Mixing up a batch: Batch mixers types and selection tips
By Warren O. Fuller
Paul O. Abbe

The right batch mixer for your process is the one that helps you produce the best product at the least expense. This article explains how to find that mixer. Sections cover batch versus continuous mixing, batch mixer types and operation, other applications, and selection advice.

In mixing, your objective is to form a homogeneous final product from two or more dissimilar raw materials. But the chemical composition, density, particle size, color, and other characteristics of each raw material can vary. And to form an acceptable product, your mixing process may need to achieve other goals, such as redistributing the raw materials or reducing larger particles.

So before you choose a batch mixer, identify your expectations. A good way to start is by having a clear understanding of how batch mixing works.

Batch Versus Continuous Mixing

In batch mixing, a volume of material is loaded into the mixer, the mixer is operated, and the final mixture is unloaded so another batch can be loaded into the vessel. By contrast, in continuous mixing, material flows steadily from an upstream process into the mixer, is retained in the vessel for a specified mixing time, and discharges at the same flowrate for downstream handling.

Typically you can choose from several mixer types that are available in both batch and continuous modes. In this case, you must determine whether a batch or continuous unit is best for your process. For instance, if your material is from a batch process upstream from the mixer, the most efficient mixer mode would be batch.

But some mixer types are available in only one mode. If you have a continuous upstream process and must choose a batch mixer, there are ways to handle the continuous flow. For instance:

• You can collect the material in a hopper and discharge it in batches to the mixer.
• You can use multiple batch mixers that are sequenced to handle the continuous flow.
• You can use a hybrid continuous-batch system in which surge hoppers hold material from a continuous upstream process to allow rapid batch mixing without lengthy downtime for material handling.

For many industries and applications batch processing is preferred, either because of the ability to trace an individual batches, without sacrificing production capacity or creating a system bottleneck, or because of other unit operations requirements.

Several types of batch mixers are available. They typically either tumble or agitate material to form the final mixed batch.

Tumble Blenders

Batch tumble blenders rotate, tumbling the material to achieve mixing. The blenders are available in various geometries, affecting each unit's material movement, mixing efficiency, and ease of between-batch cleaning.

Tumbling blenders are best suited to mixing dry powders and granules and can include liquid addition components for coating and similar applications. However, excessive moisture can cause the material to agglomerate and form lumps that will impede mixing. To break up the lumps, the units can be equipped with delumping components (such as internal baffles or agitators). From the outlet, the batch can be discharged via an optional retractable sleeve into drums or other containers for shipping.

A tumbling batch blender can be one of two basic types:

• Double-cone blender.
• V-shaped blender.

Double-cone blender (Rota-Cone)
The Rota-Cone Blender consists of two cone-shaped sections typically with 45-degree slopes. The cone sections are welded at their wide ends to a center band. The blender is mounted between two trunnions that permit the unit to tumble end over end. An opening in one cone's end can serve as both the inlet and outlet, or the inlet can be in one cone's end with the outlet in the other. Cleaning access is through the outlet. As with any tumble blender, the unit can be equipped with a spray line for liquid addition and an agitator for delumping or introduction of high shear if required (eg, for granulation). The spray line with various nozzles designed for precise atomization at different viscosities and the agitator each extend from one trunnion into the vessel. Each function is controlled independently for maximum process control.

In operation, material is loaded to fill level of typically 50 to 60 percent of the blender's capacity. The blender tumbles, and the material in the vessel spreads out. The transition area at the band between the cones prevents the material from sliding along the inner wall and instead causes the material to fold over itself. This provides gentle mixing with only very slight shear.

At the 50 to 60 percent fill level, the blender typically achieves a mixture in 5 to 10 minutes with a 95 percent or better homogeneity. But at any fill level between 35 and 75 percent (or within the blender's center band), the blender will eventually obtain an acceptable mixture. (For a batch mixer, the fill level determines the batch size.)

The blender is ideal for soft powders or granules that degrade by attrition. At a fill level of 50 percent or higher, the agitator can typically delump agglomerated material during mixing. The smooth transition area in the vessel also simplifies cleaning because there are no areas to trap particles and cross-contaminate batches.

A variant of the standard double cone blender is the Off-Set Rota-Cone Blender - utilizing the same basic geometry as the Rota-Cone but with the center-band slanted to ensure that the material along the sides is shifted away from the vessel walls to effect a "figure 8" type motion during tumbling that allows for a slightly higher fill level than in the standard Rota-Cone design. All other features are the same as the standard Rota-Cone.

In operation, material is loaded to a fill level of typically 50 to 60 percent of the blender's capacity. The unit tumbles end over end like the double-cone blender, but the mixing action is different because of the vessel shape. As the V-shaped blender tumbles, the material continuously splits and recombines, with the mixing occurring as the material free-falls randomly inside the vessel. Mixing is achieved in 5 to 15 minutes with a 95 percent or better homogeneity.

The free-falling action combined with increased frictional contact between the material and the vessel's long, straight sides result in slightly less gentle mixing than a double-cone unit. This can create slight shear, producing some particle size reduction through attrition.

The blender is suited to most powders and is often used for pharmaceuticals, but the mixing action's slight shear limits the blender's use for some very soft powders or granules. Cleaning between batches takes longer in this blender than in the double-cone unit.

