Industrial Mixers Information
Industrial mixers mix or blend a wide range of materials, or keep mixes in suspension as a secondary purpose. These devices mix liquid, dry, or slurry materials using a shaft, screw, blades, ribbons, impellers, or propellers. Mixers range in size from laboratory/table top to large production units. Materials mixed can be adhesives, chemicals, construction materials, minerals, coatings and paints. They are used for either batch or continuous processing in food, beverage, pharmaceutical, and water treatment applications. Mixer motion may consist of a shearing action created by blades moving through product or product moving through blades.
Batch mixing is a basic mode of operation in which the mixer is filled with media and product and mixing takes place. When mixing is complete, the vessel's contents are emptied for downstream processing. The mixer is then cleaned and refilled for mixing another batch. In continuous-feed mixers, the media is added continuously as mixed fluid is removed. This type of mixer is suitable for production applications because it can run for long periods of time without being shut down. Many batch mixers can be configured to run in continuous mode.
Conical screw mixers use a rotating screw that progresses around the periphery of a conical hopper. The screw lifts solids from the bottom of the hopper to the top, where the mixture flows by gravity back into the screw. Mixing occurs around the open screw, where the solids transported by the screw exit at various levels and are replaced by other solids at that level. The screw’s shearing action also mixes the various components. Gross mixing occurs by the velocity profile created in the conical hopper as it feeds the screw. This gross mixing action is most effective when the solids move along the conical hopper walls.
Paddle mixers use a horizontal rotating shaft (or shafts) with fixed arms and attached paddle-shaped feet. The paddles of the dual-shaft mixer impact the solids and force some of them onto the second shaft while pushing others toward one end of the unit. The paddles on the second shaft push the solids toward the opposite end, as well as toward the other shaft and paddle set.
Planetary mixers usually have two mixing blades that rotate around individual shafts. The two blades further rotate around a center axis. The net effect is intermixing, stirring, and shear.
Propeller and impeller mixers use vertical blades attached to a horizontal disc. The propeller or impeller pumps the media in a radial direction and generates a recirculating mixing pattern above and below the disc. This high-shear, high-power design is stable under varying liquid depths and makes an excellent choice as a rapid mixer in shallow basins and solids suspensions. Propellers are often used as the lower impeller in a multiple impeller design.
Ribbon mixers use a counter-transport mechanism that consists of an outside right-hand ribbon and an inside left-hand ribbon, each connected to the same horizontal shaft. This ribbon setup, combined with the mixer’s inability to transport an entire mass of solids, creates an extremely diverse velocity field. Such a mixer may transport a portion of the solids a short distance in both directions along the shaft’s axis, while also lifting a portion of the solids from top to bottom, allowing the mixture to slide down a repose angle. The result is a mixer capable of thorough and fast blending and mixing in the vertical plane, but is slow when mixing end to end.
Rotor-stators are single-shaft mixers with an impeller rotating in close proximity to a stationary housing. They are used to rapidly break apart solid particles in a liquid, and for emulsification. The impeller or rotor mechanically imparts high shear forces to the ingredients. Particle clumps are also broken apart by the hydraulic shearing forces generated as particles are ejected through openings in the stator into the rest of the batch. Rotor-stators are particularly effective at chopping coarse particles such as rubber or flake resin. The stator directs the flow and confines the particles, while the rotor imparts shear.
Vertical turbine mixers include a wide range of general-purpose mixing equipment. Devices operate at reduced speeds provided by an enclosed gear drive, with one or more multi-bladed impellers mounted on an overhung shaft. Vertical turbine mixers can be used on open tanks, when supported by a beam structure, or on closed tanks with a variety of seal and support arrangements. Most turbine mixers are mounted at the center-top of a vertical, cylindrical tank and used to mix low to medium viscosity liquids, slurries, dispersions, or other fluids. Because of the general-purpose capabilities of turbine mixers, at times they may be used on almost any shape tank of any size, with other drives or special impellers, for high-viscosity fluids, and for almost any mixing application.
Ultrasonic or vibrational mixers apply ultrasonic waves to a mixed medium to produce a steep gradient of acoustic pressure which moves the fluid and provides a very fine level of mixing action.
Static or motionless mixers are fins, obstructions, or channels mounted in pipes. They are designed to promote mixing as fluid flows through the device. Most static mixers use some method of first dividing the flow, then rotating, channeling, or diverting the flow, before recombining it. Other static mixers create additional turbulence to enhance mixing.
Media being mixed is an important consideration when selecting a mixer. Typical materials that are mixed and are usually covered in manufacturer specifications include:
Liquids, solutions, and emulsions: Solutions consist of a substance (called a solute) dissolved in a liquid (called a solvent). Emulsions are combinations of immiscible (unblendable) liquids, such as oil and water.
Suspensions, slurries, pastes, and colloids: Suspensions (including slurries and pastes) are liquids that contain solid particles sufficiently large for sedimentation. Colloids have smaller particles that will not settle.
Solid with liquid addition: These mixers are used to blend powders/granules and introduce liquids uniformly without allowing the solids to agglomerate. Examples include mixing clay particles with de-dusting fluids to enhance powder processing and molding.
Powders or granules: Mixers are used to process powder or granular materials such as dry cement, flour, and other foodstuffs.
Mixers can be specified with minimal functionality but most mixer manufacturers will customize mixing equipment with added features and capabilities. When specifying a mixer, heaters and coolers can be added along with timers and gauges. Programmable instrumentation or special lining materials can be added. Post mixing processes that can be integrated with the mixer include an extruder feed, an agglomerator, a coater, a dryer, milling capability, or a granulator.
Recent advances in industrial mixer design have made mixers more flexible than ever. Developments of note include new agitator designs across all mixer types. (An example agitator is shown at right.) Technology advances has allowed the combination of dissimilar agitators for better control of shear, heat transfer and flow, as well as improved dispersion, emulsification, homogenization, and powder induction.
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