The problem with particulate solids is they segregate every time we turn around. Why? Because they are a two-phase material and only about half is the solid we’re interested in. In my days as a troubleshooter at corporate engineering, the most common call I got from plants was about blenders that didn’t blend. The problems arose because people believed some myths.
The most common myth is that increased blending time results in a better blend. One of our customers mixed an inert ingredient with our product in a ribbon blender. Prior production with a different active ingredient gave an acceptable blend in 15 minutes — so the customer used that time for the new product. The resulting mix was highly variable in composition. To compensate, the plant increased the blend time to a half hour and then an hour with no improvement. In fact, the mix got worse. We found that seven minutes gave a perfect blend. What happened was that friction with the inert ingredient caused a surface charge to develop on the new active ingredient; this friction was unexpected. Most materials reach a perfect blend in a very short period of time.
Another common myth is that all blenders are created equal. We made a catalyst by an extrusion process that gave a slightly variable particle size. The catalyst was to be put into very long tubes that had to have the same pressure drop; blending was believed to smooth the distribution so the pressure drop would be uniform. Without conducting any flowability tests, the plant opted for an available twin-cone blender with 45° walls. Even after changing blend time, the product came out in linear order of size. Testing for angle-of-slide showed the 45° wall held the smaller particles and concentrated them on the top of the mix. Another twin-cone blender with a steeper cone and an internal ribbon solved the problem.
The idea that fluidization will mix solids well is another myth. Density and particle size determine how easily a material will fluidize or defluidize — a “Geldart” classification often is used as an indicator. Several ingredients were added to a blender that operated at very high speed to mix the materials. The speed was dropped to a crawl to aid in discharge. The mixture deaerated slowly and the ingredients separated almost in layers. All the ingredients but one were Geldart Group A. To demonstrate how the mixture responded, I put it in a graduated cylinder, which I shook. Then, I dropped into the mix a coin, which went all the way to the bottom. A half-hour later, I dropped another coin, which went half way down. It took two hours before I could drop a coin and have it stay on top. Jogging the blender (short fluidization times) and increasing the discharge speed maintained the blend and avoided segregation. In this case, fluidization was working well but keeping the solids fluidized was the real problem.
Many other myths exist involving agitator type, multiple ribbons and attrition in blenders. It’s hard to mechanically move particulate solids without incurring some sort of damage, so it’s often better to avoid blending. However, sometimes attrition in a blender can be put to good use. We had developed a new disinfectant product that outperformed our current offering. However, it was lighter and caused problems for the formulator. We added a twin-ribbon blender to the production line to slightly grind the product, which allowed the final material to match the density of the former product.
Not only dry blenders suffer from these types of problems. Crystallizers, solid/liquid mixers and conveyors can have similar issues. Particulate solids want to settle in a fluid whether gas or liquid, so always keep that in mind.