CEE April-June 2012

Table-2 Description of influencing factors for DoE and classical test schedule by the pressure drop on the grinding-chamber side (air inlet to classifier inlet) (Figure 6). Grinding table speed Roller contact pressure Feed rate System-air flow rate Classifier speed Material Particle size rpm m2 h H rpm 80 120 500 850 10 250 680 950 600 4000 Five machine parameters (Table 2) were shortlisted. The external influencing parameters of material type, moisture and feed Preliminary tests showed a linerarization of the pressure drop over the mill at different classifier speed settings with maximum mill throughput rates. Figure 6 shows the linearizing curve of the pressure drop across the grinding chamber at maximum feed rate, defined classifying air volume and varied classifier speed . To reach usable regulating points for the grinding chamber pressure drop at maximum throughput and changed classifying air volumes, the characteristic curves were shifted based on the corresponding no-load pressure losses. The pressure drop curve in Figure 6 for 815 m 3 /h were determined empirically, the curves for 950 m 3 /h and 680 m 3 /h were determined by shifting the curves as described above. Sub– points were determined with tests. With the use of the pressure drop as control parameter for the feed rate, only four factors had to particle size were examined in different tests. All the tests described in this article were performed using marble of a defined particle-size range (Table 2). Other parameters were initially excluded in order to restrict the necessary amount of testing. Statistical design of experiments presupposes the feasibility of all scheduled tests. Only insignificant deviation from the specified level is possible if, for process-engineering reasons, one test cannot be performed. Greater deviation would result in loss of the informational value of the plan. Some combinations of the screening schedule cannot be implemented, owing to specification of stability criteria. A high classifier speed of 4000 rpm cannot, for example, be combined with a high feed rate of 250 kg/h. Settings of 600 rpm and 20 kg/h feed rate, on the other hand, are economically irrational. Two options are available here to escape this dilemma. The use, on the one hand, of a 0-optimal screening plan, by means of which problematical factor combinations can be merged out based on the specification of boundary conditions. A large number of tests are necessary to permit formulation of such boundary conditions. This option was therefore at least temporarily rejected . In an industrial environment, on the other hand , material feed is regulated ss so 10 0 soo 0 "" flow 81SmYh o Air flow 680mYh ·--t -- . 0 .....__ - . • 6 Characteristic curves for control using the pressure drop across the grinding chamber Hall-normal plot l ~ .. :0 DA "' .I> 0 0.. 0 0 DAB -;;; e 0 .f -;;; :X: ,. ,. ,.,.._ .. 8 Classifier - • • Neog.ltr.-e E. ~t.: 1,. '--T .,....- --,-- ... ,. Standardized eHect 7 Half-normal plot for k 50 .,_ [Abscissa: effect amounts- Ordinates: cumulative frequency] 28 - .. y