Some materials and combinations of materials will blend in almost any blender, producing a consistent, uniform mixture. Other material combinations are sensitive to a variety of atmospheric influences or to particle size variations, densities, frictions, cohesions, permeability, compressibility or chemical composition. They will either mix poorly initially or eventually demix or segregate in subsequent handling steps.
Demixing mechanisms rank as the primary reasons for blending process problems and failures. Each material is unique and each blender's ability to handle a given mixture is unique. Matching the blender to your materials is key to blending success.
This article describes a proven approach for identifying the causes of demixing problems and, thus, for avoiding or addressing them. It involves four steps:
1. measuring the flow properties of the particles;
2. evaluating the mixture's combined ingredients for their demixing tendencies;
3. assessing the mixture and the process; and
4. understanding the blender's potential to demix.
Material flow properties
The first step in troubleshooting a blender for demixing problems is testing of material properties. The flow properties of each ingredient and each mixture of ingredients determine how the materials behave when combined, how they affect the blending process, and whether or not the blender your process uses will perform as expected for your specific mixture.
A number of properties such as unconfined yield strength, bulk specific weight and angle of repose affect blending. More details on these properties and what can increase their magnitude appear in Table 1.
It can be difficult to measure some of these properties. So, Dr. Jerry R. Johanson developed a simplified system to relate solids' flow properties to more practical physical measurements. His eight indices, defined in Table 2, address such issues as the potential for arching and ratholing, and solids' build-up in chutes. The Johanson Indices are useful for predicting the tendency of blended ingredients to demix and, more generally, for selecting and designing material-handling equipment. More information about them is available from the author.
The combination of materials
Using the measured values of the indices as a guideline, it is possible to evaluate the ingredients in a mixture for their susceptibility to potential demixing mechanisms. The four most-common blending demixing mechanisms are angle of repose, sifting, fluidization, and air currents. The Johanson Indices can identify a mixture's propensity to demix due to each of these mechanisms.
Vertical Screw Blender
Figure 1. Corn meal and pellets as initially fed into a vertical screw blender.
is characterized by free-flowing materials that slide on each other during the mixing action. The material with the flatter repose angle slides freely on top of the material with the steeper repose angle to the bottom of a slope or pile, creating demixing problems with either the initial fill cycle or the discharge cycle. The Arching Index (AI) identifies the potential for this type of demixing. Usually, mixtures with a major ingredient or component that has an AI greater than 0.2 but less that 0.1 and a minor ingredient that has an AI less than 0.2 will experience this type of demixing.
is caused by fines that sift through coarse particles; the Arching Index also indicates its likelihood. Such demixing often is a problem when the major ingredient contains large, free-flowing particles and comprises two-thirds or more of the mixture, and the minor ingredient is also free flowing and accounts for less than one-third of the mixture. In such cases, demixing occurs whenever the blender imposes interparticle motion. Typically, mixtures that contain a major ingredient with an AI less that 0.2 and a minor ingredient with an AI of less than 0.2 will have sifting demixing problems.
arises when a mixture contains a fine, free-flowing major ingredient that fluidizes easily and a coarse, heavier minor ingredient that can easily penetrate a layer of fluidized fines. The Arching Index combined with the Flow Rate Index (FRI) is useful in determining when a mixture is likely to demix through fluidization. A fine major ingredient with an AI less than 0.2 and an FRI less than 100 combined with a minor ingredient with an AI much less than 0.2 and an FRI much greater than 100 has potential for fluidization demixing.
Air current demixing
always involves a superfine, free-flowing ingredient. During mixing, superfines can migrate to the blender's walls or toward a dust collection system. If the superfines are a minor ingredient, this migration can be significant. A mixture containing 90% superfines with an AI less than 0.2 is likely to have the greatest problem with air current demixing.
Table 3 summarizes these four demixing mechanisms and identifies the worst and best combinations of ingredients.
The mixture and the process
Once the material flow properties for each raw ingredient are identified, then using the data from steps one and two, you should evaluate your mixture and process.
For example, if any ingredient combination shows a propensity for angle-of-repose demixing, the solution for decreasing or eliminating this problem may be as simple as increasing the mixture's cohesion to lessen angle-of-repose demixing. That does not mean adding liquid, which will make the problem worse; however, premixing liquid with coarse ingredients before adding a finer ingredient will allow the finer component to stick to the coarser particles and, thus, will minimize angle-of-repose demixing.