Tumble devices are well suited to moderately cohesive materials that do not sift or segregate. Wide variations (say, more than 3:1) in particle size of free flowing materials can create blend variations or even discharge segregation. Blend time isn’t as critical as with many other devices but the process is long.
Paddle or ribbon blenders. There are many variations, with the differences primarily in the shape of the agitator. The paddles (Figure 3) can be flat blades at an angle on the shaft or large plows at right angles to the shaft. The plow has long been thought of as very effective for cohesive materials but the mixing relies on a tumbling action as the plows or paddles lift material. The biggest problem with a plow is the limited lateral mixing along the shaft. This can be overcome by feeding at one end and emptying at the other but segregation can occur as the solids go over the discharge weir.
Figure 3. These devices often get the nod for more cohesive solids or liquid addition.
Fine solids can accumulate in the clearance between the wall and paddle so that dead areas develop where the solids don’t move at the same speed as the bulk contents. This can give wide variations in blend quality even with a continuous blender. When evaluating a paddle blender, conduct long term tests to determine the extent of this segregation. High rpm paddle blenders are a specialized version of this device, but are best used in very light loadings and short residence times. Attrition can be severe.
Ribbon blenders (Figure 4) well suit batch mixes as the ribbons can move material in opposite directions, which adds tumbling to a back-and-forth flow. The most effective blenders for wide particle-size differences use ribbons of different pitch and diameters.
Figure 4. The level of fill usually significantly impacts both performance and attrition.
Overfilling and underfilling negatively affects performance. The optimum, which varies with the type of solids, generally is around two-thirds full — but attrition is highest at that level and needs to be considered. If a dedusting occurs after the blender, this may improve product quality by eliminating fine particles that would be generated in subsequent packaging or handling. Ribbons can achieve a quicker blend, and better blend in the vertical plane, but still are slower in the lateral plane (along the shaft).
The screw blender is a variation of the horizontal ribbon blender. It utilizes a screw along the wall of the cone that rotates around the cone as it pumps solids up the wall. Some cohesiveness helps the performance, especially with the addition of liquids. While blend times can be very low, segregation on discharge and sifting of fine particles can destroy the blend. There’s often an optimum agitator speed that gives a quick blend while minimizing sifting. However, finding it may take an extensive amount of testing. Once the right set of operating conditions have been determined, the screw blender can overcome the segregation effects.
In either paddle or ribbon devices, cohesive mixtures don’t do well at the extreme end of the blender, especially if the agitator clearance is large. High rotational speeds may fluidize the solids, resulting in clumps of fine particles that slough off the shell during discharge. Consider paddle blenders for more cohesive solids or when liquids are being added to the solids and ribbons for mixtures containing minor ingredients.
Both ribbon and paddle blenders often come with high-speed wall agitators, sometimes called choppers. For very cohesive materials these prevent lumps and can impart enough shear action to the particles that the surface is more active. This highly energetic surface will bind easier to the larger particles and give a quicker end to the blend. While some attrition may occur, the shorter blend time often makes this a wash.
Specialized mechanical devices. The ideal mechanical blender would be able to separate the individual particles and put them back together in the mixture based on the overall distribution of the particles. The riffle box separates a mixture into discrete compartments and then recombines the small compartments to give a smaller mass that represents the average of the original mass of material. While this can be repeated to achieve the desired production size or consumer quantity, the method isn’t very practical for very large production rates. However, some proprietary devices such as the Diamondback hopper approach this concept.
Another variation on blending is to fluidize the particulate solids as they are loaded from the storage tank. A high-speed ribbon or paddle that doesn’t accumulate solids (no vessel holdup) will provide localized mixing and can improve the filling of tank or rail cars and avoid excessive fine particles on the top of the fill. This is important because many clients open the top hatch of a shipment to take a grab sample for analysis. If the sample doesn’t meet specifications, even though the overall contents of the shipment do, the shipment may be rejected.
Flow tube blenders (Figure 5). The riffle concept has been applied with great success on the large scale to pelletized solids. Tubes, each with a hole at a different level, are placed in a silo. Each tube primarily will move solids from its upper hole to a small discharge bin.