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16 / April 2021 powderbulk.com PBE MIXING MECHANICS James L. Davis, Powder Processing Solutions H ow well blended is your powder mixture? Is mix- ing it well even possible? Are you sure that your mixer is blending the powder properly, or is it segregating your mixture while trying to mix it? In this column, we'll talk about a good mixture. Liquids and gases are mobile. Most liquids and gases have no structure but powders do. Powders transfer stresses from one particle to the next to give them structure and less mobility than liquids and gases have. Even within powders, differences in structure and mobil- ity can lead to incomplete mixing or segregated mixing. Figures 1a and 1b represent a combination of two ingredients with an equal number of particles (represented as dots). Figure 1a shows that the ingre- dients are completely segregated before they've been mixed. Figure 1b shows a perfectly ordered mix. The square outlines in Figure 1b each represent a 16-particle sample; no matter where the 4-particle- by-4-particle square is drawn, the sample will have exactly 8 green and 8 blue particles. Unfortunately, in actual powder mixing, achieving this perfectly ordered mix is impos- sible. The best that can be achieved is a random mix. Figures 2a and 2b show two random mixes. Figure 2a shows a good random mix in which a 16-particle sample might have 10 blue particles or 6 blue particles — or even exactly 8 blue particles. But some mixers or downstream handling systems can segregate the particles, as shown in Figure 2b. Here, the blue and green particles are separated enough to yield a large majority of blue particles in some samples and a significant minority of blue particles in other samples. The key to powder mixing is to get a good random mix that doesn't segregate after mixing. Scale of scrutiny The figures also illustrate another key point: Sample size is critical to evaluating whether a powder has a random mix. If we change the sam- ple size to 100 particles (the entire number shown in each image), then both Figures 2a and 2b would actu- ally be well mixed. At a 100-particle sample size, each system has 50 blue and 50 green particles. It's not important whether the blue and green particles are mixed within the sample; we only care about the whole sample. Sample size is determined by the scale of scrutiny — that is, the sample size that must be evaluated to determine if a mix- ture meets its quality goals. Scale of scrutiny is critical to evaluating a good mix. To better understand scale of scrutiny, let's say that your company is making a sugar-free The importance of scale of scrutiny 973-261-8806 • kason.com SCALP SIFT, CLASSIFY, DEWATER & DEDUST SIFT SCALP, DEWATER AND DE-LUMP DRY COOL OR MOISTURIZE Dry bulk solids and slurries in 1 to 5 fractions—from several pounds/kilograms to 70+ tons/h—with VIBROSCREEN circular vibratory screeners. Dry bulk materials and solids- laden slurries—at rates from several pounds/kilograms to 70+ tons/h—with Quick-Clean CENTRI-SIFTER centrifugal sifters. Built to handle both batch and continuous drying, cooling and agglomerating of bulk materials—using minimal energy—with ultra-eˆ cient, self- contained, Quick- Clean, VIBRO-BED Circular Fluid Bed Systems. Solutions Sourcebook Provides Case Histories for Processing Equipment The Food and Beverage Solutions Sourcebook provides in-depth examples of challenges other processors have faced and the solutions Kason provided.