Mixing and blending of bulk solids occurs frequently in many industrial processes. Though small-scale mixing and blending functions were in use back in the early days of humankind (e.g., mixing flour, salt, yeast, and water to make bread), today’s competitive production lines necessitate robust processes capable of fast blend times, equipment flexibility, ease of cleaning, and assurances that demixing (i.e., segregation) does not result in a material that has just been blended.
The terms mixing and blending are often used interchangeably; however, they technically can be considered slightly different. Mixing is defined as the process of thoroughly combining different materials to achieve a homogenous product. In most cases, the mixture is a combination of dissimilar materials (e.g., sugar and salt); though at times, a chemically homogenous material is mixed to uniformly distribute its large range of particle sizes. Blending is also an act of combining materials; however, this operation usually occurs in a gentle fashion with multiple components (e.g., blending virgin plastic powder with impact modifiers, colorants, and flame retardants).
Industries such as pharmaceutical and food rely heavily on mixing and blending technology. In the pharmaceutical industry, small amounts of a powdered active drug are carefully blended with excipients such as starch, cellulose, lactose, and lubricants. In the food industry, many powdered consumer products result from custom mixed batches; consider cake mix, iced tea, or curry (a blend of many fine spices).
Over the past 25 years, mixing and blending technology has effectively evolved to address the following critical requirements: larger batch sizes, faster blend times, and segregation minimization.
The purpose of this brief article is to discuss the basic mixing and blending technology, factors driving technology improvements, blend sampling, and segregation.
Common Blending Technology
There are three primary mechanisms of blending: convection, diffusion, and shear. Convective blending involves gross movement of particles through the mixer either by a force action from a paddle or by gentle tumbling under rotational effects. Diffusion is a slow blending mechanism and will pace a blending process in certain tumbling mixers if proper equipment fill order and method are not utilized. Lastly, the shear mechanism of blending involves thorough incorporation of material passing along forced slip planes in a mixer. Often these mixers will infuse a liquid or powdered binder into the blend components to achieve a special consistency, such as granulates.
There are also four main types of mixing and blending equipment: tumbler, convective, hopper, and fluidization. The tumble blender is a mainstay in the pharmaceutical and food industries because of its positive features of close quality control (batch operation only), effective convective and diffusive mechanisms of blending, and gentle mixing for friable particles. This type of rotating blender comes in double-cone or vee-shaped configurations, and in some cases these geometries are given asymmetric features to reduce blend times and improve blend uniformity. Generally these blenders operate at a speed of 5 to 25 revolutions per minute with a range of 50 to 75% fill level. A variety of manufacturers can supply this equipment, including Patterson Kelly (PK) and Paul O. Abbe.
Fig. 1: Charles Ross and Son ribbon blender
Convection blenders use a fixed shell with an internal rotating element (impeller) like a ribbon, paddle, or plow (see Fig. 1). Due to the action of the impeller, the particles are moved rapidly from one location to another within the bulk of the mixture. These blenders work well with cohesive materials, which normally take substantially longer blend times in tumbling-type mixers. They also have the advantages of taking up less headroom, allowing liquid addition, and the potential for continuous operation instead of only batch mixing as with tumble blenders. Also, these blenders are less likely to experience blend segregation during discharge because the impellers typically operate during this process. Though many equipment companies can make these blenders, more-common vendors include Charles Ross & Son, Hayes & Stolz, Scott Equipment, and Forberg.
Hopper blenders are usually cone-in-cone to tube-type units, where particles flow under the influence of gravity in a contact-bed fashion. With the former unit, the inner cone produces a pronounced faster flow through the inner hopper as compared to the outer annulus section, thereby allowing moderate blending of material. These hoppers typically require two to four passes with a recirculation system to achieve proper uniformity. Tube blenders utilize open pipes within a bin; the pipes have notches in them to allow partial material flow in and out of the tubes over the height of the bin or for reintroduction into a lower portion of the bin (such as in a mixing chamber). These blenders can handle much larger volumes of material than tumbling or convective blenders since no freeboard space is required and their technology can be applied to storage bins or silos.
Fluidization mixers use high flow rates of gas to fully fluidize powders in order to rapidly blend components. The gas can also be used to process (e.g., heat, cool) the blend. Vendors like Nol-Tec or Dynamic Air can provide this technology. Not all powder blends are well-suited for fluidization mixing. For example, candidates should be fine, free-flowing powders that have a narrow size distribution and are close in particle density. The Forberg mixer (Fig. 2) combines fluidization and convective features, yielding rapid blend times with a high degree of blend uniformity.
Fig. 2: Dynamic Air’s Forberg mixer
Factors Driving Technology Improvements
In today’s competitive production environment, faster blend times can be a cure for the common process bottleneck. A faster blend time can be achieved through use of new equipment (e.g., changing from a tumble to a ribbon blender) or through modification to the blending operation (e.g., increasing blender speed, using intromitters/agitators in tumblers, reviewing the blending end-point). For years, the blender has commonly been the bottleneck in unit operations. Inherent in the blending cycle, the blender obviously cannot be filled or discharged while actively mixing, thus, two of the three processes remain in check until the blending is completed.
