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The previous article “Introduction to Mixing Technology" defined “mixing" and “blending" operations and discussed the differences between the two. The importance of mixing in industrial applications was highlighted along with a brief introduction to the different types of mixing and blending equipment.
This article highlights the differences between the mixing of liquids and the blending of solids and emphasizes the challenges associated with the blending of granular and powdered solid materials. The different types of solid blending mechanisms and the blend structures are discussed. A good understanding of these concepts is important for the selection of suitable blending equipment for solid materials.
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Differences between Liquid Mixing and Solid Blending
The mixing of low-viscosity liquids is due to the creation of flow currents that transport unmixed material to the mixing zone adjacent to the impeller. Liquid mixing has been a subject of extensive research and is well documented. The improvements in liquid mixing technology have made it possible to produce a well mixed, truly homogenous liquid mix with uniform composition.
Though the blending of solids to some extent resembles the mixing of low-viscosity liquids, there are significant differences between the two processes. A solid material cannot attain the perfect mixing that is possible with liquids. In case of solids, there are no flow currents. The three primary mechanisms of blending are diffusion, convection, and shear. These three mechanisms occur to varying extents depending on the type of mixers or blenders and the characteristics of the solids to be blended.
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Mechanisms of Solid Blending: Diffusion, Convection, and Shear
Diffusion blending is characterized by the small scale random motion of solid particles. Blender movements increase the mobility of the individual particles and thus promote diffusive blending. Diffusion blending occurs where the particles are distributed over a freshly developed interface. In the absence of segregating effects, the diffusive blending will in time lead to a high degree of homogeneity.
Tumbler blenders like the double cone blenders and v-blenders function by diffusion mixing.
For rapid blending, in addition to the fine-scale diffusion blending there should be a means by which large quantities of particles can be intermixed. This is accomplished by either convection or shear mechanisms.
Convection blending is characterized by the large scale random motion of solid particles. In convection blending groups of particles are rapidly moved from one position to another due to the action of a rotating agitator or to cascading of material within a tumbler blender.
The blending of solids in ribbon blenders, paddle blenders, and plow mixers is mainly a result of convection mixing.
Some texts define shear blending as the development of slip planes or shearing strains within a bed of material. Others define the blending mechanism of shear as high intensity impact or splitting of the bed of material to disintegrate agglomerates or overcome cohesion. For the purpose of this discussion we shall use the latter definition. Shear blending is very effective at producing small-scale uniformity generally on a localized basis.
Blenders with high speed chopper blades and intensifiers are an examples of shear blending.
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Types of Blend Structures
There are two types of blend structures: Structured and Random.
Structured, ordered, or interactive blends are observed in most industrial processes. Here, the different blend components interact with one another by physical, chemical, or molecular means or a combination of these resulting in agglomeration or coating. In granulation, large particle agglomerates are formed from smaller particles. The large agglomerates thus formed are comprised of a uniform blend of smaller building block particles. These agglomerates may either be of uniform size or of different sizes. A blend of agglomerates of uniform size will not segregate after discharge from the blender. However, when the agglomerates are of different sizes, then segregation by size may occur and result in problems like variation in bulk density and reactivity in post-blend processing. In some cases, especially in fine materials such as carbon black or fumed silica, the blend components have a tendency to adhere only to themselves, without adhering to the dissimilar components. For blending of these materials, shear blending mechanisms are adopted.
When the different blending components do not adhere or bind to each other within the blender, the result is a random blend structure. In a random blend the individual particles are free to move relative to each other and hence there is no bonding with each other. As a result, dissimilar particles readily segregate from each other under the influence of external forces like gravity and vibration and collect in zones of similar particles, e.g. a blend of salt and pepper. Completely random blends are rarely encountered in industrial applications.
Blender manufactures need to account for these material behaviors during the selection and design of blending equipment.
Blending Mechanisms and Blend Structures for Solids
This articles in this series shall focus on Industrial Mixing Technology and shall discuss the following: -Design and Construction of Mixers -Selection of Mixers for Different Applications -Case Studies of Improvements in Mixing through innovations in Mixer Design