Stirred Media Mills

Performance of Bead Mills Materials processed in stirred-media mills are listed in Table 20-17. Variables affecting the milling process are listed below. 

TABLE 20-17 Materials Processed in Stirred-Media Mills (Stehr/ /nternotiono/ J, Mineral Processing, 22 1-4, 431-4, 1988). 

The vibratoiy-tube mill is also suited to wet milhng. In fine wet milling this narrow residence time distribution lends itself to a simple open circuit with a small throughput. But for tasks of grinding to colloid-size range, the stirred media mill has the advantage. 

Particles can either be produced by bottom-up processes (e.g. precipitation) or top-down approaches (e.g. wet milling). In these processes particle-particle interactions become relevant when the particle size is below 1 pm. Engineering macroscopic product properties is then only possible through tailored surface and interfacial properties, no matter whether a bottom-up process like precipitation [11] or a top-down process such as milling in stirred media mills [12] is studied. Aggregation is an important aspect in both processes which are studied in the following. 

In precipitation, particle formation is extremely fast due to high supersaturations which in turn lead to fast nucleation. At least in the beginning, size distributions are narrow with particle sizes around one 1 nm. Nanomilling in stirred media mills is characterized by relatively slow particle formation kinetics, particle sizes ranging from several microns down to 10 nm and high sohds volume concentrations of up to 40%. Large particles may scavenge the fine fractions. The evolution of the particle size distribution can be described for both cases by population balance equations (Eq. (7)),... 

M. Sommer, N.J. Wagner, W. Peukert, J. Green, D. Spahr, Fundamental investigations on nanoparticle production by stirred media milling, in Proceedings of the AIChE Annual Meeting San Francisco, CD-ROM, 16—21 November 2003, paper 54f. 

Kwade A, Schwedes I. 2007. Wet grinding in stirred media mills. In Handbook of powder technology 229-249. 

A typical task is the milling of minerals in the pm-range with power consumption of approximately 50 Wh kg-1. A tumbling ball mill would need 1 hour of milling time at a specific power input of 50 W kg-1 for the comminution, a stirred media mill would need 2 min at 1400 W kg-1, a vibration mill 40 s at 4500 W kg-1, and a planetary mill 5 s at 36,000 W kg-1 (Fig. 14.4). 

Stirred media mills or agitated ball mills are used mainly in wet microfine and nanofme milling. They are filled to 80 % capacity with grinding media (glass beads,... 

Fl 14.T. Schematic of different stirred media mills, (a) Batch stirred mill (b) stirred ball mill. 

Example 30 Fine grinding of solids in stirred media mills... 

The above assumption [78] was convincingly confirmed in a stirred media mill with a perforated stirrer disc of given size (V = 5.54 1) at a constant energy input per unit mass, E/pV = 103 kj/kg. In batch-wise performed measurements, the kinetic energy of the beads has been varied by the tip speed of the perforated disk stirrers as well as by the density of the grinding media (glass, steel) and, in particular, by the bead diameter (dM = 97-4 000 pm). 

Fig. S5 The relationship cbo (Ekin) at E/pV = 103 kj/kg for three differently sized stirred media mills with perforated stirrer discs from [78].

In Fig. 55, the results shown were obtained in three stirred media mills with a perforated stirrer disc of different sizes (V [1] = 0.73 5.54 12.9). The results are not satisfactory with respect to the scale-up rule. Here, too, it can be seen that small mills (V < 11), under otherwise identical conditions, produce a coarser product than larger ones. A satisfying correlation is achieved by plotting the median particle size, dso, versus the mean stress intensity. This is defined as stress intensity of the grinding media multiplied by the term which takes into account the stress intensity distribution [138],... 

Walzel, P., F. Schmelz, S. Schneider, Chem.-Ing.-Tech. 73 (2001) 12,1599-1602 Herstellen monodisperser Tropfen mit pneumatischen Ziehdusen Stender, H.-H., A. Kwade, J. Schwedes, 3rd ECCE Nuremberg, 26-28 June 2001, Paper 376 Constant grinding conditions at scale-up of stirred media mills... 

Example 30 Fine grinding of solids in stirred media mills 129 Example 31 Scale-up of flotation cells for waste water purification 133 Example 32 Description of the temporal course of spin drying in centrifugal filters 140... 

Stirred media mills (batch and continuous) Stirred ball mills Stirred sand mills... 

Kwade, A. Wet comminution in stirred media mills— research and its practical application. Powder Tech. 1999, 105, 14-20. 

Garg, R. Gattinara, A. Godbole, P. Ho, J. Roberts, M. Witulski, F. Shelukar, S. Reynolds, S. Heat transfer characteristics and performance of stirred media milling for pharmaceutical applications. Int. Fine Particle Res. Inst. Ann. Con. Holland, July 2000. 

A key to the performance of media mills is the selection of an appropriate grinding medium. Jorg Schwedes and his students have developed correlations which are effective in determining optimal media size for stirred media mills [Kwade et al.. Powder Technm., 86 (1996) and Becker et ah, Int. J. Miner. Process., 61 (2001)]. Although these correlations were developed for stirred media mills, the principles developed apply to all media mills. 

Stirred media mills are also the dominant process equipment used for dispersing fine powders into liquid, e.g., pigment dispersions, and have largely displaced ball mills in these applications. In these applications, they are capable of dispersing powclers down to particle sizes below 100 nm effectively and reliably. 

Stirred media mills are used almost exclusively for wet grinding. In general, the higher the tip speed of the rotor, the lower the viscosity that can be tolerated by the mill. At high viscosity, very little bead motion occurs. Similarly, mills with lower tip speeds can tolerate the use of larger, heavier media, since gravity will cause additional motion in this case. 

Attritors In the Attritor (Union Process Inc.) a single vertical armature rotates several long radial arms. The rotation speeds are much slower than with other stirred media mills, and the grinding behavior in these mills tends to be more like that in tumbling mills than in other stirred media mills. They can be used for higher-viscosity applications. These are available in batch, continuous, and circulation t es. 

Horizontal Media Mills Horizontal media mills are the most common style of mill and are manufactured by a large number of companies. Figure 21-75 illustrates the Drais continuous stirred media mill. The mill has a horizontal chamber with a central shaft. The media are stirred by discs or pegs mounted on the shaft. The... 

Manufacturers There are many manufacturers of stirred media mills worldwide. Major manufacturers of stirred media mills include Netzsch, Buhller, Drais (now part of Buhler), Premier (now part of SPX), Union Process, and MorehouseCowles. Many of these manufacturers have devices specifically adapted for specific industries. For example, Buhler has some mills specifically designed to handle higher-viscosity inks, and Premier has a mill designed specifically for milling/flaking of metal powders. 

Reindl, A. et al.. Dispersing silicon nanoparticles with a stirred media mill and subsequent functionalization with phenyl acetylene. Colloids Sutf. A, 301, 382, 2007. 

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