parameters just a short time after a changeover, without having to stop the grinding operation. On the other hand, it shou ld not be forgotten that the grinding conditions (grinding speed, grinding pressure, water injection, grinding aids etc.) have to be optimized for each different mill feed material. Also, the height of the dam ring on the grinding table can only be optimally adjusted for one single product, or else compromises have to be made. During operation , the grinding bed depth can practically only be influenced by the grinding speed. This indicates the need for increased use of variable- speed drive units or the introduction of completely new drive solutions for vertical mills in the near future. 6