Mixing and blending processes with bulk solids are routinely carried out in all types of industrial applications. In order to be cost-competitive, blending equipment should have fast blend times (to reduce bottlenecks), batch size flexibility, easy-to-clean features, and reliable discharge without de-mixing (i.e., segregation) to prevent undoing the work of the blender.
Eric P. Maynard
Challenging bulk solids can be mixed via the basic three mechanisms of blending, namely convection, diffusion, and shear. In most cases, a combination of these mechanisms results during blending operations and optimized blend times can be achieved. Unfortunately, first-principles formulations taking into account particle and blend properties, as well as equipment configurations and operating parameters, are not currently available to allow direct calculation of cycle time. Mathematical approaches using discrete element modeling (DEM) techniques are improving, but their results to date are still based on crude particle approximations and do not yet accurately incorporate the multiphysics phenomena of collisions, electrostatics, van der Waals forces, and cohesion.
In an attempt to ensure product uniformity, most industrial operations “overblend” their mixes, thereby resulting in increased energy and labor costs, and possible undesirable phenomena like particle attrition (generation of fines), and heating. For example, consider a tumble blending operation with 300 rotations of a 75-cu-ft blender at 20 rpm. This results in 15 minutes of blend time. If only 200 rotations are required, running the blender for an additional 100 rotations not only costs 50% more electrical energy, but may also negatively affect the blend by excessive heating or particle attrition. Additionally, the cycle time is increased, thus tying up the blender and increasing overall production costs specific to the unit operation. With liquid mixing, overblending usually only results in additional energy costs; with solids overblending, often segregation of the blend can take place.
Besides the obvious costs of energy and labor with overblending, undesired particle effects can occur. Excessive particle-to-particle collisions can result in reduction of particle size (create fines) which can adversely affect blend performance. Consider lubricant addition with a direct compression blend of pharmaceutical powder. Magnesium stearate requires careful addition to the final blend, without too much dispersion. Done improperly, tablet dissolution can be affected.
So, how do you know if you are overblending? The answer is “quality sampling.” A sample thief is commonly used to collect powder samples from a blender. A thief is a metal rod with recessed cavities capable of receiving powder after being inserted into a powder bed. Care must be taken 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., insertion angle, rate, twisting, etc.).
Improvements can be achieved with stratified sampling and statistical analysis to distinguish realistic blend variability from sampling error. Instead of sampling once or twice in a blender with a thief, multiple (e.g., three) thief samples should be extracted from the same location and then repeated throughout several separate locations in the blender, especially in known “dead-zones,” like the central core or at the walls. After analysis of these multiple samples, assessments can be made to within-location versus between-location variability. If the 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 from different points in the mixer, then it is likely that the blend is not yet complete and additional time or agitation will be required. On the horizon, new 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.
Do not forget that blending and segregation 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 downstream of the blender.
Eric P. Maynard is a senior consultant at Jenike & Johanson (Tyngsboro, MA) and a member of Powder/Bulk Solids’ editorial advisory board. He has worked on hundreds of projects involving cement, minerals, chemicals, coal, resins, foods, and pharmaceuticals.