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.
Sifting demixing can be resolved by using major and minor ingredients having particles of the same size or by ensuring that the major ingredient has particles smaller than those of the minor ingredient. Another approach is to add a little liquid to the coarser ingredient or to introduce into the mixture a fine, cohesive ingredient, which may be sufficient to reduce demixing.
Anything that reduces fluidization of fines will decrease this demixing mechanism. Lowering the blender's agitation speed, cutting air or adding liquid to the mixture will help, as will pre-agglomerating fines fractions.
The best way to thwart air current demixing is to add moisture, particularly in the form of a fine spray, during the mixing cycle, or to add liquid to the coarser ingredient before introducing the finer ingredient. Avoiding dust collection during mixing or reducing the agitator speed will also improve the process.
Figure 2. After 100 revolutions in the blender, corn meal and pellets show noticeable angle-of-repose demixing.
Angle-of-repose demixing -- which is easily identifiable from the layers of coarse particles and fines, even after prolonged mixing times -- is a problem with all blenders except air-pulse units and horizontal- and vertical-shaft high-speed paddle mixers. Rotating-shell or tumble blenders are particularly susceptible to angle-of-repose demixing. Likewise, if your process uses a V-blender, angle-of-repose demixing will only worsen; if the mixture cannot be modified to include a cohesive ingredient, then modifying the outlet to cause bottom-to-top discharge will lessen angle-of-repose effects.
Unless steps are taken to optimize a mixture through ingredient modification, all blenders except the air-pulse design will produce some sifting demixing, and even the air-pulse blender will demix on discharge. For example, vertical-shaft impeller mixers create a vortex action that may leave a coarse-particle layer on top that then sifts through a finer layer. The ribbon blender may produce a high concentration of coarse particles on discharge, as will the screw mixer.
Air, plough and high-speed paddle and ribbon blenders are highly susceptible to fluidization demixing. Materials with a low FRI will fluidize in a screw mixer. If your system already uses one of these designs, then the best defense against fluidization demixing is, as previously suggested, to add liquid to the mixture or to control the rotational speed.
Though effective for sifting and angle-of-repose demixing mechanisms, air-pulse blenders are susceptible to air current demixing. Fines that strike the surface of a dust collector or are suspended in the upper portion of the blender will deposit on the top of the mixture's surface again and again. However, if the ingredient particles are nearly equal in size, then the surface will contain about the same mix as the rest of the blender's content. Vertical-shaft impeller mixers can whip up superfines that stick on the mixer's walls above the material level. Ingredients blended in high-speed plough and paddle mixers are also susceptible to air-current demixing if the mixture is free flowing and has at least one superfine ingredient. If any mixture's ingredient has a high potential for air-current demixing, you should avoid dust collection during mixing and lower the agitator speed.
Gravity-flow blenders, such as cone-in-cone units, are subject to all four demixing mechanisms; however, adjusting the flow pattern on discharge to create a first-in/first-out pattern can eliminate angle-of-repose and sifting demixing.
Table 4 summarizes typical blenders, mechanisms, demixing dangers, and flow properties. Of course, details on individual blenders and mixers and blending processes may modify these indications.
Although demixing is the main reason for blending process failure, also consider other important blender details such as flow velocity, slow-moving or dead regions, clearances between moving parts and stationary walls, ease of cleanout, and adaptability to various mixtures. Subsequent handling steps in conveying systems, feeders and discharge chutes are also critical to maintain a mixture's stability.
Figure 3. After 50 revolutions, mung beans and bone acid experience significant sifting demixing.
Regardless of the blender or blending system you now use, the only sure way to correct blending demixing problems is to analyze your mixture's flow properties. Doing so provides a sound basis for any ingredient, operation or blender modification.
Lee Dudley is president of Diamondback Technology, Inc., San Luis Obispo, Calif., a firm that specializes in equipment and consulting related to solids handling.