Some particle breakage may occur with weak organic or biologic particles. Hydraulic shear is rarely sufficient to grind or reduce the size of individual particles, but it will break agglomerates or lumps. Even within the general category of high-shear mixers, some are used primarily for rapid initial dispersion, while others use multiple recirculation passes for improved dispersion and uniformity. The primary mechanism causing dispersion is hydraulic shear, although some mechanical shear is also present, especially with rotor-stator mixers. These mixers are usually called rotor-stators, but may also be called homogenizers. Some rotating impellers are mounted inside a stationary cage having either holes or slots, through which the fluid is forced. The open impellers for high-speed dispersers may be sawtooth disks, short-bladed impellers or slotted heads. This lengthy description is needed because different processes and applications can be accomplished with variations on similar equipment (see section on Types of High-Shear Dispersers). With dispersed thickeners or emulsion formation, the fluid viscosity may increase. For processes involving soluble solids, the equipment may be called a dissolver. The purpose of dispersion is rapid formation of a suitably uniform fluid. In a few instances, gas bubbles may be dispersed in a liquid for a transport process or to create a foam. The dispersions may be powder dispersions in a low-viscosity liquid, or liquid dispersions of miscible or immiscible liquids. Most dispersers operate at a single speed, although some large dispersers may have variable speed control. The high rotational speeds of high-shear dispersers are often the motor speeds, including speeds at the electric power frequency.
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