Rotor-stator-mill

Rotor bodies, in steam turbines, 23 231 Rotor electrostatic separators, 16 643-644 Rotor rotating converter, 16 151 Rotors, molecular, 17 59-61 Rotor spinning cotton yarn, 3 17 Rotor-stator devices, 10 127 Rotor-stator disperser, 3 701 Rotor-stator mills, 13 65 Rotor-stator mixers, 16 674-675 Rotosil process, 22 412 Roughages, as ruminant feeds, 10 863 Roughing, in mineral separation, 16 604 Roughing services, magnetic drums used in, 15 446... 

Toothed Rotor Stator Mills. Rotor-stator mills (Fig. 8.4a) consist of a rotating shaft (rotor), with by an axially fixed concentric stator. Toothed rotor-stator mills have one or more rows of intermeshing teeth on both the rotor and the stator with a small gap between the rotor and stator. Variations in the number of teeth, teeth spacing, angle of incidence, etc., all impact the milling efficiency of toothed rotor-stator mills. 

Colloid Mills. Colloid mills are another form of rotor stator mill. A colloid mill is composed of a conical rotor rotating in a conical stator (Fig. 8.5). The surface of the rotor and stator can be smooth, rough, or slotted. The spacing between the rotor and stator is adjustable by varying the axial location of the rotor to the stator. The gap can be as little as a few hundred microns to a couple of millimeters.18 Varying the gap varies not only the shear imparted to the particles but also the mill residence time and the power density applied. Particle size is affected by adjusting the gap and the rotation rate. It is possible to produce particles in the 1-10 pm size range. 

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... 

Lee I, Variankaval N, Lindemann C, Starbuck C. 2004. Rotor-stator milling of APIs— emperical scale up parameters and theoretical relationships between the morphology and breakage of crystals. Am. Pharm. Rev. 7(5) 120-123. 

For this purpose a fast stirrer with zig-zag blade or a colloid mill such as a rotor-stator mill are useful. After the stirring or the passage through die mill is completed the formulation needs no further processing. Compared with the suspension/emulsion method, fewer vessels are required, low viscous fluids are handled and incorporation of the thickener is more easily achieved. 

On a manufacturing scale rotor stator mills have proven to be very efficient. Their use in the preparation of emulsions has been described in a wide body of literature [7]. When preparing suspoemulsions the same principles can be applied. 

In Rotor Stator Mills the dispersed phase is emulsified by shear stress in the conical gap between the rotor and the stator. 

Table 2 Rotor/stator milling parameters and their effect on average particle size...

For the toothed rotor/stator mills, there are localized areas of high shear where the most energy is imparted to the particles, and breakage is believed to occur, through a combination of shear and collision. Therefore, useful parameters that quantify mill performance will reflect these effects. Three typical parameters used in rotor-stator mill scale up are tip speed (rotation rate of rotor x rotor circumference), shear rate (tip speed/distance between rotor and stator), and shear frequency (rotation rate x number of slots on rotor x number of slots on stator).Rotor-stator types typically differ in the number of rows of teeth and the number of teeth in each row of rotor and stator, though other geometric differences are possible. When rotor-stators of different design are available for study, shear frequency appears to have... 

In some cases, rotor stator milling may be used to reach a particular average size that is larger than the steady-state result. In these cases, it is important to either employ some type of in-line particle size monitoring or carefully consider any differences in rotor/... 

The resulting milled particle size is much smaller for a media milling operation than for rotor/stator milling. This technique can be used to prepare material from 1-2 pm average size to nanoparticle size, in some cases, employing proprietary media or additives to the slurry.Surface areas for material of 1-2 pm size... 

High Speed Fluid Energy Mills. This type of equipment is used primarily for preparation of relatively low viscosity mill bases for inks and paints. The first is an impeller type which achieves dispersion by the appHcation of shear. The second type is in the form of a rotor—stator, and the dispersion is achieved by impingement or impact. 

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

Figure 8.4 Wet mill internals (a) and rotor-stator working principle (b). (KA.) into the product. Tip speed is determined according to the following equation ...

Liquid/liquid emulsions consist of two (or more) non-miscible liquids. Classical examples for this are oil in water (O/W) emulsions, for example milk, mayonnaise, lotions, creams, water soluble paints, photo emulsions, and so on. As appliances, teeth-rimed rotor-stator emulsifiers and colloid mills, as well as high-pressure homogenizers are used. 

Fig. 49 Relationship d32 =/(Ap) for different emulsifiers. ZKDM-teeth ringed rotor-stator emulsifier, CM - colloid mill, HPH - high pressure homogenizer. Details on the material system in text from [74]...

Emulsification Method. Emulsified sulfur-asphalt binder was prepared by combining molten sulfur and liquid asphalt in a 2Vi -in.-diameter, vertical, Gifford Wood colloid mill (1) at a rotor-stator gap setting of... 

Sulfur-asphalt binders can be prepared by various mechanical means. One conventional method is to combine liquid sulfur and asphalt at 285°-300°F in a Gifford Wood colloid mill. A rotor stator gap setting of 0.02 in. at 7000 rpm for 8 min will prepare satisfactory emulsions. This emulsion is immediately mixed with preheated aggregate. The laboratory binder was prepared by TTI (Texas Transportation Institute) scientists in cooperation with SNPA (Societe Nationale des Petroles d Aquitaine) scientists and is believed to be comparable with the binders prepared by the turbine in the field trials. The need for dispersing sulfur in the asphalt is discussed by Garrigues (9) and by Kennepohl et al., Deme, and McBee et al. elsewhere in this volume. 

Rotor-stator systems These systems apply an element that rotates quickly, very close to a corrugated element that stands still (the stator). The flow field in between the two elements becomes very intense when the distance between them is very small sometimes much less than a millimeter. There are different forms of this type of equipment. The most important ones are the stirred tank, a colloid mill, and a toothed mill. 

Colloid Mill Colloid mills are rotor-stator systems that can be used to reduce the particle size distribution of both liquid dispersions (emulsions) and solid dispersions (suspensions). The emulsion or suspension is pumped through a narrow gap that is formed by the rotating inner cone and the stationary outer cone. The width of the annulus can be adjusted by changing the relative position of the two cones. The principal size reduction in colloid mills is due to the high shear forces that are caused by the velocity difference between the rotor and the stator surfaces. To increase wall friction and reduce slip, surfaces are usually not smooth but are roughened or toothed, which, in turn, changes the flow conditions from laminar to turbulent, thereby increasing the shear forces in the annulus. 

Rotor/stator gap (gap width) Smaller gap increases shear forces, increases milling rate, and produces smaller particles. 

Rotor/stator design (number of teeth) Mainly affects milling rate, but can affect steady-state particle size achieved... 

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