Rotor stator milling particle size

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. 

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

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... 

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

Disk attrition mills have rotor-stator configurations which produce narrow gaps in the order of 30 to 500 pm, through which the suspension to be milled is forced. The milled product may have a maximum particle size as low as 10 pm. 

Pilot plant can also avoid pitfalls in equipment selection. The two rheograms on figure 11 show how a screw type pump can affect the rheological properties of silica-alumina hydrogel. The shear stress imposed by the rotor-stator couple of the pump results in an increase of the yield value of the hydrogel. In the pump, the size of the particles which compose the hydrogel is decreased by a milling effect and the forces of cohesion are increased. This can stop the flow if the pump characteristics have been underestimated. This results also in modification of the gel structure and ability to be spray dried. 

The high-speed mill (up to 6000 revolutions per minute) consists of a conic rotor and its respective stator. The gap between rotor and stator is adjustable and is normally approximately 0.25-0.50 mm. Any variation of the gap affects the sizes of the bitumen globules (particles) and, hence, the quality of the bitumen emulsion. 

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