Fig. 3: In-bin tumble blender from Paul O. Abbe
To address this inefficiency, tumbling in-bin blenders (Fig. 3) have been developed where the storage container (called an IBC) itself becomes a blender. Therefore, blend components can be loaded into the container, blended, and transferred in the container to point of use or to a storage area. This process leads to highly flexible production, and has been popular in the pharmaceutical, food, and powdered-metal industries. Vendors such as Matcon, Tote Kinetics, and LB Bohle can provide this type of technology. In-bin tumble blending is likely the foremost technology improvement that has occurred in the past 25 years. For many, the greatest benefit of this technology is its elimination of a transfer step from a blender into a container, by which segregation by various mechanisms can result.
Additional benefits include: no cleaning between batches and the blend is stored in a sealed container until use. Optimum in-bin tumble blenders incorporate mass-flow technology (all of the material is in motion whenever any is discharged) to ensure the blend does not segregate during container discharge.
Blend uniformity needs formal documentation in the pharmaceutical industry. Though other industries may not be as strict with their batch records, in most cases samples are extracted from a batch or continuous blender to ensure the mix meets critical specifications. A sample thief is commonly used to collect powder samples from a blender or container such as a drum or bin. A thief is a metal rod with recessed cavities capable of receiving powder after being inserted into a powder bed. Care must be made with thief-collected samples because this method will disturb the powder sample in-situ, and some blend components may flow preferentially or stick to the thief cavity. Studies have shown that thief sampling results can be dependent on operator technique (e.g., thief insertion angle, penetration rate, angle, twisting, etc.).
An improvement on thief sampling can be achieved with stratified (nested) sampling and statistical analysis to address realistic blend variability from sampling error (from the thief, laboratory analysis, or collection method). Instead of sampling two or three times in a blender with a thief, multiple (three to five) thief samples should be extracted from the same location and then repeated throughout several separate locations in the vessel, especially in known dead zones like the central core or at the blender walls. After analysis of these multiple samples, assessments can be made to within-location versus between-location variability. If the three to five samples collected at the same point have large degrees of variability, then questions should be raised regarding the thief or analytical testing method. If large variability exists between the samples collected around the vessel, then it is likely that the blend is not yet complete and additional time or agitation will be required.
On the horizon, new blender and process sampling techniques are being developed through the use of NIR (near infrared), NMR (nuclear magnetic resonance), and optical methods to determine degrees of blend uniformity. Though these technologies are showing promise, wide-scale industrial use has not yet been proven.
Blending and segregation (demixing) are competing processes. A general rule of thumb is that every time a transfer step is added to a process, the powder blend can segregate. Even a perfect blend does not guarantee a perfect product since segregation can, and often does, occur. Powder segregation mechanisms include sifting, fluidization, and dusting.
Fig. 4: Sifting segregation mechanism
Sifting segregation (Fig. 4) results when fine particles concentrate in the center of a bin or drum during filling, while the more coarse particles roll to the pile’s periphery. If discharge from this segregated pile occurs from the central core, then a concentration of fine particles will occur, eventually followed by the coarse material.
With fluidization segregation, finer, lighter particles can rise to the top surface of a fluidized blend of powder, while the larger, heavier particles concentrate at the bottom of the bed. In this case, the fluidizing air entrains the lower-permeability fines and carries them to the top surface. This mechanism generally only occurs with powders with an average particle size smaller than 100 US mesh.
Lastly, dusting segregation concentrates the ultrafine and fine particles at a container’s walls or at points furthest from the incoming stream of material. Dusting segregation is a common problem with fine pharmaceutical and food powders being discharged from blenders into drums, tableting press hoppers, and packaging equipment surge hoppers.
Bench scale testers (ASTM D6940-03, D6941-03) can be used to determine a material’s segregation potential, whether caused by sifting or fluidization mechanisms. Once the segregation potential has been measured, the segregation problem can be analyzed and solved.
Over the past 25 years, mixing and blending technology has greatly improved to address common requirements such as larger batch sizes, faster blend times, and segregation minimization. Although many blenders are capable of mixing all kinds of powders, the process of selecting a blender remains an art form because of the many variables involved; a first-principles (i.e., mathematical) approach is still lacking. However, knowledge gains in the area of sampling and segregation have allowed a more holistic approach to the typical blending unit operation, thereby often preventing problems with the uniformly blended material after it has been discharged from the mixer.
Eric Maynard is a senior consultant at Jenike & Johanson, an engineering consulting firm that specializes in the storage, flow, pneumatic conveying, and processing of powder and bulk solids. Maynard has designed handling systems for bulk solids, including cement and the raw materials used in its manufacture; coal; resins; foods; and pharmaceuticals. He received a BS in mechanical engineering from Villanova University (Villanova, PA) and an MS in mechanical engineering from Worcester Polytechnic Institute (Worcester, MA). He can be reached at 978-649-3300 or [email protected].