CEE April-June 2012

Vertical roller mill VRM-12M Focus on roller grinding Descriptions of the dimensioning and the estimation of the necessary drive power for roller mills based on empirical values have been contained in relevant literature for more than 45 years. The bandwidth of the advanced models based on transfer functions for parameters, which have to be acquired using special laboratory roller mills, range from power functions up to an including complex concepts taking the similarity theory as their basis. Common to all of them, however, is a great dependence on the details of the design and geometry of the mill. Modern, highly functional filler product necessitate an ever better understanding of the essential machine parameters influencing the energy input in interplay with classification. Knowledge of these parameters, combined with knowledge of the feed-material characteristics and its grindability, permit product simulation on the basis of a model. A number of methods is available for determining the influence of the fed material by means of grindability tests. It is essentially necessary to differentiate between laboratory tests and technology-centre tests performed on a semi– industrial scale. The material required for semi industrial tests with vertical mills generally significantly exceeds the amount of test material available. This problem is not insurmountable in the case of ball mills, since it is possible to obtain information using significantly less test material by means of standardized laboratory tests. The application of the BOND grindability test for the derivation of product-specific energy consumption for roller mills has only little rationality, owing to the different type of loading and the frequently inadequate generalized applicability for particle sizes other than those actually tested. An impact– compressive force effect is presupposed in grinding in ball mills, whereas a compressive-frictional force effect dominates in roller grinding. ZEISEL'S approach, thanks to the use of a ball-ring mill, matches this specific type of load better, but does have limitations with regard to the feed size and sample quantity. The influences of the machine parameters of both the grinding mechanism (grinding pressure, grinding table speed, etc.) and the classifier (speed, air flow rate, etc.) can be determined only by means of systematic tests. The new laboratory mill endeavours to combine, on the smallest possible scale, the advantages of a grindability test facility for best possible simulation of roller load, with energy take-up via a torque measurement shaft in the drive train, with those of a dassifier. This permits combined operation of the classifier and grinding unit, on the one hand, and individual operation of the machines and in - situ sampling for assessment of the system from the three sub– processes of grinding, upflow classification and air classification, on the other hand. This is intended to deepen understanding of the correlation between the fracture behaviour of the material and the influence of classification. A model set up based on these principles, for mills of different sizes, provides up-scaling know– how, enabling the design of the optimum machine for the customer, and its adaption to the needs and requirements of the service environment. The VRM200 laboratory grinding unit The VRM200 technology-centre roller mill was developed against a background of divergent economic interests, namely improvement of customer support and more precise design of machines, and the targeting of a technological lead in the field of roller grinding of a degree greater than that previously published. The Montanuniversitaet Leoben I Austria was enlisted as a scientific co– operation partner. The mill was planned with a grinding table diameter of 200 mm. This table size permits tests with comparatively small quantities of material but with nonetheless significant throughput rates by mass (up to 250 kg/h) for corresponding finenesses, and can thus be positioned in the 24