Agitated Batch Mixers

Agitatied batch mixers are available with various geometries, agitators, and agitation patterns. Depending on the type, the mixers can handle materials ranging from dry powders to non-flowing and gritty, viscous pastes. The mixers can be equipped with components for liquid addition and delumping. The batch typically discharges from the mixer outlet to a mechanical conveyor for transfer to bagging or packaging.

Some common types of agitating batch mixers are:

• Ribbon mixers.
• Plow mixers.
• Fluidizing paddle mixers.
• Sigma blade mixers.

Ribbon mixer
The ribbon mixer typically has a U-shaped trough with a rotating single-shaft, double-helix agitator and a large top cover. (A twin-trough unit with a double-shaft agitator is also available - especially for the more difficult materials that tend to "pack" during mixing.) The agitator's shaft is mounted in the trough center and has welded spokes. Inner and outer sets of ribbons (also called spirals) are welded to the spokes, conforming to the trough shape. The clearance between each ribbon's outer edge and the trough wall typically range from 1/8 to 1/4 inch. The inlet is located in the vessel's cover; and the discharge valve is typically located in the trough's bottom center, but can also be at either end of the trough.

A spray bar for adding liquids can be mounted above the ribbons. To reduce chunky feed or agglomerates that the ribbons can't break apart, the mixer walls can be fitted with spinning high-speed choppers. The cover can be taken off for cleaning access.

In operation, material is filled to the top of the outer ribbon's rotation. The shaft rotates at relatively low speed, and the moving ribbons push the material back and forth, mixing it. With a center discharge, the inner ribbons move the material toward the trough ends, and the outer ribbons push the material in the opposite direction toward the center discharge valve.

Mixing is achieved in 15 to 20 minutes with a 90 to 95 percent or better homogeneity. If so required, the high-speed choppers are operated intermittently to break down agglomerates and reduce the overall mixing time.

The mixer can handle various materials ranging from powders to some slurries. However, the mixer's pushing action can wedge particles between the ribbons and trough wall, creating shear that can reduce a soft material's particle size. The unit is also suitable to applications with limited headroom. Because of its intricate configuration, the agitator requires extensive cleaning to prevent batch cross-contamination; the agitator can remain in the mixer or be removed for cleaning.

A common variation of the ribbon mixer - the paddle mixer - has flat paddles rather than ribbons. The mixer operates similarly but the paddles tend to last longer and are suited to mixing abrasive materials. Large particles are also less likely to wedge between the paddles and trough wall, which can prevent material degradation.

Plow mixer
The plow mixer has a stationary cylindrical vessel and a rotating shaft mounted in the vessel's center. Several plows are welded to the shaft. An inlet is at the vessel's top; a discharge valve is located at the bottom center. Spray nozzles can be mounted near the vessel top. As with the ribbon mixer, the mixer walls can also be fitted with high-speed choppers. Large side doors provide cleaning access.

In operation, material is loaded through the top inlet. The shaft rotates at higher speed, producing a plow tip speed of more than 700 fpm to effect a fluidizing action. The plows throw the material upward, and the material settles in a random pattern. The plow shape aids material movement toward the center discharge. The mixer can blend the material in less than 5 minutes with a 95 to 98 percent or better homogeneity.

The mixer can handle most of the same materials as a ribbon mixer, but because the combination of high tip speed and plow shape applies more shear to the material that the ribbon mixer does, the plow mixer can't handle soft powders or granules. If so equipped, the high-speed choppers can be operated intermittently to reduce agglomerates and the overall mixing time. The mixer is also suited to applications with restricted headroom.

Sigma blade mixer
The sigma blade mixer consists of twin troughs, each fitted with a rotating agitator. Each agitator is a heavy-duty Z-shaped blade. The inlet is located in the mixer's top cover. The mixer can be tilted to one side for discharge or can be equipped with a screw discharge at the bottom between the troughs. A spray bar can be mounted above the blades and cleaning access is through the top cover.

In operation, material is loaded through the inlet to typically 40 to 65 percent of the mixer's capacity. The blades can overlap and operate at the same speed or can run tangentially at different speeds. In either case, the blades force material onto the sharp edge between the troughs, producing very high shear and reducing any large particles. The mixing action produces a mixture in 10 to 30 minutes with a 99 percent or better homogeneity. The mixing action also produces a consistent particle size distribution without requiring an additional delumping agitator.

The mixer handles pasty, sticky, and gritty slurries with viscosities up to 2,000,000 centipoise. Examples are rubber, carbide powders, silicones, caulking compounds and other industrial applications. Sanitary designs are available for food and pharmaceutical applications (chocolates, chewing gums and other pastes). When equipped with a screw discharge, difficult materials can be more easily discharged.

Other applications

Most batch mixers can be modified to dry and cool materials as well as mix them. The modifications include adding heat or cooling to the mixer vessel, operating the vessel under a vacuum, or both.

Batch mixer selection tips

When choosing a batch mixer for your application, consider these basics:

• What the material's consistency and particle size distribution will be throughout the mixing cycle: as the raw materials are fed to the mixer, during mixing, and after mixing.
• How much variation in the final mixture's homogeneity is acceptable.
• What variation in batch size the mixer must handle.
• Whether your mixture will require liquid addition.
• Whether the mixture will require mechanical agitation, shear or delumping.
• How much plant floor space and headroom are available for the mixer and related equipment.
• What handling equipment is required before and after the mixer.