Batching: Understanding Mixers
Screw MixersScrew 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 intimately mixes the various components. Gross mixing action also occurs within the mixture 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.
This requires a hopper index (HI)* less than the conical hopper angle measured from the vertical. If there is no flow at the hopper walls, a complete progression around the cone will be required to incorporate all the solids in the mixing action. Consequently, a screw mixer hopper without flow along the walls tends to mix much more slowly than mixer hoppers with flow along the walls.
Demixing possibilities include sifting and angle-of-repose demixing. Sifting produces a high concentration of coarse particles in the last discharge. A screw’s high rpm may propel superfines into the air, allowing them to resettle on the screw drive and on the top of the mixer. Additionally, using an extremely fluidizable component as the major component can cause a concentration of fines to discharge last. Angle-of-repose demixing will concentrate a low angle-of-repose solid away from the screw. Solids that do not have an HI greater than the typical cone angle of 17 degrees from the vertical will cause the lower angle-of-repose, easier-flowing solids to discharge from the mixer first.
Solids appropriate for a screw mixer include those with moderately cohesive fines, those that are not prone to angle-of-repose segregation and those that require a moderate amount of liquid addition.
Plough and Paddle MixersPlough and paddle mixers use a horizontal rotating shaft (or shafts) with fixed arms and attached plough- or paddle-shaped feet. The plough in a single-shaft mixer throws solids left, right and forward at various velocities, depending on which portion of it strikes the solid. Adjacent arms and ploughs then throw the solids partially back to the first plough and partially back to the next one, and so on. This mixing mechanism produces various progression velocities along the length of the horizontal circular mixer shell. At each end of the mixer, ploughs throw the solids only toward the center of the device.
The dual-shaft paddle mixer paddles also impact the solids and throw some of them onto the second shaft while pushing some toward one end of the unit, where the paddles on the other shaft push the solids toward the opposite end, as well as toward the other shaft and paddle set. At the mixer’s ends, the paddles throw the solid toward the other shaft.
Paddle mixers are noted for fast mixing; however, the plough mixer does not mix end-to-end very well or quickly. Demixing can occur with air currents generated by high-speed ploughs or paddles. If the mixture is free-flowing and at least one component is superfine, that component might end up coating the upper half of the mixer shell. Depending on the superfine component’s arching index,** this material can become unstable and can occasionally drop in chunks into the mixer. If this occurs near the end of the mixing cycle, a small but high concentration of superfines may occur on discharge. If the coating material is a major mixture component, a serious out-of-specification mixture can result. However, minor components are generally very fine to achieve good dispersion in the mix and are therefore more likely to drop as chunks into the mixer.
High-speed ploughs also tend to fluff solids. If a mixture containing a coarse, free-flowing component has a fluidizable component that makes up 25% or more of the mixture, then separation during mixing is likely. The fluidizable component will settle on the top and the coarse component will settle to the bottom.
The effect of top-to-bottom demixing depends on how the mixer is operated during discharge. If the mixer is jogged to keep the outlet full of solids, then it will rhythmically discharge mostly coarse solids that contain a few fines. The pulsation frequency will correspond to the jogging frequency. If the mixer runs at full speed during discharge, a significant portion of the mixture will contain coarse particles, followed by predominantly fluffed fines. Plough and paddle mixers are also susceptible to sifting demixing as a result of a component’s angle of repose.
Solids most suitable for plough and paddle mixers are those with some liquid or other cohesive component that will likely eliminate the demixing tendencies inherent with these mixers. To completely mix minor ingredients in a plough mixer, it is also important to introduce them along the entire length of the unit. Single-point introduction will not be effective.
Vertical Shaft Impeller MixersVertical shaft impeller mixers are often used for liquids but can be effective with solids, provided that the speed (rpm) is high enough to create a vortex similar to that for liquids in a vertical shaft mixer. Mixing occurs very fast, and the energy input per unit of time is large. The mixing mechanism is created by the vortex action, in which recirculating velocities vary depending on the mixture’s radical position in the vortex.
Demixing occurs through sifting, which leaves a coarse (particle) layer on top; fluidization, which deposits a fines layer on top; and air-current demixing, which results in superfines stuck to the mixer body above the solids level. Solids most appropriate for this type of mixer are those with nondegrading, fine (but not superfine), free-flowing particles that have melting or softening points larger than the temperature induced by the whirling impeller. These mixers are well suited for mixing solids with liquids.
Ribbon MixersA ribbon mixer uses 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, thus allowing the mixture to slide down a repose angle. The result is a mixer that does a thorough and fast blending/mixing job in the vertical plane, but is slow when mixing end to end.
Demixing mechanisms include sifting and angle of repose. These are manifested by a high concentration of coarse components at the end of the discharge cycle. Air currents, especially with faster rotation or a dust collection system, may create globs of minor components that build up on the mixer shell surfaces above the solids level and can break loose during the discharge cycle. Fluidization demixing can also occur but can usually be controlled by rotational speed. Ribbon mixers are used extensively for mixtures with moderately cohesive components.
