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20 / September 2020 powderbulk.com • Up-to-date News from the bulk solids industry worldwide • Ask an Expert – Experts answer problems or issues you're having with your dry bulk solids processing or handling, or concerns about a new process or equipment you're considering. • Feature – A short, practical article on a typical powder and bulk solids concern. • Read It Here First – A preview of an article published in the next issue of PBE. • Industry, Supplier, Event, and Education News. • and much more for the bulk solids industry professional. www.powderbulk.com A twice-monthly Newsletter emailed right to you. CSC Publishing Inc • 1155 Northland Drive • Saint Paul, MN 55120-1288 USA • ONLINE Visit our website powderbulk.com and click on the PBE-News link. • EMAIL Email shighum@cscpub.com with your name, company and valid email. Issues includes: Sign up and get PBE-News delivered to your inbox. for this blended product. We'll have to evaluate a different mixer. PBE For further reading Find more information on this topic in articles listed under "Mixing and blending" in Powder and Bulk Engineering's article index in the December 2019 issue or the article archive on PBE's website, www.pow derbulk.com. James L. Davis (jimdavispe@ gmail.com, 513-503-8053) is a consulting engineer specializing in solving difficult powder processing problems and optimizing com- plex powder systems for efficient operation. He has spent 12 years as president of Powder Processing Solutions and was with Procter & Gamble for 26 years, 15 of them in powder processing. He holds a BS in mechanical engineering from the University of Cincinnati and is a registered professional engineer in the state of Ohio. James L. Davis Cincinnati, OH, 513-503-8053 powderprocessingsolutions.com have a mixing index approaching 1. But even if that's the case, we may have to consider other factors before selecting this mixer. For example, a product like acet- aminophen has to meet complex and specific FDA guidelines. One com- mon guideline is that all samples have to be within ±10 percent of the claimed active ingredient content. That's for the end product. Since some segregation may occur in the handling steps after the mixer, the target mixer variation should be closer to ±5 percent. This equates to the very high mixing index of 0.95. It means that every sample of powder should have between 309 and 341 milligrams of acetaminophen. If our first batch has excessive variations, more mixing time may be needed. If the excessive vari- ations primarily show up in the samples taken at the beginning and end of the mixer discharge, it may mean that the mixing dis- charge causes segregation. If none of the batches that were run meet the required 0.95 mixing index or if excessive variation at the beginning or end of discharge occurs, it means that the mixer chosen is unsuitable to do it again for additional blend times. Ultimately, all the samples will be gathered and sent off for lab analysis. The lab technicians, too, must take care to not create segregation. For example, let's say that we sent 0.5-gram samples. If the lab analy- sis requires only 50 milligrams of powder, then the technicians have to cut the samples properly. They can't simply scoop the powder off the top of each sample because the sample may have some segregation. Technicians have to use a tool such as a chute riffler or spinning riffler (devices designed to minimize segregation while splitting samples) to separate the powder and cut it down to a representative amount for testing. Evaluating the data When all the results are in, we need to evaluate the data as it will give us the mixture's experimen- tal random variance. (We'll revisit information on how to determine this in the Mixing Mechanics col- umn in the January 2021 issue of PBE.) With these figures, we can calculate the mixing index. With luck, at least one of our batches will