(115c) The Flow Field in the Mill - a Key Factor for the Prediction of the Comminution Result

The prediction of the comminution result in technical mills often fails because the interaction between fundamental physical processes taking place inside the mill is still not well understood. Therefore it is hardly possible to quantify the grinding performance of different materials or the effect of varying operational parameters on the product quality. Factors, which characterize the solid-gas mixture leaving the mill, like particle size distribution (PSD), loading etc., are often empirically correlated with input parameters, mostly inlet PSD and supplied energy, without sufficient information about the grinding process and the material transport inside the mill. Systematic grinding experiments with different mills point out so far only influences of the operating parameters and the mill geometry on the comminution result. However, they do not provide any information about the grinding and transportation mechanisms within the mill. For selected mill constructions attempts to collect singular effects have been worked out, e.g. methods for the measurement of the breakage function by radioactive tracing or measurements of the residence time distribution within the mill.

The fundaments for the development of a generalised model for the simulation of the comminution result in hammer mills have been already set in the simulation tool for complex solid processes SolidSim. SolidSim provides a stream structure, which allows the description of solids with distributed parameters as particle colour, moisture content etc. The simulation tool enables not only the modelling of separate unit operations but also of complicated apparatus or of whole production chains. Within SolidSim the comminution is modelled by the population balance. An air classifier mill in SolidSim is built up by an interconnection of the main unit operations comminution and classification into a mill-classifier circuit according to Fig. 1.

After the particles are fed to the grinding zone of the mill, they are stressed by the rotating grinding pins and than transported by the main air to the impeller wheel classifier. The fine particles leave the mill together with the main air through the classifier, while the coarse material is rejected and transported by the internal circulation back to the grinding zone. The deduction of the comminution model was focused on a clear separation between material and operational parameters [1-2]. The size reduction in the mill is modelled by the selection and the breakage function, both determinable in single particle tests and thus being independent from mill specific features.

The hold-up inside the mill is assumed to be ideally mixed. A constant impact velocity or a theoretically deduced impact velocity distribution function can be regarded for the simulation. The free model parameter in the model correlates to the number of stressing events in the mill and shows systematic trends with the operational parameters. The quantification of this model parameter and the consideration of distributed stress intensity and impact velocity for the simulation require knowledge of the flow pattern inside the mill. Therefore the next step for the model improvement is the modelling of the comminution process by taking into account the flow field situation in the mill.

Although the air classifier mill is one of the most widely used mill types, its aerodynamic characteristics have not been investigated in detail. This type of information can be used to develop a predictive model for the comminution process on the one side and to optimize construction parameters of the air classifier mill by indicating weak points limiting efficient processing on the other side. The reasons for the lack of investigations of the flow field in this mill type are the difficult optical accessibility based on the complex geometry and the high rotational hammer and classifier velocities within the mill.

The stress conditions in the mill are of main importance for the comminution process. In contrast to single particle experiments where the particles are stressed at a constant velocity, the impact velocity and the stress intensity in technical mills are distributed parameters. The determination of these distributed factors is possible if the particle motion inside the mill is known. Therefore the focus of the presentation is set on the realistic description of the flow field inside the mill which crucial affects the comminution result. An approach to quantify the transport in the air classifier mill is presented.

Starting from the air flow visualisation measurements by Particle Image Velocimetry (PIV), the influence of the operating conditions on the flow field situation within the mill (type 100 ZPS, Hosokawa Alpine AG) is described. To determine the influence of the impact element geometry on the flow field and therefore on the comminution result cylindrical and platy impact elements were investigated. Essential information for the optimization of the mill geometry provides the measurement of the flow field in the rotating parts of the device. Therefore a mill with optical access to the grinding disc and to the deflector wheel classifier has been designed. Perpendicular to a light sheet plane a CCD-camera is positioned, so that the particle motion in the illuminated plane is recorded. The grinding disc and the classifier rotate with velocities up to 11200 rpm. By the rotation the illuminated plain changes periodically. Therefore a precise position type encoder is installed on the rotating elements. The pulsed laser and the encoder are synchronized in order to ensure successive measurements at the same defined position. This procedure enables an exact recording of the local flow field at the rotating parts.

The flow visualisation studies inside the mill have been carried out to verify a model created, which builds up the complicated flow field inside the mill. The calculation has been done by CFD (Computational Fluid Dynamics) methods (CFX) for the two geometries of impact elements used in the experiment. For the simulation of the flow field at the rotating elements of the mill the Multiple Frame of Reference (MFR) technique has been used. The comparison between the results from the simulation and the experiment is summarised and evaluated. The resulting functions for the description of the impact velocity distribution into SolidSim are presented. The extension of the verified model on the particle laden flow is discussed.

The combination of the theoretical simulations and accurate physical measurements provides a promising tool for process model development and prediction of the comminution result. The next step for the establishment of a general method for the quantification of the comminution result is the coupling of the grinding behaviour of the material under investigation with the flow field situation in the mill.

[1]        L. Vogel, W. Peukert, KONA, 21 (2003), 109-120

[2]        L. Vogel, W. Peukert, Chem. Eng. Sci., 60 (2005), 5164-5176