Air-Pulse BlenderAir-pulse blenders mix by creating air bubbles in the container that lift solids directly above the air jet, allowing the solids on the side of the bubble to flow downward to fill voids left in the air bubbles. This causes extremely fast, localized mixing. The air injected must be at a sufficient pressure to lift the entire contents of the blender and a sufficient volume to exceed the minimum fluidization velocity of the solids. Pulses often are simultaneously initiated from all vessel ports at once. However, sequential pulsing can sometimes be effective with a fine, relatively uniform-sized solid.
Due to their bottom-to-top discharge, air-pulse blenders seem to eliminate sifting and repose-angle demixing, but they are susceptible to air current demixing. Fines that are impinged on the surface of the dust collector or suspended in the upper reaches of the mixer are deposited on the top surface again. If the component particles are nearly the same size, then the surface will contain about the same mix as in the remaining portion in the vessel. An air-pulse blender with a hopper that delivers bottom-to-top discharge will work with a wide range of solids, provided that the solid contain fines with a high flow rate index (FRI)*** and the components are all about the same particle size or in the same FRI range.
Tumble MixersTumble mixers are composed of a rotating shell that contains the solids. The two most common shapes are the V-type and the twin cone. Another more recent shape is the diamondback blender. The primary mixing mechanism in a tumble mixer is the distribution of solids along the ever-changing angle-of-repose surface.
Mixing is affected by the container’s shape. The V-mixer, for example, tends to form two separate mixing chambers connected by a central cross-mixing zone. The twin-cone and diamondback blender shells converge all the solids toward the smaller diameter ends and, thus, greatly facilitate cross-mixing. The diamondback blender, with its one-dimensional convergence, tends to create a crisscross response angle as the solids slide from one portion of the blender to the other, producing additional cross-mixing.
The angle of repose is the primary demixing mechanism in tumble mixers. With a V-mixer, the low angle-of-repose component concentrates in the central cross-mixing region. The twin-cone and diamondback blenders tend to leave the lower angle-of-repose component in striations that, on discharge, may exit in high concentrations. The diamondback blender produces a bottom-to-top uniform flow across the cross-section that mixes striations effectively on discharge.
In general, tumble blenders are limited to moderately cohesive solids that do not have sifting or repose-angle demixing. However, running the diamondback blender at high speeds can extend the range of solids to those that would normally demix with sifting and repose-angle mechanisms.
Gravity-Flow BlendersAs the name implies, the only motive force in gravity blenders is the flow of a solid moving under its own weight. Mixing occurs as a result of differential blender retention times as solids exit the blender outlet. Gravity-flow blenders are especially useful when large quantities of solids must be mixed and stored.
Different types of gravity-flow blenders include tube blenders, cone-in-cone blenders and diamondback blenders.
Tube blenders derive their name from the many tubes inside a bin with entry points into the tubes at multiple elevations in the arrangement. These tubes deliver their contents from multiple elevations simultaneously to the blender outlet, thus providing the differential bin retention time required for mixing.
Cone-in-cone blenders consist of a bin with a conical hopper that houses another smaller and steeper conical hopper inside. These designs are most effective when interior cylinders above the interior cone lock in the differential flow velocities and extend that effect to the top of the outer cylinder.
The diamondback blender works the same way as a cone-in-cone blender. It derives its differential retention time from the cylinder that protrudes into the upper hopper portion of the blender, thereby producing a varying gap between the cylinder and the hopper wall. Velocity is controlled in proportion to the gap created between the cylinder and the diamondback blender.
Gravity-flow mixers are subject to all four of the demixing mechanisms previously discussed. With both the cone-in-cone and diamondback blenders, sifting and angle-of-repose demixing can be eliminated by adjusting the flow pattern on the discharge to create a first-in/first-out release during the blender’s emptying cycle. This is accomplished in the diamondback blender by raising the inner cylinder to the top of the hopper. In the cone-in-cone, the lower steeper cone must be moved down to create a vertical section between the cone-in-cone and the lower cone. Fluidization can be overcome by using a letdown chute.
Although tube blenders are restricted to extremely free-flowing mixtures, many cone-in cone and diamondback types can handle solids with severe demixing tendencies.
Problem-Free MixingThe list provided in this article is not all-inclusive—however, it does offer an overview of what happens in a number of different mixing processes. In addition to the type of equipment being used, the functionality of the solid being blended is also a key factor in the success of the mixing process. Understand the materials you are dealing with, and don’t be afraid to ask questions when you are evaluating a mixer for your facility. Additionally, pay attention to the details when evaluating a new mixer. It is fundamental that the mixer selected has the design details that will enable it to process your mixture in the most efficient way possible.
*A hopper index is the minimum half angle (the wall angle measured from the vertical) of a conical hopper required to ensure flow at a hopper’s walls.
**The arching index is the conical hopper outlet size required for unaided gravity flow after dropping the solids into an empty hopper and ensuring arch collapse in a conical bin.
***The flow rate index is the rate at which a solid will flow through a hopper outlet of diameter when totally deaerated.