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Choosing a mixer for low-shear batch mixing

A material’s characteristics dictate what type of mixer will be most efficient for batch mixing. This article covers some batch mixing basics and examines the benefits and limitations of several low-shear mixers.

When selecting a bulk solids batch mixer, you must consider your entire process. Your recipe, quality requirements, batch size, reaction time, material characteristics, and how the material is fed into and discharged from the mixer will all determine what type of mixer will give you the mixing time and consistency you require.

To determine which mixer is appropriate for your application, it's important to look at whether the material is free-flowing or cohesive. As shown in Figure 1, batch mixers can generally be categorized by the amount of shear they apply to the material during mixing. Normally, the more cohesive your material, the more shear required to mix it. Low-shear and midshear mixers are suitable for more free-flowing materials, while mid-shear, high-shear, impact, and particle-design mixers are suitable for more cohesive materials.

Selecting a batch mixer

Fig. 1: Selecting a batch mixer

Low-shear mixing is a relatively ordered process, meaning that the mixer runs at a low speed and the material's particles move in an orderly fashion throughout the mixer. For mid-shear mixing, the mixer moves at a slightly higher speed and particles move more randomly around the mixer. High-shear mixing uses a lot more energy, resulting in particles shearing against each other or against the mixer wall. Impact mixing disperses the particles in the air and impacts them with a rotor to break the particles and cause them to mix freely. Particle-design mixing is a form of high-shear mixing that uses a great deal more energy to mix very cohesive materials.

Low-shear mixing is the ideal choice for free-flowing, fragile, and heat-sensitive materials. This article will focus on selecting a low-shear batch mixer for freeflowing materials. Before discussing low-shear mixer types, however, it's important to understand some characteristics of free-flowing materials.

Characteristics of free-flowing materials

Free-flowing materials have low interparticle friction and tend to have a particle size larger than 75 microns. Free-flowing materials also tend to have low moisture content, as moisture can cause the particles to stick together. These materials typically have particles with a low aspect ratio (difference between the particle's length and width). Particles with a high aspect ratio tend to interlock and resist flowing against one another, while particles that are more rounded flow more easily against one another and are easier to mix.

In general, particles with a low aspect ratio will also have a low angle of repose. A material's angle of repose is the angle that's formed when the material piles on a flat surface. The steeper the pile, the larger the angle of repose and the more cohesive the material. Cohesive materials are typically harder to mix than free-flowing materials because more energy is needed to overcome the forces holding the particles together.

Mixing free-flowing materials presents different problems, including segregation. Segregation occurs when a material's particles naturally separate by size or some other characteristic. This can happen in several different ways, as shown in Figure 2.

Types of segregation

Fig. 2: Types of segregation

Vibration segregation. If your batch has multiple ingredients in a vessel that's subject to vibration, ingredients with coarser or lighter particles will migrate to the top of the batch.

Percolation segregation. Finer particles will naturally move through the spaces between larger particles to the bot tom of the batch, resulting in a higher concentration of course particles at the top.

Transportation segregation. Coarser particles have a higher mobility in flowing material, so when a material falls out of a spout and forms a pile, the larger particles migrate out toward the pile's perimeter. This can concentrate fines in the middle of a batch. If the material is pneumatically conveyed, coarser particles will travel farther than finer particles, so if you're using a pneumatic conveying system to fill a vessel, your fines may end up on one side of the vessel and your course particles on the other side.

Low-shear mixing modes

A totally homogeneous blend must be mixed in a way that randomizes the ingredients. The mixer moves the ingredients relative to each other until the particles of each ingredient are evenly distributed throughout the blend. The two ways (or modes) of doing this in a mixer are diffusion and convection.

In a diffusion mixer, each ingredient's particles diffuse and blend with each other randomly, assisted by gravity. A convection mixer operates more deliberately, generating specific flow patterns or currents that move particles from one area of the mixer to another.

In general, diffusion mixers tend to be less expensive and easier to maintain than convection mixers, so diffusion mixing is the best option if it works for your material. However, mixtures that are prone to segregation or have large differences in particle size or bulk density between ingredients can't be mixed using a diffusion mixer because the level of homogeneity can be capped and segregation can occur if the batch is over-mixed.

Low-shear mixer types

For low-shear mixing of free-flowing materials, the three most common mixer types are the conical screw mixer, tumble mixer, and ribbon mixer. Each has advantages and disadvantages.

Conical screw mixer
A conical screw mixer is a convection mixer that has a screw that rotates and orbits inside a stationary cone-shaped mixing chamber. Material is added into the mixer to a filling level below the screw's orbit arm so that the arm has free movement around the mixing chamber. The screw cone, and orbits around the perimeter of the chamber, ensuring a thoroughly mixed batch.

