Media mills

Rubber-mill chips are dissolved similarly to resins, to provide color concentrates. Dough mixer and chip concentrates must be diluted with solvent and other vehicles to make finished inks. Media milling is becoming a method of choice in both flexo and gravure ink manufacturing. Other high speed dispersing units, such as the Morehouse, Cowles, Kady, and others, are also used. 

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

In vertical disc-stirred mills the media should be in a fluidized condition (White, Media Milling, Premier Mill Co., 1991). Particles can pack in the bottom if there is not enough stirring action or feed flow or in the top if flow is too high. These conditions are usually detected by experiment. 

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. 

Terpenoids are susceptible to a number of alterations mediated by oxidation and reduction reactions. For example, the most abundant molecule in aged Pinus samples is dehydroabietic acid [Structure 7.10], a monoaromatic diterpenoid based on the abietane skeleton which occurs in fresh (bleed) resins only as a minor component. This molecule forms during the oxidative dehydrogenation of abietic acid, which predominates in rosins. Further atmospheric oxidation (autoxidation) leads to 7-oxodehydroabietic acid [Structure 7.11]. This molecule has been identified in many aged coniferous resins such as those used to line transport vessels in the Roman period (Heron and Pollard, 1988 Beck et al., 1989), in thinly spread resins used in paint media (Mills and White, 1994 172-174) and as a component of resin recovered from Egyptian mummy wrappings (Proefke and Rinehart, 1992). 

Figure 13.1 Schematic of the media milling apparatus and the on-line NIR implementation for particle size analysis. Reprinted with permission from Higgins et al. (2003). Copyright 2003 American Chemical Society.

Figure 9.10 Schematic of light scattering by particles near the (a) beginning (b) endpoint of the media milling process. Reprinted with permission from Higgins etal. (2003).66...

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. 

Size (P-m) Hammer Mill Universal and Pin Mill Jet Mill Jet Mills with Internal Classifier Media Mills Toothed Rotor- Stator Colloid Mill... 

Media Mills. Media mills, also referred to as pearl or bead mills, are much different in operation than a rotor-stator mill. The mill is composed of a milling chamber, milling shaft and product recirculation chamber. The milling shaft extends the... 

At the small size scales achievable by media milling, particle-particle interactions caused by van der Waals forces can begin to dominate.20 By inclusion of certain additives to the dispersion fluid, the possible agglomeration and resulting reduced efficiency and reduced effectiveness of the mill can be mitigated. Surfactants can... 

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

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