In the previous year, the influence of different grinding aid types and dosages on different process aspects, such as the residence time and material hold-up within the grinding chamber was studied. Furthermore, the impact of the various grinding aids used throughout this project (Diethylene glycol, Heptanoic acid, Hexanol) on the powder flowability within the progress of grinding process was investigated. For this purpose, a wide range of particle sizes with different grinding aid types and dosages were measured and a relation between the flowability index obtained from the ring shear tester and BET specific surface area was found, that can be used to describe this impact.
In the current year the research is focusing on studying the influence of grinding aids on various bulk material properties and how these properties can be used to describe and explain the impact of grinding aids on the various process aspects of dry grinding in tumbling ball mills. For material characterization, different techniques, like the rotating drum (dynamic angle of repose), static angle of repose, a ring shear tester, a powder rheometer, bulk density, and tap density were utilized. Material characteristics obtained from the static angle of repose and rotating drum do not deliver for the range of investigation reliable values for describing the influence of grinding aids. The results of the bulk densities and tap densities measured for various particle sizes as well as grinding aid types and dosages showed that grinding aids causing high powder flowabilities also lead to high bulk and tap densities, as particles can easily move against other particles, e.g. fine particles can slip between bigger particles creating dense particle bed. In order to account for the impact of grinding aids on the bulk density by measuring the flowability with the ring shear tester, the flowability index weighted with the bulk density results in more pronounced and reliable values of grinding aid dosages. The powder rheometer as a modern technique for material characterization was used to study the influence of grinding aids on material properties. For the application of dry grinding, the measurement of the so-called cohesion strength was found to deliver values that can be used to describe the influence of grinding aids on the process
Furthermore, it was found that measuring material under conditions similar to the conditions in the mills delivers much more reliable values. In this context the cohesion strength values measured with an aerated powder bed showed much better results for describing the dry grinding process.
For modeling the internal material transport, the axial dispersion model could deliver good approximation of the experimental data. The dimensionless equation of this model contains only one unknown, namely the Peclet Number. Comparing the resulted Peclet Numbers from the tracer tests with the cohesion strength values measured with the powder rheometer under aeration revealed a good relation between both values that can be used to estimate the influence of grinding aids on the material transport.
It could be shown in the last year that the grinding aids do not influence the cut size of the air classifier but rather the bypass as the grinding aids improve the dispersion behavior of the particles. For this reason, the Whiten model has been implemented in the flow sheet simulation tool Dyssol to describe the efficiency curve of the air classifier.