Conical screw mixer advantages
Conical screw mixers can provide a very high mixing quality, and because of the vessel's cone shape, discharging the material is easy—you open the vessel bottom, and gravity carries the material out. The screw is cantilevered, with no bearings at the bottom and no seals below the material level. The screw can very easily lift and move large amounts of material, so the mixers are available in very large sizes, up to 100,000 liters (or 3,500 cubic feet). One important caveat about using a large mixer is that a very large screw must be supported at the bottom and will require seals and bearings that will be submerged in the material during mixing, which erases some of the benefit of the mixer.

Nauta conical screw mixer

Fig. 3: The shape of the conical screw mixer allows for easy full-batch discharge

Conical screw mixer disadvantages
A disadvantage of the conical screw mixer is that cohesive materials can stick to the screw, which may result in material carryover from one batch to another or increased equipment cleaning time between batches. The conical screw mixer tends to cost more than other mixer types because of the increased number of mechanical components. Also, the conical screw mixer tends to be taller than other mixer types, which can present difficulties in buildings with height restrictions.

Tumble mixer
A tumble mixer is a diffusion mixer that's essentially a hollow vessel that rotates on its horizontal axis. The vessel is partially filled with material and then slowly rotated so that friction between the material and the vessel wall carries the material up along the wall until gravity causes the material to cascade back down onto the material bed below. Ingredients diffuse through the blend across the top surface of the material bed to provide the mixing.

A tumble mixer requires the correct rotational speed to provide efficient, effective mixing. Slower rotation provides more gentle mixing but may be inefficient, while faster rotation mixes quickly but can be more damaging to fragile particles. If the mixer rotates too fast, the material won't fall down at all, which will result in an unmixed batch.

Tumble mixer advantages
Tumble mixers are simple and cost-effective. The mixer is totally sealed and contained, with no internal moving parts or seals. This means that all bearings and lubricants are removed from material contact, which provides long operating life. A tumble mixer has low abrasion because the material just moves up against the mixer wall and falls down on itself. The mixer's shape can vary to help randomize the mix, and, for specialized applications, internal baffles or agitators can be added to help disperse the material and change the angle at which the material falls.

Tumble mixer disadvantages
The downsides to tumble mixers are few, but one is that the mixers are only suitable for non-segregative materials. For some segregative materials, mixture quality may be capped after a certain mixing time, and overmixing may cause the blend to demix.

Another disadvantage of the tumble mixer is that it must be disconnected from upstream and downstream equipment during operation. Each time material is added to or discharged from the mixer, the inlet and outlet need to be reconnected and then disconnected again to allow the mixer to tumble. When either end is disconnected, the two openings could be potential sources of leakage, which can be a safety concern if the material is toxic or hazardous. Also, the mixer's tumbling movement can potentially be dangerous to workers during operation.

Ribbon mixer
A ribbon mixer is a convective mixer that has an internal rotating ribbon (or auger) that creates convection current mixing patterns within a stationary mixing trough. The trough is partially filled with material, usually to a point just above the center line, then the auger rotates, moving the ingredients in both a circular motion around the auger's axis and in a lateral motion along the trough.

Ribbon mixers come in various styles and are easily customizable to specific applications. Options include single or twin shafts and variations to the auger design (such as paddles instead of a ribbon) depending on how vigorously you need to mix the material or your desired mixing time. Ribbon mixers can also be set to different angles to generate different types and amounts of movement inside the trough.

Ribbon mixer advantages
Ribbon mixers are especially well-suited for segregative materials or other materials with varying particle size and bulk density. The mixer is available in a wide variety of sizes and designs and the auger can be replaced depending on the application. This allows a ribbon mixer to be used for multiple applications by simply swapping out the auger as needed.

Ribbon mixer disadvantages
In a ribbon mixer, seals are submerged in the material during mixing, so with fine or abrasive materials, particles will find a way into the seals around the mixer's auger shaft. Different seal options and mechanical designs are available to help prevent this, however. The ribbon mixer can also be difficult to empty. The material is normally discharged through a central port at the trough's bottom, so all material must be moved from the corners to the center of the trough for complete discharge. This often means that material is left behind. You can install "bomb doors" in the trough to empty the entire batch at once, but you then have to transport the batch all at once, which can cause segregation.

For further reading

It is sometimes essential that the polymers being produced have a consistent quality and are completely free of contamination. In such cases, it is a significant advantage if the consecutive process steps can be performed in a single system, i.e. without having to be moved to a different machine. Hosokawa Micron has achieved such a process for a certain customer in a vacuum dryer with a special rotor. During synthesis of the polymer in question, an alcohol forms which is rinsed away using a liquid chemical. Furthermore, the mixer contains other liquids in addition to the synthesised polymer. These liquids (80% of the mixture) are heated under vacuum to evaporate them. At the end of this process, the product temperature is raised gradually which not only removes the remaining liquid but also 'cures' the product.


Author: Chris Paulsworth, application engineer, chemicals/minerals division, Hosokawa Micron Powder Systems

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